CO₂ Conversion to Dimethyl Ether on Cu/ZnO/Al₂O₃-ZSM-5 Tandem Catalysts in a Double-Bed Reactor: Tuning the ZSM-5 Catalyst Acidity and Porosity

This research evaluated the effectiveness of thermally and microwave-assisted alkali-treated HZSM-5 zeolites with varying silica/alumina ratios in catalyzing the direct one-step synthesis of dimethyl ether (DME) from CO₂. The reactions were carried out in a dual-bed reactor configuration. A primary catalyst, consisting of Cu/ZnO/Al₂O₃ (CZA) with a formulation of 10 wt % alumina, 30 wt % Zinc, and 60 wt % Copper, was used to convert CO₂ to methanol. Subsequently, zeolite or modified zeolite was utilized as a secondary catalyst for the dehydration of methanol to DME. The catalysts were extensively characterized using various advanced analytical techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), nitrogen physisorption analysis, X-ray diffraction (XRD), ammonia temperature-programmed desorption (NH₃-TPD), and hydrogen temperature-programmed reduction (H₂-TPR). The CO₂ hydrogenation study was carried out at a pressure of 40 bar and a temperature of 250 °C. The results demonstrated that alkali treatment had minimal impact on the crystallinity of the zeolites, but it greatly improved their textural properties. Moreover, the alkali modification enhanced the stability of catalysts and increased their selectivity for DME production.