Catalytic Properties of Layered Double Hydroxides and Their Calcined Derivatives

Basic Properties of MgAl-Mixed Oxides in CO2 Adsorption at High Temperature

The increase of consciousness towards global warming and the need to reduce greenhouse gas emissions lead to the necessity of finding alternative applications based on easy-to-use materials in order to control and reduce global CO2 emissions. Layered Double Hydroxides (LDHs) and LDH-derived materials are potentially good adsorbents for CO2, thanks to their low cost, easy synthesis, high sorption capacity, and surface basicity. They have been intensively studied in CO2 capture at high temperature, presenting variable sorption capacities for MgAl LDHs with the same composition, but prepared under different synthesis conditions. The ambient temperature coprecipitation synthesis method is an attractive one-step procedure to synthesize LDHs under mild conditions, with low energy consumption and short synthesis time. The present study is based on the synthesis and characterization of hydrotalcites by a mild-conditions coprecipitation process and the production of derived mixed oxides to be used as CO2 adsorbents. A critical comparison to similar materials is reported. Moreover, the effect of the surface basicity of the derived mixed oxides (measured by adsorption calorimetry) and the CO2 sorption capacity are discussed, showing a linear correlation between the amount of weak and very strong basic sites and the CO2 adsorption behavior.

Efficient degradation of organic pollutants by activated peroxymonosulfate over TiO2@C decorated Mg-Fe layered double oxides: Degradation pathways and mechanism

To activate peroxymonosulfate (PMS) is an efficient way for decomposition of non-biodegradable organic pollutants. Herein, Mg-Fe layered double oxides decorated with Ti₃C₂ MXene-derived TiO₂@C (T/LDOs) were fabricated to efficiently activate PMS for the degradation of Rhodamine B (RhB), acid red 1 (AR1), methylene blue (MB), and tetracycline hydrochloride (TC). The T/LDOs catalyst could decompose 95.8% of RhB, 94.8% of AR1, 84.9% of MB within 10 min, and 82.4% of TC within 60 min. The degradation rate constant of RhB in the optimal T/LDOs/PMS system was approximately 2.5 and 15.7 times higher than that in the Mg-Fe LDOs/PMS system and Mg-Fe LDH/PMS system, respectively. Importantly, the T/LDOs exhibited a wide working pH range (3.1-11.0) and high stability with low metal ions leaching, indicating its potential practical applications. Quenching experiments and electronic spin resonance results confirmed that both ^•O₂^- and ¹O₂ were the dominant active species in the T/LDOs/PMS system. In addition, the possible degradation pathway of RhB in the 5%-T/LDOs/PMS system was proposed. Finally, the catalytic mechanism study revealed that the T/LDOs with abundant surface hydroxyl groups and a certain amount of TiO₂@C facilitated the electron transfer between ≡Fe^(Ⅲ)‒OH complex and HSO₅^-, boosting the generation of ^•O₂^- and ¹O₂. This study provides an insight into exploiting highly efficient catalysts for PMS activation towards the degradation of organic pollutants.

Layered double hydroxide based materials applied in persulfate based advanced oxidation processes: Property, mechanism, application and perspectives

Recently, persulfate-based advanced oxidation processes (persulfate-AOPs) are booming rapidly due to their promising potential in treating refractory contaminants. As a type of popular two-dimensional material, layered double hydroxides (LDHs) are widely used in energy conversion, medicine, environment remediation and other fields for the advantages of high specific surface area (SSA), good tunability, biocompatibility and facile fabrication. These excellent physicochemical characteristics may enable LDH-based materials to be promising catalysts in persulfate-AOPs. In this work, we make a summary of LDHs and their composites in persulfate-AOPs from different aspects. Firstly, we introduce different structure and important properties of LDH-based materials briefly. Secondly, various LDH-based materials are classified according to the type of foreign materials (metal or carbonaceous materials, mainly). Latterly, we discuss the mechanisms of persulfate activation (including radical pathway and nonradical pathway) by these catalysts in detail, which involve (i) bimetallic synergism for radical generation, (ii) the role of carbonaceous materials in radical generation, (iii) singlet oxygen (¹O₂) production and several special nonradical mechanisms. In addition, the catalytic performance of LDH-based catalysts for contaminants are also summarized. Finally, challenges and future prospects of LDH-based composites in environmental remediation are proposed. We expect this review could bring new insights for the development of LDH-based catalyst and exploration of reaction mechanism.

Microwave-assisted synthesis of ZnAl-LDH/g-C3N4 composite for degradation of antibiotic ciprofloxacin under visible-light illumination

Ciprofloxacin (CIP) is a fluoroquinolone family antibiotic pollutant. CIP existence in water environment has been rising very fast in day-to-day life and subsequently, it gives enormous health issues for humans because of its potent biological activity. To encounter this, current researchers are focusing on the development of highly efficient visible light semiconductor nanocomposites with potential photocatalytic activity. In the present work, we have successfully synthesized highly efficient zinc-aluminum layered double hydroxides with graphitic carbon nitride (ZALDH/CN) composites via a simple microwave irradiation method first time for the degradation of CIP under visible light. The fabricated materials are subsequently characterized by various spectroscopic techniques. UV-Vis DRS, TRFL, XRD, FT-IR, BET, FE-SEM, TEM, and XPS for optical, crystal structure, morphological, and elemental analysis. The main reactive intermediates which are formed during the photocatalytic degradation process were analyzed by LC-MS analysis. It is worth to note that, the optimized ZALDH/CN-10 composite showed the highest photo-degradation rate constant of 1.22 × 10^-2 min^-1 with 84.10% degradation is higher than bare CN and ZALDH photocatalysts. Based on the electron-hole pair trapping experiment results, possible CIP photo-degradation mechanism was also explained in the present study. With all results, this work demonstrates the ZALDH/CN composite materials showed a high synergistic effect with more specific surface area. Highest specific surface area leads to enhanced visible light adsorption capacity. Subsequently improved number of catalytically active sites. Furthermore, as compared with pure materials, composites of ZALDH/CN are having low electron-hole pair recombination. Consequently, the composites ZALDH/CN showed superior photocatalytic activity for antibiotic pollutant CIP degradation under visible-light illumination.

Correlations of acidity-basicity of solvent treated layered double hydroxides/oxides and their CO2 capture performance

The basicity and acidity of solvent-treated layered double hydroxide (ST-LDHs) and their layered double oxides (ST-LDOs) have been fully studied using Hammett titration, in situ FTIR, CO₂-TPD and NH₃-TPD. Five solvents (ethanol, acetone, isopropanol, ethyl acetate and 1-hexanol) were selected to treat [Mg_0.72Al_0.28(OH)₂](CO₃)_0.14 (Mg_2.5Al-CO₃ LDH) and compared with traditional LDH co-precipitated from water. The Brønsted basicity strength of the ST-LDHs and ST-LDOs increased but was accompanied by a decrease in basic site density. In addition, the Lewis acidity of ST-LDOs also changes significantly, with medium strength Lewis acid sites dissapearing after solvent treatment. We found that the CO₂ capture capacity of solvent treated LDOs is 50% higher than that of traditional co-precipitated LDO sample. The ethanol treated LDO exhibited the highest CO₂ uptake of 1.01 mmol g^-1. The observed CO₂ capture performance of the ST-LDOs correlates linearly with the ratio of total acid sites to total basic sites.

Ketone Formation via Decarboxylation Reactions of Fatty Acids Using Solid Hydroxide/Oxide Catalysts

A sustainable route to ketones is described where stearone is produced via ketonic decarboxylation of stearic acid mediated by solid base catalysts in yields of up to 97%, at 250 °C. A range of Mg/Al layered double hydroxide (LDH) and mixed metal oxide (MMO) solid base catalysts were prepared with Mg/Al ratios of between 2 and 6 via two synthetic routes, co-precipitation and co-hydration, with each material tested for their catalytic performance. For a given Mg/Al ratio, the LDH and MMO materials showed similar reactivity, with no correlation to the method of preparation. The presence of co-produced oxide phases in the co-hydration catalysts had negligible impact on reactivity.

Assembly of nitroreductase and layered double hydroxides toward functional biohybrid materials

The development of new multifunctional materials integrating catalytically active and selective biomolecules, such as enzymes, as well as easily removable and robust inorganic supports that allow their use and reuse, is a subject of ongoing attention. In this work, the nitroreductase NfrA2/YncD (NR) from Bacillus megaterium Mes11 strain was successfully immobilized by adsorption and coprecipitation on layered double hydroxide (LDH) materials with different compositions (MgAl-LDH and ZnAl-LDH), particle sizes and morphologies, and using different enzyme/LDH mass ratios (Q). The materials were characterized and the immobilization and catalytic performance of the biohybrids were studied and optimized. The nitroreductase-immobilized on the nanosized MgAl-LDH displayed the best catalytic performance with 42-46% of catalytic retention and>80% of immobilization yield at saturation values of enzyme loading Cs ≈ 0.6 g NR/g LDH (Q = 0.8). The adsorption process displayed high enzyme-LDH affinity interactions yielding to a stable biohybrid material. The increase in the amount of enzyme loading favoured the catalytic performance of the biohybrid due to the better preservation of the native conformation. The biohybrid was reused several times with partial activity retention after 4 cycles. In addition, the biohybrid was successfully dried maintaining the catalytic activity for several weeks when it was stored in its dry form. Finally, thin films of NR@LDH biohybrid deposited on glassy carbon electrodes were evaluated as a modified electrode applied for nitro-compound detection. The results show that these biohybrids can be used in biotechnology applications to efficiently detect compounds such as dinitrotoluene. The search for new non-hazardous chemical designs preventing or reducing the use of aggressive chemical processes for human being and the environment is the common philosophy within sustainable chemistry.

Preparation of two dimensional layered double hydroxide nanosheets and their applications

Layered double hydroxides (LDHs) with their highly flexible and tunable chemical composition and physical properties have attracted tremendous attention in recent years. LDHs have found widespread application as catalysts, anion exchange materials, fire retardants, and nano-fillers in polymer nanocomposites. The ability to exfoliate LDHs into ultrathin nanosheets enables a range of new opportunities for multifunctional materials. In this review we summarize the current available LDH exfoliation methods. In particular, we highlight recent developments for the direct synthesis of single-layer LDH nanosheets, as well as the emerging applications of LDH nanosheets in catalyzing oxygen evolution reactions and preparing light emitting devices, supercapacitors, and flame retardant nanocomposites.

A green approach to the synthesis of a nano catalyst and the role of basicity, calcination, catalytic activity and aging in the green synthesis of 2-aryl bezimidazoles, benzothiazoles and benzoxazoles

A green synthesis of hydrotalcite (a double layered catalyst) by a grinding method using Al/Mg molar ratios of 1.0–3.0 at room temperature is described.

Eco-friendly grinding synthesis of a double-layered nanomaterial and the correlation between its basicity, calcination and catalytic activity in the green synthesis of novel fused pyrimidines

The synthesis of hydrotalcite using Al : Mg molar ratios of 1.0 : 3.0 by a grinding method at room temperature is reported.

Evaluation of Controlled-Release Property and Phytotoxicity Effect of Insect Pheromone Zinc-Layered Hydroxide Nanohybrid Intercalated with Hexenoic Acid

A controlled release formulation for the insect pheromone hexenoic acid (HE) was successfully developed using zinc-layered hydroxide (ZLH) as host material through a simple coprecipitation technique, resulting in the formation of inorganic-organic nanolayered material with sustained release properties. The release of HE from its nanohybrid was found to occur in a controlled manner, governed by a pseudo-second order kinetics model. The maximum amount of HE released from the nanocomposite into solutions at pH 4, 6.5, and 8 was found to be 84, 73, and 83% for 1100 min, respectively. The hexenoate zinc-layered hydroxide nanomaterial (HEN) was found to be nontoxic for plants when green beans and wheat seeds were successfully germinated in all HEN concentrations tested in the experiment, with higher percentage of seed germination and higher radical seed growth as compared to its counter anion, HE. ZLH can be a promising carrier for insect pheromone toward a new generation of environmentally safe pesticide nanomaterial for crop protection.

Progress on layered hydrotalcite (HT) materials as potential support and catalytic materials

The present review is focused on a recent development in the preparation of various layered hydrotalcites and their applications in environmental, catalytic and supported materials.

Layered double hydroxide composite monoliths with three-dimensional hierarchical channels: structural control and adsorption behavior

Layered double hydroxide monoliths with size-controllable macro/mesochannel for adsorbent application.

DNA-enhanced peroxidase-like activity of layered double hydroxide nanosheets and applications in H2O2 and glucose sensing

LDH nanosheets were obtained via continuous impaction and exfoliation by herring sperm DNA molecules using a constant vibration method. DNA-LDH nanohybrids were composed by electrostatic forces and they exhibited DNA-enhanced peroxidase-like activity. The morphology and structure of DNA-LDH nanohybrids were analyzed by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), X-ray diffraction (XRD), and atomic force microscopy (AFM) characterization. On the basis of the high catalytic activity of DNA/CuAl-LDH nanosheets, a rapid, sensitive, and convenient approach was developed for colorimetric detection of H2O2 and blood glucose. This method can be potentially applied in medical diagnostics and biotechnology fields.

Large continuous, transparent and oriented self-supporting films of layered double hydroxides with tunable chemical composition

Highly ordered transparent self-supporting films of layered double hydroxides (LDHs) with a size of the order of cm2 have been obtained by a simple method; the chemical composition of both the layers and interlayers can be readily tuned, as demonstrated by the preparation of [ZnAl-NO3] LDH, [NiAl-NO3] LDH and [Tb(EDTA)]- intercalated-ZnAl LDH films.

Thermal Decomposition of Crystalline NiII−CrIII 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.