Synthesis of a magnesium/aluminum/iron layered double hydroxide and its flammability characteristics in halogen‐free,flame‐retardant ethylene/vinyl acetate copolymer composites

Mg–Al–Fe ternary hydrotalcites were synthesized by a coprecipitation method and characterized with powder X‐ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The flame‐retardant effects of Mg/Al–CO₃ layered double hydroxides (LDHs) and Mg/Al/Fe–CO₃ LDHs in an ethylene/vinyl acetate copolymer (EVA) were studied with the limited oxygen index (LOI), the UL‐94 test, and the cone calorimeter test (CCT), and the thermal degradation behavior of the composites was examined by thermogravimetric analysis. The results showed that the LOI values of the EVA/(Mg/Al/Fe–CO₃ LDH) composites were basically higher than those of the EVA/(Mg/Al–CO₃ LDH) composites at the same additive level. In the UL‐94 test, there was no rating for the EVA/(Mg/Al–CO₃ LDH) composite at the 50% additive level, and a dripping phenomenon occurred. However, the EVA/(Mg/Al/Fe–CO₃ LDH) composites at the same loading level of LDHs containing a suitable amount of Fe³⁺ ion reached the V‐0 rating, the dripping phenomenon disappearing. The CCTs indicated that the heat release rate (HRR) of the EVA composites with Mg/Al/Fe–CO₃ LDHs containing a suitable amount of Fe³⁺ decreased greatly in comparison with that of the composites with Mg/Al–CO₃ LDHs. The introduction of a given amount of Fe³⁺ ion into Mg/Al–CO₃ LDHs resulted in an increase in the LOI, a decrease in the HRR, and the achievement of the UL‐94 V‐0 rating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Sorption of polycyclic aromatic hydrocarbons from aqueous solution by hexadecyltrimethylammonium bromide modified fibric peat

BACKGROUND: Fibric peat was modified by hexadecyltrimethylammonium bromide (HTAB) to improve its performance in sorption of polycyclic aromatic hydrocarbons (PAHs). The raw fibric peat (P‐R) and surfactant modified peat (P‐HTAB) were characterized by capillary rise test to determine the effect of HTAB on surface hydrophobicity. Effect of contact time was also investigated. Batch sorption data were fitted to the Freundlich model and pseudo‐second‐order model for isotherm and kinetics study. RESULTS: P‐HTAB had a more hydrophobic surface than P‐R. After modification, the sorption coefficients (K_OC) of naphthalene, phenanthrene and pyrene on fibric peat increased from 1590, 29661 and 204171 mL g^?1 to 2006, 52410 and 358569 mL g^?1, respectively; the sorption rate constants of naphthalene, phenanthrene and pyrene increased from 0.00057, 0.00036 and 0.00051 g µg^?1 min^?1 to 0.00062, 0.00140 and 0.00155 g µg^?1 min^?1, respectively. The sorption coefficients of PAHs were positively correlated with the octane‐water partition coefficients of PAHs. CONCLUSIONS: The hydrophobicity of fibric peat was enhanced through modification by HTAB, which resulted in the improved sorption rate and sorption capacity for PAHs. The sorption performance of P‐HTAB reveals that it is an effective biosorbent for PAHs and has potential for treatment of wastewater containing hydrophobic organic contaminants. Copyright © 2010 Society of Chemical Industry     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Synthesis of p-sulfonated calix[4]arene-intercalated layered double hydroxides and their adsorption properties for organic molecules

Calixarenes are macrocyclic organo anions, which cavity is capable of molecular recognition, while layered double hydroxides (LDHs) are widely known as hydrotalcite-like compounds, anion exchangers and host–guest materials. In this study, the intercalation of water-soluble p-sulfonated calix[4]arene (CS4) in the interlayer of the Mg–Al and Zn–Al LDHs (M²⁺/Al ratio = 3) by the coprecipitation method has been investigated as well as the adsorption property of the resulting CS4/LDHs for benzyl alcohol (BA) and p-nitrophenol (NP). It was found that the CS4/LDHs with the molar ratio of CS4/Al = 0.25 (Mg–Al LDH) and 0.12 (Zn–Al LDH) were obtained as a single phase. The arrangement of CS4 in the LDH interlayer was different by the kind of the host metal ions as CS4 cavity axis perpendicular (Mg–Al LDH) and parallel (Zn–Al LDH) to the basal layer, influencing strongly on the BET surface area, N₂ adsorbed volume and adsorption property for BA and NP.     

Synthesis and properties of polypropylene/layered double hydroxide nanocomposites with different LDHs particle sizes

In order to investigate whether the particle sizes of inorganic additives in polymer have an influence on the flame‐retardant and other properties of the polymer, five types of Mg₃Al–CO₃ layered double hydroxide (LDHs) with particle diameters of 80–100, 200–350, 500–550, 550–600, and 700–900 nm were synthesized using a hydrothermal method. The obtained Mg₃Al–CO₃ LDHs were treated using the aqueous miscible organic solvent treatment method to give highly dispersed platelets in Polypropylene (PP). The thermal stability, flame retardancy, and mechanical properties of the PP/AMO–LDH nanocomposites were investigated systematically. The results showed that the thermal stability and flame retardancy of PP could be improved after incorporating AMO–LDHs. The temperature at 50% weight loss (T_0.5) of PP/LDH (700–900 nm) nanocomposites with a LDH loading of 15 wt % was increased by 57 °C. When the LDHs loading was 40 wt %, the peak heat release rate (PHRR) of the PP/LDH nanocomposites with small LDHs particle sizes (<350 nm) was decreased by ca. 58%. The limiting oxygen index was increased by 5% for PP/LDH (80–100 nm) nanocomposites. The response surface regression results also indicated that both LDH particle size and loading have influence on PHRR, heat release capacity, tensile strength, and elongation at break. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46204.     

Adsorption of 2,4-D on Mg/Al–NO3 layered double hydroxides with varying layer charge density

Layered double hydroxides (LDHs) have high surface area and high anion exchange capacity, so they have been proposed to be an effective scavenger for contaminants. In this study, the adsorption of 2,4-dichlorophenoxyacetate (2,4-D) on Mg/Al–NO₃ LDHs with varying layer charge density was investigated with particular attention on the effect of the orientation of the interlayer nitrate. Three Mg/Al LDHs were synthesized with Al³⁺/(Al³⁺ + Mg²⁺) molar ratios of 3.3 (LDH3), 2.6 (LDH4) and 2.1 (LDH5). The results of adsorption experiments showed that LDH5 exhibited an S-type isotherm with a low 2,4-D adsorption capacity due to the low accessibility of 2,4-D to the interlayer space. The accessibility was restricted by the small basal spacing of LDH5 as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheet. Thus, the 2,4-D adsorption occurred mainly on the external surface of the material. On the contrary, LDH3, which has the highest layer charge density among the samples, contains nitrate with an orientation perpendicular to the hydroxide sheet of LDH3. The interlayer nitrate was readily exchanged by 2,4-D. Thus, in addition to the adsorption on the external surface, the replacement of the interlayer nitrate by 2,4-D contributed to a higher adsorbed amount of 2,4-D; the 2,4-D adsorption of LDH3 exhibited an L-type isotherm. For LDH4 that contained interlayer nitrate with both parallel and perpendicular orientations, the adsorption characteristics were between those of LDH3 and LDH5. This work has demonstrated the dependence of 2,4-D adsorption characteristics on the nitrate orientation in LDHs, as a consequence of changing layer charge density.     

Experimental investigation of sheet flexibility of layered double hydroxides: One-pot morphosynthesis of inorganic intercalates

The sheet flexibility of layered double hydroxides (LDHs) has been investigated experimentally using co-precipitation and urea hydrolysis methods in an aqueous solution of long-chain anion surfactant in this work. Using dodecylsulfate (DS) anion as morphology-controlling agent, layer-bended or contorted Mg/Al-LDH is obtained successfully. The morphology of bent layers is retained during either in situ decomposition of interlayer DS to SO₄²⁻ or ion exchange of interlayer DS by CO₃²⁻. The direct synthesis of the layer-distorted LDHs intercalated with small inorganic anions is quite difficult. It has been achieved using layer-bended LDHs pillared with bulky organic anions as precursors in this paper. The morphosynthesis is expanded to Co/Al and Ni/Al-LDHs, indicative of the general flexibility of this kind of anionic clays.     

Intermediate accumulation and efficiency of anaerobic digestion treatment of surfactant (alcohol sulfate)‐rich wastewater at increasing surfactant/biomass ratios

Textile (eg cotton) finishing industry wastewater is characterised by high concentrations of surfactants (up to 2 g dm^?3) and of readily biodegradable biopolymers (COD 5–15 g dm^?3). The anionic surfactant decyl sulfate (DS) was chosen as model surfactant and soluble starch (size) as model compound for the readily biodegradable fraction of the wastewater. Twenty‐two batch experiments with increasing DS/biomass ratio (and starch/biomass ratio) were started simultaneously. Biomass concentrations ranged from 50 to 15 000 mg dm^?3. Minor inhibition effects were found for the surfactant degradation itself at all DS/biomass ratios (maximum biodegradation rate 7.7 mg_DS g_biomass^?1 h^?1). The starch hydrolysis started without a lag‐phase at DS/biomass ratios of up to 0.15 g_DS g_biomass^?1. The lag‐phase was prolonged to about 100 h at a very high DS/biomass ratio (3 g_DS g_biomass^?1). The relative importance of the accumulated intermediates was dependent on the DS/biomass ratio. Above 0.3 g_DS g_biomass^?1 10% of the substrate organic carbon accumulated as ethanol, but no ethanol accumulation was observed at low DS/biomass ratios. Moderate DS/biomass ratios caused a considerable delay of the methanogenesis; high DS/biomass ratios prevented the methanogenesis almost completely. © 2002 Society of Chemical Industry     

On the nature of Ti(IV)‐pillared layered metal hydroxides prepared from green,water‐soluble Ti‐peroxide

Ti‐peroxide pillared layered double hydroxides (LDHs) have been prepared for the first time using water‐soluble Ti‐peroxide as an intercalating precursor. It is novel and alluring that the whole preparation procedure does not involve any usage of organic or chlorine‐containing hazards. Intercalated into the LDH interlayer region, Ti‐peroxide is prevented partially from condensation in the solvent evaporation. The interlayer Ti? O₂ unit exists in triangular (η²) structure with C_2ν symmetry in most cases, giving an interlayer gallery of 0.50–0.60 nm. But in the case of pH 4.0, monodentate (η¹) structure is also observed, giving an interlayer gallery of 0.70 nm. All the Ti‐peroxide pillared LDHs prepared in this work show catalytic activity in the selective oxidation of thioether. The Ti‐peroxide introduced into the interlayer regions of Mg/Al LDH with a particle size of around 50–120 nm exhibits better recyclability than Ti‐peroxide gel, either in bulk or adsorbed on the exterior surface of LDH particles. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Composite Fe3O4/Humic Acid Magnetic Sorbent and its Sorption Ability for Chlorophenols and some other Aromatic Compounds

A composite magnetic sorbent with a relatively high content of humic substances (above 35% of organic carbon) was prepared by co-precipitation of Fe²⁺/Fe³⁺ salts with commercially available alkaline humate concentrate. Magnetite (Fe₃O₄) was identified as the main crystalline phase bearing the magnetic properties of the sorbent. Scanning electron microscope (SEM) images revealed the presence of uniform sub-micron structures on the surface of the sorbent grains. Due to the presence of humic substances, the sorbent exhibited good sorption ability towards low-polarity organic pollutants, namely chlorophenols. The sorption efficiency increased in the order of 4-mono- < 2,4-di- < 2,4,5-trichlorophenol in accordance with growing hydrophobicity of these compounds, confirming a hydrophobic nature of the interactions involved in the sorption process. Similar trends were found in the desorption study utilizing water and methanol as leachants. Some polycyclic aromatic hydrocarbons (naphthalene, anthracene, phenanthrene, fluoranthene, pyrene) were also retained on the sorbent. The chemical composition as well as the main physical characteristics (surface area, phase composition) of the sorbent remained virtually unchanged during the sorption process. The sorbent retained its magnetic properties during the sorption of organic substances from aqueous solutions, which provides an opportunity for its regeneration.     

Effect of surfactant concentration on the stacking modes of organo-silylated layered double hydroxides

The effects of the surfactant concentration on the structure, morphology and thermal property of silylated hydrotalcites have been investigated. By in-situ coprecipitation, the surfaces of layered double hydroxides (LDHs) have been modified by using 3-aminopropyltriethoxysilane (APTS) and anionic surfactant, Na-dodecylsulfate (SDS). Two different stacking modes in the resultant materials were detected by X-ray diffraction (XRD). One has an identical structure of LDHs, in which the SDS and APTS only bond to the outside surfaces and plate edges of LDH. The other is with enlarged interlayer distance, in which SDS and APTS combined with the inside surfaces of LDH. With the increased loading of SDS and APTS, the surface of the modified LDH appeared rough as observed in the transmission electron microscopy (TEM) images. The attenuated total reflection Fourier-transform infrared (ATR FTIR) spectra of the silylated hydrotalcites showed a series of bands attributed to –NH₂ and Si–O–M (M = Mg and Al), proving that APTS has successfully been grafted onto LDH. The thermogravimetric curves (TG) showed that the silane grafted samples have less –OH concentration and less interlayer water, as a result of the –OH consumption during the condensation reaction between Si–OH and –OH on LDH surface. These nanomaterials are of potential applications including clay-based nanocomposites, adsorbents for removal of organic contaminants from water and flame retardant materials.     

Controllable bulk growth of few-layer graphene/single-walled carbon nanotube hybrids containing Fe@C nanoparticles in a fluidized bed reactor

A family of layered double hydroxides (LDHs) with varied Fe contents were employed as catalyst precursors for the controllable bulk growth of few-layer graphene/single-walled carbon nanotube (G/SWCNT) hybrids in a fluidized-bed reactor through chemical vapor deposition of methane at 950 °C. All the G/SWCNT hybrids exhibited the morphology of SWCNTs interlinked with graphene layers. The purity, thermal stability, graphitization degree, specific surface area, and total pore volume of the G/SWCNT hybrids decreased with the increasing Fe contents in the LDH precursors. A high yield of 0.97 g_G/SWCNTs/g_cat can be achieved by tuning the Fe content in the FeMgAl LDHs after a 15-min growth. After the removal of the as-calcined FeMgAl layered double oxide flakes, a high carbon purity of ca. 98.3% for G/SWCNT hybrids was achieved when the mole ratio of Fe–Al is 0.05:1. The size and density of Fe nanoparticles decorated in the as-obtained G/SWCNT hybrids depend largely on Fe content in the FeMgAl LDH precursors. Furthermore, the mass ratio of graphene materials to SWCNTs in the as-prepared G/SWCNT hybrids can be well controlled in a range of 0.4–15.1.     

Intercalation of Mg–Al layered double hydroxide by anionic surfactants: Preparation and characterization

The preparation and the characterization of intercalation compounds of Mg–Al layered double hydroxide (Mg–Al LDH) and various organic surfactants have been studied in detail. The preparation has been carried out following regeneration method. The aim of such organic modification is to prepare LDHs suitable for application in polymer-LDH nanocomposites. The LDH-surfactant hybrids have been characterized by wide angle X-ray scattering (WAXS) and Fourier transform infrared (FTIR) spectroscopy. The modified Mg–Al LDH materials show an increase in interlayer distance as compared to the unmodified Mg–Al LDH depending on the length of the surfactant anions. Thermal decomposition changed significantly after organic modification. The particle morphology was investigated by scanning electron microscopy (SEM).     

Adsorption of phosphate by layered double hydroxides in aqueous solutions

Adsorption of phosphate by various calcined layered double hydroxides (LDHs) such as Mg–Al, Zn–Al, Ni–Al, Co–Al, Mg–Fe, Zn–Fe, Ni–Fe and Co–Fe was investigated. LDHs were found to have high anion exchange capacity that enhances their capability to remove anionic contaminants from aqueous system. The nature and content of di- and trivalent cations in LDH have strong influence on the adsorption process. Calcined Mg–Al LDH (CLDH) with Mg–Al molar ratio of 2.0 showed higher adsorption capacity compared to other calcined LDHs as it possessed higher Al³⁺ content. Results indicate that the adsorption isotherm could be fitted to a linearised form of Langmuir and Freundlich equations. The adsorption phenomenon has been well supported by XRD study. The adsorption process was spontaneous and exothermic in nature and followed first order kinetics. Competitive anions were found to have detrimental effect on the percentage of adsorption.     

Promotion of the electrochemical hydrogenation of nitrobenzene at hydrogen storage alloys studied using a solid electrolyte method

The electrocatalytic properties of an AB₅-type hydrogen storage alloy towards the electrochemical hydrogenation of unsaturated organic compounds have been studied by a solid electrolyte method using electrochemical hydrogenation of nitrobenzene as a model reaction. Voltammetric studies reveal that the kinetics of the nitrobenzene electro-reduction on the hydrogen storage alloy electrode is similar to that on a Ni electrode. Aniline and p-aminophenol are produced as the reaction products. Compared to the Ni electrode, the production of aniline is considerably promoted on the hydrogen storage alloy electrode. Modifying the alloy surface with a thin layer of Cu enhances the reaction selectivity and current efficiency for aniline formation. Compared to a Cu electrode, the electrochemical hydrogenation of nitrobenzene to aniline is promoted on the Cu-modified alloy electrode. The hydrogenation promotion effect is attributed to the chemical reaction between nitrobenzene and metal hydrides that are electrochemically generated in situ. Hydrogen storage alloys therefore make it possible to intensify the electrochemical hydrogenation process of unsaturated organic compounds.     

Comparison of different synthesis routes for Mg–Al layered double hydroxides (LDH): Characterization of the structural phases and anion exchange properties

Nitrate forms of layered double hydroxides (LDHs) were synthesized based on the co-precipitation method under different synthesis conditions (aqueous ammonia solution or potassium hydroxide to control the pH of the solution; magnesium to aluminium ratios of 2:1 and 5:1). In a second step the lab procedure was up-scaled to pilot plant scale. The effects of the synthesis conditions on the structural and textural properties as well as on the anion exchange of the LDH products were investigated using different methods to evaluate the suitability as a soil conditioner. Using a Mg:Al ratio of 2:1 resulted in higher basal spacings compared to a Mg:Al ratio of 5:1, while the type of pH-controlling solution had no effect. Based on nitrogen adsorption isotherms the specific surface area of pores was calculated. The highest values for specific surface areas were found for LDHs synthesized using KOH at a Mg:Al ratio of 5:1. These products had a lower total nitrate adsorption capacity compared to LDHs synthesized at a lower Mg:Al ratio. However, nitrate exchangeability by counter anions for LDHs synthesized using KOH at a Mg:Al ratio 5:1 differed significantly (HCO₃⁻ > Cl⁻ > SO₄²⁻) indicating that these LDHs are preferable under a multi-anionic environment like the soil solution.     

Nanocomposites of polymer gel electrolyte based on poly(ethylene glycol diacrylate) and Mg–Al layered double hydroxides

The present paper describes the synthesis and characterization of nanocomposite materials composed of the polymer gel electrolyte and a layered double hydroxide (LDH) inorganic filler. Mg–Al LDHs were prepared with various ratios of Mg/Al. It was observed that the layered structure of Mg–Al LDH was totally exfoliated by the PEGDA. The ionic conductivity and mechanical strength were highly improved in the nanocomposite systems. In the case of the composite films having 4.5 wt% of the Mg–Al LDH, the ionic conductivity reached 1.6 × 10^?3 S cm^?1 at room temperature. The incorporation of nanoparticles into the gel resulted in a two‐ or threefold increase in the tensile modulus. Copyright © 2004 Society of Chemical Industry     

Nonaqueous potentiometry of coal-derived asphaltenes and model nitrogen-containing compounds in acetophenone and nitrobenzene

A series of 34 nitrogen-containing compounds with a wide range of basicities was examined under differential nonaqueous potentiometric conditions in acetophenone and nitrobenzene. These compounds could be resolved into five classes based on pK_aand half-neutralization-potential (HNP) values. Asphaltenes isolated from the vacuum still overhead (VSO) and vacuum still bottoms (VSB) fractions of the H-coal process were titrated under similar conditions. The titration results of both asphaltenes indicated the presence of two classes of nitrogen-containing compounds, a titratable class of the pyridine-ring or aniline type and a nontitratable class.     

Comparison of Mg–Al layered double hydroxides intercalated with OH− and CO32− for the removal of HCl,SO2, and NO2

Journal of Porous Materials - Two different Mg–Al layered double hydroxides (LDHs), OH?Mg–Al LDH and CO3?Mg–Al LDH, are prepared and utilized for the efficient removal...     

Factors influencing the hydration of layered double hydroxides (LDHs) and the appearance of an intermediate second staging phase

The hydration of layered double hydroxides (LDHs) was investigated by changing the interlayer anion species, the Mg/Al ratio of the LDH hosts and the relative humidity (RH). The anions were CO₃²⁻, Cl⁻, Br⁻, NO₃⁻, I⁻, SO₄²⁻, and ClO₄⁻ (listed in the order of ion size, small to large) and LDHs with Mg/Al = 1.90 (LDH2) and 2.91 (LDH3) were used. Their XRD profiles were measured by an XRD diffractometer while controlling the RH in the range 0–95% at 25 °C. Only I⁻, SO₄²⁻, and ClO₄⁻ LDH2s and SO₄²⁻ LDH3 showed a large step-wise basal-spacing expansion, 0.24–0.28 nm, under high RH conditions (> ca. 60%) probably due to the insertion of one water layer into the interlayer space. Such hydration occurred more favorably for the LDHs with larger anions and those with a higher layer charge (LDH2). Among them, I⁻ and ClO₄⁻ LDH2s exhibited the second staging – alternate stacking of hydrated (H) and non-hydrated (NH) interlayers – in the intermediate RH region.