Oxidation 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. 

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. 

FIGURE 26 Relationship between catalyst activity and surface area for standard vanadium phosphate catalysts for the oxidation of n-butane. Reproduced with permission from Ref. (33). Copyright 1997 Elsevier. 

Industrial catalysts for oxidation reactions rarely incorporate only a single bulk phase. A number of promoter elements are usually added, which can act purely as textural promoters or can enhance the activity and selectivity of the bulk catalyst. The role of promoters on vanadium phosphate catalysts has been addressed mainly in the patent literature, and Hutchings (163) has provided an extensive review of these patents. 

Bej and Rao (186-190) conducted a detailed investigation of molybdenum- and cerium-promoted vanadium phosphate catalysts. They foimd an increase in the selectivities of these catalysts as a result of incorporation of the promoters, albeit with slight decreases in activity. They attributed the improved selectivity to a role of the promoters in preventing overoxidation of the MA to carbon oxides. They also found that the promoted catalysts could withstand more severe reaction conditions than the unpromoted catalyst, and this property was also attributed to the formation of less carbon oxides, which can poison the catalyst. 

TABLE 1 Summary of Recent Results Characterizing the Achievements on the Effects of Promoters on the Performance of Vanadium Phosphate Catalysts for n-Butane Oxidation. 

The success of butane selective oxidation inevitably led to testing of vanadium phosphate catalysts for oxidation of other alkanes and alkenes. Pentane has been similarly transformed to phthalic anhydride in addition to MA (91-97). Phthalic anhydride is an important intermediate in the manufacture of plastics. Flowever, the investigation of vanadium... 

The oxidation of propane and of propene to acrylic acid has also been investigated 87,106,256-261). Vanadium phosphate catalysts that show good performance for the oxidation of C3 hydrocarbons and vanadium phosphate catalysts that are active and selective for C4 and C5 hydrocarbon oxidation have several differences in their structure and operating conditions. 

Another key difference is the need to cofeed water to achieve good selectivities for acrolein. Water is proposed to increase the crystallinity and the number of active sites for propane oxidation while at the same time decreasing the number of acid sites on the surface of the vanadium phosphate catalyst that are thought to be responsible for overoxidation of the products 258). 

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 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]. 

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. 

It is clear that vanadium phosphate catalysts are still widely studied although not widely understood. Advances in research methodology, particularly the number of complementary in situ characterization techniques now available, may be able to further the understanding of alkane achvahon and selechve oxidation. This fundamental understanding will be beneficial in the design of new catalyst systems for alkane funchonalizahon and provide new uses for this relatively unreactive, under-uhlized feedstock. 

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. 

Toluene can be readily ammoxidized to benzonitrile, usually over supported vanadium oxide and vanadium phosphate catalysts [e. g. 9,57]. Besides catalyst choice, catalytic performance mainly depends on the reaction conditions. Excess ammonia, as mentioned above, significantly increases nitrile selectivity by blocking sites responsible for consecutive oxidation ammonia also frequently reduces catalyst activity [1]. Water vapor in the reactant stream can also improve selectivity by blocking sites for total oxidation [38] or providing Brdnsted sites for the activation of ammonia [51]. 

Intercalation reactions of neutral host lattices usually occur via electron transfer between the guest species and the host lattice. Furthermore, vanadium (V) compounds are strong oxidants, and vanadium phosphate catalysts are able to promote, for example, oxidation of alcohols to ketones [12]. 

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