Interconversion between Formic Acid and H₂/CO₂ using Rhodium and Ruthenium Catalysts for CO₂ Fixation and H₂ Storage

Abstract

The interconversion between formic acid and H(2)/CO(2) using half-sandwich rhodium and ruthenium catalysts with 4,4'-dihydroxy-2,2'-bipyridine (DHBP) was investigated. The influence of substituents of the bipyridine ligand was studied. Chemical shifts of protons in bipyridine linearly correlated with Hammett substituent constants. In the hydrogenation of CO(2) /bicarbonate to formate under basic conditions, significant activations of the catalysts were caused by the electronic effect of oxyanions generated by deprotonation of the hydroxyl group. Initial turnover frequencies of the ruthenium- and rhodium-DHBP complexes increased 65- and 8-fold, respectively, compared to the corresponding unsubstituted bipyridine complexes. In the decomposition of formic acid under acidic conditions, activity enhancement by the electronic effect of the hydroxyl group was observed for the ruthenium catalyst. The rhodium-DHBP catalyst showed high activity without CO contamination in a relatively wide pH range. Pressurized H(2) can be obtained using an autoclave reactor. The highest turnover frequency and number were obtained at 80 °C. The catalytic system provides valuable insight into the use of CO(2) as a H(2) storage material by combining CO(2) hydrogenation with formic acid decomposition.

3-Methylindole-Based Tripodal Tetraphosphine Ruthenium Complexes in N2 Coordination and Reduction and Formic Acid Dehydrogenation

“…Next, we investigated the potential of these Ru-species in the catalytic dehydrogenation of formic acid (Figure 13), given our interest in the reversible storage of H 2 into liquid fuels [27][28][29][30][31][32][33]. For Ru-catalysts, formic acid dehydrogenation is accelerated when more electron-rich ligands are employed, as was shown by Himeda et al, who used a series of bipyridine ligands with various substituents at the para position (-OH, -OMe, -Me, -CO 2 H, and -H) [34]. With the various substituted tripodal tetraphosphine ligands L1 and L2 in hand, we anticipated to observe similar effects for the corresponding complexes in the formic acid dehydrogenation.…”

Recent Advances in Transition Metal-Catalysed Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media

“…In contrast to the inactivity of this kind of complexes, Himeda et al achieved significantly higher activity with 1-4 ( Figure 1 and Table 1) by introducing two strong electron-donating groups into the bipyridine ligands. (Himeda et al, 2004(Himeda et al, , 2006(Himeda et al, , 2011 More interestingly, much higher activity was obtained with the iridium analogue (vide infra).…”

“…On the other hand, the substituent effects on the rhodium and ruthenium complexes, 1 and 2, were moderate compared to the effect on iridium complex 8 (Figure 4). (Himeda et al, 2011) It is apparent that the remarkable activation of the iridium DHBP catalyst can be attributed to the strong electron-donating ability of the oxyanion. The maximal catalytic activity (TOF = 42,000 h −1 , TON = 190,000) of the iridium DHBP catalyst was obtained at 6 MPa and 120 °C.…”

Dehydrogenation of Formic Acid over a Homogeneous Ru-TPPTS Catalyst: Unwanted CO Production and Its Successful Removal by PROX

“…Although very high catalyst selectivities have been reported in literature [7,9,11], a side-reaction of FA dehydration takes usually place, next to FA dehydrogenation, due to a still insufficient catalyst selectivity and/or thermal decomposition of FA:…”

“…Moreover, FA dehydrogenation gas mixtures contain much more CO2 (~50%) which could possibly effect the conversion of CO. Recently, FA has sparked interest again, because catalysts highly effective in FA dehydrogenation in reasonable conditions have been found [5][6][7][8][9]. This fact in combination with easy and safe handling opens a route of using FA in applications such as energy storage and transport [10].…”