Experimental Evidence for the Relationship between Al Site Distribution and Catalytic Performance in Methanol-to-Olefins Reaction over ZSM-5 Zeolite

The designed synthesis of zeolite catalysts, especially in the control of the framework structure and heteroatom distribution, offers advantages to their catalytic performance. In the methanol-to-olefins reaction, the shape of intermediate species defined by the reaction space surrounding an acid site impacts the reaction mechanism and, in turn, the product selectivity. However, because of the difficulty in observing the active intermediate species, presenting a clear relationship between the catalytic site (i.e., Al atom) distribution and the catalytic property is challenging. Herein, we experimentally demonstrate a structure-catalytic performance relationship between the Al distribution in the ZSM-5 zeolite and the behavior of the methanol-to-olefins reaction, particularly in regard to propylene formation, by isotopic labeling experiments. Two ZSM-5 zeolite samples, which were synthesized with different organic structure-directing agents, were shown to possess different Al distributions based on constraint index measurements. Although their catalytic properties in the methanol-to-olefins reaction in terms of conversion and selectivity were almost identical, the transient responses of the insertion of 13C into propylene fragments showed contrasting results. The results successfully revealed via an experimental approach the relationship between the zeolite design and the catalytic performance.