MECHANISM OF THERMAL-DECOMPOSITION OF POTASSIUM AMMONIUM-SALTS OF THE12-MOLYBDOPHOSPHORIC ACID AND EFFECT ON THE CATALYTIC PERFORMANCE IN THE ISOBUTYRIC ACID OXIDEHYDROGENATION

Heteropolycompounds of the composition Kx(NH4)3-xPMo12O40 were prepare d by precipitation, through addition of HNO3 to an aqueous solution of the salts. The precipitated compounds were calcined at increasing tem peratures to evaluate the thermal behavior, and were characterized by means of X-ray diffraction, FTIR spectroscopy, surface area measuremen ts, ionic chromatography, EPR spectroscopy, and reactivity in the oxid ative dehydrogenation of isobutyric acid to methacrylic acid. A monoph asic system was obtained by calcination at 640 K, with patterns charac teristic of the Keggin-type cubic secondary structure. The cationic co mposition was found to be very close to that expected for the complete salification of the Keggin anion. The structural decomposition began at temperatures close to 693 K for the ammonium and ammonium/potassium compounds, with formation of an intermediate heteropolycompound chara cterized by crystallographic parameters and cationic composition diffe rent from the original one and more stable than the latter. The potass ium salt remained instead structurally intact up to 753 K. The data we re interpreted with the assumption that the decomposition of the heter opolycompound might occur in a stepwise way, with an initial structura l collapse of a part of the compound and displacement of some molybden um ions (originally located in peripheral position in the Keggin anion ) in the cationic position of the not yet decomposed compound. The hig hest amount of this intermediate molybdenum-salified heteropolycompoun d apparently formed during decomposition of the (NH4)3PMo12O40 compoun d. The calcination temperature remarkably affected the catalytic behav ior in the oxidative dehydrogenation of isobutyric acid to methacrylic acid. The partial structural collapse occurring by calcination at 693 K led to a decrease in the activity for the potassium/ammonium mixed salts. On the contrary, the ammonium salt increased its activity. It w as assumed that this phenomenon could be due to the formation of the i ntermediate molybdenum-salified heteropolycompound. (C) 1994 Academic Press, Inc.