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Zusammenfassung:
Due to the threat of global warming caused by carbon dioxide accumulation in the atmosphere, the separation of carbon dioxide from other gases attracted much attention in the recent literature. Because of large surface areas and the possibility to modify the surface functionality, porous silica is regarded as a potential material class to contribute to solve this problem. Amino-functionalized porous silica was already used in various studies for carbon dioxide sequestration by adsorption as well as membrane separation [1,2].
In this study, mesoporous glass membranes [3] were modified to enhance the CO2 permselectivity. 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 and inner surface areas between 40 and 100 m2/g, measured by low temperature nitrogen adsorption. Two pore diameters were set to approximately 12 and 8 nm.
Approved surface grafting methods were used to functionalize the membrane surface. Three trialkoxysilanes (3-Aminopropyl)triethoxysilane, [3-(Methylamino)propyl]trimethoxysilane and (3-Anilinopropyl)trimethoxysilane were grafted onto the membrane surface by reaction in toluene or water. Different concentrations of trialkoxysilanes were used to adjust the thicknesses of the grafted layers and therefore the surface concentrations of the amines.
Adsorption isotherms of the adsorbable gases carbon dioxide and propane were investigated at different temperatures using a volumetric method. The amino-modification led to an increased adsorption of carbon dioxide due to the formation of carbamate species.
Single gas permeation was conducted using the gases carbon dioxide, propane and nitrogen by a modified Wicke-Kallenbach-cell [4]. 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. Nitrogen is referred as a nearly nonadsorbable, inert gas and as the main component of combustion gases. Therefore, the CO2/N2 selectivity is a major criterion to evaluate the potential of possible membrane processes.
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., Ads. Funct. Mater. 15 (2005) 1635.
[3] Enke D.,Janowski F. Schwieger W., Microporous Mesoporous Mat. 60 (1-3) (2003) 19.
[4] Marković A., Stoltenberg D., Enke D., Schlünder E.-U., Seidel-Morgenstern A., J. Membrane Sci., submitted
