Ionic liquid membranes for carbon dioxide–methane separation

Abstract

The transport of carbon dioxide and methane in polymer containing ionic liquid was studied by a dynamic gas permeation method. Poly(vinylidene fluoride-co-hexafluoropropylene) polymeric membrane (Viton) and two ionic liquids 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf 2N]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf 2N]) were used for preparation of ionic liquid membranes (ILM) with different ionic liquid amounts (from 0 to 80 wt%). For the better understanding of transport mechanism through ionic liquid membranes it is very useful to know also transport properties of pure ionic liquids and pure polymeric material. The gas transport through pure ionic liquid was determined indirectly, by means of the gas transport measurement through two special types of membranes: “sandwich arrangement” and “support arrangement”. The dependencies of the separation factor and gas fluxes on the ionic liquid amounts in ionic liquid membranes were determined. The gas permeability increased with the ionic liquid [emim][Tf 2N] content in the membrane, the permeability for the membranes with [hmim][Tf 2N] exhibits the maximum for IL concentration of 70 wt%. Ideal separation factor CO 2/CH 4 was low for small contents of ILs in membranes (0–15 wt%), around 7 for [emim][Tf 2N] and 8 for [hmim][Tf 2N] but for higher contents (30–75 wt%) it was approximately constant, 15 for [emim][Tf 2N] and 12 for [hmim][Tf 2N]. Very interesting is the comparison of carbon dioxide permeability, it increases in the series: polymer, ionic liquid and ionic liquid membrane. Although the transport properties values of ILM were expected to be in the middle of the ionic liquid and the polymer from which were formed, surprisingly the obtained transport properties of ILM are much better than those for the pure components. For example, the carbon dioxide permeability for the ionic liquid membrane with 70 wt% of [hmim][Tf 2N] is almost thousand times higher than for the pure polymer, and hundred times higher than for the ionic liquid. This fact indicates that the mechanism of the transport in ionic liquid membranes has to be different from the transport mechanisms in an ionic liquid. An explanation could be that “new transport pores” are created between polymer chains and an ionic liquid.