Effect of post-synthesis hydrothermal treatments on the adsorptive volume of surfactant-templated mesostructures

Ordered aluminosilicate mesostructures of the MCM-41 family having a SiO2/Al2O3 ratio similar to 5 were prepared using cetyltrimethylammonium chloride. These mesophases were subsequently exposed to a hydrothermal treatment (110 degreesC for one week at autogeneous pressure). The effects of the hydrothermal treatment were documented by analyzing the samples before and after the treatments by scanning electron microscopy, N-2 sorption for pore volume and pore sizes, powder X-ray diffraction and solid-state, magic-angle-spinning (MAS)-NMR spectroscopy for the Al-27 and Si-29 nuclei. The post-synthesis hydrothermal treatment increased the mesostructure adsorptive volume by as much as 55% in mesostructures prepared from Na-aluminate (from 0.29 to 0.45 ml/g) and 27% in mesostructures prepared from Alhydroxide (from 0.44 to 0.56 mVg) while the average pore diameter remained essentially unchanged. The post-synthesis hydrothermal treatment caused only small changes in the SiO2/Al2O3 ratio when the source of the aluminum ion was Al-hydroxide (6.4-6.6) and Na-aluminate (5.6-5.8). The crystal symmetry did not change after successive treatments, the unit cell size of the samples prepared from Al-hydroxide increased slightly (5.5-5.7 nm) and the peak width at mid-height changed very little. The mesophases prepared from either source of aluminum showed tetrahedral (T-d) symmetry, as confirmed by Al-27-MAS NMR before the hydrothermal treatment. After two successive hydrothermal treatments, almost all the Al ions show Td symmetry together with a small population of Al ions in the octahedral (O-h) symmetry. The calcined mesostructures all showed Al ions in Td and Oh environments. (C) 2000 Elsevier Science B.V. All rights reserved.