Variance in the catalytic performance of nano-ZSM-5 zeolites during the reaction process of methanol to aromatics and its relation to the structural properties

Low selectivity to target products and poor catalytic stability remain two crucial issues for the conversion of methanol to aromatics (MTA) catalyzed by ZSM-5 zeolites. In this work, the variance in catalytic performance of ZSM-5 zeolites with the time on stream during a long term MTA test was monitored and related to the structural changes which were characterized by XRD, physisorption, NH₃-TPD, TEM, TG and ²⁷Al MAS NMR; the key structural factors affecting the catalytic performance and structure were then investigated. The results illustrate that within the early 19 h after starting the reaction, the amount of acid sites decreases significantly from 0.41 to 0.17 mmol/g due to the damage to the structure of aluminum species under high temperature hydrothermal conditions; the selectivity to light alkenes increases significantly, accompanying with a rapid increase of the liquid hydrocarbons yield from 14.7% to 19.3%. In the next stable reaction stage of 24 h, the rate of coke formation increases and the surface area decreases significantly from 340 to 275 m²/g, whilst the amount of acid sites decreases continuously to 0.10 mmol/g; the liquid hydrocarbon yield keeps above 19.5%, suggesting that a small amount of acid sites will be sufficient to stably bolster the MTA reaction. In contrast, in the next 31 h, although the rate of coke formation decreases obviously, the catalyst is deactivated gradually, dominantly by the coke deposition on the external surface; in this period, the surface area and the amount of acid sites decreases continuously and slowly and the liquid hydrocarbon yield drops to 17.3%, accompanying with a decrease in the selectivity to aromatics. At the end of the reaction of 7 h, the liquid hydrocarbon yield drops to 12.2%, due to the complete coverage of the acid sites and serious blockage of the pore by coke deposition; meanwhile, the selectivity to CH₄ increases significantly from 13.2% to 24.2%, whereas the fraction of p-xylene in xylenes increases from 24.4% to 33.1%, owing to the suppression of isomerization on the external surface acid sites which are covered by coke deposition. As the agglomerated particles are covered by coke in the deactivated catalyst, it is then proposed that a diminution of the contact between the external surfaces of particles could improve the accessibility of the reactant molecules to the active sites and enhance the coke capacity. The results may provide some relevant suggestions for the control of acidity and morphology in the preparation of MTA catalysts.