Layered double hydroxides with interlayer borate anions: A critical evaluation of synthesis methodology and pH-independent orientations in nano-galleries

Borate anions supplied by boric acid and ammonium tetraborate solutions were intercalated into Mg–Al–A-layered double hydroxides, where A = nitrate, adipate or terephtalate, by co-precipitation and ion exchange methods at varying pH values. Elemental analyses, powder X-ray diffraction, and FTIR spectra indicated that borate ion intercalating LDHs were succesfully produced by both methods. Well-textured materials were obtained by an ion-exchange technique particularly upon exchange with tetraborate solutions at pH 9. 27Al MAS NMR studies showed that the layers retained their integrity during the exchange process although there were variations in composition and crystallinity. The nature of the intercalated borate species was deduced from 11B MAS NMR data. The results showed that independently of the applied pH, observation of signals from both of BO3 and BO4 groups was consistent in all cases. The anions were intercalated as polyborate units like [B3O3(OH)4]− or [B4O5(OH)4]2−, even at pH 12 where the monoborate anions, [B(OH)4]−, are expected to exist dominantly.