Homogeneous Catalytic Dehydrogenation of Formic Acid : Progress Towards a Hydrogen-Based Economy

Um dos fatores limitantes de uma economia baseada em hidrogênio está associado à problemas de estocagem de hidrogênio. Muitas abordagens diferentes estão sendo avaliadas e uma abordagem ótima não será a mesma para todas as aplicações, i.e., necessidades estática, móvel, pequena e grande escala, etc. Neste artigo, foca-se no ácido fórmico como molécula promissora para o estoque de hidrogênio, que, em certas condições catalíticas, pode ser desidrogenado gerando hidrogênio altamente puro e dióxido de carbono, com níveis extremamente baixos de monóxido de carbônico gasoso produzido. Vários catalisadores homogêneos disponíveis que geralmente operam em soluções aquosas de ácido fórmico são descritos. Também é descrita brevemente a reação reversa que pode contribuir para tornar o uso de ácido fórmico em estoque de hidrogênio ainda mais atrativo.


Introduction
A cyclic process involving formic acid and carbon dioxide/hydrogen has been proposed as an efficient way to store and generate hydrogen when it is needed (Scheme 1). 1 Indeed, in the last few years, research on the use of formic acid as a hydrogen storage vector has grown rapidly. 2 The reason for this interest is threefold.First, formic acid contains 4.4 wt.% of H 2 , which is equivalent to 53 g hydrogen per litre and has a flash point of 69 °C, much higher than that of the gasoline (−40 °C) and methanol (12 °C).Second, carbon dioxide and carbonates can be hydrogenated to afford formic acid and formates in water and, due to the abundance of CO 2 in the atmosphere, it is an ideal C 1 building block (formic acid has other industrial uses and is therefore an interesting product beyond being a hydrogen storage molecule). 3,4Third, the reverse reaction, Scheme 1.The carbon dioxide-formic acid cycle.
i.e., the dehydrogenation of formic acid to give CO 2 and hydrogen is fast and controllable and would be ideal not only for static applications, but also potentially for mobile applications. 3

Research on Hydrogenation of Carbon Dioxide
The hydrogenation carbon dioxide and carbonates to formic acid/formates is still a challenging reaction to catalyse in an efficient manner. 4While the reaction can be catalysed with heterogeneous catalysts, 5 more effort is devoted to heterogeneous methanation catalysts instead of catalysts that give formic acid.Hence, the direct hydrogenation of carbon dioxide to formic acid/formates is usually catalysed by homogeneous catalysts in aqueous solution. 4Irrespective of the type of catalyst used the rate of this reaction depends strongly on the pH of the solution, with basic solutions resulting in highest reaction rates and conversions.The first product of the stepwise reduction of CO 2 with H 2 is the formic acid, but in gas phase this reaction does not take place, 6 as ΔG°2 98 = +32.9kJ mol −1 (equation 1): Dissolution of the gases decreases the entropy term; in aqueous solution, this reaction becomes slightly exergonic with ΔG 298 = -4 kJ mol −1 (equation 2): CO 2(aq) + H 2(aq) → HCOOH (aq) (2) Addition of a base improves the enthalpy of the reaction (ΔG°2 98 = −35.4kJ mol −1 ; ΔH°2 98 = −59.8kJ mol −1 ; ΔS°2 98 = −81 J mol −1 K −1 ), making this reaction largely available (equation 3): CO 2(aq) + H 2(aq) + NH 3(aq) → HCOO − (aq) + NH 4 A particularly well-studied class of catalyst comprises ruthenium(II) complexes with water soluble phosphine ligands (see Table 1).The most recent ruthenium(II) catalytic system reported comprises [RuCl 2 (PTA) 4 ] (PTA = 1,3,5-triaza-7-phosphaadamantane) in dimethyl sulfoxide (DMSO) and operates in the absence of any base, any additives to afford 1.9 mol L −1 formic acid solutions. 7his concentration is unprecedented and corresponds to more than two orders of magnitude higher concentration than other catalysts without base.Moreover, the catalyst is highly stable and can be recycled and reused multiple times without loss of activity.
Although water-soluble ruthenium(II) catalysts have been most extensively studied or this reaction other systems have also been investigated (see Table 2).Indeed, the highest turnover number (TON) reported for CO 2 hydrogenation in basic solution, a staggering 3.5 million, was obtained with an Ir(III) complex with a pincer-ligand. 19espite of the important goal in catalysis is to replace noble metal-based catalysts with cheap and earth abundant metals, few reports are available.The first row transition metal based catalytic systems in general have with very low activity.An interesting development in the field is  32 an iron(II)-tris[(2-diphenylphosphino)-ethyl]phosphine (PP 3 ) complex, contains a tetradentate phosphine ligand that provides stability to the more reactive (unstable) iron(II) centre.In situ multinuclear nuclear magnetic resonance (NMR) spectroscopy was used to study the iron(II)-catalysed reactions for both bicarbonate reduction and formic acid dehydrogenation and several intermediate species, notable metal-hydride species, were detected allowing catalytic cycles to be postulated (Figure 1). 29,33

Research on Dehydrogenation of Formic Acid
The most important feature of a formic acid dehydrogenation catalyst is that it must be highly selective for this reaction (equation 4), and not catalyse the  Proposed mechanism for the selective iron-catalyzed hydrogen generation from formic acid with calculated relative energies of complexes (kJ mol −1 ). 29,33Reproduced with permission of The American Association for the Advancement of Science (3470280610808).
dehydration of formic acid that results in the formation of water and carbon monoxide (equation 5).
The dehydration reaction not only reduces the amount of hydrogen produced, but the CO by-product is a poison to fuel cells and in general, the concentration of CO should remain below 10 ppm.A large number of heterogeneous catalysts have been evaluated for this reaction, but lack of selectivity tends to be a problem.Thus, there has been much recent interest in homogeneous catalysts and well-defined, immobilized heterogeneous catalysts derived from them.
Key examples of homogeneous catalysts used for the selective dehydrogenation of formic acid to CO 2 and H 2 are listed in Table 3.
In keeping with catalysts for the reverse reaction, Ru(II) complexes with water-soluble phosphine ligands have been widely explored although iron, iridium and rhodium complexes also selectively catalyse the dehydrogenation reaction.Notably, several catalysts that meet the stringent requirements for industrial applications have been developed.A high stable and selective Ru(II) catalyst is readily generated from the in situ reaction of RuCl 3 with the water-soluble m-trisulfonated triphenylphosphine (mTPPTS) ligand. 50The resulting catalyst selectively decomposes formic acid into carbon monoxide, free hydrogen and carbon dioxide in a very wide pressure range and it is undergoing commercialisation. 46The catalytic cycle has also been elucidated from in situ NMR spectroscopic studies (Figure 2).Heterogeneous catalysts based on immobilisation, have been prepared by the reaction of the ruthenium(II)-mTPPTS dimer and MCM41 silica functionalized with diphenylphosphine groups via alkyl chains.The catalytic system based on MCM41-Si-(CH 2 ) 2 PPh 2 /Ru-mTPPTS demonstrated an activity and stability comparable to those of the homogeneous catalyst: a turnover frequency of 2780 h −1 was obtained at 110 °C, and no ruthenium leaching was detected after turnover numbers of 71000. 51

Conclusions
Hydrogen is definitelly among the most promising candidates as the energy carrier in the future, though its generation from renewable sources and storage in a safe and reversible way is still challenging.Formic acid is a promising molecule for hydrogen storage and delivery.HCOOH can be generated via catalytic hydrogenation of CO 2 or bicarbonate with suitable catalysts.Under mild experimental catalytic conditions, it can be dehydrogenated to give highly pure hydrogen and carbon dioxide.We summarised here the various homogeneous catalysts available that usually operate both in aqueous and in organic formic acid solutions.The homogeneous catalytic decomposition of formic acid in aqueous solution provides an efficient in situ method for hydrogen production that operates over a wide range of pressures, under mild conditions, and at a controllable rate.On the basis of these results one can envisage the practical application of carbon dioxide as hydrogen vector: storage and delivery.

Figure 1 .
Figure1.Proposed mechanism for the selective iron-catalyzed hydrogen generation from formic acid with calculated relative energies of complexes (kJ mol −1 ).29,33 Reproduced with permission of The American Association for the Advancement of Science (3470280610808).

Table 1 .
Bicarbonate, carbonate and carbon dioxide hydrogenation into formic acid/formate or formic acid derivatives with ruthenium(II) pre-catalysts TON: turnover number; TOF: turnover frequency.there-discovery of a stable iron-based catalyst for the hydrogenation of CO 2 in basic solutions, as well as the formic acid cleavage to CO 2 and H 2 .The catalyst, first synthetized and published byBianchini et al. in 1988,

Table 2 .
Bicarbonate, carbonate and carbon dioxide hydrogenation into formic acid/formate or formic acid derivatives with other metal based catalysts TON: turnover number; TOF: turnover frequency.

Table 3 .
Selective catalytic cleavage of the formic acid into carbon dioxide and hydrogen TON: turnover number; TOF: turnover frequency.