KCS

Organic functionalization of inorganic materials is important for metal coordination, ion exchange and catalysis. Porous silicas with noncrystalline frameworks (e.g., amorphous silica, mesoporous MCM-41 and SBA-15) have been commonly used for organic functionalization in recent years, due to surface silanol groups that can be grafted with various organic functional groups. Here, we report that meso-/microporous hierarchical zeolites with crystalline microporous frameworks can be functionalized to possess a remarkably high density of organic functional groups, using silanol groups that are present at the surface of mesopore walls. Hierarchical zeolites (MFI, LTA and BEA) were obtained by following the self-assembly route using organosilane surfactants as mesopore generating agent. Mesopore diameters were tailored by a choice of the surfactants or the synthesis temperatures, in the range of 3~10 nm. Organic functionalization of the synthesized zeolites was performed with organosilanes containing various functional groups such as 3-aminopropyl, N-(2-aminoethyl)-3-aminopropyl and 3-anilinepropyl. The functionalized meso-/microporous zeolites were characterized by N2 adsorption, x-ray powder diffraction, elemental analysis and various spectroscopic techniques. We investigated the effects of solvent polarity and dielectric constants on the concentration and catalytic property of organic groups. The result shows that meso-/microporous zeolites could be functionalized with a somewhat larger number of organic groups than those for SBA-15, despite the much smaller surface area of mesopore walls (≤ 1/3). Thus, the functional group density was achieved three times higher in zeolites. In the case of functionalization with a palladium complex, zeolites exhibited very high catalytic activities for C-C coupling. Besides, the zeolites showed high stability as compared with SBA-15 or amorphous silica. The catalyst could be recycled without a significant loss of the catalytic activity and structure. We attribute this difference to hydrothermal stability of the zeolite frameworks and also high density of surface functionalization at the zeolite frameworks possessing atomic orders. We may expect that surface functionalization of meso-/microporous zeolite would provide new opportunities for the design of nanoporous, inorganic-organic hybrid materials.