Lewis Acidic Aluminosilicates: Synthesis, ²⁷Al MQ/MAS NMR, and DFT-Calculated ²⁷Al NMR Parameters

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 (δ_iso, C_Q, 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 C_Q 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.