Structural Development of Single Phase (Type I) Mullite Gels

Abstract

Single phase (type I) mullite gels were prepared by sol-gel techniques starting from alkoxides (Al-butylate, tetraethylorthosilicate) and alkoxides plus nitrates (tetraethylorthosilicate, Al(NO₃)₃·9H₂O). After drying at 150°C the aluminosilicate gels are non-crystalline and remain so up to ≈ 900°C. Above 900°C the gels transform into Al₂O₃-rich mullite plus a coexisting SiO₂ phase. Structural studies on temperature-dependent dehydroxylation and condensation of the gels were carried out by large angle X-ray scattering, by infrared spectroscopy and by²⁹ Si NMR spectroscopy. Heat-treatment (<150°C) of dried gels first causes removal of the H₂O and organic residuals weakly bound at the open pore surfaces of the gels while the stronger, structurally bound OH groups are not affected. At temperatures <600°C OH groups are released and recombine to molecular H₂O. If the temperature does not exceed 800°C the newly formed H₂O is trapped in closed nanopores of the gel-network. Corresponding electron microscopical investigations reveal agglomerates of ≈10 nm sized primary particles virtually unaffected by the heat-treatment below 900°C. NMR investigation provided a new structural model on type and distribution of coordination polyhedra in aluminium silicate gel networks. Unlike Si, which according to ²⁹Si NMR is always 4-fold coordinated with O, ²⁷Al NMR spectroscopy revealed that Al cordination is more complex and is influenced by thermal treatment. Al occurs six-fold (octahedrally) and four-fold (tetrahedrally) coordinated. A third ²⁷Al NMR signal which has been attributed to five-fold-coordinated Al in the literature increased in intensity with the heat-treatment. A comparison of NMR data of the gels with those of mullite suggests that tetrahedra triclusters (3 tetrahedra having one oxygen atom in common) occur as major structural units in aluminium silicate gels rather than five-fold-coordinated Al. Triclusters of tetrahedra may compensate the excess negative charge in the network caused by Si⁴⁺ → Al³⁺ substitution. The charge compensation model is supported by aluminosilicate gels doped with network modifiers (e.g., Na⁺). Since equimolar addition of Na⁺ compensates Si⁴⁺ → Al³⁺ substitution the formation of triclusters is no longer required which actually can be deduced from²⁷ Al NMR studies.