Abstract
Niobium pentoxides are promising acid catalysts for the conversion of biomass into fuels and chemicals. Developing new synthesis routes is essential for designing niobium pentoxide catalysts with improved activity for specific practical processes. Here we show a synthesis approach in acetophenone, which produces nanostructured niobium pentoxides with varying structure and acidity that act as efficient acid catalysts. The oxides have orthorhombic structures with different extents of distortions and coordinatively unsaturated metal atoms. A strong dependence is observed between the type and strength of the acid sites and specific structural motifs. Ultrasmall niobium pentoxide nanoparticles, which have strong Brønsted acidity, as well as Lewis acidity, give product yields of 96% (3 h, 140 °C, 100% conversion), 85% (3 h, 140 °C, 86% conversion), and 100% (3 h, 110 °C, 100% conversion) in the reactions of furfuryl alcohol, 5-(hydroxymethyl)furfural, and α-angelica lactone with ethanol, respectively.
Highlights
Niobium pentoxides are promising acid catalysts for the conversion of biomass into fuels and chemicals
Thermal treatment is accompanied by dehydration/dehydroxylation of the material, drastic decrease of the surface area, disappearance of the acid sites, and consequent decrease of the catalytic activity[2], making it difficult to correlate the type and strength of active sites with particular structures[2,12,14,15,16,17], which in turn hinders the development of high-performance catalysts for specific practical applications
We present the synthesis of niobium pentoxide acid catalysts by reaction of NbCl5 with acetophenone, which involves the aldol condensation reaction of acetophenone. It leads to highly hydroxylated ultrasmall niobium oxide nanoparticles (Nb2O5NP), spherical assemblies of nanoparticles (Nb2O5-NS), and nanoparticles supported on holey graphene oxide (Nb2O5-hGO), depending on the composition of the reaction mixture
Summary
Niobium pentoxides are promising acid catalysts for the conversion of biomass into fuels and chemicals. Hydrated amorphous niobium pentoxide (Nb2O5·nH2O, niobic acid) contains both Brønsted and Lewis acid sites, and has the ability to promote a large number of chemical reactions (e.g. hydrolysis, dehydration, isomerization, esterification, and etherification) even in the presence of water molecules[1,7,8]. Those reactions are fundamental processes in biorefineries, in which water is likely present either as solvent or reaction side-product[9], making Nb2O5 catalysts promising for biomass conversion to valuable chemicals[1]. Searching for new synthesis routes is essential for producing nanosized niobium pentoxides exhibiting strong acidity, high surface area and different structural motifs, which will allow getting insights into the structure–acidity–activity relationships
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