Mechanistic investigations of CO2 hydrogenation reactions using homogeneous/heterogeneous catalysts

Abstract

The increasing world’s population, industrialization and economic development are rapidly enhancing the worldwide energy demand. Most of the energy demands are fulfilled by conventional fossil fuel-based resources. Therefore, the fossil-fuel sources are decreasing, and these will be depleted by 2060 if it is being used at our current rate. Besides, the uses of fossil fuels increase the amount of greenhouse gases on the environment such as CO, CO2, NO, NOx, SOx among others. Therefore, it is essential to find out renewable and green carbon-neutral fuels. Recently, hydrogen fuel has gained significant attention due to its clean energy resources. But the storage and transportation of H2 fuel are very difficult. However, H2 can be stored and transported by converting H2 into some other useful chemicals known as liquid fuels (HCOOH, CH3OH, C2H5OH, C2H4 among others). In this context, CO2 conversion in the presence of H2 to value-added products and uses of these products as a liquid fuel can be an important alternate as the process is carbon neutral. Hence, the scientific community is enthusiastic towards CO2 hydrogenation reactions. Several catalytic systems have been explored for effective CO2 hydrogenation reactions. In the case of homogeneous catalysts, noble metal based systems (Ru, Rh, Ir, and so on) have been comprehensively studied, but these systems cannot be used in the industrial productions of value added products as the availability of these metals are very less and cost of these metals are very high. To circumvent these drawbacks, non-noble (Mn, Fe, Co, Ni among others) metal catalysts have been introduced. In the case of non-noble metal-based catalysts presence of the bifunctional nature of the ligand helps to activate the reaction. Side by side, Hori and co-workers have examined the uniqueness of Cu based heterogeneous catalysts for the CO2 hydrogenation to value-added products (CH3OH, CH4, C2H4, and C2H5OH). In this regard, numerous Mn-based bifunctional homogeneous catalysts and Cu-based heterogeneous catalysts have been identified for the practical utilization of various CO2 hydrogenation reactions in this thesis. vi Even though several experimental reports have reported several metal based catalysts for active CO2 hydrogenation reactions, the underlying reason behind these reactions is still unclear. Henceforth, all the possible mechanistic pathways have been investigated using computational techniques and identified the most favourable one among all the considered pathways. Moreover, some Mn and Cu-based catalytic systems have been designed and identified to be promising catalysts for CO2 hydrogenation reactions using various computational techniques.