Phenanthroline-based microporous organic materials for removal of Cu(II) from aqueous solution and reutilization of spent adsorbent as catalysts

With the development of industries such as mines, fertilizer manufacturing, battery manufacturing, electroplating, paper making and textile industries, a large number of industrial effluents and municipal effluents have increased the heavy metal contaminants in soil and groundwater. Copper is an essential nutrient for life, but excessive accumulation of copper can be pathogenic while causing serious harm to other organisms in the ecological environment. Current removal of cupric ions in water mainly include membrane separation (Nędzarek et al. 2015), biological treatment (Xiao et al. 2013), ion exchange (Fakari & Nezamzadeh-Ejhieh 2017), chemical precipitation (Wang et al. 2019), photocatalysis (Du et al. 2015) as well as adsorption. Among them, the adsorption method is widely used in wastewater treatment due to its advantages of being effective, low-cost, and easy to use on an industrial scale.

Microporous organic polymers (MOPs) are an emerging class of functional porous materials which show potential applications in various fields such as adsorption, catalysis, separation, and gas storage (Jung et al. 2019). Particularly, decorating with different kinds of functional groups enables the MOPs to interact with various heavy metals, toxic chemicals, organic solvents, and fuels as well as fullerenes, thus they have been widely used in the field of adsorption in recent years, especially for the adsorption of cupric ion from water. Importantly, the copper species is an essential component for the fabrication of useful Cu base catalysts, such as catalysts for Ullmann type C-N bond forming reactions which is capable of catalyzing the coupling of aryl halides with nitrogenous compounds.

Nitrogen-containing heterocycles are a kind of valuable compounds presenting as structural unit in many bioactive substances, materials and synthetic drugs (Chen et al. 2019). The Cu(II) catalyzed Ullmann-type reaction is an efficient and potent method for the synthesis of such compounds. However, traditional Ullmann-type reactions confront with several limitations, such as high temperature, extra ligands, limited substrate scope, and tedious separation procedures (Alelaiwi & McKee 2021). In order to improve the practicability, scientists continue to make efforts in this field such as development of heterogeneous catalysts based on hydroxyapatite (Hemmati et al. 2018), silicon-based materials (Akhavan et al. 2018), carbon nanotube materials (Veisi et al. 2017), and polymers. In this context, we conceived a 1,10-phenanthroline based microporous organic polymer (Phen-MOP) which could serve as both adsorbent and ligand for cupric ions. Herein, we wish to report our initial finds toward the fabrication of a new phenanthroline-based MOPs material that can be employed as an efficient adsorbent for removal of Cu(II) from water, and after adsorption, the resulting cupreous Phen-MOP-Cu particles act as heterogeneous catalyst for Ullmann type C-N bond forming reactions.

Sodium diethyldithiocarbamate (DDTC) could undergo a coordination reaction with Cu(II) to produce an orange yellow complex and this complex exhibits an evident absorbance signal when irradiated by light at 452 nm wavelength. Therefore, the concentration of Cu(II) could be unequivocally determined according to the absorbance change under the irradiation (Dedkova et al. 2020). CuCl₂ was selected to prepare the standard solution with DDTC and the fabricated solutions were shown in Table 1. The absorbance of each set of samples at 452 nm was detected with a UV-VIS spectrophotometer, and each sample was tested three times in parallel and the averaged results were employed.

In addition, the recyclable performance of the adsorbent was tested. The procedure was as follows: the 0.02 mL adsorbent was put into the Cu(II) solution (5 mL, 20 mg/L), stirred for 180 min, and the unit adsorption capacity and removal rate were calculated. Then the adsorbent was stirred and desorbed with 20 mL, 0.5 mol/L HCl solution for 240 min, and the desorbed adsorbent was centrifuged and washed three times and dried (Liu et al. 2016). The above steps were one cycle for a total of six cycles.

Under the condition of pH = 7, 25 °C, 0.02 mL adsorbent recorded the adsorption amount under different adsorption times (5, 10, 15, 30, 45, 60, 90, 120, 150, 180 min). Generally, the degree of consistency between the model and the experimental data could be evaluated by the coefficient (R²) value of the two kinetic models: Lagergren pseudo-first-order equation based on solid capacity and pseudo-second-order equation based on solid-phase adsorption (Ho 2006), as a way to reveal the adsorption mechanism of Phen-MOP (Zhang et al. 2016).

The thermodynamic studies were conducted according to the previous document. 0.02 mL adsorbent was added to the solutions with different concentrations of copper ion (20, 25, 30, 35, 40, 45, 50 mg/L), and the adsorption process were conducted at different temperatures (25, 35, 45 and 55 °C) (Gao et al. 2018). The detailed results were provided in Section 3.2.3.

When the adsorption reaches equilibrium, the adsorption isotherm is used to describe the relationship between the amount of ions adsorbed and the concentration of ions still in the solution. To different concentrations of copper ion solution (20, 25, 30, 35, 40, 45, 50 mg/L), 0.02 mL adsorbent was added at different pH values (4, 5, 6 and 7) and different temperatures (25, 35, 45 and 55 °C), and the adsorption was stirred for 180 min. The data were fitted by Langmuir model and Freundlich model. The degree of consistency between the model and the experimental data was evaluated by the value of the coefficient (R²) to reveal the adsorption behavior of Cu(II) on Phen-MOP (Abadian et al. 2014).

Phen-MOP (1.0 g), CuCl₂ (0.4 g), and H₂O (50 mL) were placed in a round bottom flask, and then the reaction mixture was stirred for 12 h. The resulting solid was washed three times with H₂O and three more times with MeOH, and finally dried at 60 °C to obtain the Phen-MOP-Cu catalyst.

Phen-MOP-Cu (100 mg) and Cs₂CO₃ (0.6500 g, 2 mmol) were added to a sealed test tube (25 mL). After the pipe was evacuated with an oil pump, it was filled with nitrogen, and the process should be repeated at least three times. The reaction tube was sealed after adding ethanol (1.0 mL), iodobenzene 1a (1.5 mmol, 0.3060 g) and imidazole 2a (1.0 mmol, 0.0681 g) to the tube using a syringe under N₂ environment. The system was kept to react at 100 °C for 12 h, and then the mixture was allowed to cool to room temperature. The catalyst was separated from the liquid phase using a sand core suction funnel. The product was extracted with ethyl acetate and water, then dried and concentrated. Finally, the crude product was purified by column chromatography to give the product 1-phenylimidazole 3a (0.1370 g, 95% yield).

The structure of the Phen-MOP and Phen-MOP-Cu materials were characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) measurements, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX).

In addition, nitrogen adsorption-desorption measurements were performed (SI, Figure S1). The results disclosed that the Phen-MOP exhibited mesoporous structures with an average pore size of 36.12 nm (Guayaquil-Sosa et al. 2017). The pore volume computed using the Barrett-Joyner-Halenda model was 0.69 cm³/g, and the surface area was 761.8125 m²/g. Compared with Phen-MOP, after loading Cu(II), the surface area of Phen-MOP-Cu decreased slightly, from 761.8125 m²/g to 714.4865 m²/g. But the pore volume of Phen-MOP-Cu was only slightly changed from 0.69 cm³/g to 0.70 cm³/g. Meanwhile, the pore size of Phen-MOP-Cu also increased slightly, from 36.12 nm to 39.47 nm. All results obtained from nitrogen adsorption-desorption measurements indicated that the mesoporous structure of the Phen-MOP did not change during the process of Cu(II) adsorption or loading.

The experimental results were drawn according to Equation (4). As shown in Figure 4(b), the relationship between lnK_d and 1/T was linear with the increasing of Cu(II) concentration. The thermodynamic parameters of adsorption of Cu(II) on Phen-MOP were listed in Table 3. As demonstrated in the table, the ΔH values were negative, indicating that the adsorption process was an exothermic process, which was consistent with the effect of temperature on adsorption (see Figure 3(d)). Generally, the ΔH of the physical adsorption process is less than 40 kJ/mol (Yao et al. 2010). In our study, the absolute values of ΔH were in the range of 35–42.6 kJ/mol, which indicated that there were both physical and chemical adsorptions in these adsorption processes. The ΔG values were negative in the temperature range of 298–328 K, indicating that the adsorption of Cu(II) on Phen-MOP was a spontaneous process. The ΔS values were also negative, showing that the adsorption process would lead to a lower system entropy.

The Q_m (mg/g) in the formula is the maximum adsorption capacity of Cu(II) by Phen-MOP, and K_L (L/mg) is the Langmuir equilibrium constant.

Figure 4(c)–4(f) represented the Langmuir isotherm and Freundlich isotherm models at different temperatures and pH, respectively. Comparing the values of the nonlinear correlation coefficient R² (SI, Table S1 and S2), we concluded that the adsorption process of Cu(II) on Phen-MOP was suitable to the Langmuir model and belonged to the monolayer adsorption. The maximum adsorption capacity was 128.53 mg/g and the value of the Langmuir adsorption equilibrium constant K_L is between 0.7 and 1.2. The values of R_L were all between 0 and 1, indicating that the adsorption process was favorable.

In order to investigate the catalytic performance of Phen-MOP-Cu, the optimal reaction conditions were explored using iodobenzene (1a) and imidazole (2a) as template substrates (Table 4). Notably, the yield of desired product 3a could reach 95% when the reaction was conducted at 100 °C with Cs₂CO₃ in ethanol (entry 3). Other bases such as K₂CO₃, NaOH, and KOH (entries 6–8) failed to further promote the reactivity. Decreasing the reaction temperature (entries 1–2), reducing the catalyst dosage (entry 10), and shortening the reaction time (entries 12–13) were pernicious to the reactivity. Importantly, the reaction didn't proceed without the usage of Phen-MOP-Cu testifying the vital role of this catalyst (entry 9). Furthermore, altering the ratio of reactants 1a and 2a had little effect on the reaction yield (entries 15–16). Thus, the optimal conditions for the Phen-MOP-Cu catalyzed C-N bond formation reaction were as follows: 1.5 equiv. iodobenzene, 2.0 equiv. Cs₂CO₃, 100 mg Phen-MOP-Cu and 100 °C for 12 h.

Table 4

Condition optimization

.
Entrya .	Base .	Phen-MOP-Cu (mg)b .	Temp. (°C) .	Time (h) .	Yield (%)c .
1	Cs2CO3	100	80	12	58
2	Cs2CO3	100	90	12	80
3	Cs2CO3	100	100	12	95
4	Cs2CO3	100	110	12	95
5	Cs2CO3	100	120	12	95
6	K2CO3	100	100	12	70
7	NaOH	100	100	12	75
8	KOH	100	100	12	72
9	Cs2CO3	0	100	12	—
10	Cs2CO3	75	100	12	75
11	Cs2CO3	125	100	12	95
12	Cs2CO3	100	100	6	60
13	Cs2CO3	100	100	9	85
14	Cs2CO3	100	100	15	95
15d	Cs2CO3	100	100	12	94
16e	Cs2CO3	100	100	12	95

.
Entrya .	Base .	Phen-MOP-Cu (mg)b .	Temp. (°C) .	Time (h) .	Yield (%)c .
1	Cs2CO3	100	80	12	58
2	Cs2CO3	100	90	12	80
3	Cs2CO3	100	100	12	95
4	Cs2CO3	100	110	12	95
5	Cs2CO3	100	120	12	95
6	K2CO3	100	100	12	70
7	NaOH	100	100	12	75
8	KOH	100	100	12	72
9	Cs2CO3	0	100	12	—
10	Cs2CO3	75	100	12	75
11	Cs2CO3	125	100	12	95
12	Cs2CO3	100	100	6	60
13	Cs2CO3	100	100	9	85
14	Cs2CO3	100	100	15	95
15d	Cs2CO3	100	100	12	94
16e	Cs2CO3	100	100	12	95

^aReaction conditions: 1a (1.5 mmol), 2a (1.0 mmol), Cs₂CO₃ (2.0 mmol), EtOH (1.0 mL) in N₂.

^bDosage of Cu(II) is 2 mol%.

^cIsolated yields.

^d1a (1.0 mmol).

^e1a (2.0 mmol).

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After establishing the optimal conditions, substrate generality was carried out, and the results are summarized in Table 5. Generally, all tested aryl halides could react with the corresponding N-nucleophiles to generate the desired products with 63–95% yields. Aryl halides bearing methyl, methoxy, nitro, cyano, and trifluoromethyl functional groups were tolerated in this transformation. The steric effects of iodo substituted arenes had a little effect on the reactivities (entries 2–4). What's more, substrates containing electron-withdrawing and electron-donating groups were able to transform to the desired product with 85–88% yields (entries 5–8). Besides imidazole, other N-nucleophiles such as benzimidazoles, pyrroles, pyrazoles, 1H-Benzotriazoles and anilines proved as suitable substrates in this reaction delivering the coupling products with good to high yields (entries 9–14). In addition, the reactions with cyclic amines were also feasible providing the products with 60–94% yields (entries 17–19). Finally, the reactivity between imidazoles or pyrroles with several aryl bromides were also investigated, and all these reactions proceeded smoothly with 50–74% yields (entries 20–26).

Table 5

Scope of copper catalyzed cross-coupling of aryl halides with N-nucleophiles

^aReaction conditions: 1 (1.5 mmol), 2 (1.0 mmol), Cs₂CO₃ (2.0 mmol), Phen-MOP-Cu (100 mg, dosage of Cu(II) is 2 mol%), EtOH (1.0 mL), 100 °C under N2 for 12 h. ^bIsolated yields.

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In order to evaluate the reusability of Phen-MOP-Cu, the recovery experiments were also carried out (Figure 5(b)). The Phen-MOP-Cu was washed three times with methanol after separating it from the reaction system, and then this material was employed again to catalyze the C-N bond formation reaction after drying. The results demonstrated that there was no significant loss of activity in the continuous use of the catalyst six times. In addition, several characterization studies were further performed to examine the structure of the used Phen-MOP-Cu, including FT-IR, TGA, XRD, XPS, SEM, and EDX (SI, Figures S2–S6). All the characterization results were the same as those of the unused Phen-MOP-Cu indicating the good stability of this material.

Compared with the catalysts previously reported, the fabricated material Phen-MOP-Cu exhibited good catalytic activity and selectivity in the Ullmann type C-N bond forming reaction. These results were provided in the supporting information (SI, Table S3).

A phenanthroline-based microporous organic polymer (Phen-MOP) has been synthesized through the coupling between benzene and 1,10-phenanthroline. This Phen-MOP exhibited good adsorption efficiency for removal of Cu(II) from water with high structural stability. The maximum removal efficiency could reach to 98.47% at a Cu(II) concentration of 20 mg/L, pH = 7. When the adsorption time reached 180 min, a unit adsorption Q_e of 98.475 mg/g could be achieved. Interestingly, after the adsorption of Cu(II), the resulting Phen-MOP-Cu can serve as an efficient heterogeneous catalyst for the Ullmann-type reaction. In addition, the catalytic performance of this catalyst did not significantly decrease after successive reuse multiple times. Further studies on the application of this material will be carried out.

We gratefully acknowledge the financial support from the Fundamental Research Funds for the Central Universities (N2005004, N2105005).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.