27Al NMR and Raman spectroscopic studies of alkaline aluminate solutions with extremely high caustic content – Does the octahedral species Al(OH) 63− exist in solution?

Abstract

27Al NMR and Raman spectra of alkaline aluminate solutions with 0.005 M ≤ [Al(III)] T ≤ 3 M in various M′OH solutions (M′ + = Na +, K + and Li +) were recorded and analysed. Caustic concentrations up to 20 M were used to explore whether higher aluminium hydroxo complexes are formed at extremely high concentrations of hydroxide. A single peak was observed on the 27Al NMR spectrum of each solution. The chemical shift of this peak shifts significantly upfield with increasing [M′OH] T in solutions with [Al(III)] T < 0.8 M. This variation shows a strong dependence on the cation of the solution and practically disappears in systems with [Al(III)] T ≥ 0.8 M. For Raman spectra of solutions with [Al(III)] T = 0.8 M and [NaOH] T ≥ 10 M, the peak maximum of the symmetric ν 1-AlO 4 stretching of Al(OH) 4 − shifted progressively from ∼620 to ∼625 cm −1 and decreased in intensity with increasing [NaOH] T . In parallel, modes centred at ∼720 and ∼555 cm −1 (cf. ∼705 and ∼535 cm −1 at lower [NaOH] T , ascribed to a dimeric aluminate species appeared, and their intensities increased with increasing [NaOH] T . These variations in the 27Al NMR and Raman spectra can be interpreted in terms of contact ion-pairs formed between the cation of the medium and the well-established Al(OH) 4 − or the dimeric aluminate species. Assumption of higher aluminium hydroxo complex species (e.g., Al(OH) 6 3−) is not necessary to explain the spectroscopic effects observed.