Speaker
Description
Zeolites are crystalline microporous aluminosilicates widely used as molecular sieves and catalysts in industrial chemical processes. Extra–framework cations compensate the negative charge of the microporous aluminosilicate frameworks [Sin-mAlmO2n]m- made of corner–sharing TO4 tetrahedra (T = Si, Al-). Furthermore, also electron-pair acceptor Al framework Lewis sites are often present in zeolite catalysts. The Al framework Lewis sites were suggested to correspond to Al centers tricoordinated to the zeolite framework. A typical feature of many silicon-rich zeolites is a high number of crystallographically distinguishable T sites. Since the cationic species bind to the AlO4-tetrahedra, the crystallographic position of aluminum in zeolite frameworks governs the location of the active sites, which in turn affects the catalytic activity and selectivity.
We present our results regarding the structures and NMR parameters of extra–framework Li+[1] and Na+[2] cations in the zeolites of the ferrierite structure as well as the Al framework Lewis sites in the beta zeolite.[3]
Periodic DFT calculations including extensive molecular dynamics conformational sampling of all possible Li+ and Na+ sites for all the possible distinguishable Al(T) sites were performed employing the cp2k program. The B3LYP 7Li and 23Na NMR parameters were evaluated utilizing the Gaussian program and seven coordination shell clusters and compared with our experiments. We reveal the siting of Li+ and Na+ balancing framework Al atoms located in all the distinguishable framework T sites of ferrierite.
Extensive periodic DFT calculations including molecular dynamics employing the VASP program were carried out for our plausible models of the Al framework Lewis sites in the beta zeolite. Afterwards, B3LYP 27Al NMR parameters were evaluated utilizing the Gaussian program and seven coordination shell clusters and compared with our experiments. We show the most likely structure of the Al framework Lewis sites in the beta zeolite.
References
(1) Klein, P.; Dedecek, J.; Thomas, H. M.; Whittleton, S. R.; Pashkova, V.; Brus, J.; Kobera, L.; Sklenak, S. NMR crystallography of monovalent cations in inorganic matrixes: Li+ siting and the local structure of Li+ sites in ferrierites. Chem. Commun. 2015, 51, 8962-8965.
(2) Klein, P.; Dedecek, J.; Thomas, H. M.; Whittleton, S. R.; Klimes, J.; Brus, J.; Kobera, L.; Bryce, D. L.; Sklenak, S. NMR crystallography of monovalent cations in inorganic matrices: Na+ siting and the local structure of Na+ sites in ferrierites. J. Phys. Chem. C 2022, 126, 10686-10702.
(3) Kobera, L.; Dedecek, J.; Klein, P.; Tabor, E.; Brus, J.; Fishchuk, A. V.; Sklenak, S. Formation and local structure of framework Al Lewis sites in beta zeolites. J. Chem. Phys. 2022, 156, 104702.
