Deactivation by coking of zeolite catalysts. Prevention of deactivation. Optimal conditions for regeneration

The deactivation of acid zeolite catalysts used in hydrocarbon transformations is mainly due to the deposit inside the pores of heavy secondary products generally known as coke. It is shown how the rate of coking and the deactivating effect of the coke molecules are affected by the pore structure and the acidity of the zeolites as well as by the operating conditions. Directives for minimizing the deactivation by coking are proposed: (1) choice of tridimensional zeolites without trap cavities (large cavities with small apertures); (2) adjustment of the density and strength of the acid sites to the lowest values necessary for the selective formation of the desired products; and (3) choice of operating conditions in order to avoid the formation of coke-maker molecules (alkenes, polyaromatics). The regeneration of zeolites is generally carried out through coke combustion under air or oxygen flow. The detrimental effect that water, produced by coke oxidation, has on the zeolite activity can be limited by using a two-stage generation process, the hydrogen atoms of the coke molecules being oxidized at the low temperature of the first stage.