Complete oxidation of trans-4-pentylcyclohexanecarboxylic acid by catalytic ozonation using Cu-Mg bi-metallic catalysts

Abstract

This thesis studied the use of a bi-metallic Cu and Mg catalyst dispersed on γ-Al2O3 for the complete oxidation of a naphthenic acid (NA) model compound, trans-4- pentylcyclohexanecarboxylic acid (4PCH) by ozone. Mono-metallic Cu and Mg, and bimetallic Cu-Mg catalysts were tested for the ozonation of 4PCH. The catalysts were characterized by ICP-OES, BET, SEM, TEM, XRD, NH3-TPD and XPS. It was found that bi-metallic catalysts with high amount of surface hydroxyl groups and with pHpzc close to the pH of the solution at 8.5 were suitable for the mineralization of 4PCH. The catalyst with 1 and 5 wt% of Cu and Mg loading, respectively, had the highest activity with 90% total organic carbon (TOC) removal at a dosage of 2 g/L in 120 min. The use of tert-butyl alcohol (TBA) confirmed the involvement of OH• radicals in the oxidation mechanism of 4PCH. Mg fine-tuned the pHpzc of the catalyst and provided a high concentration of neutrally charged surface hydroxyl groups to generate reactive oxygen species (ROS) to break down 4PCH in the liquid bulk. Cu participated in the complete oxidation of smaller carboxylic acid molecules through complexation and surface reactions with adsorbed ROS. The by-products of the oxidation of 4PCH were identified and a reaction network was proposed based on a meta-addition mechanism. The most active catalyst retained its TOC removal over 5 reusability cycles with an average Cu, Mg, and Al leaching of 0.07, 1.30 mg/L, and 0.2, respectively. The best performing catalyst was also proven effective for the complete oxidation of a mixture of 4 NA model compounds with an electric energy per order of 78.1 kWh/m3.