Elsevier

Journal of Membrane Science

Volume 573, 1 March 2019, Pages 157-166
Journal of Membrane Science

Friedel-Crafts alkylation route for preparation of pendent side chain imidazolium-functionalized polysulfone anion exchange membranes for fuel cells

https://doi.org/10.1016/j.memsci.2018.11.079Get rights and content

Highlights

  • Friedel-Crafts alkylation route to synthesis polysulfone based AEM.

  • A novel flexible alkylation reagent containing -(CH2)4- spacer.

  • Obvious hydrophilic/hydrophobic micro-phase separation structure.

  • Low ion conductivity-normalized swelling ratio.

Abstract

Friedel-Crafts alkylation route is proposed to synthesis polysulfone based anion exchange membrane (AEM) containing pendent imidazolium-functionalized side chain with the purpose of avoiding conventional carcinogenic chloromethylation. It has no special functional group requirements on the polymer materials, which has advantage over those reported chloromethylation free routes. A novel flexible alkylation reagent containing -(CH2)4- spacer, (1-chlorobutyl-2,3-dimethylimidazole chloride) is synthesized and promotes hydrophilic/hydrophobic micro-phase separation structure in the membranes proved by small angle X-ray scattering (SAXS) and transmission electron microscopy image (TEM). The imidazolium-functionalized polysulfone AEMs prepared by the Friedel-Crafts alkylation route in this study exhibit first level properties compared to other AEMs prepared without chloromethylation. The ion conductivity-normalized swelling ratio (0.08% cm mS−1 at 80 °C) is the lowest, while exhibits higher or equivalent IEC normalized ion conductivity (56.8 S g cm−1 mol−1 at 80 °C). The membranes prepared in this work also display good thermal, mechanical and alkaline stabilities.

Introduction

In recent years, alkaline anion exchange membrane fuel cells (AAEMFCs) have drawn increasingly attentiveness by reason of the higher reaction kinetics in basic environments [1], [2], [3], [4], [5], [6], thus non-precious metals can be utilized as catalyst and reduce commercial costs [7], [8], [9]. As the essential compound of AAEMFCs, anion exchange membranes (AEMs) act as the separator and conductive support.[10]

The conventional method to prepare AEMs involves two steps: chloromethylation and quaternary amination. Particularly, chloromethylation is conducted by using chloromethyl methyl ether (CME) or chloromethyl octylether (CMOE), which provides excellent conversions and high yields [11]. Unfortunately, CME has been restricted to use since 1970s due to its carcinogenic and hazardous, and the same problem to CMOE [12], [13]. Recent methods to avoid the use of chloromethylation reagents are as following: (1) Friedel-Crafts acylation route [14], [15], (Scheme 1(1)), unfortunately, hydroxide ions attack the ketone after acylation reaction so that the anion exchange membrane has poor alkaline stability[16], [17]; Although the reduction of the ketone group to the methylene form can effectively solve this problem [15], the step of reducing the ketone group increases the difficulty of the reaction, making the reaction process more complicated, which is not conducive to simplifying the reaction process. (2) alkalinization of polymers containing uncharged functional groups using a brominating agent or methyl iodide [12], [18], [19], [20], (Scheme 1(2)); (3) N-Bromosuccinimide (NBS) acts as a brominating agent and bromination occurs with a benzyl or allyl-containing AEM material in the backbone structure, and then the quaternization reaction with the cation is performed to prepare anion exchange membrane [5], [21], [22], [23], (Scheme 1(3)); (4) preparing the anion exchange membrane by monomer polymerization after synthesis of the quaternized functional monomer [24], [25], [26], [27], (Scheme 1(4)). However, special groups, such as uncharged functional groups, a benzyl or allyl, functional monomer on polymer materials are required in methods (2) through (4), which limits the practical applications of these methods on commonly high performance polymers.

The Friedel-Crafts alkylation reaction means the preparation of alkyl carbocation by haloalkanes under the action of Lewis acid catalysts (AlCl3, FeCl3, SnCl4, etc.), which act as electrophiles attacking the benzene ring, then benzene ring forms carbocations and losses the proton to give the alkylbenzene [28], [29]. It has no special functional group requirements on the polymer materials since Friedel-Crafts alkylation is a characteristic reaction of benzene ring. Compared with the acylation reagents, Friedel-Crafts alkylation reagents do not require the alkaline unstable ketone, and therefore improve stability of the AEMs. More importantly, it is easy to introduce a flexible pendent functional side chain in AEMs through Friedel-Crafts alkylation, by designing hydrophobic haloalkane with -(CH2)n- spacers, which could effectively promote the construction of hydrophilic/hydrophobic micro-phase separation structures. Recent studies [30], [31], [32] on AMEs morphology have shown the continuous hydrophilic domains promote OH- transfer, and the connectivity of the hydrophobic domains inhibit excessive membrane swelling. Increasing the flexibility of the functional side chains [33], [34], [35], [36] can effectively solve the challenging issues that with the growth of IECs, ion conductivities increase, however, dimensional stability decreases due to excessive swelling of the membranes, and alkaline stability declines due to the growing chance of OH- attacking on the methylene groups that connect the functional groups to the polymer backbone [37], [38], [39]. Though it is an effective way to synthesis complex organics [40], [41], [42], [43], [44], as far as we know, it has no application on the preparation of AEMs.

In this work, polysulfone based anion exchange membrane containing pendent imidazole functionalized side chain are synthesized by the novel Friedel-Crafts alkylation route (Scheme 2), which is free of toxic carcinogenic chloromethylation reagents and has advantages over the above mentioned chloromethylation free methods. A new kind of alkylation reagent with -(CH2)4- spacer, 1-chlorobutyl- 2, 3-dimethylimidazole chloride [CBDMIm] Cl is synthesized and introduces a pendent alkyl functional side chain on polysulfone. The excellent hydrophilic/hydrophobic micro-phase separation structures make the PSf-BDMImOH anion exchange membranes prepared in this study exhibit higher IEC normalized ion conductivity than other AEMs prepared without chloromethylation reagent. More delightfully is that the swelling ratio of the membranes is very low and ion conductivity-normalized swelling ratio is the lowest even at 80 °C. It also has excellent alkaline stability.

Section snippets

Material

Udel P3500 polysulfone (PSf), Potassium hydroxide (KOH), H2SO4 (92.8%), N-methyl pyrrolidone (NMP), hydrochloric acid (HCl, 12 wt%) ethanol, dichloromethane(CH2Cl2), anhydrous aluminum chloride (AlCl3), 1,2-dimethylimidazole (DmIm), 4-Chloro-1-butanol were obtained commercially. Before using the deionized water was continuous boiled to remove carbon dioxide (CO2).

Synthesis of 1- hydroxybutyl- 2, 3-dimethylimidazole chloride ([HBDMIm] Cl)

A certain proportion of DmIm and 4-chloro-1-butanol were dissolved in an appropriate amount of anhydrous ethanol, and then stirred

Structure characterization

Fig. 1 shows the 1H NMR spectra of DmIm, 4-Chloro-1-butanol, [HBDMIm] Cl, PSf and PSf-BDMImCl. In 1H NMR spectrum of DmIm the peaks of Ha, Hb, Hc, Hd, are arised at δ = 2.24, δ = 3.52, δ = 6.68, δ = 6.97 ppm, respectively. The triplets atδ= 3.80 ppm, δ = 3.62 ppm in 1 H NMR spectrum of 4-Chloro-1-butanol are attributed to the methylene protons He, Hh, respectively, and the multiple at 1.94 ppm are assigned to Hf, Hg. [HBDMIm]Cl has been synthesized after the menshutkin reaction of DmIm and

Conclusions

PSf-BDMImOH membranes with pendent imidazole functionalized side chains were prepared by Friedel-Crafts alkylation instead of chloromethylation. 1H NMR and FT-IR spectra confirmed the successful synthesis of PSf-BDMImCl. The new route is both environmentally friendly and non-toxic, avoiding the use of chloromethylation reagents. In addition, SAXS and TEM results showed that good hydrophilic/hydrophobic micro-phase separation structure has formed in the membrane. The IECs of the PSf-BDMImOH

Acknowledgement

The authors thank the National Natural Science Foundation of China (21476044, 21406031 and 21776034), the National Key Research and Development Program of China (2016YFB0101203), the Joint Funds of the National Natural Science Foundation of China (U1663223) and the Changjiang Scholars Program (T2012049) for financial support of this work.

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