Catalysts vanadium pyrophosphate

The solids analysis described above can be taken to yet another level by correlating the color measurement to chemical properties. An excellent model system is vanadium pyrophosphate (VPO), which is a well-known catalyst for butane oxidation to maleic anhydride. During the synthesis of the catalyst precursor, solid V2O5 particles are dispersed in a mixture of benzyl alcohol and i-butanol. In this slurry phase, the vanadium is partly reduced. Addition of phosphoric acid leads to a further reduction and the formation of the VPO structure. With a diffuse reflectance (DR) UV-vis probe by Fiberguide Ind., the surface of the suspended solid particles could be monitored during this slurry reaction. Four points can be noted from Figure 4.4 ... 

In industrial practice, the laboratory equipment used in chemical synthesis can influence reaction selection. As issues relating to kinetics, mass transfer, heat transfer, and thermodynamics are addressed, reactor design evolves to commercially viable equipment. Often, more than one type of reactor may be suitable for a given reaction. For example, in the partial oxidation of butane to maleic anhydride over a vanadium pyrophosphate catalyst, heat-transfer considerations dictate reactor selection and choices may include fluidized beds or multitubular reactors. Both types of reactors have been commercialized. Often, experience with a particular type of reactor within the organization can play an important part in selection. 

Another exponential relationship common to reactor engineering is that between conversion and the ratio of the reactor (or catalyst) volume, V, and the volumetric flow rate, Q. This ratio is referred to as the contact time, t. Maleic anhydride (MA) is an important chemical intermediate that is formed by the partial oxidation of butane over a vanadium pyrophosphate. Figure 2.14 plots -butane conversion, X, against contact time, r—collected in an ideal micro-reactor on ordinary rectangular co-ordinates. Since the shape of the data is concave down, we might assume that the relationship is a power law with a negative exponent. However, the data points do not fall on the trend line drawn in Figure 2.15. 

Example 9.6. Table E9.6 summarizes the particle size distribution of commercial vanadium pyrophosphate catalyst to produce maleic anhydride from butane in a circulating fluidized bed reactor. Calculate DA [l,0],Z)jv[2,0],DAf[3,0], Dat[3,2], and Z)jv[4,3]. 

TABLE E9.6S PSD for Commercial Vanadium Pyrophosphate Catalyst Solution... 

Vanadium pyrophosphate (VPO) is unique in its capacity to selectively oxidize n-butane to maleic anhydride (MA). Gas phase oxygen is made responsible for the total oxidation of butane and MA to carbon oxides. It is known that oxygen stored in the crystal structure of the VPO catalyst is required for the desired reaction. Thus it is possible to run the reaction without gas phase oxygen in order to reduce the undesired total oxidation and to increase selectivity and yield, and to reoxidize the catalyst after oxygen dq>letion. 

The work was strongly inspired by Union Carbide s Ethoxene process, a route for manufacturing ethylene from ethane and oxygen by oxidative dehydrogenation. The first catalysts consisted of molybdenum, vanadium, and niobium oxides. The selectivity for ethylene was very high but, unfortunately, the conversion of ethane was low ( 10%). Therefore, scientists at the time focused on the co-production of ethylene and acetic acid. A catalyst consisting of molybdenum, vanadium, niobium, calcium, and antimony supported on a molecular sieve was developed (63% selectivity to acetic acid, 14% selectivity to ethylene, and 3% conversion of ethane). In addition, Rhone-Poulenc (catalyst vanadium oxide or vanadyl pyrophosphate) and BP (catalyst combination of rhenium and tungsten) patented processes for the production of acetic acid from ethane. Very efficient catalysts were also disclosed by Hoechst (molybdenum vanadate, promoted with Nb, Sb, Ca, and Pd, 250-280 °C, 15 bar, 86% selectivity to acetic add at 11% conversion of ethane per pass) and Sabic (phosphorus-modified molybdenum-niobium vanadate, 260 °C, 14 bar, 50% selectivity to acetic acid at 53% conversion of ethane). 

It can be argued that some mixed metal oxides can also be technically considered as supported metal oxide catalysts because the surface is discernibly different from the underlying mixed metal oxide in terms of composition and molecular structure. For example, the vanadium phosphorus oxide (VPO) catalyst is used in the commercial production of maleic anhydride from butane [12]. The most active crystal phase is the vanadium pyrophosphate (VO)2P207, and the surface structure proposed to be the active phase is a nanometer-thick amorphous VPO layer enriched in phosphorus [12,15]. As another example, Wachs and coworkers [16]... 

The catalyst used in the production of maleic anhydride from butane is vanadium—phosphoms—oxide (VPO). Several routes may be used to prepare the catalyst (123), but the route favored by industry involves the reaction of vanadium(V) oxide [1314-62-1] and phosphoric acid [7664-38-2] to form vanadyl hydrogen phosphate, VOHPO O.5H2O. This material is then heated to eliminate water from the stmcture and irreversibly form vanadyl pyrophosphate, (V(123,124). Vanadyl pyrophosphate is befleved to be the catalyticaHy active phase required for the conversion of butane to maleic anhydride (125,126). 

The industrial catalyst for n-butane oxidation to maleic anhydride (MA) is a vanadium/phosphoras mixed oxide, in which bulk vanadyl pyrophosphate (VPP) (VO)2P207 is the main component. The nature of the active surface in VPP has been studied by several authors, often with the use of in situ techniques (1-3). While in all cases bulk VPP is assumed to constitute the core of the active phase, the different hypotheses concern the nature of the first atomic layers that are in direct contact with the gas phase. Either the development of surface amorphous layers, which play a direct role in the reaction, is invoked (4), or the participation of specific planes contributing to the reaction pattern is assumed (2,5), the redox process occurring reversibly between VPP and VOPO4. 

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. 

Vanadium phosphorus oxides (VPO) are commercially used as catalysts for the s5mthesis of maleic anhydride from the partial oxidation of n-butane. The phase constitution and the morphology of the catalyst are found to be dependent on the preparation routes and the applied solvent [78]. Recently, a method to prepare VPO catalysts in aqueous solution at elevated temperature was reported [79]. In addition to the linear relationship between specific activity and surface area, a small group of catalysts exhibit enhanced activity, which could be due to the combination of a higher proportion of V phases in the bulk of vanadyl pyrophosphate (V0)2P207 catalyst [79, 80]. With high relevance to the catalytic properties, the microstructure characterisation of VPO therefore is of great importance. 

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. 

Complex vanadium-phosphorus-oxide catalysts are the most successful industrial catalysts for the selective oxidation of /i-butane to maleic anhydride (MA) with uses in tetrahydrofurans (THE) and polyurethane intermediates. A schematic diagram of the reaction is shown in figure 3.21(a). These catalysts have been studied extensively (e.g. Centi et al 1993, Bordes 1987). In the selective catalysation of a-butane to MA, the best active phase in the V-P-0 system is identified as the vanadyl pyrophosphate, (VO)2P207 (hereafter... 

VPO catalyst, they are assumed to be V2Ok units made up of pairs of distorted edge-sharing V05 square pyramids. The assumption of these active sites, especially for the VPO catalyst, was discussed in detail in Ref. 56, in view of the fact that the most selective VPO catalyst for butane oxidation to maleic anhydride contained a slight excess of phosphorus over the stoichiometric ratio for vanadyl pyrophosphate, the phosphorus was concentrated on the surface (57-61), and the average vanadium valence of the catalyst under reaction conditions was about 4.1 (57, 58). 

In addition to the requirements with respect to size, shape, and mechanical stability, the nature of the active phase also has to be adopted when the same catalyst is applied in different reactor concepts mainly due to differing process conditions. Vanadium phosphorous oxide composed of the vanadyl pyrophosphate phase (VO)2P207 is an excellent catalyst for selective oxidation of H-butane to maleic anhydride [44-47]. This type of catalyst has been operated in, for example, fixed-bed reactors and fluidized-bed-riser reactors [48]. In the different reactor types, different feedstock is applied, the feed being more rich in //-butane (i.e. more reducible) in the riser-reactor technology, which requires different catalyst characteristics [49]. 

The most active and selective catalysts consist mainly of vanadyl pyrophosphate, which during operation has a vanadium oxidation state of close to -1-4. 

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. 

Experimental results on pure vanadium phosphate phases and active catalysts suggested that the active catalyst was vanadyl pyrophosphate with domains of on the (100) face (114). The low selectivity of side faces found by Inumaru et al., 115,116) is attributed to the difficulty of the reoxidation of the vanadium to in these planes. Hutchings et al. (117) proposed a couple as the active site, which can be present on the... 

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. 

Prior to this disclosure, Trifiro (154) proposed that the active catalyst is pure vanadyl pyrophosphate and found that the catalyst was characterized by a slight increase in the vanadium oxidation state after the equilibrium period. The small increase from -1-4.00 to -h4.03 was reproducible and attributed to the formation of isolated V " surface sites. The P/V ratio was proposed to be a key characteristic in the stabilization of V + within the catalyst, as VOPO4 formation becomes very difficult at P/V ratios >2.0. Trifiro had stated that a very high surface P/V ratio is required for an active and selective catalyst, and experimentally he has found surface P/V ratios of 10 1. 

The preparation procedure employed is known to lead to the formation of VOPO4, rather than (VO)2P207. The presence of Sb, however, may lead to a modification of the structural features. Indeed, the authors claim the presence of vanadyl pyrophosphate as the major phase present in catalysts, with a minor amount of vanadium phosphate. The atomic ratio between the components of the y-alumina-supported active phase was V/Sb/P 1/1.9/1.18. The reaction conditions were 425 °C (at which the best yields were reported), and a feed ratio of reactant/ air/ammonia of 0.6-1.0/4.2/1.5. The following results were claimed under these conditions ... 

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