JOURNAL OF CATALYSIS 174, 231–241 (1998) ARTICLE NO. CA 981983 Effect of Antimony on the Chemical–Physical Features and Reactivity in Isobutyric Acid Oxidehydrogenation of Keggin-Type Heteropolycompounds F. Cavani, A . Tanguy, F. Trifir ` o, and M. Koutrev 1 Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy E-mail: cavani@ms.fci.unibo.it Received September 23, 1997; revised November 24, 1997; accepted November 24, 1997 Potassium/ammonium salts of 12-molybdophosphoric acid were modified by the addition of Sb 3+ salt, and the chemical–physical features as well as the reactivity in isobutyric acid oxidehydrogena- tion of the compounds obtained were studied. All the salts prepared were monophasic and the secondary structure of the compounds was characterized by a cubic crystallographic cell. The addition of antimony led to a considerable increase in the thermal structural stability of the compounds. A second effect was on the average de- gree of oxidation of molybdenum, which decreased in proportion to the increase in antimony content. The results of chemical–physical characterization were interpreted by hypothesizing that antimony ions occupy a cationic position in the secondary structure of the heteropolycompound in the case of the salts containing 1 atom of potassium per Keggin unit (KU), and electronically interacts with molybdenum in the Keggin anion, leading to the formation of Mo 5+ species. In the case of samples containing three atoms of potassium per KU the antimony is present in the form of dispersed salt or oxide. The modification of the molybdenum redox properties led to considerable worsening of the catalytic performance in isobutyric acid oxidehydrogenation, with a decrease in activity (for the sam- ples with one atom of potassium per KU) and also in selectivity to methacrylic acid (for the samples with three atoms of potassium per KU). This effect was interpreted as due to the stabilization of a lower degree of oxidation of molybdenum in antimony-containing samples. c  1998 Academic Press INTRODUCTION Heteropolycompounds are finding increasing applica- tions as catalysts for both gas-phase and liquid-phase se- lective oxidation reactions (1–7). They are industrially ap- plied for the oxidation of methacrolein to methacrylic acid and are well known to be efficient also in the oxidehy- drogenation of isobutyric acid to methacrylic acid, one of the possible synthetic routes, alternatives to the acetone- cyanohydrin process for methylmethacrylate production 1 OnleavefromtheInstituteofChemicalPhysics,AcademyofSciences, Ul Kossygina 4, 117334 Moscow, Russia. (8–16). Their polyfunctionality (they are both acidic and multielectron oxidizing systems) makes them potentially interesting also for reactions of paraffin oxidation (17–32). The heteropolycompounds more frequently utilized are the Keggin-type ones, and in particular, 12-molybdophos- phoricand 12-tungstophosphoricacids,modified bythe ad- dition of different transition metal ions. These ions can either be positioned outside the Keggin unit (KU) in the secondary structure in the form of cations, or directly rep- lace molybdenum or tungsten in the anion (as in the case of vanadium). These ions sometimes have a considerable effect on the catalytic behaviour, being directly involved in the oxidation mechanism, influencing the redox properties ofthe compound (13),or modifyingitsacid-base properties (33). A thorough study of the chemical–physical features and reactivity of salts of Keggin-type heteropolycompounds was done by several authors (34–41). For instance, in the case of isobutyric acid oxidehydrogenation, the nature of the cation, when chosen among ammonium, alkali, or alka- line earth metals, was found to have a considerable effect on the catalytic activity and selectivity. Some authors pro- posed that the observed effectsare related to modifications in the microporosity and acidity of the samples (35–41); other authors (13, 14) found a correlation between the oxi- dizingactivityofthe compoundsand the standard electrode potential of the corresponding cations when these are tran- sition metal ions. Among the transition metals, the effect of vanadium (in both cationic and anionic position), zinc, chromium, nickel, copper and iron on the catalytic performance of heteropolycompoundsin gas-phase oxidation reactionshas been studied (20–25, 31–33, 42, 43). Very few studies have been devoted to the effect of antimony on the catalytic properties of heteropolycompounds in reactions of selec- tive oxidation, even though antimony is a well-known com- ponent of catalysts for this class of reactions, such as in VSbO 4 for propane ammoxidation, for alkylaromatic am- moxidation, and in systems for allylic oxidation. To our 231 0021-9517/98 $25.00 Copyright c  1998 by Academic Press All rights of reproduction in any form reserved.