A superhydrophobic and porous polymer adsorbent with large surface area

Abstract

A superhydrophobic hypercrosslinked microporous polymer adsorbent, SHMP-1, with a water contact angle of 167° and an oil contact angle < 3°, was synthesized. SHMP-1 has a specific surface area of up to 2100 m² g⁻¹, higher than the largest one (i.e. 1618 m² g⁻¹) reported for superhydrophobic materials. It is an amorphous polymer with a highly cross-linked structure of monomers linked by methylene, and consists of packed amorphous porous cells with pore channel widths of 5–19 Å. SHMP-1 can be stable up to 380 °C, and in organic solvents (e.g. ethanol, acetone, carbon tetrachloride, tetrahydrofuran and n-hexane), HCl (5 mol L⁻¹) and NaOH (5 mol L⁻¹) solution. This polymer could be a promising adsorbent for absorption of organic solvents with capacities ranging from 4–25 times its own weight and capturing CO₂ with an adsorption capacity of up to 6.02 mmol g⁻¹ (273 K, 1 bar). Moreover, the CO₂ adsorption capacity of SHMP-1 will not be decreased significantly (loss < 2.03%) by moisture even at a relative humidity of up to 90%, because of the surface superhydrophobicity.

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Superhydrophobic materials, with a water contact angle (WCA) larger than 150°,¹ have received substantial attention due to their potential applications in areas such as self-cleaning,² anti-fouling,³ microdroplet transportation,⁴ and separating organic compounds from air and water.^5,6 Superhydrophobic materials with large surface area are expected to be efficient adsorbents, especially for separation and removal of organic contaminants or oil from water^7–9 and for capture of CO₂ (ref. 10) or volatile organic compounds¹¹ by suppressing the competition of water molecules on their surface. For example, Zhang et al.¹² reported that a superhydrophobic nanoporous polymer with a surface area of 702 m² g⁻¹ displays larger adsorption capacity (8–15 g g⁻¹) for organic solvents (i.e., nitrobenzene, benzene, kerosene and n-heptane) than traditional adsorbents like active carbon or XAD-4. Moreover, Li et al.¹³ reported that microporous polymer HCMP-1 is an excellent adsorbent for organic solvents (e.g., 1,2-dichlorobenzene, nitrobenzene and chloroform) and oils (e.g., pump-oil and vegetable-oil), due to its surface superhydrophobicity (WCA = 167°) and large surface area (955 m² g⁻¹). Therefore, developing superhydrophobic adsorbents with larger surface area is of special significance but remains a great challenge.

Among the reported superhydrophobic materials, including metal oxides or non-metal oxides,¹⁴ carbon materials,^15,16 polymers,^12,13,17 metal–organic frameworks¹⁸ and other composite materials,^19,20 the largest specific surface area, 1618 m² g⁻¹, was observed for the octadecylphosphonic acid modified metal–organic framework PCN-222.²¹ In this study, a superhydrophobic hypercrosslinked microporous polymer, SHMP-1, a dark brown powder, was synthesized from 1,3,5-trinaphthylbenzene (TNP) using CH₂Cl₂ as the linker and anhydrous AlCl₃ as the catalyst based on the Friedel–Crafts alkylation reaction (Fig. 1). The detailed synthesis of SHMP-1 is described in the ESI. The yield is 123%, calculated from the mass of added TNP using the eqn (S1) listed in the ESI.† This superhydrophobic polymer is an amorphous material (identified using the XRD pattern in Fig. S1†) with a WCA of 167° (Fig. 2a), diesel contact angle < 3° (Fig. 2b), specific surface area of up to 2100 m² g⁻¹ (Fig. 2c), and a micropore volume of 0.830 cm³ g⁻¹ (Fig. 2d). The specific surface area of SHMP-1 is higher than the largest one reported for superhydrophobic materials (Table S1†), i.e., 1618 m² g⁻¹.²¹

Fig. 1 Synthesis route of SHMP-1.

Fig. 2 Water contact angle (a), diesel contact angle (b), N₂ adsorption–desorption isotherm at 77 K (c), and pore size distribution calculated from the N₂ adsorption isotherm by the NLDFT method (d) of SHMP-1.

The structure of SHMP-1, simulated by Material Studio v.8.0 software, is a highly cross-linked structure (Fig. 3a) with the molecular formula as (C₁₄₇H₈₄)_n and consists of packed porous cells with a major pore channel ranging from 5 Å to 19 Å (Fig. 3b), where the monomers are cross-linked by methylene. The simulated pore channel widths of SHMP-1 are largely consistent with the pore sizes calculated from the N₂ adsorption isotherm (at 77 K) by the NLDFT method (i.e., about 0.5, 1.0 and 2.0 nm in Fig. 2d) and that calculated from the CO₂ adsorption isotherm (at 273 K) by the GCMS method for micropores (i.e., about 0.5 and 0.8–0.9 nm Fig. S2†). The appearance of a peak at about 1690 cm⁻¹ and the disappearance of the peak at 1610 cm⁻¹ in the FTIR spectra of SHMP-1 as compared with that of TNP (Fig. 3c), indicating the changes in the vibrations of the conjugated aromatic ring of SHMP-1 compared to TNP,²² is due to the crosslinking of the naphthyl group of TNP with carbon of CH₂Cl₂. This can be also indicated by the C–H stretching vibrations of –CH₂ (ref. 23) at 2920 cm⁻¹ and 2960 cm⁻¹ which appeared in the FTIR spectra of SHMP-1 (Fig. 3c). Moreover, the resonance peak around 35 ppm in the solid-state ¹³C CP/MAS NMR spectrum of SHMP-1 (Fig. 3d), indicates that the carbon of the liquid methylene cross-linkers has been crosslinked in the solid SHMP-1.²⁴ In the NMR spectrum of SHMP-1 (Fig. 3d), the peaks with the * symbol and the resonance peaks around 138 or 130 ppm are attributed to the spinning side-bands,²⁵ and the substituted or non-substituted aromatic carbon.²²

Fig. 3 Simulated 3-dimensional array of the amorphous cell with periodic boundary conditions (a), ball-and-stick view and pore channel free dimension (Å) of the amorphous cell (b), FTIR spectra (c) and the solid ¹³C CP/MAS NMR spectrum (d) of SHMP-1.

SHMP-1 is agglomerated spheres having nano structure roughness on the sphere surface (Fig. 4a). The cooperation of roughness on the surface and the hydrophobic framework (i.e., hydrophobic conjugated aromatic rings and alkyls) is the key point responsible for the superhydrophobicity of SHMP-1.^8,13,26 Surface superhydrophobicity of SHMP-1 is also identified by the large WCA (i.e., 167° in Fig. 2a) and the low adsorption of water vapor (i.e., less than 7.8% of the SHMP-1 weight even at a relative humidity (RH) of up to 90% in Fig. 4b). The surface area normalized water adsorption capacity of SHMP-1 at 90% RH is 0.037 mg m⁻², much less than that of the reported superhydrophobic polymer PDVB-[C₁vim][SO₃CF₃] (0.138 mg m⁻², WCA = 150°)²⁷ and IISERP-COF₂ (0.115 mg m⁻², WCA = 160°).⁸ Therefore, SHMP-1, with larger surface area than the reported superhydrophobic materials, can take up polar and non-polar organic solvents, e.g., 1,2-dichlorobenzene (1,2-DCB), nitrobenzene, benzene, dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and methanol (Fig. 5a), with larger capacity than that of the reported superhydrophobic materials (Table S2†). The largest uptake capacity, 25 g g⁻¹ in Fig. 5a (i.e., 170 mmol g⁻¹ and 19.48 mL g⁻¹ in Fig. S3a and b,† respectively), observed for 1,2-DCB, is up to 25 times the SHMP-1 weight. The ultrahigh uptake of organic solvents will result in significant swelling of SHMP-1 (Fig. S4†). In addition, a linear relationship between the uptake of organic solvents on SHMP-1 and their density was observed, which accounts for the largest uptake of 1,2-DCB among these organic solvents because of the highest density of 1,2-DCB (Fig. S4†).

Fig. 4 FE-SEM image (a) and adsorption–desorption isotherm of water vapor at 298 K (b) of SHMP-1.

Fig. 5 Absorption capacity for organic solvents (a), adsorption–desorption isotherms of CO₂ at 273 K and 298 K (b), breakthrough curves of 1 bar CO₂ at 298 K with 0%, 20% and 90% RH (c), and cycles of CO₂ (1 bar) adsorption at 273 K and 298 K (d) on SHMP-1.

With large surface area and surface superhydrophobicity, SHMP-1 has a large adsorption capacity for CO₂, 6.02 mmol g⁻¹ at 273 K and 1 bar (Fig. 5b) as an example, which is comparable to the highest one (i.e., 8.51 mmol g⁻¹ at 273 K and 1 bar) reported for porous polymer TrzPOP-3,²⁸ and higher than that of other materials like porous polymers,²⁹ MOFs³⁰ and porous carbons.³¹ Moreover, the CO₂ adsorption capacity of SHMP-1 cannot be decreased significantly by moisture even at a RH of up to 90% (Fig. 5c).

For example, the calculated loss of CO₂ adsorption capacity is less than 2.03% at 90% RH (Fig. 5c). Therefore, SHMP-1 is an excellent adsorbent expected for CO₂ as well as volatile organic compounds in wet environments because the effects of competitive sorption by water molecules on the SHMP-1 surface can be largely suppressed.^8,10,11 In addition, there is almost no loss in the CO₂ adsorption capacity after 5 regenerations (Fig. 5d), indicating that SHMP-1 can be cycled and re-used for adsorption at least 5 times.

Thermal analysis (Fig. S5†) shows that SHMP-1 possesses good thermal stability in an O₂ atmosphere up to 380 °C, indicating the highly cross-linked structure nature of SHMP-1.³² Moreover, SHMP-1 can be stable in 5 mol L⁻¹ HCl and 5 mol L⁻¹ NaOH solution, showing no significant decrease in specific surface area (1970 m² g⁻¹ and 1900 m² g⁻¹ in Fig. S6†) and WCA (160° and 157° in Fig. S7†) after immersion in HCl and NaOH for 2 days. Moreover, SHMP-1 does not dissolve in common organic solvents (e.g., ethanol, acetone, carbon tetrachloride, tetrahydrofuran and n-hexane), showing no changes in its FTIR spectra (Fig. S8†) and almost no mass losses (Table S3†) after immersion in these solvents for 2 days. Therefore, SHMP-1 can be used to remove oils, CO₂ and organic compounds from harsh wastewater³³ or exhaust gas at high temperatures.

In conclusion, the novel polymer SHMP-1, synthesized in this study, is a superhydrophobic porous material with a WCA of 167° and a large specific surface area of up to 2100 m² g⁻¹. The specific surface area of SHMP-1 is the largest one for superhydrophobic materials ever reported. Moreover, this polymer has excellent chemical stability in organic solvents, and in 5 mol L⁻¹ HCl and NaOH solution, and can be stable up to 380 °C. SHMP-1 has superior absorption capacities for polar and non-polar organic solvents, up to 25 times its own weight, but limited adsorption for water (i.e., less than 7.8% of its own weight at a RH of up to 90%). The CO₂ adsorption capacity of SHMP-1 is up to 6.02 mmol g⁻¹ at 273 K and 1 bar, which is near to the largest one reported for CO₂ (i.e., TrzPOP-3 of 8.51 mmol g⁻¹ at 273 K and 1 bar) and cannot be decreased significantly by moisture (e.g., loss < 2.03% at a RH of up to 90%) due to the surface superhydrophobicity. Therefore, SHMP-1 could be a promising adsorbent for capturing of CO₂ in flue gas, oil/water separation and sorptive removal of organic contaminants from air and water under harsh conditions and at high temperatures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2017YFC0211803), the National Natural Science Foundation of China (21621005), and the Key Research and Development Program of Zhejiang Province (2019C03105).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures and tables. See DOI: 10.1039/d0ta07944h

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