The Interaction of V205 with Ti02(Anatase) II. Comparison of Fresh and Used Catalysts for o-Xylene Oxidation to Phthalic Anhydride Two types of vanadia are present in fresh V20JIi02(anatase) catalysts calcined at 450°C: surface vanadia coordinated to the TiOz

Two types of vanadia are present in fresh V20JIi02(anatase) catalysts calcined at 450°C: surface vanadia coordinated to the TiOz support and small crystallites of V205 (1, 2). The surface vanadia is the active site for the oxidation of o-xylene to phthalic anhydride (I, 3, 4). The surface vanadia is more active and selective than the VZOfi crystallites for the o-xylene oxidation reaction (I). The unique properties of the surface vanadia species are related to the vanadia-titania interaction. The influence of the o-xylene oxidation reaction upon the two types of vanadia in VzOJIiOZ(anatase) catalysts is examined here. The fresh and used V205/Ti02(anatase) catalysts for oxylene oxidation to phthalic anhydride were characterized by laser Raman spectroscopy, temperature-programmed reduction, and X-ray diffraction. The V20JTi02(anatase) catalysts were prepared by dissolving V205 in an aqueous solution of oxalic acid, and impregnating the Ti02 support (Mobay , 9 m*/g). The excess water was evaporated at -65°C the catalysts were dried overnight at llO”C, and calcined in flowing oxygen at 450°C for 2 h. These fresh catalysts were examined for their catalytic performance for the oxidation of o-xylene (1.25 mole% in air) to phthalic anhydride in a fixed-bed reactor (320-350°C) for several days (I). The reactor was purged with nitrogen upon completion of the o-xylene oxidation reaction and removed from the salt bath at the reaction temperature. Independent thermal gravimetric analysis revealed that the V205/ TiOz(anatase) system was stable in nitrogen environments up to 575°C. This removal procedure, as well as the subsequent handling in air, resulted in partial reoxidation of the used V20JIiOz(anatase) catalysts. However, as shown below, the used catalysts were still sufficiently altered by the oxylene oxidation reaction that significant differences existed between the fresh and used catalysts. The Raman data were obtained with a multichannel laser Raman spectrometer (2). An argon-ion laser (Spectra Physics, Model 1650) was tuned to the 514.5-nm line for excitation. The laser power at the sample location was set at 40 mW. The Raman spectrometer consisted of a triple monochromator (Instruments SA, Model DL203) that was coupled to an optical multichannel analyzer (Princeton Applied Research, Model OMA 2). The reducibility of the vanadia catalysts was determined by temperature-programmed reduction. About 100 mg of sample was supported on a fritted disk in a quartz tube (4 in. o.d.). The samples were reduced in a 10% H*/helium mixture flowing at 50 cm3/min. The samples were heated at -l”C/sec by a Nichrome wire wrapped around the quartz tube. The maximum reduction temperature was maintained at 700°C in order to avoid any solid-state reactions between V205 and the TiO2 support. The hydrogen consumption during the TPR experiment was monitored with a UTI-1OOC mass spectrometer. X-Ray diffraction patterns were obtained