Structural Investigation of Zn(II) Insertion in Bayerite, an Aluminum Hydroxide

Abstract

Bayerite was treated under hydrothermal conditions (120, 130, 140, and 150 °C) to prepare a series of layered double hydroxides (LDHs) with an ideal composition of ZnAl4(OH)12(SO4)0.5·nH2O (ZnAl4-LDHs). These products were investigated by both bulk techniques (powder X-ray diffraction (PXRD), transmission electron microscopy, and elemental analysis) and atomic-level techniques ((1)H and (27)Al solid-state NMR, IR, and Raman spectroscopy) to gain a detailed insight into the structure of ZnAl4-LDHs and sample composition. Four structural models (one stoichiometric and three different defect models) were investigated by Rietveld refinement of the PXRD data. These were assessed using the information obtained from other characterization techniques, which favored the ideal (nondefect) structural model for ZnAl4-LDH, as, for example, (27)Al magic-angle spinning NMR showed that excess Al was present as amorphous bayerite (Al(OH)3) and pseudoboehmite (AlOOH). Moreover, no evidence of cation mixing, that is, partial substitution of Zn(II) onto any of four Al sites, was observed. Altogether this study highlights the challenges involved to synthesize pure ZnAl4-LDHs and the necessity to use complementary techniques such as PXRD, elemental analysis, and solid-state NMR for the characterization of the local and extended structure of ZnAl4-LDHs.

Synthesis and Structural Characterization of a Pure ZnAl4(OH)12(SO4)·2.6H2O Layered Double Hydroxide

“…From our extensive optimizations (Table S1, Figure S1, and Ref 11 ), it is clear that the reaction temperature and thereby duration of the hydrothermal treatment is the most important variable.…”

“…Bayerite was prepared by the method of K. R. Poeppelmeier and S. J. Hwu 18 and similar to our earlier study except that the synthesis was scaled by a factor of five. 11 20.28 g (152.1 mmol)…”

“…ZnAl4-LDHs were prepared by hydrothermal treatment of bayerite in a zinc sulfate solution as previously described 11 using autoclaves with a volume of 40 mL and 80 mL, respectively. It is important that all zinc sulfate is dissolved prior to addition of bayerite, for this heating may be needed.…”

“…The fits yield a very good R-factor of 0.012, because the number of independent points (Nind) exceed the number of variables (Nvar) by far (see Table 1 and All spectra are uncorrected for phase shift. 27 Al NMR parameters 11 and literature values 31 for boehmite and bayerite as starting values. Figure 6.…”

“…[3][4][5][6] In contrast, incorporation of divalent cations such as Mg 2+ , Zn 2+ , Co 2+ , and Ni 2+ requires excess metal salts and hydrothermal conditions, [7][8][9][10][11][12][13][14] where dehydroxylation of the Al(OH)3 may compete with formation of the target MAl4-LDH, as observed for ZnAl4-LDH. 11 Their chemical structure is closely related to that of the minerals nickelalumite (NiAl4(OH)12SO4⋅3H2O) 2 and chalcoalumite (CuAl4(OH)12SO4⋅3H2O). 15 The challenging synthesis conditions for MAl4-LDH have hampered their structural characterization and property test as interference with both amorphous and crystalline impurities as well as the low crystallinity of the targeted MAl4-LDH has a significant impact on the analysis.…”