Effect of Steam Treatment on the Properties of Mordenite and Its Catalytic Performance in a DME Carbonylation Reaction

A series of dealuminated mordenite (MOR) samples with different acidities and various porosities were prepared through steam treatment of H⁺/Na⁺ mordenite. The obtained zeolites were elaborately characterized by X-ray diffraction, N₂ adsorption–desorption, scanning electron microscopy, ²⁹Si nuclear magnetic resonance (NMR), ²⁷Al NMR, NH₃ temperature-programmed desorption, and Fourier transform infrared spectroscopy, and the catalytic performance for dimethyl ether (DME) carbonylation was investigated. The characterization results revealed that the kind of cation balancing the zeolite charge had a great effect on the crystalline structure and acidity of mordenite zeolite during steam treatment and ion-exchange processes. Dealumination was observed both on Na-MOR and H-MOR, but it was much milder on the Na-MOR zeolite comparatively. After Na-MOR was treated with steam, its structure was well-preserved, but side pockets of mordenite were opened, resulting in an increased microporous volume and strong acid sites. Interestingly, it was found that the removal of framework Al in the 12MR channel was favored over the 8MR channel during steaming of Na-MOR, leading to the mordenite to possess more acid sites in the 8MR channel compared with those in the 12MR channel. As for the steamed H-MOR, both the crystalline structure and porosity were obviously damaged, resulting in a decreased microporous volume and acidity. The influence was more significant in the 8MR channel of H-MOR due to the obvious removal of framework Al and blockage of side pockets. The performance tests revealed that Na-MOR treated with steam was an effective way to enhance the catalytic performance for DME carbonylation reaction. The N-773 catalyst (steamed with Na-MOR at 773 K) showed a methyl acetate product rate of 0.33 gg^–1 h^–1, which was much higher than that of the parent H-MOR (0.23 gg^–1 h^–1). Moreover, the dealuminated catalyst exhibited a slow deactivation rate, which was possibly ascribed to the reduced strong acid sites in the 12MR channels. Although the steam treatment of H-MOR resulted in a lower activity in the DME carbonylation reaction, it exhibited a higher MeOAc selectivity and improved stability for its overall decreased acidity.