Ultra-Dense Supported Ruthenium Oxide Clusters via Directed Ion Exchange for Efficient Valorization of 5-Hydroxymethylfurfural

Maximizing the loadings of active centers without aggregation for a supported catalyst is a grand challenge but essential for achieving high gravimetric catalytic activity, especially toward multi-step reactions. The oxidation of 5-hydroxymethylfurfural (HMF), a key biomass-derived platform molecule, into 2,5-furandicarboxylic acid (FDCA), a promising alternative to polyester monomer, is such a multi-step reaction that involves 6 proton and electron transfers. This process often demands strong alkaline environment but also suffers from the alkali-driven polymerization side-reaction. Meanwhile, neutral media ameliorates the polymerization, but lacks efficient catalyst toward deep oxidation. Herein, we devised a strategy of creating ultra-dense supported Ru oxide clusters via directed ion exchange in a Co hydroxyanion (CoHA) support material. Pyrimidine ligands were first incorporated into the CoHA interlayers, and the subsequent evacuation of pyrimidines created porous channels for the directed ion exchange with the built-in anions in CoHA, which allowed the dense and mono-disperse functionalization of RuCl6 2- anions and their resulting Ru oxide clusters. These ultra-dense Ru oxide clusters not only enable high HMF electrooxidation currents under neutral conditions but also create microscopic channels in-between the clusters for the expedited re-adsorption and oxidation of intermediates toward highly oxidized product, such as 5-formyl-2-furoic acid (FFCA) and FDCA. A two-stage HMF oxidation process, consisting of ambient conversion of HMF into FFCA and FFCA oxidation into FDCA under 60 °C, was eventually developed to first achieve a high FDCA yield of 92.1 % under neutral media with significantly reduced polymerization.

Amorphous-crystalline heterostructure: Efficient catalyst for biomass oxidation coupled with hydrogen evolution

The development of catalysts with high activity, selectivity, and stability is critical for biomass upgrading coupled with hydrogen evolution. In this study, we present a simple method for fabricating crystalline-amorphous phase heterostructures using the etching effect of the acidic medium generated during cobalt salt hydrolysis, resulting in the formation of NiCo(OH)x-modified Ni/NiMoO4 nanosheets electrode (NiCo(OH)x/Ni/NiMoO4/NF). The nanosheets array formed during the synthesis process enlarges the surface area of the prepared catalyst, which facilitates the exposure of electrochemically active sites and improves mass transfer. Unexpectedly, the strong coupling interactions between the amorphous-crystalline heterointerface optimize the adsorption of reaction molecules and the corresponding charge transfer process, consequently boosting the catalytic activity for the 5-hydroxymethylfurfural oxidation reaction (HMFOR) and hydrogen evolution reaction (HER). Specifically, NiCo(OH)x/Ni/NiMoO4/NF catalyst requires only 1.34 V to obtain a current density of 10 mA cm-2 for HMFOR-coupled H2 evolution, and operates stably for 13 consecutive cycles with good product selectivity. This work thus provides insights into the design of efficient and robust catalysts for HMFOR-assisted H2 evolution.

Interfacial coupling effect promotes selective electrocatalytic oxidation of 5-hydroxymethylfurfural into the value-added products under neutral conditions

Owing to the sluggish reaction kinetics, it is a promising yet challenging task to achieve the adequate electricity-driven catalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) in neutral conditions. Herein, we have prepared an elelctrocatalyst with interfacial coupling effect through in-situ growth of Cu phthalocyanine (CuPc) on Co3O4 spinel (Co3O4/CuPc), which constructs an effective electrocatalytic system of HMF oxidation with overall oxidation value-added products yield and total Faraday efficiency up to 80% and 70%, respectively. The interfacial coupling effect between CuPc and Co3O4 spinel improve catalytic activity by effectively boosting the interfacial charge transfer and reducing the formation energy of key *C6H3O4 in the catalytic pathway according to the in situ Raman spectroscopy and DFT simulation. This work illustrates the significance of interfacial coupling effect for developing highly efficient electrocatalysts applied for neutral system of biomass oxidation.

Effect of K+ and Ca2+ Cations on Structural Manganese(IV) Oxide for the Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

The promising influences of K+ and Ca2+ ions in the development of effective MnO2 for the selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid (FDCA) were studied for the catalytic performance under a high-pressure reaction of aqueous O2 (0.5 MPa) in a basic system. Various oxidation states of manganese in MnO2 were able to accelerate the oxidation of 5-formyl-2-furancarboxylic acid to FDCA in the rate-determining step. The results were in good agreement that Ca2+ played a key role in the highest FDCA yield up to 85% due to the associated cations on the local coordination to enhance the high surface area and the electronic effect on the manganese ion. Both K-MnO2 and Ca-MnO2 catalysts showed excellent catalytic activities without a significant change in the efficiency in the reusability experiments.

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.

Research Progress of Highly Efficient Noble Metal Catalysts for the Oxidation of 5-Hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) is considered to be one of the most pivotal multifunctional biomass platform chemicals. This Review discusses recent advances in catalytic oxidation of HMF towards high-value products. The reaction mechanism of different noble metals and the path of HMF oxidation to high-value products have been deeply investigated in the noble metal catalytic system. The reaction mechanisms of different noble metals and HMF conversion paths were compared in detail. Moreover, the factors affecting the performance of different noble metal catalysts were summarized. Finally, effective strategies were put forward to improve the catalytic performance of noble metal catalysts. The purpose is to provide a valuable reference for the academic research on the preparation of oxidation products from biomass-based HMF and the industrial application of noble metal catalysts.