Mechanistic studies of glycine betaine catalysed CO2 reduction using density functional theory

Abstract

Fossil fuels (coal, oil and natural gas) are the main energy sources and the primary raw material sources for chemical industry. The society is facing challenging problems caused by over consumption of fossil fuels leading to serious environmental problems. For instance, the increasing concentration of CO2 in the atmosphere has a significant impact on the earth. Hence, converting CO2 to useful chemicals would be a feasible way to address global warming. Indeed, research efforts have been made to solve this challenging, by developing technology to convert atmospheric CO2 into useful fuels.
In this thesis, we first reviewed the current progress in homogeneous CO2 conversion under and catalysis of non-metal catalysts. Methods developed for converting CO2 to useful fuels include synthesis of cyclic carbonates or polycarbonates; carboxylation reactions and CO2 reduction reactions. In terms of CO2 reduction, catalytic hydrosilylation of CO2, catalytic hydroboration of CO2, hydrogenation of CO2 and catalytic amination of CO2 have all been reported. In the area of catalytic amination of CO2, one of the current problems is that aminals are difficult to trap and isolate. Other products from the catalytic amination of CO2 are formamides and methylamines and their production were widely obtained in experiments. The development of the reaction conditions and the design and discovery of new catalysts are important issues for these transformations. This research aims to study the reaction mechanisms of these reactions using DFT calculations, with the aim to help the design of potential new reactions for this transformation.
Computational chemistry aims to study chemical problems by simulating chemical systems. There are two main methodological families in computational chemistry: those based on quantum chemical (QC) methods 18 and those based on molecular mechanics (MM). In Chapter 2, a brief introduction of computational methodology was given, including Schrödinger equation, Born–Oppenheimer approximation, Hartree-Fock theory and density functional theory. The thermodynamics in computational chemistry was also introduced with conclusion remarks to finish this chapter.
The main research part lies in chapters 4 to 5, where a detailed computational studies for the reaction mechanism of a glycine betaine catalysed CO2 reduction, using PhSiH3 with the presence of N-methylaniline, was carried out. Recently, glycine betaine has been reported to be an active catalyst to produce formamides, aminals and methylamines sequentially under mild conditions. However, the reaction mechanism is unclear. To understand the reaction mechanism, we first analysed interactions between the reactants (CO2, PhSiH3 and N-methylaniline) and the catalyst (glycine betaine) in chapter 3. Then we calculated the reaction mechanism to produce the formamide, the aminal and the methylamine using SMD(CH3CN) ωB97XD/6-311++G(d,p)//ωB97XD/6- 31+G(d,p) in chapter 4 and 5. Energy profiles of the reactions in the reaction mechanism are presented. The activation free energy barriers of the rate- determining steps to produce the formamide, the aminal and the methylamine are 32.2 kcal/mol, 37.8 kcal/mol and 37.8 kcal/mol estimated with SMD(CH3CN)-ωB97XD/6-311++G(d,p)//ωB97XD/6-31+G(d,p). And when calculated in SMD(CH3CN)-M06-2X/6-311++G(d,p)//ωB97XD/6-31+G(d,p) level of theory, the activation free energy barriers of the rate-determining steps for the formation of formamide, aminal and methylamine are 20.1 kcal/mol, 28.9 kcal/mol and 28.9 kcal/mol resepctively. Since the experiments occur under mild conditions (≤100 °C, ≤0.5 MPa), the activation free energy
barriers found from my DFT calculations are reasonable. This research contributes to the understanding of the reaction mechanism of this novel CO2 conversion reaction.

Date of Award	Jul 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Binjie Hu (Supervisor), Bencan Tang (Supervisor) & Jonathon Hirst (Supervisor)