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

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 (P_CO2 > 7.4 MPa and P_methanol > 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/g_cat. 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.