Kinetic investigation for the catalytic reduction of nitrophenol using ionic liquid stabilized gold nanoparticles

We demonstrate the synthesis of gold nanoparticles (AuNP) stabilized by 1-butyl-3-hexadecyl imidazolium bromide (Au@[C(4)C(16)Im]Br) and their use as a catalyst for the reduction of nitrophenol. The AuNPs show excellent stability in presence of [C(4)C(16)Im]Br ionic liquids for the reduction of 4-nitrophenol and 2-nitrophenol using NaBH4 as a reducing agent. The detailed kinetics for the reduction of 4-nitrophenol and 2-nitrophenol were investigated and the catalytic activity of Au@[C(4)C(16)Im]Br was evaluated. The pseudo first-order rate constant (k(app)) values for 4-nitrophenol was observed to be greater than that of 2-nitrophenol and explained on the basis of hydrogen bonding present in 2-nitrophenol. Au@[C(4)C(16)Im]Br showed good separability and reusability and hence, it can be used for the complete reduction of nitrophenols in multiple cycles. The Langmuir-Hinshelwood reaction mechanism is elucidated for reduction of 4-nitrophenol by Au@[C(4)C(16)Im]Br nanocatalyst on the basis of the k(app) values. The thermodynamic activation parameters such as activation energy, enthalpy of activation and entropy of activation were determined and explained using the temperature dependent kinetics for the reduction of nitrophenol using Au@[C(4)C(16)Im]Br. The above results reveal that the Au@[C(4)C(16)Im]Br nanocatalyst demonstrates excellent catalytic performance for the reduction of nitrophenol by NaBH4 at room temperature.