Hydrogen Transfer Process Research Articles

New ruthenium(II) isocyanide catalysts for the transfer hydrogenation of ethyl levulinate to γ-valerolactone in C2-C6 alcohols

Transfer hydrogenation (TH) processes are receiving great attention for biomass valorization and ruthenium(II) complexes are renowned TH catalysts both on laboratory and industrial scale. Only a few homogeneous catalytic precursors are available in the literature for the TH of ethyl levulinate (EL) to γ-valerolactone (GVL). Herein, starting from simple, commercially available isocyanides, two classes of air-stable ruthenium(II) complexes were synthesized and tested as catalytic precursors. First, an optimized preparation of Ru(II) p-cymene isocyanide complexes was developed. Then, the thermally induced p-cymene/DMSO substitution gave access to unprecedented ruthenium isocyanide-DMSO complexes. All the complexes were characterized and tested in TH of EL to GVL showing promising performances, adopting 2-propanol as hydrogen donor, a low catalyst (Ru) and co-catalyst (KOH) amount, working under microwave heating for 1 h at 150 °C. The most selective systems were also successfully tested with different biomass-derived alcohols, including 2-butanol. Finally, the recycling of the best catalyst was also investigated, thus improving the efficiency of the entire process.

Electrochemical Dimerization of o-Aminophenols and Hydrogen Borrowing-like Cascade to Synthesize N-Monoalkylated Aminophenoxazinones via Paired Electrolysis.

A novel electrocatalytic dimerization of o-aminphenols and a hydrogen borrowing-like cascade for synthesizing N-monoalkylated aminophenoxazinones have been developed. This electrocatalytic reaction uses a constant current mode in an undivided cell and is free of metal catalysis, open to the air, and eco-friendly. In particular, this protocol exhibits a wide substrate range and provides versatile N-monoalkylated aminophenoxazinones in medium to good yields. The results of our mechanistic research reveal that this protocol involves a cascade of electrochemical cyclocondensation of o-aminphenols and the hydrogen transfer process via paired electrolysis.

Physically hybrid Zr(OH)4 + CuO catalyzed selective aniline oxidation: A new Ph‐N˙OH mediated mechanism

AbstractDeveloping the sustainable and cost‐effective heterogeneous catalytic system for controlling chemoselectivity holds substantial importance in fine organic chemicals. Herein we construct a unique Zr(OH)4 + CuO physically hybrid system for selective oxidation of anilines. Zr(OH)4 alone leads to azoxybenzene formation, and Zr(OH)4 + CuO shifts the reaction favorably toward nitrosobenzene. The proximity study indicates Zr(OH)4 + CuO outperforms its counterparts synthesized through methods like ball‐milling, loading, and coprecipitation, because the closer proximity exhibits stronger chemical interaction, restricting the activity of Zr‐OH hydroxyl sites. Through mechanistic experiments, in situ DRIFT‐IR and DFT calculations, a new Ph‐OH intermediate mechanism is firstly proposed. Two Ph‐OH self‐condensate to form azoxybenzene for only Zr(OH)4, whereas Zr(OH)4 + CuO could promote rapid transformation of Ph‐OH to nitrosobenzene on CuO through a hydrogen transfer process. Moreover, Zr(OH)4 + CuO displays good recyclability and robust scalability. This is the first report demonstrating the utilization of a physically hybrid catalyst to adjust the selectivity of the aniline oxidation reaction.

Base‐Catalyzed Domino Isomerization/Oxidant‐Free Dehydrogenative Annulation of Allylic Alcohols: Scope, Mechanism, and Application

AbstractA domino reaction comprising four consecutive steps based on the strategy of isomerization of allylic alcohols was developed. This base‐catalyzed protocol provided an approach for constructing polysubstituted quinolines without additional additives. A wide range of di‐ or trisubstituted γ‐aminoaryl allylic alcohols bearing alkyl or (hetero)aryl substituents were transformed to a range of structurally diverse quinolines. The utility of this transformation was demonstrated by the application in the concise synthesis of several polysubstituted quinoline derivatives, including natural products and pharmacological agents. Preliminary mechanistic experiments suggest that the isomerization of γ‐aminoaryl allylic alcohol proceeds via an intramolecular 1,3‐hydrogen shift whereas for the aromatization of the dihydroquinoline intermediate two possible pathways exist: acceptorless dehydrogenation and transfer hydrogenation.

Effect of Support Functionalization on Catalytic Direct Hydrogenation and Catalytic Transfer Hydrogenation of Muconic Acid to Adipic Acid

The liquid-phase hydrogenation of muconic acid (MA) to produce bio-adipic acid (AdA) is a prominent environmentally friendly chemical process, that can be achieved through two distinct methodologies: catalytic direct hydrogenation using molecular hydrogen (H2), or catalytic transfer hydrogenation utilizing a hydrogen donor. In this study, both approaches were explored, with formic acid (FA) selected as the hydrogen source for the latter method. Palladium-based catalysts were chosen for these processes. Metal’s nanoparticles (NPs) were supported on high-temperature heat-treated carbon nanofibers (HHT-CNFs) due to their known ability to enhance the stability of this metal catalyst. To assess the impact of support functionalization on catalyst stability, the HHT-CNFs were further functionalized with phosphorus and oxygen to obtain HHT-P and HHT-O, respectively. In the hydrogenation reaction, catalysts supported on functionalized supports exhibited higher catalytic activity and stability compared to Pd/HHT, reaching an AdA yield of about 80% in less than 2 h in batch reactor. The hydrogen-transfer process also yielded promising results, particularly with the 1%Pd/HHT-P catalyst. This work highlights the efficacy of support functionalization in improving catalyst performance, particularly when formic acid is used as a safer and more cost-effective hydrogen donor in the hydrogen-transfer process.

Superhydrophobic biochars as catalysts for efficient hydrogen transfer in N-heterocycles and nitrobenzene reactions

The heteroatom-doped carbon nanomaterials can be the active catalysts in hydrogen transfer reactions, however, carbon-catalyzed hydrogen transfer processes in the absence of additives and cocatalysts have been rarely described. Herein, functional biochar catalysts (FBCs) were successfully fabricated from wheat straw by a simple scalable two-step strategy via reconstructing the catalytic active sites in carbon precursors and subsequently controlled carbonization. The characterization of FBCs was discussed with regard to morphologic structures, chemical constitutions and performance, revealing that the high hydrophobicity is beneficial for obtaining high activity. Wherein the optimized FBC-850 catalyst exhibited high activity and durability toward the reaction of N-heterocycles and nitrobenzene in the aqueous systems due to the synergistic effects that were attributed to the external superhydrophobicity, internal capillary repulsion and hydrogen transfer property, eliminating the need for additional co-catalysts or additives. In the model reaction, the yields of quinoline and aniline were respectively achieved at 92% and 90%. Furthermore, the encapsulated quinone units as the active sites in FBC-850 formed the bipyramidal structure, enabling the hydrogen transfer process through a cyclical hydrogen exchange, which was proved from kinetic studies combined with spectral characterization and theoretical calculation. In addition, a possible reaction mechanism was proposed based on the observation of key intermediates and DFT calculations, also providing a rationale for the existing active site structures.

Liberating C-H Bond Activation: Achieving 56% Quantum Efficiency in Photocatalytic Cyclohexane Dehydrogenation.

The technology of liquid organic hydrogen carriers presents great promise for large-scale hydrogen storage. Nevertheless, the activation of inert C(sp3)-H bonds in hydrocarbon carriers poses formidable challenges, resulting in a sluggish dehydrogenation process and necessitating high operating temperatures. Here, we break the shackles of C-H bond activation under visible light irradiation by fabricating subnanometer Pt clusters on defective Ce-Zr solid solutions. We achieved an unprecedented hydrogen production rate of 2601 mmol gcat.-1 h-1 (turnover frequency >50,000 molH2 molPt-1 h-1) from cyclohexane, surpassing the most advanced thermo- and photocatalysts. By optimizing the temperature-dominated hydrogen transfer process, achievable by harnessing hitherto wasted infrared light in sunlight, an astonishing 56% apparent quantum efficiency and a 5.2% solar-to-hydrogen efficiency are attained at 353 K. Our research stands as one of the most effective photocatalytic processes to date, holding profound practical significance in the utilization of solar energy and the exploitation of alkanes.

Unraveling Enyne Bonding via Dehydrogenation–Hydrogenation Processes in On‐Surface Synthesis with Terminal Alkynes

AbstractOn‐surface reactions of terminal alkynes in ultrahigh vacuum have attracted widespread attention due to their high technological promise. However, employing different precursors and substrate materials often intricate reaction schemes appear far from being well‐understood. Thus, recent investigations of alkyne coupling on noble metal surfaces suggest non‐dehydrogenative scenarios, contradicting earlier reports. Herein, the study employs noncontact atomic force microscopy (nc‐AFM) with high spatial resolution to conclusively characterize exemplary alkyne coupling products. Contrary to initial interpretations proposing dehydrogenative homocoupling on Ag(111), bond‐resolved AFM imaging reveals the expression of enyne motifs. Based on complementary, extensive density functional theory calculations, the pertaining reaction mechanisms are explored. It is proposed that enyne formation initiates with a direct carbon–carbon coupling between two alkyne groups, followed by surface‐assisted dehydrogenation‐hydrogenation processes. Thereby consecutive steps of atomic hydrogen cleavage, surface migration and recombination to a different carbon atom enable bridging via carbon–carbon double bonding. The new results shed light on subtle, but crucial surface‐mediated hydrogen transfer processes involved in the chemical bond formation, which are suggested to be of general relevance in on‐surface synthesis.

Theoretical study of CO2 hydrogenation to methanol on modified Au/In2O3 catalysts: Effects of hydrogen spillover and activation energy prediction for hydrogen transfer

With the increasing attention in environmental issues caused by CO2 emissions, methanol conversion by CO2 hydrogenation is an effective strategy to solve this existing energy dilemma. The rationale behind hydrogen spillover on methanol synthesis is unraveled via density functional theory (DFT) calculations in this work, furthermore, the activation energy of hydrogen transfer process as affected by spillover is also summarized in a general paradigm for facilitating the understanding of hydrogenation characteristics. The results demonstrate that the spillover strategy significantly facilitates the hydrogenation reaction by supplying available hydrogen adatoms. This effect is particularly pronounced during the stage when OH is formed directly at the substrate site and combines with H to produce H2O, leading to a substantial reduction in activation energy from the initial 3.74 eV to 0.78 eV. In addition, a comprehensive predictive model for the kinetic characteristics of hydrogen spillover process is established based on the machine learning algorithm and SISSO guidance. By employing the combined approach of SISSO and neural network, we have achieved a stable prediction performance for activation energy with R2 = 0.99 and RMSE = 0.07 eV. The variable of ChgFSAu is identified as the most representative factor in describing the activation energy, demonstrating a correlation coefficient of -0.60. The extended multidimensional expression of DistAu further highlights its close connection to activation energy, achieving an RMSE value of 0.41 eV. To sum up, this work elucidates the possible thoughts of catalyst design with spillover effect and gives reference for the description screening towards the chemical reactions similar to hydrogen spillover.

Stabilization of unique Zr>4+ species in NiFe2O4 nanocrystals for unprecedented catalytic transfer hydrogenation reaction

Stabilization of transition metal species (like Zr>4+) with increasingly positive charge density at oxide surfaces is highly important for intrinsically improving catalytic performance for many uses like catalytic transfer hydrogenation (CTH) reaction, which however remains inaccessible. Here, we showcase a water-quenching strategy that stably achieves Zr>4+ species at surfaces of prototype spinel oxide NiFe2O4. Systematic investigation unveils that these Zr>4+species were precisely incorporated into surface octahedral Ni sites of NiFe2O4. The local electronic structure was thus regulated. As a consequence, there simultaneously appear electron depletion (Zr>4+) and accumulation (O2-) at the resulted sites, which play the distinct roles as Lewis acid-base sites in (i) facilitating O-H bond dissociation and CO bond activation, (ii) enhancing the interaction and bonding with substrate molecules, and (iii) reducing the energy barrier for hydrogen transfer process. The resulting catalysts exhibit superior catalytic performance and stability in the CTH reaction of biomass-derived carbonyls and representative aldehydes/ketones, especially the reduction of furfural to furfuryl alcohol with a yield of 90.7% under 120 ℃ for 3 h. This work provides new insights into optimizing catalytic activity in metal oxides by stabilizing transition metal species with unexpectedly positive charge density.

Theoretical insight of reactive oxygen species scavenging mechanism in lignin waste depolymerization products.

Apart from natural products and synthesis, phenolic compounds can be produced from the depolymerization of lignin, a major waste in biofuel and paper production. This process yields a plethora of aryl propanoid phenolic derivatives with broad biological activities, especially antioxidant properties. Due to its versatility, our study focuses on investigating the antioxidant mechanisms of several phenolic compounds obtained from renewable and abundant resources, namely, syringol (Hs), 4-allylsyringol (HAs), 4-propenylsyringol (HPns), and 4-propylsyringol (HPs). Employing the density functional theory (DFT) approach in conjunction with the QM-ORSA protocol, we aim to explore the reactivity of these compounds in neutralizing hydroperoxyl radicals in physiological and non-polar media. Kinetic and thermodynamic parameter calculations on the antioxidant activity of these compounds were also included in this study. Additionally, our research utilizes the activation strain model (ASM) for the first time to explain the reactivity of the HT and RAF mechanisms in the peroxyl radical scavenging process. It is predicted that HPs has the best rate constant in both media (1.13 × 108 M-1 s-1 and 1.75 × 108 M-1 s-1, respectively). Through ASM analysis, it is observed that the increase in the interaction energy due to the formation of intermolecular hydrogen bonds during the reaction is an important feature for accelerating the hydrogen transfer process. Furthermore, by examining the physicochemical and toxicity parameters, only Hs is not suitable for further investigation as a therapeutic agent because of potential toxicity and mutagenicity. However, overall, all compounds are considered potent HOO˙ scavengers in lipid-rich environments compared to previously studied antioxidants.

Rigorous design, techno-economic and environmental analysis of two catalytic transfer hydrogenation (CTH) processes to produce bio-based 2-methylfuran (2-MF)

The catalytic transfer hydrogenation (CTH) reaction has emerged as a safer method for converting biomass derivatives into value-added chemicals. This study aims to comprehensively evaluate two processes for converting furfural into 2-methylfuran (2-MF) using either methanol (or MeOH, Scheme 1) or isopropyl alcohol (or IPA, Scheme 2) as hydrogen carriers. Both processes were rigorously simulated using Aspen Plus. The reaction performance was modeled using kinetic models developed from literature data. In addition, appropriate strategies are first established for separating the reactor effluents of both processes, which contain multiple azeotropes, based on the thermodynamic parameters estimated through COSMO-based calculations. From the techno-economic analysis, the minimum required selling price (MRSP) of 2-MF is 5.15 USD/kg when produced with Scheme 1, and 3.63 USD/kg when produced with Scheme 2. The MRSP in Scheme 1 becomes 4.50 USD/kg by considering the benefits of the side product, a formalin solution. Compared to various literature data, the MRSPs of these two process schemes are significantly lower than that obtained from direct hydrogenation (6.87 USD/kg). If the cost of the starting materials can be reduced by 25% or more, the MRSP becomes comparable to that obtained from an improved direct hydrogenation process (2.20–3.30 USD/kg). Furthermore, the CO2 emission analysis reveals that Scheme 2 has a lower rate of indirect CO2 emission rate (0.926 Ton-CO2/Ton-2-MF) compared to Scheme 1 (1.088 Ton-CO2/Ton-2-MF). With thorough assessment, we have concluded that the production of 2-MF through the CTH reaction in Scheme 2 is superior. This process shows promise in both economic and decarbonization aspects and could have industrial significance.

Nickel-catalyzed direct stereoselective α-allylation of ketones with non-conjugated dienes

The development of efficient and sustainable methods for the construction of carbon-carbon bonds with the simultaneous stereoselective generation of vicinal stereogenic centers is a longstanding goal in organic chemistry. Low-valent nickel(0) complexes which promote α-functionalization of carbonyls leveraging its pro-nucleophilic character in conjunction with suitable olefin acceptors are scarce. We report a Ni(0)NHC catalyst which selectively converts ketones and non-conjugated dienes to synthetically highly valuable α-allylated products. The catalyst directly activates the α-hydrogen atom of the carbonyl substrate transferring it to the olefin acceptor. The transformation creates adjacent quaternary and tertiary stereogenic centers in a highly diastereoselective and enantioselective manner. Computational studies indicate the ability of the Ni(0)NHC catalyst to trigger a ligand-to-ligand hydrogen transfer process from the ketone α-hydrogen atom to the olefin substrate, setting the selectivity of the process. The shown selective functionalization of the α-C-H bond of carbonyl groups by the Ni(0)NHC catalyst opens up new opportunities to exploit sustainable 3d-metal catalysis for a stereoselective access to valuable chiral building blocks.

Study of Nitrogen Compound Migration during the Pyrolysis of Longkou Oil Shale with Thermal Bitumen as the Intermediate

Oil shale is an unconventional energy resource with high nitrogen content. In this study, XPS, GC–MS and ESI FT-ICR MS were carried out to investigate the nitrogen compound migration during pyrolysis, with thermal bitumen as the intermediate. The results showed that the yield of thermal bitumen was highest when the reaction temperature was 380 °C. In the process of pyrolysis, amines and some nitrides generate ammonia gas due to the hydrogen transfer process, or they generate low-grade amines, which subsequently dissolve in pyrolysis water due to bond breakage during the pyrolysis process. As determined by GC–MS analyses, the basic components in shale oil are mainly quinoline compounds. Benzocarbazole and dibenzocarbazole compounds, such as C1-benzocarbazoles, C2-benzocarbazoles and C3-benzocarbazoles, were detected via ESI FT-ICR MS in thermal bitumen.

High Alkaline Electrochemical Hydrogen Evolution on a Pt/GaN Heterostructure

AbstractThe construction of suitable metal/substrate interfaces that can synergistically optimize the water dissociation, hydrogen transfer and hydrogen adsorption processes has proved to be a promising approach to boost the alkaline electrochemical hydrogen evolution reaction (HER) but remains challenging. Herein, a Pt/GaN heterostructure electrocatalyst is constructed using the electrochemical deposition technique and used for alkaline electrochemical HER. The as‐synthesized Pt/GaN catalyst exhibits competitive HER performance, including low overpotentials of 24 and 441 mV to reach current densities of 10 and 1000 mA cm−2, respectively, and a superior mass activity (7.34 A mgPt−1 at the overpotential of 100 mV), which is 25.7 and 5.0 times higher than commercial Pt/C and as‐synthesized Pt/CP, respectively. First‐principle calculations suggest that the Pt/GaN heterostructure can greatly reduce the energy barrier for H2O dissociation, create a local acid‐like microenvironment through a H spillover process and optimize the value of ΔGH, leading to a remarkable acceleration of the alkaline HER kinetics. This discovery provides new insights for the development of alkaline HER catalysts.

Adsorption and transfer behaviors of hydrogen on Fe1-xS catalysts for direct coal liquefaction

Direct coal liquefaction (DCL) is an important and effective method to convert coal into chemicals and fuel oil. Pyrite catalysts play an important role in improving the hydrogenation and liquefaction efficiency of coal. There have been many studies on its active component Fe1-xS in promoting pyrolysis of coal and producing active hydrogen, but studies about the hydrogen migration on Fe1-xS catalyst and the interaction between hydrogen and Fe1-xS catalyst were rarely reported. It is significant to study the hydrogen transfer behaviors on Fe1-xS in DCL and understand the influence of Fe1-xS on the hydrogen migration process. In this study, the chemical adsorption and desorption of hydrogen on Fe1-xS were firstly proved experimentally. After pre-sulfidation of Fe1-xS, the maximum amount of H chemically adsorbed in Fe1-xS is 8.20 mmol/g at 360 °C and 1.5 h. The dehydrogenation of Fe1-xS in an anthracene (ANT) system and re-hydrogenation of that in the hydrogen supply systems, such as H2 and 1,2,3,4-tetrahydronaphthalene (THN), were explored at different experiment conditions, which gave some quantitative results of hydrogen transfer. Particularly, the dehydrogenation of Fe1-xS in the ANT system was analyzed in thermodynamics. The enthalpy of dehydrogenation of Fe1-xS in the ANT system is 117.6 kJ mol−1. The results break the traditional view about Fe1-xS only as a medium for active hydrogen transfer and provides a new perspective for further understanding the hydrogen transfer process.

Asymmetric Hydrogenation of Imines Using NHC‐Phosphine Iridium Complexes

Abstractα‐Chiral amines represent a frequently observed functional group in pharmaceuticals. Here, the synthesis of such motifs (up to 91% ee) is described by asymmetric hydrogenation of imines catalyzed by NHC,P‐iridium complexes. The hydrogenation proceeded smoothly, even under balloon pressure of hydrogen atmosphere. Mechanistic experiments indicated that the reduction most likely advances by a combination of direct hydrogenation and a hydrogen transfer process using either H2 or iPrOH as the reductant, respectively.

Photocatalytic Transfer Hydrogenation Reactions Using Water as the Proton Source

Transfer hydrogenation using liquid hydrogen carriers as the direct proton sources under mild conditions has received extensive attention in the research area of organic synthesis. The emerging photocatalytic water-donating transfer hydrogenation (PWDTH) is a promising alternative over the conventional hydrogenation technology due to the advantages of being eco-friendly. This paper focuses on the recent advances in the rising and rapidly developing field of PWDTH reactions, devoted to elucidating the mechanism of the hydrogen transfer process and rationalizing the design principles of efficient photocatalysts. Finally, the current challenges and future opportunities are described.

Facilely adjusting acidity features of zeolites by steaming treatment for enhanced methanol conversion to aromatics: A mechanism study

In this contribution, the effects of the steaming treatment on the acidity and aluminum structure of ZSM-5 were systematically investigated by using different types of zeolite catalysts. The relationships between the acid properties and the catalytic performance of the methanol-to-aromatics (MTA) reaction were also elucidated. The results revealed that different kinds of acid sites showed varied stability during the steaming treatment. Especially for Brønsted acid sites (BAS), the reduction of the acid site was highly related to the stability of different distributed framework Al sites (Alsingle > Alpairs) and the existence of non-framework species (“neutralization” by the EFAL and ZnO). Additionally, the presence of the extra-framework aluminum (EFAl) and metal compensation ions could enhance the stability of the zeolite framework Si-O(H)-Al. Besides, it was found that the reduction degree in the content of the Zn-LAS (LAS = Lewis acid sites) was less than the Al-LAS. The effects of steaming treatment on acid properties over zeolites with different particle sizes were also explored. Furthermore, the catalytic performance for the MTA reaction showed that the stability of the ZSM-5 catalyst was significantly enhanced after the steaming treatment. Moreover, the reduction in the proportion of the Alpairs sites may inhibit the hydrogen-transfer aromatization process of alkenes, while the increase of the Alsingle species enhanced the cracking process of higher olefins, leading to the reduction of aromatics selectivity and the increase of light olefins selectivity, respectively.

DBU‐Catalyzed Selective Isomerization of Unsaturated Alcohols to the Synthesis of Diversified Ketones

AbstractA convenient and straightforward DBU‐catalyzed selective isomerization of unsaturated alcohols for the construction of ketone scaffolds through a sequential deprotonation, hydrogen transfer and protonation process has been described. Control experiments demonstrate that the isomerization of propargyl alcohol can not be attributed to the Meyer‐Schuster rearrangement mechanism. In addition, substrates containing both allylic alcohol and propargyl alcohol structures can be selectively isomerized to afford the dibenzylideneacetones using this method.