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 (V_m) 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 (q_iso) 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 (ε, represented by the Davidson–Cole distribution parameter ε ⩽ 1) depends on the coverage θ (adsorbed gas/V_m) and is probably related to the surface heterogeneity of the main channel in Na-mordenite. The apparent molecular rotational barriers (E_a) also depend on the coverage. The E_a 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 ε and E_a suggests strongly that the favorable adsorption of fluoroalkanes occur at the Na⁺ site on the channel wall of Na-mordenite.