Article pubs.acs.org/jced
Measuring Diffusion and Solubility of Slightly Soluble Gases in [CnMIM][NTf2] Ionic Liquids Michael F. Friedrich, Simon Kokolakis, Martin Lucas, and Peter Claus* Technische Universität Darmstadt, Department of Chemistry, Ernst-Berl-Institute, Chemical Technology II, Alarich-Weiss-Straße 8, D-64287 Darmstadt, Germany S Supporting Information *
ABSTRACT: We report an advanced experimental setup for the reliable determination of diffusion and solubility, especially for slightly soluble gases in ionic liquids. We were able to improve resolution and reproducibility of an isochoric saturation method by using differential pressure measurements. It was also possible to reduce the influence of wall adsorption, too. For the first time we present diffusion coefficients of hydrogen gas (H2) in the ionic liquids [EMIM][NTf 2 ], [BMIM][NTf 2 ], [HMIM][NTf 2 ], and [OMIM][NTf2] at temperatures between 293.15 and 343.15 K. Henry’s Law constants were determined simultaneously by fitting the experimental pressure decay to a one-dimensional diffusion model. With rising temperature, diffusion coefficients and solubility of H2 were increased. Increasing the length of the alkyl side chain of the cation decreased diffusion, whereas solubility of H2 was increased. An empirical correlation equation for H2 diffusion in [CnMIM][NTf2] ionic liquids is presented. Limitations are also pointed out. Furthermore, diffusion and solubility of carbon monoxide and carbon dioxide were investigated in [BMIM][NTf2] and compared to that of H2.
1. INTRODUCTION Ionic liquids (ILs) can be defined as salts with a melting point below 100 °C.1 This definition is arbitrary but often used and usually accepted in the community. ILs are not only of academic interest. The tuneability of their properties makes ILs attractive for a wide range of applications. Some properties, such as usually negligible vapor pressure, high chemical stability, and wide liquid temperature range, distinguish them as potential solvents for separation processes, synthesis, and catalysis.2−4 They are already used in a number of industrial applications5 and inspire chemists to develop new reaction processes, such as supported ionic liquid phase (SILP)6,7 or supported catalyst with ionic liquid layer (SCILL).8 Knowledge of diffusion and solubility of reactants in ILs is crucial for design and development of these reaction processes. For this reason solubility has been the subject of numerous studies.9−17 A comprehensive summary of solubilities in ILs as well as an overview of experimental methods is given by Lei et al.18 The key factors influencing solubility are summarized in a critical review by Hu and co-workers.19 Diffusivities of gases in ILs are studied to a minor extent and are reported by a few groups.20−26 The NIST database “ILThermo”27 is a powerful tool to gain fast access to properties of ILs and their mixtures, including diffusion and solubility. For the latter, 153 entries already exist, especially for carbon dioxide, because it is highly soluble in most ionic liquids and plays a key role in gas separation. Some further solubility data are given for water, for © XXXX American Chemical Society
a few alkanes and alkenes, ammonia, oxygen, and hydrogen. However, only 48 entries can be found in the database dealing with diffusion of gases in ILs. Hydrogen is still missing in this list. To the best of our knowledge, there is only one very recent report about hydrogen diffusivity in the ionic liquid [BMIM][PF6] at this time.28 For a broad variety of possible IL−gas combinations, diffusion and solubility data are still not available. Particularly for slightly soluble gases like hydrogen, a simple and practicable determination method is lacking. Previous attempts report scattering results as summarized by Lei et al.18 or even had been unsuccessful because of the detection limit of the used measuring technique, as reported by Anthony and colleagues.9,10 In this work, we report an improved procedure for obtaining reliable experimental diffusion and solubility data for slightly soluble gases in ionic liquids. First, we describe our advanced experimental setup using differential pressure measurements. Then we show validation measurements and discuss the improvements. Moreover, new data of diffusion and solubility for hydrogen H2 in four [CnMIM][NTf2] ionic liquids are presented to show the impact of our advanced technique. H2 was chosen as a model substance because of its importance in catalysis, e.g., in hydrogenation or hydroformylation reactions. Received: November 21, 2015 Accepted: March 1, 2016
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DOI: 10.1021/acs.jced.5b00990 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Table 1. Chemicals Used in the Current Study with Suppliers and Purity Informationa
a
chemical name
CAS no.
source
purity
water
halides
[EMIM][NTf2] [BMIM][NTf2] [HMIM][NTf2] [OMIM][NTf2] hydrogen carbon dioxide carbon monoxide
174899-82-2 174899-83-3 382150-50-7 178631-04-4 1333-74-0 124-38-9 630-08-0
Aldrich Merck Merck Iolitec Air Liquide Air Liquide Westfalen
≥0.98 ≥0.98 ≥0.99 ≥0.99 0.99999 0.99995 0.9997
