Al Organization in the SSZ-13 Zeolite. Al Distribution and Extraframework Sites of Divalent Cations

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 Al-27 (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 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.