Zi-Yuan
Li
,
Jiang
Shao
,
Yi-Fei
Zhang
,
Xiao-Yu
Guo
,
De-Jiu
Wang
,
Hao
Dong
and
Ya-Wen
Zhang
*
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: ywzhang@pku.edu.cn
First published on 19th December 2024
5-Hydroxymethylfurfural (HMF), as a direct product of cellulose degradation, is an important biomass-based platform compound. The reductive amination of HMF is of significant industrial value among the upgrading reactions of HMF, which produces 5-hydroxymethylfurfurylamine (5-(aminomethyl)-2-furanmethanol, HMFA), an important intermediate of pharmaceutical and polymer materials. This work presents a facile one-pot synthesis of CuAu3@CuPd nanocubes, which demonstrate exceptional activity and selectivity in the electrochemical co-reduction of HMF and NO3− to yield HMFA. Furthermore, an optimal faradaic efficiency of over 75% was achieved by etching the nanoalloy material with a moderate concentration of NOBF4. The etching process exposed deeper CuPd active sites with a lattice distortion affected by the CuAu3 core, thereby promoting the catalytic activity. Catalytic mechanism studies indicate that the C–N coupling reaction pathway involves the in situ generation and capture of the *NH2OH intermediate. This work has paved a promising pathway for synthesizing high-value products from abundant biomass precursors utilizing the inorganic pollutant NO3− as a nitrogen source under ambient electrochemical conditions through the electrochemical co-reduction of HMF and NO3−.
Sustainability spotlightThe global shift towards sustainable energy and materials is crucial for addressing the depletion of non-renewable resources and environmental degradation. Biomass, as the only renewable organic carbon resource, offers a promising alternative. 5-Hydroxymethylfurfural (HMF), derived from the dehydration of hexose sugars, is a crucial biomass-based platform molecule with significant potential for producing high-value chemicals. This work addresses these issues by developing a novel electrochemical method for the reductive amination of HMF to produce 5-hydroxymethylfurfurylamine (HMFA), an important intermediate in pharmaceuticals and polymers. This method not only uses renewable energy sources, but also utilizes inorganic pollutant nitrate as a nitrogen source, significantly reducing the environmental impact and enhancing the value of biomass. The sustainable advancement of this work aligns with the United Nations Sustainable Development Goals (SDGs) 9 (industry, innovation, and infrastructure) and 12 (responsible consumption and production). |
5-Hydroxymethylfurfural (HMF) has been recognized as an important biomass-based platform molecule.4 HMF is primarily derived from the dehydration of hexose sugars like glucose and fructose obtained from cellulose degradation.5–7 The HMF molecule structure contains a furan ring, an aldehyde group, and a hydroxymethyl group, which confer relatively high organic reactivity, facilitating subsequent chemical development and upgrading to produce a range of high-value target products, including pharmaceutical molecules and novel polymer materials.8 Among the upgrading reactions of HMF, its reductive amination holds significant production value.9 The product, 5-hydroxymethylfurfurylamine (5-(aminomethyl)-2-furanmethanol, HMFA), is an important intermediate in pharmaceuticals and polymer materials.9,10,12 The amino functional group in the HMFA molecule provides it with more universal reactivity as a versatile molecular building block. Currently, there are two main methods of producing HMFA from HMF. One of them is the homogeneous method, which is commonly used in industrial production. However, this method involves the use of concentrated acid and therefore has its limitations, such as high corrosiveness to reaction vessels, high energy consumption and low yield.10 The other method is heterogeneous thermal catalysis, which involves using corrosive NH3 and flammable H2 in a reactor in the presence of a heterogeneous catalyst at high temperature and pressure.10–12 This method requires harsh operating conditions, leading to safety concerns, high energy consumption and challenges in catalyst separation and numerous side reactions. Therefore, it is urgent to find alternative pathways for the environmentally friendly synthesis of HMFA.
Research on electrochemical upgrading of HMF has been reported in recent years, mostly focusing on the anodic electrochemical oxidation of HMF to produce 2,5-furandicarboxylic acid.13–15 Reports on the electrochemical reductive amination of HMF are still scarce, and electrochemical synthesis of HMFA from HMF has not been reported to date. In 2016, Roylance et al. first reported the electrochemical reductive amination of HMF.16 They proposed a reaction pathway, where a primary amine captures the aldehyde group of HMF to form an imine, which is then reduced to a secondary amine on the cathode of an aqueous electrolytic reactor, using water as the proton source. In this work, methylamine was used as the nitrogen source, and the electrochemical reductive amination of HMF was carried out over an Ag dendritic electrode with a larger specific surface area, achieving >99% faradaic efficiency and selectivity at a potential of −1.1 V vs. RHE. In 2022, Zhang et al. published two consecutive studies investigating the electrochemical reductive amination of HMF using a TiS2 material with sulfur vacancies and a Ti-MOF as an electrocatalyst, respectively, with ethanolamine as the nitrogen source.17,18 All the aforementioned studies used organic amine molecules as the nitrogen sources, which brought about additional raw material costs and limited the functional group structure of the product. On the other hand, the electrocatalytic reductive amination (ERA) of carbonyl substrates (ketones or aldehydes) with inorganic ammonia or nitrate as a nitrogen source has attracted researchers' attention.19,20 Therefore, a combination of biomass-derived HMF upgrading and waste NO3− usage was explored in this work.
Herein, this work reports a novel electrochemical method for the reductive amination of HMF by coupling the electrochemical reduction products of NO3− with HMF (Fig. 1). By designing and optimizing a novel CuAu3@CuPd nanoalloy catalyst, this work achieved efficient and ambient electrosynthesis of HMFA in aqueous solution, with a faradaic efficiency of up to 75.52% and a yield of 3582.82 mg gcat−1 h−1. After over 14 hours of continuous stability testing, the catalyst activity showed only a slight decrease. A series of characterization experiments were conducted to investigate the structure and activity sites of the catalyst. In situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) and differential electrochemical mass spectrometry (DEMS) experiments were conducted to reveal the reaction mechanism.
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Fig. 1 Schematic diagram of direct electrosynthesis of HMFA from biomass-derived HMF and waste nitrate driven by renewable energy sources under ambient conditions. |
The morphology and microstructure of the synthesized catalysts were analyzed using high-resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (PXRD). In the HRTEM and STEM images, there was a significant difference between the shells and cores of individual nanoparticles, indicating the formation of a core–shell nanostructure. CPA nanoparticles (Fig. S1†) had a complete cubic morphology, while the shell of moderately etched CPA-5 (Fig. 2b and c) had many holes, and the shell of deeply etched CPA-10 samples almost completely disappeared, exposing the inner core (Fig. S2†). In the XRD patterns of the CPA sample (Fig. 2f), there were strong diffraction peaks at 2θ = 39.1°, 43.2°, and 45.4°. The diffraction peaks located at 39.1° and 45.4° correspond to the peaks of bcc CuAu3 (111) and (002) crystal planes, respectively, with a shift of 0.2° and 0.3°to a smaller angle, respectively. The diffraction peak at 43.2° corresponds to the bcc CuPd (011) peak, shifted by 0.5° to a larger angle. The shifts of diffraction peaks indicated that the Cu–Au core and Cu–Pd shell of the core–shell structure may affect the mutual lattices, causing lattice distortions in both layers.21,22 In the XRD patterns of CPA-5 and CPA-10 samples (Fig. 2f), larger shifts of diffraction peaks were observed, indicating that the inner layer with larger lattice distortion was exposed due to the etching of NOBF4. This crystal lattice distortion can cause surface strain and thus lead to a geometric effect and have a possible influence on the catalytic performances of the materials.21 Moreover, the intensity of the peak at 45.3° of CPA-10 increased, showing a larger exposure of the (001) surface of the CuAu3 core. In the HRTEM images, the lattice fringes with a spacing of 0.21 nm were measured in both CPA (Fig. S1a†) and moderately etched CPA-5 samples (Fig. 2b), matching the bcc CuPd (011) crystal plane spacing. Additionally, in the deeply etched CPA-10 samples (Fig. S2a†), lattice fringes with spacings of 0.15 nm and 0.22 nm were measured in the edge and center regions, respectively, matching the bcc CuPd (002) and fcc CuAu3 (002) crystal plane spacings.
The elemental composition and distribution of the synthesized nanoalloys were studied using scanning tunneling electron microscopy (STEM) and X-ray energy dispersive spectroscopy (EDS). The EDS mapping (Fig. S1b†) and line scan results (Fig. 2d and e) of CPA further confirmed that Pd was mainly distributed in the shell, Au was mainly distributed in the core region, and Cu was relatively evenly distributed throughout the nanoparticles. Furthermore, the EDS results of multiple CPA nanoparticles (Table 1) showed an elemental ratio of approximately Cu:
Pd
:
Au = 4
:
3
:
3 (atomic ratio), which was further confirmed by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Table 1, row 2). To make it clear, two regions of the shell (1) and core (2) on a single CPA nanoparticle were selected for EDS analysis of their elemental composition (Fig. 2d and Table 1). It was found that the shell had similar Cu and Pd content with little Au, while the core had decreased Pd content, though it was still present, due to the shell covering all surfaces of the cube. The Cu content minus the Pd content was close to 1/3 of the Au content.
Sample | Cu (atom%) | Pd (atom%) | Au (atom%) |
---|---|---|---|
CPA | 40.37 | 32.75 | 26.88 |
CPA* | 38.28 | 29.60 | 32.13 |
CPA-5 | 40.37 | 35.06 | 24.57 |
CPA-10 | 28.51 | 9.63 | 61.87 |
CPA: Area 1 | 45.61 | 47.17 | 7.22 |
CPA: Area 2 | 40.51 | 18.26 | 41.23 |
X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface elemental valence states of CPA, CPA-5 and CPA-10 samples (Fig. S3†). The peaks of Cu(0), Pd(0) and Au(0) were observed in all of the spectra, confirming that the Cu, Pd and Au elements existed in their alloy forms in CPA, CPA-5 and CPA-10 catalysts. Besides, the binding energy of Cu 2p3/2 in CPA-10 slightly increased, indicating electron transfer from Cu to Au and further proving the presence of the CuAu3 intermetallic compound in the core.21,22 In summary, the composition and structure of the synthesized nanoparticles can be denoted as CuAu3@CuPd nanocubes.
In the catalytic tests of the catalysts and control samples (commercial Cu nanoparticles (NPs), Au NPs and Pd/C catalyst) (Fig. 4a), the moderately etched CPA-5 sample showed superior FE and space-time yield compared to the original CPA and the deeply etched CPA-10 samples, corresponding to the formation and destruction of active sites, following the Sabatier principle. The FE reached 75.52%, and the yield was 3582.82 mg gcat−1 h−1 at a current density of 20 mA cm−2, the highest among all the explored electrocatalysts, with a conversion rate of 12.52% (Table S2†). The space-time yield of CPA-5 is higher than those of previously reported HMF amination catalysts running under solvothermal conditions, due to the low catalyst usage of electrocatalysis. Moreover, the electrocatalysis reaction pathway also avoids the use of high pressure NH3 and H2 gas, as well as organic solvents or acids, thus showing superior advantages in environmental protection, as well as green and sustainable chemistry. Long-term stability tests of the CPA-5 catalyst (Fig. 4b) showed that the cathodic reductive potential remained generally stable, with only a slight decline in FE during and after a 14.25 h continuous test. The HRTEM image and EDS mapping of CPA-5 after electrocatalysis (Fig. S4 and Table S1†) showed that the catalyst experienced no obvious change in morphologies and element distribution, further proving the outstanding stability. The CPA-5 catalyst after the reaction was characterized using HRTEM, EDS, PXRD and XPS measurements (Fig. S4 and S5†), and the results showed that the morphology, composition, structure, and element valence states remained consistent with those before the reaction, proving that the CPA-5 catalyst did not undergo significant changes before and after the electrocatalytic reaction, further confirming its outstanding stability.
To explain the origin of the excellent catalytic activity of CPA-5, a series of comparative experiments were designed and performed. CuPd nanocubes (Fig. S6†) were synthesized via a previously reported procedure,27 and tested under the same conditions for their catalytic performance. The results showed that the FE of the CuPd nanocubes decreased compared to commercial Cu NPs, Au NPs and Pd/C catalysts, indicating that the intrinsic activity of the pure CuPd alloy was relatively low and could not enhance the catalytic performance compared to individual metal nanoparticles. However, the situation changed when CuAu3 cores were introduced. The FE of the unetched CPA was superior to that of the CuPd nanocubes and individual metal catalysts, suggesting that the introduction of the Au-containing core and the formation of the core–shell structure helped to enhance the catalytic activity of the CuPd nanoscale alloy. Furthermore, due to appropriate etching, the deeper CuPd sites in moderately etched CPA-5 were fully exposed, which were closer to the CuAu3 core and thus experienced greater lattice distortion, thereby exhibiting superior catalytic activity. On the other hand, CPA-10 experienced a significant loss of the CuPd shell layer due to excessive etching, resulting in a decrease in the number of active sites and a consequent drop in FE. In summary, based on the comparative experiments on catalytic performance and characterization, it can be inferred that the CuPd nanoalloy phase with lattice distortion serves as the active site for the catalytic reaction.
A series of electrochemical characterization studies were performed to investigate the activity differences between the CPA-5 sample and other control samples (Fig. 4c–f). Linear sweep voltammetry (LSV) data (Fig. 4c) showed that CPA-5 had the highest current density at the same reduction potential. The Tafel slopes (Fig. 4d) were all close to the theoretical value of 118 mV dec−1, indicating that the first electron transfer step of NO3− reduction was the rate determining step.19,28 The Tafel slope of CPA-5 was 83.25 mV dec−1, the lowest among all tested catalysts and controls, indicating the highest charge transfer rate and kinetically favorable conditions for the electrocatalytic reaction. Electrochemical impedance spectroscopy (EIS) tests (Fig. 4e) showed that CPA-5 had the smallest semicircle diameter in the Nyquist plot, representing the lowest charge transfer resistance. Additionally, cyclic voltammetry (CV) tests at different scan rates (20–100 mV s−1) (Fig. S7†) showed that moderately etched CPA-5 had the largest double-layer capacitance (Cdl) (Fig. S8 and Table S2†) among all the metal nanoparticle catalysts, while the double-layer capacitance of deeply etched CPA-10 decreased, possibly because of the destruction of the “holey” shell structure. Thus, it can be indicated that moderate etching enhances the specific surface area of alloy nanocrystals, creating more active sites, consistent with previous assumptions. In addition, LSV tests were conducted on CPA-5 in electrolytes containing different substrates (Fig. 4f). The reductive current density in the electrolyte containing both HMF and NO3− is higher than those with one or no substrate, confirming the occurrence of the co-reduction reaction of HMF and NO3−.
Catalyst | C source | N source | Product |
---|---|---|---|
CPA-5 | HMF | NO2− | √ |
CPA-5 | HMF | NH2OH | √ |
CPA-5 | HMF | N2 | × |
CPA-5 | HMF | NH4+ | × |
Carbon paper | HMF | NO3− | × |
According to relevant research reports, the rate-determining step of the nitrate-involved co-reduction C–N coupling reaction is primarily the reduction of nitrate to nitrite.28,29 To demonstrate the effect of lattice distortion on the rate-determining step, we tested the LSV performance of catalysts with different degrees of lattice distortion in the CuPd surface layer (CPA-5, CPA, and CuPd NCs) under nitrate reduction conditions (Fig. S9†). The main sample CPA-5 exhibited a higher reduction current density (Fig. S9a†) and a lower Tafel slope (Fig. S9b†), further confirming that CuPd with lattice distortion serves as the active site for the catalytic reaction.
To verify the proposed reaction mechanism, differential electrochemical mass spectrometry (DEMS) and in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) were used to further characterize the intermediates under reaction conditions (Fig. 5). In the DEMS test (Fig. 5a), pulsed reduction potentials were applied to NO3− solution and (NO3− + HMF) solution, respectively, while a molecular pump applied a negative pressure on the back of the gas diffusion electrode to draw the gas-phase intermediates into the mass spectrometer, recording the signal intensity for different m/z changes over time. It was found that the peak signal intensities for m/z = 2 (corresponding to H2) and 17 (corresponding to NH3) did not change significantly before and after adding HMF to the NO3− reduction system, while the signal intensity for m/z = 33 (corresponding to NH2OH) significantly decreased in the solution containing HMF and NO3− compared to the solution containing only NO3− during electroreduction. This indicates that the *NH2OH intermediate generated during the cathodic reduction of NO3− was captured and consumed by HMF.19,30
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Fig. 5 Reaction mechanism study. (a) DEMS results and (b) in situ ATR-SEIRAS spectra of the electrocatalytic HMF-NO3− co-reduction reaction. (c) Proposed reaction mechanisms. |
In situ ATR-SEIRAS spectra (Fig. 5b) were collected by applying increasingly negative reduction potentials to the solution containing HMF and NO3−. As the applied potential became more negative, two characteristic absorption peaks appeared at 2854 and 2927 cm−1, corresponding to the characteristic vibrations of the primary amine C–N bond.30,31 Moreover, vibration peaks of the C–N bond were observed at 1558 and 1508 cm−1, confirming the presence of C–N coupling products.8 In summary, the proposed reaction pathway for the electrocatalytic HMF-NO3− co-reduction reaction can be represented as: NO3− → *NO2 → *NO → *HNO → *HNHO → *NH2OH and *NH2OH + HMF → HMF oxime → HMFA.
Footnote |
† Electronic supplementary information (ESI) available: Experimental section, additional XRD data, XPS spectra, TEM images and details of additional electrochemical tests (pdf). See DOI: https://doi.org/10.1039/d4su00700j |
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