Search

Layered double hydroxides (LDHs) are appealing nanomaterials for (bio)medical applications and their potential is threefold. One can gain advantage of the structure of LDH frame (i.e., layered morphology), anion exchanging property towards drugs with acidic character and tendency for facile surface modification with biopolymers. This review focuses on the third aspect, as it is necessary to evaluate the advantages of polymer adsorption on LDH surfaces. Beside the short discussion on fundamental and structural features of LDHs, LDH-biopolymer interactions will be classified in terms of the effect on the colloidal stability of the dispersions. Thereafter, an overview on the biocompatibility and biomedical applications of LDH-biopolymer composite materials will be given. Finally, the advances made in the field will be summarized and future research directions will be suggested., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 The Author(s). Published by Elsevier B.V. All rights reserved.)

It has been generally accepted for long that water molecules are integral parts of layered double hydroxides (LDHs). It is even thought that in the absence of water molecules, there is no LDH structure. It is shown in this contribution that via the solvolysis of Mg- or Ca-alkoxides and Al-alkoxide with methanol or propanol and co-precipitated in methanolic methoxide, water-free LDH-like structures could be constructed. These novel LDH-like materials proved to be more active and selective catalysts in a Knoevenagel condensation than the corresponding 'classical' LDHs. Synthesis, thought to be not possible to prepare, characterization and a catalytic application of water-free LDHs are described. Water molecules are replaced by small alcohol molecules in the interlayer space. [ABSTRACT FROM AUTHOR]

Ni(II)-containing layered double hydroxides (LDHs) were synthesized with various methods with the intention of placing the Ni(II) ions in different positions in/on the LDH. Ni(II) was introduced as a lattice-modifier into CaAl-, CaFe- and MgAl-LDHs or in the interlayer space in form of a complex anion or as the divalent component of NiAl-LDH or as surface dopant. The materials were structurally characterized by various instrumental methods; however, the final proof of lattice modification was proven by the Suzuki–Miyaura and the Heck coupling reactions. Each as-prepared catalyst was active and selective towards the coupling products with or without various additives. They could be recycled except the surface-doped material, in which intense leaching of the Ni(II) content was observed on reuse. Ni(II)-containing LDHs are active and recyclable catalysts in the Suzuki and Heck coupling reactions—this is the final proof that the Ni(II) ions are constituents of the lattice. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Successful synthesis of pristine and Fe(III)-amino acid intercalated CaAl-LDHs. • Models of the intercalated complexes based on the results of a range of experimental methods. • Successful applications the oxidations of cyclohexene and in Ullmann coupling. • The organic-inorganic composites proved to be efficient and recyclable catalysts. Abstract Synthesis of intercalated composites were carried out using CaAl-LDH (layered double hydroxide) as host and the anionic form of Fe(III)-amino acid complexes as guest materials. Intercalation was attempted with two methods either introducing the preformed complexes or constructing the complex among the layers of the LDH. After optimization of the synthesis parameters, structural characterization was performed by X-ray diffractometry, scanning electron microscopy as well as mid and far infrared spectroscopies. Quantitative data about the intercalated complexes were collected by chemical analysis and X-ray absorption spectroscopy investigating the near edge region as well as the extended fine structure. Structural models based on characterization measurements are also given. Catalytic activities, selectivities and recycling abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and in situ formed iodosylbenzene as oxidants in the liquid phase. The catalysts were active in the Ullmann-type etherification coupling reaction as well. The intercalated substances were found to be efficient and highly selective catalysts with very good recycling abilities. [ABSTRACT FROM AUTHOR]

An ultrasonically-enhanced mechanochemical method was developed to synthesize CaFe-layered double hydroxides (LDHs) with various interlayer anions (CO 3 2− , NO 3 − , ClO 4 − , N 3 − , F − , Cl − , Br − and I − ). The duration of pre-milling and ultrasonic irradiation and the variation of synthesis temperature in the wet chemical step were investigated to obtain the optimal parameters of preparation. The main method to characterize the products was X-ray diffractometry, but infrared and synchrotron-based X-ray absorption spectroscopies as well as thermogravimetric measurements were also used to learn about fine structural details. The synthesis method afforded successful intercalation of the anions, among others the azide anion, a rarely used counter ion providing a system, which enables safe handling the otherwise highly reactive anion. The X-ray absorption spectroscopic measurements revealed that the quality of the interlayered anions could modulate the spatial arrangement of the calcium ions around the iron(III) ions, but only in the second coordination sphere. [ABSTRACT FROM AUTHOR]

Host-guest composite materials were prepared applying the anionic forms of Ni(II)-amino acid ( l-histidine, l-cysteine, and l-tyrosine) as the guests and CaAl-layered double hydroxide (CaAl-LDH) as the host. The syntheses were performed either by introducing the amino acid anions first and then constructing the metal ion-amino acid complexes or intercalating the pre-prepared complexes in anionic forms. The pristine as well as the composite LDH samples were structurally characterized by X-ray diffractometry, mid IR spectroscopy and scanning electron microscopy. The structural features of the interlayer complexes were studied by UV-Vis, inductively coupled plasma optical emission, mid and far IR and X-ray absorption spectroscopies as well as energy-dispersed X-ray analysis. On the basis of the acquired data, structural models were constructed. The composites were applied as catalysts in the liquid-phase oxidation of cyclohexene applying peracetic acid and the in situ formed iodosyl benzene as oxidants. Using peracetic acid afforded epoxide, while applying iodosyl benzene provided cis diol as the major or exclusive oxidation product. The catalysts displayed good recycling properties. [ABSTRACT FROM AUTHOR]

L-Cysteinate-intercalated CaAl-layered double hydroxide (LDH) was prepared by the co-precipitation method producing highly crystalline hydrocalumite phase with a well-pillared interlayer gallery. The obtained materials were characterized by X-ray diffractometry, IR as well as Raman spectroscopies. By performing interlamellar oxidation reactions with peracetic acid as oxidant, oxidation of cysteinate to cystinate in aqueous and cysteinate sulfenic acid in acetonic suspensions occurred. The oxidations could be performed under mild conditions, at room temperature, under neutral pH and in air. It has been shown that the transformation pathways are due to the presence of the layered structure, that is, the confined space of the LDH behaved as molecular reactor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Intercalation of β-isocupreidinate among the layers of CaAl-LDH. • Intercalation of β-isocupreidinate among the layers of silylated CaAl-LDH. • Structural characterization (XRD, SEM, two types of IR techniques) of the composites. • Using the composites as catalysts in Michael addition producing chiral adducts. • Studying their recycling abilities and the structural changes upon reuse. β-isocupreidinate (β-iCu) anions having well-known structure and catalytic activity in various asymmetric reactions, were incorporated into the interlayer gallery of hydrocalumite (CaAl-LDH) by the partial delamination-restacking method. Silylation of the outer surface of the LDH with trimethyl silane was also employed to avoid the adsorption of β-iCu on the outer surface and to block the basic sites there. The obtained materials were characterized by a range of instrumental methods (X-ray diffractometry, scanning electron microscopy, ATR-IR and grazing incidence IR spectroscopies). The catalytic activities of the composites were tested in the asymmetric Michael addition of β-nitrostyrene and ethyl 2-fluoroacetoacetate. The β-iCu-pillared LDHs proved to be active and recyclable catalysts with very good diastereoselectivities and acceptable and in 2-propanol very good enantioselectivities. The silylated composite retained its activity and diastereoselectivity even in the third repeated run, when heptane was the solvent. [ABSTRACT FROM AUTHOR]

Abstract Layered double hydroxides (LDHs) consist of layers with two or three positively charged metal ions surrounded by hydroxide ions, water molecules and partially or fully hydrated exchangeable charge-compensating anions between the layers. These materials have many application possibilities. In this work, CaAl-LDH samples were synthesized and used as catalysts. The esterification reactions of acetate and benzoate ions were studied with benzyl bromide under solvent-free conditions over CaAl-LDH samples. The as-prepared as well as the used LDH samples were characterized with X-ray diffractometry, and the reaction products were identified by ATR-IR spectroscopy. Highlights • Acetate- and benzoate-containing CaAl-LDH samples were prepared ions by co-precipitation. • The organic anion-containing samples could be used successfully in the esterification with benzyl bromide. • The reaction and changes in the LDH structure were followed by XRD, ATR-IR and SEM–EDX. [ABSTRACT FROM AUTHOR]