Effects of Mo-Sn interactions on the performance of Mo1Sn2 catalysts for low-temperature oxidation of dimethyl ether

Dimethyl ether (DME) is a colorless and non-toxic coal-based clean fuel, which can be synthesized from coal, biomass, syngas, etc. It is also an important platform compounds for the production of high value-added oxygenated chemicals and diesel oil additives (e.g. formaldehyde, methyl formate (MF), dimethoxymethane (DMM), polyoxymethylene dimethyl ethers (DMMx), etc.). Wherein, methyl formate, as one of the downstream products of DME, is a new starting material and structural unit of C1 chemistry. Due to its molecule contains ester group and formyl group, MF can be widely used to synthesize various valuable industrial chemicals such as formic acid, N, N-dimethyl formamide (DMF), formamide and dimethyl carbonate (DMC). At present, MF is produced in industry through the liquid-phase carbonylation of methanol, where methanol and CO are used as feedstock and sodium methoxide as catalyst. The selectivity of MF obtained by this method is relatively high. However, there are some disadvantages, such as low conversion of methanol, high reaction pressure and difficult separation of catalyst. Meanwhile, the methanol dehydrogenation is another method for synthesizing MF, which includes direct dehydrogenation or oxidative dehydrogenation, and the catalysts used mainly are copper-based catalysts. However, this reaction process has high energy consumption and is controlled by thermodynamics. In recent years, the synthesis of MF from DME by low-temperature oxidation has attracted more and more attention, which has the characteristics of short process, green and good atomic economy. The synthesis of MF by low-temperature oxidation of DME requires multi-functional catalysts containing both acid-base and redox active sites. Mo-Sn catalyst is widely used in this target reaction due to its adjustable valence state. In this paper, a series of Mo1Sn2 catalysts were designed and prepared by a two-step hydrothermal method. By tuning the calcination temperatures of tin precursors, tin oxides with different surface properties were synthesized. The effects of the interaction between tin oxides and the active component molybdenum oxides on the structure-activity relationship of the Mo1Sn2 catalysts and the low-temperature oxidation performance of DME to MF were investigated. The results showed that the performance of the catalysts was closely related to the treatment conditions. When tin oxide was calcined at 80 ℃, and subjected to hydrothermal reaction with Mo, then the Mo1Sn2-80Sn-500 catalyst obtained by calcining at 500 ℃ exhibited better low-temperature oxidation performance. At the reaction temperature of 110 ℃, the selectivity of MF reached 97.7%, and the conversion of DME was 14.7%. The surface physicochemical properties of the catalysts as well as the coordination structures of molybdenum species were characterized in detail by XRD, Raman, FT-IR, low-temperature ESR, NH₃-TPD, CO₂-TPD and H₂-TPR. The results indicated that the change in calcination temperature of tin oxides affected the existence form of molybdenum oxides on the surface of SnO₂. After treating tin oxides at lower temperatures, MoO₃ and MoO_x coexisted in the Mo1Sn2 catalysts. Moreover, at the Mo-Sn interface, Mo species mainly existed in the form of Mo⁵⁺ coordination structure. More acid content on the surface of the catalysts was conducive to the oxidation reaction of DME, while strong base sites were not conducive to the formation of MF.