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Photocatalytic production of a C₁₂ liquid biofuel precursor and H₂ by Ni(OH)₂–ZnIn₂S₄ in anaerobic water†

Wanqiong Kang^a, Xiaolong Li^a, Xiongxiong Zeng^a, Han Wu^a, You Ge^a, Lan Yuan*^a, Yi Liu*^ab and Chuang Han*^c
aSchool of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, P. R. China. E-mail: yuanlan@wust.edu.cn
^bSchool of Chemistry and Materials Science, South-Central Minzu University, Wuhan 430074, P. R. China. E-mail: yiliuchem@whu.edu.cn
^cFaculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China. E-mail: hanc@cug.edu.cn

First published on 28th August 2024

Notably, most studies on generating value-added chemicals from HMF have focused on the product 2,5-furandicarboxylic acid (FDCA), through the oxidation pathway with the formation of 2,5-diformylfuran (DFF)/5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 5-formyl-2-furancarboxylic acid (FFCA) intermediates (Scheme 1).² Alternatively, recently, carbon-chain growth strategies have been devised for the self-condensation coupling of HMF to 5,5′-dihydroxymethyl furoin (DHMF), and its further selective oxidation can afford a new biomass-based polyol monomer, namely 5,5′-bihydroxymethyl furil (BHMF),³ thus upgrading HMF to high energy-density biofuels of C₁₂ compounds in the diesel or jet fuel range. However, the process conducted in the manner of conventional thermo-catalytic reactions requires complicated procedures, various organic solvents, or harsh reaction conditions.³

	Scheme 1 Typical products from HMF upgradation.

Photocatalytic upgrading of HMF has been widely investigated in terms of its green and sustainable manner.⁴ Nevertheless, most reported systems have focused on the unimolecular oxidation transformation that results in the formation of C₆ products.² Generally, the C₁₂ products generated through the dimerization path were deemed only as an undesired side products with low yield.⁵ Wang et al. reported a pioneering work for the dehydrogenative C–C coupling of methylfurans, using a Ru-doped ZnIn₂S₄ catalyst.⁶ However, the reaction system relies on the use of precious metals. Inspired by this work as well as the great advance in photocatalytic C–C coupling,⁷ we herein explored the bifunctional photocatalytic dimerization of HMF to biomass-based C₁₂ products paired with simultaneous H₂ production under noble metal- and sacrificial reagent-free conditions.

Ni(OH)₂, an efficient catalyst for H₂ evolution and HMF activation in electrocatalysis,⁸ was selected as the cocatalyst to modify ZnIn₂S₄ (ZIS) through a facile deposition–precipitation method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of bare ZIS depict its flower-like microspheres and high crystallinity (Fig. S1, ESI†). As illustrated in Fig. 1a, the flower-like microsphere structure of Ni(OH)₂–ZIS is assembled by the aggregation of the disordered nanosheets.⁹ The clear lattice fringes of 0.29 nm and 0.23 nm refer to the (104) planes of ZIS and the (101) plane of Ni(OH)₂,¹⁰ respectively (Fig. 1b). The selected area electron diffraction (SAED) pattern indicated that 2% Ni(OH)₂–ZIS were polycrystalline, and the circular rings were not uniform (Fig. 1c), confirming that Ni(OH)₂–ZIS composite materials were synthesized.¹¹ The energy-dispersive X-ray spectroscopy (EDX) spectrum (Fig. 1d) and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Fig. 1e–j) suggest that all of the elements related to ZIS and Ni(OH)₂ are evenly distributed in the composites, indicating that Ni(OH)₂ is uniformly loaded onto the surface of ZIS microspheres. For comparison, no signal for Ni can be detected in bare ZIS (Fig. S1, ESI†).

	Fig. 1 (a) TEM image, (b) HRTEM image and (c) the selected area electron diffraction (SAED) pattern of 2% Ni(OH)2–ZIS. (d) Energy-dispersive X-ray spectroscopy (EDX) spectrum of 2% Ni(OH)2–ZIS. (e) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image for the sample of 2% Ni(OH)2–ZIS, and EDX mapping images of (f) Zn, (g) In, (h) S, (i) Ni, and (j) O elements.

X-ray diffraction (XRD) patterns of the samples (Fig. 2a) show the hexagonal structure of ZIS (JCPDS No. 65-2023).^11b No diffraction peaks for Ni(OH)₂ can be observed in Ni(OH)₂–ZIS composites because of their non-crystalline character and/or low-loading content.^11b The UV-vis diffuse reflectance spectroscopy (DRS) of bare ZIS shows strong absorption in the visible light region with the absorption edge at about 550 nm (Fig. 2b). Ni(OH)₂–ZIS composites show gradually enhanced visible light absorption with increased Ni(OH)₂, due to the light absorption of pure Ni(OH)₂ corresponding to the d–d transition of Ni²⁺.¹² Compared to the survey X-ray photoelectron spectroscopy (XPS) spectrum of ZIS, besides Zn, In and S elements, Ni is observed over 2% Ni(OH)₂–ZIS (Fig. S2, ESI†), which is mainly Ni(OH)₂ with a value around 856 eV (Fig. 2c).¹³ The binding energies in 2% Ni(OH)₂–ZIS are slightly lower than ZIS (Fig. 2d–f), indicating a strong interaction formed between ZIS and Ni(OH)₂.¹⁴

	Fig. 2 (a) Powder XRD and (b) DRS patterns of ZIS and various Ni(OH)2–ZIS samples. High-resolution XPS spectra of (c) Ni 2p, (d) Zn 2p, (e) In 3d and (f) S 2p for ZIS and 2% Ni(OH)2–ZIS.

The photocatalytic upgrading of HMF under an N₂ atmosphere to produce H₂ (Fig. S3, ESI†) and organic chemicals (Fig. S4 and S5, ESI†) after 4 h light irradiation is summarized in Table 1. Specifically, the blank ZIS (entry 1) exhibits a relatively low activity for H₂ evolution (297 μmol g⁻¹). Loading of Ni(OH)₂ can greatly promote the H₂ production rate (entries 2–5), and 2% Ni(OH)₂–ZIS (entry 3) shows the highest H₂ evolution rate (2405 μmol g⁻¹), which is approximately 8 times higher than that of blank ZIS. Accordingly, the highest yield of 30% with 79% selectivity for C₁₂ products (DHMF and BHMF) was achieved, corresponding to an apparent quantum efficiency (AQE) of 0.14%.^5a,15 Compared with previous literature reports, the present photocatalytic system shows competitive catalytic performance, in terms of both H₂ evolution and liquid fuel production (Table S1, ESI†). When decreasing HMF concentration to 5 mM (entry 6), the conversion increased from 39% to 42%, while the corresponding H₂ and C₁₂ production decreased. Furthermore, increasing HMF concentration resulted in the decreased conversion as well as the yield of H₂ and C₁₂ products (entry 7). The comparison of these results suggests that varying the concentration of HMF does not lead to DFF C₆ production, but has a great impact on the photoactivity and selectivity toward C₁₂ products. Under the default conditions, the concentration of HMF was set to 10 mM to get both high conversion and production. When prolonging light irradiation, the H₂ production increased with the reaction time and reached the maximum of 4412.5 μmol g⁻¹ at 10 h (Fig. S6a, ESI†). Accordingly, 57% conversion for HMF and nearly 100% selectivity for the C₁₂ products were achieved (Fig. S6b, ESI†). The recycling test shows that HMF conversion and H₂ production appear to slightly drop, while the selectivity for the C₁₂ products retains a high value (>90%, Fig. 3a) during the 50 hours reaction. The slight activity decrease may be due to the leaching of Ni(OH)₂ from the ZIS surface, as inductively coupled plasma mass spectroscopy (ICP-MS) analysis suggests that about 3% of Ni(OH)₂ was leached from the raw catalyst during the five consecutive cycles. In contrast, much lower activity and stability were displayed by the bare ZIS sample (Fig. 3b). The XRD patterns confirm that after 5-runs of recycling reaction the samples do not show a significant change in the crystal structure (Fig. S7, ESI†), indicating the good stability of the catalysts.

	Fig. 3 Recycling experimental results for HMF upgradation with H2 evolution after each 10 h recycling test: (a) for 2% Ni(OH)2–ZIS and (b) for bare ZIS.

Next, various control experiments were performed and revealed the mechanism to simultaneously produce H₂ and C₁₂ products. When the solvent (H₂O) was changed to acetonitrile (MeCN, entry 8), both H₂ production and HMF conversion significantly reduced, while almost no product was formed in N,N-dimethylformamide (DMF, entry 9). The result indicated that protons from H₂O play a critical role in the activation and transformation of HMF. No product was formed without light irradiation or a catalyst, demonstrating the necessity for visible light and a photocatalyst to drive this reaction (Table S2, ESI†, entries 1 and 2). Note that H₂ generation in the presence of HMF was facilitated by the simultaneous oxidation of HMF by utilizing photogenerated holes, thereby suppressing the electron–hole recombination. This hypothesis can be supported by the control experiment in which only a trace amount of H₂ and no C₁₂ or C₆ product was formed without HMF (Table S2, ESI†, entry 3).

To explore the involved reactive species and reaction intermediates during the photocatalytic reforming of HMF under anaerobic conditions, free radical trapping experiments were conducted. Isopropanol (IPA) was employed to trap the possible hydroxyl radical (˙OH),¹⁶ and led to the yield of 34% for C₁₂ products (Table S2, ESI†, entry 4), which is similar to the yield delivered by the system without IPA (30%). This result suggests that ˙OH is not the active species for HMF oxidation. To elucidate the role of electrons played in the reaction, potassium persulfate (K₂S₂O₈) as an electron scavenger was added during the reaction.¹⁷ As a result, the H₂ production was totally inhibited and the major liquid products changed from the C₁₂ products to the C₆ products of DFF (Table S2, ESI†, entry 5). When triethanolamine (TEOA) was added into the system as the scavenger for holes (Table S2, ESI†, entry 6),¹⁷ the H₂ yield was enhanced while the C₁₂ products yield was drastically decreased due to the stronger electron donating ability of TEOA than that of HMF. The above results jointly suggest that both photogenerated electrons and holes are crucial for the transformation of HMF into the C₁₂ products, while electrons are responsible for H₂ production.

The in situ electron paramagnetic resonance (EPR) test was performed to detect active intermediates during the catalytic reaction utilizing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a radical scavenger. As disclosed in Fig. 4a, no signal could be detected in the dark while the spectra appeared with six characteristics under light illumination, and the intensity increased with the radiation time, indicating the formation of carbon-cantered radicals during the photocatalytic reaction. The hydrogen and nitrogen hyperfine splitting for nitroxide nitrogen was found to be 22.9 and 15.8, respectively, demonstrating the generation of DMPO carbon cantered radicals (Fig. 4b). Notably, the carbon centered radicals can be produced through the reduction of the aldehyde group or oxidation of the C–H bond in the hydroxymethyl group of HMF. To check which group is responsible for the as-formed radical, we replace HMF with furfuryl alcohol (FA) as the substrate. Although the H₂ production was considerable in the HMF cultivated system, almost no C–C coupling product was formed (Table S2, ESI†, entry 7) and no peak could be detected during the EPR test. This result implies that the carbon cantered radicals were delivered by the reduction of the aldehyde group in HMF, as shown in Fig. 4b.

	Fig. 4 (a) DMPO spin-trapping EPR spectra of 2% Ni(OH)2–ZIS suspensions in HMF water solution in the dark, 5 min and 10 min light irradiation. (b) Quantitative analysis results of the EPR spectrum of 2% Ni(OH)2–ZIS composites with visible light irradiation. (c) Proposed reaction mechanism for HMF upgradation under an O2 or N2 atmosphere over Ni(OH)2–ZIS with visible light irradiation.

We then decoded the features of the coupling reaction system for fine biofuel generation with simultaneous H₂ production. We found that the conversion of HMF was increased while the yield of C₁₂ products was decreased under aerobic conditions. Specifically, the HMF conversion without N₂ purge conditions (48%, Table 1, entry 10) is higher than that obtained in the N₂ reaction system (39%, Table 1, entry 3). However, no C₁₂ products can be detected and the yield of DFF increased to 35% with a selectivity of 90%. In addition, no H₂ can be produced under aerobic conditions. Therefore, O₂ is essential for the conversion of HMF into DFF, and the presence of O₂ could prevent the formation of C₁₂ products. In other words, different reactive species may be produced under aerobic conditions as compared to anaerobic environments. To detect the reactive species under aerobic conditions, we employed TEOA, 4-chloro-2-nitrophenol (CN), and 1,4-benzoquinone (BQ) respectively as the scavengers for holes, singlet oxygen (¹O₂) and superoxide radical (˙O₂⁻).¹⁸ The HMF conversion and DFF production reduced (Table S2, ESI†, entries 8 and 9) considerably with the addition of these additives. Particularly, no DFF could be produced in the presence of BQ and TEOA. This result is consistent with previous reports that the holes and active oxygen species (˙O₂⁻ and ¹O₂) play dominant roles in the photocatalytic oxidation of HMF to DFF (Fig. 4c, left).¹⁹ Therefore, a feasible catalytic process could be inferred for the reaction occurring in anaerobic water (Fig. 4c, right). Firstly, electrons and holes were generated from ZIS under light irradiation, and the electrons then transferred to the loaded Ni(OH)₂ cocatalyst for surface reactions. HMF was activated to the carbon-centred radical of ˙CH(OH)C₅H₅O₂ by accepting an electron and proton, which then was oxidized and dimerized to C₁₂ products with the participation of holes. Meanwhile, the electrons reduce protons to H₂ and finally complete the bifunctional coupling catalytic process.

In summary, a fully sustainable route for the self-condensation of HMF into a C₁₂ jet/kerosene fuel intermediate (60% yield, nearly 100% selectivity) with simultaneous H₂ production was accomplished by a facile noble metal-free and sacrificial reagent-free methodology.

The manuscript was written with the contributions of all authors. All authors approved the final version of the manuscript. Wanqiong Kang: investigation, writing original draft, data curation. Xiaolong Li: investigation, formal analysis. Xiongxiong Zeng: investigation, formal analysis. Han Wu: investigation, formal analysis. You Ge: validation. Lan Yuan: investigation, supervision, conceptualization, writing – review & editing. Yi Liu: investigation, funding acquisition, resource. Chuang Han: methodology, supervision, writing – review & editing.

The support from the National Natural Science Foundation of China (22102126 and 22302182), and the Natural Science Foundation of Hubei Province (2023AFB091) is gratefully acknowledged.

Data availability

Conflicts of interest

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