Nanocluster iron oxide-silica aerogel catalysts for methanol partial oxidation

Nanostructured pure and silica-supported iron oxide materials have been prepared by the aerogel approach. Pure iron oxide powder derived from sol-gel ferric acetylacetonate formed agglomerates of 5-30 nm small crystallites of hematite and maghemite according to TEM identification of crystal faces. Depositing ferric species to mesoporous silica aerogels generated 1-5 nm particles in the amorphous matrix. They were evaluated for methanol oxidation in an ambient fixed-bed flow reactor from 225 to 300 °C. Product selectivity and oxidation activity were dependent upon iron dispersion and reactor operation. The formation of dimethyl ether was mainly related to the bulk phase and Lewis acidity of iron oxide. Active catalysts that were selective to formaldehyde and methyl formate required appropriate iron dispersion on the silica surface, including a strong electronic interaction. Methoxy transformation to formaldehyde and formate species was found to be a function of surface temperature based on a Fourier transform infrared (FT-IR) analysis. Low to moderate reactor temperature and short catalyst contact time favored methanol conversion to formaldehyde. The formation of methyl formate was found to compete with that of formaldehyde. The dependence of response time on oxygen feed attenuation suggests that mobile lattice oxide ions participate in the surface reaction and that oxygen molecules help to maintain surface iron sites highly oxidized for Lewis chemisorption and redox electron transfer. A good correlation between microstructures and reaction characteristics is proposed.