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Eton, Udeme Offiong (2025) Examining cyclic carbonate synthesis under pure CO2 and simulated flue gas conditions using non-metal catalysts. PhD thesis, University of Nottingham.

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Abstract

A promising method to recycle waste CO2 in post-combustion flue gas chemically is the cycloaddition of CO2 to epoxides, which yields cyclic carbonates. The procedure can use purified CO2 collected from a capture plant or flue gas directly as an impure CO2 source. Achieving good product yields at atmospheric pressure and low temperatures is highly beneficial for both options. For direct utilization of flue gas, the process should, in addition, be chemically stable under low CO2 partial pressure and in the presence of impurities. Current industrial processes for cyclic carbonate production, however, require high CO2 pressure (30 – 100 atmospheres) and high temperature (150 – 210 oC), consuming a significant energy input for operation. In addition, they are configured to use pure CO2 gas. This research investigates the synthesis of cyclic carbonate using either pure CO2 or simulated flue gas at 1 atmosphere (atm) to simulate the two process options. It assesses the performance of metal-free catalysts (tetrabutylammonium halides or amine bases) combined with a hydrogen bond donor (HBD) as a promoter under the two process options.

Low concentrations of hydroxyl or amino-containing hydrogen bond donors (HBDs) enhanced the catalytic activity of tetrabutylammonium salts and amine bases for cyclic carbonate synthesis. The extent of enhancement depended on the type of HBD as well as the type of tetrabutylammonium salt or amine base; however, tetrabutylammonium iodide (TBAI)/H2O and 4-dimethyl aminopyridine (DMAP)/N,N’-diphenyl thiourea (DPTU) pairs exhibited high performance. Whereas the HBDs (H2O and DPTU) showed no catalytic activity when used independently, TBAI and DMAP showed moderate activity when used independently for the reaction, indicating that H2O and DPTU acted as promoters while TBAI and DMAP catalyzed the reaction. With the optimum catalyst/promoter pair in hand, reaction parameters were tuned to maximize cyclic carbonate yield and CO2 utilization. A trade-off existed between the catalyst and promoter concentration and cyclic carbonate yield and selectivity. For TBAI/H2O catalyst/promoter pair, the peak yield and selectivity was observed at a H2O and TBAI concentration of 0.15 and 0.025 mole ratio relative to the epoxide, respectively. For DMAP/DPTU catalyst/promoter pair, this was at a DMAP+DPTU/ECH mole ratio of 0.067 relative to epoxide and DMAP/DPTU mole ratio of 7.5. Catalyst and promoter concentration above this threshold was not beneficial. A CO2 feed rate of ≤ 0.161 mmol/min was advantageous as almost all the CO2 fed was utilized. Therefore, at the optimum reaction condition, CO2 utilization of ≥ 94 %, cyclic carbonate yield of ≥ 90 % and cyclic carbonate selectivity of ≥ 98 % were obtained for both catalyst/promoter pair at 60 oC and 1 atm pure CO2. This result demonstrates that low-cost and readily available H2O and DPTU could promote the catalytic activity of TBAI and DMAP, respectively under mild reaction conditions, resulting in a high yield of cyclic carbonate at a reasonable reaction time.

It is of high interest for commercial applications to examine reaction efficiency with low CO2 partial pressure and flue gas impurities. Interestingly, both catalyst/promoter pair remained active for reactions under a simulated flue gas (15 % CO2/N2) atmosphere, even at a near room temperature of 30 oC. Although the reaction rate decreased as CO2 partial pressure decreased from 1 to 0.15 atm, both catalyst/promoter pair retained sufficient activity to produce cyclic carbonate in almost quantitative yield at 50 oC in 24 hours. The selectivity remained reasonably independent of CO2 partial pressure. Furthermore, a high concentration of H2O in the reactor had a negative impact on cyclic carbonate yield and selectivity due to phase separation, dilution effect and formation of side product. The presence of O2 in the simulated flue gas at a concentration of 5 % v/v had no effect on cyclic carbonate yield and selectivity for both catalyst/promoter pairs. For actual flue gas application, it will be necessary to allow the flue gas to cool below its dew point to lower its moisture content by condensation to avoid the formation of by-products.

A kinetic study was employed to explain the influence and role of catalytic components on reaction kinetics. The HBDs (H2O or DPTU) increased the observed rate constant (kobs) while decreasing the activation energy. This result implies that the HBDs enhanced the catalytic activity of the catalysts by reducing the energy barrier for activating the reactants to the transition state. kobs was found to decrease by a factor of 0.65 or 0.84 for TBAI/H2O or DMAP/DPTU, respectively as CO2 partial pressure decreased from 1 to 0.15 atm, which explains the decrease in cyclic carbonate yield observed under simulated flue gas condition. The catalyst/promoter concentration, CO2 partial pressure, and temperature were significant factors influencing reaction kinetics and must be carefully controlled in practical application. The performance of the TBAI/H2O and DMAP/DPTU catalyst/promoter pairs was explained in terms of a previously reported synergistic mechanism in which the HBDs (H2O and DPTU) activates the epoxide and stabilizes reaction intermediates/transition states through hydrogen bond interactions, while the catalysts (TBAI and DMAP) ring opens the activated epoxide by nucleophilic attack, allowing for cyclic carbonate to be facilely obtained in high yields even at 0.15 atm CO2 partial pressure. The TBAI/H2O and DMAP/DPTU catalyst/promoter pairs exhibited a higher rate constant and lower activation energy in the reaction of ECH and CO2 at 1 atm than a previously reported catalyst based on Co/Zn bimetallic complex.

Item Type:	Thesis (University of Nottingham only) (PhD)
Supervisors:	Snape, Colin E. Meredith, William
Keywords:	CO2 Utilization; Cyclic Carbonate Synthesis; Epoxide H2O-Promoted; Flue Gas; n-tetrabutylammonium iodide; 4 dimethylaminopyridine; N,N-diphenylthiourea; Hydrogen bond donor catalysis; Conversion; Yield; Cycloaddition reaction Gas chromatography Nuclear Magnetic Resonnance
Subjects:	T Technology > TP Chemical technology
Faculties/Schools:	UK Campuses > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID:	81745
Depositing User:	ETON, UDEME
Date Deposited:	31 Dec 2025 04:40
Last Modified:	31 Dec 2025 04:40
URI:	https://eprints.nottingham.ac.uk/id/eprint/81745

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