A porphyrin-based COF to reduce CO₂ and nitrite in situ for photoelectrochemical catalysis synthesis of urea

Abstract

Photoelectrocatalytic catalysis of carbon dioxide (CO₂) and nitrite (NO₂⁻) direct coupling to synthesise urea (NH₂CONH₂) can effectively avoid the high energy consumption and pollution of the Bosch-Meiser method. COF_{_PP-TAT} was synthesised by hydroformylation of tetrabromophenyl porphyrin (TBPP), amination of trichothecenes (TC), and linking of the two compounds through Schiff base reaction. The comprehensive characterization of COF_{_PP-TAT} showed that has rich pore structure and specific adsorption of CO₂. These characteristics are favorable for COF to synthesize urea by reducing CO₂ in a green way. COF_{_PP-TAT} exhibited a good CO₂ reduction (CO₂RR) performance under photoelectrocatalytic (PEC) conditions with the stable production of urea (NH₂CONH₂) at a rate of 0.32 µmol h^-1. That process accompanied by the production of ammonia (NH₃), formic acid (HCOOH) and carbon monoxide (CO). The synthesis mechanism was discussed.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-03944-2.

Introduction

The CO₂ reduction (CO₂RR) to form valuable chemicals has been an active area of research in recent years¹. In order to achieve the goal of converting CO₂ into valuable items, photoelectrocatalytic Carbon Dioxide Reduction (CO₂R) is an effective way to convert inorganic materials into high-value organic materials^2–5.

Photosynthesis is a process used by plants to convert carbon dioxide into valuable carbohydrates⁶. Photosynthesis occurs primarily through chlorophyll, the porphyrin ring, which enables carbon dioxide reduction. Porphyrin rings have strong selective absorption of blue-violet and red light, and their large conjugated π-π system provides a transport platform for photoelectrons. Several studies have investigated the use of porphyrin materials for CO₂ electrochemical reduction. For example, a study by Ping et al. reported the use of cobalt-porphyrin complexes as electrocatalysts for the reduction of CO₂ to formic acid with high selectivity and efficiency⁷. Another study by Wu et al. investigated the use of zinc-porphyrin complexes as electrocatalysts for CO₂ reduction and demonstrated improved performance compared to traditional catalysts⁸. Whereas Zheng et al. have reported Iron (III) tetraphenylporphyrin thiocyanate having a subtle electrostatic interaction between the SCN ligand and the metal center⁹. Electrostatic interaction has led to a non-negligible effect on its electrocatalytic activity. Tridecene is a trimethylene derivative with internal free volume (IFV) channel, which has specific absorption of CO₂. COFs materials prepared based on porphyrin and trichothecene (TC) can be simultaneously photoelectrically active and efficiently adsorb CO₂.

Urea in the production process, the need to use extremely high energy consumption and flammable high-purity hydrogen, such preparation conditions will further deepen the environmental pollution^10,11. For example, the process generate large amounts of greenhouse gases (GHGs) and nitrogen oxides (NO_x) and aggravates global energy and environmental problems¹². The synthesis of NH₂CONH₂ by electrochemical reduction¹³ instead of the Bosch-Meiser method is an attractive alternative to reduce chemical energy consumption, reduce environmental pollution, and improve efficiency. The electrochemical reduction of N₂ and CO₂ to make NH₂CONH₂ has been widely used, but the inertness of the N-N bond and the higher dissociation energy were the difficulties of the reaction. The dissociation energy of the N-O bond was much lower than that of the dissociation energy of the N-N bond¹⁴, NO₃RR and NO₂RR instead of N₂RR was developed, which made the photoelectrocatalytic synthesis of NH₂CONH₂ easier. High concentration of nitrate and nitrite (NO₂⁻, NO₃⁻) leads to microbial enrichment, bacterial growth, and when absorbed by the organism it reacts with amines and causes methemoglobinemia and gastrointestinal cancers in the organism, so NO₂RR is simpler and has significance in protecting the environment for generating NH₂CONH₂.

Catalysts such as single atom catalysts (SACs) and metal-organic frameworks (MOFs) are commonly used in photoelectrocatalysis^14–21. Single-atom catalysts require fine tuning to avoid agglomeration during preparation; metal-organic frameworks not only have high morphological requirements, but also tend to deactivate during catalysis²². In addition, most of these catalysts require ionic solvents and critical conditions, leading to cumbersome post-treatments and consuming extra energy²³. In contrast, covalent organic frameworks (COFs) consist of strong covalent bonds and light elements with good water stability²⁴, which reduces the abuse of metals and lowers the difficulty of recycling. Their conjugated electronic structure allows COFs to have suitable energy levels and moderate visible light-accepting band gaps^25,26. Based on these features, COFs are considered as a promising metal-free semiconductor photocatalyst^13,27–37. The use of COFs materials for photoelectrocatalytic CO₂RR avoids complicated post-processing as well as harsh reaction conditions.

In this paper, a COFs catalyst based on porphyrin parent core and triptycene skeleton was prepared for the reduction coupling of CO₂ with NO₂⁻ to generate NH₂CONH₂, which is the first time that COF_{_PP−TAT}, a non-metallic catalyst, was applied to CO₂RR for the synthesis of NH₂CONH₂. Characterisation data illustrated the excellent optical property and electrocatalytic activity. Under the optimal conditions,

COF_{_PP−TAT} achieved continuous photoelectrocatalytic CO₂ and NO₂⁻ co-reduction to generate NH₂CONH₂ for 8 h with a yield of 0.32 µmol h^− 1. And the synthesis mechanism was described by infrared data and mechanism diagram.

Experimental section

Materials

All chemicals and reagents were used without any further purification. 4-formylphenylboronic acid, 10,15,20-tetrakis(4-bro-mophenyl)-porphyrin (TBPP), trichothecenes (TC), tetrakis(tri-phenylphosphine) palladium (10% Pd(PPh₃)₄), palladium-carbon catalyst (Pd/C), and potassium carbonate (K₂CO₃) were purchased from Anergy Chemistry, Indophenol blue reagent was purchased from Shanghai Amperexperiment Technology. Deionized water was used in all experiments.

Synthesis

Synthesis of 5, 10, 15, 20-tetrakis(4-formyl-biphenyl)-porphyrin (TBPP)

TBPP (1.20 g, 1.29 mmol) and 4-formylphenylboronic acid (TC, 1.40 g, 9.34 mmol) were dissolved in 80 mL DMF, 160 mL toluene and 40 mL H₂O in a three-necked flask, then added 10% Pd(PPh₃)₄ (0.18 g). The solution was heated to reflux and reacted for 72 h under N₂. The product was suction filtered and collected as violet black cake. The crude product was washed with in Dichloromethane (DCM). The mass of synthesized pure TBPP products is 1.21 g.

Synthesis of 2,6-Dinitrotriptycene (TAT)

Triptycene (2.54 g, 10.0 mmol) was dissolved in DCM (50 mL). Cerium ammonium nitrate (11.0 g, 20.0 mmol) was added, followed by sulfuric acid (1.92 mL, 3.53 g, 36.0 mmol). The reaction was stirred at room temperature overnight under N₂. The mixture was filtered to remove the cerium salts, and the filter-cake was washed with DCM until colorless. The comb-ined organic fractions were basified with K₂CO₃ solution (10%, 100 mL), the organic layer was taken, and the aqueous phase was extracted with further DCM (50 mL). The combined organic fractions were dried (MgSO₄), taken todryness under reduced pressure, and purified by column chromatography (gradient: 2:1 to 1:1 petrol: DCM) to give pure 2,6-dinitrotriptycene. Ethanol (100 mL) was cooled in an ice-bath under N₂. Palladium on carbon (10 wt %, 0.34 g) was added, followed by 2,6 - dinitrotriptycene (3.44 g, 10.0 mmol), and hydrazine hydrate (3.4 mL) dropwise. The reaction was heated to reflux under N₂ overnight and then cooled to room temperature. The product was collected by washing with ethanol and the mass of pure TAT products is 2.08 g.

Synthesis of COF_{_PP-TAT}

At first, TAT (0.31 g, 1105.60 µmol) was dissolved in 100 ml anhydrous DCM. A solution of TBPP (0.31 g, 280.00 µmol) was prepared in 100 ml DCM. 0.5 ml of 6 M acetic acid catalyst was added. This solution was added dropwise to the TAT solution at room temperature under Ar. Once the addition was completed the reaction mixture was stirred for 24 h. Finally, the reaction was refluxed at 50 ℃ for 48 h under O₂. This precipitate was then collected by filtration and thoroughly washed with DCM. The mass of pure COF_{_PP−TAT} products is 0.183 g. All of the synthesis routes are shown in Fig. 1.

Fig. 1.

Synthesis of COF_{_PP−TAT}.

Results and discussion

Catalyst characterization

UV spectroscopy, fluorescence spectroscopy, and photo-thermal experiments were performed to investigate the optical properties of the material. The absorption spectrum of COF_{_PP-TAT} was obtained in DCM (Fig. 2A). The spectrum in solution is characteristic of a porphyrin, featuring an intense Soret band at 413 nm and four Q bands at 512, 548, 589, and 644 nm. The UV absorption capacity of the material is interconnected with the fluorescence maximum excitation wavelength, and the fluorescence spectra show (Fig. S4) that the material has high fluorescence intensity under fluorescence excitation at 300, 450, and 650 nm, which is correspondingly linked with its UV absorption peak. The absorption peak at 670 nm is close to the region of near-infrared light, and by irradiating the samples with different concentrations by 808 nm lamp source, the rate of COF_{_PP-TAT} warming was positively correlated with its concentration (Table S1), and the above results indicate that the material can absorb and utilise photoelectrons at multiple wavelengths for photoelectrocatalysis and has the ability of photothermogenesis.

Fig. 2.

(A) UV-visible diffuse reflectance spectra of COF_{_PP−TAT}, (B) SEM spectra of COF_{_PP−TAT}, (C) CO₂ adsorption curve of COF_{_PP−TAT}, (D) COF_{_PP−TAT} sunder different atmospheres, (E) CV curve of COF_{_PP−TAT} under different wavelengths of light irradiation (F) PP, TAT and COF_{_PP−TAT} under light and dark conditions.

In order to investigate the surface morphology and adsorption of CO₂ on COF_{_PP−TAT} photocatalyst, scanning electron microscope (SEM) and BET surface area were used to characterize the photocatalysts. Materials showed that the large mesopores with large surface area (Fig. 2B), and the adsorption curve (Fig. 2C) indicates that the COF_{_PP−TAT} has a better adsorption of CO₂, which are favourable for performing the photocatalytic reduction.

Electrochemical reduction of CO₂

To evaluate the CO₂RR performance of as-prepared materials, a series of electrochemical tests were conducted in a standard H-cell. As shown in Fig. 2D, the COF_{_PP−TAT} cathode exhibited a significantly higher current density in the CO₂-saturated or N₂-saturated electrolyte than that in the Ar-saturated solution, indicating that CO₂RR or N₂RR is more favorable than HER. Furthermore, The CV curves (Fig. 2E) to evaluate the light response performance catalyzed by materials. COF_{_PP−TAT} at the 808 nm light illumination shows the highest catalytic activity of -8.4 µA cm^− 2 compared to those of 460 nm ( -6.9 µA cm^− 2) and Dark ( -5.7 µA cm^− 2), indicating its excellent photocatalytic activity of CO₂RR.

The photocatalytic activities of PP and TAT were also examined, and it can be seen from Fig. 2F that TAT has almost no photocatalytic activity. PP has a more powerful current response value under 808 nm illumination ( -35.2 µA cm^− 2), which is better than that under dark conditions ( -21.6 µA cm^− 2). COF_{_PP−TAT} showed a 15% increase in photoelectron utilisation under 808 nm light compared to the dark condition. The porphyrin-based COFs has good electrical conductivity and photocatalytic.

A series of concentrations of NaHCO₃ solutions were set to investigate the optimal CO₂RR concentration of COF_{_PP−TAT}. When the concentrations were at a low level (0.125 M to 0.5 M), the CO₂RR ability of COF_{_PP−TAT} increased with increasing concentration. The catalytic performance increased with the increase of c(NaHCO₃), indicating that COF_{_PP−TAT} has CO₂RR performance, and the optimal catalytic reduction c(NaHCO₃) is 0.5 M.

A series of concentrations of NaNO₂ solutions were set to investigate the optimal NO₂RR concentration of COF_{_PP−TAT}. When the concentrations were at a low level (0.05 M to 0.4 M), the NO₂RR ability of COF_{_PP−TAT} increased with increasing concentration. In addition, the increasing oxidation peak at 0.2 V (vs. SCE) is caused by NO₂⁻ being catalytically oxidised. Catalytic performance increased with the increase of c(NaNO₂), indicating that COF_{_PP−TAT} has NO₂RR performance, and the optimal catalytic reduction c(NaNO₂) is 0.4 M.

The photoelectrocatalytic reduction reaction of COF_{_PPTAT} was carried out in a reaction system of 0.5 m NaHCO₃ and 0.4 M NaNO₂ under the irradiation of a 300 W xenon lamp at an applied voltage of -0.78 V vs. SCE.

Online gas chromatography (GC),¹H-NMR and indophenol blue spectrophotometric method were adopted to detect gaseous and water-soluble products of CO₂RR. The electrocatalytic performance of COF_{_PP-TAT} was tested in 0.5 M mixture of NaHCO₃ and NaNO₂ using an H-type cell equipped with a cation exchange membrane to separate each compartment. The GC spectrum of the products (Fig. S5) reveals the presence of CO₂、O₂、N₂ and CO at retention time of 3.006 min, 9.126 min, 9.561 min, and 11.409 min, respectively. CO₂ is the gas supply to the system, O₂ and N₂ are from air. CO is the gaseous reduction product obtained in photoelectrocatalytic reduction. Formic acid, and urea are detected by NMR spectroscopy (Fig. 3A) at the chemical shifts of 8.33 and 5.68 ppm severally. When the liquid sample was mixed with indophenol blue colour developer, the solution changed from yellow to blue-green (Fig. S6), indicating that NH₃ was present in the system.

Fig. 3.

¹H-NMR spectrum of reduction product (A), urea synthesis mechanism (B).

Urea quantification

Set up a series of concentrations of ammonia standard solution, after the reaction with indophenol blue colour developer, take the absorbance value at 665 nm to plot the standard curve calculation (Fig. S7). The concentration of NH₂CONH₂ in the sample was calculated to be 21.3 µmol L^− 1, and the yield per unit time was 0.32 µmol h^− 1.

Catalytic properties and mechanism

The functional group changes of the electrolyte before and after catalysis were detected to illustrate the mechanism of NH₂CONH₂ synthesis. The O-H bending vibrational peak located at 2000 cm^− 1 at the electrolyte before photoelectrocatalysised (Figure S8). When the electrolyte was photoelectrocatalyzed, the peaks at 1161, 1370, 1458 and 1600 cm^− 1 in the liquid product belonged to the -NH₂ in-plane deformation vibration, stretching vibration peaks, C-N stretching vibration peak and C = O stretching vibration peak, which were the characteristic absorption peaks of NH₃ and NH₂CONH₂, respectively. It indicated that the COF_{_PP−TAT} photoelectro-catalytic CO₂ reduction to generate NH₂CONH₂ is accompanied by the by-product NH₃.The mechanism of CO₂ and NO₂⁻ synthesis of NH₂CONH₂ is as follows (Fig. 3B), the NO₂⁻ in the system adsorbed to the surface of the catalyst to form *NO₂, which was reduced to *NH by electron transfer and dehydration, and CO₂ was adsorbed, reduced, and dehydrated to obtain *CO, and the two of them were coupled in situ on the surface of the catalyst by electron transfer, and reduction and hydrogenation, which ultimately led to the formation of NH₂CONH₂.

Conclusions

In conclusion, we synthesized a non-metallic photocatalyst COF_{_PP−TAT}. It was synthesised based on porphyrin and tridecene, which can effectively activate CO₂ and NO₂⁻ and couple to synthesise urea. The synthesised COF_{_PP−TAT} possesses the large conjugated structure of PP and the IFV channel of TAT, which results in a rich pore structure and efficient adsorption of CO₂ in the material. It provides a new green way for efficient synthesis of urea and even amino acids.

Electronic supplementary material

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Author contributions

Zijian Zeng wrote the main manuscript text. All authors reviewed the manuscript.

Data availability

Data is provided within the manuscript or supplementary information files.

Declarations

Competing interests

The authors declare no competing interests.