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)(8)Si8O20 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 H-1, C-13, Al-27, Si-29, and P-31 MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry, and N2 adsorption porosimetry. The resulting aluminum centers were resolved by Al-27 TQ/MAS NMR techniques and assigned based on their spectroscopic parameters obtained by peak fitting (delta(iso), C Q, eta) 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.