Hydrogenation Reactions Research Articles

A Stable Site‐Isolated Mono(phosphine)‐Rhodium Catalyst on a Metal‐Organic Layer for Highly Efficient Hydrogenation Reactions

AbstractPhosphine‐ligated transition metal complexes play a pivotal role in modern catalysis, but our understanding of the impact of ligand counts on the catalysis performance of the metal center is limited. Here we report the synthesis of a low‐coordinate mono(phosphine)‐Rh catalyst on a metal‐organic layer (MOL), P‐MOL • Rh, and its applications in the hydrogenation of mono‐, di‐, and tri‐substituted alkenes as well as aryl nitriles with turnover numbers (TONs) of up to 390000. Mechanistic investigations and density functional theory calculations revealed the lowering of reaction energy barriers by the low steric hindrance of site‐isolated mono(phosphine)‐Rh sites on the MOL to provide superior catalytic activity over homogeneous Rh catalysts. The MOL also prevents catalyst deactivation to enable recycle and reuse of P‐MOL • Rh in catalytic hydrogenation reactions.

A Stable Site-Isolated Mono(phosphine)-Rhodium Catalyst on a Metal-Organic Layer for Highly Efficient Hydrogenation Reactions.

Phosphine-ligated transition metal complexes play a pivotal role in modern catalysis, but our understanding of the impact of ligand counts on the catalysis performance of the metal center is limited. Here we report the synthesis of a low-coordinate mono(phosphine)-Rh catalyst on a metal-organic layer (MOL), P-MOL • Rh, and its applications in the hydrogenation of mono-, di-, and tri-substituted alkenes as well as aryl nitriles with turnover numbers (TONs) of up to 390000. Mechanistic investigations and density functional theory calculations revealed the lowering of reaction energy barriers by the low steric hindrance of site-isolated mono(phosphine)-Rh sites on the MOL to provide superior catalytic activity over homogeneous Rh catalysts. The MOL also prevents catalyst deactivation to enable recycle and reuse of P-MOL • Rh in catalytic hydrogenation reactions.

Enhanced CO2 electroreduction to C2+ production on asymmetric Zn-O-Cu sites via tuning of *CO intermediate adsorption

The electrochemical CO2 reduction reaction conducted presents a promising strategy to facilitate the artificial carbon cycle. Unfortunately, the efficiency of eCO2RR-to-C2+ remains below the level required for large-scale implementation due to complex multi-electron transfer and sluggish carbon-carbon coupling. Herein, we constructed asymmetric Zn-O-Cu sites on 2.12 %Zn/CuOx, which achieving a maximum C2+ product FE of 78.77 ± 1.90 % and a high current density of 408.3 mA cm−2. Experimental and theoretical studies reveal that the O-bridged asymmetric Zn-O-Cu sites exhibit enhanced electron transfer, which plays a pivotal role in improving the coverage of *CO and adjusting the adsorption strength of the *CO. The optimal adsorption capacity of the *CO on 2.12 %Zn/CuOx facilitated the subsequent hydrogenation reaction to enhance the conversion of *CO to *COH. Consequently, the asymmetric Zn-O-Cu sites proved to be more thermodynamically favorable for the asymmetric coupling between *CO and *COH, which is conducive to the production of C2+ products.

On the Correlation between Group III‐A Elements Doping and Structure Performance of Cu/ZnO/ZrO2 Catalysts System for CO2 Hydrogenation to Methanol

AbstractA set of Cu/ZnO/ZrO2 catalysts doped with Group‐IIIA elements (M=B, Al, Ga & In) were synthesized via a facile single‐step evaporation‐induced self‐assembly (EISA) method to tune up the catalyst basicity and modulate the structure to improve the methanol yield in CO2 hydrogenation reaction. To understand the catalyst‘s textural properties and catalytic activity, prepared catalysts were exposed to several in‐situ/ex‐situ characterization techniques like Physisorption & Chemisorption studies, XRD, XPS, TEM, and in‐situ DRIFT. The addition of group IIIA elements has a significant impact on the CO2 conversion and Methanol selectivity via tailoring the important textural properties such as metallic surface area of Cu, reducibility of catalysts, particle size, controlled oxygen vacancy, and basicity of catalyst surface. CZZ doped with Al appeared to be the best catalyst, in this study. The modified Cu‐ZnO interface via density functional theory (DFT) calculations also indicated that the CO2 adsorption energy is found to be highest for CZZAl, which is concomitant with CO2‐TPD analysis results. The lowest to highest CO2 adsorption energy order over the catalyst set follows CZZIn<CZZB<CZZGa<CZZO<CZZAl. Moreover, the surface model of Cu/ZnO after doping each element was also studied to elucidate the elemental promotional effect on CO2 adsorption.

Dearomative Functionalization of Activated Quinolines: Transfer Hydrogenation/Cycloaddition Cascade to Construct α‐Tertiary Amines

Cascade dearomative functionalization is a robust protocol to convert flat arenes into medicinally relevant three‐dimensional architectures with added new functionality. Herein, a dearomative cycloaddition protocol for synthesizing tetrahydroquinoline‐embedded α‐tertiary amine scaffolds has been developed employing quinolinium salts and sulfonyl azides under metal‐free conditions. An underexplored and mechanistically distinct pathway is unveiled, creating quaternary‐center‐bearing amine skeletons by an amine group migration during the transfer hydrogenation and cycloaddition cascade reaction. This approach provided a broad substrate scope of α‐tertiary amine scaffolds from a plethora of C3‐substituted quinolinium and sulfonyl azides. The post‐synthetic modifications have further diversified the α‐tertiary amine core into interesting scaffolds. Preliminary mechanistic studies suggested the involvement of aziridine ring formation for the amine migration to the C‐3 position of quinoline to generate the α‐tertiary amine core.

Silicalite‐1 encapsulated Cu‐based catalyst for selective hydrogenation of maleic anhydride to γ‐Butyrolactone

γ‐Butyrolactone (GBL) is an important product of the hydrogenation reaction of maleic anhydride (MA). In this study, Cu@Silicalite‐1 (Cu@S1) catalyst with 20% Cu loading was synthesized using a combination of impregnation & solid‐phase synthesis. The effect of Cu and Silicalite‐1 zeolite binding methods on the MA hydrogenation was investigated methodically. MA conversion reached 96.2% and GBL selectivity reached 27.6% at 240 °C, with maintained catalytic activity after propionic acid corrosion for 150 h on the Cu@S1 catalyst. S1 zeolite as a critical factor for significant effect on MA selective hydrogenation raction over Cu‐based catalysts, it was conducive to improving GBL selectivity and reducing the generation of propionic acid (PA) by‐products. The encapsulation of Cu particles by S1 zeolite promoted metal dispersion, increased the hydrogenation activity and promoted the adsorption of reactants, which provided a reference for improving the acid and water resistance of the catalysts

Hydrogenation of inert aromatic compounds under mild conditions catalyzed by confined trimetal sites in porous metal silicate materials

Complete hydrogenation of aromatic compounds is an important subject in various industry fields. However, due to the high stabilization energy of aromatic compounds resulting from their high aromaticity and non-polarity, the hydrogenation reaction is usually performed under harsh reaction conditions in the presence of noble metal-based catalysts. We report herein a porous metal silicate (PMS) material PMS-58, which consists of Cu, Ce and Co trimetal active sites, demonstrating high catalytic activity and selectivity in hydrogenating of a series of aromatic compounds under mild conditions (60 °C and 0.5 MPa H2). PMS-58 is superior to the previously reported non-noble metal-based catalysts and could be favorably compared to noble metal-based catalysts. The unique catalytic properties of PMS-58 are derived from the synergistic work of trimetal sites with Ce species serving as strong Lewis acid sites for dissociation of dihydrogen, Co species stabilizing Cu(I) species, and Cu(I) species playing a crucial role in activating aromatic rings. Therefore, this work provides a new approach in developing highly efficient non-noble metal catalysts for complete hydrogenation of arenes under mild conditions, which could be applied in removal of aromatic compounds in petroleum products.

Regulating Local Electron Density of Cyano Sites in Graphitic Nitride Carbon by Giant Internal Electric Field for Efficient CO2 Photoreduction to Hydrocarbons.

Selective photocatalytic CO2 reduction to high-value hydrocarbons using graphitic carbon nitride (g-C3N4) polymer holds great practical significance. Herein, the cyano-functionalized g-C3N4 (CN-g-C3N4) with a high local electron density site is successfully constructed for selective CO2 photoreduction to CH4 and C2H4. Wherein the potent electron-withdrawing cyano group induces a giant internal electric field in CN-g-C3N4, significantly boosting the directional migration of photogenerated electrons and concentrating them nearby. Thereby, a high local electron density site around its cyano group is created. Moreover, this structure can also effectively promote the adsorption and activation of CO2 while firmly anchoring *CO intermediates, facilitating their subsequent hydrogenation and coupling reactions. Consequently, using H2O as a reducing agent, CN-g-C3N4 achieves efficient and selective photocatalytic CO2 reduction to CH4 and C2H4 activity, with maximum rates of 6.64 and 1.35µmol g-1h-1, respectively, 69.3 and 53.8 times higher than bulk g-C3N4 and g-C3N4 nanosheets. In short, this work illustrates the importance of constructing a reduction site with high local electron density for efficient and selective CO2 photoreduction to hydrocarbons.

Pd0–Ov–Ce3+ Interfacial Sites with Charge Redistribution for Enhanced Hydrogenation of Methyl Oleate to Methyl Stearate

Improving the efficiency of metal/reducible metal oxide interfacial sites for hydrogenation reactions of unsaturated groups (e.g., C=C and C=O) is a promising yet challenging endeavor. In our study, we developed a Pd/CeO2 catalyst by enhancing the oxygen vacancy (OV) concentration in CeO2 through high-temperature treatment. This process led to the formation of an interface structure ideal for supporting the hydrogenation of methyl oleate to methyl stearate. Specifically, metal Pd0 atoms bonded to the OV in defective CeO2 formed Pd0–OV–Ce3+ interfacial sites, enabling strong electron transfer from CeO2 to Pd. The interfacial sites exhibit a synergistic adsorption effect on the reaction substrate. Pd0 sites promote the adsorption and activation of C=C bonds, while OV preferably adsorbs C=O bonds, mitigating competition with C=C bonds for Pd0 adsorption sites. This synergy ensures rapid C=C bond activation and accelerates the attack of active H* species on the semi-hydrogenated intermediate. As a result, our Pd/CeO2-500 catalyst, enriched with Pd0–OV–Ce3+ interfacial sites, demonstrated excellent hydrogenation activity at just 30 °C. The catalyst achieved a Cis–C18:1 conversion rate of 99.8% and a methyl stearate formation rate of 5.7 mol/(h·gmetal). This work revealed the interfacial sites for enhanced hydrogenation reactions and provided ideas for designing highly active hydrogenation catalysts.

Bimetallic NiSn supported on mesoporous carbon as an efficient catalyst for selective methanol synthesis from CO2

Global warming and climate change represent the most significant environmental challenges of the 21st century, primarily attributed to the rise in CO2 emission in the atmosphere. Efforts to mitigate this increase involve exploring various methods to reduce CO2 levels, with chemical reactions, such as hydrogenation, being a prominent approach. However, the stable and inert nature of CO2 necessitates the use of catalysts to facilitate its conversion. In this research, NiSn nanoparticles with varying atomic ratios were deposited onto a mesoporous carbon support and subsequently utilized as catalysts for CO2 conversion through hydrogenation reactions at ambient pressure. The diffraction patterns of NiSn/MC reveal peaks indicating the presence of a graphitic carbon structure and the existence of a nickel-tin alloy. SEM-EDX mapping and TEM characterization demonstrate the uniform dispersion of NiSn particles on the mesoporous carbon surface, without the formation of agglomerated particles. Catalytic hydrogenation reactions indicate that the atomic ratio of Ni:Sn significantly influences the catalyst activity and selectivity for methanol formation. Among the NixSny/MC catalysts and monometallic Ni/MC, Sn/MC, and NiSn NPs without support, Ni5Sn1/MC demonstrated the highest CO2 conversion of 39.9 %. Additionally, at a reaction temperature of 175 °C and a CO2:H2 gas ratio of 1:7, Ni5Sn1/MC exhibited a methanol yield of 86.31 mmol/gcat, outperforming other catalysts in the study.

Anchoring PdOx clusters on defective alumina for improved catalytic methane oxidation

Evolution of the Pd active centers in size and spatial distribution leads to an irreversible deactivation in many high-temperature catalytic processes. This research demonstrates the use of a defective alumina (Al2O3-x) as catalyst support to anchor Pd atoms and suppress the growth of Pd clusters in catalytic methane oxidation. A combination of operando spectroscopy and density functional theory (DFT) calculations provide insights into the evolution of Pd species and reveals distinct catalytic methane oxidation mechanisms on Pd single atoms, clusters, and nanoparticles (NPs). Among these Pd species, the cluster active centers are found to be the most favorable participants in methane oxidation due to their high dispersion, high content of Pd2+ oxidation state, and resistance to deactivation by carbonates, bicarbonates, and water. The Pd/Al2O3-x catalyst shows increased stability with respect to a Pd/Al2O3 counterpart during simulated aging in alternating reducing and oxidizing conditions due to stronger interactions with the support. This study demonstrates that defect engineering of non-reducible supports can constrain the evolution of active centers, which holds promising potential for widespread utilization across diverse industrial catalytic processes, including various hydrogenation and oxidation reactions.

Acetylene Hydrogenation Processes Studied Using CFD in a Packed Bed Reactor

AbstractAcetylene hydrogenation reactions (AHR) are typically carried out in fixed bed reactors (FBRs) with high diameter‐to‐particle diameter (D/d). In this study, first, a real catalyst bed was built using the discrete element method, and the suitability of the bed was assessed using empirical equations. Second, a discrete element method‐computational fluid dynamics (DEM‐CFD) model was developed using CFD in conjunction with the reaction mechanism to simulate the AHR in the FBR and was compared with the experimental results. The results show that the DEM‐CFD can well predict the 3D interactions of velocity, temperature, and species for different D/d. The results can provide favorable theoretical guidance for the optimization of FBR and process intensification.

Mechanistic insights on the preparation of 5-methyl-2-hexanone by hydrogenation of 5-methyl-3-hexen-2-one using Pd/Al2O3 catalysts

5-methyl-2-hexanone is used as a versatile polymerization solvent for industrial preparation processes of bulk and fine chemicals. An efficient catalyst, Pd/γ-Al2O3, is reported for the preparation of 5-methyl-2-hexanone by selective hydrogenation of 5-methyl-3-hexen-2-one. The catalyst exhibits remarkable activity and selectivity even at atmospheric pressure and low temperature (1 atm, 80 °C). The influence weight of reaction conditions on the reaction process was obtained through the Artificial Neural Network model, which were reaction pressure, reaction temperature and liquid hourly space velocity in order. The reaction kinetics and mechanism of 5-methyl-2-hexanone preparation by hydrogenation over Pd/γ-Al2O3 catalyst were investigated. The hydrogenation reaction pathway of 5-methyl-3-hexen-2-one was obtained by using Density functional theory calculations, and the mechanism of selective hydrogenation of CC double bonds and CO double bonds was revealed. A kinetic model based on the LHHW model assumption was also proposed and compared with experimental results demonstrating good predictability.

Trends in the hydricities of iron, cobalt, and nickel complexes and the metal-hydride reactivities with CO2

Catalytic CO2 hydrogenation is considered one of the most efficient strategies for CO2 reduction, provided that we have efficient and selective catalysts. This paper explores a mass-spectrometric approach to rapidly compare the reactivities of 21 metal complexes (iron, cobalt, and nickel complexes with three bidentate N,N′-ligands and four bidentate P,P′-ligands) by energy-resolved MS/MS experiments. The experiments show relative hydricities of these complexes, singling out iron complexes with N,N′-ligands and cobalt complexes with P,P′-ligands as particularly reactive for hydride-donor reactions. Comparing these results with relative affinities of metal hydrides for CO2 led to the conclusion that iron hydrides should be particularly suited for CO2 hydrogenation reactions. The results, however, cannot account for the effects of the polar reaction environment in the condensed phase.

Chemo-, regio- and enantioselective hydroformylation of trisubstituted cyclopropenes: access to chiral quaternary cyclopropanes

Catalytic asymmetric synthesis of polysubstituted chiral cyclopropane presents a significant challenge in organic synthesis due to the difficulty in enantioselective control. Here we report a rhodium-catalyzed highly chemo-, regio- and enantioselective hydroformylation of trisubstituted cyclopropenes affording chiral quaternary cyclopropanes. Importantly, the easy made sterically bulky ligand L1 can effectively suppress hydrogenation and decomposition reactions and give quaternary cyclopropanes with high regio- and enantioselectivities for both aryl and alkyl functionalized substrates. Control experiments and computational studies reveal the sterically hindered well-defined chiral pocket instead of the substrates bearing electron-withdrawing diester groups is important for controlling the enantioselectivity and regioselectivity. Scale-up reaction and follow-up diverse transformations are also presented. Density Functional theory (DFT) computations suggest that the regio- and enantio-selectivities originate from the cyclopropene insertion to the Rh-H bond. The high regioselectivity is found to benefit from the presence of more efficient noncovalent interactions (NCIs) manifesting in the form of C–H···Cl, C–H···N, and l.p(Cl)···π contacts.

Dynamics of Elementary Steps on Metal Surfaces at High Coverages: The Prevalence and Kinetic Competence of Contiguous Bare-Atom Ensembles.

The rate of elementary steps on densely-covered surfaces depends sensitively on repulsive interactions within dense adlayers, situations ubiquitous in practice and with kinetic consequences seldom captured by Langmuirian treatments of surface catalysis. This study develops an ensemble-based method that assesses how such repulsion influences the prevalence and kinetic competence of bare-atom ensembles of different size. Chemisorbed CO (CO*) is used as an example because it forms dense adlayers on metal nanoparticles during CO2 hydrogenation (CO2-H2) and other reactions, leading to significant repulsion that weakens the binding of CO* and kinetically-relevant transition states (TS). This approach is enabled by density functional theory and probability formalisms and describes the prevalence of ensembles of contiguous bare atoms from their formation energy (via CO* desorption); it then determines their competence in stabilizing the TS and mediating the reaction rates. The specific conclusions reflect the extent to which a given TS and CO* desorbed to form bare ensembles "sense" repulsion and the contribution of each ensemble size to each reaction channel mediated by distinct TS structures. These formalisms are illustrated by assessing the relative contributions, kinetic relevance, and ensemble size requirements for two CO2-H2 routes (direct and H-assisted CO2 activation to CO and H2O) on Ru nanoparticles, but they are not restricted to specific bound species or reaction channels. This method is essential to assess the kinetic relevance of elementary steps in a given catalytic sequence and to determine the contributions from parallel reaction channels at the crowded surfaces that prevail in the practice of surface catalysis.

How Well Can Quantum Embedding Method Predict the Reaction Profiles for Hydrogenation of Small Li Clusters?

Quantum computing leverages the principles of quantum mechanics in novel ways to tackle complex chemistry problems that cannot be accurately addressed using traditional quantum chemistry methods. However, the high computational cost and available number of physical qubits with high fidelity limit its application to small chemical systems. This work employed a quantum-classical framework which features a quantum active space-embedding approach to perform simulations of chemical reactions that require up to 14 qubits. This framework was applied to prototypical example metal hydrogenation reactions: the coupling between hydrogen and Li2, Li3, and Li4 clusters. Particular attention was paid to the computation of barriers and reaction energies. The predicted reaction profiles compare well with advanced classical quantum chemistry methods, demonstrating the potential of the quantum embedding algorithm to map out reaction profiles of realistic gas-phase chemical reactions to ascertain qualitative energetic trends. Additionally, the predicted potential energy curves provide a benchmark to compare against both current and future quantum embedding approaches.

Covalency-Driven Differences in the Hydrogenation Chemistry of Lanthanide- and Actinide-Based Frustrated Lewis Pairs.

The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. Consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.

Enabling long-distance hydrogen spillover in nonreducible metal-organic frameworks for catalytic reaction

Hydrogen spillover is an extraordinary effect in heterogeneous catalysis and hydrogen storage, which refers to the surface migration of metal particle-activated hydrogen atoms over the solid supports. Historical studies on this phenomenon have mostly been limited to reducible metal oxides where the long-distance proton-electron coupled migration mechanism has been established, yet the key question remains on how to surmount short-distance and defect-dependent hydrogen migration on nonreducible supports. By demerging hydrogen migration and hydrogenation reaction, here we demonstrate that the hydrogen spillover in nonreducible metal-organic frameworks (MOFs) can be finely modulated by the ligand functional groups or embedded water molecules, enabling significant long-distance (exceed 50 nm) movement of activated hydrogen. Furthermore, using sandwich nanostructured MOFs@Pt@MOFs as catalysts, we achieve highly selective hydrogenation of N-heteroarenes via controllable hydrogen spillover from Pt to MOFs-shell. We anticipate that this work will enhance the understanding of hydrogen spillover and shed light on de novo design of MOFs supported catalysts for many important reactions involving hydrogen.

Silica‐Grafted Isonicotinamide to Boost Catalytic Activity of Cobalt Nanoparticles in Hydrogenation and Reductive Amination Reactions

AbstractThe functionalization of silica supports with isonicotinamide (NIC) facilitated the stabilization of small (3.5 nm), spherical cobalt nanoparticles (CoNPs) used in catalytic hydrogenation. Various silica matrices, including amorphous, mesoporous, and non‐porous ones, were synthesized and post‐grafted with NIC to serve as ligands for stabilizing CoNPs. Comprehensive characterization techniques were employed to fully analyze the prepared materials, which were then utilized in the catalytic hydrogenation of acetophenone, serving as a benchmark reaction to assess the impact of different silica supports on catalysis. The porosity and surface nature of the silica matrices played crucial roles in anchoring the amide ligand, and the presence of the ligand was essential for stabilizing the nanoparticles. Among the catalysts tested, CoNPs@SBA‐15/NIC exhibited the highest catalytic activity and demonstrated excellent performance in the hydrogenation of alkenes, nitriles, ketones, and quinoline under relatively mild conditions. No significant alteration in catalyst morphology or leaching of metal was observed after the reaction. Notably, this catalytic system achieved turnover numbers comparable to those of homogeneous cobalt catalysts. Additionally, one‐pot tandem reductive amination reactions starting from aldehydes and involving both amines and nitrobenzene yielded secondary and tertiary amines in good yields, highlighting the robustness and versatility of the catalyst.