Recently, Zhongmin Liu's team in the Department of Low Carbon Catalysis and Engineering (DNL12) has made new progress in selectively modulating the distribution of acidic sites in molecular sieves, using filamentous zeolite molecular sieve as the research target. The modified catalysts exhibited excellent performance in the carbonylation of dimethyl ether.

Molecular sieves play an indispensable role in various petrochemical processes and chemical synthesis due to their tunable active sites and shape-selective catalytic ability. The excellent catalytic performance of molecular sieves is usually attributed to their acidic site positioning and distribution, with the acidic sites at different positions exhibiting different domain-limiting effects on the reactants. For example, in the carbonylation of dimethyl ether to methyl acetate catalyzed by filamentous zeolite, the favorable active center of the reaction is located in the 8-membered ring pore channel; while the acidic sites in the larger 12-membered ring pore channel catalyze the occurrence of carbon accumulation side reactions, resulting in catalyst deactivation. Therefore, in order to improve the catalytic performance of filamentous zeolite in the carbonylation reaction of dimethyl ether, the acidic sites in the 12-membered ring pore channel need to be selectively removed.

In this work, the team proposed a method to selectively passivate the acidic sites in the 12-membered ring pore channel of filamentous zeolite by trimethylchlorosilane based on the previous study of filamentous zeolite molecular sieve and dimethyl ether carbonylation reaction (Angew. Chem. Int. Ed., 2022). The team used in situ diffuse reflectance infrared spectroscopy and online color and mass spectrometry techniques, combined with solid-state NMR, to observe the changes of multiple hydroxyl species in the molecular sieve during silylation, as well as the bonding and evolution of trimethylchlorosilane molecules on the molecular sieve backbone, thus establishing the reaction history between trimethylchlorosilane and the acidic sites of the molecular sieve during silylation. In addition, the team also combined solid-state NMR and IR spectroscopy techniques to quantitatively analyze the changes of acidic sites in different pore channels of molecular sieves after silylation modification. The experimental results showed that the trimethylchlorosilane treatment could selectively passivate the acidic sites in the 12-membered ring pore channels of the filamentous zeolite molecular sieve, and enrich about 80% of the activity located in the 8-membered ring pore channels. The modified mercerized zeolite molecular sieve showed excellent reaction performance in the carbonylation of dimethyl ether by this method, and the removal of the unfavorable acidic sites could substantially extend the catalyst lifetime. This work provides a new strategy for selectively regulating the distribution of acidic sites in molecular sieves.

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