GENESIS AND STABILITY OF SILICOMOLYBDIC ACID ON SILICA-SUPPORTED MOLYBDENUM OXIDE CATALYSTS - IN-SITU STRUCTURAL-SELECTIVITY STUDY ON SELECTIVE OXIDATION REACTIONS
Ma. Banares et al., GENESIS AND STABILITY OF SILICOMOLYBDIC ACID ON SILICA-SUPPORTED MOLYBDENUM OXIDE CATALYSTS - IN-SITU STRUCTURAL-SELECTIVITY STUDY ON SELECTIVE OXIDATION REACTIONS, Journal of catalysis, 155(2), 1995, pp. 249-255
The formation of silicomolybdic acid (SMA, H4SiMo12O40) on silica-supp
orted molybdenum oxide catalysts has been studied by in situ Raman spe
ctroscopy, by TGA measurements, and for the selective oxidation of met
hane to formaldehyde. The formation of silico-molybdic acid requires e
xposing the MoO3/SiO2 to air saturated with water for several hours at
room temperature. The large amount of water deposited on the silica s
upport allows solubilization of part of the silica support in the pres
ence of solvated heptamolybdate species, which leads to the formation
of silicomolybdic acid. Desorption of water via thermal treatments bre
aks the silicomolybdic acid into dehydrated or partial hydrated specie
s which are stable up to ca. 573 K. Above 573 K, only an isolated and
distorted mono-ore 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 o
f conventional MoO3/SiO2 catalysts for reactions taking place above 57
3 K. Unlike surface molybdenum oxide species, the surface SMA species
on SiO2 are stable during methanol oxidation at 503 K and do not trans
form into crystalline beta-MoO3 phase. The selective oxidation of meth
ane to formaldehyde (843-883 K) shows no difference between convention
al silica-supported molybdenum oxide and silica-supported silicomolybd
ic acid catalysts. In situ Raman spectroscopy studies during methanol
oxidation at temperatures above 573 K reveal that the surface silicomo
lybdic acid species are not stable and transform into crystalline beta
-MoO3. (C) 1995 Academic Press, Inc.