Fundamental Insights into the Structure and Dynamics of Confined Substrates inside Silica Nanostructures, using a combination of Molecular Dynamics and Grand Canonical Monte Carlo Simulations

  • Author / Creator
    Orupattur, Nilesh Varadan
  • Condensed phase reactions in the presence of heterogeneous catalysts are widely performed for the conversion of biomass into useful intermediates and value-added products. Specifically, nanoporous catalysts have garnered interest for liquid-phase oxidation, hydrogenation, dehydrogenation and/or isomerization reactions in the production of bio-based chemicals. Properties of molecules confined in such nanoporous materials are substantially different from those present in the bulk substrate. This is attributed to size and geometry of the confinement, and alteration in substrate-substrate interactions and substrate-surface interactions within the porous media. Changes in properties and molecular interactions can also significantly modify the reaction kinetics and thermodynamics of the confined reactants, thereby leading to an alteration in the conversion and selectivity of the heterogeneous catalytic reactions within the nanopore. Recently, Pulse-Field Gradient Nuclear Magnetic Resonance (PFG-NMR) experiments have played a crucial role in measuring intrapore diffusivities of organic substrates. In these experiments, the ratio of self-diffusivities in bulk to self-diffusivities inside the pore, also called as PFG interaction parameter, was used for the comparison of mobility between functionalized compounds like alcohols, polyols and carbonyl compounds. Polyols were specifically included due to the presence of multiple hydroxyl groups with an ability to interact well with the metal oxide surface. Interestingly, polyols showed an enhanced rate of self-diffusion within the nanostructures, whereas alcohols and carbonyl compounds showed reduced diffusivities. Anomalous behaviour of polyols was hypothesized to arise from the breakage of substrate-substrate Hydrogen bonds within the confinement, although the molecular-level explanation for disruption of Hydrogen bonding network was unaddressed.
    Inspired by the knowledge gap on the effect of confinement on the dynamics of functionalized molecules inside heterogeneous catalysts, in the present work, a combination of molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulation method was used to study the effect of confinement on mobility of nano-confined substrates within nanoporous Silica catalysts (which is commonly used as a support in the valorization of bio-based chemicals). We have considered water, glycerol, acetone, and heptane as our substrates, as representatives of different functional groups and varying molecular sizes. These substrates were confined inside Silica nanostructures of one dimension (Slit pore of 4 nm) and two dimensions (Cylindrical pores of 2, 4, and 6 nm). Repulsive beads technique was used to create the silica nanostructures, GCMC was used to obtain intrapore density of substrates, and MD was used to calculate the structural, dynamic and thermodynamic properties of the confined substrates. In this study, Heptane was used as a benchmark, due to lack of functional groups resulting in weak interactions with porous media and with other heptane molecules. Intrapore diffusivities of the substrate molecules were calculated for all the confinements to compare with the PFG-NMR experimental results. Heptane and water showed negligible impact of confinement size and geometry on the diffusivities. We observed a reduction in diffusivities of acetone with the reduction in the size of the confinement. Glycerol showed relatively enhanced diffusivities inside slit-pore of 4 nm and cylindrical pores of 6 and 4 nm, whereas it showed reduced diffusivities in the cylindrical pore of 2 nm due to stronger interactions with the pore surface. Furthermore, to attain mechanistic and thermodynamic insights into the confinement effects imposed by the silica nanostructures of varying geometry and size on the dynamics of the substrate molecules, H-bonding structure, intermolecular interactions, density profiles, and change in entropy and potential energy were analyzed. The effective pore volumes available for glycerol and water were higher than acetone and heptane, leading to an enhancement in the diffusivities of the hydroxyl-containing compounds. Reduction in the total number of hydrogen bonds was observed for glycerol inside all the confinements, confirming the experimental hypothesis of the disruption of Hydrogen bonding network of polyols inside nanoporous structures. Acetone intermolecular interactions and local densities near the center were observed to increase with the reduction in confinement size, leading to reduction in the diffusivities. In addition to intermolecular interactions, the geometry and size of the confinement and change in entropies and potential energies of substrate molecules played a crucial role in alteration of dynamics within the nanopores. Understanding these confinement effects on the intrapore dynamics of substrate molecules could aid in studying the modification of intrinsic reaction kinetics and thermodynamics of heterogeneous catalytic reactions inside nanopores.

  • Subjects / Keywords
  • Graduation date
    Fall 2021
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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