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Functionalizing polystyrene with N-alicyclic piperidine-based cations via Friedel-Crafts alkylation for highly alkali-stable anion-exchange membranes

Olsson, Joel LU ; Pham, Thanh Huong LU and Jannasch, Patric LU (2020) In Macromolecules 53(12). p.4722-4732
Abstract
Different anion-exchange membranes (AEMs) based on polystyrene (PS) carrying benzyltrimethyl ammonium cations are currently developed for use in alkaline fuel cells and water electrolyzers. However, the stability of these state-of-the-art cations needs to be further improved. Here, we introduce highly alkali-stable mono- and spirocyclic piperidine-based cations onto PS by first performing a superacid mediated Friedel-Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by quaternization of the piperidine rings either using iodomethane to produce N,N-dimethylpiperidinium cations or cyclo-quaternizations using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary... (More)
Different anion-exchange membranes (AEMs) based on polystyrene (PS) carrying benzyltrimethyl ammonium cations are currently developed for use in alkaline fuel cells and water electrolyzers. However, the stability of these state-of-the-art cations needs to be further improved. Here, we introduce highly alkali-stable mono- and spirocyclic piperidine-based cations onto PS by first performing a superacid mediated Friedel-Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by quaternization of the piperidine rings either using iodomethane to produce N,N-dimethylpiperidinium cations or cyclo-quaternizations using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary ammonium cations. Thus, it is possible to functionalize up to 27% of the styrene units with piperidine rings, and subsequently achieve complete quaternization. The synthetic approach ensures that all the sensitive β-hydrogens of the cations are present in ring structures to provide high stability. AEMs based on these polymers show high alkaline stability and less than 5% ionic loss was observed by 1H NMR spectroscopy after 30 days in 2 M aq. NaOH at 90 °C. AEMs functionalized with N,N-dimethylpiperidinium cations show a higher stability than the ones carrying N-spirocyclic quaternary ammonium. Careful analysis of the latter revealed that the rings formed in the cyclo-quaternization are more prone to degrade via Hofmann elimination than the rings introduced in the Friedel-Crafts reaction. AEMs with an ion exchange capacity of 1.5 meq g-1 reach a hydroxide conductivity of 106 mS cm-1 at 80 °C under fully hydrated conditions. The AEMs are further tuned and improved by blending with polybenzimidazole (PBI). For example, an AEM containing 2 wt% PBI shows reduced water uptake and much improved robustness during handling, and reaches 71 mS cm-1 at 80 °C. The study demonstrates that the critical alkaline stability of PS-containing AEMs can be significantly enhanced by replacing the benchmark benzyltrimethyl ammonium cations with N-alicyclic piperidine-based cations. (Less)
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publishing date
type
Contribution to journal
publication status
published
subject
in
Macromolecules
volume
53
issue
12
pages
11 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:85086249345
ISSN
0024-9297
DOI
10.1021/acs.macromol.0c00201
language
English
LU publication?
yes
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Publication Date: June 1, 2020
id
1e33ffad-9b82-475c-ae2b-fcffe3566f8d
date added to LUP
2020-05-15 18:25:44
date last changed
2020-10-07 06:58:46
@article{1e33ffad-9b82-475c-ae2b-fcffe3566f8d,
  abstract     = {Different anion-exchange membranes (AEMs) based on polystyrene (PS) carrying benzyltrimethyl ammonium cations are currently developed for use in alkaline fuel cells and water electrolyzers. However, the stability of these state-of-the-art cations needs to be further improved. Here, we introduce highly alkali-stable mono- and spirocyclic piperidine-based cations onto PS by first performing a superacid mediated Friedel-Crafts alkylation using 2-(piperidine-4-yl)propane-2-ol. This is followed by quaternization of the piperidine rings either using iodomethane to produce <i>N</i>,<i>N</i>-dimethylpiperidinium cations or cyclo-quaternizations using 1,5-dibromopentane and 1,4-dibromobutane, respectively, to obtain N-spirocyclic quaternary ammonium cations. Thus, it is possible to functionalize up to 27% of the styrene units with piperidine rings, and subsequently achieve complete quaternization. The synthetic approach ensures that all the sensitive β-hydrogens of the cations are present in ring structures to provide high stability. AEMs based on these polymers show high alkaline stability and less than 5% ionic loss was observed by <sup>1</sup>H NMR spectroscopy after 30 days in 2 M aq. NaOH at 90 °C. AEMs functionalized with <i>N</i>,<i>N</i>-dimethylpiperidinium cations show a higher stability than the ones carrying <i>N</i>-spirocyclic quaternary ammonium. Careful analysis of the latter revealed that the rings formed in the cyclo-quaternization are more prone to degrade via Hofmann elimination than the rings introduced in the Friedel-Crafts reaction. AEMs with an ion exchange capacity of 1.5 meq g<sup>-1</sup> reach a hydroxide conductivity of 106 mS cm<sup>-1</sup> at 80 °C under fully hydrated conditions. The AEMs are further tuned and improved by blending with polybenzimidazole (PBI). For example, an AEM containing 2 wt% PBI shows reduced water uptake and much improved robustness during handling, and reaches 71 mS cm<sup>-1</sup> at 80 °C. The study demonstrates that the critical alkaline stability of PS-containing AEMs can be significantly enhanced by replacing the benchmark benzyltrimethyl ammonium cations with <i>N</i>-alicyclic piperidine-based cations.},
  author       = {Olsson, Joel and Pham, Thanh Huong and Jannasch, Patric},
  issn         = {0024-9297},
  language     = {eng},
  number       = {12},
  pages        = {4722--4732},
  publisher    = {The American Chemical Society (ACS)},
  series       = {Macromolecules},
  title        = {Functionalizing polystyrene with <i>N</i>-alicyclic piperidine-based cations via Friedel-Crafts alkylation for highly alkali-stable anion-exchange membranes},
  url          = {http://dx.doi.org/10.1021/acs.macromol.0c00201},
  doi          = {10.1021/acs.macromol.0c00201},
  volume       = {53},
  year         = {2020},
}