Host effect in the sorption of ethylene by CsC24 and KC24

The host effect on the sorption behavior of the system, MC24C2H4(M = Cs, K) at 194 K was investigated. The amount of sorbed C2H4 (expressed as C2H4M) varies with the size of the natural graphite flakes used for the host material of MC24. For CsC24 prepared from natural graphite flakes with a particle size of 10 μm (abbreviated as CsC24(10 μm)) C2H4Cs = 2.0 at saturation, but it is difficult to reach saturation after C2H4Cs = 1.2 for CsC24(630 μm). In the case of KC24, C2H4K = 1.2 for KC24(10 μm) and almost no sorption is observed for KC24(630 μm). This is interpreted as a kinetical limitation due to diffusion, and the apparent diffusion coefficients of the system, KC24C2H4, are estimated to be about 10−11 to 10−13 cm2 s−1, depending on C2H4K. The diffusion rate seems to be too low for KC24(630 μm) to take up C2H4 in finite time. Even for KC24(10 μm), it is impossible to determine an isotherm because the diffusion rate decreases appreciably with increasing C2H4K. On the other hand, CsC24(10 μm) takes up C2H4 faster than KC24(10 μm): fast sorption until C2H4Cs = 1.2, followed by a relatively slow diffusion process up to 2.0 for C2H4Cs. The diffusion coefficient is about 10−11 cm2s−1, and decreases slightly with increasing C2H4Cs.