Self-assembled gold nanofilms as a simple, recoverable and recyclable catalyst for nitro-reduction

Abstract

Hexaazamacrocycle (L) stabilized gold nanoparticles (AuNPs) were prepared by combining L with HAuCl₄·3H₂O in a variety of alcohol–water (1 : 1) mixtures. The dual roles of L as a reducing and stabilizing agent were exploited for the synthesis of AuNPs under the optimized ratio of L to Au³⁺ (2 : 1). Self-assembled gold nanofilms (AuNFs) were constructed at liquid–liquid interfaces by adding equal volumes of hexane to the dispersions of AuNPs in the alcohol–water systems. The nanofilms were formed spontaneously by shaking the two-phase mixture for a minute followed by standing. The alcohols explored for the self-assembly phenomenon were methanol, ethanol, i-propanol and t-butanol. The systems containing methanol or t-butanol resulted in AuNFs at the interfaces, whereas the other two alcohols were found not suitable and the AuNPs remained dispersed in the corresponding alcohol–water medium. The AuNFs prepared under suitable conditions were coated on a variety of surfaces by the dip and lift-off method/solvent removal approach. The AuNFs were characterized by UV-vis, SEM, TEM, AFM and contact angle measurement techniques. A coated glass-vial or cuvette was used as a catalytic reservoir for nitro-reduction reactions under ambient and aqueous conditions using NaBH₄ as the reducing agent. The reduced products (amines) were extracted by aqueous work-up using ethyl acetate followed by evaporation of the organic layer; the isolated products required no further purification. The catalyst was recovered by simply decanting the reaction mixture whereupon the isolated catalyst remained coated inside the vessel. The recovered catalyst was found to be equally efficient for further catalytic cycles.