Dimethyl ether as circular hydrogen carrier: Catalytic aspects of hydrogenation/dehydrogenation steps

The intermittent nature of renewable resources requires for most applications the development of efficient and cost-effective technologies for steady supply of electrical energy. The storage of energy in the form of hydrogen chemically bound within organic molecules (rather than physically as compressed gas or cooled liquid) represents an alternative approach that is attracting great research interest. Compared to other liquid organic hydrogen carriers (LOHCs), dimethyl ether (DME) appears to have the largest potential impact on society, especially if inserted in technological chains of CO2 sequestration and utilization, so to determine an effective mitigation of environmental issues, without any net effect on the carbon footprint. Specifically, the steps of H2 storage and H2 release can take place in two coupled chemical processes, constituted by the exothermic synthesis of DME via CO2 hydrogenation and the endothermic steam reforming of DME, respectively. Herein, the latest advances in the development of heterogeneous bifunctional and hybrid catalysts for the direct hydrogenation of CO2 to DME are thoroughly reviewed, with special emphasis on thermodynamics, catalyst design and process feasibility. Despite many aspects behind the mechanism of DME synthesis from H2-CO2 streams are still to be uncovered, the recent progress in the research on H2 release by DME steam reforming is increasing the interest for effectively closing this binary H2 loop, in view of future green deals and sustainable research developments.