摘要：

Certain synthetic, anionic clays, after a carefully controlled heat activation, have been shown to promote a wide variety of base catalyzed reactions such as aldol condensations, oxirane polymerizations, carbon-bound HD exchange, and lactone polymerizations. The changes which take place during the heat activation of such a synthetic, anionic clay mineral, hydrotalcite (Mg 6Al 6(OH) 16 (CO 3 · 4H 2O), have been studied by 27Al nuclear magnetic resonance-magic angle spinning, Auger and XPS spectroscopy, transmission electron microscopy, and high resolution nitrogen desorption techniques. The heating of this material (to ~450 °C) results in the loss of interstitial water, carbon dioxide, and dehydroxylation until a catalytically active material analyzing approximately as Mg 6 Al 2O 8(OH) 2 remains. This heating does not cause a change in the crystal morphology nor in exfoliation of the layered structure. Instead, numerous fine pores (20–40 radius) form perpendicular to the crystal surface from which the gases vent. This is accompanied by an increase in the surface area from about 120 to about 230 m 2/g (N 2/BET) and a doubling of the pore volume (0.6 to 1.0 cm 3/g, Hg intrusion). The bulk aluminum changes from all octahedral to about 20% tetrahedral-80% octahedral. Auger spectroscopy did not appear to indicate changes in the surface aluminum environment. The X-ray powder pattern changes from that of a diffuse hydrotalcite to a very poor magnesium oxide pattern.

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