Gold nanoparticle-decorated halloysite nanotubes – Selective catalysts for benzyl alcohol oxidation

Abstract

Preparation of gold nanoparticle-decorated halloysite nanotubes (Au-Hal) by a deposition method and evaluation of their catalytic activity in an oxidation of benzyl alcohol are reported. An electrostatic attraction between positively charged polyethylenimine (PEI)-capped gold nanoparticles and the negatively charged external surfaces of halloysite nanotubes (Hal nanotubes) was the key factor in fabrication of Au-Hal nanotubes. Au-Hal catalysts showed good conversions and surprisingly high benzaldehyde selectivities (above 90%) in the benzyl alcohol oxidation. Influence of the amount of PEI used as a capping agent, the gold content and calcination of Au-Hal catalysts on the conversion and selectivity was investigated. A notable increase in the benzaldehyde selectivity at higher PEI contents and a significant drop in the benzaldehyde selectivity on calcination clearly indicate the central role of PEI in the selective formation of benzaldehyde. The high benzaldehyde selectivity of uncalcined catalysts are probably due to PEI donor molecules coordinating to certain surface sites on Au nanoparticles and thus blocking the catalytic sites required for further oxidation of benzaldehyde to benzoic acid. The high combined selectivity of benzoic acid and benzyl benzoate obtained with the calcined catalysts indicates that naked gold nanoparticles have the catalytic sites available for the benzoic acid formation.