Enhanced Activity and Selectivity in the One-Pot Hydroxylation of Phenol by Pd/SiO2@Fe-Containing Mesoporous Silica Core–Shell Catalyst

A novel approach to enhance the catalytic performances in one-pot hydroxylation of phenol into catechol and hydroquinone was examined using core–shell type catalysts which consist of Pd nanoparticles (NPs) for direct synthesis of H2O2 from H2 and O2 and Fe-containing mesoporous silica (Fe-MS) for hydroxylation of phenol using in situ produced H2O2. We have newly prepared three types of core–shell catalysts: (i) Pd/SiO2 core covered with Fe-MS (Pd/SiO2@Fe-MS), (ii) SiO2 core covered by Fe-MS with randomly distributed Pd NPs within the mesoporous silica structure (SiO2@Pd(R)/Fe-MS), where (R) indicates random, and (iii) SiO2 core covered by Fe-MS with Pd NPs deposited on the outer surface of the mesopores SiO2@Pd(S)/Fe-MS, where (S) indicates the surface. The effects of the relative position between the Pd NPs and Fe-MS on the catalytic activity and the selectivity were investigated. As a result, it was clearly demonstrated that the relative position significantly affects not only the reaction rate but also the product selectivity, and the Pd/SiO2@Fe-MS showed the best performance among them. The applicability of the Pd/SiO2@Fe-MS is also demonstrated in the one-pot oxidation of adamantine. The present design strategy offers a very simple and efficient methodology to realize a one-pot reaction.