Transition metal oxides are an important class of catalytic materials widely used in the chemical manufacturing and processing industry, owing to their low cost, high surface area, low toxicity, and easily tunable surface and structural properties. For these strongly correlated transition metal oxides, standard approximations in the density functional theory (DFT) exchange-correlation functional fail to describe the electron localization accurately due to the intrinsic errors arising from electron self-interactions. DFT+U method is a widely used extension of DFT, where the Hubbard U term is an onsite potential which puts a penalty on electron delocalization, successfully describing such systems at only slightly higher computational cost than standard DFT methods. The U-value is usually chosen based on its accuracy in reproducing bulk properties like lattice parameters and band structure. However, chemical reactions on transition metal oxide surfaces involve complex surface–adsorbate interactions, and using the bulk properties based U-values in a locally changing surface environment may not describe reaction energetics correctly. Hence, in the current DFT+U benchmarking work, using CuO as a model transition metal oxide, we perform DFT+U calculations to investigate the dissociative chemisorption of H2 on it. It is observed that the U-value impacts computed adsorption enthalpies by over 100 kJ mol–1. The DFT+U calculated adsorption enthalpy is compared with the experimental adsorption enthalpy, and equilibrium adsorption configurations are confirmed using infrared analysis. We reveal that the commonly used U-value of 7 eV (fitted against CuO bulk properties) overestimates the adsorption enthalpy by 20–40 kJ mol–1. The U-value between 4.5 and 5.5 eV correctly predicts the adsorption of H2 on CuO. The DFT+U benchmarking procedure elucidated in this article, encapsulates surface–adsorbate interactions, surface reactivity, and the dynamic surface reaction environment and, thus, provides an appropriate U-value to be used to model reactions on metal oxide surfaces.

Copper (Cu) is a commercial catalyst for the synthesis of methanol from syngas, low-temperature water gas shift reaction, oleo-chemical processing, and for the fabrication of graphene by chemical vapor deposition. However, high barriers for C–H bond activation and the ease of formation of carbon/graphene on its surface limits its application in the utilization and conversion of methane to bulk chemicals. In the present paper, using first-principles calculations, we predict that Cu catalyst doped with a monolayer of sub-surface boron (B–Cu) can efficiently activate the C–H bond of methane and can selectively facilitate the C–C coupling reaction. Boron binds strongest at the sub-surface octahedral site of Cu and the thermodynamic driving force for the diffusion of B from an on-surface to the sub-surface position in Cu is stronger than that for the experimentally synthesizable B–Ni (sub-surface boron in nickel) catalyst, providing a proof of concept for the experimental synthesis of this novel catalyst. Additionally, the first-principles computed free energy of the reaction to form B–Cu from boron precursor and Cu is also favorable. The presence of the monolayer sub-surface B in Cu creates a corrugated step-like structure on the Cu surface and significantly brings down the methane C–H activation barrier from 174 kJ/mol on Cu(111) to only 75 kJ/mol on B–Cu. The subsequent dehydrogenation of the adsorbed CH3* to CH2* is also kinetically and thermodynamically feasible. Our calculations also suggest that, unlike most of the transition metals, complete decomposition of methane to carbon would not be favored on B–Cu. The dissociation of the surface CH2* moiety on B–Cu is limited due to the high activation barrier of 161 kJ/mol and lower relative stability of the resultant CH* species, under reaction conditions. The coupling of CH2* fragments however is kinetically and thermodynamically favorable, with an activation barrier of only 92 kJ/mol; suggesting that B–Cu catalyst would have higher selectivity toward C2 hydrocarbons. Furthermore, the formation of carbon from the adsorbed CH* moiety has a very high activation barrier of 197 kJ/mol and the completely dehydrogenated C* is relatively much less stable than CH*, under reaction conditions; predicting that coking might not be an issue on the B–Cu catalyst. Evaluation of C–H activation on Cu(110) surface, which has a similar step-like surface structure as B–Cu, and Bader charge and density of states analyses of B–Cu reveal that the geometrical/corrugation effect and the charge transfer from B to Cu synergistically promote the C–H activation on B–Cu, making it as active as other expensive transition metals like Rh, Ru, Ir, and Pt.

Tar is the undesired viscous black liquid produced during the gasification of biomass. Catalytic elimination of tar is feasible and is economically promising. In the present investigation, we performed first-principles calculations (i) to elucidate the decomposition mechanism of tar on the popular nickel (Ni) catalyst and (ii) to reveal the promotional effect of boron (B) in improving the activity and stability of the Ni catalyst for tar decomposition. Being the most abundant component of tar, toluene was chosen as a model compound. On the Ni(111) surface, toluene adsorbs strongly in a bridge configuration and the activation barrier for methyl C–H dissociation is 72 kJ mol−1. Toluene can further decompose on the Ni(111) surface via stepwise dehydrogenation of the methyl group. The aromatic C–H bond at the ortho position could only be activated after the complete dehydrogenation of the methyl group, which is followed by subsequent ring opening (activation barriers are 112 and 84 kJ mol−1, respectively) and C–C cleavages, to generate smaller hydrocarbons. The incorporation of subsurface B into the Ni catalyst (B–Ni) results in a corrugated Ni top surface and toluene adsorbs more strongly by 11 kJ mol−1 on the B–Ni catalyst than on pure Ni. Although the mechanism of toluene decomposition remains unchanged after doping with boron, the decomposition of toluene is significantly promoted on B–Ni. The activation barrier for the first methyl C–H dissociation on B–Ni is reduced to 51 kJ mol−1. Subsequent methyl C–H activations are also promoted on the B–Ni catalyst, but to a smaller extent. The aromatic C–H activation is also strongly promoted (89 kJ mol−1vs. 112 kJ mol−1 on pure Ni). Additional calculations on stepped surface models of Ni show that the activation barriers of toluene decomposition on the B–Ni surface are very close to those on B5 and F4 step sites of pure Ni, thus suggesting that the promotional effect of B on the catalytic activity of Ni could be mainly attributed to the creation of step-like corrugations on the Ni surface. These corrugations could also inhibit the formation of graphene-like structures on B–Ni.

The activation of methane by transition metal/metal oxide catalysts is pertinent for developing/optimizing processes which help to convert this abundantly available resource to value-added chemicals. First principles calculations reveal that the under-coordinated lattice Cu–O pair on different CuO surfaces synergistically activates methane with barriers as low as 60.5 kJ mol−1 on the high-energy CuO(010) surface and 76.6 kJ mol−1 on the most stable CuO(111) surface. The significantly low activation barrier is due to (1) the stabilization of the transition state (TS) and the reduced strain on the dissociating methane molecule and (2) the stabilization of the co-adsorbed products of dissociation, resulting in favorable thermodynamics. The mechanism, which is also applicable to the chemisorbed oxygen-containing Cu(111) surface, involves simultaneous copper addition and hydrogen abstraction by the chemisorbed/lattice oxygen via a 4-centered (CH3-(Cu)–H-(O)) TS, stabilized by the Cu–CH3 and O–H dipole–dipole interaction. The activation barriers for the subsequent dissociation of surface CH3 moieties and coupling of CH3 with CH2 on the CuO(111) surface are both much higher than the barrier of the first C–H bond dissociation in methane. The mechanistic insights elucidated in this article could be applicable to methane activation by other metal–oxygen (M–O) site-pairs and thus can serve to screen potential oxide surfaces for the purpose.

Biodiesels produced from renewable sources exhibit superior fuel properties and renewability and they are more environmentally friendly than petroleum-based fuels. In this paper, a three-step transesterification, catalyzed by a pyridinium-based Brønsted acidic ionic liquid (BAIL), for biodiesel production was investigated using density functional theory (DFT) calculations at the B3LYP/6-311++G(d) level. The DFT results elucidate the detailed catalytic cycle, which involves the formation of a covalent reactant–BAIL–(methanol)n (n = 1/3) intermediate and two transition states. Hydrogen bond interactions were found to exist throughout the process of the catalytic cycle, which are of special importance for stabilizing the intermediate and transition states. Thus, a mechanism involving cooperative hydrogen bonding for BAIL-catalyzed biodiesel production was established. The Gibbs free energy profile based on the above mechanism was validated by the subsequent kinetic study. The trend of activation energy from kinetic mathematical models was reasonably consistent with that obtained from the DFT calculations.

A series of Au–M (M = Cu, Co, Ru and Pd) bimetallic catalysts were supported on TiO2via a deposition–precipitation (DP) method, using urea as a precipitating agent. The resulting catalysts were employed in the catalytic oxidation of cellobiose to gluconic acid and the properties of these catalysts were carefully examined using various characterization techniques. Cu–Au/TiO2 and Ru–Au/TiO2 catalysts demonstrated excellent catalytic activities in the oxidation of cellobiose to gluconic acid, though with contrasting reaction mechanisms. Complete conversion of cellobiose (100%) with a gluconic acid selectivity of 88.5% at 145 °C within 3 h was observed for reactions performed over Cu–Au/TiO2; whereas, a conversion of 98.3% with a gluconic acid selectivity of 86. 9% at 145 °C within 9 h was observed for reactions performed over Ru–Au/TiO2. A reaction pathway was proposed based on the distribution of reaction products and kinetic data. It is suggested that cellobiose is converted to cellobionic acid (4-O-beta-D-glucopyranosyl-D-gluconic acid) and then gluconic acid is formed through the cleavage of the β-1,4 glycosidic bond in cellobionic acid over Cu–Au/TiO2 catalysts. On the other hand, for reactions over the Ru–Au/TiO2 catalyst, glucose was observed as the reaction intermediate and gluconic acid was formed as a result of glucose oxidation. For reactions over Co–Au/TiO2 and Pd–Au/TiO2 catalysts, fructose was observed as the reaction intermediate, along with small amounts of glucose. Co and Pd remarkably promoted the successive retro-aldol condensation reactions of fructose to glycolic acid, instead of the selective oxidation to gluconic acid.

Hydride transfer changes the charge structure of the reactant and thus, may induce reorientation/reorganization of solvent molecules. This solvent reorganization may in turn alter the energetics of the reaction. In the present work, we investigate the intramolecular hydride transfer by taking Lewis acid catalyzed glucose to fructose isomerization as an example. The C2–C1 hydride transfer is the rate limiting step in this reaction. Water and methanol are used as solvents and hydride transfer is simulated in the presence of explicit solvent molecules, treated quantum mechanically and at a finite temperature, using Car–Parrinello molecular dynamics (CPMD) and metadynamics. Activation free energy barrier for hydride transfer in methanol is found to be 50 kJ mol−1 higher than that in water. In contrast, in density functional theory calculations, using an implicit solvent environment, the barriers are almost identical. Analysis of solvent dynamics and electronic polarization along the molecular dynamics trajectory and the results of CPMD-metadynamics simulation of the hydride transfer process in the absence of any solvent suggest that higher barrier in methanol is a result of non-equilibrium solvation. Methanol undergoes electronic polarization during the hydride transfer step. However, its molecular orientational relaxation is a much slower process that takes place after the hydride transfer, over an extended timescale. This results in non-equilibrium solvation. Water, on the other hand, does not undergo significant electronic polarization and thus, has to undergo minimal molecular reorientation to provide near equilibrium solvation to the transition state and an improved equilibrium solvation to the post hydride shift product state. Hence, the hydride transfer step is also observed to be exergonic in water and endergonic in methanol. The aforementioned explanation is juxtaposed to enzyme catalyzed charge transfer reactions, where the enhanced solvation of the transition and product states by enzymes, due to electrostatic interactions, reduces the activation free energy barrier and the free energy change of the reaction. Similarly, we suggest that, in the intramolecular hydride shift, improved solvation of the transition state and of the product state by water is achieved due to minimal polarization and reorientation, and (near) equilibrium solvation.

•Force field parameters for DMF, a common aprotic solvent, revisited•A partial positive charge on amide nitrogen imperative for DMF–water mixing•Positively charged nitrogen enhances water–DMF interactions.•Significant improvement in the prediction of bulk DMF properties•Reverse C–N dative bond and dual resonance structures of DMFA partial negative charge on the nitrogen atom in N,N-Dimethylformamide (DMF), as employed in all force fields, leads to the separation of water and DMF phases. Hence, based on the possibility of the existence of dual resonance structures for DMF and of the presence of a reverse carbon–nitrogen dative bond, we reparameterized DMF and demonstrate that a positively charged nitrogen atom is imperative to achieve complete DMF–water miscibility. The reversal of the charge results in enhanced water–DMF interactions. Significant improvement in the predicted bulk properties of DMF is also observed. New parameters were benchmarked using ab-initio molecular dynamics.

Cellulosic biomass derived molecules such as glucose can be converted into specific platform chemicals like 5-hydroxymethylfurfural (HMF), levulinic acid and gamma valerolactone (GVL). The solvation medium plays an important role in the selective conversion of glucose to these platform chemicals and it is shown that the addition of co-solvents increases the selectivity towards desired products and minimizes the formation of undesired condensation/polymerization products and humins. Hence, it becomes imperative to understand the implications of the solvation of glucose by co-solvents on glucose conversion reactions. In the present paper, we implement OPLS-AA force-field based molecular dynamics simulations to investigate the solvation of glucose in water, in the presence of dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and tetrahydrofuran (THF). The local arrangement of solvents around the glucose molecule is analyzed using 2-dimensional radial pair distribution functions and 3-dimensional volumetric maps. Additionally, lifetimes and activation free energies of hydrogen bonds between solvents and glucose and the tendency of glucose molecules to agglomerate were studied. It was observed that all the aforementioned co-solvents compete with water to be in the first solvation shell of glucose and significant amount of water is pushed to the second coordination shell. Though fewer water molecules are directly coordinated with glucose in the presence of co-solvents, they are bound strongly to it. Additionally, DMSO, THF and DMF tend to localize more around the hydrogen atom of the hydroxyl groups of selected carbon atoms of glucose. This preferential arrangement of co-solvents and water around glucose may play a role in facilitating the reaction pathway for the formation of HMF and levulinic acid and may reduce the likelihood of glucose' degradation to unwanted dehydration/rehydration products. Increasing the proportion of co-solvents also increases the hydrogen bond lifetimes between water and glucose and reduces the mobility of glucose molecules within the solvent. The reduced mobility of glucose molecules in the presence of co-solvents might be correlated to the experimentally observed reduction in the rate of formation of polymerization/condensation products and humins.

The adsorption of cellulosic biomass derived aldoses on heterogeneous catalysts is the first and governing step in their conversion to platform chemicals. However, there is a discrepancy between theoretical and experimental observations in determining the most stable configuration of adsorbed aldoses on transition metal surfaces: conventional density functional theory (DFT) calculations with the Perdew, Burke, and Ernzerhof (PBE) functional (Perdew et al. Phys. Rev. Lett. 1996, 77, 3865) predicted η2(C,O) as the most stable structure, whereas experiments could only detect the η1(O) configuration. Our calculations reveal that the revised PBE (Hammer et al. Phys. Rev. B 1999, 59, 7134) functional can correctly predict the most stable adsorption configuration of aldoses on transition metals. Additionally, for C6 aldoses like glucose, which exist mostly in the ring form, the open-chain adsorption configuration is a result of the transition-metal-catalyzed ring-opening process. Adsorbed glucose in the cyclic form undergoes deprotonation and ring opening, and the resultant open-chain configuration closely resembles the η1(O) structure, thus explaining why the η1(O) configuration was detected in experiments. Entropy calculations also demonstrate that the transformation from η1(O) to η2(C,O) is thermodynamically not favorable even at higher temperatures. Finally, based on the most stable η1(O) adsorbed configuration, the catalytic activity of Pd and Pt surfaces toward the decomposition, oxidation, and hydrogenation reactions is evaluated.

The mechanism of glucose ring opening and isomerization to fructose, catalyzed by the Lewis acid catalyst CrCl3 in the presence of water, is investigated using Car–Parrinello molecular dynamics with metadynamics. Minimum energy pathways for the reactions are revealed and the corresponding free energy barriers are computed. Addition of glucose replaces two water molecules in the active [Cr(H2O)5OH]+2 complex, with two hydroxyl groups of glucose taking their place. Ring opening and isomerization reactions can only proceed if the first step involving the deprotonation of glucose is accompanied by the protonation of the OH− group in the partially hydrolyzed metal center ([Cr(C6H12O6)(H2O)3OH]+2 → [Cr(C6H11O6)(H2O)4]+2). This provides further evidence that the partially hydrolyzed [Cr(H2O)5OH]+2 is the active species catalyzing ring opening and isomerization reactions and that unhydrolyzed Cr+3 may not be able to catalyze the reactions. After the ring opening, the isomerization reaction proceeds via deprotonation, followed by hydride shift and the back donation of the proton from the metal complex to the sugar. Water molecules outside the first coordination sphere of the metal complex participate in the reaction for mediating the proton transfer. The hydride shift in the isomerization is the overall rate limiting step with a free energy barrier of 104 kJ mol−1. The simulation computed barrier is in agreement with experiments.

The mechanism of glucose ring opening and isomerization to fructose, catalyzed by the Lewis acid catalyst CrCl3 in the presence of water, is investigated using Car–Parrinello molecular dynamics with metadynamics. Minimum energy pathways for the reactions are revealed and the corresponding free energy barriers are computed. Addition of glucose replaces two water molecules in the active [Cr(H2O)5OH]+2 complex, with two hydroxyl groups of glucose taking their place. Ring opening and isomerization reactions can only proceed if the first step involving the deprotonation of glucose is accompanied by the protonation of the OH− group in the partially hydrolyzed metal center ([Cr(C6H12O6)(H2O)3OH]+2 → [Cr(C6H11O6)(H2O)4]+2). This provides further evidence that the partially hydrolyzed [Cr(H2O)5OH]+2 is the active species catalyzing ring opening and isomerization reactions and that unhydrolyzed Cr+3 may not be able to catalyze the reactions. After the ring opening, the isomerization reaction proceeds via deprotonation, followed by hydride shift and the back donation of the proton from the metal complex to the sugar. Water molecules outside the first coordination sphere of the metal complex participate in the reaction for mediating the proton transfer. The hydride shift in the isomerization is the overall rate limiting step with a free energy barrier of 104 kJ mol−1. The simulation computed barrier is in agreement with experiments.

Tar is the undesired viscous black liquid produced during the gasification of biomass. Catalytic elimination of tar is feasible and is economically promising. In the present investigation, we performed first-principles calculations (i) to elucidate the decomposition mechanism of tar on the popular nickel (Ni) catalyst and (ii) to reveal the promotional effect of boron (B) in improving the activity and stability of the Ni catalyst for tar decomposition. Being the most abundant component of tar, toluene was chosen as a model compound. On the Ni(111) surface, toluene adsorbs strongly in a bridge configuration and the activation barrier for methyl C–H dissociation is 72 kJ mol−1. Toluene can further decompose on the Ni(111) surface via stepwise dehydrogenation of the methyl group. The aromatic C–H bond at the ortho position could only be activated after the complete dehydrogenation of the methyl group, which is followed by subsequent ring opening (activation barriers are 112 and 84 kJ mol−1, respectively) and C–C cleavages, to generate smaller hydrocarbons. The incorporation of subsurface B into the Ni catalyst (B–Ni) results in a corrugated Ni top surface and toluene adsorbs more strongly by 11 kJ mol−1 on the B–Ni catalyst than on pure Ni. Although the mechanism of toluene decomposition remains unchanged after doping with boron, the decomposition of toluene is significantly promoted on B–Ni. The activation barrier for the first methyl C–H dissociation on B–Ni is reduced to 51 kJ mol−1. Subsequent methyl C–H activations are also promoted on the B–Ni catalyst, but to a smaller extent. The aromatic C–H activation is also strongly promoted (89 kJ mol−1vs. 112 kJ mol−1 on pure Ni). Additional calculations on stepped surface models of Ni show that the activation barriers of toluene decomposition on the B–Ni surface are very close to those on B5 and F4 step sites of pure Ni, thus suggesting that the promotional effect of B on the catalytic activity of Ni could be mainly attributed to the creation of step-like corrugations on the Ni surface. These corrugations could also inhibit the formation of graphene-like structures on B–Ni.

A series of Au–M (M = Cu, Co, Ru and Pd) bimetallic catalysts were supported on TiO2via a deposition–precipitation (DP) method, using urea as a precipitating agent. The resulting catalysts were employed in the catalytic oxidation of cellobiose to gluconic acid and the properties of these catalysts were carefully examined using various characterization techniques. Cu–Au/TiO2 and Ru–Au/TiO2 catalysts demonstrated excellent catalytic activities in the oxidation of cellobiose to gluconic acid, though with contrasting reaction mechanisms. Complete conversion of cellobiose (100%) with a gluconic acid selectivity of 88.5% at 145 °C within 3 h was observed for reactions performed over Cu–Au/TiO2; whereas, a conversion of 98.3% with a gluconic acid selectivity of 86. 9% at 145 °C within 9 h was observed for reactions performed over Ru–Au/TiO2. A reaction pathway was proposed based on the distribution of reaction products and kinetic data. It is suggested that cellobiose is converted to cellobionic acid (4-O-beta-D-glucopyranosyl-D-gluconic acid) and then gluconic acid is formed through the cleavage of the β-1,4 glycosidic bond in cellobionic acid over Cu–Au/TiO2 catalysts. On the other hand, for reactions over the Ru–Au/TiO2 catalyst, glucose was observed as the reaction intermediate and gluconic acid was formed as a result of glucose oxidation. For reactions over Co–Au/TiO2 and Pd–Au/TiO2 catalysts, fructose was observed as the reaction intermediate, along with small amounts of glucose. Co and Pd remarkably promoted the successive retro-aldol condensation reactions of fructose to glycolic acid, instead of the selective oxidation to gluconic acid.

Hydride transfer changes the charge structure of the reactant and thus, may induce reorientation/reorganization of solvent molecules. This solvent reorganization may in turn alter the energetics of the reaction. In the present work, we investigate the intramolecular hydride transfer by taking Lewis acid catalyzed glucose to fructose isomerization as an example. The C2–C1 hydride transfer is the rate limiting step in this reaction. Water and methanol are used as solvents and hydride transfer is simulated in the presence of explicit solvent molecules, treated quantum mechanically and at a finite temperature, using Car–Parrinello molecular dynamics (CPMD) and metadynamics. Activation free energy barrier for hydride transfer in methanol is found to be 50 kJ mol−1 higher than that in water. In contrast, in density functional theory calculations, using an implicit solvent environment, the barriers are almost identical. Analysis of solvent dynamics and electronic polarization along the molecular dynamics trajectory and the results of CPMD-metadynamics simulation of the hydride transfer process in the absence of any solvent suggest that higher barrier in methanol is a result of non-equilibrium solvation. Methanol undergoes electronic polarization during the hydride transfer step. However, its molecular orientational relaxation is a much slower process that takes place after the hydride transfer, over an extended timescale. This results in non-equilibrium solvation. Water, on the other hand, does not undergo significant electronic polarization and thus, has to undergo minimal molecular reorientation to provide near equilibrium solvation to the transition state and an improved equilibrium solvation to the post hydride shift product state. Hence, the hydride transfer step is also observed to be exergonic in water and endergonic in methanol. The aforementioned explanation is juxtaposed to enzyme catalyzed charge transfer reactions, where the enhanced solvation of the transition and product states by enzymes, due to electrostatic interactions, reduces the activation free energy barrier and the free energy change of the reaction. Similarly, we suggest that, in the intramolecular hydride shift, improved solvation of the transition state and of the product state by water is achieved due to minimal polarization and reorientation, and (near) equilibrium solvation.

The activation of methane by transition metal/metal oxide catalysts is pertinent for developing/optimizing processes which help to convert this abundantly available resource to value-added chemicals. First principles calculations reveal that the under-coordinated lattice Cu–O pair on different CuO surfaces synergistically activates methane with barriers as low as 60.5 kJ mol−1 on the high-energy CuO(010) surface and 76.6 kJ mol−1 on the most stable CuO(111) surface. The significantly low activation barrier is due to (1) the stabilization of the transition state (TS) and the reduced strain on the dissociating methane molecule and (2) the stabilization of the co-adsorbed products of dissociation, resulting in favorable thermodynamics. The mechanism, which is also applicable to the chemisorbed oxygen-containing Cu(111) surface, involves simultaneous copper addition and hydrogen abstraction by the chemisorbed/lattice oxygen via a 4-centered (CH3-(Cu)–H-(O)) TS, stabilized by the Cu–CH3 and O–H dipole–dipole interaction. The activation barriers for the subsequent dissociation of surface CH3 moieties and coupling of CH3 with CH2 on the CuO(111) surface are both much higher than the barrier of the first C–H bond dissociation in methane. The mechanistic insights elucidated in this article could be applicable to methane activation by other metal–oxygen (M–O) site-pairs and thus can serve to screen potential oxide surfaces for the purpose.