Complexation of boric acid with the N-methyl-D-glucamine group in solution and in crosslinked polymer

Abstract

N-Methylglucamine resin has been shown to be a good adsorbent for borate or boric acid. Above ca. pH 6, the boron adsorbability of this resin increased with a maximum around pH 9. The maximum distribution ratio was higher than 10⁶, however, further increase in pH resulted in a decrease in the distribution ratio. The combination of distribution and ¹¹B MAS NMR measurements revealed the formation of a 1:1 tetradentate complex of B(OH)₄ ^- with the N-methylglucamine group. The calculated pH-dependence of boron adsorption agreed well with the experimental results with respect to the -log [H⁺] values in the resin phase (not in the external solution) estimated by ³¹P NMR measurements. The maximum adsorption around pH 9 of the equilibrated solution is due to the fact that the pH in the resin phase remains higher than that in the external solution. Therefore, B(OH)₃/B(OH)₄ ^- is selectively concentrated into the resin phase and by the dehydration condensation B(OH)₄ ^- forms the 1:1 tetradentate complex with the anchor group. The overall formation constant of the complex was estimated to be 10^4.0±0.1, which was almost the same magnitude as that of the 1:1 monochelate complex with an α,β-diol site of N-methyl-D-glucamine (which is an analogue of the functional group of the resin) in aqueous solutions of pH 5.1–7.4. However, the binding structures of each 1:1 complex with the N-methyl-D-glucamine group are quite different in the aqueous solution and in the crosslinked polymer.