Adjustable Functionalization of Hyper-Cross-Linked Polymers of Intrinsic Microporosity for Enhanced CO2 Adsorption and Selectivity over N2 and CH4

In this paper, we report the design, synthesis, and characterization of a series of hyper-cross-linked polymers of intrinsic microporosity (PIMs), with high CO2 uptake and good CO2/N2 and CO2/CH4 selectivity, which makes them competitive for carbon capture and biogas upgrading. The starting hydrocarbon polymers’ backbones were functionalized with groups such as −NO2, −NH2, and −HSO3, with the aim of tuning their adsorption selectivity toward CO2 over nitrogen and methane. This led to a significant improvement in the performance in the potential separation of these gases. All polymers were characterized via Fourier transform infrared (FTIR) spectroscopy and 13C solid-state NMR to confirm their molecular structures and isothermal gas adsorption to assess their porosity, pore size distribution, and selectivity. The insertion of the functional groups resulted in an overall decrease in the porosity of the starting polymers, which was compensated with an improvement in the final CO2 uptake and selectivity over the chosen gases. The best uptakes were achieved with the sulfonated polymers, which reached up to 298 mg g–1 (6.77 mmol g–1), whereas the best CO2/N2 selectivities were recorded by the aminated polymers, which reached 26.5. Regarding CH4, the most interesting selectivities over CO2 were also obtained with the aminated PIMs, with values up to 8.6. The reason for the improvements was ascribed to a synergetic contribution of porosity, choice of the functional group, and optimal isosteric heat of adsorption of the materials.


General methods and equipment
Commercially available reagents and gases were used without further purification. All reactions using air/moisture sensitive reagents were performed in oven-dried or flame-dried apparatus, under a nitrogen atmosphere. Low-temperature N2 (77 K and 298 K), CO2 (273 K and 298 K) and CH4 (298 K) adsorption/desorption measurements of PIM powders were made using a Quantachrome Nova-e. Samples were degassed for 800 min at 100 °C under high vacuum prior to analysis. The gases were supplied by BOC and used without any further purification (N2 purity > 99.999, CO2 purity > 99.995%). The specimen was measured twice after outgas in two different cells to minimize the error, providing the same results. The data were analysed with the software provided with the instrument. The BET was calculated at a relative pressure P/P0 < 0.1. NLDFT and H-K analysis were performed to calculate the pore size distribution and volume, considering a carbon equilibrium transition kernel at 273 K based on a slit-pore model; the kernel is based on a common, one centre, Lennard-Jones model. To assess the potential chemisorption, heats of adsorption were calculated from the CO2 curves measured at 237K and 298K. The data were analysed with the QuadraWin software and fitted with the Langmuir-Freundlich equation and calculated via the Clausius-Clapeyron equation. TGAs were performed using the device Thermal Analysis SDT Q600 at a heating rate of 10 °C/min from 30 to 1000 °C. Solid-state 13 C NMR spectra were recorded using a Bruker Avance III spectrometer equipped with a wide-bore 9.4 T magnet (Larmor frequencies of 100.9 MHz for 13 C). Samples were packed into standard zirconia rotors with 4 mm outer diameter and rotated at a magic angle spinning (MAS) rate of 12.5 kHz. Spectra were recorded with cross polarisation (CP) from 1H using a contact pulse (ramped for 1H) of 1.5 ms. High-power (ν1≈ 100 kHz) TPPM-15 decoupling of 1H was applied during acquisition to improve resolution. Signal averaging was carried out for 6144 transients with a recycle interval 3 of 2 s. Chemical shifts are reported in ppm relative to (CH3)4Si (TMS) using the CH3 signal of L-alanine (δ = 20.5 ppm) as a secondary solid reference.

IAST Selectivity calculation.
The ideal adsorption solution theory (IAST) of Myers and Prausnitz 1 is typically used to the selectivity of binary mixtures of gases from the single isotherms. The isotherms were fitted with Dual-Site Langmuir-Freundlich using the software IAST++ 2 and the selectivity (S) was calculated according to the formula: Were • PCO2 is the partial pressure of CO2 • PN2 is the partial pressure of N2 • QN2 is the N2 uptake • QCO2 is theCO2 uptake 3. Experimental part PIM-TPB, PIM-Tript, PIM-TPB-HSO3 and PIM-Tript-HSO3 were synthesised according to our previous paper. 3

PIM-SBF 4
SBF (1.02 g, 3.22 mmol) and AlCl3 (4.47 g, 33.5 mmol) were added to DCM (60 mL) and stirred at reflux for 24 hours under a nitrogen atmosphere. The solution was filtered, and the obtained powder washed with plenty of water and ethanol. The brown powder was washed sequentially via reflux with ethanol, chloroform, THF, acetone, and methanol, and finally dried in a vacuum oven at 100 0 C for 20 hours. The SBF-network polymer (1.40 g, 3.80 mmol, 118% yield based on ideal structure. To be considered quantitative but the complete removal of the trapped gases/moisture was impossible) was analysed by IR spectroscopy, thermogravimetric analysis (TGA) and BET. SABET= 1604 m 2 g -1 , CO2 adsorption at 273 K/1 bar= 3.28 mmol g -1 , TGA: Thermal

PIM-HPB 4
HPB (0.99 g, 1.85 mmol) and AlCl3 (3.28 g, 24.59 mmol) were added to DCM (60 mL) and stirred at reflux for 24 hours under a nitrogen atmosphere. The solution was filtered, and the obtained powder washed with plenty of water and ethanol. The powder was washed sequentially via reflux with ethanol, chloroform, THF, acetone, and methanol, and finally dried in a vacuum oven at 100 0 C for 20 hours. The HPB-network polymer (1.30 g, 2.12 mmol, 115% yield based on ideal structure. To be considered quantitative but the complete removal of the trapped gases/moisture was impossible) was analysed by IR spectroscopy, TGA, and BET.

General procedure for sulfonation of polymers-SBF-SO3H and HPB-SO3H
PIM-SBF (0.51 g, 1.38 mmol) or PIM-HPB (0.49 g, 0.79 mmol) was added to concentrated H2SO4 (20 mL) and stirred for 30 minutes, then warmed to 60 o C and stirred for more 7 hours under a nitrogen atmosphere. The solution was cooled to room temperature and poured over ice, filtered and the obtained solid was washed with plenty of water until a neutral pH of the residue was obtained. The resulting dark powder was refluxed in ethanol and methanol, filtered off and dried in a vacuum oven at 100 o C for 20 hours. PIM-HPB-HSO3 (0.57 g, 0.52 mmol, 69%): SABET= 1390 m 2 g -1 , CO2 adsorption at 273 K/1 bar= 2.92 mmol g -1 , TGA: Thermal degradation commences at 165 °C . FTIR-ATR (cm -1 ): 1039, 1175, 2980, 3420. 13

General procedure for nitration of polymers-PIM-SBF-NO2 and PIM-HPB-NO2
Under a nitrogen atmosphere, H2SO4 (50 mL) was cooled in an ice bath for 20 minutes before PIM-SBF (0.49 g, 1.33 mmol) or PIM-HPB (0.50 g, 0.82 mmol) or PIM-TPB (0.50 g, 1.44 mmol) or PIM-TRIP (0.50 g, 1.70 mmol) were added. Concentrated HNO3 (0.5 mL, 70%) was added dropwise and the solution was then stirred for 2 hours in an ice bath. The resulting solution was poured over ice, stirred for 30 minutes, filtered and the obtained solid was washed with plenty of water until a neutral pH of the residue was obtained. The