Transitions in Al Coordination during Gibbsite Crystallization Using High-Field 27Al and 23Na MAS NMR Spectroscopy

Mechanisms of nucleation and growth of Al hydroxides such as gibbsite from aqueous solution, particularly in highly alkaline conditions, remain poorly understood. In this work, quantitative Al-27 and Na-23 MAS NMR experiments were conducted on solid samples extracted from the crystallization of gibbsite from an amorphous aluminum hydroxide gel precursor. The use of a high magnetic field and a moderate sample spinning rate of 20 kHz allowed transitional tetrahedral (Al-T) and pentahedral (Al-P) aluminum species to be observed along with the octahedral aluminum (Al-O) that dominates the gibbsite product. Low-coordinated Al species could be detected at concentrations as low as 0.1% of the total Al sites. The following results have been established: (a) Al-T and Al-P coexist on the surface of growing gibbsites even with a combined percentage over the total Al sites of less than 1%. (b) Different synthesis methods generate gibbsite with varying amounts of low-coordinated Al. (c) The amorphous gel precursor contains a significant amount of low-coordinated Al sites with an Al-O:Al-P:Al-T ratio of approximately 4:2:1. (d) Upon hydration, the external, low-coordinated Al sites become 6-fold coordinated by interacting with the oxygen in H2O, and the Al-27 MAS NMR peak position shifts to that for the Al-O sites. (e) Gibbsite with increased long-range order is synthesized over longer times by gradually incorporating residual Al-P and Al-T sites into octahedrally coordinated Al-O sites. (f) Trace Na is predominantly a surface species on gibbsite particles. These findings provide a basis for understanding the gibbsite crystallization mechanism, along with a general means of characterizing gibbsite surface properties that are of equal importance for understanding related processes such as dissolution behavior.