Enhanced catalytic phenol hydroxylation by CuZnFeAl layer double hydroxides: synergistic effects of Cu+ and oxygen vacancies

In this work, a series of CuZnFeAl-LDH catalysts for phenol oxidation to dihydroxybenzene have been prepared through a co-precipitation method. Versatile characterization studies are applied to reveal electron transfer from oxygen vacancies to Cu2+ on the LDH surface. The resulting Cu+ benefits the formation of hydroxyl radicals to promote the catalytic activity. Besides, through inverse gas chromatography (IGC), the acid–base hydrotalcite surface can be quantitatively determined. Both the oxygen vacancies and acid–base ratio (Ka/Kb) abide by a volcano-like tendency with the addition of copper content, which is consistent with the catalysis result. Among all these catalysts, 15/CuZnFeAl-LDH presents the optimal conversion (66.9%), selectivity (71.3%), and stable recyclability under mild conditions (60 °C, 1.0 MPa), respectively, and is environmentally-friendly and energy efficient. The high efficiency of this catalyst is mainly attributed to the synergistic effect between Cu+ and oxygen vacancies promoted by Ka/Kb.