ABSTRACT

Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . 277

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Untreated Silica . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Chemical and H-D Exchange Probes for 1508C Activated Silica . . . . . . . . . . . . . . . . . . . . . . . . . 279

Relative Rates of Reaction of Various

Silanol Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 282

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

The accessibility of surface silanols toward hydrogen-sequestering agents with steric dimensions that

increase in the order ZnMe2, BCl3, TiCl4, AlMe3, and Me3SiNHSiMe3 (HMDS) were compared spectro-

scopically and gravimetrically for a fumed and a precipitated silica. The H-D exchange reaction was simi-

larly studied with D2O, ND3, and deuterated methanol, isopropyl alcohol, and tert-butyl alcohol. The surface

areas of the two silicas are similar, but the silanol density on the precipitated silica is about twice that on the

fumed silica. Approximately equal fractions of the total number of OH groups per square nanometer

undergo H-D exchange with a given probe, and this fraction decreases as the size of the molecule increases.