Page 398 - Adsorption by Powders and Porous Solids

CHAPTER 11. ADSORPTION BY CLAYS, PILCs AND ZEOLITES 387

Table 11.3. Henry's law constants and low coverage energies of adsorption for various alkanes on Nay
and HY
-
Adsorptive Nay HY
k, x 10' (mol g -') E,, (kJ mol-')
n-Hexane 1.7 44.2
2-Methy lpentane 1.7 44.2
3-Methy lpentane 1 -7 43.5
2.3-Dimethylbutane 1.8 43.5
2.2-Dimethylbutane 1.8 43.5
n-Heptane 3.6 50.1
2,3Dimethylpentane 3.6 50.1
n-Octane 7.9 56.0
2-Methylheptane 7.9 55.7
2 J-Diethylexane 8.3 56.0
n-Nonane 17 62.0
Fm Denaycr and Baron, (1997).

md branched paraffins by various forms of zeolite Y. By measuring the retention
times corresponding to the perturbation of the adsorption system at different load-
ings, it yas possible to derive the adsorption isotherm for each component. The mea-
surements were made over the range 275-400°C. This study of the effect of chain
length and branching of alkanes (from C, to C,,) followed earlier investigations of
the adsorption of lower hydrocarbons by the faujasite zeolites (e.g. the work of
Atkinson and Curthoys, 1981; Tharnrn et al., 1983). Our present interest is in the
behaviour of Nay and HY. Selected values of Henry's law constant, k,, and low-
coverage energy of adsorption, E,, are given in Table 11.3.
Inspection of Table 1 1.3 reveals that there are relatively small differences between
the corresponding values of kH and E, for Nay and HY. This is to be expected since
the adsorbent-adsorbate interactions are essentially non-specific (see Chapter 1).
Decationization of zeolite Y thus has a minimal effect on the energetics of adsorption
of the paraffins. The molecular shape of the adsorptive is also unimportant. In accor-
dance with the results in Figures 1.5 and 1.6, the molar mass (number of carbon
atoms) is much more important than the molecular shape. As before, there is a linear
relation between E, and N,. An exponential increase of k, with N, is of course
consistent with the form of Equation (4.3).
The interaction of polar molecules with ionic and polar surfaces was briefly dis-
cussed in Chapter 1. A simplified form of Equation (1.6) was given as

where En, represents the non-specific adsorbent-adsorbate interactions and E,, the
various specific contributions. When zeolites are used as adsorbents the E, tern
becomes exuemely important (Kiselev. 1967; Barrer, 1978). The magnitude of the
specific contributions is illustrated by the low-coverage adsorption calorimetric data
in Table 1 1.4.
Comparison of the values of E, is made in Table 1 1.4 for pairs of molecules of very