(193b) Pure and Mixed Gas Adsorption of Methane, Ethane and Nitrogen On Metal-Organic Framework MIL-53-Al

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 (Basolite^TM
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 (Basolite^TM 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.