Influence of Coke Deposition on Selectivity in Zeolite Catalysis

Abstract

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst properties and catalyst deactivation. The present work discusses how the selectivity of a complex reaction depends on the formation of coke. For zeolite catalysts, changes in selectivity can be a result of intrinsic selectivity effects or shape selectivity effects. A method is suggested to analyze a complex reaction system with deactivation caused by coke formation, and different cases of selectivity change during deactivation of zeolites are discussed. Transition-state shape selective deactivation is proposed as a mechanism in addition to the deactivation mechanisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1). By variation of the space velocity, the selectivities to the main products are measured as a function of conversion (optimum performance envelopes). Selectivities at given conversions can then be compared for results obtained with different contact time and with varying degree of catalyst deactivation (space velocity loops). Selective or nonselective deactivation is thereby distinguished. This type of selectivity plot is applied to two different types of reaction, i.e. ethene oligomerization over HZSM-5 and methanol conversion to light olefins (MTO) over SAPO-34. The selectivity of ethene oligomerization was affected only by the decrease in conversion due to coke formation; hence, this is an example of nonselective deactivation. Selective deactivation was found for methanol conversion over SAPO-34.