J. of Supercritical Fluids 56 (2011) 224–230 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu High pressure CO 2 solubility in N-methyl-2-hydroxyethylammonium protic ionic liquids Silvana Mattedi a,∗ , Pedro J. Carvalho b , João A.P. Coutinho b , Víctor H. Alvarez c , Miguel Iglesias a,d a Programa de Pós Graduac ¸ ão em Engenharia Química, Escola Politécnica, Universidade Federal da Bahia (UFBA), R. Aristides Novis 2 Federac ¸ ão 40210-630 Salvador-Bahia, Brazil b Departamento de Química, CICECO, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal c Faculdade de Engenharia Química, Universidade Estadual de Campinas (UNICAMP), Cidade Universitária “Zeferino Vaz” Av. Albert Einstein, 500, CEP 13083-852, Campinas, SP, Brazil d Departamento de Ingenería Quimica – ETSE, Universidade de Santiago de Compostela (USC), Rúa Lope Gómez de Marzoa s/n, Campus Sur, 15782 Santiago de Compostela, Spain article info Article history: Received 31 May 2010 Received in revised form 24 September 2010 Accepted 20 October 2010 Keywords: Brønsted ionic liquids Protic ionic liquids Carbon dioxide Supercritical N-methyl-2-hydroxyethylammonium acetate N-methyl-2-hydroxyethylammonium formate Solubility abstract The use of ionic liquids for CO 2 capture and natural gas sweetening is being object of intense research. Within the enormous group of existing ionic liquids, those based on conjugate bases of carboxylic acids seem to be particularly promising. This work addresses the study of the high pressure CO 2 solubility (up to 80 MPa) in two protic ionic liquids, N-methyl-2-hydroxyethylammonium formate and N-methyl- 2-hydroxyethylammonium acetate, in a wide range of temperatures (293–353 K). A thermodynamic model based on the Peng–Robinson equation of state with the Wong–Sandler mixing rule, using the NRTL model for the activity coefficients, was here adopted to describe and evaluate the thermodynamic consistency of the experimental data. Furthermore, the study of a ternary mixture of CO 2 + CH 4 + N- methyl-2-hydroxyethylammonium acetate was investigated showing a high selectivity from the IL towards these solutes. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids (ILs) are a new class of solvents composed only of ions but whose size and shape differences allow them to remain liquid at temperatures below 100 ◦ C. Among other interesting properties they present negligible vapor pressures, high thermal stabilities, large liquid phase range, no flammability and high sol- vation capacity. The possibilities of fine tune the ILs properties, through an endless combination of cations and anions, allow the design of solvents for task specific applications leading to more efficient and sustainable processes and products. Until now, studies concerning ionic liquids have been based on the imidazolium cation and, to a lesser extent, on the alkyl pyri- diniums and trialkylamines [1]. Protic ionic liquids (PILs) have, however, received limited attention from the academia. The first PIL reported, ethanolammonium nitrate, was reported by Gabriel and Weiner in 1888 [2], followed by ethylammonium nitrate (EAN), in 1914, synthesized by Walden [3]. These PILs were produced by ∗ Corresponding author. Tel.: +55 71 32839813; fax: +55 71 32839810. E-mail address: silvana@ufba.br (S. Mattedi). a stoichiometric acid–base Brønsted reaction and their main dif- ference, compared to aprotic ILs (AILs), is the presence of at least a proton, which is/are able to promote extensive hydrogen bonding [4]. These liquids present some characteristics, such as a slightly lower conductivity and stability, which may reduce, at first, their interest for a number of applications. However, their low cost, simple synthesis and purification methods, low toxicity and high biodegradation character, among other set of appealing character- istics, overcomes those limitations for many different purposes. Recently, some work has been reported on the synthesis, physicochemical and structural characterization of PILs. Bicak [5] synthesized the 2-hydroxyethylammonium formate (2-HEAF), an ionic liquid formed by the neutralization of monoethanolamine with formic acid. Greaves et al. [6] proposed different protic ionic liquids from primary amines and organic and inorganic acids. Cota et al. [7], Kurnia et al. [8] and Alvarez et al. [9] synthesized a family of these ILs by modifying the aliphatic chain of the organic acid and/or using secondary and tertiary hydroxyamines. There were also stud- ies that use PILs in catalytic reactions [7] and on the interaction with hydroxylic solvents (like water and alcohols), showing that 2-HEAF is soluble in water, ethanol and methanol in all the concentration range [10]. Moreover, a relevant aspect, considering the interest in 0896-8446/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2010.10.043