Studies on the structures of transition metal-substituted MCM-41 materialsand their catalysis for hydroxylation of benzene

Taking advantage of the substitutability of the framework and the modifiabi lity of the surface, the structural characteristics and the surface propert ies of MCM-41 were adjusted and controlled by transition-metal substitution . Owing to the accessibility of active site and lower diffusion resistance, transition-metal substituted MCM-41 materials were chosen as catalysts for the hydroxylation of aromatics. Isomorphous substituted MCM-41 materials with well-ordered structures were hydrothermally synthesized by adding Al or Ti, Fe, Cr and Ni compounds to t he reaction mixture. Not only do the long-range characteristics and local s tructures vary with the nature and amount of the heteroatom, but the charac teristics of the pore structure can also be adjusted to some extent by isom orphous substitution. Meanwhile, the surface acidity and the surface hydrop hilicity of MCM-41 increased after inserting Al or transition metal element s into the framework, while the results of benzene and methanol-TPD show th at the surface hydrophobicity of MCM-41 decreased. Transition-metal substituted MCM-41 materials were used as catalysts for th e liquid-phase hydroxylation of aromatics. The catalytic activities of subs tituted MCM-41 are influenced by both the redox properties of the transitio n elements and the surface hydrophilicity/ hydrohobicity. Owing to its Lewi s acid nature and the hydrophobic surface, Ti-MCM-41 gives high conversion of benzene and high chemical selectivity to the intermediate product-phenol . The chemical selectivity can be controlled by matching the hydrophilicity /hydrophobicity of the MCM-41 catalyst, and the polarity of the solvent to the polarities of the reacant-benzene and intermediate product-phenol. MCM- 41 exhitits some geometrical selectivity because of the narrow distribution of pore diameters. The accessible active sites, lower diffusion resistance and unique surface properties result in a better catalytic performance tha n microporous TS-1 in the hydroxylation of benzene. The MCM-41 catalysts ca n be reactived by calcination.