DEALUMINATION OF ZEOLITES .4. DEALUMINATION OF OFFRETITE THROUGH HYDROTHERMAL TREATMENT

Nine samples of dealuminated offretite have been prepared by simple or double treatment, without addition of water, at high temperature (873-1123 K) of a KHOffretite sample (K0.95H3.45Al4.4Si13.6O36). The samples were characterized by X-ray diffraction (unit cell parameters a and c, and ''crystallinity'' C(RX)), by Si-29 n.m.r. (framework Si/Al ratio and distribution of Al in T1 and T2 sites), by Al-27 n.m.r. (framework and extraframework Al species), by i.r. spectroscopy (frequencies upsilon1 and upsilon2 of the asymmetric and symmetric T-O bands), and by X.p.s. (Si/Al ratio of the outer surface). As with Y zeolites, highly dealuminated samples with a high C(RX) value can be obtained only through successive treatments with, intermediately, exchange of the K cations. The dealumination causes an increase in the Al(T1)/Al(T2) ratio and of upsilon1 and upsilon2 but has practically no effect on the c parameter. Parameter a first decreases with the number of framework Al per unit cell (N(Al)), then remains quasi-constant for N(Al) less-than-or-equal-to 2.7. Nearly linear relationships are obtained between upsilon1 and upsilon2 and N(Al), which allows an estimation of N(Al) to be made by i.r. spectroscopy.