Sorption enhanced DME synthesis by one-step CO2 hydrogenation

Abstract

DME synthesis by direct CO₂ hydrogenation was studied on physical mixture of commercial CuZnO/Al₂O₃ (CZA) and in-house synthesized PTA (phosphotungstic acid)/γ-Al₂O₃ catalysts. Effects of PTA loading (0–50 % by mass) and relative amounts of the catalysts on CO₂ conversion and DME yield were investigated. The process was intensified through integration of in-situ steam separation by Zeolite 3A (Z3A) adsorbent mixed with the catalysts. Experiments were run at 498 K, 30 bar, H₂:CO₂=3:1 and GHSV=1750 h⁻¹ (based on CZA catalyst). Optimum PTA loading and CZA:PTA/γ-Al₂O₃ mass ratio were 30 % and 1:2, respectively, that gave ∼21 % CO₂ conversion and 6.3 % DME yield. These values remained below the respective thermodynamic limits (28.5 % and 21 %), which were exceeded upon adsorbent integration. Catalytic performance depended strongly on adsorbent quantity studied at the catalyst (CZA+PTA/γ-Al₂O₃):adsorbent mass ratio range of 1:0.33–4. Catalysts and the adsorbent remained stable during the pressure swing driven adsorption-regeneration cycles. Sorption assistance at catalyst:adsorbent ratio=1:4 increased catalyst productivity from 5.5×10⁻³ to 2×10⁻² kg_DME h⁻¹ kg_cat⁻¹. The latter value was comparable to those of the sorption enhanced CO₂+CO hydrogenation systems due to the PTA-based dehydration catalyst with strong acidic features and was promising when the strong thermodynamic limitations of CO₂-DME conversion was considered.