Synthesis, characterization and catalytic applications of vanadia and silica-based materials
Glenn L. Schrader
Vanadia gels synthesized from a peroxovanadate precursor were used as catalysts for the selective oxidation of 1,3-butadiene. These vanadia gels were previously characterized using 51V and 17O MAS NMR spectroscopy [Fontenot et al., J. Phys. Chem. B 105, p10496 (2000) and J. Am. Chem. Soc. 124, p8435 (2002)]. These studies had shown the presence of an incommensurate shifted layer (+- 1.7 A° along a-axis and +- 0.5 A° along the b-axis) between two commensurate layers of the vanadia gel. This created a special site for water adsorption in which the oxygen of the water molecule was adsorbed trans to the vanadyl oxygen and the two hydrogen atoms co-ordinated with two vanadyl oxygens of the next layer. Selective oxidation studies of 1,3-butadiene were carried out with and without water addition to the feed stream to understand the role of oxygen sites and the water adsorption site in the vanadia gel structure for hydrocarbon oxidation. The reaction mechanism involved intermediates such as 3,4-epoxy-1-butetne, crotonaldehyde, 2,5-dihydrofuran, 2-butene-1,4-dial and furan. The effect of water addition on the pathway for 1,3-butadiene selective oxidation was also investigated over peroxovanadate-derived vanadia and VMoO catalysts by using 3,4-epoxy-1-butene, crotonaldehyde, 2,5-dihydrofuran and furan as feed. Addition of 0-12% water to a reactant feed of 1.4% butadiene in an air-He mixture significantly increased catalytic activity and selectivity for crotonaldehyde and furan. Competitive adsorption was believed to occur between the hydrocarbon products and water; formation of acid sites through dissociative adsorption of water was also believed to be important. Temperature programmed desorption (TPD) experiments revealed five distinct adsorption sites that could be associated with terminal V=O, corner sharing V-O-V, and edge sharing V-O oxygen. The adsorption of water trans to the vanadyl oxygen (V=O) formed an equilibrium structure resulting in the increased reactivity of the vanadyl oxygen species for 1,2-electrophilic addition across the C=C double bond in 1,3-butadiene to form 3,4-epoxy-1-butene. A proposed dissociative mechanism of adsorbed water on the catalyst surface resulted in acidic H+ species that participated in ring opening mechanisms and nucleophilic O-2 species that could easily exchange with the lattice oxygen sites, thus replenishing the catalytic activity.