| Popis: |
Oxidation of CH2Cl2 with dioxygen has been investigated on pure γ-Al2O3 and Al2O3-SiO2 system in the whole composition range. Two mechanisms of the reaction operate. At temperatures below 400°C CH3C1 is formed as intermediate, with simultaneous evolution of CO, but no HC1 appears in products. This reaction pathway may start from single-site adsorption of CH2C12 through a chlorine apex at a Lewis site, the adsorbed species transforming by SN2 reaction to chlorometoxy- and methoxy-species and then by Cannizzaro-type reaction to a formate precursor and finally CO, a hydrogen transfer to chloromethoxy-species giving CH3C1. Results of in situ IR spectroscopic studies are in line with such model. At higher temperatures 100% conversion of CH2C12 into HC1 and CO2 is observed, this reaction pathway starting from bidentate adsorption of CH2C12. Comparison of catalytic properties with the distribution of acid sites indicates that activity depends on total amount of acid sites, but the selectivity to CH3Cl is related to the concentration of Lewis acid sites. Pure silica shows no activity. |
