Niobium-containing hydroxyapatites as amphoteric catalysts: synthesis, properties, and activity

We present here a new class of Nb-containing catalysts offering both acid and base functionalities thanks to the hydroxyapatite (HAP) structure, which acts as a host matrix for guest niobium species. Incorporation of Nb has led to samples with strengthened amphoteric properties with a prevalence of acid or basic sites depending on the synthesis procedure. Nb-containing HAP samples (12−14 wt % of Nb) have been synthesized by coprecipitation in water and in alcoholic solutions and by impregnation starting from different precursors. The intrinsic and effective acid and base properties have been measured, and a series of mass and surface characterizations have been performed to determine the Nb content, its oxidation state as well as its morphologic, structural, and electronic properties. The performances of the samples have been studied in several reactions of liquid−solid heterogeneous catalysis in batch or fixed tubular reactors working in the recirculation mode on the basis of the known catalytic features of Nb, including hydrolysis, acid-catalyzed isomerization, and selective oxidation reactions.