Characterization of porous solids: Sorption and pore condensation behavior of nitrogen, argon, and krypton in ordered and disordered mesoporous silica materials at temperatures above and below the bulk triple point

摘要

The pore condensation and hysteresis behavior of nitrogen, argon and krypton was studied on well defined, ordered porous materials like MCM-48, MCM-41 silica (mode pore diameters 2―5 nm) and SBA-15 (6.7 nm) at 87 K and 77 K. A comparison with the results of similar sorption experiments carried out using more disordered adsorbents like controlled-pore glasses (mode pore diameters 11 and 16 nm), and silica gel (mode pore diameter of 11 nm) is made. The results show clearly that the shape of sorption isotherms (in particular the shape and the width of sorption hysteresis loops) depend both on temperature and pore diameter, i.e., the thermodynamic states of pore fluid and bulk fluid, but― in particular at temperatures below the bulk triple point ―also strongly on the texture (and degree of disorder) of the porous material. An analysis of nitrogen (at 77 K) and argon (at 87 K) adsorption-desorption isotherms in MCM-48 silica confirms that in this well-defined, interconnected pore network the desorption branch of the hysteresis loop represents the equilibrium transition. In addition, pore condensation of argon can still be observed at 77 K, i.e. ca. 6.5 K below the bulk triple point in MCM-48/-41 and SBA-15 silica materials with pore diameters < 8 nm. However, pore condensation vanishes at 77 K in case the pore diameter exceeds ca. 12 nm (CPG), which limits the range for mesopore-size analysis using argon sorption at 77 K.