Hydroxy-aluminium beidellites are prepared by treating Na-saturated beidellites from Taiwan, with compositions intermediate between end-member beidellite and end-member montmorillonite, with hydroxy-aluminium solutions having OH/AI ratios mainly between 2.00 and 2'50. Chemical analyses show that hydroxy-aluminium ions have compositions near AI6(OH,O)16 and charges near 1-0 in two cases, and near 2"2 in a third case. The basal spacing of the prepared clays is near 17 A and remains without collapse up to 500~ Argon adsorption measurements at liquid nitrogen temperatures, after heat-treatment up to 325~ in vaeuo overnight, give surface areas near 500 m2/g when the exchangeable Na ions are about one-third replaced by hydroxy-aluminium, but near 200 m2/g when wholly replaced by hydroxy-aluminium. In the latter case the hydroxyaluminium covers the internal surfaces of the clay almost completely prior to the heattreatment.

The adsorption of pyridine and CD3CN on silica-alumina gels covering the whole composition range (0-100 % A1203) has been studied by infrared spectroscopy, the amount adsorbed being measured by an electrobalance. The measurement of the maximum intensity reached by the bands of strongly adsorbed species has allowed us to sketch the evolution of the surface Bronsted sites and various types of Lewis sites as a function of the chemical composition of the adsorbant.The overall concentration of Lewis sites increases smoothly from 0 to 75 % A1203 ; the sharper increase in the high A1203 range is mainly due to weak sites which are revealed by a pyridine band at 1615 cm-I while pyridine coordinated with strong Lewis sites gives a band at 1625 cm-l.The concentration of acid sites associated with the mixed silica-alumina phase corresponds to the more acidic sites observed by titration in non aqueous media. A high proportion of them are strongly acidic hydroxyls which are detected by absorption of H bond acceptors.The mixed phase develops as the A1203 content increases from 0 to 25 % ; above 50 "/, A1203 it is progressively diluted in a mixture with other phases and is no longer observed above 85 % A1203.

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The properties of hydroxyl groups on mixed silica-alumina gels (0-50 % Al,O,) have been studied by adsorbing weak H bond accepting molecules onto the gels and recording the OH stretching infrared band.Additional measurements on silica gel and compilation of numerous literature data have shown that frequency shift measurements, or series of AGOH measurements condensed into BHW (Bellamy, Hallam and Williams) slopes, provide a reliable estimate of the OH acidity. The resulting scale is independent of the atom to which the hydroxyl is attached ; it may be used for surfaces of solids as well as for solutions.The results obtained for silica-alumina show that two distinct populations of hydroxyl groups are present. Hydroxyl groups of type 1 are identical with those present on the surface of silica. The spectra show the presence of hydroxyl groups of a second type, the proportion of which increases as the A1 content of the gel rises ; their acidity corresponds to a pKa between -4 and -8 and they are presumably the sites responsible for the protonation observed upon adsorption of pyridine or ammonia.

ABSTRACT. The H bonding properties of silica gel surface hydroxyl have been studied by adsorbing series of organic compounds which are weak H bond acceptors and by observing the OH stretching band. While the data are consistent with the charge transfer model of H bond, a critical examination shows that their support to this theory, as well as the support of similar data reported for solutions, is not really significant. All the results, and particularly relationships found between absorption coefficients, show a complete similarity between the characteristics of H bonds formed on the solid surface and in solutions.