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Thermogravimetric analyses (TGA) of ion-exchanged montmorillonites that were exposed to organic vapors or liquids showed multiple desorptive weight losses. Thermograms of the montmorillonite showed a low-temperature (~100°C) weight loss corresponding to the removal of physically sorbed material and further losses at higher temperatures (180°–600°C) corresponding to the removal of species chemisorbed via different mechanisms. The corresponding peaks appearing in the differential form of the thermograms (DTG) were frequently sufficiently well resolved to allow direct and independent measurement. Thus, separate isotherms relating to the individual modes of sorption could be determined simultaneously. Ethanoic acid sorbed on Al3+-exchanged bentonite was characterized by a single higher temperature DTG peak centered at about 230°C. Isotherms for vapor uptake by the clay at 35° and 60°C by both physical and chemical processes suggested that the chemisorption was rapid and provided the first layer of intercalated material. The physical sorption was slow and proceeded via the formation of a second acid layer and, at the highest relative pressures, a third. Each layer appears to have contained the same amount of acid.

The physical sorption occurred predominantly internally, and the properties of the intercalated acid were similar to those of the liquid acid. Chemisorption occurred via the formation of basic aluminum acetates, protons having been liberated in the process. This observation is of relevance to the mechanism of catalytic esterification of acid by alkenes because it establishes both the origin of the protons needed to form the alkene carbocations and that these then yield ester by attack on the basic acetate rather than acid.

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