Phthalic anhydride V2O5 catalysts

Currently, phthalic anhydride is mainly produced through catalyzed oxidation of o-xylene. A variety of metal oxides are used as catalysts. A typical one is V2O5 -1- Ti02/Sb203. Approximate conditions for the vapor-phase oxidation are 375-435°C and 0.7 atmosphere. The yield of phthalic anhydride is about 85% ... 

Phthalic anhydride will, in the presence of the V2O5 catalyst of Example 9.1, undergo complete oxidation with A772 = — 760kcal/mol. Suppose the complete oxidation is pseudo-first-order in phthalic anhydride concentration and that ln( k//) = 12.300—10,000/T. 

An alternative route to phthalic anhydride is the partial oxidation of naphthalene. The heat of reaction is — 430 kcal/mol. This reaction can be performed using a promoted V2O5 catalyst on silica, much like that considered in Example 9.1. Suppose In(fik) = 31.6800—19,100/T for the naphthalene oxidation reaction and that the subsequent, complete oxidation of phthalic anhydride follows the kinetics of Problem 9.3. Suppose it is desired to use the same reactor as in Example 9.1 but with a,>, = 53g/ m. Determine values for and T aii that maximize the output of phthalic anhydride from naphthalene. 

Three well known examples of processes employing fluidised-bed operations are the oxidations of naphthalene and xylene to phthalic anhydride using a supported V2O5 catalyst and ammoxidation of propylene utilising a mixed oxide composition containing bismuth molybdate. Typically, this latter reaction is executed by passing a mixture of ammonia, air and propylene to a fluidised bed operating at about 0.2 MPa pressure, 400—500°C and a few seconds contact time between gas and fluidised catalyst peirticles. 

When more than one reaction occurs the calculation procedures are similar to those illustrated in Example 13-6. A difference equation is written for each component, and these equations are solved simultaneously with the difference equation for the conservation of energy. Froment has used one- and two-dimensional models to predict conversion and temperatures in a fixed-bed reactor for the oxidation of o-xylene to phthalic anhydride, CO, and COj, with a V2O5 catalyst. The reaction scheme is... 

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. 

Thermal influences can often affect the catalyst compositian. In many cases one or more metastable phases are formed from the active components or the support materials. Phase changes can limit the catalyst activity or lead to catalyst- bstrate interactions. We have already dealt with the transformation of y-Al203 into a-Al203 with its lower surface area. Another example is the phase transformation of Ti02 from anatase to rutile in V2O5/ Ti02/corundum catalysts for the oxidation of o-xylene to phthalic anhydride. 

A reaction related to cleavage is aromatic ring degradation. Phthalic anhydride used to be prepared from naphthalene, and maleic anhydride from benzene, with air and V2O5 catalyst, losing 2 carbon atoms as CO2. More modern processes use o-xylene and butadiene (or -butane) respectively. However, the analogous formation of quinolinic (pyridine-2,3-dicarboxylic) acid from quinoline is still practised. Hypochlorite [94], hypochlorite or chlorite/Ru catalyst [95], or simply alkaline H2O2 [96] can be used. 

This reaction model is fairly representative of the gas-phase air oxidation of o-xylene into phthalic anhydride on V2O5 catalysts A represents o-xylene, B phthalic anhydride, and C the final oxidation products CO and CO2 lumped together. The process conditions were already described in Section 11.5.2. The purpose of this example is mainly to check whether or not radial temperature... 

In the industrial production of phthalic anhydride, the oxidation of either naphthalene or o-xylene can be utilized. The reaction mechanism in the oxidation of o-xylene over V2O5 catalyst particles can be described in a simplified maimer by a parallel-consecutive reaction, which leads to the following rate expressions ... 

Dias, C., Portela, M. and Bond, G. (1996). Oxidation of o-Xylene to Phthalic Anhydride over V2O5/T1O2 Catalysts. Part 4. Mathematical Modelling Study and Analysis of the Reaction Network, J. Catal, 164, pp. 276-287. 

Wachs, I., Chan, S. and Saleh, R. (1985). The Interaction of V2O5 with Ti02(Anatase) II. Comparison of Fresh and Used Catalysts for o-Xylene Oxidation to Phthalic Anhydride, J. Catal.,... 

The oxidation of CH3OH to HCHO is considered as a probe reaction for other selective oxidation reactions such as butane to maleic anhydride, o-xylene to phthalic anhydride, and ODH of alkanes to alkenes. Consequently, the concepts developed for the selective oxidation of methanol over vanadium oxide catalysts can be easily transferred to other catalytic reactions. Weckhuysen and Keller [82] carried out methanol oxidation as a probe reaction over various V2O5/S oxides (S = HO2, Zr02, Nb205, Ce02, and AI2O3). The relative independence of turnover frequency (TOP) to vanadia loading on amorphous oxide supports indicated that the reaction was first order with respect to surface vanadium oxide site. 

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