Effectiveness of toluene separation from gas phase using supported ammonium ionic liquid membrane

https://doi.org/10.1016/j.ces.2020.115605Get rights and content

Highlights

  • Toluene permeability in ammonium SILMs was up to 2666 barrers.

  • Toluene/N2 separation selectivity can reach 120.

  • Activation energy proved higher sensitivity to temperature of [N2228][Tf2N] and [N1114][Tf2N].

Abstract

Ammonium ionic liquids (ILs) are relatively cheap in synthesis and environmentally benign and despite that they have been very rarely used in gas separation. In this research we used several ammonium ILs as liquid membranes for removal of residual toluene from gas phase. Ionic liquids used in this study were composed of bis(trifluoromethylsulfonyl)imide anion [Tf2N] and trimethylbutylammonium [N1114], trietylbutylammonium [N2224], trietylhexylammonium [N2226], trietyloctylammonium [N2228] cations. Selected ILs formed stable membranes with limited swelling effect and good performance. The highest toluene permeability was recorded for [N2228][Tf2N] of 2666 barrers and for triethyl based ILs it was decreasing with carbon atoms number. The selectivity of toluene/N2 separation was in a range of 29–120. For better process illustration, the factors influencing the permeation process, namely viscosity and partition coefficients were examined.

Introduction

Ionic liquids are composed of a large asymmetric organic cation (ex. alkylimidazolium, alkylpirydinium, alkylammonium) and smaller organic or inorganic anion (ex. hexafluorophosphate, bis(trifluoromethylsulfonyl)imide) (Earle and Seddon, 2007). Those with a large number of fluoride atoms have low melting points (Huddleston et al., 1998). The possibility of designing the anion and cation structure results in a formation of new ionic liquids (Fredlacke et al., 2004). Due to the huge number of possibilities, they are referred to as “tunable” fluids. Ionic liquids are a substitute for toxic and volatile organic compounds due to low volatility, which prevents from pollution of gas streams. Therefore they are gladly used in research on gas separation.

Unflagging interest is given to ammonium ionic liquids. The first one was obtained in 1914 by Walden. Among many fields, they have been used in extraction (Domańska et al., 2019, Domańska et al., 2019, Królikowski et al., 2016), catalysis (Elhaj et al., 2019) and gas separation, mainly CO2 (Cichowska-Kopczynska et al., 2014, Zakrzewska et al., 2013, Yuan et al., 2007, Nkinahamira et al., 2017, Manic et al., 2012, Jacquemin et al., 2007). In organic separation they have been used rarely, mainly for extraction and separation from liquid phase, for example 1-butanol from diluted aqueous solutions (Cascon and Choudhari, 2013, Cascon et al., 2011).

Activity coefficients at infinite dilution for methyl(trioctyl)ammonium thiosalicylate ([N1888][TS]) and Ammoeng 100® reported by Reddy et al. suggest that ammonium-based ionic liquids are not well suited to aromatic-aliphatic extraction due to lower selectivity than for example imidazolium-based ionic liquids (Reddy et al., 2011). There are some additional literature data confirming this conclusion (Wolfrom et al., 2002). However, Matsumoto et al. obtained satisfying data for separation of cyclohexane and benzene through vapor permeation, they used N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate and got separation factor of 185 for 53 wt% benzene and 950 for 11 wt% benzene (Matsumoto et al., 2009). Also Domańska et al. performed separation of aromatic hydrocarbons from hexane and octane using ethyl(2-hydroxyethyl)dimetylammonium bis(trifluoromethylsulfonyl)imide and reported it as feasible (Domańska et al., 2007). They were also used to remove Bisfenol A from water (Panigrahi et al., 2013).

Ammonium ionic liquids are cheaper in production than imidazolium ones, however still the cost is high, therefore using it in absorption system is hardly considered. In present study we used them in supported ionic liquid membrane system where ammonium ionic liquids were held in the membrane pores by capillary forces (Walczyk, 2006) to create membrane feasible for separation of toluene residues from gas phase. Using ionic liquids allowed to overcome common problems of the technology like: secondary stream pollution or loss of the membrane phase caused by vaporization (Danesi et al., 1987, Neplenbroek et al., 1992, Teramoto et al., 2000). Stability of polymer supports impregnated with ionic liquids has been widely studied and it has been proven that ILs can be easily embedded in polymer or ceramic supports and form a stable system suitable for gas separation (Cichowska-Kopczynska et al., 2014, Zubielewicz Kamińska-Tarnawska E., 2005, Joskowska et al., 2012, Gan et al., 2006, Gan et al., 2006, Lee and Outcalt, 2006).

Ammonium ionic liquids haven’t been used in such systems before. In present paper we present the potential of utilization ammonium ionic liquids for toluene separation from toluene/N2 model stream. Since all the ionic liquids used in this study are based on the same anion, the effect of alkyl chain length of cation substituent is discussed.

Section snippets

Materials

Nitrogen was ultra-high purity and supplied by AirLiquid. Toluene was purchased from POCH S.A. GH Polypro (polypropylene) – PP supplied by Pall, Gelman Laboratory was used as a support with 80% porosity and average 0.2 µm pore size reported by the producer. Bulk density of the support sheet was 900 kg/m3 and the initial thickness 92 µm. However, own analysis performed with digital image processing software have shown that the average pore size was about 4.7 µm, whereas 0.2 µm was the diameter of

Physicochemical properties of ionic liquids

From the point of view of chemical technology, viscosity of ionic liquids used in gas separation processes is a key aspect due to related technical issues like pumping or mixing and also diffusion resistance. Generally, ionic liquids have relatively high viscosity, however in supported ionic liquid technology it ensures that the liquid is held in the pores of the support. In this study dynamic viscosity coefficient was determined in a range of 288–318 K in 5 K intervals. Detailed results can be

Conclusions

Ammonium ionic liquids though cheaper in production and more environmentally friendly than imidazolium ones, they are very briefly examined and rarely used in organics separation. In this research we proved that ammonium ILs can be successfully used in gas separation process with performance close to their imidazolium equivalents.

The selectivity of separation of toluene from nitrogen in a range of 29 up to 120 occurred to be promising for toluene removal, for example from industrial air.

CRediT authorship contribution statement

Iwona Cichowska-Kopczyńska: Conceptualization, Investigation, Data curation, Visualization, Writing - original draft, Writing - review & editing. Robert Aranowski: Conceptualization, provision of reagents, critical review.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was supported by the National Science Centre, Poland, grant no. 7563/B/T02/2011/40 in a  title Removal of Volatile Organic Compounds from Gas Phase using Ionic Liquids.

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