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 an alyzed. New techniques for the preparation of catalysts, all based in the impregnation method and involving the control of parameters such a s pH and ionic strength of solutions, are described; the performance o f the resulting catalysts is compared with that of catalysts prepared by other methods. Sulfur-containing substances and promoters such as A g, P, Nb, and Sb have been shown to enhance catalyst performance; stud ies of their effect on the surface area, acidic properties, and stabil ization of the oxidation state of vanadium in supported V2O5 catalysts are described. The latest attempts to correlate the physicochemical c haracteristics of the catalysts with their catalytic features are anal yzed. FTIR, Raman spectroscopy, adsorption of bases, V-51-NMR, XRD, XP S, SIMS, and electrical conductivity have been used in the study of V2 O5/TiO2 catalysts, allowing further understanding of the effects of th e 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-xylen e oxidation, the formation of involatile by-products has been establis hed as a secondary reaction, accounting for the poor carbon balances o btained under some experimental conditions. Involatile by-products, wh ose formation has been associated with the presence of strong acid sit es, can adsorb on the catalyst surface, leading to deactivation, or un dergo total combustion, acting as a source of CO2. Attempts to quantif y and characterize those by-products are described. The modeling of th e reaction using both fixed-and fluidized-bed reactors, including the study of parameters such as the inlet temperature and the bath tempera ture, is analyzed. Models considering catalyst deactivation have been also developed; for o-xylene oxidation, deactivation has been associat ed with processes both reversible, such as changes in the oxidation st ate 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 s tudy of V2O5/TiO2 EUROCAT catalysts, involving a number of European la boratories, is reviewed, and their performance is compared with that o f other V2O5/TiO2 catalysts.