This journal is © The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 707--709 | 707 Cite this: Chem. Commun., 2014, 50, 707 Base-free hydrogen generation from methanol using a bi-catalytic system† Ange ` le Monney, Enrico Barsch, Peter Sponholz, Henrik Junge, Ralf Ludwig and Matthias Beller* A bi-catalytic system, in which Ru-MACHO-BH and Ru(H) 2 (dppe) 2 interact in a synergistic manner, was developed for the base-free dehydrogenation of methanol. A total TON > 4200 was obtained with only trace amounts of CO contamination (o8 ppm) in the produced gas. Hydrogen is attracting increasing attention as an alternative energy carrier and is considered as a potential fuel of the future. 1 It is therefore conceivable that a ‘‘hydrogen economy’’ 2–6 may solve the problems related to the use of fossil fuels whose reserves are depleting. Hydrogen can be used as an energy source for fuel cells or as a direct fuel for combustion with the release of water as the only side product. An important challenge towards the implementation of a hydrogen economy is the safe, practical and economical storage of the produced hydrogen. 7 Unfortunately, the compression or liquefaction of H 2 is costly and requires substan- tial energy. Alternatively, the chemical storage of hydrogen in liquid organic compounds and its release on demand hold great promise. In this respect, methanol is an ideal candidate due to its high hydrogen content (12.6% by weight) and its liquid state at room temperature. The so-called methanol-reforming process allows for the production of H 2 from a mixture of methanol and water. 8 However, the high temperatures needed (>200 1C) and the co-production of carbon monoxide render current methanol steam reforming processes unsuitable for applications in fuel cells which only tolerate very low CO concentrations. Recently, we described an aqueous phase methanol reforming process at temperatures below 100 1C using a homogeneous ruthenium catalyst with only traces of released carbon monoxide (o10 ppm). 9 Hydrogen generation was observed with excellent catalyst turnover numbers (>350 000) and turnover frequencies (4700 h À1 ) using ruthenium complexes bearing a cooperative PNP pincer ligand 10–16 in the presence of base. From an environmental point of view and for practical applications, it would be advantageous to produce H 2 from methanol under neutral conditions. Parallel to our work, Gru ¨tzmacher et al. described another ruthenium-based catalyst capable of dehydro- genating a methanol–water mixture without base with a TOF = 50 h À1 . 17 Here, we report an improved base-free methanol dehydro- genation using the ruthenium-based PNP pincer complex Ru-MACHO-BH 18,19 (A1, see Table 1 for structure). This catalyst is comparable to Ru-MACHO (A0), 20–22 used for the base- mediated methanol reforming process, but lacks the chloride ligand and therefore does not require a base for its activation. Complex A1 showed comparable catalytic activity to A0 in the presence of a base (Table 1, entries 1 and 2). The addition of triglyme 23 as solvent was found to increase the solubility of the catalyst (cf. ESI†) and the gas generation was improved, reach- ing a gas evolution rate of 149 mL h À1 , corresponding to a Table 1 Base-free hydrogen generation from aqueous methanol a Entry Cat. Catalyst loading b [mmol]/[ppm] Base Solvent Gas evolution rate c [mL h À1 ] TON 3h 1 A0 5/23 KOH None 113 2064 2 A1 5/23 KOH None 133 2392 3 A1 5/23 KOH Triglyme 150 2743 4 A1 5/23 None Triglyme 1.7 32 5 A1 20/90 None Triglyme 17 70 6 A1 45/203 None Triglyme 37 74 7 A1 95/428 None Triglyme 61 59 a Reaction conditions: MeOH (9.0 mL), H 2 O (1.0 mL), solvent (4.0 mL) when indicated, base (80 mmol) when indicated, catalyst (5–95 mmol), T set = 93.5 1C. The gas evolution was measured manually using burettes. b ppm relative to MeOH. c Average calculated over the first 3 hours. Leibniz-Institut fu ¨r Katalyse e.V. and der Universita ¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany. E-mail: matthias.beller@catalysis.de; Fax: +49 381 1281 51113; Tel: +49 381 1281 113 † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc47306f Received 24th September 2013, Accepted 6th November 2013 DOI: 10.1039/c3cc47306f www.rsc.org/chemcomm ChemComm COMMUNICATION Published on 07 November 2013. Downloaded by University of Rostock on 16/04/2014 11:30:50. View Article Online View Journal | View Issue