Article pubs.acs.org/IECR
A Novel Copper(I)-Based Supported Ionic Liquid Membrane with High Permeability for Ethylene/Ethane Separation Yongli Sun,†,‡ Hanrong Bi,†,‡ Haozhen Dou,†,‡ Huawei Yang,†,‡ Zhaohe Huang,†,‡ Baoyu Wang,†,‡ Rong Deng,†,‡ and Luhong Zhang*,†,‡ †
School of Chemical Engineering and Technology, Tianjin University, Tianjin, China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
‡
S Supporting Information *
ABSTRACT: For the separation of an ethylene/ethane mixture, a novel copper(I)-based supported ionic liquid membrane (SILM) with high permeability has been fabricated. This SILM was prepared from a polyvinylidene fluoride microporous membrane impregnating the copper(I) based IL which formed by the cuprous chloride (CuCl) and 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]). Scanning electron microscopy, attenuated total reflection Fourier transform infrared spectroscopy, and time of flight mass spectroscopy were used to characterize the SILM. Pure and mixed gas permeation experiments were carried out to investigate the influences of ILs composition, transmembrane pressure, temperature, and time upon the separation performance. This SILM showed comparable C2H4 permselectivity but outstanding permeability with a long-term stability beyond the reported polymeric membrane upper bound. At the CuCl/[Bmim][Cl] ratio of 2, the C2H4 permeability and permselectivity reached 2653 barrer and 11.8, respectively. Furthermore, the facilitated transport effect was studied by 1H NMR and quantum mechanical calculations. The anionic species formed by sp hybridization of Cu+ possesses unfilled attachment sites to selectively complex with C2H4 and weaken the interionic hydrogen bond of [Bmim][Cl], thus lowering the system’s viscosity.
1. INTRODUCTION Light olefins are basic chemicals in the petrochemical industry which are usually obtained as a mixture of olefins and paraffins through steam cracking, catalytic cracking, or catalytic dehydrogenation of paraffins. Therefore, olefins must be separated from the gas mixture before further utilization. Unfortunately, the separation of olefins/paraffins is a challenging task because of their similar physical and chemical properties.1 Cryogenic distillation is the primary separation method in the industry. However, this technology requires huge capital and operational cost due to the very close relative volatilities between olefins and their corresponding paraffins.2 Recently, several alternative technologies have been developed, such as solvent absorption, pressure-swing adsorption, and membrane separation.3 In particular, great attention has been paid to the reactive absorption of olefins using Ag+ or Cu+, because of the formation of reversible complexation toward olefins through π-bond interaction which can easily be reversed by pressure and temperature swing.4 Nymeijer et al.5 investigated the solubility of ethylene in aqueous silver nitrate. Son et al.6 prepared the Cu(I)/pyridine/HNO3 system for selective absorption of isoprene from n-pentane mixture. However, the use of conventional aqueous and organic media often leads to the volatile loss and limited solubility for carriers, which restrict their final industrial applications.7 © XXXX American Chemical Society
Membrane separation is a promising alternative to the traditional cryogenic distillation because of its low operational cost and small-scale equipment. However, traditional polymeric membranes usually suffer from inferior permeability and selectivity because of the low solubility in the polymeric materials and similar molecular size between olefins and corresponding paraffins.8 To overcome these drawbacks, researchers focused on the facilitated transport polymer electrolyte membrane, containing Ag+ salt, in which polar polymers, such as poly(2-ethyl-2-oxazoline) (POZ),9 poly(Nvinyl pyrroli-done) (PVP),10 poly(ethylene oxide) (PEO),11 poly(vinyl m ethyl ketone) (PVMK), 1 2 or p o ly (dimethylsiloxane) (PDMS)13 were employed as solvents to dissolve silver salts and form polymer electrolyte layers.14 These polymer electrolyte membranes showed excellent selectivity but low permeability because of the low diffusion coefficient of olefins in polymer. Moreover, the Ag+ salt is uneconomical and unstable in this process.15 Recently, in order to obtain both high permeability and permselectivity with a small amount of carrier solvent, a Received: September 1, 2016 Revised: December 4, 2016 Accepted: December 29, 2016
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DOI: 10.1021/acs.iecr.6b03364 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research supported liquid membrane, in which porous support is filled with selective liquid media by capillary forces, has been widely explored in gas separation.16 Unfortunately, the permeation performance of this membrane deteriorates because of liquid depletion through evaporation. To circumvent the problem, ionic liquids (ILs), a class of green solvents with negligible vapor pressure, high thermal and chemical stability, and designable structure, were introduced to prepare the supported ionic liquid membrane (SILM).17 Ortiz and co-workers18 used AgBF4 dissolved in [Bmim][BF4] as carrier media to separate the propane/propylene gas mixtures. Sang and co-workers19 separated the propane/propylene via Cu nanoparticles positively polarized by [Moim][BF4] IL. Pitsch et al.20 prepared an adaptive self-healing ionic liquid nanocomposite membrane for olefin/paraffin separations. Although these attempts demonstrated the potential of SILMs to separate the olefins and paraffins, their practical applications were confined because of the limited solubility of metal salt in the ILs, high viscosity of ILs after dissolving the metal salt, and the high cost for metal carriers. On the other hand, some deep eutectic phenomena which can occur between many inorganic metal salts and organic salt provide novel design concepts for the fabrication of SILM. The changing of composition of deep eutectic systems brings about further variation of their physical and chemical properties.21 Thus, a high concentration of metal ions and appropriate viscosity can be simultaneously achieved at suitable component ratio. It was reported that the deep eutectic phenomenon can occur between the cuprous chloride and 1-butyl-3-methylimidazolium chloride to form the copper(I)-based ionic liquid,22 which possessed the capability to selectively absorb olefins.23 This copper(I)-based ionic liquid exhibited high carrier concentration, appropriate viscosity, low cost, and convenient preparation, and seemed to be an ideal facilitated transport medium to separate olefins and paraffins in SILM. In this work, to obtain both high ethylene permeability and permselectivity with a cheap facilitated transport carrier, a SILM was prepared by the copper(I) based ionic liquid supported in a polyvinylidene fluoride (PVDF) microporous membrane and characterized by scanning electron microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and time of flight mass spectroscopy (TOF-MS). The as-prepared SILM was employed to separate C2H4/C2H6 mixtures. Significant factors, including ILs composition, trans-membrane pressure, temperature, and time, were investigated in detail. Further, 1H NMR and quantum mechanical calculations were utilized to confirm the facilitated transport effect in this process.
Table 1. PVDF Supported Membrane’s Main Characteristics
characteristics material wettability pore size (μm) thickness (μm) membrane diameter (mm) operational temperature (°C) porosity structure pore style
polyvinylidene fluoride (PVDF) hydrophilic 0.1 100 75
