Partitioning of aluminum atoms in crystallographically non-equivalent tetrahedral sites of the zeolite offretite by 29Si MAS NMR

For the zeolite offretite, a formula is proposed which includes the framework Si/Al ratio (R), the partitioning ratio of Al over two crystallographically non-equivalent tetrahedral sites (r) and intensities of the observed peaks in the 29Si MAS NMR spectrum. By this formula, the framework Si/Al ratio of offretite can be estimated from the 29Si MAS NMR spectrum. Combined with chemical analysis of the Si/Al ratio, Al partitioning in two kinds of T sites can also be deduced. It is concluded that the TB sites are favored by Al atoms in parent offretites and Al atoms at TB sites can more easily be substituted isomorphously by Si when treated with (NH4)2SiF6. The formula proposed here is based only on experiments and may be used to testify some statistical models of Al distributions in offretites.