STUDY OF MULTIPHASIC MOLYBDATE-BASED CATALYSTS .2. SYNERGY EFFECT BETWEEN BISMUTH MOLYBDATES AND MIXED IRON AND COBALT MOLYBDATES IN MILD OXIDATION OF PROPENE

Results are reported concerning the synergy effect observed in the oxidation of propene to acrolein over bismuth and mixed iron and cobalt molybdates. The pure bismuth, iron, and cobalt molybdates and mixed cobalt and iron molybdates (solid solutions) have been prepared and individually tested as catalysts. Mechanical mixtures of these phases have been prepared and tested as catalysts. All the catalysts have been characterized before and after the catalytic reaction by several techniques such as ESR, XPS, EDX-STEM, TEM, XRD, and Mössbauer and UV spectroscopies. The synergy effect observed is tentatively explained as due to the deposition on the large bismuth molybdate particles of smaller mixed iron and cobalt molybdate particles with spreading of the bismuth molybdate over the latter particles. It is proposed that the FexCo1-xMoO4 phase plays the role of the fast electron conducting material which enhances the electron mobility and the efficiency of the redox mechanism, the active and selective phase being the overlying bismuth molybdate compounds. © 1993 Academic Press, Inc.