Elsevier

Chemical Engineering Journal

Volume 230, 15 August 2013, Pages 111-114
Chemical Engineering Journal

Highly permeable ionic liquid/Cu composite membrane for olefin/paraffin separation

https://doi.org/10.1016/j.cej.2013.06.054Get rights and content

Highlights

  • Dissociation of Cu flakes into surface activated nanoparticles.

  • Synthesis of Cu nanoparticles by utilizing 1-methyl-3-octyl imidazolium tetrafluoroborate for facilitated olefin transport.

  • Highly permeable separation performance.

Abstract

1-Methyl-3-octyl imidazolium tetrafluoroborate (MOIM+BF4-) ionic liquid was introduced for olefin/paraffin separation since it exists primarily as its free ions. It is expected that most of the free imidazolium cations will interact with olefin molecules and the free anions will positively polarize the Cu nanoparticles, resulting in enhanced olefin separation. Accordingly, when the Cu nanoparticles were generated in MOIM+BF4-, the separation performance was twice that of neat ionic liquid and the mixed gas permeance increased from 6.9 to 12 GPU due to facilitated olefin transport. This enhanced separation performance is attributed to both the free imidazolium ions and the positively polarized Cu nanoparticles, which favorably interact with the olefin molecules. The size and distribution of the generated Cu nanoparticles were confirmed by TEM and UV–vis spectra, respectively. The positive charge on the surface of the Cu nanoparticles was confirmed by investigating the binding energy using XPS.

Introduction

Facilitated transport has been widely exploited in the membrane field over the last few decades [1], [2], [3], [4], [5], [6], [7]. Facilitated transport results in the rapid transport of some molecules through specific media by both Fickian and carrier-mediated transport [1], [2], [3], [4], [5], [6], [7]. For example, olefin molecules such as propylene and ethylene can be rapidly moved by carriers, e.g., silver ions, while paraffin molecules such as propane and ethane are transferred only by Fickian diffusion. As a result, when silver ions complexed with POZ or PVP to form a polymer electrolyte, the selectivity of propylene/propane reached 56 with a permeance of 11 GPU (1 GPU = 1 × 10−6 cm3 (STP)/(cm2 s cm Hg)) [1], [7]. Recently, facilitated transport has also been applied to CO2 separation membranes. Since it is well known that amine groups can reversibly interact with CO2 molecules, they were used as CO2 carriers for separation membranes [8], [9], [10], [11], [12], [13], [14] For example, the application of an ionic liquid containing amine groups, i.e., 1-butyl--methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, as a membrane resulted in a CO2/CH4 selectivity of 6.1 with a permeance of 1.3 × 10−9 mol s−1 m−2 Pa−1 [8]. Uchytila’s group reported that the application of a poly(vinylidene fluoride-co-hexafluoropropylene) membrane containing 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid for CO2/CH4 separation resulted in a selectivity and CO2 permeability of 11.4 and 1090 × 1016 mol m m−2 s−1 Pa−1, respectively [14]. In addition to these molecules, various ionic liquids with different cations and anions have been used due to their many advantages that include thermal stability and negligible vapor pressure [9], [10], [11], [12], [13].

Very recently, various gas carriers have been reported for olefin and CO2 separation membranes. Kang’s group reported that silver nanoparticles that were positively polarized by electron acceptors such as p-benzoquinone and TCNQ could reversibly interact with propylene molecules [4], [6]; poly(ethylene-co-propylene) (EPR)/Ag nanoparticles/p-benzoquinone and PVP/Ag nanoparticles/TCNQ composite membranes showed propylene/propane selectivities of 11 and 50, respectively [4], [6].

Furthermore, our groups reported that the use of copper nanoparticles that were positively polarized by 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-) ionic liquid increased the ideal selectivity for CO2/CH4 from 4.8 to 11 with a CO2 permeance of 25 GPU [5]. These results were attributed to the presence of more free ions from the ionic liquid and polarized copper nanoparticles. For olefin separation, silver nanoparticles polarized by ionic liquid showed enhanced separation performance towards olefin/paraffin mixtures [3].

Unfortunately, even though ionic liquid/metal nanocomposite mixtures show excellent separation performance for olefin/paraffin mixtures, their permeance remains too low for practical applications due to their high viscosity. To solve this problem, the MOIM+BF4- ionic liquid was introduced for facilitated olefin transport membrane. Since the free-ion state is dominant for MOIM+BF4-, the free imidazolium ions should easily interacted with the olefin molecules as well as positively polarize the Cu nanoparticles, resulting in enhanced separation performance.

Section snippets

Experimental section

Microsized copper particles (1–5 μm, 99%, Aldrich Chemicals) were introduced into 1-methyl-3-octylimidazolium tetrafluoroborate (MOIM+BF4-, C-TRI). Transmission electron microscopy (TEM) images were obtained using a JEOL JEM-3000 operating at 300 kV. The UV–vis absorption spectrum of the MOIM+BF4-/Cu composite solution was obtained using a Beckman Coulter DU® 730UV/Vis spectrophotometer. Raman spectra were obtained using a Horiba Jobin–Yvon/LabRAM ARAMIS instrument at 785 nm (diode laser). X-ray

Results and discussion

Fig. 1 shows the appearance of the 1/0.002 MOIM+BF4-/Cu composite solution after stirring for 24 h. When the copper metal was incorporated into neat MOIM+BF4- with stirring, the solution went from colorless to deep green after 24 h, indicating that copper nanoparticles were generated in the ionic liquid. The formation of copper nanoparticles is attributed to the strong coordination between the counteranions of MOIM+BF4- and the metal surface, as reported previously [5].

TEM was used to investigate

Conclusions

To prepare a highly permeable membrane for olefin separation, MOIM+BF4- ionic liquid and Cu nanoparticles were utilized for facilitated olefin transport. The ideal selectivity of propylene over propane through neat MOIM+BF4- was 1.4 with a propylene permeance of 8.8 GPU. When copper nanoparticles were generated in MOIM+BF4-, the separation performance was greatly enhanced: The propylene permeance was 18.0 with an ideal selectivity of 2.0. Furthermore, the actual selectivity increased from 1.1 to

Acknowledgement

This work was supported by a 2012 Research Grant from Sangmyung University.

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