Synthesis and Chemical Functionalization of High Surface Area Dendrimer-Based Xerogels and Their Use as New Catalyst Supports

Second- and third-generation alkoxysilyl-terminated carbosilane dendrimers have been used as building blocks for the synthesis of high surface area xerogels, which were characterized by 29Si CP MAS NMR spectroscopy, nitrogen adsorption/desorption porosimetry, IR spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Thus, the acid-catalyzed hydrolysis of Si[CH2CH2CH2Si(CH2CH2Si(OEt)3)3]4 (G2-(OEt)36) and Si{CH2CH2CH2Si[CH2CH2CH2Si(CH2CH2Si(OEt)3)3]3}4 (G3-(OEt)108) in benzene solution yielded monolithic gels. The resulting xerogels, X-G2benz and X-G3benz, have surface areas of 600 and 800 m2/g, respectively. The surface area of these xerogels increase with increasing dendritic radii, suggesting that the dendrimer building blocks of X-G2benz are compressed onto one another to a greater extent than the corresponding dendrimers that comprise X-G3benz. The isolation of monoliths from benzene solution suggests that the hydrophobic interior of the dendrimers keeps the polymerizing species in solution. Other precursors such as TEOS, 4,4'-bis(triethoxysilyl)biphenyl, and Si(CH2CH2Si(OEt)3)4 (G1-(OEt)12) did not yield monolithic gels after hydrolysis in benzene. Dendrimers with fewer alkoxysilyl groups at the periphery (G2-(OEt)24Me12 and G3-(OEt)72Me36) were hydrolyzed in THF or benzene to materials with no appreciable surface area. After obtaining wet monolithic gels from the acid-catalyzed hydrolyses of G2-(OEt)36 and G3-(OEt)108 in benzene, postgelation processing was conducted by heating the monoliths in hot toluene for 48 h. After solvent evaporation, this procedure gave xerogels X-G2HT and X-G3HT, which possess surface areas of 1300 and 1000 m2/g, respectively. These aerogel-like gels were treated with Ti(OiPr)4 to yield X-G2Ti and X-G3Ti. Additionally, X-G2HT was treated with Ti[OSi(OtBu)3]4 to yield X-G2Ti/Si. All three gels were subsequently used as catalysts in the epoxidation of cylcohexene. These gels were shown to be very selective and significantly more active (in terms of yield and initial rate) than the Shell catalyst derived from treatment of silica with Ti(OiPr)4.