COMPARATIVE-STUDY OF THE SURFACE HYDROXYL-GROUPS OF FUMED AND PRECIPITATED SILICAS .4. INFRARED STUDY OF DEHYDROXYLATION BY THERMAL TREATMENTS

Four types of silica, fumed, precipitated, gel and ex-ethoxy, activated at different temperatures, have been studied by IR spectroscopy in the nu(OH) range in order to characterize the various types of SiOH groups, to compare their amount and their evolution with temperature, according to the nature of the sample. Fumed, precipitated, and gel samples activated at 973 K present the same amount of free hydroxyls. Taking into account gravimetric measurements, this allows the determination of the number of OH/nm2 at room temperature, 4, 14, and 12, respectively. The large values obtained for precipitated and gel silicas are explained by the presence of a great number of inner and internal hydroxyls, the decrease of which has been followed versus temperature using D2O exchange and tert-butyl alcohol retroexchange. Inner OH groups are preponderant in the precipitated sample while internal groups are more important in the gel silica. The ex-ethoxy sample presents only inner groups. Their amount cannot be related to its surface area, since the later is underestimated due to the cross section of nitrogen used for BET determinations. The study also allows determination of the dehydroxylation process; several steps occur, the first one always being the departure of the more strongly bonded vicinal hydroxyl groups. The length of the silanol chains has been differentiated according to the preparation mode of the samples.