SO2 Capture Using Porous Organic Cages

Abstract We report the first experimental investigation of porous organic cages (POCs) for the demanding challenge of SO2 capture. Three structurally related N‐containing cage molecular materials were studied. An imine‐functionalized POC (CC3) showed modest and reversible SO2 capture, while a secondary‐amine POC (RCC3) exhibited high but irreversible SO2 capture. A tertiary amine POC (6FT‐RCC3) demonstrated very high SO2 capture (13.78 mmol g−1; 16.4 SO2 molecules per cage) combined with excellent reversibility for at least 50 adsorption–desorption cycles. The adsorption behavior was investigated by FTIR spectroscopy, 13C CP‐MAS NMR experiments, and computational calculations.


Supporting Information
Synthesis of CC3: CC3-R was prepared as previously reported in its homochiral form. [1] Dichloromethane (100 ml) was layered slowly onto solid triformylbenzene (TFB, 5 g, 30.86 mmol) without stirring at room temperature. Trifluoroacetic acid (1 mL) was added directly to this solution as a catalyst for the imine bond formation. Finally, a solution of (R,R)-1,2-diaminocyclohexane (5 g, 44.64 mmol) in dichloromethane (100 mL) was added to this, again without mixing. The reaction was covered and left to stand. Over 5 days, all of the solid triformylbenzene was used up and octahedral crystals of CC3 grew on the sides of the glass reaction vessel. The crystalline product was removed by filtration and washed with 95 % ethanol / 5 % dichloromethane. A large-scale synthesis of CC3 has also been recently developed, a typical procedure is described as follows. A batch reactor was charged with 4,000 mL of isopropyl alcohol (IPA) and pre-heated to 50 °C. 1,3,5-Triformylbenzene (400g, 2.46 mol) and solid (R,R)-1,2-Diaminocyclohexane (431g, 3.77 mol) were added to the pre-heated solvent under continuous stirring. After stirring for five minutes, trifluoroacetic acid (8 mL, 0.047 mol) was added to the reaction mixture. Once the addition of the acid was complete the reaction is slowly ramped up to 78 °C and held for 17 hours with continuous stirring. After 17 hours the reactor is cooled to room temperature and the resulting off-white suspension is collected by vacuum filtration. This solid is then washed with further IPA and dried in a vacuum oven at 60 °C overnight to yield CC3 as a fine white powder. Yield 623 g, 94.7%. Note: both synthesis methods give CC3 in the same polymorph, alpha-phase; hence both products exhibit the same porosity.
Synthesis of RCC3: RCC3 was prepared and purified as previously reported. [2] The imine cage CC3-R (926 mg, 0.83 mmol) was dissolved in a CHCl3 / methanol mixture (1:1 v/v, 50 mL) by stirring. When this solution became clear, sodium borohydride (1.00 g, 26.5 mol) was added and the reaction was stirred for a further 12 hours at room temperature. Water (2 mL) was then added, and the reaction stirred for a further 12 hours. The solvent was then removed under vacuum. The resulting white solid was extracted with chloroform (2 × 50 mL) and then the combined organic phase was washed by water (2 × 100 mL). The CHCl3 phase was dried using anhydrous MgSO4 before being removed under vacuum. RCC3 (crude yield = 900 mg, 95.1 %) was obtained as a white solid. RCC3 was purified by using the reversible reaction with acetone. In a 250 mL flask, 1000 mg crude RCC3 was dissolved in acetone (100 mL). The solution was covered and left to stand. Crystals started appearing on the wall of the flask after 30 mins. The crystals (AT-RCC3) were collected after one day by filtration and were then dissolved in a CHCl3 /CH3OH mixture (1:1 v/v) by stirring. Several drops of distilled water were added to the solution and the mixture was stirred for another 12 h. After removal of the solvents, pure RCC3 (680 mg, 70.4 %) was recovered.

Synthesis of FT-RCC3.
FT-RCC3 was prepared as previously reported. [2] Paraformaldehyde (52 mg, 20 eq.) dissolved in CH3OH (10 mL) was stirred at 70 °C. To this clear solution was added RCC3 (100 mg) dissolved in CH3OH (10 mL). A white precipitate appeared upon addition of RCC3. The reaction was stirred for a further 2 h at 70 °C. The reaction was cooled to room temperature and the precipitate was collected by filtration. FT-RCC3 (52 mg, 70 %) was obtained after being washed with CH3OH (3 × 10 mL) and dried under vacuum. Heat of Adsorption. Additional 308 K SO2 adsorption isotherms were measured to estimate the heat of adsorption on all the samples (Fig. S2), a virial-type equation was used to fit the adsorption isotherms at low surface coverage, to estimate the heat of adsorption at zero coverage (Fig. S3).
N2 adsorption isotherms were recorded at 77 K up to P/P0 = 1, on a Quantachrome Autosorb MP-1 equipment under high vacuum in a clean system with a diaphragm pumping system. Powder X-ray diffraction (PXRD) patterns were measured on a Bruker D8 Advance X-ray diffractometer equipped with a LynxEye detector using CuKα radiation (λ = 1.5406 Å; monochromator: germanium) in a range 2-theta of 4−60° with a step of 0.02°. The voltage and current were 35 kV and 35 mA, respectively.
Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet 6700 spectrometer. The spectrum was generated and collected 16 times and corrected for the background noise in the wavenumber ranging from 400 to 3400 cm -1 .
The 13 C CP/MAS NMR spectra were acquired at a frequency of 75.422 MHz at a spinning rate of 6 kHz. Typical 13 C CP/MAS NMR conditions for 1 H-13 C polarization experiment used a π/2 pulse of 4 μs, contact time of 1 ms, delay time of 5 s, and at least 20 000 scans. Chemical shifts were referenced to a solid shift at 38.2 ppm relative to TMS.
Solution NMR spectra were recorder using a Bruker advance 400 NMR Ultrashield TM spectrometer, at 400 MHz for 1 H and 100 MHz for 13 C-DEPTQ.

Additional experiments
In order to evaluate the stability of 6FT-RCC3 cage in the presence of SO2 and humidity, we carried out a qualitative experiment with a homemade system previously reported (see Fig. S16). [6], [7] The system contains two principal parts: SO2 gas generator (A) dropping funnel with H2O. [1] connected to a Schlenk flask with Na2SO3(s) under stirring [2]; and the saturation chamber (B), constructed from a round flask with distilled water [3], connected to a sintered glass filter adapter (without humidity trap) [4] and to a vacuum line [5]. The activated cage sample is placed on the glass filter adapter and exposed for 3 days to SO2 and H2O vapor. After, N2 adsorption was evaluated (see Fig. S15). In comparison to the reported for 6F-RCC3 cage, [2] the BET surface area decreased from 396 to 375 m 2 g -1 after the exposure to SO2 and H2O. No apparent changes were observed in porous organic cage after exposure to SO2 and water stays intact as confirmed by 1 H NMR (Fig. S13).