Adsorption and Reaction of CO2 and CH3 OH on Ni2 ( OCH3 )2/SiO2 Catalyst

The supported dinuclear nickel methoxide complex Ni2(OCH3)2/SiO2was prepared by surface reaction modification and ion-exchange method. Its chemical composition and surface structure were characterized by element analysis and infrared spectroscopy (IR) techniques. The results showed that, in Ni2(OCH3)2/SiO2, Ni2+ bonds with surface O2-of SiO2 supported in bidentate fashion andbis-μ-OCH3bridged structure Ni2 (OCH3)2 was formed between the two metal ions. The chemisorption properties of CO2 and CH3OH on the catalyst were studied by chemisorption infrared spectroscopy and chemisorption temperature programmed desorption (TPD) techniques. The experimental results showed that CO2chemisorbs on the catalyst as two states: bridged absorption state and methyl carbonate species. At room temperature, the bridged absorption state is formed between Ni2+ and OCH3 ligand , in which metal ions adsorb with the O atom and OCH3 ligand adsorbs with the carbon atom in CO2. At higher temperature, it transforms to the methyl carbonate species, the amount of which can be increased by formation of the bridged OCH3. Whilst CH3OH only forms molecular adsorption states with O atom in CH3OH adsorbing on Ni2+ and desorbs molecularly during 110~140℃, the synergic relationship between Ni2+ and OCH3ligand determines the activation process and the adsorption state of CO2 and CH3OH. The catalytic reactivity for Ni2(OCH3)2/SiO2 was characterized by temperature programmed surface reaction mass spectroscopy(TPSR-MS) and micro-reactor evaluation techniques. The results revealed that CO2and CH3OH could react on Ni2(OCH3)2/SiO2 with good reactivity and high DMC selectivity. At 100~200℃, reaction products were mainly DMC and H2O and the reactivity was determined by surface methoxyl carbonate species and CH3OH in molecular adsorption states. Based on the results above, the activation processes of CO2 and CH3OH and the mechanism for DMC synthesis were analyzed theoretically. The formation and amount of methyl carbonate species of CO2 on Ni2(OCH3)2/SiO2 control the procedure of the reaction. The methoxylation of intermediate surface hydroxide is necessary to the catalytic circle.