LOW-TEMPERATURE PYROLYTIC TRANSFORMATIONS OF TRI-TERT-BUTOXYSILOXY DERIVATIVES OF ALUMINUM TO ALUMINOSILICATE MATERIALS

The tri-tert-butoxysiloxy complex {Me2Al[mu-OSi(OtBu)3]}2 (1) Was prepared by the reaction of [AlMe3]2 with HOSi(OtBU)3. Crystals Of 1 are tetragonal (P4(2)nm) with a = 14.684 (3) angstrom, c = 9.463 (2) angstrom, V = 2041.8 (9) angstrom3, Z = 4, and R(F) = 5.77%. Molecules of 1 in the crystal have a folded Al2O2 core (156.2-degrees), probably because of steric interactions involving the bulky siloxy groups. A trimeric derivative, [Me(tBuO)AlO-Si(OtBu)3]3 (2), was obtained by addition of tert-butyl alcohol to 1. Compounds 1 and 2 were investigated as precursors to aluminosilicate materials. In the solid state, 1 undergoes thermolysis at low temperatures (150-200-degrees-C) by cleanly eliminating isobutylene (6 equiv), along with methane and water. Clean conversion to a A12O3.2SiO2 material is observed by thermal gravimetric analysis (TGA) and elemental analysis. Differential thermal analysis (DTA) and X-ray powder diffraction (XRD) show that mullite crystallizes at about 1000-degrees-C, and at 1200-degrees-C, mullite particles of about 15 nm are produced (as calculated from the Scherrer equation). Compound 2 exhibits similar pyrolytic conversions, except that much higher surface areas are observed for the aluminosilicates produced. For example, thermolysis of 2 at 200-degrees-C gives a sample with a BET surface area of 270 m2 g-1, compared to a surface area of only 30 M2 g-1 for a sample obtained similarly from 1. Compound 2 also cleanly decomposes by elimination of isobutylene (12 equiv). The network-forming thermolyses of 1 and 2 are facile enough to be conveniently carried out in solution. For example, 1 is pyrolyzed in refluxing toluene, resulting in a polymerization that resembles the sol-gel process. A gel powder obtained from this procedure had a surface area of 210 m2 g-1 and was composed of small particles (ca. 500 nm in diameter) consisting of much smaller grains (by transmission electron microscopy, TEM). Heating this sample to 800-degrees-C increased the BET surface area to 280 m2 g-1, and further heating to 1200-degrees-C produced mullite particles (average size 18 nm, by XRD) that were densely packed into an amorphous silica matrix (by TEM).