From CO(2) to DME: Enhancement through Heteropoly Acids from a Catalyst Screening and Stability Study

[Image: see text] The direct synthesis of dimethyl ether (DME) via CO(2) hydrogenation in a single step was studied using an improved class of bifunctional catalysts in a fixed bed reactor (T(R): 210–270 °C; 40 bar; gas hourly space velocity (GHSV) 19,800 NL kg(cat)(–1) h(–1); ratio CO(2)/H(2)/N(2) 3:9:2). The competitive bifunctional catalysts tested in here consist of a surface-basic copper/zinc oxide/zirconia (CZZ) methanol-producing part and a variable surface-acidic methanol dehydration part and were tested in overall 45 combinations. As dehydration catalysts, zeolites (ferrierite and β-zeolite), alumina, or zirconia were tested alone as well as with a coating of Keggin-type heteropoly acids (HPAs), i.e., silicotungstic or phosphotungstic acid. Two different mixing methods to generate bifunctional catalysts were tested: (i) a single-grain method with intensive intra-particular contact between CZZ and the dehydration catalyst generated by mixing in an agate mortar and (ii) a dual-grain approach relying on physical mixing with low contact. The influence of the catalyst mixing method and HPA loading on catalyst activity and stability was investigated. From these results, a selection of best-performing bifunctional catalysts was investigated in extended measurements (time on stream: 160 h/7 days, T(R): 250 and 270 °C; 40 bar; GHSV 19,800 NL kg(cat)(–1) h(–1); ratio CO(2)/H(2)/N(2) 3:9:2). Silicotungstic acid-coated bifunctional catalysts showed the highest resilience toward deactivation caused by single-grain preparation and during catalysis. Overall, HPA-coated catalysts showed higher activity and resilience toward deactivation than uncoated counterparts. Dual-grain preparation showed superior performance over single grain. Furthermore, silicotungstic acid coatings with 1 KU nm(–2) (Keggin unit per surface area of carrier) on Al(2)O(3) and ZrO(2) as carrier materials showed competitive high activity and stability in extended 7-day measurements compared to pure CZZ. Therefore, HPA coating is found to be a well-suited addition to the CO(2)-to-DME catalyst toolbox.