Greener synthesis of propylene carbonate using graphene-inorganic nanocomposite catalysts

Continuous hydrothermal flow synthesis (CHFS) has been employed as rapid and cleaner route for the synthesis of a novel, highly efficient graphene-inorganic heterogeneous catalyst, denoted as Ce-La-Zr-GO nanocomposite. The catalyst was used for the direct synthesis of propylene carbonate (PC) from the cycloaddition of carbon dioxide (CO2) and propylene oxide (PO) in the absence of any organic solvents. Ce-Zr-La-GO nanocomposite was synthesized from pre-mixed aqueous solution of cerium, lanthanum, zirconium nitrate and GO (synthesized via conventional Hummers method) under alkaline conditions using the CHFS reactor. The resulting nanocomposite catalyst was heat-treated at various temperatures (773, 973 and 1173K) and their catalytic properties were assessed. The as-prepared and the corresponding heat-treated catalysts were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD) and Brunauer-Emmett-Teller (BET) surface area measurements. Effect of various parameters such as heat-treatment temperature, catalyst loading, CO2 pressure, reaction time and temperature was systematically studied to determine the optimum reaction conditions. The optimum reaction condition for the direct synthesis of PC was found at 443 K, 70 bar and 10% (w/w) catalyst loading. Catalyst reusability study was also conducted to investigate the long life stability for the synthesis of PC and it was found that the catalyst could be reused several times without losing it catalytic activity. These studies revealed that Ce-Zr-La-GO nanocomposite catalyst showed high catalytic activity as compared to other reported heterogeneous catalysts with the conversion of PO, selectivity and yield of PC. (C) 2015 Elsevier B.V. All rights reserved.