Vanadyl pyrophosphate active sites

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. 

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). 

The proposed mechanisms are linked to the researcher s hypothesis of the active site, and it is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by some authors) plays an important role in the selective oxidation of butane. The structure has been determined by X-ray diffraction and consists of edge-sharing VO5... 

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... 

Many researchers have considered that the (100) plane of crystalline vanadyl pyrophosphate is the catalytically active plane, and the mechanisms that have been proposed invoke this as their active site 11-17,19,22,24-28). However, other researchers have considered that an amorphous material supported on a (VO)2P207 matrix plays an important role in catalysis. 

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. 

Theoretical simulations of the (VO)2P207 surface have advanced greatly in recent years. Periodic calculations, cluster models, and DFT methods have been employed to describe the surface active sites. FIow-ever, in our context, these methods have been extended only to modeling of the (100) surface of vanadyl pyrophosphate (155-159) and to simulation... 

The oxidation of -butane to maleic anhydride is a 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multi-step polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed based on a variety of experimental and theoretical findings. The description of the active site is linked to the mechanism and is the subject of considerable debate in the literature. The mechanisms are linked to the researchers hypotheses of the active site, which will be discussed in a separate section in this chapter. It is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by certain authors) plays an important role in the selective oxidation of butane. 

Other proposed mechanisms involve either a direct attack of O - species at the 1,4 C atoms of n-butane (38), or an allylic oxidation of an olefinic-like C4 intermediate to crotonaldehyde, followed by internal cyclization and oxidation (39). A dienic intermediate has also been proposed by Grasselli et al. (40). In any case, the reaction patterns proposed evidence the need for different kinds of active sites on the surface of the vanadyl pyrophosphate which are able to perform with high selectivity each step in the reaction pathway. 

Literature descriptions of active sites on oxide catalysts are often speculative and very often just generate a picture of the surface active site by extrapolation of the bulk structure. In general they envisage approach of the starting material to the active site in a preferred orientation without any indication of how the preferred orientation is established. In addition, the description of the active site is usually restricted to a small number of molecules. For example, the vanadium phosphorus oxide catalysts used for n-butane oxidation to maleic anhydride is based on the vanadyl pyrophosphate structure and an active site architecture is... 

Bordes, E. (1993). Nature of the Active and Selective Sites in Vanadyl Pyrophosphate, Catalyst of Oxidation of n-Butane, Butene and Pentane to Maleic Anhydride, Catal. Today, 16, pp. 27-38. 

It is clear that in the case of selective catalysts, the presence of a second element involved in the formation of a specific crystalline phase improves the catalytic behavior of V-based materials. This is the case for V-P-0 catalysts, in which the presence of phosphorous permits the synthesis of several crystalline compounds with different compositions, in which vanadium atoms can exhibit different oxidation states. ° However, the dominant presence of the vanadyl pyrophosphate (Fig. 24.2a) phase is the key factor in the development of active and selective sites. 

It is clear that strong differences are observed between VPO and MoVTeNbO catalysts, which are responsible for their particular catalytic behavior. Thus, VPO catalysts with a vanadyl pyrophosphate structure present i) one electron redox couple (V +A " +, V +ZV b) ii) two electron redox couples, V +A + iii) bridging oxygen, V-O-V, V-O-P, or VO(P)V iv) terminal oxygen (V " " = O and V "b = O) and activated molecular oxygen, ri -peroxo and T -superoxo species and v) Lewis (probably V + and V +) and Brpnsted (probably — POH groups) acid sites. On the other hand, selective MoVTe(Sb)NbO catalysts are characterized hy b>89-93... 

In either VPO or MoVTeNbO catalysts, V-sites are directly involved in the selective oxidative activation of paraffins, while the presence of a second element is generally required in order to facilitate the formation of a defined crystalline phase (i.e. vanadyl pyrophosphate or orthorhombic metal oxide bronze, respectively) (Figs. 24.2a and 24.2c), but also for facilitating the multifuctionality of the catalysts. However, although both catalytic systems are active for the oxidative activation in alkanes, different selectivities are achieved depending on the alkane feed. Thus, a... 

Bordes, E. Nature of the active and selective sites in vanadyl pyrophosphate, catalyst of oxidation of -butane, butene and -pentane in maleic anhydride. Catal. Today 1993,16, 27-38. 

The effects of the composition and the methods of preparing V-Si-P ternary oxides on their catalytic performance in the vapor-phase aldol condensation of propionic acid with formaldehyde to form methacrylic acid were studied. The presence of both vanadyl pyrophosphate and large surface area was found to be required to achieve a good catalytic performance. Phosphorus serves to form and stabilize vanadyl pyrophosphate which is believed to be active sites and silicon serves to produce a large surface area and to modify the vanadyl pyo-phosphate. The presence of lactic acid is indispensable to produce a large surface area when the Si/V atomic ratio is in the range of 1 to 4. 

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