Selectivity 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. 

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. 

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. 

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... 

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. 

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]. 

Halogen substituted toluenes are readily converted into nitriles because electron-withdrawing substituents enhance the reactivity of such compounds in the ammoxidation reaction. The fluoro-, chloro-, bromo-, and iodo-substituted toluenes [e. g. 41,74-76] can, therefore, be converted to the corresponding nitriles. Whereas the conversion rate of /7-halotoluenes (over vanadium phosphate catalysts [41,75]) is nearly independent of the nature of the halogen substituent, the selectivity decreases in the sequence p-Cl > /7-Br >> p-l. Ammoxidation of isomeric chloro-toluenes results in different conversion p o > m) and selectivity p > o > m) sequences [41,75,76]. 

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. 

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. 

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. 

Lopez-Sanchez, J.A., Tanner, R., Collier, P., Wells, R.P.K., Rhodes, C., and Hutchings, G.J. Acetic acid stability in the presence of oxygen over vanadium phosphate catalysts comments on the design of catalysts for the selective oxidation of ethane. Appl. Catal. A Gen. 2002, 226, 323—327. 

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. 

The selective oxidation of ra-butane to give maleic anhydride (MA) catalyzed by vanadium phosphorus oxides is an important commercial process (99). MA is subsequently used in catalytic processes to make tetrahydrofurans and agricultural chemicals. The active phase in the selective butane oxidation catalyst is identified as vanadyl pyrophosphate, (V0)2P207, referred to as VPO. The three-dimensional structure of orthorhombic VPO, consisting of vanadyl octahedra and phosphate tetrahedra, is shown in Fig. 17, with a= 1.6594 nm, b = 0.776 nm, and c = 0.958 nm (100), with (010) as the active plane (99). Conventional crystallographic notations of round brackets (), and triangular point brackets (), are used to denote a crystal plane and crystallographic directions in the VPO structure, respectively. The latter refers to symmetrically equivalent directions present in a crystal. 

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... 

Vanadium Phosphate Materials as Selective Oxidation Catalysts... 

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