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We present a multistep method combining multispectroscopic experiments with DFT calculations to determine the complete Al distribution in silicon-rich zeolites, independent of the presence of AlÀOÀ(SiÀO) n ÀAl (n = 1, 2) sequences in their frameworks. 29 Si MAS NMR spectroscopy is employed to confirm the absence of AlÀOÀSiÀOÀAl in the framework of silicon-rich zeolites while 27 Al 3Q MAS NMR spectroscopy and DFT computations of 27 Al isotropic chemical shifts serve to determine the locations of isolated Al atoms. The maximum ion-exchange capacity of zeolites for [Co 2þ (H 2 O) 6 ] 2þ reveals the presence of close Al atoms (i.e., those Al atoms which are able to balance [Co 2þ (H 2 O) 6 ] 2þ ions). Then visible spectroscopy of the bare Co(II) ion in the dehydrated zeolite of the samples with close Al is utilized to identify the locations of the corresponding AlÀOÀ(SiÀO) 2 ÀAl pairs in a ring. Subsequently, their 27 Al isotropic chemical shifts are evaluated at DFT and the complete Al distribution is determined. The complete Al siting in three ferrierite samples with only isolated framework Al atoms and two ferrierites with AlÀOÀ(SiÀO) 2 ÀAl sequences was determined. Our results reveal that the Al siting in the former samples varies with the conditions of the zeolite synthesis; Al is present in three or four sites (T1b, T2, T3, and T4) depending on the sample while T1a is never occupied by Al and the concentrations of Al atoms in various T sites are very diverse. For ferrierites with both isolated and close Al atoms, isolated Al atoms occupy the T2, T3, and T4 sites and the close Al atoms are arranged in Al(T1a)ÀOÀ(SiÀO) 2 ÀAl(T1a) and Al(T2)ÀOÀ(SiÀO) 2 ÀAl(T2) sequences forming the R and β À 2 cationic sites, respectively. Isolated Al atoms do not occupy the T1b site and close Al atoms do not form Al(T4)ÀOÀ(SiÀO) 2 À Al(T4) sequences of the β À 1 site. The differences between the concentrations of Al in T sites are not as pronounced as those for the ferrierite samples with only isolated framework Al atoms. In addition, our results reveal that the Al siting in ferrierite is not random and depends on the conditions of the zeolite synthesis.

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SSZ-13 is a Si-rich (Si/Al > 5) small pore zeolite (chabazite topology) important for both acid and redox catalysis. Using a sample with Si/Al = 12, a new procedure involving 27Al (3Q) MAS NMR spectroscopy and extensive periodic DFT calculations with molecular dynamics, in addition to the standard methods based on bare Co­(II) cations as probes monitored by FTIR spectroscopy and UV–vis spectroscopy, was employed. The placement of the Al–O–(Si–O)2–Al and Al–O–(Si–O)3–Al sequences in the zeolite framework was determined (Al–O–Si–O–Al sequences are absent). 54% of the framework Al atoms correspond to Al–O–(Si–O)3–Al sequences which cannot form cationic sites for bare divalent cations but are able to accommodate divalent Co­(II) hexaaqua complexes. The corresponding Al–O–(Si–O)3–Al sequence is located in two double 6-ring cages with one Al located in the 4-ring connecting two double 6-ring units. Our study also reveals that 35% of the framework Al atoms can accommodate neither divalent Co­(II) hexaaqua complexes nor bare divalent cations. Furthermore, the siting of the Al atoms of the Al–O–(Si–O)2–Al and Al–O–(Si–O)3–Al sequences forming four cationic sites for divalent cations located in the 6-ring (Al–O–(Si–O)2–Al), 8-ring (Al–O–(Si–O)2–Al and Al–O–(Si–O)3–Al), and double 6-ring (Al–O–(Si–O)2–Al) was determined. These Al atoms correspond to a minority of the Al framework atoms.

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Zeolites are highly important heterogeneous catalysts. Besides Brønsted SiOHAl acid sites, also framework AlFR Lewis acid sites are often found in their H-forms. The formation of AlFR Lewis sites in zeolites is a key issue regarding their selectivity in acid-catalyzed reactions. The local structures of AlFR Lewis sites in dehydrated zeolites and their precursors--"perturbed" AlFR atoms in hydrated zeolites--were studied by high-resolution MAS NMR and FTIR spectroscopy and DFT/MM calculations. Perturbed framework Al atoms correspond to (SiO)3AlOH groups and are characterized by a broad (27)Al NMR resonance (δi = 59-62 ppm, CQ = 5 MHz, and η = 0.3-0.4) with a shoulder at 40 ppm in the (27)Al MAS NMR spectrum. Dehydroxylation of (SiO)3AlOH occurs at mild temperatures and leads to the formation of AlFR Lewis sites tricoordinated to the zeolite framework. Al atoms of these (SiO)3Al Lewis sites exhibit an extremely broad (27)Al NMR resonance (δi ≈ 67 ppm, CQ ≈ 20 MHz, and η ≈ 0.1).