The nature of the thermal decomposition of a catalytically active anionic clay mineral

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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 Å 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.