The Catalyst Review - March 2015 - 11

SPECIAL FEATURE
separation performances; however, it significantly affected the long-term operation stability. Additionally, UV and TEM confirmed the
long-term operational stability was strongly associated with the reduction rate of the silver ions. The reduction rate of the silver ions
in the polymer/silver salt/ionic liquid complex was observed in following order: [Bmim][BF4] > [Bmim][OTf] > [Bmim][NO3] suggesting
that among the ionic liquids investigated, [Bmim][NO3] had the most improved separation performance.
In addition to silver, copper also has been investigated in ionic liquid media. Copper nanoparticles in 1-Methyl-3-octyl imidazolium
tetrafluoroborate [Omim][ BF4] ionic liquid was introduced for olefin/paraffin separation (Hong et al. 2013). When the copper
nanoparticles were generated in the IL 1-octyl-3-methylimidazolium tetraflurorborate [Omim] [BF4] the separation performance was
twice that of the neat IL and the mixed gas permeance increased from 6.9 to 12 GPU due to facilitated olefin transport. A similar
mechanism was proposed, whereby the enhanced separation performance was attributed to both the free imidazolium ions and the
positively polarized copper nanoparticles, which favorably interact with the olefin molecules.
A more detailed study into the effects of imidazolium-based ionic liquids on dissociation of micrometer-sized copper flakes into
copper nanoparticles and on facilitated olefin transport was investigated (Han et al. 2011). Again, FT-Raman spectroscopy evidenced
that the positively charged surface on the copper was induced by interactions with anion of the IL. TEM images and UV-vis spectra
have been confirmed by the intensity of the copper-ionic liquid interactions depending on the size and distribution profiles of the
nanoparticles. The separation performance of the ionic liquid/Cu metal composite membranes for olefin/paraffin mixtures was in
the following order: [Emim][BF4]/Cu < [BMIM][PF6]/Cu < [BMIM][BF4]/Cu. Surprisingly, the binding energies of the copper atoms in
the ionic liquid/Cu metal composites were strongly correlated with the separation performance. Consequently, it was concluded that
facilitated olefin transport was consistent with the trend of increasing binding energy of copper atoms.

Silver Containing Ionic Liquids
In general, liquid membranes have higher selectivity values than the non-porous solid membranes and because of the higher liquid
phase diffusivities the economic efficiency of membrane processes is determined by the membrane selectivity and the membrane
productivity (i.e., the flux that can be realized through the membrane). For the separation of olefin/paraffins, selectivity is the most
important factor as low capacity can be compensated by a bigger membrane area but the low selectivity still requires a multicomponent process.
The use of ionic liquid-silver complexes has been recently studied and patented by Dai et al. (2011) along with the US government.
This is the only ionic liquid-based patent for olefin parrafin separation.
Agel et al. (2011) measured physical properties and complex stabilities of [Ag(C3H6)x][NTf2] to determine gas solubilities and
diffusion coefficients of propene and propane in order to calculate selectivities for membrane separation. The complex-ionic
liquid[Ag(propene)x][NTf2] formation at ambient temperatures formed a stable liquid membrane. Importantly, the system
contained no additional volatile membrane components such as molecular solvents or water that can get lost during the separation
process or contaminate the gas streams. Due to the higher silver content the propene capacity is much higher compared to those
measured for silver salt solutions in standard ionic liquids, resulting in a solubility selectivity as high as 40 at 1 bar mixed propene/
propane pressure.
Silver salt containing ionic liquids were also employed in the liquid-liquid olefin/parrafin separation. The extraction of C5-C8 linear
α-olefins from olefin/paraffin mixtures of the same carbon number using silver(I)/N,N-dimethylbenzamide bis(trifluoromethylsulfonyl)imide ([Ag(DMBA)2][Tf2N]) or silver(I)/propylamine bis(trifluoromethylsulfonyl)imide ([Ag(PrNH2)2][Tf2N]) as the extracting
agent was investigated (Wang et al. 2014). The separation performance of the system increased with increasing chain length of the
olefin/paraffin mixture with the [Ag(DMBA)2][Tf2N] ionic liquid outperforming [Ag(PrNH2)2][Tf2N] in terms of both selectivity and
distribution coefficient. Modeling using the universal quasi-chemical activity coefficient (UNIQUAC) model suggested that the silver
cation interacts more strongly with the PrNH2 compared with the DBMA, meaning in the latter IL the silver can interact more freely
with the olefin. In addition, doping of the IL with water also improved performance with selectivities as high as 38 being observed for
the octene/octane system.

Ionic Liquid Membranes
An adaptive self-healing ionic liquid nanocomposite membrane comprising a multi-layer support structure impregnated with [Ag]
[NTf2] hs been used for propene/propane separation (Pitsch et al. 2012). The ionic salt renders liquid like upon complexation with
propene increasing the olefin solubility Figure 2. This results in facilitated transport of propene over propane at benchmark-setting
selectivity and permeance levels. Importantly, the membrane showed a regenerative or self-healing when recontacted with propene
after desorption.
The Catalyst Review 										

March 2015

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The Catalyst Review - March 2015

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