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. 2025 Sep 22;10(38):44415–44424. doi: 10.1021/acsomega.5c06288

Oxidation of 5‑HMF Catalyzed by N‑Doped Carbon-Supported Non-Noble Metal Catalysts

Kaleeswari Kalimuthu 1, Thinnaphat Poonsawat 1, Peerapong Chumkaeo 1, Ekasith Somsook 1,*
PMCID: PMC12489686  PMID: 41048713

Abstract

2,5-Furandicarboxylic acid (FDCA) is an exceptionally promising biodegradable alternative to petroleum-derived terephthalic acid. The synthesis of FDCA from biomass-derived 5-hydroxymethylfurfural (HMF) has attracted significant attention. Herein, we report a straightforward method for synthesizing N-doped carbon-supported metal/metal phosphide catalysts developed using solid grinding techniques followed by pyrolysis. The synergistic effects of metal alloys and metal phosphides significantly influenced the product selectivity. The optimized catalyst (FeP-Co_0.2/NC) achieved a remarkable FDCA yield of 91.6% with complete HMF conversion at 150 °C for 24 h, using water as the solvent and O2 as the oxidant. This study presents a sustainable and efficient approach for FDCA production. This discovery represents a sustainable method for producing FDCA with a high yield by employing environmentally friendly solvents and oxidizers.

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Introduction

The swift growth of modern civilization has caused global ecological challenges affecting human quality of life, including fossil fuel depletion, greenhouse gas emission, and environmental pollution. Consequently, the pursuit of sustainable strategies and alternative resources is increasingly vital. , Biomass, a renewable resource for carbon, stands fourth in terms of energy production behind gas, oil, and charcoal, which are among the most abundant sustainable carbon resources around the world. Among biomass-derived chemicals, 2,5-furandicarboxylic acid (FDCA) stands out for its multifunctionality and potential to replace petroleum-based terephthalic acid in the production of polyethylene furanoate (PEF), a recyclable and biodegradable polymer with superior barrier properties and thermal performance compared with poly­(ethylene terephthalate) (PET). HMF, derived from C6 sugars in cellulose, is a key intermediate for FDCA production. − The oxidation of HMF to FDCA typically proceeds through intermediates, such as DFF, HMFCA, and FFCA (Scheme ). FDCA has been recognized as one of the U.S. Department of Energy’s top 12 biobased platform chemicals. − To compare with PET bottles, they exhibit superior barrier properties against water, oxygen, and carbon dioxide, along with a higher glass transition temperature and tensile modulus. −

1. Possible Reaction Pathways of the Oxidation of HMF to FDCA.

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Traditional catalytic systems generally involve precious metals (e.g., Au, Pt, Pd, Ru) because of their high stability and activity; however, their cost and scarcity make them less scalable. − However, non-noble metals, e.g., Co, Cu, Mn, and Fe, are potential alternatives for catalysis, especially in oxidations. − Cobalt-based catalysts exhibited enhanced catalytic activity for the oxidation of several organic compounds, particularly alcohols, aldehydes, and HMF, when compared to other non-noble metals. ,,−

N-doped carbon materials (NCs) have attracted significant attention as catalyst supports because of their low cost, ease of production, environmental compatibility, and enhanced catalytic performance arising from strong metal–support interactions. A significant factor influencing the catalytic performance, selectivity, and durability of N-doped carbon-supported catalysts is the presence of N species in carbon materials, which promotes the formation of metal species with a tunable electronic structure through the interaction between N atoms and metal nanoparticles. Metal nanoparticles can be deposited by using the integrated N atoms as active sites. Therefore, in comparison to N-free carbon, which is primarily in charge of the increased activity, N-doped carbon-supported metal catalysts typically exhibit higher metal nanoparticle dispersion with smaller particle sizes. − Transition metal phosphides (TMPs) represent a notable class of materials due to their excellent conductivity, natural abundance, and versatility in energy and catalytic applications. However, TMP-based catalysts are commonly synthesized using complex and waste-intensive procedures, such as coprecipitation and hydrothermal methods. − As a result, it is exceedingly necessary to develop an inexpensive and environmentally friendly catalyst production technique for the synthesis of N-doped supported non-noble metal-based catalysts for the selective oxidation of HMF to FDCA. Recently, a facile solid-state grinding approach for preparing supported metal or metal oxide catalysts for different processes was revealed. −

Solid-state grinding is an environmentally benign, time-efficient, and straightforward method for catalyst synthesis that minimizes waste and simplifies processing. In this study, FeP-Co_x/NC catalysts were prepared via solid-state grinding, followed by pyrolysis. The structure and composition of the catalysts were thoroughly characterized using XRD, XPS, TEM/EDS, SEM/EDS, BET, ICP-MS, and Raman spectroscopy. The catalytic activity, selectivity, and recyclability of FeP-Co_x/NC in HMF oxidation to FDCA were systematically investigated.

Experimental Section

Chemicals

All chemicals and solvents were of analytical grade and used as received without further purification. 5-Hydroxymethylfurfural (HMF), ethanol, ferrocenium hexafluorophosphate, and cobaltocene were purchased from Aldrich. 2,5-Diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA), 2,5-furandicarboxylic acid (FDCA), and melamine were obtained from Tokyo Chemical Industry Co., Ltd. Sodium carbonate was sourced from RCI Labscan, while acetonitrile and glacial acetic acid were supplied by Honeywell. Deionized water (resistivity ≥ 18.2 MΩ·cm) was obtained from a Nanopure analytical deionization system.

Instrumentation

The X-ray diffraction (XRD) was carried out on a Bruker D2 Phaser X-ray diffractometer. The X-ray was generated by a Cu tube for Cu Kα radiation at 40 kV and 40 mA and detected by LYNXEYE_XE-T (1D mode). The XRD patterns were recorded with the scanning angle (2θ) in the range of 10 to 90° at 0.2 s/step. Raman microscopy was performed on a Horiba XploRA PLUS system with the excitation source at a wavelength of 532 nm. The characteristic vibrational frequencies were carried out by Fourier transform infrared (FTIR) spectroscopy using α II Compact FTIR spectrometer (Bruker, Billerica, MA). Thermogravimetric analysis (DSC/TGA) was performed using a TA Instruments SDT2960 thermogravimetric analyzer simultaneously at the heating rate of 10 °C/min up to 800 °C under a nitrogen atmosphere. The elemental compositions within the materials were investigated by X-ray photoelectron spectroscopy (XPS) using an Axis Ultra DLD spectrometer (Kratos Analytical, Manchester, U.K.). The X-ray absorption measurements were performed under ultra-high vacuum conditions (∼5 × 10–9 Torr). Photoelectrons were excited using a monochromated Al Kα (Kα1,2) source (hν = 1486.6 eV), operated in hybrid mode with a 700 × 300 μm spot size. Transmission electron microscopy (TEM) and energy-dispersive spectrometry (EDS) were used to characterize the morphology and elemental mapping of the cobalt and iron metal ions on an accelerating voltage of 200 kV using a JEOL microscope (JEM-ARM 200F model), manufactured by JEOL Co., Ltd., Tokyo, Japan. The specific surface area and average particle size were measured by the Brunauer–Emmett–Teller (BET) method by using the Nova 2200e model from Quantachrome Instruments, Boynton Beach, FL, USA. Prior to N2 adsorption, 150 mg of the solid catalysts were outgassed at 300 °C for 3 h under a N2 flow. The oxidized products were quantitatively analyzed by high-performance liquid chromatography with an UV–vis detector (HPLC/UV) operating at 283 nm. The concentrations of HMF and its oxidized derivatives were detected by the WATER e2695 separation module with a UV–vis detector equipped with a Sunfire 5 μm C18 column (4.6 mm × 150 mm) by using the mixture of acetonitrile and 1% v/v glacial acetic acid as the mobile phase in the proper proportions. The contents of Co and Fe were determined by inductively coupled plasma mass spectroscopy (ICP-MS) using an Agilent 7900 model.

Synthesis of the Cobalt and Iron Phosphide-Supported N-Doped Carbon Catalyst

Briefly, 5 mmol of cobaltocene, 1 mmol of ferrocenium hexafluorophosphate, and 15 mmol of melamine were appropriately combined using a mortar and pestle for 10 to 15 min. Melamine plays the role of both the carbon and the nitrogen source, while cobaltocene and ferrocenium hexafluorophosphate provide the cobalt and iron precursors. The resulting mixture was transferred to the crucible and pyrolyzed in the tube furnace for 3 h at a heating rate of 2 °C/min from ambient temperature to 900 °C under a nitrogen flow. After cooling to room temperature, the as-synthesized mixture was collected, washed with ultrapure water to remove unreacted species, and then dried overnight at 80 °C for further analysis. The resulting catalyst was designated as FeP-Co_0.2/NC, where “0.2” indicates the Fe/Co molar ratio. FeP-Co_0.1/NC, FeP-Co_0.3/NC, and FeP-Co_0.4/NC were prepared by altering the molar ratios of metals and pyrolyzing them using the as-mentioned condition. Control catalysts were also prepared for comparison: FeP/NC (without a cobalt precursor) and Co/NC (without an iron precursor).

Study of Catalytic Activity

HMF was catalytically oxidized to FDCA in a high-pressure autoclave reactor. In a typical experiment, 25.2 mg of HMF (0.2 mmol) was dissolved in 5.00 mL of ultrapure water in the presence of 0.2 mmol of Na2CO3. It was then further mixed with 50 mg of the catalyst, and the autoclave was sealed. The system was first purged, and then, O2 (0.5 MPa) was loaded under 0.5 MPa, and the reaction was kept at 150 °C for 24 h with stirring. After the reaction, the reactor was cooled to room temperature, and the catalyst was separated from the reaction with centrifugation at 8000 rpm for 10 min. The reaction solution was filtered and washed with ethanol and water, and the volume was adjusted to 25.00 mL with a volumetric flask for HPLC analysis.

The system condition of the mobile phase was 30% of acetonitrile and 70% of acetic acid (1% v/v in DI water). During the measurement, the column temperature and flow rate were 25 °C and 0.6 mL/min, respectively. The retention times from the optimized HPLC condition were 2.74 min (HMF), 3.23 min (HMFCA), 6.72 min (FFCA), and 7.92 min (FDCA). The conversion and yield were calculated using eqs and .

conversion(%)=mole of HMF reactedmole of HMF added×100 1
yield(%)=mole of product obtainedmole of HMF added×100 2

Reusability of the Catalysts

After the reaction was completed, the catalysts were collected by centrifugation at 8000 rpm for 10 min and washed several times with ultrapure water. To fully eliminate any organic substances that may have been adsorbed on the surface of the catalyst, an ethanol solvent was utilized for washing in the final step before drying in an oven at 80 °C overnight. The used catalysts were calcined at 900 °C for 2 h before being used again, based on the moisture content that deactivated the catalyst.

Results and Discussion

The FeP-Co_x-NC catalysts were successfully synthesized by pyrolyzing the solid-ground mixture of precursors with appropriate ratios at 900 °C under a nitrogen atmosphere for 3 h. First, the XRD technique was utilized to analyze the crystal arrangement of the synthesized catalysts. The XRD patterns of FeP-Co_x-NC catalysts are exhibited in Figure a. The diffraction peak at 26.2 was attributed to the carbon (002) plane; , meanwhile, the peaks at 44.4, 51.6, and 76.1 could be attributed to the (111), (200), and (220) planes for the FCC phase of Co nanoparticles, respectively (PDF 01–077–7451). The peak around 43.5° (330) and 51.1° (420) could be represented by Co0.72Fe0.28 (PDF 00–051–740). The peak around 45.1° was reliable with the (110) crystal plane of Co–Fe alloys and the broad peak of the XRD spectrum at a position of 40.6 (111), indicating Fe2P (PDF 01–078–6749). , Importantly, both Co0.72Fe0.28 and Co–Fe alloy species were strongly identified with cobalt nanoparticles only in the FeP-Co_0.2/NC catalyst, showing that the two species were successfully doped by nitrogen with metals after pyrolysis at 900 °C. This implies that incorporating iron into the catalyst results in a composite crystalline phase in the bimetallic catalysts. Moreover, the diffraction peak intensities for Co0.72Fe0.28 improve, and Co nanoparticles decrease as the iron metal ratio rises in the catalyst. This indicates the iron to cobalt electron transfer in the successfully developed bimetal catalysts. Additionally, the Fe2P diffraction peak proved that phosphide had been brought to the catalysts. On the other hand, the FeP/NC case clearly confirms the mixture of FeP (30.8, 32.7, 34.5, 35.4, 37.1, 45.5, 46.2, 48.3, 50.3, 55.3, 56.1, 59.5, and 79.1 corresponding to (020), (011), (200), (120), (111), (201), (121), (211), (130), (141), (221), (002), and (222) Miller indexes, respectively (PDF 01–071–2262)) and Fe2P (40.2, 44.1, 47.2, 52.8, and 55.3 corresponding to (111), (110), (210), (002), and (300) Miller indexes, respectively (PDF 01–078–6749)), demonstrating the effective creation of FeP nanoparticles. Raman spectroscopy was employed, as shown in Figure b and Table S1, to verify carbon graphitization and the degree of defects. The characteristic D and G bands at 1345 and 1578 cm–1, respectively, are the centers of two prominent peaks in the Raman spectra, which corresponded to the D peak from defects in the carbon material and the G peak from the vibrations of the C–C bond. The graphitization degree and structural defects are indicated by the intensity ratio (I D/I G) of the D and G peaks. The higher graphitization is suggested by a lower I D/I G ratio, whereas the catalytic framework’s major structural defects caused by the doping of N are thus indicated by the highest I D/I G ratio. , The high I D/I G ratio of FeP-Co_0.4/NC (1.03) and FeP-Co_0.2/NC (0.98) indicates that the incorporation of iron phosphide and the formation of the Co–Fe alloy can effectively support structural defects, which is beneficial to the alcohol oxidation reaction. ,, Inductively coupled plasma mass spectrometry was used to measure the Co and Fe contents in the FeP-Co_x-NC catalysts (ICP-MS, Table S2).

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(a) XRD patterns and (b) Raman spectra of the synthesized catalysts.

The SEM pictures with different magnifications (Figure ) revealed that FeP-Co_0.2/NC has the largest contact surface, owing to the existence of Co nanoparticles and the volcano-type reliance that Fe presents. , A rougher surface with more caverns was seen when the iron concentration was raised. Further surface morphological characterizations and elemental distribution of FeP-Co_0.2/NC were carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS). The TEM observation makes it evident that the nanoparticles are encapsulated in N-doped carbon.

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SEM images of (a–c) Co/NC, (d–f) FeP-Co_0.2/NC, and (g–i) FeP/NC catalysts at the difference in magnifications.

The observations of the elemental mapping images showed that Co, Fe, N, P, and C species were well dispersible throughout the catalyst, and the corresponding high-magnification SEM image shows that the yellow dashed circles represent the tiny cobalt nanocrystals that are incorporated or embedded in the catalyst, as shown in Figure .

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(a, b) TEM images and (c–i) SEM/EDS images of FeP-Co_0.2/NC.

The BET isotherms for nitrogen adsorption obtained with FeP-Co_0.2/NC, Co/NC, and FeP/NC are presented in Figures a and S8. All three catalysts demonstrate Type IV nitrogen adsorption–desorption isotherms with H3-type hysteresis loops, indicating mesoporous structures. FeP–Co–0.2/NC outperformed Co/NC (159 m2·g–1) and FeP/NC (98 m2·g–1) in terms of the BET surface area (191 m2·g–1) and pore volume (≈0.31 cm3·g–1) while also having a larger average pore diameter (5.47 nm) (Table S3). This significant improvement in FeP–Co–0.2/NC is due to the synergistic interaction between FeP and Co on the nitrogen-doped carbon support, which promotes morphological changes and creates a more interconnected mesoporous network. This is also the case in Ni–Co phosphide systems (e.g., NiCoP showed increased active area and morphology refinement). Furthermore, mesoporous Fe–Co materials used in Fenton-like catalytic studies had similar pore sizes and surface areas, confirming the role of bimetallic loading in improving textural properties. These structural advantages like increased surface areas, pore volumes, and mesoporosity are anticipated to improve substrate access to active sites and catalytic oxidation activity toward biomass-derived HMF. The Co/NC, FeP/NC, and FeP-Co_0.2/NC catalysts’ species and chemical states were investigated using X-ray photoelectron spectroscopy (XPS).

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(a) BET analysis for the N2 adsorption–desorption isotherm of FeP-Co_0.2/NC. (b) XPS survey spectra of FeP-Co_0.2/NC. Detailed (high-resolution) scans for (c) Co 2p, (d) N 1s, (e) P 2p, and (f) Fe 2p spectra of FeP-Co_0.2/NC.

Figures S2a and S3a show the coexisting Co, Fe, C, N, and P elements’ survey spectra with the respective catalysts. The XPS spectrum of the FeP-Co_0.2/NC catalyst was analyzed in the regions Co 2p, Fe 2p, C 1s, N 1s, O 1s, and P 2p, as shown in Figure . However, the Fe 2p spectra signals were not detectable. It reveals that the amount of iron in FeP-Co_0.2/NC is very low, which is consistent with the ICP-MS analysis. As shown in Figure S2b, the Co 2p spectrum deconvoluted into four peaks for Co/NC (778.70, 780.41, 793.46, and 795.05 eV) and for FeP-Co_0.2/NC (778.67, 780.29, 793.23, and 794.66 eV), as shown in Figure c, which are ascribed to 2p3/2 and 2p1/2, which could assign to metallic Co0 species, suggesting the existence of Co nanoparticles, which corresponds with the TEM image and XRD patterns and Co2+ species. − Furthermore, the binding energy of cobalt nanoparticles and Co2+ species decreased slightly on increasing the iron content. It suggests that electron transfer from iron to cobalt occurred for the FeP-Co-0.2/NC catalyst.

As shown in Figures S2 and d, the high-resolution N 1s spectra for Co/NC and FeP-Co_0.2/NC were examined and classified into four distinctive peaks, respectively. These peaks suggest the presence of various nitrogen species in the catalyst. Peaks centered around 398.63, 399.95, 401.47, and 402.94 eV can be attributed to pyridinic N/Co–N, pyrrolic N, graphitic N, and oxidized nitrogen, respectively. ,, It is clearly shown that the FeP-Co_0.2/NC catalyst has a higher amount of pyridinic N than the Co/NC catalyst. The C–N site, which has pyridinic N over the catalyst, is typically recognized as the active site for O2 activation. It serves as a Lewis base that can strongly interact with molecular O2, weakening the O–O bond and generating activated oxygen species. These species then participate in hydrogen abstraction from HMF and intermediates, thus driving the oxidation toward FDCA. The P 2p spectrum can be deconvoluted into four peaks at 129.9, 131.1, 132.9, and 134.3 eV. These peaks are attributed to the featured P 2p3/2 and 2p1/2 peaks for metal phosphides (Fe–P) and the formation of oxidized P species (P–O, P–O–Fe), respectively, as shown in Figures f and S3d. ,, In addition, there is no peak at 285.5 eV in the C 1s spectrum, which is attributed to C–P (Figures S4 and e), and there is no band at 132.4 eV in the P 2p spectrum, which is ascribed to P–C in FeP-Co_0.2/NC. All of these suggest that phosphorus is not doped in the carbon surface, but it is bonded with iron. Meanwhile, the Fe 2p spectrum (Figure S3) deconvoluted into four peaks for FeP/NC (707.5, 708.9, 720.5, and 721.8 eV), which are ascribed to 2p3/2 and 2p1/2, which could assign metallic iron and Fe2+ species. ,

Table displays the catalytic performance, and control studies were conducted for the comparison. In the absence of the catalyst (entry 1), the results revealed that FDCA was not produced. Generally, humin was easily produced at elevated temperatures in the absence of catalysts. The catalytic performance of the catalysts was first investigated under the following conditions: 0.2 mmol of HMF, 0.2 mmol of Na2CO3, 5 mL of water, 5 bar of O2, 50 mg of the catalyst, 150 °C, 24 h. Similarly, the bimetallic catalyst demonstrated complete conversion, and interestingly, the catalyst (entry 3) showed a superior percentage of the FDCA yield with little of the intermediates, when compared to the single-metal catalysts and other bimetal catalysts. The FeP-NC catalyst (entry 6) exhibits a lower catalytic performance, as evidenced by a lower FDCA yield and the correspondingly lower consumption rates for FFCA and HMFCA. In entry 7, the single metallic catalyst (Co/NC) demonstrated complete conversion with 63% of FDCA. In addition, a physical mixture of Co/NC and FeP/NC only produces 38.8% of FDCA (entry 8), suggesting that a close interaction between Co and Fe species is essential for producing a potent synergistic effect in HMF activation.

1. Catalytic Activity of Various Types of Catalysts in HMF Oxidation,

      yield (%)
entry catalyst conversion (%) FDCA FFCA HMFCA
1   58.5     2.1
2 FeP-Co_0.1/NC 100 55.8 25.3 4.1
3 FeP-Co_0.2/NC 100 78.4 9.8 5.4
4 FeP-Co_0.3/NC 100 43.7 33.5 4.9
5 FeP-Co_0.4/NC 100 36.0 28.6 15.7
6 FeP/NC 95 4.0 29.9 18.4
7 Co/NC 100 62.8 15.6 11.5
8 Co/NC + FeP/NC (physical mixture as per the FeP-Co_0.2/NC ratio) 100 38.8 17.4 10.7
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a

The percentages of conversion and yield were calculated by the HPLC/UV technique.

b

Aerobic oxidation of HMF to FDCA by using 1.0 equiv of Na2CO3 as a base, at 150 °C and under 0.5 MPa atmospheric oxygen.

c

Physical mixture of the two catalysts (Co/NC and FeP/NC) in the same molar ratio of FeP-Co_0.2/NC.

According to the results, the FeP-Co_0.2/NC catalyst was further investigated to achieve higher yields of the product by altering the reaction parameters, with the influence of factors such as the reaction temperature, oxygen pressure, and base ratio with HMF. This is due to the high percentage of pyridinic N (38.8%) (Table S4 and Figure d) as well as the surface area of the catalyst. Next, the effect of the reaction temperature for 100, 150, and 160 °C was investigated (Table , entries 1–3). It is typically considered that higher reaction temperatures result in greater yields of products and HMF conversion. It was observed that the desired product yield is increased when the temperature increases at the same time as the intermediates are gradually falling (Table ). Nonetheless, the yield of FDCA diminishes to 71.2 from 78.4% at 160 °C, probably because of humin development from HMF at elevated temperatures. In this observation, a high reaction temperature of 150 °C was used for further investigations. The effect of oxygen pressure was investigated (Table , entries 4–6, 2) with air (without compressed O2) and 0.1, 0.3, and 0.5 bar of O2. There was a very low percentage of FDCA occurring, when the reaction proceeded in atmospheric air, whereas the HMF conversion was 100%. It was suggested that O2 played an important role during the whole catalytic process to remove the protons from the catalyst, and it is essential for the HMF oxidation to FDCA with the highest amount of the yield. Significantly, when the oxygen pressure was raised from 0.1 to 0.5 MPa, the product yield arose. Consequently, for additional research, 5 bar of oxygen was utilized.

2. Effect of Different Conditions in the Controlled Oxidation of HMF Using FeP-Co_0.2/NC .

        yield (%)
entry catalyst condition conversion (%) FDCA FFCA HMFCA
1 FeP-Co_0.2/NC 1 equiv Na2CO3, 0.5 MPa O2, 100 °C 100 63.1 14.6 10.6
2 FeP-Co_0.2/NC 1 equiv Na2CO3, 0.5 MPa O2, 150 °C 100 78.4 9.8 5.4
3 FeP-Co_0.2/NC 1 equiv Na2CO3, 0.5 MPa O2, 160 °C 100 71.2 9.4 10.7
4 FeP-Co_0.2/NC 1 equiv Na2CO3, - MPa O2, 150 °C 100 5.8 28.8 10.4
5 FeP-Co_0.2/NC 1 equiv Na2CO3, 0.1 MPa O2, 150 °C 100 45.7 8.6 3.1
6 FeP-Co_0.2/NC 1 equiv Na2CO3, 0.3 MPa O2, 150 °C 100 64.9 3.2 1.9
7 FeP-Co_0.2/NC no base, 0.5 MPa O2, 150 °C 57.0 15.3 36.3 8.6
8 FeP-Co_0.2/NC 0.5 equiv Na2CO3, 0.5 MPa O2, 150 °C 97.6 49.9 3.0 7.8
9 FeP-Co_0.2/NC 2 equiv Na2CO3, 0.5 MPa O2, 100 °C 100 91.6 3.0  
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The reaction mixture was quantitatively monitored by high-performance liquid chromatography with an UV–vis detector (HPLC/UV) operating at 283 nm.

The effect of the HMF/base ratio was investigated to reach an excellent yield (Table , entries 2, 7–9). This study, as well as previous research, supported the requirement for a soluble base in the reaction system. The oxidation of HMF and the product yield are significantly impacted by the alkalinity and its strength. , HMF was converted around 57% without a base (Table , entry 7), whereas when the base ratio increased to 1:0.5, HMF was almost converted to 98%; however, the product yield was around 50%. When increasing the base ratio to 1:1 and 1:2, the FDCA yield suddenly increased to around 78.4 and 91.6%, respectively (Table , entries 2 and 9). The effect of the reaction time was investigated from 2 to 24 h. When increasing the time from 2 to 24 h, the FDCA yield steadily increased, from around 60% at 4 h to 78.4% at 24 h under an HMF/base ratio of 1:1, in the same way the intermediates were decreased, as shown in Figure . According to these studies, the following conditions were selected as the optimized reaction conditions: HMF/base (Na2CO3) at a 1:2 ratio, 5 mL of water, O2 at 5 bar, 50 mg of the catalyst, and 150 °C for 24 h.

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Optimization of the reaction duration on the catalytic oxidation process.

As seen in Figure , the stability of FeP-Co_0.2/NC was considered when determining the reusability at 150 °C under the intended conditions. The catalyst demonstrated the capacity to generate highly selective FDCA in the recycling experiment. The reusability of FeP-Co_0.2/NC demonstrated a good performance for at least three cycles with a little reduction in FDCA. In summary, the reaction pathways indicated that the reaction was accomplished via HMFCA (Route B) due to the discouragement of the DFF product, as evidenced by the aerobic oxidation of HMF catalyzed by FeP-Co_0.2/NC. Furthermore, the catalyst’s XRD patterns (Figure b) before and after the oxidation reaction confirmed its structural stability of the catalyst; however, peak intensities slightly decreased, because of a minimal amount of metal leaching from the filtrate, as confirmed by ICP–MS analysis (Table S5). Although this partial loss of Co and Fe occurred, the catalyst maintained a high FDCA selectivity, suggesting that the remaining active sites were structurally stable and catalytically active. To further enhance the long-term durability, stronger metal–support interactions (e.g., via higher nitrogen coordination) and tailored porous architectures could be explored in future studies to suppress leaching and preserve activity. Also, the catalytic efficiency of FeP-Co_0.2/NC was extensively evaluated in comparison to other non-noble metal-supported catalysts (Table S6).

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(a) Reusability of the FeP-Co_0.2/NC catalyst. (b) XRD patterns of FeP-Co_0.2/NC before and after the HMF oxidation.

Conclusions

In summary, we have successfully developed recyclable, high-performance nitrogen-doped carbon (NC)-supported metal/metal phosphide catalysts through an environmentally friendly process. Excellent catalytic activity was demonstrated by the FeP-Co_0.2/NC catalyst, which achieved a 91.6% FDCA yield and 100% HMF conversion under the optimal conditions. The catalytic conversion of HMF to FDCA is facilitated by the nitrogen-doped carbon speciesparticularly pyridinic nitrogenas well as the Co–Fe metal alloy and FeP species. Moreover, the incorporation of FeP enhances the selectivity for the desired product. However, the synthesized catalysts exhibit certain drawbacks, including the need for elevated temperatures and extended reaction times to achieve an optimal performance. Despite these limitations, the use of nonprecious metal catalysts for HMF oxidation to produce FDCA demonstrates a strong potential for industrial application.

Supplementary Material

ao5c06288_si_001.pdf (1.2MB, pdf)

Acknowledgments

This work is funded by the National Research Council of Thailand (NRCT) and Mahidol University [Grant No. N42A650355], the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research, and Innovation [Grant No. B16F640099], and a research grant from the Center of Excellence for Innovation in Chemistry (PERCH–CIC).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c06288.

  • TEM images, XPS survey spectra, detailed (high-resolution) scans, FTIR spectra, thermogravimetric analysis, SEM/EDX analysis, BET analysis for N2 adsorption–desorption isotherms, and pore size distribution of synthesized catalysts (Figures S1–S8) and tables of the I D/I G ratios from Raman spectra, metal content (%w/w) of Co and Fe using inductively coupled plasma mass spectroscopy (ICP-MS), BET surface area and porosity measurements based on the BET method, types of nitrogen content (%atom) from XPS, leaching amounts of metal species in the filtrate based on inductively coupled plasma mass spectroscopy (ICP-MS), and comprehensive comparison of other bimetallic catalysts with FeP-Co_0.2/NC (Tables S1–S6) (PDF)

K.K.: Writingpreliminary manuscript, graphical representation, methodology, and systematic analysis. T.P.: Theoretical concept development and hypothesis validation. P.C.: Theoretical concept development and hypothesis validation. E.S.: Writingpreliminary manuscript, academic supervision, project coordination, methodological framework, experimental investigation, and conceptualization.

The authors declare no competing financial interest.

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