Temperature vanadium phosphate catalysts

A number of groups have prepared vanadium phosphate catalysts using hydrothermal s)mthesis (36,37,55-57). Using standard reaction mixtures, r)ong et al. (55) showed that at elevated temperatures and... 

However, the structure of vanadium phosphate catalysts is dependent on a number of considerations. The P/V stoichiometry, thermal treatment time, activation temperature, and gas-phase composition can all affect catalyst composition. By varying these synthesis parameters, researchers have prepared a variety of crystalline phases and identified them by X-ray diffraction in the freshly activated catalysts. 

Alfhough for fhe pasf 40 years, vanadium phosphates have been used exclusively for conversions of gas-phase reactants, these catalysts have recently been applied to low-temperature (60-140 °C) oxidations with liquid-phase reactants. It is perhaps surprising that vanadium phosphate catalysts have been used at such low temperatures because they negate one of fhe key feafures of (VO)2P207, fhe lattice oxygen mobility. The lack of... 

The structure of vanadium phosphate catalysts is dependent on a number of factors. The P/V stoichiometry, thermal treatment time, activation temperature and gas phase composition can all affect catalyst composition. By varying these factors a variety of crystaUine phases can be identified (by high-resolution transmission electron microscopy (HRTEM) [5] Figure 12.1a and X-ray diffraction Figure 12.1b) in the freshly activated catalyst [6]. ft is widely accepted that VPP plays an important role in the oxidation of butane to maleic anhydride and most hypotheses are based on the (100) face (Figure 12.2). Additionally, this phase has been reported to be an efficient catalyst for the oxyfunctionalization of light paraf-... 

A number of groups have prepared vanadium phosphate catalysts using hydro-thermal synthesis [92, 93, 128-130]. Using standard reaction mixtures, Dong and coworkers [128] showed that at elevated temperatures and pressures different materials are synthesized from those obtained under reflux conditions. Pressure did not seem to affect the product formed, but as the temperature increased to >200°C further reductions occurred and products formed. However, these materials were not found to have enhanced catalytic activity compared to traditionally prepared materials. At lower temperatures, hydrothermal syntheses have produced catalysts with comparable activity to those prepared under standard conditions [92, 93, 129, 130]. Taufiq-Yap and coworkers [129] found an enhancement in activity for hydrothermaUy prepared catalysts and suggested this was due to a modification in the redox behavior of the catalysts evidenced by TPO/TPR experiments. 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

Hutchings and coworkers (78-83) pioneered the use of supercritical antisolvent precipitation to prepare a number of catalyst and support materials including vanadium phosphates. Vanadium phosphate precursor solutions were prepared from VOCI3 and H3PO4 refluxed in isopropanol. In the supercritical antisolvent precipitation method, a solution of the material to be precipitated and supercritical CO2 are pumped through a coaxial nozzle at temperatures and pressures above the critical point of... 

Schneider proposed the use of high surface area vanadium phosphates with a PA atomic ratio of 1.0 to 1.5 that had been developed as catalysts for oxidation of n-butane to maleic anhydride. The reaction is conducted at a temperature of 360 °C, an SV of 400 h and an acetic acid/water/HCHO molar ratio of 10/2.8/1, using formalin as the source of HCHO. The single-pass yield of acrylic acid reaches 84 mol% based on the charged HCHO (8.4 mol% based on acetic acid) at the conversion of 98% selectivity is 86 mol% based on HCHO. 

Reaction over Acid Catalysts. - The reaction over a vanadium phosphate with a P/V atomic ratio of 1.06 was studied by Ai, using formalin as the source of HCHO at a temperature from 3(X) to 340 °C and a propionic acid/HCHO molar ratio of 2. Propionic acid is found to be more reactive than acetic acid. The main products are methacrylic acid, CO2, and propylene. Small amounts of methyl propionate and methyl methacrylate are obtained besides these main products. Methyl propionate is formed by esterification of propionic acid with methanol which is contained in the formalin used. Methyl methacrylate is formed by the reaction of HCHO with methyl propionate. The yield of methacrylic acid is much lower than the yield of acrylic acid obtained in the reaction with acetic acid. The single-pass yields of methacrylic acid and methyl methacrylate did not exceed about 27 and 5 mol%, respectively, based on the charged HCHO. It is also found... 

In 1975, Ohloff etal. studied the gas-phase oxidation of ot-isophorone to KIP over a vanadia/pumice catalyst modified with 1 wt% of hthium phosphate at 230°C. Under these conditions, simultaneous formation of KIP and formylisophorone occurred. More than 20 years later, Baiker et al. revisited the catalytic gas-phase oxidation of isophorone. At 200-250°C, 75% combined yields of KIP and formyhsophorone were obtained at 17% ot-isophorone conversion over vanadia/pumice impregnated with hthium phosphate j6-isophorone was found as a major by-product (18%). Bismuth molybdate or vanadium phosphate showed poor selectivity and rapid deactivation. The Ag/y-alumina-catalysed oxidation was unselec-tive and resulted mainly in isomerisation to j6-isophorone. Chromia-based catalysts led to an increased formation of 3,5-xylenol. To efficiently remove coke deposits and to re-oxidise vanadium oxides to vanacha, temperatures higher than 300°C would be needed however, under these conditions isophorone and KIP are not stable. Thus, highly selective catalysts would be required which are active at lower temperatures. 

There seems to be no literature about the direct oxidation of ethane to acetic acid over heteropolycompounds catalysts. Nevertheless, there is a limited amount of literature[10,26-28] about direct oxidation of ethane to acetic acid over oxide catalysts at low temperature (200-350 C). It seems that vanadium and molybdenum are necessary to those catalysts, and the addition of water is useful to increase the production of acetic acid. Roy et al. [10] has proved that vanadium and molybdenum phosphates supported on Ti02-anatase were effective in the direct oxidation of ethane to acetic acid. Considering previous research results, it is suggested that other promoters, such as trcmsition-metal oxides, are necessary to enhance the catalytic activity of the activated H3PMol2O40(Py) in the direct oxidation of ethane to acetic acid. 

The performance exhibited hy these catalysts was surprisingly good (above all in terms of selectivity to MAA), remarkably better them that reported in literature for heteropolycompoeinds wiihout vanadium (1,2,4,9). The reference add showed a slightly lower activity, but above all a lower selectivity to MAA. The best selectivities obtained were similar to those reported for iron phosphate-based catalyst (13) this system, however, operates at higher temperatures (400C). 

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