Genesis and Stability of Silicomolybdic Acid on Silica-Supported Molybdenum Oxide Catalysts: In-Situ Structural-Selectivity Study on Selective Oxidation Reactions

Abstract

The formation of silicomolybdic acid (SMA, H4SiMo12O40) on silica-supported molybdenum oxide catalysts has been studied by in situ Raman spectroscopy, by TGA measurements, and for the selective oxidation of methane to formaldehyde. The formation of silico-molybdic acid requires exposing the MoO3/SiO2 to air saturated with water for several hours at room temperature. The large amount of water deposited on the silica support allows solubilization of part of the silica support in the presence of solvated heptamolybdate species, which leads to the formation of silicomolybdic acid. Desorption of water via thermal treatments breaks the silicomolybdic acid into dehydrated or partial hydrated species which are stable up to ca. 573 K. Above 573 K, only an isolated and distorted mono-oxe surface molybdenum oxide species is observed by in situ Raman spectroscopy. Consequently, the silicomolybdic acid species on SiO2 should not result in catalytic behavior different from that of conventional MoO3/SiO2 catalysts for reactions taking place above 573 K. Unlike surface molybdenum oxide species, the surface SMA species on SiO2 are stable during methanol oxidation at 503 K and do not transform into crystalline β-MoO3 phase. The selective oxidation of methane to formaldehyde (843-883 K) shows no difference between conventional silica-supported molybdenum oxide and silica-supported silicomolybdic acid catalysts. In situ Raman spectroscopy studies during methanol oxidation at temperatures above 573 K reveal that the surface silicomolybdic acid species are not stable and transform into crystalline β-MoO3.