O-xylene, oxidation

The V20s/Si02 catalyst for o-xylene oxidation prepared by wet impregnation under microwave irradiation had several advantages [6] compared with that prepared by the conventional thermal method ... 

This scheme and our data agree with the product distribution in o-xylene oxidation reported by many authors (1-7) as well as with experiments of oxidation of intermediates (37) o-tolualdehyde and phthalide are observed as the main intermediates in the 523-573 K temperature range, while phthalic anhydride selectivity grows in the 473-573 K range and later only slightly decreases above 600 K when also maleic anhydride appears and conversion is very high. 

Not much is known concerning the mechanism of the oxidation of the nucleus. Complete oxidation is the main reaction while minor amounts of maleic anhydride are formed over some catalysts, in particular those based on V2Os. Blanchard and Vanhove [52] demonstrated with 14C labelling that, for o-xylene oxidation over V2Os, anhydride is exclusively formed from nuclear carbon atoms. This may be generalized to other methyl benzenes. 

The kinetics of the o-xylene oxidation appear to be rather complex due to the fact that several reaction steps seem more or less inhibited by reaction products. 

An ESR study by Yabrov et al. [355] revealed that, at least at low V205 content (0.05—5 wt. %), vanadium forms a solid solution of V4+ and V3+ in Ti02. The samples investigated were sealed in the reactor after steady state operation of the o-xylene oxidation at 350°C. The V4+ solid solution, which is considered the active phase, is not formed by the catalyst pretreatment at high temperature, but requires the interaction of the reaction mixture as was shown by the analysis of fresh catalysts. Solid state reactions between V2Os and Ti02 were also studied by Cole et al. [89]. 

Fig. 9. Selectivity and activity as a function of composition of V2Os— Ti02 catalysts for o-xylene oxidation.

Kinetic and mechanistic investigations on the o-xylene oxidation over V205—Ti02 catalysts were carried out by Vanhove and Blanchard [335, 336] using a flow reactor at 450°C. Possible intermediates like o-methyl-benzyl alcohol, o-xylene-a,a -diol, toluic acid and phthalaldehyde were studied by comparing their oxidation product distribution with that of toluene. Moreover, a competitive oxidation of o-methylbenzyl alcohol and l4C-labelled o-xylene was carried out. The compounds investigated are all very rapidly oxidized, compared with o-xylene, and essentially yield the same products. It is concluded, therefore, that these compounds, or their adsorbed forms may very well be intermediates in the oxidation of o-xylene to phthalic anhydride. The ratio in which the partial oxidation products are formed appears to depend on the nature of the oxidized compounds, i.e. o-methylbenzyl alcohol yields relatively more phthalide, whereas o-xylene-diol produces detectable amounts of phthalan. This... 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

The gas phase oxidation of naphthalene to phthalic anhydride over V2Os-based catalysts is one of the oldest successful partial oxidation processes and is still of industrial importance today. Common commercial catalysts are modified silica-supported V—K—S—O catalysts and catalysts similar to those used for benzene or o-xylene oxidation. Maximum phthalic anhydride yields of 80—85 mol. % (92—98 wt. %) at 350—400°C are reported. By-products are naphthoquinone (2—5%), maleic anhydride (2— 5%) and carbon oxides. 

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably a-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. We suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H20 (I), might also contain a contribution from benzocyclobutene from the interaction of a-bromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-o-xylene were run separately under the same conditions. 

Deactivation of catalysts is a major problem in o-xylene oxidation [6,7]. For this reaction, deactivation has been mainly attributed to the irreversible anatase - rutile transformation [2,3]. In fact, anatase was found to be the best support for vanadium pentoxide catalysts leading the presence of rutile to lower activities and selectivities [8,9], The anatase-rutile transition can take place at temperatures above 973 K [10] but it is known that the presence of vanadia promotes such transformation [11-14] which, in these conditions, can start at 773 K [14], Such temperatures are easily attained in industrial reactors due to the high exothermicity of o-xylene oxidation that can lead to the formation of temperature profiles lengthwise with pronounced maxima (hot spot) [1]. 

Reactions of organic compounds over solid catalysts are sometimes accompanied by the formation of heavy by-products which can form a deposit on the surface and lead to catalyst deactivation. For o-xylene oxidation the formation of such compounds has been frequently mentioned [15-17] but no information can be found about their influence on the catalyst deactivation. The present work reports on the formation of carbonaceous deposits over V2O5/T1O2 catalysts used for o-xyletie oxidation. Samples prepared by wet impregnation were used under operating conditions that can lead to the formation of heavy compounds. They were then collected and analysed by FTIR and TFO. The present data help to elucidate the characteristics of such compounds and their influence on the catalytic behaviour. 

The o-xylene oxidation was carried out in a continuous flow fixed bed reactor operating at atmospheric pressure. The feed mixture (0.7 mol%) was obtained injecting the organic reactant in the air flow. 

Figure 3 - TPO profiles of samples tested for o-xylene oxidation at 1,6x10-5 kgcatdm-3h and different temperatures. Time of exposure under reaction conditions 0.25 h (A), 1 h (B), 2.5 h (C), 4 h(D).

Effect of thermal deactivation of vanadium - titanium catalyst on o-xylene oxidation process yielding phthalic anhydride... 

Noble metals are preferred when non chlorinated VOCs have to be destroyed. For reasons of accessibility, FAU type zeolites are the most frequently employed catalysts and a parameter of utmost importance is the number of accessible metal sites and the reductibility. This was recently put in evidence for the o-xylene oxidation on Pd-FAU catalysts (19). Pd° was proposed as an active site, and the specific activity increased with the ratio (nPd)/nacid. A lower acidity (high Si/Al ratio) favors the Pd2 and PdO reductibility, and decreases the extent of coke formation. 

Supported vanadla used to promote the selective oxidation of hydrocarbons. Is another example. Vanadia with or without promoters, may be supported on silica (naphthalene oxidation [38]), on tltanla o-xylene oxidation [39]) or on a-alumlna (benzene oxidation [ AO]). It was believed that supports should have open porosity (and associated lower surface area) in order to minimise over oxidation to carbon oxides. However, it was shown that reasonably high activities and selectivitles could be obtained over vanadla supported on high surface area material and it was suggested that low selectivity was, in fact, primarily associated with high acidity on the support [AI]. In agreement with this, vanadla supported on 7-alumina shoved zero selectivity for the production of maleic anhydride from benzene If this Is the case, then a high surface area support with minimal acidity would be desired,... 

Air is sufficient to oxidize the methyl groups of o-xylene, under the right conditions, like it is with p-, or w-xylene just described. However, here the similarity ends since commercial o-xylene oxidation is a vapor phase process [27]. ortf o-Xylene vapor, mixed with a large excess of air to ensure operation outside the explosive range, is fed to a reactor containing a supported vanadium pentoxide catalyst and heated to about 550°C. Using about a 0.1-second contact time under these conditions produces exit gases composed of phthalic anhydride, water, and carbon dioxide (Eq. 19.68). 

Such a catalyst is well known for several reactions, such as o-xylene oxidation to phthalic anhydride and selective catalytic reduction (SCR) of NO by ammonia. The anatase form of TiO appears to be better than the rutile form. Such catalysts with 1 and 8 wt% V20s/anatase was prepared by Rhone-Poulenc (S 10 m g ) for an exercise of characterization by 25 different european laboratories. All results are assembled in one issue of Catalysis Today published in May 1994, vol. 20 n°l. Surface vanadium species were observed to exist in three different forms monomeric V04 species, polymeric vanadate species and V2O5 crystallites [27], the relative amount of which depended on initial wt % V20s/anatase and on the subsequent selective dissolution treatment. 

Therefore, the question to answer is what is the common explanation to these catalytic and non catalytic phenomena, observed as well with oxide/oxide (or chloride/chloride) as with metal/oxide The oldest and first example in selective oxidation is V205-Ti02(anatase), catalyst of o-xylene oxidation to phthalic anhydride. Nearly in the same time, three teams in Poland, Great-Britain and France [4-10] observed the catalytic synergy and studied also the... 

Table 4 results from o-xylene oxidation of a milled V205/Ti02- and a conventionally prepared V205/Ti02 catalyst. 

Multibed tubular reactors to study o-Xylene oxidation have been used by McLean (in Wainwright and Foster, 1979) the catalyst is similar to that used by Boag (in Wainwright and Foster, 1979). The network differs in that McLean shows that the phthalic anhydride is formed almost exclusively from phthalide whereas Boag has indicated that this step does not occur. 

For highly exothermic and fast reactions the catalyst is often deposited on the outer surface of the support which is usually of very low porosity (e.g. FjOj on SiC for o-Xylene oxidation) (Ellis, 1972). In other applications (e.g. ammonia oxidation converters) the catalyst is in the form of a woven wire screen (or gauze) which is usually supported on a non-catalytic pad to prevent premature ignition (Gillespie, 1970). 

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