INFLUENCE OF COKE DEPOSITION ON SELECTIVITY IN ZEOLITE CATALYSIS

The selectivity of a complex reaction depends on a number of factors, such as the reaction mechanism, operating conditions, catalyst propert ies and catalyst deactivation. The present work discusses how the sele ctivity of a complex reaction depends on the formation of coke. For ze olite catalysts, changes in selectivity can be a result of intrinsic s electivity 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 deactivat ion of zeolites are discussed. Transition-state shape selective deacti vation is proposed as a-mechanism in addition to the deactivation mech anisms suggested by Guisnet and Magnoux (Guisnet, M.; Magnoux, P. Appl . Catal. 1989, 54, 1). By variation of the space velocity, the selecti vities 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 l ight olefins (MTO) over SAPO-34. The selectivity of ethene oligomeriza tion was affected only by the decrease in conversion due to coke forma tion; hence, this is an example of nonselective deactivation. Selectiv e deactivation was found for methanol conversion over SAPO-34.