Recent developments in intercalation compounds: chemistry and applications

It is with great pleasure we welcome you to this Dalton Transactions themed issue devoted to “Recent Developments in Intercalation Compounds: Chemistry and Applications”.

Intercalation compounds are a wide class of materials resulting from reversible inclusion of guest species (with variable chemical composition, from simple ions to complex organic species) typically in a layered host matrix, having either a crystalline or amorphous structure. These materials can be considered at the intersection between chemistry, solid-state physics and materials science and interest in them is rapidly growing across multiple disciplines. They are a class of extremely flexible compounds because the appropriate choice of host and guest species results in the possibility to tune their final properties, thus extending their potential application to various technological fields.

With this themed issue we intended to meet the most recent and significant advances on intercalation compounds.

The 35 articles composing the issue can thus be considered as a representative selection of the current research on intercalation compounds. There are articles related to the synthesis and characterisation methods of intercalation materials as well as others dealing with the description of innovative properties of this class of compounds. Others report new applications of intercalation compounds in a wide range of disciplines, spanning heterogeneous catalysts, polymer science, environmental protection, biomaterials and drug delivery systems.

As far as a layered structure or even more generally a host structure is concerned, a lot of effort is devoted to (1) the preparation of a specific morphology (single platelet) and/or topology (exchange ability), (2) tuning the inside framework composition (i.e. intralayer composition/substitution for 2D) and (3) topotactic exchange reaction (i.e. interlayer composition change for 2D).

Numerous papers in this themed issue deal with layered double hydroxide (LDHs) and layered hydroxide salts (LHSs).

Regarding (1), a novel preparation way for LDH based hybrid phases has been proposed combining the polyol route with the in situ generation of inorganic platelets, in the presence of amphiphilic polymers. This was found to be an interesting new route to reach LDH exfoliation (Leroux and co-workers, DOI: 10.1039/c7dt03191b). An environmentally friendly co-hydration approach for the preparation of hybrid organic–inorganic LDH with an ultra-high aspect ratio was proposed (Greenwell et al., DOI: 10.1039/c7dt04312k), thus establishing the effect of the synthesis method on the aspect ratio of the obtained crystallites. Applications of LDHs in heterogeneous catalysis have been extended by modifying the solids with ionic liquids by adopting exfoliation/assembly strategies to prepare materials for Knoevenagel condensation of various aldehydes (Song et al., DOI: 10.1039/c7dt03665e). Moreover, an example of monodispersed hierarchical layered double hydroxides on silica spheres (LDH@SiO₂) with core–shell structures were developed for metalloporphyrin immobilization and the materials were used as effective oxidant catalysts of different substrates (Wypych et al., DOI: 10.1039/c7dt03656f). Advances in methods for the preparation of nanosheet shaped materials and their hybrids have been reported with particular attention on the solid–solid mechano-chemical methods, giving the possibility to prepare novel intercalation compounds (Ogawa et al., DOI: 10.1039/c7dt03736h). Another interesting example is the synthesis of hybrid magnesium hydroxides in organic solvents that were proven to be effective in modifying the hydroxide layers leading to a denser intercalation of ligands through bi or (tri) dentate linkages. Such organic solvent treatment was extended to a wide variety of organic functional groups (Kuroda et al., DOI: 10.1039/c7dt03699j).

Taking advantage of intralayer composition versatility (2), novel compounds have been obtained by intimately coupling Zn–Cr-LDH and a narrow band-gap semiconductor graphitic mesoporous carbon nitride thus preparing highly efficient visible light active photocatalysts with enhanced visible light absorptivity and depressed electron–hole recombination (Hwang et al., DOI: 10.1039/c7dt03466k). Highly dispersed mixed oxide was synthesized from the corresponding Cu₂Mn_0.5Al_0.5–CO₃ LDH as an excellent low-temperature NH₃-SCR (selective catalytic reduction) catalyst (Wang et al., DOI: 10.1039/c7dt02000g). Layered rare-earth hydroxides (LRH) were tested for potential use for simultaneous drug delivery and magnetic resonance imaging. The layered gadolinium hydroxide–drug intercalation compounds appear to have great potential for use in theranostic applications (Williams et al., DOI: 10.1039/c7dt03729e).

Exchange reaction (3) combined with in situ reactivity is exemplified by a carbon replica obtained from a vinylbenzenesulfonate-intercalated LDH successfully used as an adsorbent material for removing organochlorine herbicide from aqueous solution (Tronto et al., DOI: 10.1039/c7dt03720a). X. Lei et al. (DOI: 10.1039/c7dt03583g) gave an interesting example of the exchange of interlayer composition for obtaining solids with topical environmental needs. In this framework, phytate intercalated MgAl-LDH was prepared and proved to be a highly selective adsorbent for rare-earth or toxic metal ions. A combination of MgAl LDH with α-zirconium phosphate was proposed as a new cleaning agent for the removal of gypsum from ancient monuments, thus demonstrating that multipurpose materials can also be applied for the protection of artistic and historical monuments (Nocchetti et al., DOI: 10.1039/c7dt03957c). Examples of the use of LDHs for biomedical applications are also presented. Xu et al. (DOI: 10.1039/c7dt03725b) exploited LDHs with different morphologies (nanoparticles and nanosheets) as adjuvants to provoke immune responses for an anti-tumour purpose, suggesting that these materials can be potentially used for efficient protein-based anti-tumour vaccines.

Combining (1) and (3) through an aqueous miscible organic solvent treatment (AMOST), finely dispersed surfactant interleaved hydrotalcite-type materials endow polypropylene with thermal stability and an efficient flame retardancy (Wang et al., DOI: 10.1039/c7dt03477f). In situ polymerization was used for the preparation of nanocomposites based on poly(butylene succinate) and hydrotalcite-type anionic clays organo-modified with biomolecules characterized by antibacterial and/or antioxidant activities (Totaro et al., DOI: 10.1039/c7dt03914j). In that polymer nanocomposite domain, Feng et al. (DOI: 10.1039/c7dt03483k) reported the preparation of a series of μm-sized dihydrogenphosphate intercalated LDH by an anion exchange method and their selective IR absorption when dispersed into ethylene-vinyl acetate copolymer (EVA) useful for applications as agricultural films.

Besides experimental data, there are important developments in theoretical DFT methodology to understand the physical and chemical properties and to design novel materials with desirable properties, and even to assist characterization of intercalated solids. A review of the most significant DFT methods suitable for the study of intercalated layered materials (LDHs, LHSs and cationic clays) is presented by Leitao and co-workers (DOI: 10.1039/c7dt03730a).

Besides LDHs related materials, several examples mainly devoted to practical applications of smectite clay and layered silicates are given in this Issue. In the context of (1), in a short review, Brendlé (DOI: 10.1039/c7dt03902f) describes the preparation of clay-mimicking compounds with a talc-like structure and unique properties via sol–gel routes. This review also highlights the latest trends and the multifaceted applications of these functional layered compounds.

The interlayer modification (3) of natural clays is given by Bisio et al. (DOI: 10.1039/C7DT03859C). By introducing both Fe³⁺ and acid sites in natural montmorillonite, the resulting new formulation is able to transform selectively and under mild conditions toxic chemical warfare agents into products with reduced impact on the environment.

Intercalation reaction (3) was used advantageously by Ruiz-Hitzky et al. (DOI: 10.1039/c7dt04197g) to prepare a drug delivery system constituted by metformin hydrochloride (an antidiabetic drug) intercalated in natural montmorillonite clay mineral. Another application of (3) allowed the preparation of organo-modified magadiite grafted with glycidyloxypropyl and diethylenetriaminepropyl groups with an expanded interlayer space and more available N adsorption sites, which improved CO₂ adsorption properties (Pastore et al., DOI: 10.1039/c7dt03732e).

Smectites are also used for the preparation of a new class of self-assembled polymer/clay intercalates via ion-dipole bonding: materials have extraordinary gas barrier properties and could have potential in other areas such as protection for organic light emitting diodes that are very sensitive to oxidation (Beall et al., DOI: 10.1039/c7dt03900j).

This themed issue also reports recent studies on perovskite-type materials. In their perspective review, Osada et al. (DOI: 10.1039/c7dt03719h) presented an overview of recent investigations on the dielectric/ferroelectric properties of Dion–Jacobson-type perovskites, including bulk 3D layered systems and their exfoliated 2D nanosheets.

The topochemical modification (2) of layered oxide perovskites was approached by Wiley et al. (DOI: 10.1039/c7dt03865h) by exploiting microwave radiation.

The intercalation (3) of amine-related agents, from aqueous solutions, in layered perovskite niobate was examined and an osmotic swelling behavior was observed (Sasaki et al., DOI: 10.1039/c7dt03413j).

Intercalated compounds with layered phosphates and phosphonates as host are also reported.

Multiple synthetic strategies (1) were described to tether a zirconium-based catalyst to the 2D and 3D molecular sieves for olefin polymerizations (Pastore et al., DOI: 10.1039/c7dt03734a). Zima et al. (DOI: 10.1039/c7dt03728g) reviewed the intercalation chemistry (3) of layered alkaline-earth metal (phenyl)phosphonates and mixed oxides with a focus on the arrangement of the guest molecules within the interlayer space. Rodríguez-Castellón et al. (DOI: 10.1039/c7dt03717a) reported the preparation and characterization of a porous lamellar zirconium phosphate (PPH) formed from zirconium phosphate expanded with silica galleries hosting noble metals for application as a support in a catalytic hydrodeoxygenation reaction.

Other papers are related to zeolites and mesoporous materials.

Cejika et al. (DOI: 10.1039/c7dt03751a) reported the synthesis (1) of IPC-12 zeolite by using an ADOR (Assembly, Disassembly, Organization and Reassembly) approach for the structural transformation of germanosilicates with particular attention paid to the relation between the structural IPC-12 rearrangements and their properties.

With regard to interlayer modification (3) Roth et al. (DOI: 10.1039/c7dt03718j) proposed the pillaring of surfactant-swollen layered zeolite material with FER (ferrierite) topology and investigated the properties of the obtained materials as large pore catalysts and sorbents, or controlled drug delivery.

A combined experimental and computational approach was applied to study squaraine incorporated on mesoporous silica (Miletto et al., DOI: 10.1039/c7dt03735j).

Finally, the synthesis and spectroscopic characterization of alkali–metal (Li, K and Cs) doped ZrSe₂ single crystals are also reported (Gruneis et al., DOI: 10.1039/c7dt03756b).

Moreover, Sasaki et al. (DOI: 10.1039/c7dt03706f) examined the stability and quality of chemically exfoliated MoS₂ nanosheets in an aqueous dispersion from a viewpoint of their electronic nature and 2D structure.

Pralong et al. (DOI: 10.1039/c7dt04377e) investigated the possibility of generating new ordered or disordered rock salt-type structures as exemplified by the formation of Na₃V₂O₅ upon chemical reduction of V₂O₅.

Heguri et al. (DOI: 10.1039/c7dt03745g) reported the synthesis and charge transfer mechanisms of high quality bulk samples of the first four polyacenes (naphthalene, anthracene, tetracene, and pentacene) doped with alkali metals.

Of course, we wish to thank all the authors for their valuable contributions as well as the reviewers for their work and relevant input.

Finally, we would like to thank the editorial staff at Dalton Transactions for making this themed issue possible. We address a special acknowledgement to Prof. Russell Morris, Dr Mike Andrews, Dr Helen Lunn and Dr Debora Giovanelli for their precious support.

We hope this themed Dalton Transactions issue makes enjoyable reading and that it can be a source of inspiration and future fruitful collaborations.

This Issue is in honour of Prof. Umberto Costantino, who has devoted his scientific life to the deep understanding of intercalation chemistry and is an excellent example of how the values of life and science can be merged.

Finally, this issue is dedicated to the memory of Prof. Giulio Alberti and Prof. Sohrab Rabii, for their relevant and inspiring contributions in the field of intercalation compounds.

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