Comparative Study of Liquid−Solid Phase Transition of Nitrogen in Controlled Pore Glass and SBA-15:  Accurate Pore Size of Conventional Mesoporous Materials

To elucidate the origin of the discrepancies in pore size dependences of the melting temperature of the confined solids and the hysteresis temperature of the gas−liquid-phase transitions between conventional mesoporous materials and mesoporous molecular sieves, we performed measurements of the adsorption−desorption isotherm over a wide temperature range for N 2 confined in the interconnected pores of controlled pore glass with a nominal pore diameter of 11.5 nm and the cylindrical pores of SBA-15 with a mean pore diameter of 9.4 nm, as well as the X-ray diffraction pattern as a function of temperature. A hysteresis loop in the adsorption−desorption isotherm shrank with increasing temperature and eventually disappeared at a hysteresis temperature well below the bulk critical point. On the other hand, when the temperature was decreased well below the bulk triple point, the adsorption hysteresis still survived. The inflection points in the Δμ− T curve of capillary condensation and evaporation were nearly consistent with the freezing and melting temperatures determined by X-ray diffraction, respectively. The shifts of these characteristic temperatures relative to the bulk values for controlled pore glass were considerably smaller than those for SBA-15, although the nominal pore size of controlled pore glass was not very different from the pore size of SBA-15. All these results confirm the discrepancies in characteristic temperature shifts between the conventional mesoporous materials and the mesoporous molecular sieves inferred from a comparison of the literature data for the two types of adsorbents. The coherence length of the solid N 2 formed inside the interconnected pores of controlled pore glass was significantly larger than the nominal pore diameter, whereas that of the solid N 2 inside the cylindrical pores of SBA-15 was only slightly larger than the mean pore diameter. This strongly suggests that the pore sizes of controlled pore glass and other conventional mesoporous adsorbents are actually greater than specified.