Research on Direct Oxidation of Methanol to Polyoxymethylene Dimethyl Ethers over NASICON-Based Catalytic Materials

Polyoxymethylene dimethyl ethers (DMMx) are highly promising clean diesel additives. Compared to the traditional aldol condensation route, the one-step oxidative method for producing DMMx directly from methanol is a green synthesis route with significant advantages. However, due to the complexity of the reaction, a balance must be struck between oxidation depth and C–O chain growth efficiency. This imposes specific requirements on the design of catalysts with multifunctional active sites: the catalyst should possess appropriate oxidative activity, suitable acid strength distribution, and effective synergy between these two functions. To address these challenges, this study designed a sulfuric acid-modified molybdenum-doped NASICON catalyst, which demonstrated favorable catalytic performance in the one-step oxidative synthesis of DMMx from methanol. Over the NSC-Mo-0.5-30% catalyst, methanol conversion of 81.3% and DMMx selectivity of 58.7% were achieved respectively, along with the formation of heavier molecules, as evidenced by a DMM2–6 selectivity of 11.3%. The results of NH3-TPD, Py-IR, and XPS indicate that the introduction of molybdenum increase the number of weak Lewis acid sites, while sulfuric acid impregnation not only constructs gradient-distributed Brønsted acid sites but also promotes the formation of Mo5+/Mo6+ redox pairs. The cooperation of the two types of active sites significantly promotes the improvement of catalyst performance.