| Dimethyl carbonate (DMC) is an environmentally benigncompound and it is widely used in many fields, such as the reagent for carbonylation, methylation and methoxylation reaction. DMC can be also used as solvent, fuel additives etc. The cause of greenhouse gas CO2 and overcapacity methanol were used as raw materials, and dimethyl carbonate was synthesized through heterogeneous catalytic process. This is a green chemical synthetic route which is also in line with the direction of "sustainable development" principle, thus attracting more and more attention home and abroad.First, nano-CeO2 samples were prepared by different methods. The catalytic activities of these samples were tested by liquid-phase catalytic reaction and these samples were then characterized by XRD, BET, CO2-TPD and XPS. It was found that the preparation methods of nano-CeO2 have a significant impact on its catalytic activity:the nanocub prepared by the co-precipitation method is not active, whitch can be attributed to the low degree of crystallinity; the degree of crystallinity of the CeO2 nanocub prepared through hydrothermal method is high and its catalytic activity is good as well.Second, the influence of different hydrothermal conditions on nano-Ce02 and its catalytic property was investigated. The preparation method of CeO2 was optimized in term of tuning reaction parameters (such as the concentration of ammonia, CTAB adding amount, time, temperature, quenching water times and cerium nitrate concentration). The catalytic activities of the prepared catalysts were tested by the liquid-phase reaction. In the end the prepared catalysts were characterized through XRD, TEM, BET, XPS and TPD.The results show that:(1) The degree of crystallinity of nano-CeC>2 has a significant effect on its catalytic activity, the higher the degree of crystallinity is, the better the catalytic activity is; (2) The morphology of the nano-CeC>2 is related to the alkaline of the solution, as the concentration of ammonia increases, the morphology of the prepared dioxide cerium chage from spherical nanoparticles to nanocubes and nanorod; (3) The morphology of the nano-CeO2 has an impact on its catalytic activity, the catalytic activity of nanorod is better than that of nanocub, that is because the nanorod is composed of (110) plane and nanocub is composed of (100) plane, and the catalytic activity of (110) plane is better that of (100) plane; (4) the alkaline of the solution nor only affects the morphology of nano-CeO2 but also affects the crystal surface defects, high concentrations of ammonia can lead to the formation of crystal surface defects on CeO2 nanocub and the crystal surface defects can help to improve the catalytic activity of CeC>2; (5) The concentration of the cerium nitrate has a significant impact on its crystalline phase,with the increase of the concentration of the cerium nitrate, significant amorphous phase begin to appear on the CeO2 and the catalytic activity of the dioxide cerium decrease, this indicates that the catalytic activity of the amorphous phase is not so well as that of (110) and (100) plane; (6) The microporous generated by piled nano-CeO2 particles affect the catalytic reaction because the diameter of microporous has a certain geometric correspondence relationship dimensionally to the reactant molecules in the molecular size as well as to the product, the larger the micropore volume is, the better the catalytic activity of the dioxide cerium will be; (7) The strength and quantity of base sites on the nano-CeO2 have a significant influence on the catalytic activity of CeO2, the appropriate quantity of weak base sites (CO2-TPD desorption peak at 100 ℃ and 150℃) is favorable for the catalytic reaction. (8) The CTAB adding amount has an effect on the ratio of Ce(Ⅳ) to Ce(Ⅲ), the more the adding amount of CTAB is, the higher the ratio of Ce(IV)/Ce(Ⅲ) will be and the catalytic activity of CeO2 will be better, that is because the valence state of Ce element on catalyst surface has quite a significant effect on conversion of methanol and CO2 to dimethyl carbonate, high ratio of Ce(IV)/Ce(III) is favorable for activating the bond of C=O and this thereafter leds to high performance easily. |
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