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要旨:
The chemical modification of the surface of known materials is an attractive possibility to enhance the properties of well defined structures. Besides the application in the field of catalysis the use of such modified materials in gas separation attracted much attention in the recent literature. The functionalization of surfaces, for instance with amine groups, was already used in various studies for carbon dioxide sequestration by adsorption as well as membrane separation [1, 2]. Because of large surface areas and the possibility to modify the surface functionality easily, porous silica is regarded as a potential material class to benefit from the chemical modification of its internal surfaces.
In this study, mesoporous glass membranes [3] were modified with intend to enhance the CO2 selectivity over N2. Controlled pore glass was chosen due to the adjustable pore size and the known adsorption potential for carbon dioxide and propane [4]. The prepared glass membranes were characterised by a narrow pore size distribution, an average pore diameter of 5 nm and inner surface areas of approx. 140 m²/g.
Approved surface grafting methods were used to functionalize the membrane surface. Various aminopropyl-trialkoxysilanes were grafted onto the membrane surface by reaction in toluene. Several substitutions of the amino-function were used to modify the basicity and steric hindrance of the amine-group in order to adjust the strength and quantity of carbon dioxide adsorption [4]. The amine content of the modified surface was furthermore adjusted by using modifying agents containing a higher number of amine groups such as ethylenediamine and diethylenetriamine groups. The success of the grafting procedure was examined by X-ray photoelectron spectroscopy.
Adsorption isotherms of carbon dioxide were investigated at different temperatures using a volumetric method. The amino-modification led to an increased adsorption of carbon dioxide especially at elevated temperatures due to the formation of carbamate species [5].
Single and binary gas permeation was conducted using the gases carbon dioxide, propane and nitrogen by a modified Wicke-Kallenbach-cell [6] in a transient and a steady state type of diffusion experiment, respectively. Since carbon dioxide and propane have the same molecular weight, observed selectivities can be ascribed exclusively to the different interactions with the surface of the membrane. Higher surface concentrations of the adsorbable gases can result in increased surface diffusion. This effect can be exploited to generate separation performances beyond the Knudsen selectivity. Problems arise if the adsorbed species loose their mobility on the adsorbents surface. Too strong adsorption suppresses the surface diffusion at lower temperatures and reduces observed selectivities to Knudsen ratio.
For some modifications a shifting of the carbon dioxide selectivity towards higher temperatures was observed. This shifting might be useful in combination with the comparably high temperature stability of the inorganic membranes.
Literature
[1] Knowles G. P., Graham J. V., Delaney S. W., Chaffee A. L., Fuel Proc. Tech. 86 (2005) 1435.
[2] McCool B. A., DeSisto W. J., Adv. Funct. Mater. 15 (2005) 1635.
[3] Enke D., Janowski F., Schwieger W., Microporous Mesoporous Mater. 60 (1-3) (2003) 19.
[4] Zelenak V., Halamova D., Gaberova L., Bloch E., Llewellyn P., Microporous Mesoporous Mater. 116 (2008) 358.
[5] Serna-Guerrero R., Da’na E., Sayari A., Ind. Eng. Chem. Res. 47 (23) (2008) 9406.
[6] Marković A., Stoltenberg D., Enke D., Schlünder E.-U., Seidel-Morgenstern A., J. Membr. Sci. 336 (2009) 17.
