Multifunctional Polyhedral Oligomeric Silsesquioxane (POSS) Based Hybrid Porous Materials for CO2 Uptake and Iodine Adsorption

Abstract

In this study, two different types of hybrid porous organic polymers (POPs), polyhedral oligomeric silsesquioxane tetraphenylpyrazine (POSS-TPP) and tetraphenylethene (POSS-TPE), were successfully synthesized through the Friedel−Crafts polymerization of tetraphenylpyrazine (TPP) and tetraphenylethene (TPE), respectively, with octavinylsilsesquioxane (OVS) as node building blocks, in the presence of anhydrous FeCl₃ as a catalyst and 1,2-dichloroethane at 60 °C. Based on N₂ adsorption and thermogravimetric analyses, the resulting hybrid porous materials displayed high surface areas (270 m²/g for POSS-TPP and 741 m²/g for POSS-TPE) and outstanding thermal stabilities. Furthermore, as-prepared POSS-TPP exhibited a high carbon dioxide capacity (1.63 mmol/g at 298 K and 2.88 mmol/g at 273 K) with an excellent high adsorption capacity for iodine, reaching up to 363 mg/g, compared with the POSS-TPE (309 mg/g).

1. Introduction

The preparation of porous organic polymers (POPs) has become a hot topic in the academic and industry fields due to their interesting features, such as high physicochemical stability, porous character, low density, facile preparation, low regeneration energy, and good thermal and chemical stabilities [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Porous organic polymers have been applied in different applications such as light harvesting, chemical sensors, catalysis, iodine uptake, H₂ production from water, water treatment, optoelectronic devices, carbon dioxide reduction, nanofiltration, enantioseparation, energy storage, gas separation, and adsorption [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]. The synthesis of POPs was successfully achieved using various synthetic methods, including Friedel–Crafts arylation, Schiff base reactions, Suzuki reactions, Yamamoto reactions, Heck reactions, and Sonogashira reactions [30,31,32,33,34,35,36,37,38,39,40,41]. In addition, POPs can be classified into different kinds of materials, such as covalent organic frameworks (COFs), conjugated microporous polymers (CMPs), hypercrosslinked polymers (HCPs), covalent triazine-based frameworks (CTFs), metal–organic frameworks (MOFs), and polymers of intrinsic microporosity (PIMs) [37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52].

Polyhedral oligomeric silsesquioxanes (POSSs) are a kind of nanostructured compound with a diameter in the range of 1–3 nm, an empirical formula (R₈Si₈O₁₂), a well-defined structure, and a controlled porosity [53,54,55,56,57,58,59,60,61]. Recently, using an inorganic POSS as a building block for preparation, hybrid porous materials have attracted much attention because of their excellent thermal and mechanical proprieties, water retardance, and chemical stability [62,63,64,65,66]. As reported, hydrosilylation, Heck, Friedel–Crafts, Yamamoto, and Sonogashira reactions could be used for the preparation of POSS porous materials [67,68]. For example, Liu et al. successfully prepared a porous polymer (Fc-HPP) through the Friedel–Crafts reaction of ferrocene (Fc) with OVS. These materials displayed a high surface area (1015 m²/g) and excellent performance for removing heavy metals and dyes [66]. Liu et al. also prepared a series of porous polymers derived from OVS with various ethene derivates through the heck coupling reaction [67]. The same group also used a Friedel–Crafts reaction to prepare multifunctional hybrid porous materials from OVS and tetraphenylene for metal ion detection and dye adsoprtion. They revealed that these materials possess dual pore structures with a high surface area (up to 1910 m²/g) [65].

In this work, we designed, synthesized, and applied in CO₂ and I₂ the capture of two different types of hybrid porous organic polymers (POSS-TPP and POSS-TPE) derived from the Friedel−Crafts polymerization of tetraphenylpyrazine (TPP) and tetraphenylethene (TPE), respectively, with octavinylsilsesquioxane (OVS) as node building blocks in the presence of anhydrous FeCl₃ as a catalyst and 1,2-dichloroethane at 60 °C. Various instruments, such as Fourier-transform infrared spectroscopy (FTIR), solid state ¹³C cross-polarization/magic-angle spinning (CP/MAS), nuclear magnetic resonance spectroscopy (NMR), Brunauer–Emmet–Teller theory (BET), XRD, TEM, and SEM were used to examine the chemical structures, morphology, crystallinity, and porous properties for POSS-TTP and POSS-TPE, respectively. The results revealed that the as-prepared POSS-TPP exhibited a high carbon dioxide capacity (1.63 mmol/g at 298 K and 2.88 mmol/g at 273 K) with an excellent and high adsorption capacity for iodine, reaching to up to 363 mg/g. 2. Materials and Methods 2.1. Materials Octavinylsilsesquioxane (OVS, 98%), benzophenone (99%), anhydrous ferric chloride (FeCl_3, 99.99%), 1,2-dichloroethane (DCE, 99.8%), anhydrous magnesium sulfate (MgSO₄, 99.5%), ammonium acetate (CH₃COONH₄, 98%), acetic acid (CH₃COOH, 99%), potassium carbonate (K₂CO₃, 99.995%), and benzoin (98%) were purchased from Alfa Aesar (Heysham, United Kingdom). Tetrahydrofuran (THF), acetone, methanol (CH₃OH), and dichloromethane (DCM) were ordered from Aldrich (Taipei, Taiwan). 2.2. Synthesis of Tetraphenylpyrazine (TPP)

TPP was synthesized as reported in the previous study with a minor modification [69]. Benzoin (2.12 g, 10 mmol), acetic anhydride (1.45 mL, 15 mmol), ammonium acetate (2.32 g, 30 mmol), and 50 mL of acetic acid were added to a 50 mL round-bottom flask. After refluxing for 3.5 h at 90 °C, the mixture was cooled down to room temperature and then filtered. The crude products were purified with recrystallization in acetic acid to afford TPP as a white powder (yield: 34%) (¹H NMR (500 MHz, δ, ppm, CDCl₃, Figure S1): 7.65 (m, 8H), 7.33 (m, 12H); ¹³C NMR (125 MHz, δ, ppm, CDCl₃, Figure S2): 148.5, 138.5, 129.9, 128.7, 128.3).

2.3. Synthesis of Tetraphenyethene (TPE)

TPE was prepared according to our previous study [7,13]. Benzophenone (3.00 g, 16.20 mmol), zinc (3.91 g, 59.4 mmol), and dry THF (100 mL) were stirred and kept at 0 °C for 30 min. Then, TiCl₄ (3.29 mL, 29.4 mmol) was added slowly to the reaction mixture at 0 °C, and the reaction solution was refluxed at 80 °C for 18 h. After that, a 10% aqueous solution of K₂CO₃ was added dropwise to the reaction mixture at room temperature. The reaction mixture was extracted with ethylacetate (EA), dried over MgSO₄ and filtered, and the THF solution was removed to yield TPE as a white crystalline solid (yield: 97%) (¹H-NMR (500 MHz, δ, ppm, CDCl₃, Figure S3): 7.05–7.15 (m, 20H); ¹³C-NMR (125 MHz, δ, ppm, CDCl₃, Figure S4): 140.7, 141.0, 131.3, 127.7, 126.4).

2.4. Synthesis of POSS-TPP A solution of OVS (0.63 g, 1 mmol), tetraphenylpyrazine (0.32 g, 0.8 mmol), and FeCl₃ (0.9 g, 6.7 mmol) in dry 1,2-dichloroethane (20 mL) was prepared. The mixture was stirred at room temperature for 0.5 h and then heated under reflux at 60 °C for 24 h. After cooling at room temperature, the solid was filtered and washed three times with THF, methanol, chloroform, and acetone to remove the unreacted monomer and FeCl₃. Then, it was dried under a vacuum at 80 °C to get a red-brown solid (0.55 g, 87%). 2.5. Synthesis of POSS-TPE A solution of OVS (0.63 g, 1 mmol), TPE (0.28 g, 0.8 mmol), and FeCl₃ (0.9 g, 6.7 mmol) in 1,2-dry dichloroethane (20 mL) was stirred at room temperature for 0.5 h and then heated under reflux at 60 °C for 24 h. After cooling at room temperature, the solid was filtered and washed three times with THF, methanol, chloroform, and acetone to remove the unreacted monomer and FeCl₃. It was then dried under a vacuum at 80 °C to get a red-brown solid (0.59 g, 93%). 2.6. Uptake of Iodine The procedure of iodine uptake by both porous materials (POSS-TPP and POSS-TPE) was carried out as follows: POSS-TPP or POSS-TPE (5 mg) was suspended in 10 mL of an I₂ solution in hexane. The solution mixture was stirred at different times, and then the UV-Vis measurement was done. The calibration curve of the I₂ solution was recorded by using various iodine concentrations. 3. Results 3.1. Synthesis of POSS-TPP and POSS-TPE

The preparation route of the TPP and TPE monomers is shown in Scheme 1. The TPP monomer was successfully synthesized by the reaction of benzoin with ammonium acetate (CH₃COONH₄) in the presence of acetic acid at 90 °C. The TPE monomer was prepared by the reaction of benzophenone with zinc in dry THF as a solvent and the presence of TiCl₄ at 80 °C under an N₂ atmosphere. While the schematic method for the preparation of two different hybrid microporous polymers (POSS-TPP and POSS-TPE) was based on OVS, TPP and TPE are shown in Scheme 2. Firstly, POSS-TPP was prepared by a Friedel–Crafts reaction of OVS with TPP in the presence of anhydrous FeCl₃ as a catalyst and 1,2-dichloroethane as solvent at 60 °C for 24 h in an N₂ atmosphere (Scheme 2A), while POSS-TPE was synthesized by a Friedel–Crafts reaction of OVS with TPE in the presence of anhydrous FeCl₃ as a catalyst and 1,2-dichloroethane as a solvent at 60 °C for 24 h under an N₂ atmosphere (Scheme 2B). The resulting materials were successively washed several times with dimethylformamide (DMF), dimethylacetamide (DMAc), THF, DCM, and MeOH to remove FeCl₃ and unreacted OVS, as well as other monomers. In addition, the POSS-TPP and POSS-TPE were not soluble in acetone, THF, DMF, DCM, or MeOH. Figure 1 shows the ¹H-NMR spectra of TPP and TPE with CDCl₃ as the solvent. The proton signals were located at 7.64 ppm and 7.35 ppm for the aromatic rings in the TPP (Figure 1A). The ¹H-NMR spectrum (Figure 1B) of TPE showed a characteristic proton signal centered at 8.04–7.04 ppm for the aromatic proton signals. Furthermore, we used ¹³C-NMR measurements to confirm the chemical structures of the TPP and TPE, as displayed in Figure 1. As shown in Figure 1C,D, the signals of the aromatic carbon nuclei in the TPP and TPE moieties appeared in the range of 149.64–129.29 ppm for the TPP and 141.06–126.40 ppm for the TPE.

In addition, FTIR analyses were used to investigate the functional groups in OVS, TPP, TPE, POSS-TPP, and POSS-TPE, respectively (Figure 2). The absorption bands of Si–O–Si, C=C–H, and C=C in the OVS (Figure 2A) were located at 1107 cm⁻¹, 1600 cm⁻¹, and 3066 cm⁻¹, respectively. The FTIR spectra of TPP, TPE, POSS-TPP, and POSS-TPE displayed a characteristic absorption band in the range of 3048–3052 cm⁻¹ for the stretching C–H aromatic groups and at 1599 cm⁻¹ for the C=C bonds (Figure 2B–E). In addition, the absorption signals appeared at 3450 cm⁻¹, 2950 cm⁻¹, and 1457 cm⁻¹ in the FTIR spectra of POSS-TPP and POSS-TPE (Figure 2D,E), representing adsorbed water by our porous materials and the presence of CH₂-CH₂ linkage in both the POSS-TPP and POSS-TPE polymer frameworks.

The chemical structures of POSS-TPP and POSS-TPE were confirmed through the solid-state ¹³C CP/MAS analyses (Figure 3). The solid-state ¹³C CP/MAS profiles (Figure 3) of POSS-TPP and POSS-TPE features carbon nuclei signals in the range 138.11–129.77 ppm and 41.08–20.80 pm for POSS-TPP (Figure 3A) which corresponds to aromatic ring in POSS-TPP, unreacted vinyl carbons, and Si–CH₂–CH₂-TPP or Si–CH(CH₃)-TPP units, respectively. While the signals appearing in the range 140.94–129.17 and 39.96–15.17 pm for POSS-TPE (Figure 3B) arising from TPE units, unreacted vinyl carbons and Si–CH₂–CH₂-TPE or Si–CH(CH₃)-TPE units, respectively.

The solid state ²⁹Si MAS NMR measurement was done to confirm the presence of POSS moiety in both POSS-TPP and POSS-TPE, as presented in Figure 4. In the displayed solid state ²⁹Si MAS NMR spectra of POSS-TPP and POSS-TPE, two major signals at −66.95 ppm and −78.66 ppm were attributed to the silicon atom of CSi(OSi)₃ (T₃) from the SiCH₂ CH₂TPP, Si¬CH(CH₃) TPP units and the unreacted SiCH=CH₂ group, respectively, and for POSS-TPP and the silicon atom of CSi(OSi)₃ (T₃) from the SiCH₂CH₂TPE, SiCH(CH₃) TPE units and unreacted SiCH=CH₂ group, respectively. In addition, the disappearance of signals arising from the T₁ or T₂ units (T_n = CSi (OSi)_n(OH)_3−n) indicated both the remaining structure and the lack of cleavage of the cage structure in the POSS-TPP and POSS-TPE hybrid polymers.

3.2. Thermal Stability and Porosity Properties of POSS-TPP and POSS-TPE

The thermal stabilities of OVS, TPP, TPE, POSS-TPP, and POSS-TPE were studied using TGA analysis under an N₂ atmosphere, as shown in Figure 5. As displayed in Figure 5, the degradation temperatures (T_d5 and T_d10) and char yields were 242 °C, 255 °C, and 3.6% for OVS; 296 °C, 312 °C, and 0% for TPP; 246 °C, 263 °C, and 0% for TPE; 267 °C, 398 °C, and 71% for POSS-TPP; and 378 °C, 475 °C, and 73% for POSS-TPE, respectively. Interestingly, both POSS-TPP and POSS-TPE hybrid polymers displayed high thermal stabilities compared with their corresponding monomers, which was attributed to the introduction of POSS units, which enhanced the highly crosslinked network in the system, preventing the rapid degradation of hybrid polymers and also increasing the cleavage degradation of organic groups in the POSS-TPP and POSS-TPE. The values of T_d5 and T_d10 and the char yield for the OVS, TPP, TPE, and their corresponding polymers are summarized in Table 1.

The wide-angle powder X-ray diffraction (WAXD) patterns of POSS-TPP and POSS-TPE (Figure 6) showed a broad peak at 2θ ≈ 22°, indicating that both of these two porous materials possessed amorphous characteristics, with no long-range order compared with the highly crystalline of TPE and OVS. These results displayed the random cross-linking between the TPP and TPE molecules with POSS cage structures [70,71].

Nitrogen adsorption–desorption experiments at 77 K were measured to examine the porosity parameters for our new POSS-TPP and POSS-TPE (Figure 7). Both of the nitrogen adsorption–desorption profiles of POSS-TPP and POSS-TPE (Figure 7A,B) revealed a rapid N₂ uptake at a low P/P₀ pressure that continued to increase at a high P/P₀ pressure, indicating the coexistence of mesopores and micropores in the hybrid polymer network. Based on the International Union of Pure and Applied Chemistry (IUPAC) classification for BET curves, both POSS-TPP and POSS-TPE featured type IV adsorption isotherms. In addition, there were the hysteresis loops in both curves which also confirmed the presence of micropores and mesopores in the polymer framework. The Brunauer–Emmet–Teller (BET) surface area and total pore volume of the POSS-TPP and POSS-TPE were calculated to be 270 m²/g and 0.22 cm³/g, respectively, for POSS-TPP and 741 m²/g, 0.58 cm³/g, respectively, for POSS-TPE. The pore size distributions of POSS-TPP and POSS-TPE were calculated by using non-local density functional theory (NL-DFT), as elucidated in Figure 7C,D. The pore size diameters of the POSS-TPP were 1.04 nm and 1.82 nm (Figure 7C), while the POSS-TPE featured pore size diameters at 1.08 nm and 2.09 nm (Figure 7D). The porosity parameters of the POSS-TPP and POSS-TPE are provided in Table 1. In addition, TEM images (Figure 7E,F and Figure S5) of the POSS-TPP and POSS-TPE featured a microporous structure with no long-range order. The TEM results were consistent with the XRD pattern (Figure 6).

3.3. CO₂ Capture and Iodine (I₂) Adsorption Performance

We applied POSS-TPP and POSS-TPE for CO₂ capture and iodine adsorption due to their high total pore volumes, BET surface areas, and the presence of mesoporous and microporous structures in the polymer networks. We performed CO₂ uptake measurements for POSS-TPP and POSS-TPE at 298 K and 273 K to examine their CO₂ uptake abilities, as presented in Figure 8. The results revealed that POSS-TPP displayed the highest CO₂ adsorption capacity, reaching 1.63 mmol/g at 298 K and 2.88 mmol/g at 273 K, while POSS-TPE showed CO₂ capture values reaching 0.99 mmol/g at 298 K and 1.97 mmol/g at 273 K. As previously reported, the enhancement of polymeric porous materials for the adsoprtion of acidic CO₂ gas can be achieved through the incorporation of basic nitrogen functionalities and oxygen groups into the polymeric network. Interestingly, even POSS-TPP possesses a low BET surface area (270 m²/g) compared with the POSS-TPE. POSS-TPP displayed a higher CO₂ uptake than that other porous materials, such as poly(1,1-dimethyl-3,4-diphenyl-2,5-bis(4’,4’-diphenylaminophenyl)) silole (PDMTPAS), (1.02 mmol/g at 298 K), An-CPOP-1 (1.30 mmol at 298 K and 1.40 mmol/g at 273 K), An-CPOP-2 (1.40 mmol/g at 298 K and 1.52 mmol/g at 273 K), and C2M1-Al (1.44 mmol/g at 273 K) [5,72,73], due to the POSS-TPP containing N-atoms, which can lead to effectively increasing the dipole–quadrupole interactions between the CO₂ molecules and the POSS-TPP. The CO₂ adsoprtion capacity performance of POSS-TPP and POSS-TPE, compared with other porous materials, is provided in Table S1.

The developing porous polymer absorbents featuring good porosities and unique structures to decrease environmental pollution, such as I₂, have attracted much intention in both academics and industry [19]. As reported by different groups, the performance of the porous organic polymers as adsorbents for iodine uptake strongly depends on the interaction of the iodine molecule with the polymer framework and does not depend on the pore size or volume of the polymer network [19]. Figure 9A,B presents the UV absorbance spectra of the suspension solutions of the POSS-TPP and POSS-TPE in ethanol with an I₂ solution (10⁻⁴ M) at different interval times (0 h to 24 h). As shown in Figure 9A,B, the maximum intensity of the adsorption peak of I₂ was detected at λ_max = 523 nm. After the addition of 5 mg of POSS-TPP or POSS-TPE into the iodine solution in hexane, the intensity of the absorption bands of the I₂ gradually decreased when the time increased from 0 to 24 h. These results suggest that both POSS-TPP and POSS-TPE could act as good precursors for I₂ adsoprtion. The calibration curve (Figure 9C) of the iodine was measured through the recording of UV-Vis spectra of different I₂ concentrations in hexane. We observed that the maximum adsorption capability value (Figure 9D) of the POSS-TPP (363 mg/g) was higher than that of POSS-TPP (309 mg/g). From the adsoprtion experiments, the POSS-TPP was considered a better adsorbent material for iodine than POSS-TPE due to the presence of N-atoms in the POSS-TPP framework, leading to strong interaction with the I₂ molecules. POSS-TPP and POSS-TPE showed excellent iodine uptake performance compared with other POP adsorbents, as presented in Table S2. In addition, the synthesis of hybrid porous organic polymers based on the POSS unit through a simple Friedel–Crafts reaction with large-scale applications was considered as an effective synthetic method [36]. For recycling experiments, the iodine-loaded POSS-TPP and POSS-TPE samples were washed with ethanol to remove the I₂ molecules and dried at 100 °C. As shown in Figure S6, after recycling the measurements six times, the iodine capture capacities of the POSS-TPP and POSS-TPE were found to drop from 363 mg/g to 362 mg/g and 309 mg/g to 308 mg/g, respectively. From these results, our new adsorbent materials were used as good precursors for reversible and recyclable iodine uptake.

4. Conclusions In summary, two different kinds of hybrid porous materials (POSS-TPP and POSS-TPE) were easily synthesized via simple Friedel−Crafts polymerization of cubic octavinylsilsesquioxane as node building blocks with tetraphenylpyrazine and tetraphenylethene, respectively, in the presence of FeCl₃ as a catalyst. The synthesized POSS-TPP and POSS-TPE hybrid porous polymers exhibited high surface areas of 270 m²/g and 741 m²/g, respectively, with mesoporous structures. In addition, the POSS-TPP displayed the highest CO₂ adsorption capacity, reaching 1.63 mmol/g at 298 K and 2.88 mmol/g at 273 K, with a maximum adsorption capacity for I₂ reaching 363 mg/g, compared with the POSS-TPE and other porous materials.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4360/13/2/221/s1. Figure S1: 1H NMR spectrum of TPP. Figure S2: 13C NMR spectrum of TPP. Figure S3: 1H NMR spectrum of TPE. Figure S4: 13C NMR spectrum of TPE. Figure S5: TEM images of POSS-TPP (A,B,C) and POSS-TPE (D,E,F). Figure S6: Repeated I2 uptake experiments for (A) POSS-TPP and (B) POSS-TPE. Table S1: Performance data of POSS-TPP and POSS-TPE compared with those of other previously porous materials. Table S2: Iodine uptake properties of POSS-TPP, POSS-TPE and other porous materials.

Author Contributions

M.G.M. and M.-Y.T. did the experiment, C.-F.W., C.-F.H., T.C., and M.D. helped to design the reaction, L.D. helped to design the POSS composite, and S.-W.K. contributed to the literature review and to the writing of this paper. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology, Taiwan, under contracts MOST109-2221-E-110 -067 -MY3 and MOST 109-2923-E-110 -001 -MY3, This work was also supported by the National Natural Science Foundation of China (51773214) and by SAS-MOST Joint Research Project "Tough-VigNet SAS-MOST JRP 2019/07". We also thank Hsien-Tsan Lin of the Regional Instruments Center at National Sun Yat-Sen University for help with the TEM measurement.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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