J. Cent. South Univ. (2020) 27: 1247-1261

DOI: https://doi.org/10.1007/s11771-020-4364-4

Ferrocenyl building block constructing porous organic polymer for gas capture and methyl violet adsorption

HUANG Jin(黄晶)^{1, 3}, TAN Zhi-qiang(谭志强)^{1, 2}, SU Hui-min(苏惠敏)^{1, 2}, GUO Yi-wen(郭翼雯)^{1, 2},

LIU Huan(刘欢)^{1, 2}, LIAO Bo(廖博)^{1, 2}, LIU Qing-quan(刘清泉)^{1, 2, 3}

1. Schoolof Materials Science and Engineering, Hunan University of Science and Technology,Xiangtan 411201, China;

2. Hunan Provincial Key Lab of Advanced Materials for New Energy Storage and Conversion,Xiangtan 411201, China;

3. Hunan Provincial Key Lab of Controllable Preparation and Functional Application of Fine Polymers, Xiangtan 411201, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Ferrocene-based porous organic polymer (FcPOP) was constructed with ferrocene and porphyrin derivatives as building blocks via Schiff-base coupling. FcPOP was well characterized, and exhibited good thermal stability, high porosity, microporous structure, and homogeneous pore size distribution. Ferrocene blocks with highly electron-rich characteristics endowed FcPOP with excellent adsorption capacity of CO₂ and methyl violet. The kinetic study indicated adsorption of methyl violet onto FcPOP mainly complied with pesudo-second order model. The maximum adsorption capacity of FcPOP derived from Langmuir isotherm model reached up to 516 mg/g. More importantly, FcPOP could be easily regenerated and repeatedly employed for removal of methyl violet with high efficiency. Overall, FcPOP in the present study highlighted prospective applications in the field of gas capture and dyeing wastewater treatment.

Key words: ferrocene; porous organic polymer; gas capture; dyeing wastewater

Cite this article as: HUANG Jin, TAN Zhi-qiang, SU Hui-min, GUO Yi-wen, LIU Huan, LIAO Bo, LIU Qing-quan. Ferrocenyl building block constructing porous organic polymer for gas capture and methyl violet adsorption [J]. Journal of Central South University, 2020, 27(4): 1247-1261. DOI: https://doi.org/10.1007/s11771-020-4364-4.

1 Introduction

The design, synthesis and application of porous polymers have generated enormous interest and been research hot spots during the past decades due to their wide application in the field like gas capture [1], water purification [2], metal ion enrichment [3], separation [4], and heterogeneous catalysis [5]. Several kinds of porous polymers have been developed such as covalent organic frameworks (COFs) [6], polymer with intrinsic microporosity (PIMs) [7], hypercrosslinked polymers (HCPs) [8], and porous organic polymers (POPs) [5, 9], porous coordination polymers (PCPs) [10, 11]. Typically, structure diversity of building monomer and extensively active groups dramatically improved design flexibility of pore channel and skeleton of POPs, which made it one of the most important platforms to developing novel porous polymers [12]. In comparison with PCPs, POPs showed excellent chemical stability under harsh environment. Research progress [13-15] in recent years suggests that taking full advantage of nanoporous channels of POPs can possess great potential in developing novel functional materials to solve some challenging issues in the field of energy and environment.

Metal-containing POPs often displayed unique properties due to introducing metal elements into polymer networks. For example, theoretical and experimental studies elucidated that doping the host-material surface with metals such as Li, Na, K, Ca was a promising choice to enhance gas uptake of host materials [16, 17]. Furthermore, introducing transition metal ions opened the possibility to generate nanoporous materials with additional electrical, optical, catalytic active center, or magnetic properties. There are two synthetic routes to produce metal-containing POPs. One is post-metallization of POPs via introducing carboxyl group or sulfonic group into the networks firstly, and then metallizing the networks via metal ion exchange [17]. Post-metallization of POPs usually resulted in partial loss of surface area. The other is the preparation of POPs via metal-containing building block like metalloporphyrin derivative [18]. Various organometallics with at least two active groups can be used as a building block, and greatly facilitate the generation of metal-containing POPs.

Ferrocene and its derivatives are a kind of typical organometallic compounds, and have sandwich structure with highly electron-rich and excellent thermal stability [19]. Ferrocene derivatives have been used as a rigid building block to construct POPs, which displayed excellent gas storage capacity [20-22]. In fact, highly electron- rich ferrocene is expected to have strongly interaction with cationic organic compound such caitonic dyes, and can be applied in the field of dyeing wastewater purification. With the background in mind, we prepared ferrocenyl POP (FcPOP) with 1,1'-ferrocenedicarboxaldehyde and 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H- porphine as rigid building blocks via Schiff-base reaction, and attempted to systematically investigate adsorption behavior of methyl violet onto FcPOP.

2 Experiments

2.1 Materials

4-nitrobenzaldehyde(97%), pyrrole(99%) and SnCl₂·2H₂O(98%) were purchased from Aladdin (China). Dimethylsulfoxide (DMSO), methanol, acetone, tetrahydrofuran(THF), aceticanhydride, propionic acid, pyridine, dichloromethane, and methyl violet were obtained from Sinopharm Chemical Reagent Co. Ltd. (China). Anhydrous DMSO was refluxed by CaH₂ and distilled in reduced pressure. 5,10,15,20-tetrakis(4-aminophenyl)- 21H,23H-porphine was synthesized by the method described in the literature with slight modifications [18]. 1,1’-Ferrocenedicarboxaldehyde was also provided by Aladdin (China).

2.2 Synthesis of 5,10,15,20-tetrakis(4- aminophenyl)-21H,23H-porphine

4-nitrobenzaldehyde (22.0 g, 0.145 mol) and aceticanhydride (24.0 mL, 0.254 mol) were dissolved in propionic acid (600 mL). The solution was then refluxed, to which pyrrole (10.0 mL, 0.144 mol) was slowly added. After refluxing for 30 min, the mixture was cooled to give a precipitate which was collected by filtration, washed with H₂O and methanol, and dried under vacuum. The resulting powder was dissolved in pyridine (160 mL), and then was heated to reflux for 1 h. After being cooled to room temperature, the precipitate was collected by filtration and washed with acetone to give 5,10,15,20-tetrakis(4- nitrophenyl)-21H, 23H-porphine as a purple crystal in 14% yield. The product (4.13 g, 5.19×10^-3 mol) was dissolved in hot dilute HCl (500 mL) at 70 °C, to which SnCl₂·2H₂O (18.0 g, 7.97×10^-2 mol) was added. The resulting mixture was stirred at 70 °C for 30 min and then cooled to 0 °C. After neutralized with aqueous ammonium, the gray precipitate was collected as the crystalline product by filtration. The product was dissolved in THF, and the purple crystal of 5,10,15,20-tetrakis(4- aminophenyl)-21H,23H-porphine was obtained by rotary evaporation and vacuum drying. ¹H NMR (300 MHz, CDCl₃) δ (ppm): 2.7 (s, 2H), 4.0 (s, 8H), 7.1–8.0 (m, 16H), 8.9 (s, 8H)

2.3 Synthesis of FcPOP

In a 100 mL three-neck flask with magnetic stirring bar, 5,10,15,20-tetrakis(4-aminophenyl)- 21H,23H-porphine (276.8 mg, 0.41 mmol) and 1,1'-ferrocenedicarboxaldehyde (198.5 mg,0.82 mmol) were suspended in anhydrous dimethyl sulfoxide (40 mL). The flask was then degassed by freeze-pump-thaw technique three times in a liquid nitrogen bath. Finally, the flask was evacuated to an internal pressure of 150 mtorr. The reaction was carried out at 180 °C for 72 h to obtain a black solid, which was filtered and washed with methanol, acetone, and THF, respectively. The black solid was further extracted by Soxhlet extractor with THF for 24h, and methanol for 24h, respectively. The final product was dried in a vacuum oven for 12 h at 100 °C to afford black powder (64.8 mg, 64.8% yield based on 1,1'-ferrocenedicarboxaldehyde).

2.4 Adsorption of methyl violet (MV)

The adsorption property of FcPOP was evaluated by adsorption experiments using methyl violet (MV), which was dissolved in water to obtain 65.8 mg/L solution. All adsorption experiments were stirred at the speed of 300 r/min and 298 K.

Adsorption kinetics can reveal the relation between adsorbent structure and its adsorption property, and the adsorption process and result can be analyzed and predicted according to the kinetic model. The adsorption kinetic experiments were carried out with 5.0 mL MV solution and 10 mg FcPOP, and MV concentration was detected with a UV spectrophotometer at an interval of 5 min. The pseudo-first order and pseudo-second order models rearranged by ZHAN et al [23] are employed to investigate adsorption kinetics of MV onto FcPOP, and the models are provided as follows:

　　　　　　　　(1)

　　　　　　　　(2)

Eqs. (1) and (2) are pseudo-first order and pseudo-second order models of adsorption kinetics, respectively, where C₀ and C_t (mg/L) are the initial MV concentration and the concentration at time t; m_s (g/L) is the concentration of adsorbent; k₁ and k₂ are the rate constants of pseudo-first order and pseudo-second order models, respectively; and Q_e is the equilibrium adsorption capacity.

The equilibrium adsorption isotherm was implemented with 5 mg FcPOP and 8 mL MV solution with the initial concentration varied from 65.8 to 394.8 mg/L. The mixtures were stirred for 6 h to assure that the adsorption reached equilibrium. Equilibrium adsorption capacity and equilibrium MV concentration were calculated from the initial and final MV concentrations. Langmuir [24], Freundlich [25], and Redlich-Peterson [26] models were employed to fit the experimental data of adsorption isotherm.

Langmuir isotherm model:

                             (3)

where Q_m (mg/g) is the maximum adsorption capacity based on monolayer adsorption; b is the Langmuir constant; C_e (mg/L) is the equilibrium MV concentration.

Freundlich isotherm model.

                              (4)

where K_F and n are the Freundlich constants in relationship with the sorption capacity and sorption intensity, respectively.

Redlich-Peterson isotherm model:

                             (5)

where K_R and α are the Redlich-Peterson constants, and β is the coefficient from 0 to 1.

The cyclic adsorption/desorption experiments were conducted with 5.0 mL MV solution (65.8 mg/L) and 10 mg FcPOP. The mixtures were stirred at the speed of 300 r/min for 6 h, and then centrifugal separation was employed to remove MV solution. FcPOP were regenerated by methonal/ water co-solvent with NaCl [27].

2.5 Characterization

FT-IR spectrum was collected in transmission mode in KBr pellets at room temperature on a FTIR-2000 spectrometer (Perkin-Elmer in America). ¹H-NMR Characterization was performed on a Varian 400-MR spectrometer, operating at 400 MHz. ¹³C cross-polarization, magic angle spinning (CP/MAS) nuclear magnetic resonance spectra were recorded on a AVANCE III 400 MHz spectrometer (BRUKER Inc., Switzerland) equipped with a 4-mm HXY T3 PENCIL probe.

The thermogravimetric analyses (TGA) were carried out under N₂ flow on a Mettler TGA/ SDTA851 thermogravimetric analysis instrument. Sample was heated at a rate of 5 °C/min from 35 to 1000 °C under a nitrogen atmosphere.

Microstructures of FcPOP were examined by high resolution transmission electron microscopy (HR-TEM, JEOL JEM-2000EX). Surface morphology was observed by scanning electron microscopy (SEM, JSM-6380 LV).

The X-ray photoelectron spectroscopy (XPS) experiment was performed on a K-Alpha 1063 spectrometer (Thermo Fisher Scientific) and the core level spectrum was measured using a monochromatic Al K_α X-ray source (hv=1386.6 eV).

Nitrogen sorption porosimetry was performed on a micromeritics ASAP 2020 volumetric adsorption analyzer. The experiments were carried out at the temperature of liquid nitrogen (77.3 K). The samples were first heated in a tube under vacuum at 120 °C for 20 h to remove adsorbed materials. Pore size distribution curve and the pore volume were derived from the adsorption branches of the isotherms using the nonlocal density functional theory (NLDFT). CO₂ and CH₄ adsorption experiments were conducted at 273 K and 298 K, respectively.

MV concentration was determined by UV751GW ultraviolet spectrophotometer based on a standard linear curve, which was constructed by the relationship between UV adsorption intensity at 569 nm and a series of MV solution with known different concentration.

3 Results and discussion

3.1 Synthesis and characterization of FcPOP

Synthesis route ¹HNMR spectrum of 5,10,15,20-tetrakis(4-aminophenyl)-21H, 23H-porphine was introduced in supporting information (Scheme S1 and Figure S1). Ferrocene- based porous organic polymer (FcPOP) was synthesized by Schiff-base coupling of 1,1'-ferrocenedicarboxaldehyde and 5,10,15,20-tetrakis (4-aminophenyl)-21H,23H- porphine, and the synthetic route is shown in Scheme 1.

The chemical structure of FcPOP was investigated by FTIR (Figure S2) and ¹³C CP/MAS NMR (Figure 1). FT-IR spectrum of FcPOP exhibited a characteristic stretching vibration of C=N at 1600 cm^-1, which was characteristic of v_C=N moieties. The absorption at 2970 cm^-1 and 1410 cm^-1 was assigned to C—H stretching of ferrocene and the cyclopentadienyl(Cp) rings bending, respectively. In addition, the stretching vibration of C=O in 1,1’-ferrocene- dicarboxaldehyde at 1680 cm^-1 disappeared in the spectrum of FcPOP. The solid-state ¹³C CP/MS NMR spectrum displayed multiple broad peaks with different strength. In particular, the signals at 61.12ppm, 65.09ppm, 68.53ppm, 69.97ppm and 76.12ppm were ascribed to five kinds of carbon in cyclopentadienyl(Cp) rings; and signals at 118.906ppm, 141.82ppm and 152.73ppm were derived from the carbons of porphyrin rings. The characteristic peaks of carbons in C=N appeared at 156.45ppm.

Scheme 1 Synthetic route for ferrocene-based porous organic polymer (FcPOP)

Figure 1 Solid-state ¹³C CP/MAS NMR spectrum of FcPOP

XPS analysis can provide more information about the elemental composition of FcPOP surfaces (Figure 2). The transitions energy of around 707.4 eV and 720 eV corresponded in shape and position to Fe 2p 3/2 and Fe 2p 1/2 bands in ferrocenyl blocks, respectively. XPS result of FcPOP was in accordance with the previous XPS study of ferrocene [28]. The C/Fe molar ratio from XPS spectrum was calculated to be 47/1, which was higher than the theoretical value of 34/1 derived from the feed ratio of raw materials. We speculated that XPS method only analyzed the elemental composition of FcPOP surface, which was different from that of FcPOP bulk.

Figure 2 X-ray photoelectron spectroscopy (XPS) graph of FcPOP

Overall, the results of FT-IR, ¹³C CP/MS NMR and XPS analysis demonstrated that ferrocenyl units were successfully incorporated into polymer networks as a building block.

The morphology of FcPOP was investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM), as shown in Figure 3. SEM image revealed that FcPOP was random granules with particle size of 1-3 μm. HR TEM image displayed FcPOP was amorphous, which was further demonstrated by XRD curves (Figure S3). Interesting, a number of ultra-micropores could be observed in TEM image (the red circles in Figure 3(b)). Furthermore, elemental distribution maps of carbon, nitrogen, and iron were also obtained by energy dispersive spectrometer (Figures 3(c)-(e)). Expectedly, carbon, nitrogen, and iron elements were uniformly distributed in FcPOP.

As-prepared FcPOP was found to be insoluble in any organic solvent, such as dimethyl sulphoxide, dimethylformamide, and tetrahydrofuran. Moreover, there was no mass loss or BET surface area decrease after FcPOP was soaked in HCl or NaOH solution (~10 wt%) for at least 12 h. The facts suggested that FcPOP has an excellent physicochemical stability. The thermal stability was investigated by thermogravimetric analysis under nitrogen atmosphere (Figure S4). The results indicated that FcPOP remained stable up to approximately 230 °C, and the mass loss below 230 °C was believed to be water or/and residual solvent loss. The thermal stability of FcPOP was similar to previous porous organic polymers, such as CMPs [4], HCPs [29], COFs [6], and PIMs [30].

Figure 3 SEM (a), TEM(b), and elemental images (c, d, e) (from 100 μm×100 μm region) of FcPOP

3.2 Pore structure and gas capture of FcPOP

To demonstrate porous architecture of FcPOP effectively constructed with ferrocene and porphyrin derivatives as building blocks, nitrogen adsorption/desorption measurement was carried out to evaluate the permanent porosity of FcPOP, and the isotherm is provided in Figure 4(a). Obviously, the isotherm exhibited type I profile according to IUPAC classifications [31]. The N₂ uptake at low pressure (p/p₀=0-0.01) experienced a sharp increase, suggesting a large number of permanent micropores in FcPOP. The surface area of FcPOP was calculated to be 602 m²/g (Figure S5), which was comparable to similar POPs like PECONF-2 (637 m²/g) [32] and PTPA-3 (530 m²/g) [33], but lower than that of COFs, such as COF-5 (1590 m²/g) [34] and COF-10 (1760 m²/g) [35]. In addition, the pore volume of FcPOP was calculated to be 0.4 cm³/g at relative pressure of p/p₀=0.99, and the micropore volume was 0.12 cm³/g, which was derived from t-plot method. Pore size distribution curve of FcPOP (Figure 4(b)) was calculated using nonlocal density functional theory (NLDFT). The network exhibited a narrow pore size distribution and a large number of pores in the micropore region extending to the lower part of the mesopore region. The average pore diameter of 1.6 nm could also demonstrate the predominant microporous nature in FcPOP.

The excellent porosity of metal-doping FcPOP promoted us to estimate their gas storage property. Determination of CO₂, CH₄ and N₂ uptake was carried out at 273 K/1.0 bar and 298 K/1.0 bar, respectively, and the isotherms are shown in Figure 4(c). The volumetric CO₂ uptake of FcPOP was 2.54 mmol/g (11.2 wt%) at 273 K and 1.63 mmol/g (7.2 wt%) at 298 K. The value at 273 K was higher than PPAF-34 (1.39 mmol/g, 953 m²/g) [36] and CMP-SO-1B2 (2.13 mmol/g, 1085 m²/g) [37] under the same conditions although the surface area of FcPOP was lower than that of PPAF-34 or CMP-SO-1B2. The excellent CO₂ capacity of FcPOP may be attributed to highly electron-rich ferrocene blocks, which could enhance affinity of FcPOP for electron-poor adsorbate of CO₂.

Figure 4 Nitrogen adsorption (filled symbols) and desorption (empty symbols) isotherm measured at 77 K (a); Pore size distribution curve for FcPOP calculated by NL-DFT (b); Gas uptake performance of FcPOP (c); CO₂ and CH₄ isosteric heats of FcPOP (d)

FcPOP exhibited moderate CH₄ capture capacity of 0.7 mmol/g at 273 K and 0.42 mmol/g at 298 K, respectively. To thoroughly investigate the host-guest interaction, Q_st (isosteric heat of adsorption) value of CO₂ and CH₄ were calculated using the virial model [38]. The virial equation fitting curves of CO₂ and CH₄ are provided in Figures S6 and S7, respectively. The CO₂ and CH₄ isosteric heats of FcPOP are shown in Figure 4(d). At low gas loading, the Q_st value mainly reflected the interaction strength between adsorbate and adsorbent, and was found to be 22.4 and 16.3 kJ/ mol for CO₂ and CH₄, respectively. The Q_st value of CO₂ was the same level to many other porous polymers, such as porous electron-rich covalent organic frameworks [32], PTPA-3 [33], and PAFs [37]. Interestingly, Q_st only decreased slightly with increasing CO₂ loading of FcPOP. This fact indicated that comparatively strong interaction of CO₂ with FcPOP are kept during adsorption of CO₂ by adsorbate. The Q_st value of CH₄ was lower than that of CO₂, indicating weaker host-guest interaction.

To assess the potential application of FcPOP in the field of gas separation, selectivities of CO₂ over CH₄ and N₂ were investigated, respectively. The ideal adsorption of solution theory (IAST) model was applied to calculating CO₂/CH₄ and CO₂/N₂ ideal selectivities based on the initial slopes of adsorption isotherms derived from 273 K and 298 K, and the results are shown in Figure S8 and Table S1. FcPOP exhibited moderate selectivities for CO₂/CH₄ (5.39/273 K, 4.38/298 K), and excellent selectivities for CO₂/N₂ (29.81/273 K, 24.85/298 K), which was comparable to the results reported by COOPER et al [39] and JIANG et al [40]. The highly elcetron-rich ferrocenyl blocks in FcPOP may be favorable to improve the interaction between polarizable CO₂ molecules via hydrogen bonding and/or dipole-quadrupole interactions. The fact can be responsible for the excellent CO₂ ideal selectivity.

3.3 Removal of methyl violet from aqueous solution

The as-prepared FcPOP have excellent physicochemical stability, a narrow pore size distribution, and electron-rich conjugated structure with ferrocene and porphyrin units, which is favorable to improve the interaction with small organic molecules, and can find application in removal of organic molecules from wastewater. Consequently, in the present manuscript, methyl violet (MV) is used as a target dye to investigate the adsorption capacity and adsorption rate of FcPOP. MV is a typical cationic dye, and widely used as coloring agents for textile and leather materials. The experimental data, the adsorption kinetic curves derived from pesudo-first order and pesudo-second order model [23] are shown in Figure 5, and the parameters calculated from model fitting are provided in Table 1.

In the experiment of adsorption kinetics, FcPOP (10 mg) was employed to treat 5.0 mL MV aqueous solution with initial concentration of 65.8 mg/L. MV concentration of aqueous solution was monitored by UV absorption intensity at 569 nm according to the equation from the fitted standard curve (Figure S9), and UV curves recorded at an interval of 5 min were provided in Figure S10. MV was rapidly adsorbed by FcPOP, and the adsorption equilibrium reached within 30 min. The removal efficiency of MV was about 98.9%, and the purple solution gradually turned to colourless.

Figure 5 Adsorption experimental data, and kinetic curves derived from pesudo-first and pesudo-second order models

Table 1 Pesudo-first and pesudo-second order parameters and correlation coefficient

The equilibrium adsorption capacity Q_e values derived from pesudo-first and pesudo-second order models were 32.58 and 37.18 mg/g, respectively. The values were close to the experimental data, which are calculated to be 32.72 mg/g from Figure 5. The fact illustrated that Q_e could be predicted by the kinetic models. Similar to chemical reaction order, there are two factors like dye concentration and adsorbent property having effect on the adsorption rate of pesudo-second order model. However, only one fact such as dye concentration or adsorbent property, has impact on the adsorption rate of pesudo-first order model. Moreover, a plateau could be observed in the fitting curve of pesudo-first order model with decreasing MV concentration, but there was still a reduced trend with increasing adsorption time in the curve from pesudo-second order model. The correlation coefficient from pesudo-second order model was slightly higher than that of the former, indicating that MV adsorbed by FcPOP mainly complied with pesudo-second order model. The interaction between electron-rich conjugated structure in FcPOP with cationic MV molecules prompted that both MV concentration and FcPOP adsorption property had important effect on the adsorption rate.

Experimental data of equilibrium adsorption isotherm are shown in Figure 6. The equilibrium adsorption capacity of FcPOP dramatically improved with increasing MV concentration. However, the rising trend diminished, and gradually, the adsorption capacity of FcPOP became saturated. Non-linear models of Langmuir, Freundlich, and Redlich-Peterson [24-26] were employed to fit the experimental data of MV adsorbed onto FcPOP. The fitting curves are also provided in Figure 6. Parameters and the correlation coefficient of various models are provided in Table 2.

The fitting results displayed that the correlation coefficient of Redlich-Peterson model was higher than that of Langmuir or Freundlich models. The fact suggested that the adsorption process of MV onto FcPOP was more accordant with the adsorption isotherm of Redlich-Peterson model, which is in noncompliance with the ideal monolayer adsorption of Langmuir model, and more applicable to physical and chemical adsorption of inhomogeneous surface. Consequently, it can be concluded that the main adsorption site of FcPOP was the inhomogeneous pore and surface, and MV failed to be adsorbed by monolayer. The maximum adsorption capacity of FcPOP could be calculated from Langmuir model to be 516 mg/g. The value is much higher than that of active carbon or most common adsorbents, and but lower than that of functional porous polymers with carboxyl group, which has strong electrostatic interaction with cationic MV. Comparison of adsorption capacities of various adsorbents for MV is provided in Figure 7(a). The interaction between electron-rich groups in FcPOP and MV, homogeneous pore size distribution, and small size of MV molecules (11.3 , Figure S11) could be responsible for excellent adsorption property of FcPOP.

Figure 6 Experimental data of equilibrium adsorption isotherm, and fitting curves derived from Langmuir, Freundlich, and Redlich-Peterson models

Table 2 Parameters and correlation coefficient R² of adsorption isotherm models

Figure 7 Comparison of adsorption capacities of various adsorbents for MV (a), and cyclic adsorption efficiency of MV onto FcPOP (b)

Cyclic regeneration and resuability are significantly important property of adsorbents. Cyclic adsorption efficiency of FcPOP was investigated with water/methanol co-solvent with NaCl as an eluent, which has proved excellent capacity to recover cationic dyes from porous polymers in our previous work [27]. Cyclic adsorption experiment was the same to that of adsorption kinetics. FcPOP were separated centrifugally, and then extracted with the eluent. Eight adsorption/desorption cycles were performed to examine MV removal efficiency of FcPOP, and the results are shown in Figure 7(b). Obviously, the adsorption capacity of FcPOP was only slightly decreased after 8 cycles, and the removal efficiency of MV still exceeded 96%, indicating an excellent stable adsorption capacity. A slightly declining of adsorption efficiency could be resulted by the progressive loss of pore structure during cyclic adsorption/desorption process. The high removal efficiency of dye, simple regeneration process, and excellent reusability of FcPOP in cyclic adsorption/desorption highlighted the prospective application in field of dyeing wastewater treatment.

4 Conclusions

Ferrocene-based porous organic polymer (FcPOP) was constructed by Schiff-base reaction of 1,1'-ferrocenedicarboxaldehyde and 5,10,15,20- tetrakis(4-aminophenyl)-21H,23H-porphine.FcPOP exhibited excellent thermal stability and high porosity. The incorporation of ferrocene blocks was favorable to strengthen the interaction with CO₂, and improve the CO₂ uptake and ideal selectivity. Excellent physicochemical stability, narrow pore size distribution, and highly electron- rich conjugated blocks prompted us to extend FcPOP application for removal of MV from aqueous solution. Nonlinear pesudo-second order model exhibited better fitting effect in comparison with pesudo-first order model. Investigation of adsorption isotherm indicated that main adsorption sites of FcPOP were the inhomogeneous pore and surface. Furthermore, the high removal efficiency of dye, simple regeneration process, and excellent reusability of FcPOP in cyclic adsorption/ desorption highlighted the prospective application in field of dyeing wastewater treatment.

Supporting information

Scheme S1 Synthesis route of 5,10,15,20-tetrakis(4-aminophenyl)-21H, 23H-porphine

S1 ¹H-NMR spectrum of 5,10,15,20-tetrakis(4- aminophenyl)-21H, 23H-porphine

Figure S1 ¹H-NMR spectrum of 5,10,15,20-tetrakis(4- aminophenyl)-21H, 23H-porphine

S2 FT-IR spectrum of FcPOP

Figure S2 FT-IR spectrum of FcPOP

S3 XRD pattern of FcPOP

Figure S3 XRD pattern of FcPOP

S4 TGA curve of FcPOP

Figure S4 TGA curve of FcPOP

S5 BET transform plot

Figure S5 BET transform plot of FcPOP

S6 Q_st simulation

The isosteric heat of adsorption was evaluated by the binding affinity single-component adsorption isotherms based on virial equation.

1) Virial equation

A virial-type expression in the following form can be used to fit the experimental isotherm data for a given material at different temperatures.

                (I)

where N is the amount adsorbed at a pressure P and a temperature T; m and n determine the number of terms which are required to adequately describe the isotherm; both a_i and b_i are temperature- independent empirical parameters. The isosteric heat of adsorption as a function of uptake was calculated by the virial coefficients from a₀ to a_m:

                         (II)

where R is the universal gas constant,8.314 J/(K·mol).

Figure S6 CO₂ isotherms at 273 K and 298 K (symbols) and virial equation fits (lines) for FcPOP lnp= lnN+(-2701.01+153.81N-25.49N²+3.09N³)/273+14.6

S7 Selectivity of FcPOP

Figure S7 CH₄ isotherms at 273 K and 298 K (symbols) and virial equation fits (lines) for FcPOP lnp=lnN+ (-1970.2094+181.7674N-60.8063N²)/273+13.8742

S8 Removal of MV from aqueous solution

Figure S8 CO₂/N₂ and CO₂/CH₄ selectivities derived from IAST calculation at 273 K and 298 K (CO₂(273 K):V=103.216p/p₀+1.314; CO₂ (298 K): V=49.506p/p₀+ 0.640; CH₄ (273 K): V=19.160p/p₀+0.093; CH₄ (298 K): V=11.303p/p₀+0.039; N₂ (273 K): V=3.462p/p₀+0.00068 N₂ (298 K): V=1.992p/p₀+0.00009

Table S1 CO₂ and CH₄ uptake and selectivities of FcPOP at 273 K and 298 K

Figure S9 Linear fitting between MV concentration and UV absorption

Figure S10 UV absorption spectra of aqueous MV solution at an interval of 5 min

Figure S11 Molecular size of MV2B optimized by MM2

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(Edited by HE Yun-bin)

中文导读

用于气体捕获与甲基紫吸附的二茂铁基多孔有机聚合物

摘要：以二茂铁与卟啉衍生物为构筑单元，通过希夫碱偶合反应制备了二茂铁基多孔有机聚合物(FcPOP)。系统表征了FcPOP的结构与形貌，FcPOP表现出良好的热稳定性、高孔隙率、微孔结构以及均匀的孔径分布。二茂铁单元具有高度富电子特性，赋予FcPOP优秀的CO₂和甲基紫吸附能力。甲基紫吸附动力学研究表明，FcPOP对甲基紫的吸附符合准二级动力学模型。以Langmuir等温线模型计算，得到FcPOP的最大吸能力达到516 mg/g。更重要的是，FcPOP可被简单再生，再重复应用于高效除去印染废水中的甲基紫。总的来说，制备的FcPOP在气体捕获和印染废水处理领域中表现出良好的应用前景。

关键词：二茂铁；有机多孔聚合物；气体捕获；印染废水

Foundation item: Project(51778226) supported by the National Natural Science Foundation of China; Project(2018JJ3159) supported by the Hunan Provincial Natural Science Foundation for Youths, China

Received date: 2019-04-01; Accepted date: 2020-02-14

Corresponding author: LIU Qing-quan, PhD, Professor; Tel: +86-731-58290782; E-mail:qqliu@hnust.edu.cn; ORCID: 0000-0003-3846- 4268