Performance and kinetic investigations on the CeO2 catalyzed direct synthesis of dimethyl carbonate from CO2 and methanol in dual supercritical conditions

Abstract

In this study, the direct synthesis conditions of dimethyl carbonate (DMC) from methanol and CO2 were controlled under the dual supercritical conditions, whose temperature was higher than the maximal critical temperature of CO2 and methanol (>239 ℃), and the partial pressure for each reactant was larger than its partial critical pressure (PCO2 > 7.4 MPa and Pmethanol > 8.1 MPa). A series of CeO2 catalysts with different morphologies including traditional nanorod, amorphous and flower structures were synthesized by hydrothermal method. The prepared catalysts were characterized by the BET, XRD, SEM, NH3/CO2-TPD, XPS, and H2-TPR, and the results indicated that flower CeO2 performed the largest acid-base sites and oxygen vacancies as expected, proving its superior physicochemical properties and catalytic activity. Moreover, the catalytic activity synthesis of DMC from CO2 and methanol was investigated, confirming that the flower CeO2 owned the highest catalytic performance, and the maximum yield of DMC was 3.11 mmol/gcat. under the reaction conditions (16 MPa, 250 °C, reaction time of 1 h without stirring). Importantly, the synthesis kinetic mechanism of DMC in dual supercritical systems was experimentally studied using the synthetic CeO2 catalysts, and the flower CeO2 catalyst possessed the lowest apparent activation energy of 45.9 kJ/mol, meanwhile, the initial reaction rate equation obtained from the experiment can be represented as: Rate = k [*] [CH3OH] [CO2]1/2, which was consistent with the reaction mechanism of Langmuir–Hinshelwood type.