(70ap) Direct deposition of airborne V2O5/TiO2 on ceramic foams for phthalic anhydride catalysis

The partial oxidation of o-xylene to phthalic anhydride is a highly exothermic reaction. The evolving heat has to be transferred effectively out of the catalyst bed, because hot spots deactivate the catalyst irreversibly and lead to an increased risk of thermal reactor runaway. Ceramic foams of Al₂O₃ can improve the heat transfer compared packed beds of spheres. The open-pore structure and the high void fraction lead to a lower pressure drop over the foam height compared to packed beds. The possible formation of a turbulent gas flow can increase the heat and mass transfer compared to a laminar flow in honeycombs. In addition, thermal conductivity and surface properties can be modified by a large variety of foam materials. Thus, it can be expected that foams can combine properties of packed beds and honeycombs in a beneficial way. In commonly used vanadia/titania catalysts, anatase as titania crystal structure showed higher catalyst activity than other crystal structures. A high dispersion of vanadia on titania is crucial for high activity but selectivity is mainly influenced by the chemical characteristics of vanadia itself. Flame synthesis has recently been used for the production of vanadia/titania mixed oxide nanoparticles, containing large anatase fractions. The vanadia content has been varied from 0 ? 10 wt% and the materials exhibited a high vanadia dispersion on titania nanoparticles. No evidence for interstitial vanadia/titania solutions have been found. Production rates ranged from 4 ? 200 g/h and specific surface areas from 23 ? 120 m²/g. Flame-made particles exhibit, beside the high specific surface area, an open-pore structure that might facilitate mass transfer limited reactions compared to wet-phase made catalysts. The direct deposition of airborne nanoparticles on foam supports led to a different agglomerate and pore size structure than obtained by common multi-step coating techniques in the wet-phase. The open-pore structure was retained, promoting the gas penetration into the active layer. Powders were characterized by various techniques including nitrogen adsorption, X-ray diffraction, temperature programmed reduction and Raman spectroscopy and electron microscopy. Powder properties of flame-made and wet-made powders were compared. The influence of key production parameters was investigated. Flame-made powders directly deposited on ceramic foams exhibited enhanced catalytic activity with comparable selectivity at high conversions compared to wet-phase made catalysts.