A Rational Approach to CO 2 Capture by Imidazolium Ionic Liquids: Tuning CO 2 Solubility by Cation Alkyl BranchingChemSusChem

About

Authors
Marta C. Corvo, João Sardinha, Teresa Casimiro, Graciane Marin, Marcus Seferin, Sandra Einloft, Sonia C. Menezes, Jairton Dupont, Eurico J. Cabrita
Year
2015
DOI
10.1002/cssc.201500104
Subject
Environmental Chemistry / Materials Science (all) / Energy (all) / Chemical Engineering (all)

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Text

A Rational Approach to CO2 Capture by Imidazolium Ionic

Liquids: Tuning CO2 Solubility by Cation Alkyl Branching

Marta C. Corvo,*[a] Jo¼o Sardinha,[a] Teresa Casimiro,[a] Graciane Marin,[b] Marcus Seferin,[c]

Sandra Einloft,[c] Sonia C. Menezes,[d] Jairton Dupont,[b, e] and Eurico J. Cabrita*[a]

Introduction

The prospect of fine-tuning the physical and chemical properties of ionic liquids (ILs) has motivated continuously growing research interest due to their wide range of applications, from alternative solvents to reactive catalytic supports, spatial devices, and in biotransformations.[1]

One possible application for ILs is in the field of gas capture and storage. ILs are considered to be alternative materials for

CO2 capture because CO2 solubility and selectivity in ILs can be controlled by the correct combination of specially tailored anions and cations. In this context, and mainly due to the global warming effect, the development of efficient IL-based methodologies to capture CO2 from industrial flue gases has become an important issue.[2] Nowadays, CO2 capture is mostly based on the reaction with alkanolamines, which form ammonium carbamates in aqueous solutions. This process requires high energy and suffers from solvent loss and corrosion.[3] ILs offer an alternative to this process, through either chemical or physical absorption.[3b–5] The possibility of performing chemisorption is closely related to the basicity of ILs,[6] the use of superbases as additives,[7] or the presence of primary or secondary amine functionalities in the IL structure.[8] The formation of carbene adducts has been particularly highlighted to explain the mechanism of CO2 chemisorption in basic imidazolium (Im)

ILs.[9] Although chemical absorption falls outside the scope of this paper, it is interesting to note that significant cation effects were found under these conditions.[10]

In an IL-based separation or capture process by physical absorption, after CO2 has been removed from the gas mixture, it can be released by either pressure or temperature changes, and the IL can be reused. This requires low energy and reduces concerns of solvent loss due to the extremely low vapor pressures of ILs.[4] Physical absorption of ILs is particularly attractive for applications such as natural gas sweetening and precombustion gas separation, in which the CO2 content is of higher concentrations and pressures are up to tens of bar (1 bar=1 105 Pa).[4, 5]

To optimize IL-based CO2 capture technology, accurate knowledge of the factors that control gas absorption at the molecular level is needed.

Several reports have documented that functionality on the anion and cation, as well as the IL free volume, can improve the solubility of CO2 in the IL. [11] Higher absorptions are usually attained with fluorination of the anion and also with cations with longer alkyl chains or non-fluorinated substituents with carbonyls, esters, or ether groups.[12] Fluorination at the cation is also a successful strategy for improving CO2 solubility in

ILs.[13] However, this additional fluorination brings even more concerns regarding biodegradability and toxicity. General molecular criteria for enhancing gas solubility and selectivity in ILs

Branching at the alkyl side chain of the imidazolium cation in ionic liquids (ILs) was evaluated towards its effect on carbon dioxide (CO2) solubilization at 10 and 80 bar (1 bar=110 5 Pa).

By combining high-pressure NMR spectroscopy measurements with molecular dynamics simulations, a full description of the molecular interactions that take place in the IL–CO2 mixtures can be obtained. The introduction of a methyl group has a significant effect on CO2 solubility in comparison with linear or fluorinated analogues. The differences in CO2 solubility arise from differences in liquid organization caused by structural changes in the cation. ILs with branched cations have similar short-range cation–anion orientations as those in ILs with linear side chains, but present differences in the long-range order. The introduction of CO2 does not cause perturbations in the former and benefits from the differences in the latter.

Branching at the cation results in sponge-like ILs with enhanced capabilities for CO2 capture. [a] Dr. M. C. Corvo, Dr. J. Sardinha, Dr. T. Casimiro, Prof. Dr. E. J. Cabrita

REQUIMTE, UCIBIO, LAQV, Dep. Qumica

Fac. CiÞncias e Tecnologia, UNL 2829-516 Caparica (Portugal)

E-mail : marta.corvo@fct.unl.pt ejc@fct.unl.pt [b] Dr. G. Marin, Prof. Dr. J. Dupont

Institute of Chemistry, UFRGS

Av. Bento GonÅalves 9500 91501-970 Porto Alegre, RS (Brazil) [c] Prof. Dr. M. Seferin, Prof. Dr. S. Einloft

PUCRS, Fac. Qumica

BR-90619900 Porto Alegre, RS (Brazil) [d] Dr. S. C. Menezes

PETROBRAS/CENPES 21941-915 Rio de Janeiro, R.J. (Brazil) [e] Prof. Dr. J. Dupont

School of Chemistry, University of Nottingham

University Park, Nottingham, NG7 2RD (UK)

ChemSusChem 0000, 00, 0 – 0  0000 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim1 &

These are not the final page numbers! 

Full PapersDOI: 10.1002/cssc.201500104 have also been suggested: 1) the use of anions with either smaller molar volumes and greater charge densities or a compromise between greater molar volumes and greater anion– gas interactions, 2) the use of cation cores with acidic hydrogen atoms, and 3) the use of ILs with substituent groups that allow for stronger interactions between the ILs and gas molecules.[14] These empirical molecular criteria are a reflection of the as-yet not fully understood solubility mechanism of CO2 in

ILs. In a recent review, Chen et al. compiled several theories for this mechanism: the anion effect theory, in which the anion has a stronger effect on determining the CO2 solubility than that of the cation; the IL free volume effect as the main prevailing influence; and the Lewis acid–base interaction effect, which links solubility to the interaction between the anions and CO2, if CO2 acts as a Lewis acid and the anion as a Lewis base.[15] Despite these studies, all theories have pitfalls and, even though the IL free volume effect seems to be more promising, no theory alone can explain all experimental data concerning CO2 solubility in ILs. There is therefore still a lack of understanding with regard to the molecular mechanism of absorption, the CO2 effect on the IL structure and properties, and the exact correlation between the structure of the IL structure and CO2 absorption ability. Thus, the quest for the rational design of CO2-philic ILs remains a challenge.