| Abstract | In the last two decades, several attempts have been made to develop new catalytic systems for the chemical fixation of CO2, both homogeneous and heterogeneous catalysis. However, these attempts have failed to yield satisfactory results as most of these catalysts requires high temperature and/or pressure (usually around 453 K and pressure higher than 8 atm), further separation and purification steps, many of these catalysts deactivate after few recycle experiments and worse of all, low product yield. Hence, this research is focused on the use of metal-organic frameworks (MOFs) catalysts as a relatively new and promising candidate that addresses these aforementioned shortfalls.
The development of a novel Zr/ZIF-8 catalyst via a simple low-cost solvothermal method, easy separation by centrifugation, and its excellent recyclability properties have demonstrated that the catalyst could be viable for large-scale industrial applications. The heterogeneity of the catalyst has been proven by recovering and reusing the catalyst for up seven times without any significant loss in catalytic activity. The powder x-ray diffraction (XRD), fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) of the recycled catalyst shows that its framework is quite stable after reusability performance.
Furthermore, the catalyst shows high substrates tolerance towards different epoxides including epichlorohydrin (ECH), styrene oxide (SO) and butylene oxide (BO). The reaction has been carried out under solvent-free and cocatalyst conditions. The catalytic properties of the novel catalyst have been satisfactorily consistent with pristine ZIF-8 catalyst using multiple physicochemical characterisation techniques. We believe that this work could provide a new direction for designing more sustainable, non-toxic catalysts for the transformation of CO2 and other substrates.
The comparison of catalytic activity of both the pristine ZIF-8 and the novel Zr/ZIF-8 catalysts have been drawn based on the effect of various reaction conditions such as reaction temperature, CO2 pressure, catalyst loading, reaction time, stirring speed and reusability studies. Zr/ZIF-8 catalyst has
been assessed as a suitable heterogeneous catalyst outperforming the catalytic activity of pristine ZIF-8 catalyst with respect to the conversion of epoxide, selectivity and yield of the desired carbonates.
In addition, experimental design, modelling and optimisation techniques via response surface methodology (RSM) have also been implemented for different process responses. The experimental results have been employed to design and simulate chloromethyl ethylene carbonate (CMEC) and styrene carbonate (SC) using batch experimental studies. The adequacy of the models has been validated by the correlation between the experimental and predicted values of the responses using an Analysis of Variance (ANOVA) method. Therefore, statistical modelling using RSM can be used as a reliable prediction tool for system optimisation for greener synthesis of both chloromethyl ethylene carbonate and styrene carbonate.
In conclusions, the catalyst has displayed high epoxide conversion and high carbonate selectivity. The optimum experimental conditions and results for the synthesis of chloromethyl ethylene carbonate were found to be 353 K, 8 bar of CO2 pressure and 8 h using fresh 10% (w/w) Zr/ZIF-8 catalyst loading for a 93% ECH conversion, 86% and 76% of CMEC selectivity and yield, respectively. While the optimum experimental conditions and results for the synthesis of styrene carbonate were found to be 353 K, 6 bar of CO2 pressure and 8 h using fresh 6% (w/w) Zr/ZIF-8 catalyst loading for a 98% SO conversion, 72% and 68% of SC selectivity and yield, respectively. Similarly, the optimised reaction conditions and results using RSM techniques for the synthesis of chloromethyl ethylene carbonate were found at 353 K, 11 Bar of CO2 pressure 12 h using 12% (w/w) fresh catalyst loading for a 96% ECH conversion and 68% CMEC yield while 353 K, 6.1 Bar of CO2 pressure and 8.2 h using fresh 6% (w/w) Zr/ZIF-8 catalyst loading for 98% SO conversion and 68% SC yield for the synthesis of styrene carbonate. |
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