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

Abstract

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−AlO 3 ] (L = THF, Et 3 N, pyridine, triethylphosphine oxide (TEPO)) and [AlO 4 ] − centers were prepared through nonhydrolytic sol−gel condensation reactions of the spherosilicate building block (Me 3 Sn) 8 Si 8 O 20 with L−AlX 3 (X = Cl, Me, Et) and [Me 4 N] [AlCl 4 ] 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 1 H, 13 C, 27 Al, 29 Si, and 31 P MAS NMR and FTIR spectroscopies, ICP-OES, gravimetry, and N 2 adsorption porosimetry. The resulting aluminum centers were resolved by 27 Al 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 [AlO 4 ] − to 15.1 MHz for [THF−AlO 3 ]. Products containing exclusively [TEPO−AlO 3 ] or [AlO 4 ] − centers could be obtained (single-site materials). For L = THF, Et 3 N, and pyridine, the [AlO 4 ] − centers were formed together with the expected [L−AlO 3 ] species, and a viable mechanism for the unexpected emergence of [AlO 4 ] − was proposed.