Structure-function relationships in controlled morphology molybdenum trioxide/silicon dioxide for methanol oxidation.

摘要

Molybdenum based catalysts have ample application in the petro-chemical industries, especially in hydrotreating and hydrocracking of petroleum feedstock. They are also used for hydro-processing of coal liquids and hydrogenation of coal-derived gases. The rarely studied hexagonal phase of MoO{dollar}sb3{dollar} can be produced with a low temperature calcination of the ammonium heptamolybdate (AHM) precursor.; Several series of controlled morphology {dollar}rm MoOsb3/SiOsb2{dollar} catalysts were prepared, characterized, and tested for methanol oxidation activity. Molybdenum surface loadings of 0.2, 0.8, 2.0, and 4.0 atoms Mo/nm{dollar}sp2{dollar} were prepared as sintered hexagonal and sintered orthorhombic, as well as a novel "well dispersed hexagonal" phase. Methanol oxidation in excess oxygen was performed in a differential reactor at 1 atm over the temperature range 150-290{dollar}spcirc{dollar}C. Characterization by XRD, Raman, TPR and O{dollar}sb2{dollar} chemisorption reveal that the dispersion of the "well dispersed hexagonal" form is much higher than normally thought achievable at high loadings. Activity data fully supports this observation. Selectivity trends also reflect the morphological trends and further reveal production of methyl formate can be the gauge of MoO{dollar}sb3{dollar} - SiO{dollar}sb2{dollar} interaction.; At the low loading, 0.2 atom Mo/nm{dollar}sp2,{dollar} final morphologies are independent of preparation technique. A corrected interpretation of MoO{dollar}sb3{dollar} TPR has been established: bulk and dispersed microcrystalline of MoO{dollar}sb3{dollar} reduce to MoO{dollar}sb2{dollar} at 650{dollar}spcirc{dollar}C and to Mo metal at 1000{dollar}spcirc{dollar}C; morphology can be inferred from the peak width of the lower temperature peak. Activity of MoO{dollar}sb3{dollar}/SiO{dollar}sb2{dollar} toward methanol oxidation is proportional to dispersion in the order of; sintered hexagonal {dollar}<{dollar} sintered orthorhombic {dollar}<{dollar} dispersed hexagonal with the exception of low loadings. TOFs of the crystalline phases appear higher than that of the dispersed phase. However, the predominant factor for activity per gram of MoO{dollar}sb3{dollar} for methanol oxidation is dispersion; the higher activity of the crystallites does not compensate their lesser dispersion. The suggested methanol oxidation mechanism in the temperature range 150-290{dollar}spcirc{dollar}C is largely consistent with the literature, and involves formaldehyde production over MoO{dollar}sb3{dollar} and SiO{dollar}sb2,{dollar} CO and CO{dollar}sb2{dollar} production over SiO{dollar}sb2{dollar} at higher temperatures, and methyl formate production at the MoO{dollar}sb3{dollar} - SiO{dollar}sb2{dollar} adlineation.