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A Novel Supported Liquid Membrane Based on Binary Metal Chloride Deep Eutectic Solvents for Ethylene/Ethane Separation Bin Jiang, Wenjun Tao, Haozhen Dou, Yongli Sun, Xiaoming Xiao, Luhong Zhang, and Na Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03843 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017
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Industrial & Engineering Chemistry Research
A Novel Supported Liquid Membrane Based on Binary Metal Chloride Deep Eutectic Solvents for Ethylene/Ethane Separation Bin Jiang †, Wenjun Tao†, Haozhen Dou†, Yongli Sun†,‡, Xiaoming Xiao†, LuhongZhang†,‡, Na Yang*,† † School of Chemical Engineering and Technology, Tianjin University, Tianjin, China ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
* Corresponding author. E-mail addresses:
[email protected], Tel/Fax: +86 22 27400199, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China ACS Paragon Plus Environment
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ABSTRACT Binary metal chloride deep eutectic solvents (DESs) were first prepared by adding different metal chlorides into 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)/CuCl DESs to improve the activity of the Cu+ carrier for ethylene/ethane separation. The binary metal chloride DESs were impregnated in the polyvinylidene fluoride microporous membrane to prepare the supported liquid membranes (SLMs). The performance of the SLMs based on binary metal chloride DESs was evaluated by the ethylene/ethane separation experiments. For the SLM based on 0.5[BMIM]Cl-CuCl-1/15ZnCl2 DES, the selectivity of ethylene/ethane mixture increased from 10.7 to 17.8, and the ethylene permeability decreased slightly from 28.6 barrer to 25.8 barrer, compared to 0.5[BMIM][Cl] SLM without ZnCl2. Furthermore, it exhibited long-term stability for more than 150 h. The beneficial role of ZnCl2 for Cu+ carrier was analyzed by ESI-MS, Raman spectroscopy and quantum mechanical calculation. The Raman spectrum showed that the Cu-Cl bond was weakened with the addition of ZnCl2. Quantum mechanical calculation revealed that the electron transferred from a Cl- ion of CuCl-2 to the Zn2+ ion, which led to the weakening of the Cu-Cl bond and the activation of the Cu+ carrier. Key words: Ethylene/ethane separation, Supported liquid membrane, Deep eutectic solvents, Binary metal chloride, Membrane separation, Activated carrier, Facilitated transport
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1. INTRODUCTION In recent years, increasing attention has been devoted to the topic of separating ethylene (C2H4) and ethane (C2H6), which is a vital but challenging process in the petrochemical industry. Due to their similar physicochemical properties, there is an increasing need to develop approaches for effective ethylene/ethane separation. Traditional low-temperature distillation, which is a typical separation technique for C2H4 and C2H6, is limited by its high cost and energy consumption. Therefore, several alternative techniques such as extractive distillation, physical adsorption and membrane separation have been developed. Among these, the membrane separation technique is considered to show the greatest potential for C2H4/C2H6 separation due to its energy and capital savings1-4. Traditional polymeric membranes suffer several drawbacks, such as low permeability and selectivity caused by similar molecular sizes and solubility of olefin and paraffin5. In the early 21st century, some researchers turned their attention to facilitated transport mechanisms by using a carrier such as Ag+ or Cu+ within the solid membrane to enhance the olefin and paraffin selectivity6-9. Ag+ or Cu+ ions can react reversibly and specifically with the olefin molecules, and the olefin molecules permeate through the membrane faster than the paraffin molecules by the means of carrier mediated transport and Fickian transport10. However, the gas permeability of the solid membrane was quite low compared to the supported liquid membrane containing aqueous silver salt solutions11,
12
. Azizi et al.13 reported a kind of n-methyl pyrrolidone/AgNO3 supported liquid membrane.
However, the main problem is the latent evaporation of n-methyl pyrrolidone with the sweep of the mixed gas during a long time. To prevent the loss of the component in the membrane, an ionic liquid (IL) consisting only of ions with negligible vapor pressure14 is employed for injection into the membrane15, and the carriers exist in the IL. Recently, deep eutectic solvents (DESs) have been reported as promising green solvents16 that not only exhibit most of the advantages of IL such as negligible vapor pressure but also overcome some of their limitations17. Usually, DESs are obtained by the combination of a quaternary ammonium salt with a metal salt or hydrogen bond donor18. Additionally, some transition metal salts for which the cation shows complexing action can also react with IL (solid state or liquid state at room temperature) to form DESs to separate mixed gas. Zarca et al.19 reported a copper(I)-containing supported liquid membranes for CO/N2 separation. The copper(I) as the carrier can form a CO-copper(I) complex reversibly with CO. Recently, SLMs based on silver-based DESs constructed from silver trifluoromethanesulfonate (AgCF3SO3) and acetamide exhibited good separation performance for olefin/paraffin mixtures20. However, the high cost of AgCF3SO3 and potential degradation21, 22 are the major concerns with this approach and have motivated researchers to turn their attention to the alternative Cu salts. Sun et al.23 reported SLMs based on [BMIM][Cl]/CuCl DESs for the separation of ethylene/ethane mixtures, and these SLMs showed a permselectivity of 11.8, which still needs to be improved for future industrial use. In our previous work24, novel CuCl/DESs-SLMs were prepared for the separation of ethylene/ethane mixtures, exhibiting a permselectivity of 20, and the activity of Cu(I)-containing anionic species was enhanced through ACS Paragon Plus Environment
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the hydrogen-bond interactions between the anionic species and DESs. Although the CuCl/DESs-SLMs exhibited a permselectivity of 20, the permeability was much lower than that of [BMIM][Cl]/CuCl. Yu et al. investigated the effect of the carrier anion species on the selectivity of propylene and propane25,26 and noted that the Cu+ ligand in the absorption system plays a significant role in absorption selectivity and that different Cu+ ligands will cause the Cu-ligand bond strength to change, thereby influencing the Cu+ activity. Kim10 reported a polymer/silver complex membranes containing mixed salts for the propylene/propane separation; it was found that the membrane reached a higher selectivity than the membranes containing a single salt due to the increased transient cross-links of the polymer chains by the coordination of the small size AgNO3. In the field of the catalysis, bimetallic catalysts have occasionally showed better performance than single metal catalysts have. For a bimetallic catalyst consisting of a main catalytic metal and a dopant metal, the catalytic activity of the main catalytic metal (possesses catalytic function) is improved with the addition of the second, dopant metal (which possesses no or poor catalytic activity). The beneficial role of the dopant metal for the main catalytic metal can be suggested to be due to electron transfer between the main catalytic metal and the dopant metal contributing to the improvement of the activity of the main catalytic metal27-30. In this work, inspired by the bimetallic catalyst mechanism, we developed binary metal chloride deep eutectic solvents system by adding different metal chlorides (MCln, M= Zn, Co, Ni, Al) into 0.5[BMIM][Cl]-CuCl DESs to tune the bond strength of Cu-Cl by electron transfer, and further improve the activity of Cu+ carrier to enhance the permselectivity of ethylene, while not reducing the permeability of ethylene significantly. It is the first study of the binary metal chloride deep eutectic solvents applied to gas separation. We discovered that the addition of ZnCl2 would markedly improve the permselectivity of the Cu-based SLMs. We also investigated the effects of the MCln/CuCl ratio, CuCl concentration, temperature, trans-membrane pressure difference and time on permselectivity. The binary metal chloride deep eutectic solvents were characterized by ESI-MS and Raman spectroscopy. Additionally, quantum mechanical calculations were employed to explain the electron transfer mechanism.
2. EXPERIMENTAL SECTION 2.1. Materials Ethylene (≥99.5%) and ethane (≥99.5%) were purchased from Tianjin Shengtang Gas, Ltd. Nitrogen gas (5N) was purchased from Tianjin Dongxiang Gas Ltd. 1-butyl-3-methylimidazoliumchloride ([BMIM][Cl]) (≥99.5%, melting point: 72 °C) was supplied by Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. Cuprous monochloride (CuCl) (≥99%), zinc chloride (ZnCl2) (≥99%), cobalt chloride (CoCl2) (≥99%), nickel chloride
(NiCl2)
(≥99%)
and
aluminum
chloride
(AlCl3)
(≥99%)
were
purchased
from
Shanghai Aladdin Bio-Chem Technology, Ltd. The polyvinylidene fluoride (PVDF) supported membrane was obtained from Zhonglihaining Filtration Equipment Corporation with the porosity of 70%, average thickness of 100 μm and pore size of 0.1μm. ACS Paragon Plus Environment
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2.2. Synthesis of Binary Metal Chloride DESs The synthesis procedures for [BMIM][Cl]/ZnCl2 DESs and [BMIM][Cl]/CuCl DESs were similar. [BMIM][Cl]/CuCl DESs were prepared by mixing the CuCl powder and the [BMIM][Cl] crystal (melting point: 72 °C) with a certain mole ratio. Then, the mixture was stirred at 333K in vacuum (
