Long-Chain Carboxylate Ionic Liquids Combining High Solubility and

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Long-chain Carboxylate Ionic Liquids Combining High Solubility and Low Viscosity for Light Hydrocarbon Separations Yi Zhang, Xu Zhao, Qiwei Yang, Zhiguo Zhang, Qilong Ren, and Huabin Xing Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00660 • Publication Date (Web): 15 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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Long-chain Carboxylate Ionic Liquids Combining High Solubility and Low Viscosity for Light Hydrocarbon Separations

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Yi Zhang , Xu Zhao , Qiwei Yang , Zhiguo Zhang , Qilong Ren , and Huabin Xing *.

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† Key Laboratory of Biomass Chemical Engineering of Ministry of Education,

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College of Chemical and Biological Engineering, Zhejiang University, Hangzhou

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310027, China

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‡ The Institute of Seawater Desalination and Multipurpose Utilization, State Oceanic

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Administration, Tianjin 300192, China

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ABSTRACT: Ionic liquids (ILs) have been proposed as promising solvents for

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hydrocarbon separations, but designing an industrially attractive IL combining high

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solubility and low viscosity remains challenging. Here we synthesized three new

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long-chain carboxylate ILs with asymmetric phosphonium cations that had relatively

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low viscosity and good thermal stability, and exhibited very high solubility and

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excellent selectivity for hydrocarbons with different carbon number at ambient

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condition. The solubilities of propane, ethane, methane, and nitrogen in these

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tributylethyl-phosphonium

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temperature of 298.1 to 313.1 K and pressure of 20 to 150 kPa. The effects of

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molecular structure on the properties of ILs and their absorption performance were

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investigated. It was found that the introduction of asymmetric cation with short alkyl

1 2



†‡





long-chain

carboxylate



ILs

1

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determined

at

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chains significantly reduced the viscosity of carboxylate ILs, while an extension on

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the alkyl chain of carboxylate anions enhanced the solubility. At 298.1 K and 150 kPa,

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the solubilities of propane, ethane, and methane in tributylethylphosphonium stearate

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reach 0.408, 0.133 and 0.009 mmol/g with selectivities of propane/methane and

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methane/nitrogen up to 16.92 and 2.72, respectively. This study demonstrates the

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great potential of long-chain carboxylate ILs as novel solvents for separating light

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hydrocarbons.

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KEYWORDS: Ionic liquids, solubility, hydrocarbons, gas absorption, natural gas.

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INTRODUCTION

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Separation of light hydrocarbons is a critical process in petrochemical industry for

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the production of high-purity chemicals and clean energy.1-3 Especially, the rapid

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development of shale gas in recent years has greatly spurred research on hydrocarbons

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separation.4,5 Shale gas is a new kind of natural gas that consists primarily of methane

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(CH4), ethane (C2H6), propane (C3H8), other alkanes and nitrogen (N2).4 Effective

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utilization of these hydrocarbon resources requires energy-saving separation

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technologies for hydrocarbons with different carbon number. In addition, removing

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nitrogen from natural gas is a key step for liquefied natural gas (LNG).6,7 Solvent

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absorption is one of the most economical methods for hydrocarbon separation in

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industry. However, traditional absorption processes with organic solvents as

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adsorbents are facing the drawbacks of difficulty of efficiently regenerating

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absorbents, and absorbents loss due to the volatility of organic solvents.8 Therefore, it

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is necessary to design novel absorbents for the separation of light hydrocarbons with 2

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different carbon number.

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Ionic liquids (ILs) are novel solvents with unique physical properties. They have

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negligible vapor pressure near ambient temperature, are nonflammable, and have

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tunable structures and properties for the vast number of possible combination of

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cations and anions.9-14 More importantly, ILs generally possess multiple solvation

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interactions,15 enhanced H-bond basicity,16 and H-bonding interaction.17,18 Owing to

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these attributes, ILs have been widely studied in the fields of extraction,19,20

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catalysis,11 biomass processing and conversion,21,22 energy storage.23 and so on. ILs

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have also been investigated as novel absorbents for various gas separation

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applications, such as CO2 capture,24-27 SO2 removal,28 paraffin/olefin separation,29-33

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and acetylene/ethylene separation,8,34,35 while researches on the separation of light

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hydrocarbons with different carbon number are limited.36-41 Common low-viscosity

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ILs with fluorinated anions exhibited good selectivity to different saturated

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hydrocarbons however the gas solubilities in these ILs are moderated.40,42-45 For

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example, the Henry’s law constants (KH) of CH4, C2H6, and C3H8 in

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1-ethyl-3-methylimidazolium bis(trifluoromethane)sulfonamide ([Emim][NTf2]) are

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546, 169, and 85 bar at 313.1 K, respectively.39 Very recently, Prausnitz and

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co-workers39,46-48

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bis(2,4,4-trimethyl-pentyl) phosphinate [P(14)666][TMPP] demonstrated record high

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solubility for light hydrocarbons with KH value of C2H6 and C3H8 being 16 and 5.1

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bar at 303.1 K, respectively. However, the high viscosity of TMPP-based ILs

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([P(14)666][TMPP]: 1004 mPa·s at 298 K) limited their application.48 In order to

reported

that

trihexyl

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tetradecylphosphonium

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decrease the viscosity of TMPP-based ILs while maintain the high solubility of light

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hydrocarbons,

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([Bhmim][AC]) was used as diluents.48 This strategy gives a significant drop on the

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viscosities of these IL mixtures and maintains high solubilities for light hydrocarbons

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simultaneously; however, these IL mixtures bear the drawback of low thermal

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stability and relative high volatility due to the nature of protic ILs (Figure S3).14,49

low-viscosity

protic

IL

of

1-butyl-3-H-imidazolium

acetate

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It was the first time for us to report the selective separation of CH4, C2H6, C3H8,

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and N2 using long-chain carboxylate ILs (LCC-ILs, molecular structures see Scheme

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1). The designed new phosphonium LCC-ILs have flexible and highly asymmetric

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molecular structures, possess low viscosity and excellent thermal stability, and exhibit

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very high solubility and excellent selectivity for light hydrocarbons at ambient

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condition. The solubilities of CH4, C2H6, C3H8, and N2 in LCC-ILs were determined at

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temperature of 298.1 to 313.1 K and pressure from 20 to 150 kPa, and their

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thermodynamics and separation performance were discussed. The physic properties of

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prepared LCC-ILs have also been investigated.

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EXPERIMENTAL SECTION

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Materials and reagents

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The 40% (wt) aqueous solution of tetrabutylphosphonium hydroxide ([P4444][OH])

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was purchased from Tokyo Chemical Industry Co. Ltd., tributylethylphosphonium

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bromine ([P4442][Br], ≥99.0%) was obtained from Green Chemistry and Catalysis,

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LICP, CAS (China), n-hexanoic acid (≥99.0%) were purchased from J&K scientific

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(China), lauric acid (≥98.0%) and octadecanoic acid (≥98.0%) were purchased from 4

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Aladdin Reagent Co. Ltd., China. Strongly basic anion-exchange resin Dowex

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Monosphere 550A UPW (OH) was obtained from Aldrich Co. Ltd. The gases of C3H8,

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C2H6, CH4, and N2 were all purchased from Hangzhou Jin Gong Materials Co. Ltd.

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with a purity of ≥99.9%.

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Synthesis of LCC-ILs

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The

LCC-ILs

of

tributylethylphosphonium

phosphonium

laurate

caproate

[P4442][C5H11COO],

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tributylethyl-

[P4442][C11H23COO],

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tributylethylphosphonium stearate [P4442][C17H35COO] (scheme 1) were prepared by

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the similar method reported in the literature,50 through neutralizing [P4442][OH] with

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equimolar n-hexanoic acid, lauric acid or octadecanoic acid, respectively, at room

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temperature for about 12 h. After the neutral reaction, water was distilled off at 328.1

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K under reduced pressure. The ILs obtained were further dried under a high vacuum

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at 328.1 K for at least 24 h in a freeze drier. The [P4442][OH] aqueous solution was

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obtained by eluting the [P4442][Br] aqueous solution through anion-exchange resin.

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The water contents of the ILs were determined by Karl-Fisher’s titration and their

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values were all below 0.3%. The chemical structure and the purity of synthesized ILs

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were confirmed by 1H NMR (supporting information). The acid values of synthesized

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ILs were determined by acid-base titration with 0.01mol/L NaOH aqueous solution.

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ILs with different acid/alkali contents were also synthesized to evaluate the effect of

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residual acid/alkali on absorption by controlling the amount of fatty acids and

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[P4442][OH] aqueous solution.

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Scheme 1. The structures of LCC-ILs investigated in this work. Characterization

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Viscosity measurements were performed with a Brookfield LVDV-II+Pro

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Cone/Plate programmable viscometer at least three times. Thermal gravimetric

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analysis (TGA) was performed with a PerkinElmer Pyris 1 TGA instrument from

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323.1 to 773.1 K under a nitrogen atmosphere at the heating rate of 10 K/min.

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Differential scanning calorimetry (DSC) measurements were conducted with a TA

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Q200 differential scanning calorimeter, under a temperature range from 198.1 to

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423.1 K at the scanning rate of 10 K/min, with a nitrogen atmosphere.

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Measurements of Gas solubilities

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The illustrative diagram of the solubility measurement device consisted of an

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equilibrium cell, an isothermal oven, and a gas reservoir is shown in Figure. S1. The

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pressures were measured by a pressure transducer (Druck RPT 350, 3.5-350 kPa)

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whose accuracy is ± 0.01% for full scale, and the temperatures were determined by

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K-type thermocouples with an accuracy of ± 0.15 K. The volumes of the equilibration

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cell and the gas reservoir were determined with an accuracy of ± 0.01 mL.

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The solubilities of methane, ethane, propane, and nitrogen in LCC-ILs

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([P4442][C5H11COO], [P4442][C11H23COO] and [P4442][C17H35COO]) were determined

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by the isochoric saturation technique.8,51,52 Typical procedures for solubility

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measurement are as follows.8 The determined gas was fed from the supply cylinder to

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the gas reservoir, the valve V4 was closed afterwards (Figure. S2). A known volume

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of 7.0 mL IL was put into the equilibrium cell and degassed for at least 12 h ( 339.1 K)

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and a very wide liquid range. It was found that the design of asymmetric cation with

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short alkyl chains significantly reduced the viscosity of LCC-ILs. The solubilities of

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C3H8, C2H6, CH4, and N2 in these LCC-ILs were determined at temperature of 298.1

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to 313.1 K and at pressure of 20 to 150 kPa. The LCC-ILs exhibited very high

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solubility and excellent selectivity for hydrocarbons with different carbon number at

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ambient condition, and results indicated that the extension of alkyl chain of

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carboxylate anions was an efficient strategy to enhance the solubility. At 298.1 K and

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150 kPa, the solubilities of C3H8, C2H6, and CH4 in [P4442][C17H35COO] reach 0.408,

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0.133 and 0.009 mmol/g with selectivities of C3H8/CH4 and CH4/N2 up to 16.92 and

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2.72, respectively, significantly better than common ILs. Therefore, this study not

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only demonstrates the great potential of LCC-ILs as novel solvents for hydrocarbon

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separations, but also facilitates a molecular design to the development of novel ILs for

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other gas separations.

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ASSOCIATED CONTENT

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Supporting Information

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Experimental procedures and data. This material is available free of charge via the

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Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author

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*

Huabin Xing Tel/Fax: +86 571 87952375. E-mail: [email protected]. 18

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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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This research was supported by the Natural Science Foundation of Zhejiang

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Province of China (LR13B060001), the Natural Science Foundation of China

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(21436010), and the National Program for Support of Top-notch Young Professionals

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(H. X.).

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