Process for producing aromatic carbonates

A process for producing aromatic carbonates, which comprises transesterifying, in the presence of a metal-containing catalyst, a starting material selected from a dialkyl carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant selected from an aromatic monohydroxy compound, an alkyl aryl carbonate and a mixture thereof, characterized in that: at least one type of catalyst-containing liquid is taken out, wherein the catalyst-containing liquid is selected from a portion of a high boiling point reaction mixture obtained by the above transesterification and containing the desired aromatic carbonate and the metal-containing catalyst, and a portion of a liquid catalyst fraction obtained by separating the desired aromatic carbonate from the high boiling point reaction mixture, wherein each portion contains (A) high boiling point substance having a boiling point higher than the boiling point of the produced aromatic carbonate and (B) the metal-containing catalyst; (C) a functional substance capable of reacting with at least one component selected from high boiling point substance (A) and metal-containing catalyst (B) is added to the taken-out catalyst-containing liquid; and the (B)/(C) reaction product is recycled to the reaction system, while withdrawing the (A)/(C) reaction product. By the process of the present invention, the desired aromatic carbonates having high purity can be produced stably for a prolonged period of time.

BACKGROUND OF THE INVENTION
 1. Field of The Invention
 The present invention relates to a process for producing aromatic
 carbonates. More particularly, the present invention is concerned with a
 process for producing aromatic carbonates, which comprises
 transesterifying, in the presence of a metal-containing catalyst, a
 starting material selected from the group consisting of a dialkyl
 carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant
 selected from the group consisting of an aromatic monohydroxy compound, an
 alkyl aryl carbonate and a mixture thereof, characterized in that:
 at least one type of catalyst-containing liquid is taken out,
 the catalyst-containing liquid being selected from the group consisting of:
 a portion of a high boiling point reaction mixture obtained by the above
 transesterification and containing the desired aromatic carbonate and the
 metal-containing catalyst, and
 a portion of a liquid catalyst fraction obtained by separating the desired
 aromatic carbonate from the high boiling point reaction mixture,
 each portion containing (A) high boiling point substance having a boiling
 point higher than the boiling point of the produced aromatic carbonate and
 containing (B) the metal-containing catalyst;
 (C) a functional substance capable of reacting with at least one component
 selected from the group consisting of the high boiling point substance (A)
 and the metal-containing catalyst (B) is added to the taken-out
 catalyst-containing liquid, to thereby obtain at least one reaction
 product selected from the group consisting of an (A)/(C) reaction product
 and a (B)/(C) reaction product; and
 the (B)/(C) reaction product is recycled to the reaction system, while
 withdrawing the (A)/(C) reaction product.
 According to the process of the present invention, disadvantageous
 phenomena, such as the accumulation of the high boiling point substance
 (A) in the reaction system which causes the discoloration of an ultimate
 aromatic polycarbonate (which is produced from an aromatic carbonate), can
 be prevented without withdrawing the catalyst from the reaction system so
 that the desired aromatic carbonates having high purity can be produced
 stably for a prolonged period of time.
 2. Prior Art
 An aromatic carbonate is useful as a raw material for, e.g., the production
 of an aromatic polycarbonate (whose utility as engineering plastics has
 been increasing in recent years) without using poisonous phosgene. With
 respect to the method for the production of an aromatic carbonate, a
 method for producing an aromatic carbonate or an aromatic carbonate
 mixture is known, in which a dialkyl carbonate, an alkyl aryl carbonate or
 a mixture thereof is used as a starting material and an aromatic
 monohydroxy compound, an alkyl aryl carbonate or a mixture thereof is used
 as a reactant, and in which a transesterification reaction is performed
 between the starting material and the reactant.
 However, since this type of transesterification is a reversible reaction in
 which, moreover, not only is the equilibrium biased toward the original
 system but the reaction rate is also low, the production of an aromatic
 carbonate by the above-mentioned method on an industrial scale is
 accompanied with great difficulties.
 To improve the above-mentioned method, several proposals have been made,
 most of which relate to the development of a catalyst for increasing the
 reaction rate. As a catalyst for use in the method for producing an alkyl
 aryl carbonate, a diaryl carbonate or a mixture thereof by reacting a
 dialkyl carbonate with an aromatic hydroxy compound, there have been
 proposed various metal-containing catalysts, which include for example, a
 Lewis acid, such as a transition metal halide, or compounds capable of
 forming a Lewis acid, [see Unexamined Japanese Patent Application
 Laid-Open Specification No. 51-105032, Unexamined Japanese Patent
 Application Laid-Open Specification No. 56-123948 and Unexamined Japanese
 Patent Application Laid-Open Specification No. 56-123949 (corresponding to
 West German Patent Application Publication No. 2528412, British Patent No.
 1499530 and U.S. Pat. No. 4,182,726)], a tin compound, such as an
 organotin alkoxide or an organotin oxide [Unexamined Japanese Patent
 Application Laid-Open Specification No. 54-48733 (corresponding to West
 German Patent Application Publication No. 2736062), Unexamined Japanese
 Patent Application Laid-Open Specification No. 54-63023, Unexamined
 Japanese Patent Application Laid-Open Specification No. 60-169444
 (corresponding to U.S. Pat. No. 4,554,110 and West German Patent
 Application Publication No. 3445552), Unexamined Japanese Patent
 Application Laid-Open Specification No. 60-169445 (corresponding to U.S.
 Pat. No. 4,552,704 and West German Patent Application Publication No.
 3445555), Unexamined Japanese Patent Application Laid-Open Specification
 No. 62-277345, and Unexamined Japanese Patent Application Laid-Open
 Specification No. 1-265063 (corresponding to European Patent Publication
 No. 338760 and U.S. Pat. No. 5,034,557)], salts and alkoxides of an alkali
 metal or an alkaline earth metal (Unexamined Japanese Patent Application
 Laid-Open Specification No. 56-25138), lead compounds (Unexamined Japanese
 Patent Application Laid- Open Specification No. 57-176932), complexes of a
 metal, such as copper, iron or zirconium (Unexamined Japanese Patent
 Application Laid-Open Specification No. 57-183745), titanic acid esters
 [Unexamined Japanese Patent Application Laid-Open Specification No.
 58-185536 (corresponding to U.S. Pat. No. 4,410,464 and West German Patent
 Application Publication No. 3308921)], a mixture of a Lewis acid and
 protonic acid [Unexamined Japanese Patent Application Laid-Open
 Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501 and
 West German Patent Application Publication No. 3445553)], a compound of
 Sc, Mo, Mn, Bi, Te or the like [Unexamined Japanese Patent Application
 Laid-Open Specification No. 1-265064 (corresponding to European Patent
 Publication No. 0 338 760 A1 and U.S. Pat. No. 5,034,557)], and ferric
 acetate (Unexamined Japanese Patent Application Laid-Open Specification
 No. 61-172852).
 As a catalyst for use in the method for producing a diaryl carbonate by a
 same-species intermolecular transesterification, wherein an alkyl aryl
 carbonate is disproportionated to a dialkyl carbonate and a diaryl
 carbonate, there have been proposed various catalysts, which include for
 example, a Lewis acid and a transition metal compound which is capable of
 forming a Lewis acid [see Unexamined Japanese Patent Application Laid-Open
 Specification No. 51-75044 (corresponding to West German Patent
 Application Publication No. 2552907 and U.S. Pat. No. 4,045,464)], a
 polymeric tin compound [Unexamined Japanese Patent Application Laid-Open
 Specification No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110 and
 West German Patent Application Publication No. 3445552)], a compound
 represented by the formula R--X(=O)OH (wherein X is selected from Sn and
 Ti, and R is selected from monovalent hydrocarbon residues) [Unexamined
 Japanese Patent Application Laid-Open Specification No. 60-169445
 (corresponding to U.S. Pat. No. 4,552,704 and West German Patent
 Application Publication No. 3445555)], a mixture of a Lewis acid and
 protonic acid [Unexamined Japanese Patent Application Laid-Open
 Specification No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501 and
 West German Patent Application Publication No. 3445553)], a lead catalyst
 [Unexamined Japanese Patent Application Laid-Open Specification No.
 1-93560 (corresponding to U.S. Pat. No. 5,166,393)], a titanium or
 zirconium compound (Unexamined Japanese Patent Application Laid-Open
 Specification No. 1-265062), a tin compound [Unexamined Japanese Patent
 Application Laid-Open Specification No. 1-265063 (corresponding to U.S.
 Pat. No. 5,034,557 and European Patent Publication No. 0 338 760)], and a
 compound of Sc, Mo, Mn, Bi, Te or the like [Unexamined Japanese Patent
 Application Laid-Open Specification No. 1-265064 (corresponding to U.S.
 Pat. No. 5,034,557 and European Patent Publication No. 0 338 760)].
 Another attempt for improving the yield of aromatic carbonates in these
 reactions consists in biasing the equilibrium toward the product system as
 much as possible, by modifying the mode of the reaction process. For
 example, there have been proposed a method in which by-produced methanol
 is distilled off together with an azeotrope forming agent by azeotropic
 distillation in the reaction of a dimethyl carbonate with phenol [see
 Unexamined Japanese Patent Application Laid-Open Specification No.
 54-48732 (corresponding to West German Patent Application Publication No.
 2736063 and U.S. Pat. No. 4,252,737) and Unexamined Japanese Patent
 Application Laid-Open Specification No. 61-291545], and a method in which
 by-produced methanol is removed by adsorbing the same onto a molecular
 sieve [Unexamined Japanese Patent Application Laid-Open Specification No.
 58-185536 (corresponding to U.S. Pat. No. 4,410,464 and West German Patent
 Application Publication No. 3308921)].
 Further, a method is known in which an apparatus comprising a reactor
 having provided on the top thereof a distillation column is employed in
 order to separate and distill off alcohols (by-produced in the course of
 the reaction) from a reaction mixture obtained in the reactor. [With
 respect to this method, reference can be made to, for example, Unexamined
 Japanese Patent Application Laid-Open Specification No. 56-123948
 (corresponding to U.S. Pat. No. 4,182,726 and West German Patent
 Application Publication No. 2528412), Unexamined Japanese Patent
 Application Laid-Open Specification No. 56-25138, Unexamined Japanese
 Patent Application Laid-Open Specification No. 60-169444 (corresponding to
 U.S. Pat. No. 4,554,110 and West German Patent Application Publication No.
 3445552), Unexamined Japanese Patent Application Laid-Open Specification
 No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704 and West German
 Patent Application Publication No. 3445555), Unexamined Japanese Patent
 Application Laid-Open Specification No. 60-173016 (corresponding to U.S.
 Pat. No. 4,609,501 and West German Patent Application Publication No.
 3445553), Unexamined Japanese Patent Application Laid-Open Specification
 No. 61-172852, Unexamined Japanese Patent Application Laid-Open
 Specification No. 61-291545, and Unexamined Japanese Patent Application
 Laid-Open Specification No. 62-277345.]
 As more preferred methods for producing an aromatic carbonate, the present
 inventors previously developed a method in which a dialkyl carbonate and
 an aromatic hydroxy compound are continuously fed to a continuous
 multi-stage distillation column to effect a continuous transesterification
 reaction in the distillation column, while continuously withdrawing a low
 boiling point reaction mixture containing a by-produced alcohol from an
 upper portion of the distillation column by distillation and continuously
 withdrawing a high boiling point reaction mixture containing a produced
 alkyl aryl carbonate from a lower portion of the distillation column [see
 Unexamined Japanese Patent Application Laid-Open Specification No.
 3-291257 (corresponding to U.S. Pat. No. 5,210,268 and European Patent
 Publication No. 0 461 274)], and a method in which an alkyl aryl carbonate
 is continuously fed to a continuous multi-stage distillation column to
 effect a continuous transesterification reaction in the distillation
 column, while continuously withdrawing a low boiling point reaction
 mixture containing a by-produced dialkyl carbonate by distillation and
 continuously withdrawing a high boiling point reaction mixture containing
 a produced diaryl carbonate from a lower portion of the distillation
 column [see Unexamined Japanese Patent Application Laid-Open Specification
 No. 4-9358 (corresponding to U.S. Pat. No. 5,210,268 and European Patent
 Publication No. 0 461 274)]. These methods for the first time realized
 efficient, continuous production of an aromatic carbonate. Thereafter,
 various methods for continuously producing an aromatic carbonate have
 further been developed, based on the above-mentioned methods developed by
 the present inventors. Examples of these methods include a method in which
 a catalytic transesterification reaction is performed in a column reactor
 [see Unexamined Japanese Patent Application Laid-Open Specification No.
 6-41022 (corresponding to U.S. Pat. No. 5,362,901 and European Patent
 Publication No. 0 572 870), Unexamined Japanese Patent Application
 Laid-Open Specification No. 6-157424 (corresponding to U.S. Pat. No.
 5,334,724 and European Patent Publication No. 0 582 931), Unexamined
 Japanese Patent Application Laid-Open Specification No. 6-184058
 (corresponding to U.S. Pat. No. 5,344,954 and European Patent Publication
 No. 0 582 930)], a method in which use is made of a plurality of reactors
 which are connected in series [Unexamined Japanese Patent Application
 Laid-Open Specification No. 6-234707 (corresponding to U.S. Pat. No.
 5,463,102 and European Patent Publication No. 0 608 710 A1), and
 Unexamined Japanese Patent Application Laid-Open Specification No.
 6-263694], a method in which a bubble tower reactor is used [Unexamined
 Japanese Patent Application Laid-Open Specification No. 6-298700
 (corresponding to U.S. Pat. No. 5,523,451 and European Patent Publication
 No. 0 614 877)], and a method in which a vertically long reactor vessel is
 used (Unexamined Japanese Patent Application Laid-Open Specification No.
 6-345697).
 Also, there have been proposed methods for producing an aromatic carbonate
 stably for a prolonged period of time on a commercial scale. For example,
 Unexamined Japanese Patent Application Laid-Open Specification No.
 6-157410 (corresponding to U.S. Pat. No. 5,380,908 and European Patent
 Publication No. 0 591 923 A1) discloses a method for producing aromatic
 carbonates from a dialkyl carbonate and an aromatic hydroxy compound,
 which comprises continuously supplying a mixture of raw materials and a
 catalyst to a reactor provided with a distillation column thereon to
 effect a transesterification reaction in the reactor, while continuously
 withdrawing a by-produced aliphatic alcohol from the reactor through the
 distillation column by distillation so as to keep the aliphatic alcohol
 concentration of the reaction system at 2% by weight or less. This prior
 art document describes that, by this method, continuous production of an
 aromatic carbonate can be performed in a stable manner. The object of this
 method is to avoid the deposition of the catalyst in the distillation
 column. Further, Patent Application prior-to-examination Publication
 (Kohyo) No. 9-11049 (corresponding to WO 97/11049) discloses a process for
 producing an aromatic carbonate, in which the transesterification is
 conducted while maintaining a weight ratio of an aromatic polyhydroxy
 compound and/or a residue thereof to the metal component of the
 metal-containing catalyst at 2.0 or less, with respect to a
 catalyst-containing liquid-phase mixture in a system for the
 transesterification, so that the desired aromatic carbonates can be
 produced stably for a prolonged period of time without suffering
 disadvantageous phenomena, such as the deposition of the catalyst.
 On the other hand, it is known that when an aromatic carbonate is produced
 by transesterification, high boiling point substances are likely to be
 by-produced. For example, Unexamined Japanese Patent Application Laid-Open
 Specification No. 61-172852 discloses that when diphenyl carbonate is
 produced by a transesterification of dimethyl carbonate with phenol, an
 impurity having a boiling point equal to or higher than the boiling point
 of the produced diphenyl carbonate is by-produced, and that the impurity
 is caused to enter the diphenyl carbonate and causes the discoloration of
 an ultimate product, such as an aromatic polycarbonate. This prior art
 document does not dis-close an example of the impurity having a boiling
 point equal to or higher than the boiling point of the produced diphenyl
 carbonate; however, as an example of the impurity, there can be mentioned
 an aryloxycarbonyl-(hydroxy)-arene which is produced as an isomer of a
 diaryl carbonate by Fries rearrangement. More specifically, when diphenyl
 carbonate is produced as the diaryl carbonate, a phenyl salicylate can be
 mentioned as an example of the aryloxycarbonyl-(hydroxy)-arene. Phenyl
 salicylate is a high boiling point substance whose boiling point is 4 to
 5.degree. C. higher than the boiling point of the diphenyl carbonate.
 In this case, when the transesterification is conducted for a long period
 of time, the above-mentioned high boiling point substance accumulates in
 the reaction system and the amount of the impurity mixed into the ultimate
 aromatic carbonate tends to increase, so that the purity of the ultimate
 aromatic carbonate is lowered. Further, as the amount of the high boiling
 point substance in the reaction mixture increases, the boiling point of
 the reaction mixture rises, so that the by-production of the high boiling
 point substance is accelerated, thus rendering it difficult to produce
 desired aromatic carbonates stably for a prolonged period of time. As a
 measure for solving the problems, it is conceivable to withdraw a high
 boiling point substance-containing reaction mixture from the reaction
 system, thereby preventing the accumulation of the high boiling point
 substance in the reaction system. However, by this measure, a disadvantage
 is brought about in that, when a catalyst which is soluble in the reaction
 liquid is used, both the catalyst and the high boiling point substance are
 present in a state dissolved in the reaction mixture, so that, for
 separating the catalyst from the high boiling point substance by a
 conventional distillation method, it is necessary to heat the reaction
 mixture at high temperatures, leading to a further increased formation of
 by-products. Therefore, it is difficult to separate the catalyst from the
 high boiling point substance. This means that the withdrawal of the high
 boiling point substance from the reaction system is inevitably accompanied
 by the discharge of the catalyst. Accordingly, for continuing the
 reaction, it is necessary to supply a fresh catalyst to the reaction
 system. As a result, a large quantity of the catalyst is needed.
 SUMMARY OF THE INVENTION
 In this situation, for solving the above-mentioned problems accompanying
 the prior art, the present inventors have made extensive and intensive
 studies. As a result, it has been found that:
 in a process for producing aromatic carbonates which comprises
 transesterifying, in the presence of a metal-containing catalyst, a
 starting material selected from the group consisting of a dialkyl
 carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant
 selected from the group consisting of an aromatic monohydroxy compound, an
 alkyl aryl carbonate and a mixture thereof,
 when use is made of a process characterized in that:
 at least one type of catalyst-containing liquid is taken out,
 the catalyst-containing liquid being selected from the group consisting of
 a portion of a high boiling point reaction mixture obtained by the above
 transesterification and containing the desired aromatic carbonate and a
 metal-containing catalyst, and a portion of a liquid catalyst fraction
 obtained by separating the desired aromatic carbonate from the high
 boiling point reaction mixture, wherein each portion containing high
 boiling point substance (A) having a boiling point higher than the boiling
 point of the produced aromatic carbonate and containing the
 metal-containing catalyst (B);
 a functional substance (C) capable of reacting with at least one component
 selected from the group consisting of the high boiling point substance (A)
 and the metal-containing catalyst (B) is added to the taken-out
 catalyst-containing liquid, to thereby obtain at least one reaction
 product selected from the group consisting of an reaction product (A)/(C)
 and a reaction product (B)/(C); and
 the reaction product (B)/(C) is recycled to the reaction system directly or
 indirectly, while withdrawing the high boiling point substance without
 withdrawing the catalyst from the reaction system,
 disadvantageous phenomena, such as the accumulation of the high boiling
 point substance (A) in the reaction system which causes the discoloration
 of an ultimate aromatic polycarbonate (which is produced from an aromatic
 carbonate), can be prevented, so that a high purity aromatic carbonate can
 be stably produced for a prolonged period of time. The present invention
 has been completed, based on the above finding.
 Accordingly, it is a primary object of the present invention to provide an
 improved process for producing an aromatic carbonate, which comprises
 transesterifying, in the presence of a metal-containing catalyst, a
 starting material selected from the group consisting of a dialkyl
 carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant
 selected from the group consisting of an aromatic monohydroxy compound, an
 alkyl aryl carbonate and a mixture thereof, wherein the desired high
 purity aromatic carbonates can be produced stably for a prolonged period
 of time without suffering above-mentioned disadvantageous phenomena.
 That is, according to the process of the present invention, the high
 boiling point substance can be selectively discharged from the reaction
 system, so that the concentration of the high boiling point substance in
 the reaction system can be maintained at a level below a certain value and
 hence aromatic carbonates having high purity can be produced. Further,
 since the catalyst can be recycled, not only can the necessary amount of
 the catalyst be remarkably reduced, but also the occurrence of a
 catalyst-containing waste containing a high boiling point substance, which
 used to occur in the conventional technique for the withdrawal of a high
 boiling point substance out of the reaction system, can be prevented.
 The foregoing and other objects, features and advantages of the present
 invention will be apparent from the following detailed description and
 appended claims taken in connection with the accompanying drawings.

DESCRIPTION OF REFERENCE NUMERALS
 1, 101, 201: continuous multi-stage distillation column
 2, 102, 202: top of the continuous multi-stage distillation column
 3, 5, 7, 9, 10, 12, 13, 15, 15', 16, 18, 19, 20, 20', 21, 23, 25, 27, 28,
 29, 30, 32, 34, 35, 37, 39, 40, 41, 44, 45, 46, 48, 48', 49, 51, 53, 55A,
 56, 59A, 58, 60, 61, 63, 105, 113, 115, 115', 116, 118, 119, 120, 121,
 124, 125, 127, 128, 129, 130, 132, 149, 224, 225, 227, 228, 229, 230, 232,
 233, 235: conduit
 4: preheater
 6, 106, 206: bottom of the continuous multi-stage distillation column
 8: evaporator
 11, 22, 26, 127, 226, 234: condenser
 14, 114: evaporator
 17, 31, 117, 131: reboiler
 24, 43, 54, 62: distillation column
 33: thin-film evaporator
 36, 47, 59: storage vessel
 38: electric furnace
 42, 50, 55, 100: reaction vessel
 DETAILED DESCRIPTION OF THE INVENTION
 In the present invention, there is provided a process for producing
 aromatic carbonates, which comprises:
 (1) transesterifying a starting material selected from the group consisting
 of a dialkyl carbonate represented by the formula (1)
 ##STR1##
 an alkyl aryl carbonate represented by the formula (2)
 ##STR2##
 and a mixture thereof with a reactant selected from the group consisting of
 an aromatic monohydroxy compound represented by the formula (3)
EQU Ar.sup.1 OH (3),
 an alkyl aryl carbonate represented by the formula (4)
 ##STR3##
 and a mixture thereof,
 wherein each of R.sup.1, R.sup.2 and R.sup.3 independently represents an
 alkyl group having 1 to 10 carbon atoms, an alicyclic group having 3 to 10
 carbon atoms or an aralkyl group having 6 to 10 carbon atoms, and each of
 Ar.sup.1, Ar.sup.2 and Ar.sup.3 independently represents an aromatic group
 having 5 to 30 carbon atoms,
 in the presence of a metal-containing catalyst which is soluble in a
 reaction system comprising the starting material and the reactant and
 which is present in a state dissolved in the reaction system, to thereby
 obtain a high boiling point reaction mixture comprising the
 metal-containing catalyst and at least one aromatic carbonate which is
 produced by the transesterification and which corresponds to the starting
 material and the reactant and is selected from the group consisting of an
 alkyl aryl carbonate represented by the formula (5)
 ##STR4##
 and a diaryl carbonate represented by the formula (6)
 ##STR5##
 wherein R and Ar are, respectively, selected from the group consisting of
 R.sup.1, R.sup.2 and R.sup.3 and selected from the group consisting of
 Ar.sup.1, Ar.sup.2 and Ar.sup.3 in correspondence to the starting material
 and the reactant,
 while withdrawing a low boiling point reaction mixture which contains a low
 boiling point by-product comprising an aliphatic alcohol, a dialkyl
 carbonate or a mixture thereof corresponding to the starting material and
 the reactant and represented by at least one formula selected from the
 group consisting of ROH and
 ##STR6##
 wherein R is as defined above,
 (2) separating the high boiling point reaction mixture into a product
 fraction comprising the produced aromatic carbonate and a liquid catalyst
 fraction comprising the metal-containing catalyst, and
 (3) recycling the liquid catalyst fraction to the reaction system while
 withdrawing the product fraction,
 characterized in that the process further comprises:
 (1') taking out at least one type of catalyst-containing liquid which is
 selected from the group consisting of:
 a portion of the high boiling point reaction mixture before the separation
 of the high boiling point reaction mixture into the product fraction and
 the liquid catalyst fraction, and
 a portion of the separated liquid catalyst fraction,
 each portion containing (A) at least one high boiling point substance
 having a boiling point higher than the boiling point of the produced
 aromatic carbonate and containing (B) the metal-containing catalyst,
 (2') adding to the taken-out catalyst-containing liquid a functional
 substance (C) capable of reacting with at least one component selected
 from the group consisting of the component (A) and the component (B), to
 thereby obtain at least one reaction product selected from the group
 consisting of an (A)/(C) reaction product and a (B)/(C) reaction product,
 and
 (3') recycling the (B)/(C) reaction product to the reaction system directly
 or indirectly, while withdrawing the (A)/(C) reaction product.
 For an easy understanding of the present invention, the essential features
 and various preferred embodiments of the present invention are enumerated
 below.
 1. A process for producing aromatic carbonates, which comprises:
 (1) transesterifying a starting material selected from the group consisting
 of a dialkyl carbonate represented by the formula (1)
 ##STR7##
 an alkyl aryl carbonate represented by the formula (2)
 ##STR8##
 and a mixture thereof with a reactant selected from the group consisting of
 an aromatic monohydroxy compound represented by the formula (3)
EQU Ar.sup.1 OH (3),
 an alkyl aryl carbonate represented by the formula (4)
 ##STR9##
 and a mixture thereof,
 wherein each of R.sup.1, R.sup.2 and R.sup.3 independently represents an
 alkyl group having 1 to 10 carbon atoms, an alicyclic group having 3 to 10
 carbon atoms or an aralkyl group having 6 to 10 carbon atoms, and each of
 Ar.sup.1, Ar.sup.2 and Ar.sup.3 independently represents an aromatic group
 having 5 to 30 carbon atoms,
 in the presence of a metal-containing catalyst which is soluble in a
 reaction system comprising the starting material and the reactant and
 which is present in a state dissolved in the reaction system, to thereby
 obtain a high boiling point reaction mixture comprising the
 metal-containing catalyst and at least one aromatic carbonate which is
 produced by the transesterification and which corresponds to the starting
 material and the reactant and is selected from the group consisting of an
 alkyl aryl carbonate represented by the formula (5)
 ##STR10##
 and a diaryl carbonate represented by the formula (6)
 ##STR11##
 wherein R and Ar are, respectively, selected from the group consisting of
 R.sup.1, R.sup.2 and R.sup.3 and selected from the group consisting of
 Ar.sup.1, Ar.sup.2 and Ar.sup.3 in correspondence to the starting material
 and the reactant,
 while withdrawing a low boiling point reaction mixture which contains a low
 boiling point by-product comprising an aliphatic alcohol, a dialkyl
 carbonate or a mixture thereof corresponding to the starting material and
 the reactant and represented by at least one formula selected from the
 group consisting of ROH and
 ##STR12##
 wherein R is as defined above,
 (2) separating the high boiling point reaction mixture into a product
 fraction comprising the produced aromatic carbonate and a liquid catalyst
 fraction comprising the metal-containing catalyst, and
 (3) recycling the liquid catalyst fraction to the reaction system while
 withdrawing the product fraction,
 characterized in that the process further comprises:
 (1') taking out at least one type of catalyst-containing liquid which is
 selected from the group consisting of:
 a portion of the high boiling point reaction mixture before the separation
 of the high boiling point reaction mixture into the product fraction and
 the liquid catalyst fraction, and
 a portion of the separated liquid catalyst fraction,
 each portion containing (A) at least one high boiling point substance
 having a boiling point higher than the boiling point of the produced
 aromatic carbonate and containing (B) the metal-containing catalyst,
 (2') adding to the taken-out catalyst-containing liquid a functional
 substance (C) capable of reacting with at least one component selected
 from the group consisting of the component (A) and the component (B), to
 thereby obtain at least one reaction product selected from the group
 consisting of an (A)/(C) reaction product and a (B)/(C) reaction product,
 and
 (3') recycling the (B)/(C) reaction product to the reaction system directly
 or indirectly, while withdrawing the (A)/(C) reaction product.
 2. The process according to item 1 above, wherein the portion of the high
 boiling point reaction mixture is from 0.01 to 10% by weight, based on the
 weight of the high boiling point reaction mixture, and wherein the portion
 of the separated liquid catalyst fraction is from 0.01 to 40% by weight,
 based on the weight of the separated liquid catalyst fraction.
 3. The process according to item 1 or 2 above, wherein the high boiling
 point substance (A) originates from at least one compound selected from
 the group consisting of the starting material, the reactant, impurities
 contained in the starting material and the reactant, and by-products of
 the transesterification reaction.
 4. The process according to item 3 above, wherein the high boiling point
 substance (A) is at least one substance selected from the group consisting
 of an aromatic hydroxy compound (7), a compound (8) derived from the
 compound (7), an aromatic carboxy compound (9), a compound (10) derived
 from the compound (9), and xanthone,
 wherein:
 compound (7) is represented by the formula (7):
 ##STR13##
 wherein Ar.sup.4 represents an aromatic group having a valence of m, m
 represents an integer of 2 or more, and each --OH group is independently
 bonded to an arbitrary ring-carbon position of the Ar.sup.4 group,
 compound (8) contains a residue represented by the formula (8):
 ##STR14##
 wherein Ar.sup.4 and m are as defined for formula (7), n represents an
 integer of from 1 to m, and each of the --OH group and the --O-- group is
 independently bonded to an arbitrary ring-carbon position of the Ar.sup.4
 group,
 compound (9) is represented by the formula (9):
 ##STR15##
 wherein Ar.sup.5 represents an aromatic group having a valence of r, r
 represents an integer of 1 or more, s represents an integer of from 0 to
 (r-1), and each of the --OH group and the --COOH group is independently
 bonded to an arbitrary ring-carbon position of the Ar.sup.5 group, and
 compound (10) contains a residue represented by the formula (10):
 ##STR16##
 wherein Ar.sup.5, r and s are as defined for formula (9), t is an integer
 of from 0 to s, u is an integer of from 0 to (r-s), with the proviso that
 t and u are not simultaneously 0, and each of the --OH group, the --COOH
 group, the --O-- group and the --(COO)-- group is independently bonded to
 an arbitrary ring-carbon position of the Ar.sup.5 group.
 5. The process according to any one of items 1 to 4 above, wherein the
 functional substance (C) is an oxidizing agent, so that the (A)/(C)
 reaction product is a low boiling point oxidation product and the (B)/(C)
 reaction product is a metal oxide.
 6. The process according to any one of items 1 to 4 above, wherein the
 functional substance (C) is a precipitant, so that the (B)/(C) reaction
 product is a metal-containing substance which precipitates.
 7. The process according to item 6 above, wherein the metal-containing
 substance is a metal compound selected from the group consisting of a
 metal carbonate, a metal hydroxide, a metal oxide, a metal sulfide and a
 metal sulfate.
 8. The process according to any one of items 1 to 4 above, wherein the
 functional substance (C) is a reactive solvent, so that the (A)/(C)
 reaction product is a low boiling point product obtained by the solvolysis
 of component (A).
 9. The process according to item 8 above, wherein the reactive solvent is
 water, so that the (A)/(C) reaction product is an aromatic monohydroxy
 compound obtained by the hydrolysis of component (A).
 10. The process according to any one of items 1 to 9 above, wherein the
 steps (1), (2) and (3) are continuously performed, thereby continuously
 producing an aromatic carbonate.
 11. The process according to item 10 above, wherein the starting material
 and the reactant are continuously fed to a continuous multi-stage
 distillation column to effect a transesterification reaction therebetween
 in at least one phase selected from the group consisting of a liquid phase
 and a gas-liquid phase in the presence of the metal-containing catalyst in
 the distillation column, while continuously withdrawing a high boiling
 point reaction mixture containing the produced aromatic carbonate in a
 liquid form from a lower portion of the distillation column and
 continuously withdrawing a low boiling point reaction mixture containing
 the low boiling point by-product in a gaseous form from an upper portion
 of the distillation column by distillation.
 12. A process for producing aromatic polycarbonates, which comprises
 subjecting to transesterification polymerization an aromatic carbonate
 produced by the process according to any one of items 1 to 11 above and an
 aromatic dihydroxy compound.
 The process of the present invention for producing an aromatic carbonate
 from the above-mentioned starting material and reactant by
 transesterification in the presence of a metal-containing catalyst is
 characterizd in that:
 at least one type of catalyst-containing liquid is taken out,
 the catalyst-containing liquid being selected from the group consisting of:
 a portion of a high boiling point reaction mixture obtained by the above
 transesterification and containing the desired aromatic carbonate and a
 metal-containing catalyst, and
 a portion of a liquid catalyst fraction obtained by separating the desired
 aromatic carbonate from the high boiling point reaction mixture,
 each portion containing high boiling point substance (A) having a boiling
 point higher than the boiling point of the produced aromatic carbonate and
 containing the metal-containing catalyst (B);
 a functional substance (C) capable of reacting with at least one component
 selected from the group consisting of the high boiling point substance (A)
 and the metal-containing catalyst (B) is added to the taken-out
 catalyst-containing liquid, to thereby obtain at least one reaction
 product selected from the group consisting of an (A)/(C) reaction product
 and a (B)/(C) reaction product; and
 the (B)/(C) reaction product is recycled to the reaction system, while
 withdrawing the (A)/(C) reaction product.
 As described above, when the metal-containing catalyst soluble in the
 reaction system is used, the separation of the high boiling point reaction
 mixture into the catalyst (B) and the high boiling point substance (A) by
 the conventional techniques is difficult.
 Therefore, the recycling of only the catalyst (B) to the reaction system
 was conventionally impossible.
 In the process of the present invention, by reacting the
 catalyst-containing liquid containing a high boiling point substance (A)
 and a metal-containing catalyst (B) with a functional substance (C), an
 (A)/(C) reaction product and/or a (B)/(C) reaction product can be
 obtained. The separation between the (A)/(C) reaction product and the
 (B)/(C) reaction product can be easily performed. Thus, it has for the
 first time been possible to withdraw the high boiling point substance (A)
 out of the reaction system, while recycling the catalyst (B) to the
 reaction system.
 The present invention is described below in detail.
 The dialkyl carbonate used as a starting material in the present invention
 is represented by formula (1):
 ##STR17##
 wherein R.sup.1 represents an alkyl group having 1 to 10 carbon atoms, an
 alicyclic group having 3 to 10 carbon atoms or an aralkyl group having 6
 to 10 carbon atoms. Examples of R.sup.1 include an alkyl group, such as
 methyl, ethyl, propyl (isomers), allyl, butyl (isomers), butenyl
 (isomers), pentyl (isomers), hexyl (isomers), heptyl (isomers), octyl
 (isomers), nonyl (isomers), decyl (isomers) and cyclohexylmethyl; an
 alicyclic group, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl
 and cycloheptyl; and an aralkyl group, such as benzyl, phenethyl
 (isomers), phenylpropyl (isomers), phenylbutyl (isomers) and methylbenzyl
 (isomers). The above-mentioned alkyl group, alicyclic group and aralkyl
 group may be substituted with a substituent, such as a lower alkyl group,
 a lower alkoxy group, a cyano group and a halogen atom, as long as the
 number of carbon atoms of the substituted group does not exceed 10, and
 may also contain an unsaturated bond.
 As a dialkyl carbonate having such R.sup.1, there may be mentioned for
 example, dimethyl carbonate, diethyl carbonate, dipropyl carbonate
 (isomers), diallyl carbonate, dibutenyl carbonate (isomers), dibutyl
 carbonate (isomers), dipentyl carbonate (isomers), dihexyl carbonate
 (isomers), diheptyl carbonate (isomers), dioctyl carbonate (isomers),
 dinonyl carbonate (isomers), didecyl carbonate (isomers), dicyclopentyl
 carbonate, dicyclohexyl carbonate, dicycloheptyl carbonate, dibenzyl
 carbonate, diphenethyl carbonate (isomers), di(phenylpropyl) carbonate
 (isomers), di(phenylbutyl) carbonate (isomers), di(chlorobenzyl) carbonate
 (isomers), di(methoxybenzyl) carbonate (isomers), di(methoxymethyl)
 carbonate, di(methoxyethyl) carbonate (isomers), di(chloroethyl) carbonate
 (isomers) and di(cyanoethyl) carbonate (isomers). These dialkyl carbonates
 can also be used in mixture.
 Of these dialkyl carbonates, a dialkyl carbonate containing as R.sup.1 a
 lower alkyl group having 4 or less carbon atoms is preferably used. Most
 preferred is dimethyl carbonate.
 The alkyl aryl carbonate used as the starting material in the present
 invention is represented by the following formula (2):
 ##STR18##
 wherein R.sup.2 may be identical with or different from R.sup.1, and
 represents an alkyl group having 1 to 10 carbon atoms, an alicyclic group
 having 3 to 10 carbon atoms or an aralkyl group having 6 to 10 carbon
 atoms; and Ar.sup.2 represents an aromatic group having 5 to 30 carbon
 atoms. As R.sup.2, there may be mentioned, for example, the same groups as
 set forth above for R.sup.1.
 Illustrative examples of Ar.sup.2 in formula (2) include:
 a phenyl group and various alkylphenyl groups, such as phenyl, tolyl
 (isomers), xylyl (isomers), trimethylphenyl (isomers), tetramethylphenyl
 (isomers), ethylphenyl (isomers), propylphenyl (isomers), butylphenyl
 (isomers), diethylphenyl (isomers), methylethylphenyl (isomers,),
 pentylphenyl (isomers), hexylphenyl (isomers) and cyclohexylphenyl
 (isomers);
 various alkoxyphenyl groups, such as methoxyphenyl (isomers), ethoxyphenyl
 (isomers) and butoxyphenyl (isomers);
 various halogenated phenyl groups, such as fluorophenyl (isomers),
 chlorophenyl (isomers), bromophenyl (isomers), chloromethylphenyl
 (isomers) and dichlorophenyl (isomers);
 various substituted phenyl groups represented by the formula (11):
 ##STR19##
 wherein A represents a single bond, a divalent group, such as --O--, --S--,
 --CO-- or --SO.sub.2 --, an alkylene group, a substituted alkylene group
 of the following formula:
 ##STR20##
 wherein each of R.sup.4, R.sup.5, R.sup.6 and R.sup.7 independently
 represents a hydrogen atom; or a lower alkyl group, a cycloalkyl group, an
 aryl group or an aralkyl group, which may be substituted with a halogen
 atom or an alkoxy group,
 or a cycloalkylene group of the following formula:
 ##STR21##
 wherein k is an integer of from 3 to 11, and the hydrogen atoms may be
 replaced by a lower alkyl group, an aryl group, a halogen atom or the
 like, and the aromatic ring in formula (2) may be substituted with a
 substituent, such as a lower alkyl group, a lower alkoxy group, an ester
 group, a hydroxyl group, a nitro group, a halogen atom and a cyano group;
 a naphthyl group and various substituted naphthyl groups, such as naphthyl
 (isomers), methylnaphthyl (isomers), dimethylnaphthyl (isomers),
 chloronaphthyl (isomers), methoxynaphthyl (isomers) and cyanonaphthyl
 (isomers); and
 various unsubstituted or substituted heteroaromatic groups, such as pyridyl
 (isomers), cumaryl (isomers), quinolyl (isomers), methylpyridyl (isomers),
 chloropyridyl (isomers), methylcumaryl (isomers) and methylquinolyl
 (isomers).
 Representative examples of alkyl aryl carbonate having these R.sup.2 and
 Ar.sup.2 include methyl phenyl carbonate, ethyl phenyl carbonate, propyl
 phenyl carbonate (isomers), allyl phenyl carbonate, butyl phenyl carbonate
 (isomers), pentyl phenyl carbonate (isomers), hexyl phenyl carbonate
 (isomers), heptyl phenyl carbonate (isomers), octyl tolyl carbonate
 (isomers), nonyl ethylphenyl carbonate (isomers), decyl butylphenyl
 carbonate (isomers), methyl tolyl carbonate (isomers), ethyl tolyl
 carbonate (isomers), propyl tolyl carbonate (isomers), butyl tolyl
 carbonate (isomers), allyl tolyl carbonate (isomers), methyl xylyl
 carbonate (isomers), methyl trimethylphenyl carbonate (isomers), methyl
 chlorophenyl carbonate (isomers), methyl nitrophenyl carbonate (isomers),
 methyl methoxyphenyl carbonate (isomers), methyl cumyl carbonate
 (isomers), methyl naphthyl carbonate (isomers), methyl pyridyl carbonate
 (isomers), ethyl cumyl carbonate (isomers), methyl benzoylphenyl carbonate
 (isomers), ethyl xylyl carbonate (isomers), benzyl xylyl carbonate
 (isomers). These alkyl aryl carbonates can also be used in mixture. Of
 these alkyl aryl carbonates, one containing as R.sup.2 an alkyl group
 having 1 to 4 carbon atoms and as Ar.sup.2 an aromatic group having 6 to
 10 carbon atoms is preferably used, and methyl phenyl carbonate is most
 preferred.
 The starting material used in the present invention is selected from the
 group consisting of a dialkyl carbonate represented by formula (1) above,
 an alkya aryl carbonate represented by formula (2) above and a mixture
 thereof.
 The aromatic monohydroxy compound used as the reactant in the present
 invention is represented by formula (3):
EQU Ar.sup.1 OH (3)
 wherein Ar.sup.1 may be identical with or different from Ar.sup.2,
 represents an aromatic group having 5 to 30 carbon atoms, and the type of
 the compound is not limited as long as the hydroxyl group is directly
 bonded to the aromatic group. As Ar.sup.1, there may be mentioned, for
 example, the same groups as set forth above for Ar.sup.2.
 Preferred examples of aromatic monohydroxy compounds of formula (3) include
 phenol; various alkylphenols, such as cresol (isomers), xylenol (isomers),
 trimethylphenol (isomers), tetramethylphenol (isomers), ethylphenol
 (isomers), propylphenol (isomers), butylphenol (isomers), diethylphenol
 (isomers), methylethylphenol (isomers), methylpropylphenol (isomers),
 dipropylphenol (isomers), methylbutylphenol (isomers), pentylphenol
 (isomers), hexylphenol (isomers) and cyclohexylphenol (isomers); various
 alkoxyphenols, such as methoxyphenol (isomers) and ethoxyphenol (isomers);
 various substituted phenols represented by the following formula (12):
 ##STR22##
 wherein A is as defined above; naphthol (isomers) and various substituted
 naphthols; and heteroaromatic monohydroxy compounds, such as
 hydroxypyridine (isomers), hydroxycumarine (isomers) and hydroxyquinoline
 (isomers). These aromatic monohydroxy compounds can also be used in
 mixture.
 Of these aromatic monohydroxy compounds, an aromatic monohydroxy compound
 containing as Ar.sup.1 an aromatic group having 6 to 10 carbon atoms is
 preferably used in the present invention, and phenol is most preferred.
 The alkyl aryl carbonate used as the reactant in the present invention is
 represented by the following formula (4):
 ##STR23##
 wherein R.sup.3 may be identical with or different from R.sup.1 and
 R.sup.2.sub.7 and represents an alkyl group having 1 to 10 carbon atoms,
 as alicyclic group having 3 to 10 carbon atoms or an aralkyl group having
 6 to 10 carbon atoms; and Ar.sup.3 may be identical with or different from
 Ar.sup.1 and Ar.sup.2, and represents an aromatic group having 5 to 30
 carbon atoms. As R.sup.3, there may be mentioned, for example, the same
 groups as set forth above for R.sup.1. As Ar.sup.3, there may be
 mentioned, for example, the same groups as set forth above for Ar.sup.2.
 As alkyl aryl carbonates having these R.sup.3 and Ar.sup.3, there may be
 mentioned for example, those which are set forth above for alkyl aryl
 carbonates represented by the above-mentioned formula (2).
 Of these alkyl aryl carbonates, one containing as R.sup.3 an alkyl group
 having 1 to 4 carbon atoms and as Ar.sup.3 an aromatic group having 6 to
 10 carbon atoms is preferably used, and methyl phenyl carbonate is most
 preferred.
 The reactant used in the present invention is selected from the group
 consisting of a aromatic monohydroxy compound represented by formula (3)
 above, an alkyl aryl carbonate represented by formula (4) above and a
 mixture thereof.
 The typical reactions, which are involved in the process of the present
 invention for producing an aromatic carbonate or an aromatic carbonate
 mixture by transesterifying a starting material with a reactant in the
 presence of a metal-containing catalyst, are represented by the following
 formulae (E1), (E2), (E3) and (E4):
 ##STR24##
 wherein R.sup.1, R.sup.2, R.sup.3, Ar.sup.1, Ar.sup.2 and Ar.sup.3 are as
 defined above, each of Ar's appearing in formula (E4) independently
 represents Ar.sup.2 or Ar.sup.3, and each of R's appearing in formula (E4)
 independently represents R.sup.2 or R.sup.3, and wherein when
 R.sup.2.dbd.R.sup.3 and Ar.sup.2.dbd.Ar.sup.3 in formula (E4), the
 reaction is a same-species intermolecular transesterification reaction
 generally known as a disproportionation reaction.
 When each of the reactions of formulae (E1), (E2), (E3) and (E4) is
 performed according to the process of the present invention, dialkyl
 carbonates or alkyl aryl carbonates as the starting materials for the
 reaction can be used individually or in mixture and aromatic monohydroxy
 compounds or alkyl aryl carbonates as the reactants for the reaction can
 be used individually or in mixture.
 When R.sup.2.dbd.R.sup.3.dbd.R and Ar.sup.2.dbd.Ar.sup.3.dbd.Ar in the
 transesterification reaction of formula (E4), a diaryl carbonate and a
 dialkyl carbonate can be obtained by a same-species intermolecular
 transesterification reaction of a single type of alkyl aryl carbonate.
 This is a preferred embodiment of the present invention.
 Further, when R.sup.1.dbd.R.sup.2.dbd.R.sup.3.dbd.R and
 Ar.sup.1.dbd.Ar.sup.2.dbd.Ar.sup.3.dbd.Ar in formulae (E1) and (E4), by
 combining the reaction of formula (E1) with the reaction of formula (E4),
 a diaryl carbonate can be obtained from a dialkyl carbonate and an
 aromatic monohydroxy compound through an alkyl aryl carbonate as shown in
 formulae (E5) and (E6). This is an especially preferred embodiment of the
 present invention.
 ##STR25##
 Recycling of the dialkyl carbonate by-produced in the reaction of formula
 (E6) as the starting material for the reaction of formula (E5) results in
 the formation of 1 mol of a diaryl carbonate and 2 mol of an aliphatic
 alcohol from 1 mol of a dialkyl carbonate and 2 mol of an aromatic
 monohydroxy compound.
 When R.dbd.CH.sub.3 and Ar.dbd.C.sub.6 H.sub.5 in the above formulae (E5)
 and (E6), diphenyl carbonate, which is an important raw material for a
 polycarbonate and an isocyanate, can be readily obtained from dimethyl
 carbonate, which is the simplest form of a dialkyl carbonate, and phenol.
 This is especially important.
 The metal-containing catalyst used in the present invention is one capable
 of promoting the reactions of formulae (E1) to (E4). As such
 metal-containing catalysts, there may be mentioned for example:
 &lt;lead compounds&gt;
 lead oxides, such as PbO, PbO.sub.2 and Pb.sub.3 O.sub.4 ; lead sulfides,
 such as PbS and Pb.sub.2 S; lead hydroxides, such as Pb(OH).sub.2 and
 Pb.sub.2 O.sub.2 (OH).sub.2 ; plumbites, such as Na.sub.2 PbO.sub.2,
 K.sub.2 PbO.sub.2, NaHPbO.sub.2 and KHPbO.sub.2 ; plumbates, such as
 Na.sub.2 PbO.sub.3, Na.sub.2 H.sub.2 PbO.sub.4, K.sub.2 PbO.sub.3, K.sub.2
 [Pb(OH).sub.6 ], K.sub.4 PbO.sub.4, Ca.sub.2 PbO.sub.4 and CaPbO.sub.3 ;
 lead carbonates and basic salts thereof, such as PbCO.sub.3 and
 2PbCO3.Pb(OH).sub.2 ; lead salts of organic acids, and carbonates and
 basic salts thereof, such as Pb(OCOCH.sub.3).sub.2, Pb(OCOCH.sub.3).sub.4
 and Pb(OCOCH.sub.3).sub.2.PbO.3H.sub.2 O; organolead compounds, such as
 Bu.sub.4 Pb, Ph.sub.4 Pb, Bu.sub.3 PbC.sub.1, Ph.sub.3 PbBr, Ph.sub.3 Pb
 (or Ph.sub.6 Pb.sub.2), Bu.sub.3 PbOH and Ph.sub.3 PbO wherein Bu
 represents a butyl group and Ph represents a phenyl group; alkoxylead
 compounds and aryloxylead compounds, such as Pb(OCH.sub.3).sub.2,
 (CH.sub.3 O)Pb(OPh) and Pb(OPh).sub.2 ; lead alloys, such as Pb--Na,
 Pb--Ca, Pb--Ba, Pb--Sn and Pb--Sb; lead minerals, such as galena and zinc
 blende; and hydrates of these lead compounds;
 &lt;copper family metal compounds&gt;
 salts or complexes of copper family metals, such as CuCl, CuCl.sub.2, CuBr,
 CuBr.sub.2, CuI, CuI.sub.2, Cu(OAc).sub.2, Cu(acac).sub.2, copper oleate,
 Bu.sub.2 Cu, (CH.sub.3 O).sub.2 Cu, AgNO.sub.3, AgBr, silver picrate,
 AgC.sub.6 H.sub.6 ClO.sub.4, Ag(bullvalene).sub.3 NO.sub.3,
 [AuC.ident.C--C(CH.sub.3).sub.3 ].sub.n and [Cu(C.sub.7 H.sub.8)Cl].sub.4
 wherein Ac represents an acetyl group and acac represents an acetylacetone
 chelate ligand;
 &lt;alkali metal complexes&gt;
 alkali metal complexes, such as Li(acac) and LiN(C.sub.4 H.sub.9).sub.2 ;
 &lt;zinc complexes&gt;
 zinc complexes, such as Zn(acac).sub.2 ;
 &lt;cadmium complexes&gt;
 cadmium complexes, such as Cd(acac).sub.2 ;
 &lt;iron family metal compounds&gt;
 iron family metal complexes, such as Fe(C.sub.10 H.sub.8)(CO).sub.5,
 Fe(CO).sub.5, Fe(C.sub.3 H.sub.6)(CO).sub.3, Co(mesitylene).sub.2
 (PEt.sub.2 Ph).sub.2, CoC.sub.5 F.sub.5 (CO).sub.2, Ni-.pi.-C.sub.5
 H.sub.5 NO and ferrocene;
 &lt;zirconium complexes&gt;
 zirconium complexes, such as Zr(acac).sub.4 and zirconocene;
 &lt;Lewis acids and Lewis acid-forming compounds&gt;
 Lewis acids and Lewis acid-forming transition metal compounds, such as
 AlX.sub.3, TiX.sub.3, TiX.sub.4, VOX.sub.3, VX.sub.5, ZnX.sub.2, FeX.sub.3
 and SnX.sub.4 wherein X represents a halogen atom, an acetoxy group, an
 alkoxy group or an aryloxy group; and
 &lt;organotin compounds&gt;
 organotin compounds, such as (CH.sub.3).sub.3 SnOCOCH.sub.3, (C.sub.2
 H.sub.5).sub.3 SnOCOC.sub.6 H.sub.5, Bu.sub.3 SnOCOCH.sub.3, Ph.sub.3
 SnOCOCH.sub.3, Bu.sub.2 Sn(OCOCH.sub.3).sub.2, Bu.sub.2 Sn(OCOC.sub.11
 H.sub.23).sub.2, Ph.sub.3 SnOCH.sub.3, (C.sub.2 H.sub.5).sub.3 SnOPh,
 Bu.sub.2 Sn(OCH.sub.3).sub.2, Bu.sub.2 Sn(OC.sub.2 H.sub.5).sub.2,
 Bu.sub.2 Sn(OPh).sub.2, Ph.sub.2 Sn(OCH.sub.3).sub.2, (C.sub.2
 H.sub.5).sub.3 SnOH, Ph.sub.3 SnOH, Bu.sub.2 SnO, (C.sub.8 H.sub.17).sub.2
 SnO, Bu.sub.2 SnCl.sub.2 and BuSnO(OH) wherein Ph represents an phenyl
 group.
 These catalysts are effective even when they are reacted with an organic
 compound present in the reaction system, such as an aliphatic alcohol, an
 aromatic monohydroxy compound, an alkyl aryl carbonate, a diaryl carbonate
 and a dialkyl carbonate. Those which are obtained by heat-treating these
 catalysts together with a starting material, a reactant and/or a reaction
 product thereof prior to the use in the process of the present invention
 can also be used.
 It is preferred that the metal-containing catalyst have high solubility in
 the liquid phase of the reaction system. Preferred examples of
 metal-containing catalysts include Pb compounds, such as PbO, Pb(OH).sub.2
 and Pb(OPh).sub.2 ; Ti compounds, such as TiCl.sub.4 and Ti(OPh).sub.4 ;
 Sn compounds, such as SnCl.sub.4, Sn(OPh).sub.4, Bu.sub.2 SnO and Bu.sub.2
 Sn(OPh).sub.2 ; Fe compounds, such as FeCl.sub.3, Fe(OH).sub.3 and
 Fe(OPh).sub.3 ; and those products which are obtained by treating the
 above metal compounds with phenol or a liquid phase of the reaction
 system.
 There is no particular limitation with respect to the type of the reactor
 to be used in the process of the present invention, and various types of
 conventional reactors, such as a stirred tank reactor, a multi-stage
 stirred tank reactor and a multi-stage distillation column, can be used.
 These types of reactors can be used individually or in combination, and
 may be used either in a batchwise process or a continuous process. From
 the viewpoint of efficiently biasing the equilibrium toward the product
 system, a multi-stage distillation column is preferred, and a continuous
 process using a multi-stage distillation column is especially preferred.
 There is no particular limitation with respect to the multi-stage
 distillation column to be used in the present invention as long as it is a
 distillation column having a theoretical number of stages of distillation
 of two or more and which can be used for performing continuous
 distillation. Examples of such multi-stage distillation columns include
 plate type columns using a tray, such as a bubble-cap tray, a sieve tray,
 a valve tray and a counterflow tray, and packed type columns packed with
 various packings, such as a Raschig ring, a Lessing ring, a Pall ring, a
 Berl saddle, an Intalox saddle, a Dixon packing, a McMahon packing, a Heli
 pack, a Sulzer packing and Mellapak. In the present invention, any of the
 columns which are generally used as a multi-stage distillation column can
 be utilized. Further, a mixed type of plate column and packed column
 comprising both a plate portion and a portion packed with packings, can
 also be preferably used.
 In one preferred embodiment of the present invention, in which the
 continuous production of an aromatic carbonate is conducted using a
 multi-stage distillation column, a starting material and a reactant are
 continuously fed to a continuous multi-stage distillation column to effect
 a transesterification reaction there-between in at least one phase
 selected from a liquid phase and a gas-liquid phase in the presence of a
 metal-containing catalyst in the distillation column, while continuously
 withdrawing a high boiling point reaction mixture containing a produced
 aromatic carbonate or aromatic carbonate mixture in liquid form from a
 lower portion of the distillation column and continuously withdrawing a
 low boiling point reaction mixture containing a by-product in gaseous form
 from an upper portion of the distillation column by distillation.
 The amount of the catalyst used in the present invention varies depending
 on the type thereof, the types and weight ratio of the starting material
 and the reactant, the reaction conditions, such as reaction temperature
 and reaction pressure, and the like. Generally, the amount of the catalyst
 is in the range of from 0.0001 to 30% by weight, based on the total weight
 of the starting material and the reactant.
 The reaction time (or the residence time when the reaction is continuously
 conducted) for the transesterification reaction in the present invention
 is not specifically limited, but it is generally in the range of from
 0.001 to 50 hours, preferably from 0.01 to 10 hours, more preferably from
 0.05 to 5 hours.
 The reaction temperature varies depending on the types of the starting
 material and reactant, but is generally in the range of from 50 to
 350.degree. C., preferably from 100 to 280.degree. C. The reaction
 pressure varies depending on the types of the starting material and
 reactant and the reaction temperature, and it may be any of a reduced
 pressure, an atmospheric pressure and a superatmospheric pressure.
 However, the reaction pressure is generally in the range of from 0.1 to
 2.0.times.10.sup.7 Pa.
 In the present invention, it is not necessary to use a reaction solvent.
 However, for the purpose of facilitating the reaction operation, an
 appropriate inert solvent, such as an ether, an aliphatic hydrocarbon, an
 aromatic hydrocarbon or a halogenated aromatic hydrocarbon, may be used as
 a reaction solvent.
 As mentioned above, the process of the present invention is characterized
 by taking out the catalyst-containing liquid containing the high boiling
 point substance (A) and a metal-containing catalyst (B); adding a
 functional substance (C) capable of reacting with the high boiling point
 substance (A) and/or the metal-containing catalyst (B) to the taken-out
 catalyst-containing liquid, to thereby obtain an (A)/(C) reaction product
 and/or a (B)/(C) reaction product; and recycling the (B)/(C) reaction
 product directly or indirectly to the reaction system, while withdrawing
 the (A)/(C) reaction product.
 In the process of the present invention, the passage "recycling the (B)/(C)
 reaction product directly or indirectly to the reaction system" means
 "recycling the (B)/(C) reaction product to the reactor directly, or
 recycling the (B)/(C) reaction product to the reactor indirectly through a
 pipe and a device which communicate with the inlet of the reactor or which
 are used for recovering the catalyst".
 The "catalyst-containing liquid containing the high boiling point substance
 and a metal-containing catalyst" means at least one type of
 catalyst-containing liquid which is selected from the group consisting of
 a portion of the high boiling point reaction mixture before the separation
 of the high boiling point reaction mixture into the product fraction and
 the liquid catalyst fraction, and a portion of the separated liquid
 catalyst fraction. More specifically, the above-mentioned
 catalyst-containing liquid means, for example, a catalyst-containing
 liquid which is selected from the group consisting of a portion of the
 reaction mixture (containing the metal-containing catalyst (B) and the
 high boiling point substance (A)) which is withdrawn from the reactor, or
 a portion of a liquid material (having increased concentrations with
 respect to the catalyst and the high boiling point substance) which is
 obtained by subjecting to evaporation a part of the catalyst-containing
 reaction mixture withdrawn from the reactor. In the catalyst-containing
 liquid, the catalyst may be completely dissolved, or may be in the form of
 a slurry in which insoluble matters are formed by the reaction between the
 catalyst and the high boiling point substance. In the present invention,
 when the catalyst-containing liquid is in the form of a slurry, a portion
 in the slurry which is present in a non-dissolved state is also included
 in the "catalyst-containing liquid containing the high boiling point
 substance (A) and a metal-containing catalyst (B)". The
 catalyst-containing liquid may be taken out continuously or
 intermittently.
 In the process of the present invention, the "high boiling point substance
 (A)" means a substance having a boiling point higher than the boiling
 point of the produced aromatic carbonates, wherein such a substance
 originates from at least one compound selected from the group consisting
 of the starting material, the reactant, impurities contained in the
 starting material and the reactant, and by-products of the
 transesterification reaction. Examples of such high boiling point
 substances (A) include an aromatic hydroxy compound, a compound containing
 a residue of the aromatic hydroxy compound, an aromatic a compound, a
 compound containing a residue of the aromatic carboxy compound, and
 xanthone. Those by-products having a high molecular weight which are
 produced, by reaction, from the aromatic hydroxy compound, a compound
 containing a residue of the aromatic hydroxy compound, an aromatic carboxy
 compound, a compound containing a residue of the aromatic carboxy
 compound, and xanthone can also be mentioned as examples of
 above-mentioned high boiling point substances (A).
 In the present invention, the aromatic hydroxy compound is represented by
 the following formula (7):
 ##STR26##
 wherein Ar.sup.4 represents an aromatic group having a valence of m, m
 represents an integer of 2 or more, and each --OH group is individually
 bonded to an arbitrary ring-carbon position of the Ar.sup.4 group.
 The residue of the aromatic hydroxy compound is represented by the
 following formula (8):
 ##STR27##
 wherein Ar.sup.4 and m are as defined above, n represents an integer of
 from 1 to m, and each of the --OH group and the --O-- group is
 individually bonded to an arbitrary ring-carbon position of the Ar.sup.4
 group.
 The residue (8) of the aromatic hydroxy compound is present in such a form
 as chemically bonded to at least one member selected from the group
 consisting of the metal of the metal-containing catalyst, an
 alkoxycarbonyl group derived from the dialkyl carbonate or the alkyl aryl
 carbonate, an aryloxycarbonyl group derived from the alkyl aryl carbonate
 or the diaryl carbonate, and a carbonyl group derived from the dialkyl
 carbonate, the alkyl aryl carbonate or the diaryl carbonate.
 Illustrative examples of the Ar.sup.4 groups in formulae (7) and (8) above
 include aromatic groups represented by the following formulae (13), (14),
 (15), (16) and (17):
 ##STR28##
 wherein Y.sup.1 represents a single bond, a divalent alkane group having 1
 to 30 carbon atoms or a divalent group selected from --O--, --CO--, --S--,
 --SO.sub.2 --, --SO-- and --COO--,
 ##STR29##
 wherein each of two Y.sup.1' s is as defined above, and two Y.sup.1' s may
 be the same or different;
 ##STR30##
 wherein Z represents a trivalent group, such as a C.sub.1 -C.sub.30
 trivalent alkane group or a trivalent aromatic group; and at least one
 hydrogen atom of each aromatic ring may be replaced with a substitutent,
 such as a halogen atom, a C.sub.1 -C.sub.30 alkyl group, a C.sub.1
 -C.sub.30 alkoxy group, a phenyl group, a phenoxy group, a vinyl group, a
 cyano group, an ester group, an amido group, a nitro group or the like;
 and
 ##STR31##
 Examples of these aromatic polyhydroxy compounds include hydroquinone,
 resorcin, catechol, trihydroxybenzene (isomers), bis(hydroxyphenyl)propane
 (isomers), bis(hydroxyphenyl)methane (isomers), bis(hydroxyphenyl)ether
 (isomers), bis(hydroxyphenyl)ketone (isomers), bis(hydroxyphenyl)sulfone
 (isomers), bis(hydroxyphenyl)sulfide (isomers), dihydroxy diphenyl
 (isomers), bis(dihydroxyphenyl)methane (isomers), 2-hydroxyphenyl
 hydroxypropyl phenol, dihydroxy (hydroxyphenyl diphenyl) (isomers),
 tri-(hydroxyphenyl)ethane (isomers), tri-(hydroxyphenyl)benzene (isomers),
 dihydroxynaphthalene (isomers) and trihydroxynaphthalene (isomers).
 Of these aromatic hydroxy compounds and compounds having a residue of the
 aromatic hydroxy compounds, attention should be made to those compounds
 which are likely to be present in the system for the transesterification
 for the production of an aromatic carbonate. As such a compound, there can
 be mentioned at least one member selected from the group consisting of:
 (a) an oxidation product of an aromatic monohydroxy compound as the
 reactant,
 (b) at least one member selected from the group consisting of a product
 produced by the Fries rearrangement of a diaryl carbonate obtained by the
 transesterification and oxidation products of the product, and
 (c) at least one member selected from the group consisting of aromatic
 dihydroxy compounds derived from phenol as the reactant and represented by
 the following formula (18):
 ##STR32##
 wherein Y.sup.1 is as defined above, and oxidation products of the aromatic
 dihydroxy compounds.
 As examples of oxidation products (a) of an aromatic monohydroxy compound,
 compounds represented by the following formulae (19) and (20) can be
 mentioned.
 ##STR33##
 As examples of products (b) produced by the Fries rearrangement of a diaryl
 carbonate, compounds represented by the following formulae (21), (22) and
 (23) can be mentioned.
 ##STR34##
 As examples of oxidation products of the above-mentioned product (b)
 produced by the Fries rearrangement of a diaryl carbonate and represented
 by formula (21), compounds represented by the following formulae (24) and
 (25) can be mentioned. Also, as examples of respective oxidation products
 of the above-mentioned products (b) represented by formulae (22) and (23),
 compounds represented by the following formulae (26) and (27) can be
 mentioned.
 ##STR35##
 As an example of aromatic dihydroxy compounds (c) represented by formula
 (18), a compound represented by the following formula (28) can be
 mentioned.
 ##STR36##
 As examples of oxidation products of the above-mentioned aromatic dihydroxy
 compounds (c) represented by formula (28), compounds represented by the
 following formulae (29) and (30) can be mentioned.
 ##STR37##
 wherein Y.sup.1 is as defined above.
 The reason why the above-mentioned oxidation product (a) of an aromatic
 monohydroxy compound is likely to be present in the system for the
 transesterification for the production of an aromatic carbonate, for
 example, is that such an oxidation product is formed by the oxidation of
 an aromatic monohydroxy compound with a very small amount of oxygen which
 occasionally enters the system for the transesterification, or that such
 an oxidation product is occasionally present as a contaminant of an
 aromatic monohydroxy compound as a raw material and enters the system
 together with the raw material. Representative examples of type (a)
 oxidation products, namely, oxidation products of aromatic monohydroxy
 compounds include dihydroxybenzene (isomers), dihydroxy diphenyl
 (isomers), and the like.
 Product (b) produced by the Fries rearrangement of a diaryl carbonate is
 likely to be formed as a by-product in the production of the diaryl
 carbonate. Examples of products (b) include 2,2'-dihydroxybenzophenone,
 2,4'-dihydroxybenzophenone and 4,4'-dihydroxybenzophenone.
 The aromatic dihydroxy compound (c) is a compound which is usually used as
 a monomer for the production of an aromatic polycarbonate. An aromatic
 polycarbonate can be produced by a transesterification of the
 above-mentioned aromatic dihydroxy compound (c) with a diaryl carbonate,
 wherein an aromatic monohydroxy compound is by-produced. When such a
 by-produced aromatic monohydroxy compound is used as a raw material in the
 process of the present invention, the aromatic dihydroxy compound (c) is
 likely to be introduced into the system for the transesterification for
 the production of an aromatic carbonate. Examples of aromatic dihydroxy
 compounds (c) include 2,2-bis-(4-hydroxyphenyl)propane, and the like.
 Further, 2,2-bis-(4-hydroxyphenyl)propane usually contains aromatic
 polyhydroxy compounds represented by the following formulae, which
 compounds are also included in the aromatic polyhydroxy compound defined
 in the present invention.
 ##STR38##
 In the present invention, the aromatic carboxy compound, which is one of
 the high boiling point substances, is represented by the following formula
 (9):
 ##STR39##
 wherein Ar.sup.5 represents an aromatic group having a valence of r, r
 represents an integer of 1 or more, s represents an integer of from 0 to
 r-1, and each of the --OH group and the --(COOH) group is individually
 bonded to an arbitrary ring-carbon position of the Ar.sup.5 group, and
 The residue of the aromatic carboxy compound is represented by the
 following formula (10):
 ##STR40##
 wherein Ar.sup.5, r and s are as defined above, t represents an integer of
 from 0 to s, u represents an integer of from 0 to r--s, with the proviso
 that t and u are not simultaneously O, and each of the --OH group, the
 --(COOH) group, the --O-- group and the --(COO)-- group is individually
 bonded to an arbitrary ring-carbon position of the Ar.sup.5 group.
 The residue of the aromatic carboxy compound, which is represented by
 formula (10), is present in such a form as chemically bonded to at least
 one member selected from the group consisting of a metal of the
 metal-containing catalyst, an alkoxycarbonyl group derived from the
 dialkyl carbonate or the alkyl aryl carbonate, an alkyl group formed by
 the decarboxylation reaction of the alkoxycarbonyl group, an
 aryloxycarbonyl group derived from the alkyl aryl carbonate or the diaryl
 carbonate, an aryl group formed by the decarboxylation reaction of the
 aryloxycarbonyl group, and a carbonyl group derived from the dialkyl
 carbonate, the alkyl aryl carbonate or the diaryl carbonate.
 Examples of these aromatic carboxy compounds and compounds having residues
 of such aromatic carboxy compounds include aromatic carboxylic acids, such
 as benzoic acid, terephthalic acid, isophthalic acid and phthalic acid;
 aromatic carboxylic acid esters, such as methyl benzoate, phenyl benzoate
 and dimethyl terephthalate; hydroxyaromatic carboxylic acids, such as
 salicylic acid, p-hydroxybenzoic acid, m-hydroxybenzoic acid,
 dihydroxybenzoic acid (isomers), carboxydiphenol (isomers) and
 2-(4-hydroxyphenyl)-2-(3'-carboxy-4'-hydroxyphenyl)propane;
 aryloxycarbonyl-(hydroxy)-arenes, such as phenyl salicylate, phenyl
 p-hydroxybenzoate, tolyl salicylate, tolyl p-hydroxybenzoate, phenyl
 dihydroxybenzoate (isomers), tolyl dihydroxybenzoate (isomers), phenyl
 dihydroxybenzoate (isomers), phenoxycarbonyldiphenol (isomers) and
 2-(4-hydroxyphenyl)-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)propane;
 alkoxycarbonyl-(hydroxy)-arenes, such as methyl salicylate, methyl
 p-hydroxybenzoate, ethyl salicylate, ethyl p-hydroxybenzoate, methyl
 dihydroxybenzoate (isomers), methoxycarbonyldiphenol (isomers) and
 2-(4-hydroxyphenyl)-2-(3'-methoxycarbonyl-4'-hydroxyphenyl)propane;
 aryloxycarbonyl-(alkoxy)-arenes, such as phenyl methoxybenzoate (isomers),
 tolyl methoxybenzoate (isomers), phenyl ethoxybenzoate (isomers), tolyl
 ethoxybenzoate (isomers), phenyl hydroxy-methoxybenzoate (isomers),
 hydroxy-methoxy-(phenoxycarbonyl)-diphenyl (isomers),
 2-(4-methoxyphenyl)-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)propane and
 2-(4-hydroxyphenyl)-2-(3'-phenoxycarbonyl-4'-methoxyphenyl)propane;
 aryloxycarbonyl-(aryloxy)-arenes, such as phenyl phenoxybenzoate
 (isomers), tolyl phenoxybenzoate (isomers), tolyl tolyloxybenzoate
 (isomers), phenyl hydroxy-phenoxy-benzoate (isomers),
 hydroxyphenoxy-(phenoxycarbonyl)-diphenyl (isomers),
 2-(4-phenoxyphenyl)-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)propane and
 2-(4-hydroxyphenyl)-2-(3'-phenoxycarbonyl-4'-phenoxyphenyl)propane;
 alkoxycarbonyl-(alkoxy)-arenes, such as methyl methoxybenzoate (isomers),
 ethyl methoxybenzoate (isomers), methyl ethoxybenzoate (isomers), ethyl
 ethoxybenzoate (isomers), methyl hydroxy-methoxybenzoate (isomers),
 hydroxy-methoxy-(methoxycarbonyl)-diphenyl (isomers),
 2-(4-methoxyphenyl)-2-(3'-methoxycarbonyl-4'-hydroxyphenyl)propane and
 2-(4-hydroxyphenyl)-2-(3'-methoxycarbonyl-4'-methoxyphenyl)propane;
 alkoxycarbonyl-(aryloxy)-arenes, such as methyl phenoxybenzoate (isomers),
 ethyl phenoxybenzoate (isomers), methyl tolyloxybenzoate (isomers), ethyl
 tolyloxybenzoate (isomers), phenyl hydroxy-methoxy-benzoate (isomers),
 hydroxy-methoxy-(phenoxycarbonyl)-diphenyl (isomers),
 2-(4-methoxyphenyl)-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)propane and
 2-(4-hydroxyphenyl)-2-(3-phenoxycarbonyl-4'-methoxyphenyl)propane
 (isomers); aryloxycarbonyl-(aryloxycarbonyloxy)-arenes, such as phenyl
 phenoxycarbonyloxybenzoate (isomers), tolyl phenoxycarbonyloxybenzoate
 (isomers), tolyl tolyloxycarbonyloxybenzoate (isomers), phenyl
 hydroxy-phenoxycarbonyloxybenzoate (isomers),
 hydroxy-phenoxycarbonyloxy-(phenoxycarbonyl)-diphenyl (isomers),
 2-[4-(phenoxycarbonyloxy)phenyl]-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)pr
 opane and
 2-(4-hydroxyphenyl)-2-[3'-phenoxycarbonyl-4'-(phenoxycarbonyloxy)phenyl]pr
 opane; aryloxycarbonyl-(alkoxycarbonyloxy)-arenes, such as phenyl
 methoxycarbonyloxybenzoate (isomers), tolyl methoxycarbonyloxybenzoate
 (isomers), phenyl ethoxycarbonyloxybenzoate (isomers), tolyl
 ethoxycarbonyloxybenzoate (isomers), phenyl
 hydroxy-methoxycarbonyloxybenzoate (isomers),
 hydroxy-methoxycarbonyloxy-(phenoxycarbonyl)-diphenyl (isomers),
 2-[4-(methoxycarbonyloxy)phenyl]-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)pr
 opane and
 2-(4-hydroxyphenyl)-2-[3'-phenoxycarbonyl-4'-(methoxycarbonyloxy)phenyl]pr
 opane; alkoxycarbonyl-(aryloxycarbonyloxy)-arenes, such as methyl
 phenoxycarbonyloxybenzoate (isomers), ethyl phenoxycarbonyloxybenzoate
 (isomers), methyl tolyloxycarbonyloxybenzoate (isomers), ethyl
 tolyloxycarbonyloxybenzoate (isomers), methyl
 hydroxy-phenoxycarbonyloxy-benzoate (isomers),
 hydroxy-phenoxycarbonyloxy-(methoxycarbonyl)-diphenyl (isomers),
 2-[4-(phenoxycarbonyloxy)phenyl]-2-(3'-methoxycarbonyl-4'-hydroxyphenyl)pr
 opane and
 2-(4-hydroxyphenyl)-2-[3'-methoxycarbonyl-4'-(phenoxycarbonyloxy)phenyl]pr
 opane; and alkoxycarbonyl-(alkoxycarbonyloxy)-arenes, such as methyl
 methoxycarbonyloxybenzoate (isomers), ethyl methoxycarbonyloxybenzoate
 (isomers), methyl ethoxycarbonyloxybenzoate (isomers), ethyl
 ethoxycarbonyloxybenzoate (isomers), methyl
 hydroxy-methoxycarbonyloxybenzoate (isomers),
 hydroxy-methoxycarbonyloxy-(methoxycarbonyl)-diphenyl (isomers),
 2-[4-(methoxycarbonyloxy)phenyl]-2-(3'-methoxycarbonyl-4'-hydroxyphenyl)pr
 opane and
 2-(4-hydroxyphenyl)-2-[3'-methoxycarbonyl-4'-(methoxycarbonyloxy)phenyl]pr
 opane.
 Of these aromatic carboxy compounds and compounds having residues of such
 aromatic carboxy compounds, attention should be made to those which are
 likely to be present in the system for the transesterification for the
 production of an aromatic carbonate. As such an aromatic carboxy compound
 and a compound having a residue of such an aromatic carboxy compound,
 there can be mentioned at least one member selected from the group
 consisting of:
 (d) at least one member selected from the group consisting of a product
 produced by the Fries rearrangement of an aromatic carbonate obtained by
 the transesterification and a derivative of the product and
 (e) at least one member selected from the group consisting of a product
 produced by the Fries rearrangement of a reaction product obtained by the
 transesterification of the aromatic polyhydroxy compound and a derivative
 of the product.
 As mentioned above, in the process for producing aromatic carbonates of the
 present invention, the reactions of producing methyl phenyl carbonate and
 diphenyl carbonate from dimethyl carbonate and phenol are especially
 important. Therefore, taking these reactions as examples, examples of
 aromatic carboxy compounds and compounds having residues of such aromatic
 carboxy compounds, which are included in (d) and (e) above are enumerated
 below.
 Examples of (d) include salicyclic acid, p-hydroxybenzoic acid, phenyl
 salicylate, phenyl p-hydroxybenzoate, methyl salicylate, methyl
 p-hydroxybenzoate, phenyl methoxybenzoate (isomers), phenyl
 phenoxybenzoate (isomers), phenyl phenoxycarbonyloxybenzoate (isomers),
 methyl phenoxycarbonyloxybenzoate (isomers), methyl
 methoxycarbonyloxybenzoate (isomers).
 Examples of (e) include dihydroxybenzoic acid (isomers), phenyl
 dihydroxybenzoate (isomers), phenoxycarbonyldiphenol (isomers),
 2-(4-hydroxyphenyl)-2-(3'-phenoxycarbonyl-4'-hydroxyphenyl)propane.
 Examples of xanthones belonging to the high boiling point substances in the
 present invention include xanthone and those in which the aromatic ring of
 xanthone is substituted with at least one substituent selected from the
 group consisting of an alkyl group, such as methyl, ethyl, propyl,
 isopropyl, butyl, iso-butyl and the like; a hydroxy group; an alkoxy
 group, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy and the like;
 aryloxy group, such as phenoxy, tolyloxy and the like; an
 alkoxycarbonyloxy group, such as methoxycarbonyloxy, ethoxycarbonyloxy,
 propoxycarbonyloxy, butoxycarbonyloxy and the like; an aryloxycarbonyloxy
 group, such as phenoxycarbonyloxy, tolyloxycarbonyloxy and the like; a
 carboxy group; an alkoxycarbonyl group, such as methoxycarbonyl,
 ethoxycarbonyl and the like; an aryloxycarbonyl group, such as
 phenoxycarbonyl, tolyloxycarbonyl and the like; an arylcarbonyloxy group,
 such as benzoyloxy, tolylcarbonyloxy and the like.
 The functional substance (C) used in the present invention is a substance
 which is capable of reacting with at least one component selected from the
 group consisting of the high boiling point substance (A) and the
 metal-containing catalyst (B). There is no particular limitation with
 respect to the functional substance (C), as long as the substance is
 capable of forming at least one reaction product selected from the group
 consisting of an (A)/(C) reaction product {which is a reaction product of
 the functional substance (C) with the high boiling point substance (A)}
 and a (B)/(C) reaction product {which is a reaction product of the
 functional substance (C) with the metal-containing catalyst (B)}. Examples
 of such a functional substance (C) include oxidizing agents, reducing
 agents, precipitants, adsorbents and reactive solvents. Of these,
 oxidizing agents, precipitants and reactive solvents are preferred.
 Further, these functional substances may be used individually, or at least
 two different functional substances may be simultaneously or stepwise
 added to the taken-out catalyst-containing liquid. Further, the reaction
 of the functional substance (C) with the high boiling point substance (A)
 and/or the metal-containing catalyst (B) can be carried out in a batchwise
 or a continuous manner.
 In the present invention, when the functional substance (C) is capable of
 reacting with the high boiling point substance (A), the (A)/(C) reaction
 product is a product formed by the reaction between the high boiling point
 substance (A) and the functional substance (C). However, when the
 functional substance (C) is not capable of reacting with the high boiling
 point substance (A), the unreacted high boiling point substance (A) is
 regarded as the (A)/(C) reaction product. On the other hand, when the
 functional substance (A) is capable of reacting with the metal-containing
 catalyst (B), the (B)/(C) reaction product is a product formed by the
 reaction between the metal-containing catalyst (B) and the functional
 substance (C). However, when the functional substance (C) is not capable
 of reacting with the metal-containing catalyst (B), the unreacted
 metal-containing catalyst (B) is regarded as the (B)/(C) reaction product.
 In the present invention, the (A)/(C) reaction product is withdrawn from
 the production system for the desired aromatic carbonates, whereas the
 (B)/(C) reaction product is recycled to the reaction system comprising the
 starting material and the reactant. The withdrawal of the (A)/(C) reaction
 product can be carried out, for example, by separating the (A)/(C)
 reaction product from the (B)/(C) reaction product during and/or after the
 reaction of the high boiling point substance (A) with the functional
 substance (C).
 With respect to the method for separating the (A)/(C) reaction product from
 the (B)/(C) reaction product, any methods can be employed as long as the
 catalyst-containing liquid can be separated into a component which is
 composed mainly of the (A)/(C) reaction product and a component which is
 composed mainly of the (B)/(C) reaction product. Examples of such
 separation methods include a gas phase-condensed phase separation method,
 such as a gas phase-liquid phase separation method, a gas phase-solid
 phase separation method or a gas phase-solid/liquid mixed phase separation
 method; a solid phase-liquid phase separation method, such as
 sedimentation, centrifugation or filtration; a distillation method; an
 extraction method; and an adsorption method. Of these, the sedimentation,
 the distillation and the adsorption method are preferred. These separation
 methods can be employed individually, or at least two of such separation
 methods can be simultaneously or stepwise employed.
 With respect to the combination of the functional substance (C) and the
 separation method, there is no particular limitation. However, examples of
 the preferred modes usable for practicing the method of the present
 invention, in which specific combinations of the functional substance (C)
 and the separation method are used, include:
 (I) a mode in which the functional substance (C) is an oxidizing agent, so
 that an oxidation reaction is performed with respect to the
 catalyst-containing liquid, in which the (A)/(C) reaction product is a low
 boiling point oxidation product and the (B)/(C) reaction product is a
 metal oxide; and the separation method is the gas phase-condensed phase
 separation method,
 (II) a mode in which the functional substance (C) is a precipitant, so that
 a precipitation reaction is performed with respect to the
 catalyst-containing liquid, in which the (B)/(C) reaction product is a
 metal-containing substance which precipitates; and the separation method
 is the solid-liquid separation method,
 (III) a mode in which the functional substance (C) is a reactive solvent,
 so that a solvolysis reaction is performed with respect to the
 catalyst-containing liquid, in which the (A)/(C) reaction product is a
 low-boiling point solvolysis product; and the separation method is the
 distillation method.
 When the preferred mode of item (I) above is employed, use is made of an
 oxidizing agent which not only can oxidize the high boiling point
 substance (A) to form a low boiling point oxidation product as the (A)/(C)
 reaction product, but also can oxidize the metal-containing catalyst (B)
 to form a metal oxide as the (B)/(C) reaction product. Examples of
 oxidizing agents include air; molecular oxygen; ozone; hydrogen peroxide;
 silver oxide; organic peroxides, such as peracetic acid, perbenzoic acid,
 benzoyl peroxide, tert-butyl hydroperoxide and cumyl hydroperoxide;
 oxo-acids, such as nitrous acid, nitric acid, chloric acid, hypochlorous
 acid; and salts thereof. Of these, air, molecular oxygen, ozone, hydrogen
 peroxide, nitrous acid and nitric acid are preferred, and air and
 molecular oxygen are more preferred.
 The type of the reaction performed in the catalyst-containing liquid using
 the oxidizing agent varies depending on the type of the oxidizing agent
 and the reaction conditions. However, the reaction is performed in a phase
 selected from the group consisting of a liquid phase, a gas-liquid mixed
 phase and a gas-liquid/solid mixed phase. The reaction temperature varies
 depending on the type of the oxidizing agent; however, the reaction
 temperature is generally in the range of from -30 to 2,000.degree. C.,
 preferably from 0 to 1,200.degree. C., more preferably from 0 to
 900.degree. C. The reaction time varies depending on the type of the
 oxidizing agent and the reaction temperature; however, the reaction time
 is generally in the range of from 0.001 to 100 hours, preferably from 0.1
 to 20 hours. The reaction pressure is generally in the range of from 10 to
 10.sup.7 Pa, preferably 10.sup.2 to 3.times.10.sup.6 Pa. The reaction can
 be performed in either a batchwise or a continuous manner.
 In the preferred mode of item (I) above, the gas phase-condensed phase
 separation method is employed to separate the (A)/(C) reaction product
 from the (B)/(C) reaction product. The condensed phase means a liquid
 phase, a solid phase or a solid/liquid mixed phase. In the case where the
 oxidation reaction mixture obtained at the completion of the oxidation
 reaction forms a liquid phase, a gas/liquid mixed phase or a
 gas/solid/liquid mixed phase, the reaction mixture is separated into a gas
 phase composed mainly of a low boiling point oxidation product and a
 condensed phase containing a metal oxide. Then, by distilling off or
 evaporating the low boiling point oxidation product from the separated
 condensed phase, a metal oxide-rich condensed phase (composed mainly of
 the metal oxide) can be obtained. Alternatively, when the metal oxide
 (formed by the oxidation of the metal-containing catalyst (B) which is
 conducted with respect to the catalyst-containing liquid) forms a solid
 phase during the oxidation reaction, it is possible to obtain a reaction
 mixture in the form of a liquid-solid mixture. Further, during the
 oxidation reaction, the low boiling point oxidation product formed by
 oxidation of the high boiling point substance (A) may be evaporated
 together with the volatile components of the liquid reaction system, to
 thereby obtain a solid reaction mixture. This method is preferred, because
 it becomes possible to separate the oxidation reaction system into the
 solid phase composed mainly of the metal oxide and the gas phase
 containing the low boiling point oxidation product while performing the
 oxidation reaction.
 The "low boiling point oxidation product" means compounds having a boiling
 point lower than that of the high boiling point substance (A), which are
 formed by oxidation of the high boiling point substance (A) using the
 oxidizing agent. The type of the low boiling point oxidation product
 varies depending on the type of the oxidizing agent and the type of the
 high boiling point substance (A). Examples of low boiling point oxidation
 products include carbon dioxide, water, carbon monoxide, oxygen-containing
 organic compounds, unsaturated organic compounds, compounds formed by the
 decomposition of the high boiling point substance.
 The "metal oxide" means an oxide of the metal of the metal-containing
 catalyst (B). A single type of the metal-containing catalyst (B) may form
 different metal oxides depending on the oxidation reaction conditions and
 the type of the metal contained in the catalyst (B). Specific examples of
 metal oxides include PbO, PbO.sub.2, Pb.sub.3 O.sub.4, CuO, Cu.sub.2 O,
 Li.sub.2 O, ZnO, CdO, FeO, Fe.sub.3 O.sub.4, Fe.sub.2 O.sub.3, CoO,
 Co.sub.3 O.sub.4, Co.sub.2 O.sub.3, CoO.sub.2, NiO, ZrO.sub.2, Al.sub.2
 O.sub.3, TiO, Ti.sub.2 O.sub.3, TiO.sub.2, SnO and SnO.sub.2. When the
 metal-containing catalyst (B) contains a plurality of different metals,
 there is or are obtained a mixture of metal oxides corresponding to the
 metals contained in the catalyst (B) or/and a compound metal oxide.
 When the preferred mode of item (II) above is employed, there is no
 particular limitation with respect to the metal-containing substance
 formed as the (B)/(C) reaction product, as long as the metal-containing
 substance is present in a solid state in the precipitation reaction
 mixture, and contains the metal. Examples of metal-containing substances
 include metal hydroxides; metal chalcogenides, such as a metal oxide and a
 metal sulfide; salts of inorganic acids, such as a metal carbonate and a
 metal sulfate; metal salts of organic acids; metal complexes; and metal
 double salts.
 Of these, from the viewpoint of the low solubility in the reaction mixture,
 a metal carbonate, a metal hydroxide, a metal oxide, a metal sulfide and a
 metal sulfate are preferred. Each of the metal-containing substances may
 contain other substance (such as the reactant, the starting material and
 the high boiling point substance) coordinated thereto.
 With respect to the precipitant, there is no particular limitation, as long
 as the precipitant can react with the metal-containing catalyst (B) to
 form the above-mentioned metal-containing substance. For example, for
 precipitating metal hydroxides, use can be made of inorganic hydroxides
 (such as a hydroxide of an alkali metal or an alkaline earth metal) and
 water; for precipitating metal oxides, use can be made of inorganic oxides
 (such as an oxide of an alkali metal or an alkaline earth metal) and
 oxidizing agents (such as hydrogen peroxide); for precipitating metal
 sulfides, use can be made of inorganic sulfides (such as a sulfide of an
 alkali metal or an alkaline earth metal) and hydrogen sulfide; for
 precipitating metal carbonates, use can be made of inorganic carbonates
 (such as a carbonate of an alkali metal or an alkaline earth metal),
 carbonic acid and carbon dioxide with water; for precipitating metal
 sulfates, use can be made of inorganic sulfates (such as a sulfate of an
 alkali metal or an alkaline earth metal), sulfuric acid and sulfur
 trioxide with water.
 The type of the reaction between the metal-containing catalyst (B) and the
 precipitant varies depending on the type of the catalyst, the type of the
 precipitant, the reaction conditions and the like. However, the reaction
 is generally performed in a phase selected from the group consisting of a
 liquid phase, a liquid-gas mixed phase, a gas-liquid-solid mixed phase and
 a solid-liquid mixed phase. The reaction temperature varies depending on
 the type of the precipitant; however, the reaction temperature is
 generally in the range of from -70 to 600.degree. C., preferably from -30
 to 400.degree. C., more preferably from -10 to 250.degree. C. The reaction
 time varies depending on the type of the precipitant and the reaction
 temperature; however, the reaction time is generally in the range of from
 0.001 to 100 hours, preferably from 0.1 to 20 hours. The reaction pressure
 is generally in the range of from 10 to 10.sup.7 Pa. The above-mentioned
 reaction can be performed in either a batchwise manner or a continuous
 manner.
 In the present invention, it is preferred to add a substance which serves
 as a crystal nucleus to the precipitation reaction system. At the time of
 the separation of the metal-containing substance from the precipitation
 reaction mixture, the metal-containing substance needs to be in a solid
 state. However, the metal-containing substance need not be in a solid
 state during the precipitation reaction, as long as the metal-containing
 substance becomes a solid by a cooling operation, etc. after the
 completion of the reaction.
 In the preferred mode of item (II) above, the solid phase-liquid phase
 separation method is employed to separate the (A)/(C) reaction product
 from the (B)/(C) reaction product. Specifically, the precipitation
 reaction mixture is separated into a solid phase composed mainly of a
 metal-containing substance and a liquid phase composed mainly of
 substances originating from a high boiling point substance. The solid
 phase-liquid phase separation method is generally conducted by
 sedimentation, centrifugation, filtration or the like.
 Further, in the preferred mode of item (II) above, the high boiling point
 substance (A) contained in the catalyst-containing liquid does not undergo
 the precipitation reaction with the functional substance (C) [therefore,
 in this preferred mode, the unreacted component (A), which is not
 precipitated when the functional substance (C) is added, is regarded as
 the (A)/(C) reaction product]; however, the high boiling point substance
 (A) may undergo a reaction other than the precipitation reaction during
 the precipitation reaction of the metal-containing catalyst (B).
 When the preferred mode of item (III) above is employed, there is no
 particular limitation with respect to the reactive solvent, as long as the
 reactive solvent can react with the high boiling point substance (A) to
 form compounds having a boiling point lower than the boiling point of the
 high boiling point substance (A). Examples of reactive solvents include
 water; lower alcohols, such as methanol, ethanol, propanol (isomer) and
 butanol (isomer); lower carboxylic acids, such as formic acid, acetic acid
 and propionic acid; and carbonates, such as dimethyl carbonate and diethyl
 carbonate. Of these, water, methanol, ethanol, acetic acid, methyl
 acetate, ethyl acetate, dimethyl carbonate, diethyl carbonate and the like
 are preferred, and water is more preferred.
 In the present invention, the "solvolysis" means the decomposition reaction
 of the high boiling point substance (A) with the reactive solvent. The
 reaction product obtained by the solvolysis may be subjected to further
 reaction other than the solvolysis, such as the decarboxylation and the
 like.
 With respect to the low boiling point product obtained by the solvolysis,
 there is no particular limitation, as long as the low boiling point
 product has a boiling point lower than the boiling point of the high
 boiling point substance (A). The type and structure of the low boiling
 point product vary depending on the type of the reactive solvent and the
 type of the high boiling point substance (A). With respect to the
 relationship between the reactive solvent, the high boiling point
 substance (A) and the low boiling point product, specific explanation is
 made below, taking as an example the case where the high boiling point
 substance (A) is phenyl salicylate which is one of the aromatic carboxy
 compounds.
 (i) When the reactive solvent is water, phenol and salicylic acid are
 formed by the hydrolysis, and the formed salicylic acid undergoes
 decarboxylation to form phenol and carbon dioxide.
 (ii) When the reactive solvent is an alcohol, an alkyl salicylate and
 phenol are formed by alcoholysis.
 (iii) When the reactive solvent is a carboxylic acid, salicylic acid and a
 phenyl carboxylate are formed by transesterification, and the formed
 salicylic acid undergoes decarboxylation to form phenol and carbon
 dioxide.
 As mentioned above, the above explanation is made, taking as an example
 phenyl salicylate, which has a relatively simple structure as an aromatic
 carboxy compound. However, also in the case of an aromatic carboxy
 compound having a more complicated structure, the same types of reactions
 as mentioned in items (i) to (iii) above occur. Therefore, as the reaction
 products corresponding to those mentioned in items (i) to (iii) above,
 there can be obtained, for example, an aromatic hydroxy compound, such as
 an aromatic monohydroxy compound; a lower carboxylic acid ester of an
 aromatic monohydroxy compound; an ester of an aromatic carboxy compound
 with a lower alcohol; and carbon dioxide. Of the above-mentioned reaction
 products obtained by the solvolysis, the aromatic monohydroxy compound is
 especially preferred, because this product is a reactant used in the
 present invention so that this product can be recycled.
 The catalyst-containing liquid contains the metal-containing catalyst (B),
 and the catalyst (B) generally also serves as a catalyst for the
 solvolysis. Therefore, it is not necessary to specifically use a catalyst
 for the solvolysis, but such a catalyst for the solvolysis can be used for
 the purpose of improving the reaction rate, etc.
 The type of the reaction between the high boiling point substance (A) and
 the reactive solvent varies depending on the reaction conditions; however,
 the reaction is generally performed in a phase selected from the group
 consisting of a liquid phase and a solid-liquid mixed phase. The reaction
 temperature varies depending on the type of the reactive solvent; however,
 the reaction temperature is generally in the range of from -30 to
 400.degree. C., preferably from -10 to 300.degree. C., more preferably
 from 0 to 250.degree. C. The reaction time varies depending on the type of
 the reactive solvent and the reaction temperature; however, the reaction
 time is generally in the range of from 0.001 to 100 hours, preferably from
 0.1 to 20 hours. The reaction pressure is generally in the range of from
 10 to 10.sup.7 Pa. The reaction can be performed in either a batchwise
 manner or a continuous manner.
 The metal-containing catalyst (B) may or may not undergo the solvolysis
 [therefore, in this preferred mode, when the metal-containing catalyst (B)
 does not undergo the solvolysis, the unreacted component (B), which is not
 solvolyzed with the functional substance (C), is regarded as the (B)/(C)
 reaction product]. In the case where water or an alcohol is used as a
 reactive solvent so as to solvolyze an aromatic carboxy compound contained
 as the high boiling point substance (A) in the catalyst-containing liquid,
 a decarboxylation reaction occurs simultaneously with the solvolysis, so
 that carbon dioxide is formed as one of the reaction products originating
 from the high boiling point substance (A). Therefore, it is possible that
 the formed carbon dioxide serves as a precipitant and reacts with the
 metal-containing catalyst (B) to there-by form a metal-containing
 substance (such as a metal carbonate) in the form of a solution thereof
 and/or in the form of a solid.
 In the preferred mode of item (III) above, the separation of the (A)/(C)
 reaction product from the (B)/(C) reaction product is conducted by a
 distillation method, wherein a low boiling point product formed as the
 (A)/(C) reaction product by the solvolysis is removed from the solvolysis
 reaction mixture as a distillate. The (B)/(C) reaction product is
 contained in the liquid remaining in the distillation column employed. The
 distillation temperature is generally in the range of from 10 to
 300.degree. C., preferably from 50 to 250.degree. C., in terms of the
 temperature of the liquid in the distillation column. The distillation
 pressure is generally in the range of from 0.1 to 1.0.times.10.sup.6 Pa,
 preferably from 1.0 to 1.0.times.10.sup.5 Pa. The distillation can be
 conducted either in a batchwise manner or a continuous manner.
 The recycling of the (B)/(C) reaction product to the reaction system can be
 conducted by a method in which the (B)/(C) reaction product, which has
 been separated from the (A)/(C) reaction product and which is in the form
 of a liquid, a solid or a liquid-solid mixture, as such, is recycled to
 the reaction system. Alternatively, when the (B)/(C) reaction product is
 obtained in such a form as contains other components than the reaction
 product, the recycling of the (B)/(C) reaction product can be conducted by
 a method in which a part or all of such other components are separated
 from the other components-containing (B)/(C) reaction product, and the
 resultant is recycled to the reaction system. Further, the recycling of
 the (B)/(C) reaction product can be conducted by a method in which the
 separated (B)/(C) reaction product is mixed and/or reacted with the
 starting material or the reactant, and the resultant (i.e., a liquid
 reaction mixture, a slurry, etc.) is recycled to the reaction system. This
 method is advantageous when the (B)/(C) reaction product is in the form of
 a solid or a solid-liquid mixture. The recycling of the (B)/(C) reaction
 product to the reaction system can be conducted in either a batch-wise
 manner or in a continuous manner.
 As mentioned above, the method of the present invention comprises:
 taking out at least one type of catalyst-containing liquid which is
 selected from the group consisting of:
 a portion of the high boiling point reaction mixture obtained by the
 transesterification reaction before the separation of the high boiling
 point reaction mixture into the product fraction and the liquid catalyst
 fraction, and
 a portion of the separated liquid catalyst fraction,
 each portion containing at least one high boiling point substance (A)
 having a boiling point higher than the boiling point of the produced
 aromatic carbonate and containing the metal-containing catalyst (B); and
 adding to the taken-out catalyst-containing liquid a functional substance
 (C) capable of reacting with at least one component selected from the
 group consisting of the component (A) and the component (B).
 With respect to the amount of the portion of the high boiling point
 reaction mixture, which is taken out as the catalyst-containing liquid,
 the amount is from 0.01 to 10% by weight, preferably from 0.1 to 5% by
 weight, more preferably from 0.3 to 1% by weight, based on the weight of
 the high boiling point reaction mixture. On the other hand, with respect
 to the amount of the portion of the separated liquid catalyst fraction,
 which is taken out as the catalyst-containing liquid, the amount is from
 0.01 to 40% by weight, preferably from 0.1 to 20% by weight, more
 preferably from 1 to 10% by weight, based on the weight of the separated
 liquid catalyst fraction.
 With respect to the concentration of the high boiling point substance (A)
 in the taken-out catalyst-containing liquid, the concentration varies
 depending on the type of the high boiling point substance (A). However,
 too low a concentration of the high boiling point substance (A) is not
 preferable, since the amount of the taken-out catalyst-containing liquid
 becomes too large. On the other hand, too high a concentration of the high
 boiling point substance (A) is also not preferable, since the boiling
 point and viscosity of the taken-out catalyst-containing liquid become too
 high, so that the handling of the taken-out catalyst-containing liquid
 becomes difficult. Therefore, the concentration of the high boiling point
 substance (A) in the taken-out catalyst-containing liquid is generally
 from 0.01 to 99% by weight, preferably from 0.1 to 95% by weight, more
 preferably from 1 to 90% by weight.
 Further, when the high boiling point substance (A) is an aromatic
 polyhydroxy compound, for preventing the catalyst from depositing on or
 adhering to the inner walls of the reactor, the pipes and the like, it is
 preferred that the taken-out catalyst-containing liquid contains the
 aromatic polyhydroxy compound and the metal-containing catalyst in amounts
 such that the weight ratio of the aromatic polyhydroxy compound to the
 metal of the catalyst becomes 2.0 or less.
 With respect to the separation of the desired aromatic carbonate from the
 product fraction (separated from the high boiling point reaction mixture
 obtained by the transesterification reaction) comprising the aromatic
 carbonate, the unreacted starting material and the unreacted reactant, the
 separation can be easily conducted by a conventional separation method,
 such as a distillation method.
 In the present invention, the purity of the aromatic carbonate which has
 been separated from the product fraction can be calculated by the
 following formula:
 ##EQU1##
 The purity of the aromatic carbonate obtained by the process of the present
 invention is generally 99% or more, preferably 99.5% or more, most
 preferably 99.8% or more.
 In a further preferred aspect of the present invention, there is provided a
 mode of the above-mentioned process of the present invention, in which the
 above-mentioned steps (1), (2) and (3) are continuously conducted. That
 is, in this mode of the process, the following steps are continuously
 conducted:
 (1) transesterifying, in the presence of a metal-containing catalyst, a
 starting material selected from the group consisting of a dialkyl
 carbonate, an alkyl aryl carbonate and a mixture thereof with a reactant
 selected from the group consisting of an aromatic monohydroxy compound, an
 alkyl aryl carbonate and a mixture thereof, to thereby obtain a high
 boiling point reaction mixture comprising the metal-containing catalyst
 and at least one aromatic carbonate, while withdrawing a low boiling point
 reaction mixture which contains a low boiling point by-product comprising
 an aliphatic alcohol, a dialkyl carbonate or a mixture thereof,
 (2) separating the high boiling point reaction mixture into a product
 fraction comprising the produced aromatic carbonate and a liquid catalyst
 fraction comprising the metal-containing catalyst, and
 (3) recycling the liquid catalyst fraction to the reaction system while
 withdrawing the product fraction, thereby enabling continuous production
 of the aromatic carbonate.
 In this preferred mode for continuously producing the aromatic carbonate,
 it is especially preferred that the step (1) of the process of the present
 invention is performed as follows: the starting material and the reactant
 are continuously fed to a continuous multi-stage distillation column to
 effect a transesterification reaction therebetween in at least one phase
 selected from the group consisting of a liquid phase and a gas-liquid
 phase in the presence of a metal-containing catalyst, wherein a high
 boiling point reaction mixture containing the produced aromatic carbonate
 is withdrawn in a liquid form from a lower portion of the distillation
 column, while continuously withdrawing a low boiling point reaction
 mixture containing the low boiling point by-product in a gaseous form from
 an upper portion of the distillation column by distillation.
 In another aspect of the present invention, there is provided a process for
 producing an aromatic polycarbonate, which comprises polymerizing the high
 purity diaryl carbonate obtained by the process of the present invention
 with an aromatic dihydroxy compound by transesterification.
 With respect to the method for producing the aromatic polycarbonate by
 transesterification, reference can be made to, for example, U.S. Pat. No.
 5,589,564. By the use of the diaryl carbonate obtained by the process of
 the present invention, it has become possible to perform the
 polymerization at a high rate. Further, the aromatic polycarbonate
 obtained by the transesterification reaction between the aromatic
 dihydroxy compound and the diaryl carbonate obtained by the process of the
 present invention is a high quality aromatic polycarbonate which is free
 from the discoloration.
 The aromatic dihydroxy compound, which can be used for producing the
 aromatic polycarbonate by transesterification, can be represented by the
 following formula:
EQU HO--Ar'--OH
 wherein Ar' represents a divalent aromatic group having from 5 to 200
 carbon atoms.
 Preferred examples of divalent aromatic groups Ar' having from 5 to 200
 carbon atoms include an unsubstituted or substituted phenylene group, an
 unsubstituted or substituted naphthylene group, an unsubstituted or
 substituted biphenylene group and an unsubstituted or substituted
 pyridylene group. Further examples of such divalent aromatic groups
 include divalent groups, each represented by the following formula:
EQU --Ar.sup.1' --Y --Ar.sup.2'
 wherein each of Ar.sup.1' and Ar.sup.2' independently represents a divalent
 carbocyclic or heterocyclic aromatic group having from 5 to 70 carbon
 atoms, and Y' represents a divalent alkane group having from 1 to 30
 carbon atoms.
 In the divalent aromatic groups Ar.sup.1' and Ar.sup.2', at least one
 hydrogen atom may be substituted with a which does not adversely affect
 the reaction, such as a halogen atom, an alkyl group having from 1 to 10
 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a phenyl
 group, a phenoxy group, a vinyl group, a cyano group, an ester group, an
 amide group and a nitro group.
 Illustrative examples of heterocyclic aromatic groups include an aromatic
 group having at least one hetero atom, such as a nitrogen atom, an oxygen
 atom or a sulfur atom.
 Examples of divalent aromatic groups Ar.sup.1' and Ar.sup.2' include an
 unsubstituted or substituted phenylene group, an unsubstituted or
 substituted biphenylene group and an unsubstituted or substituted
 pyridylene group. Substituents for Ar.sup.1' and Ar.sup.2' are as
 described above.
 Examples of divalent alkane groups Y' include organic groups respectively
 represented by the following formulae:
 ##STR41##
 wherein each of R.sup.3', R.sup.4', R.sup.5' and R.sup.6' independently
 represents a hydrogen atom, an alkyl group having from 1 to 10 carbon
 atoms, an alkoxy group having from 1 to 10 carbon atoms, a cycloalkyl
 group having from 5 to 10 ring-forming carbon atoms, a carbocyclic
 aromatic group having from 5 to 10 ring-forming carbon atoms and a
 carbocyclic aralkyl group having from 6 to 10 ring-forming carbon atoms;
 k' represents an integer of from 3 to 11; each X' represents a carbon atom
 and has R.sup.7' and R.sup.8' bonded thereto; each R.sup.7' independently
 represents a hydrogen atom or an alkyl group having from 1 to 6 carbon
 atoms, and each R.sup.8' independently represents a hydrogen atom or an
 alkyl group having from 1 to 6 carbon atoms, wherein R.sup.7' and R.sup.8'
 are the same or different;
 wherein at least one hydrogen atom of each of R.sup.3', R.sup.4', R.sup.5',
 R.sup.6', R.sup.7' and R.sup.8' may be independently replaced by a
 substituent which does not adversely affect the reaction, such as a
 halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy
 group having from 1 to 10 carbon atoms, a phenyl group, a phenoxy group, a
 vinyl group, a cyano group, an ester group, an amide group and a nitro
 group.
 Specific examples of divalent aromatic groups Ar' include groups
 respectively represented by the following formulae:
 ##STR42##
 wherein each of R.sup.9' and R.sup.10' independently represents a hydrogen
 atom, a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an
 alkoxy group having from 1 to 10 carbon atoms, a cycloalkyl group having
 from 5 to 10 ring-forming carbon atoms, or an allyl group having from 6 to
 30 carbon atoms; each of m' and n' independently represents an integer of
 from 1 to 4, with the proviso that when m' is an integer of from 2 to 4,
 R.sup.9' 's are the same or different, and when n' is an integer of from 2
 to 4, R.sup.10' 's are the same or different.
 Further, examples of divalent aromatic groups Ar' also include those which
 are represented by the following formula:
EQU --Ar.sup.1' --Z--Ar.sup.2' --
 wherein Ar.sup.1' and Ar.sup.2' are as defined above; and Z' represents a
 single bond or a divalent group, such as --O--, --CO--, --S--, --SO.sub.2,
 --SO--, --COO--, or --CON(R.sup.3')--, wherein R.sup.3' is as defined
 above.
 Examples of such divalent aromatic groups Ar' include groups respectively
 represented by the following formulae:
 ##STR43##
 wherein R.sup.9', R.sup.10', m' and n' are as defined above.
 The above-mentioned aromatic dihydroxy compounds can be used individually
 or in combination. Representative examples of aromatic dihydroxy compounds
 include bisphenol A.
 With respect to the material of an apparatus used for producing the
 aromatic polycarbonate, there is no particular limitation. However,
 stainless steel, glass or the like is generally used as a material for at
 least the inner walls of the apparatus.
 BEST MODE FOR CARRYING OUT THE INVENTION
 Hereinbelow, the present invention will be described in more detail with
 reference to the following Examples and Comparative Examples, but they
 should not be construed as limiting the scope of the present invention.
 In the following Examples and Comparative Examples, various measurements
 were conducted in accordance with the following methods.
 The metal concentration of a metal-containing catalyst was measured by
 means of an ICP (inductively coupled plasma emission spectral analyzer)
 (JY38PII: manufactured and sold by Seiko Electronics Co, Ltd., Japan).
 The concentration of an organic matter in a liquid was measured by gas
 chromatography.
 The concentration of a high boiling point substance (A) coordinated to a
 metal-containing catalyst in a catalyst-containing liquid was measured by
 a method in which a ligand exchange with trifluoroacetic acid is
 conducted, followed by analysis by gas chromatography.
 The total concentration of both high boiling point substances (A)
 coordinated to and not coordinated to a metal-containing catalyst in a
 catalyst-containing liquid was determined as follows. The
 catalyst-containing liquid was subjected to distillation using a small
 size distillation column and the total of the weight of a fraction having
 a boiling point higher than that of a desired aromatic carbonate and the
 weight of an organic matter contained in the distillation residue
 remaining in the distillation column was calculated. Then, the weight
 percentage of the thus calculated total weight, based on the weight of the
 catalyst-containing liquid, was obtained, and the obtained weight
 percentage was taken as the total concentration of both high boiling point
 substances (A) coordinated to and not coordinated to a metal-containing
 catalyst in the catalyst-containing liquid.
 The number average molecular weight of a produced aromatic polycarbonate
 was measured by gel permeation chromatography (GPC) (apparatus: HLC-8020,
 manufactured and sold by Tosoh Corp., Japan; column: TSK-GEL, manufactured
 and sold by Tosoh Corp., Japan; solvent: tetrahydrofuran).
 All of the concentrations are indicated by weight percentages.
 EXAMPLE 1
 Preparation of Catalyst
 A mixture of 40 kg of phenol (hereinafter, frequently referred to as
 "PhOH") and 8 kg of lead monoxide was heated to and maintained at
 180.degree. C. for 10 hours, thereby performing a reaction. After that
 period of time, water formed in the resultant reaction mixture was
 distilled off together with unreacted phenol, to thereby obtain catalyst
 I.
 Production of Aromatic Carbonate
 The production of an aromatic carbonate was conducted using the system as
 shown in FIG. 1, which comprises continuous multi-stage distillation
 column 1 having a height of 6 m and a diameter of 6 inches and equipped
 with 20 sieve trays.
 A mixture of dimethyl carbonate (hereinafter, frequently referred to as
 "DMC"), phenol (which contains, as an impurity, 30 ppm by weight of
 4,4'-dihydroxydiphenyl which is a high boiling point substance) and
 catalyst I was continuously fed in liquid form from conduit 3 through
 preheater 4 and conduit 5 into continuous multi-stage distillation column
 1 at a position of 0.5 m below top 2 thereof at a rate of 32 kg/hr, and
 was allowed to flow down inside multi-stage distillation column 1, thereby
 performing a reaction. The weight ratio of the dimethyl carbonate to the
 phenol in the mixture was 62/38, and catalyst I was used in an amount such
 that the Pb concentration of the reaction mixture in conduit 13 became
 0.038% by weight, wherein the Pb concentration can be confirmed using a
 sample withdrawn through a sampling nozzle (not shown) provided on conduit
 13. Dimethyl carbonate was fed from conduit 7 into evaporator 8 thereby
 forming a gas and the formed gas of dimethyl carbonate was fed through
 conduit 9 to bottom 6 of continuous multi-stage distillation column 1 at a
 rate of 26 kg/hr. The reaction conditions of the above reaction were such
 that the temperature at the bottom of continuous multi-stage distillation
 column 1 was 203.degree. C. and the pressure at the top of continuous
 multi-stage distillation column 1 was 7.4.times.10.sup.5 Pa. Gas distilled
 from column top 2 was led through conduit 10 into condenser 11, in which
 the gas was condensed. The resultant condensate was continuously withdrawn
 at a rate of 25 kg/hr through conduit 12. A reaction mixture [containing
 methyl phenyl carbonate (as a desired reaction product) (hereinafter,
 frequently referred to as "MPC"), the catalyst, and high boiling point
 substances] was continuously withdrawn from column bottom 6 at a rate of
 34 kg/hr and led into evaporator 14 through conduit 13, from which an
 evaporated gas containing the methyl phenyl carbonate was withdrawn and
 led through conduit 21 into condenser 22, in which the gas was condensed.
 The resultant condensate was withdrawn from condenser 22 through conduit
 23, wherein the condensate withdrawal rate during the period of time of
 400 hours from the start of the operation, the condensate withdrawal rate
 during the period of time of from 400 hours to 600 hours after the start
 of the operation and the condensate withdrawal rate during the period of
 time of from 600 hours to 5,000 hours after the start of the operation
 were 32.95 kg/hr, 32.99 kg/hr and 33 kg/hr, respectively. On the other
 hand, an evaporation-concentrated liquid containing the catalyst and high
 boiling point substances was formed in evaporator 14. A portion of the
 concentrated liquid was led into reboiler 17 through conduits 15 and 16
 and recycled into evaporator 14 through conduit 18. The remainder of the
 concentrated liquid in evaporator 14 was recycled into continuous
 multi-stage distillation column 1 at a rate of 1 kg/hr through conduits
 15, 19 and 3. After the start of the recycling of the concentrated liquid
 into continuous multi-stage distillation column 1 through conduits 15, 19
 and 3, the feeding rate of the mixture of dimethyl carbonate, phenol and
 catalyst I through conduit 3 into continuous multi-stage distillation
 column 1 was appropriately controlled according to the recycling rate of
 the concentrated liquid.
 During the period of time of from 400 hours to 5,000 hours after the start
 of the operation, a portion of the concentrated liquid formed in
 evaporator 14 was continuously withdrawn through conduit 20 at a rate of
 0.05 kg/hr and led into thin-film evaporator 33 thereby forming an
 evaporated gas. At a point in time of 400 hours after the start of the
 operation, a sample (of the concentrated liquid withdrawn from evaporator
 14) was taken through a sampling nozzle (not shown) provided on conduit
 15', and was analyzed to determine the composition of the concentrated
 liquid by the above-mentioned methods. The concentrated liquid had the
 following composition: Pb (which is the metal component of catalyst I):
 1.3% by weight; the total concentration of high boiling point substances:
 1.7% by weight; and 4,4'-dihydroxydiphenyl (which is a high boiling point
 substance): 0.7% by weight. The evaporated gas formed in thin-film
 evaporator 33 was continuously withdrawn therefrom through conduit 35 at a
 rate of 0.04 kg/hr and recycled through conduit 49 into the system for the
 transesterification. On the other hand, an evaporation-concentrated liquid
 containing the catalyst and high boiling point substances was continuously
 withdrawn from the bottom of thin-film evaporator 33 through conduit 34 at
 a rate of 0.01 kg/hr and led into storage vessel 36. A sample (of the
 evaporation-concentrated liquid withdrawn from thin-film evaporator 33)
 was taken through a sampling nozzle (not shown) provided on conduit 34 at
 a point in time of 400 hours after the start of the operation, and was
 analyzed to determine the composition of the evaporation-concentrated
 liquid by the above-mentioned methods. The evaporation-concentrated liquid
 had the following composition: Pb (which is the metal component of
 catalyst I): 6.5% by weight; the total concentration of high boiling point
 substances: 8.6% by weight; and 4,4'-dihydroxydiphenyl (which is a high
 boiling point substance): 3.6% by weight. At a point in time of 550 hours
 after the start of the operation, 1 kg of the concentrated liquid stored
 in storage vessel 36 was withdrawn and led into electric furnace 38
 through conduit 37. In electric furnace 38, the concentrated liquid was
 heated to and maintained at 700.degree. C. for 8 hours while introducing
 air into electric furnace 38 from conduit 39, to thereby oxidize the
 concentrated liquid under atmospheric pressure. The resultant oxidation
 products (i.e., carbon dioxide, water and low boiling point organic
 compounds) derived from organic matter contained in the concentrated
 liquid were withdrawn through waste product conduit 40. The oxidation
 products remaining in electric furnace 38 were allowed to cool and, then,
 a sample of the remaining oxidation products was taken out from electric
 furnace 38 and analyzed. By the analysis, only lead monoxide, derived from
 catalyst I, was detected. This means that, by the oxidative reaction of
 the concentrated liquid, the organic matter in the concentrated liquid was
 changed to volatile oxidation products having a low boiling point.
 0.07 kg of the oxidation product remaining in electric furnace 38 (i.e.,
 lead monoxide) was charged into reaction vessel 42 provided with
 distillation column 43 and a jacket (not shown) for circulating a heating
 medium, and 1.2 kg of phenol was introduced into reaction vessel 42 from
 conduit 45, to thereby obtain a mixture. The obtained mixture was heated
 to and maintained at 160.degree. C. (as measured at the heating medium)
 for 6 hours under atmospheric pressure, thereby performing a reaction.
 Then, the heating temperature was elevated to 200.degree. C. (as measured
 at the heating medium) so as to cause both the water formed by the
 reaction and unreacted phenol to be distilled off from the top of
 distillation column 43 through conduit 44, wherein the total amount of the
 water and the unreacted phenol both distilled off was 0.277 kg. A sample
 was taken from the reaction mixture remaining in reaction vessel 42 and
 analyzed. The results of the analysis show that the remaining reaction
 mixture is a solution of lead(II) diphenoxide [Pb(OPh).sub.2 ] in phenol.
 1 kg of the remaining reaction mixture was withdrawn from reaction vessel
 42 and transferred through conduit 46 and introduced into storage vessel
 47. Thereafter, every 100 hours after the point in time of 550 hours from
 the start of the operation (i.e., the point in time at which 1 kg of the
 concentrated liquid was withdrawn from storage vessel 36 and led into
 electric furnace 38 as mentioned above), a sequence of the above
 operations using storage vessel 36 (from which 1 kg of the concentrated
 liquid was withdrawn), electric furnace 38, reaction vessel 42 and storage
 vessel 47 (into which 1 kg of the remaining reaction mixture obtained in
 reaction vessel 42 was introduced) was repeated in the same manner as
 described above. On the other hand, from a point in time of 600 hours
 after the start of the operation, the reaction mixture stored in storage
 vessel 47 was continuously withdrawn at a rate of 0.01 kg/hr through
 conduit 48, and the reaction mixture withdrawn from storage vessel 47 was
 caused to meet the evaporated gas which was withdrawn from thin-film
 evaporator 33 and which was led through conduit 35, and the resultant
 mixture (i.e., a mixture of the products withdrawn through conduits 48 and
 35) was recycled into the system for the transesterification through
 conduit 49. As mentioned above, the condensate withdrawal rate from
 condenser 22 through conduit 23 during the period of time of from 400
 hours to 600 hours after the start of the operation was 32.99 kg/hr, and
 the condensate withdrawal rate from condenser 22 through conduit 23 during
 the period of time of from 600 hours to 5,000 hours after the start of the
 operation was 33 kg/hr. During the period of time of from 400 hours to 600
 hours after the start of the operation, catalyst I was added to
 distillation column 1 through conduit 3 at such a feeding rate as to
 compensate for the catalyst withdrawal rate at which the catalyst was
 withdrawn through conduit 20, i.e., catalyst I was added through conduit 3
 at a feeding rate such that the above-mentioned Pb concentration of 0.038%
 by weight in conduit 13 was able to be maintained. The operation was
 conducted for 5,000 hours. From the point in time of 600 hours after the
 start of the operation, i.e., from the point in time at which the
 recycling of the catalyst into the system for the transesterification
 through conduit 49 was started, there was no need for introducing a fresh
 catalyst into the system for the transesterification. In addition, since
 the catalyst-containing liquid containing both the catalyst and high
 boiling point substances was withdrawn from the system for the
 transesterification and subjected to the above-described treatments
 according to the present invention, a waste liquid containing a spent
 catalyst did not occur at all. From the evaporation-concentrated liquid
 which was formed in evaporator 14 and which contained the catalyst and
 high boiling point substances, samples were taken through the
 above-mentioned sampling nozzle provided on conduit 15', wherein the
 samples were, respectively, withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation. The
 determination of the total concentration of the high boiling point
 substances in each sample was conducted by the above-mentioned method.
 With respect to these samples withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation, the total
 concentrations of the high boiling point substances were 1.7% by weight,
 1.8% by weight and 1.8% by weight, respectively.
 During the 5,000 hour operation time, the operation could be stably
 conducted (for example, both the flow and the composition in each conduit
 were stable) without suffering disadvantageous phenomena, such as the
 deposition of the catalyst from a catalyst-containing liquid and the
 adherence of the deposited catalyst to the inside surfaces associated with
 the equipment employed for the operation. During the operation, samples of
 the reaction mixture withdrawn from the bottom of continuous multi-stage
 distillation column 1 were taken through the above-mentioned sampling
 nozzle provided on conduit 13, and the samples were analyzed. With respect
 to the reaction mixture which was taken from conduit 13 at a point in time
 of 3,000 hours after the start of the operation, the composition of the
 reaction mixture was as follows: phenol (PhOH): 31% by weight; methyl
 phenyl carbonate (MPC): 9% by weight; diphenyl carbonate (hereinafter,
 frequently referred to as "DPC"): 0.5% by weight; anisole (hereinafter,
 frequently referred to as "ANS"): 0.1% by weight; and Pb: 0.038% by
 weight. The purity of the aromatic carbonate (which was a mixture of MPC
 and DPC) in the condensate withdrawn from condenser 22 through conduit 23
 was 99.99% or more, and no high boiling point substance was detected in
 the condensate. After the operation was terminated, the inside surfaces
 associated with the equipment employed for the operation were examined. No
 adherence of the catalyst to any of the inner walls of continuous
 multi-stage distillation column 1, evaporator 14, reboiler 17, conduits
 and the like was observed.
 Comparative Example 1
 Substantially the same procedure as in Example 1 was repeated, except that
 the withdrawal of a portion of the evaporation-concentrated liquid (which
 was formed in evaporator 14 and which contained the catalyst and high
 boiling point substances) out of the production system through conduit 20
 was not conducted, and the introduction of the fresh catalyst into the
 system for the transesterification from conduit 3 through preheater 4 and
 conduit 5 into continuous multi-stage distillation column 1 (which was
 conducted in Example 1 during the period of time of from 400 hours to 600
 hours after the start of the operation) was not conducted. With respect to
 the samples withdrawn at points in time of 1,000 hours, 2,500 hours and
 5,000 hours after the start of the operation, the total concentrations of
 the high boiling point substances were 5.2% by weight, 14.6% by weight and
 32.0% by weight, respectively. With respect to the reaction mixture which
 was taken from conduit 13 at a point in time of 3,000 hours after the
 start of the operation, the composition of the reaction mixture was as
 follows: PhOH: 33% by weight; MPC: 6.5% by weight; DPC: 0.2% by weight;
 ANS: 0.1% by weight; and Pb: 0.038% by weight. The purity of the aromatic
 carbonate (which was a mixture of MPC and DPC) in the condensate withdrawn
 from condenser 22 through conduit 23 was 97%, and the total concentration
 of the high boiling point substances in the above-mentioned condensate was
 1.5% by weight. After the operation was terminated (the operation was
 conducted for 5,000 hours), the inside surfaces associated with the
 equipment employed for the operation were examined. The adherence of the
 catalyst to a part of the inner wall of each of continuous multi-stage
 distillation column 1, evaporator 14 and the conduits was observed.
 Comparative Example 2
 Substantially the same procedure as in Example 1 was repeated, except that,
 after an evaporation-concentrated liquid containing the catalyst and high
 boiling point substances was withdrawn from the bottom of thin-film
 evaporator 33, the evaporation-concentrated liquid was introduced into and
 accumulated in a waste catalyst storage vessel (not shown) instead of
 leading the evaporation-concentrated liquid to storage vessel 36, so that
 the sequence of the operations using storage vessel 36, electric furnace
 38, reaction vessel 42 and storage vessel 47 was not conducted; and not
 only during the period of time of from 400 hours to 600 hours after the
 start of the operation, but also after the point in time of 600 hours
 after the start of the operation, catalyst I was added to distillation
 column 1 through conduit 3 at such a feeding rate as to compensate for the
 catalyst withdrawal rate at which the catalyst was withdrawn through
 conduit 20, i.e., catalyst I was added through conduit 3 at a feeding rate
 such that the Pb concentration of 0.038% by weight in conduit 13 was able
 to be maintained. The operation was conducted for 5,000 hours. During the
 period of time of from 600 hours to 5,000 hours after the start of the
 operation, in order to maintain the above-mentioned Pb concentration of
 0.038% by weight in conduit 13, it was necessary to add fresh catalyst I
 to continuous multi-stage distillation column 1 through conduit 3 in an
 amount as large as 2.86 kg, in terms of the weight of Pb in the catalyst.
 During the period of time of from 600 hours to 5,000 hours after the start
 of the operation, the amount of the evaporation-concentrated liquid
 (containing the catalyst and high boiling point substances) which was
 withdrawn from the bottom of thin-film evaporator 33 and introduced into
 and accumulated in the waste catalyst storage vessel reached a level as
 large as 44 kg.
 EXAMPLE 2
 The production of diphenyl carbonate (DPC) from methyl phenyl carbonate
 (MPC) was conducted using catalyst I prepared in Example 1, and the system
 as shown in FIG. 2, which comprises continuous multi-stage distillation
 column 1 having a height of 6 m and a diameter of 4 inches and equipped
 with 20 sieve trays.
 A mixture of MPC and catalyst I was continuously fed in liquid form from
 conduit 3 through preheater 4 and conduit 5 into continuous multi-stage
 distillation column 1 at a position of 2.0 m below top 2 thereof at a rate
 of 8 kg/hr, and was allowed to flow down inside multi-stage distillation
 column 1, thereby performing a reaction. Catalyst I was used in an amount
 such that the Pb concentration of the reaction mixture in conduit 13
 became 0.19% by weight, wherein the Pb concentration can be confirmed
 using a sample withdrawn through a sampling nozzle (not shown) provided on
 conduit 13.
 The reaction conditions of the above reaction were such that the
 temperature at the bottom of continuous multistage distillation column 1
 was 195.degree. C. and the pressure at the top of continuous multi-stage
 distillation column 1 was 2.59.times.10.sup.4 Pa. Gas distilled from top 2
 of continuous multi-stage distillation column 1 was led through conduit 25
 into condenser 26, in which the gas was condensed. A portion of the
 resultant condensate was recycled into top 2 of continuous multi-stage
 distillation column 1 through conduits 27 and 28, and the remainder of the
 condensate was continuously withdrawn at a rate of 2.4 kg/hr through
 conduits 27 and 29. A portion of the reaction mixture at bottom 6 of
 continuous multi-stage distillation column 1 was led into reboiler 31
 through conduit 30, and recycled into column bottom 6 through conduit 32,
 and the remainder of the reaction mixture was led into evaporator 14
 through conduit 13 at a rate of 7.6 kg/hr. From evaporator 14, an
 evaporated gas containing DPC was withdrawn and led through conduit 21
 into condenser 22, in which the gas was condensed. The resultant
 condensate was withdrawn from condenser 22 through conduit 23 at a rate of
 5.6 kg/hr. On the other hand, an evaporation-concentrated liquid
 containing the catalyst and high boiling point substances was formed in
 evaporator 14. A portion of the concentrated liquid was led into reboiler
 17 through conduits 15 and 16 and recycled into evaporator 14 through
 conduit 18. The remainder of the concentrated liquid in evaporator 14 was
 recycled into continuous multi-stage distillation column 1 through
 conduits 15, 19 and 3 at a rate of 2 kg/hr. After the start of the
 recycling of the concentrated liquid into continuous multi-stage
 distillation column 1 through conduits 15, 19 and 3, the feeding rate of
 the mixture of MPC and catalyst I through conduit 3 into continuous
 multi-stage distillation column 1 was appropriately controlled according
 to the recycling rate of the concentrated liquid.
 During the period of time of from 400 hours to 5,000 hours after the start
 of the operation, a portion of the concentrated liquid formed in
 evaporator 14 was continuously withdrawn through conduit 20 at a rate of
 0.05 kg/hr and led into thin-film evaporator 33. At a point in time of
 1,000 hours after the start of the operation, a sample (of the
 concentrated liquid withdrawn from evaporator 14) was taken through a
 sampling nozzle (not shown) provided on conduit 15', and was analyzed to
 determine the composition of the concentrated liquid by the
 above-mentioned methods. The concentrated liquid had the following
 composition: Pb (which is the metal component of catalyst I): 0.7% by
 weight; the total concentration of high boiling point substances: 5.0% by
 weight; and phenyl salicylate (which is a high boiling point substance):
 0.25% by weight. The evaporated gas formed in thin-film evaporator 33 was
 continuously withdrawn therefrom through conduit 35 at a rate of 0.04
 kg/hr and recycled through conduit 49 into the system for the
 transesterification. On the other hand, an evaporation-concentrated liquid
 containing the catalyst and high boiling point substances was continuously
 withdrawn from the bottom of thin-film evaporator 33 through conduit 34 at
 a rate of 0.01 kg/hr and led into storage vessel 36. A sample (of the
 evaporation-concentrated liquid withdrawn from thin-film evaporator 33)
 was taken through a sampling nozzle (not shown) provided on conduit 34 at
 a point in time of 1,000 hours after the start of the operation, and was
 analyzed to determine the composition of the evaporation-concentrated
 liquid by the above-mentioned methods. The evaporation-concentrated liquid
 had the following composition: Pb (which is the metal component of
 catalyst I): 3.5% by weight; the total concentration of high boiling point
 substances: 24.8% by weight; and phenyl salicylate (which is a high
 boiling point substance): 1.3% by weight.
 At a point in time of 550 hours after the start of the operation, 1 kg of
 the concentrated liquid stored in storage vessel 36 was withdrawn through
 conduit 37 and led into reaction vessel 50 which had a capacity of 10
 liters and which was provided with distillation column 54, a jacket (not
 shown) for circulating a heating medium, and an agitator. The temperature
 of reaction vessel 50 was elevated to 180.degree. C. (as measured at the
 jacket). Then, both a feeding of carbon dioxide into reaction vessel 50 at
 a flow rate of 3.9 NL/hr [NL means L (liter) as measured under the normal
 temperature and pressure conditions, namely at 0.degree. C. under 1 atm.]
 and a feeding of water into reaction vessel 50 at a flow rate of 3.1 g/hr
 were conducted for 2 hours while stirring, to thereby effect a reaction,
 thus obtaining a reaction mixture containing lead(II) carbonate as a
 reaction product. This reaction was conducted under atmospheric pressure.
 After the lapse of the 2-hour reaction time, the stirring was stopped so
 as to allow the solids [containing the lead(II) carbonate] in the obtained
 reaction mixture to be precipitated. After the precipitation, the
 resultant supernatant in the reaction mixture was withdrawn through
 conduit 53. The concentration of Pb in the withdrawn supernatant was 400
 ppm by weight.
 Then, 1.021 kg of PhOH was charged into reaction vessel 50 and stirred at
 180.degree. C. (as measured at the jacket) under atmospheric pressure, to
 thereby effect a reaction. During the reaction, unreacted PhOH was
 distilled off from the top of distillation column 54 disposed on reaction
 vessel 50 at a rate of 0.1 kg/hr through conduit 55A. Thus, in reaction
 vessel 50, a reaction proceeded in which lead(II) carbonate reacts with
 PhOH to form diphenoxy lead, carbon dioxide and water. The carbon dioxide
 and water formed in the above reaction were withdrawn from the reaction
 vessel together with the unreacted PhOH distilled off. A reaction mixture,
 which remained in reaction vessel 50 after performing the above reaction
 for 2 hours, was withdrawn from reaction vessel 50 and transferred through
 conduit 46 and introduced into storage vessel 47.
 Thereafter, every 100 hours after the point in time of 550 hours from the
 start of the operation (i.e., the point in time at which 1 kg of the
 concentrated liquid was withdrawn from storage vessel 36 and led into
 reaction vessel 50 as mentioned above), a sequence of the above operations
 using storage vessel 36 (from which 1 kg of the concentrated liquid was
 withdrawn), reaction vessel 50 and storage vessel 47 (into which the
 remaining reaction mixture obtained in reaction vessel 50 was introduced)
 was repeated in the same manner as described above. On the other hand,
 from a point in time of 600 hours after the start of the operation, the
 reaction mixture stored in storage vessel 47 was continuously withdrawn at
 a rate of 0.01 kg/hr through conduit 48, and the reaction mixture
 withdrawn from storage vessel 47 was caused to meet the evaporated gas
 which was withdrawn from thin-film evaporator 33 and which was led through
 conduit 35, and the resultant mixture (i.e., a mixture of the products
 withdrawn through conduits 48 and 35) was recycled into the system for the
 transesterification through conduit 49. The condensate withdrawal rate
 from condenser 22 through conduit 23 during the period of time of from 400
 hours to 600 hours after the start of the operation was 5.55 kg/hr, and
 the condensate withdrawal rate from condenser 22 through conduit 23 during
 the period of time of from 600 hours to 5,000 hours after the start of the
 operation was 5.6 kg/hr. During the period of time of from 400 hours to
 600 hours after the start of the operation, catalyst I was added to
 distillation column 1 through conduit 3 at such a feeding rate as to
 compensate for the catalyst withdrawal rate at which the catalyst was
 withdrawn through conduit 20, i.e., catalyst I was added through conduit 3
 at a feeding rate such that the above-mentioned Pb concentration of 0.19%
 by weight in conduit 13 was able to be maintained.
 The operation was conducted for 5,000 hours. From the point in time of 600
 hours after the start of the operation, i.e., from the point in time at
 which the recycling of the catalyst into the system for the
 transesterification through conduit 49 was started, the feeding rate of
 catalyst I into the system for the transesterification through conduit 3
 was as small as 0.0033 g/hr, in terms of the weight of Pb contained in
 catalyst I. Further, during the operation, the above-mentioned supernatant
 (containing Pb) withdrawn from reaction vessel 50 through conduit 53 was
 subjected to burning to thereby obtain lead monoxide and the obtained lead
 monoxide was used for producing catalyst I. The amount of catalyst I which
 was prepared from the thus obtained lead monoxide (recovered Pb) was
 sufficient to be used as catalyst I which was to be introduced in an
 amount as small as 0.0033 g/hr through conduit 3 (from the point in time
 of 600 hours after the start of the operation, i.e., from the point in
 time at which the recycling of the catalyst into the system for the
 transesterification through conduit 49 was started). Therefore, from the
 point in time of 600 hours after the start of the operation, all need for
 the catalyst was met by both the recycled catalyst and the catalyst
 prepared from the Pb recovered from the supernatant withdrawn from
 reaction vessel 50.
 In addition, as mentioned above, the supernatant withdrawn from reaction
 vessel 50 was subjected to burning to obtain lead monoxide, and the
 obtained lead monoxide was recovered and used for preparing catalyst I.
 Therefore, a waste liquid containing a spent catalyst did not occur at
 all.
 From the evaporation-concentrated liquid which was formed in evaporator 14
 and which contained the catalyst and high boiling point substances,
 samples were taken through a sampling nozzle provided on conduit 15',
 wherein the samples were, respectively, withdrawn at points in time of
 1,000 hours, 2,500 hours and 5,000 hours after the start of the operation.
 The determination of the total concentration of the high boiling point
 substances in each sample was conducted by the above-mentioned method.
 With respect to these samples withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation, the total
 concentrations of the high boiling point substances were 5.0% by weight,
 5.1% by weight and 5.1% by weight, respectively, and the phenyl salicylate
 concentrations were 0.25% by weight, 0.25% by weight and 0.26% by weight,
 respectively.
 During the 5,000 hour operation time, the operation could be stably
 conducted (for example, both the flow and the composition in each conduit
 were stable) without suffering disadvantageous phenomena, such as the
 deposition of the catalyst from a catalyst-containing liquid and the
 adherence of the deposited catalyst to the inside surfaces associated with
 the equipment employed for the operation. During the operation, samples of
 the reaction mixture withdrawn from the bottom of continuous multi-stage
 distillation column 1 were taken through the above-mentioned sampling
 nozzle provided on conduit 13, and the samples were analyzed. With respect
 to the reaction mixture which was taken from conduit 13 at a point in time
 of 3,000 hours after the start of the operation, the composition of the
 reaction mixture was as follows: MPC: 23.8% by weight; DPC: 74.6% by
 weight; and Pb: 0.19% by weight. The purity of the aromatic carbonate
 (which was a mixture of MPC and DPC) in the condensate withdrawn from
 condenser 22 through conduit 23 was 99.99% or more, and no high boiling
 point substance was detected in the condensate. After the operation was
 terminated, the inside surfaces associated with the equipment employed for
 the operation were examined. No adherence of the catalyst to any of the
 inner walls of continuous multi-stage distillation column 1, evaporator
 14, reboiler 17, conduits and the like was observed.
 Comparative Example 3
 Substantially the same procedure as in Example 2 was repeated, except that,
 after an evaporation-concentrated liquid containing the catalyst and high
 boiling point substances was withdrawn from the bottom of thin-film
 evaporator 33, the evaporation-concentrated liquid was introduced into and
 accumulated in a waste catalyst storage vessel (not shown) instead of
 leading the evaporation-concentrated liquid to storage vessel 36, so that
 the sequence of the operations using storage vessel 36, electric furnace
 38, reaction vessel 42 and storage vessel 47 was not conducted; and not
 only during the period of time of from 400 hours to 600 hours after the
 start of the operation, but also after the point in time of 600 hours
 after the start of the operation, catalyst I was added to distillation
 column 1 through conduit 3 at such a feeding rate as to compensate for the
 catalyst withdrawal rate at which the catalyst was withdrawn through
 conduit 20, i.e., catalyst I was added through conduit 3 at a feeding rate
 such that the Pb concentration of 0.19% by weight in conduit 13 was able
 to be maintained. The operation was conducted for 5,000 hours. During the
 period of time of from 600 hours to 5,000 hours after the start of the
 operation, in order to maintain the above-mentioned Pb concentration of
 0.19% by weight in conduit 13, it was necessary to add fresh catalyst I to
 continuous multi-stage distillation column 1 through conduit 3 in an
 amount as large as 1.54 kg, in terms of the weight of Pb in the catalyst.
 During the period of time of from 600 hours to 5,000 hours after the start
 of the operation, the amount of the evaporation-concentrated liquid
 (containing the catalyst and high boiling point substances) which was
 withdrawn from the bottom of thin-film evaporator 33 and introduced into
 and accumulated in the waste catalyst storage vessel reached a level as
 large as 44 kg.
 EXAMPLE 3
 The production of diphenyl carbonate was conducted using catalyst I
 prepared in Example 1, and the system as shown in FIG. 3.
 A mixture of dimethyl carbonate, PhOH (which contains, as an impurity, 200
 ppm by weight of 4,4'-dihydroxydiphenyl which is a high boiling point
 substance) and methyl phenyl carbonate was continuously fed in liquid form
 from conduit 3 through preheater 4 and conduit 5 into continuous
 multi-stage distillation column 1 at a position of 0.5 m below the top 2
 thereof (which column was comprised of a plate column having a height of
 12 m and a diameter of 8 inches and provided with 40 sieve trays) at a
 rate of 31 kg/hr, thereby allowing the mixture to flow down inside
 continuous multi-stage distillation column 1 so as to perform a reaction.
 The composition of the mixture fed from conduit 3 was so controlled that
 the mixture flowing through conduit 5 during the operation (the mixture
 flowing through conduit 5 was comprised of a liquid introduced from
 conduit 19, which was recycled from evaporator 14; a liquid introduced
 from conduit 129, which was recycled from continuous multi-stage
 distillation column 101; and the above-mentioned mixture fed from conduit
 3) had a composition of 49.9% by weight of DMC, 44.7% by weight of PhOH
 and 4.9% by weight of MPC. DMC was fed through conduit 7 to evaporator 8,
 in which the DMC was subjected to evaporation. The resultant gas was fed
 to bottom 6 of continuous multi-stage distillation column 1 through
 conduit 9 at a rate of 55 kg/hr. Catalyst I was fed from conduit 224 in
 such an amount that the Pb concentration at conduit 13 became 0.042% by
 weight, wherein the Pb concentration can be confirmed using a sample
 withdrawn from a sampling nozzle (not shown) provided on conduit 13.
 Continuous multi-stage distillation column 1 was operated under conditions
 such that the temperature at the column bottom was 203.degree. C. and the
 pressure at the column top was 7.4.times.10.sup.5 Pa. Continuous
 multi-stage distillation column 1 was clad with a heat insulating material
 and a part of the column was heated by a heater (not shown). Gas distilled
 from top 2 of the column was led through conduit 10 into condenser 11, in
 which the gas was condensed. The resultant condensate was continuously
 withdrawn at a rate of 55 kg/hr from conduit 12. A reaction mixture was
 withdrawn continuously from bottom 6 at a rate of 31 kg/hr, and was led to
 evaporator 14 through conduit 13. In evaporator 14, an
 evaporation-concentrated liquid containing the catalyst and high boiling
 point substances was formed. A portion of the concentrated liquid was led
 into reboiler 17 through conduits 15 and 16 and recycled into evaporator
 14 through conduit 18. The remainder of the concentrated liquid in
 evaporator 14 was recycled into continuous multi-stage distillation column
 1 at a rate of 1 kg/hr through conduits 15, 19 and 3. During the period of
 time from 400 hours to 5,000 hours after the start of the operation, a
 portion of the concentrated liquid formed in evaporator 14 was
 continuously withdrawn from conduit 20 at a rate of 0.05 kg/hr and
 introduced into thin-film evaporator 33.
 Catalyst I was fed from conduit 224 at such a feeding rate as to compensate
 for the catalyst withdrawal rate at which the catalyst was withdrawn
 through conduit 20, i.e., catalyst I was fed from conduit 224 at a feeding
 rate such that the above-mentioned Pb concentration of 0.042% by weight in
 conduit 13 was able to be maintained. On the other hand, an evaporated gas
 formed in evaporator 14 was fed through conduits 21 and 105 into
 continuous multi-stage distillation column 101 at a position of 2.0 m
 below top 102 thereof, which column was comprised of a plate column having
 a height of 6 m and a diameter of 10 inches and provided with 20 sieve
 trays, thereby performing a reaction. The composition of the mixture in
 conduit 105 was as follows: DMC: 43.1% by weight; PhOH: 24.5% by weight;
 MPC: 27.1% by weight; and DPC: 4.5% by weight (the mixture in conduit 105
 was comprised of a gas introduced through conduit 21 and a liquid
 introduced from conduit 119, which was recycled from evaporator 114).
 Catalyst I was fed from conduit 124 in such an amount that the Pb
 concentration at conduit 113 became 0.16% by weight, wherein the Pb
 concentration can be confirmed using a sample withdrawn from a sampling
 nozzle (not shown) provided on conduit 113. Continuous multi-stage
 distillation column 101 was operated under conditions such that the
 temperature at the column bottom was 198.degree. C. and the pressure at
 the column top was 3.7.times.10.sup.4 Pa. Gas distilled from column top
 102 was led through conduit 125 to condenser 126, in which the gas was
 condensed. A portion of the resultant condensate was recycled into column
 top 102 through conduit 128, and the remainder of the condensate was
 recycled into continuous multi-stage distillation column 1 through
 conduits 127 and 129, preheater 4 and conduit 5. After the start of the
 recycling of the condensate into continuous multi-stage distillation
 column 1 through conduit 129, PhOH (containing 200 ppm by weight of
 4,4'-dihydroxydiphenyl which is a high boiling point substance) was added
 to the mixture fed from conduit 3 in such an amount that the
 above-mentioned composition of the mixture at conduit 5 can be maintained.
 A portion of the reaction mixture at bottom 106 of continuous multi-stage
 distillation column 101 was led into reboiler 131 through conduit 130, and
 recycled into column bottom 106 through conduit 132, and the remainder of
 the reaction mixture was led to evaporator 114 through conduit 113 at a
 rate of 8.8 kg/hr. In evaporator 114, an evaporation-concentrated liquid
 containing the catalyst and high boiling point substances was formed. A
 portion of the concentrated liquid was led into reboiler 117 through
 conduits 115 and 116 and recycled into evaporator 114 through conduit 118.
 The remainder of the concentrated liquid in evaporator 114 was recycled
 into continuous multi-stage distillation column 101 through conduits 115,
 119 and 105 at a rate of 2 kg/hr. During the period of time of from 400
 hours to 5,000 hours after the start of the operation, a portion of the
 concentrated liquid formed in evaporator 114 was continuously withdrawn at
 a rate of 0.05 kg/hr from the system for the transesterification through
 conduit 120, and was caused to meet the concentrated liquid led through
 conduit 20. The resultant liquid mixture (i.e., a mixture of the liquid
 products withdrawn from the system for the transesterification through
 conduits 120 and 20) was led into thin-film evaporator 33 through conduit
 20'. At a point in time of 1,000 hours after the start of the operation, a
 sample (of the above-mentioned liquid mixture) was taken through a
 sampling nozzle (not shown) provided on conduit 20', and was analyzed to
 determine the composition of the liquid mixture by the above-mentioned
 methods. The liquid mixture had the following composition: Pb (which is
 the metal component of catalyst I): 1.0% by weight; the total
 concentration of high boiling point substances: 3.3% by weight;
 4,4'-dihydroxydiphenyl (which is a high boiling point substance): 1.8% by
 weight; and phenyl salicylate: 0.13% by weight. The evaporated gas formed
 in thin-film evaporator 33 was continuously withdrawn therefrom through
 conduit 35 at a rate of 0.09 kg/hr and recycled through conduit 149 into
 the system for the transesterification. On the other hand, an
 evaporation-concentrated liquid containing the catalyst and high boiling
 point substances was continuously withdrawn from the bottom of thin-film
 evaporator 33 through conduit 34 at a rate of 0.01 kg/hr and led into
 storage vessel 36. A sample (of the evaporation-concentrated liquid
 withdrawn from thin-film evaporator 33) was taken through a sampling
 nozzle (not shown) provided on conduit 34 at a point in time of 1,000
 hours after the start of the operation, and was analyzed to determine the
 composition of the evaporation-concentrated liquid by the above-mentioned
 methods. The evaporation-concentrated liquid had the following
 composition: Pb (which is the metal component of catalyst I): 9.9% by
 weight; the total concentration of high boiling point substances: 33.4% by
 weight; 4,4'-dihydroxydiphenyl (which is a high boiling point substance):
 3.7% by weight; and phenyl salicylate: 1.3% by weight.
 At a point in time of 550 hours after the start of the operation, 1 kg of
 the concentrated liquid stored in storage vessel 36 was withdrawn through
 conduit 37 and led into reaction vessel 50 which had a capacity of 10
 liters and which was provided with distillation column 54, a jacket (not
 shown) for circulating a heating medium, and an agitator. The temperature
 of reaction vessel 50 was elevated to 180.degree. C. (as measured at the
 jacket). Then, both a feeding of carbon dioxide into reaction vessel 50 at
 a flow rate of 11 NL/hr and a feeding of water into reaction vessel 50 at
 a flow rate of 8.7 g/hr were conducted for 2 hours while stirring, to
 thereby effect a reaction, thus obtaining a reaction mixture containing
 lead(II) carbonate as a reaction product. This reaction was conducted
 under atmospheric pressure. After the lapse of the 2-hour reaction time,
 the stirring was stopped so as to allow the solids [containing the
 lead(II) carbonate] in the obtained reaction mixture to be precipitated.
 After the precipitation, the resultant supernatant in the reaction mixture
 was withdrawn through conduit 53. The concentration of Pb in the withdrawn
 supernatant was 400 ppm by weight.
 Then, 0.620 kg of PhOH was charged into reaction vessel 50 and stirred at
 180.degree. C. (as measured at the jacket) under atmospheric pressure, to
 thereby effect a reaction. During the reaction, unreacted PhOH was
 distilled off from the top of distillation column 54 disposed on reaction
 vessel 50 at a rate of 0.1 kg/hr through conduit 55A. Thus, in reaction
 vessel 50, in the same manner as in Example 2, a reaction proceeded in
 which lead(II) carbonate reacts with PhOH to form diphenoxy lead, carbon
 dioxide and water. The carbon dioxide and water formed in the above
 reaction were withdrawn from the reaction vessel together with the
 unreacted PhOH distilled off. A reaction mixture, which remained in
 reaction vessel 50 after performing the above reaction for 2 hours, was
 withdrawn from reaction vessel 50 and transferred through conduit 46 and
 introduced into storage vessel 47.
 Thereafter, every 100 hours after the point in time of 550 hours from the
 start of the operation (i.e., the point in time at which 1 kg of the
 concentrated liquid was withdrawn from storage vessel 36 and led into
 reaction vessel 50 as mentioned above), a sequence of the above operations
 using storage vessel 36 (from which 1 kg of the concentrated liquid was
 withdrawn), reaction vessel 50 and storage vessel 47 (into which the
 remaining reaction mixture obtained in reaction vessel 50 was introduced)
 was repeated in the same manner as described above. On the other hand,
 from a point in time of 600 hours after the start of the operation, the
 reaction mixture stored in storage vessel 47 was continuously withdrawn at
 a rate of 0.01 kg/hr through conduit 48. A portion of the reaction mixture
 withdrawn from storage vessel 47 was recycled into the system for the
 transesterification through conduit 49 at a rate of 0.0065 kg/hr. The
 remainder of the reaction mixture withdrawn from storage vessel 47 was led
 through conduit 48' at a rate of 0.0035 kg/hr and caused to meet the
 evaporated gas which was withdrawn from thin-film evaporator 33 and which
 was led through conduit 35, and the resultant mixture (i.e., a mixture of
 the products withdrawn through conduits 48'and 35) was recycled into the
 system for the transesterification through conduit 149.
 During the period of time of from 400 hours to 600 hours after the start of
 the operation, catalyst I was added to distillation column 1 through
 conduit 224 and to distillation column 101 through conduit 124 both at
 such a feeding rate as to compensate for the catalyst withdrawal rate at
 which the catalyst was withdrawn through conduits 20 and 120, i.e.,
 catalyst I was added through conduits 224 and 124 both at a feeding rate
 such that both of the above-mentioned Pb concentration of 0.042% by weight
 in conduit 13 and the above-mentioned Pb concentration of 0.16% by weight
 in conduit 113 were able to be maintained. An evaporated gas formed in
 evaporator 114 was fed through conduit 121 into continuous multi-stage
 distillation column 201 at a position of 2.0 m below top 202 thereof,
 which column was comprised of a plate column having a height of 6 m and a
 diameter of 6 inches and provided with 20 sieve trays, thereby separating
 DPC from the fed gas. Continuous multi-stage distillation column 201 was
 operated under conditions such that the temperature at the column bottom
 was 184.degree. C. and the pressure at the column and top was
 2.times.10.sup.3 Pa. Gas distilled from top 202 of the column was led
 through conduit 225 to condenser 226, in which the gas was condensed. A
 portion of the resultant condensate was recycled into top 202 of the
 column through conduit 228, and the remainder of the condensate was
 recycled into continuous multi-stage distillation column 101 through
 conduits 227 and 229. A gas was withdrawn from continuous multi-stage
 distillation column 201 through conduit 233 provided at a position of 4.0
 m below column top 202 and was led to condenser 234, in which the
 withdrawn gas was condensed. The resultant condensate was withdrawn at a
 rate of 6.7 kg/hr through conduit 235.
 The operation was conducted for 5,000 hours. From the point in time of 600
 hours after the start of the operation, i.e., from the point in time at
 which the recycling of the catalyst into the system for the
 transesterification through conduits 49 and 149 was started, the total
 feeding rate of catalyst I into the system for the transesterification
 through conduits 124 and 224 was as small as 0.0032 g/hr, in terms of the
 weight of Pb contained in catalyst I. Further, during the operation, the
 above-mentioned supernatant (containing Pb) withdrawn from reaction vessel
 50 through conduit 53 was subjected to burning to thereby obtain lead
 monoxide and the obtained lead monoxide was used for producing catalyst I.
 The amount of catalyst I which was prepared from the thus obtained lead
 monoxide (recovered Pb) was sufficient to be used as catalyst I which was
 to be introduced in the above-mentioned amount of 0.0032 g/hr through
 conduits 124 and 224 (from the point in time of 600 hours after the start
 of the operation, i.e., from the point in time at which the recycling of
 the catalyst into the system for the transesterification through conduits
 49 and 149 was started). Therefore, from the point in time of 600 hours
 after the start of the operation, all need for the catalyst was met by
 both the recycled catalyst and the catalyst prepared from the Pb recovered
 from the supernatant withdrawn from reaction vessel 50.
 In addition, as mentioned above, the supernatant withdrawn from reaction
 vessel 50 was subjected to burning to obtain lead monoxide, and the
 obtained lead monoxide was recovered and used for preparing catalyst I.
 Therefore, a waste liquid containing a spent catalyst did not occur at
 all.
 From the evaporation-concentrated liquid which was formed in evaporator 14
 and which contained the catalyst and high boiling point substances,
 samples were taken through a sampling nozzle provided on conduit 15',
 wherein the samples were, respectively, withdrawn at points in time of
 1,000 hours, 2,500 hours and 5,000 hours after the start of the operation.
 The determination of the total concentration of the high boiling point
 substances in each sample was conducted by the above-mentioned method.
 With respect to these samples withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation, the total
 concentrations of the high boiling point substances were 2.2% by weight,
 2.3% by weight and 2.3% by weight, respectively. Further, from the
 evaporation-concentrated liquid which was formed in evaporator 114 and
 which contained the catalyst and high boiling point substances, samples
 were taken through a sampling nozzle provided on conduit 115', wherein the
 samples were, respectively, withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation. With respect
 to these samples withdrawn at points in time of 1,000 hours, 2,500 hours
 and 5,000 hours after the start of the operation, the total concentrations
 of the high boiling point substances were 5.0% by weight, 5.1% by weight
 and 5.1% by weight, respectively, and the phenyl salicylate concentrations
 were 0.25% by weight, 0.26% by weight and 0.26% by weight, respectively.
 During the 5,000 hour operation time, the operation could be stably
 conducted (for example, both the flow and the composition in each conduit
 were stable) without suffering disadvantageous phenomena, such as the
 deposition of the catalyst from a catalyst-containing liquid and the
 adherence of the deposited catalyst to the inside surfaces associated with
 the equipment employed for the operation. At a point in time of 3,000
 hours after the start of the operation, the purity of the aromatic
 carbonate (which was DPC) in the condensate withdrawn from condenser 234
 through conduit 235 was 99.99% or more, and no substance other than DPC
 was detected in the condensate. After the operation was terminated, the
 inside surfaces associated with the equipment employed for the operation
 were examined. No adherence of the catalyst to any of the inner walls of
 continuous multi-stage distillation column 1, evaporator 14, reboiler 17,
 conduits and the like was observed.
 Comparative Example 4
 Substantially the same procedure as in Example 3 was repeated, except that,
 with respect to the evaporation-concentrated liquid (containing the
 catalyst and high boiling point substances) which was formed in evaporator
 14 and the evaporation-concentrated liquid (containing the catalyst and
 high boiling point substances) which was formed in evaporator 114, the
 withdrawal of a portion of each of these evaporation-concentrated liquids
 out of the production system through conduits 20 and 120 was not
 conducted, and that the introduction of the fresh catalyst into the system
 for the transesterification from conduits 224 and 124 into continuous
 multi-stage distillation columns 1 and 101 (which was conducted in Example
 3 during the period of time of from 400 hours to 5,000 hours after the
 start of the operation) was not conducted. With respect to the samples
 withdrawn through the sampling nozzle provided on conduit 115' at points
 in time of 1,000 hours, 2,500 hours and 5,000 hours after the start of the
 operation, the total concentrations of the high boiling point substances
 were 12.5% by weight, 30.4% by weight and 52.3% by weight, respectively,
 and the phenyl salicylate concentrations were 0.62% by weight, 1.7% by
 weight and 2.9% by weight, respectively.
 With respect to the aromatic carbonate (which was DPC) in the condensate
 withdrawn from condenser 234 through conduit 235 at a point in time of
 3,000 hours after the start of the operation, the purity thereof was
 98.7%. Further, the concentration of phenyl salicylate in the
 above-mentioned condensate was 12 ppm by weight, and the total
 concentration of the high boiling point substances in the above-mentioned
 condensate was 0.06% by weight. The operation was conducted for 5,000
 hours. After the operation was terminated, the inside surfaces associated
 with the equipment employed for the operation were examined. The adherence
 of the catalyst to a part of the inner wall of each of continuous
 multi-stage distillation column 1, evaporator 14 and the conduits was
 observed.
 EXAMPLE 4
 The production of an aromatic carbonate was conducted in substantially the
 same manner as in Example 2, except that the system as shown in FIG. 4 was
 used instead of the system as shown in FIG. 2. As shown in FIGS. 4 and 2,
 the difference between the system as shown in FIG. 4 and the system as
 shown in FIG. 2 resides in the region into which the concentrated liquid
 stored in storage vessel 36 is introduced through conduit 37 and from
 which a reaction mixture obtained from the above concentrated liquid is
 transferred to storage vessel 47 through conduit 46.
 During the period of time of 400 hours to 5,000 hours after the start of
 the operation, a portion of the concentrated liquid formed in evaporator
 14 was continuously withdrawn through conduit 20 at a rate of 0.05 kg/hr
 and led into thin-film evaporator 33. At a point in time of 1,000 hours
 after the start of the operation, a sample (of the concentrated liquid
 withdrawn from evaporator 14) was taken through a sampling nozzle (not
 shown) provided on conduit 15', and was analyzed to determine the
 composition of the concentrated liquid by the above-mentioned methods. The
 concentrated liquid had the following composition: Pb (which is the metal
 component of catalyst I): 0.7% by weight; the total concentration of high
 boiling point substances: 4.0% by weight; and phenyl salicylate (which is
 a high boiling point substance): 0.15% by weight. The evaporated gas
 formed in thin-film evaporator 33 was continuously withdrawn therefrom
 through conduit 35 at a rate of 0.04 kg/hr and recycled through conduit 49
 into the system for the transesterification. On the other hand, an
 evaporation-concentrated liquid containing the catalyst and high boiling
 point substances was continuously withdrawn from the bottom of thin-film
 evaporator 33 through conduit 34 at a rate of 0.01 kg/hr and led into
 storage vessel 36. A sample (of the evaporation-concentrated liquid
 withdrawn from thin-film evaporator 33) was taken through a sampling
 nozzle (not shown) provided on conduit 34 at a point in time of 1,000
 hours after the start of the operation, and was analyzed to determine the
 composition of the evaporation-concentrated liquid by the above-mentioned
 methods. The evaporation-concentrated liquid had the following
 composition: Pb (which is the metal component of catalyst I): 3.5% by
 weight; the total concentration of high boiling point substances: 19.8% by
 weight; and phenyl salicylate (which is a high boiling point substance):
 0.75% by weight.
 At a point in time of 550 hours after the start of the reaction, 1 kg of
 the concentrated liquid stored in storage vessel 36 was withdrawn through
 conduit 37 and led into reaction vessel 55 which had a capacity of 10
 liters and which was provided with distillation column 62, a jacket (not
 shown) for circulating a heating medium, and an agitator. 5 kg of water
 was introduced into reaction vessel 55, and the temperature of reaction
 vessel 55 was elevated to and maintained at 200.degree. C. (as measured at
 the jacket) while stirring. The internal pressure of reaction vessel 55
 rose to 3.0.times.10.sup.6 Pa. After continuing the stirring at
 200.degree. C. for 4 hours, the stirring was stopped, and the temperature
 of reaction vessel 55 (as measured at the jacket) was lowered to
 100.degree. C. and allowed to stand for 1 hour. The internal pressure of
 reaction vessel 55 was lowered to atmospheric pressure by discharging gas
 from reaction vessel 55 through conduit 63. From the resultant reaction
 mixture in reaction vessel 55, a liquid phase was withdrawn and led to
 storage vessel 59 through conduit 58, leaving a white precipitate in
 reaction vessel 55. The white precipitate left in reaction vessel 55 was
 analyzed, and the results of the analysis showed that the white
 precipitate was a solid comprised mainly of lead(II) carbonate. On the
 other hand, when the liquid phase introduced into storage vessel 59 was
 allowed to cool to room temperature, it was separated into upper and lower
 liquid layers. The upper layer had the following composition: water: 93.5%
 by weight; PhOH: 6.5% by weight; and no high boiling point substance was
 detected. The lower layer had the following composition: PhOH: 57.3% by
 weight; the total concentration of high boiling point substances: 14.3% by
 weight; Pb: 100 ppm by weight; and phenyl salicylate was not detected at
 all. From the mass balance of PhOH, phenyl salicylate and high boiling
 point substances, it was found that phenyl salicylate had been converted
 into PhOH by hydrolysis and decarboxylation. The lower layer in storage
 vessel 59 was withdrawn therefrom through conduit 60. The weight of the
 lower layer withdrawn from storage vessel 59 was 603 g. The lead(II)
 carbonate in reaction vessel 55 was converted into diphenoxy lead in
 substantially the same manner as in Example 2, i.e., by a method in which
 PhOH is introduced into reaction vessel 55 through conduits 56 and 59A,
 and the resultant mixture in reaction vessel 55 is subjected to a reaction
 while stirring at 180.degree. C. (as measured at the jacket) and
 distilling off by-produced water and carbon dioxide together with
 unreacted PhOH. 1 kg of a reaction mixture, which remained in reaction
 vessel 55 after performing the above reaction for 2 hours, was withdrawn
 from reaction vessel 55 and transferred through conduit 46 and introduced
 into storage vessel 47.
 Thereafter, every 100 hours after the point in time of 550 hours from the
 start of the operation (i.e., the point in time at which 1 kg of the
 concentrated liquid was withdrawn from storage vessel 36 and led into
 reaction vessel 55 as mentioned above), a sequence of the above operations
 using storage vessel 36 (from which 1 kg of the concentrated liquid was
 withdrawn), reaction vessel 55, storage vessel 59 and storage vessel 47
 (into which 1 kg of the remaining reaction mixture obtained in reaction
 vessel 55 was introduced) was repeated in the same manner as described
 above. With respect to each of the second-time to last-time practices of
 the above-mentioned sequence of the operations using storage vessel 36,
 reaction vessel 55, storage vessel 59 and storage vessel 47, as the 5 kg
 of water which is introduced into reaction vessel 55 (so as to be mixed
 with 1 kg of the concentrated liquid transferred from storage vessel 36),
 use was made of an aqueous mixture obtained by a method in which the
 above-mentioned upper layer obtained in storage vessel 59 is taken out and
 water is added thereto in an amount such that the weight of the resultant
 aqueous mixture becomes 5 kg.
 On the other hand, from a point in time of 600 hours after the start of the
 operation, the reaction mixture stored in storage vessel 47 was
 continuously withdrawn at a rate of 0.01 kg/hr through conduit 48, and the
 reaction mixture withdrawn from storage vessel 47 was caused to meet the
 evaporated gas which was withdrawn from thin-film evaporator 33 and which
 was led through conduit 35, and the resultant mixture (i.e., a mixture of
 the products withdrawn through conduits 48 and 35) was recycled into the
 system for the transesterification through conduit 49.
 The condensate withdrawal rate from condenser 22 through conduit 23 during
 the period of time of from 400 hours to 600 hours after the start of the
 operation was 5.55 kg/hr, and the condensate withdrawal rate from
 condenser 22 through conduit 23 during the period of time of from 600
 hours to 5,000 hours after the start of the operation was 5.6 kg/hr.
 During the period of time of from 400 hours to 600 hours after the start
 of the operation, catalyst I was added to distillation column 1 through
 conduit 3 at such a feeding rate as to compensate for the catalyst
 withdrawal rate at which the catalyst was withdrawn through conduit 20,
 i.e., catalyst I was added through conduit 3 at a feeding rate such that
 the Pb concentration of 0.19% by weight in conduit 13 was able to be
 maintained.
 The operation was conducted for 5,000 hours. From the point in time of 600
 hours after the start of the operation, i.e., from the point in time at
 which the recycling of the catalyst into the system for the
 transesterification through conduit 49 was started, the feeding rate of
 catalyst I into the system for the transesterification through conduit 3
 was as small as 0.0006 g/hr, in terms of the weight of Pb contained in
 catalyst I. Further, during the operation, the above-mentioned lower layer
 (containing Pb) withdrawn from storage vessel 59 through conduit 60 was
 subjected to burning to thereby obtain lead monoxide and the obtained lead
 monoxide was used for producing catalyst I. The amount of catalyst I which
 was prepared from the thus obtained lead monoxide (recovered Pb) was
 sufficient to be used as catalyst I which was to be introduced in an
 amount as small as 0.0006 g/hr through conduit 3 (from the point in time
 of 600 hours after the start of the operation, i.e., from the point in
 time at which the recycling of the catalyst into the system for the
 transesterification through conduit 49 was started). Therefore, from the
 point in time of 600 hours after the start of the operation, all need for
 the catalyst was met by both the recycled catalyst and the catalyst
 prepared from the Pb recovered from the lower layer withdrawn from storage
 vessel 59 (wherein the lower layer withdrawn from storage vessel 59 is a
 portion of the liquid phase withdrawn from reaction vessel 55).
 In addition, as mentioned above, the lower layer withdrawn from storage
 vessel 59 through conduit 60 was subjected to burning to obtain lead
 monoxide, and the obtained lead monoxide was recovered and used for
 preparing catalyst I. Therefore, a waste liquid containing a spent
 catalyst did not occur at all.
 From the evaporation-concentrated liquid which was formed in evaporator 14
 and which contained the catalyst and high boiling point substances,
 samples were taken through a sampling nozzle provided on conduit 15',
 wherein the samples were, respectively, withdrawn at points in time of
 1,000 hours, 2,500 hours and 5,000 hours after the start of the operation.
 The determination of the total concentration of the high boiling point
 substances in each sample was conducted by the above-mentioned method.
 With respect to these samples withdrawn at points in time of 1,000 hours,
 2,500 hours and 5,000 hours after the start of the operation, the total
 concentrations of the high boiling point substances were 4.0% by weight,
 4.1% by weight and 4.1% by weight, respectively.
 During the 5,000 hour operation time, the operation could be stably
 conducted (for example, both the flow and the composition in each conduit
 were stable) without suffering disadvantageous phenomena, such as the
 deposition of the catalyst from a catalyst-containing liquid and the
 adherence of the deposited catalyst to the inside surfaces associated with
 the equipment employed for the operation. During the operation, samples of
 the reaction mixture withdrawn from the bottom of continuous multi-stage
 distillation column 1 were taken through the sampling nozzle provided on
 conduit 13, and the samples were analyzed. With respect to the reaction
 mixture which was taken from conduit 13 at a point in time of 3,000 hours
 after the start of the operation, the composition of the reaction mixture
 was as follows: MPC: 23.9% by weight; DPC: 74.8% by weight; and Pb: 0.19%
 by weight. The purity of the aromatic carbonate (which was a mixture of
 MPC and DPC) in the condensate withdrawn from condenser 22 through conduit
 23 was 99.99% or more, and no high boiling point substance was detected in
 the condensate. After the operation was terminated, the inside surfaces
 associated with the equipment employed for the operation were examined. No
 adherence of the catalyst to any of the inner walls of continuous
 multi-stage distillation column 1, evaporator 14, reboiler 17, conduits
 and the like was observed.
 EXAMPLE 5
 Preparation of Catalyst
 A mixture of 30 kg of PhOH, 10 kg of methyl phenyl carbonate and 8 kg of
 dibutyltin oxide was heated to and maintained at 180.degree. C. for 10
 hours, there-by performing a reaction. After that period of time, water
 formed in the resultant reaction mixture was distilled off together with
 unreacted PhOH. Then, most of the remaining PhOH and methyl phenyl
 carbonate were distilled off from the reaction mixture under reduced
 pressure, and the resultant mixture was allowed to cool in a nitrogen
 atmosphere, to thereby obtain catalyst II.
 Production of Aromatic Carbonate
 The production of an aromatic carbonate was conducted using the system as
 shown in FIG. 5, which comprises distillation column 24 having a height of
 1 m and a diameter of 4 inches and containing Dixon packing (6 mm.phi.),
 and reaction vessel 100 having a capacity of 200 liters and equipped with
 an agitator.
 A mixture of dimethyl carbonate, PhOH and catalyst II was continuously fed
 in liquid form from conduit 3 into reaction vessel 100 at a rate of 20
 kg/hr, thereby performing a reaction. The weight ratio of the dimethyl
 carbonate to the PhOH in the mixture was 50/50, and catalyst II was used
 in an amount such that the Sn concentration of the reaction mixture in
 conduit 13 became 0.4% by weight, wherein the Sn concentration can be
 confirmed using a sample withdrawn through a sampling nozzle (not shown)
 provided on conduit 13. The reaction conditions of the above reaction were
 such that the temperature in reaction vessel 100 was 204.degree. C. and
 the pressure at the top of distillation column 24 was 7.5.times.10.sup.5
 Pa. Gas (containing methanol and dimethyl carbonate) formed in reaction
 vessel 100 was led into distillation column 24 through conduit 30. From
 distillation column 24, dimethyl carbonate was recycled to reaction vessel
 100 through conduit 32, while the gas (containing methanol and dimethyl
 carbonate) distilled from the top of distillation column 24 was led
 through conduit 25 into condenser 26, in which the gas was condensed. A
 portion of the resultant condensate was recycled into distillation column
 24 at a reflux ratio of 5.0 through conduits 27 and 28, and the remainder
 of the condensate was continuously withdrawn at a rate of 2.3 kg/hr
 through conduit 29. A reaction mixture [containing methyl phenyl carbonate
 (as a desired reaction product), the catalyst, and high boiling point
 substances] was continuously withdrawn from the bottom of reaction vessel
 100 at a rate of 17.7 kg/hr through conduit 13 and led into evaporator 14,
 from which an evaporated gas containing the methyl phenyl carbonate was
 withdrawn and led through conduit 21 into condenser 22, in which the
 evaporated gas was condensed. The resultant condensate was withdrawn from
 condenser 22 through conduit 23 at a rate of 16.7 kg/hr. On the other
 hand, an evaporation-concentrated liquid containing the catalyst and the
 high boiling point substances was formed in evaporator 14. A portion of
 the concentrated liquid was led into reboiler 17 through conduits 15 and
 16 and recycled into evaporator 14 through conduit 18. The remainder of
 the concentrated liquid in evaporator 14 was recycled into reaction vessel
 100 at a rate of 1 kg/hr through conduits 15, 19 and 3. During the period
 of time of from 400 hours to 2,000 hours after the start of the operation,
 a portion of the concentrated liquid formed in evaporator 14 was
 continuously withdrawn through conduit 20 at a rate of 0.05 kg/hr and led
 into storage vessel 36 having a capacity of 10 liters. At a point in time
 of 1,000 hours after the start of the operation, a sample (of the
 concentrated liquid withdrawn from evaporator 14) was taken through a
 sampling nozzle (not shown) provided on conduit 15', and was analyzed to
 determine the composition of the concentrated liquid by the
 above-mentioned methods. The concentrated liquid had the following
 composition: Sn (which is the metal component of catalyst II): 6.7% by
 weight; the total concentration of high boiling point substances: 2.2% by
 weight; and phenyl salicylate (which is a high boiling point substance):
 0.7% by weight.
 At a point in time of 500 hours after the start of the operation, 2 kg of
 the concentrated liquid stored in storage vessel 36 was withdrawn through
 conduit 37 and led into reaction vessel 55 which had a capacity of 10
 liters and which was equipped with distillation column 62, a jacket (not
 shown) for circulating a heating medium, and an agitator. 4 kg of dimethyl
 carbonate was introduced into reaction vessel 55 from conduit 56, and the
 temperature of reaction vessel 55 was elevated to and maintained at
 200.degree. C. (as measured at the jacket) while stirring. The pressure in
 reaction vessel 55 rose to 7.2.times.10.sup.5 Pa. After continuing the
 stirring at 200.degree. C. for 4 hours, the temperature of reaction vessel
 55 (as measured at the jacket) was lowered to 80.degree. C. Then, the
 composition of the resultant reaction mixture in reaction vessel 55 was
 analyzed. The analysis of the composition showed that phenyl salicylate
 was not present at all and, instead, methyl salicylate was present
 (wherein the methyl salicylate is presumed to have been formed by the
 reaction of phenyl salicylate with dimethyl carbonate). Thereafter,
 distillation was started by elevating the temperature of reaction vessel
 55 to 200.degree. C. (as measured at the jacket) under atmospheric
 pressure, and a distillate begun to come out through conduit 63. The
 distillation was continued while gradually lowering the pressure in
 reaction vessel 55 from atmospheric pressure to reduced pressure. When the
 amount of the distillate obtained through conduit 63 became 4.32 kg, the
 distillation was terminated. Subsequently, the pressure in reaction vessel
 55 was adjusted to atmospheric pressure by introducing nitrogen gas, and
 the weight of the reaction mixture in reaction vessel 55 was adjusted to 2
 kg by introducing PhOH. The reaction mixture, which remained in reaction
 vessel 55 after performing the above distillation, was withdrawn from
 reaction vessel 55 and transferred through conduit 46 and introduced into
 storage vessel 47 having a capacity of 10 liters. The composition of the
 reaction mixture was analyzed. The analysis of the composition showed that
 phenyl salicylate was not present at all and the total concentration of
 high boiling point substances had decreased to 0.8% by weight.
 Thereafter, every 40 hours after the point in time of 500 hours from the
 start of the operation (i.e., the point in time at which 2 kg of the
 concentrated liquid was withdrawn from storage vessel 36 and led into
 reaction vessel 55 as mentioned above), a sequence of the above operations
 using storage vessel 36 (from which 2 kg of the concentrated liquid was
 withdrawn), reaction vessel 55 and storage vessel 47 (into which the
 remaining reaction mixture obtained in reaction vessel 55 was introduced)
 was repeated in the same manner as described above.
 On the other hand, from a point in time of 600 hours after the start of the
 operation, the reaction mixture stored in storage vessel 47 was
 continuously withdrawn at a rate of 0.05 kg/hr through conduit 48 and
 recycled into the system for the transesterification through conduit 49.
 The condensate withdrawal rate from condenser 22 through conduit 23 during
 the period of time of from 400 hours to 600 hours after the start of the
 operation was 16.65 kg/hr, and the condensate withdrawal rate from
 condenser 22 through conduit 23 during the period of time of from 600
 hours to 2,000 hours after the start of the operation was 16.7 kg/hr.
 During the period of time of from 400 hours to 600 hours after the start
 of the operation, catalyst II was added to reaction vessel 100 through
 conduit 3 at such a feeding rate as to compensate for the catalyst
 withdrawal rate at which the catalyst was withdrawn through conduit 20,
 i.e., catalyst II was added through conduit 3 at a feeding rate such that
 the above-mentioned Sn concentration of 0.4% by weight in conduit 13 was
 able to be maintained.
 The operation was carried out for 2,000 hours. From the period of time of
 from 600 hours after the start of the operation, i.e., from the point in
 time at which the recycling of the catalyst into the system for the
 transesterification through conduit 49 was started, there was no need for
 introducing a fresh catalyst into the system for the transesterification.
 In addition, since the catalyst-containing liquid containing both the
 catalyst and high boiling point substances was withdrawn from the system
 for the transesterification and subjected to the above-described
 treatments according to the present invention, a waste liquid containing a
 spent catalyst did not occur at all.
 From the evaporation-concentrated liquid which was formed in evaporator 14
 and which contained the catalyst and high boiling point substances,
 samples were taken through the above-mentioned sampling nozzle provided on
 conduit 15', wherein the samples were, respectively, withdrawn at points
 in time of 1,000 hours, 1,500 hours and 2,000 hours after the start of the
 operation. The determination of the total concentration of the high
 boiling point substances in each sample was conducted by the
 above-mentioned method. With respect to these samples withdrawn at points
 in time of 1,000 hours, 1,500 hours and 2,000 hours after the start of the
 operation, the total concentrations of the high boiling point substances
 were 2.2% by weight, 2.2% by weight and 2.2% by weight, respectively.
 During the 2,000 hour operation time, the operation could be stably
 conducted (for example, both the flow and the composition in each conduit
 were stable) without suffering disadvantageous phenomena, such as the
 deposition of the catalyst from a catalyst-containing liquid and the
 adherence of the deposited catalyst to the inside surfaces associated with
 the equipment employed for the operation. During the operation, samples of
 the reaction mixture withdrawn from the bottom of reaction vessel 100 were
 taken through the above-mentioned sampling nozzle provided on conduit 13,
 and the samples were analyzed. With respect to the reaction mixture which
 was taken from conduit 13 at a point in time of 2,000 hours after the
 start of the operation, the composition of the reaction mixture was as
 follows: PhOH: 51% by weight; methyl phenyl carbonate (MPC): 6% by weight;
 diphenyl carbonate (DPC): 0.4% by weight; anisole (ANS): 0.6% by weight;
 and Sn: 0.4% by weight. The purity of the aromatic carbonate (which was a
 mixture of MPC and DPC) in the condensate withdrawn from condenser 22
 through conduit 23 was 99.99% or more, and no high boiling point substance
 was detected in the condensate. After the operation was terminated, the
 inside surfaces associated with the equipment employed for the operation
 were examined. No adherence of the catalyst to any of the inner walls of
 reaction vessel 100, evaporator 14, reboiler 17, conduits and the like was
 observed.
 EXAMPLE 6
 Preparation of Catalyst
 A mixture of 40 kg of PhOH and 8 kg of titanium tetrachloride was heated to
 and maintained at 50.degree. C. for 10 hours under a flow of nitrogen gas,
 thereby performing a reaction. After that period of time, hydrogen
 chloride formed in the resultant reaction mixture was distilled off
 together with unreacted PhOH. Then, most of the remaining PhOH was
 distilled off from the reaction mixture under reduced pressure, and the
 resultant mixture was allowed to cool in a nitrogen atmosphere, to thereby
 obtain catalyst III.
 Production of Aromatic Carbonate
 Substantially the same procedure as in Example 5 was repeated, except that
 catalyst III was used in an amount such that the Ti concentration of the
 reaction mixture in conduit 13 became 0.2% by weight. The operation was
 continued for 2,000 hours. During the 2,000 hour operation time, the
 operation could be stably conducted (for example, both the flow and the
 composition in each conduit were stable) without suffering disadvantageous
 phenomena, such as the deposition of the catalyst from a
 catalyst-containing liquid and the adherence of the deposited catalyst to
 the inside surfaces associated with the equipment employed for the
 operation. From the evaporation-concentrated liquid which was formed in
 evaporator 14 and which contained the catalyst and high boiling point
 substances, samples were taken through the sampling nozzle provided on
 conduit 15', wherein the samples were, respectively, withdrawn at points
 in time of 1,000 hours, 1,500 hours and 2,000 hours after the start of the
 operation. The determination of the total concentration of the high
 boiling point substances in each sample was conducted by the
 above-mentioned method. With respect to these samples withdrawn at points
 in time of 1,000 hours, 1,500 hours and 2,000 hours after the start of the
 operation, the total concentrations of the high boiling point substances
 were 2.8% by weight, 2.9% by weight and 2.9% by weight, respectively.
 During the operation, samples of the reaction mixture withdrawn from the
 bottom of reaction vessel 100 were taken through the sampling nozzle
 provided on conduit 13, and the samples were analyzed. With respect to the
 reaction mixture which was taken from conduit 13 at a point in time of
 2,000 hours after the start of the operation, the composition of the
 reaction mixture was as follows: PhOH: 51% by weight; methyl phenyl
 carbonate (MPC): 6% by weight; diphenyl carbonate (DPC): 0.4% by weight;
 anisole (ANS): 0.4% by weight; and Ti: 0.2% by weight. The purity of the
 aromatic carbonate (which was a mixture of MPC and DPC) in the condensate
 withdrawn from condenser 22 through conduit 23 was 99.99% or more, and no
 high boiling point substance was detected in the condensate. After the
 operation was terminated, the inside surfaces associated with the
 equipment employed for the operation were examined. No adherence of the
 catalyst to any of the inner walls of reaction vessel 100, evaporator 14,
 reboiler 17, conduits and the like was observed.
 EXAMPLE 7
 235 g of diphenyl carbonate obtained in Example 3 and 228 g of bisphenol A
 were placed in a vacuum reaction apparatus equipped with an agitator, and
 the resultant mixture was heated to 180.degree. C. while stirring and
 gradually evacuating the reaction apparatus with nitrogen gas. Then, the
 temperature of the mixture was slowly elevated from 180 to 220.degree. C.
 while stirring and evacuating the reaction apparatus with nitrogen gas.
 Subsequently, the reaction apparatus was sealed, and a polymerization was
 effected at 8,000 Pa for 30 minutes while stirring at 100 rpm, and then,
 at 4,000 Pa for 90 minutes while stirring at 100 rpm. Thereafter, the
 temperature of the reaction apparatus was elevated to 270.degree. C., and
 a polymerization was effected at 70 Pa for one hour, thereby obtaining an
 aromatic polycarbonate. The obtained aromatic polycarbonate was colorless
 and transparent, and had a number average molecular weight of 10,200.
 Comparative Example 5
 Substantially the same procedure as in Example 7 was repeated, except that
 the diphenyl carbonate obtained in Comparative Example 4 was used (instead
 of the diphenyl carbonate obtained in Example 3). The obtained aromatic
 polycarbonate had suffered yellowing and had a number average molecular
 weight of 8,800.
 INDUSTRIAL APPLICABILITY
 By the process of the present invention, an aromatic carbonate having high
 purity can be produced stably for a prolonged period of time. Therefore,
 the process of the present invention can be advantageously employed in a
 commercial-scale mass production of an aromatic carbonate. An aromatic
 carbonate produced by the process of the present invention is used as a
 raw material for producing aromatic polycarbonates, use of which as
 engineering plastics has been increasing in recent years.