Direct hydroxylation of aromatics to their corresponding phenols catalysed by H-[Al]ZSM-5 zeolite

The direct hydroxylation of benzene and its derivatives with N2O is studied using H-[Al]ZSM-5 zeolite as catalyst. The activity of the H-[Al]ZSM-5 zeolite is increased by hydrothermal treatment. This treatment dealuminates the zeolitic framework and creates Lewis acidic extra framework alumina (EFA) which controls the catalytic activity. It is shown by 27Al MAS NMR and IR measurements that the degree of dealumination correlates with the amount of EFA species formed and the yield of phenol obtained. Simple calcination of the H-[Al]ZSM-5 zeolite for a longer time (18 h/550°C) increases the selectivity whereas conversion remains unchanged. Furthermore, benzene derivatives, naphthalene and biphenyl were used as substrates in the direct hydroxylation with N2O. The selectivity to the corresponding phenols is lower than in the case of benzene because the functional groups participate in the reaction mechanism. No correlation is found between the activation or deactivation of the aromatic ring by the functional group and the yield of the corresponding hydroxylated product. In case of naphthalene and biphenyl the yield is limited by substrate decomposition.