A High-Performance Base-Metal Approach for the Oxidative Esterification of 5-Hydroxymethylfurfural

Benzenoid Aromatics from Renewable Resources

In this Review, all known chemical methods for the conversion of renewable resources into benzenoid aromatics are summarized. The raw materials that were taken into consideration are CO2; lignocellulose and its constituents cellulose, hemicellulose, and lignin; carbohydrates, mostly glucose, fructose, and xylose; chitin; fats and oils; terpenes; and materials that are easily obtained via fermentation, such as biogas, bioethanol, acetone, and many more. There are roughly two directions. One much used method is catalytic fast pyrolysis carried out at high temperatures (between 300 and 700 °C depending on the raw material), which leads to the formation of biochar; gases, such as CO, CO2, H2, and CH4; and an oil which is a mixture of hydrocarbons, mostly aromatics. The carbon selectivities of this method can be reasonably high when defined small molecules such as methanol or hexane are used but are rather low when highly oxygenated compounds such as lignocellulose are used. The other direction is largely based on the multistep conversion of platform chemicals obtained from lignocellulose, cellulose, or sugars and a limited number of fats and terpenes. Much research has focused on furan compounds such as furfural, 5-hydroxymethylfurfural, and 5-chloromethylfurfural. The conversion of lignocellulose to xylene via 5-chloromethylfurfural and dimethylfuran has led to the construction of two large-scale plants, one of which has been operational since 2023.

Phosphazene-Based Covalent Organic Framework as an Efficient Catalyst (COF-1) for the Dehydration of Fructose to 5-HMF

5-Hydroxymethylfurfural (HMF) is a promising organic platform for producing value-added chemicals. In this work, we focused on using a covalent organic framework (COF-1) as a heterogeneous catalyst for the dehydration of fructose to 5-HMF. The unique phosphazene unit-functionalized pores of COF-1 are essential active sites for catalytic performance. The results show that under the optimized reaction conditions, a maximum yield of 90% was obtained within 1.5 h at 120 °C. Furthermore, the effects of the catalyst load, reaction temperature, and usage of solvents for the improvement of reaction yield were investigated. The catalyst recyclability results showed that the yield of HMF did not change appreciably (90-82%) over five consecutive recycling runs. This work provides a viable strategy by applying phosphazene-based COF-1 for the efficient synthesis of HMF from renewable biomass. The synthesized HMF was further used for the synthesis of the biopolymer monomer furan-2,5-dimethylcarboxylate (FDMC) through N-heterocyclic carbene (NHC)-catalyzed oxidative esterification.

Control in Local Coordination Environment Boosting Activating Molecular Oxygen with an Atomically Dispersed Binary Mn-Co Catalyst

Activation of molecular oxygen plays a crucial role in natural organisms and the modern chemical industry. Herein, we report a Mn-Co dual-single-atom catalyst that exerts a specific synergy in boosting O₂ activation by collaboration between two distinct types of activation sites. Taking the oxidative esterification of the biomass platform 5-hydroxymethylfurfural (HMF) as the model reaction, the activation of O₂ is demonstrated through transforming O₂ into a reactive superoxide anion radical (O₂^•-) on Co-N₄ sites and, meanwhile, by reversible consumption and supplement of coordinated surface oxygen as a new type of reactive oxygen species (ROS) on N,O-coordinated single-atom Mn sites (Mn-N_xO_y). EXAFS analysis results show a longer average Mn-O bond distance at near 2.19 Å, which makes the breaking and formation of surface Mn-O bonds easier to cycle. Control experiments support that such Mn-O bonding conditions could facilitate H-elimination of C-H in HMF. The co-existence of two types of ROS effectively matches the oxidation of hydroxyl and aldehyde groups, and thus, the overall reaction is boosted in excellent yield of diester (95.8%) with an extremely high carbon balance. This study represents a rare example of taking advantage of the synergy of the diatomic catalyst for activating O₂ by two types of activation pathways.

N-doped carbon nanotube encapsulated cobalt for efficient oxidative esterification of 5‑hydroxymethylfurfural

The cobalt nanoparticles embedded into graphitic nitrogen-rich carbon nanotube (Co/GCN) was prepared with a facile method and employed as an efficient catalyst for oxidative esterification of 5-hydroxymethylfurfural (HMF). The introduction...

Hydrogen-Binding-Initiated Activation of O-H Bonds on a Nitrogen-Doped Surface for the Catalytic Oxidation of Biomass Hydroxyl Compounds

Hydrogen binding of molecules on solid surfaces is an attractive interaction that can be used as the driving force for bond activation, material-directed assembly, protein protection, etc. However, the lack of a quantitative characterization method for hydrogen bonds (HBs) on surfaces seriously limits its application. We measured the standard Gibbs free energy change (ΔG⁰ ) of on-surface HBs using NMR. The HB-accepting ability of the surface was investigated by comparing ΔG⁰ values employing the model biomass platform 5-hydroxymethylfurfural on a series of Co-N-C-n catalysts with adjustable electron-rich nitrogen-doped contents. Decreasing ΔG⁰ improves the HB-accepting ability of the nitrogen-doped surface and promotes the selectively initiated activation of O-H bonds in the oxidation of 5-hydroxymethylfurfural. As a result, the reaction kinetics is accelerated. In addition to the excellent catalytic performance, the turnover frequency (TOF) for this oxidation is much higher than for reported non-noble-metal catalysts.

Earth-abundant 3d-transition-metal catalysts for lignocellulosic biomass conversion

Transformation of biomass to chemicals and fuels is a long-term goal in both science and industry. However, high cost is one of the major obstacles to the industrialization of this sustainable technology. Thus, developing catalysts with high activity and low-cost is of great importance for biomass conversion. The last two decades have witnessed the increasing achievement of the use of earth-abundant 3d-transition-metals in catalysis due to their low-cost, high efficiency and excellent stability. Here, we aim to review the fast development and recent advances of 3d-metal-based catalysts including Cu, Fe, Co, Ni and Mn in lignocellulosic biomass conversion. Moreover, present research trends and invigorating perspectives on future development are given.

Ultrafine Copper Oxide Particles Dispersed on Nitrogen-Doped Hollow Carbon Nanospheres for Oxidative Esterification of Biomass-Derived 5-Hydroxymethylfurfural

One-pot synthesis of furan-2,5-dimethylcarboxylate (FDMC) from 5-hydroxymethylfurfural (HMF) is highly demanding for the commercial production of polyethylene furanoate (PEF). Herein, a direct synthesis of FDMC is reported from oxidative esterification of HMF using ultrafine CuO particles dispersed on nitrogen-doped hollow carbon nanospheres (CuO/N-C-HNSs) as a catalyst and tert-butyl hydroperoxide (TBHP) as an oxidizing and methylating reagent. The CuO/N-C-HNSs was prepared through a template protection-sacrifice strategy using SiO₂ as a sacrificial template and histidine as the precursor for N and C. N-doping facilitated a strong interaction between the support and copper species, affording formation of CuO nanoparticles of less than 10 nm in size. By virtue of the highly dispersed CuO nanoparticles and a high BET surface area 373 m² /g, the CuO/N-C-HNSsshows excellent catalytic performance in the selective conversion of HMF into FDMC affording 93 % yield of the desired product with a TON value of 49. Furthermore, the oxidative esterification involving SP³ C-H bond functionalization is also demonstrated using the same catalyst.

Oxidative Esterification of 5-Hydroxymethylfurfural with an N-doped Carbon-supported CoCu Bimetallic Catalyst

The direct fabrication of furan-2,5-dimethylcarboxylate (FDMC), a promising renewable monomer, from biomass-derived 5-hydroxymethylfurfural (HMF) is a cutting-edge process. In this contribution, an elaborately designed N-doped carbon-supported CoCu bimetallic catalyst (Co_x Cu_y -NC; x/y=9:1, 7:3, 4:6, which represents the designed molar ratio of Co and Cu in the catalyst), which could offer a desirable FDMC yield of 95 % under mild and base-free conditions (Co₇ Cu₃ -NC, 2 bar O₂ , 80 °C, 4 h) is described for the oxidative esterification of HMF. Notably, an FDMC formation rate of 6.1 mol_FDMC  mol_Co ^-1  h^-1 was achieved over Co₇ Cu₃ -NC, which represents the highest catalytic efficiency so far among Co-based catalytic systems. It has been demonstrated that Cu-doping in Co₇ Cu₃ -NC catalyst brings about more active sites (Co-N_x species) with stronger molecular oxygen activation ability. The increase of surface N content of Co₇ Cu₃ -NC also improves basicity of the catalyst, which favors the hydrogen abstraction process during the HMF oxidative esterification reaction. These findings may pave an efficient and green way for the synthesis of sustainable bio-based polymer monomers.

AuPd-Fe3 O4 Nanoparticle-Catalyzed Synthesis of Furan-2,5-dimethylcarboxylate from 5-Hydroxymethylfurfural under Mild Conditions

Efficient one-pot oxidative esterification of 5-hydroxymethylfurfural (HMF) to furan-2,5-dimethylcarboxylate (FDMC) was achieved under extremely mild reaction conditions by using AuPd alloy nanoparticles (NPs) supported on Fe₃ O₄ . A high yield of FDMC (92 %) was obtained at room temperature under atmospheric O₂ . The reaction proceeded through the synergistic effects of the AuPd heterobimetallic catalyst system. The most effective molar ratio of noble metal contents for HMF oxidation was 1.00:1.18. If Au-Fe₃ O₄ NPs were used as the catalyst, selective synthesis of 5-hydroxymethylfuroic acid methyl ester (HMFE) was achieved. Additionally, the AuPd-Fe₃ O₄ catalyst could be successfully reused.