Chemistry of the Silica Surface: Liquid-Solid Reactions of Silica Gel with Trimethylaluminum

摘要

The reaction of trimethylaluminum and dry,high-surface-area (500 m~2/g) silica gel in a mixed slurry was studied using multinuclear,solid-state NMR spectroscopy.The products of the initial reaction were characterized,and their progress through subsequent washing with diethyl ether and reactions with measured amounts of water was followed.The quantitative distribution of different chemical forms of carbon deposited on the silica surface by the initial reaction was measured.The products of the initial reaction are dominated by methyl species of the types AI(CH_3)_n (with Si-O-Al linkages),Si-O-CH_3,and (Si-O)_(4-n)-Si(CH_3)_n; aluminum is seen to exist predominantly as a five-coordinate species.Subsequent treatment with diethyl ether fails to remove any surface species,but instead the ether becomes strongly associated with the surface and highly resistant to removal.Stepwise additions of water hydrolyze the Al-CH_3 and Si-O-CH_3 moieties,leading to conversion of five-coordinate aluminum to four- and six-coordinate aluminum,and affect the partial release of the surface-associated diethyl ether; Si-CH_3 moieties remain.The effect of aromatic and saturated solvents on the initial reaction was examined and found to cause a small but significant change in the distribution of products.Structures of aluminum-centered species on the silica surface consistent with the spectroscopic data are proposed.