Microporous organic polymers (MOPs) possessing large specific surface area with high stability are suitable adsorbent to remove contaminants from water, such as organic pollutant and heavy metal contaminants. Herein, a phenanthroline-based microporous organic polymer (Phen-MOP) has been synthesized through the coupling between benzene and 1,10-phenanthroline. The adsorption kinetics and thermodynamics were investigated. This Phen-MOP exhibited good adsorption efficiency for removal of Cu(II) from water with high structural stability and reusability. The maximum removal efficiency could reach to 98.47% at a Cu(II) concentration of 20 mg/L, pH = 7, 25 °C. It was found by investigating the adsorption isotherms that the maximum adsorption capacity Qm was 128.53 mg/g. Interestingly, after the adsorption of Cu(II), the resulting Phen-MOP-Cu can serve as an efficient heterogeneous catalyst for the Ullmann-type reaction. The structure and composition of the Phen-MOP-Cu were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The results indicated that this catalyst possessed immense specific surface area, large pore volume and high stability. The catalyst was easily recyclable and did not significantly lose catalytic activity after being reused six times.

  • A phenanthroline-based microporous organic polymer (Phen-MOP) has been synthesized.

  • The fabricated Phen-MOP possessed large pore volume and high stability.

  • The Phen-MOP exhibited good adsorption capacity and high removal rate for Cu(II).

  • After adsorption, the resulting Phen-MOP-Cu could be used as recyclable catalyst for organic synthesis.

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.

Synthesis of Phen-MOP

The synthesis of Phen-MOP and the adsorption of Cu2+ were illustrated in Figure 1. AlCl3 (0.25 mol) was added to a mixed solution of benzene (0.03 mol) and 1,10-phenanthroline (0.025 mol) in 80 mL of chloroform. After the reaction mixture was mixed at 0 °C, it was stirred at 50 °C for 2 h to form the original network structure, and then heated to 80 °C for 40 h. The resulting pellet was washed with methanol (20 mL) and then three more times with dilute hydrochloric acid (3 × 40 mL). The obtained solid reacted with aqueous KOH at 50 °C for 40 min, and the obtained solid was washed with methanol three more times. They were then washed with methanol in Soxhlet extraction solution for 24 h. Finally, the obtained solid was dried under reduced pressure at 60 °C for 24 h to obtain Phen-MOP as a black powder (5.5625 g).
Figure 1

Synthesis of Phen-MOP and the adsorption of Cu2+.

Figure 1

Synthesis of Phen-MOP and the adsorption of Cu2+.

Close modal

Adsorption experiments

Establishment of the standard curve for Cu(II) adsorption

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.

Table 1

Standard Cu(II) solutions

Sample1234567891011
ρ(Cu(II))/(mg·L−110 12 14 16 18 20 
V(Cu(II))/mL 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 
V(H2O)/mL 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 
V(DDTC)/mL 
Sample1234567891011
ρ(Cu(II))/(mg·L−110 12 14 16 18 20 
V(Cu(II))/mL 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 
V(H2O)/mL 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 
V(DDTC)/mL 
Table 2

Adsorption kinetic parameters for the adsorption of Cu(II) onto the Phen-MOP

ModelsParametersValue
Pseudo-first-order R2 0.94994 
K1 (1/min) 0.06126 
Qe (mg/g) 92.20306 
Pseudo-second-order R2 0.99667 
K2 [g/(mg·min)] 7.57343 × 10−4 
Qe (mg/g) 103.9535 
ModelsParametersValue
Pseudo-first-order R2 0.94994 
K1 (1/min) 0.06126 
Qe (mg/g) 92.20306 
Pseudo-second-order R2 0.99667 
K2 [g/(mg·min)] 7.57343 × 10−4 
Qe (mg/g) 103.9535 

Cu(II) adsorption experiments

The Phen-MOP were dispersed in deionized water and used as the adsorbent (50 mg/mL). 0.02 mL of adsorbent was added to the standard Cu(II) solution (5 mL, 20 mg/L), and after the adsorption was completed, the solution was centrifuged. 1 mL of the supernatant was collected, 2 mL of chromogen (DDTC, 100 mg/L) was added, and the absorbance at 452 nm was measured by UV-VIS spectrophotometer to determine the concentration of Cu(II) in solution after the adsorption was completed according to the Cu(II) standard curve. At time t, the amount of Cu(II) adsorbed by Phen-MOP Qt (mg/g) was obtained from the following equation:
formula
(1)
where V (L) is the volume of Cu(II) solution, and m (g) is the mass of Phen-MOP. C0 and Ct (mg/L) are the concentrations of Cu(II) in solutions initially and at time t, respectively. Just as the adsorption process reaches equilibrium, the Cu(II) in the system reached equilibrium concentration (Ce). At this time, the equilibrium adsorption amount (Qe) of Cu(II) can be calculated with Equation (1).

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.

Kinetic studies

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).

Thermodynamic studies

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.

Adsorption isotherm studies

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).

Synthesis of Phen-MOP-Cu catalyst

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 catalyzed C-N bond formation reactions

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).

Characterization of Phen-MOP and Phen-MOP-Cu

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 Figure 2(a), the peak near 3,416 cm−1 was related to the O-H stretching vibration of residual water in the system (Pazoki et al. 2019). The peak around 2,969 cm−1 was matched to the C-H stretching band and in-of-plane bending vibrations of the aryl rings (Kundu & Bhaumik 2015). Owing to the -C = N- stretching band, the peak near 1602 cm−1 could be observed (Xu & Hedin 2013). The backbone stretching caused by the binding of benzene and 1,10-phenanthroline leads to a signal around 1,384 cm−1; while the C-H out-of-plane bending vibration of aromatic rings gives rise to a peak signal around 707 cm−1 (Xu et al. 2019). The TGA curve shown in Figure 2(b) was related to the thermal stability of Phen-MOP and Phen-MOP-Cu. It was obvious to see that, when the temperature was below 250 °C, the mass loss of both was less than 3%. These results demonstrated that Phen-MOP and Phen-MOP-Cu exhibited good thermal stability under N2 atmosphere. The Figure 2(c) presented the XRD patterns of Phen-MOP, Phen-MOP-Cu and Cu (PDF#04-0836). It could be found that the peaks and relative intensities of the Phen-MOP-Cu were in accordance with the standard XRD pattern of Cu(II), which indicated that Cu(II) had been successfully loaded into the pores of Phen-MOP. Furthermore, the XPS was used to measure the surface composition of Phen-MOP after adsorption. As depicted in Figure 2(d), a binding energy band indicating Cu2p3/2 and Cu2p1/2 could be observed, which disclosed that Cu(II) was adsorbed on Phen-MOP (Dalmieda et al. 2021). Figure 2(e) and 2(f) were SEM images of Phen-MOP and Phen-MOP-Cu. It showed that both materials had little microstructural gap and appeared as irregular porous structures, together with the results of XPS analysis, we were convinced that the microstructure of the Phen-MOP remained intact after adsorption of Cu(II). From Figure 2(g)–2(j), the elemental mapping with EDX indicated that a uniformly distributed Cu presented by the well dispersed N in the framework of Phen-MOP after reaching the adsorption equilibrium, and the content of Cu(II) was 2 mol%.
Figure 2

(a)–(c) FT-IR spectra, TGA curves and XRD patterns of Phen-MOP and Phen-MOP-Cu; (d) XPS image of Phen-MOP-Cu; (e) SEM image of Phen-MOP; (f) SEM image of Phen-MOP-Cu; (g)–(j) EDX elemental mapping of Phen-MOP-Cu.

Figure 2

(a)–(c) FT-IR spectra, TGA curves and XRD patterns of Phen-MOP and Phen-MOP-Cu; (d) XPS image of Phen-MOP-Cu; (e) SEM image of Phen-MOP; (f) SEM image of Phen-MOP-Cu; (g)–(j) EDX elemental mapping of Phen-MOP-Cu.

Close modal

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.

Adsorption of Cu(II)

Effects of adsorption conditions

Through the systemic investigation of the relationship between the concentration of Cu(II) and the absorbance at 452 nm, the standard curve was established and the results were shown in Figure 3(a). The Cu(II) concentration was linear with the absorbance of the solution, and the fitted equation could be obtained as A = 0.05485c + 0.00127 (where A represents absorbance, c represents the concentration). Figure 3(b)–3(d) plotted the adsorption capacity Qt (mg/g) varied with the altering of the pH, adsorption time and temperature, respectively. When pH < 7.5, the copper ions in the solution mainly existed in the form of Cu2+, and the removal of copper ions was mainly accomplished by the adsorption process (Sheng et al. 2010). From Figure 3(b), it could be seen that the adsorption capacity of Phen-MOP was inferior under strong acid environment (pH = 1–3). This illustrated that increasing the amount of H+ led to deterioration of the adsorption of Cu(II). The adsorption capacity was high in both weak acids (pH = 4–6) and neutral environments and elevating the pH was beneficial to the adsorption capacity. It could reach a maximum value under neutral conditions. Then, the influence of adsorption time on the adsorption capacity was tested at pH = 7, 25 °C. Figure 3(c) depicted that Phen-MOP reached equilibrium at 180 min with a maximum unit adsorption of 98.475 mg/g and a maximum adsorption of 98.47%. Figure 3(d) described the influence of temperature and the adsorption capacity remained high in a wide range of temperature change. However, further increasing the temperature was detrimental to the adsorption capacity, and the adsorption capacity reached a maximum value at ambient temperature.
Figure 3

(a) Cu(II) standard curve, inset: UV-vis spectroscopy of Cu(II) solutions; (b) effects of pH on the adsorption performance at 25 °C; (c) effects of contact time on the adsorption performance at pH = 7, 25 °C; (d) effects of temperature on the adsorption performance at pH = 7.

Figure 3

(a) Cu(II) standard curve, inset: UV-vis spectroscopy of Cu(II) solutions; (b) effects of pH on the adsorption performance at 25 °C; (c) effects of contact time on the adsorption performance at pH = 7, 25 °C; (d) effects of temperature on the adsorption performance at pH = 7.

Close modal

Adsorption kinetic

The pseudo-first-order model hypothesizes that the rate of adsorption is most affected by diffusion. The adsorption rate is directly proportional to the number of unoccupied adsorption sites. The nonlinear equation of the pseudo-first-order model is as follows:
formula
(2)
where K1 (1/min) is the pseudo-first-order rate constant.
The pseudo-second-order model assumes that the process by which ions are adsorbed is the step that most affects the adsorption rate. The adsorption rate is proportional to the square of the number of unoccupied adsorption points. The nonlinear equation of the pseudo-first-order model is as follows:
formula
(3)
where K2 [g/(mg·min)] is pseudo-second-order rate constant (Liu & Wang 2017).
Figure 4(a) shown the nonlinear plots of the pseudo-first-order and pseudo-second-order kinetic models for Cu(II) adsorption onto Phen-MOP, respectively. From the R2 values displayed in Table 2, it was clear that the adsorption process more followed the pseudo-second-order kinetic model, evincing that the adsorption of Cu(II) by Phen-MOP was dominated by chemisorption.
Figure 4

(a) The pseudo-first-order and pseudo-second-order kinetic models; (b) Van't Hoff equation plots at 298–328 K; (c) Langmuir isotherms at 25, 35, 45 and 55 °C; (d) Freundlich isotherms at 25, 35, 45 and 55 °C; (e) Langmuir isotherms at pH = 4, 5, 6 and 7; (f) Freundlich isotherms at pH = 4, 5, 6 and 7.

Figure 4

(a) The pseudo-first-order and pseudo-second-order kinetic models; (b) Van't Hoff equation plots at 298–328 K; (c) Langmuir isotherms at 25, 35, 45 and 55 °C; (d) Freundlich isotherms at 25, 35, 45 and 55 °C; (e) Langmuir isotherms at pH = 4, 5, 6 and 7; (f) Freundlich isotherms at pH = 4, 5, 6 and 7.

Close modal

Adsorption thermodynamic

The adsorption enthalpy (ΔH, kJ/mol) and entropy (ΔS, [J/(mol·K)]) are calculated according to the slope and intercept of the fitting curve based on the linear fitting of 1/T by lnKd according to Van't Hoff equation depicted as follows:
formula
(4)
where Kd (L/g) is the distribution coefficient of adsorbate. R = 8.314 J/(mol·K) is the gas constant and T (K) is the absolute temperature (Bouguettoucha et al. 2016).
Then the Gibbs free energy ΔG (kJ/mol) is calculated by:
formula
(5)

The experimental results were drawn according to Equation (4). As shown in Figure 4(b), the relationship between lnKd 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.

Table 3

Adsorption thermodynamic parameters for the adsorption of Cu(II) onto the Phen-MOP

ΔG (kJ/mol)
C0 (mg/L)ΔH (kJ/mol)ΔS [J/(mol·K)]298 K308 K318 K328 K
20 −42.563 −94.946 −14.269 −13.320 −12.370 −11.421 
25 −41.624 −100.683 −11.621 −10.614 −9.607 −8.600 
30 −40.023 −101.680 −9.722 −8.705 −7.688 −6.671 
35 −38.487 −99.519 −8.831 −7.836 −6.840 −5.845 
40 −38.216 −99.851 −8.460 −7.461 −6.463 −5.464 
45 −35.652 −94.530 −7.482 −6.537 −5.591 −4.646 
50 −35.799 −96.526 −7.034 −6.069 −5.104 −4.139 
ΔG (kJ/mol)
C0 (mg/L)ΔH (kJ/mol)ΔS [J/(mol·K)]298 K308 K318 K328 K
20 −42.563 −94.946 −14.269 −13.320 −12.370 −11.421 
25 −41.624 −100.683 −11.621 −10.614 −9.607 −8.600 
30 −40.023 −101.680 −9.722 −8.705 −7.688 −6.671 
35 −38.487 −99.519 −8.831 −7.836 −6.840 −5.845 
40 −38.216 −99.851 −8.460 −7.461 −6.463 −5.464 
45 −35.652 −94.530 −7.482 −6.537 −5.591 −4.646 
50 −35.799 −96.526 −7.034 −6.069 −5.104 −4.139 

Adsorption isotherm

The adsorption process was nonlinear fitted by Langmuir model and Freundlich model. The Langmuir isotherm model assumes that the adsorbed ions are adsorbed by a uniform monolayer onto the adsorbent surface without any interaction between the adsorbed molecules (Kim et al. 2019). The nonlinear equation of the Langmuir isotherm model is as follows:
formula
(6)

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

The dimensionless separation factor (RL) is a key feature of the Langmuir isotherm model (Rowe & Schiessler 1966), and the value of RL demonstrated that the adsorption process is favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1), or irreversible (RL = 0), respectively (Belhamdi et al. 2016). Its calculation is represented as follows:
formula
(7)
where C0 (mg/L) is the highest original Cu(II) concentration value in solution.
The heterogeneity of the adsorbent surface was represented by the Freundlich isotherm model. This model illustrates the multilayer adsorption characteristic of an adsorbent with an energetically uneven distribution (Kumar et al. 2016). Its nonlinear equation can be expressed as:
formula
(8)
where KF [(mg/g)/(mg/L)−1/n] and 1/n are constants related to the adsorption capacity and intensity, respectively (Hidayat et al. 2021).

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 KL is between 0.7 and 1.2. The values of RL were all between 0 and 1, indicating that the adsorption process was favorable.

Catalytic application of spent Phen-MOP-Cu adsorbent

Optimization of reaction conditions

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


EntryaBasePhen-MOP-Cu (mg)bTemp. (°C)Time (h)Yield (%)c
Cs2CO3 100 80 12 58 
Cs2CO3 100 90 12 80 
Cs2CO3 100 100 12 95 
Cs2CO3 100 110 12 95 
Cs2CO3 100 120 12 95 
K2CO3 100 100 12 70 
NaOH 100 100 12 75 
KOH 100 100 12 72 
Cs2CO3 100 12 — 
10 Cs2CO3 75 100 12 75 
11 Cs2CO3 125 100 12 95 
12 Cs2CO3 100 100 60 
13 Cs2CO3 100 100 85 
14 Cs2CO3 100 100 15 95 
15d Cs2CO3 100 100 12 94 
16e Cs2CO3 100 100 12 95 

EntryaBasePhen-MOP-Cu (mg)bTemp. (°C)Time (h)Yield (%)c
Cs2CO3 100 80 12 58 
Cs2CO3 100 90 12 80 
Cs2CO3 100 100 12 95 
Cs2CO3 100 110 12 95 
Cs2CO3 100 120 12 95 
K2CO3 100 100 12 70 
NaOH 100 100 12 75 
KOH 100 100 12 72 
Cs2CO3 100 12 — 
10 Cs2CO3 75 100 12 75 
11 Cs2CO3 125 100 12 95 
12 Cs2CO3 100 100 60 
13 Cs2CO3 100 100 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).

Substrate scope

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.

Regeneration of Phen-MOP and recyclability of Phen-MOP-Cu

The regeneration performance of the adsorbent was studied. As shown in Figure 5(a), after being reused for six times, the adsorption capacity of the adsorbent accounted for 94.29% of the initial adsorption capacity 98.475 mg/g. The decrease of adsorption capacity might be due to the inactivation of a small number of adsorption sites and the loss of adsorbents during recycling experiments. The above results shown that Phen-MOP exhibited good reusability and stability.
Figure 5

(a) Regeneration performance of the Phen-MOP adsorbent; (b) recyclability of the Phen-MOP-Cu catalyst.

Figure 5

(a) Regeneration performance of the Phen-MOP adsorbent; (b) recyclability of the Phen-MOP-Cu catalyst.

Close modal

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 Qe 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.

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Supplementary data