Chromatographic Characterization Of Adsorption Properties Of Mcm-41 C16
Chromatographic Characterization of Adsorption Properties of MCM-41 C16
H. Grajek1, J. Choma1, M. Jaroniec2
1Military Technical Academy, Institute of Chemistry,
Kaliski st. 2, 00-908 Warsaw, Poland
2Department of Chemistry, Kent State University, Kent, OH 44242, USA
A mesoporous siliceous adsorbent of hexagonally ordered mesostructure has been synthesized using alkyltrimethylamonium surfactants. Its synthesis has been carried out in the presence of cetyltrimethylamonium bromide by applying a relatively long hydrothermal formation of siliceous material MCM-41 C16 at 373 K. This material was used to measure adsorption isotherms for benzene and ethanol at the temperature range from 343 to 393 K (Fig. 1.) by employing gas chromatography.
Table 1. The parameters of the Unilan equation calculated on the basis of ethanol and benzene adsorption isotherms at different temperatures and the corresponding enthalpies of adsorption
Adsorbate
Temp.; K
am ; mmol/g
s ; -
c ; kPa
B2S ; cm3/g
-DHads ; kJ/mol
Ethanol
343
0.67
6.28
8.4
9667.2
43.1
363
0.56
6.77
29.0
3750.5
393
0.16
7.65
45.0
1595.1
Benzene
343
0.47
6.33
50.5
1176.5
37.6
363
0.37
6.20
74.2
598.1
393
0.20
6.46
128.6
251.4
The Unilan equation [1] was employed to analyze the chromatographically obtained adsorption isotherms for ethanol and benzene on the MCM-41 C16 material. This equation was chosen because it is usually used to characterize the energetically heterogeneous adsorbents, it predicts the Henry law behavior, contains only three constants and gives analytical expressions for the amount adsorbed. The values of these constants for the
systems studies are listed in Table 1. The temperature–dependent constants changed regularly for ethanol and irregularly for benzene. In addition, Table 1 contains the values of the adsorption second virial coefficient predicted by the Unilan equation as well as the corresponding values of the adsorption enthalpy evaluated by using Antoine-type plot.
Figure 1. Benzene and ethanol adsorption isotherms.
The higher values of the adsorption enthalpy for ethanol are probably connected with the higher interaction energy between ethanol molecules and surface hydroxyl groups in comparison to that between the surface hydroxyls and the p-electrons ring of benzene molecule. The delocalized p-electrons in the benzene ring are polarizable and control interactions between aromatic rings and surface hydroxyls of the MCM-41 material. In contrast to benzene, ethanol molecules are able to create a non-stable and stable hydrogen bonds of energy about 22-42 kJ/mol. Surface hydroxyl groups are probably unaffected by heating of the MCM-41 samples at the temperatures studied.
References
1. D.P. Valenzuela and A.L. Myers, Adsorption Equilibrium Data Handbook, Prentice Hall, Englewood Cliffs, New Jersey (1989).
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