Simple preparation method for Mg–Al hydrotalcites as base catalysts

Recent progress in ZnCr and NiCr layered double hydroxides and based photocatalysts for water treatment and clean energy production

In pursuit of advancing photocatalysts for superior performance in water treatment and clean energy generation, researchers are increasingly focusing on layered double hydroxides (LDHs) which have garnered significant attention due to their customizable properties, morphologies, distinctive 2D layered structure and flexible options for modifying anions and cations. No review has previously delved specifically into ZnCr and NiCr LDH-based photocatalysts and therefore, this review highlights the recent surge in ZnCr and NiCr-based LDHs as potential photocatalysts for their applications in water purification and renewable energy generation. The structural and fundamental characteristics of layered double hydroxides and especially ZnCr-LDHs and NiCr-LDHs are outlined. Further, the various synthesis techniques for the preparation of ZnCr-LDHs, NiCr-LDHs and their composite and heterostructure materials have been briefly discussed. The applicability of ZnCr-LDH and NiCr-LDH based photocatalysts in tackling significant issues in water treatment and sustainable energy generation is the main emphasis of this review. It focuses on photocatalytic degradation of organic pollutants in wastewater, elucidating the principles and advancements for enhancing the efficiency of these materials. It also explores their role in H2 production through water splitting, conversion of CO2 into valuable fuels and NH3 synthesis from N2, shedding light on their potential for clean energy solutions. The insights presented herein offer valuable guidance for researchers working towards sustainable solutions for environmental remediation and renewable energy generation.

Transfer Hydrogenation of Furfural to Furfuryl Alcohol Under Microwave Irradiation Using Mixed Oxides

The reaction to obtain furan alcohols is one of the most important in the upgrading of furan derivates. An attractive route is the transfer hydrogenation of furfural using acidic-basic catalysts. In this work, mixed oxides derived from ternary hydrotalcites were employed to obtain furfuryl alcohol from furfural assisted by microwave irradiation. These materials were characterized via X-ray diffraction (XRD), N2 adsorption-desorption isotherms, Fourier-transform infrared (FTIR) and the CO2 temperature-programmed desorption (CO2 -TPD) analyses. The lamellar structure of hydrotalcite-type materials collapses during the calcination process, resulting in the loss of carbonate anions and hydroxyl groups, present in the interlayer space. This leads to the formation of mixed oxides that exhibit larger surface areas. Furthermore, these changes alter the basic nature of these materials, giving rise to the formation of strong basic sites. The reaction was studied using containing Co2+ and Ni2+ in their structure and was then optimized using distinct primary and secondary alcohols as hydrogen donor sources, as well as distinct temperatures and initial concentrations of furfural. The yields to furfuryl alcohol are strongly dependent on the type of Me2+ in layered oxides mainly due to higher basicity and to the donor employed in the reaction. The mixed oxide containing Co2+ showed complete conversion of furfural and higher yields to furfuryl alcohol (>95 %) at short times of reaction (<1 h).

Layered Double Hydroxide Materials: A Review on Their Preparation, Characterization, and Applications

Layered double hydroxides (LDHs), a type of synthetic clay with assorted potential applications, are deliberated upon in view of their specific properties, such as adsorbent-specific behavior, biocompatibility, fire-retardant capacity, and catalytic and anion exchange properties, among others. LDHs are materials with two-dimensional morphology, high porosity, and exceptionally tunable and exchangeable anionic particles with sensible interlayer spaces. The remarkable feature of LDHs is their flexibility in maintaining the interlayer spaces endowing them with the capacity to accommodate a variety of ionic species, suitable for many applications. Herein, some synthetic methodologies, general characterizations, and applications of LDHs are summarized, encompassing their broader appliances as a remarkable material to serve society and address several problems viz. removal of pollutants and fabrication of sensors and materials with multifaceted useful applications in the medical, electrochemical, catalytic, and agricultural fields, among others.

Insights on Guerbet Reaction: Production of Biobutanol From Bioethanol Over a Mg–Al Spinel Catalyst

The production of biobutanol from bioethanol by the Guerbet reaction is an alternative pathway to renewable sources. The commercial viability of this green route requires improvements in the process development. This study experimentally examines the influence of operating conditions on the performance of a Mg–Al spinel catalyst prepared from hydrotalcite precursors. This catalyst demonstrates an exceptional performance in the Guerbet reaction with a promising activity/butanol selectivity balance, excellent long-term stability, and very-low-carbon footprint (CO2 generation as by-products is minimal). This study showcases a systematic strategy to optimize the reaction parameters in the Guerbet reaction for biobutanol production using an advanced spinel catalyst. Upon carefully adjusting temperature, pressure, space velocity, and reactants co-feeding, very promising conversion (35%) and butanol selectivity values (48%) were obtained.

Layered double hydroxides and LDH-derived materials in chosen environmental applications: a review

With increasing global warming awareness, layered double hydroxides (LDHs), hydrotalcites, and their related materials are key components to reduce the environmental impact of human activities. Such materials can be synthesized quickly with high efficiency by using different synthesis processes. Moreover, their properties' tunability is appreciated in various industrial processes. Regarding physical and structural properties, such materials can be applied in environmental applications such as the adsorption of atmospheric and aqueous pollutants, hydrogen production, or the formation of 5-hydroxymethylfurfural (5-HMF). After the first part that was dedicated to the synthesis processes of hydrotalcites, the present review reports on specific environmental applications chosen as examples in various fields (green chemistry and depollution) that have gained increasing interest in the last decades, enlightening the links between structural properties, synthesis route, and application using lamellar materials.

Effect of the Solvent on the Basic Properties of Mg–Al Hydrotalcite Catalysts for Glucose Isomerization

We suggested the existence of a relationship between the base properties of Mg–Al hydrotalcite catalysts and the solvents employed in the industrially important isomerization of glucose produce fructose. We prepared Mg–Al hydrotalcite catalysts with different Mg/Al atomic ratios to tune the basic properties of the catalyst. The prepared catalysts were used in the glucose isomerization conducted in various solvents. Experimental results confirmed that the catalysts exhibited different activities in the different solvents. We also implemented the Hammett indicator method, which allows to analyze the basic properties of the catalysts in various solvents. According to evidence, the basic properties of the catalysts varied substantially in different solvents. Notably, increases in the catalysts’ base properties matched the observed increases in fructose yield of the glucose isomerization. Consequently, we suggested that, in order to prepare efficient Mg–Al hydrotalcite catalysts for glucose isomerization, the interaction between the solvent used to conduct the reaction and the basic properties of the catalyst, which are in turn influenced by the solvent, should be considered.

Isomerization of Glucose to Fructose in Hydrolysates from Lignocellulosic Biomass Using Hydrotalcite

The isomerization of glucose-containing hydrolysates to fructose is a key step in the process from lignocellulosic biomass to the platform chemical hydroxymethylfurfural. We investigated the isomerization reaction of glucose to fructose in water catalyzed by hydrotalcite. Catalyst characterization was performed via IR, XRD, and SEM. Firstly, glucose solutions at pH-neutral conditions were converted under variation of the temperature, residence time, and catalyst loading, whereby a maximum of 25 wt.% fructose yield was obtained at a 38 wt.% glucose conversion. Secondly, isomerization was performed at pH = 2 using glucose solutions as well as glucose-containing hydrolysates from lignocellulosic biomass. Under acidic conditions, the hydrotalcite loses its activity for isomerization. Consequently, it is unavoidable to neutralize the acidic hydrolysate before the isomerization step with an inexpensive base. As a neutralizing agent NaOH is preferred over Ba(OH)2, since higher fructose yields are achieved with NaOH. Lastly, a pH-neutral hydrolysate from lignocellulose was subjected to isomerization, yielding 16 wt.% fructose at a 32 wt.% glucose conversion. This work targets the application of catalytic systems on real biomass-derived samples.

Ternary Hydrotalcites in the Multicomponent Synthesis of 4H-Pyrans

Lamellar double hydroxides (LDH) with double divalent cations were synthesized by the co-precipitation method and studied in the multicomponent synthesis of 4H-pyrans. The solids obtained were characterized by X-ray diffraction (XRD), infrared spectroscopy (FTIR), N2 adsorption isotherms, and thermogravimetric analysis (TGA). The XRD patterns confirmed the formation of LDHs in which the incorporation of Ni2+ or Co2+ improves their crystalline and textural properties. The results of catalytic activity showed that the synthesis of 4H-pyrans is favored in solvent-free conditions with the LDH–Ni catalyst, avoiding calcination processes. In addition, it was found that hydrotalcite with double divalent cations can conduct this reaction through multicomponent synthesis or by the Michael addition reaction, which can be performed by different types of basicity that depend on the composition of another divalent cation in the brucite layer or a calcination process.

Highly Efficient Lithium Recovery from Pre-Synthesized Chlorine-Ion-Intercalated LiAl-Layered Double Hydroxides via a Mild Solution Chemistry Process

Lithium extraction from salt lake brine is critical for satisfying the increasing demand of a variety of lithium products. We report lithium recovery from pre-synthesized LiAl-layered double hydroxides (LDHs) via a mild solution reaction. Lithium ions were released from solid LiAl-LDHs to obtain a lithium-bearing solution. The LiAl-LDHs phase was gradually transformed into a predominantly Al(OH)₃ phase with lithium recovery to the aqueous solution. The lithium recovery percentage and the concentration of the lithium-bearing solution were dependent on the crystallinity of LiAl-LDHs, the initial concentration of the LiAl-LDHs-1 slurry, the reaction temperature, and the reaction time. Under optimized conditions, the lithium recovery reached 86.2% and the Li⁺ concentration in the filtrate is 141.6 mg/L. Interestingly, no aluminum ions were detected in the filtrate after solid–liquid separation with high crystallinity LiAl-LDHs, which indicated the complete separation of lithium and aluminum in the liquid and solid phases, respectively. The ²⁷Al NMR spectra of the solid products indicate that lithium recovery from the lattice vacancies of LiAl-LDHs affects the AlO₆ coordination in an octahedral configuration of the ordered Al(OH)₃ phase. The XPS O 1s spectra show that the O_ad peak intensity increased and the O_L peak intensity decreased with the increasing lithium recovery, which indicated that the Al-OH bond was gradually formed and the metal–oxygen–metal bond was broken.

Construction of a MnCo2O4@NiyMx (S and P) crosslinked network for efficient electrocatalytic water splitting

Using MnCo₂O₄@Ni₃S₂ as a bifunctional water splitting catalyst, an overpotential of ∼370 mV is obtained at a very low cell voltage of 1.60 V with a current density of 10 mA cm⁻² in 1.0 M KOH.

Synthesis of Mg-Al layered double hydroxides by electrocoagulation

Recently, layered double hydroxides (LDHs) have attracted much consideration due to their versatility and easily manipulating properties and their potential applications such as anion exchangers, support of catalysts, flame retardants, biomedical drug delivery. A novel method for the in-situ preparation in situ of LDHs, using electrocoagulation (EC) processes was developed, the EC process was performed under two different conditions, at 5 mA m⁻², changing polarity of the electrodes to find out the composition that leads to LDHs generation. The final product was characterized using XRD, BET and FTIR techniques.

This method presented the following advantages: (1) Simultaneously LDHs synthesis and wastewater treatment by ion removal; (2) Polarity control allows to manipulate the M²⁺/M³⁺ molar ratio, LDHs properties and its potential applications; (3) The method spent less time to carry out the synthesis and; (4) it did not need complicated solid-liquid separation processes.

Preparation of a palygorskite supported KF/CaO catalyst and its application for biodiesel production via transesterification

This study aimed to explore base catalysts with high transesterification efficiencies to be used for heterogeneous biodiesel production. Palygorskite, a promising low-cost clay mineral, served as a support for KF/CaO to prepare a base catalyst. The base catalyst was prepared via a facile impregnation method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and N₂ adsorption. Subsequently, the prepared catalyst was used for producing biodiesel via the transesterification of commercial soybean oil with methanol. The effects of the catalyst preparation conditions as well as the transesterification parameters, such as the loading amount of KF, calcination temperature, catalyst amount, methanol to oil molar ratio, reaction temperature and water content, on the biodiesel yield were investigated. Results showed that the metal oxide species dispersed on the surface of palygorskite, and the formed KCaF₃ was the main active component for the activity of the catalyst. A maximum biodiesel yield of 97.9% was obtained under the optimal conditions. The separated catalyst could be directly used in the next round of reactions and gave a satisfactory yield. The biodiesel yields decreased from 97.9% to 91.3% from the first to the tenth use of the catalyst particles.

This study aimed to explore base catalysts with high transesterification efficiencies to be used for heterogeneous biodiesel production.