From CO₂ to DME: Enhancement through Heteropoly Acids from a Catalyst Screening and Stability Study

Abstract

The direct synthesis of dimethyl ether (DME) via CO2 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 kgcat –1 h–1; ratio CO2/H2/N2 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 kgcat –1 h–1; ratio CO2/H2/N2 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 Al2O3 and ZrO2 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 CO2-to-DME catalyst toolbox.

CO2 to dimethyl ether (DME): structural and functional insights of hybrid catalysts

“…Authors have employed two mixing methods to produce bi-functional catalysts; (a) single-grain process: agate motor mixing of Cu/ZnO/ZrO 2 with dehydration catalyst to have close intra-contact and (b) dual-grain process: physical mixing to have minimal contact. 160 On the other hand, as dehydration catalysts, ferrierite, β-zeolite, Al 2 O 3 , and ZrO 2 with silicotungstic or phosphotungstic acid coatings are selected to test the effect of heteropoly acid coating on DME synthesis. Silicotungstic acid-coated bifunctional catalysts prepared by the single-grain method or homogeneous mixture experience initial catalytic deactivation, while catalysts prepared by the dual-grain method or heterogeneous mixture results in stable activity.…”

Does Direct Dimethyl Ether Synthesis from Carbon Dioxide and Hydrogen Yield More Valuable Products than the Equivalent Methanol Production? A Review on Recent Work

“…An alternative approach coats strong Brønsted acids with low surface area onto high surface area materials [51]. Multiple reports about using heteropolyacids as promising coating materials for MtD are published [52][53][54][55][56][57].…”

“…The Keggin-structured heteropolyacids have a well-defined structure covering approximately an area of 1 nm 2 per Keggin unit (KU) [54]. In addition, the process is sensitive to the KU loading and on high-surface materials the optimal KU loading is one layer [51]. Favorably, heteropolyacid-coated supporting materials are comparably easy to synthesize and the process may also be up-scaled [21,51].…”

“…In addition, the process is sensitive to the KU loading and on high-surface materials the optimal KU loading is one layer [51]. Favorably, heteropolyacid-coated supporting materials are comparably easy to synthesize and the process may also be up-scaled [21,51].…”

“…Hence, a series of multiple Brønsted and Lewis acidic MtD catalysts with different mixing and coating methods (over 40 combinations) was tested in the direct DME synthesis [51]. The CtM catalyst CZZ and the different MtD catalysts were either combined by mortaring (single-grain method) or physical mixing (dual-grain method).…”