Development of heterogeneous catalysts for CO2 fixation under atmospheric pressure

Mitra, Antarip (2023) Development of heterogeneous catalysts for CO2 fixation under atmospheric pressure. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Antarip Mitra (18RS049)) 18RS049.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

The carbon dioxide (CO₂) level in the earth's atmosphere has increased since the industrial revolution, and the CO₂ level has risen from 278 ppm in the 1780s to 410 ppm in the 21st century. Therefore, we can conclude that there has been 50% rise in global CO2 content since the last couple of centuries. The principal source of CO2 is the combustion of fossil fuels in automobiles and other industries. The rise of CO₂ levels in the atmosphere has resulted in many adverse effects like ocean acidification, acid rain, and global warming. Thus, it is very important to develop ways to decrease the concentration of CO₂ in the atmosphere. In this context, direct CO₂ capture from the atmosphere is a key method. The captured CO₂ can be temporarily stored in cylinders for commercial applications or permanently disposed of in marine aquifers. The chemical fixation of CO₂ into commercially important value-added chemicals is considered as another important strategy both from environmental and commercial points of view. In this context, coupling CO₂ with epoxides to prepare cyclic carbonates is interesting because of the atom economic and non-reductive features of the pathway. Additionally, cyclic carbonates have widespread commercial applications as polar high boiling solvents, electrolytes for lithium-ion batteries, precursors of dialkyl carbonates and polyurethane and more. However, the major bottleneck of this reaction is the high activation energy of CO₂, resulting in the requirement of drastic conditions for successful conversion. The activation barrier can be reduced by performing the reaction in the presence of suitable catalysts. Nevertheless, most of the reported catalysts are either homogenous or operate under high pressure of CO₂. To overcome the above-mentioned challenges associated with catalysts for CO₂ fixation, this thesis aims towards •Development of heterogeneous catalysts using cheap, non-toxic earth-abundant metals like iron and aluminium. •Perform CO₂ fixation under atmospheric pressure. •Avoid use of solvents and external additives (e.g., TBAI or KI). The entire thesis has been separated into five chapters. Chapter 1 gives a general introduction and discusses the harmful effects related to the rise of CO₂ in the atmosphere, especially climate change and global warming. The different techniques which can be adopted to mitigate the problem of excess CO₂ in the atmosphere is illustrated in this chapter. The techniques involve the use of renewable energy sources and reduced dependence of fossil fuels. Additionally, there must be efforts of afforestation. Moreover, industrially CO₂ can be captured and stored. This chapter discusses reports on different redox and non-redox pathways that can be adopted to utilize CO₂ to prepare commercially important compounds. Particularly, emphasis was given to the preparation of cyclic carbonates from epoxides utilizing COv with the aid of different catalysts. This chapter ends with basic challenges associated with the reported catalysts and motivations to develop new catalysts, which are discussed in this thesis. Chapter 2 discusses the development of iron-based heterogeneous catalysts by reacting catacol and guanidine containing ligand (L1) and Fe(III) ions. The catalysis was performed under the atmospheric pressure of CO₂. The results revealed the as-prepared material (Fe-L1) to be a heterogeneous catalyst with negligible loss of catalytic activity after six catalytic cycles. The catalyst was able to convert multiple epoxides in the absence of solvent and additives like TBAB or KI. The investigation of the mechanistic pathway indeed validates the synergistic effect of metal and halide ions that leads to the efficient conversion of different epoxides into their corresponding cyclic carbonates. Chapter 3 illustrates the synthesis of γ-Al₂O₃ followed by functionalization with guanidine hydrochloride (Gh) to prepare the active catalyst Al-Gh. The incorporation of N-rich guanidine moieties enhanced the CO₂ adsorption of Al-Gh by 10 times compared to γ-Al₂O₃. Green solvents like ethanol and water were used for the preparation of Al-Gh, which efficiently converts multiple epoxides into their corresponding cyclic carbonates with satisfactory yields and high selectivity. The material was able to prepare styrene carbonate in gram scale. The cycloaddition mechanism was supported by DFT, which proves the role of alumina in stabilizing the intermediates and substrates. This resulted in the enhancement of the overall catalytic activity. Chapter 4 reports the preparation of a halide-free Aluminium containing coordination polymer (Al-PDC) to catalyse the reaction under atmospheric pressure in absence of solvents and cocatalysts. The catalyst was prepared by the reaction of 2,5-pyridine carboxylic acid and Al (III) ions which was found to be an excellent catalyst for the conversion of multiple terminal epoxides. In addition, gram scale synthesis of styrene carbonate was achieved under optimized conditions. The material retained its catalytic activity for six catalytic cycles and post catalytic characterization confirmed the robust nature of the catalyst. The role of nitrogen atom of the pyridine ring to open the epoxide ring is also experimentally studied in this chapter. Chapter 5 describes the facile one-pot synthesis of diaspore [α-AlO(OH)] in aqueous medium. In the presence of minute quantities of amides like DMF, DMAc, etc. diaspore was able to convert multiple epoxides into their corresponding cyclic carbonates utilizing CO2 under atmospheric pressure in the absence of halide-containing cocatalysts. Hardly any change in the catalytic activity was observed even after five cycles of catalysis. The DFT calculations confirm the spontaneity of the cycloaddition reaction by stabilization of the intermediates and substrates on diaspore. Finally, the summary of this thesis has been discussed. The outlook section describes the future directions and scopes of research in this field.

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