HYDROGENATION OF CITRIC ACID AND SUBSTITUTED CITRIC ACIDS TO 3-SUBSTITUTED TETRAHYDROFURAN, 3- AND 4-SUBSTITUTED BUTYROLACTONES AND MIXTURES THEREOF

Abstract: A process is disclosed for preparing 3-substituted tetrahydrofuran, 3- and 4-substituted butyrolactones and mixtures thereof comprising hydrogenating a hydrogenatable precursor in the presence of an aqueous reaction medium and a catalyst comprising palladium and rhenium and at least one support selected from the group consisting of titanium oxide, zirconium oxide, and carbon.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for making 3-substituted tetrahydrofuran, 3- and 4-substituted butyrolactones and mixtures thereof by hydrogenation of citric acid and substituted citric acids. Background of the Invention Butyrolactone is a known compound which is employed in the synthesis of pyrrolidone, glutaric acid, and many other compounds. 3-Substituted tetrahydrofuran is employed as monomer in copolymers. Improved processes for preparing butyrolactone and 3-substituted tetrahydrofuran are of interest to the chemical industry. The following references disclose known processes for making butyrolactone. U.S. Patent 4,096,156 discloses a process for the preparation of gamma-butyrolactone by catalytic hydrogenation of maleic acid, maleic acid anhydride, succinic acid, succinic acid anhydride, or fumaric acid, or of a mixture of two or more of these compounds. The catalyst contains a mixture of an element of the VIII th subgroup of the periodic system of elements or of one of its compounds with an element of group IB of the periodic system of elements or of one of its compounds. U.S. 4,105,674 discloses a process for producing gamma-butyrolactone by hydrogenating a feed compound selected from the group consisting of maleic acid, succinic acid, maleic anhydride, succinic anhydride, and mixtures of any of the foregoing in the vapor phase. The reaction is conducted in the presence of a Cu-Pd or Cu-Pt catalyst. U.S. 3,113,138 discloses a process for preparing butyrolactone which comprises hydrogenating succinic anhydride in the liquid phase in the presence of a solvent selected from the group consisting of lower alkyl acetates and dioxane, and a catalyst comprising palladium deposited on a carrier selected from the group consisting of activated carbon and activated alumina, at a pressure of at least 200 p.s.i.g., while maintaining the temperature in the range of 30-95°C, until at least a major portion of the reaction has taken place. EP-A-147,219 describes a process for preparing tetrahydrofuran, 1,4-butanediol or mixtures thereof comprising hydrogenating a hydrogenatable precursor in the presence of a solvent medium and a catalyst, the catalyst being a composite of palladium and rhenium on a carbon support, the composite comprising about 0.5% to 10% of palladium and about 1% to 10% of rhenium by total weight, the palladium being present in the form of crystallites having an average size of about 10 nm to 25 nm and the rhenium being present in the form of a highly dispersed phase of crystallites having an average size of less than about 2.5 nm. SUMMARY OF THE INVENTION The present invention provides a process for preparing 3-substituted tetrahydrofuran, 3- and 4-substituted butyrolactones and mixtures thereof comprising contacting a hydrogenatable precursor selected from the group consisting of citric acid, substituted citric acid and mixtures thereof with a catalyst consisting essentially of palladium and rhenium and at least one support selected from the group consisting of titanium dioxide, zirconium oxide and carbon in the presence of an aqueous reaction medium and hydrogen. DETAILED DESCRIPTION OF THE INVENTION The invention provides a process for making 3-substituted tetrahydrofuran, 3- and 4-substituted butyrolactones and mixtures thereof by hydrogenating citric acid, substituted citric acid, or mixtures thereof. Preferably, the hydrogenatable precursor is citric acid. The processes of the invention are characterized in that they can be oriented, as desired, toward high ratios of 3-substituted tetrahydrofurans to 3- and 4-substituted butyrolactones or vice versa. The catalyst of this invention consists essentially of palladium and rhenium and at least one support selected from the group consisting of titanium oxide, zirconium oxide, and carbon. Preferably, the support is titanium oxide or carbon, and most preferably, the catalyst consists essentially of from about 1 to about 19 weight percent palladium and from about 4 to about 76 weight percent rhenium. The catalyst provides high conversion of the precursor, high selectivity to and yield of product, and the advantage of being able to control the product ratio of 3-substituted tetrahydrofuran/3- and 4-substituted butyrolactone by varying the support. It has been found that carbon supported catalysts favor the production of 3-substituted tetrahydrofurans and that titanium oxide and zirconium oxide supported catalysts favor production of 3- and 4-substituted butyrolactones. Preferably, the process is conducted at a temperature of from about 100°C to about 300°C, and most preferably from about 150°C to about 250°C. Preferably, the process is conducted at a pressure of from about 3.5 MPa (500 psig) to about 27.6 MPa (4000 psig), and most preferably from about 3.5 MPa (500 psig) to about 17.3 MPa (2500 psig). The process is conducted in the presence of an aqueous reaction medium which can be water or an aqueous solution containing water soluble substances such as methanol, ethanol, tetrahydrofuran or 1,4-dioxane. Preferably, the aqueous reaction medium is water. The concentration of precursor is not critical. The precursor can be employed in dilute solutions to near the maximum solubility level, typically from about 1 to about 50 weight percent. The liquid phase hydrogenation of the invention can be run using conventional apparatus and techniques in a stirred-tank reactor or in a fixed-bed reactor. Hydrogen is fed continuously, generally in considerable stoichiometric excess with no inert diluent gases. Unreacted hydrogen can be returned to the reactor as a recycle stream. The reaction can be run in the batch or continuous mode. The invention is further described in the following examples wherein all parts and percentages are by weight and degrees are Celsius. Catalysts used in the Examples were prepared according to the following general procedure. Catalyst Preparation The catalysts were prepared by adding 20 g of support to a solution containing 0.33 g of PdCl₂, 3 mL of concentrated hydrochloric acid and 15 mL of distilled water. The resultant slurry was stirred for three hours at ambient temperature and dried at 110° for 18 hours. The resulting supported catalyst was then heated in a furnace for one hour at 150° in an atmosphere of helium (flow rate 100 mL/min), followed by heating at 150° for one hour and 300° for three hours in an atmosphere of helium (flow rate 100 mL/min) and hydrogen (flow rate 100 mL/min). While still maintaining the hydrogen helium atmosphere, the catalyst was cooled to ambient temperature and passivated at ambient temperature with an atmosphere of 1.8% oxygen in nitrogen for 18 hours. The resulting reduced catalyst was added to a solution containing the desired amount of ammonium perrhenate and 6 mL of distilled water. The resultant slurry was treated as described above. The catalysts so prepared were granulated to 0.92-1.16mm (16 to 19 mesh; U.S Standard Sieve Units) and charged into the reactor employed in the Examples. Prior to start of the liquid feed the granulated catalysts were reduced in the reactor for about two hours each at 100°, 150°, and 200° in a stream of hydrogen. EXAMPLE 1 Hydrogenation of Citric Acid to 3-Methyltetrahydrofuran A fixed bed reactor having a volume of 6mL was charged with 1.73 g of 1% palladium and 4% rhenium on carbon catalyst. A 5% solution of citric acid monohydrate in water was passed through the reactor at a flow rate of 11.2 mL/minute along with hydrogen at a flow rate of 50 mL/minute. Optimum performance was obtained at an operating pressure of 6.9 MPa (1000 psig) and a temperature of 250°. Contact time was approximately 30 minutes based on the empty reactor volume. The citric acid conversion was greater than 99%. The product leaving the reactor was analysed by gas chromatography. The selectivity to 3-methyl tetrahydrofuran was 70.4%. EXAMPLE 2 Hydrogenation of Citric Acid to 3 & 4-Methylbutyrolactones A Hastelloy reactor having a volume of 6mL was charged with 1.73 g of 1% palladium and 4% rhenium on titanium oxide catalyst. A 5% solution of citric acid monohydrate in 98% water and 2% dioxane was passed through the reactor at a flow rate of 12 mL/hour along with hydrogen at a flow rate of 100 mL/minute. Near complete conversion of citric acid was achieved at an operating pressure of 6.9 MPa (1000 psig), a temperature of 250° and a contact time of about 30 minutes, based on an empty reactor volume. The product leaving the reactor was analysed by gas chromatography. The analysis showed 55.1% of 3 & 4-methylbutylrolactones and 8.5% of 3-methyltetrahydrofuran.

Claims: A process for preparing 3-substituted tetrahydrofuran, 3- and 4-substituted butyrolactones and mixtures thereof comprising contacting a hydrogenatable precursor selected from the group consisting of citric acid, substituted citric acid and mixtures thereof with a catalyst consisting essentially of palladium and rhenium and at least one support selected from the group consisting of titanium dioxide, zirconium oxide and carbon in the presence of an aqueous reaction medium and hyrogen. A process according to Claim 1, wherein the hydrogenatable precursor is citric acid. A process according to Claim 1, wherein the support is titanium oxide or carbon. A process according to Claim 3, wherein the catalyst comprises from about 1 to about 19 weight percent palladium and from about 4 to about 76 weight percent rhenium. A process according to Claim 4, wherein the process is conducted at a temperature of from about 100°C to about 300°C. A process according to Claim 5, wherein the process is conducted at a temperature of from about 150°C to about 250°C. A process according to Claim 6, wherein the process is conducted at a pressure of from about 3.5 MPa to about 27.6 MPa. A process according to Claim 7, wherein the process is conducted at a pressure of from about 3.5 MPa to about 17.3 MPa. A process according to Claim 8, wherein the aqueous reaction medium is water.

Inventors: RAO, VELLIYUR NOTT MALLIKARJUNA
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1988
Main classification: C07D307
IPC codes: C07D307, C07D315, C07B61, B01J23