Aluminium Partition on Crystallographically Unequivalent Tetrahedral Sites in Zeolite Offretite Studied by ²⁹Si MAS NMR.

In zeolite off retite, there are two kinds of crystallographically unequivalent tetrahedral sites （12 T_A and 6 T_B sites per unit cell）. the chemical shift difference due to site inequivalence（5. 3ppm） is almost the same as the effect of aluminium in the first coordination sphere of silicon（5. 5ppm）, so it is a very good approximation to consider a complete overlap of the two sets of ²⁹Si MAS NMR signals corresponding to T_A and T_B sites. A relationship based on Si MAS NMR spectrum between framework Si/ Al ratio（R） and the aluminium partition ratio（r） is proposed. When the framework Si/Al ratio of the sample is obtained by chemical analysis, the aluminium partition in parent and （NH₄）₂SiF₆ treated zeolite offretite can be calculated by the relationship mentioned above. It is concluded that for parent samples, Al atoms prefer T_B sites, while dealuminated with （NH₄）₂SiF₆, Al on T_B sites are more easily to be substituted by Si.