Hydrogen gas is a storable form of chemical energy that could complement intermittent renewable energy conversion. One of the main disadvantages of hydrogen gas arises from its low density, and therefore, efficient handling and storage methods are key factors that need to be addressed to realize a hydrogen-based economy. Storage systems based on liquids, in particular, formic acid and alcohols, are highly attractive hydrogen carriers as they can be made from CO or other renewable materials, they can be used in stationary power storage units such as hydrogen filling stations, and they can be used directly as transportation fuels. However, to bring about a paradigm change in our energy infrastructure, efficient catalytic processes that release the hydrogen from these molecules, as well as catalysts that regenerate these molecules from CO and hydrogen, are required. In this review, we describe the considerable progress that has been made in homogeneous catalysis for these critical reactions, namely, the hydrogenation of CO to formic acid and methanol and the reverse dehydrogenation reactions. The dehydrogenation of higher alcohols available from renewable feedstocks is also described. Key structural features of the catalysts are analyzed, as is the role of additives, which are required in many systems. Particular attention is paid to advances in sustainable catalytic processes, especially to additive-free processes and catalysts based on Earth-abundant metal ions. Mechanistic information is also presented, and it is hoped that this review not only provides an account of the state of the art in the field but also offers insights into how superior catalytic systems can be obtained in the future.

Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable processes are developed that directly utilize it as feedstock. Two of its reduction products are formic acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and the disproportionation of formic acid into methanol can be realized at ambient temperature and in aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without additives, while acidification results in complete (98 %) and selective (96 %) formic acid disproportionation into methanol. These promising features in combination with the low reaction temperatures and the absence of organic solvents and additives are relevant for a sustainable hydrogen/methanol economy.

Production of methanol (MeOH) from CO 2 is strongly desired as a key chemical feedstock and a fuel. However, the conventional process requires elevated temperature and pressure, and high temperature restricts the productivity of MeOH due to equilibrium limitations between CO 2 and MeOH. This paper describes the efficient hydrogenation/disproportionation of formic acid (FA) to MeOH by using iridium catalysts with electronically tuned ligands and by optimizing reaction conditions. An iridium complex bearing 5,5′-dimethyl-2,2′bipyridine in FA hydrogenation achieved MeOH selectivity with H 2 of up to 47.1% for FA hydrogenation under 4.5 MPa of H 2 in the presence of H 2 SO 4 . The final concentration of MeOH of 3.9 M and a TON of 1314 were obtained in 12 M FA aqueous solution including 10 mol % of H 2 SO 4 at 60 °C under 5.2 MPa of H 2 . Even under atmospheric pressure without introduction of external hydrogen gas, the FA disproportionation under deuterated conditions produced MeOH with 15.4% selectivity. Furthermore, the isotope effect and NMR studies revealed mechanistic insight into the catalytic hydrogenation of FA to MeOH.

The selective dehydrogenation of aqueous solutions of HCOOH/HCOONa to H 2 and CO 2 gas mixtures has been investigated using RuCl 3 •3H 2 O as a homogeneous catalyst precursor in the presence of different monoaryl-biaryl or alkyl-biaryl phosphines and aryl diphosphines bearing sulfonated groups. All catalytic systems were used in water without any additives and proved to be active at 90 °C, giving high conversions and good TOF values. As an alternative Ru(II) metal precursor, the known dimer [Ru(η 6 -C 6 H 6 )Cl 2 ] 2 was also tested as in situ catalyst with selected phosphines as well as an isolated Ru(II)-catalyst with one of them. By using high-pressure NMR (HPNMR) techniques, indications on the nature of the active species involved in the catalytic cycles were obtained.

Methanol is produced from the formic acid disproportionation reaction with unprecedented yields under mild conditions.

The front cover artwork for Issue 15/2015 is provided by the GCEE-Groupo fC atalysis for Energy and Environment, LCOM, EPFL, Switzerland. The image shows as afe and effective method for storing hydrogeni nc esium formate (CsOOCH) solutions. The oppositer eactiont akesp lace upon hydrogenation of cesium bicarbonate (CsHCO 3), requiringn oc hange of solvent , catalyst, or pH. See the Full Paper itself at http://dx.

A proof-of-concept prototype of a heterogeneous catalytic reactor has been developed for continuous production of hydrogen via formic acid (FA) dehydrogenation. A laboratory-type polymer electrolyte fuel cell (PEFC) fed with the resulting reformate gas stream (H 2 + CO 2 ) was applied to convert chemical energy to electricity. To implement an efficient coupling of the reactor and PEFC, research efforts in interrelated areas were undertaken: (1) solid catalyst development and testing for H 2 production; (2) computer modeling of heat and mass transfer to optimize the reactor design; (3) study of compatibility of the reformate gas fuel (H 2 + CO 2 ) with a PEFC; and (4) elimination of carbon monoxide impurities via preferential oxidation (PROX). During the catalyst development, immobilization of the ruthenium(II)−meta-trisulfonated triphenylphosphine, Ru-mTPPTS, catalyst on different supports was performed, and this complex, supported on phosphinated polystyrene beads, demonstrated the best results. A validated mathematical model of the catalytic reactor with coupled heat transfer, fluid flow, and chemical reactions was proposed for catalyst bed and reactor design. Measured reactor operating data and characteristics were used to refine modeling parameters. In turn, catalyst bed and reactor geometry were optimized during an iterative adaptation of the reactor and model parameters. PEFC operating conditions and fuel gas treatment/purification were optimized to provide the best PEFC efficiency and lifetime. The low CO concentration (below 5 ppm) in the reformate was ensured by a preferential oxidation (PROX) stage. Stable performance of a 100 W PEFC coupled with the developed reactor prototype was successfully demonstrated.

The equilibrium position in formic acid/amine-CO 2 systems has been examined as a function of pressure and temperature under isochoric conditions. The homogeneous ruthenium(II)-1,2-bis(diphenylphosphino)ethane catalyst was active in both reactions, that is, in formic acid cleavage producing pure hydrogen and CO 2 , as well as in carbon dioxide hydrogenation under basic conditions. High yields of formic acid dehydrogenation into H 2 and CO 2 are favored by low gas pressures and/or high temperatures, and H 2 uptake is possible at elevated H 2 -CO 2 pressures. These results take us one step closer to the realization of a practical H 2 storage-discharge device.