Formic Acid as a Potential On-Board Hydrogen Storage Method: Development of Homogeneous Noble Metal Catalysts for Dehydrogenation Reactions

Ru-Nitrosyl Complex Salts as Efficient Catalysts for the Reversible CO2 Hydrogenation/FA Dehydrogenation in Ionic Liquids

We demonstrate that bench-stable ruthenium-PNP nitrosyl complex salts with different counteranions (Cl-, BF4 -, BPh4 -, PF6 -, and OTs-) and a ruthenium-POP nitrosyl complex are competent (pre)-catalysts for the CO2 hydrogenation to formic acid (FA) at low temperatures in ionic liquids. Only a minor effect of variation of the counteranion was observed, and weakly basic ionic liquids such as EMIM OAc, BMIM OAc, and EMIM HCO2 were suitable for this transformation, affording conversions up to 94 mol % formic acid compared to the ionic liquid (FA/IL) and turnover numbers (TONs) up to 1305. Importantly, the same catalytic system was also efficient for the dehydrogenation of formic acid back to CO2 and H2, affording conversions up to >95% (949 TON) after 3 h at 95 °C. To investigate the application of such protocols for hydrogen storage and transportation purposes, hydrogenation/dehydrogenation cycles were performed, showing that this new catalytic system can promote up to 10 reversible CO2 hydrogenation/FA dehydrogenation cycles before losing its activity.

Unveiling the Origin of pH-Dependent Catalytic Performance of Bi2O3 Nanostructure for Electrochemical CO2 Reduction

Over the past decade, the electrochemical conversion of CO2 into valuable chemicals and fuels has garnered increasing interest as a promising pathway toward a carbon-neutral circular economy. This study investigates Bi2O3 gas diffusion electrodes (Bi-GDEs) for the conversion of CO2 to formic acid/formate (HCOOH/HCOO-), which demonstrate excellent selectivity at high current densities. The catalyst is synthesized through a one-pot microwave-assisted process that is rapid, energy-efficient, and scalable, utilizing the green solvent ethylene glycol. The resulting Bi2O3 nanostructure achieves near-unit selectivity for CO2 conversion, with a faradaic efficiency exceeding 95% for HCOOH/HCOO- formation across a wide pH range. The catalytic activity is strongly pH-dependent, with an increase in the pH reducing the overpotential at a given current density. To elucidate the origin of this pH-dependent activity, operando Raman spectroscopy, post-mortem scanning electron microscopy (SEM), and electrical double layer characterization were performed. Operando Raman results reveal that Bi2O3 undergoes reduction more readily in highly acidic or basic electrolytes, whereas its reduction is inhibited near neutral pH. However, at highly negative potentials relevant to CO2RR, cationic Bi species fully convert to metallic Bi. Despite structural variations at different electrolyte pH values, metallic Bi remains the active phase, explaining the high selectivity of Bi-GDEs across a broad pH range. Post-mortem SEM images highlight the influence of electrolyte pH on morphological evolution under CO2RR conditions. At the highest pH of 14, a hierarchical dendritic structure emerges, showing an increase of 100% in double layer capacitance, which evidences the significant enhancement in the electrochemical active surface area and consequently the CO2RR activity.

Formic acid dehydrogenation catalysed by a novel amino-di(N-heterocyclic carbene) based Ru-CNC pincer complex

A new Ru(II) complex featuring a novel amino-di(N-heterocyclic carbene) CNC pincer ligand, iPrCNC-RuCl2(CO) (Ru-1), has been developed and characterised in depth. Ru-1 forms an efficient and durable catalytic formic acid dehydrogenation system in combination with the ionic liquid 1-ethyl-3-methylimidazolium diethylphosphate (EMIM PO2(OEt)2).

Formic acid dehydrogenation using Ruthenium-POP pincer complexes in ionic liquids

Formic acid is one of the most promising candidates for the long-term storage of hydrogen in liquid form. Herein, we present a new collection of ruthenium pincer complexes of the general formula [RuHCl(POP)(PPh3)] using commercially available or easy-to-synthesize tridentate xantphos-type POP pincer ligands. We applied these complexes in the dehydrogenation of formic acid to CO2 and H2 using the ionic liquid BMIM OAc (1-butyl-3-methylimidazolium acetate) as solvent under mild, reflux-free conditions. The best performing catalyst with respect to maximum turnover frequency, the literature-known complex [RuHCl(xantphos)(PPh3)] Ru-1, produced a maximum turnover frequency of 4525 h-1 with 74% conversion after 10 min at 90 °C and complete conversion (> 98%) occurring within 3 h. On the other hand, the best overall performing catalyst, the novel complex [RuHCl(iPr-dbfphos)(PPh3)] Ru-2, facilitated full conversion within 1 h leading to an overall turnover frequency of 1009 h-1. Moreover, catalytic activity was observed at temperatures as low as 60 °C. Only CO2 and H2 are observed in the gas phase, with no CO detected. High-resolution mass spectrometry suggests the presence of N-heterocyclic carbene complexes in the reaction mixture.

Catalytic dehydrogenative coupling and reversal of methanol-amines: advances and prospects

The development of efficient hydrogen release and storage processes to provide environmentally friendly hydrogen solutions for mobile energy storage systems (MESS) stands as one of the most challenging tasks in addressing the energy crisis and environmental degradation. The catalytic dehydrogenative coupling of methanol and amines (DCMA) and its reverse are featured by high capacity for hydrogen release and storage, enhanced capability to purify the produced hydrogen, avoidance of carbon emissions and singular product composition, offering the environmentally and operationally benign strategy of overcoming the challenges associated with MESS. Particularly, the cycle between these two processes within the same catalytic system eliminates the need for collecting and transporting spent fuel back to a central facility, significantly facilitating easy recharging. Despite the promising attributes of the above strategy for environmentally friendly hydrogen solutions, challenges persist, primarily due to the high thermodynamic barriers encountered in methanol dehydrogenation and amide hydrogenation. By systematically summarizing various reaction mechanisms and pathways involving Ru-, Mn-, Fe-, and Mo-based catalytic systems in the development of catalytic DCMA and its reverse and the cycling between the two, this review highlights the current research landscape, identifies gaps, and suggests directions for future investigations to overcome these challenges. Additionally, the critical importance of developing efficient catalytic systems that operate under milder conditions, thereby facilitating the practical application of DCMA in MESS, is also underscored.

Formic Acid Dehydrogenation through Ligand Design Strategy of Amidation in Half-Sandwich Ir Complexes

A series of Cp*Ir (Cp* = pentamethylcyclopentadienyl) complexes with amidated 8-aminoquinoline ligands were synthesized and tested for formic acid (FA) dehydrogenation. These complexes showed improved activities compared to pristine 8-anminquinoline (L1). Specially, amidation changed the outer coordination sphere of the complex (3) bearing N-8-quinolinylformamide (L3), and 3 was proved to be a proton-responsive catalyst. Our experimental results and DFT calculations demonstrated that the deprotonated carbanion in L3 could interact with a water molecule to stabilize the transition states and lower the reaction energy barrier, which improved the reaction activity. A turnover frequency of 206250 h-1 was achieved by 3 under optimized conditions. This study presents a method to develop new ligands and modify the existing ligands for efficient FA dehydrogenation.

On the Demise of PPP-Ligated Iron Catalysts in the Formic Acid Dehydrogenation Reaction

The PPP-ligated iron complexes, cis-(iPrPPRP)FeH2(CO) [iPrPPRP = (o-iPr2PC6H4)2PR (R = H or Me)], catalyze the dehydrogenation of formic acid to carbon dioxide but lose their catalytic activity over time. This study focuses on the analysis of the species formed from the degradation of cis-(iPrPPMeP)FeH2(CO) over its course of catalyzing the dehydrogenation reaction. These degradation products include species both soluble and insoluble in the reaction medium. The soluble component of the decomposed catalyst is a mixture of cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x], protonated iPrPPMeP, and oxidation products resulting from adventitious O2. The precipitate is solvated Fe(OCHO)2. Further mechanistic investigation suggests that cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x] displays diminished but measurable catalytic activity, likely through the displacement of a CO ligand by the formate ion. The formation of Fe(OCHO)2 along with the dissociation of iPrPPMeP is responsible for the eventual loss of catalytic activity.

Liberation of carbon monoxide from formic acid mediated by molybdenum oxyanions

Multistage mass spectrometry experiments, isotope labelling and DFT calculations were used to explore whether selective decarbonylation of formic acid could be mediated by molybdate anions [(MoO3)x(OH)]- (x = 1 and 2) via a formal catalytic cycle involving two steps. In step 1, both molybdate anions undergo gas-phase ion-molecule reactions (IMR) with formic acid to produce the coordinated formates [(MoO3)x(O2CH)]- and H2O. In step 2, both coordinated formates [(MoO3)x(O2CH)]- undergo decarbonylation under collision-induced dissociation (CID) conditions to reform the molybdate anions [(MoO3)x(OH)]- (x = 1 and 2), thus closing a formal catalytic cycle. In the case of [MoO3(O2CH)]- an additional decarboxylation channel also occurs to yield [MoO3(H)]-, which is unreactive towards formic acid. The reaction between [Mo18O3(18OH)]- and formic acid gives rise to [Mo18O3(O2CH)]- highlighting that ligand substitution occurs without 18O/16O exchange between the coordinated 18OH ligand and HC16O2H. The reaction between [(MoO3)x(OD)]- (x = 1 and 2) and DCO2H initially produces [(MoO3)x(OH)]- (x = 1 and 2), indicating that D/H exchange occurs. DFT calculations were carried out to investigate the reaction mechanisms and energetics associated with both steps of the formal catalytic cycle and to better understand the competition between decarbonylation and decarboxylation, which is crucial in developing a selective catalyst. The CO and CO2 loss channels from the monomolybdate anion [MoO3(O2CH)]- have similar barrier heights which is in agreement with experimental results where both fragmentation channels are observed. In contrast, the dimolybdate anion is more selective, since the decarbonylation pathway of [(MoO3)2(O2CH)]- is both kinetically and thermodynamically favoured, which agrees with experimental observations where the CO loss channel is solely observed.

A ligand design strategy to enhance catalyst stability for efficient formic acid dehydrogenation

New Ir complexes bearing N-(methylsulfonyl)-2-pyridinecarboxamide (C1) and N-(phenylsulfonyl)-2-pyridinecarboxamide (C2) were employed as catalysts for aqueous formic acid dehydrogenation (FADH). The ligands were designed to maintain the picolinamide skeleton and introduce strong sigma sulfonamide moieties. C1 and C2 exhibited good stability towards air and concentrated formic acid (FA). During 20 continuous cycles, C1 and C2 could achieve the complete conversion of FA with TONs of 172 916 and 172 187, respectively. C1 achieved a high TOF of 19 500 h^-1 at 90 °C and an air-stable Ir-H species was observed by ¹H NMR spectroscopy.

Versatile CO2 Hydrogenation-Dehydrogenation Catalysis with a Ru-PNP/Ionic Liquid System

High catalytic activities of Ru-PNP [Ru = ruthenium; PNP = bis alkyl- or aryl ethylphosphinoamine complexes in ionic liquids (ILs) were obtained for the reversible hydrogenation of CO₂ and dehydrogenation of formic acid (FA) under exceedingly mild conditions and without sacrificial additives. The novel catalytic system relies on the synergic combination of Ru-PNP and IL and proceeds with CO₂ hydrogenation already at 25 °C under a continuous flow of 1 bar of CO₂/H₂ (1:5), leading to 14 mol % FA with respect to the IL. A pressure of 40 bar of CO₂/H₂ (1:1) provides 126 mol % of FA/IL corresponding to a space-time yield (STY) of FA of 0.15 mol L^-1 h^-1. The conversion of CO₂ contained in imitated biogas was also achieved at 25 °C. Furthermore, the Ru-PNP/IL system catalyzes FA dehydrogenation with average turnover frequencies up to 11,000 h^-1 under heat-integrated conditions for proton-exchange membrane fuel cell applications (<100 °C). Thus, 4 mL of a 0.005 M Ru-PNP/IL system converted 14.5 L FA over 4 months with a turnover number exceeding 18,000,000 and a STY of CO₂ and H₂ of 35.7 mol L^-1 h^-1. Finally, 13 hydrogenation/dehydrogenation cycles were achieved with no sign of deactivation. These results demonstrate the potential of the Ru-PNP/IL system to serve as a FA/CO₂ battery, a H₂ releaser, and a hydrogenative CO₂ converter.

Near thermal, selective liberation of hydrogen from formic acid catalysed by copper hydride ate complexes

A near thermal two-step catalytic cycle for the selective release of hydrogen from formic acid by mononuclear cuprate anions was revealed using multistage mass spectrometry experiments, deuterium labelling and DFT calculations. In gas-phase ion-molecule reactions, mononuclear copper hydride anions [(L)Cu(H)]^- (where L = H^-, O₂CH^-, BH₄^- and CN^-) were found to react with formic acid (HCO₂H) to yield [(L)Cu(O₂CH)]^- and H₂. The copper formate anions [(L)Cu(O₂CH)]^- can decarboxylate via collision-induced dissociation (CID) to reform the copper hydride [(L)Cu(H)]^-, thereby closing the two-step catalytic cycle. Analogous labelling experiments with d₁-formic acid (DCO₂H) reveal that the decarboxylation process also occurs spontaneously. A kinetic study was carried out to provide further insights into the species involved in this reaction. Energetics from density functional theory (DFT) calculations show that the key decarboxylation step can occur without CID, thus in support of experimental observations.

Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics

As the world pledges to significantly cut carbon emissions, the demand for sustainable and clean energy has now become more important than ever. This includes both production and storage of energy carriers, a majority of which involve catalytic reactions. This article reviews recent developments of homogeneous catalysts in emerging applications of sustainable energy. The most important focus has been on hydrogen storage as several efficient homogeneous catalysts have been reported recently for (de)hydrogenative transformations promising to the hydrogen economy. Another direction that has been extensively covered in this review is that of the methanol economy. Homogeneous catalysts investigated for the production of methanol from CO2, CO, and HCOOH have been discussed in detail. Moreover, catalytic processes for the production of conventional fuels (higher alkanes such as diesel, wax) from biomass or lower alkanes have also been discussed. A section has also been dedicated to the production of ethylene glycol from CO and H2 using homogeneous catalysts. Well-defined transition metal complexes, in particular, pincer complexes, have been discussed in more detail due to their high activity and well-studied mechanisms.

H2 Production from Formic Acid Using Highly Stable Carbon-Supported Pd-Based Catalysts Derived from Soft-Biomass Residues: Effect of Heat Treatment and Functionalization of the Carbon Support

The production of hydrogen from liquid organic hydrogen carrier molecules stands up as a promising option over the conventional hydrogen storage methods. In this study, we explore the potential of formic acid as a convenient hydrogen carrier. For that, soft-biomass-derived carbon-supported Pd catalysts were synthesized by a H₃PO₄-assisted hydrothermal carbonization method. To assess the impact of the properties of the support in the catalytic performance towards the dehydrogenation of formic acid, three different strategies were employed: (i) incorporation of nitrogen functional groups; (ii) modification of the surface chemistry by performing a thermal treatment at high temperatures (i.e., 900 °C); and (iii) combination on both thermal treatment and nitrogen functionalization. It was observed that the modification of the carbon support with these strategies resulted in catalysts with enhanced performance and outstanding stability even after six consecutive reaction cycles, thus highlighting the important effect of tailoring the properties of the support.

Impact of Green Cosolvents on the Catalytic Dehydrogenation of Formic Acid: The Case of Iridium Catalysts Bearing NHC-phosphane Ligands

The catalysts [Ir(COD)(κ3-P,C,P'-PCNHCP)]BF4 and [Ir(COD)(κ2-P,C-PCNHCO)]BF4 proved to be active in the solventless dehydrogenation of formic acid. The impact of various cosolvents on the activity was evaluated, showing an outstanding improvement of the catalytic performance of [Ir(COD)(κ2-P,C-PCNHCO)]BF4] in "green" organic carbonates: namely, dimethyl carbonate (DMC) and propylene carbonate (PC). The TOF1h value for [Ir(COD)(κ2-P,C-PCNHCO)]BF4 increases from 61 to 988 h-1 upon changing from solventless conditions to a 1/1 (v/v) DMC/HCOOH mixture. However, in the case of [Ir(COD)(PCNHCP)]BF4, only a marginal improvement from 156 to 172 h-1 was observed under analogous conditions. Stoichiometric experiments allowed the identification of various key reaction intermediates, providing valuable information on their reactivity. Experimental data and DFT calculations point to the formation of dinuclear species as the catalyst deactivation pathway, which is prevented in the presence of DMC and PC.