Study Of Bifunctional Catalyst For Single-Step Synthesis Of Dimethyl Ether (DME) From Biomass-Derived Syngas

The biomass-derived energy has been the most promising alternative for fossil energy because it is abundant, renewable and carbon neutral. The synthesis of dimethyl ether （DME） from biomass-derived syngas has been an important route for the utilization of biomass energy. Varies of bifunctional catalysts were prepared and tested for single-step synthesis of dimethyl ether from biomass-derived syngas. The main results are as follows:1. Single-step synthesis of DME from bio mass-derived syngas over CuO-ZnO-MO_x （M = Zr, Al, Cr, Ti）/HZSM-5 hybrid catalyst: Effects of MO_xA series of CuO-ZnO-MO_x （M = Zr, Al, Cr, Ti） catalysts were prepared by co-precipitation method. The CuO-ZnO-ZrO₂ catalyst exhibits the highest BET surface area and Cu surface area. For all the CuO-ZnO-MO_x catalysts, Cu0 was the predominant copper species detectable on the surface of both reduced and spent samples. As-prepared CuO-ZnO-MO_x catalysts were mixed physically with HZSM-5 zeolite to synthesize DME via biomass-derived syngas. The highest CO conversion and DME yield were obtained over a CuO-ZnO-ZrO₂/HZSM-5 hybrid catalyst. The CO conversion increases with the increase in the Cu surface area, but the relationship between them is not linear. Due to the H2-deficient characteristic of biomass-derived syngas, the water-gas shift reaction, by which H2 can be produced in-situ for the hydrogenation of CO, plays an important role in the direct DME synthesis.2. The effects of combination method on CuO-ZrnO-Al₂O₃/HZSM-5 hybrid catalysts for one-step synthesis of DME from biomass-derived syngasThe CuO-ZrnO-Al₂O₃/HZSM-5 hybrid catalysts were prepared by four different combination methods （dual-bed, granule-mixing, powder-mixing and core-shell）. The influence of combination methods of hybrid catalyst on the activity, selectivity and DME yield has been evaluated. The results show that the combination methods affect the properties of active sites and the macrostructure of the hybrid catalyst, which further affect the synergistic reaction. A superior CO conversion and DME yield was obtained over the hybrid catalyst combined by granule-mixing, whereas, the dual-bed system exhibits the lowest CO conversion and DME yield. The effects of combination method for hybrid catalysts validated the role of the water-gas shift reaction. The deactivation of hybrid catalyst was mainly ascribed to the aggregation of Cu nanoparticles. Moreover, the influences of experimental conditions on CO conversion and product selectivity were investigated in terms of temperature, pressure, and space velocity.3. The effect of MgO additives on the stability of CuO-ZnO-Al₂O₃/HZSM-5 hybrid catalysts for single step synthesis of DME from biomass-derived syngasThe MgO modified CuO-ZrnO-Al₂O₃ catalysts were prepared using incipient wetness method. As-prepared MgO-CuO-ZnO-Al₂O₃ catalysts were mixed physically with HZSM-5 zeolite to synthesize DME via biomass-derived syngas. The decrease in CO conversion is mainly ascribed to the deactivation of the methanol synthesis catalyst. The aggregation of Cu nanoparticles is responsible for the deactivation of Cu-based catalyst. The coke formation hardly affects dehydration properties of HZSM-5. MgO additives results in higher reduction temperature. The reason for the significantly improved stability of hybrid catalysts can be attributed to inhibitory effect of MgO on the aggregation of Cu nanoparticles.