HYDROGENATION CATALYST AND METHOD FOR PREPARING TETRAHYDROFURAN

Abstract: Hydrogenation in a back-mixed reactor or plug flow reactor of a hydrogenatable precursor such as maleic acid to tetrahydrofuran in the presence of a tri- or polymetallic catalytic composite consisting essentially of a combination of a catalytically effective amount of palladium, rhenium and one or more metals selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium deposited on a support such as an activated, porous carbon carrier; and the continuous hydrogenation of a hydrogenatable precursor to tetrahydrofuran with hydrogen in a back mixed reactor in the presence of a suitable hydrogenation catalyst with continuous tetrahydrofuran removal while maintaining the concentration of acid within a predetermined range.

Description: TITLE HYDROGENATION CATALYST AND METHOD FOR PREPARING TETRAHYDROFURAN CROSS-REFERENCE TO EARLIER FILED APPLICATION This application is a continuation-in-part of application Serial No. 07/558,991, filed July 27, 1990. BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to the hydrogenation of hydrogenatable precursors to tetrahydrofuran in a back-mixed or a plug flow reactor in the presence of a novel tri- or polymetallic catalytic composite consisting essentially of a combination of: (1) a catalytically effective amount of palladium (Pd) ; (2) a catalytically effective amount of rhenium (Re) ; and, (3) a catalytically effective amount of one or more metals selected from rhodium (Rh) , cobalt (Co) , platinum (Pt) , ruthenium (Ru) , iron (Fe) , thulium (T ) , cerium (Ce) , yttrium (Y) , neodymium (Nd) , aluminum (Al) , praesodymiu (Pr) holmiu (Ho) , copper (Cu) , samarium (Sm) , europium (Eu) , hafnium (Hf) , manganese (Mn) , vanadium (V) , chromium (Cr) , gold (Au) , terbium (Tb) , lutetium (Lu) , nickel (Ni) , scandium (Sc) and niobium (Nb) deposited on a support, i.e., carrier. In another aspect, the present invention relates to an efficient aqueous process for the manufacture of high purity tetrahydrofuran comprising continuous hydrogenation of a hydrogenatable tetrahydrofuran precursor in the presence of a suitable hydrogenation catalyst in a back mixed reactor and maintaining the concentration of the acid in the reaction mixture within a predetermined range. DESCRIPTION OF RELATED ART Numerous catalysts are disclosed in the art as being useful for preparing tetrahydrofuran (*THF") and 1,4-butanediol ("BDO") by the hydrogenation of suitable THF precursors such as maleic acid, maleic anhydride, fumaric acid, succinic acid, malic acid, dimethyl succinate and ga ma-butyrolactone. Many of these catalysts incorporate the metals palladium and rhenium on a suitable support. For example, U.S. Patent 4,609,636 describes the use of a catalyst composite comprising palladium and rhenium on a carbon support for making THF, BDO or mixtures thereof from a variety of hydrogenatable precursors. U.S. Patent 4,973,717 discloses the batchwise or continuous production of an alcohol and/or ether from a carboxylic acid ester using, for example, a palladium based catalyst and further discloses the important effect of a metal capable alloying with palladium. The.use of these alloyed catalysts for the direct, selective production of THF/BDO from precursors containing one or more carboxylic acid groups is not disclosed in U.S. Patent 4,973,717. Methods are known in the art for the selective production of THF by the catalytic reduction of hydrogenatable precursors. For example, U.S. Patent 4,609,636 teaches that the relative ratio of THF to BDO can be increased by increasing one or more variables selected from operating temperature, contact time, and hydrogen spacetime. It is also known from numerous references, such as U.S. Patent 3,726,905, that the dehydration of BDO to give THF is catalyzed by acid and that increasing the acid concentration results in an increase in the relative ratio of THF to BDO. However, it is also known that rhenium containing hydrogenation catalysts are inhibited by acids in the presence of water. Bulletin of The Japan Petroleum Institute, Volume 12, pages 89 to 96 (1970) describes a "kinetic study of the hydrogenation of maleic anhydride and intermediates using nickel- rhenium catalyst on kieselguhr" and reports that "water and succinic acid may be considered as the chief inhibitor components" of the step involving the reduction of the intermediate succinic anhydride to gamma-butyrolactone. The authors conclude that "in order to make the THF production rate greater, it is necessary to decrease the concentration of succinic acid as much as possible". Other known catalysts and processes for the production of THF from hydrogenatable precursors are cited in the above mentioned teachings, and are useful for their intended purposes, however, all are subject to improvement. One important area subject to improvement is catalyst performance, i.e., selectivity, space time yield and activity. Selectivity is defined herein to refer to a measure of the percentage of the exit stream composed of THF/BDO/gammabutyrolactone ("GBL") in a plug flow reactor or a back-mixed reactor. Space time yield (STY) is defined herein to refer to the amount of grams of THF/kilogram catalyst/hour. Activity is defined herein to refer to the percent acid converted at a given hold up time in a plug-flow reactor. Another important area subject to improvement is the preferential production of THF using catalysts that give both high selectivity and high space time yield. Such improvements are of great commercial significance since they allow for the more economical production of THF which is an item of commerce with a plurality of uses. For example, tetrahydrofuran is a useful solvent for high polymers, such as polyvinyl chloride and as a monomer in polyether polyols. It has been discovered in the present invention that the process for making tetrahydrofuran in a back-mixed or plug-flow reactor using a highly active polymetallic palladium-rhenium catalytic composite including one or more metals selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium produces high space time yields of THF while maintaining high selectivity in a back mixed reactor or produces a high acid conversion in a plug flow reactor. It has also been discovered in the present invention that, in the continuous manufacture and removal by vapor take off of THF under back-mixed conditions with a highly active catalyst in an aqueous medium, maintaining the concentration of carboxylic acids within a predetermined range results in high selectivity to THF with very little over reduction and surprisingly little loss in catalyst activity. SUMMARY OF THE INVENTION According to the present invention a hydrogenatable precursor such as maleic acid, maleic anhydride, fumaric acid, succinic acid, succinic anhydride, malic acid, or mixtures thereof, i.e., these precursors can be described as dicarboxylic acids, or anhydrides, or mixtures of said acids and/or anhydrides, is reacted with hydrogen in a back-mixed reactor or in a plug flow reactor at a temperature of about 150*C to 300*C at a pressure of about 1000 to 3000 psig in the presence of a novel tri- or polymetallic catalytic composite comprising a combination of a catalytically effective amount of palladium, rhenium and one or more metals (M) , i.e., a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium deposited on a support such as an activated, porous carbon carrier with a surface area m excess of about 650 m 2/g or a refractory oxide carrier, e.g., alumina, zirconia, titania, hafnium oxide, silica or barium carbonate and the like, to produce high space time yields of THF while maintaining high selectivity in a back mixed reactor or to produce high acid conversion in a plug flow reactor. The polymetallic catalytic composites of this invention consist by total weight of: (1) from about 0.1 to 10 weight percent of a palladium component; (2) from about 1 to 20 weight percent of a rhenium component; and, (3) from about 0.01 to 1.0 weight percent of the metal component with the remaining weight comprising either an activated, porous carbon carrier or a refractory oxide carrier. In another aspect of this invention, a hydrogenatable precursor is reacted on a continuous basis in an aqueous medium with hydrogen in a back-mixed reactor at a temperature of about 150*C to 300*C at a pressure of about 1000 to 3000 psig in the presence of a suitable hydrogenation catalyst with continuous THF removal by vapor take off, and the concentration of acid in the reaction mixture is maintained within a predetermined range. DETAILED DESCRIPTION OF THE INVENTION Catalyst and Preparation One aspect of the invention relates to a novel tri- or polymetallic catalytic composite consisting essentially of a combination of: (1) a catalytically effective amount of palladium (Pd) ; (2) a catalytically effective amount of rhenium (Re) ; and, (3) a catalytically effective amount of one or more metals (M) , i.e., a metal component selected from rhodium (Rh) , cobalt (Co) , platinum (Pt) , ruthenium (Ru) , iron (Fe) , thulium (T ) , cerium (Ce) , yttrium (Y) , neodymium (Nd) , aluminum (Al) , praesodymium (Pr) , holmium (Ho) , copper (Cu) , samarium (Sm) , europium (Eu) , hafnium (Hf) , manganese (Mn) , vanadium (V) , chromium (Cr) , gold (Au) , terbium (Tb) , lutetium (Lu) , nickel (Ni) , scandium (Sc) and niobium (Nb) deposited on a support. The tri- or polymetallic catalytic composite consists essentially of about 0.1 to 10 weight percent of palladium, about 1 to 20 weight percent of rhenium and about 0.01 to 1.0 weight percent of the metal or metals M by total weight. It is the addition of the metal or metals M that unexpectedly improves catalytic performance. Suitable supports include activated, porous carbons and refractory oxide carriers, e.g., alumina, zirconia, titania, hafnium oxide, silica or barium carbonate and the like. The preferred carrier is an activated, porous carbon carrier. Suitable carbon supports have 2 a surface area in excess of about 650 /g (measured ising standard N_ BET techniques) , typically in excess of about 1000 m 2/g, preferably m excess of abou-tjr 1500 m 2/g' The catalyst carbon support is fine powder particles for use in a slurry reactor or larger support granules for use in a fixed bed reactor. The tri- or polymetallic catalytic composite of this invention can be prepared in any one of a number of different methods known in the art. How¬ ever, a preferred step in the method for preparing said catalyst involves sequential deposition of the palladium component and the rhenium component as described in greater detail in U.S. Patent 4,609,636, the teachings of which are incorporated herein by reference. For example, a method for preparing said catalyst includes, in sequence, the steps of: (a) impregnating a carbon support with a solution containing sources of the metal M and palladium and removing the solvent; (b) drying the metal M and palladium impregnated carbon at a temperature in the range of from 100'C to 500*C, under reducing conditions for about 0.5 to 24 hours; (c) applying to the metal M and palladium impregnated carbon a source of rhenium in solution and removing the solvent to form said catalyst; and (d) drying the metal M/palladium/rhenium impregnated carbon at a temperature in the range of from 100°C to 500'C, under reducing conditions for about 0.5 to 24 hours. It will be appreciated by those skilled in the art that variants of this method in which the source of metal (M) is applied to the support and reduce*, prior to the addition of the palladium, after the add- .ion of the palladium, after the addition of the rhenium or simultaneously with the rhenium can also be be ^ficially employed to prepare the catalysts f this inven t ion. This is illustrated by Examples 65-67 which show that a catalyst prepared by an alternate sequence of metal deposition (Rh deposited and reduced before Pd, Example 30) gives similar performance properties. The solution containing the palladium compound is typically an aqueous medium containing an amount of palladium compound to yield a catalyst product with the requisite amount of palladium. The palladium compound is typically PdCl- and can also be, but is not limited to, a palladium compound such as PdBr 2 , Pd(N0 3 ) 2 , Pd(C 2 H 3 0 2 ) 2 (wherein C 2 H 3 0 2 denotes acetate), Pd(C 5 H_0 2 ) 2 (wherein C_H_0 2 denotes acetylacetonate) , and coordination compounds such as (NH 3 ) 4 PdCl 2 or (NH.) 2 PdClg. The solution containing the rhenium compound is typically an aqueous one containing an amount of rhenium compound to yield a catalyst product with the requisite amount of rhenium. The rhenium compound is typically Re 2 ° 7 but can ^ e perrhenic acid or a perrhenate of ammonium or of an alkali metal, K 2 ReCl 6 , (C 2 H 3 0 2 ) 2 ReCl, or (NH 4 ) 2 Re 2 Clg, etc. The solution containing the metal compounds M are typically aqueous and contain an amount of metal sufficient to yield a catalyst product with the requisite metal loading. By way of example, when the metal is r odium, the rhodium compound is typically RhCl *xH,0 but can also be a rhodium compound such as RhBr 3 *xH 2 0, R 2 (C 2 H 3 0 2 ) 4 , Rh 6 (C0) 16 , Rh 4 (C0) 12 , (Rh(C0) 2 Cl) 2 , Rh(C 5 H 7 C 2 ) 3 or Rh(N0 3 ) 3 *2H 2 0, Rh 2 (S0 4 ) 3 , as well as salts thereof exemplified by Na 3 RhCl g , and (C 4 H g ) 4 NRh(C0)_Cl 2 , and coordination compounds where Rh is ligated, for example by amines, halides, carboxylates, cε bon monoxide, etc ) ., exempli: » d by Rh(NH 3 ) g Cl 3 . Whfc the netal is iron, the ir-vn compound is typica * .y FeCl *6H 2 0 but can a' _*o be an iron compound such as Fed *xH 2 0, FeBr 2 , Fe(N0 3 ) 3 *9H 2 0, Fe(S0 4 )*7H 2 0, Fe 2 (S0 4 ) 3 , Fe(C 5 H ? 0 2 ) 3 , (C 5 H 5 ) 2 Fe (wherein C 5 H g denotes cyclopentadienyl) , Fe(C0) 5 , Fe 2 (C0) , as well as salts and coordination compounds thereof. When the metal is cobalt, the cobalt compound is typically CoCl *6H 2 0 but can also be a cobalt compound such as CoBr *xH_0, Co(OH)., Co(N0 3 ) 2 *6H 2 0, CoS0 4 *7H 2 0, Co(C 2 H 3 0 2 ) 2 , Co 3 0 4 , Co(C 5 H ? 0 2 ) 2 , Co(C 5 H ? 0 2 ) 3 , Co 2 (C0) 8 , coordination compounds such as Co(NH 3 ) 6 Cl 3 , and salts such as Co(C10 4 ) 2 . When the metal is platinum, the platinum compound is typically H 2 PtCl *6H 2 0, but can also be a platinum compound such as PtCl 2 , Na 2 PtCl 4 , PtCl 4 , PtBr 2 , PtBr 4 , H 2 PtBr 6 , H 2 Pt(0H) 6 , Pt(C 5 H ? 0 2 ) 2 , coordination compounds such as (NH 4 ) 2 PtCl 4 , (C 2 H g N 2 ) 3 PtCl 4 , or (NH 3 ) 4 Pt(N0 3 ) 2 , and organometallic precursors such as (n-C 4 H g ) 4 NPtBr 3 (CO) . When the metal is ruthenium, the ruthenium compound is typically RuCl *3H 2 0, but can also be a ruthenium compound such as RuBr * H 2 0, RuNO(N0 3 ) 3 , RuO *xH 2 0, Ru(C 5 H 7 0 2 ) 3 , Ru 2 (C 2 H 3 0 2 ) 4 C1, coordination compounds such as Ru(NH 3 ) 5 Cl 3 and (NH 4 ) 2 Ru(H 2 0)Cl 5 , (NH 4 ) 2 RuCl g , and organometallic compounds such as Ru 3 (C0) 12 « When the metal is thulium, the thulium compound is typically TmCl *7H. > 0, but can also be a thulium compound such as πsBr *xH 2 0, TmF.., Tml 3 , Tm_0 3 , Tm(C 2 H 3 0 2 ) 3 *xH 2 0, Tm(C 5 H 7 0 2 ) 3 , m 2 (C0 3 ) 3 *xH 2 0, and Tm(N0 3 ) 3 *5H 2 0. When the metal is cerium, the cerium compound is typically CeCl *xH 2 0, but can also be a cerium compound such as CeBr *6H 2 0, CeF 3 , Cel 3 , Ce 2 (C 2 H 3 0 2 ) 3 *3H 2 0, Ce 2 (C0 3 ) 3 *5H 2 0, Ce(N0 3 ) 3 *6H 2 0, Ce(C_H_0 2 ) 3 , as well a salts such as (NH 4 )2Ce(N0 3 )_. When the mt'al is yttri, , the yttrium compound is typically YC"< *6H 2 0, but :an also be a yttrium compound such as YBr *xH 2 0, Y(C 2 H 3 0 2 ) 3 *4H 2 0, Y(C 5 H 7 0 2 ) 3 , (C 5 H 5 ) 3 Y, Y 2 (C0 3 ) 3 *3H 2 0, and Y(N0 3 ) 3 *6H 2 0. When the metal is neodymium, the neodymium compound is typically NdCl *6H 2 0, but can also be a neodymium compound such as NdBr 3 *xH 2 0, NdF 3 , Ndl 3 , Nd 2 0 3 , Nd(C 2 H 3 0 2 ) 3 *H 2 0, Nd(C 5 H ? 0 2 ) 3 , Nd(N0 3 ) 3 *6H 2 0 and d 2 (C0 3 ) 3 * H 2 0. When the metal is aluminum, the aluminum compound is typically A1C1 *6H O, but can also be an aluminum compound such as A1C1 3 , AlBr 3 and hydrates, A1F, and hydrates, A1I 3 , Al(OH) 3 , A1(C 3 H ? 0) 3 (wherein C 3 H ? is isopropoxide) , A1(C..H 7 0 2 ) 3 , and A1(N0 3 ) 3 *9H 2 0. When the metal is praesodymiu , the praesodymiu compound is typically PrCl *7H 2 0, but can also be a praesodymium compound such as PrBr 3 and hydrates, PrF 3 , Prl 3 , Pr 6 o 11# Pr(C 2 H 3 0 2 ) 3 *3H 2 0, Pr(C 5 H 7 0 2 ) 3 , Pr 2 (C0 3 ) 3 *8H 2 0 and Pr(N0 3 ) 3 *6H 2 0. If the metal is holmium, the holmium compound is typically HoCl *6H 2 0, but can also be a holmium compound such as HoBr 3 and hydrates, HoF 3 , HoI 3 , Ho 2 0 3 , Ho(C 5 H 7 0 2 ) 3 , Ho(C 2 H 3 0 2 ) 3 * H 2 0, Ho 2 (C0 3 ) 3 *xH 2 0 and Ho(N0 3 ) 3 *5H 2 0. The M precursor can be any M compound with properties suitable for the catalyst preparation, e.g. , soluble in the solvent of choice. Suitable compounds include oxides, carbonates, alkoxides, -diketonates, halides, nitrates, sulfates, hydroxides, carboxylates, carbonyls, coordination compounds and combinations of the above as well as solvates and salts thereof. A preferred M compound is of the general formula MCI *xH,0. The preparation of the catalyst composite may be carried out in the presence of Group IA or IIA metals, which may be present in the carbon as obtaine or may be added J .w For examp -e, t .-- beneficial effect of the additionof potassi im is t own in Example 70. It is believed that addition of po "ssium to the slurry catalyst of the present invent i o can be beneficially employed in a slurry reactor. The fixed bed catalyst support carbon contains potassium as obtained. Catalytic Process for Preparing THF Another aspect of the present invention involves the catalytic process for preparing THF, employing the tri- or polymetallic catalytic composite described above, in a back-mixed reactor to achieve a high space time yield, e.g., in excess of about 280 g THF/kg catalyst/hr in a back-mixed reactor while maintaining high selectivity, e.g., up to about 90%. Alternatively, the process can be carried out in a plug flow reactor with a catalyst of the present invention exhibiting high activity, e.g., in excess of 58% acid conversion of a 5% by weight maleic acid feed at 250°C, 2000 psig total pressure and a contact time of 0.016 hour. More preferred composites employed in the process are those wherein the r zal component is selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium and holmium. The hydrogenatable precursors, i.e., starting reactants useful for carrying out the invention are, for example, maleic acid, maleic anhydride, fumaric acid, succinic acid, succinic anhydride, malic acid, or mixtures thereof. These precursors can be described as dicarboxylic acids, or anhydrides, or mixtures of said acids and/or anhydrides. Preferred hydrogenatable precursors include maleic acid and malϊic anhydride. For example, using these precursors in aqueou solution, the process is believed to pi'ceed in a si pwise manner with maleic acid (MAC) ; ~ *rst being r '.uced to succinic acid (SAC) which is further reduced tc reduced directly to THF, but is also reduced to BDO which is finally dehydrated to THF. By products include alcohols (1-propanol (PrOH) and 1-butanol (BuOH) ) and alkanes (primarily butane, methane). It will be appreciated by those skilled in the art that the process of this invention is equally applicable to the production of 3-methyltetrahydrofuran. Suitable hydrogentable precursors to 3-methylTHF include, but are not limited to, itaconic acid, itaconic anhydride, citraconic acid, citraconic anhydride, mesaconic acid, citric acid and aconitic acid. Itaconic acid is a preferred precursor due to cost and ease of reduction. It is further recognized that analogous precursors to other substituted THFs, such as 3-ethylTHF and 3-propylTHF can be beneficially employed in the process of this invention. Production f THF includes the hydrogenation of the hydrogentable precursor in an aqueous or organic solvent medium, i.e., the precursor solution is reacted with hydrogen in a back-mixed reactor or in a plug flow reactor. A preferred solvent for the process of this invention is water. The hydrogenation conditions include a reaction temperature in the range of about 150°C to 300°C, preferably about 250*C and a hydrogen pressure of about 1000 to 3000 psig, preferably about 2000 psig, a hydrogen spacetime of about 1 to 14 minutes and a liquid contact time of about 0.5 to 7 hours in the presence of the tri- or polymetallic catalytic composite described above. The hydrogenation of this invention can be run using conventional apparatus and techniques in a back-s- or plug flc reactor. Hydrogen is fed continuous-! -, generally X'Λ considerable stoichiometric excess. Unreacted hydrogen can be returned to the reactor as Λ recycled stream. The precursor solution, e.g., maleic acid-water solution, is fed continuously at concentrations ranging from dilute solutions to near the maximum solubility level; typically the concentration is between about 30 to 40 weight percent. The catalyst carbon support is fine powder particles for use in a slurry reactor or larger support granules for use in a fixed bed reactor. The amount of catalyst required will vary widely and is dependent upon a number of factors such as reactor size and design, contact time and the like. One method for carrying out the invention is in a plug flow reactor as described in greater detail in the examples. The selectivity was a measure of what percent of the exit stream is composed of THF, BDO and GBL. The highly active catalysts of this invention, when tested in a plug flow reactor, typically exhibit higher activity, and comparable selectivity, than a similarly prepared bimetallic Pd,Re/C catalyst. The loss of selectivity observed in some cases may be caused by over hydrogenation. If the catalyst is highly active but unselective at a certain temperature, decreasing its activity by lowering the temperature increases selectivity. Operating at lower temperatures can be an advantage in itself. A preferred method of preparing THF is in a back-mixed reactor such as, ' for example, a continuous slurry reactor. It has been discovered in the present invention that the higher activity of the tri- or polymetallic catalytic composite described above can be most effectively utilized to give high STY and selectivity to THF in this type of reactor. hile this reactor configuration results in a high concentration of aqueous carboxylic acids in th*. reactor, it has surprisingly been found that hignly active palladium/rhenium-on-carbon catalysts, such as those described herein and in U. S. Patent 4,609,636, still perform well in a reaction mixture containing high concentration of acid. One major advantage of the back-mixed reactor, which is particularly well-suited for producing THF from maleic acid, is vapor take off of THF, i.e., the THF can be purged from the reactor shortly after being formed, thus minimizing "over-hydrogenation", i.e., further reduction of the desired THF product, to form less desirable alcohols and alkanes. A second advantage of the back-mixed reactor is that acid in the feed is distributed throughout the reaction mass, and is thus available to catalyze the last step in the maleic acid to THF sequence, i.e., the ring closing of BDO to THF. This is of critical importance since it has been found that the BDO is also subject to over hydrogenation and a rapid conversion of BDO to THF serves to minimize yield losses due to over reduction of the BDO. It will be appreciated by those skilled in the art that these two features of a back-mixed reactor contribute to the ability to use the catalyst of higher activity without loss in selectivity. Alternatively, a fixed bed reactor with adequate recycle such that it approximates a back-mixed reactor can be used to carry out the invention. Tetrahydrofuran and 1,4-butanediol are the products produced by the process of this invention in a plug flow reactor or in a back-mixed reactor. The catalysts and processes of this invention are particularly well suited for the manufacture of THF. Another aspect of the invention is the production of THF in an essentially back-mixed reactor using a continuous process which provides definite advantages for separation and recovery of THF, for example: (1) THF and over reduced by products are volatile and can be distilled out of a back mixed reactor as they are formed and, if necessary, the THF further purified using conventional procedures; (2) the maleic acid starting material and the intermediates (up to and including BDO) are less volatile and tend to remain behind in the reactor; and (3) small amounts of THF precursors or intermediates, such as GBL, which are swept out with the THF can be separated and recycled to the reactor. In the continuous hydrogenation process of this invention, the BDO and THF are initially produced and the relative amounts obtained are dependent upon the nature of the catalyst employed as well as other factors such as those described in U.S. patent 4,609,636. The conversion of BDO produced to THF does not require further hydrogenation, but only an acid-catalyzed ring closure. This conversion is readily achieved in a back mixed reactor where the concentration of the carboxylic acids in the reaction mixture is maintained within a predetermined range, e.g., between about 1% and 10% by weight (calculated as succinic acid) of the reaction mixture. Preferably the concentration of carboxylic acids in the reaction mixture is maintained in excess of about 3% by weight of the reaction mixture. Better STY and selectivity is obtained above 3% by weight while fewer operational problems associated with acid solidification in process equipment are encountered below about 10%, typically about 8% by weight. This continuous process is particularly well suited for use with certain highly active hydrogenation catalysts which comprise a bi-, tri-, or polymetallic catalytic composite of fine metallic particles on an activated porous carbon support. For example, catalysts for use in this continuous process are comprised of, by total weight, from about 0.1 to 10 weight percent of palladium, about 1 to 20 weight percent of rhenium and optionally from about 0.01 to 1.0 weight percent of a component containing one or more of the metals selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium on a carbon support having a surface area in excess of about 1000 m 2/g. A-preferred hydrogenatable precursor for use in this continuous process is aqueous maleic acid. In the continuous process of the present invention, the required acid is provided by maleic, succinic, and to a lesser extent, other acids distributed throughout the reaction mixture in the back-mixed reactor. Thus, by controlling acid levels according to the process of this invention, high selectivity to THF can be obtained regardless of the relative amounts of BDO and THF initially produced with very little over reduction and surprisingly little loss in catalyst activity. It will be appreciated by those skilled in the art that the continuous process of this invention is equally applicable to the production of 3-methyltetrahydro- furan. Suitable hydrogentable precursors to 3-methyl- THF include, but are not limited to, itaconic acid, itaconic anhydride, citraconic acid, citraconic anhydride, esaconic acid, citric acid and aconitic acid. Itaconic acid is a preferred precursor due to cost and ease of reduction. It is further recognized that analogous precursors to other substituted THFs, such as 3-ethylTHF and 3-propylTHF can be beneficially employed in the continuous process of this invention. The following examples serve to illustrate the invention, but are not intended to limit the scope of the invention. EXAMPLE 1 An example of a preparation of a catalyst suitable for use in a slurry reactor is presented. Note that the items in parentheses refer to the parameter headings in Table 1. The remaining preparations of slurry catalysts, i.e.. Examples 2-4 were done in the same way; the parameters used in the preparation of Examples 2-4 are listed in Table 1. This example describes the preparation of a Rh+Pd,Re/C trimetallic catalytic composite of the present invention, suitable for use in a slurry reactor (type) . The notation M+pd,Re/C is meant to imply that M and Pd were codeposited and reduced followed by Re deposition and reduction. 0.24 g RhCl *H 2 0 (M precursor) containing 0.10 g Rh(Wt M) was dissolved in 115 ml H 2 0. 3.30 ml (vol 1 = 115+3.3) PdCl 2 -HCl stock solution containing 0.5 g Pd (Wt Pd) was added. The solution was added to 50g (wt Cl) Darco KBB ® carbon, commercially available, and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115*C. The Rh,Pd/C powder was recovered and then reduced- at 300'C in flowing H 2 ~He (1:1) for eight hours. The reduced powder was cooled to 50*C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% 02 in N . A solution was prepared by adding 13.1 ml of a Re O.-H.O stock solution containing 1.03 g Re (wt Re) to 77ml (vol 2=13.1+77) H 2 0. This solution was added to 36.4 g (wt C,) of reduced Rh,Pd/C and stirred occasionally for 3 hours at room temperature. The Rh,Pd,Re/C sample was dried, reduced and passivated as above. Nominal loadings are 0.2% Rh(% M) , 1.0% Pd(% Pd) , and 2.8% Re(% Re). Nominal loading is defined as 100*wt metal/wt support composite(support + any metals previously deposited). Example 5 This example describes the preparation of a trimetallic Rh+Pd,Re/C catalytic composite of the present invention suitable for use in the fixed bed reactor. Note that the items in parentheses refer to the parameter headings in Table 1. The preparations of fixed bed catalysts of Examples 5-12 were done in the same way; the parameters used in the preparation of Examples 5-12 are listed in Table l. 0.064 g RhCl *xH 2 0(M precursor) containing 0.027 g Rh(Wt M) was added to 0.65 ml PdCl -HC1 stock solution containing 0.099 g Pd(Wt Pd) . 18ml H 2 0(vol 1= 0.65+18) was added and mixed well. The solution was added to 10g(Wt Cl) Calgon PCB ® 12x30 carbon (commercially available) , which had been calcined at 400 β C for two hours in air and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115*C. The Rh,Pd/C powder was recovered and then reduced at 300' C in flowing H_-He (1:1) for eight hours. The reduced powder was cooled to 50"C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% 0 2 in N«. A solution was prepared by adding 3.76 ml of a Re 2 0 7 ~H 2 0 stock solution containing 0.29 g Re(Wt Re) to 10 ml H 2 0(vol 2=3.76+10). This solution was added to 8.0 g of reduced Rh,Pd/C(Wt C ) and stirred occasionally for 3 hours at room temperature. The Rh+Pd,Re/C sample was dried, reduced and passivated as above. Nominal ' loadings are 0.3% Rh(% M) , 1.0% Pd(% Pd) , and 3.6% . Re(% Re) . The catalysts of Examples 13-29 were prepared similarly, but a slightly different reduction 5 protocol was used. The preparation of Example 13 is described in detail to indicate these slight differences. The preparation parameters for Examples 13-29 are listed in Table 1. 0 Example 13 0.22 g TmCl 3 * H 2 0(M precursor) containing 0.093g Tm(Wt M) was added to a solution of 1.00 ml PdCl -HC1 stock solution containing 0.147 g Pd(Wt Pd) in 29ml H 2 0 (vol 1= 1.00+29) and mixed well. 15g(Wt 15 Cl) Calgon PCB* 12x30 carbon (commercially available) which had been calcined at 400*C in air for 2 hours, was added to the solution and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115'C. 20 The Tm,Pd/C powder was recovered and then reduced at 300'C in flowing H -He (3:97) for 5.8 hours. The reduced powder was purged with He at 300'C for 0.5 hour, cooled to room temperature overnight (>5 hours) in flowing He. A solution was prepared by adding 6.5 2.5 ml of a e 2 0 7 ~H 2 0 stock solution containing 0.52 g Re(Wt Re) to 19.5 ml H 2 0 (vol 2=6.5+19.5). To this solution was added 13.0 g of reduced Tm,Pd/C(Wt C«) and stirred occasionally for 3 hours at room temperature. The Tm+Pd,Re/C sample was dried and 30 reduced as above. Nominal loadings are 0.6% Tm(% M) , τ.98% Pd(% Pd) , and 4.0% Re(% Re). The Rh,Pd,Re/C catalyst of Example 30 was prepared using a different sequence of metal dep* ^ition from previous examples. Rh was deposited "i5 on the carbon support and reduced. Pd was next deposited and reduced, and then Re was deposited and reduced. Example 30 0.094 g RhCl *xH 2 0 containing 0.040 g Rh was added to 36ml H 2 0 and mixed well. To this solution was added 20g of Calgon PCB ® 12x30 carbon (commercially available) which had been calcined at 400 'C in air for 2 hours, and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115*C. The Rh/C powder was recovered and then reduced at 300"C in flowing H 2 -He (1:1) for eight hours. The reduced powder was cooled to 50"C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% O in K.. A solution was prepared by adding 1.12 ml of a PdCl_-HCl stock solution containing 0.17 g Pd to 30 ml H 2 0. To this solution was added 16.8 g of reduced Rh/C and stirred occasionally for 3 hours at room temperature. The Rh,Pd/C sample was dried, reduced and passivated as above. A solution was prepared by adding 4.80 ml of a Re 2 0 ? -H 2 0 stock solution containing 0.38 g Re to 15 ml H 2 0. To this solution was added to 8.0 g of reduced Rh,Pd/C and stirred occasionally for 3 hours at room temperature. The Rh,Pd,Re/C sample was dried,, reduced and passivated as above. Nominal loadings are 0.2% Rh, 1.0% Pd, and 4.8% Re. Comparative Example A Prepar v .ion of a Bimetallic Pd,Re/C Slurry Catalyst 6.30 ml PdCl -HC1 stock solution containing 0.96 g Pd was added to 224 cc water. T e solution wat. dded to 77g Darco KBB ® garbon, commercially available, and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115*C. The Pd/C powder was recovered and then reduced at 300*C in flowing H 2 -He (1:1) for eight hours. The reduced powder was cooled to 50*C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% 0 2 in N.. A solution was prepared by adding 36.3 ml of a Re 2 0.--H 2 Q stock solution containing 2.78g Re to 200ml H-O. This solution was added to 71.Og of reduced Pd/C and stirred occasionally for 3 hours at room temperature. The Pd,Re/C sample was dried, reduced and passivated as above. Nominal loadings are 1.25% Pd and 3.9% Re. Comparative Example B Preparation of a Bimetallic Pd,Re/C Fixed Bed Catalyst 6.80 ml PdCl 2 -HCl stock solution containing 1.01 g Pd was added to 172cc water and mixed well. The solution was added to lOOg Calgon PCB* 12x30 carbon (commercially available) which had been calcined at 400*C in air for 2 hours and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight a 115*C. The Pd/C powder was recovered and then reduced at 300*C in flowing H 2 -He (1:1) for eight hours. The reduced powder was cooled to 50*C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% 0 2 in K.. A solution was prepared by adding 45.1 ml of a Re 2 07-H 2 0 stock solution containing 3.60 g Re(Wt Re) to 116 ml H 2 0. ''his solutic was added to 90 g of reduced Pd/C and st;rred occasio, ally for 3 hours at room temperature. The Pd-^e/C sample was dried, reduced SUB T and passivated as above. Nominal loadings are 1.0% Pd and 4.0% Re. Comparative Example C Preparation of a Bimetallic Pd,Re/C Slurry Catalyst Containing Potassium 1.91g KC1 was dissolved in 230cc DI water and mixed well. This mixture was added to lOOg Darco KBB ® carbon, commercially available, and stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115'C. 5.4 ml PdCl 2 -HCl stock solution containing 0.82 g Pd was added to 192cc water and mixed well. The solution was added to 85.lg K/C and the resulting slurry was stirred occasionally over a 3 hour period at room temperature. The slurry was dried overnight at 115*C. The K,Pd/C powder was recovered and then reduced at 300 ' C in flowing H 2 -He (1:1) for eight hours. The reduced powder was cooled to 50°C, purged with He, cooled to room temperature in flowing He, and then passivated 30 minutes in flowing 1% 0 2 in N,. A solution was prepared by adding 36.2 ml of a Re 2 0 7 -H_0 stock solution containing 2.84g Re to 200ml H 2 0. This solution was added to 70.7g of reduced K,Pd/C and stirred occasionally for 3 hours at room temperature. The K,Pd,Re/C sample was dried, reduced and passivated as above. Nominal loadings are 1.0%K, .96% Pd and 4.0% Re. Catalyst Performance in a Back-Mixed Reactor The catalyst fo. a back-mixed reactor was tested by charging 7-lf g iry basis) of slurry catalyst in 150 ml wa^er to \ 300 ml Hastaloy C autoclave, equipped with an a itator, a thermocouple, feed lines for hydrogen and male'.c acid, and an exit line through which the product was swept out with the excess hydrogen and water. The catalyst was activated by heating at 250*C under a 1000 ml/min hydrogen flow at 2000 psig for one hour. The maleic acid was fed as a 40% by weight aqeous solution at feed rates ranging from 18 to 36 ml/min, and the reactor was maintained at 2000 psig and 250'C. The volatile products and water were swept out of the reactor at a rate controlled by the hydrogen feed rate. The hydrogen feed rate was adjusted so that the amount of water carried out with the exiting hydrogen gas balances the amount of water added with the maleic acid feed and the amount produced by the reaction; the reactor level was maintained at 100-2OOcc. Note that in all cases a very large excess of hydrogen was fed, compared to the amount consumed by the reaction; thus, the hydrogen feed rate does not affect catalyst performance. Several different feed rates of maleic acid were used in a catalyst test. Typically, the feed rate was increased from run to run until optimum performance was achieved. The reproducibility of this optimum performance was then checked over two or more runs at the same conditions. A catalyst test was made up of several runs. Typically, each run lasts 8-12 hours, with the reactor in steady state operation for 6-10 hours. The product composition data generated during steady state operation was averaged to give the average production rates(g/hr) of THF, BDO, GBL, PrOH, BuOH, and alkanes (primarily butane and methane) . The product composition data were measured in .he following way. A portion of the volati'ized produc. -/water in the exit gas stream is condensed and col. scted as "liquid product". The volume of the liquid pi luct collected each hour was measured, and its compositi <n analyzed using a calibrated gas chromatograph (GC) equipped with a flame ionization detector. The remaining uncondensed product(THF and alkanes) still in the exit gas stream was analyzed by measuring the gas flow rate, then analyzing the gas stream every two hours, using procedures similar to the one used for liquid analysis. The reactor contents are sampled every four hours and analyzed by GC and titration. The GC analysis was carried out using a Supelcowax 10 capillary column (30 m X 0.052 milliliter) which was maintained at 75*C for 5 minutes after injection and then heated at 10 β C per minute to a final temperature of 200'C. The acid level in the mixture was measured by titration with sodium hydroxide and reported as % by weight of succinic acid. The combination of these three analyses permits calculation of the catalyst's performance (STY and -selectivity) and the mass balance for each run. THF STY = wt THF produced hr 1 kg catalyst Selectiyity=(moles/hr of (THF(gas) + THF(liq) + GBL(liq) + BDO(liq))) / (moles/hr of (THF(gas) + THF(liq) + GBL(liq) + BD0(liq)+ PrOH(liq) + BuQH(liq) + alkane(gas) ) ) The maximum observed THF STY for the 1%, 4% Pd f Re/C slurry catalyst of comparative example A was 280 g THF/kg catalyst/hr. The trimetallic catalyst of this invention gave a THF STY in excess of 280 THF/kg _-catalyst/hr. Th. selectivity was a measure .<f what percentage of t'.e exit stream was made up oi THF, BDO and GBL. The addition of rhodium greatly imp. x ved STY, while maintaining high selectivity. The back-mixed run results are summarized in Table 2. Another aspect of the invention is control of the acid concentration in the reaction mixture within the.range of about 1% and 10% by weight (calculated as succinic acid) of the reaction mixture. The bimetallic catalyst composite of Comparative Example A was run in several days in the backmixed reactor to demonstarate the beneficial effects of acid control. The results, listed in Table A, show the beneficial effects on STY and selectivity, e.g., increasing the acid concentration from about 3% to about 10% benefits STY and selectivity. Table A CATALYST PERFORMANCE: BACKMIXED SLURRY REACTOR" 1 Selec- tivity Prepar- Acid Acid THF (THF+ Catalyst ation Feed Rate Cone. (STYΪ GBL+BDOΪ Pd,Re/C Ex.A 18 cc/hr 3.1% 216 83 25 4.5 276 91 31 8.0 280 91 30 9.9 282 93 1 Hydrogenation of 40% maleic acid at 250*C in excess flowing H 2 , 2000 psig total pressure. 2 Feed rate of 40% maleic acid/water in cc/hr. 3 Concentration of acid in the reactor, reported as wt% succinic acid, measured by acid-base titration. The beneficial effects of increasing the concentration of acid in the reactor on STY and selectivity are apparent. Although not listed here, lo =tr acid fee rates result in even lower acid concentra ion, STY a- ^ selectivity. Catalvsf Performance in a Pluσ Flow Reactor One method of preparing THF is liquid phase hydrogenation using conventional apparatus and techniques in a plug flow reactor as described in greater detail in U.S. Patent 4,609,636, the teaching of which are incorporated herein by reference. The catalyst for a plug flow reactor was tested by charging 3 g of catalyst to a 1/4 inch diameter Hastaloy U-tube reactor which was immersed i a heated sand bath for temperature control. The catalyst was activated by heating for one hour at 250 β C in a 100 ml/min hydrogen flow at 2000 psig. Hydrogen and maleic acid were co-fed to one end of the reactor, and the liquid/gas stream exited from the reactor through a pressure let-down valve which was set to control the pressure at the desired level. The excess gas was disengaged from the liquid in a chilled separator held at 90 psig pressure. In these experiments, the maleic acid was fed as a 5% aqueous solution at flow rates ranging from 6 to 300 ml/hr. Hydrogen flow was maintained in large excess at 100 to 200 ml/min. The temperature was maintained at 250 β C and pressure at 2000 psig. Each catalyst was evaluated at four different maleic acid feed rates to adequately map its performance characteristics. The gas and liquid was analyzed by GC as described above. Since neither maleic nor succinic acid is detected on the GC, the concentration of (maleic + succinic) acids in the product was determined directly by acid-base titration and reported as weight % succinic acid. The fixed bed performance data are summarized in Table 3. The percent of acid converted at a given hold up time is a measure of catalyst activity. The selectivity to (THF + BDO + GBL) is the maximum observed in a fitted plot of selectivity vs contact time for the four different flow rates. The THF STY(g THF/kg catalyst/hr) is the curve maximum from a fitted plot of STY vs contact time for the four different flow rates. The catalyst of this invention exhibited higher activity, and comparable selectivity, than the bimetallic Pd,Re/C catalyst of comparative Example B. The loss of selectivity observed in some cases was caused by "over-hydrogenation", i.e. reaction that goes beyond the desired THF product, to form alcohols and alkanes. Although the catalyst was too active at this temperature, resulting in over-hydrogenation, decreasing its activity by lowering the temperature increased selectivity, as shown in Table 4. Operating at lower temperatures can be an advantage in itself. M Pre¬ EX. Catalyst Type cursor 1 Rh+Pd,Re/C slurry RhCl *H O 2 Ru+Pd,Re/C slurry RuCl^*3H_0 Co+Pd,Re/C slurry CoCl *6H O 4 Fe+Pd ,Re/C slurry 5 Rh+Pd,Re/C fixed bed 5 6 Ru+Pd,Re/C fixed bed 7 Co+Pd,Re/C fixed bed 8 Fe+Pd,Re/C fixed bed 9 Ni+Pd,Re/C fixed bed 10 Au+Pd,Re/C fixed bed 11 Pt+Pd,Re/C fixed bed 12 Mn+Pd,Re/C fixed bed 13 Tm+Pd,Re/C fixed bed 14 Ce+Pd,Re/C fixed bed 15 Y+Pd,Re/C fixed bed 16 Nd+Pd,Re/C fixed bed 17 Al+Pd,Re/C fixed bed 18 Pr+Pd,Re/C fixed bed 19 Ho+Pd,Re/C fixed bed 20 Cu+Pd,Re/C fixed bed 21 Sm+Pd,Re/C fixed bed 22 Eu+Pd,Re/C fixed bed 23 Hf+Pd,Re/C fixed bed 24 V+Pd,Re/C fixed bed 25 Cr+Pd,Re/C fixed bed 26 Tb+Pd,Re/C fixed bed 27 Lu+Pd,Re/C fixed bed 28 Sc+Pd,Re/C fixed bed 29 Nb+Pd,Re/C fixed bed TABLE 2 CATALYST PERFORMANCE: BACK MIXED SLURRY REACTOR -1 CATALYST THF STY 1 Hydrogenation of 40% maleic acid at 250*C in excess flowing H 2 , 2,000 psig total pressure. TABLE 3 CATALYST PERFORMANCE: PLUG FLOW FIXED BED REACTOR 1 1 Hydrogenation of 5% maleic acid at 250"C in excess flowing H 2 , 2,000 psig total pressure- 2 Percent (maleic+succinic) acids converted at contact time of 0.016 hour. 3 Maximum percent selectivity to (THF + BDO + GBL) . 4 Percent (maleic+succinic) acids converted at contact time of 0.016 hour; determined by difference, not by titration. TABLE 4 CATALYST PERFORMANCE VS. TEMPERATURE: PLUG-FLOW FIXED BED REACTOR 1 67 Rh,Pd,Re/C EX.30 200 76 68 Pd+Re,Rh EX.5 250 97 69 Pd+Rh,Re EX.5 200 79 1 Hydrogenation of 5% maleic acid in excess flowing H_, 2,000 psig total pressure. 2 Percent (maleic+succinic) acids converted at contact time of 0.03 hour; determined by difference, not by titration; 3 Maximum percent selectivity to (THF + BEX) + GBL) . TABLE B CATALYST PERFORMANCE: BACKMIXED SLURRY REACTOR Example Catalyst Preparation Acid Feed Rate 70 K,Pd,Re/C Ex.C 24 cc/hr 32 31 Example Acid Cone' THF STY Selectivity THF+BDO+GBL 70 4.9% 285 85% 6.4 355 85 5.6 324 85 1 Hydrogenation of 40% maleic acid at 250C in excess flowing H_, 2,000 psig total pressure. 2 Feed rate of 40% maleic acid/water in cc/hr. 3 Concentration of acid in the reactor, reported as weight % succinic acid, measured by acid-base titration. The beneficial effect of addition of potassium to the bimetallic Pd,Re/C catalyst are apparent- STY increased from 280 to approximately 335.

Claims: CLAIMS What is claimed: 1. A polymetallic catalytic composite consisting essentially of a combination of a catalytically effective amount of fine particles of a (a) palladium component; (b) rhenium component; and (c) one or more of a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, ' neodymium, aluminum, praesodymium and holmium deposited on a support. 2. The polymetallic catalytic composite of Claim 1 wherein the palladium component ranges from about 0.1 to 10 weight percent, the rhenium component ranges from about 1 to 20 weight percent and the metal component ranges from about 0.01 to 1.0 weight percent by total weight. 3. The polymetallic catalytic composite of Claim 1 or Claim 2 wherein the support is selected from the group consisting essentially of an activated, porous carbon carrier having a surface area in excess of 650 m 2/g or a refractory oxide carrier. 4. A hydrogenation catalyst consisting essentially of a polymetallic catalytic composite of fine particles of palladium, rhenium and one or more of . metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium,!yttrium, neodymium, aluminum, praesodymium, holmium,-' copper, samarium europium, hafnium, manganese, vanadium. chromium, gold, terbium, lutetium, nickel, scandium and niobium on an activated porous carbon support having a surface area in excess of 650 m 2/g and comprising about 0.1 to 10 weight percent of palladium, about 1 to 20 weight percent of rhenium and about 0.01 to 1.0 weight percent of the metal component by total weight to achieve increased space time yield of product while maintaining high selectivity in a back mixed reactor or to achieve high activity in a plug flow reactor. 5. The catalyst of Claim 4 wherein the metal component is selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium and holmium. 6. A process for preparing tetrahydrofuran or substituted tetrahydrofuran from a hydrogenatable precursor, the process comprising contacting the hydrogenatable precursor with hydrogen at a temperature of about 150*C to about 300*C, and a pressure of about 1000 psig to about 3000 psig in the presence of a polymetallic catalytic composite comprising a combination of a catalytically effective amount of fine particles of a (a) palladium component; (b) rhenium component; and (c) one or more of a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, eodymiuπ., aluminum, praesodymium, holmium, c oper, samarium, europium, hafnium, ma ganese, vanadium, chromium, gold, ten 'urn, lutetium, nickel, scandium and niobiur deposited on a support. 7. The process of Claim 6 wherein the hydrogentable precursor is selected from itaconic acid, itaconic anhydride, citraconic acid, citraconic anhydride, mesaconic acid, citric acid and aconitic acid or mixtures thereof. 8. The process of Claim 6 wherein the metal component is selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium and holmium. 9. The process of Claim 6 or Claim 8 wherein the support is selected from the group consisting of an activated, porous carbon carrier having a surface area m excess of 650 m 2/g or a refractory oxide carrier. 10. The process of 9 wherein the palladium component comprises about 0.1 to 10 weight percent, the rhenium component comprises about 1 to 20 weight percent and the metal component comprises about 0.01 to 1.0 weight percent of the total weight. 11. The process of Claim 10 wherein the hydrogenatable precursor is selected from the group consisting of maleic acid, maleic anhydride, fumaric acid, succinic acid, succinic anhydride, malic acid, or mixtures thereof and is in an aqueous or organic solvent medium. a * 12. The p-, Claim 1.'- wherein the hydrogenatable precursor is selected from maleic acid and the solvent medium is water. 13. A process for preparing tetrahydrofuran from an aqueous solution of maleic acid to achieve increased space time yield while maintaining high selectivity or exhibiting high activity, the process comprising contacting said maleic acid with hydrogen at a temperature of about 150'C to about 300'C, and a pressure of about 1000 psig to about 3000 psig in the presence of a trimetallic catalytic composite comprising a combination of fine particles of a (a) about 0.9-1.0 weight percent palladium component; (b) about 2.1-4.5 weight percent rhenium component; and (c) about 0.1 -0.6 weight percent of a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium and holmium deposited on an activated porous carbon support having a surface area m excess of 650 m 2/g. 14. A process for preparing tetrahydrofuran from a hydrogenatable precursor in a back-mixed reactor, the process comprising contacting the hydrogenatable precursor with hydrogen at a temperature of about 150*C to about 300*C, and a pressure of about 1000 psig to about 3000 psig in the presence of a polymetallic catalytic composite comprising a combination of a catalytically effective amount of fine particles of a (a) pεlladium component; (b) rhe.ium compont .t ; and (c) one ϊr more of ι metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium cerium, yttrium. neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganese, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium deposited on a support. 15. A process for preparing tetrahydrofuran from a hydrogenatable precursor in a plug flow reactor, the process comprising contacting the hydrogenatable precursor with hydrogen at a temperature of about 150"C to about 300 * C, and a pressure of about 1000 psig to about 3000 psig in the presence of a polymetallic catalytic composite comprising a combination of a catalytically effective amount of fine particles of a (a) palladium component; (b) rhenium component; and (c) one or more of a metal selected from one or more of a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganesiu , vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium deposited on a support. 16. The process of Claim 14 or Claim 15 wherein the hydrogenatable precursor is selected from the group consisting essentially of maleic acid, maleic anhydride, fum ric acid, succ Λc acid, succinic anhydride, i;.-leic acid or mi., .ures thereof. 17. The process of Claim 16 further including the hydrogenatable precursor in an aqueous or organic solvent medium. 18. The process in Claim 13 or Claim 14 or Claim 15 wherein the support is an activated, porous carbon carrier having a surface area in excess of 2 650 m /g or a refractory oxide carrier. 19. The process of Claim 14 further comprising an aqueous, continuous process wherein the hydrogenatable precursor is maleic acid and the concentration of carboxylic acids in the reaction mixture is 1% to 10% by weight of the reaction mixture to accomplish high selectivity to tetrahydrofuran while minimizing over reduction of tetrahydrofuran. 20. An aqueous process for the manufacture of high purity tetrahydrofuran or substituted tetrahydrofuran by the continuous hydrogenation with vapor take off of tetrahydrofuran of a hydrogenatable tetrahydrofuran precursor selected from the group consisting of maleic acid, maleic anhydride, fumaric acid, succinic acid, succinic anhydride, malic acid, and mixtures thereof, and itaconic acid, itaconic anhydride, citraconic acid, citraconic anhydride, mesaconic acid, citric acid, aconitic acid or mixtures thereof, in the presence of a suitable hydrogenation catalyst and maintaining the concentration of carboxylic acids in the reaction mixture in a predetermined range. 21. The process of Clai 20 whereii the precursor is selected from maleic acid or itacc c acid. 22. The process of Claim 20 or Claim 21 wherein the suitable hydrogenation catalyst comprises fine metallic particles on an activated porous carbon support comprising by total weight of from about 0.1 to 10 wt % of palladium, about 1 to 20 wt % of rhenium and optionally from about 0.01 to 1.0 wt % of a metal component selected from rhodium, cobalt, platinum, ruthenium, iron, thulium, cerium, yttrium, neodymium, aluminum, praesodymium, holmium, copper, samarium, europium, hafnium, manganesium, vanadium, chromium, gold, terbium, lutetium, nickel, scandium and niobium. 23. The process of Claim 19 or Claim 20 or Claim 21 wherein the concentration of acids is 3% to 8% by weight. 24. The process of Claim 19 or Claim 20 wherein the catalyst is of Claim 4 and the support is an activated porous carbon carrier having a surface area in excess of 1000 2/g.

Inventors: ERNST RICHARD EDWARD, MICHEL JOHN BYRNE, ERNST, RICHARD, EDWARD, MICHEL, JOHN, BYRNE
Applicants: DU PONT, E.I. DU PONT DE NEMOURS COMPANY
Application year: 1991
Main classification: B01J23
IPC codes: B01J23, C07B61, C07D307