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Sulfolane alters the mechanism and energetics of dissociation of H2O2, thereby controlling the active oxygen species in the Fe-ZSM-5 catalyst.

The aim of this work is to develop insights into adsorption of hydrogen and carbon dioxide in a zeolitic imidazolate framework, ZIF-8, using high-pressure adsorption studies, adsorption isotherm model fitting, and DFT investigation of preferential adsorption sites and binding energies. The robustness of ZIF series metal–organic frameworks has drawn interest towards its utility in large scale applications in gas storage and separation. We use room temperature synthesis of ZIF-8 using DMF as a solvent, and benchmarked it against typical solvothermal synthesis. The resulting material is characterized using XRD, SEM, TG–DSC and N2 adsorption isotherm. High-pressure volumetric adsorption of the activated materials is conducted to analyze the hydrogen and carbon dioxide storage capacities up to 50 and 40 bar, respectively. ZIF-8 shows maximum H2 storage capacity of 3.13 wt% at 50 bar and 77 K, and CO2 storage capacity of 46 wt% at 40 bar and 300 K. The parameters of Unilan adsorption isotherm are estimated from the equilibrium adsorption data and isosteric heats of adsorption for H2 and CO2 on ZIF-8 are computed. DFT calculations are used to obtain preferential adsorption sites of H2 and CO2. Adsorption enthalpy values were calculated from DFT as − 7.08 and − 25.98 kJ/mol, respectively for H2 and CO2 at the most preferred sites. We found a close agreement between isosteric heat of adsorption of hydrogen (− 4.68 kJ/mol) and the enthalpy of hydrogen adsorption from DFT (− 6.04 kJ/mol) at 77 K.

Catalytic upgrading of biowaste to chemicals and fuels is a key step towards circular economy. Computational investigations of catalytic upgrading of biowaste have made valuable contributions towards catalysts design. Recent applications of 1) Density Functional Theory (DFT) and molecular dynamics (MD) simulations, and microkinetic modelling to derive insights on active sites, reactions mechanisms, solvation and catalyst deactivation, 2) DFT and MD simulations, multiscale models, and statistical methods for identification of structure of catalyst nanoparticles, 3) DFT calculations and statistical methods for identification of catalytic descriptors and development of scaling relations, are discussed. A recent trend is to exploit advances in data science and data repositories for catalysis. There is immense potential for combined multiscale computational techniques and machine learning to enable in silico design of catalysts for the upgradation of biowaste.

Solvents are crucial components in specialty chemical and pharmaceutical industries and in electrochemical and photoelectrochemical processes, and are increasingly being used in catalytic reactions. Solvents significantly influence the kinetics and thermodynamics of reactions and can alter product selectivity markedly. While such solvent effects are observed routinely, identification of the root causes of such effects is less frequent. Solvents can influence reaction rates, conversion and product selectivity by 1) directly participating in the reaction steps and opening alternate reaction pathways, 2) competing with the reactant for interaction with the catalysts, 3) changing the relative stabilization of the reactant, the transition state (TS) and/or the product, 4) altering intra-pore diffusion characteristics in porous catalysts, 5) exhibiting entropic confinement effects altering free energy barriers of reactions, 6) changing the solubility of different components in the reaction mixture, and 7) inhibiting undesired reactions. Their indirect influences may be due to 1) changes brought on to active sites on catalysts and 2) altered structure/stability of catalysts. This article discusses these fundamental reasons behind observed solvent effects with suitable examples. Advances in computational chemistry have led to the development of multiple tools and techniques, considering solvents either as implicit or as explicit molecules, providing molecular insights into complex solvent effects in catalysis. This article provides an overview of some of these methods with suitable examples to demonstrate their application and potential. This mapping of the solvent effects and their origins is believed to aid in the rational selection of solvents for catalytic reactions. The description of the computational tools, their application and their potential is likely to encourage widespread use of these techniques to investigate solvent effects.

Quantum chemical computations predict that compression of the methane dimer to an inter-nuclear separation lower than 2 A facilitates a concerted coupling and dissociation of C H bonds of the molecules to form ethane/ethylene. In this bimolecular, concerted mechanism, ethane formation is accompanied by production of H radicals from each methane moiety that may further abstract hydrogen atoms to lead to ethylene formation. Alternatively, transformation to ethane and ethylene proceeds via stepwise molecular hydrogen elimination, with the first eliminated hydrogen molecule originating from one of the methane moieties, accompanied by an intermolecular hydrogen transfer, and the second originating from both methyl groups.

Methane is the primary component of natural gas, coal bed gases, shale gas and natural gas hydrates, and its commercially recoverable amounts exceed those of petroleum and coal by a few orders of magnitude. The strong C-H bonds of methane make its selective and controlled activation/dissociation difficult and this impedes the widespread utilization of this small alkane as a chemical feedstock to make value added chemicals, fuels and materials. Thus, developing alternate techniques, synthesising new catalytic materials and gaining fundamental mechanistic insights into C-H bond activation on known catalytic surfaces are pivotal to making ‘methane to chemicals and fuels’ a reality. Using state of the art quantum chemical simulations, it is shown for the first time that mechanical impact of methane molecules induces the dissociation of their strong C-H bonds. This novel bimolecular ‘coupling induced dissociation mechanism’ is demonstrated to convert methane molecules undergoing mechanical impact, directly to C2 hydrocarbons like ethane and ethylene. The impact induced vibrational excitation of the molecules allow successive transitions to lower potential energy states, leading to the concerted dissociation of C-H bonds of the molecules and coupling of the carbon atoms. Using ab initio molecular dynamics and Metadynamics simulations, it is shown that the mechanical impact of methane on an inactive transition metal like copper, in the form of nanoclusters, can dissociate it with activation free energy barriers between 60 and 90 kJ mol-1. The impact induced vibrational excitation of the molecule and the formation of a collision complex, which serves as a precursor for the dissociation of methane are key reasons for the low free energy barriers. Additionally, the small 3-dimensional copper nanoclusters have electronic structure and morphological features which favour the impact induced dissociation of methane while subsequent dehydrogenation steps have high barriers. The stabilization of the transition state and the products of dissociation (CH3 and H) by the under-coordinated Cu-O pair on the CuO surfaces, enable the dissociation of methane with an activation energy barrier as low as 76 kJ mol-1 on the most stable CuO(111) surface, compared to the barrier of 170 kJ mol-1 on Cu(111) surface. Thus, oxygen enhances the activity of copper in methane dissociation. The stronger the H chemisorption on the surface lattice oxygen on the different CuO surfaces, the more active is the surface in dissociating methane, making H chemisorption energy a descriptor for the reactivity of the surface. The weaker the copper- lattice oxygen interaction on the surface, the stronger is the H chemisorption on the lattice oxygen. The difference in the intrinsic metal- oxygen interaction on the different CuO surfaces is the root cause for the differences in their reactivity. In contrast to CuO, the 5-coordinated surface lattice oxygen on the most stable NiO surface, NiO(100), makes the surface inactive for methane dissociation with activation barrier of 136 kJ mol-1 compared to 70 kJ mol-1 on Ni(100) surface. NiO surfaces are as active as metallic Ni for methane dissociation when 1) the surface lattice oxygen is 4 coordinated as in the less stable NiO(110) surface; 2) NiO has surface Ni vacancies; and 3) NiO is doped with low valent dopants like Li. On comparing un-doped and doped surfaces, the H chemisorption energy, the descriptor for the reactivity of the lattice oxygen on different NiO surfaces, was found to have a weaker correlation with the intrinsic binding strength of the lattice oxygen on the surfaces. Low valent dopants like Li increase the activity of surface lattice oxygen for methane dissociation by lowering the binding strength of the surface lattice oxygen. In summary, this thesis reveals unusually high reactivity of methane resulting from collisional impact. The findings have implications for understanding of certain planetary phenomena, offer an explanation for the genesis of ‘abiogenic petroleum’ on the earth and also indicate opportunities for the commercial exploitation of methane. Insights into the role of lattice oxygen, vacancies and dopants on metal oxides in methane dissociation, and H chemisorption energy as a descriptor for reactivity can serve as guidelines in screening and designing efficient multifunctional catalysts for methane dissociation and oxidation reactions. DOCTOR OF PHILOSOPHY (SCBE)

The utilization of low frequency ultrasound (US) offers a straightforward and powerful tool for the production of nanostructured materials, in particular for structurally stable, highly crystalline, and shape-controlled catalytic materials. Herein, we report an unconventional strategy for the synthesis of CuO nanoleaves within 5 min of US irradiation. The as-obtained CuO nanoleaves were found to be selective in the base-free aqueous oxidation of glycerol to dicarboxylic acids (78% yield of tartronic and oxalic acids), in the presence of hydrogen peroxide (H2O2) and under mild reaction conditions. Density Functional Theory (DFT) investigations revealed a synergy between the CuO catalyst and H2O2 in maintaining the structural integrity of the catalyst during the reaction, creating alternative efficient pathways for the selective formation of dicarboxylic acids. Isotope labeling experiments using H218O2 further confirmed that oxygen from hydrogen peroxide, not from CuO, was preferentially incorporated into the dicarboxylic acid, significantly preserving the monoclinic structure of the CuO catalyst.

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 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.

Small metal clusters exhibit unique size and morphology dependent catalytic activity. The search for alternate minimum energy pathways and catalysts to transform methane to more useful chemicals and carbon nanomaterials led us to investigate collision induced dissociation of methane on small Cu clusters. We report here for the first time, the free energy barriers for the collision induced activation, dissociation, and coupling of methane on small Cu clusters (Cu n where n = 2–12) using ab initio molecular dynamics and metadynamics simulations. The collision induced activation of the stretching and bending vibrations of methane significantly reduces the free energy barrier for its dissociation. Increase in the cluster size reduces the barrier for dissociation of methane due to the corresponding increase in delocalisation of electron density within the cluster, as demonstrated using the electron localisation function topology analysis. This enables higher probability of favourable alignment of the C–H stretching vibration of methane towards regions of high electron density within the cluster and makes higher number of sites available for the chemisorption of CH3 and H upon dissociation. These characteristics contribute in lowering the barrier for dissociation of methane. Distortion and reorganisation of cluster geometry due to high temperature collision dynamics disturb electron delocalisation within them and increase the barrier for dissociation. Coupling reactions of CH x (x = 1–3) species and recombination of H with CH x have free energy barriers significantly lower than complete dehydrogenation of methane to carbon. Thus, competition favours the former reactions at high hydrogen saturation on the clusters. Published version