Hydrogen Migration Reactions Research Articles

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

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

Theoretical kinetic studies on intramolecular H-migration reactions of peroxy radicals of diethoxymethane.

Diethoxymethane (DEM), a promising carbon-neutral fuel, has high reactivity at low temperatures. The intramolecular hydrogen migration reaction of the DEM peroxy radicals can be viewed as a critical step in the low temperature oxidation mechanism of DEM. In this work, multistructural transition state theory (MS-TST) was utilized to calculate the high-pressure limit rate constants of 1,5, 1,6 and 1,7 H-migration reactions for DEM peroxy radicals. In addition to the tunneling effects and anharmonic effects, the intramolecular effects, including steric hindrance, intramolecular hydrogen bonding and conformational changes in reactants and transition states, are also considered in the rate constant calculations. The calculated energy barriers and rate constants demonstrated the substantial impact of intramolecular effects on the kinetics of H-migration reactions in DEM peroxy radicals. Specifically, the distinct configurations of transition states could potentially influence the reaction kinetics. The pressure-dependent rate constants are computed using system-specific quantum RRK theory. The calculated results show that the falloff effect of 1,5 and 1,6 H-migration reactions is more pronounced than that of the 1,7 H-migration reaction. The thermodynamics and kinetics presented in this study could be instrumental in understanding the low-temperature oxidation mechanism of DEM and might prove crucial for future research on comprehensively analyzing the autoignition behavior.

Exploring low-temperature oxidation chemistry of 2- and 3-pentanone

The low-temperature oxidation chemistry of 2- and 3-pentanone was investigated over a wide range of conditions using a jet-stirred reactor (JSR) and a rapid compression machine (RCM). The JSR oxidation experiment was performed at the pressure of 93.3 kPa over a temperature range of 600-1000 K. Detailed speciation information was obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. Ignition delay times (IDTs) of 2- and 3-pentanone were measured in an RCM from 640 to 820 K at pressures of 15 and 25 bar and an equivalence ratio of 1.0. The two C5 ketones showed NTC behavior and two-stage ignition phenomena. Mole fraction time histories of intermediate species during the two-stage ignition process of both ketones were obtained using a fast-sampling system coupled with gas chromatography. There are distinct differences between 2- and 3-pentanone in species concentration profiles and IDTs. A kinetic mechanism for the low-temperature oxidation of 2- and 3-pentanone was developed, which can satisfactorily predict all available measurements. The reaction path analyses indicate that the intramolecular hydrogen migration reaction of ROO radicals tends to produce resonance-stabilized QOOH radical structures. The secondary oxygen addition reaction of resonance-stabilized QOOH radicals thus is the most important source of OH radicals in the low-temperature oxidation of ketone fuels. The intramolecular hydrogen migration reactions are slowed down in the presence of the carbonyl functional group, which makes the low-temperature reactivity of the two C5 ketones lower than that of n-pentane. The position of the carbonyl functional group affects the species pools during the oxidation of the two ketones to a great extent. Larger production of CH4, C3H6, CH3COCH3, and C2H5CHO were observed in 2-pentanone oxidation, while the production of CH3CHO was favored during 3-pentanone oxidation both in the JSR and RCM experiments. The different lengths of the carbon chain on both sides of the carbonyl group in 2- and 3-pentanone resulted in the difference in the species distribution.

The kinetic model of ethylcyclohexane combustion over a wide temperature range and its comprehensive validation

An experimental and modeling study is performed to develop a high accuracy combustion model of ethylcyclohexane (ECH) over a wide temperature range. The shock tube experiments are conducted to determine ignition delay times for ECH/air mixtures at 678–1287 K. Extensive quantum-chemical calculations concerning the low temperature kinetics of ECH including oxygen addition to hydroperoxyalkyl (·QOOH) and the following decomposition of hydroperoxyalkylperoxy radicals (·OOQOOH) have been investigated at the level of CBS-QB3//M06-2X/6-311++G(d,p). Due to the mutual influence of hydrogen bond and ring structure, the species configuration needs to be carefully determined in order to obtain thermodynamic data and then kinetic parameters. Different temperature- and pressure-dependent rate constants are computed for the side chain of ECH, the ring part, and the part where the ring and the side chain intersect. The rate constants of the hydrogen migration reaction are compared with the values estimated by analogizing with linear alkanes. Obvious deviations are observed especailly at low temperature. A detailed kinetic model is developed based on the existing low temperature model of ECH with the replacement of reaction calculated in present work and the core mechanism of AramcoMech 3.0. The model is validated comprehensively with experimental results of ignition delay time, laminar flame speed and species concentration profiles in jet-stirred reactor (JSR), which shows a good prediction over a wide temperature range. The results of sensitivity analyses show that the computed reactions of ·QOOH and ·OOQOOH have a great impact on the low temperature oxidation of ECH, which help to accurately reproduce the negative-temperature-coefficient behavior of ECH ignition. The kinetic parameters obtained in this work are conductive to improve the combustion models of substituted cycloalkanes with alkyl side chains, and the obtained kinetic model with high accuracy of ECH can be used as a basis to develop model of jet fuel.

Generative adversarial networks for transition state geometry prediction.

This work introduces a novel application of generative adversarial networks (GANs) for the prediction of starting geometries in transition state (TS) searches based on the geometries of reactants and products. The multi-dimensional potential energy space of a chemical reaction often complicates the location of a starting TS geometry, leading to the correct TS combining reactants and products in question. The proposed TS-GAN efficiently maps the space between reactants and products and generates reliable TS guess geometries, and it can be easily combined with any quantum chemical software package performing geometry optimizations. The TS-GAN was trained and applied to generate TS guess structures for typical chemical reactions, such as hydrogen migration, isomerization, and transition metal-catalyzed reactions. The performance of the TS-GAN was directly compared to that of classical approaches, proving its high accuracy and efficiency. The current TS-GAN can be extended to any dataset that contains sufficient chemical reactions for training. The software is freely available for training, experimentation, and prediction at https://github.com/ekraka/TS-GAN.

Effect of alkali metal ions on the formation mechanism of HCN during pyridine pyrolysis

The catalytic effects of alkali metal ions (Na+ and K+) on NOx precursor formation during coal pyrolysis were investigated using the N-containing compound pyridine as a model compound. Density functional theory calculations at the B3LYP/6-31G (d, p) level of theory were conducted to elucidate the mechanism of pyridine pyrolysis and the pathways for HCN formation. The calculation results indicate that Na+ and K+ have distinct influences on different pyrolysis reactions; these alkali metal ions facilitate the initial hydrogen transfer from C1 to N and C2, whereas they hinder the other hydrogen migration reactions. Both Na+ and K+ significantly reduce the activation energies for C–C bond breakage and triple-bond formation, whereas they increase the activation energies for the isomerization reactions. The different effects essentially result from the distinct charge distributions induced by the two ions. Due to the distinct influences on the different reactions, the rate-determining steps are modulated, affecting the competitiveness of the different possible pathways of HCN formation. The formation of HCN from pyridine is promoted in the presence of Na+ and K+ because all the overall activation energies are decreased for different pathways. The calculation results agree well with previous experimental studies. Thus, the findings offer a new and promising approach to reveal the formation mechanism of NOx and facilitate the control of NOx for coal utilization.

Al2O3 ALD films grown using TMA + rare isotope 2[formula omitted]O and 1[formula omitted]O precursors

In this work hydrogen and oxygen migration and exchange reactions in the atomic layer deposited (ALD) Al2O3 thin films were studied together with hydrogen incorporation by varying deposition parameters. Al2O3 films deposited at low temperatures can contain more than 20 at.% of hydrogen. Both higher temperature and longer purge length decrease the hydrogen and carbon concentrations significantly. In order to track the hydrogen and oxygen movement in the films, heavy water (2H216O) and oxygen-18 enriched water (1H218O) were used as precursors in combination with trimethylaluminium (TMA). Different isotopes of the same element were quantified by means of time-of-flight elastic recoil detection analysis (ToF-ERDA). It was found out that 1H/2H exchange reactions take place even at room temperature if the hydrogen concentration is high enough. On the other hand, oxygen atoms in the films do not migrate notably.

Effect of SiC nano-size fillers on the aging resistance of XLPE insulation: A first-principles study

Effective strategies to prevent the electrical tree growth and extend the lifetime of cross-linked polyethylene (XLPE) insulation are of crucial importance for the development of power cable industry. The first-principles study with DFT method was performed to understand the mechanism of aging resistance for SiC/XLPE nanocomposite. The Bader charge redistribution calculations between different types of silicon carbide and XLPE suggest that SiC fillers have the ability of capturing hot electrons to suppress the accumulation of space charges because of their unsaturated electronic structures and magnetism on the Si- or C-terminated surfaces. The interfacial behavior between SiC and XLPE was investigated based on the physical interaction and chemical reactivity. The 3C, 4H and 6H-type SiC with flat Si-terminated surfaces are proposed to be favorable additives by showing relatively strong physical interaction with XLPE to limit its movement, and high activation energy for the hydrogen migration reaction to protect the XLPE.

Production of Benzene from 2‐Hydroxybenzaldehyde by Various Reaction Paths using IRC Calculations within a DFT framework

AbstractThe bio‐oil produced from pyrolysis of lignocellulosic biomass comprises of several oxy‐catalogues such as acids, alcohols, aldehydes, furans, phenols, etc. The oxy‐functionals in the bio‐oils vitiate the quality of bio‐oil, therefore, the unprocessed bio‐oils cannot be used directly to transportation vehicles. In this work, 2‐hydroxybenzaldehyde (2‐HB) has been taken as bio‐oil model component and carried out seven reaction pathways to produce benzene and other important intermediate products such as phenol, benzaldehyde, etc. The first reaction pathway is about the cleavage of formyl group of 2‐HB followed by single step hydrogenation reaction to produce phenol which further produces benzene component. The reaction pathway 2 is the hydrogen migration reaction followed by cleavage of formyl group and the reaction pathway 3 is decarbonylation reaction of 2‐hydroxybenzaldehyde to produce phenol component. The reaction pathway 4 is single atom hydrogenation at the aromatic carbon of Caromatic‐CHO sigma bond followed by the removal of formyl group to produce phenol. Similarly, the reaction pathway 5 starts with a single atom hydrogenation at the aromatic carbon of Caromatic‐OH sigma bond followed by hydroxyl group removal to produce benzaldehyde component. The reaction pathway 6 is the dehydrogenation reaction of formyl group followed by CO removal and a single step hydrogenation to produce phenol. The reaction pathway 7 directly cleaves the hydroxyl functional followed by hydrogenation reaction to produce benzaldehyde which further follows two sub‐pathways to produce benzene. The direct cleavages of hydroxyl and formyl functionals are not favourable as they require very high amount of energetics. Instead, the single atom hydrogenation to the aromatic carbon of Caromatic‐X (X=•OH or •CHO) followed by the oxy‐functional removal is found to be favourable reaction pathway.

Role of Carbon-Addition and Hydrogen-Migration Reactions in Soot Surface Growth.

Using density functional theory and master equation modeling, we have studied the kinetics of small unsaturated aliphatic molecules reacting with polycyclic aromatic hydrocarbon (PAH) molecules having a diradical character. We have found that these reactions follow the mechanism of carbon addition and hydrogen migration (CAHM) on both spin-triplet and open-shell singlet potential energy surfaces at a rate that is about ten times those of the hydrogen-abstraction-carbon-addition (HACA) reactions at 1500 K in the fuel-rich postflame region. The results also show that the most active reaction sites are in the center of the zigzag edges of the PAHs. Furthermore, the reaction products are more likely to form straight rather than branched aliphatic side chains in the case of reacting with diacetylene. The computed rate constants are also found to be independent of pressure at conditions of interest in soot formation, and the activation barriers of the CAHM reactions are linearly correlated with the diradical characters.

Reactions of Imines, Nitriles, and Isocyanides with Pentaphenylborole: Coordination, Ring Expansion, C–H Bond Activation, and Hydrogen Migration Reactions

The reactions of pentaphenylborole with imines, isocyanides, and acetonitrile were investigated experimentally and theoretically. On the basis of literature precedent, we envisioned that the dipolar substrates would undergo facile ring expansion reactions to yield new BNC5 heterocycles. For acetonitrile and one particular imine, this ring expansion process was observed. However, in many cases, unexpected reactivity occurred. This included hydride migration of an imine ring expanded product and the ortho C-H bond activation of an aryl group of an imine if two phenyl groups were present on the α-carbon. A bulky group on the nitrogen atom of an imine prevented coordination to the boron center, and no reaction was observed, indicating that coordination to the borole is a critical step for any type of reaction to occur. Isocyanides made coordination complexes, but heating to induce further reactivity resulted in mixtures. The mechanisms were elucidated via DFT calculations, which complement the experimental findings.

Calculation of Rate Constants for Intramolecular Hydrogen Migration Reactions of Alkylperoxy Radicals

Calculation of Rate Constants for Intramolecular Hydrogen Migration Reactions of Alkylperoxy Radicals

Theoretical studies of ruthenium hydride-catalyzed addition reactions of benzaldehydes to isoprenes leading to β,γ-unsaturated ketones: The role of the ligands hydride, carbonyl, chloride, and triphenylphosphine of the catalyst

Density functional theory (DFT) was used to investigate ruthenium hydride-catalyzed addition reactions of benzaldehydes to isoprenes. Calculation indicated that two π bonds of isoprene had different reactivity, and the less substituted one had better reactivity. The role of the ligands hydride, carbonyl, chloride, and triphenylphosphine of RuHCl(CO)(PPh3)3 were studied in our present work. The hydride was active, and promoted this reaction. The carbonyl or chloride was not used as a carrier of hydrogen migration reaction, so they could not change the reaction channels. The only role of them was the transformation of the electron and geometry structures of those intermediates and transition states. The triphenylphosphine decreased generally the free energies of the intermediates and transition states. The di-ruthenium complexes were not an efficient catalyst.

Hydrogen Migrations in Alkylcycloalkyl Radicals: Implications for Chain-Branching Reactions in Fuels

A thorough understanding of the oxidation chemistry of cycloalkanes is integral to the development of alternative fuels and improving current fuel performance. An important class of reactions essential to this chemistry is the hydrogen migration; however, they have largely been omitted from the literature for cycloalkanes. The present work investigates all of the hydrogen migration reactions available to methylcyclopentane, ethylcyclopentane, methylcyclohexane, and ethylcyclohexane. The kinetic and thermodynamic parameters have been studied by a combination of computational methods and compared to their corresponding n-alkyl and methylalkyl counterparts to determine the effect that the cycloalkane ring has on these reactions. In particular, although the alkylcycloalkyl activation energies for the dominant 1,4, 1,5, and 1,6 H-migration are higher than in n-alkyl and methylalkyl radicals, because several of the rotors needed to form the transition state are locked into place as part of the cycloalkane ring, the A-factors are higher for the alkylcycloalkyl reactions, making the rates closer to the noncyclic systems, at higher temperatures. The results presented here suggest that the relative importance of each H-migration pathway differs from the trends predicted by either the n-alkyl or methylalkyl radical systems. Of particular interest is the observation that since the barrier height of the 1,4 H-migration is only 3-5 kcal mol(-1) higher than the 1,5 H-migration in the methyl and ethylcycloalkyl radicals, compared to a difference of roughly 7 kcal mol(-1) in similar reactions for both the n-alkyl and methylalkyl radicals, the 1,4 H-migrations in alkylcycloalkyl radicals will be more important in the overall mechanism than would be predicted based on the n-alkyl and methylalkyl radicals. These results have important combustion model implications, particularly for fuels with high cycloalkane content.

Ab Initio Study of Key Branching Reactions in Biodiesel and Fischer–Tropsch Fuels

Many biologically and Fischer-Tropsch synthesized fuels contain branched alkanes which, during their combustion and atmospheric oxidation mechanism, produce methylalkyl radicals. As a result, an accurate description of the chemistry of these species is essential to integrating these fuels into our energy systems. Even though branched alkanes make up roughly one-third of the compounds in gasoline and diesel fuels, both experimental and theoretical data on methylalkyl radicals and their reactions are scarce, especially for larger chain systems and combustion conditions. The present work investigates all the hydrogen migration reactions available to the n-methylprop-1-yl through n-methylhept-1-yl radicals, for n = 2-6, using the CBS-Q, G2, and G4 composite computational methods, over a wide temperature range. The resulting thermodynamic and kinetic parameters are used to determine the effect that the presence of the methyl group has on these important unimolecular, chain branching reactions, for the reactions involving not only a tertiary abstraction site but also all the primary and secondary sites. The activation energies of hydrogen migration reactions with the methyl group, either within or immediately outside the ring, are found to be roughly 0.8-1.6 kcal mol(-1) lower in energy than expected on the basis of analogous reactions in n-alkyl radicals. An important implication of this result is that the current method of using rate parameters derived from n-alkyl radicals to predict the chain branching characteristics of methylated alkyl radicals significantly underpredicts the importance of these reactions in atmospheric and combustion processes. Discussion of a possible cause for this phenomenon and its effect on the overall combustion mechanism of branched hydrocarbons is presented. Of particular concern is that 2,2,4,4,6,8,8-heptamethylnonane, which is currently used to model branched alkanes in diesel fuel surrogates, is predicted to have a much lower activation energy, by as much as 3 kcal mol(-1), for the dominant, 1,5 H-migration reactions than for the same reaction in many of the less branched alkanes that it is meant to represent. Also, counter to the currently accepted theory, the 3-methylalk-1-yl radical 1,4 and 1,5 H-migrations involving the movement of a secondary hydrogen are found to have higher rate coefficients than the corresponding reactions involving a tertiary hydrogen. The results presented here have significant implications for future experimental, computational, and combustion modeling studies.

Ab Initio Study of Hydrogen Migration in 1-Alkylperoxy Radicals

Alkylperoxy and hydroperoxyalkyl radicals are key reactive intermediates in hydrocarbon oxidation mechanisms. An understanding of the interconversion of these two species via a hydrogen migration reaction is of fundamental importance to the prediction of chain branching reactions and end product composition. An extensive ab initio investigation of the hydrogen migration reaction in 1-ethyl, 1-propyl, 1-butyl, 1-pentyl, and 1-hexylperoxy radicals is conducted to assess the validity of using cycloalkanes to model the ring strain of their transition states as well as the effect of both location of the migrating hydrogen and directionality of the remaining alkyl chain in the transition state of the reaction involving a secondary hydrogen. The G2 and CBS-Q composite methods are used to determine the activation energy and enthalpy of reaction relative to the alkylperoxy radical. Both methods show good agreement with five experimentally determined reaction enthalpies, having root mean squared deviations of 0.7 and 1.3 kcal mol(-1) for the CBS-Q and G2 methods, respectively. The effect of hydrogen abstraction site and transition state geometry, particularly axial and equatorial geometries of the remaining alkyl chain, on the activation energy, Arrhenius A-factor, tunneling, and rate coefficient are discussed. Differences between terminal adjacent and nonterminal adjacent secondary sites result in small but consistent differences in barrier height. Failure of key assumptions within the cycloalkane based estimation method leads to the break down in the accuracy for both small and large transition states. For large transition states, the breakdown of these assumptions also results in the failure of the current cycloalkane method as a conceptual model. Of great interest is the observed alteration in the preferred H-migration from the 1,5 to the 1,6 H-migration within the temperature region where these reactions are particularly important to the combustion mechanism.

Intramolecular Hydrogen Migration in Alkylperoxy and Hydroperoxyalkylperoxy Radicals: Accurate Treatment of Hindered Rotors

We have calculated the thermochemistry and rate coefficients for stable molecules and reactions in the title reaction families using CBS-QB3 and B3LYP/CBSB7 methods. The accurate treatment of hindered rotors for molecules having multiple internal rotors with potentials that are not independent of each other can be problematic, and a simplified scheme is suggested to treat them. This is particularly important for hydroperoxyalkylperoxy radicals (HOOQOO). Two new thermochemical group values are suggested in this paper, and with these values, the group additivity method for calculation of enthalpy as implemented in reaction mechanism generator (RMG) gives good agreement with CBS-QB3 predictions. The barrier heights follow the Evans-Polanyi relationship for each type of intramolecular hydrogen migration reaction studied.

Rearrangemements in Piperidine-Derived Nitrogen-Centered Radicals. A Quantum-Chemical Study

Hydrogen migration reactions in piperidine radicals and their protonated counterparts were studied by quantum chemical calculations. G3B3 and G3(MP2)-RAD levels of theory were used as reference procedures in order to evaluate the efficiency of other computational models. In the gas phase, the 1,4-[N<-->C]-H and 1,4-[C<-->C]-H shifts are the most feasible rearrangements in the piperidine radical cation and neutral piperidine radical, respectively. However, if one explicit water molecule is well placed to facilitate hydrogen migrations, the 1,2-[N<-->C]-H shift becomes the most favorable process in both cases. Three different water-catalyzed [N<-->C]-H shift mechanisms were considered for piperidine radical cation, and only one is found to be operative in the case of neutral piperidine radical. We found that explicit solvation and protonation of piperidine-derived radicals strongly influence the overall mechanism of hydrogen migration reactions.

Density functional theory study on the pyrolysis mechanism of thiophene in coal

The pyrolysis mechanism of thiophene in coal has been investigated with a density functional theory method. Three intramolecular hydrogen migration reaction paths leading to the formation of H 2S were designed. It can be concluded that the favorable energy path by the kinetic analysis is that the α-H migrates to S firstly; then the β-H migrates to the α-C followed by a concerted C–S bond cleavage resulting in the cyclic structure of thiophene turning into a chain structure; finally H 2S and butadiyne are formed via H migrating twice. In the favorable energy path, the rate determining step is the α-H migration to S, and the activation energy is about 351.63 kJ mol −1 at 298.15 K.

Photoisomerization Reactions of Cyclopropene and 1,3,3-Trimethylcyclopropene: A Theoretical Study.

The mechanisms of the photochemical isomerization reactions were investigated theoretically using two model systems, cyclopropene and 1,3,3-trimethylcyclopropene with the CASSCF/6-311G(d) (six-electron/six-orbital active space) and MP2-CAS/6-311G(d,p)/CASSCF/6-311G(d) methods. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for both cyclopropene and 1,3,3-trimethylcyclopropene is as follows: reactant → Franck-Condon region → local minimum → transition state → conical intersection → local intermediate → transition state → photoproduct. The theoretical findings suggest that the conical intersection mechanism found in this work gives a good explanation and supports the experimental observations. We also investigated the thermal (dark) reaction mechanisms for the hydrogen migration reactions. Again, all the relative yields of final products predicted based on the present work are in good agreement with the available experimental findings.