Catalysts vanadium phosphate

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. 

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. 

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... 

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. 

Because of the various compositions of vanadium phosphate catalysts, there is debate as to whether vanadyl pyrophosphate is indeed the active catalyst or whether a combination of phases is responsible for the catalysis. The key features of the catalyst are discussed in the following sections. 

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 19 Scheme of the proposed evolution of the vanadium phosphate catalyst with time. Reproduced with permission from Ref. (772). Copyright 1995 Elsevier. 

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. 

In summary, both amorphous and crystalline material is found in these vanadium phosphate catalysts, and it cannot be stated with any certainty whether or not the amorphous phase is the active phase. However, experimental observations have added weight to the postulate that amorphous material is the catalytically active material. 

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. 

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

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. 

When considering the roles of promoters in vanadium phosphate catalysts, care must be taken to decouple the effects of structural promotion which increases the surface area, and hence the activity of catalysts. A vast array of transition metals, alkali metals, alkali earth metals, and lanthanides have all been studied as promoters but only a handful have been found to have promotional effects that are independent of the surface area of the catalyst. 

FIGURE 31 Comparison of reaction rates and surface areas of promoted vanadium phosphate catalysts. Reproduced with permission from Ref (770). Copyright 1993 Elsevier. 

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. 

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. 

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). 

Interest in the industrial production of nitriles has increased, and vanadium phosphate catalysts have shown great promise, giving high selectivities and yields in the conversion of halogenated methyl... 

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... 

---