The effect of Zn, Al layered double hydroxide on thermal decomposition of poly(vinyl chloride)

Poly(vinyl chloride)/layered double hydroxide (LDH) composite was prepared by mixing 4 wt% Zn₂Al-CO₃-LDH with PVC and fluxing at 180 °C. The thermal decomposition behaviour of the LDH + PVC composite in air and nitrogen environments was systematically investigated. We found that mixing Zn₂Al-CO₃-LDH into PVC facilitates dehydrochlorination from ca. 300 to 270 °C but reduces the reaction extent to leave more chlorine on the polyene backbones both in air and N₂. We have also found that at 400-550 °C, both in air and N₂, LDH assists the formation of char-like materials and decreases the release of volatile hydrocarbons. From 550 to 800 °C, the char-like materials are mostly retained in N₂ while they are almost completely thermo-oxidized (burned) in air. Thus, addition of Zn₂Al-CO₃-LDH to PVC does not increase the thermal stability, but does promote charring to retard the generation of flame. The influence of LDH on PVC thermal properties has been also addressed mechanically.     

Electrochemical synthesis of Zn-Al layered double hydroxide (LDH) films

A new electrodeposition condition to produce Zn-Al LDH films was developed using nitrate solutions containing Zn (2+) and Al (3+) ions. Deposition was achieved by reducing nitrate ions to generate hydroxide ions on the working electrode. This elevates the local pH on the working electrode, resulting in precipitation of Zn-Al LDH films. The effect of deposition potential, pH of the plating solution, and the Zn (2+) to Al (3+) ratio in the plating solution on the purity and crystallinity of the LDH films deposited was systematically studied using X-ray diffraction and energy dispersive spectroscopy (EDS). The optimum deposition potential to deposit pure and well-ordered Zn-Al LDH films was E = -1.65V versus a Ag/AgCl in 4 M KCl reference electrode at room temperature using a solution containing 12.5 mM Zn(NO 3) 2.6H 2O and 7.5 mM Al(NO 3) 3.9H 2O with pH adjusted to 3.8. The resulting film contained 39 atomic %Al (3+) ions replacing Zn (2+) ions, leading to a composition of Zn 0.61Al 0.39(OH) 2(NO 3) 0.39. xH 2O. Increasing or decreasing the aluminum concentration in the plating solution resulted in the formation of aluminum- or zinc-containing impurities, respectively, instead of varying aluminum content incorporated into the LDH phase. Choosing an optimum deposition potential was important to obtain LDH as a pure phase in the film. When the potential more negative than the optimum potential is used, zinc metal or zinc hydroxide was deposited as a side product, whereas making the potential less negative than the optimum potential resulted in the formation of zinc oxide as the major phase. The pH condition of the plating solution was also critical, as increasing pH destabilizes the formation of the LDH phase while decreasing pH promoted deposition of other impurities.     

Suppression of the reversible thermal behavior of the layered double hydroxide (LDH) of Mg with Al: stabilization of nanoparticulate oxides

The layered double hydroxide of Mg with Al decomposes below 600 degrees C with the loss of nearly 48% mass, resulting in the formation of an oxide residue having the rock salt structure and nanoparticulate morphology. However, this product reconstructs back into the parent LDH, owing to its compositional and morphological metastability. The oxide can be kinetically stabilized within an amorphous phosphate network built up through an ex situ reaction with a suitable phosphate source such as (NH4)H2PO4. This oxide transforms into a thermodynamically more stable phase with a spinel structure on soaking in an aqueous medium. The oxide residue has a nanoparticulate morphology as revealed by the Scherrer broadening of the Bragg reflections as well as by electron microscopy. This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of Mg2+ ions from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of a nanostructured morphology for the spinel. Another way of stabilizing the oxide is by incorporating the thermally stable borate anion into the LDH. This paves the way for an in situ reaction between the cations of the host LDH and the borate guest. The in situ reaction directly leads to the formation of an oxide with a spinel structure.     

Morphology and fracture behaviour of polyethylene/Mg-Al layered double hydroxide (LDH) nanocomposites

Fracture behaviour of polyethylene (PE)/Mg-Al layered double hydroxide (LDH) based nanocomposites has been studied by essential work of fracture (EWF) approach. Transmission electron microscopy (TEM and X-ray diffraction (XRD) analysis have been used to investigate the morphological features of these nanocomposites. A maximum in the non-essential work of fracture was observed at 5 wt.% LDH demonstrating enhanced resistance to crack propagation compared to pure PE. Morphological analyses of the nanocomposites show that the dispersed LDH platelets are partially exfoliated and also forms clusters with polymer chains remaining entrapped within. Rheological analyses show that the typical low-frequency Newtonian flow behaviour, as observed in unfilled polymer, shifts to shear-thinning behaviour with increasing LDH concentration. At 5 wt.% LDH a ductile-to-brittle transition has been observed. Fracture surface investigation by SEM reveals the arresting of the plastic crack growth by the LDH particle clusters, which is more significant at 5 wt.% LDH content. At higher LDH concentrations, the number of such particle clusters increases causing decrease in the average distance between them. As a result large-scale plastic deformation of the matrix at higher LDH concentration is effectively arrested favouring small strain failure and this in turn reaffirms the possible existence of a ductile-to-brittle transition. The study in general reveals that the resistance against crack initiation (essential work of fracture: EWF) and crack propagation (non-essential work of fracture: βw_p) in these nanocomposites are structurally correlated with the matrix behaviour and the morphology (state of LDH particle dispersion) respectively.     

Solution decomposition of the layered double hydroxide of Co with Fe: phase segregation of normal and inverse spinels

The nitrate-intercalated layered double hydroxide of Co with Fe decomposes on hydrothermal treatment to yield an oxide residue at a temperature as low as 180 degrees C. The oxide product is phase segregated into a Co(3)O(4)-type normal spinel and a CoFe(2)O(4)-type inverse spinel. Phase segregation is facilitated as decomposition in a solution medium takes place by dissolution of the precursor hydroxide followed by reprecipitation of the oxide phases. In contrast, thermal decomposition takes place at 400 degrees C. This temperature is inadequate to induce diffusion in the solid state whereby phase segregation into the thermodynamically stable individual spinels is suppressed. The result is a single-phase metastable mixed spinel oxide. This is rather uncommon in that a hydrothermal treatment yields thermodynamically stable products where as thermal decomposition yields a metastable product.     

Structural characterization and thermal decomposition of layered double hydroxide/poly(p-dioxanone) nanocomposites

Nanocomposites based on layered double hydroxides (LDH) and poly(p-dioxanone) (PPDO) were prepared by melt processing using dodecylbenzene sulfonate (DBS) and 4-hydroxybenzene sulfonate (HBS) as organic modifiers. The incorporation of organic anions in LDH was demonstrated by wide-angle X-ray scattering (WAXS) and Fourier transform infrared (FTIR). The dispersion degree of the organically modified LDHs in the PPDO matrix was analyzed by WAXS, indicating that only the LDH modified with HBS was exfoliated. The effect of the organically modified LDHs on the thermal stability of PPDO was studied using thermogravimetric analysis (TGA). The thermal stability of PPDO matrix was enhanced by the incorporation of the LDH modified with HBS due to the shielding effect of the exfoliated layers. In contrast, the LDH modified with DBS produced a decrease of the thermal stability of PPDO, probably due to hydrolytic decomposition of ester group. The thermogravimetric analysis also showed that the organo-modified LDH did not modify the thermal decomposition mechanism of the polymer, but had an effect on the thermal stability.     

Preparation of ultrathin membranes by layer-by-layer deposition of layered double hydroxide (LDH) and polystyrene sulfonate (PSS)

Nanofilms were prepared by consecutively alternating adsorption of Mg–Al (2:1) layered double hydroxide (LDH) and polysodium 4-styrenesulfonate (PSS). The charge density of oppositely charged materials strongly affect film properties like thickness and ordering. The specific charge of the colloid particles (LDH) and macromolecules was determined with the particle charge detector. The sequential build-up of the thin films was followed by spectrophotometry and X-ray diffraction. The surface morphology of the multilayers was characterized by atomic force microscopy. The influence of the charge density of the applied materials and of the mass ratio of LDH/PSS on the film thickness were studied.     

基于层状氢氧化物衍生复合氧载体的生物质化学链气化实验研究

通过低饱和共沉淀法合成了类水滑石结构的层状氢氧化物(Layered Double Hydroxide,LDH)前驱体,经煅烧获得衍生Cu/Al/Zn、Cu/Al/Ni、Cu/Al/Ni/Zn高分散复合氧载体。采用XRD、XRF、H2-TPR、SEM及BET等分析手段对氧载体的结构及反应性能进行了表征,并通过固定床反应器开展了氧载体与生物质化学链气化实验。结果表明,合成的三种前驱体都具有典型的水滑石特征衍射峰,且层板稳定。Cu/Al/Zn前驱体层间厚度为0.264 2 nm,Ni2+引入后,层间距减小。前驱体煅烧后形成的复合氧载体中元素含量与制备试剂基本一致。氧载体中Zn、Ni元素的引入可提升Cu O的反应活性,降低H2还原的反应温度,Zn元素与Cu具有更好的协同作用。Cu/Al/Ni/Zn氧载体在固定床化学链气化中具有较好的碳转化率和气体产率,其碳转化率为82.03%。反应后氧载体比表面积为5.995 m2/g,具有较好的可再生性与抗烧结性,是生物质化学链气化反应较为理想的氧载体。     

Thermally induced polytype transformations among the layered double hydroxides (LDHs) of Mg and Zn with Al

The hydrotalcite-like layered double hydroxide (LDH) of Mg with Al shows dramatic changes in the peaks arising from the (h0l)/(0kl) family of reflections in its powder X-ray diffraction pattern during thermal treatment. DIFFaX simulations show that these changes arise due to the transformation of the disordered 3R1 polytype into the 1H polytype on dehydration. The 1H polytype is an essential precursor to the decomposition reaction, which results in the formation of an oxide residue with the rock salt structure. In contrast, the LDH of Zn with Al does not undergo any such transformation, retaining the structure of the 3R1 polytype until decomposition into the oxide residue. Given the poor octahedral site preference of the Zn2+ ion, the 1H polytype is neither structurally stable nor is it topochemically necessary for the thermal decomposition of the Zn-Al LDH, the end product of the decomposition reaction being an oxide with the wurtzite structure.     

Synergistic effects of layered double hydroxide with hyperfine magnesium hydroxide in halogen-free flame retardant EVA/HFMH/LDH nanocomposites

The synergistic effects of layered double hydroxide (LDH) with hyperfine magnesium hydroxide (HFMH) in halogen-free flame retardant ethylene-vinyl acetate (EVA)/HFMH/LDH nanocomposites have been studied by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), mechanical properties' tests, and dynamic mechanical thermal analysis (DMTA). The XRD results show that the exfoliated EVA/HFMH/LDH can be obtained by controlling the LDH loading. The TEM images give the evidence that the organic-modified LDH (OM-LDH) can act as a disperser and help HFMH particles to disperse homogeneously in the EVA matrix. The TGA data demonstrate that the addition of LDH can raise 5-18 °C thermal degradation temperatures of EVA/HFMH/LDH nanocomposite samples with 5-15 phr OM-LDH compared with that of the control EVA/HFMH sample when 50% weight loss is selected as a point of comparison. The LOI and mechanical tests show that the LDH can act as flame retardant synergist and compatilizer to apparently increase the LOI and elongation at break values of EVA/HFMH/LDH nanocomposites. The DMTA data verify that the T_g value (−10 °C) of the EVA/HFMH/LDH nanocomposite sample with 15 phr LDH is much lower than that (T_g = −2 °C) of the control EVA/HFMH sample without LDH and approximates to the T_g value (−12 °C) of pure EVA, which indicates that the nanocomposites with LDH have more flexibility than that of the EVA/HFMH composites.     

Thermogravimetry study of the ester interchange of sunflower oil using Mg/Al layered double hydroxides (LDH) impregnated with potassium

Mg/Al layered double hydroxides (LDH) containing KI were synthesized and tested as basic heterogeneous catalysts for transesterification of sunflower oil, in order to obtain biodiesel. The process was carried out using reflux with 15:1 molar ratio of methanol to sunflower oil, and catalyst concentration of 2 mass%. The characterization of sunflower oil and biodiesel was accomplished according to ASTM and EN standard methods. The gas chromatographic and TG/DTG profiles were evaluated, and the results of yield and conversions were compared. The gas chromatographic analysis showed that the catalysts were effective in converting vegetable oil into biodiesel, specially using LDH catalysts modified with KI and molar ratio Mg/Al = 1, with conversions higher than 99 % indicating the strong influence of the chemical composition and controlled basicity, due to the presence of potassium in the structure of the catalyst.     

镁铝水滑石的有机改性实验研究

采用共沉淀法合成了对甲基苯磺酸有机插层改性的镁铝水滑石,用XRD、FTIR、TG-DSC等技术表征产物特征.研究表明,合成的对甲基苯磺酸插层LDH物相纯净、结晶度高,层间无硝酸根,而且2:1型有更好的晶形;2:1型对甲基苯磺酸插层LDH在450-500℃层状结构逐渐被破坏,而3:1型在400-460℃0层状结构被破坏....     

Thermal decomposition of crystalline Ni(II)-Cr(III) layered double hydroxide: a structural study of the segregation process

A structural study of the thermal evolution of Ni(0.69)Cr(0.31)(OH)(2)(CO(3))(0.155) x nH(2)O into NiO and tetragonal NiCr(2)O(4) is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns. The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides. It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of the parent Cr(III)-doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations play in the segregation of crystalline spinels is also discussed.     

Hollow nanoshell of layered double hydroxide

Hollow nanoshells of layered double hydroxide (LDH) have been fabricated using exfoliated LDH nanosheets as a shell building block and polystyrene beads as a sacrificial template.     

Immobilization of uranium(VI) onto Mg2Al layered double hydroxide: role of key geochemical parameters

In this study, the sorption of U(VI) from aqueous solution on Mg₂Al layered double hydroxide (Mg₂Al LDH) was studied as a function of various water quality parameters such as contact time, pH, ionic strength, soil fulvic acid (FA), solid content and temperature by using a batch technique. The sorption of U(VI) on Mg₂Al LDH was dependent on pH. The presence of FA increased U(VI) sorption at low pH, whereas decreased U(VI) sorption at high pH. Both kinetics and thermodynamic parameters of the sorption process were evaluated. It was found that the pseudo-second-order model was more suitable for our experiment. The Langmuir model fitted the sorption isotherms of U(VI) better than the Freundlich and D-R model at three different temperatures of 298, 303 and 313 K. The thermodynamic parameters (ΔH°, ΔS°, and ΔG°) were calculated from the temperature dependent sorption isotherms, and the results suggested that U(VI) sorption was a spontaneous and endothermic process. The results demonstrate that Mg₂Al LDH is a promising sorbent material for the preconcentration and separation of uranium pollution from large volumes of aqueous solutions.     

Synthesis and characterization of layered double hydroxides (LDH) intercalated with non-steroidal anti-inflammatory drugs (NSAID)

Layered double hydroxides (LDHs) with the hydrotalcite type structure and a Mg:Al ratio of two have been prepared, with salicylate or naproxen in the interlayer. Two synthetic routes have been used: reconstruction from a mildly calcined hydrotalcite-CO₃ precursor, and a coprecipitation method with chlorides of the metals. The solids have been characterized using several physicochemical techniques, i.e., powder X-ray diffraction, FTIR and ¹³C CP/MAS NMR spectroscopies and thermal analysis (thermogravimetric and differential thermal analyses). The gallery height determined is in all cases larger than the size of the drug, 11.5 Å for salicylate and 15.8 and 16.6 Å for naproxen, depending on the specific synthesis route followed. Experimental data suggest the anion molecules form a tilted bilayer, with the carboxylate groups pointing towards the brucite-like layers. The solids are stable up to 230 °C and their evolution from 350 °C upwards is very similar to that observed for a carbonate-containing hydrotalcite, forming mostly amorphous solids with a large specific surface area.     

Removal of radiocobalt from aqueous solution by Mg2Al layered double hydroxide

The adsorption behavior of radiocobalt by Mg₂Al layered double hydroxide (Mg₂Al LDH) was studied as a function of contact time, pH, ionic strength, foreign ions, FA and temperature under ambient conditions. The results showed that the kinetic adsorption could be described by a pseudo-second order model very well. The adsorption of Co(II) on Mg₂Al LDH was strongly dependent on pH and ionic strength. The presence of FA enhanced the adsorption of Co(II) on Mg₂Al LDH at low pH, whereas reduced Co(II) adsorption at high pH. The Langmuir model fitted the adsorption isotherms of Co(II) better than the Freundlich and D–R model at three different temperatures of 303, 323 and 343 K. The thermodynamic parameters (ΔH°, ΔS° and ΔG°) calculated from the temperature dependent adsorption isotherms indicated that the adsorption process of Co(II) on Mg₂Al LDH was endothermic and spontaneous. The results show that Mg₂Al LDH is a promising material for the preconcentration and separation of pollutants from large volumes of aqueous solutions.     

Dispersion control and nematic ordering of Ni/Al layered double hydroxide suspensions

In this paper, we report the preparation of aqueous suspensions of Ni/Al layered double hydroxide (LDH) nanoparticles by a non-steady co-precipitation followed by peptization. By choosing suitable peptization temperature and time, well-dispersed suspensions were obtained. Meanwhile, the particle size, shape and size polydispersity can be efficiently controlled. Nematic ordering is observed in colloidal Ni/Al LDH suspensions and confirmed by birefringence observations and SAXS measurements. Furthermore, we showed that the sol-gel transition takes place after a liquid crystalline phase transition in concentrated Ni/Al LDH suspensions. The absence of isotropic-nematic phase separation can be attributed to the fact that the nematic phase droplets are too small to settle to the bottom of the cuvette.     

Synthesis of layered zinc hydroxide chlorides in the presence of Al(III)

Zinc hydroxide chloride particles were synthesized by hydrolysis of ZnCl₂ solutions dissolving AlCl₃ at different atomic Al/Zn ratios from 0 to 1.0 and characterized by various techniques. Increasing Al/Zn ratio changed the crystal phases of the products as ZnO→ZnO+ZHC (Zn₅(OH)₈Cl₂·H₂O)→ZHC→LDH (layered double hydroxides, Zn-Al-Cl) and the particle morphology as agglomerates (ZnO)→fine particles (ZnO)→plates (ZHC)+rods (ZnO)→plates (ZHC)→plates (LDH). The atomic Cl/Zn ratios of LDH particles formed at Al/Zn?0.3 were ca. 0.3 despite the increase of Al/Zn ratio, being due to the intercalation of CO₃²⁻ into the LDH crystal. The OH⁻ content of LDH estimated by TG was reduced by the deprotonation of OH⁻ to counteract the excess positive charge produced by replacing Zn(II) with Al(III). ZHC exhibited a high adsorption selectivity of H₂O.