Browsing by Subject "Heterogeneous Catalysis"

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    Computational Modeling and Predictive Design of Metal-organic Frameworks for Catalysis and Adsorption
    (2023-08) Chheda, Saumil
    Metal-organic frameworks (MOFs) are structurally well-defined nanoporous materials built from inorganic metal oxide nodes connected by organic linkers. The high porosity, surface area, and chemical and thermal stability of MOFs have attracted interest to use these materials in catalysis, gas adsorption and storage, and separation. Furthermore, the modularity of MOFs allows to tailor their nanoscale pore environment for enhanced performance in a desired application. This thesis utilizes different computational modeling techniques to provide fundamental mechanistic insights into catalysis and adsorption phenomena occurring in MOFs which can be used to design better-performing systems. Chapter 1 highlights the suitability of MOFs for catalysis and adsorption and briefly discusses the computational methods useful for modeling MOFs for these applications. In Chapter 2, the reactivity of CAU-1, an Al-based MOF, is investigated for the dehydration of methanol to dimethylether. Density functional theory (DFT) studies in conjuction with experimental reactivity measurements are used to elucidate the reaction mechanism occurring on active sites constituted by the nodes and linkers of the MOF. In Chapter 3 and Chapter 4, the catalytic activity of several single-atom transition metals deposited on the nodes of UiO-66 (a Zr-based MOF) through post-synthetic modifications (Mn+-UiO-66), is investigated for the dimerization of 1-butene to linear octenes. The Cossee-Arlmann reaction mechanism is found to be the energetically most favorable reaction mechanism occurring on the undercoordinated metal sites of Mn+-UiO-66 catalysts investigated with Ni-UiO-66 outperforming the other metalated catalysts. Chapter 5 and Chapter 6 demonstrate the use of Al-rod-based MOFs for adsorption-assisted atmospheric water harvesting (AWH). In Chapter 5, insights into the primary adsorption sites and water uptake mechanism in MOF-303 obtained from periodic DFT optimization and ab initio molecular dynamics (MD) simulations in concert with single crystal X-ray diffraction measurements are used to design a linker-variate analogue of MOF-303, MOF-333, with increased water throughput. Furthermore, the water adsorption behavior predicted in these MOFs using force-field-based Gibbs ensemble Monte Carlo (GEMC) simulations are shown to achieve good agreement with experimental data after a careful choice of the rigid framework structures and force field parameters used for the MOF. In Chapter 6, a novel linker extension strategy in MOFs is used to enhance the water harvesting characteristics of MOF-303. Remarkably, the new MOF, MOF-LA2-1, shows a 50% increase in the water uptake capacity compared to MOF-303. Finally, in Chapter 7, the effect of the structural flexibility of IRMOFs on the adsorption and self-diffusion behavior of DMF is investigated using GEMC and MD simulations in the constant-stress ensemble using flexible force fields for the MOF.
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    Enabling the selective conversion of biomass-derived oxygenates to C4-C5 dienes
    (2021-05) Kumar, Gaurav
    The catalytic conversion of biomass-derived saturated furans over zeotype solid acids affords a potentially renewable route to access conjugated C4-C5 dienes — commodity monomers in tires, plastics, adhesives, and resins. A lack of fundamental understanding of reaction mechanisms and pathways coupled with existing trial-and-error catalyst design approaches have limited diene yields to <60%. Poor catalyst lifetimes, attributed to rapid coking typical for oxygenate conversion reactions, have also remained a challenge. Improving the diene yields and mitigating catalyst deactivation are the first key steps to engender industrial interest in the resulting process technology. In this dissertation,we first highlight the mechanistic details of the tandem-ring opening and dehydration of tetrahydrofuran (THF) to butadiene on the aluminosilicate H-ZSM-5, which enable the formulation of the relative ratio of C-O to C-C scission rates as the diene selectivity descriptor. By considering aluminum-, and boron-substituted zeolites in 2-methyltetrahydrofuran (2-MTHF) dehydration to pentadienes, we demonstrate the weakening of solid acid strength as a strategy to tune this descriptor towards dienes’ production. By exploiting the thermodynamic stability of the desirable C5 conjugated diene (1,3-pentadiene), we further explicate strategies harnessing diffusional hurdles to suppress the production of its non-conjugated isomer (1,4-pentadiene). Combined, these insights lead to ~30% improvement in 1,3-pentadiene yield. Having discovered the utility of mild solid acids, we focus the rest of the dissertation on investigating the broad implications of weak surface binding in dehydration catalysis. Using two distinct classes of solid acid zeotype materials with weak Brønsted acidity (namely, borosilicates, and phosphorous-modified zeosils), we detail how these materials can potentially improve dehydration selectivity and stability, albeit often at a cost of lower overall turnover rates. Tying this discussion back to renewable dienes production on these materials, we conclude this work by underscoring the technological and economic improvements still required to achieve competitive diene prices from this process technology.
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    Supporting data for "Catalysis-in-a-Box: Robotic Screening of Catalytic Materials in the Times of COVID-19 and Beyond"
    (2020-05-29) Kumar, Gaurav; Bossert, Hannah; McDonald, Dan; Chatzidimitriou, Anargyros; Ardagh, Alexander M; Pang, Yutong; Lee, ChoongSze; Tsapatsis, Michael; Abdelrahman, Omar A; Dauenhauer, Paul; hauer@umn.edu; Dauenhauer, Paul, J; Dauenhauer Research Group
    The emergence of a viral pandemic has motivated the transition away from traditional, labor-intensive materials testing techniques to new automated approaches without compromising on data quality and at costs viable for academic laboratories. Reported here is the design and implementation of an autonomous micro-flow reactor for catalyst evaluation condensing conventional laboratory-scale analogues within a single gas chromatograph (GC), enabling the control of relevant parameters including reactor temperature and reactant partial pressures directly from the GC. Inquiries into the hydrodynamic behavior, temperature control, and heat/mass transfer were sought to evaluate the efficacy of the micro-flow reactor for kinetic measurements. As a catalyst material screening example, a combination of four Brønsted acid catalyzed probe reactions, namely the dehydration of ethanol, 2-propanol, 1-butanol, and the dehydra-decyclization of 2-methyltetrahydrofuran on a solid acid HZSM-5 (Si/Al 140), were carried out in the temperature range 403-543 K for the measurement of apparent reaction kinetics. Product selectivities, proton-normalized reaction rates, and apparent activation barriers were in agreement with measurements performed on conventional packed bed flow reactors. Furthermore, the developed micro-flow reactor was demonstrated to be about ten-fold cheaper to fabricate than commercial automated laboratory-scale reactor setups and is intended to be used for kinetic investigations in vapor-phase catalytic chemistries, with the key benefits including automation, low cost, and limited experimental equipment instrumentation.