Synthesis Of Hierarchically Porous Ti-MCM-41 And Its Catalytic Performances In Oxidation

Since the discovery of Ti-MCM-41, porous titanosilicate materials with a variety of pore sizes and synthesized by diverse methods have been brought about the extensive concern due to their promising applications in the selective oxidation of alcohols, aromatic compounds, alkenes and other substrates as catalyst. The outstanding catalytic activity of mesoporous titanosilicate derives from their high surface areas and tetrahedrally coordinated Ti species. More recently, the study on the epoxidation of a phenyl butadienyl-substituted boron dipyrromethene difluoride probe with tert-butyl hydroperoxide investigated that only the titanium sites in the outer 300 nm of common micrometer-sized Ti-MCM-41 particles contribute to the observed activity. The work opened a door to fully evaluate the accessibility of active sites in a mesoporous material and suggested that they might suffer from intraparticle diffusion limitation in some cases. Up to now, designing hierarchically porous mesoporous titanosilicates containing framework mesopores and secondary macropores is recognized as an effective strategy to minimize the diffusion limitation in mesoporous particles.We synthesized the hierarchically porous Ti-MCM-41 by employing evaporation induced self-assembled, and the cetyltrimethylammonium bromide(CTAB) serve as stencil for trellis mesopores and used the method preparation of emulsion polymerization for polystyrene(PS) spheres as agent for creating ordered macropore architecture. We have determined the best silicon titanium mole ratio for 40 : 1, and the ratio of CTAB and TEOS is 2 : 10. X-ray diffraction,UV-Visible spectra and the isotherms of nitrogen were measured to indicate the presence of ordered 2D hexagonal(p6mm) framework mesopores and the presence of isolated and tetrahedrally concerted Ti species in the trellis. The average pore size of hierarchically porous Ti-MCM-41 is at 2.23 nm. The mesopore volume of Ti-MCM-41 is 0.59 cm3/g and the surface area of hierarchical porous Ti-MCM-41 is 1281.55 cm2/g. The coexistence of macropores and mesopores in hierarchically porous Ti-MCM-41 was established by the photographs of SEM and TEM.In our case, catalysts was applied into several epoxidations of bulky alkenes including cis-cyclooctene, cyclohexene, cyclododecatriene and cyclododecene. By the comparation of Ti-MCM-41, hierarchically porous Ti-MCM-41 showed higherturnover frequency(TOF) values based on the epoxidation conversions of various bulky alkenes due to the advantage of the hierarchical macro-mesoporous architecture in catalysis. Furthermore, the hierarchically porous Ti-MCM-41 is a remarkably stable catalyst immune to titanium leaching and can be used in successive catalytic cycles attributed to its no significant damage of activity and selectivity in the epoxidation.