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The thermal oxidative decomposition of dimethyl methylphosphonate (DMMP) has been studied over amorphous manganese oxide (AMO) and Al2O3-supported manganese oxide catalysts. The reaction was carried out using air as the oxidant at temperatures between 200 and 400°C. The highest reaction rates occurred using temperatures of 400°C. Gas chromatography (GC) was used to examine reactant DMMP and other gas phase products. DMMP was found to oxidatively decompose over AMO and Al2O3-supported manganese oxide catalysts. The highest activity was observed using a catalyst prepared by precipitation of AMO on Al2O3. During the initial stages of reaction, DMMP was completely removed from the gas phase. During this period DMMP was oxidized to CO2, with no other gas phase products being observed. After a certain period of time (5 min–8 h), DMMP reappeared in the gas phase. The CO2 concentration then decreased and MeOH began to form, indicative of hydrolysis of DMMP. These results indicate that deactivation of catalysts occurs due to adsorbed P-species. Fourier transform infrared (FTIR) spectroscopy and ion chromatography (IC) were used to examine adsorbed products on the surface of the catalysts. The IC analyses indicated that several products accumulate on the surface of the catalysts, including methyl methylphosphonate, methylphosphonic acid, and phosphoric acid. FTIR analyses showed that DMMP bonds strongly to Mn Lewis acid sites on the manganese oxide surface via phosphoryl oxygen. The bare Al2O3 support was also examined in DMMP decomposition reactions and showed high activity, with 100% DMMP removal from the gas stream for over 15 h. The major products observed over Al2O3 were dimethyl ether and MeOH. No CO2 was observed, indicating that DMMP is not oxidized over Al2O3. The GC, IC, and FTIR results suggest that DMMP is dissociatively adsorbed over Al2O3. Finally, the results for the thermal oxidation of DMMP over AMO are compared to results previously obtained using photo-assisted oxidative methods. |
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