Effect of pore size on the adsorption of xenon on mesoporous MCM-41 and on the ¹²⁹Xe NMR chemical shifts: A variable temperature study

A comprehensive study of the effect of pore size on the adsorption of xenon on mesoporous MCM-41 molecular sieves and ¹²⁹Xe NMR chemical shifts has been made. ¹²⁹Xe NMR spectra of MCM-41 samples (Si/Al = 37; pore size 1.8-3.0 nm) with varied xenon loading were obtained at different temperatures (140-340 K). The observed ¹²⁹Xe NMR chemical shifts were fitted by regressional nonlinear least-squares fitting based on a two-site exchange model. As a result, the temperature variation of ¹²⁹Xe chemical shifts at zero xenon loading, i.e. δ_s(ρ = 0) which arise mainly from xenon-wall interactions, were obtained. The pore size (d) and δ_s can be correlated by an empirical relations: δ_s(T, d) = A(T)/(d + B(T)). The two parameters, A(T) and B(T), are found to have nearly the same temperature dependence. At low temperature (T < 190 K), the two parameters both increase abruptly with decreasing temperature. Whereas at high temperature (T > 250 K), they were found to slowly decrease with increasing temperature.