PREPARATION OF ALUMINA-PILLARED MONTMORILLONITES WITH HIGH THERMAL-STABILITY, REGULAR MICROPOROSITY AND LEWIS BRONSTED

Alumina-pillared montmorillonites containing 1.58 and 1.84 pillaring aluminum ions per unit cell and exhibiting unusual thermal stability, microporosity and Lewis/Bronsted acidity have been prepared by the reaction of the Na+- exchanged forms of the clay with Al13 oligomers formed by basic hydrolysis of aluminum trichloride. The exceptional stability of the products is attributable to synthesis procedures that minimize the hydrolysis of the Al13 oligomers bound to the gallery surfaces. Replacing the initial Na+ ions with protons prior to pillaring diminishes the performance properties of the pillared forms. Nitrogen BET surface areas of 393-458 m2/g are obtained upon calcination of the products at 150-degrees-C. Even after calcination at 800-degrees-C, where previously reported pillared montmorillonites are collapsed, surface areas of approximately 215 m2/g are observed. The reduction in surface area at high temperature is accompanied by a decrease in gallery height from approximately 10.2+/-0.2 angstrom (150-degrees-C) to approximately 7.6+/-0.1 angstrom (800-degrees-C). Approximately 70% of the total surface area occurs in the micropore range (<20 angstrom). Relatively broad bimodal micropore distributions with maxima near 5.5 and 7.5 angstrom are observed for products calcined at 150-degrees-C. Owing to a lateral redistribution and restructuring of the pillars, the micropore structure becomes more uniform at higher calcination temperatures. Fourier transform infrared (FTIR) studies of pyridine chemisorption confirm the presence of both Lewis and Bronsted acid sites for samples calcined at 400-800-degrees-C.