Direct DME production from CO₂ hydrogenation and MeOH dehydration over CZA & HZSM-5 bifunctional catalyst bed in fixed bed reactor

The direct synthesis of dimethyl ether (DME) from carbon dioxide (CO₂) and hydrogen (H₂) offers a promising carbon capture and utilization (CCU) route for clean fuel production. This study evaluates a bifunctional catalyst system comprising commercial CuO-ZnO-Al₂O₃ (CZA) and HZSM-5 pellets in a fixed-bed reactor (FBR). Catalyst characterization was performed using XRD, FESEM, EDX, BET, mercury intrusion, TGA, H₂-TPR, and NH₃-TPD. CZA showed high hydrogen uptake (4900 µmol·g⁻¹), confirming active Cu⁺/Cu⁰ surface sites, while HZSM-5 exhibited moderate acidity with NH₃ desorption peaking near 217 °C. Pre-liminary tests compared catalyst form (powder vs pellet), packing structure (mixed vs sequential), and CZA:HZSM-5 mass ratios (1:1 to 4:1) under baseline conditions (200 °C, 40 bar, 500 ml g⁻¹ h⁻¹ GHSV). The optimal configuration—pellet form, sequential packing, 3:1 ratio—was selected for operating condition optimization. Temperature (150–250 °C), pressure (20–60 bar), and GHSV (250–583 ml g⁻¹ h⁻¹) were varied using a one-factor-at-a-time approach. The best performance was achieved at 225 °C, 50 bar, and 500 ml g⁻¹ h⁻¹, yielding 85.8% CO₂ conversion, 84.0% DME selectivity, and 97.9% DME yield.