Roles of acidity and pore structure in the deactivation of zeolites by carbonaceous deposits

Abstract

Deactivation of zeolite acid catalysts during hydrocarbon transformations is mainly due to the formation and the retention inside the pores of heavy secondary products. The effects of pore structure of zeolites and of their acidity on the rate of formation of these carbonaceous compounds, on their composition and on their deactivating effect are examined. The roles of strength and density of active acid sites (often protonic sites) are generally limited in comparison to the roles played by size and shape of cavities (or channel intersections) and by the size of their apertures. The formation of carbonaceous compounds (coke) and the deactivation that they cause are clearly shape selective processes. Their formation occurs through a nucleation-growth pathway. In the range of temperatures of refining and petrochemical processes nucleation is due to trapping in the cavities of coke precursors. Condensation and hydrogen transfer reactions are involved both in the formation of coke precursors and in their growth. Four modes of deactivation can be distinguished: (1) limitation or (2) blockage of the access of the reactant to the active sites of a cavity in which one coke molecule is located, due to steric reasons or to a competition for adsorption between reactant or coke molecules and (3) steric limitation or (4) blockage of the access of the reactant to the active sites of cavities or of channels in which no coke molecule is located. The deactivation of monodimensional zeolites and of zeolites with trap cavities (large cavities with small apertures) only occurs through Modes 3 and 4, hence is very rapid. With the other types of zeolites, deactivation passes successively from Mode 1 to Mode 4 as the coke content increases; Modes 3 and 4 are due to the coke molecules which overflow onto the outer surface of zeolite crystallites.