Layered Double Hydroxide-Based Catalysts for Advanced Chemical Technologies

More than 180 years after their discovery in Sweden (1842) [1,2], and 80 after their first synthesis by W. Feitknecht [3], hydrotalcites (or layered double hydroxides (LDHs)), continue to intrigue the scientific world, as do their corresponding mixed oxides, obtained by thermal treatment. Such interest in these materials is explained by their peculiar physicochemical properties, which make them ideal candidates in many fields, as catalysts [4], supporting materials [5], adsorbents [6], molecular sieves [7], ion exchangers [8], flame retardants [9], stabilizers [10], and medicine [11], among other applications. The general formula for these materials is [M²⁺_1−xM³⁺_x(OH)₂]^x+(Aⁿ⁻_x/n)·mH₂O, where M²⁺ and M³⁺ are di- and tri-valent cations, respectively, in the octahedral positions of brucite-type layers; Aⁿ⁻ represents the interlayer anions that compensate for the positive charge of the layers; x is the fraction of the trivalent cations (M³⁺/M²⁺+M³⁺); while m is the number of crystallization water molecules, usually lower than 4. These solids are ditopic materials possessing both acid and base sites, whose strength, dispersion, and type can be tailored according to the requirements of their application. Moreover, these structures can include various types of cations, such as reducible cations, and their anions can be well dispersed.

Their synthesis is simple and can be achieved via several methods, depending on the nature of the cations/anions considered: these include co-precipitation, reconstruction (based on their structural memory effect), sol–gel, and hydro-thermal, microwave-assisted, and mechanochemical syntheses, for example. [12]. However, new discoveries in the field of composite/hybrid materials have also modified the synthesis approaches to LDHs: layer-by-layer assembly, exfoliation restacking, in situ synthesis, electrochemical methods, pulsed laser ablation, plasma exfoliation, etc., are all viable candidates [13,14]. Layered double hydroxides and their related materials require typical characterization techniques for evaluating their physical–structural characteristics, including the following: X-ray diffraction, Fourier Transform Infrared Spectroscopy, thermogravimetric analysis, Diffuse Reflectance UV–vis spectroscopy, Transmission and Scanning Electron Microscopies, X-ray photoelectron spectroscopy, NH₃/CO₂-Temperature Programmed Desorption, Temperature-Programmed Reduction, and Temperature-Programmed Oxidation, among others [1,15].

Further, these materials are well adapted to catalysis, finding applications as active catalytic materials in different reactions, such as oxidation, reduction, addition, alkylation, acylation, decarboxylation, polymerization, and hydrogenation [16].

This Special Issue, entitled “Layered Double Hydroxide-Based Catalysts for Advanced Chemical Technologies”, comprises six contributions presenting advanced strategies for the synthesis, characterization, and use of LDH-type materials based on tailored physicochemical properties.

LDH-based hierarchical materials with adjustable electronic structures for the hydrogen evolution reaction (i.e., NiCoP nanowire@NiCoP nanosheet on nickel foam (NW-NiCoP@NS-NiCoP/NF)) were synthesized [17] by growing NiCo-LDH nanosheets on nickel foam under hydrothermal conditions, followed by the in situ growth of NiCo-LDH nanowires at different temperatures. Phosphorus was added by anion exchange with different dosages of NaH₂PO₂·H₂O at 300 °C to adjust the electronic structure of the transition metals. This synthesis approach led to a uniform distribution of Ni, Co, and P elements within the obtained hierarchical structure, which has a large surface area, improved gas diffusion, and low charge transfer resistance. The optimized hierarchical system showed excellent hydrogen evolution activity and noticeable stability.

Li et al. [18] synthesized a series of RuNi catalysts deposited on mixed metal oxide supports, MMO-C and MMO-N, obtained by the calcination of NiAl-CO₃-LDH and NiAl-NO₃-LDH precursors, respectively. Ru deposition on the mixed oxide support was achieved by incipient wetness impregnation using RuCl₃·nH₂O as a precursor. The reduction under hydrogen led to bimetallic catalysts with Ni and Ru nanoparticles 4.8 nm and 2.7 nm in size for the RuNi/MMO-C and RuNi/MMO-N, respectively. RuNi/MMO-N showed better activity and selectivity in CO selective methanation in H₂-rich gas mixtures due to a more uniform surface distribution of Ni and Ru particles on the support, the presence of a significant number of acid sites, which played a positive role in CO adsorption and a negative role in CO₂ dissociation, and a higher number of electron-rich Ni sites favoring CO dissociation.

Tannous et al. [19] synthesized MnCoAl mixed oxides with different Mn/Co ratios and tailored physicochemical properties for the total oxidation of ethanol. LDH synthesis was achieved by classical co-precipitation, either under air or nitrogen, followed by calcination at 500 °C. Under the latter, it has been shown that the formation of metal carbonates and Mn segregation are avoided. The mixed oxide catalysts containing Mn₂CoO₄ and lamellar Mn₅O₈ phases showed better redox and catalytic properties, while a high Mn/Co ratio led to an increase in activity. Among the investigated catalysts, Mn₅CoAl₂ and Mn₆Al₂ mixed oxides precipitated under air and under nitrogen, respectively, yet calcined under the former, leading to a complete ethanol conversion to CO₂ at temperatures as low as 160 °C.

The presence of acid–base sites is crucial in carbon-enrichment reactions in fine chemistry. With this in mind, Stoylkova et al. [20] considered LDH-derived acid–base M²⁺MgAlO and M²⁺AlO mixed oxide (M²⁺ = Mg, Cu, Co, Zn, Ni) catalysts in the Claisen–Schmidt condensation between cyclohexanol and benzaldehyde, under solvent-free conditions, to obtain 2,6-dibenzylidene-cyclohexanone (di-condensed product). The synthesis of the catalytic materials was achieved via the co-precipitation of the LDH precursors, followed by their thermal decomposition at 500 °C for M²⁺MgAlO and MgAlO, and 350 °C for M²⁺AlO (M²⁺ = Co, Zn, Ni). Their conversion activity after a 2 h reaction at 150 °C, which was correlated with their basicity, followed the order CoMgAlO (97%) > CuMgAlO (83%) > NiMgAlO > ZnMgAlO > MgAlO > CoAlO > NiAlO > ZnAlO. No side reactions were evidenced for MgAlO and M²⁺MgAlO catalysts, confirming their specificity for the di-condensation product.

LDH-derived mixed oxide catalysts were shown to be viable candidates for the simultaneous oxidation of toluene and CO released from the biomass combustion process [21]. Thus, Cu₆Al_2−xCe_x mixed oxides with x = 0–0.8 were prepared by the thermal decomposition of their corresponding LDH precursors at 500 °C, characterized and tested in the simultaneous oxidation of toluene and CO. Their catalytic performance is strongly influenced by the value of x, with the best system identified being Cu₆Al_1.2Ce_0.8. This composition was also prepared through different mechanical mixing approaches, with the obtained mixed oxides being less effective than the ex-LDH material. The latter consists of highly dispersed small crystallites, has higher surface area, and shows an enhanced Cu-Ce synergistic interaction.

Al Hasnawi et al. [22] considered the use of LDH for solid iron-based magnetic composites modified with Cu and Ni phthalocyanines for water photocatalytic decontamination. LDH material synthesis was achieved using both traditional and non-traditional methods, while the phthalocyanines were introduced using the memory effect of the calcined LDH, a specific property allowing for the reconstruction of the layered structure by hydrating the ex-LDH mixed oxides obtained at temperatures not exceeding 500–600 °C, depending on their composition. The best material for the decontamination of water polluted with amoxicillin and ampicillin was that containing Mg_0.325Fe_0.325Al_0.25-LDH prepared using a traditional method of co-precipitation. It showed both high adsorption capacity and high photocatalytic activity, and demonstrated that the cation in the phthalocyanine does not play a key role. No antibiotic mineralization reached after 2 h of irradiation. However, the synthesis of magnetic composites incorporating LDH represents an important research direction in decontamination processes and beyond.

These contributions demonstrate the incredible flexibility of LDH and related materials, resulting in broad catalytic applications. They offer valuable data, innovative ideas, and directions for future research.