Oxidation of aromatic hydrocarbons with hydrogen peroxide over Zn,Cu,Al-layered double hydroxides

Layered double hydroxides (LDHs) with general formula [(Zn1−yCuy)1−xAlx(OH)2](NO3)x·nH2O (x≅0.33, 0.6 ≤y≤1.0, n≅0.5) have been synthesized and characterized by XRD, chemical analysis, and ESR. According to ESR, copper ions exist both as isolated species and as clusters, their relative contribution depending on the overall Cu content. The tendency to form clusters is observed even at a very low absolute Cu concentration. The materials are active in the low temperature liquid phase oxidation of toluene, o-, m-, and p-xylene with hydrogen peroxide. The catalytic reaction involves both the side chain oxidation and the hydroxylation/oxidation of the aromatic ring. The catalytically active centres are formed by isolated copper species. The clustered copper ions are inactive in the oxidation reaction but accelerate the non-productive decomposition of hydrogen peroxide. The catalytic performance is discussed in terms of competition between the process of H2O2 decomposition and its consumption in the oxidation reaction.