Browsing by Subject "Metal Organic Framework"

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    Modeling nanoporous materials for applications in catalysis, adsorption, and separations
    (2023-01) Patel, Roshan Ashokbhai
    Crystalline nanoporous materials with well-defined pore topologies, such as metal-organic frameworks (MOFs) and zeolites, are effective materials for applications in catalysis, adsorption, and separations. The work in this thesis utilizes computational modeling by employing density functional theory (DFT) and force-field-based methods to gain fundamental molecular-level insights that complement the findings from the experiments. Further, we utilize the improved understanding of the processes to develop better-performing systems such as by improving product selectivity via rational selection of MOF-catalyst/solvent system and predicting separation performance of zeolite membranes at high temperatures and pressures.In the first part of this work, we investigate liquid-phase biomass conversion reactions – glucose isomerization and etherification in MOF Zr-UiO-66 and demonstrate the effect of polar protic and aprotic solvents on reaction kinetics. We use DFT calculations and ab initio molecular dynamics (AIMD) simulations to examine the critical steps of the competing glucose isomerization and etherification reactions. Using these approaches, we investigate the important role of the methanol and 1-propanol solvation media in affecting fructose (isomerization product) selectivity. Experiments and DFT calculations show that in 1-propanol solvent, isomerization is favored, and fructose is obtained as the dominant product, whereas, in methanol, methyl glucoside resulting from the etherification of glucose was found to be the major product. Our computations show that the higher hydrogen bonding acidity provided by the methanol solvation environment compared to 1-propanol stabilizes transition states involving the basic alkoxide group on glucose in the etherification pathway. Based on the insights from this work, we propose the use of polar aprotic solvents that lack hydrogen bonding acidity to minimize the etherification of glucose. In particular, we use tetrahydrofuran (THF)/alcohol solvent mixtures and perform DFT calculations to show that THF (aprotic solvent) is incapable of stabilizing the basic etherification transition state because of a lack of acidic character. Experimental evidence validates our findings, wherein a 1:3 methanol:THF solvent mixture enables a significant enhancement in fructose formation, leading to an unprecedented selectivity of 90% at 80% conversion. The results from these works underscore the importance of solvents and the utilization of MOF/solvent systems to tune reaction kinetics and product selectivities. We further use force-field-based NpT Gibbs ensemble Monte Carlo simulations to study the adsorption of polar sorbates – water and ethanol, in MOF UiO-66 to improve our understanding of the solvent structure and hydrophilic domains in the framework. In the next part of this thesis, the focus shifts to the tunable nature of MOFs. Herein, we modify the MOF Zr-NU-1000 and incorporate sulfate moieties to tune the Brønsted acidity of the MOF. Using DFT calculations, we determine the reaction energetics for the dynamic nature of the sulfate binding motif in Zr-NU-1000-SO4, which is found to transition between monodentate, bidentate, and tridentate upon dehydration/hydration steps. In all cases, increased Brønsted acidity compared to the parent Zr-NU-1000 MOF was observed upon sulfation, and computations were used to elucidate the acidity of different protic sites by calculating the deprotonation energies for different proton-bearing ligands. The MOF Zr-NU-1000-SO4 not just acts as a better Brønsted acid catalyst, but also proves to be a well-defined support for immobilizing other organometallic catalysts. In the final part of this thesis, we shift gears to modeling gas mixture separations using zeolite membranes. We evaluate the performance of bulk all-silica MFI zeolite and a 3 nm thick all-silica MFI nanosheet for ammonia/nitrogen/hydrogen separations at moderate temperature and pressure conditions (T = 373 K, p = 5 bar) as well as conditions relevant for a membrane-based reactor–separator process (T = 523 or 623 K, p = 80 bar), the latter is challenging to probe via lab-scale experiments. Force-field-based isobaric–isothermal Gibbs ensemble Monte Carlo (GEMC) simulations and NpT molecular dynamics simulations were carried out to understand the selective adsorption behavior and competitive transport for mixtures with ammonia, nitrogen, and hydrogen. Our simulations show that bulk MFI and MFI nanosheets are highly selective toward ammonia adsorption, and that overall membrane selectivity is favorable toward ammonia in all cases considered, showing that the MFI zeolite nanosheet membrane holds promise for separation of ammonia/nitrogen/hydrogen at industrially relevant conditions. In summary, we used DFT and force-field-based modeling of MOFs and zeolites for applications in catalysis, adsorption, and separations, viz. MOF/solvent systems for improved reaction rates and product selectivities, MOF as adsorbents, MOF as a tunable material for imparting Brønsted acidity and as support for other catalysts, and zeolites as promising materials for membrane-based separations at process-relevant conditions.