Copper-based ternary hydrotalcite as a catalyst for hydroxylation of phenolic compounds

Hydrotalcites with different cations were synthesized by co-precipitation method and characterized using different techniques. The catalytic activity of the hydrotalcites was studied using hydroxylation of phenol by H2O2 as a model reaction. The CuNiAl ternary hydrotalcite was found to be the best catalyst in this work; and the effects of reaction temperature, ratio of substrate:catalyst, H2O2 concentration, solvent and pH value of the medium were investigated. The results showed that under optimal reaction conditions, the conversion of phenol can reach up to 62.8%, with catechol and hydroquinone as the main products. The substrate scope was expanded to other phenol-derivatives, such as m-cresol and 3-ethylphenol. A plausible reaction mechanism was proposed based on the control reactions and XPS analysis. The CuNiAl hydrotalcite shows high toxicity (the 48 h LC50 value of CuNiAl hydrotalcite was 2.12 mg/L) in the acute toxicity tests of Daphnia magna. However, after simple filtration of the solid catalyst, the 48 h LC50 value of the remaining solution was increased to 83.46 mg/L. In all, the as-synthesized CuNiAl35 hydrotalcite offers a novel biodegradation pathway for in situ treatment of phenol-contaminated water or other phenolic pollutions when applied together with dioxygenases for further degradation of resulting catechols or hydroquinones.