The significant increase of carbon dioxide (CO2) into the atmosphere is alarming since the industrial revolution and this has resulted to prevailing environmental challenges such as global warming experienced in recent times. There is an urgency to reduce CO2 emissions to a sustainable level in order to prevent global warming and climate change. Several methods of reducing CO2 emissions have been identified. However, greener synthesis of organic carbonates such as 1,2-butylene carbonate (BC) and styrene carbonate (SC) through the utilisation of CO2 have been identified to be valuable chemicals in chemical industry. The utilisation of CO2 to produce value-added chemicals is considered as one of the promising technological advancements targeted at reducing CO2 emissions to a sustainable level.
1,2-Butylene carbonate and styrene carbonate are promising green chemicals, which find their applications in chemical and pharmaceutical industries. These organic carbonates exhibit excellent chemical properties and can be used widely as intermediates to synthesise other chemicals, electrolyte to power lithium batteries and fuel additives.
The syntheses of 1,2-butylene carbonate and styrene carbonate using conventional approaches involve the use of phosgene, a toxic feedstock and produce acid waste, which is highly toxic and environmentally unfriendly. The application of solvent-free heterogeneous catalytic processes promote green processes and offer more sustainable process for the syntheses of organic carbonates.
In this work, batch experimental studies have been conducted using several commercially available heterogeneous catalysts such as ceria and lanthana doped zirconia (Ce–La–Zr–O), ceria doped zirconia (Ce–Zr–O), lanthana doped zirconia (La–Zr–O), lanthanum oxide (La–O), zirconium oxide (Zr–O) and graphene oxide supported inorganic nanocomposites where graphene oxide (GO) has been used as a suitable support and metal oxide catalyst (Ce-La-Zr/GO) has been extensively assessed for the synthesis of 1,2-butylene carbonate and styrene carbonate. Ceria, lanthana, zirconia doped graphene nanocomposites (Ce-La–Zr/GO) have been synthesised using a new innovative approach known as a continuous hydrothermal flow synthesis (CHFS) reactor. Copper, zirconia doped graphene oxide (Cu-Zr/GO) and copper, zirconia oxide/graphene composite (HTR450) have been synthesised using conventional wet impregnation methods and assessed as suitable heterogeneous catalysts for the synthesis of 1,2-butylene carbonate via a facile direct route. These catalysts have been characterised using various analytical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and Brunauer-Emmett-Teller (BET) surface area measurement.
The use of a solvent-free heterogeneous catalytic process for the direct syntheses of BC and SC have been conducted in a high-pressure reactor. Various reaction parameters such as the effect of reaction temperature, CO2 pressure, reaction time, catalyst loading, and stirring speed have been investigated to achieve the optimum reaction conditions on the conversion of epoxides and yield of cyclic carbonates. The long-term stability of the heterogeneous catalysts has been evaluated by conducting reusability studies. Copper, zirconia doped graphene nanaocomposite catalyst (HTR450) and ceria, lanthana, zirconia doped graphene oxide catalyst (Ce-La-Zr/GO) have been found to be the most active and selective for the synthesis of BC as compared to other commercial catalysts evaluated in this research work. For the direct synthesis SC, ceria, lanthana doped zirconia (Ce-La-Zr-O) has been found to be the best-performed catalyst as compared to other used catalysts. The reusability studies of these HTR450, Ce-La-Zr/GO and Ce-La-Zr-O catalysts have evidently shown the long-term stability without any significant reduction in their performances.
Response Surface Methodology (RSM) in the design of experiment is used for modelling and optimisation of experiments in the process industry, catalysis and chemical reaction engineering. The application of RSM minimise the number of experiments thereby saving time and materials. Hence, RSM using box-Behnken design (BBD) has been explored to evaluate and optimise multiple responses (output variables), which are influenced by several independent variables such as catalyst loading, temperature, CO2 pressure, and reaction time. BBD model has been developed for the direct synthesis of SC via cycloaddition reaction of CO2 to styrene oxide (SO) and direct synthesis of BC through reaction of butylene oxide (BO) and CO2. The developed models have been used to compare the experimental results and the predicted results. Regression analyses have been carried out to establish the optimum reaction parameters for a maximum yield of SC and BC. The predicted values of BBD model are in good agreement with the experimental results with <1.5% error.