Zeolites are one of the shape-selective catalysts. The characteristics of zeolites, which come from the structural confinement on the molecular dimensions, are crucial for shape-selective catalysis.
The catalytically acid sites at different positions of zeolites show a distinct confinement effect for reactant molecules, especially reflected in mordenite (MOR) zeolite catalyzing a dimethyl ether (DME) carbonylation reaction.
Recently, a research team led by Prof. LIU Zhongmin from the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences (CAS) developed a new strategy to preferentially remove the acid sites in the 12-membered ring (12-MR) channels of MOR zeolite by a trimethylchlorosilane (TMCS) silylation treatment, which could improve the performance of DME carbonylation.
This study was published in ACS Catalysis on April 1.
The 8-membered ring (8-MR) channels of MOR zeolite are preferred for the selective carbonylation of DME, while the larger 12-MR channels can accommodate more reaction routes, which will cause rapid deactivation of the MOR zeolite. Therefore, it is necessary to selectively remove the acid sites in 12-MR channels of MOR to improve its catalytic performance in the DME carbonylation reaction.
The researchers used in situ diffuse reflectance infrared Fourier transform (DRIFT) and 29Si cross-polarization (CP) Nuclear Magnetic Resonance (NMR) spectroscopic techniques to investigate the interaction of TMCS molecules with the bridging hydroxyl groups within different positions of the H-mordenite (HMOR) zeolite.
They found that TMCS molecules bridged the HMOR framework via a hydrolysis reaction between chloro groups and Brønsted H+ atoms to cover the acid sites. Due to the space limitation, the silylation treatment selectively retained most of the acid sites (80%) in the desired position by TMCS replacing the Brønsted H+ atoms within 12-MR channels in the HMOR zeolite, leading to better selectivity and a much longer lifetime in the DME carbonylation reaction.
This work was supported by the National Natural Science Foundation of China, the International Partnership Program of CAS, and the Key Research Program of Frontier Sciences of CAS. (Text by LIU Rongsheng)