10
M. B. Shiflett et al.
to several hundred bars. Therefore, an ionic liquid suitable for this application must
display good tribological behavior, minimum hydrogen solubility, suitable viscosity
and density, as well as high thermal stability, conductivity, and heat capacity at the
operational conditions. There must be no reactivity between the ionic liquid and
hydrogen (or any other gas that may be present). This requires a certain defined
quality of the ionic liquid. Ideally, the ionic liquid and hydrogen should be mutually
immiscible.
Hydrogen solubility (in contrast to, for example, carbon dioxide) is extremely low
in most ionic liquids. The temperature dependence of solubility is typically the opposite of that observed with carbon dioxide. Actually, hydrogen solubility increases with
increasing temperature. This behavior was observed during the early stage of systematic solubility investigations on ionic liquids of the second generation [54]. Solubility
data with a particularly low uncertainty over an extended p, T-range were reported for
[C 4 C 1 im][PF 6 ] and the IUPAC-proposed reference [C 6 C 1 im][NTf 2 ] using a staticcell technique [55, 56]. Calculating the enthalpy sol H and entropy sol S of gas
dissolution provides information about the molecular interaction between the gas
and the ionic liquid and about the degree of ordering that determines the solubility.
In the case of hydrogen, the Henry’s Law constant decreases at higher temperatures
which results in a positive dissolution enthalpy [57, 58]. From a thermodynamic
consideration, the solubilities of different gases converge to the same value at the
solvent critical temperature, an imaginary point for most ionic liquids due to their
limited stability [59]. Figure 1.1 illustrates this particular behavior of the Henry’s
Law constant based on solubility data for [C 6 C 1 im][NTf 2 ]. The low-soluble hydrogen shows the inverse temperature dependence in the region relevant to technical
applications.
In 2002, a company
12 started to pursue the idea of developing a commercial
ionic compressor with an ionic liquid as the operational fluid [61]. A patent for this
Fig. 1.1 Gas solubility as
represented by Henry’s Law
constant (approaching zero
pressure and on the molality
scale) of various gases in the
ionic liquid [C 6 C 1 im][NTf 2 ]
with extrapolation to a
hypothetical critical
temperature. For the
corresponding data, see the
paper by Kumełan et al. and
the references cited therein
[60]
12 Linde GmbH.
M. B. Shiflett et al.
to several hundred bars. Therefore, an ionic liquid suitable for this application must
display good tribological behavior, minimum hydrogen solubility, suitable viscosity
and density, as well as high thermal stability, conductivity, and heat capacity at the
operational conditions. There must be no reactivity between the ionic liquid and
hydrogen (or any other gas that may be present). This requires a certain defined
quality of the ionic liquid. Ideally, the ionic liquid and hydrogen should be mutually
immiscible.
Hydrogen solubility (in contrast to, for example, carbon dioxide) is extremely low
in most ionic liquids. The temperature dependence of solubility is typically the opposite of that observed with carbon dioxide. Actually, hydrogen solubility increases with
increasing temperature. This behavior was observed during the early stage of systematic solubility investigations on ionic liquids of the second generation [54]. Solubility
data with a particularly low uncertainty over an extended p, T-range were reported for
[C 4 C 1 im][PF 6 ] and the IUPAC-proposed reference [C 6 C 1 im][NTf 2 ] using a staticcell technique [55, 56]. Calculating the enthalpy sol H and entropy sol S of gas
dissolution provides information about the molecular interaction between the gas
and the ionic liquid and about the degree of ordering that determines the solubility.
In the case of hydrogen, the Henry’s Law constant decreases at higher temperatures
which results in a positive dissolution enthalpy [57, 58]. From a thermodynamic
consideration, the solubilities of different gases converge to the same value at the
solvent critical temperature, an imaginary point for most ionic liquids due to their
limited stability [59]. Figure 1.1 illustrates this particular behavior of the Henry’s
Law constant based on solubility data for [C 6 C 1 im][NTf 2 ]. The low-soluble hydrogen shows the inverse temperature dependence in the region relevant to technical
applications.
In 2002, a company
12 started to pursue the idea of developing a commercial
ionic compressor with an ionic liquid as the operational fluid [61]. A patent for this
Fig. 1.1 Gas solubility as
represented by Henry’s Law
constant (approaching zero
pressure and on the molality
scale) of various gases in the
ionic liquid [C 6 C 1 im][NTf 2 ]
with extrapolation to a
hypothetical critical
temperature. For the
corresponding data, see the
paper by Kumełan et al. and
the references cited therein
[60]
12 Linde GmbH.