Design and Development of heterogeneous catalyst for the conversion of CO₂ to cyclic carbonates

Paliwal, Khushboo S. (2024) Design and Development of heterogeneous catalyst for the conversion of CO₂ to cyclic carbonates. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Khushboo S. Paliwal (17IP014)) 17IP014.pdf - Submitted Version Restricted to Repository staff only Download (11MB)

Abstract

The anthropogenic CO₂ concentration has been increasing very rapidly since the Industrial Revolution in the 18th century. Ocean acidification, acid rain, and global warming are some of the major effects caused by the rise in the CO₂ concentration in the earth’s atmosphere. Global warming is defined as the increase in the average temperature of the earth’s atmosphere, and it is mostly caused by greenhouse gases (GHGs) like CO₂, methane, etc. The adverse effects of global warming are among the prime problems of the scientific community. Burning of fossil fuels for electricity generation and transportation are the major contributor to the release of CO₂ into the atmosphere. Therefore, it is vital to reduce the level of CO₂ in the earth’s atmosphere. However, cutting the need for electricity and transportation is impractical because of rapid population growth and urbanization. In this context, the utilization of CO₂ as a C1 feedstock to synthesize commodity chemicals is the most promising path to follow. This way, anthropogenic CO₂ levels can be modulated and further CO₂ can be converted into value-added chemicals. One of the promising strategies is to prepare cyclic carbonates by the reaction of CO2 with epoxides. This cyclo-addition reaction of CO₂ with epoxide is an atom-economic and non-redox procedure. The products cyclic carbonates are an interesting class of chemicals that have many commercial applications like solvents in Li-ion batteries, methylating agents, monomers for polycarbonates, monomers for polyurethanes, and many more. Moreover, cyclic carbonates have the longest life span compared to other products (CH₃OH, CO, HCOOH, CH₃CH₂OH, CH₄, etc.) making them the best candidate for reducing the concentration of CO₂. The kinetic inertness and thermodynamic stability of CO₂ molecules can be overcome by the use of catalysts and energy, respectively. The employed catalyst can be either of homogeneous or partially heterogeneous or fully heterogeneous nature. Despite having better catalytic activity homogeneous catalysts suffer from issues related to tedious product separation and recycling. On the other hand, heterogeneous catalyst offers an easy product separation route and long cycle life. The employment of heat energy for this transformation is well known, however, there are relatively few reports on the utilization of light energy for the CO2 fixation reactions. This thesis is aimed to address the knowledge gaps in the CO₂ to cyclic carbonate conversion field and it is divided into five chapters. Chapter 1 gives a broad introduction about Global warming, its causes, and its harmful effects on biological, ecological and economic structure. Furthermore, this chapter discusses about the different pathways to reduce the anthropogenic concentration of CO₂ in the earth’s atmosphere and the details of techniques used to achieve the aforementioned task. The different techniques discussed involve the use of renewable energy sources, afforestation, carbon capture and storage (CCS), and carbon capture and utilization (CCU). Special emphasis is given to the CCU techniques since the works reported in this thesis are based on the CO₂ utilization technique. First, an overview of different organic transformation reactions of CO₂ into commodity chemicals through redox and non-redox paths is provided, followed by a detailed analysis of cyclic carbonate preparation with the aid of various catalysts. The current obstacles and motivation behind this thesis are discussed in the conclusion section of the chapter. Chapter II illustrates the design and development of ionic liquid (IL) grafted biopolymer chitosan (CS) based multifunctional catalysts for the efficient reaction between CO₂ and epoxides to obtain cyclic carbonates under ambient pressure. This work demonstrates the structure-activity relationship between different catalysts having a varying range of hydrophobicity, CO₂ absorption ability, surface area, and IL content. The aforementioned study reveals that the optimum hydrophilic/hydrophobic structurally balanced catalyst provides best yield for most of the epoxides ranging from polar to non-polar epoxides. The optimized catalysts IL-CS-8 (1-octylpyridin-1-ium iodide IL grafted chitosan) have shown excellent activity in converting a range of epoxides to corresponding cyclic carbonate products along with good recyclability up to four cycles of catalysis. The excellent activity of IL-CS-8 is attributed to the octyl group attached to IL moieties which aids in the effective interaction with the hydrophobic part of the epoxide substrates. Chapter III demonstrates the development of graphitic carbon nitride (g-C₃N₄)/AlOOH composite catalysts for the thermal-mediated fixation of CO₂ into epoxide to generate cyclic carbonates under 1 bar pressure. The composites were prepared in a simple hydrothermal route using green solvent, water. A thorough characterization has revealed that under hydrothermal conditions g-C₃N₄ sheets break down into smaller pieces with simultaneous deposition of AlOOH on them. This not only resulted in the homogeneous distribution of AlOOH on the g-C3N4 sheets but also expose number of active sites such as -NH2 and C-OH. The synergistic interaction between these basic (-NH, guanidine N, etc.) and acidic (-OH of AlOOH) sites drives the CO₂ cycloaddition reaction effectively. The optimized catalyst has displayed very good efficiency for 13 different epoxides with minimum amount of cocatalyst (TBAB, 0.3 mol%) reported so far for g-C₃N₄-based catalytic systems. Moreover, the catalyst is efficient in converting 6 different epoxides to respective CCs under cocatalyst-free conditions. Furthermore, the composite catalyst displayed retention of catalytic activity and chemical integrity up to six cycles of catalysis. Chapter IV illustrates the application of light energy instead of heat energy in catalyzing the CO₂ fixation reactions into epoxides to generate corresponding cyclic carbonates. In this regard, biopolymer CS was calcined at different temperatures under an ambient atmosphere to obtain N-doped carbon materials. The as-prepared materials showed excellent photothermal conversion ability and the structure-activity relationship study demonstrates that the highest activity observed for Cal-CS-300 (prepared at 300°C) is attributed to its disordered graphite-like structure and the presence of amine groups. The good photothermal conversion ability of this material is harnessed for the light-driven reaction of multiple epoxides with CO₂ to generate corresponding cyclic carbonates in good yields. Recyclability experiment shows that the catalyst retains its catalytic activity up to five cycles of catalysis indicating excellent chemical stability and structural robustness. Chapter V demonstrates the design and development of x% Co-Al₂O₃ (Al₂O₃/CoAl₂O₄) composite photocatalysts in a simple hydrothermal route followed by calcination. Evaluation of catalytic activity of the composites reveals that 15% Co-Al₂O₃ composite displays best catalytic activity for the light-driven CO₂ fixation into multiple epoxides to synthesize cyclic carbonate products with very high selectivity and yield. The material retains its catalytic activity up to eight cycles of catalysis along with the retention of its chemical structure. Mechanistic investigation suggests that the CoAl₂O₄ component of the composite primarily assists in the photothermal conversion whereas Al2O3 aids in the substrate activation. Furthermore, several control experiments confirm that the entitled reaction occurs through both photothermal and photocatalytic pathways.

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