Article Cite This: Inorg. Chem. 2018, 57, 7299−7313 pubs.acs.org/IC Influences of Cation Ratio, Anion Type, and Water Content on Polytypism of Layered Double Hydroxides † § † § ‡ † § † § Meng Chen,*, , Runliang Zhu, , Xiancai Lu, Jianxi Zhu, , and Hongping He , † CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou 510640, China ‡ State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China § University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China *S Supporting Information ABSTRACT: Layered double hydroxides (LDHs) are a signifi- 2− 2− − − cant sink of anions (CO3 ,SO4 ,NO3 ,Cl, etc.) and divalent transition-metal cations in soil. The anion exchange capacity gives rise to functional materials. The stability of LDHs is determined by the interaction between cation-bearing layers and intercalated water and anions, which is correlated with polytypism and coordination structure. A systematic investigation is performed to show the influence of cation ratio, anion type, and water content on polytypism, swelling behavior, and interlayer structure of Mg− Al-LDHs using molecular dynamics simulations. LDHs interca- − lated with NO3 ions exhibit a polytype transition from 3R1 (three- layer rhombohedral polytype) to 1T (one-layer trigonal polytype) with increasing water content. NO − ions exhibit a D point group symmetry at low water contents. The polytype transition 3 3h − coincides with the complete transformation into tilted NO3 ion with a C2v point group symmetry. The transition appears at a 2− fi lower water content when the Mg/Al ratio is lower. LDHs with SO4 ions exhibit a three-stage polytypism. The rst and last ff stages are 3R1. The intermediate stage could be 1T or a mixture of di erent O(octahedra)-type interlayers, which depends on the 2− cation ratio. The relative popularity of SO4 ions with a Cs point group symmetry is characteristic for the intermediate stage, 2− while mostly SO4 ions exhibit a C3v symmetry. There is no clear relevance between cation ratio and water content at which a fi − 2− polytype transition happens. The con gurational adjustments of NO3 and SO4 ions facilitate the swelling behavior of LDHs. 2− − fi LDHs with CO3 or Cl ions always maintain a 3R1 polytype irrespective of water content and hardly swell. The con gurations of anions and water reflect local coordination structure due to hydrogen bonds. The layer-stacking way influences long-ranged Coulombic interactions. Hydrogen-bonding structure and long-ranged Coulombic interactions collectively determine polytypism and stability of LDHs. 1. INTRODUCTION LDHs.21 Calorimetric studies disclosed the thermodynamic See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. ff Downloaded via GUANGZHOU INST OF GEOCHEMISTRY on October 5, 2018 at 11:09:13 (UTC). stability of LDHs intercalated with di erent anions follows the Layered double hydroxides (LDHs), with the general chemical − 2 2− 21,25 II III n− · order NO3 <SO4 <CO3 , consistent with the order of formula [M 1−xM x(OH)2](A )x/n mH2O, are a class of − − 2 2− 24 anion exchange capacity (NO3 >Cl >SO4 >CO3 ) and natural and synthetic inorganic compounds with layered − − that of the magnitude of the binding energy (NO3 <Cl < structure derived from that of brucite Mg(OH)2. Natural 2 2− 26 SO4 <CO3 ). Based on thermodynamic data and measured occurrences of LDHs are generally associated with serpentine − − 2 1 3 concentrations of ions in soil, LDHs with CO3 were shown to and carbonate. Although natural stocks of them are limited, − 7 they are commonly precipitated in contaminated soil, as a be more preferable than those with Cl . A dissolution kinetics significant sink of transition-metal ions (Zn(II), Ni(II), Co(II), study showed, with a silicate-for-nitrate exchange in the − 16 and Fe(II)) in the environment.4 10 Divalent metal cations are interlayer, LDHs become much more stable. These studies adsorbed onto surfaces of clay minerals or gibbsite in soil, with show the importance of anions in stability of LDHs. dissolution of Al from minerals, which leads to nucleation and Besides anions, intercalated water is also relevant to the 11−19 structure and properties of LDHs. The behavior of it is growth of mixed-metal(II)-Al LDHs. Thermodynamic 20,27−30 studies showed the formation of LDHs is favored over that intermediate between solid ice and liquid water. Water − − fi of pure metal hydroxides.20 23 Anions (An ) intercalated in molecules are xed in sites of the interlayer like atoms in the 2− 2− − − LDHs can be CO3 ,SO4 ,NO3 ,Cl, etc., which are exchangeable.24 It was suggested anions are much more Received: April 10, 2018 important than cations in determining the solubility of Published: June 4, 2018 © 2018 American Chemical Society 7299 DOI: 10.1021/acs.inorgchem.8b00949 Inorg. Chem. 2018, 57, 7299−7313 Inorganic Chemistry Article solid lattice, and they diffuse through hopping between adjacent verifying experimental results. In principle, if a simulation is sites.31,32 With moveable water and anions in the interlayer long enough, thermodynamically stable polytype structure can galleries, LDHs exhibit characteristics such as anion ex- be achieved independent of the initial model. In this study, we − − changes,33 35 hydration,36 dehydration,37 39 and protonic use the simulated-annealing79 method in order to accelerate the conduction.40 Such characteristics are relevant to their roles equilibration process. MD simulations could be performed with as catalysts, catalyst supports, adsorbing agents, electrode ab initio methods29,35,80,81 or empirical force fields.27,28,31,82 − modifiers, and so on.41 45 Organic compounds can be Because the later requires much less computational power, it is intercalated into LDHs through anion exchanges, giving rise superior in predicting polytype-transition behaviors. Although to functional materials with characteristic optical properties or interlayer swelling and the structure of water and anions have − − − photochemical behaviors.46 52 Especially, LDHs have been been investigated in previous simulation studies,27 29,31,35,80 82 speculated as a possible inorganic catalyst for the origin of the dependence of polytype on compositions has not been organic life.53,54 It has been supposed, through anion disclosed explicitly to our knowledge. In this paper, we give a exchanges, that phosphates entering the interlayer of LDHs detailed and clear picture of the polytype-transition and on the primitive Earth became concentrated, favoring the interlayer-swelling behaviors and their relationship with subsequent synthesis of polyphosphate, the proto-biomole- compositions of water, anions, and cations. Through comparing cules.53 our results with available experimental observations, they are in Anions and water influence properties of LDHs through general agreement. Moreover, since simulations are performed interacting with rigid brucite-like layers. Depending on water with continuous water contents, how a polytype transition is content and the composition of anions and cations, rigid layers induced or not is shown through the variation of the exhibit characteristics such as swelling,36,55 undulations,56 and coordination structure of water or anions. Not only a more polytype-transformations.57,58 Polytypism, referred to as one- detailed picture compared to experimental observations is dimensional polymorphism, accounting for different layer- provided here, but also the factors determining a polytype are stacking ways, is a characteristic property of layered inorganic pointed out. We deduce a polytype is collectively determined − compounds.59 62 Polytypes are coined based on the space- by the local coordination structure and long-ranged Coulombic group symmetry and number of layers in a minimal repeating interactions. The local coordination structure is formed due to unit. Reported polytypes for LDHs are 1T (one-layer trigonal hydrogen bonds (HB) between anions, water, and layers, i.e., fi polytype), 2H1 (two-layer hexagonal polytype), 3R1 (three- short-ranged electrostatic interactions. The clari cation of layer rhombohedral polytype), 3R2 (another three-layer polytypism paves the way for understanding the relationship rhombohedral polytype), etc.63 Detailed characterizations of between stability of LDHs in environment and their micro- different polytypes will be shown in Section 2.2. LDHs scopic structure. It also sheds light on synthesizing materials intercalated with CO 2− generally exhibit 3R and 2H derived from LDHs through anion exchanges or exfoliation. − 3 1 1 46 83 polytypes,3,64 66 while those with Cl− were just reported to Intercalating large organic compounds or delamination 67,68 requires increasing the interlayer spacing at first, which is be of a 3R1 polytype. The polytype of LDHs intercalated − − 2− 69 2 with SO4 depends on cation ratio and water content. For probably more appropriate for LDHs with NO3 or SO4 .As · instance, zincowoodwardite (Zn1−xAlx)(OH)2(SO4)x/2 nH2O) will be shown in this paper, LDHs with those anions exhibit exists as a 1T polytype when x ranges from 0.32 to 0.33, while it interlayer swelling and a polytype transition with increasing 70 fl exists as a 3R1 polytype when x ranges from 0.35 to 0.50. In water content, which correlates with the exible coordination another instance, honessite and hydrohonessit are two mineral structure of anions. In this study, LDHs intercalated with CO 2−,NO−,SO2−, species with almost the same chemical formula − 3 3 4 ([Ni FeIII (OH) ](SO )·mH O) except that water content m and Cl anions are investigated independently, with continuous 6 2 2 4 2 II III is higher in the latter.71 Honessite was suggested to be a 3R water contents and the most reported cation ratios (M :M = 1 63 II III 2+ 3+ polytype,58 while hydrohonessit was thought to be a 1T72 or 3:1 and 2:1).