High resolution adsorption isotherms of N2 and Ar for nonporous silicas and MFI zeolites

The high resolution adsorption isotherms of N-2 (77.4 K) and Ar (87.3 K) have been measured for two nonporous silicas with different silanol contents (3.3 and 0.35 OH/ nm(2)) and for two MFI zeolite with different Al contents (Si/Al = 12.5 and 500). Silanol groups and Al sites ( acid sites) gives the significant effect on the N-2 isotherms at submonolayer, but the Ar isotherms are independent of silanols and Al sites. The Ar isotherms, therefore, are preferable in calculation of microporosity of zeolites. The N-2 and Ar isotherms for MFI zeolite (Si/Al = 500) have been measured at temperatures of 77-94 K, from which the differential adsorption energies of N-2 and Ar are calculated. The interaction of N-2 with channel surface of MFI zeolite is greater than that of Ar in the range of as = 0.1-0.7. The hystereses are detected for the N-2 isotherm in p/p(o) = 0.1-0.3 at 77.4 K and for the Ar isotherm in p/p(o) = 3 x 10(-4) -2 x 10(-3) at 87.3 K. However, it is difficult to explain the hysteresis phenomenon using differential adsorption energy.