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The Effect of Crystallization Conditions on the Properties Of ZAM-5 Zeolite and its Performance in MTO Reaction

The demand for light olefins such as ethylene and propylene continues to grow and the analysis of the market shows that this trend will continue in the future. The synthesis of olefins from methanol (MTO reaction) is one of the industrially important processes over zeolite catalysts (1). Chabazite (CHA) type material SAPO-34 has shown excellent performance in light olefin synthesis and reached industrial application. However, this catalyst gets quickly deactivated due to the coke deposition (2). On the other hand, the catalyst life of zeolite ZSM-5, which is also used in the same reaction, is much longer. The main drawback in ZSM-5 application for MTO reaction is lower olefin yield. Hence, the design of ZSM-5 catalyst with improved stability and selectivity towards olefins would overcome the main drawbacks in the use of ZSM-5 in the MTO process. In this work the effect of crystallization temperature on the properties of nanosized ZSM-5 crystals and their activity in MTO reaction is studied. Nanosized ZSM-5 samples were synthesized at 170 °C, 150 °C, 120 °C and 100 °C. The series of nanosized samples were compared with an industrial ZSM-5 of similar Si/Al ratio. A set of complementary characterization techniques was employed (XRD, chemical analysis, SEM, N2 adsorption, NMR and in situ IR) in order to compare the properties of zeolite catalysts. According to the obtained results, changing of the synthesis temperature gives rise to notable differences in zeolite physicochemical properties particularly in terms of their relative crystallinity, external surface area and acidity. Namely, the nanosized ZSM-5 samples have significantly lower number of acid sites than the reference industrial material as the number of acid sites is a function of synthesis temperature. Decrease of synthesis temperature results in ZSM-5 of lower crystallinity, lower strength of acid sites and larger external surface area. The set of catalytic data revealed that the changes in the physicochemical properties of zeolite crystals modify substantially the catalytic performance. The selectivity to light olefins and the ratio of propylene to ethylene increases with the decrease of crystallization temperature. The propylene-to-ethylene ratio above 6 with the highest selectivity to propylene of 53 % is obtained over a ZSM-5 zeolite catalyst prepared at 100 °C. Moreover, this sample exhibits three times longer catalyst lifetime in MTO than the studied industrial sample. The observed performance is attributed to lower strength of acid sites and smaller size of the crystals. Therefore, the modulation in the crystallization temperature can be used for the preparation of efficient ZSM-5 catalyst for propylene synthesis from methanol.

zeolite, ZSM-5, MTO, catalysis

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