Structure of the Carbonate-Intercalated Layered Double Hydroxides: A Reappraisal Shivanna Marappa* and P. Vishnu Kamath* Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India * S Supporting Information ABSTRACT: Carbonate-intercalated layered double hydroxides have hitherto been thought to crystallize with rhombohedral symmetry (space group R3̅m). This widely accepted structure model comprises positively charged metal hydroxide layers wherein the cations are disordered. However, spectroscopic studies and simple chemical considerations militate against the possibility of cation disorder. This study shows that the observed powder X-ray diffraction pattern can indeed be fit to a cation- ordered crystal with monoclinic symmetry. With use of the carbonate-intercalated layered double hydroxide of Zn and Al as an illustration, the structure of the layered double hydroxide is refined by the Rietveld method. The resulting structure (space group C2/m, a = √3 × a 0 ; b =3 × a 0 ; c ∼ c 0 /3; β ∼ 103°, where a 0 and c 0 are cell parameters of the cation-disordered structure) resolves many of the anomalies of the cation-disordered structure. 1. INTRODUCTION Amelioration of atmospheric CO 2 is one of the biggest challenges facing human kind today. 1 Large quantities of CO 2 are dissolved in natural water bodies, wherein it is transformed into carbonate ions as CO 2 +H 2 O → CO 3 2- + 2H + . CO 2 dissolution causes the acidification of natural water sources. There is an urgent need to mineralize the dissolved carbonate ions, in the form of insoluble inorganic carbonates. 2 If this is not done, even a slight increase in the average ambient temperature has the potential to release massive amounts of dissolved CO 2 back into the atmosphere. Layered double hydroxides (LDHs) are among the most important candidate materials for CO 2 amelioration. 3-6 LDHs are obtained by the partial, isomorphous substitution of M(II) ions by M′(III) ions in the structure of M(II)(OH) 2 . The resulting metal hydroxide layer, [M(II) 1-x M′(III) x (OH) 2 ] x+ , has a positive charge, to compensate which anions, A n- , are included in the interlayer galleries. 7,8 Mineral LDHs as well as laboratory-synthesized samples are generally obtained from the aqueous medium. Natural water bodies, as well as laboratory water, unless specially treated, are rich in dissolved CO 2 . LDHs therefore crystallize with carbonate ions in the interlayer region. 9 LDHs comprising other anions are known to readily exchange their anions for incoming carbonates. 10 The carbonate ions once incorporated into the LDHs cannot be exchanged for other anions, unless they are discharged first by the use of a mineral acid. 11,12 On the basis of these empirical observations, it is suggested that the LDHs have a high affinity for carbonate ions. 12 The origin of this affinity is traced to the crystal structure of LDHs. Numerous crystal structures have been refined in this diverse family of hydroxides with M(II) = Mg, Ca, Co, Ni, Cu, Zn; M′(III) = Al, Cr, Fe, V, Ga, In; A n- = Cl - , Br - , NO 3 - , CO 3 2- , SO 4 2- , among others. 13-15 A recent detailed review makes a critical appraisal of these reported structures. 13 All the structures reviewed by Richardson 14,15 comprise a cation-disordered metal hydroxide layer. The metal hydroxide layer is obtained from a hexagonally packed array of hydroxyl ions, with the cations, M(II) and M′(III), occupying alternative layers of octahedral sites randomly. A single-metal hydroxide layer can be described as AbC within the Bookin and Drits scheme. 16 Here A and C represent the close-packed positions of hydroxyl ions and “b” represents the octahedral interstitial site occupied by the cations statistically. Carbonate-containing LDHs are shown to have the sequence AC CB BA AC·······, wherein successive metal hydroxide layers are translated by ( 2 / 3 , 1 / 3 ) relative to one another. Such a stacking sequence yields a three-layer cell of rhombohedral crystal symmetry. Taylor 9 suggested that this mode of stacking is most conducive to the incorporation of carbonate ions in the interlayer gallery. The interlayer gallery lined by identical arrays of hydroxyl ions include prismatic interlayer sites (local site symmetry D 3h ). The molecular symmetry of the carbonate ions is also D 3h . The coincidence of the molecular symmetry of the carbonate ion with the local symmetry of the interlayer site maximizes the hydrogen-bonding interactions, leading to a tight packing of the interlayer space, greater thermodynamic stability, and enhanced affinity for CO 3 2- ions. In opposition to Taylor’s views, there are contrarian studies that show the heat of formation of carbonate LDHs to be nearly zero. 17,18 There are other suggestions that CO 3 2- LDHs are kinetically nonlabile (metastable) but thermodynamically unstable, 19 to account for the poor leaving nature of intercalated carbonate ions. Evidence is now emerging which militates against the widely accepted cation-disordered structure model: (1) X-ray diffraction studies of single crystals of mineral LDHs reveal weak reflections arising out of a supercell of a cation-ordered metal hydroxide layer. 20,21 (2) EXAFS spectra 22 of Mg-Fe Received: August 31, 2015 Revised: October 20, 2015 Accepted: October 22, 2015 Published: November 2, 2015 Article pubs.acs.org/IECR3 © 2015 American Chemical Society 11075 DOI: 10.1021/acs.iecr.5b03207 Ind. Eng. Chem. Res. 2015, 54, 11075-11079