Potential of Supported Gold Bimetallic Catalysts for Green Synthesis of Adipic Acid from Cyclohexane

Abstract

Adipic acid (AA) is the main raw material for making nylon 6,6 fiber, which is further used in the production of different commercially important products (e.g. coatings, polymers etc.). Herein, we demonstrated a green chemistry route for the one-step oxidation of cyclohexane (CH) to AA using supported bimetallic catalysts containing gold and palladium or silver nanoparticles. The catalysts were prepared in two steps using sol–gel and impregnation techniques and were then characterized by different techniques such as ICP, BET surface area, XRD, TEM etc. Activity tests were carried out using Parr autoclave under the following optimum conditions (400 mg of catalysts, acetonitrile as solvent (5 ml), 0.1 g of TBHP as initiator, 10 bar of O2, 150 °C). XRD patterns revealed that no reflections corresponding to either Au or Pd are present in Au–Pd/TiO2 system, while a weak reflection belonging to metallic Au phase could be seen in Au–Ag/TiO2 catalyst. In terms of catalytic activity, Pd/TiO2 exhibited somewhat inferior performance (X–CH = 16 %, S–AA = 18 %) compared to Au/TiO2 solid (X–CH = 25 %, S–AA = 26 %). Interestingly, the combination of Au and Pd (Pd–Au/TiO2) markedly improved the selectivity of AA from 26 to ca. 34 %. In case of Au–Ag/TiO2, the combination of Ag and Au (i.e. 1 % Au–1 % Ag/TiO2) enhances the conversion of CH and selectivity of cyclohexanone (X–CH = 33 %, S–One = 41 %); compared to monometallic Ag/TiO2 solid. The results showed that the nature of second metal has shown a clear influence on the conversion of CH as well as the product distribution.