1. Introduction

The growing use of fossil fuels has highly promoted the development of industrial revolution, but it also caused rapid increase of CO₂ emissions in the atmosphere and led to serious global climate warming. Since the past decade, scientists over the world have devoted great efforts to investigating series of porous materials for CO₂ capture and storage (CCS) [1-3]. As an important family of porous materials, porous organic frameworks (POFs) which usually constructed from wholly light elements (C, H, O, B, etc.) had attracted scientists wide attention and became one of alternative materials for CCS, because of their large surface areas and good mechanical properties. Most POF materials, including polymers of intrinsic microporosity (PIMs) [4, 5], conjugated microporous polymers (CMPs) [6, 7], hyper-crosslinked polymers (HCPs) [8], triazine-based organic frameworks (CTFs) [9, 10], and porous aromatic frameworks (PAFs) [11-13] have been used for CCS [14, 15]. Besides, the variety of synthetic routes and ease of controlling pore properties in skeletons make POF materials been utilized in many fields, such as gas storage [16], separation [17], heterogeneous catalysis [18, 19], sensors [20] and electricity [21]. However, most of these materials are synthetized via noble metals involved reactions such as palladium catalyzed Sonogashira-Hagihara cross-coupling reaction [22], palladium catalyzed Suzuki cross-coupling reaction [23] and nickel catalyzed Yamamoto-type Ullmann cross-coupling reaction [24] or under high temperatures [9, 10]. These catalysts are very expensive and usually oxygen-sensitive which limited the large-scale development of the resulting POF materials. Recently, Cooper and Tan group reported the preparation of a series of HCPs with small heterocyclic molecular as monomers (pyrrole, thiofuran, furan, benzene and biphenyl, etc.) by Friedel-Crafts alkylation reaction [25-27]. These HCPs materials exhibited certain CO₂ sorption abilities. Yuan and Jiang group have prepared a mass of polymers used planar monomers (1, 3, 5-triphenylbenzene, cyanuric chloride, tetraphenylethylene and 1, 1, 2, 2-tetraphenylethane-1, 2-diol, etc.) as building units [28-30]. Cubic units (such as octavinylsilsesquioxane) and tetrahedral molecules were also used to synthesize POF materials via Friedel-Crafts alkylation reaction [31]. This kind of reaction is catalysed by low-cost anhydrous FeCl₃ or AlCl₃. In the presence of Lewis acid, aromatic nuclei of monomers become active to achieve polymerization to product POF materials. Meanwhile, these materials polymerized by Friedel-Crafts alkylation reaction have many applications in electricity chemistry [32] and gas separation [33]. From the above, it suggested that Friedel-Crafts alkylation reaction could be an ideal candidate reaction for preparing cost-effective POF materials.

It is well known that diamond is one of the most stable compounds in nature. Tetrahedral organic molecules as components have many applications in design and preparation of supramolecular architectures and covalent polymers [34]. As a representable POF material with diamond-like structure, PAF-1 built from tetraphenylmethane (TPM) has ultra-high BET surface area (5600 m² g^-1) and good thermal and chemical stability [11]. Then our group promoted to synthesize three PAFs with TPM units and successfully incorporated functional groups (hydroxyl and amino group) in the skeleton with the alkylation reaction [35]. After that, Marchese group reported the synthesis of mPAF based on TPM building unit and carried out CO₂ adsorption simulation with theoretical modeling, which confirmed that the POFs derived from tetrahedral monomers were expected to perform well in the gas uptake, especially at low pressures [36].

Herein, we presented the synthesis of a novel PAF material, PAF-8, choosing diamond-like structural monomer tetraphenylsilane, and using Friedel-Crafts alkylation reaction catalysed by anhydrous FeCl₃ under mild condition. Adequate characterizations have been employed to confirm the structural features and pore properties of the resulting PAF material, and it turned out that PAF-8 was synthesized as desired. Beside from the good stability and high surface area (785 m² g^-1), PAF-8 also presented good performance on gas and liquid sorption such as CO₂, CH₄, water and methanol. In addition, gas chromatography experiment was carried out based on the resulting data and PAF-8 material exhibited good separation abilities for CH₄-CO₂ and N₂-CO₂ mixture.

2. Experimental 2.1. Chemicals

Tetraphenylsilane and formaldehyde dimethyl acetal (FDA) were purchased from Alfa Aesar. CaH₂ was purchased from Aladdin. Other chemicals and reagents were purchased from commercial suppliers without further purification unless otherwise stated. Methylene dichloride (CH₂Cl₂) was dehydrated with CaH₂.

2.2. Synthesis of PAF-8

Tetraphenylsilane (1 mmol, 0.336 g) and FeCl3 (4 mmol, 0.65 g) were added into a 50 mL round-bottom-flask and then this system was purged with Ar by three freeze-pump-thaw cycles. FDA (4 mmol 0.36 mL) and CH₂Cl₂ (10 mL) were injected into the flask with a syringe. The polymerization fused aromatic monomers with 4 equivalents of FDA (depending on the number of aromatic reaction sites available). This mixture was stirred at 45 ℃ for 24 h to produce the PAF material. After cooling down to room temperature, the resulting powders were collected by filtration and washed with 1 mol L^-1 hydrochloric acid solution, acetone and methanol until the filtrate liquid was nearly colorless. Then the product was Soxhlet extracted for 24 h with methanol, tetrahydrofuran and dichloromethane, respectively to remove the monomers and catalysts thoroughly and then dried under vacuum to give PAF-8 as brown powders (96% yield, Fig. 1).

2.3. Physical measurements

The Fourier transform infrared spectroscopy (FTIR) spectra (film) were measured using IFS 66V/S Fourier transform infrared spectroscopy. Solid-state ¹³C CP/MAS NMR measurement was performed on a BrukerAvance Ⅲ model 400 MHz spectrometer at a MAS rate of 5 kHz. The Element analysis (for C and H) were measured using a Perkin Elmer 2400 Series CHNS/O Analyzer. The powder X-ray diffraction (PXRD) was performed by a Riguku D/ MAX 2550 diffractometer using Cu-Kα radiation, 50 kV, 200 mA with scanning rate of 4° min^-1 (2θ). Scanning electron microscopy (SEM) analysis was performed on a JEOS JSM 6700. Transmission electron microscopy (TEM) was recorded using a JEOL JEM 3010 with an acceleration voltage of 300 kV. Thermogravimetric analysis (TGA) was performed using a Netzch Sta449c thermal analyzer system at a heating rate of 10 ℃ min^-1 in air. The gas adsorption-desorption isotherms were measured on a Quantachrome Autosorb-iQ2 analyzer. Gas chromatography (GC) separation of selected light gases using a column packed with PAFs was performed with a SP-2100A GC system equipped with a thermal conductivity detector.

3. Results and discussion

The structure properties of PAF-8 were characterized by various spectroscopic measurements. FTIR spectra, ¹³C CP/MAS NMR spectrum and elemental analysis were performed to confirm the structural features and the polymerization degree. PXRD, SEM and TEM were carried out to discover the morphology and crystallinity. The pore character was studied by N₂ sorption isotherm measurement. Because of the permanent porosity and high surface area of PAF-8, CO₂ and CH₄ adsorption and separation were also carried out. And the results indicated this PAF material had good gas sorption and separation abilities. Additionally, PAF-8 exhibited high water and methanol sorption abilities.

Comparisons of the FTIR spectra of PAF-8 and its tetrahedral monomer were performed to confirm the structural features and reaction degree of polymerization. As shown in Fig. 2, the appearance of intense saturated C-H bands (at 2903 cm^-1) of PAF-8 demonstrated the networks were linked by methylene group as desired. The peaks at ca. 3000 cm^-1 belong to aromatic C-H stretching vibrations and the peaks at ca. 1500 cm^-1 belong to aromatic C-H bending vibrations. The reduction in peak intensity of aromatic C-H bending vibrations was affected by the introduction of methylene group which indicated that reaction degree of polymerization was complete relatively. The aromatic C＝C stretching vibrations at 1630 cm^-1 indicated the existence of phenyl rings in the PAF-8 network. These results displayed the success of Friedel-Crafts alkylation reaction and the integrity of the aromatic skeleton. The local structure of PAF-8 was further investigated by ¹³C CP/MAS NMR spectrum. In the ¹³C CP/MAS NMR spectrum of PAF-8 (Fig. 3), three different types of resonance peaks were clearly observed. The strong signals in the range of 140 ppm-125 ppm were attributed to the aromatic carbon atoms. The peak at 127 ppm belonged to the aromatic C (C₂, C₃) connected with H and the peak at 138 ppm belonged to the aromatic C (C₁, C₄) connected with other atom or perssad. Moreover, the signal near 37 ppm was related to the methylene carbon atoms (C₅), thus further indicating the FDA participated in the Friedel-Crafts alkylation reaction. From the results of FTIR and ¹³C CP/MAS NMR, PAF-8 was synthetized successfully as expected. Elemental analysis (for C and H. Table 1) were measured to determine their content quantitatively and the result confirmed the methylene group participated in the alkylation. Because of the introduction of methylene group, the C/H ratio decreased slightly. It was consistence with the result of elemental analysis measurement. All of these showed that success of Friedel-Crafts alkylation reaction and the networks were linked by methylene group as desired.

In order to confirm the long-range structure of PAF-8, PXRD was carried out to discover the crystallinity. There were no intense peaks but only a broad peak indicating that the texture of the polymer was amorphous which might be caused by the irreversibility of Friedel-Crafts alkylation reaction and introducing of flexible perssad (methylene). This is common in most POF materials which synthesized via kinetics-controlled irreversible coupling processes [22, 35]. SEM image (Fig. 4a) showed that PAF-8 was amorphous and most particles were micro-sized irregular blocks. And its dimensions ranged from 200 nm to 800 nm. From the TEM (Fig. 4b), no ordered and obvious pore structure could be discovered which indicated the arrangement of the pores was random. This result was consistent with PXRD.

TGA under air condition was employed to investigate the structure stability of the polymers (Fig. 5). PAF-8 displayed high thermal stability with decomposition temperature above 350 ℃. Because of the existence of Si element, there was 16% oxide residue. It was consistent with the Si element content in PAF-8. In addition, PAF-8 material cannot be dissolved and decomposed in common organic solvents and acid solution, such as DMF, THF, CHCl₃, 1 mol L^-1 HCl acid solution and so on, which indicated its high chemical stability.

The pore character of PAF-8 was studied by N₂ sorption isotherm measured at 77 K under 1 atm. As shown in Fig. 6, there was a sharp uptake at low relative pressures, indicating PAF-8 possessed microporous texture. A distinct hysteresis loop was also clearly observed in the desorption branch, which is a feature of classic type IV isotherm, suggesting the presence of mesopores. The apparent surface area of PAF-8 calculated from the Brunauer-Emmett-Teller (BET) model is 785 m² g^-1. The pore size distribution was calculated from nonlocal density functional theory (NLDFT). It gave two narrow peaks at 1.69 nm and 3.80 nm agreed with the N₂ sorption isotherm. The high surface area and permanent porosity make PAF-8 an excellent candidate for gas sorption.

Gas sorption measurements were carried out because of the high surface area and permanent porosity of PAF-8. Industrial production CO₂ and CH₄ sorption were studied in our paper. CO₂ and CH₄ adsorption capacities of PAF-8 were measured at 273 K and 298 K, respectively. In Fig. 7a, CO₂ adsorption quantity of PAF-8 at 273 K and 298 K are 35.5 cm³ g^-1 and 19.4 cm³ g^-1, respectively. The heat of adsorption (Q_st) of PAF-8 was calculated from adsorption data collected from 273 and 298 K, using the Clausius-Clapeyron equation and the Q_st of CO₂ is 34.5 kJ mol^-1. Its Q_st of CO₂ is higher than many other POF materials such as HCP materials (20-24 kJ mol^-1) [37], polycarbazole CPOP-1 (27 kJ mol^-1) [38], polybenzimidazole BILP-1 (26.5 kJ mol^-1) [39]. The CH₄ adsorption quantity of PAF-8 at 273 K and 298 K are 10.7 cm³ g^-1 and 5.8 cm³ g^-1 (Fig. 7c). The Q_st of CH₄ is 24.5 kJ mol^-1, which is lower than Q_st of CO₂, thus indicating PAF-8 material possessed stronger sorption ability for CO₂ compared with CH₄. Due to the comparative CO₂ and CH₄ sorption capacity and different sorption ability, the separation measurement was also performed. To validate the sieving effect of PAF-8, the gas-chromatographic (GC) separation was employed at room temperature. PAF-8 was grinded and then packed tightly into the column (30 cm long×2 mm internal diameter). The lower end of the column was filled with glass wool during the column packing. GC with a SP-2100A GC system equipped with thermal conductivity detector was used to characterize the kinetic separation of CO₂-CH₄ mixture. The collected separation curve was shown in Fig. 7e. The result indicated that PAF-8 had very good separation ability of CO₂ and CH₄. Then, another nonpolar molecule N₂ was chosen for the separation measurement. It happened that there was a similar case that CO₂ and N₂ mixture were also separated through PAF-8. The retention times of CH₄, N₂ and CO₂ were 1.6, 2.6 and 12.5 min, respectively. The reason might be van der Waals interactions between skeleton and weak nonpolar CO₂ were stronger than that between skeleton and strong nonpolar CH₄ and N₂, so CH₄ and N₂ flowed more quickly than CO₂. The results suggested the promising application of PAF-8 in selective adsorption and separation of CO₂.

The inherent porosity of PAF-8 allows potential access by small solvent molecules, such as water and methanol. As shown in Fig. 8, PAF-8 material can adsorb certain amounts of water and methanol with value of 72 mg g^-1 and 308 mg g^-1 at 298 K. It indicated that PAF-8 material had high liquid vapor adsorption abilities. The adsorption amount of methanol is higher than water which showed that this PAF material may have potential application in H₂O-CH₃OH separation.

4. Conclusion

In summary, the PAF material (PAF-8) based on tetrahedral building blocks was successfully synthesized by simple one-step Friedel-Crafts alkylation reaction in the presence of inexpensive catalyst FeCl₃. The properties of PAF-8 material were well investigated and discussed. FTIR spectra and ¹³C CP/MAS NMR spectrum confirmed the structural features and the polymerization degree and the methylene group was introduced into the polymer skeletons successfully. TGA characterizations indicated that the PAF-8 material displayed high thermal stability. Meanwhile, PAF-8 material cannot be dissolved and decomposed in common organic solvents and acid solution. The apparent BET surface area of PAF-8 is 785 m₂ g^-1. It possessed both micropores and mesopores at 1.69 and 3.80 nm. PAF-8 also exhibited the great potential application in CO₂ and CH₄ capture and especially possessed high Q_st of CO₂ (34.5 KJ mol^-1), which was higher than many other POF materials. N₂-CO₂ and CH₄-CO₂ mixture can also be separated through GC separation. Additionally, PAF-8 material had good water and methanol vapor sorption abilities. All of it indicated that PAF-8 material had good application foreground for CO₂ capture and separation as well as for liquid vapor adsorption.

Acknowledgment

We are grateful for the financial support of National Basic Research Program of China (973 Program, Nos. 2012CB821700 and 2014CB931804), Major International (Regional) Joint Research Project of NSFC (No. 21120102034) and NSFC Project (Nos. 21531003 and 21503038).