V-P-Oxide catalysts

An iron phosphate catalyst with a P/Fe atomic ratio of 1.2 used in this study was prepared according to the procedures described in the previous studies [6-8]. On the other hand, a V-P oxide catalyst with a P/V atomic ratio of 1.06 and pumice supported 12-molybdophosphoric acid (H3PM012O40) and its cesium salt (CS2HPM012O40) catalysts were the same as used in a previous study [9]. Pumice supported W03-based mixed oxide catalysts were the same as used in a previous study [10]. 

Direct evidence about the first step of activation of butane was obtained on a V-P oxide catalyst in the butane oxidation to maleic anhydride based on deuterium kinetic isotope effect (34). It was found that when a butane molecule was labeled with deuterium at the second and third carbon, a deuterium kinetic isotope effect of 2 was observed. No kinetic isotope effect was observed, however, if the deuterium label was at the first or fourth carbon. By comparing the observed and theoretical kinetic isotope effects, it was concluded that the first step of butane activation on this catalyst was the cleavage of a secondary C—H bond, and this step was the rate-limiting step. 

The ammoxidation of isobutene has not received much attention. The only contribution in this field is by Onsan and Trimm [2.44] for a rather unusual catalyst, a mixture of the oxides of Sn, V and P (ratio 1/9/3) supported on silica. At 520 C, a maximum selectivity to methacrylonitrile + methacrolein of 80% was reached with a Sn—V—P oxide catalyst (ratio 1/9/3), an isobutene/ammonia/oxygen ratio of 1/1.2/2.5 and a contact time of 120 g sec l ]. The kinetics are very similar to those for the pro-pene ammoxidation. Again, the data are initially analysed by means of (parallel) power rate equations, for which the parameters were calculated, while a more detailed analysis proves that a Langmuir—Hinshelwood model with surface reaction as the rate-controlling step provides the best fit with regard to the two main products. At 520° C, the equation which applies for the production of methacrolein plus methacrylonitrile is... 

V2O5 promoted by P or Mo is widely used for selective oxidation. V P oxide catalysts are effective for the selective oxidation of n-butane to maleic anhydride. The reaction is a 14-electron oxidation involving the abstraction of eight hydrogen atoms and insertion of three oxygen atoms (equation 9). 

We conclude therefore, that the Nb-Ti-V-P-oxide catalyst investigated is relatively active for the oxidation of paraffins, since it has a V/P ratio of only 1/12 as compared to a 1/1 ratio for (VO)2P207. Of course, we had hoped that the V in the NASICON structure would be sufficiently site isolated to yield products less oxidized than maleic anhydride from n-butane. However, unfortunately that does not appear to be the case. One explanation for this might be that there are still too many adjacent V atoms, i.e., (V-0-V) moieties, where n > 0. Nonetheless, the NASICON structure provides for some desired V site isolation, however, apparently not complete and hence not sufficient to achieve our desired catalytic goal. Another observed fact is, that the Nb-Ti-V-P-oxide under investigation shows an amorphous overlayer via TEM which is enriched in vanadium. The (V/P)s ,face > (V/P)pa icie- One can reason that at the temperature of 900 °C required to obtain the NASICON structure, the more... 

Acrylic acid is more stable than acetic acid and HCHO is much more stable than acetic acid over the V-P oxide catalysts. It is therefore concluded that CO2 observed in the reaction of HCHO with acetic acid is mainly formed by the decomposition of acetic acid but not by that of HCHO. It is also found that the decarboxylation of acetic acid over the V-P oxide catalysts is suppressed markedly by the presence of water and HCHO. ... 

The reaction was also tested over a V-P oxide catalyst with a P/V atomic ratio of 1.06 consisting of (VO)2P207 at a temperature of 320 °C and a methyl propionate/HCHO molar ratio of 2.0, using methylal as the source of HCHO. The yields of methyl methacrylate, acrylic acid, propionic acid reach 25, 6, and 20 mol%, respectively, based on the charged HCHO the sum of the yields of methyl methacrylate and methacrylic acid reaches 31 mol%. By the combination of Si with the V phosphate, the yield of methyl methacrylate is markedly improved. For example, over the VSi8P2,g catalyst, the yields of methyl methacrylate, methacrylic acid, and propionic acid reach 42, 6, and 20 mol%, respectively, based on the charged HCHO the sum of yields of methyl methacrylate and methacrylic acid reaches 48 mol% based on HCHO. The yield of methyl methacrylate increases as the methyl propionate/HCHO molar ratio is increased, for example, with the molar ratio of 4.0, the sum of yields of methyl methacrylate and methacrylic acid reaches 68 mol% based on the charged HCHO. As for the selectivity, the selectivity based on HCHO increases as the methyl propionate/HCHO ratio is increased, while the selectivity based on methyl propionate decreases. With a methyl propionate/HCHO molar ratio of 4.0, the selectivity based on HCHO is 100 mol%. On the other hand, when the molar ratio is 1.2, the selectivity based on methyl propionate becomes 100 mol%. 

V + P oxide catalysts. They suggest no effect of V/P ratio on the yield of MA. The main factors affecting the latter were time and temperature of calcination. 

It is clear that the V-P oxide and Mo-P heteropoly compound catalysts are not effective for the production of citraconic anhydride. The acidic catalysts such as... 

V-P oxides are known to be efficient for the production of maleic anhydride (abbreviated as MA) from n-butane (1-3). A single-phase (VO2P2O7 has been inferred to be the active catalyst phase (4-6). In addition, the catalytic properties of (VO)2P207 varied depending on the microstructure of (VO)2P207 particles (7,8). Some claimed that the... 

In the process based on n-butane feedstock, vanadium phosphorous oxides (V-P-O) catalysts are mainly used.1010-1012 Processes for the oxidation of low-cost C4 fraction from naphtha cracker consisting mainly of butenes have also been developed.1013,1014 In contrast with benzene oxidation where two carbon atoms are lost in the form of ethylene no carbon is lost in the oxidation of C4 hydrocarbons ... 

It has been reported that Mo-P oxides show a good performance in oxidation of butenes to maleic anhydride [26]. On the other hand, Bordes et al. [27] have reported that U-Mo oxides with Mo-rich compositions are effective as catalysts for oxidation of butenes to maleic anhydride. These findings suggest that the functions required for oxidation of toluene are similar to those required for oxidation of butenes to maleic anhydride. However, the V-P oxides are not effective for toluene oxidation. Possibly, the consecutive oxidation of benzaldehyde cannot be suppressed with V Og-containing catalysts. Even over the Mo-P and U-Mo oxides, benzaldehyde is degraded, to a certain extent. 

Waugh et al.131 discussed the selective oxidation of benzene to maleic anhydride on the basis of a detailed study of maleic anhydride and benzene adsorption on a V-Mo oxide catalyst supported on a-Al203. Hydroquinone is found to be an intermediate in this reaction and p-benzoquinone, formed from the hydroquinone, is the main intermediate in the non-selective pathway. The maleic anhydride is observed to be immobile adsorbed and the surface oxidation reaction has a relatively low activation energy. From this the authors conclude that it is not lattice oxygen but weakly bound molecular 02 which is responsible for the selective oxidation and a detailed mechanism, in which use is made of orbital symmetry arguments, is presented. 

Tufan and Akgerman138 studied the same reaction using a V-Mo oxide catalyst modified with P and Sb and found the highly selective reaction to follow a two-step redox mechanism. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

VO)2P207 is superior for the selective dehydrogenation of n-butane to butene to other crystalline V-P oxides, while few differences exist between the oxidation of butene and butadiene, which are considered reaction intermediates. The abstraction of methylene hydrogen from n-butane is the slowest step. Hence this step determines the overall catalytic activity." 2) Selectivity in forming anhydride from C4 and C5 alkanes, but not in selective oxidation of lower (C2 and C3) or higher (Ce-Cg) alkanes." 3) The number of surface layers involving the catalytic reactions is limited to 2-3 in contrast to Bi-Mo-O catalysts. 

This volume consists of reviews devoted to a range of important subjects. Vadim Guliants and Moises Carreon (University of Cincinnati) review the selective oxidation of butane. This is an excellent example of a catalytic process designed to add value to an inexpensive raw material, and is the only vapor phase selective oxidation of an alkane that is practiced industrially. This process also avoids the use of benzene, which eliminates the risk of handling this carcinogenic compound. The authors review the synthesis, activation, and mechanism of this reaction on V-P-O catalysts. 

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