The effects of intralayer metal composition of layered double hydroxides on glass transition, dispersion, thermal and fire properties of their PMMA nanocomposites

A series of aluminum-containing layered double hydroxides (LDHs), containing Mg, Ca, Co, Ni, Cu and Zn as the divalent metals, have been prepared by the co-precipitation method and used to prepare nanocomposites of PMMA by in situ bulk polymerization. The additives were characterized by Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy (XRD) and thermogravimetric analysis while the polymer composites were characterized by XRD, transmission electron microscopy, differential scanning calorimetry and cone calorimetry. Polymerization of methyl methacrylate in the presence of these undecenoate LDHs results in composites with enhanced thermal stability. The glass transition temperatures of the composites and the pristine polymers are found to be around 110 °C; this suggests that the presence of these additives has little effect on the polymer. It is found that the additive composition and the dispersion state of LDHs agglomerates in the polymer matrix influence the fire properties of composites as measured by cone calorimetry.     

The role of the trivalent metal in an LDH: Synthesis, characterization and fire properties of thermally stable PMMA/LDH systems

Two layered double hydroxides (LDHs), calcium aluminum undecenoate (Ca₃Al) and calcium iron undecenoate (Ca₃Fe), have been prepared by the co-precipitation method. XRD analysis of these LDHs reveals that they are layered materials and FT-IR and TGA confirmed the presence of the undecenoate anions in the material produced. The PMMA composites were prepared by bulk polymerization and the samples were characterized by XRD, TEM, TGA and cone calorimetry. Both additives greatly enhance the thermal stability of PMMA, while the calcium aluminum LDH gives better results when the fire properties were examined using the cone calorimeter.     

Thermal and photocatalytic behavior of Ti/LDH nanocomposites

The development of nanocomposite photocatalyst based on layered double hydroxides (LDHs) associated with TiO₂ was the subject of this research. The thermally activated Zn–Al LDHs were selected as catalyst support precursor because of their proven photocatalytic activity and therefore their possible contribution to overall activity of novel Ti–Zn–Al nanocomposite. The catalyst precursor (Zn–Al LDH) was synthesized by low supersaturation coprecipitation method, and its association with active TiO₂ component targeting the formation of novel Ti–Zn–Al nanocomposite was achieved by wet impregnation. Simultaneous thermal analysis (TG–DTA) was used to investigate the thermal behavior of Zn–Al LDH and Ti–Zn–Al LDHs. Complementary, morphology, texture, and structure characterization was carried out. The photocatalytic test reaction was performed under UV light using the methylene blue degradation. The results confirmed a successful impregnation of TiO₂ on catalyst support precursor Zn–Al–LDH followed by considerable change in morphology and structure of Zn–Al LDH precursor. It was concluded that the synergic effect between TiO₂ and Zn–Al LDH precursor contributes to the overall photocatalytic activity.     

Effect of Electrolytes and Polymers on the Thixotropy of Mg‐Al‐Layered Double Hydroxides/Kaolinite Dispersions

The effect of electrolytes (NaCl and CaCl₂) and polymers (CPAM and HPAM) on the thixotropy of Mg‐Al‐layered double hydroxide (LDHs)/kaolinite dispersions has been investigated. It was observed that the type of thixotropy in LDH/kaolinite dispersions may be affected by NaCl, but not by CaCl₂ in range of concentration of interest. The type of thixotropy in LDH/kaolinite dispersion with R=0 transformed from positive thixotropy to complex thixotropy and at last positive thixotropy again with the concentration of NaCl in range of 0.00–0.10 mol·L⁻¹; the type of thixotropy in LDHs/kaolinite dispersions with R=0.25 transformed from complex thixotropy to positive thixotropy and then complex thixotropy again with the concentration of NaCl in range of 0.00–0.10 mol·L⁻¹. The type of thixotropy in LDH/kaolinite dispersion with R=0 may be not affected by cationic polyacrylamide (CPAM) and hydrolyzed polyacrylamide (HPAM); but the LDHs/kaolinite dispersions with R=0.25 transformed from complex thixotropy to positive thixotropy with the both polymers concentration in range of interest, which indicated that the microstructure of the dispersion changed from weak folc sediments structure to steric network structure.     

Amine‐Assisted Syntheses of Carbonate‐Free and Highly Crystalline Nitrate‐Containing Zn‐Al Layered Double Hydroxides

Zn‐Al layered double hydroxides (LDHs), with nitrate as the charge balancing anion in the interlayer space, were synthesized by precipitation from homogeneous solution containing different amines [e.g., hexamethylenetetraamine (HMTA), diethylenediamine (DEDA), trimethylamine (TMA) and dimethylamine (DMA)]. The applied method does not require nitrogen atmosphere. The solution pH and concentration of different amines were varied in order to identify the controlling parameters and whether nitrate or carbonate are the interlayer anion. Particularly, the addition of amines turns out to be an effective tool for the synthesis of nitrate containing Zn‐Al LDHs independent from the nitrogen atmosphere. The structure, textural, composition, and morphological properties were investigated using the powder X‐ray diffraction (PXRD), thermogravimetric analysis (TGA), FT‐IR spectroscopy, and scanning electron microscopy (SEM). The analyses showed that the samples had high crystallinity and purity. The NO₃‐ZnAl LDHs samples show that LDH sheets are predominantly smooth textured and the thickness of LDH sheets are found to be around 23 nm. The results also indicate that this method successfully produces a NO₃^– form Zn‐Al LDH that is almost identical to the one synthesized by conventional methods.     

Calcined Mg-Al Layered Double Hydroxide as a Heterogeneous Catalyst for the Synthesis of Urea Derivatives from Amines and CO2

The calcined Mg‐Al layered double hydroxides (LDHs) with a Mg/Al molar ratio of 3:1 were synthesized and characterized thoroughly by X‐ray diffraction (XRD), temperature‐programmed desorption (TPD) of CO₂, and thermogravimetric analysis (TGA). Thus the calcined Mg‐Al LDHs were used as catalyst for the catalytic synthesis of disubstituted ureas from amines and CO₂. The effects of reaction time, reaction temperature, pressure, solvent and calcined temperature on activity have been investigated. The results indicated that aliphatic amines, cyclohexylamine and benzylamine can be converted to the corresponding ureas selectively over the calcined Mg‐Al LDHs catalysts with N‐methyl‐2‐pyrrolidone (NMP) as solvent without using any dehydrating regent. The catalyst can be recycled several times with only slight loss of activity.     

The effect of OCoAl‐LDH and OCoFe‐LDH on the combustion behaviors of polyvinyl chloride

The effects of the modified layered double hydroxide (LDH) of Co/Al (OCoAl‐LDH) and the modified LDH of Co/Fe (OCoFe‐LDH) on the combustion behaviors of polyvinyl chloride (PVC) during pyrolysis processes were compared and investigated. The thermal degradation and combustion behavior of the PVC composites were investigated by thermogravimetric analysis (TGA), microscale combustion calorimetry (MCC), and cone calorimetry (CONE). The results indicate that the incorporation of LDHs brought about the improved thermal stability and reduced heat release of PVC composites at a high temperature. The smoke‐suppression properties of the composites are investigated by steady‐state tube furnace (SSTF), and the results indicated that the toxic gases such as CH₄, CO, and N_xO were inhibited by both of the two LDHs, but the OCoFe‐LDH has a better effect on the smoke suppression. Subsequently, the char layer was investigated by scanning electron microscopy–energy‐dispersive spectrometry (SEM‐EDS) and Raman analysis. The results indicate that the LDHs can promote the dechlorination of PVC during the thermal oxidation process and can inhibit the production of HCl in inert gas. Generally, OCoAl‐LDH and OCoFe‐LDH can be potential catalysts for waste disposal and can improve the fire safety of PVC.     

Solution decomposition of the layered double hydroxide (LDH) of Zn with Al

The layered double hydroxides (LDH) of Zn with Al containing intercalated CO₃²⁻ and NO₃⁻ ions undergo solution decomposition to yield a highly crystalline oxide mixture comprising ZnO and ZnAl₂O₄ at temperatures as low as 150–180 °C under hydrothermal conditions. In contrast solid-state decomposition takes place at a much higher temperature (240–315 °C) in air. Solution decomposition is not only guided by the low octahedral crystal field stabilization energy of Zn²⁺ ions, a factor that also affects solid-state decomposition, but also by solubility considerations. The LDHs of Mg and Ni with Al do not undergo solution decomposition.     

Thermally Induced Growth of ZnO Nanocrystals on Mixed Metal Oxide Surfaces

An in situ method for the growth of ZnO nanocrystals on Zn/Al mixed metal oxide (MMO) surfaces is presented. The key to this method is the thermal treatment of Zn/Al layered double hydroxides (Zn/Al LDHs) in the presence of nitrate anions, which results in partial demixing of the LDH/MMO structure and the subsequent crystallization of ZnO crystals on the surface of the forming MMO layers. In a first experimental series, thermal treatment of Zn/Al LDHs with different fractions of nitrate and carbonate in the interlayer space was examined by thermogravimetry coupled with mass spectrometry (TG‐MS) and in situ XRD. In a second experimental series, Zn/Al LDHs with only carbonate in the interlayer space were thermally treated in the presence of different amounts of an external nitrate source (NH₄NO₃). All obtained Zn/Al MMO samples were analysed by electron microscopy, nitrogen physisorption and powder X‐ray diffraction. The gas phase formed during nitrate decomposition turned out to be responsible for the formation of crystalline ZnO nanoparticles. Accordingly, both interlayer nitrate and the presence of ammonium nitrate led to the formation of supported ZnO nanocrystals with mean diameters between 100 and 400 nm, and both methods offer the possibility to tailor the amount and size of the ZnO crystals by means of the amount of nitrate.     

Effect of hydrothermal treatment on properties of Ni-Al layered double hydroxides and related mixed oxides

The Ni-Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by coprecipitation and treated under hydrothermal conditions at 180 °C for times up to 20 h. Thermal decomposition of the prepared samples was studied using thermal analysis and high-temperature X-ray diffraction. Hydrothermal treatment increased significantly the crystallite size of coprecipitated samples. The characteristic LDH diffraction lines disappeared completely at ca. 350 °C and a gradual crystallization of NiO-like mixed oxide was observed at higher temperatures. Hydrothermal treatment improved thermal stability of the Ni2Al and Ni3Al LDHs but only a slight effect of hydrothermal treatment was observed with the Ni4Al sample. The Rietveld refinement of powder XRD patterns of calcination products obtained at 450 °C showed a formation of Al-containing NiO-like oxide and a presence of a considerable amount of Al-rich amorphous component. Hydrothermal aging of the LDHs resulted in decreasing content of the amorphous component and enhanced substitution of Al cations into NiO-like structure. The hydrothermally treated samples also exhibited a worse reducibility of Ni²⁺ components. The NiAl₂O₄ spinel and NiO still containing a marked part of Al in the cationic sublattice were detected in the samples calcined at 900 °C. The Ni2Al LDHs hydrothermally treated for various times and related mixed oxides obtained at 450 °C showed an increase in pore size with increasing time of hydrothermal aging. The hydrothermal treatment of LDH precursor considerably improved the catalytic activity of Ni2Al mixed oxides in N₂O decomposition, which can be explained by suppressing internal diffusion effect in catalysts grains.     

Photooxidation of polypropylene/layered double hydroxide nanocomposites: Influence of intralamellar cations

The influence of layered double hydroxides (LDHs) on the photooxidation of polypropylene (PP)/LDH nanocomposites was studied under irradiation at long wavelengths (λ > 300 nm, 60 °C and in the presence of oxygen). The influence of hybrid LDHs containing different divalent cations (Mg, Zn or both Mg and Zn) on the photooxidation mechanism of PP and on the rates of oxidation of the matrix was characterised based on infrared analysis. The presence of LDHs modifies the photoproducts accumulating in the PP and the rates of oxidation of PP were changed depending on the divalent cations in the LDH layers. Whereas natural clays, such as montmorillonite (MMt), can lead to a faster degradation of the materials, LDHs (Zn₂-Al-DS, for example) appear to have no inductive effect on polymer oxidation.     

Synthesis and Characterization of [Zn‐Al] Layered Double Hydroxides: Effect of the Operating Parameters

In this study, [Zn‐Al] layered double hydroxides (LDHs ) were prepared using the coprecipitation method at constant pH . The synthesis parameters (including the aging time, synthesis pH , nature of the alkali, concentration of metallic salts, and the cationic molar ratio R = Zn/Al) were varied in order to elucidate their effect on the properties of the obtained materials. Different characterization techniques, namely X‐ray diffraction, Fourier transform infrared, thermogravimetric analysis‐differential thermogravimetry, transmission electron microscopy, energy‐dispersive X‐ray, inductively coupled plasma, and photoluminescence, were used in this study. It was found that obtaining well‐crystallized LDHs requires (i) an aging time of 24 h and (ii) a synthesis pH value in the range 8–12. Our results also show that the concentration of the cationic metallic salts has no influence on the structural properties of the LDHs . The use of NH₄OH as alkali for adjusting the pH value during the synthesis favors the formation of nitrated LDH phases while NaOH gives rise to carbonated ones. Moreover, it was found that irrespective of the molar cationic ratio used (between 1 and 5), [Zn‐Al] LDHs could be obtained. The sample synthesized at R = 2 exhibited the best crystallinity.     

Preparation of Mg–Al layered double hydroxides intercalated with alkyl sulfates and investigation of their capacity to take up N,N-dimethylaniline from aqueous solutions

This study examined the effect of the interlayer spacing of a Mg–Al layered double hydroxide (Mg–Al LDH) on the ability of the Mg–Al LDH to take up a nonionic organic material. Mg–Al LDHs, intercalated with 1-propanesulfonate (PS^?), 1-hexanesulfonate (HS^?), and 1-dodecanesulfonate (DS^?), were prepared by coprecipitation, yielding PS·Mg–Al LDH, HS·Mg–Al LDH, and DS·Mg–Al LDH, respectively. The increase in the alkyl chain lengths of the Mg–Al LDHs (PS^? < HS^? < DS^?) resulted in the perpendicular orientation of the organic acid anions in the interlayer of Mg–Al LDH, which in turn resulted in more organic acid anions being accommodated in the interlayer space. An organic acid anion with a large molecular length was more easily intercalated in the interlayer of Mg–Al LDH than one with a small molecular length. This was attributed to the hydrophobic interaction between the alkyl chains, affecting the intercalation of the organic acid anions. The uptake of N,N-dimethylaniline (DMA) by Mg–Al LDHs increased in the order PS·Mg–Al LDH < HS·Mg–Al LDH < DS·Mg–Al LDH. The uptake was attributed to the hydrophobic interactions between DMA and the intercalated PS^?, HS^?, and DS^?. Thus, Mg–Al LDH, which has a lot of large interlayer spacings when intercalated with organic acid anions, can take up a large number of DMA molecules from an aqueous solution.     

Controllably Confined ZnO on USY Zeolites (USY@ZnO/Al2O3) as Efficient Lewis Acid Catalysts for Baeyer–Villiger Oxidation

A ZnAl‐LDHs (layered double hydroxides) phase was readily formed on the surface of a USY zeolite through a distinctive in situ growth method that benefitted from the interaction of the added Zn source and aluminum species extracted from the Al‐rich USY zeolite crystals. The migration of aluminum and simultaneous interaction with the external Zn source took place in one pot to form a ZnAl‐LDHs phase coated on the surface of the USY crystals. Upon calcination, the ZnAl‐LDHs phase was transformed into a ZnO/Al₂O₃ composite that was still firmly anchored on the USY zeolite, without sacrificing the core–shell structure. The resultant USY@ZnO/Al₂O₃ materials gave rise to unique Lewis acidity and hierarchical porosity, which endowed the catalyst with promising performance in the Baeyer–Villiger oxidation of ketones with H₂O₂ or bulky tert‐butyl hydroxide as an oxidant.     

Catalytic transfer hydrogenation of furfural into furfuryl alcohol over Ni–Fe‐layered double hydroxide catalysts

Layered double hydroxides (LDHs) and their derivatives have been reported to be widely used as heterogeneous catalysts in various reactions. Herein, Ni‐Fe LDHs with the controlled Ni/Fe molar ratios (2:1, 3:1, 4:1) were synthesized via an easy hydrothermal method, which were used to catalyze the selective reduction of biomass‐derived furfural into furfuryl alcohol using 2‐propanol as a H‐donor under autogenous pressure and characterized using FT‐IR, XRD, TGA, BET, SEM, NH₃‐TPD, and CO₂‐TPD. It was found that the LDH with a Ni/Fe molar ratio of 3:1 demonstrated the best catalytic activity among the LDHs with different Ni/Fe molar ratios, which showed 97.0% conversion of furfural and 90.2% yield of furfuryl alcohol at 140°C for 5 hr. This was attributable to the synergistic effect of acidic sites and basic sites of the catalyst.     

One Step Fabrication of Au Nanoparticles‐Ni‐Al Layered Double Hydroxide Composite Film for the Determination of L‐Cysteine

This paper reports the fabrication of Au nanoparticles (Au NPs)‐Ni‐Al layerd double hydroxide (LDH) composite film by one step electrochemical deposition on the surface of a glass carbon electrode from the mixture solution containing HAuCl₄ and nitrate salts of Ni²⁺ and Al³⁺. Improved conductivity was obtained by Au NPs codeposited on LDH film. The synergic effect of LDHs and Au NPs dramatically improves the performance of L ‐cysteine electro‐oxidation, displaying low oxidation peak potential (0.16 V) and high current response. Thus the electrode was used to sense L ‐cysteine, showing good sensitivity and selectivity.     

NMR Crystallography Reveals Carbonate Induced Al-Ordering in ZnAl Layered Double Hydroxide

Layered double hydroxides (LDHs) serve a score of applications in catalysis, drug delivery, and environmental remediation. Smarter crystallography, combining X-ray diffraction and NMR spectroscopy revealed how interplay between carbonate and pH determines the LDH structure and Al ordering in ZnAl LDH. Carbonate intercalated ZnAl LDHs were synthesized at different pH (pH 8.5, pH 10.0, pH 12.5) with a Zn/Al ratio of 2, without subsequent hydrothermal treatment to avoid extensive recrystallisation. In ideal configuration, all Al cations should be part of the LDH and be coordinated with 6 Zn atoms, but NMR revealed two different Al local environments were present in all samples in a ratio dependent on synthesis pH. NMR-crystallography, integrating NMR spectroscopy and X-ray diffraction, succeeded to identify them as Al residing in the highly ordered crystalline phase, next to Al in disordered material. With increasing synthesis pH, crystallinity increased, and the side phase fraction decreased. Using ¹H−¹³C, ¹³C−²⁷Al HETCOR NMR in combination with ²⁷Al MQMAS, ²⁷Al-DQ-SQ measurements and Rietveld refinement on high-resolution PXRD data, the extreme anion exchange selectivity of these LDHs for CO₃²⁻ over HCO₃⁻ was linked to strict Al and CO₃²⁻ordering in the crystalline LDH. Even upon equilibration of the LDH in pure NaHCO₃ solutions, only CO₃²⁻ was adsorbed by the LDH. This reveals the structure directing role of bivalent cations such as CO₃²⁻ during crystallization of [M²⁺₄M³⁺₂(OH)₂]²⁺[A²⁻]₁⋅yH₂O LDH phases.     

Synthesis of Mg–Al–Fe–NO3 layered double hydroxides via a mechano-hydrothermal route

Mg–Al–Fe–NO₃ layered double hydroxides (LDHs) with a constant Mg²⁺/(Al³⁺ + Fe³⁺) molar ratio but varying Al³⁺/Fe³⁺ molar ratios were successfully synthesized by a mechano-hydrothermal (MHT) method from Mg(OH)₂, Al(OH)₃ and Fe(NO₃)₃·9H₂O or Mg(NO₃)₂·6H₂O as starting materials. The resulting LDHs (MHT-LDHs) were characterized by XRD, TEM, SEM, FT-IR, and zeta potential, size distribution and specific surface area analyses. It was found that pre-milling played a key role in the LDH formation during subsequent hydrothermal treatment. The MHT route is advantageous in terms of low reaction temperature compared with the conventional hydrothermal method, and the target products are of high crystallinity and good dispersion compared with the conventional mechanochemical (MC) method. The MHT-LDHs had higher specific surface area and zeta potential, and lower hydrodynamic diameter than LDHs obtained by MC method (MC-LDHs). Furthermore, the removal of Cr(VI) from aqueous solutions using the LDHs was examined, showing that the MHT-LDHs are of higher removal efficiency than MC-LDHs for the heavy metal pollutant.     

A flame‐retardant DOPO‐MgAl‐LDH was prepared and applied in poly (methyl methacrylate) resin

A novel inorganic and organic composite flame retardant (9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide [DOPO]–layered double hydroxide [LDH]) was synthesized via grafting DOPO with organic‐modified Mg/Al‐LDH, which was introduced into poly (methyl methacrylate) (PMMA) resin to prepare the flame‐retardant PMMA composites. Thermogravimetric analyzer (TGA) showed that the T_‐50% of DOPO‐LDH/PMMA composites enhanced by about 20°C, and with the 20% flame retardant, the residual char content can be increased by 39.8% in the air atmosphere compared with LDH/PMMA composites. In the UL‐94 and the limiting oxygen index (LOI) tests, it can be found that compared with LDH/PMMA composites, the LOI value of DOPO‐LDH/PMMA composites were raised evidently with the increased flame retardants, and the droplet combustion was greatly improved. These results could be ascribed to the action of DOPO free‐radical, catalytic charring of polymer and the effect of LDH physical barrier. Moreover, the novel DOPO‐LDH not only given PMMA a good flame‐retardant property and thermal stability, but also have higher visible light transmittance, ultraviolet‐shielding effect, and low loss of mechanical properties, which could further facilitate the wide application of inorganic environment‐friendly flame retardants in general resins and engineering resins and broaden the application of polymers.     

Synthesis and Characterization of Magnetic Composites: Intercalation of Naproxen into Mg‐Al Layered Double Hydroxides Coated on Fe3O4

In this work the synthesis and characterization of a magnetic composite is described. It was prepared by deposition of Nap‐Mg‐Al‐LDH on Fe₃O₄ by a coprecipitation method. The structures of the obtained products were studied by powder X‐ray diffraction, FT‐IR spectroscopy, elemental analysis, and thermogravimetric analysis. From the X‐ray diffraction results, it was found that Fe₃O₄ was successfully distributed into the layered double hydroxide (LDH) phase. Furthermore, the increase in the basal spacing of LDHs from 0.881 nm to 2.157 nm shows that naproxen was successfully intercalated into the interlayer space. The particle size was estimated using transmission electron microscopy. Thermogravimetric studies indicate that the thermal stability of naproxen was enhanced after intercalation. Moreover, in vitro drug release experiments in phosphate buffer solution (pH = 7.4) were investigated.