Abstract:
The invention is a liquid-phase process for epoxidizing an olefin with hydrogen and oxygen in the presence of a supported catalyst comprising palladium on niobium-containing support. The process exhibits good productivity and selectivity for olefin epoxidation with hydrogen, and oxygen. This is particularly surprising because typical palladium-containing epoxidation catalysts require the presence of a titanium zeolite.

Description:
FIELD OF THE INVENTION 
     This invention relates to a liquid-phase epoxidation process using a supported catalyst to produce epoxides from hydrogen, oxygen, and olefins. The supported catalyst contains palladium on a niobium containing support. Surprisingly, this supported catalyst is active in liquid phase direct epoxidation. 
     BACKGROUND OF THE INVENTION 
     Many different methods for the preparation of epoxides have been developed. Generally, epoxides are formed by the reaction of an olefin with an oxidizing agent in the presence of a catalyst. The production of propylene oxide from propylene and an organic hydroperoxide oxidizing agent, such as ethyl benzene hydroperoxide or tert-butyl hydroperoxide, is commercially practiced technology. This process is performed in the presence of a solubilized molybdenum catalyst, see U.S. Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, see U.S. Pat. No. 4,367,342. Hydrogen peroxide is another oxidizing agent useful for the preparation of epoxides. Olefin epoxidation using hydrogen peroxide and a titanium silicate zeolite is demonstrated in U.S. Pat. No. 4,833,260. One disadvantage of both of these processes is the need to pre-form the oxidizing agent prior to reaction with olefin. 
     Another commercially practiced technology is the direct epoxidation of ethylene to ethylene oxide by reaction with oxygen over a silver catalyst. Unfortunately, the silver catalyst has not proved very useful in epoxidation of higher olefins. Therefore, much current research has focused on the direct epoxidation of higher olefins with oxygen and hydrogen in the presence of a catalyst. In this process, it is believed that oxygen and hydrogen react in situ to form an oxidizing agent. Thus, development of an efficient process (and catalyst) promises less expensive technology compared to the commercial technologies that employ pre-formed oxidizing agents. 
     Many different catalysts have been proposed for use in the direct epoxidation of higher olefins. For liquid-phase reactions, the catalysts typically contain palladium on a titanium zeolite support. For example, JP 4-352771 discloses the epoxidation of propylene oxide from the reaction of propylene, oxygen, and hydrogen using a catalyst containing a Group VIII metal such as palladium on a crystalline titanosilicate. For vapor-phase epoxidation of olefins, gold supported on titanium oxide (Au/TiO 2  or Au/TiO 2 —Si 0   2 ), see for example U.S. Pat. No. 5,623,090, and gold supported on titanosilicates, see for example PCT Intl. Appl. WO 98/00413, have been disclosed. 
     One disadvantage of the described direct epoxidation catalysts is that they all show either less than optimal selectivity or productivity. As with any chemical process, it is desirable to develop new direct epoxidation methods and catalysts. 
     In sum, new processes and catalysts for the direct epoxidation of olefins are needed. I have discovered an effective, convenient epoxidation process that gives good productivity and selectivity to epoxide. 
     SUMMARY OF THE INVENTION 
     The invention is an olefin epoxidation process that comprises reacting an olefin, oxygen, and hydrogen in a solvent in the presence of a catalyst comprising palladium on a niobium-containing support. The new catalyst is surprisingly useful in the epoxidation of olefins with hydrogen and oxygen. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The process of the invention employs a catalyst comprising palladium and a niobium-containing inorganic oxide support. Suitable niobium-containing inorganic oxide supports include niobium oxides and niobium mixed oxides. Niobium oxides include oxides of niobium wherein the valency of niobium is 2 to 5. Suitable niobium oxides include such oxides as NbO, Nb 2 O 3 , NbO 2 , and Nb 2 O 5 . Niobium mixed oxides such as niobium oxide-silica, niobium oxide-alumina, and niobium oxide-titania may also be used. The amount of niobium present in the support is preferably in the range of from about 0.1 to about 86 weight percent. Preferred niobium-containing inorganic oxide supports include Nb 2 O 5  and niobium oxide-silica. 
     The catalyst employed in the process of the invention also contains palladium. The typical amount of palladium present in the catalyst will be in the range of from about 0.01 to 20 weight percent, preferably 0.01 to 10 weight percent. The manner in which the palladium is incorporated into the catalyst is not considered to be particularly critical. For example, the palladium (for example, Pd tetraamine bromide) may be supported on the niobium-containing inorganic oxide support by impregnation, adsorption, ion-exchange, precipitation, or the like. 
     There are no particular restrictions regarding the choice of palladium compound used as the source of palladium. For example, suitable compounds include the nitrates, sulfates, halides (e.g., chlorides, bromides), carboxylates (e.g. acetate), and amine complexes of palladium. 
     Similarly, the oxidation state of the palladium is not considered critical. The palladium may be in an oxidation state anywhere from 0 to +4 or any combination of such oxidation states. To achieve the desired oxidation state or combination of oxidation states, the palladium compound may be fully or partially pre-reduced after addition to the catalyst. Satisfactory catalytic performance can, however, be attained without any pre-reduction. 
     After catalyst formation, the catalyst may be optionally thermally treated in a gas such as nitrogen, helium, vacuum, hydrogen, oxygen, air, or the like. The thermal treatment temperature is typically from about 50 to about 550° C. 
     The catalyst may be used in the epoxidation process as a powder or as a pellet. If pelletized or extruded, the catalyst may additionally comprise a binder or the like and may be molded, spray dried, shaped or extruded into any desired form prior to use in epoxidation. 
     Examples of catalysts comprising palladium and a niobium-containing inorganic oxide support are known. For instance, palladium on niobia catalysts have been disclosed for production of hydrogen peroxide (see, for example, U.S. Pat. No. 5,496,532). 
     The process of the invention comprises contacting an olefin, oxygen, and hydrogen in an oxygenated solvent in the presence of the catalyst. Suitable olefins include any olefin having at least one carbon-carbon double bond, and generally from 2 to 60 carbon atoms. Preferably the olefin is an acyclic alkene of from 2 to 30 carbon atoms; the process of the invention is particularly suitable for epoxidizing C 2 -C 6  olefins. More than one double bond may be present, as in a diene or triene for example. The olefin may be a hydrocarbon (i.e., contain only carbon and hydrogen atoms) or may contain functional groups such as halide, carboxyl, hydroxyl, ether, carbonyl, cyano, or nitro groups, or the like. The process of the invention is especially useful for converting propylene to propylene oxide. 
     The process of the invention also requires the use of a solvent. Suitable solvents include any chemical that is a liquid under reaction conditions, including, but not limited to, oxygen-containing hydrocarbons such as alcohols, aromatic and aliphatic solvents such as toluene and hexane, chlorinated aromatic and aliphatic solvents such as methylene chloride and chlorobenzene, and water. Preferred solvents are oxygenated solvents that contain at least one oxygen atom in its chemical structure. Suitable oxygenated solvents include water and oxygen-containing hydrocarbons such as alcohols, ethers, esters, ketones, and the like. Preferred oxygenated solvents include lower aliphatic C 1 -C 4  alcohols such as methanol, ethanol, isopropanol, and tert-butanol, or mixtures thereof, and water. Fluorinated alcohols can be used. A particularly preferred solvent is water. It is also possible to use mixtures of the cited alcohols with water. 
     Preferably, the process of the invention will also use buffers. If used, the buffer will typically be added to the solvent to form a buffer solution. The buffer solution is employed in the reaction to inhibit the formation of glycols during epoxidation. Buffers are well known in the art. 
     Suitable buffers include any suitable salts of oxyacids, the nature and proportions of which in the mixture, are such that the pH of their solutions may range from 3 to 10, preferably from 4 to 9 and more preferably from 5 to 8. Suitable salts of oxyacids contain an anion and cation. The anion portion of the salt may include anions such as phosphate, carbonate, acetate, citrate, borate, phthalate, silicate, aluminosilicate, or the like. The cation portion of the salt may include cations such as ammonium, alkylammoniums (e.g., tetraalkylammoniums), alkali metals, alkaline earth metals, or the like. Cation examples include NH 4 , NBu 4 , Li, Na, K, Cs, Mg, and Ca cations. More preferred buffers include alkali metal phosphate buffers. Buffers may preferably contain a combination of more than one suitable salt. Typically, the concentration of buffer is from about 0.0001 M to about 1 M, preferably from about 0.001 M to about 0.1 M, and most preferably from about 0.005 M to about 0.05 M. 
     Oxygen and hydrogen are also required for the process of the invention. Although any sources of oxygen and hydrogen are suitable, molecular oxygen and molecular hydrogen are preferred. The molar ratio of hydrogen to oxygen can usually be varied in the range of H 2 :O 2 =1:100 to 5:1 and is especially favorable at 1:5 to 2:1. The molar ratio of oxygen to olefin is usually 1:1 to 1:20, and preferably 1:1.5 to 1:10. Relatively high oxygen to olefin molar ratios (e.g., 1:1 to 1:3) may be advantageous for certain olefins. 
     In addition to olefin, oxygen and hydrogen, an inert gas carrier may be preferably used in the process. As the carrier gas, any desired inert gas can be used. Suitable inert gas carriers include noble gases such as helium, neon, and argon in addition to nitrogen and carbon dioxide. Saturated hydrocarbons with 1-8, especially 1-6, and preferably with 1-4 carbon atoms, e.g., methane, ethane, propane, and n-butane, are also suitable. Nitrogen and saturated C 1 -C 4  hydrocarbons are the preferred inert carrier gases. Mixtures of the listed inert carrier gases can also be used. The molar ratio of olefin to carrier gas is usually in the range of 100:1 to 1:10 and especially 20:1 to 1:10. 
     Specifically in the epoxidation of propylene according to the invention, propane can be supplied in such a way that, in the presence of an appropriate excess of carrier gas, the explosive limits of mixtures of propylene, propane, hydrogen, and oxygen are safely avoided and thus no explosive mixture can form in the reactor or in the feed and discharge lines. 
     The amount of catalyst used may be determined on the basis of the molar ratio of the palladium contained in the catalyst to the olefin that is supplied per unit time. Typically, sufficient catalyst is present to provide a palladium/olefin per hour molar feed ratio of from 0.0001 to 0.1. 
     For the liquid-phase process of the invention, the catalyst is preferably in the form of a suspension or fixed-bed. The process may be performed using a continuous flow, semi-batch or batch mode of operation. It is advantageous to work at a pressure of 1-100 bars. Epoxidation according to the invention is carried out at a temperature effective to achieve the desired olefin epoxidation, preferably at temperatures in the range of 0-250° C., more preferably, 20-200° C. 
     The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims. 
    
    
     EXAMPLE 1 
     Preparation of Pd/Nb 2  O 5  Catalyst 
     Catalyst 1A: 1 wt. % Palladium on Niobium Oxide 
     In a glass beaker, Pd(NH 3 ) 4 Br 2  (0.64 g) is dissolved in 40 grams of deionized water. In a separate beaker, niobium oxide powder (20 g, from Reference Metals) is slurried in 90 grams of deionized water. The palladium salt solution is added to the niobium oxide slurry with stirring over a 10-minute period. The resulting slurry is stirred at 23° C. for two hours, then the solids are separated by centrifuge. The solids are washed four times by slurrying in 80 grams of water and centrifuging. The solids are then dried in a vacuum oven (1 torr) at 50° C. for 4 hours to give 14.6 grams of Catalyst 1A. Elemental analysis showed palladium=1.01 wt. %, bromide=1.6 wt. %, nitrogen 0.22 wt. % and niobium=68 wt. %. 
     Catalyst 1B: 0.5 wt. % Palladium on Niobium Oxide The procedure of Catalyst 1A is repeated except that 0.24 grams of Pd(NH 3 ) 4 Br 2  (in 30 grams of deionized water) and 15 grams of niobium oxide powder are used. The preparation resulted in the isolation of 9.6 grams of Catalyst 1B. Elemental analysis showed palladium=0.51 wt. %, bromide=0.72 wt. %, nitrogen&lt;0.1 wt. % and niobium=68 wt. %. 
     EXAMPLE 2 
     Epoxidation of Propylene Using Pd/Nb 2 O 5  Catalyst 
     Run 2A: Catalyst 1A with a Cesium Phosphate Buffer Cesium phosphate buffer is first produced according to the following procedure. Cesium hydroxide (22.12 g) is dissolved in deionized water (17.25 g) in a plastic beaker. In a separate container, 85% phosphoric acid (5.85 g) is added with cooling to 400 grams of deionized water. Twenty-five grams of the cesium hydroxide solution is carefully added to the phosphoric acid solution. After the addition, enough deionized water is added to the cesium phosphate buffer to give a volume of 500 mL. The pH of the solution is measured to be 6.9. Two hundred and twenty grams of the above solution (pH=6.9) is then treated with 85% phosphoric acid (1.01 g) to give a cesium phosphate buffer solution with a pH=6.02. 
     A 300 cc stainless steel reactor is charged with Catalyst 1A (0.6 g), deionized water (117 g), and 13 grams of a buffer (0.1 molar cesium phosphate, pH=6 as prepared above). The reactor is then charged to 200 psig of a feed consisting of 4% hydrogen, 4% oxygen, 5% propylene, 0.5% methane and the balance nitrogen. The pressure in the reactor is maintained at 200 psig via a backpressure regulator with the feed gases passed continuously through the reactor at 1480 cc/min (measured at 23° C. and one atmosphere pressure). In order to maintain a constant solvent level in the reactor during the run, the oxygen, nitrogen and propylene feeds are passed through a two-liter stainless steel vessel, preceding the reactor, containing 1.5 liters of water. The reactor is stirred at 1600 rpm. The reaction mixture is heated to 60° C. and the gaseous effluent is analyzed by an online GC every hour and the liquid analyzed by offline GC at the end of the 18 hr run. The GC analyses showed a total of 6 millimoles of propylene oxide in the gas phase and 2 millimoles of PO in the form of propylene glycol is formed in the liquid phase. 
     Run 2B: Catalyst 1B with a Potassium Phosphate Buffer 
     Potassium phosphate buffer solution is prepared by the following procedure. Potassium dihydrogen phosphate (6.8 g) is dissolved into 500 grams of deionized water. Potassium hydroxide (1.68 g) is dissolved in 300 mL of deionized water in a plastic beaker. A pH=7 buffer is obtained by adding 232 grams of the potassium hydroxide solution to 400 grams of the potassium dihydrogen phosphate solution. The pH of the mixed solution is 6.97. A pH=6 buffer is obtained by adding 11.2 grams of the potassium hydroxide solution to 100 grams of the potassium dihydrogen phosphate solution. The pH of the mixed solution is 6.03. 
     A 300 cc stainless steel reactor is charged with 0.6 grams of Catalyst 1B, 117 grams of deionized water and 13 grams of a buffer (0.1 molar potassium phosphate, pH=6). The epoxidation is conducted as in Run 2A. The GC analyses showed a total of 2.5 millimoles of PO in the gas phase and 0.92 millimoles of PO in the form of PG in the liquid phase. 
     EXAMPLE 3 
     Epoxidation of Propylene Using Pd/Nb 2  O 5  Catalyst Without Buffer 
     A 300-cc stainless steel reactor is charged with 0.6 grams of Catalyst 1A and 130 grams of deionized water. Epoxidation is run according to the same procedure as Example 2, except that the water does not contain a buffer. The GC analyses showed a total of 0.77 millimoles of PO and 0.16 millimoles of acetone in the gas phase and 2.7 millimoles of PO in the form of PG in the liquid phase. 
     The epoxidation results show that the use of a Pd/Nb 2 O 5  catalyst surprisingly leads to the production of propylene oxide (PO) and PO equivalents in the form of propylene glycol (PG) in the epoxidation of propylene with H 2  and O 2 . The use of a buffered solution improves the selectivity to propylene oxide, with less unwanted glycol formation.