In order to utilize biogas and natural gas as energy resource, a separation of undesired components is recommended. Nitrogen and carbon dioxide decrease the heating value, sulphur-containing compounds are often corrosive. A possibility to upgrade biogas or natural gas is the separation by adsorption on porous materials. Usually zeolites and activated carbons were used for such separation processes. In the last few years a new class of porous materials, the so called MOFs (metal-organic frameworks) were developed, which have mostly high specific surface areas, and pore volumes. One of the commercially available MOF's is MIL-53-Al (BasoliteTM A100). MIL-53-Al is a microporous material with a specific surface area of 1100-1500 m2 g-1 and a total pore volume of about 0.4 cm3 g-1. However, in contrast to other microporous adsorbents, this material exhibits a high framework flexibility after solvent removal. As a result, a hysteresis loop is observed in the adsorption isotherms for subcritical fluids like xenon or carbon dioxide at temperatures below 263 K.
Adsorption measurements of pure gases on porous materials are a starting point to characterize the potential to separate gas mixtures, e.g., natural gas mixtures like methane/nitrogen, methane/carbon dioxide or methane/ethane. Binary adsorption data can be easily obtained from the pure gas isotherms by using the IAST model. Furthermore, separation factors for binary mixtures can be determined. This is less time-consuming than the measurement of mixed gas adsorption by volumetric-chromatographic methods. The disadvantage of this method is the assumption of ideal behavior of the system.
Another method to measure binary adsorption isotherms is the gravimetric method by using a constant flow at different pressures. Here, only the total adsorbed amount of a mixture can be measured. The adsorbate concentrations are unknown and can be determined by the van-Ness Method.
By combining the gravimetric and volumetric method or by using the volumetric method coupled with gas chromatography it is possible to get all data experimentally, which is necessary to describe the whole system accurately.
In this presentation, we will show pure gas adsorption of methane, nitrogen and ethane in the temperature range of 273 to 323 K and pressures up to 5 MPa, which were measured by the gravimetric method on the commercially available MOF material MIL-53-Al (BasoliteTM A100). From these isotherms, the binary adsorption isotherms for mixtures containing 10, 20 and 40 vol.-% nitrogen or ethane in methane were calculated using the IAST model. Secondly, binary adsorption of these mixtures was measured in the same temperature ranges and pressures up to 2 MPa by the gravimetric method. Also, the prediction of the IAST model was evaluated. Finally, selected binary adsorption points were proven by volumetric-chromatographic method.