303 V 2 O 5 -ZrO 2 catalyst for selective oxidation of o-xylene to phthalic anhydride: I. Catalyst preparation, catalytic activity and selectivity measurements L. Makedonski* Medical University, 55 Marin Drinov St., 9002 Varna, Bulgaria Received May 16, 2008; Revised January 15, 2009 The thermal stability, activity and selectivity of a series of V 2 O 5 -ZrO 2 catalyst samples towards o-xylene oxidation to phthalic anhydride have been investigated. It has been established that the phthalic anhydride content yield reaches about 55 mol.% in samples containing 7 and 10 wt.% V 2 O 5 . The selectivity of the catalyst with 7 wt.% V 2 O 5 -ZrO 2 has been compared to that of commercially available V 2 O 5 -TiO 2 (anatase). The investigations have been performed within two temperature ranges. At low temperatures (below 410°C) the V 2 O 5 -TiO 2 (anatase) catalyst shows a better selectivity to phthalic anhydride than the V 2 O 5 -ZrO 2 catalyst does. At high temperatures, the V 2 O 5 -ZrO 2 catalyst is more selective with respect to the main product of partial oxidation due to the positive effect of the ZrO 2 support. Even when the temperature rises up to 550°C and the time of exploitation becomes 50 h, the selectivity of V 2 O 5 -ZrO 2 decreases slightly and remains above 50 mol.%, whereas with the V 2 O 5 -TiO 2 (anatase) sample a significant decrease in selectivity (below 45 mol.%) is observed. The results from experiments on the activity and selectivity of a model mixture of 7 wt.% V 2 O 5 - ZrO 2 and pure ZrO 2 taken in a ratio of 1:1 show that high catalyst selectivity is achieved when the zirconia surface is completely covered by VO x -phases. Studies by different physicochemical analysis methods have shown that V 2 O 5 -ZrO 2 used in a high-temperature regime undergoes no significant phase and structural changes, which is an indication of its good thermal stability. The specific surface area decreases a little, the monoclinic zirconia support exhibits no phase changes and the fine structure of the V 2 O 5 active phase displays no substantial alterations. Key words: V 2 O 5 -ZrO 2 catalyst, V 2 O 5 -TiO 2 (anatase) catalyst, partial selective oxidation, o-xylene oxidation, phthalic anhydride, maleic anhydride. INTRODUCTION Vanadium oxide catalysts are frequently used for the oxidation of aromatic compounds [1, 2]. As the specific surface area and catalytic activity are usually low, the active phase is fixed on a support. The role of the support is not only to increase the surface area and to improve the mechanical strength. It might also modify the active surface due to an interaction of the support and the active phase. Among the multitude processes of partial oxidation of aromatic compounds, the process of oxidation of o-xylene is most interesting both in scientific and industrial aspects. Usually such catalysts are sup- ported on TiO 2 in the form of anatase [3] and SiO 2 [2]. In comparison with titanium and silica, much less attention has been paid to zirconium as a sup- port. In our opinion the use of zirkonia is interesting because of its high thermal stability and acid-base character, which is very close to that of TiO 2 . Having in mind that the process of partial oxidation is highly exothermic, these properties of the support are of great significance. In the scientific and patent literature there are few communications concerning the use of ZrO 2 as a support of vanadium oxide catalysts for partial oxidation of o-xylene. There are no published data regarding the selectivity of V 2 O 5 - ZrO 2 catalysts to the valuable side product, maleic anhydride, as well as to the undesired substance phthalide. Supported metal oxides exhibit interesting cata- lytic properties depending on the kind of the sup- port, the active component content and the prepara- tion method [5−7]. In particular, vanadium oxide catalysts combinated with various promoters are widely used for several reactions including oxida- tion of hydrocarbons [8], ammoxidation of aromatics and methylaromatics [9], olefins [10], ethanol and ethane, ammoxidation of 3-picoline and toluene, oxidation of methanol [11] and selective catalytic reduction of NO x by NH 3 [12, 13]. All partial oxidation processes are highly exo- thermic, due to which the support of the V 2 O 5 active component should be carefully chosen. Among the mentioned partial oxidation processes and oxidation ammonolysis, the partial oxidation of o-xylene to phthalic anhydride is most interesting both from industrial and scientific viewpoint [5, 6, 14]. The V 2 O 5 -TiO 2 (anatase) system is the basis of modern catalysts for partial oxidation of o-xylene to phthalic Bulgarian Chemical Communications, Volume 41, Number 3 (pp. 303–312) 2009 © 2009 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria * E-mail: lubomir60@yahoo.com