Vanadium - porous titania glass catalysts

Abstract

Porous Titania Glass (PTG) is a novel meso-porous form of TiO2, which can have a surface area as high as 300 m2 g-1, depending on thermal pretreatment. Such high surface areas have given PTG the potential to be a better catalytic support than other forms of titania for many industrially important catalysts. To investigate a specific system which exploits this advantage, a variety of vanadium-PTG catalysts have been prepared, characterized and tested for their activity in the oxidation of o-xylene. A transmission electron microscopic study indicates that PTG annealed at 573 K consists of grains and grain boundaries. The grains consist primarily of anatase, with small quantities of randomly distributed minor phases. Subsequent ultraviolet-visible and resonance Raman spectroscopic studies suggest that the overall structure of PTG is distorted giving rise to intra-granular defects. Raman and infrared studies indicate that the wet impregnation method deposits vanadium species, which are similar to those found by other workers on V2O5/TiO2, onto the PTG surface. Nitrogen physisorption and x-ray photoelectron spectroscopic experiments indicate that, below the theoretical monolayer coverage, surface vanadium forms an overlayer several molecules thick on the exterior surface, as well as within the pores, of PTG. This overlayer converts the mesopores into micropores. These surface vanadium species display Raman and infrared bands which have been attributed to polymeric and monomeric vanadyl species. Above the loading required for a monolayer coverage, vanadium crystallizes as V2O5, predominantly on the exterior surface of PTG. The interactions of these vanadium species with gas phase o-xylene have also been followed by electron spin resonance (ESR) spectroscopy, and V4+ ions in four types of coordination environments have been identified. ESR studies performed under photolytic conditions indicated that the redox mechanisms for surface and interstitial vanadium ions are different. Subsequent catalytic tests show that, for the oxidation of o-xylene to phthalic anhydride, the vanadium-PTG catalysts exhibit similar levels of selectivity and conversion to conventional anatase- or rutile-supported catalysts, but at significantly lower temperatures. The activity for phthalic anhydride production increases with vanadium loading. The best performance of 50% selectivity to phthalic anhydride at 100% o-xylene conversion was achieved at 553 K with a catalyst containing 5.6% (w/w) vanadium.