Heterogeneous Catalytic Systems For Direct Synthesis Of Dimethyl Carbonate From Carbon Dioxide And Methanol

Dimethyl carbonate(DMC) is an important non-toxic organic chemical intermediate. Since there are a carbonyl group and two methyl groups or two methoxyl groups or a methoxycarbonyl group in a DMC molecule, it can replace severely toxic phosgene, dimethyl sulfate, or methyl halide to develop a series of green chemical technologies. Moreover, DMC can be used as a green solvent and a potential gasoline additive. Above all, DMC has broad application prospects. DMC was traditionalty synthesized by phosgene route and has been replaced by non-phosgene routes for it took toxic phosgene as a starting material and gernerated a corrosive hydrochloric acid as a by-produce. Recently, one-step synthesis of DMC from CO2 and CH3 OH has received more and more attention due to the consumption of the greenhouse gas CO2 as the starting material. It is more preferred non-phosgene route for it matches the view of sustainable development. Herein we investigated several heterogeneous catalytic systems for the synthesis of DMC from CO2 and methanol.1. Fe2O3 catalyst was prepared by precipitation method. Combined with the characterization results from IR, TG-DTA, N2 adsorption-desorption, SEM, TEM, and XRD and the catalytic activity evaluation results on an autocalve, we investigated the influence of preparation conditions and structure on the catalytic activity of iron oxide. When prepared using NH3.H2 O as a precipitant, the concentration of Fe3+ = 0.625 mol/L,aging time of 12 h, and calcined by 350℃ for 2h, the resultant Fe2O3 sample shows the highest catalytic activity. The uniform distribution of the nanoparticles, larger surface area and the α-Fe2O3 crystal type integrity are responsible for the high catalytic activity. When N,N’-dicyclohexylcarbodiimide(DCC) was added as a dehydrant, DMC yield increased linearly with the amount of DCC. In addition, Fe2O3 can be reused for three times without a significant loss in its catalytic performance.2. Fe-Zr bimetallic oxide catalyst was prepared by the sol-gel method and was characterized by IR, N2 adsorption-desorption, XRD, TPD, and XPS. Its catalytic performance was evaluated on the autoclave. The catalytic activity of Fe-Zr bimetallic oxide catalyst with Fe-Zr molar ratio of 5/1 is two times higher than that of ZrO2. Fe2O3 mainly exists in rhombohedral phase and ZrO2 in tetragonal phase. There occurs an interaction between iron oxide and zirconium oxide. The surface of Fe-Zr bimetallic oxide catalyst has stronger Lewis acid sites and the generation of some Bronsted acid sites, causing the increase of the catalytic activity. Additionally a possiable catalytic mechanism was suggested.3. Fe-Zr bimetallic oxide catalyst was also prepared by the co-precipitation method. The relationship among the preparation conditions, structure and catalytic activity was discussed. The synthesized Fe-Zr bimetallic oxide catalysts were fully characterized by SEM, TEM and N2 adsorption-desorption. The results reveal that it has a uniform nano-scale size particle, large specific surface area, and mesoporous channels beneficial for the mass transfer of the substrates and products. The analyses of XRD and XPS show that there exists an interaction between iron and zirconium. A slight increase in iron valence state leads to the enhancement of the corresponding Lewis acid. Iron and zirconium can enter into each other’s oxide lattice, leading to the change of crystal phase.4. Fe-Zr-Si ternary oxide catalyst was prepared by the sol-gel method. The effect of incorporation of Si on the structure, acidity and catalytic activity of Fe-Zr oxide was discussed. The catalyst with a molar ratio of iron/zirconium/silicon = 5/1/2 has the highest catalytic activity. N2 adsorption- desorption, NH3-TPD, XRD and SEM-EDX were used to characterize the textural properties and acidity of catalysts. The results show that the BET specific surface area increases and the pore diameter become small and concentrated with increasing silicon content. The biggest pore volume and the pores of 5~11nm beneficial for mass transport appear at the molar ratio of Fe/Zr/Si = 5/1/2. The bigger specific surface area and suitable pore size may be the key reason for the high catalytic activity of Fe-Zr-Si oxide. In addition, the synergistic effect of the solid solution of zirconium-silicon, iron-silicon and iron-zirconium promote the catalytic performance of Fe-Zr-Si oxide.5. Activated-carbon supported K2CO3 catalyst was prepared. The effect of support and water on the catalytic performance of K2CO3 was discussed. The results demonstrate that K2CO3 supported on the columnar activated carbon derived from coal shows the highest catalytic activity, about 2 times DMC yield per mass K2CO3 than that of unsupported one. Forthermore the DMC synthesis reaction can occur at a more modest reaction condition. The effect of water was studied by adding different volume of water into the system. The result reveals that a little water can increase the catalytic activity of K2CO3 by increasing its solubility in methanol. A modified reaction mechanism catalyzed by K2CO3 was proposed accordingly.