Search Results (71)

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

5-aminotetrazole (5AT)-based gas generators, particularly the 5AT/NaIO₄ system, have garnered interest for their high gas production and energy potential. This study investigates the impact of various metal oxides (MnO₂, Al₂O₃, TiO₂, CuO, Fe₂O₃, MgO, ZnO, and MoO₃) on the thermal decomposition and combustion performance of 5AT/NaIO₄. The REAL calculation program was used to infer reaction products, which indicated that the gas products are almost all harmless, with negligibly low percentages of NO and CO. Thermogravimetric analysis revealed that metal oxides, especially MoO₃, significantly advance the decomposition process above 400 °C, reducing the activation energy by 130 kJ/mol and lowering critical ignition and thermal explosion temperatures. Combustion performance tests and closed bomb tests confirmed MoO₃’s positive effect, accelerating reaction rates and enhancing decomposition efficiency. The system’s high Gibbs free energy indicates non-spontaneous reactions. These findings provide valuable insights for designing environmentally friendly gas generators, highlighting MoO₃’s potential as an effective catalyst. Full article

Global municipal solid waste (~2B tons/year) affects sustainability, as landfill and incineration face persistent leachate contamination, demanding effective management to advance water recycling and circular economies. Accelerated investigation of hybrid biocatalytic ozonation systems is imperative to enhance contaminant removal efficiency for stringent discharge compliance. This study investigates the catalytic ozonation effects of γ-Al₂O₃-based catalysts loaded with different metals (Cu, Mn, Zn, Y, Ce, Fe, Mg) on the biochemical effluent of landfill leachate. The catalysts were synthesized via a mixed method and subsequently characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Pseudo-second-order kinetics revealed active metal loading’s impact on adsorption capacity, with Cu/γ-Al₂O₃ and Mg/γ-Al₂O₃ achieving the highest Q_e (0.85). To elucidate differential degradation performance among the catalysts, the ozone/oxygen gas mixture was introduced at a controlled flow rate. Experimental results demonstrate that the Cu/γ-Al₂O₃ catalyst, exhibiting optimal comprehensive degradation performance, achieved COD and TOC removal efficiencies of 84.5% and 70.9%, respectively. UV–vis absorbance ratios revealed the following catalytic disparities: Mg/γ-Al₂O₃ achieved the highest aromatic compound removal efficiency; Ce/γ-Al₂O₃ excelled in macromolecular organics degradation. EEM-PARAFAC analysis revealed differential fluorophore removal: Cu/γ-Al₂O₃ exhibited broad efficacy across all five components, while Mg/γ-Al₂O₃ demonstrated optimal removal of C2 and C4, but showed limited efficacy toward C5. These findings provide important insights into selecting catalysts in practical engineering applications for landfill leachate treatment. This study aims to elucidate catalyst formulation-dependent degradation disparities, guiding water quality-specific catalyst selection to ultimately enhance catalytic ozonation efficiency. Full article

This study focuses on optimizing the Reverse Water Gas Shift (RWGS) reaction system using a membrane reactor to improve CO₂ conversion efficiency. A one-dimensional simulation model was developed using FlexPDE Professional Version 8.01/W64 software to analyze the performance of ZSM-5 membranes integrated with 0.5 wt% Ru-Cu/ZnO/Al₂O₃ catalysts. The results show that the membrane reactor significantly outperforms the conventional Packed Bed Reactor by achieving higher CO₂ conversion (0.61 vs. 0.99 with optimized parameters), especially at lower temperatures, due to its ability to remove H₂O and shift the reaction equilibrium selectively. Key operational parameters, including temperature, pressure, and sweep gas flow rate, were optimized to maximize membrane reactor performance. The ZSM-5 membrane showed strong H₂O selectivity, with an optimum operating temperature of around 400–600 °C. The problem is that many reactants permeate at higher temperatures. Subsequently, a Half-MPBR design was introduced. This design was able to overcome the reactant permeation problem and increase the conversion. The conversion ratios for PBR, MPBR, and Half-MPBR are 0.71, 0.75, and 0.86, respectively. This work highlights the potential of membrane reactors to overcome the thermodynamic limitations of RWGS reactions and provides valuable insights to advance Carbon Capture and Utilization technologies. Full article

The thermocatalytic hydrogenation of CO₂ to methanol offers a promising route for reducing greenhouse gas emissions (GHG) and producing valuable chemicals and fuels. In this study, copper–zinc bimetallic catalysts supported on a zirconium-based MOF-808 framework were synthesized via a facile deposition–precipitation method and compared to a conventional Cu/ZnO/Al₂O₃ (CZA) catalyst. MOF-808 was selected due to its high surface area and porous structure, which enhance metal dispersion. Characterization through X-ray diffraction (XRD) and N₂ physisorption showed significant changes in surface area and pore structure after Cu-Zn incorporation and calcination. The 50-CuZn MOF-808 catalyst achieved the best catalytic performance at 260 °C and 40 bar, demonstrating a high STY of 193.32 gMeOH·Kgcat⁻¹ h⁻¹ and a turnover frequency (TOF) of 47.44 h⁻¹, surpassing traditional CZA catalysts. The strong Cu-Zn-Zr interactions within the MOF-808 framework played a crucial role in promoting CO₂ activation and methanol formation. This study underscores the potential of MOF-808-supported Cu-Zn catalysts as viable alternatives to traditional systems for CO₂ hydrogenation to methanol. Full article

Layered double hydroxide (LDH)-derived mixed oxides offer a promising approach for CO₂ hydrogenation to light hydrocarbons. Herein, we explore the impact of various transition metals (X = Mn, Co, Cu, and Zn) incorporated into the M-Al or M-(Al+Fe) LDH structures, with the aim of exploring possible synergistic effects. Structural and compositional analyses reveal that an abundance of Fe over Al (Fe/Al ratio ~4) leads to the formation of mixed oxide crystalline phases attributed to CoFe₂O₄, CuFe₂O₄, and ZnFe₂O₄. Catalytic evaluation results demonstrate that the X-Al LDH-derived oxides exhibit high CO₂ conversion yet are selective to CH₄ or CO. In contrast, Fe incorporation shifts selectivity toward higher hydrocarbons. Specifically, the yield to higher hydrocarbons (C₂⁺) follows the order Ζn-Al-Fe > Cu-Al-Fe > Mn-Al-Fe > Co-Al-Fe >> Mn-Al, Co-Al, Zn-Al, Cu-Al, highlighting the pivotal role of Fe. Moreover, Zn-Al-Fe and Mn-Al-Fe catalysts have been shown to be the most selective towards light olefins. Zn-based systems also exhibit high thermal and structural stability with minimal coke formation, whereas Co-, Cu-, and Mn-based catalysts, when modified with Fe, experience increased carbon deposition or structural changes that may impact long-term stability. This work provides insights into the combined role of Fe and a second transition metal in LDHs for modulating catalytic activity, phase transformations, and stability, underscoring the need for further optimization to balance selectivity and catalyst durability in CO₂ hydrogenation applications. Full article

The CuZnOAl₂O₃ catalyst shows excellent activity and selectivity in the reaction of CO₂ hydrogenation to methanol as a consequence of its controllable physicochemical properties, which is expected to offer an efficient route to renewable energy. In this study, CuZnOAl₂O₃ catalysts are engineered by a special pretreatment, constructing a carbonate structure on the surface of the catalyst. Compared to the unmodified catalyst, the optimized catalyst (CZA-H-C1) not only exhibits an improved methanol selectivity of 62.5% (250 °C and 3 MPa) but also retains a minimal degree of deactivation of 9.57% over a 100 h period. By characterizing the catalysts with XRD, TEM, XPS and in situ DRIFTS spectroscopy, it was found that the surface carbonate species on Cu-based catalysts could significantly enhance the reaction and shield the active sites. This study offers theoretical insights and practical strategies for the rational design and optimization of high-performance heterogeneous catalysts. Full article

Compared with the traditional granular catalyst, a mesh-type structured CuZn-based catalyst prepared by the self-growth ordered nanoporous carrier γ-Al₂O₃/Al can adapt to the MSR reaction system with high flux, strong mass transfer and heat transfer, and can promote the practical process. In this study, the kinetic experiment using a CuZn-based structured catalyst was carried out, and a nonlinear single-rate PL model was established. Based on this, a 3D theoretical model of the MSR reformer was drawn in COMSOL Multiphysics. By changing the stacking form and mesh shape of the catalyst to construct the flow channel, the chemical reaction process, heat transfer process, and mass transfer process of the CuZn structured catalyst in the reformer under different flow channels are further simulated, which provides a theoretical basis for studying the enhancement of the MSR reaction and the development of a supporting micro hydrogen production system. Full article

Water can cause the growth and oxidation of Cu nanoparticles on the surface of Cu-based catalysts, leading to their deactivation. However, during methanol synthesis process from syngas on Cu-based catalysts, water is inevitably produced as a by-product due to the presence of CO₂. Therefore, enhancing the stability of Cu-based catalysts during the reaction, particularly in the presence of water, is crucial. In this study, Cu/ZnO/Al₂O₃ was first subjected to wet etching and then hydrophobically modified using the sol–gel method with methyltrimethoxysilane (MTMS) and the grafting method with 1H,1H,2H,2H-perfluoroalkyltriethoxysilanes (PFOTES) as modifiers. These modifications aimed to mitigate the impact of water on the catalyst and improve its stability. After modification, the catalysts exhibited excellent hydrophobicity and enhanced catalytic activity in the methanol synthesis process. The surface physical properties, composition, and thermal stability of the catalysts before and after hydrophobic modification were characterized by SEM, FT-IR, BET, XRD and TGA. Additionally, molecular dynamics simulations were employed to compare the diffusion behavior of water molecules on the catalyst surfaces before and after hydrophobic modification. The results indicated that the modified catalyst surface formed a micro/nano structure composed of nanosheets and nanosheet clusters, while the hydrophobic modification did not alter the structure of the catalyst. According to the results of simulations, the hydrophobic layers on the modified catalysts were able to expel water quickly from the surfaces and reduce the relative concentration of water molecules at the active sites, thereby improving the stability of the catalyst. Notably, the thermal stability and hydrophobicity of the PFOTES-modified catalyst were superior to those of the MTMS-modified catalyst, resulting in a more significant enhancement in catalyst stability, which aligned with the experimental results. Full article

In recent decades, with the rapid development of the inorganic synthesis and the increasing discharge of pollutants in the process of industrialization, hollow-structured metal oxides (HSMOs) have taken on a striking role in the field of environmental catalysis. This is all due to their unique structural characteristics compared to solid nanoparticles, such as high loading capacity, superior pore permeability, high specific surface area, abundant inner void space, and low density. Although the HSMOs with different morphologies have been reviewed and prospected in the aspect of synthesis strategies and potential applications, there has been no systematic review focusing on the structures and compositions design of HSMOs in the field of environmental catalysis so far. Therefore, this review will mainly focus on the component dependence and controllable structure of HSMOs in the catalytic elimination of different environmental pollutants, including the automobile and stationary source emissions, volatile organic compounds, greenhouse gases, ozone-depleting substances, and other potential pollutants. Moreover, we comprehensively reviewed the applications of the catalysts with hollow structure that are mainly composed of metal oxides such as CeO₂, MnO_x, CuO_x, Co₃O₄, ZrO₂, ZnO, Al₃O₄, In₂O₃, NiO, and Fe₃O₄ in automobile and stationary source emission control, volatile organic compounds emission control, and the conversion of greenhouse gases and ozone-depleting substances. The structure–activity relationship is also briefly discussed. Finally, further challenges and development trends of HSMO catalysts in environmental catalysis are also prospected. 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

The performance of bifunctional hybrid catalysts based on phosphotungstic acid (H₃PW₁₂O₄₀, HPW) supported on TiO₂ combined with a Cu-ZnO(Al) catalyst in the direct synthesis of dimethyl ether (DME) from syngas has been investigated. In this work, different types of TiO₂ were used as a support to study the effect of changes in the structure of the TiO₂ support on the acidity and dispersion of HPW. Various TiO₂ supports with different structural and surface characteristics have been studied and the results indicate that: (i) the crystallinity and crystallite size of the primary particles of the HPW units depend on the TiO₂ support; (ii) the pore size distribution of the TiO₂ support affects the surface segregation of the heteropolyacids; and (iii) changes in the supported HPW acid catalysts do not significantly alter the crystal structure of the CuO and ZnO phases after contact with CZA in bifunctional catalysts. The activity results indicate that the variation in the intrinsic activity of the Cu-ZnO_x centers in the bifunctional catalysts for direct DME synthesis is minimal due to the limited alteration of the crystal structure of the centers. Full article

Methanol synthesis from CO₂ can be made in the presence of a sorbent to increase the achievable yield. If the fresh sorbent is continuously fed to a fluidized bed and separated from the catalyst bed by segregation, a steady-state operation can be achieved. The objective of the present work is to provide insight on the suitable operating conditions for such a fluidized bed reactor system. For this, a conventional CuO/ZnO/Al₂O₃ was selected as the catalyst, and the SiOLITE^® zeolite was selected as the sorbent. Different particle sizes were used to be tested in various proportions to perform the fluidized bed segregation study. The fluid dynamics and segregation of the catalyst–sorbent binary mixtures were the most critical points in the development of this proof of concept. A good bed segregation with a mixing index of 0.31 was achieved. This fact favors the correct operation of the system with the continuous addition of adsorbent, which had hardly any catalyst losses during the tests carried out, achieving a loss of 0.005 g/min under optimal conditions. Continuous feeding and removal of sorbent with a low loss of catalyst was observed. Reactor simulations with MATLAB provided promising results, indicating that the addition of sorbent considerably improves the methanol yield under some operating conditions. This makes it more viable for industrial scaling, since it allows us to considerably reduce the pressure used in the methanol synthesis process or to increase the yield per step, reducing the recirculation of unconverted reactants. Full article

The quest for efficient catalysts based on abundant elements that can promote the selective CO₂ hydrogenation to green methanol still continues. Most of the reported catalysts are based on Cu/ZnO supported in inorganic oxides, with not much progress with respect to the benchmark Cu/ZnO/Al₂O₃ catalyst. The use of carbon supports for Cu/ZnO particles is much less explored in spite of the favorable strong metal support interaction that these doped carbons can establish. This manuscript reports the preparation of a series of Cu-ZnO@(N)C samples consisting of Cu/ZnO particles embedded within a N-doped graphitic carbon with a wide range of Cu/Zn atomic ratio. The preparation procedure relies on the transformation of chitosan, a biomass waste, into N-doped graphitic carbon by pyrolysis, which establishes a strong interaction with Cu nanoparticles (NPs) formed simultaneously by Cu²⁺ salt reduction during the graphitization. Zn²⁺ ions are subsequently added to the Cu–graphene material by impregnation. All the Cu/ZnO@(N)C samples promote methanol formation in the CO₂ hydrogenation at temperatures from 200 to 300 °C, with the temperature increasing CO₂ conversion and decreasing methanol selectivity. The best performing Cu-ZnO@(N)C sample achieves at 300 °C a CO₂ conversion of 23% and a methanol selectivity of 21% that is among the highest reported, particularly for a carbon-based support. DFT calculations indicate the role of pyridinic N doping atoms stabilizing the Cu/ZnO NPs and supporting the formate pathway as the most likely reaction mechanism. Full article

This paper presents the preparation of heterogeneous catalysts for the direct hydrogenation process of CO₂ to methanol. The development of the modern chemical industry is inextricably linked to the use of catalytic processes. As a result, currently over 80% of new technologies introduced in the chemical industry incorporate catalytic processes. Since the basic factor of catalytic processes is the catalysts, the studies for the deepening of the knowledge regarding the nature of the action of the catalysts, for the development of new catalysts and catalytic systems, as well as for their improvement, represent a research priority of a fundamental or applied nature. The Cu/ZnO/Al₂O₃ catalyst for the synthesis of green methanol, using precursors of an inorganic (copper nitrate, denoted by Cu/ZnO/Al₂O₃-1) and organic (copper acetate, denoted by Cu/ZnO/Al₂O₃-2) nature, are obtained by chemical impregnation that includes two stages: preparation and one of calcination. The preparation methods and conditions, as well as the physico-chemical properties of the catalyst precursor, play a major role in the behavior of the catalysts. The prepared catalysts were characterized using atomic adsorption analysis, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, specific surface area and pore size analyses, adsorption, and the chemisorption of vapor (BET). Full article