ADSORPTION ON MCM-41 MESOPOROUS MOLECULAR-SIEVES .1. NITROGEN ISOTHERMS AND PARAMETERS OF THE POROUS STRUCTURE

Nitrogen adsorption isotherms have been measured on a series of alumin osilicate and titanosilicate MCM-41 molecular sieves, whose mean pore radius varied from ca. 0.9 to ca. 2 nm. By means of comparison plots t he nature of the adsorption on these materials was found to depend str ongly on their pore size. With pores of radius of around 1 nm multilay er coverage of the pore walls occurs. If the radius is increased to 1. 5-1.8 nm the mechanism of adsorption changes into a two-stage one, i.e . the multilayer coverage of the pore walls is succeeded by the sponta neous filling of the pore volume by capillary condensation without hys teresis. In even greater pores the usual capillary condensation with h ysteresis occurs. The estimation of pore structure parameters was base d on standard methods of adsorption isotherm processing, including the calculation of the pore size distribution from the desorption branch of the hysteresis loop. With the smaller pore materials, where the Kel vin equation does not hold, the estimation was based on the cylindrica l pore model. A detailed knowledge of the pore structure of MCM-41 mat erials was thus obtained.