CH Hydrogen Research Articles

Asymmetric Coupled Binuclear‐Site Catalysts for Low‐Temperature Selective Acetylene Semi‐Hydrogenation

AbstractHeterogeneous metal catalysts with bifunctional active sites are widely used in chemical industries. Although their improvement process is typically based on trial‐and‐error, it is hindered by the lack of model catalysts. Herein, we report an effective vacancy‐pair capturing strategy to fabricate 12 heterogeneous binuclear‐site catalysts (HBSCs) comprising combinations of transition metals on titania. During the synthesis of these HBSCs, proton‐passivation treatment and step‐by‐step electrostatic anchorage enabled the suppression of single‐atom formation and the successive capture of two target metal cations on the titanium–oxygen vacancy‐pair site. Additionally, during acetylene hydrogenation at 20 °C, the HBSCs (e.g., Pt1Pd1−TiO2) consistently generated more than two times the ethylene produced by their single‐atom counterparts (e.g., Pd1−TiO2). Furthermore, the Pt1Pd1 binuclear sites in Pt1Pd1−TiO2 were demonstrated to catalyze C2H2 hydrogenation via a bifunctional active‐site mechanism: initially C2H2 chemisorb on the Pt1 site, then H2 dissociates and migrates from Pd1 to Pt1, and finally hydrogenation occurs at the Pt1−Pd1 interface.

Asymmetric Coupled Binuclear-Site Catalysts for Low-Temperature Selective Acetylene Semi-Hydrogenation.

Heterogeneous metal catalysts with bifunctional active sites are widely used in chemical industries. Although their improvement process is typically based on trial-and-error, it is hindered by the lack of model catalysts. Herein, we report an effective vacancy-pair capturing strategy to fabricate 12 heterogeneous binuclear-site catalysts (HBSCs) comprising combinations of transition metals on titania. During the synthesis of these HBSCs, proton-passivation treatment and step-by-step electrostatic anchorage enabled the suppression of single-atom formation and the successive capture of two target metal cations on the titanium-oxygen vacancy-pair site. Additionally, during acetylene hydrogenation at 20 °C, the HBSCs (e.g., Pt1Pd1-TiO2) consistently generated more than two times the ethylene produced by their single-atom counterparts (e.g., Pd1-TiO2). Furthermore, the Pt1Pd1 binuclear sites in Pt1Pd1-TiO2 were demonstrated to catalyze C2H2 hydrogenation via a bifunctional active-site mechanism: initially C2H2 chemisorb on the Pt1 site, then H2 dissociates and migrates from Pd1 to Pt1, and finally hydrogenation occurs at the Pt1-Pd1 interface.

Research on modeling the lignin waste molecular structure and pyrolytic reactions using ReaxFF MD method

The lack of clarity in characterizing the chemical structure model of lignin waste hinders the comprehensive understanding of its pyrolysis reaction mechanism. The characterization results revealed that the de–alkalized lignin sample contained larger amounts of aliphatic and aromatic carbons compared to carbonyl carbons. Additionally, the aliphatic carbon molecules exhibited a higher presence of oxygenated carbons. The degree of aromaticity (fa) was determined to be 63.93 %. The resulting single–molecule structural model exhibited a molecular formula of C33H26O11 and a molecular weight of 598.56. Furthermore, the evolutions patterns and formation pathways of primary volatile components (H2, CH4, CO, and CO2) in pyrolysis reactions were revealed. The number of H2 molecules exhibited a positive correlation with the rise in pyrolysis reaction temperature. CH4 molecules exhibited a pattern of initially increasing and subsequently decreasing. The number of CO molecules exhibited a positive correlation with the rise in pyrolysis temperature. Conversely, the number of CO2 molecules demonstrated an initial increase followed by a decrease as the pyrolysis temperature increased. Finally, the generation pathway analysis of pyrolysis products shows that the H2 molecule is composed of two hydrogen atoms bonded together that have been dissociated from the carboxyl group. Alternatively, it can be produced through hydrogenation reactions involving hydrogen atoms detached from the carboxyl group and methyl groups. CH4 is primarily generated through the reaction between –CH3 and free hydrogen ions. CO is primarily produced through the cleavage of the carbonyl group. CO2 is primarily produced through the cleavage of carboxyl and ester groups.

Optimization of activity and selectivity for syngas-to-ethanol conversion on metal-based catalysts via the first-principles microkinetic simulations

Achieving high selectivity in converting syngas to ethanol has long been a challenging task, requiring identification of key factors favoring ethanol production over other by-products such as methane, methanol, and acetaldehyde. Herein, we perform systematic density functional theory (DFT) calculations and microkinetic simulations to shed light on the activity and selectivity trends of ethanol relative to three competing by-products in syngas conversion, which quantitatively unveils the key determining factors on the transition metal-based catalysts. The scaling relations involved are revealed as functions of the adsorption energies of C and O species (EC and EO), and the three-dimensional activity and selectivity surfaces are constructed quantitatively, indicating that the optimal condition (peak position) correspond to EC = -6.10 eV and EO = -5.70 eV. Furthermore, we find that ethanol activity at the peak is primarily constrained by the kinetic barriers of CH3O dissociation and the coupling reaction of CH3 and CO, while its selectivity can be enhanced by increasing the energy barrier of CH3 hydrogenation and strengthening the adsorption of CH3CHO. More significantly, following these rules, the single-atom metal alloy catalysts (i.e., Cu1Co, Cu1Ni) are identified that could serve as promising candidates for ethanol synthesis, which are superior to the common alloy-type catalysts. We believe that these insights further deepen the understanding of theoretical design of metal alloy catalysts in ethanol production.

Microwave spectra of two carboxylic acid anhydrides: Acetic anhydride and acetic difluoroacetic anhydride

Microwave spectra of acetic anhydride, D6-acetic anhydride, and acetic difluoroacetic anhydride have been observed in a supersonic jet. In conjunction with accompanying DFT and MP2 calculations, these systems are shown to adopt a nonplanar configuration in which the C=O groups point in approximately orthogonal directions. Methyl group internal rotation was fully analyzed for both species. The observed conformation of these systems appears to result from an interaction between a CH3 hydrogen (in acetic anhydride) or the CF2H hydrogen (in acetic difluoroacetic anhydride) with the carbonyl group to which it is not directly bound, forming a six-membered ring. The fitted rotational constants for both systems are in reasonably good agreement with calculated values, but for acetic anhydride, the agreement is somewhat worse than that previously observed for a series of syn anhydrides. The calculations indicate a pronounced flexing of the heavy atom frame as the CH3 group in the six-membered ring undergoes internal rotation, and this likely influences the level of agreement between the theoretical and vibrationally averaged experimental constants. The other CH3 group does not interact with a carbonyl oxygen because of its orientation in the molecule, and its internal rotation does not induce similar changes in the molecular frame. In the acetic difluoroacetic anhydride, it is the CF2H hydrogen that interacts with its remote carbonyl oxygen, leaving the internally rotating CH3 group unaffected by participation in a six-membered ring and giving rise to much smaller deviations in the rotational constants as it moves along its internal rotation coordinate. Correspondingly better agreement between experimental and theoretical spectroscopic constants is obtained.

Neighboring Effects of Sulfur Oxidation State on Dynamic Covalent Bonds and Assemblies.

The impact of a varied sulfur oxidation state (sulfide, sulfoxide, and sulfone) on imine dynamic covalent chemistry is presented. The role of noncovalent interactions, including chalcogen bonds and CH hydrogen bonds, on aldehyde/imine structures and imine exchange reactions was elucidated through experimental and computational evidence. The change in the sulfur oxidation state and diamine linkage further allowed the regulation of imine macrocycles, providing a platform for controlling molecular assemblies.

Conformational Analysis Explores the Role of Electrostatic Nonclassical CF···HC Hydrogen Bonding Interactions in Selectively Halogenated Cyclohexanes.

The conformational equilibria of selectively halogenated cyclohexanes are explored both experimentally (VT-NMR) for 1,1,4,-trifluorocyclohexane 7 and by computational analysis (M06-2X/aug-cc-pVTZ level), with the latter approach extending to a wider range of more highly fluorinated cyclohexanes. Perhaps unexpectedly, 7ax is preferred over the 7eq conformation by ΔG = 1.06 kcal mol-1, contradicting the accepted norm for substituents on cyclohexanes. The axial preference is stronger again in 1,1,3,3,4,5,5,-heptafluorocyclohexane 9 (ΔG = 2.73 kcal mol-1) as the CF2 groups further polarize the isolated CH2 hydrogens. Theoretical decomposition of electrostatic and hyperconjugative effects by natural bond orbital analysis indicated that nonclassical hydrogen bonding (NCHB) between the C-4 fluorine and the diaxial hydrogens at C-2 and C-6 in cyclohexane 7 and 9 largely accounts for the observed bias. The study extended to changing fluorine (F) for chlorine (Cl) and bromine (Br) at the pseudoanomeric position in the cyclohexanes. Although these halogens do not become involved in NCHBs, they polarize the geminal -CHX- hydrogen at the pseudoanomeric position to a greater extent than fluorine, and consequent electrostatic interactions influence conformer stabilities.

Bridging the size gap between experiment and theory: large-scale DFT calculations on realistic sized Pd particles for acetylene hydrogenation.

Metal nanoparticles, often supported on metal oxide promoters, are a cornerstone of heterogeneous catalysis. Experimentally, size effects are well-established and are manifested through changes to catalyst selectivity, activity and durability. Density Functional Theory (DFT) calculations have provided an attractive way to study these effects and rationalise the change in nanoparticle properties. However such computational studies are typically limited to smaller nanoparticles (approximately up to 50 atoms) due to the large computational cost of DFT. How well can such simulations describe the electronic properties of the much larger nanoparticles that are often used in practice? In this study, we use the ONETEP code, which is able to achieve more favourable computational scaling for metallic nanoparticles, to bridge this size gap. We present DFT calculations on entire Pd and Pd carbide nanoparticles of more than 300 atoms (approximately 2.5 nm diameter), and find major differences in the electronic structure of such large nanoparticles, in comparison to the commonly investigated smaller clusters. These differences are also manifested in the calculated chemical properties such as adsorption energies for C2H2, C2H4 and C2H6 on the pristine Pd and PdC x nanoparticles which are significantly larger (up to twice in value) for the ∼300 atoms structures. Furthermore, the adsorption of C2H2 and C2H4 on PdC x nanoparticles becomes weaker as more C is introduced in the Pd lattice whilst the impact of C concentration is also observed in the calculated reaction energies towards the hydrogenation of C2H2, where the formation of C2H6 is hindered. Our simulations show that PdC x nanoparticles of about 5% C per atom fraction and diameter of 2.5 nm could be potential candidate catalysts of high activity in hydrogenation reactions. The paradigm presented in this study will enable DFT to be applied on similar sized metal catalyst nanoparticles as in experimental investigations, strengthening the synergy between simulation and experiment in catalysis.

Dual Pt atoms stabilized by an optimized coordination environment for propane dehydrogenation

Tailoring atomically dispersed catalysts by various methods has been the subject of intense current research in heterogeneous catalysis. In this work, the structural stability and propane dehydrogenation performance of Pt-TiO2 catalysts with single, dual, and triple Pt atoms doped on the reduced TiO2 surface have been examined. Experimental results indicate the atomically dispersed Pt is thermally very stable and shows exceptionally high activity (TOFpropane = 4.93 s−1) and selectivity (∼98.5 %) for the dehydrogenation reaction. The recently developed machine-learning-based SSW-NN method is then employed to explore the global potential energy surface of the atomically dispersed Pt atom-doped TiO2, which suggests that the single and dual Pt atoms can be tightly held by the vacancies created on the reduced transition-metal oxide, as confirmed by DFT + U calculated energetics. After that, advanced characterization techniques are used in conjunction with microkinetic analysis to establish the quantitative structure–activity relationships. It is found that the dual-Pt-atom structure with two Pt atoms embedded in adjacent Ti and O vacancies is the only active ensemble that dramatically promotes the CH bond activation and hydrogen recombination, as compared to single Pt atoms accommodated by either Ti or O vacancies. These results provide new and direct evidence in support of the idea that specific function can be imparted to specific active sites by tuning their coordination environments, which makes it possible to tailor the catalytic properties of metal oxides to a particular reaction such as propane dehydrogenation.

Tuning CH Hydrogen Bond-Based Receptors toward Picomolar Anion Affinity via the Inductive Effect of Distant Substituents.

Inspired by nature, artificial hydrogen bond-based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3 )-H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron-withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013 M-1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron-withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

Tuning CH Hydrogen Bond‐Based Receptors toward Picomolar Anion Affinity via the Inductive Effect of Distant Substituents

AbstractInspired by nature, artificial hydrogen bond‐based anion receptors have been developed to achieve high anion selectivity; however, their binding affinity is usually low. The potency of these receptors is usually increased by the introduction of aryl substituents, which withdraw electrons from their binding site through the resonance effect. Here, we show that the polarization of the C(sp3)‐H binding site of bambusuril receptors, and thus their potency to bind anions, can be modulated by the inductive effect. The presence of electron‐withdrawing groups on benzyl substituents of bambusurils significantly increases their binding affinities to halides, resulting in the strongest iodide receptor reported to date with an association constant greater than 1013 M−1 in acetonitrile. A Hammett plot showed that while the bambusuril affinity toward halides linearly increases with the electron‐withdrawing power of their substituents, their binding selectivity remains essentially unchanged.

Syngas Conversion over Co4 Cluster Grafted on HZSM‐5 Zeolite: Mechanistic Insights from DFT Modeling

AbstractWe present a detailed DFT‐based mechanistic investigation of syngas conversion mechanism over Co4 cluster grafted onto HZSM‐5 zeolite, [Co4H], employing a QM/MM embedded cluster approach. Starting from the [Co4H] complex, our results show that a favorable coordination of CO over H2, followed by CO hydrogenation leads to a stable −CH2O complex, [Co4(CH2O)(H)]. Coordination of a second CO molecule to [Co4(CH2O)(H)] complex, followed by CH2−O bond activation, and subsequent removal of CO as CO2 results in the formation of crucial methylene complex [Co4(CH2)(H)], serving as a branching point for the pathways leading to methane, ethene, and ethane. On the pathway to ethene formation, coordination of a third CO molecule to [Co4(CH2)(H)] complex yields the active [Co4(CH2)(CO)(H)] complex, which is 16.0 kcal mol−1 more stable than the methyl complex [Co4(CH3)] on the pathway to methane. From the active species [Co4(CH2)(CO)(H)], we demonstrate that the pathways to both methane and ethene are competing in nature, with the −CH3 hydrogenation barrier, 35.1 kcal mol−1, is lower by only 1.3 kcal mol−1 than the competing C−O bond activation barrier on the pathway to ethene, 36.4 kcal mol−1. However, the significant stability of the active species [Co4(CH2)(CO)(H)] effectively compensates for this minor difference in barriers, ultimately favoring the formation of ethene over methane. Finally, the ethene desorption barrier is 4.1 kcal mol−1 lower than the ethene hydrogenation barrier on the pathway to ethane, indicating the ease of ethene removal from the system. Overall, our DFT study describes that the syngas conversion mechanism catalyzed by [Co4H] system produces ethene selectively via 4CO+2H2→C2H4+2CO2.

Tuning of catalysts for the selective hydrogenation of acetylene with high stability and selectivity: Ligand coordination effects

Selectivity of acetylene over CeO2 supported single Pd catalyst (Pd1/CeO2(111)) and 1,10-phenanthroline-5,6-dione (PDO)-liganded Pd single catalyst (Pd1-PDO/CeO2(111)) was studied by density functional theory calculations combined with microkinetic modeling as well as ab initio molecular dynamics (AIMD) in this work. The present results show that the PDO-ligand can stabilize the single Pd catalyst through strong Pd-O/Pd-N interaction, which is further conformed by AIMD simulation. In the presence of PDO ligand, which contains Pd-O/Pd-N bonds, the single atom Pd is oxidized and becomes more stable. It was also found that the adsorption strength of C2H2 is weaker than C2H4 on single Pd, whereas opposite trend holds for PDO-liganded Pd single catalyst, indicating potential higher C2H4 formation electivity for Pd1-PDO/CeO2(111). Reaction mechanism analysis and microkinetic modeling demonstrate that the catalytic activity of C2H2 hydrogenation decreases but the C2H4 formation selectivity increases when PDO ligand was introduced, which agrees with the experimental observation. Moreover, the free energy change of C2H4 desorption as well as its further hydrogenation was quantitative studied by AIMD simulation, verifies that the PDO ligand indeed helps ethene desorption at high reaction temperature. It is hoped the present work may extend to other ligands which contain -O or -N groups that can bind with single metal atom strongly like –OH groups.

Single-atom alloy Pd1Ag10/Al2O3 catalyst: Effect of CO-induced Pd surface segregation on the structure and catalytic performance in the hydrogenation of C2H2

The effective surface concentration of Pd1 sites composed of individual palladium atoms isolated from each other by Ag atoms in the Pd1Ag10/Al2O3 single-atom alloy (SAA) catalyst can be significantly enhanced by the adsorption-induced surface segregation upon a special pretreatment of the catalyst in a CO-containing atmosphere at 200 °C. The catalyst activity towards acetylene hydrogenation gets tripled without sacrificing selectivity for ethylene. The segregated surface of the catalyst is thus preserved under target reaction conditions.

C2H2 semi-hydrogenation over Pdn/TiO2 and PdnCO/TiO2 catalysts: Probing into the roles of Pd cluster size and pre-adsorbed CO in tuning catalytic performance

The size effect of Pd catalyst and CO as a selective accelerator in C2H2 semi-hydrogenation over Pd catalyst are two main factors to affect catalytic performance. In this work, density functional theory calculations were used to unravel the roles of Pd cluster size and introduced CO over the anatase TiO2 supported Pdn (n = 2, 3, 4, 7, 13) clusters in tuning catalytic performance of C2H2 semi-hydrogenation. The results show that as the increasing of Pdn cluster size over Pdn/TiO2 catalysts, C2H4 selectivity generally presents a volcanic-type curve, and the activity of C2H4 and green oil presents an inverted volcanic-type curve. Compared with Pdn/TiO2 catalysts, the introduction of CO greatly improves C2H4 selectivity over Pd2/TiO2 and Pd3/TiO2 catalysts, enhances C2H4 formation activity over Pdn/TiO2 (n = 3, 4, 7, 13), and decreases green oil production over Pd2/TiO2 and Pd13/TiO2 catalysts. Meanwhile, the activation free energy of C2H4 hydrogenation and C2H3 coupling reaction can be used as the evaluate index to quantitatively predict the selectivity of C2H4 and green oil, respectively. The different geometric and electronic effects induced by the introduction of CO could tune catalytic performance. The screened Pd4/TiO2 and Pd2CO/TiO2 catalysts are the most suitable catalysts in the Pdn/TiO2 and PdnCO/TiO2 catalysts, respectively. The relationship of Pdn cluster size and CO introduction with C2H4 selectivity and formation activity would provide valuable structural clue for the construction of C2H2 semi-hydrogenation catalyst with highly catalytic performance.

Kinetic parameters for H abstraction from the serine amino acid molecule

The Serine compound signifies an important model entity of polar amino acids species in various categories of biomass. The presence of adjacent amino and hydroxyl groups is expected to afford this molecule distinct fragmentation pathways. Comprehending the decomposition chemistry of serine (and other amino acids) is of a crucial important in the pursuit to minimize emission of notorious nitrogen-bearing species during thermal treatment of biomass. This contribution provides thermo-kinetic parameters for H abstraction from the five distinct sites in the serine molecule; namely amine’s, secondary’s, hydroxyl’s, and α CH hydrogen atoms by the abundant radical species in the pyrolytic/oxidative environments (H, CH3, NH2, OH, and HO2). For all considered radicals, it is illustrated that reactions of these five radicals predominantly ensue through abstraction of the α CH’s hydrogen atoms. Nonetheless, computed reaction rate constants also disclose that abstraction from the secondary’s H site becomes important as the temperature increases. Outcomes provided herein could be deployed in formulating robust kinetic models that account for the nitrogen transformation chemistry in energy recovery of biomass.

Selectivity Rule of Cryptands for Anions: Molecular Rigidity and Bonding Site.

Cryptands utilize inside CH or NH groups as hydrogen bond (H-bond) donors to capture anions such as halides. In this work, the nature and selectivity of confined hydrogen bonds inside cryptands were computationally analyzed with the energy decomposition scheme based on the block-localized wavefunction method (BLW-ED), aiming at an elucidation of governing factors in the binding between cryptands and anions. It was revealed that the intrinsic strengths of inward hydrogen bonds are dominated by the electrostatic attraction, while the anion preferences (selectivity) of inner CH and NH hydrogen bonds are governed by the Pauli exchange repulsion and electrostatic interaction, respectively. Typical conformers of cages are classified into two groups, including the C3(h) -symmetrical conformers, in which all halide anions are located near the centroids of cages, and the "semi-open" conformers, which exhibit shifted bonding sites for different halide anions. Accordingly, the difference in governing factors of selectivity is attributed to either the rigidity of cages or the binding site of anions for these two groups. In details, the C3 conformers of NH cryptands can be enlarged more remarkably than the C3(h) -symmetrical conformers of CH cryptands as the size of anion (ionic radius) increases, resulting in the relaxation of the Pauli repulsion and a dramatic reduction in electrostatic attraction, which eventually rules the selectivity of NH cryptands for halide anions. By contrary, the CH cryptands are more rigid and cannot effectively reduce the Pauli repulsion, which subsequently governs the anion preference. Unlike C3 conformers whose rigidity determines the selectivity, semi-open conformers exhibit different binding sites for different anions. From F- to I- , the bonding site shifts toward the outside end of the pocket inside the semi-open NH cryptand, leading to the significant reduction of the electrostatic interaction that dominates the anion preference. Differently, binding sites are much less affected by the size of anion inside the semi-open CH cryptand, in which the Pauli exchange repulsion remains the key factor for the selectivity of inner hydrogen bonds.

Molecular Pincers Using a Combination of N-H and C-H Donors for Anion Binding.

A naphthalene imide (1) and a naphthalene (2) bearing two pyrrole units have been synthesized, respectively, as anion receptors. It was revealed by 1H NMR spectral studies carried out in CD3CN that receptors 1 and 2 bind various anions via hydrogen bonds using both C-H and N-H donors. Compared with receptor 2, receptor 1 shows higher affinity for the test anions because of the enhanced acidity of its pyrrole NH and naphthalene CH hydrogens by the electron-withdrawing imide substituent. Molecular mechanics computations demonstrate that the receptors contact the halide anions via only one of the two respective available N-H and C-H donors whereas they use all four donors for binding of the oxyanions such as dihydrogen phosphate and hydrogen pyrophosphate. Receptor 1, a push-pull conjugated system, displays a strong fluorescence centered at 625 nm, while receptor 2 exhibits an emission with a maximum peak at 408 nm. In contrast, upon exposure of receptors 1 and 2 to the anions in question, their fluorescence was noticeably quenched particularly with relatively basic anions including F-, H2PO4-, HP2O73-, and HCO3-.

Efficient carbon monoxide electroreduction on two-dimensional transition metal phosphides: A computational study

Carbon monoxide electroreudction (COER) has been emerging as a key component for tandem carbon dioxide (CO2) electrolysis, in which engineering COER electrocatalysts with good catalytic performance is still a huge challenge. In this work, we performed comprehensive density functional theory (DFT) computations to explore the feasibility of a new class of two-dimensional transition metal phosphides (TM2P) as COER catalysts. Our simulations demonstrated that these TM2P candidates possess rather high stability and excellent electrical conductivity. Remarkably, Nb2P monolayer was revealed to exhibit high COER catalytic activity for CH4 production due to its small limiting potential (−0.51 V) from the CH3⁎ hydrogenation step. Interestingly, Nb2P monolayer was predicted to exhibit highest COER catalytic activity among these TM2P candidates due to its moderate interaction with CH3⁎ intermediate, which may be attributed to its optimal d-band center in β-spin orbitals. Our work not only enriches the applications of 2D metal phosphides, but also offers an in-deep insight into the rational design of efficient COER catalysts.

Nano-sized Metallic Nickel Clusters Stabilized on Dealuminated beta-Zeolite: A Highly Active and Stable Ethylene Hydrogenation Catalyst

Supported Ni catalysts were synthesized using the beta-zeolite framework, with and without the framework Al, as a platform for dispersing Ni. The silanol nest sites of dealuminated zeolite beta provide isolated cationic Ni sites that can be reduced under relatively mild conditions to create highly dispersed metal clusters. Compared to the Ni sites present in Ni-[Al]-beta-19, Ni-[DeAl]-beta exhibit a 20-fold increase in the apparent reaction rate for C2H4 hydrogenation and is stable, with little deactivation over 16 h of catalysis. Ni K-edge X-ray absorption spectroscopy (XAS), as well as CO adsorption monitored with Fourier transform infrared spectroscopy, shows that in the oxidized Ni-[DeAl]-beta catalyst Ni reoccupies vacant silanol nests produced from dealumination. After reductive treatment, XAS shows that approximately 50% of Ni is reduced to metallic Ni, forming clusters that are approximately 1 nm in size. Scanning transmission electron microscopy images are consistent with the absence of large (>1 nm) metallic Ni clusters. These results indicate that [DeAl]-beta can be used to synthesize isolated cationic Ni sites as well as stabilize highly dispersed metal clusters that can be used as a highly active and stable C2H4 hydrogenation catalyst.