STUDY OF SI AND AL ORDERING IN THE FRAMEWORK OF SILICEOUS CUBIC AND HEXAGONAL FAUJASITE ZEOLITES

Cubic faujasites of Si/Al = 1.7 to 3.7 and hexagonal faujasites of Si/Al = 3.3 and 4.0 were synthesized hydrothermally and were compared with secondary synthetic faujasites of Si/Al = 2.7 to 7.9 derived from SiCl4 or (NH4)2SiF6 treatment. XRD, IR and solid-state NMR (Si-29, Al-27 and Na-23) were used to characterize the materials. Both the unit cell size, a0, and the framework vibration frequencies of faujasites obey a linear relationship with the content of framework aluminum as determined by Si-29 MAS NMR. Comparisons of experimental Si-29 NMR with the theoretical models of Si and Al ordering suggest that the beta-cage model can be applied to fit the aluminum distribution in directly synthesized cubic and hexagonal faujasites, while the aluminum distribution in the framework of the (NH4)2SiF6-treated samples, CSY, is based on the random siting model with Loewenstein's rule.