Highly effective ammonia removal in a series of Brønsted acidic porous polymers: investigation of chemical and structural variations

Efficient removal of ammonia from air is demonstrated in a series of Brønsted acidic porous polymers under dry and humid conditions. The impact of acidic group strength and their spatial distribution on the ammonia uptake is investigated systematically.


General Methods
Starting materials and reagents were purchased from Sigma-Aldrich and used as received without further purification. Tetrakis(4-bromophenyl)methane, 1 S1, 2 S2, 3 PAF-1, 4 PAF-1-CH 2 Cl, 5 P1-NH 3 Cl, 6 P1-SO 3 H, 7 P2-CO 2 C 9 H 19 , 6 and P2-CO 2 H 6 were prepared following the procedures reported in the literature. All reactions were performed under a nitrogen or argon atmosphere and in dry solvents, unless otherwise stated. Analytical thin-layer chromatography (TLC) was performed on aluminum sheets, precoated with silica gel 60-F 254 (Merck 5554). Flash column chromatography was carried out using silica gel 60 (Silicycle) as the stationary phase. Deuterated solvents were purchased from Cambridge Isotope Laboratories (Andover, MA) and used without further purification. 1 H and 13 C NMR spectra were recorded on a Bruker AV-400 and Bruker AMX 400 spectrometers (400.132 MHz for 1 H and 100.623 MHz for 13 C) at ambient temperature. 1 H NMR data are reported as follows: chemical shift (multiplicity (br s = broad singlet, s = singlet, d = doublet, dd = doublet of doublets), coupling constants, and integration). Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents. Electrospray Ionization (ESI) mass spectra were obtained from QB3/Chemistry Mass Spectrometry Facility at the University of California, Berkeley.
Thermal gravimetric analysis (TGA) data was collected at ramp rates of 5 ºC/min under flowing nitrogen using a TA Instruments TGA Q5000. Infrared spectra were obtained on a Perkin-Elmer Spectrum 100 Optica FTIR spectrometer furnished with an attenuated total reflectance accessory. Carbon, hydrogen, nitrogen, and sulfur elemental analyses were obtained from the Microanalytical Facility at the University of California, Berkeley. Elemental analyses for chlorine, phosphorus, and oxygen were performed at Galbraith Laboratories.
Scanning electron microscopy (SEM) samples of polymers were prepared by dispersing fine powders into methanol and drop casting onto a silicon chip. To dissipate charge, the samples were sputter coated with approximately 3 nm of Au (Denton Vacuum). Polymers were imaged at 5 keV and 12 µA by field emission SEM (JEOL FSM6430).
Solid-state 1 H-13 C cross-polarization (CP) spectra were collected on a 7.05 Tesla magnet at 13 C frequency of 75.5 MHz under 10 kHz magic-angle spinning (MAS) condition. A Chemagnetics 4 mm H/X probe and a Tecmag Discovery spectrometer were used. The Hartmann-Hahn condition for CP experiments was obtained on solid adamantane, which is also a secondary reference of 13 C chemical shift (the methylene signal of adamantane was set to 38.48 ppm relative to TMS). Two pulse phase modulation (TPPM) proton decoupling scheme was used. The TPPM angle was 15 degrees and the decoupling field strength was ~60 kHz. A contact time of 10 ms and a pulse delay of 4 s were used in CP experiments. Solid-state 1 H NMR spectra were also collected using the same instrument under 13.5 kHz MAS condition. The 1 H chemical shift was calibrated on adamantine (1.74 ppm relative to TMS). A 90-degree pulse of 3.3 µs and a pulse delay of 4 s were used in 1 H MAS experiments. Experimental 1 H MAS NMR spectra were deconvoluted to show individual peaks.

S3
Gas adsorption isotherms were measured using a Micromeritics ASAP 2020/2420 or 3Flex instruments. Samples were transferred to a pre-weighed glass analysis tube, which was capped with a Transeal, and were evacuated on the degas ports until the outgas rate was less than 3 µbar/min. Ultrahigh-purity grade (99.999%) nitrogen and anhydrous ammonia (99.999%) was used for gas adsorption measurements. Ammonia and water isotherms were obtained at 25 ºC and 20 ºC, respectively, using a water circulator. Nitrogen isotherms were obtained using a 77 K liquid-N 2 bath and used to determine the surface areas and pore volumes using the Micromeritics software, assuming a value of 16.2 Å 2 for the molecular cross-sectional area of N 2 . Pore-size distributions were calculated using the density functional theory method with a QSDFT adsorption branch model of N 2 at 77 K adsorbed in carbon with slit/cylindrical/spherical pores, as implemented in the Quantachrome VersaWin software. The activation temperatures for porous polymers, except those mentioned in the following Synthesis section, were: 150 ºC for PAF-1, 120 ºC for PAF-1-CH 2 Cl, 110 ºC for P1-NH 3 Cl, 120 ºC for P1-SO 3 H, and 110 ºC for P2-CO 2 H.

Synthesis
Scheme S1 Synthesis of compound 1a Synthesis of 1a: A 100-mL 3-neck round bottomed flask was charged with compound S1 (2 g, 5.7 mmol), bis(pinacolato)diboron (4.4 g, 17.3 mmol), KOAc (3.4 g, 34.6 mmol), and Pd(dppf)Cl 2 ·CH 2 Cl 2 (450 mg, 0.55 mmol) and then equipped with a reflux condenser and rubber stoppers. The solid mixture was subjected to three cycles of brief vacuum/argon (degas/backfill) with no stirring. Anhydrous 1,4-dioxane (50 mL) was quickly transferred into the flask using a syringe under argon atmosphere and the suspension was stirred for 12 h at 90 ºC. After confirming the completion of reaction by TLC (EtOAc/Hexanes 5:1), the reaction mixture was cooled down to room temperature and concentrated in vacuo. The residual solid was redissolved in CHCl 3 (50 mL), washed with H 2 O (50 mL), dried over MgSO 4 , and suspended over activated carbon for 1 h, after which was filtered through a pad of Celite. The solvent was removed under reduced pressure and remaining solid was briefly washed with EtOAc and MeOH to deliver compound 1a as a pale yellow powder (1.5 g, 59%). 1

S5
Scheme S2 Synthesis of compound 1b Synthesis of S3: The procedure was adopted from a literature report. 8 Compound S2 (5 g, 15.0 mmol) and neopentyl alcohol (1.58 g, 17.9 mmol) were dissolved in CH 2 Cl 2 (25 mL) and cooled to 0 ºC in an ice-water bath. Then, pyridine (2.5 mL, 30.0 mmol) was added dropwise over a period of 30 min. The reaction mixture was allowed to stir at room temperature for 12 h and diluted with Et 2 O. The organic layer was washed with 0.1% HCl followed by brine and then dried over MgSO 4 . After removing the solvent in vacuo, recrystallization of the crude product in EtOH provided S3 as a colorless crystalline solid (3.7 g, 64%). 1

Synthesis of 1b:
A 100-mL 3-neck round bottomed flask were charged with compound S3 (1.5 g, 3.9 mmol), bis(pinacolato)diboron (2.96 g, 11.7 mmol), KOAc (2.25 g, 22.9 mmol), and Pd(dppf)Cl 2 ·CH 2 Cl 2 (350 mg, 0.43 mmol) and then equipped with a reflux condenser and rubber stoppers. The solid mixture was subjected to three cycles of brief vacuum/argon (degas/backfill) with no stirring. Anhydrous 1,4-dioxane (40 mL) was quickly transferred into the flask using a syringe under argon atmosphere and the suspension was stirred for 48 h at 90 ºC. After confirming the completion of reaction by TLC, the reaction mixture was cooled down to room temperature and concentrated in vacuo. The residual solid was redissolved in CHCl 3 (50 mL), washed with H 2 O (50 mL), dried over MgSO 4 , and suspended over activated carbon for 1 h, after which was filtered through a pad of Celite. The solvent was removed under reduced pressure, followed by the addition of MeOH and the flask was placed in a −30 ºC freezer to induce precipitation. The product 1b was collected by filtration as a yellow powder (0.8 g, 43%). 1

Synthesis of P1-PO 3 H 2 :
An oven-dried 100-mL round bottomed flask charged with P1-PO 3 Et 2 (300 mg) was sealed with a rubber septum, purged with argon, and then filled with 15 mL of anhydrous CH 2 Cl 2 . Bromotrimethylsilane (Me 3 SiBr, 3 mL, 20 equiv. of phosphonate esters based on elemental analysis data) was added dropwise over 10 min. The resulting suspension was stirred at 40 ºC for 24 h, after which the mixture was filtered to remove solvent and unreacted Me 3 SiBr, and then washed with CH 2 Cl 2 . The isolated solid was transferred into a 100-mL round bottomed flask again containing 50 mL of MeOH and was stirred for 6 h at room temperature. After removing solvent by filtration, the remaining solid, 10 mL of H 2 O, and 10 mL of concentrated HCl was mixed in a flask and refluxed for 12 h in order to ensure the complete hydrolysis of phosphonate esters. Synthesis of P2-NHBoc: Tetrakis(4-bromophenyl)methane (500 mg, 0.79 mmol), 1a (770 mg, 1.73 mmol), and chloro(2-dicyclohexylphosphino-2',6'-dimethoxy-1,1'-biphenyl)[2-(2'-amino-1,1'-biphenyl)]palladium(II) (SPhos Pd G2) (45 mg, 0.062 mmol) were charged into a 100-mL 3-neck round bottomed flask, which was equipped with a reflux condenser and rubber stoppers. The solid mixture was subjected to three cycles of brief vacuum/argon (degas/backfill) with no stirring. Anhydrous THF (25 mL) and degassed aqueous K 2 CO 3 (2 M, 2.5 mL) were transferred into the flask using a syringe under argon atmosphere and the solution was then stirred for 3 days at 70 ºC. The reaction became an extremely viscous gel during polymerization. The reaction mixture was cooled down to room temperature and insoluble product was isolated by filtration. Gel-like product turned into powder upon drying and was washed with THF, hot H 2 O, hot EtOH, and hot CHCl 3 . The product was further purified by Soxhlet extraction with THF for 24 h. The isolated solid was activated at 100 ºC under vacuum to yield P2-NHBoc as a beige powder (500 mg, 91%).
Synthesis of P2-NH 3 Cl: P2-NHBoc (400 mg) was placed into a 100-mL round-bottomed flask containing 30 mL of HCl (4 N in 1,4-dioxane) solution. The suspension was stirred at room temperature for 18 h. The solid was isolated by filtration and washed extensively with 1,4,dioxane and EtOH. The resulting powder was activated under vacuum at 100 ºC for 24 h, resulting in P2-NH 3 Cl as a yellow/pale brown solid (300 mg).

Breakthrough Experiments
Breakthrough testing was conducted on porous polymers using a microbreakthrough setup (Fig.  S36) that has been described previously. [16][17][18] Briefly, neat ammonia was injected into a steel canister, which was then pressurized to approximately 15 psig. A stream from this ballast was delivered via mass flow controller and mixed with humidity-controlled stream at a rate necessary to achieve 2000 mg/m 3 . The mixed stream was delivered at a total flow rate of 20 mL/min to a glass-fritted tube submerged in a temperature-controlled bath at 20 °C. Within the 4 mm ID tube, polymers were packed to a bed depth of approximately 4 mm, resulting in a residence time of approximately 0.15 s. Breakthrough was measured on the effluent side of the bed using HP5890 Series II gas chromatographs equipped with a photoionization detector. The effluent curve was integrated to calculate the loading of ammonia. dination bonds of the structure may provide additional reactive centers for ammonia removal. In this work we present a detailed study of the ammonia removal properties of Cu 3 (BTC) 2 through breakthrough analysis, nitrogen isotherm data, PXRD, and MAS NMR. relativ flow ra was co throug detecto A C dried a conditi challen then pu once-e nitroge the sam of the

In Situ Infrared Spectroscopy
FTIR spectra were collected at 2 cm −1 resolution on a Bruker Vertex 70 spectrophotometer, equipped with a MCT cryodetector, at "beam temperature"-i.e., the temperature reached by samples under the IR beam. The samples were examined in the form of self-supporting pellets mechanically protected with a pure gold frame (P1-SO 3 H and P1-PO 3 H 2 ) or in the form of thin layer depositions on Si wafers, starting from aqueous suspensions (PAF-1 and P2-CO 2 H). Before NH 3 adsorption, all samples were activated in controlled atmosphere, at the corresponding activation temperature, using a home-made quartz IR cell equipped with KBr windows and characterized by a small optical path (2 mm). The cell was connected to a conventional highvacuum glass line, equipped with mechanical and turbo molecular pumps (capable of a residual pressure p < 10 −4 mbar), which allows performing in situ adsorption/desorption experiments of molecular probes.