Direct oxidation of benzene to phenol by molecular oxygen over catalytic systems containing Pd(OAc)2 and heteropolyacid immobilized on HMS or PIM

Heteropolyacid (HPA) and Pd(OAc)2 were immobilized through chemical bonds onto the functional organic groups modified solid surfaces of hexagaonal mesoporous silica (HMS) and polyimine (PIM). The obtained hybrid samples were used as heterogeneous catalysts for the oxidation of benzene to phenol by molecular oxygen and compared to the homogeneous catalytic system containing HPA and Pd(OAc)2 (denoted by HPA + Pd(OAc)2). The activity of various catalytic systems was in the order of HPA + Pd(OAc)2 > HMS–HPA + Pd(OAc)2 > PIM–HPA + Pd(OAc)2 > HPA + HMS–Pd(OAc)2 > HMS–HPA + HMS–Pd(OAc). Although the benzene conversions over the heterogeneous catalytic systems containing solid catalysts were lower than that over the homogeneous catalytic system HPA + Pd(OAc)2, HMS–HPA + Pd(OAc)2 gave the same maximum achievable yield of phenol as that over HPA + Pd(OAc)2 by extending the reaction time or increasing the catalyst amount. Moreover, HMS–HPA + Pd(OAc)2 was able to be reused after simple filtration without leaching HPA. The benzene conversion and the selectivity to phenol over the heterogeneous system HMS–HPA + Pd(OAc)2 did not decrease even after reused five times.

Graphical abstractHeteropolyacid and Pd(OAc)2 were immobilized on the surfaces of HMS and PIM for the benzene oxidation by O2. The active components of the solid catalysts did not leach to solvent after reaction. Although the activity of heterogeneous systems were lower than that of the homogeneous system, heterogeneous systems achieved the same maximum phenol yield as that obtained in the homogeneous system. Figure optionsDownload full-size imageDownload as PowerPoint slide