Abstract
A method for enhancing para-xylene selectivity during toluene disproportionation over H-ZSM-5 zeolite is reported. The coke selectivation process was divided into five separate stages, each under either N2 or H2 carrier gases and at different reaction temperatures (480℃ or 540℃) for varied period of retaining times. The spent sample obtained during each stage was characterized by the combination of different techniques, namely thermogravimetric analysis, temperature programmed desorption of ammonia, xenon adsorption, X-ray photoelectron and 129Xe NMR spectroscopy. In the presence of N2 carrier gas, the carbonaceous deposits formed within the intracrystalline channels of the H-ZSM-5 catalyst are mostly light weighted, volatile soft coke which can be effectively removed by succeeding treatment by H2 gas. On the other hand, the bulkier, more condensed hard cokes are formed on the external surfaces of the zeolite crystallites and are more difficult to remove by simple hydrogen treatment. After the five-stage selective coking pretreatment, most of the cokes were found to deposit on the external surface of the catalyst. As a result, the isomerization of the primary products, para-xylene, was retarded. The para-selectivity was found to increase from ca. 24% up to 49% at a mild expanse of conversion. A detailed reaction mechanism liable for the para-selectivity enhancement during the five-stage coke selectivation process is proposed.