Direct epoxidation catalyst and process

#The invention relates to a catalyst comprising a transition metal zeolite, a noble metal, and a thiol. The catalyst is used to produce an epoxide by reacting an olefin, hydrogen, and oxygen.

#Direct epoxidation of higher olefins (containing three or more carbons) such as propylene with oxygen and hydrogen has been the focus of recent efforts. For example, the reaction may be performed in the presence of a catalyst comprising gold and a titanium-containing carrier (see, e.g., U.S. Pat. Nos. 5,623,090, 6,362,349, and 6,646,142), or a catalyst containing palladium and a titanium zeolite (see, e.g., JP 4-352771; U.S. Pat. Nos. 6,008,388, 6,498,259). Unfortunately, these catalyst systems also produce alkanes from the corresponding olefins. U.S. Pat. No. 6,005,123 discloses that a sulfur compound may be used to inhibit propane formation in the epoxidation of propylene, but it also inhibits the epoxides formation. There is a continued need to find new catalysts that inhibit hydrogenation of olefin without decreasing its epoxidation activity.

#Thiol-functionalized mesoporous silicas are reported to selectively adsorb Pt2+ and Pd2+ (see Ind. Eng. Chem. Res. 43 (2004) 1478). On the other hand, the hydrogenation activity of Pt or Pd is strongly inhibited by the thiol functionality (see J. Chem. Eng. of Japan 35(3) (2002) 255).

#In one aspect, the invention is a catalyst comprising a transition metal zeolite, a noble metal, and a thiol. Zeolites are porous crystalline solids with well-defined structures. Generally they contain one or more of Si, Ge, Al, B, P, or the like, in addition to oxygen. Many zeolites occur naturally as minerals and are extensively mined in many parts of the world. Others are synthetic and are made commercially for specific uses. Zeolites have the ability to act as catalysts for chemical reactions which take place mostly within their internal cavities. Transition metal zeolites are zeolites comprising transition metals in framework. A transition metal is a Group 3-12 element. The first row of transition metals are from Sc to Zn. Preferred transition metals are Ti, V, Mn, Fe, Co, Cr, Zr, Nb, Mo, and W. More preferred are Ti, V, Mo, and W. Most preferred is Ti.

#Preferred titanium zeolites are titanium silicates (titanosilicates). Preferably, they contain no element other than titanium, silicon, and oxygen in is the lattice framework (see R. Szostak, “Non-aluminosilicate Molecular Sieves,” in Molecular Sieves: Principles of Synthesis and Identification (1989), Van Nostrand Reinhold, pp. 205-82). Small amounts of impurities, e.g., boron, iron, aluminum, phosphorous, copper, and the like, and mixtures thereof, may be present in the lattice. The amount of impurities is preferably less than 0.5 wt. %, more preferably less than 0.1 wt. %. Preferred titanium silicates will generally have a composition corresponding to the following empirical formula: xTiO2.(1−x)SiO2, where x is between 0.0001 and 0.5000. More preferably, the value of x is from 0.01 to 0.125. The molar ratio of Si to Ti in the lattice framework of the zeolite is advantageously from 9.5:1 to 99:1, most preferably from 9.5:1 to 60:1. Particularly preferred titanium zeolites are titanium silicalites (see Catal. Rev. Sci. Eng., 39(3) (1997) 209). Examples of these include TS-1 (titanium silicalite-1, a titanium silicalite having an MFI topology analogous to that of the ZSM-5 aluminosilicate), TS-2 (having an MEL topology analogous to that of the ZSM-11 aluminosilicate), and TS-3 (as described in Belgian Pat. No. 1,001,038). Titanium zeolites having framework structures isomorphous to zeolite beta, mordenite, ZSM-12, MCM-22, MCM-41, and MCM-48 are also suitable for use. Examples of MCM-22, MCM-41, and MCM-48 zeolites are described in U.S. Pat. Nos. 4,954,325, 6,077,498, and 6,114,551; Maschmeyer, T., et al, Nature 378(9) (1995) 159; Tanev, P. T., et al., Nature 368 (1994) 321; Corma, A., J. Chem. Soc. Chem. Commun. (1998) 579; Wei D., et al., Catal. Today 51 (1999) 501). The most preferred is TS-1.

#Transition metal zeolites may be formed into particles. Spray drying is a preferred forming technique. Spray drying is a suspended particle processing system that utilizes liquid atomization to create droplets that are dried to individual particles while moving in a gaseous medium (see K. Maters, Spray Drying In Practice, SprayDryConsultant International ApS (2002) pp. 1-15). Spray drying is known in forming zeolites, including titanium zeolites (see, e.g., U.S. Pat. Nos. 4,954,653, 4,701,428, 5,500,199, 6,524,984, and 6,106,803).