Synthesis and usage of common and functionalized ionic liquids for biogas upgrading

Halide imidazolium-based ionic liquids 1-butyl-3-methylimidazolium bromide [bmim][Br] and 1-propylamine-3-methylimidazolium bromide [pamim][Br] were synthesised and tested experimentally as absorbents for biogas upgrading. The former solvent is a conventional off-the-shelf room temperature ionic liquid (RTIL), which is soluble in water and expected to have a high CO2 solubility. The second solvent is a new type of task-specific ionic liquid (TSIL) and has a straight amine-alkyl substitute incorporated to enhance CO2 solubility. Experimental evidence suggests that the majority of known imidazolium-based ionic liquids are good CO2 absorbents; adding these to a designed solvent that combines a long amino-alkyl lineal group is expected to improve the performance of CO2 absorption over other alternatives under discussion.CO2 absorption experiments were conducted in an absorbing column packed with randomly placed Raschig rings. A biogas model consisting of CO2 (43% v/v) diluted in N2 was used to test CO2 uptake with aqueous solutions of the above-mentioned ILs at 5%, 10% and 15% (w/w) concentrations. The gaseous and liquid streams were operated under concurrent flow with a gas–liquid volumetric ratio of 1:2. Additionally, absorption tests were conducted with aqueous solutions of monoethanolamine (MEA) mixed with the ILs under investigation to elucidate potential activations of these amino solutions. In this experiment, the ILs used for biogas upgrading do not present a higher absorption than amino solutions, and there were no significant synergy results from mixing them with MEA, which could have enhanced the CO2 uptake.

► [bmim][Br] and [pamim][Br] tested as CO2 uptake for biogas upgrading.
► These ILs do not present better CO2 uptake than solutions like MEA or DEA.
► The TSIL does not exhibit a better CO2 uptake than the TSIL for biogas upgrading.
► The results can be attributed to the increase in viscosity during the absorption.
► There is slight synergy in CO2 uptake in mixtures of these ILs and MEA solutions.