Metal Hydroxide Layers Research Articles

Ultrathin layered double hydroxide nanosheets prepared from a water-in-ionic liquid surfactant-free microemulsion for phosphate removal from aquatic systems

A water-in-(ionic liquid) surfactant-free reverse microemulsion were used for the controllable preparation of the small sized Mg2Al-layered double hydroxide (LDH) nanosheets. The metallic salts and alkali aqueous solutions were respectively employed as water phase for preparation of BmimPF6-dimethylformamide-water reverse microemulsions. Then the uncontaminated LDH sheets (LDH-M) have been afforded with a narrow diameter distribution (∼10–35nm). The LDH nanoplatelets present a uniform lateral dimension (∼31nm) and a small thickness (∼0.71nm), which may probably be composed of single brucite-like layer. They display larger surface areas (110–291m2/g) and smaller pore sizes (17.57nm and ∼3.83nm) than LDH based on traditional co-precipitation technique. The treatment of its calcined product (LDO) in water results in the flower-like LDH nanosheets with obvious aggregation and larger size. The phosphate adsorption experiments show that the small sized LDH nanosheets perform superior removal ability toward low concentration phosphate to the large sized LDH sheets. However, the LDO product exhibits further excellent adsorption efficiency due to losing the interaction of carbonate with metal hydroxide layers. After five recycle usage, the ultrathin LDH-M nanoflakes and the calcined product LDO keep about 88.7–93.5% uptake efficiency of the original adsorbents, indicating a good reusability.

Lignin Depolymerization with Nitrate-Intercalated Hydrotalcite Catalysts

Hydrotalcites (HTCs) exhibit multiple adjustable parameters to tune catalytic activity, including interlayer anion composition, metal hydroxide layer composition, and catalyst preparation methods. Here, we report the influence of several of these parameters on β-O-4 bond scission in a lignin model dimer, 2-phenoxy-1-phenethanol (PE), to yield phenol and acetophenone. We find that the presence of both basic and NO3– anions in the interlayer increases the catalyst activity by 2–3-fold. In contrast, other anions or transition metals do not enhance catalytic activity in comparison to blank HTC. The catalyst is not active for C–C bond cleavage on lignin model dimers and has no effect on dimers without an α-OH group. Most importantly, the catalyst is highly active in the depolymerization of two process-relevant lignin substrates, producing a significant amount of low-molecular-weight aromatic species. The catalyst can be recycled until the NO3– anions are depleted, after which the activity can be restored by re...

Interaction of pristine hydrotalcite-like layered double hydroxides with CO2: a thermogravimetric study

Metal oxides in general have surface acidic sites, but for exceptional circumstances, are not expected to mineralize CO2. Given their intrinsic basicity and an expandable interlayer gallery, the hydrotalcite-like layered double hydroxides (LDHs) are expected to be superior candidate materials for CO2 mineralization. However, the incorporation of Al3+ adversely impacts the ability of the metal hydroxide layer to interact with CO2 in the gas phase in comparison with the unitary Mg(OH)2. Thermogravimetric analysis shows that the decomposition reaction of the [Mg–Al–CO3] LDH is only marginally delayed in flowing CO2 in comparison with flowing N2, showing only an apparent marginal CO2 uptake. Al3+ ion severely attenuates the surface basicity of the LDHs, as the unitary Al(OH)3 is acidic in comparison with Mg(OH)2 and shows little or no interaction with CO2 in the gas phase.

Structure of the Carbonate-Intercalated Layered Double Hydroxides: A Reappraisal

Carbonate-intercalated layered double hydroxides have hitherto been thought to crystallize with rhombohedral symmetry (space group R3m). 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 × a0; b = 3 × a0; c ∼ c0/3; β ∼ 103°, where a0 and c0 are cell parameters of the cation-disordered structure) resolves many of the anomalies of the cation-disordered structure.

Layered Double Hydroxides: Proposal of a One-Layer Cation-Ordered Structure Model of Monoclinic Symmetry.

Layered double hydroxides are obtained by partial isomorphous substitution of divalent metal ions by trivalent metal ions in the structure of mineral brucite, Mg(OH)2. The widely reported three-layer polytype of rhombohedral symmetry, designated as polytype 3R1, is actually a one-layer polytype of monoclinic symmetry (space group C2/m, a = 5.401 Å, b = 9.355 Å, c = 11.02 Å, β = 98.89°). This structure has a cation-ordered metal hydroxide layer defined by a supercell a = √3 × a0; b = 3 × a0 (a0 = cell parameter of the cation-disordered rhombohedral cell). Successive layers are translated by (1/3, 0, 1) relative to one another. When successive metal hydroxide layers are translated by (2/3, 0, 1) relative to one another, the resultant crystal, also of monoclinic symmetry, generates a powder pattern corresponding to the polytype hitherto designated as 3R2. This structure model not only removes all the anomalies intrinsic to the widely accepted cation-disordered structure but also abides by Pauling's rule that forbids trivalent cations from occupying neighboring sites and suggests that it is unnecessary to invoke rhombohedral symmetry when the metal hydroxide layer is cation ordered. These results have profound implications for the correct description of polytypism in this family of layered compounds.

Electrochemical Oxygen Separation Using Layered Double Hydroxides As Hydroxide Ion Conductor

Oxygen separation membranes using oxide ion conducting materials have been studied for oxygen separation from air. However, the membrane can effectively separate oxygen from the air only at high temperatures higher than several hundred degrees, because the oxide ion conductor shows high ionic conductivity at only elevated temperatures. On the other hand, we have been studied layered double hydroxide (LDH) as a hydroxide ion conducting material [1-3]. LDHs are consisting of positively charged metal hydroxide layer and interlayer anions for charge compensation of cationic layer. The general formula for LDHs is [MII 1−x MIII x(OH)2][(An−)x/n • mH2O], where MII is a divalent cation such as Ni2+, Mg2+, Zn2+, etc., and MIII is a trivalent cation such as Al3+, Fe3+, Cr3+, etc., and An− is an anion such as CO3 2−, Cl−, OH−, etc. We have recently reported that LDHs intercalated with CO3 2- showed high ionic conductivity of the order of 10-3 S cm-1under humidification, and the LDHs are solid hydroxide ion conductors [1-3]. Here we report the development of an electrochemical oxygen separation process using a hydroxide ion conductor [4]. The electrochemical oxygen separation cell consists of pelletized OH− conductive solid electrolyte and electrodes. At the air feed side, oxygen in air, H2O and electrons produces OH- ions at the electrode with the electrochemical oxygen reduction reaction. At the same time, oxygen, H2O and electrons are generated from OH− ions at the other electrode with the electrochemical oxygen evolution reaction. Since H2O will easily be separated with O2, oxygen separation will be achieved. In this study, Ni-Fe LDH intercalated with CO3 2− (Ni-Fe CO3 2− LDH) was used as a hydroxide ion conductive material. The pellet of Ni-Fe CO3 2− LDH as an electrolyte was sandwiched with electrodes using Pt/C as a catalyst and Ni-Fe CO3 2− LDH as an ionomer. The oxygen concentration at the O2 product side was determined using a gas chromatography system. The electrochemical oxygen product flow was confirmed to be proportional to the external current density at 50oC under 70% of relative humidity, indicating that the membrane functioned as electrochemical oxygen separator. [1] K. Tadanaga, Y. Furukawa, A. Hayashi, M. Tatsumisago, Adv. Mater., 22 (2010) 4401-4404. [2] Y. Furukawa, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics, 192(2011) 185-187. [3] D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago, J. Electroanal. Chem., 671 (2012) 102-105. [4] Y. Arishige, D. Kubo, K. Tadanaga, A. Hayashi, M. Tatsumisago, Solid State Ionics, 262 (2014) 238-240. Figure 1

Water Molecules in Hydrotalcite‐like Layered Double Hydroxides: Interplay between the Hydration of the Anions and the Metal Hydroxide Layer

AbstractLayered double hydroxides comprise positively charged metal hydroxide layers and intercalated anions. These materials are obtained from aqueous medium both in nature and in the laboratory. Consequently the layered double hydroxides include a considerable amount of water. The presented study was designed to determine the proportion of water associated with the hydration sphere of the anion as opposed to that of the metal hydroxide slab. Among the two differently bound water species observed in all layered double hydroxides, the weakly bound water is associated with the metal hydroxide layer and is lost at 100 °C, whereas the strongly bound water is in the hydration sphere of the anion and is lost at higher temperatures (100 °C ≤ T ≤ 250 °C). This is in contrast to the better known cationic clays, wherein all the intercalated water is generally found to be in the hydration sphere of the cations. Further the water molecules in layered double hydroxides also bond to each other, leading to the incorporation of water in excess of what is predicted by the Miyata formula (Miyata, 1975) based on crystal chemical considerations. The excess water is one of the reasons for the poor crystallinity of layered hydroxides.

Acid Treatment of Layered Double Hydroxides Containing Carbonate

AbstractCarbonate‐containing Mg–Al layered double hydroxides (LDHs) with two different crystallinities were treated with acidic solutions that contain Cl–. Upon treatment with an acetate buffer/NaCl aqueous solution, low‐crystallinity LDH consumed more protons for dissolution of the metal hydroxide layers than did high‐crystallinity LDH, and it showed a lower extent of exchange of interlayer anions for Cl– than high‐crystallinity LDH. Varying washing procedures also resulted in differences in the characteristics of the washed products, such as the presence of Al(OH)3 and the extent of exchange for Cl–. When chloride‐containing LDHs were dispersed in water, Mg hydroxide species in the LDHs were found to be dissolved to some extent even in water, and Cl– ions in the LDHs were replaced with OH– ions that were formed by the dissolution.

Influences of Structural Differences of Layered Double Hydroxides on Their Hydroxide Ion Conductivities

Anion-exchange membrane fuel cells (AEMFCs) have attracted much attention since they can potentially overcome the stumbling blocks of proton-exchange membrane fuel cells. There are many research approaches to improve the performance of AEMFCs, and one of them is to seek new ion conductors that effectively transport hydroxide ions at the ambient temperature. Ion conductors with not only high ion conductivities but also high robustness are strongly required to realize the high-efficiency and long-life performance of AEMFCs. Among such ion conductors, we have focused on layered double hydroxides (LDHs). The basic layer structure of LDHs is based on that of brucite [Mg(OH)2]. In the brucite lattice, magnesium ions are surrounded octahedrally with hydroxide ions, and these octahedral units form infinite plane of the 2D layers. LDHs are derived by the substitution of a fraction of divalent cations in brucite by trivalent cations, which lead to positive charges in hydroxide layers. These charges are balanced by intercalation of anions and water between the positively charged layers. The possibility of varying the kinds and relative proportions of the di- and trivalent cations as well as the interlayer anions gives rise to the large variety of LDHs with a general formula [M2+ 1−xM3+ x(OH)2]x+[An−]x/n·yH2O, where M2+ and M3+ are di- and trivalent metal cations respectively, and An− is an anion. This flexibility in composition of LDHs has induced an increase in interest for many applications, such as catalyst supports, ion-exchange and adsorption, drug-delivery system, etc. In addition, some research groups including our group recently reported that LDHs operated as an OH−conductor, which could be applied for alkaline fuel cells, metal-air secondary batteries, and alkaline secondary batteries. In this study, we focus on the nature of ion conductivities in LDHs. To understand the mechanism of ion conductivities in LDHs in detailed, we synthesized Mg-Al and Mg-Ga LDHs with different relative proportions of di- and trivalent metal cations, and evaluated the influence of the proportion of trivalent cation on their ion conductivities by electrochemical measurements. We chose Mg-Al and Mg-Ga LDHs since they are widely used in LDH studies. We shed light on the structural relationship, which could guide the design of improved or novel hydroxide-conductive materials. Using the co-precipitated method, we successfully synthesized Mg-Al-CO3 and Mg-Ga-CO3 LDHs with various proportions of trivalent cations. In the results of X-ray diffraction, as the Mg/Al ratio decreased from 4 to 2, the 003 diffraction peak positively shifted, which indicates that layer height between metal hydroxide layers were diminished. This decrease in the layer height was due to an increase in electrostatic attraction between positive and negative layers. In contrast, resulted LDHs were characterized in similar BET surface areas and mean diameter in spite of changing the proportion of trivalent cations. Both series of LDHs did not displayed a linear increase in ion conductivities as a function of the proportion of trivalent cations, but a sharp raise appeared only for Mg2+/Al3+ = 2 and Mg2+/Ga3+ = 3. Based on the results of electron-diffraction analysis, Mg-Al and Mg-Ga LDHs, showing the highest ion conductivities had the ordered honeycomb cation arrangement within some parts of (0001) hydroxide layers. Other LDHs had complete random cation distribution. The ordered honeycomb structure was a cation distribution that can elude direct contact of M3+−M3+ and diminish the distance of the nearest trivalent cations; “neither near nor far” for individual interlayer anions. These things for the ordered cation arrangements not only demonstrate a helpful strategy for improving ion conductivities of LDHs, but also present new insights into structural relationship between immobile cationic charge centers and mobile interlayer anions.

Mg-Al-Fe-containing layered hydroxides

Magnesium-aluminum-iron layered hydroxides with hydrotalcite-like structure have been synthesized, and their chemical composition and crystal lattice parameters have been elucidated. Thermal decomposition of the iron-containing layered hydroxides has been studied by X-ray diffraction, thermogravimetry, and IR spectroscopy; the decomposition starts with loss of interlayer water, followed by dehydroxylation of metal hydroxide layers and decomposition of anions at higher temperature. Onset of the both decomposition stages has been shifted to lower temperature with increasing iron fraction in the sample. Magnesium-aluminum-iron hydroxides restore their structure after annealing and subsequent rehydration.

ChemInform Abstract: Topochemical Synthesis of Alkali‐Metal Hydroxide Layers within Double‐ and Triple‐Layered Perovskites.

AbstractAlkali metal hydroxide layers within lamellar perovskites are formed by a two‐step topochemical synthesis.

Relative Humidity-Induced Reversible Hydration Of Sulfate-Intercalated Layered Double Hydroxides

AbstractLayered double hydroxides (LDH) are extremely important materials for industrial processes and in the environment, and their physical-chemical behavior depends in large part on their hydration state, but the characterization of these hydration effects on their properties are incomplete. The present study was designed to explore the interpolytype transitions induced by variation in the ambient humidity among LDHs. The cooperative behavior of intercalated water molecules resulted in a sudden, single-step, reversible dehydration of the [Zn-Cr-SO4] LDH. The [Zn-Al-SO4] LDH provided an interesting contrast with (1) the coexistence of the end members of the hydration cycle over the 40-20% relative humidity range during the dehydration cycle, and (2) a random interstratified intermediate in the hydration cycle. These observations showed that the [Zn-Al-SO4] LDH offered sites having a range of hydration enthalpies, whereby, at critical levels of hydration (20–40%), the non-uniform swelling of the structure resulted in an interstratified phase. The variation in domain size during reversible hydration was also responsible for the differences observed in the hydration vs. the dehydration pathways. This behavior was attributed to the distortion in the array of hydroxyl ions which departs from hexagonal symmetry on account of cation ordering as shown by structure refinement by the Rietveld method. This distortion was much less in the [Zn-Cr-SO4] LDH, whereby the nearly hexagonal array of hydroxyl ions offered sites of uniform hydration enthalpy for the intercalated water molecules. In this case, all the water molecules experienced the same force of attraction and dehydrated reversibly in a single step. The changes in basal spacing were also accompanied by interpolytype transitions, involving the rigid translations of the metal hydroxide layers relative to one another.

The Influence of Oxalate-Promoted Growth of Saponite and Talc Crystals on Rectorite: Testing the Intercalation-Synthesis Hypothesis of 2:1 Layer Silicates

AbstractThe intercalating growth of new silicate layers or metal hydroxide layers in the interlayer space of other clay minerals is known from various mixed-layer clay minerals such as illite-smectite (I-S), chlorite-vermiculite, and mica-vermiculite. In a recent study, the present authors proposed that smectite-group minerals can be synthesized from solution as new 2:1 silicate layers within the low-charge interlayers of rectorite. That study showed how oxalate catalyzes the crystallization of saponite from a silicate gel at low temperatures (60ºC) and ambient pressure. As an extension of this work the aim of the present study was to test the claim that new 2:1 silicate layers can be synthesized as new intercalating layers in the low-charge interlayers of rectorite and whether oxalate could promote such an intercalation synthesis. Two experiments were conducted at 60ºC and atmospheric pressure. First, disodium oxalate solution was added to a suspension of rectorite in order to investigate the effects that oxalate anions have on the structure of rectorite. In a second experiment, silicate gel of saponitic composition (calculated interlayer charge -0.33 eq/O10(OH)2) was mixed with a suspension of rectorite and incubated in disodium oxalate solution. The synthesis products were extracted after 3 months and analyzed by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The treatment of ultrathin sections with octadecylammonium (nC =18) cations revealed the presence of 2:1 layer silicates with different interlayer charges that grew from the silicate gel. The oxalate-promoted nucleation of saponite and talc crystallites on the rectorite led to the alteration and ultimately to the destruction of the rectorite structure. The change was documented in HRTEM lattice-fringe images. The crystallization of new 2:1 layer silicates also occurred within the expandable interlayers of rectorite but not as new 2:1 silicate layers parallel to the previous 2:1 silicate layers. Instead, they grew independently of any orientation predetermined by the rectorite crystal substrate and their crystallization was responsible for the destruction of the rectorite structure.

Structure of the Chloride‐ and Bromide‐Intercalated Layered Double Hydroxides of Li and Al – Interplay of Coulombic and Hydrogen‐Bonding Interactions in the Interlayer Gallery

AbstractRietveld refinement of the structures of the [LixAl(OH)3]Xx·yH2O (X = Br, Cl; y ≈ 1/2, 2/3) layered double hydroxides (LDHs) shows that the halide ion occupies two distinct positions in each phase. One position (2a), promoted by Coulombic attraction, is proximal to the Li+ ion, which is the seat of the positive charge in the metal hydroxide layer. The other position (6h) is within a close hydrogen‐bonding distance from the hydroxy group of the metal hydroxide layer as well as the intercalated water molecule. The LDH with intercalated Br– ions crystallizes in two different structures, one of which is isostructural with the Cl– analogue. In the other structure obtained at a higher relative humidity (≥76 %; y ≈ 2/3), the Br– ion occupies a site proximal to the Al3+ ion (2b), and intercalated water molecules are proximal to the Li+ ion. In this structure, the hydration of the Li+ ion in a distant coordination shell overcomes Coulombic interactions with Br– ions. On partial dehydration (ambient humidity 29 %; y ≈ 1/2), this Br– ion reversibly reverts back to the site (2a) proximal to the Li+ ion, which results in a humidity‐induced structure change without any variation in the basal spacing. Thereby, the position of the halide ion in the interlayer gallery is determined by the interplay of Coulombic and hydrogen‐bonding interactions.

Polytypism in Sulfate-Intercalated Layered Double Hydroxides of Zn and M(III) (M = Al, Cr): Observation of Cation Ordering in the Metal Hydroxide Layers

The as-precipitated sulfate-intercalated layered double hydroxide of Zn and Al crystallizes in the structure of the 3R1 polytype. On hydrothermal treatment, this 3R1 polytype transforms into the somewhat rare 3H and 3R2 polytypes at different temperatures. Observation of the 3R2 polytype distinct from the 3R1 polytype is evidence for the lack of cation ordering in the [Zn-Al-SO4] system. The layered double hydroxide of Zn and Cr (polytype, 1H) on hydrothermal treatment in mother liquor yields a cation-ordered phase also having the structure of the 1H polytype. Direct evidence of cation ordering is found by the appearance of weak supercell reflections corresponding to a = √3 × a(o) (a(o) is the a parameter of the cation-disordered phase). The same precursor under other conditions yields the cation-disordered 3R1 polytype. In this work, the structures of both the cation-ordered and the cation-disordered phases with similar states of hydration are refined and compared.

Nitrate‐Intercalated Layered Double Hydroxides – Structure Model, Order, and Disorder

AbstractLayered double hydroxides with a high layer charge (+0.33 per empirical formula unit) intercalate nitrate ions with the molecular plane of the NO3– ion inclined at ca. 70° to the metal hydroxide layer, which results in a basal spacing of 8.8 Å. Three different N–O bond lengths are observed and yield Cs coordination symmetry. At lower charge (0.165 ≤ x ≤ 0.2), a basal spacing of 8.0 Å is observed, which indicates that the nitrate ion is intercalated with its molecular plane parallel to the metal hydroxide layer (coordination symmetry D3h) in a manner isostructural with carbonate‐intercalated layered double hydroxides. Consequently, crystal chemical considerations predict the layer charge corresponding to the idealized composition of the nitrate‐intercalated phase to be +0.165 per empirical formula unit. The [Mg–Al–NO3]0.165 phase is highly ordered with robust crystal growth along [001] and is stable to hydrothermal treatment, in contrast to the carbonate analogue of the same layer charge. Phases obtained with intermediate layer charge (+0.20 to 0.25 per empirical formula unit) exhibit structural disorder arising out of (i) planar faults and (ii) random interstratification of layers comprising nitrate ions in different orientations.

Observation of cation ordering and anion-mediated structure selection among the layered double hydroxides of Cu(ii) and Cr(iii)

Highly ordered Cl(-) and SO4(2-)-intercalated layered double hydroxides (LDHs) of Cu(II) and Cr(III) are obtained when coprecipitation is carried out at low pH ~ 5 and elevated temperature (60-80 °C). Precipitation under other conditions results in the formation of a gel. The SO4(2-)-LDH exhibits weak reflections which could be indexed to the 100 and 101 planes of a supercell corresponding to a = √3 ×a(o), providing direct evidence for cation ordering among LDHs by X-ray diffraction. The ordering of the M(II) and M'(III) in the metal hydroxide layer has been a subject of considerable debate in the LDH literature for the past several years and was earlier probed using short-range techniques such as NMR and EXAFS. Rietveld refinement indicates that the cation-ordered LDH adopts the structure of the 1H polytype (space group P3] a = 5.41 Å, c = 11.06 Å). In contrast the Cl(-)-intercalated LDH adopts the cation-disordered structure of the 3R1 polytype (space group R3[combining macron]m, a = 3.11 Å, c = 23.06 Å). The Cl(-)-LDH was used as a precursor to synthesize LDHs with other anions. While Br(-) and CO3(2-) (molecular symmetry, D3h) select for the 3R1 polytype, the XO3(-) (X = Br, I) ions (molecular symmetry, C3v) select for the rare 3R2 polytype. This work demonstrates the role of the intercalated anion in structure selection of the LDH.

Strategy for Synthesizing Quantum Dot-Layered Double Hydroxide Nanocomposites and Their Enhanced Photoluminescence and Photostability

Layered double hydroxide-quantum dot (LDH-QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields and thermal- and photostabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites.

Identification of Cation Clustering in Mg–Al Layered Double Hydroxides Using Multinuclear Solid State Nuclear Magnetic Resonance Spectroscopy

A combined X-ray diffraction and magic angle spinning nuclear magnetic resonance (MAS NMR) study of a series of layered double hydroxides (LDHs) has been utilized to identify cation clustering in the metal hydroxide layers. High resolution (multiple quantum, MQ) 25Mg NMR spectroscopy was successfully used to resolve different Mg local environments in nitrate and carbonate-containing layered double hydroxides with various Al for Mg substitution levels, and it provides strong evidence for cation ordering schemes based around Al–Al avoidance (in agreement with 27Al NMR), the ordering increasing with an increase in Al content. 1H MAS double quantum NMR spectroscopy verified the existence of small Mg3OH and Mg2AlOH clusters within the same metal hydroxide sheet and confirmed that the cations gradually order as the Al concentration is increased to form a honeycomb-like Al distribution throughout the metal hydroxide layer. The combined use of these multinuclear NMR techniques provides a structural foundation wit...

Molecular Modeling of Hydrotalcite Structure Intercalated with Transition Metal Oxide Anions: CrO42– and VO43–

Molecular dynamics (MD) simulations are used to study the interlayer structure, hydrogen bonding, and energetics of hydration of Mg/Al (2:1 and 4:1) layered double hydroxide (LDH) or hydrotalcite (HT) intercalated with oxymetal anions, CrO(4)(2-), and VO(4)(3-). The ab initio forcefield COMPASS is employed for the simulations. The charge on the oxymetal anions is determined by quantum mechanical density functional theory. The structural behavior of the oxymetal anions in LDH directly relates to the energetic relationships, with electrostatic and H-bonding interactions between the anions, hydroxide sites of the metal hydroxide layers, and the interlayer water molecules. Distinct minima in the hydration energy indicate the presence of energetically well-defined structural states with specific water content. The experimentally identified variability in the retention of the CrO(4)(2-) and VO(4)(3-) is well reflected in the calculations and self-diffusion coefficients obtained from the simulations give insight into the mobility of the intercalated species.