AL-27 AND NA-23 MAS NMR AND POWDER X-RAY-DIFFRACTION STUDIES OF SODIUM ALUMINATE SPECIATION AND THE MECHANISTICS OF ALUMINUM HYDROXIDE PRECIPITATION UPON ACID-HYDROLYSIS

Al-27 and Na-23 MAS NMR, powder X-ray diffraction, and infrared spectroscopic investigations of freeze-dried sodium aluminates and aluminum hydroxides formed through acid hydrolysis have been undertaken, with OH/Al hydrolysis ratios between 5.3 and 2.8 being analyzed. Numerous Al-27 NMR resonances were observed, the intensities of which vary as a function of OH/Al ratio, and these have been assigned to four-, five-, and six-coordinate aluminum species constituting a variety of structural moieties. The dominant species at an OH/Al ratio above 4.4 appears to be a Q(0) Na[Al(OH)(4)] salt, as indicated by a Al-27 resonance at 86.6 ppm. In addition, a second, broader resonance at 71.3 ppm demonstrates the simultaneous existence of further four-coordinate aluminum species linked through oxo bonds to other four-coordinate aluminums (e.g., Q(2) [Al(OH)(2)(OH)(2)(OAl)(2)](x-)), although a range of other polyoxoanion species (e.g., Q(1) [Al2O(OH)(6)(2-)] and Q(4) [AlO2-]) would almost certainly be present concurrently, also contributing to this vicinity of the Al-27 chemical shift range. At an OH/Al ratio between 4.4 and 4.1, a water-soluble phase forms that contains both four- and six-coordinate aluminum. At OH/Al ratios of 4.0 and below, a water-insoluble phase exists possessing four-, five-, and six-coordinate aluminum, the relative ratios of which vary with the degree of acid hydrolysis. The insoluble portion of the samples in the OH/Al less than or equal to 3.9 range exhibits Al-27 chemical shifts similar to those reported for transitional aluminas such as gamma-, eta-, and theta-Al2O3 and an infrared spectrum similar to pseudo-spinel gels, suggesting that a pseudo-spinel intermediate is the first phase involved in the crystallization of gibbsite. The resonance assigned to five-coordinate aluminum probably results from species involved in the transformation of the pseudo-spinel phase to pseudo-boehmite. The formation of gibbsite upon the acid hydrolysis of alkaline sodium aluminate solutions thus appears to follow the pathway pseudo-spinel --> pseudo-boehmite --> bayerite --> gibbsite.