Synthesis And Characterization Of MCM-41Mesoporous Molecular Sieves

As classic representative of M41S mesoporous materials, MCM-41molecularsieves are identified to possess excellent properties such as uniform hexagonal passage-way channels, highly selective adsorption, uniformly distributed pore diameter, largerspecific surface areas and thermal stability, as such have been widely applied in variousindustries. To stabilize the skeleton structure and improve the catalytic performance,MCM-41mesoporous molecular sieves are developed with doping of rare earthelements, and thus is the focus of this study.In this study, mesoporous molecular sieves Ln-MCM-41(Ln=La, Ce, Pr)(x=Ln/Si=0.00,0.01,0.03,0.05) were synthesized by hydrothermal crystallization. Thethermal stability, chemical composition, morphology and microstructure of the sampleswere analyzed using X-Ray Diffraction, Thermogravimetric Analysis, DifferentialThermal Analysis, Scanning Electron Microscope and Transmission ElectronMicroscope. The effect of the synthesis procesure, dopant moiety and concentration ofrare earths Ln (Ln=La, Ce, Pr) and the ratio of x=Ln/Si on the structure and propertiesof MCM-41were investigated. The results indicate that the rare earth dopants Ln arewell wedged in the lattice framework of the nanosized mesoporous molecular sieves.The mesoporous sieves present typically ordered hexagonal arrays of mesoporestructure with a uniform pore size.The results also reveal that mesoporous molecular sieves Ln-MCM-41(Ln=La, Ce,Pr)(x=Ln/Si=0.01) doped with small-sized rare earths result in small interplanarspacing d100and lattice constant, and thermal stability and long-distance ordering areenhanced. On the other hand, the thermal stability decreases with increase in interplanarspacing d100and lattice constant a0, and the size of the grains becomes unevenlydistributed with irregular shapes. The long-distance ordering becomes worse ordamaged with an increment with the dopant concentration. After the dopantconcentration added in MCM-41increases up to a threshold value, the grain size thenbecomes even larger and the shape grows to be more irregular. the lattice framework ofthe nanosized mesoporous molecular sieves Ln-MCM-41are eventually lapsed.The TG-DTA results indicate that the rare earth dopants Ln (Pr) are well wedged inthe lattice skeleton of the MCM-41. This not only leads to stronger interaction betweenthe organic template and inorganic framework wall that has been visulized in the TG curves, but also results in an enhancement on the thermal stability of the skeletonstructure at temperature as high as1120℃. It is evident that the thermal stability of theLn-MCM-41can be largely improved by rare earth dopants Ln. In addition, theobservation of SEM and TEM reveals that the spherical particles about0.5~2.0μm areactually composed by the clusters of nano-scale (4~7nm) ordered hexagonal arraysalong preferred orientation.