Bond Hydrogenolysis Research Articles

Construction of Cu/ZrO2/Al2O3 composites for ethanol synthesis: Synergies of ternary sites for cascade reaction

The hydrogenation of dimethyl oxalate (DMO) to ethanol involves the stepwise hydrogenation of CO bonds and hydrogenolysis of CO bonds. In the present work, the assembly of ternary active sites (Cu, ZrO2, Al2O3 sites) for this reaction was investigated. The calcination temperature was demonstrated to have profound influences on the catalytic performance, evolutions of texture and structure properties and functionality of active phases of the ternary catalysts with similar compositions. The Cu/ZrO2/Al2O3 catalyst calcined at a high temperature of 750°C exhibited an ethanol yield up to 97.4% and stable performance (>200h). Key to success is the concurrence of the stable metallic Cu sites with more proportions of Cu+ sites, active crystalline ZrO2 phases and sufficient acid sites which were generated upon high temperature calcination. The high-temperature calcination annealed Cu particles into large ones, facilitating the high stability. Another factor for the good stability is the formation of CuAl2O4 spinel and the resulted strong interactions between Cu and Al2O3 after reduction. Overall, the design of efficient multifunctional catalysts for hydrogenation/hydrogenolysis of CO/CO bonds lies in the adequate assembly and modulation of textural, structural and surface properties of catalysts. The structure-performance relationships were well elucidated by the analysis of structural and surface properties of active phases and catalytic performance, which provide more rational choices for making high-performance catalysts for CO hydrogenation and CO hydrogenolysis reactions.

Base promoted hydrogenolysis of lignin model compounds and organosolv lignin over metal catalysts in water

Herein we present a systematic investigation on the promotional effect of base in metal catalysed hydrogenolysis of lignin model compounds and organosolv lignin. The research started with the evaluation of pH effects (pH 1–14) on the hydrogenolysis of a lignin β-O-4 model compound over a Ru catalyst (a stable catalyst over a wide pH range), from which a significantly increased selectivity towards monomeric compounds was observed in the presence of base. This promotional effect was studied in detail over bimetallic Ni7Au3 nanoparticles. Addition of a strong base such as NaOH significantly enhanced the activity and selectivity for C-O bond hydrogenolysis over undesired hydrogenation reactions, not only in lignin model compounds but also in real lignin conversion. Notably, the yield for monomeric aromatic compounds from lignin over Ni7Au3 catalyst increased ca. 100% after adding NaOH as a promoter, under the same reaction conditions. Mechanistic study suggest that addition of base significantly reduced the benzene ring hydrogenation activity of the metal catalysts. The effect of adding different bases over various metal catalysts were also investigated.

Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts

H/D isotope effects on methylcyclohexane hydrogenolysis over Ir/Al2O3 catalysts were examined by combining measured rates with theoretical estimates provided by partition function based analyses. Normal H/D isotope effects (r(H)/r(D) > 1) were observed for endocyclic and exocyclic C-C bond hydrogenolysis. Hydrogenolysis is concluded to occur via stepwise dehydrogenation followed by cleavage of the C-C bond and subsequent hydrogenation of the cleaved entities. The so-called multiplet mechanism (i.e., the C-C bond of a flat-lying physisorbed cyclic molecule is cleaved upon the attack of a coadsorbed H atom) is unequivocally excluded. For ring-opening, either C-C bond cleavage or C-H(D) bond reformation may be rate-determining, due to their indistinguishable isotope effects under the studied conditions. C-H(D) bond dissociation does not control the rate of C-C bond hydrogenolysis. For the exocyclic cleavage of the methyl group, a higher degree of unsaturation of the surface intermediate and the potential impact of mobile H atoms on large Ir particles are noted.

Catalytic C–F Bond Hydrogenolysis of Fluoroaromatics by [(η5-C5Me5)RhI(2,2′-bipyridine)

A new class of efficient catalyst, the Rh(I) complex [(η5-C5Me5)RhI(bpy)] (1; bpy = 2,2′-bipyridine), for the C–F bond hydrogenolysis of fluoroaromatics (C6F5CF3, C6F6, C6F5H, and C6F5CH3) is presented. The best turnover number of 380 for C6F6 is afforded by using 0.1 mol % of 1, 0.8 MPa of H2, and 2 equiv of Et2NH in CH3CN at 25 °C. The successful isolation of the C–F bond cleavage product [(η5-C5Me5)RhIII(bpy)(C6F5)](F) as a plausible intermediate of the catalytic hydrogenolysis of C6F6 by 1 is also described.

ChemInform Abstract: Catalytic Conversion of Nonfood Woody Biomass Solids to Organic Liquids

AbstractReview: 51 refs.

One-Pot Conversion of Cellulose into n-Hexane over the Ir-ReOx/SiO2 Catalyst Combined with HZSM-5

One-pot conversion of cellulose to n-hexane was carried out over the Ir-ReOx/SiO2 (Re/Ir = 2) catalyst combined with HZSM-5 as cocatalyst in a biphasic reaction system (n-dodecane + H2O). The yield of n-hexane reached 83% from ball-milled cellulose and 78% from microcrystalline cellulose. Even using a high weight ratio of cellulose to water (1:1), a 71% yield of n-hexane could be obtained from ball-milled cellulose. The yield of n-hexane was almost maintained during three repeated tests when the catalyst was calcined again. The transformation of cellulose to n-hexane consists of the hydrolysis of cellulose to glucose via water-soluble oligosaccharides, hydrogenation of glucose to sorbitol, and successive hydrogenolysis of sorbitol to n-hexane. The Ir-ReOx/SiO2 catalyst promotes a hydrogenation and hydrogenolysis step. HZSM-5 enhanced the hydrolysis of cellulose in hot water and C–O bond hydrogenolysis activity of the Ir-ReOx/SiO2 catalyst.

A Series of NiM (M = Ru, Rh, and Pd) Bimetallic Catalysts for Effective Lignin Hydrogenolysis in Water

In this paper, NiRu, NiRh, and NiPd catalysts were synthesized and evaluated in the hydrogenolysis of lignin C–O bonds, which is proved to be superior over single-component catalysts. The optimized NiRu catalyst contains 85% Ni and 15% Ru, composed of Ni surface-enriched, Ru–Ni atomically mixed, ultrasmall nanoparticles. The Ni85Ru15 catalyst showed high activity under low temperature (100 °C), low H2 pressure (1 bar) in β-O-4 type C–O bond hydrogenolysis. It also exhibited significantly higher activity over Ni and Ru catalysts in the direct conversion of lignin into monomeric aromatic chemicals. Mechanistic investigation indicates that the synergistic effect of NiRu can be attributed to three factors: (1) increased fraction of surface atoms (compared with Ni), (2) enhanced H2 and substrate activation (compared with Ni), and (3) inhibited benzene ring hydrogenation (compared with Ru). Similarly, NiRh and NiPd catalysts were more active and selective than their single-component counterparts in the hydrogenolysis of lignin model compounds and real lignin.

Catalytic conversion of nonfood woody biomass solids to organic liquids.

This Account outlines recent efforts in our laboratories addressing a fundamental challenge of sustainability chemistry, the effective utilization of biomass for production of chemicals and fuels. Efficient methods for converting renewable biomass solids to chemicals and liquid fuels would reduce society's dependence on nonrenewable petroleum resources while easing the atmospheric carbon dioxide burden. The major nonfood component of biomass is lignocellulose, a matrix of the biopolymers cellulose, hemicellulose, and lignin. New approaches are needed to effect facile conversion of lignocellulose solids to liquid fuels and to other chemical precursors without the formation of intractable side products and with sufficient specificity to give economically sustainable product streams. We have devised a novel catalytic system whereby the renewable feedstocks cellulose, organosolv lignin, and even lignocellulose composites such as sawdust are transformed into organic liquids. The reaction medium is supercritical methanol (sc-MeOH), while the catalyst is a copper-doped porous metal oxide (PMO) prepared from inexpensive, Earth-abundant starting materials. This transformation occurs in a single stage reactor operating at 300-320 °C and 160-220 bar. The reducing equivalents for these transformations are derived by the reforming of MeOH (to H2 and CO), which thereby serves as a "liquid syngas" in the present case. Water generated by deoxygenation processes is quickly removed by the water-gas shift reaction. The Cu-doped PMO serves multiple purposes, catalyzing substrate hydrogenolysis and hydrogenation as well as the methanol reforming and shift reactions. This one-pot "UCSB process" is quantitative, giving little or no biochar residual. Provided is an overview of these catalysis studies beginning with reactions of the model compound dihydrobenzofuran that help define the key processes occurring. The initial step is phenyl-ether bond hydrogenolysis, and this is followed by aromatic ring hydrogenation. The complete catalytic disassembly of the more complex organosolv lignin to monomeric units, largely propyl-cyclohexanol derivatives is then described. Operational indices based on (1)H NMR analysis are also presented that facilitate holistic evaluation of these product streams that within several hours consist largely of propyl-cyclohexanol derivatives. Lastly, we describe the application of this methodology with several types of wood (pine sawdust, etc.) and with cellulose fibers. The product distribution, albeit still complex, displays unprecedented selectivity toward the production of aliphatic alcohols and methylated derivatives thereof. These observations clearly indicate that the Cu-doped solid metal oxide catalyst combined with sc-MeOH is capable of breaking down the complex biomass derived substrates to markedly deoxygenated monomeric units with increased hydrogen content. Possible implementations of this promising system on a larger scale are discussed.

Mechanism of Ni N-heterocyclic carbene catalyst for C-O bond hydrogenolysis of diphenyl ether: a density functional study.

Catalysts for aromatic C-O bond activation can potentially be used for the lignin degradation process. We investigated the mechanisms of C-O bond hydrogenolysis of diphenyl ether (PhOPh) by the nickel N-heterocyclic carbene (Ni-SIPr) complex to produce benzene and phenol as products. Our calculations revealed that diphenyl ether is not only a substrate, but also serves as a ligand to stabilize the Ni-SIPr complex. The Ni(SIPr)(η(6)-PhOPh) complex is initially formed before rearranging to Ni(SIPr)(η(2)-PhOPh), the active species for C-O bond activation. The catalytic reaction has three steps: (i) oxidative addition of Ni(SIPr)(η(2)-PhOPh) to form [Ni(SIPr)(OPh)(Ph)](0), (ii) σ-complex-assisted metathesis, in which H2 binds to the nickel to form [Ni(SIPr)(OPh)(Ph)(H2)](0), and then benzene (or phenol) is eliminated, and (iii) reductive elimination of phenol (or benzene) and the binding of PhOPh to regenerate Ni(SIPr)(η(2)-PhOPh). As the rate determining step is the oxidative addition step (+24 kcal mol(-1)), we also calculated the free energy barriers for the oxidative addition of diaryl ether containing a trifluoromethyl electron withdrawing group (PhOC6H4CF3) and found that C-O bond activation at the carbon adjacent to the aryl ring that contains the electron withdrawing substituent is preferred. This is in agreement with the experimental results, in that the major products are phenol and trifluoromethylbenzene. Moreover, the hydrogenation of benzene via Ni(SIPr)(η(2)-C6H6) requires a high energy barrier (+39 kcal mol(-1)); correspondingly, the hydrogenation products, e.g. cyclohexane and cyclohexadiene, were not observed in the experiment. Understanding the reaction mechanisms of the nickel catalysts for C-O bond hydrogenolysis of diphenyl ether will guide the development of catalytic systems for aromatic C-O bond activation to achieve the highest possible selectivity and efficiency.

Rapid Ether and Alcohol C–O Bond Hydrogenolysis Catalyzed by Tandem High-Valent Metal Triflate + Supported Pd Catalysts

The thermodynamically leveraged conversion of ethers and alcohols to saturated hydrocarbons is achieved efficiently with low loadings of homogeneous M(OTf)n + heterogeneous Pd tandem catalysts (M = transition metal; OTf = triflate; n = 4). For example, Hf(OTf)4 mediates rapid endothermic ether ⇌ alcohol and alcohol ⇌ alkene equilibria, while Pd/C catalyzes the subsequent, exothermic alkene hydrogenation. The relative C-O cleavage rates scale as 3° > 2° > 1°. The reaction scope extends to efficient conversion of biomass-derived ethers, such as THF derivatives, to the corresponding alkanes.

ChemInform Abstract: Nickel‐Catalyzed Hydrogenolysis of Unactivated Carbon—Cyano Bonds.

Selective hydrogenolysis of C–CN bonds can allow chemists to take advantage of ortho-directing ability, α-C–H acidity and electron withdrawing ability of the cyano group for synthetic manipulations. We have discovered hydrogenolysis of aryl and aliphatic cyanides under just 1 bar of hydrogen by using a nickel catalyst. This protocol was applied in the aryl cyanide directed functionalization reaction and α-substitution of benzyl cyanides.

A copper-phyllosilicate core-sheath nanoreactor for carbon–oxygen hydrogenolysis reactions

Hydrogenolysis of carbon-oxygen bonds is a versatile synthetic tool in organic synthesis. Copper-based catalysts have been intensively explored as the copper sites account for the highly selective hydrogenation of carbon-oxygen bonds. However, the inherent drawback of conventional copper-based catalysts is the deactivation by metal-particle growth and unstable surface Cu(0) and Cu(+) active species in the strongly reducing hydrogen and oxidizing carbon-oxygen atmosphere. Here we report the superior reactivity of a core (copper)-sheath (copper phyllosilicate) nanoreactor for carbon-oxygen hydrogenolysis of dimethyl oxalate with high efficiency (an ethanol yield of 91%) and steady performance (>300 h at 553 K). This nanoreactor, which possesses balanced and stable Cu(0) and Cu(+) active species, confinement effects, an intrinsically high surface area of Cu(0) and Cu(+) and a unique tunable tubular morphology, has potential applications in high-temperature hydrogenation reactions.

Stability and activity of Pt and Ni catalysts for hydrodeoxygenation in supercritical water

We determined the stability, activity, and selectivity of Pt/C, Ni/SiO2Al2O3, Raney Ni 2800, and Raney NiCu for the hydrodeoxygenation (HDO) of o-cresol in supercritical water at 380°C. We also demonstrate the feasibility of using a renewable chemical, formic acid, for the in situ production of the H2 needed for HDO. Pt/C, Raney Ni, and Raney NiCu were stable in supercritical water, but only Pt/C and Raney NiCu were selective to deoxygenated products. The Raney NiCu catalyst provided a 3.4-fold increase in yield of deoxygenated products when compared with the Raney Ni catalyst by decreasing the CC bond hydrogenolysis activity. The Raney NiCu catalyst was stable throughout the 24h time on stream that was examined and appears to be a promising catalyst for further study and optimization.

Modifier–substrate interactions of various types in the Orito reaction: Reversal of the enantioselection in the hydrogenation of ketopantolactone on Pt modified by β-isocinchonine and O-phenylcinchonidine

The enantioselective hydrogenation of ketopantolactone (KPL) on Pt–alumina catalyst modified by β-isocinchonine (β-ICN) and O -phenylcinchonidine (PhOCD) in toluene, acetic acid and their mixtures under otherwise identical experimental conditions was studied. Reversal of the enantioselection was obtained dependent on the concentration of acetic acid (ee max = 17% ( S ) on Pt–PhOCD and 50% ( R ) on Pt–β-ICN, respectively). The possible role in enantioselection of adducts forming in the reaction mixture and the stability of PhOCD under the conditions of the hydrogenation was investigated by ESI-MS. The results of the nonlinear phenomenon measurements on β-ICN + PhOCD mixtures suggest that the intermediate surface complexes β-ICN–KPL and PhOCD–KPL responsible for the opposite enantioselection include different types of interactions and the enantioselection is directed by the competition between these interactions. ► Hydrogenation of KPL over Pt modified β-ICN and PhOCD was studied. ► The reversal of the enantioselection was dependent on the AcOH concentration. ► PhOCD did not suffered C–O bond hydrogenolysis in the presence of AcOH. ► Competing intermediate complexes involving different interactions were suggested.

Accuracy of Calculations of Heats of Reduction/Hydrogenation: Application to Some Small Ring Systems

The enthalpies of reduction of carbonyl compounds and hydrogenation of alkenes have been calculated at the HF, B3LYP, M06, MP2, G3, G4, CBS-QB3, CBS-APNO, and W1BD levels and, in the case of the first four methods, using a variety of basis sets up to aug-cc-pVTZ. The results are compared with the available experimental data, and it is found that the compound methods are generally more satisfactory than the others. Large basis sets are usually needed in order to reproduce experiments. Some C-C bond hydrogenolysis reactions also have been examined including those of bicycloalkanes and propellanes. In addition, the dimerization of the remarkably strained bicyclo[2.2.0]hex(1,4)ene was studied. The reaction forming a pentacyclic propellane was calculated to have ΔH = -57 kcal/mol, and the cleavage of the propellane to give a diene had ΔH = -71 kcal/mol. The strain energies of these compounds were estimated.

Etheric C–O Bond Hydrogenolysis Using a Tandem Lanthanide Triflate/Supported Palladium Nanoparticle Catalyst System

Selective hydrogenolysis of cyclic and linear ether C-O bonds is accomplished by a tandem catalytic system consisting of lanthanide triflates and sinter-resistant supported palladium nanoparticles in an ionic liquid. The lanthanide triflates catalyze endothermic dehydroalkoxylation, while the palladium nanoparticles hydrogenate the resulting intermediate alkenols to afford saturated alkanols with high overall selectivity. The catalytic C-O hydrogenolysis is shown to have significant scope, and the C-O bond cleavage is turnover-limiting.

C–O bond hydrogenolysis of cyclic ethers with OH groups over rhenium-modified supported iridium catalysts

Hydrogenolysis of tetrahydrofurfuryl alcohol to 1,5-pentanediol and other related substrates such as 3-hydroxytetrahydrofuran and 1,2-cyclohexanediol proceeds over Ir–ReOx/SiO2 catalyst. TOF values are higher than those of Rh–ReOx/SiO2, which has been reported to be an effective catalyst. The selectivity to the product, where the C–O bond neighboring the C–OH group in the substrate is dissociated, is comparable to or higher than that of Rh–ReOx/SiO2. Hydrogenolysis of most substrates except 1,2-cyclohexanediol proceeds via the direct mechanism where hydride species formed from hydrogen molecule attacks the anti-position of C–O bond. In the case of hydrogenolysis of 1,2-cyclohexanediol where attack of anti-position of C–O bond is unfavorable, indirect mechanism involving dehydrogenation to 2-hydroxycyclohexanone is responsible for the formation of cyclohexanol.

Novel synthesis of 8-deaza-5,6,7,8-tetrahydroaminopterin analogues via an aziridine intermediate.

An efficient method for the construction of the tetrahydrofolate skeleton is described. Starting from pterin analogues and aromatic amines, 8-deaza-5,6,7,8-tetrahydroaminopterin derivatives and the heterocyclic benzoyl isosteres were synthesized via a novel aziridine intermediate. Following this method, the byproducts of carbon-nitrogen bond hydrogenolysis in traditional synthetic strategy can be completely avoided.

Gas-phase hydrodeoxygenation of guaiacol over Fe/SiO2 catalyst

Lignin could be an important green source for aromatic hydrocarbon production (benzene, toluene and xylenes, BTX). Catalytic hydrodeoxygenation (HDO) of guaiacol was studied over Fe/SiO2 as a model reaction of lignin pyrolysis vapours hydrotreatment. The catalytic conditions were chosen to match with the temperature of never-condensed lignin pyrolysis vapours. The catalyst was characterised by XRD, Mössbauer spectroscopy, N2 sorption and temperature programmed oxidation. A comparison is made with a commercial cobalt-based catalyst. Cobalt-based catalyst shows a too high production of methane. Fe/SiO2 exhibits a good selectivity for BT production. It does not catalyse the aromatic ring hydrogenation. Temperature (623–723K) and space time (0.1–1.5gcath/gGUA) influence the aromatic carbon–oxygen bond hydrogenolysis reaction whereas H2 partial pressure (0.2–0.9bar) has a minor influence. 38% of BT yield was achieved under the best investigated conditions. Reaction mechanisms for guaiacol conversion over Fe/SiO2 are discussed.

Evolution and interlayer dynamics of active sites of promoted transition metal sulfide catalysts under hydrodesulfurization conditions

A new mechanistic model for molybdenum sulfide catalysts with cobalt and nickel promoters under hydrodesulfurization reaction conditions is proposed, which includes dynamic migration of sulfur and promoter atoms between adjacent sulfide layers. These migrations are caused by heterolytic dissociation of gas-phase hydrogen and formation of hydride hydrogen on a promoter atom. Hydride hydrogen adsorbed on a promoter induces electron density transfer from the bonding promoter to Mo, changing its catalytic activity. The proposed concept of dynamic migrations provides new interpretation for the “rim-edge” model, which is based on the formation of two types of active sites: highly active (“rapid”) and less active (“slow”) sites. The “slow” sites are proposed to be associated with a non-promoted cluster on the edge of a MoS2 slab layer (“rim”), which catalyzes hydrogenation. The “rapid” sites are proposed to be associated with a combination of promoted and non-promoted clusters on edges of the same (type I) or adjacent (type II) MoS2 layers. The “rapid” sites catalyze C–S bond hydrogenolysis. Adsorbed hydrogen induces migration of sulfur and promoter atoms between adjacent clusters. This dynamic migration causes a transformation of “rapid” into “slow” sites and vice versa and, therefore, influences catalytic activity. Migration of Co or Ni promoter atoms to Mo sites on the rim is more likely than to Mo sites on the edge of Co(Ni)MoS layers because the former can accept promoter atoms from both upper and lower layers. The proposed model provides critical information for rational design of improved hydrotreating catalysts and selection of preferred catalytic reaction conditions for various types of hydrocarbon feedstocks through optimization of the density and ratio of “rapid” and “slow” catalyst active sites.