A homogenous catalytic system has been developed that efficiently and selectively decomposes formic acid into hydrogen and carbon dioxide. [Ru(H(2)O)(6)](2+), [Ru(H(2)O)(6)](3+) and RuCl(3) x xH(2)O are all excellent pre-catalysts in presence of TPPTS (TPPTS = meta-trisulfonated triphenylphosphine), the formic acid decomposition taking place in the aqueous phase, under mild conditions and over a large range of pressures. Optimisation of the reaction conditions is described together with a detailed mechanistic study leading to a tentative catalytic cycle. The performance of the catalytic system for continuous hydrogen generation is presented. Overall, the method proposed overcomes the limitations of other catalysts for the decomposition of formic acid making it a viable hydrogen-storage material.

Hydrogen represents an important alternative energy feedstock for both environmental and economic reasons, and when combined with fuel-cell technology, very efficient energy conversion can be achieved.[1] Although the advantages of hydrogen over fossil fuels are numerous, the actual use of hydrogen as a transportation fuel is limited mainly because of storage and delivery problems. Conventional hydrogen-storage methods, such as high-pressure gas containers and cryogenic liquid/gas containers, have weight and safety issues.[2] Consequently, a great deal of research is being undertaken to develop new materials, such as metal hydrides [2,3] and carbon nanostructures, [4] that store hydrogen efficiently, although no entirely satisfactory options have been found so far. Formic acid containing 4.4 wt % of hydrogen, as well as its conjugate base, formate salt, are well known sources of hydrogen [5][6][7] and have previously been reported as potential hydrogen-storage material.[8] Formic acid has the advantage over other substrates that only gaseous products are formed (H 2 /CO 2 ), hence preventing the accumulation of by-products, which is a limitation for mobile applications. However, until now potential applications have been limited by catalyst regeneration requirements, by harsh reaction conditions, and by poor selectivity. We present herein an efficient, completely selective, and robust system for hydrogen production from formic acid using water-soluble homogeneous catalysts. [9] Decomposition of formic acid was carried out in aqueous solution using hydrophilic ruthenium-based catalysts, generated from the highly water-soluble ligand meta-trisulfonated triphenylphosphine (TPPTS) with either [Ru(H 2 O) 6 ] 2+ or, more conveniently, commercially available RuCl 3 .The catalysts were activated prior to use by reaction with sodium formate and formic acid and the catalytic decomposition of formic acid performed under a wide range of pressures and temperatures. The generated H 2 /CO 2 pressure was typically between 1 and 220 bar, but no inhibition of catalytic activity was observed up to a pressure of 750 bar (see the Supporting Information for details). The rate of formic acid decomposition increased with temperature, and at all temperatures a conversion of 90-95 % can be achieved (Figure 1). The total conversion does not reach 100 % because the formate salt added for the activation of the catalyst is not converted; however, all the formic acid is consumed.[10] In addition, the catalyst was stable up to 170 8C and remained active after one year in solution.The thermal decomposition of formic acid into CO and H 2 O, which depends on the temperature and the formic acid concentration, becomes nonnegligible at elevated temperatures. [11] No traces of CO, which is known to poison some fuel cells, [12] could be detected by FTIR spectroscopy (detection limit of 3 ppm) in a sample of gases generated by using this catalyst system at 100 8C (see the Supporting Information), because of the rapidity of the reaction and, hence, the ...

A new range of pendent diphosphine (η 6 -p-cymene)ruthenium(II) complexes, [RuCl(PPh 3 )(η 1 -(P-P))-(η 6 -p-cymene)]PF 6 (P-P ) dppm, cis-PPh 2 CHCHPPh 2 (dppv), dppe, dppp, dppf), have been prepared by substitution of the labile acetonitrile ligand in [RuCl(CH 3 CN)(PPh 3 )(η 6 -p-cymene)]PF 6 . The formation of chelate complexes, [RuCl(η 2 -(P-P))(η 6 -p-cymene)] + , from these pendent phosphine complexes and from the related neutral complexes, [RuCl 2 (η 1 -(P-P))(η 6 -p-cymene)] (P-P ) dppm, dppv), has been investigated, including determination of activation enthalpies (∆H q ) and entropies (∆S q ). A concerted substitution mechanism is proposed for the latter complexes, in which methanol plays an important role in the ring-closing process by formation of hydrogen bonds with the chloride ligands. This proposal is supported by volumes of activation (∆V q ) determined by variable-pressure UV-visible spectroscopy. In contrast, a dissociative mechanism is proposed for the series of cationic pendent phosphine complexes, which generally require higher temperatures to effect ring closure. Secondary reaction pathways can be observed in some cases and are discussed in terms of differences between the phosphine complexes and supplemented by investigations using electrospray ionization mass spectrometry (ESI-MS). The X-ray structures of [RuCl(PPh 3 )(η 1 -(P-P))(η 6 -p-cymene)]PF 6 (P-P ) dppm, dppv, dppp) and [RuCl(η 2 -dppv)-(η 6 -p-cymene)]PF 6 are also reported.

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An indirect hydration process route for the production of cyclohexanol was proposed by Steyer et al., which uses formic acid as a reactive entrainer (