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

In order to investigate the nature of the surface potential inside,the micr opore of Na-mordenite at the molecular level, the temperature dependence of F-19 spin-lattice relaxation times (T-1) and the adsorption isotherms were measured for adsorbed CF4 and C2F6. The adsorption isotherms can be interp reted 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 isos teric heat of adsorption (q(iso)) of 25 +/- 2 kJ mol(-1) for CF4 and 37 +/- 2 kJ mol(-1) for C2F6. The T-1 of F-19 is governed by the rotation of the guest fluoroalkane below ca. 150 K and assumes a single minimum in the vici nity of 50 K for CF4 and ca. 60 K for C2F6, respectively at 40.4 MHz. The s lopes on both sides of the T-1 minimum are different for each guest compoun d, suggesting some distribution of the correlation times for the molecular reorientation. The degree of the distribution (epsilon, represented by the Davidson-Cole distribution parameter epsilon less than or equal to 1) depen ds on the coverage theta (adsorbed gas/V-m) and is probably related to the surface heterogeneity of the main channel in Na-mordenite. The apparent mol ecular rotational barriers (E-a) also depend on the coverage. The E-a for C F4 and C2F6 are estimated to be 3.4 and 4.0 kJ mol(-1), respectively, at lo w coverage (< 0.3) and 3.0 and 3.3 kJ mol(-1), respectively at high coverag e (greater than or equal to 1). Such a close correlation between epsilon an d E-a suggests strongly that the favorable adsorption of fluoroalkanes occu r at the Na+ site on the channel wall of Na-mordenite.