Utilisation de composés de coordination en vue d'un meilleur contrôle de la synthèse de catalyseurs molybdates de bismuth supportés sur silice

Doctorat en sciences chimiques -- UCL, 199

Bismuth and Mixed Bismuth-lanthanide Carboxylates As Precursors for Pure and Ln-promoted Bismuth Molybdate Catalysts

Several bismuth(III) carboxylates (formate, acetate, propionate, oxalate, lactate) and the analogous tervalent lanthanum, samarium and praseodymium formates and acetates were synthesized and characterized in view of their use as precursors for the formation of bismuth- or lanthanide-based molybdate catalysts upon controlled thermal decomposition on MoO3 support. Mixed Bi-Ln carboxylates were prepared and used in parallel as precursors for Ln-promoted bismuth molybdate catalysts. Both the nature of the lanthanide element and the calcination temperature were found to determine the formation of specific molybdate-type phases on the support. Detailed examination of these catalysts by X-ray diffraction and FTIR spectroscopy identified alpha-M(2)Mo(3)O(12) phases in Bi- and Pr-containing samples calcined at 823 K, and hexamolybdate phases M(2)Mo(6)O(21) in La- or Sm-containing catalysts calcined at 973 K

Bismuth carboxylates as precursors for the incorporation of bismuth in oxide-based materials

The use of bismuth(III) carboxylates (acetate, oxoacetate, lactate, oxalate) as precursors for the incorporation of this element on oxide supports is evaluated as a new tool to generate bismuth-based oxide phases at the surface of MoO3 or WO3 supports. These insoluble precursors are deposited as small particles from a slurry in a liquid hydrocarbon under appropriate experimental conditions, Bismuth molybdate and tungstate phases are produced by solid-state reactions between in situ generated Bi2O3 and the supporting oxide at 673 K. The samples are characterized by specific surface area measurements, X-ray diffractometry, and X-ray photoelectron spectroscopy, The selective oxidation of isobutene to methacrolein is employed as a reaction test to demonstrate the applicability of this approach, The use of precursors containing lactate or acetate-type ligands is found to generate materials displaying enhanced specific surface area and high bismuth-to-molybdenum surface atomic ratios. This bismuth enrichment of the surface is indicative of the presence of dispersed crystallites of either Bi2O3 itself or ternary Bi-Mo-O or Bi-W-O phases, which are known to promote partial oxidation of alkenes, The performances of these materials in the isobutene to methacrolein conversion are indeed in most cases definitely better than those of the so-called reference materials obtained from the simple mixtures of the same oxides prepared separately. (C) 1996 Academic Press, Inc