Sol-gel preparation of mesoporous sodium aluminosilicate glasses: mechanistic and structural investigations by solid state nuclear magnetic resonance

Abstract

A new sol-gel route for the preparation of sodium aluminosilicate glasses along the composition line (NaAlO₂)_x–(SiO₂)_1−x (0.1 ≤ x ≤ 0.66) has been developed using aluminium lactate, sodium acetate and tetraethylorthosilicate as precursors. Preparation conditions (pH, gelation temperature) were optimized based on liquid state NMR spectra and the structural evolution from solution to gel and to glass was monitored by ²⁷Al, ²⁹Si and ²³Na magic angle spinning (MAS) NMR. At the gel stage aluminium is mostly present as a six coordinated (Al^VI) complex attached to H₂O and lactate ligands with only small amounts of four coordinated (Al^IV) aluminium bound to silica. Si–O–Al bond formation proceeds at annealing temperatures above ∼350 °C producing fully polymerized sodium aluminosilicate tetrahedral networks. Glasses annealed to 600 °C show mesoporous characteristics with surface areas up to 335 m²/g. Both solid state NMR and X-ray photoelectron spectroscopy (XPS) results reveal that the short range order of these materials is very similar to that of analogous glasses prepared viamelt-cooling.