Synthesis and characterization of novel silver based catalysts for selective oxidation of olefins.

摘要

A new method was developed to produce a silica-supported, silver catalyst using nanometer-size, silver clusters synthesized by gas aggregation techniques. The steps in this method are: (1) production of nanometer diameter silver clusters using a multiple expansion cluster source (MECS), (2) capture and stabilization of the clusters as a colloidal suspension in dodecane using a metal cluster colloid reactor, (3) adsorption of the colloidal silver onto fumed silica, (4) removal of the support particles from the solvent, and (5) oxidation/reduction treatment to remove residual solvent and surfactant. The size distribution of silver clusters as characterized by transmission electron microscopy (TEM) is essentially constant from step 1 to step 5. Silica-supported, silver catalysts with mean cluster diameters of 2.4, 4.4, 7.8, 8, and 11 nm were produced by controlling the size of the silver clusters synthesized in the MECS.; Nanometer-size, silver clusters coated with a silicon layer, having a thickness of a few Angstroms, were also synthesized by gas aggregation techniques. A silica-supported, catalyst was produced by adsorption of nanometer-size, silicon-coated, silver clusters onto fumed silica using the method described above and then oxidation of the silicon coating to silica.; The two nanometer-size, silica-supported, catalysts were tested for ethylene epoxidation and propylene epoxidation reactions and compared to a silver powder catalyst with micron diameter grains and a silica-coated, silver powder formed by impregnation of the silver powder with methyl tri-methoxy silane, hydrolysis of the silane, and oxidation of the cross-linked silane to SiO{dollar}sb2.{dollar}; The activities of the nanometer-size, silica-supported, catalysts are lower than those of the micron-size, powders, and the activities of the silica-coated, catalysts are lower than those without the silica coating. The most interesting result is that, while the selectivity of each of the four catalysts for ethylene epoxidation is essentially the same (30-40%), the selectivity of the two nanometer-size, silica-supported, catalysts for propylene epoxidation increase with a decrease in reaction temperature and have values at 483 K of about 40%. The selectivity of the micron-size, powder for this reaction is less than 5% and for the silica-coated, micron-size powder is less than 15%.