Selective oxidation of o-xylene to phthalic anhydride over vanadia/titania: A comparative study of integral and recycle reactor operation

Two vanadia/titania catalysts, containing 1 and 8 wt% vanadia on anatase, have been investigated for the selective oxidation of o-xylene to phthalic anhydride. In a comparative study, activity and selectivity in o-xylene oxidation were measured using an integral reactor and a reactor with external product recirculation, with the objective of studying the influence of backmixing in the latter on the reaction behaviour. Adsorbed surface species were investigated by means of in-situ diffuse reflectance FT-IR spectroscopy, using an environmental chamber which corresponds to an integral reactor. No difference in activity and selectivity in o-xylene oxidation was observed between the two catalysts. FT-IR studies showed adsorbed o-xylene, benzoate and a high concentration of phthalic anhydride on the surface of both catalysts. o-Xylene oxidation follows a sequential reaction path, with o-tolualdehyde as the first intermediate and phthalide, phthalic anhydride and maleic anhydride as sequential products. No evidence was found for a direct oxidation-path of o-xylene to phthalic anhydride. Total oxidation products CO and CO₂ are formed by direct oxidation of the o-xylene and by oxidation of the reaction products. Due to greatly reduced concentration and temperature gradients, the recycle reactor proved to be highly suitable for studying the extremely exothermic oxidation of o-xylene.     

Oxidation of o-Xylene to Phthalic Anhydride on Sb-V/ZrO2 Catalysts

Zirconia-supported and bulk-mixed vanadiumantimonium oxide catalysts were used for the oxidation of o-xylene to phthalic anhydride. X-ray diffraction, Raman spectroscopy and photoelectron spectroscopy were used for characterization. It was found that vanadium promotes the transition of tetragonal to monoclinic zirconia. The simultaneous presence of Sb and V on zirconia at low coverage led to a preferential interaction of individual V and Sb oxides with the zirconia surface rather than the formation of a binary Sb-V oxide, while at higher Sb-V contents the formation of SbVO₄ took place. Sb-V/ZrO₂ catalysts showed high activity for o-xylene conversion and better selectivity to phthalic anhydride as compared to V/ZrO₂ catalysts. However, their selectivity to phthalic anhydride was poor in comparison to a V/TiO₂ commercial catalyst. The improved selectivity of the Sb-containing catalysts is attributed to the blocking of non-effective surface sites of ZrO₂, the decrease of the total amount of acid sites and the formation of surface V-O-Sb-O-V structures.     

Facile and not facile reactions for the production of maleic and phthalic anhydrides with vanadium mixed oxides based catalysts

Vanadium is a key element in the formulation of catalysts for the production of anhydrides by selective vapor oxidation with molecular oxygen. A study of the oxidations ofo-xylene,n-butane and benzene on V-P, V-Mo and V-Ti mixed oxides, the commercial catalysts for the synthesis of anhydrides, was carried out. The oxidation ofo-xylene on the three catalysts was the most facile reaction leading to high yields in phthalic anhydride; on the contrary in the oxidation ofn-butane only V-P mixed oxide was active and selective. V-P mixed oxide is active and selective in the oxidation of all the feedstocks and can be identified as the most polyfunctional catalyst. V-Mo is active and selective in botho-xylene and benzene oxidation and V-Ti only ino-xylene oxidation. Moreover the lesser selectivity in phthalic anhydride observed with V-P and V-Mo mixed oxides has been attributed to parallel oxidation reactions to the formation of phthalic anhydride, evidenced by a higher amount of phthalide.     

Anatase-supported vanadium oxide catalysts for o-xylene oxidation: From consolidated certainties to unexpected effects

In this paper some aspects of the reaction of o-xylene oxidation, catalysed by anatase-supported vanadium oxide, are re-examined and compared with the scientific and patent literature. Specifically, the effect of vanadia loading on turnover frequency and on the distribution of products has been investigated, using catalysts having sub-monolayer and above-monolayer vanadium oxide deposited on a high-purity anatase support. It was found that even catalysts having less-than-monolayer vanadia loading, containing isolated or oligomeric VO₄ species, may give good selectivity to phthalic anhydride, provided the support does not catalyze the formation of heavy compounds. Moreover, the effect of water on the reactivity and chemical–physical characteristics of catalysts was studied by means of in situ Raman spectroscopy. Steam promoted dynamic reversible phenomena occurring at the catalyst surface, by facilitating the dispersion of bulk vanadium oxide and therefore increasing the catalyst activity.     

What Limits the Selectivity Attainable in the Catalysed Oxidation of o-Xylene to Phthalic Anhydride?

The degree of selectivity obtained in the selective oxidation of o-xylene to phthalic anhydride by V₂O₅/TiO₂ catalysts under industrial conditions is no more than 75–80%: the mechanism of the non-selective oxidation has commanded little interest. Principal features of the reaction and the catalyst are reviewed: the formation of polymeric by-products at low conversion, when the surface is not highly oxidised, is noted, as these will lead to carbon oxides, but at high conversion it is probable that adsorbed intermediate species may undergo side-reactions with reduced V sites, or exposed Ti ions. The resulting strongly-held species can only lead to carbon oxides, so that limiting factors may be (i) degree of completeness of the V₂O₅ monolayer, and (ii) the rate of re-oxidation of reduced surface. © 1997 SCI.     

Investigation of the oxidation of o-Xylene over a Vanadia/Titania catalyst by means of the TAP reactor

The oxidation of o-xylene over a commercial V₂O₅/TiO₂ (anatase) catalyst was studied in the TAP (Temporal Analysis of Products) reactor system. The transient method with high time resolution has led to new insights into the reaction mechanism. The selective oxidation is shown to consist of a sequence of steps, involving intermediates which desorb as o-toluolaldehyde and phtalide. The key reaction step in the non-selective oxidation is shown to be a decar-bonylation of a surface species, leading to a surface benzoate. The kinetics of the elementary steps in the reaction were derived from a quantitative analysis of the transient responses on o-xylene pulses.     

Selective oxidation of toluene ando-xylene on MgF2-supported monolayer vanadium oxide catalyst

Information on the reactions of selective oxidation of toluene ando-xylene on monolayer V₂O₅/MgF₂ catalyst was obtained using IR spectroscopy. Also adsorption of aldehyde derivatives of these reagents, i.e. benzaldehyde ando-tolualdehyde, was investigated in order to identify intermediate species, which were formed during the oxidation process. The measurements were carried out at different temperatures, under conditions similar to that of the catalytic reaction. On the surface of the catalyst many intermediate species, as aldehyde-like, keton-like and benzoate species, were detected. On the basis of the data obtained the mechanism of these reactions has been proposed.     

Synthesis of Phthalic Anhydride: Catalysts,Kinetics, and Reaction Modeling

Information concerning the oxidation of o-xylene and naphthalene, the two main processes for producing phthalic anhydride, is updated and analyzed. New techniques for the preparation of catalysts, all based in the impregnation method and involving the control of parameters such as pH and ionic strength of solutions, are described; the performance of the resulting catalysts is compared with that of catalysts prepared by other methods. Sulfur-containing substances and promoters such as Ag, P, Nb, and Sb have been shown to enhance catalyst performance; studies of their effect on the surface area, acidic properties, and stabilization of the oxidation state of vanadium in supported V₂O₅ catalysts are described.

The latest attempts to correlate the physicochemical characteristics of the catalysts with their catalytic features are analyzed. FTIR, Raman spectroscopy, adsorption of bases, ⁵¹V-NMR, XRD, XPS, SIMS, and electrical conductivity have been used in the study of V₂O₅/TiO₂ catalysts, allowing further understanding of the effects of the properties such as acidity and the state of oxidation of the surface. Particular emphasis has been given to the presence of V^IV, which is thought to cause lower selectivity to phthalic anhydride.

For o-xylene oxidation, the formation of involatile by-products has been established as a secondary reaction, accounting for the poor carbon balances obtained under some experimental conditions. Involatile by-products, whose formation has been associated with the presence of strong acid sites, can adsorb on the catalyst surface, leading to deactivation, or undergo total combustion, acting as a source of CO₂. Attempts to quantify and characterize those by-products are described.

The modeling of the reaction using both fixed- and fluidized-bed reactors, including the study of parameters such as the inlet temperature and the bath temperature, is analyzed. Models considering catalyst deactivation have been also developed; for o-xylene oxidation, deactivation has been associated with processes both reversible, such as changes in the oxidation state of vanadium, deposition of involatile compounds, and irreversible, such as structural changes, decrease in surface area, sintering, and variation of the promoter concentration at the catalyst surface.

The study of V₂O₅/TiO₂ EUROCAT catalysts, involving a number of European laboratories, is reviewed, and their performance is compared with that of other V₂O₅/TiO₂ catalysts.     

Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds

Catalytic oxidations of volatile organic compounds (VOCs) (benzene, toluene and o-xylene) over 1 wt% Pd/γ-Al₂O₃ catalyst were carried out to assess the properties and performance of the Pd based catalyst. The properties of the prepared catalysts were characterized by the Brunauer Emmett Teller (BET) surface area, H₂ chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The experimental results revealed a significant increase in VOCs conversion with the lapse of the reaction time at certain reaction temperatures. On the other hand, the hydrogen pretreated 1 wt% Pd/γ-Al₂O₃ catalyst, whose shape of conversion curve is similar to the non pretreated catalyst, led the conversion curves for the total oxidation of VOCs to be shifted to lower temperature. It was also found that such increases in VOCs conversion were highly dependent on the oxidation state of Pd and the growth of Pd particles in the catalyst. In addition, in the case of the catalyst consisting of the same oxidation state (PdO/Pd²⁺ or Pd⁰), the particle sizes possibly play a more important role in the catalytic activity. The activity order of 1 wt% Pd/γ-Al₂O₃ catalyst with respect to the VOC molecule was o-xylene > toluene > benzene.     

Effect of the reaction bath temperature in a fixed-bed reactor for oxidation of oxylene over V2O5/TiO2 catalysts

The effects of the reaction variables in the operation of a fixed-bed reactor for oxidation ofo-xylene over V₂O₅/TiO₂ catalysts were studied experimentally using a bench reactor. Reaction temperature, feed flow rate and feed concentration ofo-xylene were found to have significant effects on the product distribution and the temperature profile in the reactor. Drastic enhancements ofo-xylene oxidation reaction were observed at some conditions, which was ascribed to the effect of heat accumulated in the bed and indicated a possible way to increase the productivity in the industrial condition. This paper was presented at the 8th APCChE (Asia Pacific Confederation of Chemical Engineering) Congress held at Seoul between August 16 and 19, 1999.     

The contribution of homogeneous and non-oxidative side reactions in the performance of vanadyl pyrophosphate,catalyst for the oxidation of n-butane to maleic anhydride,under hydrocarbon-rich conditions

The reactivity of vanadyl pyrophosphate, catalyst for the selective oxidation of n-butane to maleic anhydride, was examined under n-butane-rich conditions, simulating a feed in which oxygen is the limiting reactant, and a process in which the unconverted n-butane is recycled. A lower selectivity to maleic anhydride was found with respect to the hydrocarbon-lean conditions, due to the higher selectivity to carbon oxides and to the formation of C₈ by-products: tetrahydrophthalic and phthalic anhydrides. The latter compounds formed by a consecutive reaction of maleic anhydride with the unsaturated C₄ intermediates. This occurred under conditions of total oxygen conversion, due to the decreased catalyst oxidizing property. A relevant contribution of radicalic, homogeneous reactions was also observed, which mainly led to the formation of carbon oxides and olefins. This contribution decreased in the presence of the catalyst, which acted as a radical scavenger, but nevertheless remained important at temperatures higher than 400 °C. When conditions were used under which the conversion of oxygen was not total, olefins generated in the gas phase reacted at the catalyst surface yielding maleic anhydride. This homogeneously initiated heterogeneous process led to an unusual effect, of a relevant increase of maleic anhydride yield over 400 °C.     

Oxidation of o-xylene on mesoporous Ti-phosphate-supported VOx catalysts and promoter effect of K+ on selectivity

The selective oxidation of o-xylene on catalysts based on mesoporous titanium phosphate-supported vanadium oxide has been studied. The catalysts were characterized by different physico-chemical techniques (XRD, XPS, N₂ isotherms, TPD of chemisorbed NH₃ and Raman spectroscopy). The conversion and yield to phthalic anhydride increased with vanadium oxide loading up to one theoretical monolayer. Beyond this point, no substantial improvement was achieved. Moreover, a significant improvement in the yield of phthalic anhydride, coinciding with a decrease in the formation of carbon oxides, was observed when potassium was incorporated to the vanadium catalysts. The enhancement of the phthalic anhydride yield was correlated with the neutralization of the stronger surface acid centers by the K⁺ ions, providing evidence for the hypothesis that stronger acid sites are involved in the formation of total oxidation products (carbon oxides).     

The prediction of the performance of packed-bed catalytic reactors in the air-oxidation of o-xylene

The kinetics of the catalytic air-oxidation of o-xylene to phthalic anhydride over various commercial V₂O₂ and TiO₂-V₂O₂ catalysts are extended and a model derived which correctly predicts the temperature and yield profiles in a quasi-isothermal fixed-bed reactor of commercial dimensions. The model predicts the relative freedom from thermal “run-away” of commercial fixed-bed reactors and is used to demonstrate the possibilities of operation at elevated Xylene concentrations.In what follows, XH, TA, PI and PA refer to o-xylene o-tolualdehyde, phthalide and phthalic anhydride respectively, sometimes abbreviated to A, B, C and D.     

A new approach to the oxidation of o-xylene into phthalic anhydride in a fixed bed of vanadium-titanium catalyst: Ein neues Verfahren der in einem stationären Festbettreactor an einem Vanadium—Titan katalysierten Oxidation von o-Xylol zu Phthalsäureanhydrid

A new approach is proposed to feeding in the input mixture for the highly exothermic processes of partial oxidation. It is shown experimentally, by oxidating o-xylene into phthalic anhydride in a pilot non-isothermal integral reactor with a fixed bed of catalyst, that an additional feed-in of o-xylene beyond the hot spot considerably increases the productivity of the process. It is also shown that, if there is more than one feed-in, the use of two beds of industrial catalysts of different activity also increases the productivity with a lower expenditure on catalyst.     

n-octane dehydrocyclisation on monofunctional and bifunctional Pt/Al2O3 catalyst

The dehydrocyclisation of n-octane to iso-octane, ethylbenzene and o-, m- and p-xylene was investigated on monofunctional (non-acidic) and bifunctional (acidic) Pt/Al₂O₃ catalyst in a microcatalytic reactor with hydrogen as carrier at 1.8 atm and 563–673 K. On bifunctional Pt/Al₂O₃, the total conversion of n-octane started from a high value and decreased with increasing temperature for all pulse sizes investigated. The primary product of n-octane conversion on acidic Pt/Al₂O₃ was iso-octane. The product yield-temperature profiles showed a large initial production of iso-octane which decreased to a minimum as the catalyst temperature increased due to its conversion to ethylbenzene and o-xylene. On non-acidic Pt/Al₂O₃, the total conversion of n-octane increased initially and then went through a maximum as the catalyst temperature increased. The primary products of the reaction were found to be ethylbenzene and o-xylene, indicative of the activity of the metal to effect these ring closure reactions.     

The partial oxidation of crotonaldehyde over V–Mo–O catalyst

The partial oxidation of crotonaldehyde, furan, crotonic acid and maleic anhydride was carried out over V–Mo–O _x catalyst in the temperature range 260–360 °C. The dependence of selectivity to main reaction products on the conversion of crotonaldehyde confirmed the parallel character of furan and maleic anhydride formation. The effect of water vapor on furan formation was explained by ability of the catalyst to isomerize crotonaldehyde. The reaction scheme of the oxidation of crotonaldehyde was suggested based on the experimental data.     

The catalytic performance of Cs-doped V/Ti/O catalysts in the oxidation of o-xylene to phthalic anhydride: a TPR/TPO and reactivity study

Titania-supported vanadia (V/Ti/O) systems were modified by addition of cesium oxide for application as catalysts in the selective oxidation of o-xylene to phthalic anhydride (PA). Catalytic tests demonstrated that cesium is a strong promoter of the activity and selectivity to PA, but this effect is evident only under well-defined reaction conditions. Samples with a Cs content lower than 0.35 wt.% Cs₂O exhibited a considerable increase in conversion as compared with the undoped V/Ti/O system. Catalytic tests made with varying o-xylene and oxygen concentrations in the feed demonstrated that in Cs-doped V/Ti/O catalysts the rate-determining step is the re-oxidation of vanadium by molecular oxygen. Thermal-programmed reduction (TPR) and thermal-programmed re-oxidation (TPO) tests evidenced that the addition of Cs decreases the vanadium reducibility and increases the re-oxidizability of the reduced vanadium sites. The positive effect of Cs on selectivity to PA was evident only for o-xylene concentrations in feed lower than 1.5 mol%.     

Modified Mo/V/W‐Mixed Oxides for Catalytic Tar Removal from Biosyngas via Oxidation

In view of the growing energy demand and the current climate problems, renewable energy sources are becoming increasingly popular. The so‐called biosyngas obtained from the gasification of dry biomass can be used to synthesize fuels and basic chemicals. Besides gaseous by‐products and salts, this gas contains tar which needs to be removed before downstream processing. Catalysts like Mo/V/W‐oxides were found to be inert towards the oxidation of CO and H₂ but are able to activate the tar model compound naphthalene highly selective. Unfortunately, besides the total oxidation, the partial oxidation to the undesired intermediates phthalic acid anhydride and maleic acid anhydride takes place. Modification of the catalyst with bases leads to synergetic effects on the catalyst surface, the total oxidation is promoted, and the formation of intermediates decreases.     

Kinetics of o-Xylene oxidation over sintered vanadium pentoxide in a spinning catalyst basket reactor

Kinetics of vapor phase oxidation of o-xylene has been studied over sintered and compacted vanadium pentoxide in a continuous stirred tank catalytic reactor in the temperature range of 450–517°C at atmospheric pressure. The major product obtained is phthalic anhydride. The other products are maleic anhydride and carbon dioxide. The reaction rate data are well represented (with average absolute deviation less than 6%) by the following expression derived by applying the steady-state oxidation-reduction model of Mars and van Krevelen to a parallel reaction scheme and assuming first order with respect to both o-xylene and oxygen: Significantly, the activation energies for all three postulated reactions with rate constants k₁, k₂, and k₃ turn out to be identical having a value of 14.8 kcal/mole, which may be taken to imply that there is only one rate-influencing reaction step for all the products and not three as assumed in deriving this equation.