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Greim, Dominik ; Milius, Wolfgang ; Bojer, Beate ; Siegel, Renée ; Breu, Josef ; Senker, Jürgen:
Structure of Na₃[Al(L-Lactate)₃]₂.6H₂O Crystallized from Lohtragon AL 250 : A Commercial Cement Adjuvant.
In: Crystal Growth & Design.
Bd. 19
(2019)
Heft 8
.
- S. 4557-4563.
ISSN 1528-7505
DOI: https://doi.org/10.1021/acs.cgd.9b00394
Abstract
Aqueous aluminum(III) solutions at medium pH are used for industrial applications such as cement adjuvants. The necessary high Al3+ concentrations are achieved by using complexing ligands, such as lactic acid, to prevent precipitation. Here, we grew crystals from commercially available neutralized aluminum lactate solutions to derive an understanding of the underlying aluminum lactate chemistry. Using X-ray diffraction in combination with multinuclear NMR spectroscopy and quantum mechanical calculations, we found that the crystals have a stoichiometry of Na-3Al(L-lactate)(3)](2)center dot 6H(2)O and consist of negatively charged Al(L-lactate)(3)](2)(3-) bis-clusters. The remarkable stability of these clusters is achieved by linking both Al(L-lactate)(3) subunits via three very short symmetrical hydrogen bonds between the alpha-hydroxy groups with half of them being deprotonated. The trends for both the Al-27 chemical shift and the spin-spin relaxation for the Lohtragon-type solutions as a function of pH suggest the bis-cluster to be the origin for the stability of highly concentrated Al3+ solutions at medium pH. It may inspire alternative synthesis routes to extend the stability of concentrated Al3+ solutions over a broad pH range in the future.