On the design of mesostructured acidic catalysts for the one-pot dimethyl ether production from CO2

Dimethyl ether (DME) production from hydrogenation of CO2 based on two-function (redox and acidic) catalysts is receiving increasing attention due to the high demand for alternative and green fuels. In this work, we propose different mesostructured acidic metal oxides as methanol dehydration catalysts to be used as physical mixtures in combination with a commercial Cu-based redox catalyst (CZA) for the CO2-to-DME one-pot production. Al-MCM41, TiO2 and TiO2-ZrO2 mixed oxides, obtained through Sol-Gel methods, either in a conventional or Evaporation-Induced Self-Assembly approach were selected as mesostructured acidic systems and compared with a commercial zeolite (ferrierite). The regular mesoporous structure should render the active sites of the acidic catalyst easily accessible for CO2 and H2 and allow a homogeneous dispersion of the redox phase inside the mesopores in view of a possible development of bifunctional catalysts (redox + acidic). With the aim of understanding how the textural and acidic properties can be correlated with the performances and eventually design efficient dehydration catalysts, a careful study on the acidic sites was performed by both adsorption microcalorimetry with ammonia and FTIR-monitored adsorption of pyridine. The results of the performances highlighted a higher activity toward methanol dehydration for catalysts featured by Bronsted sites (zeolite and Al-MCM-41); as for catalysts with Lewis sites only (TiO2, Ti0.77Zr0.23O2) better performances were shown in case of systems presenting sites of moderate strength (Ti0.77Zr0.23O2). In the light of the above, Al-MCM-41 and TiO2ZrO2 demonstrated to be the most promising mesostructured dehydration catalysts in terms of selectivity to DME.