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 xe rogels, which were characterized by Si-29 CP MAS NMR spectroscopy, nitrogen adsorption/desorption porosimetry, IR spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Thus, the acid-ca talyzed 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-G 2(benz) and X-G3(benz), have surface areas of 600 and 800 m(2)/g, respectiv ely. The surface area of these xerogels increase with increasing dendritic radii, suggesting that the dendrimer building blocks of X-G2(benz) are comp ressed onto one another to a greater extent than the corresponding dendrime rs that comprise X-G3(benz). The isolation of monoliths from benzene soluti on suggests that; the hydrophobic interior of the dendrimers keeps the poly merizing species in solution. Other precursors such as TEOS, 4,4'-bis(triet hoxysilyl)biphenyl, and Si(CH2CH2Si(OEt)(3))(4) (G1-(OEt)(12)) did not yiel d monolithic gels after hydrolysis in benzene. Dendrimers with fewer alkoxy silyl groups at the periphery (G2-(OEt)(24)Me-12 and G3-(OEt)(72)Me-36) wer e hydrolyzed in THF or benzene to materials with no appreciable surface are a. After obtaining wet monolithic gels from the acid-catalyzed hydrolyses o f G2-(OEt)(36) and G3-(OEt)(108) in benzene, postgelation processing was co nducted by heating the monoliths in hot toluene for 48 h. After solvent eva poration, this procedure gave xerogels X-G2(HT) and X-G3(HT), which possess surface areas of 1300 and 1000 m(2)/g, respectively. These aerogel-like ge ls were treated with Ti((OPr)-Pr-i)(4) to yield X-G2(Ti) and X-G3(Ti). Addi tionally, X-G2(HT) was treated with Ti[OSi((OBu)-Bu-t)(3)](4) to yield X-Ga -Ti/Si. All three gels were subsequently used as catalysts in the epoxidati on of cylcohexene. These gels were shown to be very selective and significa ntly more active (in terms of yield and initial rate) than the Shell cataly st derived from treatment of silica with Ti((OPr)-Pr-i)(4).