Preparation and Characterization of Hydroxysilicoaluminum Pillared Clays

Solutions containing hydroxy-SiAl(HSA) oligocations were prepared by two procedures: (1) treatment of a mixture of silicic acid, which was prepared by dispersing silica sol in an ultrasonic generator, and A1C1₃ with aqueous NaOH, followed by aging of the product and (2) preliminary preparation and aging of hydroxy-Al₁₃ oligocations followed by reaction of the latter with silicic acid. Exchange of Na-montmorillonite with HSA oligocations yielded pillared clays (designated as SiAl-PILC obtained using method 1, and GSiAl-PILC obtained using method 2) showing mixed maximum d(001) values of 2.65 and 1.90 nm for air-dried samples and maximum surface areas of ~267 m²/g after outgassing at 200 °C/101 kPa. Elemental analysis showed a gradual increase in the Si/Al ratio in the intercalated HSA oligocations with increasing Si/Al ratio in the pillaring solution. The pillaring structure, thermal stability, and surface acidity of the pillared clay products have been determined using ²⁷A1-NMR, X-ray diffraction (XRD), XPS, DTA, IR, and temperature-programmed desorption (TPD) measurements. The results suggest that the thermal stability of the pillared clays was improved by incorporating silica. Hydroxy-SiAl pillared clays show sharply higher concentrations of strong acid sites, especially Bronsted acid sites, compared with reference Al-PILC obtained by pillaring with hydroxy-Al₁₃ oligocations. This increased acidity is probably due to the presence of acidic, surface silanol groups in the HSA oligocations. The cracking of cumene and the disproportionation and alkylation of 1,2,4-trimethylbenzene were carried out in a pulse reaction system at 400 °C over the pillared clay catalysts. Incorporating silica into an alumina pillar increased the cracking activity of cumene and the activity and selectivity for the disproportionation of 1,2,4-trimethylbenzene compared to those observed for Al-PILC.