Preparation 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... 

Hutchings and coworkers (78,149,150) prepared vanadium phosphate catalysts by using supercritical antisolvent precipitation. These materials were found to be amorphous by XRD and by electron diffraction, but they showed activity about twice as high as that of the standard vanadium phosphate catalysts. 

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. 

Vanadium phosphate catalysts were prepared by heating V2O4, phosphorus acid, either H3PO4 or H4P2O7, and water together in an autoclave at 145°C for 72 hours. Afterwards, the solid produced was recovered, washed with distilled water and dried in air at 120°C for 16 hours. Detailed preparation procedure is described in [79]. Such prepared precursors were activated in n-butane/air at 400°C to form the final catalysts. TEM and EELS are used to study the catalysts in Philips CM200 PEG microscope. 

Ballarini et al. (8) posed the question of whether vanadium phosphate catalysts for n-butane oxidation offer the scope for further improvements. They concluded that as a consequence of the complexity of the dynamic surface species present on the catalyst, optimization of such material will not be forthcoming without further fundamental investigations. Previous investigations have involved probing of a number of catalyst parameters, including the V P ratio, the content of metal ion dopants, and the method of preparation. These and related topics are evaluated in detail below. 

In this review, we discuss how the methods of preparation of vanadium phosphafe materials can influence greatly their behavior as catalysts, and we describe the characterization of the various vanadium phosphates that can be made. Furthermore, we describe in detail the mechanism of selective n-butane oxidation and the emerging trend of applying vanadium phosphate catalysts to other oxidation reactions. 

Novel methods of preparation of vanadium phosphate catalysts have been explored by several groups these methods include hydrothermal synthesis, gas-phase s)mthesis, supercritical antisolvent precipitation, and the use of templates and structure-directing agents to modify the bulk... 

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. 

A series of vanadium phosphate catalysts prepared by various routes and containing various phases were examined by Guliants et al. (105). From this investigation, it was concluded that the catalytically active phase is an active surface layer on vanadyl pyrophosphate. The experimental results showed VOPO4 phases to be detrimental to the performance of the catalyst. 

FIGURE 28 TEM and electron diffraction pattern (insert) of the vanadium phosphate catalyst prepared via supercritical antisolvent precipitation. 

Zazhigalov et al. (209) investigated cobalt-doped vanadium phosphate catalysts prepared by coprecipitation and impregnation methods. The performance of catalysts prepared by both methods was improved as a consequence of the promotion. The cobalt is thought to have been present as cobalt phosphate, which is considered to stabilize excess phosphorus at the surface, which has previously been foimd to be an important characteristic of active catalysts. 

The selective oxidation of n-butane and 1-butene on vanadium phosphate catalysts prepared via different routes was investigated by Cavani and coworkers [77] Precursors prepared in aqueous medium were found to have greater crystallinity than those prepared in organic solvents (the activity and selectivity of which was the same for 1-butene oxidation). However, for butane activation, the crystalline catalyst was considerably less active than the organically prepared catalyst, which had an XRD pattern showing some disorder in the (100) plane. 

Vanadium phosphate catalysts are obtained by activating the catalyst precursor in the reaction feedstock. After pre-treatment, the catalyst is equilibrated and catalytic activity remains consistent throughout the lifetime of the catalyst. The activated catalysts are formed topotactically from the precursors [86]. For this reason, a great deal of research is based around the preparation of catalyst precursors with well defined, favorable morphologies. 

Vanadium phosphate catalysts are obtained from precursors prepared by a two-step sjmthesis. In the first step, a V0P04-mixed isobutanol-water intercalate was obtained by precipitation from a solution containing vanadyl isobutoxide, H3PO4 and carefully adjusted water content (precursor A). In the second step, precursor B was formed by reflux of precursor A in (i) an inert (n-octane) or (ii) reductive (isobutanol) medium. By such a procedure, precursors and catalysts (with PA atomic ratio equsd to 1.05) displaying widely different structural defects (XRD, IR) were prepared. Catalysts were tested in the oxidation of n-pentane into maleic (AM) and phthalic (PA) anhydrides. Formation of PA demands a highly ordered structure, while AM could be formed on a highly defective VPO catalyst. 

Use of 31p NMR by Spin Echo Mapping to prepare precursors of Vanadium Phosphate catalysts for n-Butane oxidation to Maleic Anhydride... 

Hutchings G.J., OUer R., Sanan s M.T. and Volta J.-C. (1994). Vanadium Phosphate Catalysts Prepared by the Reduction of VOPO4, 2H2O , in Cortes Corberan V. and Vic BeUon S. (eds). New Developments in Selective Oxidation II, Stud. Surf. Sci. Catal., 82, 213-220. 

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... 

Understanding the role of cafalysf promofers is not a simple matter. Confusion in the interpretation of promofer effects has resulted because different groups have reported contrasting results for the same promoters. The effects of promofers on vanadium phosphate performance was summarized by Ballarini ef al. (8) (Table 1). As illustrated by the work by Sananes-Schulz et al. (196), the catalyst preparation method can alter the effect of the promoter, as can the method of doping. Hutchings and... 

The preparation procedure employed is known to lead to the formation of VOPO4, rather than (VO)2P207. The presence of Sb, however, may lead to a modification of the structural features. Indeed, the authors claim the presence of vanadyl pyrophosphate as the major phase present in catalysts, with a minor amount of vanadium phosphate. The atomic ratio between the components of the y-alumina-supported active phase was V/Sb/P 1/1.9/1.18. The reaction conditions were 425 °C (at which the best yields were reported), and a feed ratio of reactant/ air/ammonia of 0.6-1.0/4.2/1.5. The following results were claimed under these conditions ... 

A patent (726) has described the preparation of 2methyl-pyrazine by reaction with ammonia and air at 350° over a catalyst containing vanadium pentoxide and potassium sulfate a series of cyanomethylpyrazines has been prepared from the corresponding methylpyrazines by reaction with sodium amide in liquid ammonia followed by Af-methyl-A -phenylcyanamide in dioxane (644). 2-Hydroxyiminomethylpyrazine has been prepared from 2-methylpyrazine, sodium amide, and liquid ammonia with butyl nitrite (727, 728), and 2-hydroxy-iminomethyl-3,6-dimethyI-5-pentylpyrazine similarly from 2,3,5-trimethyl-6-pentylpyrazine (648). Nitrones (28) have been prepared from 23-and 2,5-dimethyl-and tetramethylpyrazine through the substituted methylpyridinium (perchlorates) (27) by reaction with p-nitroso-A, fV-dimethylaniline (729). Dehydrogenation of ethylpyrazine at 600° over a calcium cobaltous phosphate catalyst gives 2-vinyl-pyrazine (658). 

Vanadium Phosphate Oxide catalysts are well known to perform the mild oxidation of n-butane to maleic anhydride. The preparation of the precursor of this catalyst, the vanadyl phosphate hemihydrate VOHPO4, 0.5 H2O appears to be very important to control the final properties of the VPO catalyst since the transformation precursor/vanadyl pyrophosphate (VO)2P2C>7 which corresponds to the final active phase is topotactic. It thus appears that it is possible to control the morphology of the final catalyst by the control of the morphology of its precursor (1-4). 

The ion-exchange technique allows to prepare VO modified titanium phosphates. Different vanadium loadings can be obtained by properly controlling the operating exchange conditions and precursor phase. Vanadyl modified titanium phosphates catalysts were found active and selective towards SCR reaction, either as hydrogen or pyrophosphate phase. The results obtained in this paper indicate that the activity of the materi s can be relate to vanadyl species whose redox properties affect the catalytic behaviour. 

Particles are the bed material employed in fluidized bed reactors and can be reactants (e.g., coal and limestone), products (e.g., polyethylene), catalysts, or inert. The choice of particle size, in general, affects the hydrodynamics, transport processes, and hence the extent of reactor conversion. Particles experience particle particle collisions, friction between particles and walls or internals, and cyclones. In some cases, the catalyst material is inherently susceptible to attrition, and special preparation to enhance the attrition resistance is required. For example, the vanadium phosphate metal oxide (VPO) catalysts developed for butane oxidation... 

The catalyst used in the production of maleic anhydride from butane is vanadium—phosphoms—oxide (VPO). Several routes may be used to prepare the catalyst (123), but the route favored by industry involves the reaction of vanadium(V) oxide [1314-62-1] and phosphoric acid [7664-38-2] to form vanadyl hydrogen phosphate, VOHPO O.5H2O. This material is then heated to eliminate water from the stmcture and irreversibly form vanadyl pyrophosphate, (V(123,124). Vanadyl pyrophosphate is befleved to be the catalyticaHy active phase required for the conversion of butane to maleic anhydride (125,126). 

It should be mentioned finally that acetonitrile could be also prepared by the catalytic ammoxidahon of bioethanol over vanadium-alumino-phosphate (VAPO) catalysts [138], which is an alternative starting from biomass-derived raw materials. 

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