Heterogeneous Catalytic Conversion of Carbon Dioxide to Value Added Chemicals

Carbon dioxide (CO2) emissions have increased to unsustainable levels in the atmosphere, which has led to the current environmental problems such as climate change and global warming. The reduction of CO2 emissions has become a global environmental challenge. Greener chemical process for utilisation of CO2 in the synthesis of valuable chemicals such as organic carbonates is one of technological advancements aimed at reducing CO2 emission into atmosphere. Organic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), 4-vinyl-1-cyclohexene carbonate (VCHC), (chloromethyl)ethylene carbonate (CMEC) and styrene carbonate (SC) have been used widely as intermediates in the synthesis of chemicals, pharmaceuticals and fuel additives. The conventional method of organic carbonates syntheses employs homogeneous catalysts, solvents as co-catalysts and toxic raw materials including phosgene (COCl2) and iso-cyanates (RNCO), and produces carcinogenic by-products that could have serious impact on the environment and human health. Hence, there is a need for an environmentally benign green process for the synthesis of organic carbonate from CO2. Catalysis is an important tool in designing a greener process for the synthesis of valuable chemicals. In this work, several heterogeneous catalysts have been synthesised using a continuous hydrothermal flow synthesis (CHFS) reactor and their catalytic activity evaluated for the synthesis of organic carbonates by cycloaddition reaction of epoxides and CO2 under a solvent free system. The catalysts were characterised using various analytical characterisation techniques including Raman spectroscopy (RS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and x-ray powder diffraction (XRD). The Brunauer-Emmett-Teller (BET) surface area and porosity measurements were conducted using micrometrics analyser. Solvent free heterogeneous catalytic process for organic carbonate synthesis has been investigated using a high pressure Parr reactor. The effect of various parameters such as heat-treatment temperature, catalyst loading, CO2 pressure, reaction time and reaction temperature on the conversion of epoxides, selectivity and yield of organic carbonates was studied for optimisation of reaction conditions. Batch experimental studies were conducted to investigate the long term stability of the catalysts by reusing the catalysts several times for the syntheses of organic carbonates. Ceria, lanthana and zirconia graphene oxide (Ce-La-Zr-GO) nanocomposite catalyst has been the most active and selective for the cycloaddition reaction as compared to other heterogeneous catalysts such as commercially available catalysts (Zr-O, Ce-Zr-O, La-Zr-O and Ce-La-ZrO catalysts were supplied by the Magnesium Elecktron Limited (MEL) Chemicals) used in this research work. The reusability studies showed that Ce-La-Zr-O and Ce-La-Zr-GO catalysts could be reused several times without losing catalytic activity. Keywords: Carbon dioxide (CO2), ceria and lanthana doped zirconia (CeLa-Zr-O), ceria, lanthana and zirconia graphene oxide (Ce-La-Zr-GO), ceria doped zirconia (Ce-Zr-O), lanthana doped zirconia (La-Zr-O), heterogeneous catalysts, propylene carbonate (PC), propylene oxide (PO), butylene carbonate (BC), butylene oxide (BO), 4-vinyl-1- cyclohexene carbonate (VCHC), 4-vinyl-1-cyclohexene 1,2-epoxide (VCHE), (chloromethyl)ethylene carbonate (CMEC), epichlorohydrin (ECH), catalyst characterization, zirconia (Zr-O)