Wet-impregnated niobosilicate catalysts for glycerol conversion into solketal

Novel mesoporous niobosilicate materials with a homogeneous dispersion of niobium species were synthesized via impregnation of (NH4)3[Nb(O2)2(edtaO2)]·H2O·H2O2 on different hydrophilic surfaces of porous siliceous supports: extra small silica nanoparticles (XS-SiO2) and silica nanotubes (NTs-SiO2) bearing three different Si/Nb molar ratios of 74, 37 and 18. The Nb-silica catalysts obtained were calcined at 600 °C and then fully characterized by different physico-chemical techniques. The niobium precursor was found to be successfully loaded on the two silica support surfaces without noticeable modification of the silicate structural integrity. The Nb-containing catalysts were applied as active and selective heterogeneous catalysts for acid-catalyzed condensation of glycerol with acetone yielding solketal (2,2-dimethyl-1,3-dioxolane-4-methanol). The catalytic performance of these materials is ascribed to the enhanced accessibility of their active sites given by their morphology and to the suitable combination of acid sites. Moreover, no leaching of active sites was evidenced and the catalyst reusability studies indicated that the Nb-XS-74 and Nb-NTs-74 catalysts were successfully recyclable and highly stable in the acetalization of glycerol. The robustness and stability of the mesoporous Nb-silicate materials were also supported via characterization of the spent catalysts after the fifth recycling.