Catalytic Transfer Hydrogenation Research Articles

Ethanol as hydrogen donor: An efficient transfer hydrogenation of aldehydes, ketones, and nitroarenes with H-bonded Ru(II)-N-heterocyclic iminium complex

Isopropanol and formic acid are predominantly utilized as hydrogen sources in catalytic transfer hydrogenation reactions. However, the utilization of primary alcohols in such reactions has been sparsely documented, with only a limited number of reports available. In this article, we hereby present findings that underscore the prospective application of ecologically sustainable and economically viable ethanol, alongside a ruthenium(II) catalyst that exhibits remarkable resilience in the presence of moisture and oxygen. The air-stable Ru(II) complex (I) has been synthesized by non-covalently interacting anionic Ru(p-cymene)Cl3 and 1,3-diisopropyl-1H-perimidin-2(3H)-iminium (LH2+). The compound was subjected to experimental evaluation to conduct transfer hydrogenation reactions on aldehydes, ketones, and nitroarenes. The process entailed the utilization of ethanol, a hydrogen source that is both economically advantageous and ecologically benign. The hydrogen bond interaction existing between the Ru(p-cymene)Cl3 and LH2+ species plays a crucial in facilitating the exceptional catalytic efficiency exhibited by the Ru(II) complex (I). The reaction exhibits a wide range of substrate compatibility, enabling the hydrogenation of ketones, aldehydes, and nitroarenes. The results obtained from the control experiments have provided evidence for the formation of Ru − H species. A plausible mechanism has been hypothesized, relying on stoichiometric investigations.

Heteropolyacids promoted MOF-derived high-performance Co(H4)@C-HPW0.25 catalysts for catalytic transfer hydrogenation of vanillin in mild condition

Catalytic transfer hydrogenation (CTH) was recognized as a promising technique for converting various lignin derivatives to fuels and high-value compounds in heterogeneous systems. Herein, Co@C catalysts with diverse organic ligands, Keggin-type heteropolyacids, acid addition quantities, and metal/organic ligand molar ratios were synthesized via one-pot hydrothermal processes and carbonization methods. The optimized catalyst demonstrated highly efficient active hydrogen generation from isopropanol and the best conversion (100 %) for vanillin (VAN) hydrodeoxygenation (HDO) with 90 % 2-methoxy-4-methylphenol (MMP) yield at 180 °C for one hour under mild conditions of 0 MPa N2 (hydrogen-free circumstance), which were strongly connected to the higher specific surface area (154.45 m2/g), acidic site density (4.67 mmol/g), and stronger metal-P interactions of the Co(H4)@C-HPW0.25 catalyst. The findings in this work provided an advanced thinking for designing high-performance non-noble-metal catalysts that could be applied in the catalytic HDO transformation of various biomass derivatives into green fuel and chemicals under mild circumstances.

Exploring Hydrogen Sources in Catalytic Transfer Hydrogenation: A Review of Unsaturated Compound Reduction.

Catalytic transfer hydrogenation has emerged as a pivotal chemical process with transformative potential in various industries. This review highlights the significance of catalytic transfer hydrogenation, a reaction that facilitates the transfer of hydrogen from one molecule to another, using a distinct molecule as the hydrogen source in the presence of a catalyst. Unlike conventional direct hydrogenation, catalytic transfer hydrogenation offers numerous advantages, such as enhanced safety, cost-effective hydrogen donors, byproduct recyclability, catalyst accessibility, and the potential for catalytic asymmetric transfer hydrogenation, particularly with chiral ligands. Moreover, the diverse range of hydrogen donor molecules utilized in this reaction have been explored, shedding light on their unique properties and their impact on catalytic systems and the mechanism elucidation of some reactions. Alcohols such as methanol and isopropanol are prominent hydrogen donors, demonstrating remarkable efficacy in various reductions. Formic acid offers irreversible hydrogenation, preventing the occurrence of reverse reactions, and is extensively utilized in chiral compound synthesis. Unconventional donors such as 1,4-cyclohexadiene and glycerol have shown a good efficiency in reducing unsaturated compounds, with glycerol additionally serving as a green solvent in some transformations. The compatibility of these donors with various catalysts, substrates, and reaction conditions were all discussed. Furthermore, this paper outlines future trends which include the utilization of biomass-derived hydrogen donors, the exploration of hydrogen storage materials such as metal-organic frameworks (MOFs), catalyst development for enhanced activity and recyclability, and the utilization of eco-friendly solvents such as glycerol and ionic liquids. Innovative heating methods, diverse base materials, and continued research into catalyst-hydrogen donor interactions are aimed to shape the future of catalytic transfer hydrogenation, enhancing its selectivity and efficiency across various industries and applications.

Heterocyclic compounds: their potential application as antimicrobials of health importance and in the catalytic hydrogen transfer reaction

Heterocyclic compounds: their potential application as antimicrobials of health importance and in the catalytic hydrogen transfer reaction

Confinement effect induced Pd/UiO-66-rGO for efficient aromatic alkenes catalytic hydrogenation

A simple and adaptable synthetic method is reported here for synthesizing UiO-66-GO nanohybrid materials by modulating the growth and dispersion of UiO-66 on GO surfaces, using PdCl2 as the Pd source with thermal treatment at 120 °C without additional reduction to obtain Pd/UiO-66-rGO composite catalyst. Pd/UiO-66-rGO demonstrates excellent activity in catalytic transfer hydrogenation for C═C bond of aromatic alkenes by using HCOONH4 and water as hydrogen donor and solvent at 25 °C, with 99.6 % styrene conversion and 99.9 % selectivity to ethylbenzene. The confinement effect leads to ultra-small Pd NPs active sites evenly dispersed on Pd/UiO-66-rGO, which is the good catalytic performance of Pd/UiO-66-rGO results from. Additionally, UiO-66-rGO makes a protection effect to stabilize and prevent the Pd active sites leaching out, ensuring the cyclic stability of this catalyst.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes.

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Mechanistic Studies on the Bismuth‐Catalyzed Transfer Hydrogenation of Azoarenes

AbstractOrganobismuth‐catalyzed transfer hydrogenation has recently been disclosed as an example of low‐valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi‐catalyzed transfer hydrogenation of azoarenes using p‐trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6‐bis[N‐(tert‐butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p‐CF3−C6H4) is observed, and its thermodynamic parameters are established through variable‐temperature NMR studies. Additionally, pKa‐gated reactivity is observed, validating the proton‐coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate‐limiting transition state in which the initial N−H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen‐bonded adduct of azobenzene and 4. These studies guided the discovery of a second‐generation Bi catalyst, the rate‐limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Catalytic transfer hydrogenation of CO2 to formic acid with glycerol as the hydrogen donor over Ru-REUSY catalysts

In this work, a "hydrogen-free" strategy based on the ruthenium (Ru) modified REUSY molecular sieve catalysts was proposed, which can promote CO2 catalytic transfer hydrogenation using endogenous hydrogen induced by glycerol dehydrogenation. As the active component, Ru can not only act on the dehydrogenation reaction with glycerol, but also promote the hydrogenation reaction of CO2. Comparison experiments confirmed that under conditions of 200 °C, 36 h, and 7.92 g KHCO3, the reaction could obtain the highest yields over 10 wt.% Ru-deAl(5 h)-REUSY. The yields of formic acid and lactic acid were 19.81 % and 6.13 % respectively. In addition, when replacing the carbon source from KHCO3 to CO2 (dry ice), formic acid and lactic acid yields over 10 wt.% Ru-deAl(5 h)-REUSY catalysts were further improved, reaching 64.14 % and 17.13 %, respectively. This work demonstrated the feasibility of the "hydrogen-free" upgrade process realizing the catalytic transfer hydrogenation of CO2 into formic acid with glycerol as hydrogen donor.

Microwave-Induced Catalytic Transfer Hydrogenation in Different Solvents Toward Optically Active Hydroxy β-Lactams: Effects of Penetration Depth

In present work, the stereospecific synthesis of optically active cis hydroxy β-lactams by catalytic transfer hydrogenation under diversemicrowave-induced conditions is invstigated. The effects of the penetration depth of the solvents are found to be more crucial than solvents with high dipole moments and dielectric constants. Despite significant progress of microwave-induced reactions, no reports have examined the penetration depth of the solvents used in these processes

Conversion of xylose into D-xylitol using catalytic transfer hydrogenation with formic acid as H-donor

d-Xylitol, a biomass-derived sweetener, is increasingly used in cosmetics and pharmaceutical products. The raw material for d-xylitol production, d-xylose, is easily accessible from dissolving pulp production. d-xylitol production involves the heterogeneously catalyzed hydrogenation of d-xylose; this process is energy intensive, as the use of H2 requires high pressure and temperature. This work examined catalytic transfer hydrogenation for xylose conversion into xylitol. Formic acid (FA) was used to replace H2 as the H-donor, as it is easily available, inexpensive, may be obtained from renewable sources, and it avoids the risks associated with the use of high-pressure inflammable gas. A variety of commercially available catalysts were screened to reveal the one enabling the highest yield. The experiments were performed at 40, 80, and 140 °C, with pure xylose as a model compound. Triethylamine (Et3N) was added to ensure sufficient conversion rates. Based on the preliminary studies an experimental design was created (Design Expert®), including the two best performing catalysts Ru/Al2O3 and Ru/C, to investigate the influence of temperature and H-donor and base concentration on xylitol yield. Ru/C resulted in maximum d-xylitol yield of 73.2 % at 100 °C, FA to d-xylose ratio 5:1 and Et3N to FA ratio 0.4.

Enantioselective synthesis of the bioactive flavanones lonchocarpol A and 6-(1,1-dimethylallyl)naringenin by catalytic asymmetric transfer hydrogenation

Practical enantioselective syntheses of the natural products lonchocarpol A and 6-(1,1-dimethylallyl)naringenin as well as their optical antipodes are reported. A rhodium-catalyzed asymmetric transfer hydrogenation of readily available racemic substrates with virtually complete kinetic resolution served as the key step. Solvent-controlled, clay-catalyzed [1,3] or [3,3] shifts of prenyl aryl ethers were used to install the C-6 prenyl or 1,1-dimethylallyl substituent of the target flavanones.

Structure-activity relationships of ZrO2 crystalline phases in the catalytic transfer hydrogenation of methyl levulinate with ethanol

The depletion of fossil resources is driving the research towards renewable alternatives, like lignocellulosic biomass. Therefore, the development of efficient continuous-flow processes, allowing to achieve better productivity compared to batch processes, will play a crucial role in promoting a sustainable transition. In this context, we report on the continuous-flow, gas-phase, catalytic transfer hydrogenation (CTH) of methyl levulinate and ethanol over zirconia catalysts, in particular focusing on the effect of two different crystalline phases (i.e. monoclinic, m-ZrO2, and tetragonal, t-ZrO2) on catalyst performance. An in-depth catalyst characterisation was coupled with both computational and 1H NMR relaxation studies to assess the structure-activity relationship, providing fundamental insights into the catalytic process and future catalyst optimization. The results, indicate that the higher Lewis acidity and basicity along with the lower affinity with ethanol of m-ZrO2 with respect to t-ZrO2 are responsible for the promotion of undesired oligomerisation reactions of angelica lactones responsible for catalyst deactivation.

Highly active, ultra-low loading single-atom iron catalysts for catalytic transfer hydrogenation

Highly effective and selective noble metal-free catalysts attract significant attention. Here, a single-atom iron catalyst is fabricated by saturated adsorption of trace iron onto zeolitic imidazolate framework-8 (ZIF-8) followed by pyrolysis. Its performance toward catalytic transfer hydrogenation of furfural is comparable to state-of-the-art catalysts and up to four orders higher than other Fe catalysts. Isotopic labeling experiments demonstrate an intermolecular hydride transfer mechanism. First principles simulations, spectroscopic calculations and experiments, and kinetic correlations reveal that the synthesis creates pyrrolic Fe(II)-plN3 as the active center whose flexibility manifested by being pulled out of the plane, enabled by defects, is crucial for collocating the reagents and allowing the chemistry to proceed. The catalyst catalyzes chemoselectively several substrates and possesses a unique trait whereby the chemistry is hindered for more acidic substrates than the hydrogen donors. This work paves the way toward noble-metal free single-atom catalysts for important chemical reactions.

High Selectivity Cofactor NADH Regeneration Organic Iridium Complexes Used for High-Efficiency Chem-Enzyme Cascade Catalytic Hydrogen Transfer.

Our research demonstrated that novel pentamethylcyclopentadienyl (Cp*) iridium pyridine sulfonamide complex PySO2NPh-Ir (7) could highly specifically catalyze nicotinamide adenine dinucleotide (NAD+) into the corresponding reducing cofactor NADH in cell growth media containing various biomolecules. The structures and catalytic mechanism of 7 were studied by single-crystal X-ray, NMR, electrochemical, and kinetic methods, and the formation of iridium hydride species Ir-H was confirmed to be the plausible hydride-transfer intermediate of 7. Moreover, benefiting from its high hydrogen-transfer activity and selectivity for NADH regeneration, 7 was used as an optimal metal catalyst to establish a chem-enzyme cascade catalytic hydrogen-transfer system, which realized the high-efficiency preparation of l-glutamic acid by combining with l-glutamate dehydrogenase (GLDH).

Catalytic transfer hydrogenation of furfural via metal-organic framework-derived high entropy alloy catalysts at room pressure

Catalytic transfer hydrogenation (CTH), as an environmentally friendly selective hydrogenation method, has attracted great interest in the preparation of high value-added products from biomass-derived resources. Herein, the carbon-based high-entropy alloy (HEA) catalysts (PtCoNiCuZn@NC-X) were prepared via MOF derivatization, which exhibited good catalytic behavior for the CTH of furfural (FF) with isopropyl alcohol (IPA) as the hydrogen donor. The conversion of FF and selectivity of furfuryl alcohol (FOL) reached 100% at 100 °C without additional pressure via PtCoNiCuZn@NC-600 catalyst. Systematic characterization suggests that the abundance of strong acidic sites, high content of high-valent metals and appropriate alloy surface composition play a crucial role in determining the catalytic behavior. This work confirms the multi-sites synergistic catalysis of HEA, establishing structure-property correlations and revealing reaction pathways that can be extended to other CTH reactions in biomass upgrading processes.

Piano‐stool dinuclear ruthenium (II) complexes of pyrazine‐carboxylate/carboxamide ligands: Structural studies and catalytic transfer hydrogenation of ketones

Reactions of ligand pyrazine‐2‐carboxylic acid (HL1) with [Ru(η6‐p‐cymene)Cl2]2 precursor gave the dinuclear piano‐stool ruthenium (II) complex [{Ru(η6‐p‐cymene)Cl2}‐μ‐(L1)‐{Ru(p‐cymene)Cl}] (Ru1). Separately, reactions of N‐(quinolin‐8‐yl) pyrazine‐2‐carboxamide (HL2), 5‐methyl‐N‐(−(quinolin‐8‐yl) pyridine‐2‐carboxamide (HL3) and 5‐chloro‐N‐(quinolin‐8‐yl) pyridine‐2‐carboxamide (HL4) with [Ru(η6‐p‐cymene)Cl2]2 dimer in the presence of KPF6 afforded the cationic dinuclear complexes [{Ru(η6‐p‐cymene)Cl}2‐μ‐(L2)][PF6] (Ru2), [{Ru(η6‐p‐cymene)Cl}2‐μ‐(L3)][Ru(L3)Cl3] (Ru3) and [{Ru(η6‐p‐cymene)Cl}2‐μ‐(L4)][PF6] (Ru4). The Ru (II) complexes were analysed using FT‐IR, 1H, 13C{1H}, 31P{1H} (Ru2 and Ru4) and 19F (Ru2 and Ru4) NMR spectroscopic techniques, micro‐analyses and mass spectrometry. Molecular structures of complexes Ru1 and Ru3 were confirmed to display piano‐stool coordination nature using single‐crystal X‐ray crystallography analyses. All the complexes (Ru1–Ru4) mediated the transfer hydrogenation (TH) of a broad spectrum of ketones in isopropanol in the presence of a base and demonstrated high catalytic activities (TON of 24,000) at catalyst concentrations of 0.002 mol%. In general, the catalytic performance of these Ru (II) complexes depended on the identity of the ligands, coordination chemistry and ketone substrates.

Pt/CeCrO2−x: A multifunctional catalyst for tandem catalysis of lignocellulose hydro-liquefaction and Guerbet reaction

Catalytic transfer hydrogenation (CTH) of lignocellulose liquefaction using renewable ethanol presents a promising alternative to conventional hydrogenation techniques. However, the development of efficient and stable multifunctional catalysts remains a significant challenge. Herein, a Pt/Ce9Cr1O2−x catalyst was developed to facilitate the CTH reactions and achieve nearly complete utilization of lignocellulose. Isotope labeling experiments were conducted to elucidate the mechanism of CTH. The introduction of Cr3+ increased the density of surface oxygen vacancies, consequently enhancing the hydrodeoxygenation ability and metal anchoring. The solid solution structure promotes the electrons transfer from Cr atoms to Pt atoms, forming electron-rich Ptδ+ species that promote ethanol dehydrogenation and lignocellulose hydrodeoxygenation. Furthermore, Pt/Ce9Cr1O2−x catalyst provided optimal acid-base sites and hydroxyl groups, thereby improving ethanol upgrading efficiency and wettability.

Oxygenated groups enriched polyolefin-Zr(IV) complexes catalyzing low-temperature transfer hydrogenation of biomass-derived furfural with 2-propanol

Zr4+−O2− pairs in MOFs and macromolecule − Zr(IV) complexes have proved one of the ideal catalytic components for catalytic transfer hydrogenation of biomass-derived furfural. However, restricted by limited adjustability of Zr4+−O2− coordination environment and the complex design and creation of ligand defects, currently, there are only a few efficient low-temperature (<100 °C) catalytic systems. In this work, oxygenated groups enriched polyolefin − Zr(IV) complexes were prepared and fixed on/in mesoporous silica aerogel support. The complexes possessed a stable structure with CC bonds as the connecting skeleton of each oxygenated group, and the types and densities of oxygenated groups (COOH, SO3H, and OH) could be easily adjusted by using different monomers. In the catalytic transfer hydrogenation reaction of furfural and 2-propanol, the catalyst synthesized using dicarboxylic maleic acid as a monomer of polyolefin exhibited the best catalytic performance in low temperatures, the main product furfuryl alcohol with a high yield of 96.0 % and a high selectivity of 98.2 % was obtained at 85 °C for 8 h. In 6 cycles of reactions at 85 °C, the furfuryl alcohol yield decreased gradually from 96 % in the 1st cycle to 84.0 % in the 4th cycle and further restored to 90.9 % and 89.3 % in the 5th and 6th cycles, respectively, by refreshing the used catalyst with methanol washing, and there was almost no leaching of Zr4+ species in the reaction medium. The catalyst poisoning experiment with acidic and basic sites shielded by pyridine and boric acid, respectively, showed that the catalytic activity was more sensitive to the shielding of basic sites, and the isotope experiments revealed that two H of the product furfuryl alcohol are transferred from 2-propanol. Hereby, the oxygenated group-bearing polyolefin − Zr(IV) complexes with stable structure, variable functional groups and the corresponding various Zr4+−O2– coordination pairs, are ideal catalyst candidates for the selective transfer hydrogenation of furfural to furfuryl alcohol.

Trimetallic NiZr/CoOx catalyst for the selective hydrogenation of furfural into furfuryl alcohol

Catalytic transfer hydrogenation has attracted great interest as a safe and green method to achieve biomass-based hydrogenation. In this study, a NiZr1/CoOx-400 catalyst was fabricated by in-situ structure topological transformation of layered double hydroxides (LDHs) precursors, which exhibited excellent performance in the in-situ hydrogenation of furfural (FF) using isopropanol as the hydrogen donor, with a maximum FF conversion of 83.5% in only 2 h. According to XRD, XPS and H2-TPR analyses, there was a strong interaction between different metal species in NiZr/CoOx, which increased the surface charge heterogeneity and promoted the phase transition of metal oxides, thereby favoring the generation of oxygen vacancies. Among them, LDHs structure of NiZr1/CoOx-400 catalyst was conducive to expose oxygen vacancies, which could facilitate its interaction with FF. And the Co in the catalyst similarly promoted the adsorption of FF through the coordination of the catalyst surface with the –CHO group in FF, thus contributing to its conversion. The electron-rich Zr repelled the furan ring and prevented the excessive hydrogenation, which improved its selectivity. In addition, the FF hydrogenation followed the quasi-primary kinetics with a low apparent activation energy of 18.4 kJ/mol, which was comparable to that of noble metal catalysts.

Reactivation of Ni-TiO2 catalysts in hydrogen flow and in supercritical 2-propanol-Comparative study by electron spin resonance in situ.

Highly dispersed Ni-TiO2 catalyst has been studied in the process of preparation and under catalytic transfer hydrogenation reaction conditions in supercritical 2-propanol (250°C, 70 bar) using electron spin resonance in situ. Electron spin resonance in situ has been used to study the process of the catalyst passivation and subsequent reduction of the oxide layer in the gas flow. Reduction of the NiO layer on the surface of passivated Ni nanoparticles has been detected in supercritical 2-propanol, which is in agreement with kinetic modeling data. It has been found that the reduction of the nickel oxide layer in supercritical 2-propanol occurs at a lower temperature compared with the reduction in hydrogen flow, according to in situ electron spin resonance study.