19F NMR study on CnF2n+2 (n=1 and 2) adsorbed in Na-mordenite: Dynamic behaviour and host–guest interaction

Abstract

In order to investigate the nature of the surface potential inside the micropore of Na-mordenite at the molecular level, the temperature dependence of 19F spin–lattice relaxation times (T1) and the adsorption isotherms were measured for adsorbed CF4 and C2F6. The adsorption isotherms can be interpreted by the Langmuir plot, giving the monolayer capacity (Vm) of 23 ± 2 ml(STP) g−1 for CF4 and of 18 ± 2 ml(STP) g−1 for C2F6, and the isosteric heat of adsorption (qiso) of 25 ± 2 kJ mol−1 for CF4 and 37 ± 2 kJ mol−1 for C2F6. The T1 of 19F is governed by the rotation of the guest fluoroalkane below ca. 150K and assumes a single minimum in the vicinity of 50K for CF4 and ca. 60 K for C2F6, respectively at 40.4 MHz. The slopes on both sides of the T1 minimum are different for each guest compound, suggesting some distribution of the correlation times for the molecular reorientation. The degree of the distribution (e, represented by the Davidson–Cole distribution parameter e ⩽ 1) depends on the coverage θ (adsorbed gas/Vm) and is probably related to the surface heterogeneity of the main channel in Na-mordenite. The apparent molecular rotational barriers (Ea) also depend on the coverage. The Ea for CF4 and C2F6 are estimated to be 3.4 and 4.0 kJ mol−1, respectively, at low coverage (<0.3) and 3.0 and 3.3 kJmol−1, respectively at high coverage (1). Such a close correlation between e and Ea suggests strongly that the favorable adsorption of fluoroalkanes occur at the Na+ site on the channel wall of Na-mordenite.