Thermocatalytic oxidation of dimethyl methylphosphonate on supported metal oxides

Thermocatalytic oxidation of dimethyl methylphosphonate (DMMP) was carried out on nickel, iron, copper, and vanadium oxides supported on gamma-Al2O3. The vanadium catalyst was found to exhibit exceptional catalytic activity, even better than platinum catalysts. Varying the vanadium loading from 1 to 15% by weight indicated that 10% vanadium on Al2O3 was an optimal content. In conjunction with XRD patterns, monolayer dispersion of V2O5 on Al2O3 was considered to be beneficial to the longevity of these catalysts. Different supports, including Al2O3, SiO2, and TiO2, were examined and SiO2 was the optimum support because of its large surface area and the ability to resist poisoning by P2O5. On 10% V/SiO2 catalysts, 100% (to our limit of detection of 0.1%) conversion of DMMP was reached for more than 100 h at 723 K. IR, X-ray powder diffraction, ion chromatography, and XPS results illustrated that the used catalysts contained phosphorus species. The presence of methylphosphonic acid on the catalyst surface and downstream of the packed bed reactor demonstrated the difficulty of P-CH3 cleavage. The deposition of coke in the catalyst bed and along the reactor wall resulted from the dehydration of methanol and DMMP on P2O5. Accumulation of phosphorus species and coke on catalysts gave rise to a tremendous loss of surface area. However, P2O5 itself was observed to catalyze the decomposition of DMMP. A mechanism for this reaction was proposed to explain these experimental observations. C) 2000 Academic Press.