Process for the preparation of 2-hydroxybenzonitrile

2-Hydroxybenzonitrile is prepared by passing 2-hydroxybenzamide in the gas phase over a heterogeneous catalyst thereby effecting dehydration of the 2-hydroxybenzamide to produce 2-hydroxybenzonitrile.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The present invention relates to a process for the preparation of
 2-hydroxybenzonitrile by dehydration of 2-hydroxybenzamide in the gas
 phase.
 DESCRIPTION OF THE BACKGROUND
 2-Hydroxybenzonitrile is an important intermediate for the preparation of
 biologically active substances.
 The preparation of 2-hydroxybenzonitrile from 2-hydroxybenzamide is known
 in principle. However, only expensive reagents are described as
 dehydrating agents, which, moreover, must further be used in
 stoichiometric or superstoichiometric amounts.
 Japanese patent application JP 12 301/66 discloses the reaction of, for
 example, 2-hydroxybenzamide with phosphorus chloride nitrides at 100 to
 600.degree. C.
 DE-A 25 33 245 teaches the preparation of 2-hydroxybenzonitrile by reaction
 of the amide with phosgene in a nonpolar solvent. The nitrile is obtained
 in this reaction in a yield of &gt;90%. Apart from the fact that phosgene
 must be used here in superstoichiometric amounts, the use of phosgene in
 industrial processes is associated with considerable difficulties because
 of its extreme toxicity.
 The use of thiophosphoric diamide (Bull. Soc. Chim. Belg. 86, 4 (1977)),
 thionyldiimidazole (Heterocycles 12, 1285 (1979)) and trichloromethyl
 chloroformate (Tetrahedron Lett. 2203 (1986)) as dehydrating agents is
 also known for the preparation of 2-hydroxybenzonitrile from
 2-hydroxybenzamide.
 All of the methods described supra are only suitable for preparation of
 dehydrated products on the laboratory scale, since the reagents required
 are expensive and, furthermore, must be used in stoichiometric or
 superstoichiometric amounts.
 Another reaction which is known is the catalytic reaction of methyl
 2-hydroxybenzoate with ammonia to give 2-hydroxybenzonitrile (Izv. Akad.
 Nauk. Kaz. SSR, Ser. Khim. 1987 (2), 62). The catalyst used in this case
 is boron phosphate. A problem in this process is the formation of
 methylamine from ammonia and methanol which originates from the methyl
 ester. In addition, 2-hydroxybenzonitrile has the tendency to convert
 irreversibly at temperatures of &gt;100.degree. C. in the liquid phase into a
 high-melting triazine. That is, the 2-hydroxybenzonitrile which is formed
 reacts further in the hot reactor predominantly to give triazine. The
 product yield is therefore poor. Furthermore, this procedure is not
 suitable as an industrial process, since the reactor clogs within a short
 time. A need therefore continues to exist for an improved, industrially
 acceptable method of dehydrating amide compounds to the corresponding
 nitrites.
 SUMMARY OF THE INVENTION
 Accordingly, one object of the present invention is to provide a simple and
 economical process of dehydrating 2-hydroxybenzamide to
 2-hydroxybenzonitrile in good yields.
 Briefly, this object and other objects of the invention as hereinafter will
 become more readily apparent can be attained in a method of preparing
 2-hydroxybenzonitrile by passing 2-hydroxybenzamide in the gas phase over
 a solid catalyst, thereby effecting dehydration of the 2-hydroxybenzamide
 to the corresponding nitrile product.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Surprisingly, it has been found that the dehydration of 2-hydroxybenzamide
 to 2-hydroxybenzonitrile can be carried out by heterogeneous catalysis in
 the gas phase with particularly good success. A particularly suitable
 catalyst is a silica gel impregnated with phosphoric acid. In a suitable
 manner, the catalyst impregnated with phosphoric acid is dried before use.
 A particularly suitable support is, for example, AF 125 (Kali-Chemie).
 However, other silica gel quality grades of comparable specification can
 also be used.
 The dehydration reaction of the invention is preferably carried out at a
 temperature ranging from 200 to 400.degree. C. The reaction is carried out
 in a suitable manner under reduced pressure. A surprising discovery is
 that the secondary reaction of 2-hydroxybenzonitrile to give a
 high-melting triazine compound does not take place if the reaction is
 preferably carried out at a pressure ranging from 5 to 100 mbar,
 particularly preferably from 20 to 40 mbar. Preferably, the reaction is
 carried out at an LHSV (liquid hourly space velocity) between 0.05 and 1
 h.sup.-1, particularly preferably between 0.1 and 0.5 h.sup.-1. The crude
 output from the dehydration reaction, in addition to reaction water,
 contains more than 90% by weight of 2-hydroxybenzonitrile. Pure
 2-hydroxybenzonitrile is obtained in a suitable manner by distillation,
 with vacuum distillation being preferred.
 The process of the present invention is characterized by the use of a
 heterogeneous catalyst to promote the dehydration of 2-hydroxybenzamide to
 2-hydroxybenzonitrile in the gas phase. The catalyst, which is used in the
 process of the invention, can be any one of a variety of shapes including
 spheres, beads, chips, strands, tubes or even as a monolith, for example,
 in a honeycomb shape. However, the catalyst can also be employed in powder
 form. Generally, the catalyst employed is an oxide catalyst comprised of
 aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, silicates
 and various combinations of the oxides. Such oxide materials are then
 normally treated with an inorganic acid such as phosphoric acid, and then
 subjected to a thermal post-treatment.
 The reaction of the invention is expediently conducted in a tubular
 reactor. In a suitable manner, the dehydration reaction is conducted in
 such a way that the catalyst, a silica gel impregnated with phosphoric
 acid, preferably with aqueous 5 to 85% strength by weight phosphoric acid,
 is first heated to a temperature of 200 to 500.degree. C., preferably 300
 to 400.degree. C., particularly preferably 380 to 390.degree. C. The
 2-hydroxybenzamide can be added to the catalyst in the molten state, in
 which case the LHSV should be between 0.05 to 1 h.sup.-1, preferably
 between 0.1 and 0.5 h.sup.-1.
 The present dehydration reaction is conducted in a suitable manner under
 reduced pressure, preferably at a pressure ranging from 5 to 100 mbar,
 particularly preferably at a pressure ranging from 20 to 40 mbar. The
 crude product thus obtained, also termed the crude output, is generally
 purified by distillation, preferably under vacuum, in which case the
 distillation should proceed without high thermal stressing of the
 2-hydroxybenzonitrile, since the product has a tendency to form a
 high-melting triazine.
 The FIGURE of the present application is a diagram of the experimental
 apparatus employed to conduct the dehydration reaction of the invention.
 In the process 2-hydroxybenzamide starting material, in starting material
 reservoir 1, which can be heated, is passed through line 7 into tubular
 reactor 2 which contains catalyst 3. The tubular reactor 1 is provided
 within heating jacket 4. Dehydration of the 2-hydroxybenzamide starting
 material occurs as the starting material, in the gas phase, contacts the
 solid catalyst in the reactor. The product produced is discharged from the
 reactor through line 8 into product receiving vessel 5. A reduced pressure
 atmosphere is provided in the system by vacuum pump 6.
 The process of the present invention provides the possibility of preparing
 2-hydroxybenzonitrile in a simple and economical manner.
 Having generally described this invention, a further understanding can be
 obtained by reference to certain specific examples which are provided
 herein for purposes of illustration only and are not intended to be
 limiting unless otherwise specified.
 EXAMPLE 1
 Preparation of the Dehydration Catalyst:
 A 200 g amount of silica gel (AF 125 from Kali-Chemie, Hannover) is stirred
 with 500 g of 30% strength by weight phosphoric acid for 2 h at room
 temperature. The solids are removed by filtration and then dried at
 110.degree. C. for 30 h.
 Carrying out the Dehydration Reaction
 Referring to the apparatus shown in the FIGURE discussed above, an 80 g
 (0.58 mol) amount of 2-hydroxybenzamide is melted in the heatable starting
 material reservoir (inlet temperature: 150.degree. C.). At a pressure of
 30 mbar and a catalyst temperature of 380 to 390.degree. C., the liquid
 amide is metered (approximately 15 ml/h) into the tubular reactor onto the
 catalyst (catalyst volume: 100 ml). The LHSV is 0.15 h.sup.-1. A 76 g
 amount of crude output is obtained from the experimental system.
 The crude output obtained was distilled in a conventional distillation
 apparatus in order to purify the product. The data obtained are shown in
 Table I.
 TABLE I
 BT TT P m m.p.
 Fraction [.degree. C.] [.degree. C.] [mbar] (g) [.degree. C.]
 1 60 -- 0.07 9.5 --
 2 120-173 46-173 0.1 62.3 80-84
 RS -- -- 2.4 &gt;140
 74.2
 BT = bottom temperature
 CT = cold trap
 TT = top temperature
 P = pressure
 RS = residue
 m.p. = melting point
 m = mass
 Fraction 2 comprises 97% by weight of 2-hydroxybenzonitrile. The yield of
 2-hydroxybenzonitrile is thus 87%.
 EXAMPLE 2
 A 210 g (1.53 mol) amount of 2-hydroxybenzamide is melted in a heatable
 reservoir (inlet temperature: 150.degree. C.). At a pressure of 20 mbar
 and a catalyst temperature of 380 to 400.degree. C., the liquid amide is
 metered (approximately 20 ml/h) onto the catalyst (catalyst volume: 100
 ml, catalyst preparation cf. Example 1). The LHSV is 0.2 h.sup.-1. A 202 g
 amount of a crude output was obtained from the catalyst furnace.
 Distillation of the crude output is conducted in a simple distillation
 apparatus and the data obtained are shown in Table II.
 TABLE II
 BT TT P m m.p.
 Fraction [.degree. C.] [.degree. C.] [mbar] (g) [.degree. C.]
 1 60 -- 0.08 10 --
 2 120-175 50-172 0.1 163 80-85
 RS -- -- -- 20 --
 CT -- -- -- 5 --
 198
 Fraction 2 comprises 97% by weight of 2-hydroxybenzonitrile. The yield of
 2-hydroxybenzonitrile is thus 87%.
 EXAMPLE 3
 A 150 g (1.09 mol) amount of 2-hydroxybenzamide is melted in a heatable
 reservoir (inlet temperature: 150.degree. C.). At a pressure of 30 mbar
 and a catalyst temperature of 380 to 390.degree. C., the liquid amide is
 metered (approximately 10 ml/h) onto the catalyst (catalyst volume: 100
 ml, catalyst preparation cf. Example 1). The LHSV is 0.1 h.sup.-1. A 145 g
 amount of crude output was obtained.
 Distillation of the crude output is then conducted in a simple distillation
 apparatus and the results obtained are shown in Table III.
 TABLE III
 BT TT P m m.p.
 Fraction [.degree. C.] [.degree. C.] [mbar] (g) [.degree. C.]
 1 65 -- 0.1 6 --
 2 125-175 50-170 0.1 114 79-83
 RS -- -- -- 15 --
 CT -- -- -- 5 --
 140
 Fraction 2 comprises 97% by weight of 2-hydroxybenzonitrile. The yield is
 thus 85%.
 Having now fully described the invention, it will be apparent to one of
 ordinary skill in the art that many changes and modifications can be made
 thereto without departing from the spirit or scope of the invention as set
 forth herein.