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

Abstract

A comprehensive study of the effect of pore size on the adsorption of xenon on mesoporous MCM-41 molecular sieves and 129 Xe NMR chemical shifts has been made. 129 Xe 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 129 Xe 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 129 Xe 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.