Downloaded from orbit.dtu.dk on: Oct 02, 2021 Towards a methanol economy based on homogeneous catalysis: methanol to H2 and CO2 to methanol Alberico, E.; Nielsen, Martin Published in: Chemical Communications Link to article, DOI: 10.1039/c4cc09471a Publication date: 2015 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Alberico, E., & Nielsen, M. (2015). Towards a methanol economy based on homogeneous catalysis: methanol to H and CO to methanol. Chemical Communications, 51(31), 6714-6725. https://doi.org/10.1039/c4cc09471a 2 2 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. 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Nielsen*cd Received 26th November 2014, The possibility to implement both the exhaustive dehydrogenation of aqueous methanol to hydrogen and Accepted 6th February 2015 CO2 and the reverse reaction, the hydrogenation of CO2 to methanol and water, may pave the way to a DOI: 10.1039/c4cc09471a methanol based economy as part of a promising renewable energy system. Recently, homogeneous catalytic systems have been reported which are able to promote either one or the other of the two www.rsc.org/chemcomm reactions under mild conditions. Here, we review and discuss these developments. the atmosphere. Moreover, it is well known that oil sources are likely to Creative Commons Attribution 3.0 Unported Licence. Introduction be near-depleted by the end of this century if we maintain or increase More than 80% of the total worldwide energy consumption is today the present rate of their consumption. Hence, there is an urgency to 1 based on fossil fuels, resulting in an increase of anthropogenic CO2 in develop techniques for exploiting alternative energy resources such as wind, sunlight, and biomass in order to generate electricity and/or H2, the latter being equivalent to chemically stored electrical energy. The a Istituto di Chimica Biomolecolare, CNR, tr. La Crucca 3, 07100 Sassari, Italy. E-mail: [email protected] capability of storing electricity is important because of the intermittent 2–4 b Leibniz-Institut fu¨r Katalyse an der Universita¨t Rostock, supply of most renewable energy sources. Albert-Einstein-Strasse 29a, 18059 Rostock, Germany Hydrogen is an ideal fuel as its combustion releases energy c This article is licensed under a Department of Chemistry and Chemical Biology, Harvard University, and water as the sole by-product. However, mainly because of 12 Oxford Street, Cambridge, Massachusetts 02138, USA its physical and chemical properties, hydrogen is not an ideal d Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark, Kemitorvet 207, 2800 Kgs. Lyngby, Denmark. energy vector: it is a gas with a limited volumetric energy density E-mail: [email protected] which, as fuel, especially in the field of automotive applications, Open Access Article. Published on 16 February 2015. Downloaded 14/12/2015 13:59:19. Elisabetta Alberico obtained her Martin Nielsen obtained his PhD degree in Chemistry (Laurea) from at Aarhus University in Denmark the University of Sassari, Italy, and under the supervision of Prof. Karl her PhD under the supervision of Anker Jørgensen. He then worked Prof. Albrecht Salzer at the as a postdoc on a Alexander von Rheinisch-Westfa¨lische Technische Humboldt Research Fellowship at Hochschule (RWTH), Aachen, the Leibniz Institute for Catalysis Germany. Since 2001 she holds a (LIKAT), Rostock, in the group of permanent position as researcher Prof. Matthias Beller. Hereafter, at the Institute of Biomolecular he moved to Harvard University, Chemistry of the National USA, where he did his next Research Council in Sassari. She postdoc with Prof. Theodore A. E. Alberico is currently a visiting researcher at M. Nielsen Betley on a Danish Council for the Leibniz Institute for Catalysis Independent Research Fellowship (LIKAT), Rostock, in the group of Prof. Matthias Beller. Her research followed by a Marie Curie International Outgoing Fellowship. interests are in the fields of organometallic chemistry, asymmetric Currently, he is a senior researcher at the Technical University of homogeneous hydrogenation and transfer hydrogenation and Denmark. His research interests include catalysis and sustainable application of catalytic methods to the synthesis of molecules chemistry. endowed with biological activity. 6714 | Chem. Commun., 2015, 51, 6714--6725 This journal is © The Royal Society of Chemistry 2015 View Article Online Feature Article ChemComm has to be either compressed at very high pressure (350–700 bars) MeOH dehydrogenation pathway by homogeneous catalysis com- or liquefied at very low temperature (À253 1C). It is flammable mences with the formation of formaldehyde, which is then con- and can diffuse through several metals and materials. verted to formic acid promoted by a H2O molecule. The final step is The chemical storage of hydrogen in solid or liquid com- CO2 production from formic acid. In each of these three steps, a H2 pounds from which it can be released as gas through a suitable molecule is liberated. The CO2 hydrogenation pathway is envisioned (and ideally fully reversible) dehydrogenation reaction has been to follow the reversed sequence of the same reaction steps. intensively investigated as a possibility to overcome some of Intheabsenceofwaterandinthepresenceofsuitablecatalysts, these limitations.5 Several criteria have to be considered when methanol may decompose to hydrogen and formaldehyde (the latter evaluating the potential of a chemical substance as hydrogen may further react to afford other products, depending on the catalyst carrier, among others, its storage capacity (H2 wt%), its volumetric and the reaction conditions) (eqn (1)) or be fully dehydrogenated to À3 hydrogen content (kg H2 m carrier) and the energy efficiency of hydrogen and carbon monoxide (eqn (2)). The catalysts which were the whole process of carrier hydrogenation–dehydrogenation, from found able to promote such processes in the absence of a hydrogen hydrogen production to final hydrogen utilization. Methanol acceptor in almost all cases were applied with much greater success belongs to the broader class of liquid storage compounds which to the dehydrogenation of ethanol and iso-propanol, the latter being includes liquid organic hydrogen carriers (LOHC i.e. methylcyclo- often the substrate of choice to test new catalysts, because of the hexane and N-ethylperhydrocarbazole),6a alcohols6b–d and formic more favourable thermodynamics and the possibility of a higher acid.6d,e It is a key platform chemical for existing fuel and chemical operational temperature.6b–d,10,11 infrastructures and contains 12.6% w/w hydrogen, which can be DH ¼ 129:8kJmolÀ1 released through aqueous reforming.7 Heterogeneous catalysts, 298 CH3OHðlÞ!H2COðgÞþH2ðgÞ 7 À1 which promote this reaction and the reverse one, the conversion DG298 ¼ 63:5kJmol 8 1 of CO2 and H2 to MeOH, operate at high temperatures (4200 C) (1) and/or applied pressures (425 bar). Indeed the high temperature Creative Commons Attribution 3.0 Unported Licence. required for dehydrogenation is one of the main factors which so DH ¼ 127:9kJmolÀ1 far has made MeOH less suitable as energy carrier in the field of 298 CH3OHðlÞ!COðgÞþ2H2ðgÞ (2) À1 portable applications. Therefore, developing milder routes for these DG298 ¼ 29:0kJmol two conversions is highly desirable in order to reach a viable H2 energy system based on a CO2–MeOH cycle. Moreover, the carbon dioxide released in the process and from any other source might be Methanol dehydrogenation hydrogenated back to methanol using hydrogen obtained from renewable sources, ideally from water electrolysis powered by solar Partial methanol conversion energy, thus completing a carbon neutral cycle. The first examples of homogeneously catalysed methanol dehydro- This article is licensed under a The advantages and limitations of a methanol based economy, genation were published approximately 30 years ago. However, how far this is from being implemented, especially in relation to because reactions were carried out in the absence of water, the technologies currently available for carbon dioxide capture and possibility for the full conversion of methanol to CO2 and three H2 Open Access Article. Published on 16 February 2015. Downloaded 14/12/2015 13:59:19. sequestration and water electrolysis for hydrogen production, have was precluded. Yet they will be briefly discussed here as they set the been reviewed in excellent monographs and articles and the reader very early stage for future developments towards the successful is referred to them for an in-depth acquaintance with the topic.9 sustainable production of hydrogen from alcohols. In this review we would like to draw the readers’ attention to Thermal decomposition of methanol. Early accounts on the recent reports concerning the development and application of thermal dehydrogenation of methanol with hydrogen evolution homogenous catalysts for MeOH dehydrogenation (r95 1C and promoted by a homogeneous catalyst were reported by the 12 13 14 15 atmospheric pressure) and CO2 hydrogenation to MeOH groups of Saito, Maitlis, Shinoda and Cole-Hamilton. (r145 1C and r60 bars) which are active under comparatively These reactions were carried out using ruthenium catalyst mild conditions.