PREPARATION AND CHARACTERIZATION OF MICROPOROSITY AND ACIDITY OF SILICA-ALUMINA PILLARED CLAYS

The preparation and characterization of silica-alumina pillared clays have been studied. The formation and aging of hydroxy-alumina oligocations followed by reaction with Si(OEt)(4) have been studied in order to obtain hydroxy-SiAl oligocations. Two solutions of Al-13 (Keggin cation) containing an OH/Al mole ratio equal to 2 have been used in the preparation of the hydroxy-silica-alumina solutions: solution A aged at room temperature for 48 h, and solution B aged at 393 K: for 4 h. Different Si/Al ratios (0, 0.5, 1, and 2) and Al/clay ratios (7, 11, and 22 mM Al (g of clay)(-1)) have been used to obtain clays with different accessibility and acidic properties. The Al/clay ratio played an important role in the properties of the samples obtained. Micropore size distributions obtained with the Horvath-Kawazoe approach confirmed the existence of two sizes of micropores for samples (solution A or B) with a Si/Al less than or equal to 1. On the other hand, measurable differences in the micropore distributions were observed when the Al/clay ratio was varied. The acidity of hydroxy-alumina and hydroxy-silica-alumina pillared clays observed is mainly Lewis type, but the nature of the acid sites detected by pyridine adsorption seems to be independent of Si/Al ratio.