Research on CO2 capture by imidazolium and alkali metal salt hybrid ionic liquids

Abstract

In this work, several alkali metal salts, shortened by MBF4 and CH3OOM(MAc, M = Li+, Na+, K+), were mixed with [Bmim][BF4] or [Bmim][Ac] separately to prepare kinds of hybrid absorbents (i.e., [Bmim][BF4]-LiBF4, [Bmim][BF4]-NaBF4 and [Bmim][BF4]-KBF4 with the same anion of [BF4]-, [Bmim][Ac]-LiAc, [Bmim][Ac]-NaAc and [Bmim][Ac]-KAc with the same anion of acetate) to absorb CO2. The results show that the hybrid absorbents based on [Bmim][BF4] has a relatively enhanced absorption effect on CO2, while the hybrid absorbent based on [Bmim][Ac] has an inhibitory effect. When a low concentration of MBF4 was applied, LiBF4-[Bmim][BF4] shows higher CO2 capacity than NaBF4-[Bmim][BF4] and KBF4-[Bmim][BF4] solutions. With the concentration of MBF4 increase, NaBF4-[Bmim][BF4] and KBF4-[Bmim][BF4] become more favorable for CO2 absorption, which also appeared in the MAc-[Bmim][Ac] systems. According to the MD simulation results, Na+ and K+ have stronger attraction forces with CO2, and Na+ is easier to make the [BF4]- anions to aggregate around it. According to the MD calculations, for the [Bmim][BF4]-MBF4 hydrid systems, Na+ has the weakest ability to aggregate and form M+ layer hindering the contact of CO2 and anion. Combining with its strongest ability to attract the [BF4]- anion, a largest space of [Bmim][BF4] to accomodate more CO2 molecules is created. In comparison, for the physical–chemical absorbent [Bmim][Ac], the strong M+ aggregation and [Ac]- attraction by M+ are the main reasons for the decreased CO2 capacity, due to the reduced number of [Ac]- responsible for the chemically binding with CO2 during absorption.