catalysts Review Direct Synthesis of Dimethyl Ether from CO 2 : Recent Advances in Bifunctional/Hybrid Catalytic Systems Noelia Mota, Elena Millán Ordoñez ,Bárbara Pawelec , José Luis G. Fierro and Rufino M. Navarro *   Citation: Mota, N.; Millán Ordoñez, E.; Pawelec, B.; Fierro, J.L.G.; Navarro, R.M. Direct Synthesis of Dimethyl Ether from CO 2 : Recent Advances in Bifunctional/Hybrid Catalytic Systems. Catalysts 2021, 11, 411. https://doi.org/10.3390/ catal11040411 Academic Editor: Javier Ereña Loizaga Received: 4 March 2021 Accepted: 22 March 2021 Published: 24 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Instituto de Catálisis y Petroleoquímica, The Sustainable Energy and Chemistry Group, Consejo Superior de Investigaciones Científicas, 28049 Madrid, Spain; noelia.mota@icp.csic.es (N.M.); elena.millan.ordonez@csic.es (E.M.O.); bgarcia@icp.csic.es (B.P.); jlgfierro@icp.csic.es (J.L.G.F.) * Correspondence: r.navarro@icp.csic.es; Tel.: +34-915854774 Abstract: Dimethyl ether (DME) is a versatile raw material and an interesting alternative fuel that can be produced by the catalytic direct hydrogenation of CO 2 . Recently, this process has attracted the attention of the industry due to the environmental benefits of CO 2 elimination from the atmosphere and its lower operating costs with respect to the classical, two-step synthesis of DME from syngas (CO + H 2 ). However, due to kinetics and thermodynamic limits, the direct use of CO 2 as raw material for DME production requires the development of more effective catalysts. In this context, the objective of this review is to present the latest progress achieved in the synthesis of bifunctional/hybrid catalytic systems for the CO 2 -to-DME process. For catalyst design, this process is challenging because it should combine metal and acid functionalities in the same catalyst, in a correct ratio and with controlled interaction. The metal catalyst is needed for the activation and transformation of the stable CO 2 molecules into methanol, whereas the acid catalyst is needed to dehydrate the methanol into DME. Recent developments in the catalyst design have been discussed and analyzed in this review, presenting the different strategies employed for the preparation of novel bifunctional catalysts (physical/mechanical mixing) and hybrid catalysts (co-precipitation, impregnation, etc.) with improved efficiency toward DME formation. Finally, an outline of future prospects for the research and development of efficient bi-functional/hybrid catalytic systems will be presented. Keywords: CO 2 hydrogenation; dimethyl ether; DME; bifunctional catalyst; hybrid catalyst 1. Introduction Despite the increasing use of renewable energy sources, fossil fuels (oil, coal, and natural gas) continue to be used in the short and medium term [1]. Unfortunately, the combustion of carbon-based fossil fuels is accompanied by huge emissions of CO 2 (in 2019, global energy-related CO 2 emissions reached around 33 gigatons (Gt)), disrupting the Earth’s natural carbon cycle and causing global warming, ocean acidification, sea-level rise, and climate change [2]. Therefore, the use of fossil fuels in the near future should include their efficient transformation and the carbon capture and utilization (CCU) of the CO 2 produced. By way of example, the 2014 European Council committed EU member countries to achieve a national economy-wide target of at least a 40% reductions in greenhouse gas (GHG) emissions by 2030, which is in line with a cost-effective path of reducing at least 80% of greenhouse gas emissions by 2050 [3] In addition to the challenges to achieve effective CO 2 capture, its use as raw material for the production of chemical building blocks and synthetic fuels is also a major techno- logical challenge [4]. There are several C-rich chemical products that could be synthesized from CO 2 (Figure 1). However, the industrial use of CO 2 in the synthesis of chemicals is currently limited to a few processes, such as the synthesis of urea and its derivatives and the synthesis of salicylic acid and carbonates, with an annual consumption of CO 2 of around 140 MTm/year [5]. The conversion of CO 2 into chemicals can be achieved Catalysts 2021, 11, 411. https://doi.org/10.3390/catal11040411 https://www.mdpi.com/journal/catalysts