An Update on Formic Acid Dehydrogenation by Homogeneous Catalysis

Ultrasmall Pd nanoparticles supported on a metal-organic framework DUT-67-PZDC for enhanced formic acid dehydrogenation

The highly dispersed ultrasmall palladium nanoparticles (Pd NPs) (1.7 nm) were successfully immobilized on a N-containing metal-organic framework (MOF, DUT-67-PZDC) using a co-reduction method, and it is used as an excellent catalyst for formic acid dehydrogenation (FAD). The optimized catalyst Pd/DUT-67-PZDC(10, 10 wt% Pd loading) shows 100% hydrogen (H2) selectivity and formic acid (FA) conversion at 60 °C, and the commendable initial turnover frequency (TOF) values of 2572 h-1 with the sodium formate (SF) as an additive and 1059 h-1 even without SF, which is better than most reported MOF supported Pd monometallic heterogeneous catalysts. The activation energy (Ea) of FAD is 43.2 KJ/mol, which is lower than most heterogeneous catalysts. In addition, the optimized catalyst Pd/DUT-67-PZDC(10) maintained good stability over five consecutive runs, demonstrating only minimal decline in catalytic activity. The outstanding catalytic performance could be ascribed to the synergistic corporations of the unique structure of DUT-67-PZDC carrier with hierarchical pore characteristic, the metal-support interaction (MSI) between the active Pd NPs and DUT-67-PZDC, the highly dispersed Pd NPs with ultrafine size serve as the catalytic active site, as well as the N sites on the support could act as the proton buffers. This work provides a new paradigm for the efficient H2 production of FAD by constructing highly active heterogeneous Pd-based catalysts using MOF supports.

Iridium Catalyst Immobilized on Crosslinked Polyethyleneimine for Continuous Hydrogen Production Using Formic Acid

Hydrogen is an alternative fuel that can play a critical role in achieving net zero emissions, leading to global environment sustainability. An iridium-immobilized catalyst based on polyethyleneimine (PEI) was synthesized and utilized for hydrogen production via formic acid dehydrogenation (FADH). Iridium complex is cross-linked with its ligand and PEI to form the immobilized catalyst, where the iridium content could be easily varied in the range of 1-10 %. The structure of the iridium-immobilized catalyst was confirmed using solid-state NMR, DNP NMR, and FTIR spectroscopies. The iridium-immobilized catalyst with PEI showed excellent catalytic activity for FADH, exhibiting the catalyst's highest turnover frequency (TOF) value of 73 200 h-1 and a large turnover number (TON) value of over 1 130 000. The catalyst could be used for continuous hydrogen production via FADH, exhibiting high durability for over 2 000 h with TON value of 332 889 without any degradation in catalytic activity. The obtained hydrogen gas was evaluated for power generation using a standard fuel cell, as well as achieved 5 h of stable power generation.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

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.

Recent Developments in Reversible CO2 Hydrogenation and Formic Acid Dehydrogenation over Molecular Catalysts

Carbon dioxide (CO2), a valuable feedstock, can be reutilized as a hydrogen carrier by hydrogenating CO2 to formic acid (FA) and releasing hydrogen by FA dehydrogenation in a reversible manner. Notably, FA is liquid at room temperature and can be stored and transported considerably more safely than hydrogen gas. Herein, we extensively reviewed transition-metal-based molecular catalysts explored for reversible CO2 hydrogenation and FA dehydrogenation. This Review describes different approaches explored for carbon-neutral hydrogen storage and release by applying CO2 hydrogenation to FA/formate and the subsequent release of H2 by the dehydrogenation of FA over a wide range of molecular catalysts based on noble and non-noble metals. Emphasis is also placed on the specific catalyst-to-substrate interaction by highlighting the specific role of the catalyst in the CO2 hydrogenation-FA dehydrogenation pathway.

Carbon neutral hydrogen storage and release cycles based on dual-functional roles of formamides

The development of alternative clean energy carriers is a key challenge for our society. Carbon-based hydrogen storage materials are well-suited to undergo reversible (de)hydrogenation reactions and the development of catalysts for the individual process steps is crucial. In the current state, noble metal-based catalysts still dominate this field. Here, a system for partially reversible and carbon-neutral hydrogen storage and release is reported. It is based on the dual-functional roles of formamides and uses a small molecule Fe-pincer complex as the catalyst, showing good stability and reusability with high productivity. Starting from formamides, quantitative production of CO-free hydrogen is achieved at high selectivity ( > 99.9%). This system works at modest temperatures of 90 °C, which can be easily supplied by the waste heat from e.g., proton-exchange membrane fuel cells. Employing such system, we achieve >70% H₂ evolution efficiency and >99% H₂ selectivity in 10 charge-discharge cycles, avoiding undesired carbon emission between cycles.

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.

Hydrogen Production from Formic Acid by In Situ Generated Ni/CdS Photocatalytic System under Visible Light Irradiation

Simple and practical noble-metal-free catalyzed hydrogen production from sustainable resources, such as renewable formic acid, is highly desirable. Herein, the development of an efficient photocatalytic hydrogen production from aqueous solution of formic acid using in situ generated Ni/CdS photocatalytic system was described. CdS-Cys (Cys=l-cysteine) quantum dots (QDs) acting as photocatalyst with Ni(OAc)₂ as H₂ production catalyst precursor, a 94 % yield was obtained within 5 h under visible light irradiation at 50 °C. The average rate of H₂ production reached up to 282 μmol mg^-1  h^-1 with 99.8 % H₂ selectivity. Mechanistic studies indicate cooperation of dynamic quenching and static quenching of CdS-Cys QDs by Ni(OAc)₂ . Especially, Ni⁰ , generated in the dynamic quenching, accelerated the electron transfer by acting as an electron outlet and enhancing the stability of CdS to slow down the photocorrosion distinctly, delivering efficient H₂ production with high selectivity. Our study will inspire exploration of various efficient non-noble-metal catalysts for practical H₂ production from bio-based formic acid.

Iridium-(κ2-NSi) catalyzed dehydrogenation of formic acid: effect of auxiliary ligands on the catalytic performance

The iridium(III) complexes [Ir(H)(Cl)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (2) and [Ir(H)(OTf)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (3) (NSi^tBu₂ = {4-methylpyridine-2-yloxy}ditertbutylsilyl) have been synthesized and characterized including X-ray studies of 3. A comparative study of the catalytic activity of complexes 2, 3, [Ir(H)(OTf)(κ²-NSi^tBu₂)(coe)] (4), and [Ir(H)(OTf)(κ²-NSi^tBu₂)(PCy₃)] (5) (0.1 mol%) as catalysts precursors for the solventless formic acid dehydrogenation (FADH) in the presence of Et₃N (40 mol%) at 353 K has been performed. The highest activity (TOF_{5 min} ≈ 3260 h^-1) has been obtained with 3 at 373 K. However, at that temperature the FTIR spectra show traces of CO together with the desired products (H₂ and CO₂). Thus, the best performance was achieved at 353 K (TOF_{5 min} ≈ 1210 h^-1 and no observable CO). Kinetic studies at variable temperature show that the activation energy of the 3-catalyzed FADH process is 16.76 kcal mol^-1. Kinetic isotopic effect (5 min) values of 1.6, 4.5, and 4.2 were obtained for the 3-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, at 353 K. The strong KIE found for DCOOH and DCOOD evidenced that the hydride transfer from the C-H bond of formic acid to the metal is the rate-determining step of the process.

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.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)₂-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time viaoperando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·g_cat^–1·h^–1) under ambient conditions. These performance indicators make UiO-66(COOH)₂-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)₂-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)₂-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)₂-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or ^iPrPP^HP) is used to react with FeCl₂ under a carbon monoxide (CO) atmosphere to yield trans-(^iPrPP^HP)Fe(CO)Cl₂. A subsequent reaction with NaBH₄ produces syn/anti-(^iPrPP^HP)FeH(CO)Cl or cis,anti-(^iPrPP^HP)Fe(CO)H₂, depending on the amount of NaBH₄ employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH₂OH (under H₂) or PhCO₂CH₂Ph (under Ar). It also catalyzes the dehydrogenation of PhCH₂OH to PhCHO and PhCO₂CH₂Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO₂), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H₂ while forming anti-(^iPrPP^HP)FeH(CO)(OCHO)·HCO₂H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO₂ under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Photochemical hydrogen production based on the HCOOH/CO2 cycle promoted by a pentanuclear cobalt complex

The first catalytic cycle for hydrogen production based on the photochemical two-electron reduction of carbon dioxide (CO₂) and the dehydrogenation of formic acid at ambient temperature was demonstrated using a pentanuclear cobalt complex (Co5). A series of mechanistic studies were performed to elucidate the mechanism responsible for the promotion of the photocatalytic cycle by Co5.

Sustainable catalysis with fluxional acridine-based PNP pincer complexes

Because of the widespread use of fossil fuels and the resulting global warming, development of sustainable catalytic transformations is now more important than ever to obtain our desired fuels and building materials with the least carbon footprint and waste production. Many sustainable (de)hydrogenation reactions, including CO2 reduction, H2 carrier systems, and others, have been reported using molecular pincer complexes. A specific subset of pincer complexes containing a central acridine donor with flanking CH2PR2 ligands, known as acridine-based PNP pincer complexes, exhibit special reactivities that are not imitable by other PNP pincer complexes such as pyridine-based or (R2PCH2CH2)2NH type ligands. The goal of this article is to highlight the unique reactivities of acridine-based complexes and then investigate how these reactivities allow these complexes to catalyse many sustainable reactions that traditional pincer complexes cannot catalyse. To that end, we will initially go over the synthesis and structural features of acridine complexes, such as the labile coordination of the central N donor and the observed fac-mer fluxionality. Following that, distinct reactivity patterns of acridine-based complexes including their reactivity with acids and water will be discussed. Finally, we will discuss the reaction systems that have been developed with acridine complexes thus far, including the notable selective transformations of primary alcohols to primary amines using ammonia, N-heteroaromatic synthesis from alcohols and ammonia, oxidation reactions with water with H2 liberation, development of H2 carrier systems, and others, and conclude the article with future possible directions. We hope that the systemic study presented here will aid researchers in developing further sustainable reactions based on the unique acridine-based pincer complexes.

Highly Efficient Hierarchical Porous Carbon Supported Pd-Based Catalysts for Additive-Free Dehydrogenation of Formic Acid

Formic acid (FA) is one of the most prospective hydrogen carriers for renewable energy transformation. In this context, the addition of extra-amine is always required for promoting the reactivity of FA, which is still a key challenge. Herein, we report a simple but effective strategy to synthesize Pd nanoparticles, supported on NH2-functionalized, phosphorous-doped glucose-based porous carbon (NH2-P-GC). The introduction of NH2- groups on the support acts as an immobilized amine-additive for FA dehydrogenation, while phosphorus not only serves as an electronic promoter to keep Pd in the electronic deficient state for FA dehydrogenation, but also as an enlarger of the aperture size of the carbon. As a result, the Pd/NH2-P-GC has exceptional catalytic activity, 100% H2 selectivity, CO generation that is undetectable, and good reusability for hydrogen production from FA. In the additive-free dehydrogenation of aqueous FA solution, the initial turnover frequency (TOF) can reach 5126 h−1 at room temperature, which is substantially higher than the best heterogeneous catalyst so far recorded. Overall, the system’s high activity, selectivity, stability, and simplicity in producing CO-free H2/CO2 gas from FA, without the need for any additive, makes it attractive for practical deployment.

Dehydrogenation of Formic Acid Using Iridium-NSi Species as Catalyst Precursors

Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-iloxy)diisopropylsilyl and [{Ir(κ2-NSiMe)2}2(µ-CF3SO3)2] (2) (NSiMe = (4-methylpyridin-2-iloxy)dimethylsilyl) in presence of Et3N, it has been possible to achieve the...

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.

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.

Advances in Nonprecious Metal Homogeneously Catalyzed Formic Acid Dehydrogenation

Formic acid (FA) possesses a high volumetric concentration of H2 (53 g L−1). Moreover, it can be easily prepared, stored, and transported. Therefore, FA stands out as a potential liquid organic hydrogen carrier (LOHC), which allows storage and transportation of hydrogen in a safe way. The dehydrogenation to produce H2 and CO2 competes with its dehydration to give CO and H2O. For this reason, research on selective catalytic FA dehydrogenation has gained attention in recent years. Several examples of highly active homogenous catalysts based on precious metals effective for the selective dehydrogenation of FA have been reported. Among them are the binuclear iridium-bipyridine catalysts described by Fujita and Himeda et al. (TOF = 228,000 h−1) and the cationic species [IrClCp*(2,2′-bi-2-imidazoline)]Cl (TOF = 487,500 h−1). However, examples of catalytic systems effective for the solventless dehydrogenation of FA, which is of great interest since it allows to reduce the reaction volume and avoids the use of organic solvents that could damage the fuel cell, are scarce. In this context, the development of transition metal catalysts based on cheap and easily available nonprecious metals is a subject of great interest. This work contains a summary on the state of the art of catalytic dehydrogenation of FA in homogeneous phase, together with an account of the catalytic systems based on non-precious metals so far reported.

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.

Current Developments in the Chemical Upcycling of Waste Plastics Using Alternative Energy Sources

The management of plastics waste is one of the most urgent and significant global problems now. Historically, waste plastics have been predominantly discarded, mechanically recycled, or incinerated for energy production. However, these approaches typically relied on thermal processes like conventional pyrolysis, which are energy-intensive and unsustainable. In this Minireview, some of the latest advances and future trends in the chemical upcycling of waste plastics by photocatalytic, electrolytic, and microwave-assisted pyrolysis processes are discussed as more environmentally friendly alternatives to conventional thermal reactions. We highlight how the transformation of different types of plastics waste by exploiting alternative energy sources can generate value-added products such as fuels (H₂ and other carbon-containing small molecules), chemical feedstocks, and newly functionalized polymers, which can contribute to a more sustainable and circular economy.

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.

Deciphering the Mechanistic Details of Manganese-Catalyzed Formic Acid Dehydrogenation: Insights from DFT Calculations

A comprehensive density functional theory investigation has been carried out to unravel the complete mechanistic landscape of aqueous-phase formic acid dehydrogenation (FAD) catalyzed by a pyridyl-imidazoline-based Mn(I) catalyst [Mn(PY-^NHIM)(CO)₃Br], which was recently reported by Beller and co-workers. The computed free energy profiles show that for the production of a Mn-formate intermediate [Mn(HCO₂^-)], a stepwise mechanism is both kinetically and thermodynamically favorable compared to the concerted mechanism. This stepwise mechanism involves the dissociation of a Br^- ion from a Mn-bromide complex [Mn(Br)] to create a vacant site and coordination of water solvent to this vacant site, followed by the dissociative exchange of the aqua ligand with the formate ion to form Mn(HCO₂^-). Non-covalent interaction analysis revealed that the steric hindrance at the transition state is the cardinal reason for the preference to a stepwise mechanism. The β-hydride elimination process was estimated to be the rate-determining step with a barrier of 19.0 kcal/mol. This confirms the experimental observation. The generation of a dihydrogen-bound complex was found to occur through the protonation of Mn-hydride by a hydronium ion instead of formic acid. The mechanistic details and insights presented in this work would promote future catalytic designing and exploration of earth-abundant Mn-based catalytic systems for potential applications toward FAD.

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.

Oxy-tethered Cp*Ir(III) complex as a competent catalyst for selective dehydrogenation from formic acid

A bifunctional tethered iridium catalyst containing a 1,2-diphenylethylenediamine framework was synthesised for the first time. The ethereal tether chain was easily constructed via the intramolecular oxydefluorination of a perfluorophenylsulfonyl substituent by using a modified 1,2,3,4,5-pentamethylcyclopentadienyl ligand with a hydroxyalkyl chain. The conformationally constrained structure could hamper deactivation pathways in the catalytic hydrogen generation from formic acid, leading to advanced durability and complete conversion.

Resin-supported iridium complex for low-temperature vanillin hydrogenation using formic acid in water

Biorefinery seeks to utilize biomass waste streams as a source of chemical precursors with which to feed the chemical industry. This goal seeks to replace petroleum as the main feedstock, however this task requires the development of efficient catalysts capable of transforming substances derived from biomass into useful chemical products. In this study, we demonstrate that a highly-active iridium complex can be solid-supported and used as a low-temperature catalyst for both the decomposition of formic acid (FA) to produce hydrogen, and as a hydrogenation catalyst to produce vanillyl alcohol (VA) and 2-methoxy-4-methylphenol (MMP) from vanillin (V); a lignin-derived feedstock. These hydrogenation products are promising precursors for epoxy resins and thus demonstrate an approach for their production without the need for petroleum. In contrast to other catalysts that require temperatures exceeding 100 °C, here we accomplish this at a temperature of <50 °C in water under autogenous pressure. This approach provides an avenue towards biorefinery with lower energy demands, which is central to the decentralization and broad implementation. We found that the high activity of the iridium complex transfers to the solid-support and is capable of accelerating the rate determining step; the decomposition of FA into hydrogen and carbon dioxide. The yield of both VA and MMP can be independently tuned depending on the temperature. The simplicity of this approach expands the utility of molecular metal complexes and provides new catalyst opportunities in biorefinery.

Solid-supported molecular catalysis for biorefinery. Hydrogenation using formic acid in water at low temperature.

Highly efficient additive-free dehydrogenation of neat formic acid

Formic acid (FA) is a promising hydrogen carrier which can play an instrumental role in the overall implementation of a hydrogen economy. In this regard, it is important to generate H₂ gas from neat FA without any solvent/additive, for which existing systems are scarce. Here we report the remarkable catalytic activity of a ruthenium 9H-acridine pincer complex for this process. The catalyst is unusually stable and robust in FA even at high temperatures and can catalyse neat FA dehydrogenation for over a month, with a total turnover number of 1,701,150, while also generating high H₂/CO₂ gas pressures (tested up to 100 bars). Mechanistic investigations and DFT studies are conducted to fully understand the molecular mechanism to the process. Overall, the high activity, stability, selectivity, simplicity and versatility of the system to generate a CO-free H₂/CO₂ gas stream and high pressure from neat FA makes it promising for large-scale implementation.

Synergistic Effect of Pendant N Moieties for Proton Shuttling in the Dehydrogenation of Formic Acid Catalyzed by Biomimetic IrIII Complexes

Formic acid (FA) is among the most promising hydrogen storage materials. The development of efficient catalysts for the dehydrogenation of FA via molecular-level control and precise tuning remains challenging. A series of biomimetic Ir complexes was developed for the efficient dehydrogenation of FA in an aqueous solution without base addition. A high turnover frequency of 46510 h^-1 was achieved at 90 °C in 1 m FA solution with complex 1 bearing pendant pyridine. Experimental and mechanistic studies revealed that the integrated pendant pyridine and pyrazole moieties of complex 1 could act as proton relay and facilitate proton shuttling in the outer coordination sphere. This study provides a new strategy to control proton transfer accurately and a new principle for the design of efficient catalysts for FA dehydrogenation.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.