N-Pentane, 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. 

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. 

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. 

On heating S9O decomposes at 32-34 °C with melting and SO2 evolution. At 20 °C the solid oxide decomposes quantitatively within 2 h to SO2 and a polymeric sulfuroxide (S 0)x with n>9. Even dissolved in carbon disulfide S9O decomposes within 20 min to a large extent with formation of SO2 as can be seen from the decrease of the infrared absorption intensity at 1134 cm (S9O) and the intensity increase at 1336 cm (SO2). The solubihty of S9O in CS2 (>21 g r at 0 °C) is much higher than in CH2CI2 (260 mg at 0 °C) while the substance is practically insoluble in n-pentane, n-hexane and tribromomethane. At -80 °C, S9O can be stored for longer periods of time without decomposition. 

A mixture of 0.79 g. (a slight excess over 0.001 mol) of [MoS6C6(CF3)6] and 1.54 g. (0.001 mol) of [(C6H6)4As]2[MoS6-C8(CF3)6] in 100 ml. of dry dichloromethane is refluxed for 3 hours with careful exclusion of atmospheric moisture. The mixture becomes very dark blue, almost black. The solvent is removed in vacuo to leave a blue oil. This oil is dissolved in 30 ml. of dry dichloromethane and carefully triturated with dry n-pentane, which causes the product to crystallize. The product is collected on a sintered-glass funnel and washed with two 25-ml. portions of dry n-pentane. The product while still moist with n-pentane is dried over phosphorus(V) oxide in a vacuum desiccator. The yield is 2.1 g. (91%) of very dark crystals, m.p. 189 to 192.5° (checkers report m.p. 190 to 191°). The product is sensitive to reduction by atmospheric moisture. 

The multi-functionality of metal oxides1,13 is one of the key aspects which allow realizing selectively on metal oxide catalysts complex multi-step transformations, such as w-butane or n-pentane selective oxidation.14,15 This multi-functionality of metal oxides is also the key aspect to implement a new sustainable industrial chemical production.16 The challenge to realize complex multi-step reactions over solid catalysts and ideally achieve 100% selectivity requires an understanding of the surface micro-kinetic and the relationship with the multi-functionality of the catalytic surface.17 However, the control of the catalyst multi-functionality requires the ability also to control their nano-architecture, e.g. the spatial arrangement of the active sites around the first centre of chemisorption of the incoming molecule.1... 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

The Trcinsformation of Light Alkanes to Chemicals Mechcmistic Aspects of the Gas-phase Oxidation of n-Pentane to Mcdeic Anhydride and... 

Figure 4 shows the catalytic performance of sample oxlsp the conversion of n-pentane, and the selectivity to the main products, MA, PA and carbon oxides, are reported as functions of the reaction temperature. The selectivity to MA increased on increasing the reaction temperature, and correspondingly the selectivity to PA decreased. The overall selectivity to PA and MA was approximately constant up to 400°C, but then decreased, due to the preferred formation of carbon oxides. 

Figure 5 summarizes the results obtained for fresh and spent samples the selectivity to MA and that to PA at approximately 43-50% n-pentane conversion are plotted as a function of the average oxidation state of V. The most oxidized the catalyst was, the most preferred was the formation of MA with respect to that of PA, with a MA/PA selectivity ratio equal to 7 at 50% n-pentane conversion for sample 0x3. The opposite was true for most reduced samples, with a MA/PA selectivity ratio... 

Figure 5. Selectivity to MA and to PA at 43-50% n-pentane conversion as functions of the average oxidation state of V in fresh and spent samples.

Data reported in the present work demonstrate that the degree of crystallinity and the acid properties are related the amount of present at the surface of VPP. When the VPP is not fully equilibrated, and hence may contain discrete amounts of it is more selective to MA and less to PA. The reason is that in oxidized catalysts, the olefmic intermediate is preferentially oxidized to MA, rather then being subjected to the acid-catalyzed condensation with a second unsaturated molecule, to yield the precursor of PA. When instead the catalyst is more crystalline, and hence it does contain less oxidized V sites, its surface acid properties predominate over O-insertion properties, and the catalyst becomes more effective in PA formation. In this case, the selectivity to PA at 50% n-pentane conversion becomes comparable to that one of MA. 

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