| Abstract | 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). |
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