VPO activation

It is also well known that pure VOPO4 phases do not perform as well as "reactor equilibrated" (VO)2P207 for n-butane oxidation [6]. This fact indicates that a VOPO4 surface by itself cannot be the active phase. Consequently, a number of workers have suggested that the VPO active site is situated at a (V0)2P207/V0P04 interface [5,7,8],... 

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). 

Promoters are sometimes added to the vanadium phosphoms oxide (VPO) catalyst during synthesis (129,130) to increase its overall activity and/or selectivity. Promoters may be added during formation of the catalyst precursor (VOHPO O.5H2O), or impregnated onto the surface of the precursor before transformation into its activated phase. They ate thought to play a twofold stmctural role in the catalyst (130). First, promoters facilitate transformation of the catalyst precursor into the desired vanadium phosphoms oxide active phase, while decreasing the amount of nonselective VPO phases in the catalyst. The second role of promoters is to participate in formation of a soHd solution which controls the activity of the catalyst. 

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

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. 

Fig. 20. (a) Active sites observed by in situ atomic-resolution ETEM structural modification of VPO in n-butane along (201) indicates the presence of in-plane anion vacancies (active sites in the butane oxidation) between vanadyl octahedra and phosphate tetrahedra. (b) Projection of (010) VPO (top) and generation of anion vacancies along (201) in n-butane. V and P are denoted. Bottom model of novel glide shear mechanism for butane oxidation catalysis the atom arrowed (e.g., front layer) moves to the vacant site leading to the structure shown at the bottom. 

Samples 1 and 2 correspond to VPO treated in steam for 92 and 312 h, respectively. Samples 3 and 4 are N2-treated and activated base VPO catalysts, respectively. MA capacities represent the total amount of MA liberated by reduction in 1.5% butane/N2 at the reaction temperature. Table II shows that the base and N2-treated catalyst have nearly equal activities in the presence of air in the reactant stream and continue to operate. 

Samples 1-4 correspond to VPO treated in steam for 92, 312h, in N2 and activated base catalysts, respectively, k, are pseudo-first-order rate constants for the disappearance of butane. The constants are measured in a microreactor on a larger amount ( 1 g) of catalyst at 633 K. k (intrinsic) are based on the BET surface area. 

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. 

Elemental and Structural Characterization Many oxidation reactions occur on mixed oxides of complex composition, such as SbSn(Fe)0, VPO, FePO, heteropolycompounds, etc. Very often the active surfaces are not simple terminations of the three dimensional structure of the bulk phases. There is need to extensively apply structural characterization techniques to the study of catalysts, if possible in their working state. 

This study has resulted in interesting informations concerning the active sites of the VPO catalysts for n-butane oxidation to maleic anhydride being obtained. The study of VPO catalysts in the course of n-butane oxidation by an in-situ Raman cell has shown... 

The procedures for synthesizing the catalysts have a marked effect on their activity. The active VPO catalyst phases are produced as follows. 

For high performance industrial catalysts capable of activating butane, organic media are used for the synthesis of VPO, which results in a large area of active (010) orientations. VPO catalysts synthesized in organic media are therefore used in the present chapter to elucidate dynamic butane catalysis by in situ ETEM. The other methods of preparation can be multi-phasic, which can result in the modification of the reactivity of the catalyst (Centi 1993, Kiely et al 1996). 

Precursor to active catalyst transformation VHPO to active VPO catalysts and dynamic electron diffraction... 

Dynamic atomic-resolution ETEM and diffraction studies provide fundamental insights into the catalyst precursor transformation mechanism. The studies reveal that the temperature regimes are critical to the formation of active catalysts. They show that the nature of the VHPO -> VPO transformation is topotac-tic. Topotaxy is defined as the conversion of a single crystal to a pseudomorph... 

Figure 3.22. Dynamic electron diffraction (ED) image of the topotactic transformation of the VHPO precursor to active VPO catalyst in N2 (a) (010) VHPO at room temperature (b) physical mixture of VHPO and VPO at 425 °C (c) final VPO in the (010) active plane and (d) VPO microcrystals (V) and cracks (arrowed) on the precursor surface.

Figure 3.23. Model for topotactic transformation of the precnrsor to the active catalyst (a) face-sharing VOe octahedra and H-phosphate tetrahedra (H atoms are shaded) (b) removal of bonded water (resulting in VO5 which rotate, and transfer of H-atoms from H-phosphates) and (c) final reconnected VPO after removal of water.

An SEM image of a rosette-shaped, well-calcined and activated VPO catalyst is shown in in figure 3.24(a). VPO catalyst structure at room temperature and the corresponding ED patterns are shown in figures 3.24(b) and (c), respectively. 

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

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). 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

The concept of structural dynamics is clearly demonstrated in the VPO catalyst system. The high-resolution TEM in Figure 1.10 shows an activated VPO catalyst (no promoters, made by the alcohol route) that exhibits a typical [215, 225, 244] termination with little structural order supported on a perfectly ordered crystal consistent with the pyrophosphate structure (and with other structures of the VPO family [245, 246]). The lack of long-range order is not seen due to the operation... 

Figure 1.12 Wiring diagram of the mode of operation of the VPO system under steady state operation. The central shaded part represents the catalytic operation, the outer parts highlight the complexity of the structural dynamics creating a steady state abundance of isolated active sites.

---