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

Abstract

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.

Tuning the Aluminum Distribution in Zeolites to Increase their Performance in Acid‐Catalyzed Reactions

“…Whereas the presence of AlSi 2 Al sequences has been confirmed for a number of zeolites, the formation of AlSi 3 Al sequences is rare. Recently, they have been proven for SSZ‐13, but they represent only a minor Al species in that zeolite . We can also speculate on their formation in zeolites in other frameworks with Al pairs (sites for divalent cations) located in the 8‐ring, for example, mordenite.…”

“…The methods described in this chapter result from the need to analyze the organization of Al in some zeolites (ZSM‐5, ferrierite, mordenite, beta zeolite, TNU‐9, and SSZ‐13) that have appeared during the last decades . The developed methods are, therefore, without doubt affected by the nature of the investigated matrices.…”

“…Close unpaired Al atoms are able to accommodate Co 2+ hexaaqua complexes, but they are unable to form cationic sites for bare Co 2+ cations . We can speculate that acid sites related to these Al atoms can cooperate in acid‐catalyzed reactions.…”

“…Note that the above classification was first developed for ZSM‐5 zeolite and was then continuously extended to other pentasil‐ring zeolites . Recently, the synthesis of Si‐rich zeolites of a family other than the pentasil‐ring family (ABC‐6 family) was reported, and the Al distribution in these materials was analyzed . The study required actualization of the definition of the individual types of Al species, as shown below.…”

“…Owing to the odd number of Al atoms in this sequence, the isolated Al triad is able to accommodate only one divalent cation. Conversely, the acid sites related to all three Al atoms have to be close enough to cooperate in acid‐catalyzed reactions …”