Is Direct DME Synthesis Superior to Methanol Production in Carbon Dioxide Valorization? From Thermodynamic Predictions to Experimental Confirmation
Dustin Kubas, Malte Gierse, Ouda Salem, Ingo Krossing
Abstract
CO 2 valorization is a key measure to reach climate neutrality. Thermodynamics suggest direct conversion of CO 2 into dimethyl ether (DME) to have greater potential than the indirect route via methanol synthesis, followed by purification of methanol and then a separate dehydration into DME. In this work, we introduce heteropoly acids as a capable class of dehydration catalysts for direct DME synthesis from CO 2 and H 2 . To clarify if direct DME synthesis is in fact superior to sole MeOH synthesis, accurate thermodynamic equilibrium calculations are performed and pose as the base of our argumentation. An efficient Cu/ZnO/ZrO 2 (CZZ) methanol catalyst is used to compare the methanol synthesis using a feedstock with stoichiometric CO 2 /3H 2 mixture against bifunctional catalysts, containing CZZ and a dehydration component. The dehydration components include a commercial ferrierite (FER) and heteropoly acid (HPA) coated alumina and zirconia. For direct DME synthesis, the CO 2 feed gas at gas hourly space velocities (GHSVs) between 1,250 and 158,400 NL kg cat –1 h –1, at temperatures of 210–270 °C and 40 bar pressure, was investigated. Based on the wide parameter window investigated, the following can be concluded at 250 °C: under a thermodynamic regime, CO 2 conversion is close to the theoretical limit (30% exp /32% theo ) and the direct DME synthesis is superior to sole methanol synthesis (+20% exp /+33% theo ). The amount of valuable products (methanol, DME) profits significantly more (+70% exp /+88% theo ) from the direct DME synthesis than CO 2 conversion indicates. Under a kinetic regime, HPA-coated catalysts show superior apparent activation energies for DME production than the widely used ferrierite (HPA: 45 kJ mol –1 /FER: 80 kJ mol –1 ), making HPA coatings a great option for highly capable dehydration catalysts under CO 2 - and water-rich conditions.