One step oxidative esterification of methacrolein with methanol over Au-CeO2/γ-Al2O3 catalysts

Preparation of Hydrophobic Au Catalyst and Application in One-Step Oxidative Esterification of Methacrolein to Methyl Methacrylate

The water produced during the oxidative esterification reaction occupies the active sites and reduces the activity of the catalyst. In order to reduce the influence of water on the reaction system, a hydrophobic catalyst was prepared for the one-step oxidative esterification of methylacrolein (MAL) and methanol. The catalyst was synthesized by loading the active component Au onto ZnO using the deposition-precipitation method, followed by constructing the silicon shell on Au/ZnO using tetraethoxysilane (TEOS) to introduce hydrophobic groups. Trimethylchlorosilane (TMCS) was used as a hydrophobic modification reagent to prepare hydrophobic catalysts, which exhibited a water droplet contact angle of 111.2°. At a temperature of 80 °C, the hydrophobic catalyst achieved a high MMA selectivity of over 95%. The samples were characterized using XRD, N2 adsorption, ICP, SEM, TEM, UV-vis, FT-IR, XPS, and water droplet contact angle measurements. Kinetic analysis revealed an activation energy of 22.44 kJ/mol for the hydrophobic catalyst.

Ionic liquid-mediated hexagonally porous ZnO nanocrystal-supported Au catalysts: highly stable materials for aldehyde oxidative esterification

Ionic liquid-mediated hexagonally porous ZnO nanocrystal-supported Au catalysts: highly efficient and stable materials for oxidative esterification of methylacrolein.

Controlled Metal-Support Interactions in Au/CeO2-Mg(OH)2 Catalysts Activating the Direct Oxidative Esterification of Methacrolein with Methanol to Methyl Methacrylate

The strong metal-support interaction (SMSI) between the three components in Au/CeO₂-Mg(OH)₂ can be controlled by the relative composition of CeO₂ and Mg(OH)₂ and by the calcination temperature for the direct oxidative esterification of methacrolein (MACR) with methanol to methyl methacrylate (MMA). The composition ratio of CeO₂ and Mg(OH)₂ in the catalyst affects the catalytic performance dramatically. An Au/CeO₂ catalyst without Mg(OH)₂ esterified MACR to a hemiacetal species without MMA production, which confirmed that Mg(OH)₂ is a prerequisite for successful oxidative esterification. When Au/Mg(OH)₂ was used without CeO₂, the direct oxidative esterification of MACR was successful and produced MMA, the desired product. However, the MMA selectivity was much lower (72.5%) than that with Au/CeO₂-Mg(OH)₂ catalysts, which have an MMA selectivity of 93.9-99.8%, depending on the relative composition of CeO₂ and Mg(OH)₂. In addition, depending on the calcination temperature, the crystallinity of the CeO₂-Mg(OH)₂ and the surface acidity/basicity can be remarkably changed. Consequently, the Au-nanoparticle-supported catalysts exhibited different MACR conversions and MMA selectivities. The catalytic behavior can be explained by the different metal-support interactions between the three components depending on the composition ratio of CeO₂ and Mg(OH)₂ and the calcination temperature. These differences were evidenced by X-ray diffraction, X-ray photoelectron spectroscopy, and CO₂ temperature-programmed desorption. The present study provides new insights into the design of SMSI-induced supported metal catalysts for the development of multifunctional heterogeneous catalysts.

Zn Promoted Mg-Al Mixed Oxides-Supported Gold Nanoclusters for Direct Oxidative Esterification of Aldehyde to Ester

The synthesis of ester compounds is one of the most important chemical processes. In this work, Zn-Mg-Al mixed oxides with different Zn²⁺/Mg²⁺ molar ratios were prepared via co-precipitation method and supported gold nanoclusters to study the direct oxidative esterification of aldehyde and alcohol in the presence of molecular oxygen. Various characterization techniques such as N₂-physical adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and CO₂ temperature programmed desorption (TPD) were utilized to analyze the structural and electronic properties. Based on the results, the presence of small amounts of Zn²⁺ ions (~5 wt.%) provoked a remarkable modification of the binary Mg-Al system, which enhanced the interaction between gold with the support and reduced the particle size of gold. For oxidative esterification reaction, the Au₂₅/Zn_0.05MgAl-400 catalyst showed the best performance, with the highest turnover frequency (TOF) of 1933 h⁻¹. The active center was believed to be located at the interface between metallic gold with the support, where basic sites contribute a lot to transformation of the substrate.

A Multifunctional Au/CeO2-Mg(OH)2 Catalyst for One-Pot Aerobic Oxidative Esterification of Aldehydes with Alcohols to Alkyl Esters

Au nanoparticles bound to crystalline CeO2 nanograins that were dispersed on the nanoplate-like Mg(OH)2, denoted as Au/CeO2-Mg(OH)2, were developed as the highly active and selective multifunctional heterogeneous catalyst for direct oxidative esterification of aldehydes with alcohols to produce alkyl esters under base-free aerobic conditions using oxygen or air as the green oxidants. Au/CeO2-Mg(OH)2 converted 93.3% of methacrylaldehyde (MACR) to methyl methacrylate (MMA, monomer of poly(methyl methacrylate)) with 98.2% selectivity within 1 h, and was repeatedly used over eight recycle runs without regeneration. The catalyst was extensively applied to other aldehydes and alcohols to produce desirable alkyl esters. Comprehensive characterization analyses revealed that the strong metal-support interaction (SMSI) among the three catalytic components (Au, CeO2, and Mg(OH)2), and the proximity and strong contact between Au/CeO2 and the Mg(OH)2 surface were prominent factors that accelerated the reaction toward a desirable oxidative esterification pathway. During the reaction, MACR was adsorbed on the surface of CeO2-Mg(OH)2, upon which methanol was simultaneously activated for esterifying the adsorbed MACR. Hemiacetal-form intermediate species were subsequently produced and oxidized to MMA on the surface of the electron-rich Au nanoparticles bound to partially reduced CeO2-x with electron-donating properties. The present study provides new insights into the design of SMSI-induced supported-metal-nanoparticles for the development of novel, multifunctional, and heterogeneous catalysts.

Efficient Synthesis of Methyl Methacrylate by One Step Oxidative Esterification over Zn-Al-Mixed Oxides Supported Gold Nanocatalysts

Methyl methacrylate (MMA) is an important monomer in fine chemicals. The synthesis of MMA by one-step oxidative esterification from methacrolein with methanol over a heterogeneous catalyst with high activity, selectivity and stability is highly desirable. Herein, Zn-Al-hydrotalcites (HTs)-supported atomically precise Au25 nanoclusters with different molar ratios of Zn2+/Al3+ were prepared and used as the precursors for this reaction. They exhibited good performances in comparison with the gold catalysts prepared by the deposition precipitation method. The structural and electronic properties were evaluated by various characterization technologies, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of CO adsorption, X-ray photoelectron spectroscopy (XPS), and CO2 temperature-programmed desorption (TPD). The combined characterization results suggested that the adsorption property of gold and the basicity of the catalyst contributes to their high activities. Substrates extended experiments and stability tests implied the potential application of Zn-Al-mixed oxides supported gold catalysts, which paves a new way for supported gold catalyst in the one-step oxidation esterification reaction.

Waste to Catalyst: Synthesis of Catalysts from Sewage Sludge of the Mining, Steel, and Petroleum Industries

The generation of sewage sludge presents a problem for several manufacturing companies as it results from industrial processes or effluent treatment systems. The treatment of this type of waste requires high economic investment, for this reason, it is necessary to find alternatives to recover the valuable materials of the sludges. In this study, metal catalysts were synthesized using waste sludge from the steel, mining, and hydrocarbon industries. The waste sludge was subjected to thermal treatments for the removal of organic content and the reduction of metals with hydrogen current to activate their catalytic properties. The sludge and synthesized catalysts were analyzed to determine their physical, chemical, thermoenergetic, and catalytic properties. Catalytic activity was evaluated using CO chemisorption and by thermal–catalytic decomposition of crude oil. The best conditions for synthesizing the catalysts were a calcination temperature between 300 and 500 °C and a reduction temperature between 300 and 900 °C. The catalysts presented a specific surface between 2.33 and 16.78 m2/g. The catalytic material had a heat capacity between 0.7 and 1.2 kJ/kg∙K. The synthesized materials presented catalytic activity comparable to that of commercial catalysts. With this recovery technique, the industrial waste can be valorized, obtaining catalyst derived from the sludges and promoting the circular economy of manufacturing companies.