Activation vanadium phosphate catalysts

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. 

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. 

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 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. 

Further evidence for the catalytic importance of amorphous material comes from experiments carried out with cobalt-doped catalysts. Hutchings et al. (217) found that doping of the catalysts with cobalt improved their performance. Moreover, Sajip et al. (148) found that the cobalt-promoted catalysts are far more disordered than the undoped catalysts. In the doped catalysts, the promoter is dispersed in the amorphous phase, and cobalt is not found in the vanadyl pyrophosphate crystals. It is thought that one of the properties of the cobalt promoter is to stabilize the disordered phase and V -containing phases in the final catalysts, which leads to improved performance. This suggestion implies that the disordered material is the catalytically active vanadium phosphate phase. 

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. 

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. 

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. 

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). 

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-... 

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. 

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. 

A number of other groups have also found that zirconium enhances the activity of vanadium phosphate catalysts [11, 56, 146, 148, 150, 154, 156-163]. Zeyss and coworkers [158] investigated catalysts doped with 5 to 15% zirconium. Unlike the... 

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