Adsorption, acid and catalytic changes induced in ZSM-5 by coking with different hydrocarbons

Abstract

Abstract Controlled coking of ZSM-5 has been carried out using mesitylene, toluene and isobutene as coke precursors. The coked samples have been characterized by thermogravimetry in air. X-ray diffraction, IR, adsorption-diffusion experiments, ammonia temperature-programmed desorption and catalytic measurements. It has been found that the amount, nature and location of the carbonaceous residues strongly depend on the hydrocarbon used as coke precursor. Coke formed from mesitylene is deposited on the external zeolite surface whereas coking with isobutene leads to deposits located within the internal pore structure and mainly of paraffinic nature. Coke from toluene is polyaromatic, being deposited both on the external and the internal zeolite surface. The catalytic properties of the coked ZSM-5 samples have been tested in toluene disproportionation. Coke formed from isobutene causes a decrease of toluene conversion almost without changing p -selectivity which has been assigned to the relative enhancement of the role of the external zeolite surface as the internal volume is filled by coke. On the other hand, p -selectivity is increased by coking with mesitylene because of the shape-selectivity enhancement produced by the deposition of the coke species on the external surface of the ZSM-5 zeolite. Coke from toluene shows an intermediate effect on the catalytic properties leading to a p -selectivity/toluene conversion relationship similar to that of the uncoked ZSM-5.