A physicochemical study of coke formation on [AI]-ZSM-5 and its gallium(III) and iron(III) isomorphs during methanol conversion

Abstract

The progress of coke formation on samples of the zeolite H-[AI]-ZSM-5, Si/Al ratios of 27 and 39, during their use in the conversion of methanol to hydrocarbons has been monitored by measurement of (a) the coke content of the zeolite as a function of the quantity of methanol feed, (b) zeolitic ion-exchange capacities at various coke loadings, and (c) the adsorption of methylene blue on the surface of the zeolite containing various amounts of coke. Similar measurements have been made on the coking of the gallium(III) and iron(III) isomorphs of ZSM-5 which occurs during methanol conversion and on the coking of H-[Al]-ZSM-5 during its use in mesitylene cracking. The results of such measurements show that the acid site coverage per unit quantity of coke in the early stages of coking follows the order of increasing Brønsted acidity, namely Fe(III) < Ga(III) < Al(III); coke formation involves some aggregation of coke material on the surface of the zeolite; the coking of H[Al]-ZSM-5 is greatly influenced by its Al(III) content; and coking occurs only to a minor extent as a result of mesitylene cracking on H-[Al]-ZSM-5.