Porous acid–base hybrid polymers for enhanced NH3 uptake with assistance from cooperative hydrogen bonds

Carboxylic acid-modified materials are a common means of achieving efficient NH3 adsorption. In this study, we report that improved NH3 adsorption capacity and easier desorption can be achieved through the introduction of substances containing Lewis basic groups into carboxylic acid-modified materials. Easily synthesized mesoporous acid–base hybrid polymers were constructed with polymers rich in carboxylic acid and Lewis base moieties through cooperative hydrogen bonding interactions (CHBs). The hybrid polymer PAA–P4VP presented higher NH3 capacity (18.2 mmol g−1 at 298 K and 1 bar NH3 pressure) than PAA (6.0 mmol g−1) through the acid–base reaction and the assistance from CHBs with NH3, while the NH3 desorption from PAA–P4VP was easier for the reformation of CHBs. Based on the introduction of CHBs, a series of mesoporous acid–base hybrid polymers was synthesized with NH3 adsorption capacity of 15.8–19.3 mmol g−1 and high selectivity of NH3 over CO2 (SNH3/CO2 = 25.4–56.3) and N2 (SNH3/N2 = 254–1068), and the possible co-existing gases, such as SO2, had a lower effect on NH3 uptake by hybrid polymers. Overall, the hybrid polymers present efficient NH3 adsorption owing to the abundant acidic moieties and CHBs, while the concomitant Lewis bases promote NH3 desorption.


Synthesis of hybrid polymers
The synthesis and characterization of P4VP and PVIm were described in our previous work in detail. 1 The molecular weight of P4VP and PVIm are about 116900 and 49400 g/mol, respectively, according to the viscosity method.1.2.1.Synthesis of PVBA.4-Vinylbenzoic acid (VBA, 10.0 g) and 50 mL deionized water were put in a 150 mL three-seater flask, 0.16g AIBN was added in the solution after bubbling through N 2 flow for 0.5 hour to drive the oxygen in the solution away.The solution was stirred under protection of N 2 for 12 hours at 60 o C, the obtained white solid powder was filtrated and dried in 80 o C vacuum.Yeild, 86.3%.The IR and 1 H NMR spectra of the synthesized PVBA were exhibited in Figure S1a and Figure S1b with the comparison of VBA.IR (cm -1 ): 3500-3100 (υ OH ), 3022 (υ CH of benzene ring), 2917, 2663, and 2536 (υ CH of methylene and methyne), 1687 and 1279 (υ COOH ), 1607 and 1575 (υ benzene ring ). 1 H NMR (d 6 -DMSO, ppm): 1.24 (1H, CH), 1.44 (2H, CH 2 ), 6.63 (2H, CH in benzene ring), 7.65 (2H, CH in benzene ring), 12.79 (1H, COOH). he molecular weight of PVBA is about 306000 g/mol according to the viscosity method.1.2.2.Synthesis of PAA-P4VP.1.44 g PAA (containing 20 mmol carboxylate acid) was dissolved in 40 mL mixed solvent while 2.10 g P4VP (containing 20 mmol pyridine) was dissolved in another 40 mL mixed solvent, where the mixed solvent is methanol and deionized water (V:V=1:1).Add the PAA solution into P-4VP solution drop by drop and then stirring for 3 h at room temperature, there was white floccules precipitated out.The white floccules were obtained through centrifugation and dealt with freeze drying, and then placed into the vacuum drying oven at 80℃ for 24 h.Yield, 87.8%.Synthesis of PAA-PVIm, PVBA-P4VP, PVBA-PVIm, and PAA-BPY were like PAA-P4VP, their yield was 92.5%, 51.4%, 60.8%, 81.3%, 76.4%, and 84.7%, respectively.1.2.3.Synthesis of PAA-PS.1.44g PAA was dissolved in 40 mL H 2 O while 2.08 g PS (containing 20 mmol benzene ring) was dissolved in 40 mL tetrahydrofuran.Add the PAA aqueous solution into PS solution drop by drop and then stirring for 3 h at room temperature.Most of the solvent in the solution was removed by rotary evaporation, and the obtained white solid was dried at 80 o C in a vacuum oven for 24 h.Yield, 96.4%.

Characterization of acid frustrated hybrid polymers.
1 H NMR spectra was measured using a Bruker spectrometer (500 MHz) with tetramethyl silane (TMS, 0.00 ppm) as an internal standard.Fourier transform infrared spectroscope (Nicolet IS 50 FT-IR) was recorded with the attenuated total reflectance (ATR) module in the range of 400-4000 cm -1 .2D correlation FT-IR spectra were got by 2D software based on FT-IR spectra and the analysis was according to the socalled Noda's rule. 2 Scanning Electron Microscopy (SEM, JSM-7610F Plus) were used for morphological characterization of the gold-sprayed sample.Thermogravimetric analysis (TGA) was performed with a ramp rate of 10 °C/min from 60 o C to 600℃under Ar atmosphere by using Shimadzu DTG-60H.Elemental analyses for C, H, and N were performed at the Vario EL Cube Element analyzer (EA).The NH 3 -TPD of hybrid polymers was detected by PCA-1200 chemical adsorption instrument with the nitrogen flow of 30 mL/min at the programmed heating rate of 2 ℃/min, where the samples were NH 3 saturated.N 2 adsorption isotherm measurements were performed on a micromeritics 3FLEX surface characterization measurement at 77 K.

NH 3 absorption and desorption experiment
NH 3 and CO 2 uptake performance of the hybrid polymers were measured by weighing method through open method as presented in Figure S14.The open method was operated with controlled ammonia or CO 2 gas flow of 40 sccm through the polymers until the weight of the sample keep steady within 5 min.The effect of NH 3 partial pressure for open uptake was investigated by altering the composition of NH 3 and N 2 .The NH 3 desorption from the hybrid polymers were carried out under vacuum at 80 o C and 0.1 kPa until the weight maintained constant.The coefficient of selectivity for NH 3 /CO 2 and NH 3 /N 2 were calculated according to the equation , where 1 and 2 are two gas, S 1/2 is the adsorption selectivity of gas 1 over gas 2, c 1 and c 2 is the gas adsorption capacity of 1 and 2 (mmol/g) at 1bar and 25 o C. Thereinto, the CO 2 and N 2 adsorption were measured on a micromeritics 3FLEX surface characterization measurement at 25 o C. The effect of the containing water on NH 3 uptake of hybrid polymers was tested in the apparatus as shown in our previous work. 330 sccm N 2 flow passed through 25 o C water (saturated vapor pressure is 3.169 kPa) and hybrid polymer in turn, where the mixture gas passed through the hybrid polymer was moist N 2 steam containing 3.1% H 2 O.The weight increase of hybrid polymer was considered as H 2 O adsorption.The SO 2 uptake of hybrid polymer was tested using the apparatus shown in Figure S15, 30 sccm mixture flow with 5% SO 2 and 95% N 2 passed through hybrid polymer at 25 o C, the weight increase of hybrid polymer was considered as SO 2 adsorption.The data in the bracket is the calculated element content corresponding to the molar ratio of acid and base moieties as 1:1.

.
Figure S1.The comparison of IR and 1 H NMR spectra of VBA and PVBA.

Figure S2 .
Figure S2.Optimized structure of the unit of hybrid polymers.

Figure S3 .
Figure S3.Comparison of the IR spectra of hybrid polymers with PAA and PVBA.

Figure S6
Figure S6 TGA curves of hybrid polymers from 60 to 600 o C with temperature increase ratio of 10 o C/min under Ar flow.

Figure S8 .
Figure S8.(a) Structure of the agents used for synthesizing hybrid polymers.(b) SEM images of PAA, PAA-PS, and PAA-BPY.

Figure S9 .
Figure S9.Comparison of partial IR spectra of PAA-BPY, and PAA-PS with their NH 3 saturated and desorbed states.

Figure S10 .
Figure S10.(a) The NH 3 uptake and desorption of PAA-PS and PAA-BPY.NH 3 uptake and release at 25 o C, 1bar and 80 o C vacuum, respectively.

Figure S12. 6
Figure S12.6 consecutive NH 3 uptake capacity of PAA-PVIm at 25 o C and desorption under 80 o C vacuum for 90 min.

Figure S13. 6
Figure S13.6 consecutive NH 3 uptake capacity of PVBA-P4VP at 25 o C and desorption under 80 o C vacuum for 90 min.

Figure S14. 6
Figure S14.6 consecutive NH 3 uptake capacity of PVBA-PVIm at 25 o C and desorption under 80 o C vacuum for 90 min.

Table S1 .
The element content of acid-basic hybrid complexes.

Table S2 .
Comparison of NH 3 uptake properties of various sorbents with acidic sites.

Table S2 .
Comparison of NH 3 uptake properties of various sorbents with acidic sites.