Multi-hydroxyl-containing porous organic polymers based on phenol formaldehyde resin chemistry with high carbon dioxide capture capacity

Abstract

Three multi-hydroxyl groups containing porous organic polymers (PFPOP-1–3) based on phenol formaldehyde resin chemistry have been reported. FT-IR and solid-state ¹³C NMR spectra confirm the polymerization between 1,2,4,5-tetrahydroxyl benzene and trialdehydes, and SEM as well as TEM observations reveal the spherical morphologies of PFPOPs. Nitrogen sorption isotherm measurements suggest that PFPOPs possess considerable surface area with narrow pore size distribution in micropore region. On the basis of their microporous properties and hydroxyl-rich structures, PFPOPs present high potentials in the applications for small gas adsorption and separation. Consequently, PFPOP-3 shows a high carbon dioxide capture capability (17.3 wt% at 273 K and 1.0 bar) and considerable CO₂/N₂ selectivity (56.5, IAST at 273 K and 1.0 bar).

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Introduction

In recent years, porous organic polymers (POPs) have been developed rapidly in expanding the scopes of internal structure and reaction type.¹ Crystalline covalent organic frameworks (COFs) with regular porosity are synthesized through highly reversible polycondensation reactions.² A well-established pathway for the generation of microporosity in amorphous polymers relies on a multiple Friedel–Crafts alkylation for the synthesis of hypercrosslinked polymers (HCPs).³ By connecting sterically demanding building blocks, polymers of intrinsic microporosity (PIMs) are obtained.⁴ Conjugated microporous polymers (CMPs) which combine high surface area with π-conjugation are mainly synthesized by cross-coupling reactions.⁵ Owing to their exceptional performance in structures and functionalities, POPs are promised to be utilized in wide applications such as gas adsorption, catalysis, sensing, photoelectricity, and energy storage.

The diversity of organic building blocks endows POPs with a high flexibility in the molecular design of both skeletons and pore structure. However, the weak thermal, chemical stabilities, or the requirements of metal catalysts that limited cost effective preparation and entangled in post-purification from unavoidable catalyst residues, drive scientists to explore new methodologies to solve these problems. Phenol formaldehyde resin (PF) as a class of commonly useful synthetic polymers is generated by the reaction of phenolic molecules with aldehydes. One reaction sites at ortho-carbon of phenolic ring can combine with two carbonyl groups, hence the monomers with multi-functional groups supply the feasibility to construct POPs.⁶ Although these reactions are generally catalyzed by acids or bases, phenol formaldehyde resin-based porous polymers can be synthesized under catalyst-free conditions, and the formed carbon–carbon covalent bonds make these materials possess highly thermal and chemical stabilities.

Carbon dioxide (CO₂) is an important greenhouse gas and strongly influences the global climate.⁷ Its concentration in the atmosphere has increased rapidly owing to industrial consumption of carbon-based fuels. Therefore, high efficient CO₂ adsorbent should be developed to deal with this crucial environmental issue.⁸ In general, the adsorption of CO₂ on porous material skeletons is mainly weak physisorptive processes that relatively limit their CO₂ capture capability. The surface modification of POPs by introducing polar groups can significantly enhance their CO₂ binding energy by dipole–quadrupole interactions, and then increase their CO₂ adsorption capability.⁹ According to the recent research,¹⁰ considerable microporosity and polar functional groups such as amine, hydroxyl, and halogen are of benefit to CO₂ capture and separation. Therefore, synthesis of new POPs with plenty of polar groups in their frameworks is an applicable methodology to achieve this target. Here we report three phenol formaldehyde resin-based porous organic polymers (PFPOP-1–3) with multi-hydroxyl groups prepared by combining four –OH containing phenol with trialdehydes. The PFPOPs exhibit high thermal stabilities, spherical morphologies, considerable BET specific surface area, and narrow pore size distribution in micropore region. Their microporous properties and internal structures with abundant hydroxyl groups make these materials suitable for small gas capture and separation. The high CO₂ adsorption capacity of PFPOP-3 is up to 17.3 wt% at 273 K and 1.0 bar. Meanwhile, PFPOPs show considerable CO₂ over N₂ selectivity (43.7–56.5, IAST at 273 K and 1.0 bar) in the flue gas composition (CO₂/N₂ = 15/85).

Experimental section

Materials and methods

Tetrahydrofuran (THF), anhydrous potassium carbonate, sulfurous dichloride, and acetic acid were purchased from the Beijing Chemical Reagent Company. 1,3,5-Tribromobenzene, 4-formylphenylboronic acid, bis(triphenylphosphine)palladium(II), triphenylamine, p-bromoaceto phenone, and potassium iodide were purchased from Aldrich. Ethanol, dichloromethane, ethyl acetate, petroleum ether, and acetone were commonly used in our laboratory. 1,2,4,5-Tetrahydroxyl benzene (THB),¹¹ 1,3,5-tris(4-formylphenyl)benzene (M1),¹² 1,3,5-tris(4-formylbiphenyl)benzene (M2),¹² and tris(4-formylbiphenyl)amine (M3)¹² were prepared following the reported literature.

Preparation of PFPOPs

A mixture of M1 (39.0 mg, 0.1 mmol) and THB (42.6 mg, 0.3 mmol) was dissolved in dioxane (4 mL). After ultrasonication for 10 min, the mixture was degassed in a Pyrex tube (10 mL) by three freeze–pump–thaw cycles. The sealed tube was cooled to room temperature after reacting at 220 °C for 96 h, and then the precipitate was collected by centrifugation, washed with dichloromethane, water, THF, ethanol, and acetone for several times, and dried at 120 °C under vacuum overnight to obtain PFPOP-1 as dark brown powder in 70% isolated yield.

PFPOP-2 and PFPOP-3 were obtained using M2 and M3 in a similar procedure in 65% and 73% isolated yield, respectively.

Instrumental characterization

The ¹H and ¹³C NMR spectra of all organic compounds were obtained on a Bruker DMX400 NMR spectrometer with tetramethylsilane as an internal reference. Solid-state ¹³C CP/MAS NMR spectra of the polymers PFPOPs were recorded on a Bruker Avance III 400 spectrometer (Bruker, Germany). Elemental analysis was performed on a Thermo Finnigan EA 1112 Series Flash Elemental Analyzer (Thermo Finnigan, USA). Infrared spectra were recorded by using a Perkin-Elmer Spectrum One Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer Instruments Co. Ltd, USA). The sample was prepared by mixing the polymers with KBr and pressing to form disks, and the measurements were carried out from 4000 to 400 cm⁻¹. Thermogravimetric analysis (TGA) was applied on a Pyris Diamond thermogravimetric/differential thermal analyzer (Perkin-Elmer Instruments Co. Ltd, USA) by heating the samples (10 mg) to 800 °C at rate of 10 °C min⁻¹ under nitrogen atmosphere. Field emission scanning electron microscopy (FE-SEM) measurements were performed in a Hitachi S-4800 microscope (Hitachi Ltd., Japan) operating at an accelerating voltage of 6.0 kV. The samples were prepared by drop-casting an ethanol suspension onto silicon wafer and then coated with gold. Nitrogen sorption isotherm measurements and hydrogen adsorption experimentations were carried out by ASAP 2020M + C surface area and porosimetry analyzers (Micromeritics Instrument Corporation, USA) at 77 K. Carbon dioxide uptake isotherms under low pressure (0–1.06 bar) were collected with TriStar II 3020 accelerated surface area and porosity analyzer (Micromeritics Instrument Corporation, USA) at 273 K. Before the measurement, all the samples were degassed for 12 h at 120 °C under vacuum. The specific surface area and micropore surface area were evaluated based on the obtained sorption isotherms with the application of the Brunauer–Emmett–Teller (BET) model and t-plot method, respectively. The pore size distribution (PSD) profiles of obtained materials were calculated from the related adsorption branch by the non-local density function theory (NLDFT) approach. Total pore volume was calculated from nitrogen sorption isotherms at P/P₀ = 0.99.

Results and discussions

We utilized phenol formaldehyde resin-inspired chemistry to obtain PFPOP-1, PFPOP-2, and PFPOP-3, respectively. Typically, THB and trialdehydes (M1, M2, and M3, Scheme 1a) in molar ratio of 1 : 3 were polymerized at 220 °C for 4 days under solvothermal condition. In this reaction system, four hydroxyl groups give THB high reactivity for electrophilic aromatic substitution owing to the electron donating resonance effect, and the electron-rich benzene ring can attack the carbonyl group easily. After elimination of one molecule of H₂O, a carbonyl group and two THB molecules are connected by carbon–carbon bonds. A trialdehyde molecule can reacted with six THB as well as a THB has two electrophilic aromatic substitution positions, hence they can be employed to construct porous network (Scheme 1b). TGA measurements were utilized to investigate their thermal stabilities (Fig. 1). The results exhibit that PFPOP-1–3 are highly stable materials with slightly decomposing up to 400 °C and more than 60% remaining mass at 800 °C.

Scheme 1 (a) Molecule structures of trialdehydes. (b) Schematic description of synthesis route to PFPOP-1–3.

Fig. 1 TGA curves of PFPOP-1–3.

The structures of polymeric PFPOP-1–3 were investigated by FT-IR spectrum measurements and solid-state ¹³C CP/MAS NMR spectroscopy. The FT-IR spectra of the monomers and related polymers are shown in Fig. 2. The disappeared vibration bands at 1690 cm⁻¹ (C=O stretching) and 2820 cm⁻¹ (C–H stretching) assigning to the carbonyl group of the trialdehyde in the spectra of all PFPOPs confirm high conversion of the functional groups in monomers. The spectra of PFPOPs also exhibit broad peaks in the range of 3000–3500 cm⁻¹, which can be attributed to O–H stretching. The ¹³C CP-MAS NMR spectra of PFPOPs show several resonance peaks (Fig. S1, ESI†). As shown in Fig. S1b,† spectrum of PFPOP-2 exhibits the sharp peaks at 140 and 127 ppm corresponding to the phenoxy carbons and aromatic carbons respectively, and a shoulder peak at 116 ppm attributing to ortho-carbon atoms of THB. The tertiary carbon formed in polymerization is established by the presence of the peak at 34 ppm. Meanwhile, the signal at 146 ppm in the spectrum of PFPOP-3 originates from the aromatic carbons connected with nitrogen atom (Fig. S1c, ESI†). The contents of C, H, and N evaluated by the elemental analysis results are close to the theoretical values of PFPOP-1–3 (Table S1, ESI†). To investigate the morphology of PFPOPs, FE-SEM observation was carried out and SEM images show that all of PFPOPs exhibit regular and smooth spherical particles with 2–3 μm diameters (Fig. 3). The spherical particles of PFPOPs were also confirmed TEM measurement (Fig. S2, ESI†), whereas the small diameter of the pores did not allow for the direct observation of any additional details.

Fig. 2 FT-IR spectra of PFPOP-1–3 compared with their monomers.

Fig. 3 SEM images of (a) PFPOP-1, (b) PFPOP-2, and (c) PFPOP-3.

Nitrogen sorption isotherm measurements at 77 K were performed to investigate the porous property of PFPOP-1–3. As shown in Fig. 4a, isotherms of all PFPOPs present type I characters with a steep uptake at low relative pressure (P/P₀ < 0.1), suggesting the microporous property of these materials. The sorption hysteresis existing between adsorption and desorption branches can be explained as the swelling of soft framework or the restricted access of nitrogen molecules to a narrow pore opening. The curves show gradual increase in the range of medium and high relative pressure, and these results are in accord with the morphology investigations and demonstrate that the PFPOPs possess no macroporosity characters. The surface area results are evaluated with the application of the Brunauer–Emmett–Teller (BET) model and found to be 570, 630, and 530 m² g⁻¹ for PFPOP-1, PFPOP-2, and PFPOP-3, respectively (Table 1 and Fig. S3, ESI†). Meanwhile, the micropore surface area results of PFPOP-1–3 are calculated to be 350–410 m² g⁻¹ using the t-plot method, accounting for 65% of the BET surface area. The total pore volumes are calculated to be 0.30–0.35 cm³ g⁻¹ according to single point adsorption at P/P₀ = 0.99. The pore size distribution profiles of PFPOPs based on the non-local density function theory (NLDFT) method display the main peaks centered at 0.67–0.73 nm accompanied by several small peaks between 1–2 nm (Fig. 4b). These predominant micropore structures with intrinsic multi-hydroxyl involved in PFPOPs lead them to be promising candidates in gas adsorption towards small molecules.

Fig. 4 (a) Nitrogen sorption isotherms measured at 77 K of PFPOP-1–3. (b) Pore size distribution profiles calculated using the NLDFT method of PFPOP-1–3.

Table 1 Porosity properties of PFPOP-1–3

PFPOPs	SBET^a (m^2 g^−1)	Smicro^b (m^2 g^−1)	Vmicro^c (cm^3 g^−1)	Vtotal^d (cm^3 g^−1)	H2 uptake^e (wt%)
a Specific surface area calculated from the nitrogen adsorption isotherm using the BET method in the relative pressure (P/P0) range from 0.01 to 0.10.b Micropore surface area calculated from the nitrogen adsorption isotherm using the t-plot method.c Micropore volume calculated from the nitrogen adsorption isotherm using the t-plot method.d Total pore volume at P/P0 = 0.99.e Hydrogen uptake at 1.0 bar and 77 K.
PFPOP-1	570	380	0.19	0.32	1.79
PFPOP-2	630	410	0.19	0.35	1.20
PFPOP-3	530	350	0.16	0.30	1.58

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Adsorption properties for carbon dioxide and hydrogen are shown in Fig. 5. Hydrogen uptake capacity of PFPOP-1–3 is in the region of 1.2–1.8 wt% at 1.0 bar and 77 K (Fig. 5a). To our surprise, in spite of the unobvious difference in specific surface area, hydrogen storage capacity of PFPOPs is much better than many other POPs,¹³ which might be owing to the microporosity of the aromatic networks of PFPOPs. Carbon dioxide adsorption experiments were carried out by volumetric methods at 273 and 298 K (Fig. 5b and c). Fig. 5b exhibits that the carbon dioxide capture capacities are 12.2, 15.0, and 17.3 wt% at 273 K and 1.0 bar for PFPOP-1, PFPOP-2, and PFPOP-3, respectively, and the values are even comparable with the porous organic polymers with high BET specific surface area such as PAF-3 (15.3 wt%; S_BET = 2932 m² g⁻¹),^14a ALP-3 (16.6 wt%; S_BET = 975 m² g⁻¹),^14b Zn/Ni-ZIF-8-1000 (18.7 wt%; S_BET = 750 m² g⁻¹),^14c and BILP-7 (19.2 wt%; S_BET = 1122 m² g⁻¹).^14d To further investigate the binding affinity of PFPOPs for CO₂, we calculated the corresponding isosteric heat (Q_st) of carbon dioxide derived from adsorption data recorded at 273 and 298 K from 0 to 1.06 bar by using the Clausius–Clapeyron equation, and the initial Q_st results of PFPOP-1–3 toward CO₂ are found to be close to each other in the range between 26.9 and 32.5 kJ mol⁻¹ (Table 2 and Fig. S4, ESI†), which show a strong affinity between CO₂ and porous polymer skeletons.^15a These values are comparable to that of the most reported POPs with high CO₂ uptakes such as PPFs (22–29 kJ mol⁻¹),^9d PI-1 (34 kJ mol⁻¹),^15b PECONFs (26–34 kJ mol⁻¹)^15c or TBILP-1 (35 kJ mol⁻¹),¹⁶ and reveal that microporous and hydroxyl-rich structure contribute to stronger binding affinities of polymers toward CO₂. Interestingly, the CO₂ capture capacities of PFPOPs present good correlations with their Q_st. It is noteworthy that although PFPOPs have similar micropore structures with approximate pore size distributions, PFPOP-3 presents the highest CO₂ capture capability and isosteric heat owing to the polar influences of nitrogen atoms.^9d,16

Fig. 5 (a) Hydrogen adsorption isotherms for PFPOP-1–3 at 77 K. Carbon dioxide sorption isotherms for PFPOP-1–3 at (b) 273 K and (c) 298 K. (d) CO₂/N₂ selectivity of PFPOP-1–3 for a molar ratio of 15/85 at 273 K.

Table 2 Corresponding data of CO₂ capture and separation for PFPOP-1–3

PFPOPs	CO2 at 1.0 bar
	273 K^a	298 K^a	Qst^b	Selectivity^c
a CO2 uptake in wt%.b The isosteric enthalpies of adsorption (Qst) in kJ mol^−1.c Selectivity (mol mol^−1, 1.0 bar) was calculated by IAST method at mole ratio of 15/85 for CO2/N2 (flue gas model) at 273 K.
PFPOP-1	12.2	5.1	26.9	43.7
PFPOP-2	15.0	6.8	30.2	52.1
PFPOP-3	17.3	7.5	32.5	56.5

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In order to investigate the potential use of PFPOPs in gas separation applications, single component adsorption isotherms for CO₂ and N₂ were collected at 273 K, 1.0 bar. CO₂/N₂ selectivity was evaluated by using the results from the ideal adsorbed solution theory (IAST), which has been reported to provide good predictions of gas mixture behavior from single-component isotherms¹⁷ and widely used in various porous materials such as zeolites,¹⁸ metal–organic frameworks,¹⁹ and porous organic polymers.²⁰ To evaluate the CO₂ over N₂ selectivity of the PFPOPs under post combustion flue gas conditions, we used the IAST model along with experimental data and fitting of the single-component gas isotherm to evaluate the uptakes in the mixture for specified partial pressures in the bulk gas phase. Under simulated flue gas conditions (CO₂/N₂, 15/85), the experimental CO₂ and N₂ isotherms collected at 273 K for PFPOPs were fitted to the dual-site Langmuir model and the single-site Langmuir model, respectively (Fig. S5, ESI†). The calculated IAST data for PFPOPs are shown in Table S2 (ESI†). At 273 K and 1.0 bar, PFPOP-1–3 exhibit an appreciably high selectivity of CO₂ over N₂ under flue gas conditions (43.7–56.5) (Fig. 5d and Table 2). Importantly, these values are almost similar to the performance of reported high CO₂/N₂ selective porous materials, such as ALPs (34–44)^14b and Zn/Ni-ZIF-8-1000 (30).^14c

Conclusions

In summary, we have synthesized three kinds of multi-hydroxyl groups containing porous organic polymers based on phenol formaldehyde resin chemistry. THB and trialdehydes have been employed to polymerize to achieve PFPOP-1–3 in catalyst-free solvothermal conditions. The formations of these polymers were confirmed by FT-IR spectra and solid-state ¹³C NMR measurements, and their spherical morphologies were investigated via SEM and TEM observations. Nitrogen sorption isotherm measurements reveal that PFPOPs possess considerable surface area and exhibit narrow pore size distribution in micropore region. The highest CO₂ capture capability among them is up to 17.3 wt% for PFPOP-3. Furthermore, PFPOPs show considerable CO₂ over N₂ selectivity (43.7–56.5, IAST at 273 K and 1.0 bar) in a specified flue gas composition (CO₂/N₂ = 15/85). These properties suggest that PFPOPs can be considered as suitable candidates for CO₂ adsorption and separation owing to their microporous properties and intrinsic hydroxyl-rich structures.

Acknowledgements

The financial support of the National Natural Science Foundation of China (Grants no. 21304022 and 21374024) is acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details regarding solid-state ¹³C NMR spectra, elemental analysis data, TEM images, and corresponding data for gas adsorption and selectivity. See DOI: 10.1039/c5ra13405f

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