29Si{27Al} TRAPDOR MAS NMR to distinguish Qn(mAl) sites in aluminosilicates. Test case: Faujasite-type zeolites

Si-29{Al-27} TRAPDOR MAS NMR was applied to two faujasite-type zeolites with Si/Al ratios of 1.3 (Na-X) and 2.7 (Na-Y). The aim of this test study is to show that different Q(4)(mAl) sites (m = 4, 3, 2, 1) can be distinguished by differently strong TRAPDOR effects (Delta S/S-0). Indeed, it was found that the TRAPDOR effect depends on the number m of AlO4 units connected to the Q(4) silicon tetrahedrons. For Na-X, the measured Delta S/S-0 values are 1 : 0.81 : 0.56 for Q(4)(4Al), Q(4)(3Al) and Q(4)(2Al), respectively (normalized to Q(4)(4Al)). The corresponding Delta S/S-0 values are the same for Na-Y within the error bars, although the silicon sites are different: Q(4)(3Al), Q(4)(2Al) and Q(4)(1Al) and now normalized to Q(4)(3Al) as no Q(4)(4Al) is present. Nevertheless, the proposed method opens up the possibility to distinguish overlapping Si-29 NMR signals of the Q(n)(mAl) sites in amorphous materials as the main goal of these investigations.