Interaction of water vapour at 298 K with Al-MCM-41 materials synthesised at room temperature

The interaction of water vapour with Al-MCM-41, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, aluminium sulfate, hexadecyltrimethylammonium bromide and ammonia, and its effect on the pore structure were studied in order to investigate the stability towards prolonged exposure to water vapour and the influence of the aluminium content. With this purpose two consecutive water adsorption isotherms were determined at 298 K on samples with Si/Al ratio between 15 and 100. The samples were characterised by X-ray diffraction and adsorption of nitrogen at 77 K and toluene at 298 K, prior to and after exposure to water vapour. Pore size distributions were calculated from nitrogen, toluene and water adsorption isotherms using, respectively, the NLDFT method, a recently developed hybrid MC-DBdB method and the DBdB macroscopic approximation. It was found that Al-MCM-41 samples are significantly stable and that the stability improves as the amount of aluminium increases. Upon prolonged exposure to water vapour, there is a small decrease in pore size (3–5%), pore volume (8–16%) and total surface area (3–7%). The structural changes are essentially a consequence of the surface hydroxylation that occurred and not a result of a partial collapse of the pore structure. Although the presence of some extraframework Al can contribute to the improvement of the stability by protecting the surface, it was concluded that tetracoordinated Al plays an important role. The stabilizing effect of the Al incorporated in the walls can result from a higher degree of condensation on the surface of the pore walls and from the mild acidity generated.