Amino-Functionalized Phosphotungstic Acid-Anchored Mesoporous Molecular Sieves: Highly Efficient Catalysts for Selective Epoxidation of Cyclohexene

Highly ordered hexagonal mesoporous silicate (IITM-41) and aluminosilicate (AlIITM-41) molecular sieves synthesized using long-chain ionic liquid (1-hexadecyl-3-methyl imidazolium chloride) which exhibit high surface area, moderate pore size, large pore volume and thicker pore wall structure. These materials were chemically modified using 3-chlorotriethoxypropylsilane and imidazole (or) triethoxy-3-(2-imidazolin-1-yl) propylsilane (TEIPS) for effective anchoring of phosphotungstic acid (PTA). The resulting catalysts, viz., IITM-41-TEIPS-PTA and AlIITM-41-TEIPS-PTA, were characterized by XRD, BET, FT-IR, DRUV-VIS and MAS-NMR. The diffraction and N-2 sorption studies reveal that there is no loss of mesostructural ordering even after grafting/anchoring procedures. FT-IR and DRUV-VIS spectral studies confirm the existence of propyl group and imidazole moiety on the surface modified catalysts. Further, the nature of bonding of PTA with imidazole and silica surface was investigated using C-13, Si-29 and P-31 MAS-NMR. Finally, the well-characterized catalysts were tested for cyclohexene epoxidation reaction using H2O2 as oxidant. The results indicate that surface-modified catalysts, in particular AlIITM-41-TEIPS-PTA, show excellent performance owing to a combination of both PTA and the Bronsted acidity originating from the silica/alumina matrix. In addition, large hydrophilic imidazole group enhances the accessibility of the oxidant, which in turn increases the catalytic activity. Further, these catalysts can easily be recovered and recycled without leaching and loss of activity.