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. 2023 Oct 19;3(11):2987–2992. doi: 10.1021/jacsau.3c00537

Formamide Electrosynthesis from Methanol and Ammonia in Water over Pr-Doped MnO2

Jiewei Zhu †, Jiang Shao ‡, Bai-An Shen ‡, Jixiang Chen †, Yifu Yu ‡, Shuyan Song §, Xin-Bo Zhang §, Bin Zhang †,‡,*, Bo-Hang Zhao ‡,*
PMCID: PMC10685412  PMID: 38034964

Abstract

graphic file with name au3c00537_0005.jpg

A rare earth element doping strategy is reported to boost the activity and enhance the stability of MnO2 for selective formamide production through electrocatalytic oxidation coupling (EOC) of methanol and ammonia. MnO2 doped with 1% Pr was selected as the best candidate with an optimized formamide yield of 211.32 μmol cm–2 h–1, a Faradaic efficiency of 22.63%, and a stability of more than 50 h. The easier formation of Mn6+ species and the lower dissolution rate of Mn species over Pr-doped MnO2 revealed by in situ Raman spectra were responsible for the boosted formamide production and enhanced stability. In addition, a two-electrode flow electrolyzer was developed to integrate EOC with C2H2 semihydrogenation for simultaneously producing value-added products in both the anode and cathode.

Keywords: electrosynthesis, formamide, water, rare earth, doping

Amides, as a very important class of compounds in chemistry, medicine, and the agriculture industry, have been studied extensively over the past century.1−4 Formamide, for example, is produced and consumed in millions of tons worldwide every year and is widely used in pesticides, dyes, pigments, flavors, organic synthesis, and pharmaceuticals.5,6 Currently, formamide is mostly synthesized from CO and ammonia (NH3), which not only requires high-pressure conditions but also suffers from byproducts such as sodium formate and sodium carbonate, which crystallize in the pipeline and cause blockages.7 In addition, this technique is energy-consuming and environmentally unfriendly.8 Recently, an electrocatalytic refinery strategy under ambient conditions as an eco- and energy-friendly alternative has gained much attention. Qiao et al. synthesized 2-chloroethanol through ethylene electro-oxidation with natural seawater as the electrolyte.9,10 Accordingly, our proposed electrocatalytic oxidation coupling (EOC) strategy for the reaction of methanol and NH3 provides a new and sustainable approach toward formamide synthesis (Figure 1a). However, both the activity and durability of catalytic materials under harsh oxidative conditions are still critical issues for EOC strategies, and the high dependence on noble metals to avoid dissolution of the catalyst further hinders their practical application.11−13 Thus, it is highly desirable to develop functional transition metal (TM) materials with outstanding activity and stability for the EOC strategy, but this is still challenging.

Figure 1.

Figure 1

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(a) Sustainable method to synthesize formamide from methanol and ammonia. (b) Catalyst screening.

Among the general methods for promoting the activity and stability of electrooxidation catalysts, including forming a protection layer, constructing superaerophobic nanoarrays, and doping heteroatoms, the doping strategy is the most facile and feasible due to its convenience and adjustability.14−16 It has been reported that the unique electronic structure of rare earth elements enables highly active outermost f electrons and second-outermost d electrons, making them outstanding dopants.17,18 Recently, rare earth element-doped materials have greatly increased the catalytic activity, energy conversion, and storage efficiency in multitudinous fields.19−22 MnO2 is particularly suitable as a catalytic material for electrooxidation due to its low price, eco-friendliness, and abundant resources.23−25 Moreover, the electron configuration of manganese in MnO2 is 3d54s2, showing five electrons in unsaturated d orbitals, which is easy to electronically modulate and thus improves the catalytic activity and stability.26,27 Therefore, rare earth element-doped MnO2 is supposed to be a promising candidate for the EOC process to produce formamide but has rarely been reported.

Herein, various rare earth element-doped MnO2 catalysts were synthesized by the pyrolysis method and used as electrooxidation catalysts for the C–N coupling of methanol and ammonia through the EOC process to synthesize formamide, among which Pr-doped MnO2 (1%Pr-MnO2) showed the best performance and stability enhancement. The optimized formamide production Faradaic efficiency (FE) and yield were 22.63% and 211.32 μmol cm–2 h–1, respectively, which were able to maintain stability for more than 8 h. The key intermediates in the reaction process were captured by electrochemical online differential mass spectrometry (DEMS) and in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). A reasonable reaction path was deduced. Our work not only demonstrates an efficient and stable non-noble catalyst for formamide electrosynthesis but also provides new insight into the application of rare earth metals as dopants to enhance the activity and stability of TM materials in electrooxidation reactions.

First, the performance of different catalysts for the EOC process to obtain formamide from methanol and ammonia was evaluated by electrolysis in a single-chamber electrolytic cell at a current density of 100 mA cm–2 for 3 h. Among the 11 screened materials, Pr-MnO2 exhibited the highest FE of formamide, ∼23%, which is approximately 2 times higher than that of the single MnO2 counterpart, demonstrating that rare earth element doping could promote the EOC process toward formamide synthesis and that Pr is the best dopant from the perspective of formamide synthesis (Figure 1b). Thus, Pr-MnO2 was chosen as the template material for further research.

To understand the physicochemical information on the chosen template material, a series of characterizations were performed by field-emission scanning electron microscopy (SEM), field-emission transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). As shown in Figures 2a and S1, the sintered Pr-MnO2 and MnO2 catalysts did not retain the lamellar structure but presented a stacked three-dimensional configuration on top of each other. The higher-magnification TEM (Figures 2b and S2) showed that the Pr-MnO2 edge structure is well-defined and that the internal structure is built up by lamellar layers. The lattice stripes of Pr-MnO2 were characterized by HRTEM (Figure 2c), indicating that the exposed 101 crystal planes of Pr-MnO2 possess larger crystal plane spacing compared to MnO2(101) (Figure S2c), also suggesting that Pr is doped into the lattice rather than loaded on the surface of MnO2. In addition, the uniform elemental distributions of Pr-MnO2 and MnO2 are confirmed by energy-dispersive spectroscopy (EDS) mapping (Figures 2d–g and S3).

Figure 2.

Figure 2

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(a) SEM, (b) TEM, (c) HRTEM, and (d–g) EDS mapping of the Pr-MnO2 catalyst. (h) XRD patterns of x%Pr-MnO2. (i) XPS spectra of Pr-MnO2 with different Pr loadings.

To further illustrate the present form of Pr in the catalyst, we synthesized x%Pr-MnO2 catalysts with different rare earth element contents (including x = 0, 0.5, 1, and 2) and characterized them by XRD and XPS. As shown in Figure 2h, the characteristic peaks in these four samples all originate from β-MnO2 (JCPDS no. 24-0735), indicating that the introduced Pr does not affect the formation of the MnO2-based material. Note that although the overall peaks remain unchanged, the main peak located at 37.5° shifts to a small angle with the introduction of Pr, and the shift becomes more obvious with the increased ratio of introduced Pr, further confirming that Pr with a larger atomic radius is doped into the MnO2 crystal lattice.28 Moreover, the XPS results (Figure 2i) show that the doping of Pr into MnO2 results in a positive shift of binding energy in Mn4+ species with the doping amount of Pr, which reflects a deficiency of electrons on the Mn atoms in the doped MnO2 materials, suggesting the existence of electronic interactions between Pr and MnO2.29,30 Although the XPS signal of Pr is relatively weak, the negative shift in binding energy further verifies the existence of electronic interactions and the high dispersion of the Pr dopant (Figure S4). All of these aforementioned results demonstrate that Pr is successfully doped into MnO2.

Before evaluating the durability of the as-prepared Pr-MnO2, we first optimized the reaction system by screening the volume ratio of methanol and ammonia at a current density of 100 mA cm–2 and found that the optimum performance was attained when the volume ratio was 2:1 (Figures 3a, S5, and S6). In addition, the performance of the catalysts with different rare earth dopings was also compared with that of their MnO2 counterparts, and it was discovered that the 1%Pr-MnO2 catalyst demonstrates the highest selectivity toward formamide synthesis (Figure 3b). Then we conducted performance tests using 1%Pr-MnO2 as the electrocatalyst at different current densities and compared the results with those for the control sample MnO2. The optimal current densities for formamide production of 1%Pr-MnO2 and MnO2 were 100 and 80 mA cm–2, with FEs of 22.63% and 15.13%, respectively (Figure 3c,d). The FE of all oxidation products under the optimized conditions is displayed in Figure S7. In addition, no-electricity control experiments were carried out to exclude the electrothermal effect (Figure S8). The stability evaluation of formamide electrosynthesis through the EOC process was performed under the optimal experimental conditions (CH3OH:NH3 = 2:1, 100 mA cm–2). As shown in Figure S9, the FE and formamide yield of MnO2 decrease immediately with the reaction time within 7.5 h, indicating the poor durability of the MnO2 catalyst. Note that the formamide synthesis activity and selectivity of Pr-MnO2 are retained after seven cycles of an 8 h continuous test (more than 50 h in total) without an obvious decay (Figures 3e, S10, and S11), confirming the enhanced catalytic durability after Pr doping. Moreover, MnO2 needed a higher potential for stability testing under the same conditions, a significant potential rise occurred with increasing test time, and the catalyst was deactivated after less than 5 h of electrolysis, also indicating the poor conductivity and stability of the catalyst. The FE of formamide over MnO2 gradually decreased and was less than one-half of the performance of Pr-MnO2, further illustrating the importance of Pr doping for enhancing the activity and stability of MnO2 used for formamide electrosynthesis (Figure S9). Additionally, we performed ICP analyses on the electrolyte following stability tests and discovered that the amount of Mn dissolved in Pr-MnO2 is less than that of MnO2 (Figure S12), which further reinforces that the doping of Pr is favorable for enhancing the durability of MnO2. Moreover, a two-electrode flow electrolysis, which couples EOC with cathodic acetylene (C2H2) semihydrogenation to ethylene (C2H4) reaction,31 was built to demonstrate the advantage of the electrocatalytic strategy. As shown in Figure 3f, C2H4 could be produced with a selectivity of ∼100% and a C2H2 conversion of 37.21% under the optimized conditions of the EOC process, further enhancing the additional value of our proposed electrocatalytic strategy.

Figure 3.

Figure 3

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(a) Optimization of the volume ratio of methanol and ammonia. (b) Influence of different catalysts on formamide electrosynthesis. (c) Performance of 1% Pr-MnO2. (d) Performance of MnO2 for formamide synthesis. (e) Stability test of 1% Pr-MnO2. (f) Illustration and performance of two-electrode electrolysis.

To illustrate the changes in the catalyst during the reaction, we characterized the catalyst by using in situ Raman spectroscopy. As shown in Figure 4a, three distinct peaks located at 273, 367, and 646 cm–1 were attributed to MnO2,32,33 and with the application of current, the characteristic peak of MnO2 disappeared and was subsequently replaced by a peak at 1014 cm–1 for Mn6+ species, which were regarded as the reactive sites.34 Given the appearance of the surface-adsorbed *NH2 peak signal (∼1450 cm–1) at a relatively higher current density than that of the Mn6+ species, we rationally deduced that Mn6+ acted as a trigger for formamide electrosynthesis. To verify this assumption, an electricity cutoff control experiment was conducted. As shown in Figure S13, the intensity of the Mn6+ peak decreases quickly when the electricity is cut off, and the signal of *NH2 disappears along with the Mn6+ species, confirming that the high-valence Mn6+ species play a key role in the oxidative coupling process, which is similar to Ni3+ in the oxygen evolution reaction.35,36 Then, in situ Raman tests were also performed on MnO2, and Mn6+ appeared only at a higher current density compared to Pr-MnO2, indicating that the doping of Pr could favor the generation of Mn6+ during electrocatalysis. To further confirm the accuracy of the peak attribution, ex situ Raman tests of MnO2, Pr-MnO2, and carbon paper were carried out (see Figure S14 for details). Next, to reveal the reaction pathway of methanol and ammonia C–N coupling to formamide, we adopted online DEMS to trap the intermediate species of the reaction and found that the signal of the imine (H2C=NH2) intermediate with m/z = 29 was successfully captured with the application of current, which played a key role in the presumption of the reaction pathway (Figures 4b and S15).37,38 To further investigate the changes in the intermediates during the reaction, we also performed in situ ATR-FTIR with the current density applied in the range of 10–100 mA cm–2, during which the spectral signals were collected and the C–O (1245 cm–1), C–N (1445 cm–1), and C≡N (2183 cm–1) signals of key intermediate species were successfully traced (Figure 4c).39−41 Then a series of control experiments demonstrated that the HCOH intermediate and NH3 reactant are crucial to the oxidative C–N strategy (Table S1). Thus, we propose a reasonable reaction pathway in which ammonia nucleophilically attacks the aldehyde intermediate produced by methanol oxidation, followed by dehydration to produce H2C=NH2, which is subsequently oxidized to produce HC≡N, hydrolysis of which yields formamide (Figure 4d).42−44

Figure 4.

Figure 4

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(a) In situ Raman, (b) DEMS, and (c) in situ ATR-FTIR spectra of 1% Pr-MnO2. (d) Plausible reaction pathway.

In conclusion, to overcome the noble metal dependence of the proposed ECO process, rare earth element-doped MnO2-based materials were demonstrated as efficient, selective, and durable non-noble catalysts for the EOC process to form formamide with commendable activity and stability. The study demonstrates that the 1% Pr-doped MnO2 catalyst can generate formamide at a current density of 100 mA cm–2 with a Faradaic efficiency of 22.63%, a yield of 211.32 mol cm–2 h–1, and stability for 8 h. The easier formation of Mn6+ species and the lower dissolution rate of Mn species after Pr doping are revealed to be responsible for the boosted formamide production and enhanced stability. Additionally, we proposed a plausible C–N coupling reaction path and detected the key intermediates in the reaction process using methods such as DEMS and ATR-FTIR. This study not only offers a integrated method for the environmentally friendly synthesis of formamide and ethylene but also provides a fresh perspective into the application of rare earth metals as dopants to enhance the activity and stability of TM materials in electrooxidation reactions.

Methods

Electrochemical Measurements

To begin the performance evaluation, pre-experiments on the electrolyte choice were conducted first. The target product was strongly suppressed in both strongly alkaline and acidic electrolytes. Thus, CH3OH and NH3 with different volume ratios were added to a 0.5 M NaHCO3 aqueous solution. [Re]-MnO2, Pt foil, and a saturated calomel electrode were used as the working, counter, and reference electrodes, respectively. Different current densities from 20 to 150 mA cm–2 were determined using an electrochemical workstation (CS150H, Wuhan CorrTest Instruments Co., Ltd.), and the reaction time was set as 3 h. The influence of the reactant ratio on the catalytic performance was determined by controlling the total volume of 15 mL (a mixture of CH3OH and NH3), and other experimental conditions were kept consistent. The two-electrode flow electrolyte electrochemical measurements were carried out in a typical membrane electrode reactor consisting of 1% Pr-MnO2 as the working electrode and a gas diffusion layer (GDL) loaded with Cu nanoparticles as the counter electrode. The cathode cell and anode cell were separated by a Nafion 117 proton exchange membrane. A schematic illustration of the flow cell is shown in Figure 3f. A peristaltic pump was applied to implement circulation of the liquid phase. The flow rate of C2H2 gas was controlled at 15 mL min–1 by a mass flow meter, and the gas at the flow cell outlet was directly introduced into the gas chromatograph for the analysis of the products.

In Situ ATR-FTIR Measurements

In situ ATR-FTIR was carried out to trace the signals of the intermediates using a Nicolet Nexus 670 spectrometer equipped with a liquid-nitrogen-cooled mercury–cadmium–telluride detector. An ECIR-II cell equipped with a Pike Veemax III ATR accessory in a three-electrode system was provided by Shanghai Linglu Instrument & Equipment Co. The data were collected through Pt-covered monocrystal silicon. To improve the signal intensity, monocrystalline silicon was initially coated with a layer of Au using the chemical plating method.

Acknowledgments

We acknowledge the National Natural Science Foundation of China (22271213 and 22209120) and the Open Funds of the State Key Laboratory of Rare Earth Resource Utilization (RERU2022005).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.3c00537.

  • Additional experimental details, material characterizations, and methods, including the calibration curves (PDF)

Author Contributions

CRediT: Jiewei Zhu data curation, formal analysis, investigation, methodology; Jiang Shao formal analysis, methodology, validation, writing-original draft; Bai-An Shen formal analysis, investigation; Jixiang Chen formal analysis; Yifu Yu funding acquisition, resources, visualization, writing-review & editing; Shuyan Song data curation, investigation, project administration; Xin-Bo Zhang data curation, investigation, project administration; Bin Zhang conceptualization, formal analysis, supervision, writing-review & editing; Bo-Hang Zhao data curation, formal analysis, investigation, methodology, project administration, supervision, writing-original draft.

The authors declare no competing financial interest.

Supplementary Material

au3c00537_si_001.pdf (1.6MB, pdf)

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