Layered Double Hydroxides Phase Research Articles

Ca‐Rich Ca–Al‐Oxide, High‐Temperature‐Stable Sorbents Prepared from Hydrotalcite Precursors: Synthesis, Characterization, and CO2 Capture Capacity

We present the design and synthesis of Ca-rich Ca-Al-O oxides, with Ca(2+)/Al(3+) ratios of 1:1, 3:1, 5:1, and 7:1, which were prepared by hydrothermal decomposition of coprecipitated hydrotalcite-like Ca-Al-CO(3) precursors, for high-temperature CO(2) adsorption at 500-700 °C. In situ X-ray diffraction measurements indicate that the coprecipitated, Ca-rich, hydrotalcite-like powders with Ca(2+)/Al(3+) ratios of 5:1 and 7:1 contained Ca(OH)(2) and layered double hydroxide (LDH) phases. Upon annealing, LDH was first destroyed at approximately 200 °C to form an amorphous matrix, and then at 450-550 °C, the Ca(OH)(2) phase was converted into a CaO matrix with incorporated Al(3+) to form a homogeneous solid solution without a disrupted lattice structure. CaO nanocrystals were grown by thermal treatment of the weakly crystalline Ca-Al-O oxide matrix. Thermogravimetric analysis indicates that a CO(2) adsorption capacity of approximately 51 wt. % can be obtained from Ca-rich Ca-Al-O oxides prepared by calcination of 7:1 Ca-Al-CO(3) LDH phases at 600-700 °C. Furthermore, a relatively high CO(2) capture capability can be achieved, even with gas flows containing very low CO(2) concentrations (CO(2)/N(2) = 10 %). Approximately 95.6 % of the initial CO(2) adsorption capacity of the adsorbent is retained after 30 cycles of carbonation-calcination. TEM analysis indicates that carbonation-promoted CaCO(3) formation in the Ca-Al-O oxide matrix at 600 °C, but a subsequent desorption in N(2) at 700 °C, caused the formation CaO nanocrystals of approximately 10 nm. The CaO nanocrystals are widely distributed in the weakly crystalline Ca-Al-O oxide matrix and are present during the carbonation-calcination cycles. This demonstrates that Ca-Al-O sorbents that developed through the synthesis and calcination of Ca-rich Ca-Al LDH phases are suitable for long-term cyclic operation in severe temperature environments.

Structural and functional characterisation of a biohybrid material based on acetylcholinesterase and layered double hydroxides

This work describes the use of layered double hydroxides (LDHs) for the immobilisation of acetylcholinesterase (AChE) on insulator/semiconductor solid supports. Different LDHs have been synthesised by a co-precipitation method. Afterwards, biohybrid materials based on AChE–LDH mixtures have been produced using wild and recombinant enzymes. Spectroscopic techniques have confirmed the LDH phase identity and the links created between the LDH and AChE. Spectrophotometric assays have demonstrated that most of the biohybrid materials are functional and stable. Several configurations have been used for AChE immobilisation. The highest catalytic responses have been observed when using wild enzyme and immobilising AChE–LDH mixtures on LDHs previously deposited on the solid supports. LDHs have been demonstrated to be suitable host matrices for AChE immobilisation on electrodes for the subsequent development of electrochemical biosensors.

Synthesis and characterisation of exfoliated layered double hydroxide (LDH) / sugar nanocomposites

Abstract : In this study the preparation of layered double hydroxide (LDH)/sugar nanocomposites using an exfoliated MgAl-lactate LDH (Mg: Al = 3:1) and a sugar (sucrose and maltose) as precursors is described. The products have been characterised by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The properties of the composite materials have been further studied using differential scanning calorimetry (DSC). The expanded interlayer spacing of the lactate LDH suggests the intercalation of lactate ions within the gallery region. Stirring of the MgAl-lactate LDH in water for 24 hours at room temperature resulted in a clear colloidal dispersion. Upon solvent evaporation, the resulting material consisted of a disorganized restacked LDH phase. The most significant evidence for the formation of nanocomposites from the exfoliated clay materials is suggested by the absence of reflections associated with the LDH in the PXRD patterns. DSC results suggest that the glass transition temperatures of the composite materials with 5%, 10% and 20% LDH loading are superior to the parent maltose. Therefore, the LDH nanolayers give the LDH filled sugar nanocomposites better thermal stability than the pristine sugar, thus these composites could be used as drug excipients in the pharmaceutical industry. Keywords : Amorphous sugar, exfoliation, layered double hydroxides, nanocomposites, thermal stability. Doi: 10.4038/jnsfsr.v39i2.3171 J.Natn.Sci.Foundation Sri Lanka 2011 39 (2): 113 - 119

Experimental (X-Ray Photoelectron Spectroscopy) and theoretical studies of benzene based organics intercalated into layered double hydroxide

The present paper deals with a fine physico-chemical analysis of some hybrid materials combining an inorganic layered double hydroxide phase (LDH) with an organic benzene derivative entity R′-C 6H 4-R with R = –SO 3 −, –CO 2 − and R′ = –H, –OH. The main topic of this work is, in a nanoscale, to propose a way to approach the understanding of the interactions between inorganic and organic sub-systems. The role of the anionic headgroup R is discussed in term of reactivity with the mineral support. The classical expertise of materials via the PXRD technique puts into light an effective LDH interlayer space enlargement with the organics incorporation and a minimum angle of inclination for every organics within the LDH matrix. The originality of this study is to use the X-Ray Photoelectron spectroscopy (XPS) as a local probe of the chemical environments of the headgroup of the organic entities. In a parallel way, some quantum calculations (by using molecular and periodical codes) are carried out to best appreciate the electronic and structural evolutions before and after the intercalation step. A specific reactivity of the –SO 3 − group compared with the –CO 2 − one is evidenced. Moreover, a correlation at the first order is then set up between the net charges of atoms and the XPS binding energies of their core levels.

Exfoliation and liquid crystal phase formation of layered double hydroxide into waterborne polyurethane coatings

Waterborne polymer coatings based on polymer composites were prepared by using layered double hydroxides (LDH) as reinforcing inorganic particles. Starting with a model system we demonstrated the impact of the nature of the charge balancing anions of the LDH phase on the kinetics of the polymer intercalation and the film building as well as on the degree of particle dispersion. A broader scope of coating morphologies, covering particle arrangement and heterogeneities on extended length scales resulted from specific polymer–LDH particle interactions. From a combination of characterization techniques using small angle X-ray scattering, transmission electron microscopy and confocal laser scanning microscopy and by using a mixture of compatible polymers it was found that textured coatings were obtained via a liquid crystal phase of polymer stabilized LDH platelets whereas the LDH particles impact on the phase separation of incompatible polymers resulted in disperse, bicontinuous or self-stratified coatings. This novel innovative approach was found to be promising for coatings domains where a high impact resistance is needed.

Supported layered double hydroxide-related mixed oxides and their application in the total oxidation of volatile organic compounds

Structured mixed oxide catalysts were prepared by the calcination of layered double hydroxides (LDHs) deposited on Al 2O 3/Al supports (anodized aluminum foil). The deposition of LDH precursors on the supports was carried out under hydrothermal conditions at 140 °C in aqueous solutions of Ni, Co, Cu, and Mn nitrates. M II-(Mn)-Al LDHs (M II = Ni, Co, Ni-Co, Ni-Cu, and Co-Cu) with only slight Mn contents were obtained. An increased pH of the solutions used for deposition enhanced the formation of LDH phases. After heating at 500 °C, spinel-like and/or NiO-like oxides were detected in the supported mixed oxides. Compared to the mixed oxides obtained by calcination of the coprecipitated LDH precursors, the supported mixed oxides exhibited worse reducibility and lower catalytic activity in the total oxidation of ethanol; both the formation of spinel-like phases and high structural ordering of the products deposited on the Al 2O 3/Al supports could explain the poor reducibility. Among the supported catalysts, the Ni-Cu-(Mn)-Al mixed oxide was the most active in the total oxidation of ethanol. Increasing the pH of solutions used during the hydrothermal deposition of the LDH precursors resulted in improved catalytic activity and selectivity of the supported mixed oxides.

Mg-Fe-mixed oxides derived from layered double hydroxides: A study of the surface properties

The influence of surface properties on the selectivity of the synthesized catalysts was studied, considering that their selectivity towards particular hydrocarbons is crucial for their overall activity in the chosen Fischer- -Tropsch reaction. Magnesium- and iron-containing layered double hydroxides (LDH), with the general formula: [Mg1-xFex(OH)2](CO3)x/2?mH2O, x = = n(Fe)/(n(Mg)+n(Fe)), synthesized with different Mg/Fe ratio and their thermally derived mixed oxides were investigated. Magnesium was chosen because of its basic properties, whereas iron was selected due to its well-known high Fischer-Tropsch activity, redox properties and the ability to form specific active sites in the layered LDH structure required for catalytic application. The thermally less stable multiphase system (synthesized outside the optimal single LDH phase range with additional Fe-phase), having a lower content of surface acid and base active sites, a lower surface area and smaller fraction of smaller mesopores, showed higher selectivity in the Fischer-Tropsch reaction. The results of this study imply that the metastability of derived multiphase oxides structure has a greater influence on the formation of specific catalyst surface sites than other investigated surface properties.

Plane-Wave Density Functional Theory Study on the Structural and Energetic Properties of Cation-Disordered Mg−Al Layered Double Hydroxides

Periodic solid models have been employed to investigate the structural and energetic properties of cation-disordered Mg-Al layered double hydroxides (LDHs), including composition, ordering, and stacking pattern of layers. The geometry, lattice energy and density of states (DOS) of the periodic models were computed using the plane-wave pseudopotential implementation of density functional theory (DFT) with the virtual crystal approximation (VCA). The calculation results for the MgAl-Cl-LDH model show that a pure LDH phase is difficult to sustain when the Mg/Al ratio (R) is R > or = 4. The stability of the cation-disordered MgAl-Cl-LDH unit cell increases upon increasing R, owing to the decrease in the number of Al(III)-O-Al(III) linkages in the LDH sheets. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of MgAl-Cl-LDH receive contributions from the 3s and 3p orbitals of Mg and Al and the 2p orbitals of O and Cl, respectively, implying significant host-guest interactions. The increase of R leads to the decrease in electron density of 3s and 3p of metal cation and that of 2p in Cl. Consequently, the HOMO-LUMO energy gap as well as the systematical stability increase upon increasing R. From this point of view, it has been demonstrated that the cations distribute in an ordered arrangement with the absence of Al(III)-O-Al(III) linkages if R is in the low range (R = 2-3). It was also found that the 3R polytype is the most stable stacking pattern with the same Mg/Al ratio, owing to the fact that the conductive band energy levels decline with the increase of electron density of 3s and 3p of the metal cation in the 3R-stacking system. These findings agree well with the experimental results and provide a profound understanding of critical factors influencing the structure of LDHs, including metal composition and ordering, the stacking sequence, as well as the host-guest interactions.

Structural and microstructural changes during anion exchange of CoAl layered double hydroxides: anin situX-ray powder diffraction study

Anion-exchange processes in cobalt–aluminium layered double hydroxides (LDHs) were studied byin situsynchrotron X-ray powder diffraction (XRPD). The processes investigated were CoAl–CO3→ CoAl–Cl → CoAl–CO3, CoAl–Cl → CoAl–NO3and CoAl–CO3→ CoAl–SO4. The XRPD data show that the CoAl–CO3→ CoAl–Cl process is a two-phase transformation, where the amount of the CoAl–CO3phase decreases exponentially while that of the CoAl–Cl phase increases exponentially. Energy-dispersive X-ray spectroscopy (EDXS) studies of a partially chloride-exchanged CoAl–CO3LDH sample along within situXRPD data suggested that the individual particles in the CoAl–CO3sample are generally anion-exchanged with chloride one at a time. In contrast with the CoAl–CO3→ CoAl–Cl transformation, the XRPD data show that the reverse CoAl–Cl → CoAl–CO3process is a one-phase transformation. Rietveld refinements indicate that the occupancy factors of the carbon and oxygen sites of the carbonate group increase, while that of the chloride site decreases. In the CoAl–Cl → CoAl–NO3anion-exchange reaction, the XRPD patterns reveal the existence of two intermediate phases in addition to the initial CoAl–Cl and final CoAl–NO3phases. Thein situdata indicate that one of these intermediates is a mixed nitrate- and chloride-based LDH phase, where the disorder decreases as the nitrate content increases. The XRPD data of the partial CoAl–CO3→ CoAl–SO4anion-exchange reaction show that the process is a two-phase transformation involving a sulfate-containing LDH with a 1H polytype structure.

Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag

The morphology of outer-product (Op) C–S–H in 20-year-old slag–cement pastes appeared in most blends to be finer than at younger ages. The Ca/Si and Ca/(Si + Al) ratios of the Op C–S–H decreased with increasing slag content, and the Al/Si ratio increased. The Ca/Si ratio of C–S–H in the slag-containing pastes was lower at 20 years than at 14 months and the amount of Ca(OH) 2 was reduced indicating that additional slag must have reacted. The mean aluminosilicate chain length of the C–S–H was very long in all the samples and would be expected to have increased with age. The TEM-EDX and NMR data are consistent with nanostructural models for C–S–H. The Mg/Al ratio of the Mg–Al layered double hydroxide phase (LDH) was lower at 20 years than at 14 months in all cases except for the neat slag paste; aluminium hydroxide-based structure might be interstratified with those of the Mg–Al LDH.

Proposed General Sol−Gel Method to Prepare Multimetallic Layered Double Hydroxides: Synthesis, Characterization, and Envisaged Application

A versatile sol−gel method for the synthesis of multimetallic layered double hydroxides (LDHs) is presented. The success of this method is due to the accurate control over crucial synthesis parameters, which are thoroughly analyzed, and to the complexation of metallic precursors in order to equalize their hydrolysis and condensation rates to obtain transparent gels. XRD patterns show pure LDH phase, with broad peaks indicative of small crystal size. Solids thus obtained show nanocapsular morphology, instead of the platelet-like particles obtained by coprecipitation and other methods. Thermal decomposition was analyzed by TGA, DTA, and FTIR, revealing that a variety of interlayer anions are present in different proportions, mainly carbonates, nitrates, acetates, and alkoxy groups. This observation was corroborated by the expansion in the interlayer region observed by XRD. N2 physisorption confirmed that all calcined sol−gel LDHs have high specific surface areas (up to 290 m2 g−1) and very narrow pore size ...

Method for Large-Scale Production of Multimetallic Layered Double Hydroxides: Formation Mechanism Discernment

A simple, economical, and environmentally friendly method for the production of layered double hydroxides (LDHs) is presented. The synthesis procedure is based on dispersing insoluble metal oxides, adjusting the pH by adding an optimum amount of metal nitrates, and dispersing and aging the product for a short time (6−8 h). The final product does not require washing, opposite to the traditional coprecipitation synthesis procedure. A dissolution−precipitation−recrystallization mechanism is proposed for the formation of LDHs, based on particle size measurements, XRD analyses, radial distribution functions, and 27Al MAS NMR studies. Solids were characterized by XRD, N2 physisorption, TGA-DTA, SEM, and TEM, revealing that both LDHs and their calcination products have very similar properties to those prepared by conventional procedures. Pure LDH phase is obtained after 6−8 h; a large, uniform particle size that would usually require prolonged hydrothermal treatments is attained. Surface areas ranged from 32 to ...

Comparative study of the synthesis of layered transition metal molybdates

Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T–Al containing MMOs ( T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT 2(OH)(MoO 4) 2·H 2O, where A=NH 4 +, Na + or K +. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu–Al and Zn–Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products.

Synthesis of crystalline layered double hydroxides: Precipitation by using urea hydrolysis and subsequent hydrothermal reactions in aqueous solutions

The urea method is frequently used for synthesis of highly crystalline layered double hydroxides (LDHs). However, in the Mg–Al system, Mg/Al ratios in products have been often found to be smaller than those of starting solutions. In this study, we emphasized monitoring of Mg/Al ratios to investigate the process of LDH formation, while setting the Mg/Al ratio of the starting solutions at 2. LDHs were synthesized by using the urea hydrolysis reaction under hydrothermal conditions. From a starting solution with high metal ion concentration (0.5 M), by-products formed when Mg/Al ratios of products reached about 2; these by-products were most likely hydrated magnesium carbonate hydroxide phases (HM). In contrast, from a low metal ion concentration starting solution (0.01 M), a boehmite-like by-product was detected at an early stage of product formation, but prolonged reaction time or increasing temperature led to product conversion to the LDH phase. Particle size was mainly influenced by metal ion concentration. However, in this study, formation of HM by-products from relatively high metal ion concentration starting solution could not be avoided in one step. Therefore, in order to convert the products to pure LDH phase, product containing HM was reacted hydrothermally in various solutions, yielding almost pure LDH phase. The size of relatively large platelets remained almost the same as that before the hydrothermal reactions, but some or many small broken pieces were present in products after the hydrothermal reactions. Moreover, Mg/Al ratio slightly increased when Na 2CO 3 or NaOH solution was used; with NaOH solution, (00 l) reflections split.

Adsorption and photocatalytic degradation of phenol and 2,4 dichlorophenoxiacetic acid by Mg–Zn–Al layered double hydroxides

Mg–Zn–Al layered double hydroxides (LDHs) with varying amounts of zinc were prepared by the coprecipitation method. Solids were analyzed by XRD and N2 physisorption, confirming the formation of pure LDH phase; and the production of mixed oxides with high specific surface areas (182–276m2g−1) after calcination. Band gap energy was also determined, presenting the expected decreasing tendency on increasing zinc amounts. These mixed oxides were tested both for the adsorption of 2,4 dichlorophenoxiacetic acid (2,4-d) and for the photocatalytic degradation of 2,4-d and phenol. Nearly total (97%) degradation of initial 1.45mmolL−1 of 2,4-d, with 1g calcined LDH per liter, was accomplished in 9h, while phenol half-life was as short as 3.5h, with the catalyst with lowest zinc amount (5wt.%). Langmuir adsorption isotherms are presented. Solids were also characterized by XRD and FTIR analysis after photocatalytic and adsorption activity, to determine the presence of 2,4-d. The versatility of LDH decomposition products in the elimination of different contaminants by different mechanisms puts them forward as a viable alternative for environmental remediation.

Preparation of Co1−zAlz(OH)2(NO3)z Layered Double Hydroxides and Li(Co1−zAlz)O2

The precipitate formed by the addition of an aqueous solution of Co(NO3)2 and Al(NO3)3 to LiOH solution has been thoroughly studied as a function of the Al:(Al+Co) ratio, z, for 0 ≤ z ≤ 1, using X-ray diffraction and thermogravimetric analysis. For 0 ≤ z ≤ 0.2, the precipitate was a mixed phase of Co(OH)2 and the layered double hydroxide (LDH) Co0.8Al0.2(OH)2(NO3)0.2·nH2O. For 0.2 ≤ z ≤ 0.4 the precipitate was a single LDH phase containing both NO3− and CO32− ions to compensate the Al3+ charge. As z increases in this range, the amount of NO3− decreases and the amount of CO32− increases. For z > 0.4, the precipitate was a two phase mixture of an LDH phase and Al(OH)3. Li[Co1−zAlz]O2 samples were made from the coprecipitated products for 0 ≤ z ≤ 0.5, by calcining the coprecipitate and Li2CO3. These Li[Co1−zAlz]O2 samples were compared to samples of the same stoichiometries made by the direct solid state reaction of Co3O4, Al(OH)3, and Li2CO3. The lattice constants varied smoothly with z for the Li[Co1−zAlz]...

Phases Formed in Al-Doped Ni1 / 3Mn1 / 3Co1 / 3 ( OH ) 2 Prepared by Coprecipitation: Formation of Layered Double Hydroxide

The coprecipitation process, using either mixed nitrate or sulfate solutions, to produce Al-doped has been studied in detail. Using X-ray diffraction (XRD) and thermal gravimetric analysis (TGA), it has been determined that charge-compensating or ions accompany the incorporated ions into the structure to form layered double hydroxides (LDHs). The phases formed were determined for in and . For , two phase mixtures between (, Mn, and Co) and the LDH phase with were formed. For aluminum contents greater than , XRD and TGA experiments suggest that the excess Al is present as and that the product is a mixture of or and , where M represents the mixture of Ni, Mn, and Co. Alternatively, the data can also be explained assuming that samples with are single phase . TGA experiments show that the in the -containing LDH is eliminated at lower temperature than the in the -containing LDH. Thus coprecipitations using nitrates may be preferred in situations where excellent cation mixing is desired in the production of Al-doped .

Synthesis of layered double hydroxides (LDHs) with varying pH: A valuable contribution to the study of Mg/Al LDH formation mechanism

Very few papers deal with comprehension of mechanisms leading to the formation of layered double hydroxide (LDH) compounds which are usually obtained by coprecipitation at constant pH value. In this paper, we study the influence of pH values of coprecipitation on the formation of LDH phases, their crystallinity and chemical composition, and then discuss about LDH formation mechanisms, using a synthesis route with varying pH. We clearly show that LDH phases can be obtained by such a method, at pH values from 10 to 13.2. A rich aluminium specie is firstly obtained which tends to continuously incorporate Mg until LDH species with the targeted Mg/Al ratio are obtained.

Competitive Intercalation of Sulfonates into Layered Double Hydroxides (LDHs): the Key Role of Hydrophobic Interactions

Layered double hydroxides (LDHs) Mg:Al 2:1 intercalated with straight-chain hydrocarbon sulfonates have been synthesized by anion exchange and characterized by powder XRD, FTIR, and elemental (C,H and metals) analysis. We find that the interlayer spacing of these LDHs can be explained by an interpenetrating packing model with the hydrocarbon chain tilting at an angle of 55° to the metal hydroxide layers. When two such sulfonate anions differ by only one methylene group, their simultaneous intercalation results in a single uniform LDH phase, in which the interlayer spacing varies linearly with the ratio between the two anions. In competitive intercalation experiments, the longer-chain sulfonate is preferentially or even exclusively intercalated. Semiquantitative analysis shows that the affinity of C8H17SO3- (or C7H15SO3-) for LDH is 6 kJ mol-1 stronger than that of C7H15SO3- (or C6H13SO3-). We attribute this increase to an increase in hydrophobic interactions between hydrocarbon chains. Hence, we estimate that the total contribution of hydrophobic interactions between hydrocarbon chains to the affinity of C8H17SO3- for Mg2Al−LDH is around 40−50 kJ mol-1.

Synthesis of magnesium aluminium layered double hydroxides in reverse microemulsions

Reverse microemulsions have been used to control the nucleation and growth of magnesium aluminium layered double hydroxides (LDH). The reactants were prepared in aqueous solution and then dispersed in an isooctane–sodium dodecyl sulfate (DDS) microemulsion. Under these conditions magnesium aluminium LDH phases were obtained containing DDS as the charge-balancing interlayer anion. By adjusting the water : surfactant molar ratio (w = [H2O]/[Surf.]) of the microemulsion we were able to control the nucleation and growth of the LDH phases. In some cases LDH nanoplatelets exhibiting a uniform 14 Å monolayer thickness could be isolated and characterized by AFM.