Hydrogenation Of Acetylene Research Articles

Rational Design of Metallylenes for Hydrogenation Reactions

AbstractThe hydrogenation of alkenes, alkynes, carbonyls, and imines by H2‐activated metallylenes (H3C−EH2−L, CELH2; E=Si, Ge, Sn and L=NMe2, PMe2) was studied using relativistic density functional theory at ZORA‐BP86/TZ2P. By means of activation strain and Kohn–Sham molecular orbital analyses, the physical factors underlying the trends in reactivity were identified and quantified. Firstly, the hydrogenation reactivity increases on descending Group 14, that is from silylenes (E=Si) to stannylenes (E=Sn). This reactivity trend originates primarily from a reduced energy required to break the bonds between the Group 14 atom and the hydrogen atoms because the strength of these bonds decreases from Si−H to Ge−H to Sn−H. Secondly, the reactivity decreases as the Group 15 ligand changes from nitrogen to phosphorus (L=NMe2, PMe2) which stems from the trigonal pyramidal PMe2 geometry compared to the trigonal planar NMe2 geometry. The latter can, therefore, effectively engage in stronger orbital interactions with the substrate than the former, due to a more efficient HOMO–LUMO orbital overlap. By combining our insights, we rationally designed an optimally tuned catalyst (Group 14 atom and ligand) considering the overall catalytic cycle involving H2 activation of the metallylene and subsequent hydrogenation.

Efficient utilization of glycerol as a hydrogen source for partial hydrogenation of alkynes over a Cu-loaded TiO2 photocatalyst

The utilization of biomass glycerol would contribute to the fight against global warming by reducing greenhouse gas emissions; however, efficient utilization of glycerol in water is still a challenge. In this study, partial hydrogenation of alkynes over a titanium(IV) oxide photocatalyst with a copper cocatalyst (Cu/TiO2) was investigated in the presence of glycerol in an acetonitrile-water solvent. Glycerol worked efficiently as a hydrogen source and alkynes were converted to corresponding alkenes. Glycolic acid and formic acid, which are formed as oxidation products of glycerol, cause a decrease of recyclability due to the partial dissolution of Cu nanoparticles. The stability of the Cu/TiO2 photocatalyst is improved by the addition of NaOH, although the reaction rate decreases. A slight increase in the reaction temperature restores the reaction rate of partial hydrogenation, and preferably, the H2 yield becomes smaller. The results provide a new application of biomass glycerol in water, i.e., a hydrogen source for photocatalytic reactions.

Unique Possibilities of Dynamic Metal–Polymer Interactions in Selective Hydrogenation

AbstractRationally designed polymers can function as supports or promoters for metal catalysts, imparting distinct catalytic properties in selective hydrogenation. With strongly metal–ligating functional groups, mobile polymer chains can spontaneously decorate the metal catalyst surfaces under mild conditions, forming stable metal–polymer interfaces. We have termed this phenomenon ‘dynamic metal–polymer interaction (DMPI),’ which can be roughly considered as an organic version of the strong metal–support interaction (SMSI) concept. The polymer chains that dynamically interact with the metal surface can control the adsorption of reactants and products through competitive adsorption, significantly improving selectivity and catalyst stability. One of the remarkable advantages of using polymers as catalytic materials is that their molecular structures, such as molecular weight, crystallinity, and chemical functionality, can be tailored using rich organic chemistry. This, in turn, allows us to precisely tune the metal–polymer interactions and catalytic properties. In this Concept, we will discuss how metal–polymer interfaces can be designed and utilized for selective hydrogenation, with a particular emphasis on the industrially relevant acetylene partial hydrogenation reaction.

Selective Hydrogenation of Alkyne by Atomically Precise Pd6 Nanocluster Catalysts: Accurate Construction of the Coplanar and Specific Active Sites

Selective Hydrogenation of Alkyne by Atomically Precise Pd₆ Nanocluster Catalysts: Accurate Construction of the Coplanar and Specific Active Sites

Partial carbon encapsulation synthesis of separated Ni-Ag catalyst for efficient selective hydrogenation of acetylene: Synergizing hydrogen spillover and steric hindrance

The selective hydrogenation of acetylene is a vital process to for the purification of crude ethylene in petroleum industry. Achieving high activity and selectivity simultaneously over Ni-based catalysts is still challenging. Herein, a partial encapsulation strategy is proposed by fabricating carbon-encapsulated Ag nanoparticles and un-encapsulated Ni nanoparticles on Al2O3 surface (Ni-Ag@C/Al2O3) toward the enhanced selectivity without loss of activity. In particular, the Ni-Ag@C/Al2O3 exhibits higher ethylene selectivity of 81% at the complete acetylene conversion along with excellent stability for 30-h, in sharp with the Ni/Al2O3 (6%) and Ni-Ag/Al2O3 (54%) catalyst. The enhanced selectivity is attributed to the promotion of hydrogen spillover by the carbon species on Al2O3 surface and the inhibition of oligomer formation by a steric hindrance formed by the carbon layer on the Ag surface. This study provides a new structural design avenue of bifunctional bimetallic catalyst for selective hydrogenation reaction.

Subsurface carbon modification of Ni-Ga for improved selectivity in acetylene hydrogenation reaction

Subsurface carbon modification of Ni-Ga for improved selectivity in acetylene hydrogenation reaction

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO2 thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions. The hydrogenation is performed under isothermal reaction conditions using a pulsed molecular beam reactive scattering (pMBRS) technique. Besides the activity and selectivity of clean Pd particles also the impact of dehydrogenated species intentionally introduced a priori is elucidated, whereas the active phase of the catalyst is additionally characterized by CO infrared reflection-absorption spectroscopy (IRRAS) and post-mortem temperature-programmed reaction (TPR). Metal-support interactions are found to influence the catalytic properties of Pd particles by charge-transfer, where positive charging leads to increased activity for acetylene hydrogenation. However, the increased activity is accompanied by formation of undesired byproducts. The active sites for acetylene and ethylene hydrogenation are shown to be different as previously proposed by the A and E model. The availability of the two different active sites on the Pd nanoparticles is determined by dehydrogenated species, whose nature and stability can be tuned by metal-support interactions. Based on these findings an electronic model is proposed how selectivity for acetylene hydrogenation can be steered solely by metal-support interactions leading to blocking of unselective sites in situ.

Selective hydrogenation of high concentration acetylene over the well-defined crystalline sites of Pd1Cu1 and Pd1Cu3

The Pd1Cu1 crystalline site inhibits excessive hydrogenation and oligomerization and exhibits superior ethylene selectivity and stability.

Aluminium porphyrins catalyse the hydrogenation of CO2 with H2.

Boron-based frustrated Lewis pairs (FLPs) have become well-established catalysts for the hydrogenation of a wide range of functional groups. Conversely, aluminium-based FLP hydrogenation catalysts are less common, especially for CO2 reduction. They are mostly confined to the hydrogenation of imines, alkenes, and alkynes even though aluminium is much more abundant than boron and forms structurally related compounds. Moreover, aluminium forms penta- and hexa-coordinated complexes, which remain untested in FLP hydrogenation catalysis. Herein, we demonstrate that cationic, hexa-coordinated diaqua-meso-tetraphenylporphyrin aluminium complexes [Al(TPP)(OH2)2]X and [Al( t BuTPP)(OH2)2]X (X = Cl-, OTf-, ClO4 -) form FLPs with nitrogen bases, activate H2, and reductively couple CO2 to amines, yielding N-formylamines and water. Our experimental results and DFT analysis indicate that H2 activation involves the formation of an FLP, base-promoted CO2 reduction and formate salt elimination from the FLP, as proposed for transition metal-catalyzed N-formylations. These similarities in the reaction mechanism and structure of aluminium complexes brings Al-based FLPs closer to transition metal catalysis and may enable us to apply this knowledge to ligand design to enhance main group metal-promoted hydrogenations.

Semi-hydrogenation of alkynes to alkenes over Pd nanoparticles embedded in Mo2C prepared from a high-temperature pulse.

Pd nanoparticles embedded in Mo2C were successfully prepared from a high-temperature pulse, and are efficient for semi-hydrogenation of alkynes to alkenes, where the embedded Pd sites possessed appropriate alkyne adsorption but suppressed alkene adsorption, thus exhibiting both high activities and good alkene selectivities in the hydrogenation of various alkynes.

Fabrication of a PdCu@SiO2@Cu core-shell-satellite catalyst for the selective hydrogenation of acetylene.

Pd25Cu75@SiO2 core-shell and PdCu@SiO2@Cu core-shell-satellite architectures were fabricated by silica-coating of Pd25Cu75 colloids in a reverse microemulsion. Hydrolysis of tetraethylorthosilicate in the reverse microemulsion containing hydrazine and ammonia yielded a core-shell structure, while the use of ammonia only, instead of a mixture of hydrazine and ammonia, formed a core-shell-satellite structure. The ammonia-leached copper species migrated onto the developing silica shell and formed smaller Cu clusters. Air-calcination at 673 K followed by H2-reduction at 773 K of the as-synthesized samples removed the organic surfactants and generated the permeable porous silica shells. The core-shell catalyst consisted of a metal core (8.5 nm) and a silica shell (7.8 nm), while the core-shell-satellite catalyst was composed by a metal core (7.0 nm), a silica shell (8.0 nm), and satellite Cu clusters (1.4 nm) on the silica shell. When used to catalyze the selective hydrogenation of acetylene to ethylene, the core-shell-satellite catalyst showed substantially enhanced activity and stability because of the synergetic catalysis between the metal core and the surrounding Cu clusters.

Thermally induced intermetallic Rh1Zn1 nanoparticles with high phase-purity for highly selective hydrogenation of acetylene.

Ordered M1Zn1 intermetallic phases with structurally isolated atom sites offer unique electronic and geometric structures for catalytic applications, but lack reliable industrial synthesis methods that avoid forming a disordered alloy with ill-defined composition. We developed a facile strategy for preparing well-defined M1Zn1 intermetallic nanoparticle (i-NP) catalysts from physical mixtures of monometallic M/SiO2 (M = Rh, Pd, Pt) and ZnO. The Rh1Zn1 i-NPs with structurally isolated Rh atom sites had a high intrinsic selectivity to ethylene (91%) with extremely low C4 and oligomer formation, outperforming the reported intermetallic and alloy catalysts in acetylene semihydrogenation. Further studies revealed that the M1Zn1 phases were formed in situ in a reducing atmosphere at 400 °C by a Zn atom emitting-trapping-ordering (Zn-ETO) mechanism, which ensures the high phase-purity of i-NPs. This study provides a scalable and practical solution for further exploration of Zn-based intermetallic phases and a new strategy for designing Zn-containing catalysts.

Nickel-catalyzed tandem conversion of paraformaldehyde : methanol to hydrogen and formate/chemo- and stereoselective hydrogenation of alkynes under neutral conditions

A tandem dihydrogen generation from a mixture of paraformaldehyde–methanol under base-free conditions followed by semihydrogenation of alkynes is reported.

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.

Synthesis and reactivity of a heteroleptic magnesium hydride on a dearomatized picolyl-based NNN-chelator.

Heteroleptic magnesium hydrides are important for their stoichiometric and catalytic reduction chemistry. Their primary nucleophilic site is typically the hydride, while the ancillary ligands commonly used are mostly spectators. Chemically non-innocent ligands in comparison are rarely applied on MgH as their reactivity can be complex. Milstein et al. have recently reported Mg-mediated alkyne hydrogenation by using their metal-ligand cooperation (MLC) concept on a dearomatized picolyl-based PNP pincer that is non-innocent with a nucleophilic nature. A '(PNP)MgH' is noted as the active catalyst in hydrogenation but without structural validation. Inspired by the same, we report herein a novel NNN-chelator (MesL) with a dearomatized picolyl moiety and its well-defined MgH. Having two prominent nucleophilic sites, the present MgH shows metal-ligand competition while reacting with certain electrophiles. It also distinguishes nonpolar alkynes and polar carbonyls by cleanly inserting itself into the former but not the latter. The nucleophilicities of the two sites are also probed by DFT methods and compared with Milstein's (PNP)MgH. Although the present system shows no MLC-type H2 activation, the addition of a CS2 molecule in that way is realized.

Hydrogenation of Acetylene over Pd–Ag/Sibunit Catalysts: Effect of the Deposition Sequence of Active Component Precursors

The study investigates interactions between palladium and silver in Pd–Ag bimetallic catalysts supported on a mesoporous carbon material Sibunit and, in particular, the dependence of these interactions on the deposition sequence of the metal precursors. Using XRD and TEM results, it was shown that impregnating the support with an aqueous solution that contained nitrate salts of both metals, followed by hydrogen treatment at 500°C, generates uniformly sized Pd0.6Ag0.4 particles (dav = 5.6 nm). These particles exhibit high selectivity (79%) in the reaction of acetylene hydrogenation to ethylene. The catalysts synthesized by sequential impregnation of the support with solutions of Pd and Ag nitrates interleaved with heat treatment in H2 exhibited a lower selectivity (68–73%) due to the formation of particles non-uniform both in composition and size (about 4 to 60 nm). The IR spectroscopy data suggest this effect is presumably associated with the removal of O-containing functional groups from the carbon surface during the reduction of the supported precursor. Given that O-groups act as anchoring sites for the precursors of active components and suppress the ability of Sibunit to reduce metals from their salt solutions, the subsequent deposition of the second metal salt causes a non-uniform distribution of this metal on the surface and the generation of larger particles.

Fabrication of stable and selective non‐noble metal catalysts for selective alkyne hydrogenation

AbstractModification of Ni/Al2O3 catalysts with Sn is appealing for tuning catalyst activity and selectivity for hydrogenation reactions. However, a strong interaction between the Al2O3 and Sn ions can lead to problems when a traditional impregnation strategy is employed. For example, tin aluminate complexes can form and uneven distribution of the metals can occur. In this work, a support coordination induction strategy is developed, in which the use of a rod‐shaped Al2O3 with a high number penta‐coordinated Al3+ surface species prepared from a rotating liquid film reactor was used. The unsaturated Al3+ coordination sites on this support induce the formation of a stable and uniform Ni‐Sn intermetallic phase without the appearance of either monometallic Ni or SnO2 particles. This uniform Ni‐Sn intermetallic phase was then shown to be responsible for enhanced activity, selectivity, and stability for alkyne hydrogenation relative to samples which lacked the same level of Ni‐Sn uniformity.

Challenges and Opportunities for Exploiting the Role of Zeolite Confinements for the Selective Hydrogenation of Acetylene.

Zeolites, with their ordered crystalline porous structure, provide a unique opportunity to confine metal catalysts, whether single atoms (e.g., transition metal ions (TMIs)) or metal clusters, when used as a catalyst support. The confined environment has been shown to provide rate and selectivity enhancement across a variety of reactions via both steric and electronic effects, such as size exclusion and transition state stabilization. In this review, we provide a survey of various zeolite confined catalysts used for the semihydrogenation of acetylene highlighting their performance, defined by ethylene selectivity at full acetylene conversion, in relationship to the synthesis technique employed. Synthesis methods that ensure confinement with the catalyst transition metal location in the extra-framework positions are reported to have the highest selectivity to ethylene. However, the underlying molecular factors responsible for selective catalysis within confinement remain elusive due to the difficulty in deconvoluting individual effects. Through the careful use of a combination of characterization and spectroscopic methods, insights into the relationship between the properties of zeolite confined catalysts and their performance have been explored in other works for a variety of reactions. More specifically, operando spectroscopy studies have revealed the dynamic behavior of zeolite confined catalysts under various conditions implying that the structure and properties observed ex situ do not always match those of the active catalyst under reaction conditions. Applying this type of analysis to acetylene semihydrogenation, a simple gas phase reaction, can help elucidate the structure-function relationship of zeolite confined catalysts allowing for more informed design choices and consequently their application to a wider variety of more complex reactions such as the liquid phase hydrogenation of alkynols where solvent effects must also be considered in addition to those of confinement.

Hydrogenation of Alkynes and Olefins Catalyzed by Quaternary Ammonium Salts.

Catalytic hydrogenation of unsaturated hydrocarbons to alkenes and alkanes using molecular hydrogen is one of the most fundamental transformations in organic synthesis. While methodologies involving transition metals as catalysts in homogeneous and heterogeneous processes have been well developed, metal-free catalytic hydrogenation offers an ideal approach for future chemistry. Herein, the common and inexpensive quaternary ammonium salts are first introduced as catalysts in the catalytic hydrogenation system for the transformations from alkynes or olefins into the corresponding olefins or alkanes. Interestingly, the hydrogenation process of alkynes can be controlled to selectively produce alkenes or alkanes under different conditions. Moreover, the possible mechanism is discussed in new insights into the catalytic behavior of quaternary ammonium salts.

Influence of Pd Concentration in Au–Pd Nanoparticles for the Hydrogenation of Alkynes

Influence of Pd Concentration in Au–Pd Nanoparticles for the Hydrogenation of Alkynes