Search Results (78)

The production of biodiesel from microalgae presents a sustainable and renewable solution to the growing global energy demands, with catalysts playing a critical role in optimizing the transesterification process. This study examines the emerging catalysts and innovative techniques utilized in converting microalgal lipids into fatty acid methyl esters, emphasizing their impact on reaction efficiency, yield, and environmental sustainability. Sulfuric acid demonstrates excellent performance in in situ transesterification, while NaOH/zeolite achieves high biodiesel yields using ultrasound- and microwave-assisted methods. Metal oxides such as CuO, NiO, and MgO supported on zeolite, as well as ZnAl-layered double hydroxides (LDHs), further enhance reaction performance through their high activity and stability. Enzymatic catalysts, particularly immobilized lipases, provide a more environmentally friendly option, offering high yields (>90%) and the ability to operate under mild conditions. However, their high cost and limited reusability pose significant challenges. Ionic liquid catalysts, such as tetrabutylphosphonium carboxylate, streamline the process by eliminating the need for drying and lipid extraction, achieving yields as high as 98% from wet biomass. The key novelty of this work lies in its detailed focus on the use of ionic liquids and nanocatalysts in microalgae-based biodiesel production, which are often underrepresented in previous reviews that primarily discuss homogeneous and heterogeneous catalysts. Full article

The urgent need to mitigate rising global CO₂ emissions demands the development of efficient carbon capture technologies. This study addresses the persistent challenge of sintering-induced performance degradation in CaO-based sorbents during high-temperature CO₂ capture. A novel solvent/nonsolvent synthetic strategy to fabricate CaO/CaAl-layered double oxide (LDO) composites was developed, where CaAl-LDO serves as a nanostructural stabilizer. The CaAl-LDO precursor enables atomic-level dispersion of components, which upon calcination forms a Ca₁₂Al₁₄O₃₃ “rigid scaffold” that spatially confines CaO nanoparticles and effectively mitigates sintering. Thermogravimetric analysis results demonstrate exceptional cyclic stability; the composite achieves an initial CO₂ uptake of 14.5 mmol/g (81.5% of theoretical capacity) and retains 87% of its capacity after 30 cycles. This performance significantly outperforms pure CaO and CaO/MgAl-LDO composites. Physicochemical characterization confirms that structural confinement preserves mesoporous channels, ensuring efficient CO₂ diffusion. This work establishes a scalable, instrumentally simple route to high-performance sorbents, offering an efficient solution for carbon capture in energy-intensive industries such as power generation and steel manufacturing. Full article

Cu-based layered double hydroxides (LDH) have been extensively employed as catalyst precursors. However, due to the Jahn–Teller effect of copper ions, it is a challenge to synthesize well-crystallized LDH with a high Cu content, which usually contains considerable CuO impurity. By adding competitive ligands during the coprecipitation process, such as glycine, a well-crystallized Cu-rich LDH with less CuO impurity was successfully synthesized. The Cu-Mg-Al mixed oxides derived from the well-crystallized Cu-rich LDH have relatively high S_BET, large pore volume, and well dispersion of Cu nanoparticles. The derived catalyst exhibited unexpectedly high catalytic activity in the water–gas shift (WGS) reaction, and the mass-specific reaction rate was reached as high as 33.5 μmol_CO${·\mathrm{g}}_{\mathrm{cat}}^{-1}$·s⁻¹ at 200 °C. The high catalytic activity of this catalyst may originate from the high S_BET and well dispersion of Cu particles and metal oxides. Moreover, the derived catalyst also displayed outstanding long-term stability in the WGS reaction, which should benefit from the enhanced metal–support interaction. Full article

Mixed oxides were obtained via calcination at 550 °C from layered double hydroxides (LDHs), which were synthesized in a previous study via co-precipitation and co-precipitation followed by hydrothermal treatment using aluminum residues as the source of this element. After characterization, these oxides (Mg-Al-_LDH-CP and Mg-Al-_LDH-H, named according to the synthesis methods of the precursor LDHs) were applied as heterogeneous catalysts in the methyl transesterification of ethyl acetate (EA). The formation of mixed oxides was confirmed by the absence of basal peaks associated with the layered LDH structure in the XRD analysis, due to calcination. Further characterization revealed that Mg-Al-_LDH-CP exhibited the highest number of acidic sites, while Mg-Al-_LDH-H had the highest number of basic sites. The transesterification activity was evaluated in the reaction between ethyl acetate (EA) and methanol (MeOH). The best result, obtained under a molar ratio of 1:5:0.005 (EA:MeOH:catalyst) at 120 °C, was a 63% conversion after 360 min of reaction for the Mg-Al-_LDH-CP catalyst, which had a higher number of acidic sites and fewer basic sites. Additionally, the catalysts demonstrated robustness, maintaining catalytic activity over four cycles without a significant decrease in performance. These results indicate the feasibility of using mixed oxides derived from LDH, synthesized from aluminum residues, as heterogeneous catalysts in transesterification reactions, highlighting their potential for advancing more sustainable catalyst development. Full article

Metal carbonyl clusters, which can be seen as monodispersed and atomically defined nanoparticles stabilized by CO ligands, were used to prepare Ru-based catalysts with tuned basic properties to conduct the 5-hydroxymethylfurfural (HMF) aerobic oxidation to produce 2,5-furandicarboxylic acid (FDCA) in base-free conditions. The controlled decomposition of the carbonyl cluster [HRu₃(CO)₁₁]⁻, a methodology not yet applied to Ru catalysts for this reaction, on different supports focusing on controlling and tuning the basic properties of support allowed the formation of small Ru nanoparticles with a mean diameter of around 1 nm. The catalytic systems obtained resulted in more activity in the HMF oxidation than those prepared through a more common salt-impregnation technique, and the deposition of Ru nanoparticles on materials with basic functionalities has allowed avoiding the use of basic solutions in the reaction. The characterization by CO₂-TPD of Mg(Al)O catalysts obtained from decomposition of layered double hydroxide hydrotalcites with different composition and activation has allowed disclosure of an important correlation between the selectivity of FDCA and the fraction of weak basic sites, which is decreased by the calcination treatment at increased temperature. Full article

Given the pressing demand for efficient uranium (U(VI)) enrichment and its elimination from wastewater to curtail the risks of radioactive contamination inherent in nuclear energy applications, it is crucial to design materials with high removal efficiency and straightforward separation processes. In the current study, we incorporated konjac gum (KGM) into MgAl-double oxide (MgAl-LDO) and synthesized an innovative, economical, and environmentally friendly LDO-KGM material by using the freeze-drying-calcination (FDC) method, which provided a solution for U(VI) concentration from aqueous solutions. The nanoflower structures LDO-KGM with abundant pore structure and high specific surface area exhibited an optimal U(VI) adsorptive capacity (3019.56 mg·g⁻¹) at pH = 6.0 and 293 K, which was 2.3 times greater than that of MgAl-LDO (1296.39 mg·g⁻¹). LDO-KGM also showed great adaptability for the immobilization of U(VI) over a broad pH range (4.0 to 9.0) and coexisting ions. U(VI) adsorption onto LDO-KGM adhered to the pseudo-second-order kinetic model (R² ≥ 0.99) and the Langmuir isotherm model (R² ≥ 0.99). The analysis of thermodynamic parameters derived from isotherms at varying temperatures revealed that U(VI) adsorption onto LDO-KGM was an endothermic and spontaneous process. The mechanism underlying U(VI) adsorption by LDO-KGM was mainly complexation, carbonate co-precipitation, and electrostatic adsorption. Furthermore, the adsorption efficiency of LDO-KGM for U(VI) could still retain more than 84.5% after five cycles. The findings indicate that the synthesized LDO-KGM exhibits potential as an exceptionally potent adsorbent for the purification of wastewater contaminated with U(VI). Full article

Layered double hydroxides (LDH) containing various exchangeable anions were studied to show how X-ray Photoelectron Spectroscopy (XPS) can provide information on the local environments of the different elements within the interlayer anionic groups and their possible influence on the LDH interlayer hydroxide surfaces. As such, XPS can potentially provide additional information about these systems that cannot be obtained by other common spectroscopic methods, such as infrared and Raman spectroscopy. A Mg₆Al₂X(OH)₁₆. 4H₂O with X representing interlayer anions CO₃²⁻, PO₄³⁻, SO₄²⁻, MoO₄²⁻, CrO₄³⁻, Fe(CN)₆⁴⁻, and Fe(CN)₆³⁻ was studied. The hydroxide layer structure is characterized by the Mg 2p and Al 2p with a binding energy of around 50.1 and 74.5 eV for the normal CO₃²⁻ containing LDH. The O 1s contained three peaks related to the layer OH-groups at 531.6 eV, interlayer CO₃²⁻ at 530.5 eV and interlayer water at 532.4 eV. Similar observations were made for the other interlayer anions showing characteristic P 2p, S 2p, and Mo 3d peaks. Intercalation with CrO₄³⁻ shows that a significant amount of the Cr⁶⁺ has been reduced to Cr³⁺. Finally, the intercalation of hexacyanoferrate in hydrotalcite showed the potential of XPS in detecting changes in the oxidation state of Fe upon intercalation in the LDH with a change in the Fe 2p peaks with a shift in binding energy and the possibility of determining the amount of reduction of Fe(III) to Fe(II). In general, the XPS high-resolution scans of P 2p, S 2p, Mo 3d, and Cr 2p show that slightly lower binding energies are observed compared to the binding energy values for the corresponding anionic groups as part of a rigid crystal structure, such as in minerals. Overall, the influence of the nature of the interlayer anion on the binding energy of the elements (Mg, Al, O) in the layered double hydroxide structure is minimal and considered to be within the experimental error of XPS. A detailed analysis of XPS data in combination with infrared and Raman spectroscopy shows how XPS can provide additional information that is not readily available via vibrational spectroscopy. XPS can simultaneously account for both surface and bulk properties of LDH that are not available through common vibrational spectroscopic methods. Full article

A layered double hydroxide (LDH) containing Mg and Al was synthesized from a nitrate solution using a coprecipitation method. The resulting material exhibited a homogeneous structure, which, upon calcination at 450 °C, was converted into a layered double oxide (LDO). When rehydrated in a fluoride-containing aqueous solution, the original hydroxide structure was successfully regenerated, demonstrating the LDH’s memory effect. During this transformation, fluoride anions from the solution were incorporated into the interlayer galleries to maintain electroneutrality, as confirmed by energy-dispersive X-ray spectroscopy (EDS) analysis. Separately, the process was tested in the presence of ethanol, which significantly enhanced the incorporation of fluoride ions into the interlayer spaces. The material’s potential for controlled fluoride release was evaluated by monitoring its release into demineralized water. For comparison, a simple ion-exchange process was carried out using the as-synthesized MgAl LDH. The memory effect mechanism displayed a notably higher fluoride incorporation capacity compared to the ion-exchange process. Among all the specimens, the sample reconstructed in the presence of ethanol exhibited the highest fluoride ion content. Fluoride release studies revealed a two-phase pattern: an initial rapid release within the first three hours, followed by a substantially slower release over time. Full article

Anion Exchange Membranes (AEMs) are promising materials for electrochemical devices, such as fuel cells and electrolyzers. However, the main drawback of AEMs is their low durability in alkaline operating conditions. A possible solution is the use of composite ionomers containing inorganic fillers stable in a basic environment. In this work, composite anion exchange membranes are prepared from poly (2,6-dimethyl-1,4-phenylene oxide) with quaternary ammonium groups on long-side chains (PPO-LC) and exfoliated Mg/Al lamellar double hydroxide (LDH) as inorganic filler added in different percentages (2, 5, and 10%). The mechanical stiffness of the membranes increases significantly by the addition of exfoliated LDH up to 5%. The ionic conductivity is measured as a function of the temperature in fully humidified conditions and as a function of relative humidity (RH). The maximum conductivity is observed for 5% LDH. The average activation energy for conductivity amounts to 0.20 ± 0.01 eV in fully humidified conditions and >50% RH. Thermogravimetric analysis of membranes before and after alkaline degradation tests (2 M KOH @ 80 °C, 48 h) reveals that the sample with 5% LDH has improved stability (19% vs. 36% of degradation). The stability tests are also investigated, measuring the ionic conductivity and the water uptake. A protective effect of LDH on the alkaline degradation of quaternary ammonium groups is clearly evidenced and opens the way to the use of different compounds and exfoliation methods in the LDH family. Full article

Pollution in ecosystems has increased, especially in water, due to the pollutant agents that alter their chemical, physical and biological characteristics. This requires actions to resolve or at least reduce the harmful effects generated on the environment and people’s health. Many of the contaminants present in water come from the industrial sector, with the textile industry being one of the most impactful as it uses mostly synthetic dyes, which are characterized as being recalcitrant and toxic, so they cannot be degraded by conventional water treatment methods. Advanced oxidation processes have a great potential for application, especially those that use heterogeneous photocatalysis. The present research evaluates the efficiency in the adsorption and degradation of the triazoic Direct Blue 71 dye in aqueous mediums at concentrations of 600 ppm by the heterogeneous photocatalysis method. The photocatalysts used are layered double hydroxides (LDHs) with a Mg/Al = 3 ratio and are thermally activated and doped with Fe at 1, 3 and 5% w/w. The most efficient materials achieved removal percentages greater than 80% by means of a second-order kinetic model with a DB71 half-life decolorization of less than one hour; as shown by an HPLC study, the absence of intermediate products would confirm the mineralization of the dye. Full article

A series of M(x)CoCeMgAlO mixed oxides with different transition metals (M = Cu, Fe, Mn, and Ni) with an M content x = 3 at. %, and another series of Fe(x)CoCeMgAlO mixed oxides with Fe contents x ranging from 1 to 9 at. % with respect to cations, while keeping constant in both cases 40 at. % Co, 10 at. % Ce and Mg/Al atomic ratio of 3 were prepared via thermal decomposition at 750 °C in air of their corresponding layered double hydroxide (LDH) precursors obtained by coprecipitation. They were tested in a fixed bed reactor for complete methane oxidation with a gas feed of 1 vol.% methane in air to evaluate their catalytic performance. The physico-structural properties of the mixed oxide samples were investigated with several techniques, such as powder X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental mappings, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction under hydrogen (H₂-TPR) and nitrogen adsorption–desorption at −196 °C. XRD analysis revealed in all the samples the presence of Co₃O₄ crystallites together with periclase-like and CeO₂ phases, with no separate M-based oxide phase. All the cations were distributed homogeneously, as suggested by EDX measurements and elemental mappings of the samples. The metal contents, determined by EDX and ICP-OES, were in accordance with the theoretical values set for the catalysts’ preparation. The redox properties studied by H₂-TPR, along with the surface composition determined by XPS, provided information to elucidate the catalytic combustion properties of the studied mixed oxide materials. The methane combustion tests showed that all the M-promoted CoCeMgAlO mixed oxides were more active than the M-free counterpart, the highest promoting effect being observed for Fe as the doping transition metal. The Fe(x)CoCeMgAlO mixed oxide sample, with x = 3 at. % Fe displayed the highest catalytic activity for methane combustion with a temperature corresponding to 50% methane conversion, T₅₀, of 489 °C, which is ca. 40 °C lower than that of the unpromoted catalyst. This was attributed to its superior redox properties and lowest activation energy among the studied catalysts, likely due to a Fe–Co–Ce synergistic interaction. In addition, long-term tests of Fe(3)CoCeMgAlO mixed oxide were performed, showing good stability over 60 h on-stream. On the other hand, the addition of water vapors in the feed led to textural and structural changes in the Fe(3)CoCeMgAlO system, affecting its catalytic performance in methane complete oxidation. At the same time, the catalyst showed relatively good recovery of its catalytic activity as soon as the water vapors were removed from the feed. Full article

Current global challenges associated with energy security and climate emergency, caused by the combustion of fossil fuels (e.g., jet fuel and diesel), necessitate the accelerated development and deployment of sustainable fuels derived from renewable biomass-based chemical feedstocks. This study focuses on the production of long-chain (straight and branched) ketones by direct α-alkylation of short chain ketones using both homogenous and solid base catalysts in water. Thus, produced long-chain ketones are fuel precursors and can subsequently be hydrogenated to long-chain alkanes suitable for blending in aviation and liquid transportation fuels. Herein, we report a thorough investigation of the catalytic activity of Pd in combination with, (i) homogenous and solid base additives; (ii) screening of different supports using NaOH as a base additive, and (iii) a comparative study of the Ni and Pd metals supported on layered double oxides (LDOs) in α-alkylation of 2-butanone with 1-propanol as an exemplar process. Among these systems, 5%Pd/BaSO₄ with NaOH as a base showed the best results, giving 94% 2-butanone conversion and 84% selectivity to alkylated ketones. These results demonstrated that both metal and base sites are necessary for the selective conversion of 2-butanone to alkylated ketones. Additionally, amongst the solid base additives, Pd/C with 5% Ba/hydrotalcite showed the best result with 51% 2-butanone conversion and 36% selectivity to the alkylated ketones. Further, the screening of heterogenous acid-base catalysts 2.5%Ni/Ba_1.2Mg₃Al₁ exhibited an adequate catalytic activity (21%) and ketone selectivity (47%). Full article

Acid–base M²⁺MgAlO and M²⁺AlO mixed oxides (where M²⁺ = Mg, Cu, Co, Zn, and Ni) were obtained by thermal decomposition of the corresponding layered double hydroxide (LDH) precursors and used as catalysts for cyclohexanol and benzaldehyde condensation under solvent-free conditions. The catalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and temperature-programmed desorption of CO₂ (TPD-CO₂). Gas chromatography–mass spectroscopy (GC/MS) was used for the identification and quantification of the product mixtures. In the reaction of cyclohexanol and benzaldehyde on M²⁺MgAlO and MgAlO catalysts, a 2,6-dibenzylidene-cyclohexanone was obtained as the main product as a result of consecutive one-pot dehydrogenation of cyclohexanol to cyclohexanone and subsequent Claisen–Schmidt condensation. In the reaction mixture obtained in the presence of NiAlO, CoAlO, and ZnAlO catalysts, a cyclohexyl ester of 6-hydroxyhexanoic acid was detected together with the main product. This is most likely a by-product obtained after the oxidation, ring opening, and subsequent esterification of the cyclohexanol. Full article

Ni-based catalysts have been widely used for the CO₂ reforming of methane (CRM) process, but deactivation is their main problem. This study created an alternative electronic Ni-NiO-CeO₂ interaction on the surface of 5 wt% Ni-5 wt% CeO₂/Al₂O₃-MgO (5Ni5Ce(xh)/MA) catalysts to enhance catalytic potential simultaneously with coke resistance for the CRM process. The Ni-NiO-CeO₂ network was developed on Al₂O₃-MgO through layered double hydroxide synthesis via our ammonia vapor diffusion impregnation method. The physical properties of the fresh catalysts were analyzed employing FESEM, N₂ physisorption, and XRD. The chemical properties on the catalyst surface were analyzed employing H₂-TPR, XPS, H₂-TPD, CO₂-TPD, and O₂-TPD. The CRM performances of reduced catalysts were evaluated at 600 °C under ambient pressure. Carbon deposits on spent catalysts were determined quantitatively and qualitatively by TPO, FESEM, and XRD. Compared to 5 wt% Ni-5 wt% CeO₂/Al₂O₃-MgO prepared by the traditional impregnation method, the electronic interaction of the Ni-NiO-CeO₂ network with the Al₂O₃-MgO support was constructed along the time of ammonia diffusion treatment. The electronic interaction in the Ni-NiO-CeO₂ nanostructure of the treated catalyst develops surface hydroxyl sites with an efficient pathway of OH* and O* transfer that improves catalytic activities and coke oxidation. Full article

Growing research activity on layered double hydroxide (LDH)-based materials for novel applications has been increasing; however, promoting LDH layer growth and examining its morphologies without resorting to extreme pressure conditions remains a challenge. In the present study, we enhance LDH growth and morphology examination without extreme pressure conditions. By synthesizing Mg-Al LDH directly on plasma electrolytic oxidation (PEO)-treated Mg alloy surfaces and pores at ambient pressure, the direct synthesis was achieved feasibly without autoclave requirements, employing a suitable chelating agent. Additionally, enhancing corrosion resistance involved incorporating electron donor–acceptor compounds into a protective layer, with 8-Hydroxyquinoline-5-sulfonic acid (HQS) that helps in augmenting Mg alloy corrosion resistance through the combination of LDH ion-exchange ability and the organic layer. DFT simulations were used to explain the mutual interactions in the LDH system and provide a theoretical knowledge of the interfacial process at the molecular level. Full article