Hydrogen Transfer Mechanism Research Articles

Tuning catalytic selectivity of liquid-phase hydrogenation of furfural via synergistic effects of supported bimetallic catalysts

Bimetallic catalysts supported over TiO2–ZrO2 binary oxides were prepared by co-impregnation methods and used for catalyzing liquid-phase hydrogenation of furfural. Highly selective hydrogenation catalysts can be developed based on bimetallic synergistic effect. The coexistence of small proportion of palladium with supported nickel species greatly improves the catalytic performance and transfer the reaction selectivity from partial hydrogenation to total hydrogenation. The catalyst with Ni–Pd mole ratio of 5:1 shows the best performance. The yield of tetrahydrofurfuryl alcohol (THFA) reaches 93.4%. Ni–Pd synergistic effect is interpreted through XPS measurement and a hydrogen-transfer mechanism is proposed. Pt–Re bimetallic catalyst is an excellent partial hydrogenation catalyst for furfural conversion. Furfural can be totally converted and the selectivity of partial hydrogenation product (FA) reaches 95.7%. When rhenium oxide species are located on the Pt surface, the hydrogen species on Pt are transferred to adsorbed CO bond to achieve selective hydrogenation.

ChemInform Abstract: Radical Reactions of Borohydrides

AbstractReview: [58 refs.

Hydrogen generation from alcohols (α-hydroxy carboxylic acids) and alcohol–ammonia coupling in aqueous media catalysed by water-soluble bipyridine-Cp*Ir (Rh or Os) catalyst: a computational mechanism insight

Density functional theory (DFT) calculations were performed to elucidate the mechanism of the dehydrogenative oxidation of various primary alcohols (or α-hydroxy carboxylic acids) and the dehydrogenative coupling of alcohols with ammonia catalysed by the same water-soluble Cp*Ir complex bearing a 2-pyridonate-based ligand (A–Ir). Another two new catalysts A–Rh and A–Os are computationally designed for the dehydrogenative oxidation of alcohols. The plausible pathway for alcohol dehydrogenation includes three steps: alcohol oxidation to aldehyde (step I); the generation of dihydrogen in the metal coordination sphere (step II); and the liberation of dihydrogen accompanied with the regeneration of active catalyst A (step III). Among them, the step I follows bifunctional concerted double hydrogen transfer mechanism rather than the β-H elimination. For step II, the energy barriers involving the addition of one or two water molecules are higher than in absence of water. Our results also confirm that A–Ir can be applied in the dehydrogenation of various α-hydroxy carboxylic acids by the similar mechanism. Remarkably, A–Ir is also found to be efficient for the coupling reactions of various primary benzyl alcohols with ammonia to afford amides.

Chemo- and diastereoselectivities in the electrochemical reduction of maleimides.

The electrochemical cathodic reduction of cyclic imides (maleimides) to succinimides can be achieved chemoselectively in the presence of alkene, alkyne, and benzyl groups. The efficiency of the system was demonstrated by using a 3D electrode in a continuous flow reactor. The reduction of 3,4-dimethylmaleimides to the corresponding succinimides proceeds with a 3:2 diastereomeric ratio, which is independent of the nitrogen substituent and electrode surface area. The stereoselectivity of the process was rationalized by using DFT calculations, involving an acid-catalyzed tautomerization of a half-enol occurring through a double hydrogen-transfer mechanism.

Vacancy-induced initial decomposition of condensed phase NTO via bimolecular hydrogen transfer mechanisms at high pressure: a DFT-D study.

Density functional theory with dispersion-correction (DFT-D) was employed to study the effects of vacancy and pressure on the structure and initial decomposition of crystalline 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (β-NTO), a high-energy insensitive explosive. A comparative analysis of the chemical behaviors of NTO in the ideal bulk crystal and vacancy-containing crystals under applied hydrostatic compression was considered. Our calculated formation energy, vacancy interaction energy, electron density difference, and frontier orbitals reveal that the stability of NTO can be effectively manipulated by changing the molecular environment. Bimolecular hydrogen transfer is suggested to be a potential initial chemical reaction in the vacancy-containing NTO solid at 50 GPa, which is prior to the C-NO2 bond dissociation as its initiation decomposition in the gas phase. The vacancy defects introduced into the ideal bulk NTO crystal can produce a localized site, where the initiation decomposition is preferentially accelerated and then promotes further decompositions. Our results may shed some light on the influence of the molecular environments on the initial pathways in molecular explosives.

Ethanol Dehydration and Dehydrogenation on γ-Al2O3: Mechanism of Acetaldehyde Formation

Steady state kinetics and measured pyridine inhibition of ethanol dehydration and dehydrogenation rates on γ-alumina above 623 K show that ethanol dehydrogenation can be described with an indirect hydrogen transfer mechanism to form acetaldehyde and ethane and that this mechanism proceeds through a shared surface intermediate with ethylene synthesis from ethanol dehydration. Ethane is produced at a rate within experimental error of acetaldehyde production, demonstrating that ethane is a coproduct of acetaldehyde synthesis from ethanol dehydrogenation. Steady state kinetic measurements indicate that acetaldehyde synthesis rates above 623 K are independent of co-fed water partial pressure up to 1.7 kPa and possess an ethanol partial pressure dependence between 0 and 1 (Pethanol = 1.0–16.2 kPa), consistent with ethanol dehydrogenation rates being inhibited only by ethanol monomer surface species. The surface density of catalytically active sites for ethylene and diethyl ether production were estimated from i...

Evaluating the antioxidant capacity of polyphenols with an off–on fluorescence probe and the mechanism study

A spin-labeled fluorescence probe (RNO˙) was employed to evaluate the antioxidant capacity of polyphenols. The hydrogen-transfer mechanism was proposed by kinetic studies using fluorescence spectroscopy, and the product was confirmed to be RNOH by HPLC-MS with little side products. The HPLC-UV/vis method devoted to the simultaneous monitoring of the RNO˙ and RNOH at 556 nm was developed, which can distinguish the antioxidant capacity of polyphenols with similar structure. The method has the potential to be used to screen antioxidants when modifying or optimizing natural antioxidants.

Experimental and theoretical insights into photochemical transformation kinetics and mechanisms of aqueous propylparaben and risk assessment of its degradation products.

The kinetics and mechanisms of ultraviolet photochemical transformation of propylparaben (PPB) were studied. Specific kinetics scavenging experiments coupled with quantum yield determinations were used to distinguish the roles of various reactive species induced by self-sensitized and direct photolysis reactions, and the excited triplet state of PPB ((3) PPB*) was identified as the most important species to initiate the photochemical degradation of PPB in aquatic environments. The computational results of time-resolved absorption spectra proved that (3) PPB* is a highly reactive electron acceptor, and a head-to-tail hydrogen transfer mechanism probably occurs through electron coupled with proton transfer. Physical quenching by, or chemical reaction of (3) PPB* with, O2 was confirmed as a key step affecting the initial PPB transformation pathways and degradation mechanisms. The transformation products were identified and the toxicity evolutions of PPB solutions during photochemical degradation under aerobic and anaerobic conditions were compared. The results indicate that anaerobic conditions are more likely than aerobic conditions to lead to the elimination and detoxification of PPB but less likely to lead to PPB mineralization.

Computational study on the hydrogen transfer of H2O and O2 on Fe(II) porphyrazine

Computational study is performed to probe the hydrogen transfer mechanism of O2 and H2O catalyzed by the Fe(II) porphyrazine on the lowest singlet, triplet and quintet states. Results suggest that Fe(II) prefers to bind H2O with higher adsorption energy than that of O2. The hydrogen transfer from H2O to O2 is hindered by the high energy barrier and reaction heat in the ground singlet and quintet states, which becomes easier in the excited triplet state. Both the explicit and implicit solvent effects of water favor the hydrogen transfer. The energy barriers are linearly correlated with the reaction heats according to the classic Brønsted–Evans–Polanyi relations.

Lipoic acid and dihydrolipoic acid. A comprehensive theoretical study of their antioxidant activity supported by available experimental kinetic data.

The free radical scavenging activity of lipoic acid (LA) and dihydrolipoic acid (DHLA) has been studied in nonpolar and aqueous solutions, using the density functional theory and several oxygen centered radicals. It was found that lipoic acid is capable of scavenging only very reactive radicals, while the dehydrogenated form is an excellent scavenger via a hydrogen transfer mechanism. The environment plays an important role in the free radical scavenging activity of DHLA because in water it is deprotonated, and this enhances its activity. In particular, the reaction rate constant of DHLA in water with an HOO(•) radical is close to the diffusion limit. This has been explained on the basis of the strong H-bonding interactions found in the transition state, which involve the carboxylate moiety, and it might have implications for other biological systems in which this group is present.

DFT Study on Mechanism of N-Alkylation of Amino Derivatives with Primary Alcohols Catalyzed by Copper(II) Acetate

DFT calculations have been carried out to study the mechanism of Cu(AcO)2-catalyzed N-alkylation of amino derivatives with primary alcohols. The calculations indicate that tBuOK is necessary for the generation of the active catalyst from Cu(AcO)2 and that the catalytic cycle involves three sequential steps: (1) Cu-catalyzed alcohol oxidation to give the corresponding aldehyde and copper hydride, (2) aldehyde-amine condensation to generate an imine, (3) imine reduction to yield the expected N-alkylation secondary amine product and to regenerate the active catalyst. Based on the comparison of different reaction pathways, we conclude that the outer-sphere hydrogen transfer in a stepwise manner is the most favorable pathway for both alcohol oxidation and imine reduction. Thermodynamically, alcohol oxidation and imine formation are all uphill, but imine reduction is downhill significantly, which is the driving force for the catalytic transformation. Using the energetic span model, we find that the turnover frequency-determining transition state (TDTS) and the turnover frequency-determining intermediate (TDI) are the hydride transfer transition state for imine reduction and the active catalyst, respectively. The calculated turnover frequency (TOF) roughly agrees with the experimental observation and, therefore, further supports the validity of the proposed hydrogen transfer mechanism.

Rhenium-catalyzed amination of alcohols by hydrogen transfer process

The rhenium heptahydride complex [ReH7(PCy3)2] was found to be an effective homogeneous catalyst for amination of various alcohols through hydrogen transfer mechanism. Under carbon monoxide atmosphere, a variety of primary and secondary alcohols could directly undergo the CN coupling process.

Coupling Between Hydrogen Atoms Transfer and Stacking Interaction in Adenine-Thymine/Guanine-Cytosine Complexes: A Theoretical Study

Four different complexes of two base pairs, an adenine-thymine and a guanine-cytosine one, have been studied in order to understand the modifications induced by the staking interaction between the two base pairs on the hydrogen atoms transfers between the bases in either base pair. The inclusion of these two kinds of interactions allows us to clarify if some properties, as the mechanism of hydrogen transfer, is exclusively a local effect of a base pair or can be modified by a more long-range interaction between the base pairs. The results on these four complexes are compared with those of the monomeric systems, the A-T and G-C base pair, and with those of the A-T and G-C dimers. The specificity of each complex and of each hydrogen bond has been analyzed.

Comparative study of the performance of an anaerobic rotating biological contactor and its potential to enrich hydrogenotrophic methanogens

BACKGROUND Process upset due to volatile fatty acid accumulation is one of the major drawbacks of anaerobic treatment of wastewater. The performance of an anaerobic rotating biological contactor (AnRBC) was studied under normal operating conditions as well as shock loadings of propionic acid, for its ability to enhance the conversion of hydrogen to methane. These results were compared with those of a reactor lacking the enhanced gas-liquid hydrogen transfer mechanism, a conventional anaerobic digester. RESULT The AnRBC exhibited lower concentrations of hydrogen in the headspace (300–600 ppm) under both normal and shock loading conditions, and also showed more hydrogenotrophic methanogens on the denaturing gradient gel electrophoresis (DGGE) profile of 16S rRNA gene amplicons, both in terms of intensity and band numbers (4 in the AnRBC and 2 in the anaerobic digester). CONCLUSION This study showed that the AnRBC is able to maintain lower hydrogen partial pressure at both normal operating and shock loading conditions due to the abundance of hydrogenotrophic methanogens and good gas–liquid hydrogen transfer efficiency. This finding should encourage the commercial exploitation of AnRBC for the treatment of wastewaters, especially that containing easily acidifiable organic compounds, which causes instability in operation through souring. © 2014 Society of Chemical Industry

Quantum-chemical study on relationship between structure and antioxidant properties of hepatoprotective compounds occurring in Cynara scolymus and Silybum marianum

Accurate quantum computations based on the density functional theory have been performed to study the relationship between the electronic geometry and antioxidant capacity of chlorogenic acid, silybin and all geometric stereoisomers of cynarine, isolated from plant extract of Cynara scolymus and Silybum marianum. To elucidate their antioxidant activity, the HOMO orbital distribution, adiabatic ionization potential (AIP), spin density in free radicals, homolytic dissociation enthalpies (BDE), and proton dissociation enthalpies (PDE) of the O – H bonds have been calculated. For minimum energy conformations, the antioxidative parameters were quantitatively analyzed at the B3LYP/6-311G(d,p) level of theory. The results have shown that the hydrogen transfer mechanism is more preferable in nonpolar medium than in water. From the results obtained we can conclude that SET-PT (single electron transfer followed by proton transfer) is the most preferred mechanism in water medium. The catechol moiety and planar geometry of trans-stilbene and cis-stilbene moieties in cynarine stereoisomers, chlorogenic acid and their phenoxy radicals strongly contribute to enhancement of the antioxidant activity of these compounds. Trans,trans-cynarine appears to be the best candidate for proton and hydrogen atom donor. It has been predicted that cis,cis, trans,cis and cis,trans stereoisomers of cynarine show antioxidant capacity. Our study shows that all of the investigated compounds reveal strong antioxidant activity.

Deep hydrogenation of coal tar over a Ni/ZSM-5 catalyst

We have developed a Ni/ZSM-5 catalyst and utilised it to completely hydrogenate a series of condensed arenes, including naphthalene, anthracene and phenanthrene, under relatively mild conditions. The yields of decalin, perhydroanthracene and perhydrophenanthrene reached 100%, 98.8% and 25.8%, respectively. By analyzing the isomer distribution of the perhydroarenes, we proposed a mechanism of biatomic hydrogen transfer, which was further proven via the hydrogenation of 9,10-diphenylanthracene. Through hydrotreatment over the Ni/ZSM-5 catalyst, both high-temperature coal tar rich in condensed arenes and low-temperature coal tar mixing arenes with alkanes were greatly upgraded. The majority of arenes in the coal tars were deeply and even completely hydrogenated, which may open up a route for further processing and application of coal tar as clean fuels.

Radical reactions of borohydrides.

Borohydrides are an important class of reagents in both organic and inorganic chemistry. Though popular as hydride-transfer reagents for reduction, since earlier work from the 1970s, borohydride reagents have also been known to serve as hydrogen-transfer reagents. In pursuit of greener tin hydride substitutes, recent progress has been made to mediate radical C-C bond forming reactions, including Giese reactions, radical carbonylation and addition to HCHO reactions, with borohydride reagents. This review article focuses on state-of-the-art borohydride based radical reactions, also covering earlier work, kinetics and some DFT calculations with respect to the hydrogen transfer mechanism.

Mechanism of the Formation of Carboxylate from Alcohols and Water Catalyzed by a Bipyridine-Based Ruthenium Complex: A Computational Study

The catalytic mechanism for oxidizing alcohols to carboxylate in basic aqueous solution by the bipyridine-based ruthenium complex 2 (BIPY-PNN)Ru(H)(Cl)(CO) (Nat. Chem. 2013, 5, 122) is investigated by density functional theory (DFT) with the ωB97X-D functional. Using water as the oxygen donor with liberation of dihydrogen represents a safe and clean process for such oxidations. Under NaOH, the active catalyst is 3 (BIPY-PNN)Ru(H)(CO). Four steps are involved: dehydrogenation of alcohol to aldehyde (Step 1); coupling of aldehyde and water to form the gem-diol (Step 2); dehydrogenation of gem-diol to carboxylic acid (Step 3); and deprotonation of carboxylic acid to carboxylate anion under base (Step 4). The dehydrogenations of alcohol (Step 1) and gem-diol (Step 3) prefer the double hydrogen transfer mechanism to the β-H elimination mechanism. The coupling of aldehyde and water (Step 2) proceeds through cleavage of water by catalyst 3 followed by concerted hydroxyl and hydrogen transfer to the aldehyde. The formation of the carboxylate anion occurs via direct deprotonation of the carboxylic acid under base (Step 4), while in the absence of base a stable carboxylic acid-addition complex 6 was formed. Added base was found to play important roles in the generation of catalyst 3 from both the stable carboxylic acid-addition complex 6 and its chloride precursor complex 2. The chemoselectivity for the formation of carboxylic acid rather than ester is ascribed to the favorable cleavage of water and the subsequent generation of the stable carboxylate anion that leads to carboxylic acid upon acidification.

Hydration Dynamics for Vanadia/Titania Catalysts at High Loading: A Combined Theoretical and Experimental Study

Periodic DFT calculations are used to simulate the early stages of hydration of dehydrated high-loading vanadia/titania catalysts. The hydration of molecularly dispersed vanadia is studied by successive additions of water molecules to initially dehydrated models. Special attention is paid to the formation and transformation between different surface species, monovanadates and polyvanadates, and the role of V–OH in the hydration process. It is found that two physical surface processes occur at high vanadia coverage and that their importance depends on the surface water content. First, under mild hydration conditions, a polymerization process increases the number of polyvanadates chains. Polyvanadates formation is preceded by an initial generation of monomers with OV(OH)O2 and OVO3 pyramidal structures. Second, with higher number of water molecules, a solvation process increases the coordination number of vanadium. The interconversion between different surface vanadium oxide species occurs through a fast hydrogen transfer mechanism and depends on water content. Theoretical results are combined with in situ Raman spectra acquired at several temperatures in dry and humid environment during stepwise dehydration. The experimental data indicate that the redshift of the vanadyl band upon exposure to increasing humidity and decreasing temperature is associated to such progressive interaction with water molecules, which weakens the vanadyl V═O bond.

Zirconia-supported niobia catalyzed formation of propanol from 1,2-propanediol via dehydration and consecutive hydrogen transfer

Vapor-phase catalytic dehydration of 1,2-propanediol was investigated over Zirconia-supported niobia catalysts. The catalysts exhibit selectivity favoring propanol (approximately 39%) at 85.0% 1,2-propanediol conversion at 290°C under 1atm N2. The ZrNbO catalysts were analyzed by various techniques; the results indicated that the active sites were weak Brønsted acid sites. A dehydration and hydrogen transfer mechanism was also proposed.