Zeolites are microporous crystalline material with well-defined channels or cavities, and it has been widely utilized in petrochemical industry during the past decades. The uniform pore structure endows its unique shape-selective catalytic performance. How to realize the Al siting on specific T sites in zeolite channels is a hot topic in zeolite field. Specially, FER- and MOR-type zeolites have proved to be efficient catalysts in the DME carbonylation reaction. The MOR-type zeolite displays a high activity but a short lifetime. In contrast, the FER-type zeolite shows good catalytic stability but relatively low activity. How to improve the carbonylation activity for FER-type zeolite though controlling the Al siting on specific T site has attracted more and more attention. Although much work has been done for FER zeolite in this reaction, the improvements to the catalytic performance are still limited. Moreover, precisely identifying and controlling more Al atoms on different T sites of FER-type zeolite are also of great challenge.
Recently, Prof. Xiangxue Zhu, Peng Guo and Xiujie Li from Dalian Institute of Chemical Physics, Chinese Academy of Sciences unraveled the constrained Al crystallographic sites in FER-type zeolites by Rietveld refinement against the powder X-ray diffraction (PXRD) data. In this work, they deliberately synthesized a series of FER-type zeolites using cyclic (cyclohexylamine, piperidine, pyrrolidine, and pyridine) and linear (ethylenediamine) organic structure-directing agents in the hydroxide and fluoride medium, respectively. Rietveld refinement combined with simulated annealing algorithms was applied to investigate Al sites in the as-made and pyridine adsorbed FER-type zeolite samples at the atomic level. Interestingly, Al atoms of FER-type zeolites were constrained at T1 and/or T3 sites regardless of the types of structure-directing agents and synthetic medium utilized. Unfortunately, Brønsted acid sites associated with T1/T3 sites were inactive centers in the DME carbonylation reaction. Therefore, FER-type zeolites showed a lower carbonylation activity compared with MOR-type zeolite. And this well explained why the improvement of catalytic performance over FER-type zeolites was limited through different synthetic methodologies. Their unprecedented findings and characterization methodologies may shed lights on the investigations of structure-activity relationships in the zeolite-related research fields. The detailed results were published in Chinese Journal of Catalysis (https://doi.org/10.1016/S1872-2067(21)63884-6).
