Enhanced para-selectivity by selective coking during toluene disproportionation over H-ZSM-5 zeolite

A coke selectivation process, i.e., modification of selectivity, was divided into five separate stages. Each stage was under either N-2 or H-2 as the carrier gas and at different reaction temperatures for various time-on-stream. The spent sample obtained during each stage was characterized by a combination of different techniques, namely thermogravimetric analysis, temperature programmed desorption of ammonia, xenon adsorption, X-ray photoelectron, and Xe-129 NMR spectroscopy. In the presence of N-2 carrier gas, the carbonaceous deposits formed within the intracrystalline channels were mostly light volatile soft coke, which was effectively removed by a succeeding treatment with H-2 gas. On the other hand, bulkier, more condensed hard coke was formed on the external surface of the zeolite crystallites and is more difficult to be removed by simple hydrogen treatment. After the five-stage selectivation process, most of the coke is found to be on the external surface of the catalyst. As a result, the isomerization of the primary product, para-xylene, was retarded. The para-selectivity was found to increase from ca. 24 to 49% at a slight expense of conversion. A detailed reaction mechanism for the para-selectivity enhancement during the five-stage coke selectivation process is proposed. (C) 1999 Academic Press.