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

Abstract

The phase purity of a series of ZnAl4(OH)12SO4•nH2O layered double hydroxides (ZnAl4-LDH) obtained from a reaction of bayerite (Al(OH)3) with an excess of zinc(II) sulfate under hydrothermal conditions was investigated as a function of the reaction temperature, the duration of the hydrothermal treatment, and the zinc(II) concentration. The product quality, i.e., crystalline impurities, Al impurities and bulk Zn:Al ratio, were assessed by powder X-ray diffraction (PXRD), 27 Al MAS NMR, and elemental analysis. Structural characterization of a stoichiometric ZnAl4-LDH (120 °C, 9 days, and 2.8 M Zn(II)) showed a well-defined structure of the metal ion layer as evident by a single, well-defined Zn environment, i.e., no Zn substitution on the Al sites according to Zn K-edge EXAFS and PXRD. Furthermore, nearly all of the twelve different 1 H in the-OH groups and 4 27 Al resonances could be assigned using 1 H, 27 Al NMR correlation experiments recorded with ultra-fast MAS. The interlayer water content is variable based thermogravimetric analysis and changes in the 1 H MAS NMR spectra with temperature. A composition of ZnAl4(OH)12(SO4)⋅2.6H2O was obtained from combination of these techniques and confirmed that ZnAl4-LDH is isostructural to the mineral nickelalumite (NiAl4(OH)12SO4⋅3H2O).

Dritsite, Li2Al4(OH)12Cl2·3H2O, a New Gibbsite-Based Hydrotalcite Supergroup Mineral

“…This is because the hypothetical occupancy of the Li site of 80% in dritsite does not lead to the formation of a new mineral species, but results in the formation of 20% vacancies in the metal-hydroxide layers. The structural similarity between dritsite and chalcoalumite group minerals indicates that the place of the chalcoalumite group members in mineral nomenclature may be reconsidered with their insertion into the hydrotalcite supergroup, i.e., recognized as LDHs [54]).…”

Synthesis and Thermal Degradation of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, Cu2+, and Zn2+

“…16,17,21,23−25 Procedures for incorporation into gibbsite have been reported for MgCl 2 , CoCl 2 , NiCl 2 , Co(NO 3 ) 2 , Ni(NO 3 ) 2 , Cu(NO 3 ) 2 , Zn(NO 3 ) 2 , and ZnSO 4 salts, 21,24,25 whereas incorporation into bayerite has only been reported with Co(NO 3 ) 2 , Ni(NO 3 ) 2 , and ZnSO 4 solutions. 16,17,23 The same MAl 4 -A-LDH is obtained irrespective of the aluminum hydroxide reagent (bayerite or gibbsite). 27 However, the bayerite route works under milder operating conditions, requires less metal salt, and is more robust than the gibbsite route based on our experience.…”

“…1 The second LDH class, reported herein, are the much less-studied aluminum hydroxide [Al(OH) 3 ] type, 16,17 which is obtained by incorporation of lithium or divalent cations into the bayerite or gibbsite Al(OH) 3 polymorphs. 16,17 Both starting materials consist of dioctahedral Al(OH) 3 sheets, i.e., one third of the octahedral Al sites are vacant, and they differ only in their stacking sequences. For Li + , all Al-vacancies are occupied, 18 whereas only half of the vacancies are filled by divalent cations due to charge−charge repulsion.…”

“…MAl 4 -A-LDH is prepared by hydrothermal treatment of bayerite or gibbsite powder in a highly concentrated aqueous solution of a metal salt with the desired M(II) and anion. 16,17,21,25,26 The synthesis of pure products is challenging as either incomplete reaction (unreacted Al(OH) 3 ) or formation of byproducts is common. 16,17,21,23−25 Procedures for incorporation into gibbsite have been reported for MgCl 2 , CoCl 2 , NiCl 2 , Co(NO 3 ) 2 , Ni(NO 3 ) 2 , Cu(NO 3 ) 2 , Zn(NO 3 ) 2 , and ZnSO 4 salts, 21,24,25 whereas incorporation into bayerite has only been reported with Co(NO 3 ) 2 , Ni(NO 3 ) 2 , and ZnSO 4 solutions.…”

M(II)Al4 Type Layered Double Hydroxides—Preparation Using Mechanochemical Route, Structural Characterization and Catalytic Application

“…In the synthesis procedures, the application of mechanochemical pretreatment is not a necessity [11][12][13][14]16,18,20]; however, in several cases, mechanically activated aluminum hydroxide particles were used for obtaining these LDHs [10,15,17,19,21]. Recently, a large number of simple-to-set-up, moderately priced instruments with specific operating characteristics have become commercially available, making it possible to carry out various mechanochemical treatments for the preparation of well-known [22] as well as unique LDHs [23].…”