New insights into the degermanation process of ITQ-17 zeolites

Abstract

ITQ-17 zeolites have been degermanated under highly alkaline (NH4OH, 1M) or acidic (HCl, 10−2M) conditions in the presence of organic molecules in the pores. Relatively high degermanation levels can be obtained both at high and low pH values, without observing a significant decrease in textural and structural properties of the zeolite. Up to 35–40% degermanation, the structure remains intact and the composition of the framework is modified without extraction of the molecules occluded in the pores. More severe conditions (higher temperature, lower pH values, longer treatments…) increase the extent of degermanation but the structure is severely damaged, suggesting that high-silica ITQ-17 zeolites cannot be obtained following this procedure. On the basis of the evolution of 19F NMR spectra with the framework Ge content, a degermanation scheme has been proposed in which Ge extraction initially takes place preferentially at T2 and T3 sites of the structure. Then, T1 sites located in D4R units are degermanated and the structure gradually collapses due to the accumulation of framework connectivity defects. Degermanated zeolites are thus heterogeneous, with a Ge-rich core similar in composition to the original zeolite surrounded by an almost totally degermanated layer, in which most of D4R units are not fully connected. Degermanation significantly improves the stability of calcined zeolites under ambient atmosphere, particularly after an additional treatment with NH4F.