Browsing by Subject "Reaction Engineering"

Now showing 1 - 4 of 4
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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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    Finding the Chemistry in Biomass Pyrolysis: Millisecond Chemical Kinetics and Visualization
    (2016-06) Krumm, Christoph
    Biomass pyrolysis is a promising thermochemical method for producing fuels and chemicals from renewable sources. Development of a fundamental understanding of biomass pyrolysis chemistry is difficult due to the multi-scale and multi-phase nature of the process; biomass length scales span 11 orders of magnitude and pyrolysis phenomena include solid, liquid, and gas phase chemistry in addition to heat and mass transfer. These complexities have a significant effect on chemical product distributions and lead to variability between reactor technologies. A major challenge in the study of biomass pyrolysis is the development of kinetic models capable of describing hundreds of millisecond-scale reactions of biomass into lower molecular weight products. In this work, a novel technique for studying biomass pyrolysis provides the first- ever experimental determination of kinetics and rates of formation of the primary products from cellulose pyrolysis, providing insight into the millisecond-scale chemical reaction mechanisms. These findings highlight the importance of heat and mass transport limitations for cellulose pyrolysis chemistry and are used to identify the length scales at which transport limitations become relevant during pyrolysis. Through this technique, a transition is identified, known as the reactive melting point, between low and high temperature depolymerization. The transition between two mechanisms of cellulose decompositions unifies the mechanisms that govern low temperature char formation, intermediate pyrolysis conditions, and high temperature gas formation. The conditions under which biomass undergoes pyrolysis, including modes of heat transfer, have been shown to significantly affect the distribution of biorenewable chemical and fuel products. High-speed photography is used to observe the liftoff of initially crystalline cellulose particles when impinged on a heated surface, known as the Leidenfrost effect for room-temperature liquids. Order-of-magnitude changes in the lifetime of cellulose particles are observed as a result of changing modes in heat transfer as cellulose intermediate liquid droplets wet and de-wet polished ceramic surfaces. Introduction of surface macroporosity is shown to completely inhibit the cellulose Leidenfrost effect, providing avenues for surface modification and reactor design to control particle heat transfer in industrial pyrolysis applications. Cellulosic particles on surfaces consisting of microstructured, asymmetric ratchets were observed to spontaneously move orthogonal to ratchet wells above the cellulose reactive Leidenfrost temperature (>750 °C). Evaluation of the accelerating particles supported the mechanism of propelling viscous forces (50-200 nN) from rectified pyrolysis vapors, thus providing the first example of biomass conveyors with no moving parts driven by high temperature for biofuel reactors. Combined knowledge of pyrolysis chemistry, kinetics, and heat and mass transport effects direct the design of the next generation pyrolysis reactors for tuning bio- oil quality and design of improved catalytic upgrading 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.
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    Theoretical Insights into the Effects of Interfacial Electrolytes and Catalyst Characteristics on Reduction Chemistries
    (2022-05) Gorthy, Sahithi
    The limited reserves of fossil fuels, and rising concerns about global warming and climate change, have motivated the development of sustainable methods for catalytic systems. This thesis focuses on the electrocatalytic reduction of carbon dioxide (CO2) and the catalytic reduction of oxygen (O2) to value-added chemicals using environmentally-friendly processes.The electrochemical reduction of CO2 to energy-dense chemicals using renewable energy resources is attractive; however, lowering the associated overpotentials and improving selectivity at high current density outputs is imperative to become carbon-neutral. The work presented herein uses potential-dependent ab initio molecular dynamics and density functional theory methods to explore the role of the local reaction environment and the metal catalyst on CO2 reduction. Specifically, we examine the role of ionic liquids and alkaline electrolytes on CO2 activation and subsequent reduction. The calculations with ionic liquids reveal that their cations can stabilize negatively charged surface intermediates through hydrogen bonding, thereby lowering CO2 onset potential. Our simulations in potassium hydroxide solutions reveal that the hydroxide anion can adsorb on the cathode to promote electron transfer to the adsorbed CO2 radical, improving the reduction current density. The analysis of alkaline electrolytes with different anions indicates that the anion can play a dual role by promoting charge transfer and directly interacting with the adsorbed intermediates through hydrogen bonding or electrostatic interactions, thus changing reduction overpotential and current density. Further, we also study the formation of multi-carbon products on different copper facets under different operating conditions.Finally, we investigate the effect of bimetallic catalysts of gold and palladium on reducing oxygen to hydrogen peroxide selectively in aqueous environments. Theoretical calculations and experimental rate measurements indicate that solvent water molecules mediate oxygen reduction through proton-electron transfer steps and that the difference in the structural sensitivity for the formation of peroxide vs. water results in increased selectivity as palladium is isolated in gold. This thesis shows that both the solvent environment and the active catalyst play critical roles in determining the activity and selectivity of reduction reactions, and explicit solvent modeling is essential to accurately capture the interactions and understand the distinct roles played by each component.