Two kinds of FSM-type mesoporous silicas with different OH densities were prepared by calcination and acid treatment and modified with various silylating agents. The density of silanol groups in acid-treated FSM was about 1.5 times as large as that in calcined FSM (2.4 OH groups nm-2), determined by thermogravimetry and 29Si MAS NMR. When calcined FSM was trimethylsilylated, 73% of the surface was covered with trimethylsilyl (TMS) groups. With the increase in the alkyl chain length of chloroalkyldimethylsilanes, the pore diameter of silylated FSM-type mesoporous silicas decreased. The silylated FSM-type mesoporous silicas possessed extremely hydrophobic pores. The hydrophobicity of the surfaces was also changed by using aminopropylethoxysilanes. In contrast, when the acid-treated FSM was silylated with chlorotrimethylsilane, TMS groups came into contact with one another (surface coverage ca. 100%) because of the presence of a larger amount of silanol groups. Moreover, the sensitive control of the pore size by silylation reaction was achieved by varying a grafted number of octyldimethylsilyl (ODMS) groups. ODMS groups were located near the inner surfaces in the mesoporous silica silylated in a lesser degree, and the groups were directed toward the center of mesopores in the fully silylated mesoporous silica.
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