Effect of Support in Heterogeneous Ruthenium Catalysts Used for the Selective Aerobic Oxidation of HMF in Water

Au-Based Bimetallic Catalysts for Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid under Base-Free Reaction Conditions

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) plays a pivotal role in the synthesis of renewable, biodegradable plastics and sustainable chemicals. Although supported gold nanoclusters (NCs) exhibit significant potential in this process, they often suffer from low selectivity. To address this challenge, a series of gold-M (M means Ni, Fe, Cu, and Pd) bimetallic NCs catalysts were designed and synthesized to facilitate the selective oxidation of HMF to FDCA. Our findings indicate that the introduction of doped metals, particularly Ni and Pd, not only improves the reaction rates for HMF tandem oxidation but also promotes high yields of FDCA. Various characterizations techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (CO-DRIFTS), and temperature-programmed desorption of oxygen (O2-TPD), were employed to scrutinize the structural and electronic properties of the prepared catalysts. Notably, an electronic effect was observed across the Au-based bimetallic catalysts, facilitating the activation of reactant molecules and enhancing the catalytic performance. This study provides valuable insights into the alloy effects, aiding in the development of highly efficient Au-based bimetallic catalysts for biomass conversions.

A simple and efficient synthesis of 5-hydroxymethylfurfural from carbohydrates using acidic ionic liquid grafted on silica gel

The catalytic application of 3-(4-sulfobutyl)-1H-imidazole-3-ium chloride immobilized on activated silica gel (SiO2-Imi-SO3H) for the production of 5-hydroxymethylfurfural is described here for the first time. This material was synthesized using a three-step method involving the grafting of chloropropyl groups onto activated silica gel, the substitution of zwitterions, and the acidification of zwitterions to form silica-supported ionic liquid. The successful immobilization of the IL on silica gel was confirmed through energy-dispersive X-ray (EDX) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and elemental mapping. SiO2-Imi-SO3H-2 demonstrated good catalytic activity and recycling ability in fructose dehydration to 5-HMF. Several conditions for reaction were investigated, and an excellent 5-HMF yield (94.1%) was obtained after 4 h at 160 °C in dimethyl sulfoxide (DMSO) from fructose. Furthermore, a mechanism was proposed, the catalyst's reusability was investigated, and the catalyst was applied for the conversion of glucose to 5-HMF with other metal salts.

Enhanced Basicity of MnOx-Supported Ru for the Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

The present study focused on developing a stable basic MnOx support for Ru (RuMn) for the efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water in the absence of an external base. A series of MnOx supports, synthesized via hydrothermal approach using urea as precipitant, was prepared by thermal treatment at various temperatures (300-800 °C) before doping with Ru. The RuMn-2 (1 wt % Ru, MnOx calcined at 400 °C) possessed a large number of basic sites (1.72 mmol g^-1 ) based on CO₂ temperature-programmed desorption analysis, affording an FDCA yield of 87 % with a turnover frequency of 22 h^-1 . Transmission electron microscopy energy-dispersive X-ray spectroscopy elemental mapping of RuMn-2 showed a high dispersion of Ru over the surface of MnOx, contributing to the efficient HMF oxidation. Moreover, X-ray diffraction, X-ray photoelectron spectroscopy, and H₂ temperature-programmed reduction indicated that the predominant MnO₂ phase (ϵ-MnO₂ ) played a vital role in HMF oxidation. RuMn-2 was recyclable for up to four runs without significant loss in the activity and retained its structural integrity.

Metal vs. Metal-Free Catalysts for Oxidation of 5-Hydroxymethylfurfural and Levoglucosenone to Biosourced Chemicals

Lignocellulosic feedstocks, such as forestry biomass and agricultural crop residues, can be utilized to generate biofuels and biochemicals. Converting these organic waste materials into biochemicals is widely regarded as a remedial approach to develop a sustainable, clean, and green energy source. Nevertheless, are these methods sustainable and clean? Prior studies have shown that most such conversions use metals - including heavy metals or noble metals - as catalysts. In addition to the fact that many metals (e. g., aluminum, cobalt, titanium, platinum) have been listed as critical minerals, these methods suffer from high cost, deactivation, and leakage problems and the release of toxic wastes. This Review summarizes catalytic methods using metal and metal-free catalysts for the oxidation of the platform molecules 5-hydroxymethylfurfural and levoglucosenone and demonstrates the potential and effectiveness of metal-free catalysts.

Challenges of Green Production of 2,5-Furandicarboxylic Acid from Bio-Derived 5-Hydroxymethylfurfural: Overcoming Deactivation by Concomitant Amino Acids

The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is highly attractive as FDCA is considered as substitute for the petrochemically derived terephthalic acid. There are only few reports on the direct use of unrefined HMF solutions from biomass resources and the influence of remaining constituents on the catalytic processes. In this work, the oxidation of HMF in a solution as obtained from hydrolysis and dehydration of saccharides in chicory roots was investigated without intermediate purification steps. The amount of base added to the solution was critical to increase the FDCA yield. Catalyst deactivation occurred and was attributed to poisoning by amino acids from the bio-source. A strong influence of amino acids on the catalytic activity was found for all supported Au, Pt, Pd, and Ru catalysts. A supported AuPd(2 : 1)/C alloy catalyst exhibited both superior catalytic activity and higher stability against deactivation by the critical amino acids.

Aerobic Oxidation of 5-Hydroxymethylfurfural over Ag Nanoparticle Catalysts Stabilized by Polyvinylpyrrolidone with Different Molecular Weights

The metal–support interaction (MSI) has a remarkable effect on the catalytic properties, but how to precisely modulate its degree remains a huge challenge. Herein, polyvinylpyrrolidone (PVP) with three different molecular weights (MWs) (24, 58, and 130 kDa) was used as a capping agent to fabricate Ag nanoparticles (NPs) supported on ZrO₂. The physiochemical properties of the catalysts were characterized by X-ray diffraction (XRD), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS), and Fourier transform infrared (FT-IR) techniques. The impacts of MSI on the catalytic activity and reaction kinetics for aerobic oxidation of 5-hydroxymethylfurfural (HMF) were investigated. The results showed that the introduction of PVP with various MWs could efficiently tailor the interfacial interactions and charge transfers (CT) among PVP, the support, and Ag NPs, thereby affecting the oxidation activity of HMF. The turnover number (TON) for HMF oxidation decreases in the order of unsupported colloidal Ag clusters > Ag/ZrO₂ (58,000) > Ag/ZrO₂ (130,000) > Ag/ZrO₂ (24,000) > Ag/ZrO₂. The reason for this large difference in the catalytic activity for HMF oxidation is that various MWs of PVP result in a change of MSI, thereby facilitating CT from PVP to Ag metal sites. This study offers a new strategy for modulating MSI by varying the MW of capping agents, thereby tuning the catalytic properties in the oxidation of HMF.

Nickel(ii)-modified covalent-organic framework film for electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF)

Nickel(ii)-modified covalent-organic framework (COF) film exhibited electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) with ∼96% conversion.

Oxidation of lignocellulosic platform molecules to value-added chemicals using heterogeneous catalytic technologies

This minireview gives an overview about heterogeneous catalytic technologies for the oxidation of key platform molecules (glucose, 5-hydroxymethylfurfural, furfural and levulinic acid) into valuable chemicals.

Ru-Catalyzed Oxidative Cleavage of Guaiacyl Glycerol--Guaiacyl Ether-a Representative -O-4 Lignin Model Compound

The introduction of efficient and selective catalytic methods for aerobic oxidation of lignin and lignin model compounds to aromatics can extend the role of lignin applications in biorefineries. The current study focussed on the catalytic oxidative transformation of guaiacyl glycerol--guaiacyl ether (GGGE)–a -O-4 lignin model compound to produce basic aromatic compounds (guaiacol, vanillin and vanillic acid) using metal-supported catalysts. Ru/Al2O3, prepared with ruthenium(IV) oxide hydrate, showed the highest yields of the desired products (60%) in acetonitrile in a batch reactor at 160 C and 5-bar of 20% oxygen in argon. Alternative catalysts containing other transition metals (Ag, Fe, Mn, Co and Cu) supported on alumina, and ruthenium catalysts based on alternative supports (silica, spinel, HY zeolite and zirconia) gave significantly lower activities compared to Ru/Al2O3 at identical reaction conditions. Moreover, the Ru/Al2O3 catalyst was successfully reused in five consecutive reaction runs with only a minor decrease in catalytic performance.

Selective oxidation of glycerol to tartronic acid over Pt/N-doped mesoporous carbon with extra framework magnesium catalysts under base-free conditions

N-Doped mesoporous carbons (NMCs) with extra framework magnesium were prepared by a one-pot method and used as supports for Pt catalysts. The surface basicity of NMC improved in the presence of extra framework magnesium (e.g., -Ph-O-Mg), meanwhile, the electron density of Pt was enriched by the electron transfer from graphitic N in NMC to Pt. As a result, the catalytic activity of the supported Pt catalyst was improved to be able to selectively oxidize glycerol (GLY) to tartronic acid (TA) under base-free conditions.

An Eco-Friendly Method to Get a Bio-Based Dicarboxylic Acid Monomer 2,5-Furandicarboxylic Acid and Its Application in the Synthesis of Poly(hexylene 2,5-furandicarboxylate) (PHF)

Recently, we have developed an eco-friendly method for the preparation of a renewable dicarboxylic acid 2,5-furandicarboxylic acid (FDCA) from biomass-based 5-hydroxymethylfrufural (HMF). In the present work, we optimized our reported method, which used phosphate buffer and Fe(OH)₃ as the stabilizer to improve the stability of potassium ferrate, then got a purified FDCA (up to 99%) in high yield (91.7 wt %) under mild conditions (25 °C, 15 min, air atmosphere). Subsequently, the obtained FDCA, along with 1,6-hexanediol (HDO), which was also made from HMF, were used as monomers for the synthesis of poly(hexylene 2,5-furandicarboxylate) (PHF) via direct esterification, and triphenyl phosphite was used as the antioxidant to alleviate the discoloration problem during the esterification. The intrinsic viscosity, mechanical properties, molecular structure, thermal properties, and degradability of the PHFs were measured or characterized by Koehler viscometer, universal tensile tester, Nuclear Magnetic Resonance (NMR), Fourier-transform Infrared (FTIR), X-ray diffraction (XRD), Differential Scanning Calorimeter (DSC), Derivative Thermogravimetry (DTG), Scanning Electron Microscope (SEM), and weight loss method. The experimental evidence clearly showed that the furan-aromatic polyesters prepared from biomass-based HMF are viable alternatives to the petrochemical benzene-aromatic polyesters, they can serve as low-melting heat bondable fiber, high gas-barrier packaging material, as well as specialty material for engineering applications.

Selective Oxidation of HMF via Catalytic and Photocatalytic Processes Using Metal-Supported Catalysts

In this study, 5-hydroxymethylfurfural (HMF) oxidation was carried out via both the catalytic and the photocatalytic approach. Special attention was devoted to the preparation of the TiO₂-based catalysts, since this oxide has been widely used for catalytic and photocatalytic application in alcohol oxidation reactions. Thus, in the catalytic process, the colloidal heterocoagulation of very stable sols, followed by the spray-freeze-drying (SFD) approach, was successfully applied for the preparation of nanostructured porous TiO₂-SiO₂ mixed-oxides with high surface areas. The versatility of the process made it possible to encapsulate Pt particles and use this material in the liquid-phase oxidation of HMF. The photocatalytic activity of a commercial titania and a homemade oxide prepared with the microemulsion technique was then compared. The influence of gold, base addition, and oxygen content on product distribution in the photocatalytic process was evaluated.

Pt Nanoparticles Supported on Nitrogen-Doped-Carbon-Decorated CeO2 for Base-Free Aerobic Oxidation of 5-Hydroxymethylfurfural

Currently, the base-free aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to produce 2,5-furandicarboxylic acid (FDCA) is attracting intense interest due to its prospects for the green, sustainable, and promising production of biomass-based aromatic polymers. Herein, we have developed a new Pt catalyst supported on nitrogen-doped-carbon-decorated CeO₂ (NC-CeO₂ ) for the aerobic oxidation of HMF in water without the addition of any homogeneous base. It was demonstrated that the small-sized Pt particles could be well dispersed on the surface of the hybrid NC-CeO₂ support, and the activity of the supported Pt catalyst depended strongly on the surface structure and properties of the catalysts. The as-fabricated Pt/NC-CeO₂ catalyst, with abundant surface defects, enhanced basicity, and favorable electron-deficient metallic Pt species, enabled an almost 100 % yield of FDCA in water with molecular oxygen (0.4 MPa) at 110 °C for 8 h without the addition of any homogeneous base, which is indicative of exceptional catalytic performance. Furthermore, this Pt/NC-CeO₂ catalyst also showed good stability and reusability owing to strong metal-support interactions. An understanding of the role of surface structural defects and basicity of the hybrid NC-CeO₂ support provides a basis for the rational design of high-performance and stable supported metal catalysts with practical applications in various transformations of biomass-derived compounds.

Ruthenium Supported on High-Surface-Area Zirconia as an Efficient Catalyst for the Base-Free Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Several ZrO₂ -supported ruthenium catalysts were prepared and utilized in the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) under base-free conditions. Full conversion of HMF and almost perfect selectivity towards FDCA (97 %) were achieved after 16 h by using pure O₂ as an oxidant and water as a solvent. The catalytic tests show that the size of the Ru particles is crucial for the catalytic performance and that the utilization of high-surface-area ZrO₂ leads to formation of very small Ru particles. Superior activity was obtained for catalysts based on ZrO₂ that had been synthesized by a surface-casting method and has high surface areas up to 256 m²  g^-1 . In addition to good activity and selectivity, these catalysts show also high stability and constant activity upon recycling, confirming the suitability of Ru/ZrO₂ in the base-free oxidation of HMF.

Toward biomass-derived renewable plastics: Production of 2,5-furandicarboxylic acid from fructose

We report a process for converting fructose, at a high concentration (15 weight %), to 2,5-furandicarboxylic acid (FDCA), a monomer used in the production of polyethylene furanoate, a renewable plastic. In our process, fructose is dehydrated to hydroxymethylfurfural (HMF) at high yields (70%) using a γ-valerolactone (GVL)/H₂O solvent system. HMF is subsequently oxidized to FDCA over a Pt/C catalyst with 93% yield. The advantage of our system is the higher solubility of FDCA in GVL/H₂O, which allows oxidation at high concentrations using a heterogeneous catalyst that eliminates the need for a homogeneous base. In addition, FDCA can be separated from the GVL/H₂O solvent system by crystallization to obtain >99% pure FDCA. Our process eliminates the use of corrosive acids, because FDCA is an effective catalyst for fructose dehydration, leading to improved economic and environmental impact of the process. Our techno-economic model indicates that the overall process is economically competitive with current terephthalic acid processes.

Catalytic Organic Reactions in Water toward Sustainable Society

Traditional organic synthesis relies heavily on organic solvents for a multitude of tasks, including dissolving the components and facilitating chemical reactions, because many reagents and reactive species are incompatible or immiscible with water. Given that they are used in vast quantities as compared to reactants, solvents have been the focus of environmental concerns. Along with reducing the environmental impact of organic synthesis, the use of water as a reaction medium also benefits chemical processes by simplifying operations, allowing mild reaction conditions, and sometimes delivering unforeseen reactivities and selectivities. After the "watershed" in organic synthesis revealed the importance of water, the development of water-compatible catalysts has flourished, triggering a quantum leap in water-centered organic synthesis. Given that organic compounds are typically practically insoluble in water, simple extractive workup can readily separate a water-soluble homogeneous catalyst as an aqueous solution from a product that is soluble in organic solvents. In contrast, the use of heterogeneous catalysts facilitates catalyst recycling by allowing simple centrifugation and filtration methods to be used. This Review addresses advances over the past decade in catalytic reactions using water as a reaction medium.

Direct catalytic effect of nitrogen functional groups exposed on graphenic materials when acting cooperatively with Ru nanoparticles

Various carbon materials (including doped graphenic materials) have been used as supports of Ru nanoparticles to determine their catalytic effects in the base-free aqueous-phase oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid.

Base-Free Aqueous-Phase Oxidation of 5-Hydroxymethylfurfural over Ruthenium Catalysts Supported on Covalent Triazine Frameworks

The base-free aqueous-phase oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxilic acid (FDCA) was performed at 140 °C and 20 bar of synthetic air as the oxidant. Ru clusters supported on covalent triazine frameworks (CTFs) enabled superior conversion (99.9%) and FDCA yields in comparison to other support materials such as activated carbon and γ-Al2O3 after only 1 h. The properties of the CTFs such as pore volume, specific surface area, and polarity could be tuned by using different monomers. These material properties influence the catalytic activity of Ru/CTF significantly as mesoporous CTFs showed superior activity compared to microporous materials, whereas high polarities provide further beneficial effects. The recyclability of the prepared Ru/CTF catalysts was comparable to that of Ru/C at high conversions and product yields. Nevertheless, minor deactivation in five successive recycling experiments was observed.

Recent progress in the development of solid catalysts for biomass conversion into high value-added chemicals

In recent decades, the substitution of non-renewable fossil resources by renewable biomass as a sustainable feedstock has been extensively investigated for the manufacture of high value-added products such as biofuels, commodity chemicals, and new bio-based materials such as bioplastics. Numerous solid catalyst systems for the effective conversion of biomass feedstocks into value-added chemicals and fuels have been developed. Solid catalysts are classified into four main groups with respect to their structures and substrate activation properties: (a) micro- and mesoporous materials, (b) metal oxides, (c) supported metal catalysts, and (d) sulfonated polymers. This review article focuses on the activation of substrates and/or reagents on the basis of groups (a)-(d), and the corresponding reaction mechanisms. In addition, recent progress in chemocatalytic processes for the production of five industrially important products (5-hydroxymethylfurfural, lactic acid, glyceraldehyde, 1,3-dihydroxyacetone, and furan-2,5-dicarboxylic acid) as bio-based plastic monomers and their intermediates is comprehensively summarized.

Selective aerobic oxidation of 5-HMF into 2,5-furandicarboxylic acid with Pt catalysts supported on TiO2 - and ZrO2 -based supports

Pt catalysts prepared over different metallic oxide supports were investigated in the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in alkaline aqueous solutions with air, to examine the combined effect of the support and base addition. The base (nature and amount) played a significant role in the degradation or oxidation of HMF. Increasing amounts of the weak NaHCO3 base improved significantly the overall catalytic activity of Pt/TiO2 and Pt/ZrO2 by accelerating the oxidation steps, especially for the aldehyde group. This was highlighted by a proposed kinetic model that gave very good concentration-time fittings. Moreover, the promotion of the catalyst with bismuth yielded a PtBi/TiO2 catalytic system with improved activity and stability. Y2 O3  and La2 O3 ZrO2 -supported catalysts exhibited lower activity than Pt/ZrO2 , which suggests no cooperative effect of the weakly basic properties introduced and the homogeneous base. Quantitative oxidation of HMF (0.1 M) and high yields of FDCA (>99 %) were obtained in less than 5 h by using an HMF/Pt molar ratio of 100 and Na2 CO3 as a weak base over PtBi/TiO2 (Bi/Pt=0.22).

Immobilization of imidazolium ionic liquids on hydrotalcites using silane linkers: retardation of memory effect

We report a new covalent surface immobilization of silane-modified imidazolium ionic liquids on hydrotalcite-like materials (HTs) and provide detailed characterization of the resulting surface chemistry using PXRD, CP-MAS, TGA and FT-IR.