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First authors: Ruixiang Ge, Ye Wang, Zezhou Li
Corresponding author: Duan Haohong
Communication unit: Tsinghua University
DOI: 10.1002/anie.202200211
Full text at a glance
Biomass-derived alcohol oxidation reactions (BDAOR) have excellent prospects in sustainable chemical production; however, selective electrooxidation to prepare high value-added aldehyde compounds remains challenging. In this paper, the authors successfully developed a nickel oxide-loaded single-atom ruthenium catalyst (Ru1-NiO) that electrocatalyzes BDAORs to selectively generate aldehyde compounds under a neutral medium. For the electrooxidation reaction of 5-hydroxymethylfurfural (HMF), Ru1-NiO exhibits a low potential of 1.283 V at a current density of 10 mA cm-2 and up to 90% optimal 2,5-dicarbon (DFF) selectivity. Experimental studies have shown that neutral media play a key role in achieving high aldehyde product selectivity, while single atom Ru provides OH* by promoting water dissociation to facilitate the oxidation of HMF in neutral media. In addition, the Ru1-NiO catalyst can be extended to a series of biomass-derived alcohols for the selective electrooxidation of the corresponding aldehyde compounds, which are often difficult to obtain in alkaline media.
Background
Replacing traditional fossil resources with renewable carbon sources is essential for the sustainable production of chemicals. Biomass materials are ideal candidates for their richness and carbon neutrality. Therefore, the development of advanced biomass value technology is crucial. Electrocatalysis offers many unique advantages over traditional thermocatalytic methods because electrochemical reactions can be carried out efficiently under benign conditions (room temperature and ambient pressure) and do not require an external oxidant/reducing agent. In addition, by coupling with renewable electricity, electrochemical processes can be carried out in a sustainable manner. Thanks to the unique advantages mentioned above, a growing number of studies are working on converting biomass derivatives into high-value fuels and chemicals through electrochemical reactions. Among them, the electrocatalytic biomass-derived alcohol oxidation reaction (BDAOR) has received widespread attention, and this process can produce high value-added products. In addition, BDAOR can be used as a thermodynamically more advantageous replacement reaction to replace the slow oxygen evolution reaction (OER), thereby increasing reactivity and lowering the starting potential when coupled to important cathodic processes such as hydrogen evolution (HER) or CO2 reduction.
Over the past few years, the development of the BDAOR process has made great progress, in which the efficient preparation of carboxylic acid compounds has been widely reported, including the oxidation of 5-hydroxymethylfural (HMF) to 2,5-furandicarboxylic acid (FDCA), and glycerol oxidation to formic acid. Currently, alkaline electrolytes are the most commonly used medium for BDAOR processes because non-precious metal-based catalysts are typically active and durable in such electrolytes. However, there are still few studies on biomass-derived alcohol-derived selective oxidation of aldehyde-based products.
The use of alkaline media seriously hinders the synthesis of aldehyde compounds. On the one hand, the alkali-catalyzed dimer reaction or hydroxyaldehyde condensation reaction of the aldehyde compound under high pH conditions; on the other hand, the base can promote the conversion of aldehyde into a reactive kaidiol, thereby promoting the subsequent electrocatalytic dehydrogenation to form carboxylic acid compounds. In view of the above situation, the use of neutral/ near-neutral electrolyte will bypass the above problems and selectively prepare aldehyde compounds from BDAOR. In fact, as previous studies have shown, oxidation of aldehyde groups in HMF can be largely inhibited under neutral pH conditions, as evidenced by the formation of 2,5-dicarbonate (DFF) intermediates and the accumulation of formyl-2-furancarboxylic acid (FFCA), suggesting that neutral pH conditions are more favorable for the synthesis of aldehydes. However, the activity of BDAOR in neutral/near-neutral media is several orders of magnitude lower than in alkaline media, from becoming a major obstacle to the synthesis of aldehyde compounds by BDAOR at neutral pH.
In this paper, the authors developed a nickel oxide-loaded single-atom ruthenium catalyst (Ru1-NiO) that electrocatalyzes the oxidation of biomass-derived alcohols into aldehyde products under a neutral medium. For the HMF oxidation reaction (HMFOR) in a 1.0 M phosphate buffer solution (PBS), Ru1-NiO exhibited a low potential of 1.283 V at a current density of 10 mA cm-2. In addition, 2,5-dicarbon (DFF) is an important pharmaceutical and chemical intermediate with an optimal selectivity of 90% at 1.5 V. Further results show that the selectivity of HMFOR is affected by the pH of the electrolyte, and the neutral medium is more conducive to the formation of aldehyde compounds than the alkaline medium. Analysis combined with cyclic voltammetry (CV), Raman spectroscopy, and operando electrochemical impedance spectroscopy (EIS) found that the high catalytic activity of Ru1-NiO stemmed from a significantly enhanced water dissociation process at the single-atom Ru site. In addition, Ru1-NiO can also be generalized to the electrocatalytic oxidation reactions of furfuryl alcohol, ethylene glycol, 1,3-propanediol, glycerol and benzyl alcohol, and the corresponding aldehyde products can be generated. The study reveals the important role of water dissociation in neutral BDAOR and creates new opportunities for the field of biomass value.
Graphic and text analysis
Figure 1. Microscopic topography of Ru1-NiO: (a) SEM plot of Ru1-NiO; (b) TEM plot of Ru1-NiO; (c) map of EDS elements of Ru1-NiO; (d) atomic-level resolution HAADF-STEM plot of Ru1-NiO, where red circles show monodispersed Ru atoms; HAADF-STEM plot of NiO obtained from the (e) [110] and (g) [211] directions; (f) and (h) correspond to the figure ( Intensity distribution of marker lines in e) and (g).
Figure 2. Spectroscopic characterization of the catalyst: (a) Ru 3p XPS spectra; (b) Ni 2p XPS spectra; (c) Ru K-edge XANES spectra; (d) Ni K-edge XANES spectra; (e) Fourier conversion Ru K-edge EXAFS spectra; (f) Fourier conversion Ni K-edge EXAFS spectra.
Figure 3. Electrochemical property characterization: (a) LSV curves of Ru1-NiO in a 1.0 M PBS solution containing and without 50 mM HMF; (b) LSV curves of NiO, Ru1-NiO and RuO2 in a 1.0 M PBS solution containing 50 mM HMF; (c) product distribution of Ru1-NiO at different potentials in a 1.0 M PBS solution; and (d) Ru1-NiO at 1.0 M (e) Possible reaction paths of HMFOR in neutral and alkaline media, (f) product distribution of Ru1-NiO at different potentials in 1.0 M KOH solution, and (g) HMF conversion and product yield in HMFOR process catalyzed by Ru1-NiO in 1.0 M KOH solution.
Figure 4. HMFOR mechanism analysis of Ru1-NiO: (a) CV curves of NiO and (b) Ru1-NiO in a 1.0 M PBS solution containing and without 50 mM HMF; (c) non-in situ Raman spectra of Ru1-NiO after 5 min of electrocatalysis; and (d) calculated surface pH of Ru1-NiO.
Figure 5. Operando EIS analysis: (a) Ru1-NiO and (b) Bode plots of Ru1-NiO in 1.0 M PBS solutions at different potentials; (c) equivalent circuit models fitted from impedance data; (d) Cφ vs. potential diagrams of catalysts in 1.0 M PBS solutions; and (e) HMFOR mechanism of Ru1-NiO in neutral media.
Figure 6. Substrate expansion studies.
Summary and outlook
In summary, a Ru1-NiO electrocatalyst was synthesized, which exhibited significantly enhanced activity during the electrocatalytic selective oxidation of HMF to generate DFF. Studies have shown that neutral media are more conducive to the formation of aldehyde products than alkaline media, and single atom Ru can significantly improve HMFOR activity in neutral media by promoting water dissociation. In addition, the Ru1-NiO catalyst can also be used for the electro-oxidation of various biomass derivation alcohols to form aldehyde compounds. This work provides a direction for the rational design of highly selective BDAOR electrocatalysts in neutral media.
Literature source
Ruixiang Ge, Ye Wang, Zezhou Li, Ming Xu, Si-Min Xu, Hua Zhou, Kaiyue Ji, Fengen Chen, Jihan Zhou, Haohong Duan. Selective Electrooxidation of Biomass-Derived Alcohols to Aldehydes in a Neutral Medium: Promoted Water Dissociation over a Nickel-Oxide-Supported Ruthenium Single-Atom Catalyst. Angew.Chem. Int. Ed. 2022. DOI: 10.1002/anie.202200211.
Literature links: https://doi.org/10.1002/anie.202200211