NMR Evidence for Specific Germanium Siting in IM-12 Zeolite

Ge-rich ITQ-13 and ITQ-22 zeolites were degermanated by acid leaching of as-made—still containing organic template—and calcined forms. These two zeolites could be almost completely degermanated without modifying the framework structure. This behavior is different from IM-12 zeolite, which undergoes framework transformation from UTL to OKO type upon degermanation. All three zeolite types contain double four-ring (D4R) units populated by germanium atoms. The UTL to OKO transformation involves the elimination of Germanate 4 rings (Ge 4R) from double four-ring (D4R) units connecting the layers. The differences of Si and Ge atom siting in D4R units in the three zeolites were probed using 19 F MAS NMR after post synthetic incorporation of fluoride and 1 H– 29 Si CP/MAS NMR. In IM-12, NMR suggests that Ge atoms are all located in one of the faces of D4R units and form 4Rs that connect Si-rich layers together. In the degermanation process, the 4 Ge atoms of the Ge 4Rs are dislodged simultaneously, in excellent agreement with earlier EXAFS data. In ITQ-13 and ITQ-22 zeolites, Ge distributions are more complex, leading a progressive degermanation of the framework and to the presence of Si–O–Si bridges that prevents the structure from collapsing upon Ge extraction. Data suggest that IM-12 is unique in the family of Ge-containing zeolites.