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 aluminosilicate and titanosilicate MCM-41 molecular sieves, whose mean pore radius varied from ca. 0.9 to ca. 2 nm. By means of comparison plots the nature of the adsorption on these materials was found to depend strongly on their pore size. With pores of radius of around 1 nm multilayer 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 spontaneous filling of the pore volume by capillary condensation without hysteresis. In even greater pores the usual capillary condensation with hysteresis occurs. The estimation of pore structure parameters was based 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 Kelvin equation does not hold, the estimation was based on the cylindrical pore model. A detailed knowledge of the pore structure of MCM-41 materials was thus obtained.