Pentan oxidation

Much less studied has been the role of V species in n-pentane oxidation to MA and PA. Papers published in this field are aimed mainly at the determination of the reaction mechanism for the formation of PA (2-4,11). Moreover, it has been established that one key factor to obtain high selectivity to PA is the degree of crystallinity of the VPP amorphous catalysts are not selective to PA, and the progressive increase of crystallinity during catalyst equilibration increases the formation of this compound at the expense of MA and carbon oxides (12). Also, the acid properties of the VPP, controlled by the addition of suitable dopants, were found to play an important role in the formation of PA (2). 

Catalytic tests of n-pentane oxidation were carried out in a laboratory glass flow-reactor, operating at atmospheric pressure, and loading 3 g of catalyst diluted with inert material. Feed composition was 1 mol% n-pentane in air residence time was 2 g s/ml. The temperature of reaction was varied from 340 to 420°C. The products were collected and analyzed by means of gas chromatography. A FlP-l column (FID) was used for the separation of C5 hydrocarbons, MA and PA. A Carbosieve Sll column (TCD) was used for the separation of oxygen, carbon monoxide and carbon dioxide. 

Some tubes, termed chemical shock tubes, have been designed so that conventional chemical analysis can be performed on the reaction products, by use of gas chromatography, or related, techniques. One example of the application of the chemical shock tube, perhaps unique in the low-temperature oxidation of hydrocarbons, is the study by Dahm and Voer-hook [83] of n-pentane oxidation. They measured the organic peroxide yields over the shocked gas temperature range 570-630 K at reaction times of 1-3 s. 

Normal pentane oxidation was also the subject of an investigation by Cullis and Hirschler [189] with respect to the formation of 1- and 2-pentene, and the effect of their addition to pentane + oxygen mixtures. Mechanisms of reaction were interpreted from radioactive isotope tracer studies by addition of substituted pentenes to the reactants [189], albeit under conditions involving appreciable temperature change. In an earlier study, Chung and Sandler [26] showed that the proportion of pentenes formed during pentane oxidation in a flow system reached a maximum at a reactor temperature of about 750 K. 

The most selective reactions are those which lead to the formation of stable oxygenated compounds, such as anhydrides (this is the case of the n-butane and n-pentane oxidation). On the contrary, products such as the unsaturated aeids and aldehydes are unstable under reaction conditions which are necessary for the activation of the paraffin and hence, low selectivities are observed. 

Table 3 shows the catalytic properties of these samples. One can see from the data, that all catalysts synthesized on the basis of the mechanochemically treated V2O5 show an increased selectivity towards maleic anhydride and higher specific rate of n-butane and n-pentane oxidation as compared to those obtained in traditional synthesis. The best effect in the improvement of selectivity can be reached by increase of the relative exposure of (001) plane at the VOHPO4.O.5.H2O surface which is known to be transformed into (200) plane of (VO)2P207. The low paraffins conversion over VPO-E samples at the given reaction conditions can be directly connected with their low specific surface area. The comparison made between samples VPO-El and VPO-E2 shows that the precursor synthesis using V2O5-E needs to be optimized in order to improve the catalytic performance. 

Sample SsAI m /g n-Butcme oxidation n-Pentane oxidation Propane oxidation ... 

Addition of MTBE substantially suppressed the n-pentane oxidation, with the onset of n-pentane conversion shifting by about 50 °C to a higher temperature. The typical result of such experiment is depicted in Fig. 

Figure 6. Response curves of n-pentane oxidation (a) and formation of maleic (b) or phthalic (c) anhydrides after stopping MTBE addition. (Conversion of n-pentane, and the amount of MA or PA formed are normalised to the values before MTBE addition). Experimental conditions n-pentane 1.5 vol.%, oxygen 20 vol.%, and 0.6 vol.% of MTBE reaction temperature 365 C, void section 7 cm. ti/2 is half time, namely the time necessary after stopping of MTBE to reach one half of its steady-state value before the MTBE was injected.

The objective of this paper is to study the influence of Uie conditions during VPO preparation on the properties of ffie final precursor and to correlate the solid state properties of these precursors with the catal3rtic activity in n-pentane oxidation and in partic ar in the selectivity to PA. Our attention focused mainly on the extent of structural defects observed in precursors and catalysts. 

Clearly two regions are observed one in which both short and long range order are present, and the region in which no or only short range order is observed. MA was the only product of selective oxidation over catalysts produced from Bo or Bi by via solution preparation route, i.e. starting precursor A with low content of water. Catalysts obtained by the via solid route produced by n-pentane oxidation both MA and PA and the PA/MA ratio was the highest for samples with the maximum of water used in the synthesis of precursor. 

By comparing the selectivity of the different catalysts for PA and MA formation in n-pentane oxidation over VPO catalysts, we concluded that PA formation displayed a different sensitivity to the nature of the structural defects of the cat yst. 

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. 

It has been well established that, for butane selective oxidation, both the precursor formation step and its activation are of crucial importance for the final performance of the catsdyst. The type of solvent and the kind of reducing agent used have been correlated with e catalytic activity. Apparently the use of organic media is regarded as more convenient than other methods. However, very limited information is obtained firom the literature with respect to the influence of these two steps in the oxidation of n-pentane. Only the surface topology of the catalyst was discussed as controlling the PA/MA ratio at n-pentane oxidation (8). 

In the case of n-pentane oxidation, even if the preparation via the precursor N promotes higher surface area catalysts, our results show that such preparation method favours the adequate solid state transformation of the precursor into the catalyst which is optimal for the foimation of PA. 

---