Hydrogen Production from Formic Acid and Formaldehyde over Ruthenium Catalysts in Water

Pyrrolic Nitrogen Boosted H2 Generation from an Aqueous Solution of HCHO at Room Temperature by Metal-Free Carbon Catalysts

Hydrogen production from organic hydrides represents a promising strategy for the development of safe and sustainable technologies for H2 storage and transportation. Nonetheless, the majority of existing procedures rely on noble metal catalysts and emit greenhouse gases such as CO2/CO. Herein, we demonstrated an alternative N-doped carbon (CN) catalyst for highly efficient and robust H2 production from an aqueous solution of formaldehyde (HCHO). Importantly, this process generated formic acid as a valuable byproduct instead of CO2/CO, enabling a clean H2 generation process with 100% atom economy. Mechanism investigations revealed that the pyrrolic N in the CN catalysts played a critical role in promoting H2 generation via enhancing the transformation of O2 to generate •OO- free radicals. Consequently, the optimized CN catalysts achieved a remarkable H2 generation rate of 13.6 mmol g-1 h-1 at 30 °C. This finding is anticipated to facilitate the development of liquid H2 storage and its large-scale utilization.

Recent progress in molecular transition metal catalysts for hydrogen production from methanol and formaldehyde

Hydrogen is considered as a potential alternative and sustainable energy carrier, but its safe storage and transportation are still challenging due to its low volumetric energy density. Notably, C1-based substrates, methanol and formaldehyde, containing high hydrogen contents of 12.5 wt% and 6.7 wt%, respectively, can release hydrogen on demand in the presence of a suitable catalyst. Advantageously, both methanol and aqueous formaldehyde are liquid at room temperature, and hence can be stored and transported considerably more safely than hydrogen gas. Moreover, these C1-based substrates can be produced from biomass waste and can also be regenerated from CO2, a greenhouse gas. In this review, the recent progress in hydrogen production from methanol and formaldehyde over noble to non-noble metal complex-based molecular transition metal catalysts is extensively reviewed. This review also focuses on the critical role of the structure-activity relationship of the catalyst in the dehydrogenation pathway.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.

Diruthenium Catalyst for Hydrogen Production from Aqueous Formic Acid

Diruthenium complexes [{(η⁶-arene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ containing the bridging bis-imidazole methane-based ligand {1,4-bis(bis(2-ethyl-5-methyl-1H-imidazol-4-yl)methyl)benzene} (benztetraimd) are synthesized for catalytic formic acid dehydrogenation in water at 90 °C. Catalyst [{(η⁶-p-cymene)RuCl}₂(μ-κ²:κ²-benztetraimd)]²⁺ [1-Cl₂] exhibited a remarkably high turnover frequency (1993 h^-1 per Ru atom) and long-term stability over 60 days for formic acid dehydrogenation, while the analogous (η⁶-benzene)diruthenium and mononuclear catalysts displayed low activity with poor long-term stability. Notably, catalyst [1-Cl₂] also displayed an appreciably high turnover number of 93 200 for the bulk-scale reaction. In addition, the in-depth mass and nuclear magnetic resonance investigations under the catalytic and control experimental conditions revealed the active involvement of several crucial catalytic intermediate species, such as Ru-aqua species [{(η⁶-p-cymene)Ru(H₂O)}₂(μ-L)]²⁺ [1-(OH₂)₂], Ru-formato species [{(η⁶-p-cymene)Ru(HCOO)}₂(μ-L)] [1-(HCOO)₂], and Ru-hydrido species [{(η⁶-p-cymene)Ru(H)}₂(μ-L)] [1-(H)₂], in the catalytic formic acid dehydrogenation reaction.

Pincer Platinum(II) Hydrides: High Stability Imparted by Donor-Flexible Pyridylidene Amide Ligands and Evidence for Adduct Formation before Protonation

Donor-flexible ligands are an emerging class of noninnocent ligands. Their ability to adapt their donating strength toward a metal center has had numerous catalytic advantages yet has never been utilized to stabilize and isolate intermediate complexes within these processes. We demonstrate through the use of a pincer ligand containing two donor-flexible pyridylidene amide (PYA) arms in coordination with platinum(II) that this ligand adaptability revealed remarkably stable hydride and formate complexes. These are typically fleeting catalytic intermediates within formic acid dehydrogenation and CO₂ hydrogenation catalytic cycles. The PYA platinum hydride complexes are indefinitely stable in air, while formate complexes show no sign of β-hydrogen elimination. This robustness allowed us to investigate hydride protonation as a seemingly simple reaction, though in-depth kinetic analysis reveals a pre-equilibrium step prior to platinum hydride protonation. This initial step has been attributed to adduct formation and is slower than the protonation, and therefore a relevant aspect when designing catalytic cycles for hydrogen release and its microscopic reverse, viz., hydrogen uptake.

Hydrogen Production from Formaldehyde and Paraformaldehyde in Water under Additive-Free Conditions: Catalytic Reactions and Mechanistic Insights

Efficient catalytic systems based on arene-Ru(II) complexes bearing bis-imidazole methane-based ligands were developed to achieve additive-free hydrogen generation from formaldehyde and paraformaldehyde in water. Our findings inferred the influential role of bis-imidazole methane ligands in the observed catalytic performance of the studied catalysts. Among the screened complexes, [(η⁶-p-cymene)RuCl(L)]⁺Cl^- ([Ru]-2) (L = 4,4'-((2-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole) outperformed others to generate hydrogen gas from paraformaldehyde in water with an exceptionally high turnover number (TON) of >20,000. A detailed mechanistic pathway for hydrogen gas generation from formaldehyde has been proposed on the basis of identified several crucial catalytic intermediate species involved in the hydrogen production process.

Formic acid dehydrogenation by [Ru(η6-benzene)(L)Cl] catalysts: L = 2-methylquinolin-8-olate and quinolin-8-olate

Two Ru-arene-based catalysts are employed for the dehydrogenation of formic acid. The mechanism has also been interpreted. The catalytic activity of the complexes was also compared with previously reported ruthenium arene complexes using manometry.

Water-induced gaseous formaldehyde decomposition using ruthenium organic crystalline particles

Novel ruthenium organic crystalline particles are prepared for providing two distinctive approaches for formaldehyde decomposition: catalytic oxidation or water-induced formaldehyde decomposition.

DNA binding, antitubercular, antibacterial and anticancer studies of newly designed piano-stool ruthenium(ii) complexes

The chemotherapeutic potential of ruthenium(ii) complexes as DNA binding, antitubercular, antibacterial, and anticancer agents.

Metal-Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO2 Hydrogenation under Ambient Conditions

Interconversion between CO₂ + H₂ and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO₂ hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)]ⁿ⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H₂O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H₂O catalyzes FA dehydrogenation at 90 °C with a TOF_max of 45 900 h^-1. Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO₂ hydrogenation in the presence of base to formate under atmospheric pressure (CO₂/H₂ = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h^-1 in aqueous solution and with a TOF value of 29 h^-1 in a methanol/H₂O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO₂ hydrogenation.

Bis-Imidazole Methane Ligated Ruthenium(II) Complexes: Synthesis, Characterization, and Catalytic Activity for Hydrogen Production from Formic Acid in Water

A series of half sandwich arene-ruthenium complexes [(η⁶-arene)RuCl(κ²-L)]⁺ ([Ru]-1-[Ru]-10) containing bis-imidazole methane-based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {4,4'-((4-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2), {4,4'-((2-methoxyphenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3), {4,4'-((4-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L4), and {4,4'-((2-chlorophenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L5) are synthesized. The synthesized and purified complexes ([Ru]-1-[Ru]-10) are further employed for hydrogen production from formic acid in aqueous medium. Among the investigated complexes, [(η⁶-p-cymene)RuCl(κ²-L2)]⁺ [Ru]-2, having Ru(II) coordinated 4-methoxy phenyl substituted bis-imidazole methane ligand (L2), outperformed over others, displaying a higher catalytic turnover of 8830 and high efficiency (TOF = 1545 h^-1) with appreciably high long-term stability for formic acid dehydrogenation in water.

A CuNi Alloy-Carbon Layer Core-Shell Catalyst for Highly Efficient Conversion of Aqueous Formaldehyde to Hydrogen at Room Temperature

A copper (Cu) material is catalytically active for formaldehyde (HCHO) dehydrogenation to produce H₂, but the unsatisfactory efficiency and easy corrosion hinder its practical application. Alloying with other metals and coating a carbon layer outside are recognized as effective strategies to improve the catalytic activity and the long-term durability of nonprecious metal catalysts. Here, highly dispersed CuNi alloy-carbon layer core-shell nanoparticles (CuNi@C) have been developed as a robust catalyst for efficient H₂ generation from HCHO aqueous solution at room temperature. Under the optimized reaction conditions, the CuNi@C catalyst exhibits a H₂ evolution rate of 110.98 mmol·h^-1·g^-1, which is 1.5 and 4.9 times higher than those of Cu@C and Ni@C, respectively, which ranks top among the reported nonprecious metal catalysts for catalytic HCHO reforming at room temperature to date. Furthermore, CuNi@C also displays excellent stability toward the catalytic HCHO reforming into H₂ in tap water owing to the well-constructed carbon sheath protecting CuNi nanocrystals from oxidation in an alkaline medium. Combined with density functional theory calculations, the superior catalytic efficiency of CuNi@C for H₂ generation results from the synergistic contribution between the massive active species from HCHO decomposition on the Cu sites and the remarkable H₂ evolution activity on Ni sites. The improved performance of CuNi@C highlights the enormous potential of advancing noble-metal-free nanoalloys as cost-effective and recyclable catalysts for energy recovery from industrial HCHO wastewater.

A Process for Hydrogen Production from the Catalytic Decomposition of Formic Acid over Iridium-Palladium Nanoparticles

The present study investigates a process for the selective production of hydrogen from the catalytic decomposition of formic acid in the presence of iridium and iridium-palladium nanoparticles under various conditions. It was found that a loading of 1 wt.% of 2% palladium in the presence of 1% iridium over activated charcoal led to a 43% conversion of formic acid to hydrogen at room temperature after 4 h. Increasing the temperature to 60 °C led to further decomposition and an improvement in conversion yield to 63%. Dilution of formic acid from 0.5 to 0.2 M improved the decomposition, reaching conversion to 81%. The reported process could potentially be used in commercial applications.

Low-temperature hydrogen production from methanol over a ruthenium catalyst in water

Efficient conversion of methanol to hydrogen gas and formate with an appreciably high TOF and TON is achieved over the in situ generated ruthenium catalyst in water at low temperature.

Effect of formaldehyde properties on SnO2 clusters gas sensitivity: A DFT study

Formaldehyde (CH₂O) properties such as flash point and autoignition temperature have a great effect on the temperature range of sensitivity of sensors applied to detect CH₂O gas. Tin dioxide nanocrystal interaction with formaldehyde is investigated from room temperature to 500 °C using transition state and density functional theory. Gibbs free energy, enthalpy, and entropy of activation and reaction are evaluated as a function of temperature. The sensitivity and response time of SnO₂ clusters towards formaldehyde are evaluated. Results show that the activation energy of SnO₂ clusters with formaldehyde increases with the rise of temperature while the reaction energy decreases (in negative value) with the rise of temperature. Response time is inversely proportional to formaldehyde concentration. The highest CH₂O gas-sensitive range of SnO₂ is confined between the formaldehyde flash point at 64 °C and the autoignition temperature at 430 °C. The effect of partial oxidation and dissociation of formaldehyde is discussed.

Hydrogen Generation from Catalytic Reforming of Paraformaldehyde and Water by Polymeric Bifunctional Catalysts Comprising Ruthenium and Sulfonic Acid Units

As a clean and sustainable source of energy, hydrogen shows great potential to be the ultimate energy source in future. In this research, paraformaldehyde is used as hydrogen carrier. Several bifunctional catalysts are prepared for the hydrogen generation from paraformaldehyde. The bifunctional catalysts contain two catalytically active sites. One is a sulfonic acid group for paraformaldehyde hydrolysis, and the other is an organometallic group that catalyzes the hydrogen release from formaldehyde. Bifunctional iridium catalysts and bifunctional rhodium catalysts could only generate traces of hydrogen in the initial phase of paraformaldehyde decomposition. Only the bifunctional ruthenium catalyst shows high activity due to its bifunctional catalytically active sites, thus more than 98.0 % of the initially produced gas contains hydrogen. The initial TOF is 685 h^-1 at 363 K when the paraformaldehyde concentration is 20 wt%. A reaction mechanism is proposed for the hydrogen generation from paraformaldehyde in which formaldehyde and formic acid are intermediates Formic acid decomposition is the rate-determining step in the later phase of paraformaldehyde decomposition.