Sorption and pore condensation behavior of pure fluids in mesoporous MCM-48 silica, MCM-41 silica, SBA-15 silica and controlled-pore glass at temperatures above and below the bulk triple point

The pore condensation and hysteresis behavior of nitrogen and argon 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 and 77 K. A comparison with the results of similar sorption experiments carried out using more disordered adsorbents like controlled-pore glasses (CPG) (mode pore diameters, 11 and 16 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. Analyses of nitrogen (at 77 K) and argon (at 87 K) adsorption–desorption isotherms in MCM-48 silica lead to the conclusion 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 of argon at 77 K vanishes in case the pore diameter exceeds ca. 12 nm (based on BJH method), which limits the range for mesopore-size analysis of silica materials using argon sorption at 77 K.