Maleic vanadium phosphate catalysts

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

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 series of vanadium phosphate catalysts prepared by different routes and containing different phases were examined by Guliants and coworkers [23]. From this study it was concluded that the catalytically active phase is an active surface layer on VPP. Their experimental results showed VOPO4 phases to be detrimental to the performance of the catalyst. This was confirmed by Cavani and Trifiro, who suggested that V sites are responsible for the over-oxidation of maleic anhydride to carbon oxides [24]. 

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. 

Recently, vanadium phosphate catalysts have been found to be effective catalysts for the oxidation of other alkanes, for example, propane ammoxidation and pentane oxidation to phthalic anhydride and maleic anhydride. However, these reactions are not commercialized and the oxidation of n-butane to maleic anhydride represents the only industrial, large-scale selective oxidation of an alkane currently in operation. 

Vanadium phosphate catalysts can also be used for the selective oxidation of butene to maleic anhydride, and Bordes (192) compared the results for butene and butane oxidation. Vanadium phosphates have been extensively studied as catalysts and several hundred papers and patents have been published. Much of this has been considered in several major reviews (193-198). In this section, the structure of these catalysts is described and discussed. 

Mechanism of n-Butane to Maleic Anhydride over Vanadium Phosphate Catalysts. The oxidation of n-butane to maleic anhydride occurs via 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multistep polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed on the basis of a variety of experimental and theoretical findings. 

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

Contractor RM, Bergna HE, Horowitz HS, Blackstone CM, Malone B, Torardi CC, Griffiths B, Chowdry U, Sleight AW. Butane oxidation to maleic anhydride over vanadium phosphate catalysts. Catalysis Today 1 49-58, 1978. 

Briickner, A., Kubias, B., Liicke, B.,etal. (1996). In Situ — ESR Study of Vanadium Phosphate Catalysts (VPO) during the Selective Oxidation of n-Butane to Maleic Anhydride (MA), Colloids Surf. A, 115,pp.l79-186. 

Maleic anhydride production. The oxidation of benzene to maleic anhydride over a vanadium pentoxide electrode has been studied by Pizzini et ai 90,91 Unfortunately, the quantities of benzene and maleic anhydride were not determined experimentally. Breckner et al. have studied the partial oxidation of butene to produce maleic anhydride over a vanadium phosphate catalyst. Reaction rate and oxygen activity were monitored in order to correlate catalyst selectivity with oxygen activity. The selectivity of the catalyst was found to increase as the oxygen activity of the catalyst decreased. Both the catalyst reactivity and oxygen activity were found to be dependent upon prior treatments. 

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. 

Vanadyl Phosphate Catalysts. — For the oxidation of C4-hydrocarbons to maleic anhydride, vanadyl phosphate catalysts with a variety of V P ratios and different additives have been proposed. Nakamura etal.123 observed for V P = 1 2, an average oxidation number of four for vanadium, highly aggregated vanadium ions and a high selectivity. Varma and Saraf124 also studied this reaction and on the basis of kinetic results propose a two-stage redox mechanism. They also concluded that maleic anhydride is hardly oxidized to carbon oxides, which are mainly formed in a side reaction from the original... 

Another example is the activation of vanadium phosphate (VPO) catalysts in the oxidation of butane to maleic anhydride the fixed-bed VPO catalyst has to be contacted for several days with air-butane mixture before reaching normal activity. This case and that of Raney nickel (both unsupported) fall outside the scope of the present discussion... 

Hutchings GJ. Vanadium phosphate a new look at the active components of catalysts for the oxidation of butane to maleic anhydride. J. Mater. Chem. 2004 14(23) 3385-3395. 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

Air oxidation of n-butane to maleic anhydride is possible over vanadium phosphate and, remarkably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in contrast to the situation found in the liquid, n-alkanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

Vanadium phosphates have been applied to a number of selective oxidation and ammoxidation reactions, although the partial oxidation of n-butane to maleic anhydride remains the most widely studied reaction for these catalysts. Even though the first patent for this reaction was filed over 40 years ago, there are stQl a large number of papers published on this system every year. 

In cases where the oxidation state of the solid changes during the reaction, the active solid state phase present at steady state must be characterized. For example, the reduced vanadium phosphate phase forms during the oxidation of butene to maleic anhydride (28,29). And for supported catalysts such as V Oc-/TiO anatase. 

Vanadium phosphate (VPO) compounds are important industrial catalysts used in the conversion of n-butane to maleic anhydride[l]. Although it is now generally accepted that the reaction requires the presence of both V and V cations in close proximity, the precise nature of the active site in this catalyst is still a matter for... 

Vanadium phosphates (VPO) of different structure are suitable precursors of veiy active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Va H2O is transformed into (V0)2P207 applied as the n-butane oxidation catalyst. Otherwise, if VOHPO4 V2 H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(V0)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(V0)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product ( 4.11 [7]) a second, mixed-valent (V 7v ) vanadium-rich phase must be formed. 

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. 

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