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. 2025 Aug 25.
doi: 10.1039/d5sc05036g. Online ahead of print.

Molecular functionalization of Ni(OH)2 promotes electrosynthesis of adipic acid

Rui Yang  1 , Yuanhao Li  2 , Haonan Xu  3 , Qicheng Zhang  4 , Shufan He  1 , Tao Shen  1 , Xiaobin Fan  4 , Tao Wu  3 , Yifan Sun  1
Affiliations

Molecular functionalization of Ni(OH)2 promotes electrosynthesis of adipic acid

Rui Yang et al. Chem Sci. .

Abstract

Adipic acid is an essential platform molecule for polymer production and is industrially manufactured by thermochemical oxidation of the cyclohexanone/cyclohexanol mixture (KA oil). Alternatively, electrifying provides a green and sustainable route to synthesizing adipic acid, but has been restricted by the low catalytic efficiency. Herein, we report that a nickel hydroxide electrocatalyst functionalized with 4,4'-bipyridine (Bipy-Ni(OH)2) delivers a 3-fold greater productivity compared with that of pristine Ni(OH)2, achieving an excellent yield (90%) towards efficient adipic acid electrosynthesis. The experimental and molecular dynamics (MD) simulation results show that Bipy serves as a reservoir to accumulate cyclohexanone, which has low solubility in aqueous solutions. Molecular probe analysis coupled with density functional theory (DFT) calculations demonstrates that Bipy functionalization promotes formation of the key intermediate (2-hydroxycyclohexanone) via modulating the surface electronic characteristics. A Bipy-Ni(OH)2//Ru electro-reforming system in a two-electrode configuration was further constructed to enable concurrent hydrogen and adipate production, revealing its potential for practical applications. Our report demonstrates the efficacy of grafting judicious ligands to electrocatalysts to harness mass transfer and optimize active sites, and the insights can be useful for electrooxidation of a wider scope of organic molecules.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Scheme 1
Scheme 1. Illustration of the (a) thermochemical and (b) proposed electrochemical routes of KA oil oxidation to produce adipic acid.
Fig. 1
Fig. 1. (a) Schematic illustration for the preparation of the Bipy-Ni(OH)2 catalyst. (b) XRD patterns of the Bipy-Ni(OH)2 and Ni(OH)2, and the α-Ni(OH)2 (JCPDS: no. 38-0715) reference pattern. (c) TEM image of Bipy-Ni(OH)2. (d) HAADF-STEM image and corresponding STEM-EDS element maps of Bipy-Ni(OH)2, indicating a uniform distribution of Ni, O and N. (e) Raman spectra of Bipy and Bipy-Ni(OH)2. (f) Normalized XANES spectra and (g) k3-weighted EXAFS profiles of Bipy-Ni(OH)2, Ni(OH)2 and the Ni foil reference.
Fig. 2
Fig. 2. (a) Reaction scheme for the anodic oxidation of cyclohexanone to adipate (1), glutarate (2) and succinate (3). (b) Adipic acid productivity (bar chart, left axis) and FE (line graph, right axis) over Bipy-Ni(OH)2 and Ni(OH)2 at different potentials in 1.5 M NaOH with 100 mM cyclohexanone. (c) i–t curves of Bipy-Ni(OH)2 and Ni(OH)2 at 1.53 V vs. RHE in 1.5 M NaOH with 100 mM cyclohexanone. (d) Yield of adipate as a function of reaction time over Ni(OH)2 with different Bipy contents at 1.53 V vs. RHE in 1.5 M NaOH with 100 mM cyclohexanone. Bipy-Ni(OH)2-11, Bipy-Ni(OH)2-12, and Bipy-Ni(OH)2-13 denote the mole ratio between Bipy and Ni for the electrodeposition as 1 : 1, 1 : 2 and 1 : 3, respectively. (e) 1H NMR spectra of the sample before and after electrolysis of Bipy-Ni(OH)2 at 1.53 V vs. RHE, respectively, and pure adipate for comparison. (f) Comparison of the yield and selectivity of adipate of Bipy-Ni(OH)2 with those in previous reports (Table S1). (g) Adipate productivity of Bipy-Ni(OH)2 and Ni(OH)2 by normalization with ECSA.
Fig. 3
Fig. 3. (a) The snapshots from time-dependent MD simulations showing the spatial distribution of cyclohexanone molecules near the Bipy-Ni(OH)2 surfaces. (b) The productivity and corresponding FE of products over Bipy-Ni(OH)2 and Ni(OH)2 with C4–C8 cyclic ketones with low aqueous solubility at 1.53 V vs. RHE in 1.5 M NaOH with 100 mM substrate. (c) Structural illustration of various N,N-containing molecular ligands. (d) The productivity and corresponding FE of adipic acid electrosynthesis over bare Ni(OH)2 and functionalized Ni(OH)2 with different N,N-containing ligands at 1.53 V vs. RHE in 1.5 M NaOH with 100 mM cyclohexanone.
Fig. 4
Fig. 4. (a) Possible reaction pathways of cyclohexanone oxidation to adipic acid in the literature. (b) EPR signal over Bipy-Ni(OH)2 captured in 1.5 M NaOH with 0.1 M cyclohexanone. (c) Schematic of Path 2 with 2-hydroxycyclohexanone as the key intermediate. (d) Yield of adipate electrosynthesis using cyclohexanone and 2-hydroxycyclohexanone as the substrate molecule after 4 h of electrolysis (electrolyte composition: 100 mM substrate molecule + 1.5 M NaOH). (e) DFT-calculated energy diagram of cyclohexanone to 2-hydroxycyclohexanone over the pristine and Bipy-functionalized Ni(OH)2 surface. The nickel, oxygen, carbon, hydrogen and nitrogen atoms are in grey, red, brown, pink and blue, respectively. (f) In situ Raman spectra of Bipy-Ni(OH)2 in the 1.5 M NaOH aqueous solution without and (g) with cyclohexanone.
Fig. 5
Fig. 5. (a) Linear sweep voltammetry (LSV) curves of the Bipy-Ni(OH)2//Ru couple in 1.5 M NaOH with and without KA oil (3 : 2 of cyclohexanol and cyclohexanone) at a scan rate of 5 mV s−1. (b) Yield and selectivity of adipate using the Bipy-Ni(OH)2//Ru couple with a two-electrode configuration at 1.8 V. (c) The long-term i–t curve of KA oil oxidation over Bipy-Ni(OH)2//Ru at 1.80 V, with the electrolyte (1.5 M NaOH with KA oil) refreshed every 4 h.
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