Patent US9,845,450 issued on Dec 26, 2017. The patent is a continuation of our efforts to demonstrate that tetramethylimidazolium (TMI) functionalized styrene alkaline anion exchange membranes are useful for alkaline water and alkaline CO₂ electrolysis. This patent shows the importance of having extra vinylbenzyl chloride (VBC) units in the polymer to promote stability. TMI, itself is not stable in alkaline solutions. The methyls in tetramethylimidazolium protect the imidazolium group from attack with by OH¯ , but the methyls can react with the OH¯ to form CH2 groups leading to degradation. But surprisingly TMI is stable for thousands of hours in 1 M KOH when the TMI is incorporated into our Sustainion® 37 membranes. Evidently, the presence of styrene somehow stabilizes the TMI.
Patent US9,845,450 shows that we get extra stability when we add excess VBC to the polymer. This suggests that the VBC can react with the CH2 groups to produce a CH2-CH2-Phenyl linkage. The phenyls can then protect the TMI in a manner similar to that proposed by Hugar et al. J. Am. Chem. Soc., 2015, 137 (27), pp 8730–8737. Click here to try the membranes yourself