Computational Study and Manipulation of Local Chemical Environment in heterogeneous Catalysis and Separation.
Abstract
Reactive separation is proposed as a less energy-intensive chemical process with functions of catalytic reaction and separation. It offers an active chemical conversion combined with selective separation to obtain the desirable products. Catalytic membranes are one of the successful examples and the design of it requires understanding the structure-activity relationship between catalyst and membranes.
To guide the process design, this thesis gives computational insight on the following aspects: catalyst design focused on selectivity and activity in aqueous environments, and a porous structure suitable for incorporating catalytic active sites and responsible for separation. For catalyst design, the aim is to understand the role of the local chemical environment on reaction activity and selectivity. It is promising to tune the reaction activity by manipulating its solvation environments and coordination environment for the catalyst. By understanding the interplay between catalytic performance and the local chemical environment, a catalytic active site with a well-defined structure is proposed for further catalytic membrane design. Additionally, a well-established membrane for gas separation is studied to characterize cavity formation with tunable membrane contents. The study encompasses fundamental aspects of catalysis, including electronic structure, kinetic principles, and temperature-dependent separation processes, while analyzing detailed polymer structures.
Firstly, the aldol condensation reaction of cyclopentanone (CPO) over metal oxide MgO is investigated. Functionalized groups grown on an oxide catalyst highlights the tunable kinetic performance and promotional role of water in association with functionalized groups. Density Functional Theory (DFT) calculations show the strong adsorbate-adsorbate interaction results in hindered C-C coupling step, which contributes to low reaction rates observed on the functionalized surface. The role of water is identified in facilitating CPO adsorption and reducing the kinetic barrier of the kinetic-relevant step. Water thereby promotes the reaction rate by alleviating serious adsorbate-adsorbate interactions on the functionalized surface. This study paves a way to understand the solvent effect on the heterogeneous catalyst.
With an emphasis on denitrification in wastewater treatment, palladium has been proposed as an active catalyst for wastewater treatment, but the detailed mechanism and the solvent effect are not well understood. To address this, DFT calculations were employed to investigate the detailed reaction mechanism and kinetics for nitrite reduction. This study revealed that the overall activity is primarily determined by the activity of NO and a comprehensive kinetic model is developed to explain the observed reaction order. Interestingly, water played a promotional role in the nitrite hydrogenation reaction over the palladium catalyst. DFT analysis demonstrates that water facilitates a lower-energy pathway for hydrogenation by shuttling surface hydrogen to the reactants. The tunned free energy pathway aligns with the experimental findings on the reaction order.
The solvent effect is associated with the micro-solvation environments created by a polymer (n-isopropyl acrylamide) overlayer that shows temperature-dependent behavior. Growing this NIPAM overlayer on the catalyst support alters the selectivity and activity of palladium along with the tuned solvation environment. Molecular dynamics simulations propose a lower activation barrier introduced by NIPAM due to polymer-water interactions. The presence of polymer in the aqueous phase forms a structured water layer on the palladium, favoring the reaction pathway through the proton-shuttling mechanism. These findings enhance our understanding of designing and manipulating the micro-solvation environments for a tunable solvent effect.
As revealed on metal catalysts, the binding energy of NO provides crucial insights to promote reaction activity and selectivity. The binding energy determines the activation of NO bond hydrogenation is highly associated with the structure of the active sites. The investigation extends to the activity of single atoms in nitrite reduction powered by renewably generated electricity and examines the denitrification mechanisms across dispersed Co and La-based catalysts. The enhanced charge transfer is identified on NO bonded with La. and different mechanisms are investigated including entropic stabilization and the solvent effect. The study suggests that atom-dispersed metal is one of the promising catalysts toward a high activity and selectivity of NO reductions which dominates the denitrification reaction.
The catalytic membrane requires a stable polymer structure to selectively filter out the ions. We conducted a simulation on membrane synthesis using a triptycene-based membrane (TPBO) with tunable cavity distribution based on the content of triptycene. By manipulating the content of the triptycene unit, the free volume distribution corresponding to the triptycene molecule content is obtained. These structure-properties relationships reveal the flexibility of TPBO-based polymers for gas separation. Our study on polymer structure provides a foundation acknowledge for further membrane design which is critical to catalytic membranes.
Overall, this dissertation suggests a synergetic combination that takes advantage of polymer structures and single-atom catalysis to achieve enhanced activity and selectivity in reactive separation. This approach holds great promise for advancing catalytic technologies and addressing challenges in fields such as separation science and environmental sustainability. This study on catalysis research has been extensively focused on understanding the fundamental principles governing catalyst performance, with a growing recognition of the significance of the local chemical environment. Fine-tuning and manipulating this environment presents novel avenues for catalyst design.
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