Total oxidation of naphthenic acids by catalytic ozonation

Abstract

This study investigated the mineralization of a model naphthenic acid compound, 4-methyl-1-cyclohexane carboxylic acid (4MCH), by catalytic ozonation. A catalyst screening was conducted using Cu, Ce, and Mn dispersed on γ-Al2O3, TiO2, ZrO2, and MgO to formulate an active catalyst for total oxidation of 4MCH. Cu/MgO (1 and 5 wt%) catalysts had the highest total organic carbon (TOC) removal. The catalysts maintained their TOC removal over five reusability cycles. The effect of various operating parameters, such as the pH of the reaction media, inlet gas phase ozone concentration, and type of buffer, was studied on the TOC removal of the Cu/MgO catalysts. To improve the activity of the Cu/MgO catalysts, novel copper-magnesium phosphate catalysts (CuxPyMg) were synthesized with different molar ratios of Cu to Mg (i.e., x) and P to Mg (i.e., y). The crystalline structure of the CuxPyMg catalysts was made of Mg(OH)2, Mg3(PO4)2·zH2O (z = 5 and 8), and MgNH4PO4·H2O. Cu atoms partially substituted Mg atoms and formed bonds with phosphorus atoms via lattice oxygen atoms. The Cu0.01P0.1Mg catalyst had the highest TOC removal with the lowest leaching of Cu and P. The P=O functional group of the Cu0.01P0.1Mg catalyst, combined with the atomic dispersion of Cu, resulted in 12 times faster TOC removal rates when compared to those of the Cu/Mg catalyst. A probe-based kinetic study was conducted to quantify the contributions of various reactive oxygen species involved in the oxidation of 4MCH. 57% of 4MCH was oxidized by hydroxyl radicals, while the contributions of ozone and superoxide radicals were 33% and 10%, respectively. Singlet oxygen had no reactivity with 4MCH. The by-products of the oxidation of 4MCH were identified, and a reaction network was proposed for the oxidation of the molecule. It was proposed that the early stages of 4MCH oxidation involved the formation of oxygenated and hydroxylated intermediates, which ultimately decomposed into carbon dioxide and water. In conclusion, this research established that Cu/MgO and CuxPyMg catalysts were suitable catalysts for total oxidation of recalcitrant naphthenic acids.