(719a) Using a Gravimetric Method to Measure the Solubility and Diffusivity of H2 and CO2 in the Ionic Liquid [Bmim][PF6]

Using a Gravimetric Method to Measure
the Solubility and Diffusivity of H₂ and CO₂
in the Ionic Liquid [bmim][PF₆]

Seyed Hamed Barghi, Theodore T. Tsotsis, and Muhammad
Sahimi[1]

Mork Family Department of Chemical Engineering &
Materials Science, University of Southern California, Los Angeles, California
90089-1211
 
Abstract

In
this talk, we will report on the measurement of the solubility and diffusivity
of H₂ and CO₂ in the ionic liquid 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF₆]) over the temperature range 25 ^oC
- 55 ^oC and for pressures of up to 10 MPa. The data were obtained
using a magnetic suspension balance, a gravimetric instrument that is capable
of simultaneously and accurately measuring the gas solubility and diffusivity
in such liquids. To our knowledge, this is the first time that the H₂ diffusivity
has been measured in an ionic liquid. While solubility data for H₂ have
been previously reported, they vary widely. The cause for such variations will
be discussed as well. The diffusivity data for both H₂ and CO₂
follow an Arrhenius-like dependence on
temperature:

where (cm²/s) is
the diffusivity, T (K),  the pre-exponential factor (cm²/s),  (kJ/mol) the activation energy, and  the gas constant (0.008314 kJ/mol. K). For H₂,
=146 and =42.43. For CO₂, =0.64 and =32.

The
diffusivity data for both gases indicate very little, if any, dependence on the
pressure, indicative of Fickian-like diffusion and the lack of swelling
effects. The solubility data for both H₂ and CO₂ in the
same ionic liquid indicate a Henry-like dependence for the entire ranges of
pressures and temperatures studied. Both Henry's constants increase with
increasing temperature, as one may have expected. The success of the technique
of measuring the transport of thermodynamic properties of this important gas
pair will, hopefully, motivate its use for measuring the transport and
thermodynamic properties of other important gas mixtures in this and other ionic
liquids.