Carboxylate-Substituted Alumoxanes as Processable Precursors to Transition Metal−Aluminum and Lanthanide−Aluminum Mixed-Metal Oxides: Atomic Scale Mixing via a New Transmetalation Reaction

Abstract

Carboxylate−alumoxanes, [Al(O)x(OH)y(O2CR)z]n, have been uniformly doped with a transition metal or lanthanide. Thermolysis of the doped alumoxanes yields homogeneous mixed-metal oxides. The carboxylate−alumoxanes were synthesized by the reaction of boehmite, [Al(O)(OH)]n, with either hexanoic, octanoic, methacrylic, or [(methoxyethoxy)ethoxy]acetic acids. Reaction of the carboxylate alumoxane with either a metal acetylacetonate complex, M(acac)3 (M = Mn, La, Er), V(O)(acac)2, or a salt, [NH4]2[Ce(NO3)6] and [NH4]4[Ce(SO4)4], results in transmetalation and the formation of a doped-alumoxane. Upon thermolysis these doped-alumoxanes result in homogeneous mixed-metal oxides. The formation of single-phase materials is similar to that of traditional sol−gel methodologies; however, the alumoxane precursors are indefinitely stable, in solid and solution. In addition, these precursors show no propensity to segregation or polymerization and are readily processed in aqueous or hydrocarbon medium. The stability of the doped alumoxanes and uniformity of the subsequent oxide is a consequence of the atomic level mixing produced by a unique substitution reaction. A transition-metal or lanthanide ion is exchanged for an aluminum ion within the alumoxane nanoclusters and the elimination of Al3+ as Al(acac)3 or [Al(H2O)6]3+. Evidence for this reaction is obtained from NMR and IR spectroscopy. The doped-alumoxane precursors have been characterized by IR and NMR spectroscopy and TG/DTA. The resulting mixed-metal oxide ceramics have been characterized by scanning electron and scanning electron microscopy (SEM), ESR, microprobe analysis, and X-ray diffraction.