Kinetics of chlorine deposition and removal over promoted silver catalysts during ethylene epoxidation

Abstract

High partial oxidation selectivity in ethylene oxidation over Ag catalysts is achieved in part via the presence of adsorbed chlorine promoters. Chlorine coverage during ethylene oxidation is maintained by co-feeding organic chlorides in ppm levels and alkane moderators in concentrations of ~0.5 mol%. The relative efficacy of organic chlorides in Cl deposition and of alkanes in Cl removal is typically estimated using empirical correlations. Apparent kinetics of Cl removal were measured by feeding ethyl chloride and propane during ethylene oxidation catalysis over a highly-promoted Ag/α-Al2O3 catalyst and quantifying the rate of chloropropane formation during steady state reaction. Propane oxychlorination forms 1- and 2-chloropropane in a ~1:2 ratio, as expected given the lower bond dissociation enthalpy of methylene than methyl CH bonds in propane, and at total rates on the order of 10-4 mol C3H7Cl (mol Agsurf)-1 s−1. Concurrent ethylene oxidation rates of order 10-1 mol (mol Agsurf)-1 s−1 provide evidence that Cl moderation occurs in a distinct catalytic cycle over promoted Ag catalysts. Cl removal has a supralinear dependence on propane pressure (PC3H8) and an inverse dependence on ethyl chloride pressure (PC2H5Cl) at high PC2H5Cl/PC3H8, while Cl removal is positive order in pressures of both C2H5Cl and C3H8 at low PC2H5Cl/PC3H8, as expected for surfaces that are increasingly covered in Cl as PC2H5Cl/PC3H8 increases. Chloropropane formation rates were positive order in dioxygen pressure for all PC2H5Cl/PC3H8 ratios. Taken together, the kinetics of Cl removal require both rate determining deposition of Cl from an alkyl chloride and kinetically relevant removal of Cl by an alkane. We develop a kinetic model that describes these trends and allows for quantification of rate and equilibrium parameters, and provide evidence for the elementary steps enumerated by measuring a kinetic isotope effect and assessing the fate of alkyl fragments from chloroethane and chloropropane in gas-phase batch reactions. Rate and equilibrium constants regressed from this kinetic model are used to develop an isotherm for Cl coverage as a function of the ratio of organic chloride promoter to alkane moderator pressures. This isotherm quantitatively predicts Cl coverages measured in situ after steady state reactions with feedstreams containing both ethyl chloride-ethane and ethyl chloride-propane mixtures. Measured reaction orders for chloropropane formation with respect to ethylene and carbon dioxide pressure reflect surface ethylene and carbon dioxide coverages that are lower for sites that form chloropropane than for sites that form ethylene oxide, suggesting that site ensembles for Cl deposition and removal vary from those involved in selective oxidation of ethylene.