A Precious Catalyst: Rhodium-Catalyzed Formic Acid Dehydrogenation in Water

Highly-Efficient Reusable [Silica@Iminophosphine-FeII] Hybrids for Hydrogen Production via Formic Acid and Formaldehyde Dehydrogenation

The use of hybrids, developed by grafting homogeneous catalysts onto supporting materials, has already demonstrated significant potential in various catalytic processes. These systems combine the advantages of homogeneous catalysts, such as high activity and selectivity, with those of solid supports, including enhanced recyclability. Catalytic hydrogen (H2) production via dehydrogenation of C1 organic molecules targeting its use in fuel cells is a contemporary scientific issue directly connected with climate crisis. Here, Iminophospine hybrid [SiO2@benzNP] and its reduced analogue [SiO2@benzNHP] were synthesized, covalently grafted on colloidal SiO2, fully characterized (FT-IR, RAMAN, TGA, ssNMR, BET), and used for in-situ synthesis of [SiO2@benzNP-FeII] and [SiO2@benzNHP-FeII] catalytic complexes for H2 production from formic acid (HCOOH) and formaldehyde (HCHO), at 80 °C. In HCOOH, both heterogenized catalysts exhibit high selectivity, producing H2 and CO2 in a 1 : 1 ratio, without CO contamination, making them ideal for fuel cell applications. [SiO₂@benzNHP-FeII] catalyst demonstrated superior performance in both substates. In HCOOH dehydrogenation, over 82,000 turnover number (TONs) were achieved and retained its efficiency for over five cycles, without any further metal addition. In HCHO dehydrogenation, it showed excellent efficiency as well, achieving 1.3 L of pure H2 with TONs exceeding 7,000, in 3 consecutive uses. Advanced spectroscopic analysis confirmed the stability and structural integrity of the catalysts, linking the Schiff base reduction and N-H groups to enhanced activity, durability and reusability. This study demonstrates the potential of hybrid materials with non-noble metals for cost-effective and sustainable H2 production, paving the way for scalable renewable energy solutions.

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.

Boosting Effect of Sterically Protected Glucosyl Substituents in Formic Acid Dehydrogenation by Iridium(III) 2-Pyridineamidate Catalysts

[Cp*Ir(R-pica)Cl] (Cp*=pentamethylcyclopentadienyl anion, pica=2-picolineamidate) complexes bearing carbohydrate substituents on the amide nitrogen atom (R=methyl-β-D-gluco-pyranosid-2-yl, 1; methyl-3,4,6-tri-O-acetyl-β-D-glucopyranosid-2-yl, 2) were tested as catalysts for formic acid dehydrogenation in water. TOFMAX values over 12000 h-1 and 50000 h-1 were achieved at 333 K for 1 and 2, respectively, with TON values over 35000 for both catalysts. Comparison with the simpler cyclohexyl-substituted analogue (3) indicated that glucosyl-based complexes are much better performing under the same experimental conditions (TOFMAX=5144 h-1, TON=5000 at pH 2.5 for 3) owing to a lower tendency to isomerize to the less active k2-N,O isomer upon protonation. The 5-fold increase in TOFMAX observed for 2 with respect to 1 is reasonably due to an optimal steric protection by the acetyl substituent, which may prevent unproductive inner-sphere reactivity. These results showcase a powerful strategy for the inhibition of the common deactivation pathways of [Cp*Ir(R-pica)X] catalysts for FA dehydrogenation, paving the way for the development of better performing hydrogen storage systems.

Formic acid stability in different solvents by DFT calculations

CONTEXT

New technologies have been developed toward the use of green energies. The production of formic acid (FA) from carbon dioxide (CO[Formula: see text]) hydrogenation with H[Formula: see text] is a sustainable process for H[Formula: see text] storage. However, the FA adduct stabilization is thermodynamically dependent on the type of solvent and thermodynamic conditions. The results suggest a wide range of dielectric permittivity values between the dimethyl sulfoxide (DMSO) and water solvents to stabilize the FA in the absence of base. The thermodynamics analysis and the infrared and charge density difference results show that the formation of the FA complex with H[Formula: see text]O is temperature dependent and has a major influence on aqueous solvents compared to the FA adduct with amine, in good agreement with the experiment. In these conditions, the stability thermodynamic of the FA molecule may be favorable at non-organic solvents and dielectric permittivity values closer to water. Therefore, a mixture of aqueous solvents with possible ionic composition could be used to increase the thermodynamic stability of H[Formula: see text] storage in CO[Formula: see text] conversion processes.

METHODS

Using the Quantum ESPRESSO package, density functional theory (DFT) calculations were performed with periodic boundary conditions, and the electronic wave functions were expanded in plane waves. For the exchange-correlation functional, we use the vdW-DF functional with the inclusion of van der Waals (vdW) forces. Electron-ion interactions are treated by the projector augmented wave (PAW) method with pseudopotentials available in the PSlibrary repository. The wave functions and the electronic densities were expanded employing accurate cut-off energies of 6.80[Formula: see text]10[Formula: see text] and 5.44[Formula: see text]10[Formula: see text] eV, respectively. The electronic density was computed from the wave functions calculated at the [Formula: see text]-point in the first Brillouin-zone. Each structural optimization was minimized according to the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm, with force and energy convergence criteria of 25 meV[Formula: see text]Å[Formula: see text] and 1.36 meV, respectively. The electrostatic solvation effects were performed by the [Formula: see text] package with the Self-Consistent Continuum Solvation (SCCS) approach.

Formate dehydrogenase activity by a Cu(II)-based molecular catalyst and deciphering the mechanism using DFT studies

Due to the requirement to establish renewable energy sources, formic acid (FA), one of the most probable liquid organic hydrogen carriers (LOHCs), has received great attention. Catalytic formic acid dehydrogenation in an effective and environmentally friendly manner is still a challenge. The N3Q3 ligand (N3Q3 = N,N-bis(quinolin-8-ylmethyl)quinolin-8-amine) and the square pyramidal [Cu(N3Q3)Cl]Cl complex have been synthesised in this work and characterised using several techniques, such as NMR spectroscopy, mass spectrometry, EPR spectroscopy, cyclic voltammetry, X-ray diffraction and DFT calculations. This work investigates the dehydrogenation of formic acid using a molecular and homogeneous catalyst [Cu(N3Q3)Cl]Cl in the presence of HCOONa. The mononuclear copper complex exhibits catalytic activity towards the dehydrogenation of formic acid in H2O with the evolution of a 1 : 1 CO2 and H2 mixture. The activation energy of formic acid dehydrogenation was calculated to be Ea = 86 kJ mol-1, based on experiments carried out at various temperatures. The Gibbs free energy was found to be 82 kJ at 298 K for the decomposition of HCOOH. The DFT studies reveal that [Cu(N3Q3)(HCOO-)]+ undergoes an uphill process of rearrangement followed by decarboxylation to generate [Cu(N3Q3)(H-)]+. The initial uphill step for forming a transition state is the rate-determining step. The [Cu(N3Q3)(H-)]+ follows an activated state in the presence of HCOOH to liberate H2 and generate the [Cu(N3Q3)(OH2)]2+.

Efficient [Fe-Imidazole@SiO2] Nanohybrids for Catalytic H2 Production from Formic Acid

Three imidazole-based hybrid materials, coded as IGOPS, IPS and impyridine@SiO₂ nanohybrids, were prepared via the covalent immobilization of N-ligands onto a mesoporous nano-SiO₂ matrix for H₂ generation from formic acid (FA). BET and HRTEM demonstrated that the immobilization of the imidazole derivative onto SiO₂ has a significant effect on the SSA, average pore volume, and particle size distribution. In the context of FA dehydrogenation, their catalytic activity (TONs, TOFs), stability, and reusability were assessed. Additionally, the homologous homogeneous counterparts were evaluated for comparison purposes. Mapping the redox potential of solution E_h vs. SHE revealed that poly-phosphine PP₃ plays an essential role in FA dehydrogenation. On the basis of performance and stability, [Fe²⁺/IGOPS/PP₃] demonstrated superior activity compared to other heterogeneous catalysts, producing 9.82 L of gases (VH₂ + CO₂) with TONs = 31,778, albeit with low recyclability. In contrast, [Fe²⁺/IPS/PP₃] showed the highest stability, retaining considerable performance after three consecutive uses. With VH₂ + CO₂ = 7.8 L, [Fe²⁺/impyridine@SiO₂/PP₃] activity decreased, and it was no longer recyclable. However, the homogeneous equivalent of [Fe²⁺/impyridine/PP₃] was completely inactive. Raman, FT/IR, and UV/Vis spectroscopy demonstrated that the reduced recyclability of [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/impyridine@SiO₂/PP₃] nanohybrids is due to the reductive cleavage of their C-O-C bonds during catalysis. An alternative grafting procedure is proposed, applying here to the grafting of IPS, resulting in its higher stability. The accumulation of water derived from substrate's feeding causes the inhibition of catalysis. In the case of [Fe²⁺-imidazole@SiO₂] nanohybrids, simple washing and drying result in their re-activation, overcoming the water inhibition. Thus, the low-cost imidazole-based nanohybrids IGOPS and IPS are capable of forming [Fe²⁺/IGOPS/PP₃] and [Fe²⁺/IPS/PP₃] heterogeneous catalytic systems with high stability and performance for FA dehydrogenation.

Efficient solventless dehydrogenation of formic acid by a CNC-based rhodium catalyst

A CNC based rhodium complex is an efficient catalyst for formic acid dehydrogenation under solventless conditions.

DFT Probe into the Mechanism of Formic Acid Dehydrogenation Catalyzed by Cp*Co, Cp*Rh, and Cp*Ir Catalysts with 4,4'-Amino-/Alkylamino-Functionalized 2,2'-Bipyridine Ligands

The mechanistic landscape of H₂ generation from formic acid catalyzed by Cp*M(III) complexes (M = Co or Rh or Ir) with diamino-/dialkylamino-substituted 2,2'-bipyridine ligand architectures have been unveiled computationally. The calculations indicate that the β-hydride elimination process is the rate-determining step for all the investigated catalysts. The dialkylamino moieties on the 2,2'-bipyridine ligand were found to reduce the activation free energy required for the rate-limiting β-hydride elimination step and increase the hydridic nature of the Ir-hydride bond, which accounts for the experimentally observed enhanced catalytic activity. Furthermore, the protonation by H₃O⁺ ion was found to be the kinetically most favorable route than the conventional protonation by formic acid. The origin for this preference lies in the increased electrophilicity of the proton from hydronium ion which facilitates easy protonation of the metal-hydride with low activation energy barrier. The Co and Rh analogues of the chosen iridium catalyst were computationally designed and were estimated to possess a rate-determining activation barrier of 16.9 and 14.5 kcal/mol, respectively. This illustrates that these catalysts are potential candidates for FAD. The insights derived in this work might serve as a vital knowledge that could be capitalized upon for designing cost-effective catalyst for FAD in future.

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

Hydrogen can be used as an energy carrier for renewable energy to overcome the deficiency of its intrinsically intermittent supply. One of the most promising application of hydrogen energy is on-board hydrogen fuel cells. However, the lack of a safe, efficient, convenient, and low-cost storage and transportation method for hydrogen limits their application. The feasibility of mainstream hydrogen storage techniques for application in vehicles is briefly discussed in this Review. Formic acid (FA), which can reversibly be converted into hydrogen and carbon dioxide through catalysis, has significant potential for practical application. Historic developments and recent examples of homogeneous noble metal catalysts for FA dehydrogenation are covered, and the catalysts are classified based on their ligand types. The Review primarily focuses on the structure-function relationship between the ligands and their reactivity and aims to provide suggestions for designing new and efficient catalysts for H₂ generation from FA.

Recent Developments in Rh Heterogeneous Catalysts

Rh-based catalysts successfully catalyze bond making and bond breaking reactions in most cases [...]

Dehydrogenation of Formic Acid to CO2 and H2 by Manganese(I)–Complex: Theoretical Insights for Green and Sustainable Route

In this work, a detailed computational study on a recently synthetized Mn(I)-dependent complex [(tBuPNNOP)Mn(CO)2]+ is reported. This species promotes the dehydrogenation of formic acid to carbon dioxide and hydrogen. The here proposed catalytic cycle proceeds through the formation of stabilized adduct between [(tBuPNNOPtBu)Mn(CO)2]+ and formate and the progressive release of CO2 and H2, mediated by the presence of trimethylamine. In order to evaluate the influence of the environment on the catalytic activity, different solvents have been taken into account. The computed barriers and the geometrical parameters account well for the available experimental data, confirming the robustness of the complex and reproducing its good catalytic performance. Outcomes from the present investigation can stimulate further experimental works in the design of new more efficient catalysts devoted to H2 production.

Crystal structure of chlorido-(η 5-pentamethylcyclopentadienyl)-((bis-pyrazol-1-yl)methane-κ2 N,N′) rhodium(III) hexafluorophosphate. (C17H23ClN4RhF6P)

C17H23ClN4RhF6P, monoclinic, P21/c (no. 14), a = 10.6954(2) Å, b = 15.3868(3) Å, c = 12.6130(2) Å, β = 90.5590(10)°, V = 2075.60(7) Å3, Z = 4, R gt(F) = 0.0296, wR ref(F 2) = 0.0727, T = 100(2) K.