Abstract
The synthesis and characterization of several dispersed molybdena catalysts on silica support (MoO3-SiO2) prepared from a variety of precursors (Mo(VI)-acetylacetonate, oxo-peroxo Mo-species, hydrated ammonium heptamolybdate) and preparation methods (deposition of the Mo-phase on finite SiO2 support by aqueous and methanol impregnations, by adsorption, by oxo-peroxo route-like, and by one-step synthesis of MoO3-SiO2 system with molecular precursors) are presented. The molybdena concentration on silica was comprised in a large interval (1.5 - 14 wt%) depending on the preparation method which governed the Mo-loading on silica. Convenient comparisons among samples at similar Mo-concentration have been made discussing the morphologic-structural (XRD, XPS, UV-vis-DRS, and N2-adsorption) and physicochemical (TG-DTG, TPR, and n-butylamine-TPD) sample properties. Polymeric octahedral polymolybdate aggregates predominated in the samples prepared by aqueous and methanol impregnations, which were at high Mo-concentration. On the contrary, isolated Mo(VI) species in distorted Td symmetry predominated in the sample prepared by adsorption which was at very low Mo-concentration. The sample acidity was composed of a weak acidy site population, associated with the silica support, and a strong acid site population associated with the Mo-dispersed phase. Oxidation tests of formaldehyde, an oxygen-containing VOC (Volatile Organic Compound), were performed to determine the prevalent redox or acidic function of the Mo-species at the surface of the catalysts.
Highlights
Molybdenum containing catalysts are receiving great interest because of their importance both in environmental catalysis, such as combustion of soot, and in many industrial reactions, such as hydrodesulfuration, dehydrogenation of alkanes, partial oxidation of methanol to formaldehyde, and metathesis of olefins, among others [1,2,3]. The efficiency of these catalysts is strongly related to the amount and, in particular, the dispersion of the Mo-phase [4,5] which in its turn can be very influenced by the preparation method of the sample [6,7]
The mesoporous silica (SIM) support was synthesized by a modification of the procedure described by Huh et al [19] which consists of a condensation method based on sodium hydroxide-catalyzed reaction of TEOS, in the presence of low concentration of CTAB, followed by acid extraction of the as-made-product in a methanol mixture of hydrochloric acid
The two samples prepared by metal adsorption and oxo-peroxo route like had lowest amount of MoO3 loaded on SIM, 1.2 and 3.8 wt%, respectively
Summary
Molybdenum containing catalysts are receiving great interest because of their importance both in environmental catalysis, such as combustion of soot, and in many industrial reactions, such as hydrodesulfuration, dehydrogenation of alkanes, partial oxidation of methanol to formaldehyde, and metathesis of olefins, among others [1,2,3] The efficiency of these catalysts is strongly related to the amount and, in particular, the dispersion of the Mo-phase [4,5] which in its turn can be very influenced by the preparation method of the sample [6,7]. Another successful synthesis proposed to develop highly dispersed Mo supported catalysts is via the so called oxoperoxo route [14,15] running through formation of low nuclearity oxo-peroxo species
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