Characterization of Al₃₀ in commercial poly-aluminum chlorohydrate by solid-state ²⁷Al NMR spectroscopy

Investigation of commercially produced hydrolysis salts of aluminum by solid-state ²⁷Al NMR spectroscopy and size-exclusion chromatography (SEC) reveals well-defined and distinct Al environments that can be related to physicochemical properties. ²⁷Al MAS and MQ-MAS NMR spectroscopic data show that the local structure of the solids is dominated by moieties that closely resemble the Al₃₀ polyoxocation (Al₃₀O₈(OH)₅₆(H₂O)₂₆¹⁸⁺), accounting for 72-85% of the total Al. These Al₃₀-like clusters elute as several size fractions by SEC. Comparison of the SEC and NMR results indicates that the Al₃₀-like clusters includes intact isolated clusters, moieties of larger polymers or aggregates, and possibly fragments resembling δ-Al₁₃ Keggin clusters. The coagulation efficacy of the solids appears to correlate best with the abundance of intact Al₃₀-like clusters and of smaller species available to promote condensation reactions.