posted on 2024-01-25, 20:09authored byMartin Kejik, Jiri Brus, Lukas Jeremias, Lucie Simonikova, Zdenek Moravec, Libor Kobera, Ales Styskalik, Craig E. Barnes, Jiri Pinkas
Porous aluminosilicates are functional materials of paramount
importance
as Lewis acid catalysts in the synthetic industry, yet the participating
aluminum species remain poorly studied. Herein, a series of model
aluminosilicate networks containing [L–AlO3] (L
= THF, Et3N, pyridine, triethylphosphine oxide (TEPO))
and [AlO4]− centers were prepared through
nonhydrolytic sol–gel condensation reactions of the spherosilicate
building block (Me3Sn)8Si8O20 with L–AlX3 (X = Cl, Me, Et) and [Me4N] [AlCl4] compounds in THF or toluene. The substoichiometric
dosage of the Al precursors ensured complete condensation and uniform
incorporation, with the bulky spherosilicate forcing a separation
between neighboring aluminum centers. The materials were characterized
by 1H, 13C, 27Al, 29Si,
and 31P MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry,
and N2 adsorption porosimetry. The resulting aluminum centers
were resolved by 27Al TQ/MAS NMR techniques and assigned
based on their spectroscopic parameters obtained by peak fitting (δiso, CQ, η) and their correspondence
to the values calculated on model structures by DFT methods. A clear
correlation between the decrease in the symmetry of the Al centers
and the increase of the observed CQ was
established with values spanning from 4.4 MHz for distorted [AlO4]− to 15.1 MHz for [THF–AlO3]. Products containing exclusively [TEPO–AlO3]
or [AlO4]− centers could be obtained
(single-site materials). For L = THF, Et3N, and pyridine,
the [AlO4]− centers were formed together
with the expected [L–AlO3] species, and a viable
mechanism for the unexpected emergence of [AlO4]− was proposed.