GENESIS AND STABILITY OF SILICOMOLYBDIC ACID ON SILICA-SUPPORTED MOLYBDENUM OXIDE CATALYSTS - IN-SITU STRUCTURAL-SELECTIVITY STUDY ON SELECTIVE OXIDATION REACTIONS

Citation
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
Citations number
31
Categorie Soggetti
Chemistry Physical
Journal title
ISSN journal
00219517
Volume
155
Issue
2
Year of publication
1995
Pages
249 - 255
Database
ISI
SICI code
0021-9517(1995)155:2<249:GASOSA>2.0.ZU;2-G
Abstract
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.