Reversible Construction of Ionic Networks Through Cooperative

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Reversible Construction of Ionic Networks Through Cooperative Hydrogen Bonds for Efficient Ammonia Absorption Xiaoyan Luo, Rongxing Qiu, Xiaoyan Chen, Baoyou Pei, Jinqing Lin, and Congmin Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00554 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Sustainable Chemistry & Engineering

Reversible Construction of Ionic Networks Through Cooperative

Hydrogen

Bonds

for

Efficient

Ammonia Absorption Xiaoyan Luo†, Rongxing Qiu†, Xiaoyan Chen†, Baoyou Pei†, Jinqing Lin*†, and Congmin Wang*‡ †

College of Materials Science and Engineering, Huaqiao University, No. 668, Jimei Road,

Xiamen 361021 (P.R. China) ‡

Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, No. 38, Zheda

Road, Hangzhou 310027 (P. R. China) * E-mail: [email protected]; [email protected]. Keywords: ammonia absorption, sigmoidal absorption isotherm, ionic networks, reversible construction, energy-saving desorption.

ACS Paragon Plus Environment

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Abstract: ILs with reversible construction of ionic networks, which mainly consist of cooperative hydrogen bonds (CHBs) were designed for sigmoidal ammonia (NH3) absorption isotherm, which leads to efficient absorption, energy-saving desorption and high reversibility. Combined with NH3 absorption-desorption experiments, spectroscopic investigations, NH3-TPD measurement and quantum-chemical calculations, NH3 absorption mechanism was proposed as hydrogen bond interaction with IL by overcoming the heat for disorganizing ionic networks, including CHBs breakage and the phase change of IL from solid to liquid. Reversely, the NH3 desorption would be promoted by the heat release for the reformation of ionic networks. Thereinto, [BzAm][Tf2N] with ionic networks showed NH3 absorption with threshold pressure at 0.28 bar and NH3 capacity of 2.8 mol NH3/mol IL at 1 bar as well as be desorpted completely just through pressure swing, calorimetric test indicated the exothermic reformation of ionic networks provided 31.8% of energy for NH3 desorption from [BzAm][Tf2N]. Furthermore, the ammonia capacity as well as the threshold pressure would be changed by varying the CHBs interaction in IL, that [2PyH][Tf2N] with weaker interaction of CHBs indicating decreased threshold pressure at 0.04 bar and enhanced NH3 capacity of 3.8 mol NH3/mol IL at 1 bar. We believe this highly efficient and reversible process by reversible construction of absorbents can provide a potential alternative for NH3 as well as other gas absorption.

Introduction Basic gases such as ammonia (NH3) lead to significant environmental and industrial concerns even at small concentrations. Actually, NH3 released into the air is a threat to the environment through combination with NOx or SOx into ammonium salts, a main component of


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PM2.5 as well as threaten the health of human health.1-4 Traditionally, water and acidic aqueous solutions are widely used NH3 absorbents, however the high volatility of water and strong reactivity between acid and NH3 make NH3 desorption and absorbents recycling to be an energy consuming and resource wasting process. For this situation, different approaches avoiding aqueous solutions have been proposed to remove and reuse NH3 from gas effluents on the basis of the lone pair of electrons as showed in Fig. 1. (1) NH3 as a base could be fixed by reacting with acid to form ammonium salt5-8. (2) NH3 as a ligand with affinity of metal complex with vacancies, and ammonia could be absorpted by providing lone pair electrons9-15. (3) NH3 as hydrogen acceptor could interact with hydrogen donor through hydrogen bond formation8, 16-18. As can be seen, ammonia could be fixed via diverse methods as showed in Fig. 1 (a).

Fig. 1 Ammonia absorption properties. (a) Absorption methods from references; (b) the comparation of Langmuir and sigmoidal ammonia absorption isotherm; (c) energy change in the process of ammonia absorption and desorption; (d) ammonia capture by cooperative hydrogen bonding agent.


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It’s reported that high energy as above 120 kJ for per mol NH3 should be provided for NH3 desorption from Brønsted acidic agents and metal complexes with unoccupied d orbital.12, 19 In recent years, NH3 absorption through hydrogen bond interaction received attention for its low energy consumption for ammonia desorption. Some deep eutectic solvents (DESs) mixed with hydrogen-bond acceptor and various hydrogen-bond donor18,

20, 21,

functionalized ionic liquids (ILs)17,

26,

22-24,

metal-containing IL25,

hydroxyl group

and some solid

adsorbents8 were reported for efficient and reversible NH3 absorption. The studies available reveal the great potential of absorbents with hydrogen bond to absorb NH3 efficiently. Alternatively, sigmoidal absorption isotherm is recognized as efficient and energy-saving for gas separation, such as carbon dioxide adsorption by metal-organic frameworks27-29 and porous coordination polymers PCP-N30. It’s reported the gas desorption would be realized by facile small pressure swing as ΔP, the energy consumption for desorption would be less than Langmuir adsorption isotherm corresponding to Path A showed in Fig. 1 (b). As we known that high enthalpy change of NH3 absorption is required to achieve meaningful capacity for ΔG=ΔH-TΔS and the entropic decrease of NH3 from gas to fixed state.31-33 Simultaneously, the desorption of NH3 and the reuse of ad-/absorption agents are as important as the fixation of ammonia, but corresponding energy (Q1) should be provided as showed in Path A, Fig. 1 (c). We know cooperative hydrogen bonds (CHBs) are naturally present in base pairs, self-assembled materials, sensors, and so on34-36, meanwhile, the breakage of CHBs was considered as endothermic37. Considering the advantage of NH3 absorption by hydrogen bond and the flexible designability of the structure of ILs, ILs designed with CHBs to form ionic networks as showed in Fig. 1 (d) were considered as


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ammonia capture agent in this work accordingly. Thanks to the heat absorption of the breakage of ionic networks, NH3 absorption would realize with sigmoidal isotherm for the interaction energy should enough to overcome the energy (Q in Fig. 1(c)) for disorganization of ionic networks to expose NH3 absorption sites. Conversely, the energy provided for NH3 desorption would decrease to Q2 through a concomitant endothermic ionic networks reformation as Path B in Fig. 1 (c). Based on this idea, [BzAm][Tf2N] containing benzyl amidinium ([BzAm]) with lots of hydrogen donor and bistrifluoromethanesulfonimide ([Tf2N]) with lots of hydrogen acceptor such as N and O atom (Chart 1) was synthesized and applied for ammonia absorption. The NH3 absorption mechanism was proposed as hydrogen bond interaction between ammonia and amidino-group with the breakage of CHBs of [BzAm][Tf2N] and reverse, while the endothermic breakage of CHBs benefits to sigmoidal NH3 absorption isotherm, and the exothermic reformation of CHBs made the energy for NH3 desorption decrease. [EtAm][Tf2N] and [MeOBzAm][Tf2N] with acetamidinium and p-methoxyl benzyl amidinium cation were also synthesized as contrast and [2PyH][Tf2N] was further synthesized to improve NH3 absorption properties. The existence of CHBs in ILs were investigated through XRD characterization, NH3 absorption mechanism was investigated by combination of IR and NMR spectroscopies and theoretical calculations. Furthermore, the energy consumption for ammonia desorption were tested by DSC measurement, the results showed the breakage and reformation of CHBs played an important role both in ammonia absorption and desorption.


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Chart 1 Structures of cations and anion of ILs used as NH3 absorption agent.

Experimental Section Chemicals. NH3 (99.999%) was purchased from Foushan Kodi Gas Chemical Industry Co., LTD. Benzamidine Hydrochloride ([BzAm][Cl]), acetamidine hydrochloride ([EtAm][Cl]), 4methoxybenzamidine hydrochloridewere ([MeO-BzAm][Cl]), 2-aminopyridine (2Py), and bistrifluoromethanesulfonimide Lithium ([Li][Tf2N]) were purchased from Shanghai Aladdin biochemical technology Co. LTD. These reagents were used without further treatment. Preparation and measurement. The target ILs were prepared by mixturing with derivative amidine hydrochloride aqueous solution with LiTf2N aqueous solution in the ratio of 1:1 (mol/mol), the products could be got from the mixture solution. For [BzAm][Tf2N] and [2PyH][Tf2N], it would crystallize from the solution, the crystal product was filtered from the solution and then placed into the vacuum drying oven at 80oC for 24h. [MeO-BzAm][Tf2N] and [EtAm][Tf2N] should be extracted by ethyl acetate from the mixture solution several times, the products were obtain through evaporating ethyl acetate, and then placed into the vacuum drying oven at 80 oC for 24h. Their structures were characterized by IR and 1H NMR spectrograph, the results showed the expected ILs were obtained and no impurity was detected. NMR spectra was obtained using a 500 MHz Bruker Avance III spectrometer in either deuterated dimethyl sulfoxide (d6-DMSO) or in drive pipe inside d6-DMSO using tetramethylsilane as the standard.


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FTIR spectra was obtained using a Nicolet IS50 spectrometer. The crystal of [BzAm][Tf2N] and [2PyH][Tf2N] were characterized by X-ray diffractometer using Aglient Gemini EGemini E. The DSC detection was operated by NETZSCT DSC 200 calorimeter with temperature various from 25 oC to 200 oC by heating ratio of 10 oC/min. The TGA measurement of NH3 saturated complexes were measured by Shimadzu DTG-60H with temperature various from 25 oC to 600 oC

by heating ratio of 10 oC/min. The acid site was detected by PCA-1200 chemical adsorption

instrument. Data. IR and 1H NMR spectra data of [BzAm][Tf2N], [2PyH][Tf2N], [EtAm][Tf2N] and [MeOBzAm][Tf2N] as well as their ammonia saturated complexes were listed as following. [BzAm][Tf2N]. IR (cm-1): 3422, 3404, 3372 and 3268 (νNH2), 3079 (νC-H, Ph), 1682, 1669, 1608, 1593, 1551, 1527, 1481, 1450, 1339, 1189, 1131, 1059, 790, 765, 740, 687. 1H NMR (d6-DMSO, ppm): 9.308 (s, 2H, NH2), 8.888 (s, 2H, NH2), 7.82-7.78 (m, 2H, Ph), 7.77-7.73 (m, 1H, Ph), 7.66-7.60 (m, 2H, Ph). NH3 saturated [BzAm][Tf2N]. IR (cm-1): 3397, 3282(νNH2…NH3), 3066 (νC-H,

Ph),

1685, 1636,

1609, 1566, 1527, 1480, 1448, 1346, 1326, 1180, 1131, 1051, 787, 764, 741, 698. 1H NMR (d6DMSO): 7.14-6.97 (m, 2H, Ph), 6.97-687 (m, 1H, Ph), 6.87-6.70 (m, 2H, Ph), 3.442 (s, 12.9H, NH2…NH3). [2PyH][Tf2N]. IR (cm-1): 3452 (νN-H (Py)), 3356, 3228 (νNH2), 3112 (νC-H (Py)), 1663, 1627, 1545, 1477, 1355, 1339, 1320, 1186, 1130, 1047, 1032, 793, 774, 743, 724, 613, 571, 512. 1H NMR (d6-DMSO, ppm): 13.265 (s, H, NH+), 7.925 (m, 4H, Py and NH2), 6.974 (d, 1H, Py), 6.869 (t, 1H, Py).


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NH3 saturated [2PyH][Tf2N]. IR (cm-1): 3483 (νN-H…NH3 (Py)), 3395, 3291 (νNH2…NH3), 3028 (νC-H (Py)),

1622, 1607, 1507, 1488, 1446, 1346, 1325, 1182, 1131, 1052, 791, 775, 741, 655, 613, 600,

570, 510. 1H NMR (d6-DMSO): 7.355 (d, 2H, Py), 6.846 (t, 1H, Py), 6.033 (m, 1H, Py), 3.540 (s, 2H, NH2), 3.173 (s, 11.4H, NH+…NH3). [EtAm][Tf2N]. IR: 3425, 3370, 3277 and 3211 (νNH2), 1688, 1351, 1330,1192, 1124, 1051, 787, 765, 739. 1H NMR (d6-DMSO, ppm): 2.108 (s, 3H, CH3), 8.338 (s, 2H, NH2), 8.870 ppm (s, 2H, NH2). NH3 saturated [EtAm][Tf2N]. IR: 3399 (νNH2…NH3), 1697, 1345, 1325, 1181, 1130, 1051, 791, 765, 741. 1H NMR (d6-DMSO, ppm): 1.774 (s,3H,CH3), 3.872 (s,13.0H,NH2…NH3) [MeO-BzAm][Tf2N]. IR: 3419, 3365, 3276(νNH2), 3020, 2977, 2948, 2911, 2849 and 2757 (νC-H (Ph)

and νC-H

(CH3)),

1676, 1607, 1586, 1551, 1486, 1443, 1344, 1314, 1271, 1181, 1127, 1050,

1025, 842, 815, 792, 765. 1H NMR (d6-DMSO): 3.872 (s, 3H, CH3), 7.179-7.161 (m, 2H, Ph), 7.827-7.809 (m, 2H, Ph), 8.660 (s, 2H, NH2), 9.127 (s, 2H, NH2). NH3 saturated [MeO-BzAm][Tf2N]. IR: 3367, 3284 (νNH2…NH3), 3020, 2977, 2948, 2911, 2849 and 2757 (νC-H (Ph) and νC-H (CH3)), 1674, 1603, 1588, 1549, 1488, 1444, 1341, 1308, 1272, 1183, 1126, 1051, 1025, 842, 820, 794, 766. 1H NMR (d6-DMSO): 3.116 (s, 3H, CH3), 3.458 (s, 13.2H, NH2…NH3), 6.333 (s, 2H, Ph), 7.059 (s, 2H, Ph).

Results and Discussion Characterization of cooperative hydrogen bonds. The ionic networks of [BzAm]Tf2N] was characterized by XRD, the result in Fig. 2 indicates cooperative hydrogen bonds (the detail date about hydrogen bond length less than 2.8Å


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and hydrogen bond angle more than 120o were listed in Table S1 and Fig. S1) form between cation and anion. The hydrogen bond formation contributes to [BzAm][Tf2N] as orthorhombic crystal, its melting point was detected as 105 oC, which is a little higher than common ionic liquids. There is no crystal water exist in [BzAm][Tf2N], which might be generated to the wellorganized [BzAm][Tf2N] and thanks to the hydrophobic phenyl group. Its hygroscopicity was detected with bubbling N2 flow with 4.2% water vapor, [BzAm][Tf2N] was nonabsorbent for water from the result in Fig. S2 for no weight increased after 30 min.

Fig. 2 The crystal structure of [BzAm][Tf2N] characterized by XRD. The yellow dash line represents hydrogen bond with bond length less than 2.8Å and bond angle larger than 120o.

NH3 absorption experiment. [BzAm][Tf2N] was applied to NH3 absorption and the equilibrium reached balance within 20 min with high capacity as 2.8 mol NH3 per mol IL at 1 bar and 30 oC (Fig. 3). The NH3 capacity decreased with temperature increase and pressure decrease as showed in Fig. S3 and Fig. 3 (b), respectively. It’s interesting that there was a sharp increase of capacity from 0 to 2.0 mol/mol IL on account of NH3 pressure increased from 0.28 to 0.5 bar, following a slow increase of capacity to 2.8 mol/mol with NH3 pressure increase to 1.0 bar. Thus, the NH3 absorption isotherm was sigmoid type, which means the captured NH3 might easily be desorpted under low pressure. Intriguingly, the experiment results


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showed the fixed NH3 could be desorpted completely just by reducing pressure to 1kPa for 2 hours as seen in Fig. S4 or desorpted rapidly at 50 oC and 1kPa within 30 min as showed in Fig. 3 (a). The IR spectra of the NH3 desorpted sample in Fig. 3 (c) was consistent with the fresh [BzAm][Tf2N], which indicated the captured NH3 was in fact desorpted easily with pressure and temperature swings as expected. But did the ionic networks generated from the CHBs between cation and anion prompt ammonia absorption with sigmoidal isotherm and improve NH3 desorption? Proposed ammonia absorption mechanism. [BzAm][Tf2N] and its NH3 saturated complex were characterized by IR and 1H NMR spectroscopies as show in Fig. 3 (c) and (d), respectively. The IR spectra of [BzAm][Tf2N] in Fig. 3 (c) indicates two stretching vibration bands at about 3270 and 3410 cm-1 belonged to the absorption of N-H in amidium group disappear with uptaking of NH3, simultaneously, a new absorption appears at about 3390 cm-1, which means NH3 fixed by amidium moiety. 1H NMR spectra in Fig. 3 (d) indicates that the signal of H in amidinium at 9.31 and 8.89 ppm shift to 3.44 ppm with reaction of ammonia company with the proton number increases from 4 to 12.9, which demonstrates 3.0 mol NH3 absorpted by per mol amidinium group, it's consistent with the ammonia absorption experiment.


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Fig. 3 Ammonia absorption properties by amidinium based ILs. (a) Ammonia absorption by [BzAm][Tf2N] (circle), [EtAm][Tf2N] (square), and [MeO-BzAm][Tf2N] (pentagon) at 30 oC and 1 bar ammonia pressure while desorption at 50 oC and 1 kPa. (b) Ammonia absorption varied with pressure at 30 oC by [BzAm][Tf2N] (circle), [EtAm][Tf2N] (square), and [MeO-BzAm][Tf2N] (pentagon). (c) IR spectra of [BzAm][Tf2N], its NH3 saturated complex and the NH3 desorpted sample, (d) 1H NMR spectra of [BzAm][Tf2N] compared with its NH3 saturated complex.

The reaction enthalpy of NH3 with [BzAm] was calculated through Gaussian process at the b3lyp/6-31g** level. The results showed in Fig. 4 indicated amidinium reacted with NH3 through hydrogen bond in the form of NH…NH3, and the proposed reaction pathway indicated there were three potential active sites to capture NH3 with reaction enthalpies as -72.16, -55.10, and -44.62 kJ/mol.38 As seen from their structures, the planar of [BzAm] would be destroyed along with rotation of phenyl group after reacting with NH3 from Fig. 4, and combined with experiment we noticed phase change of [BzAm][Tf2N] from solid to liquid. Specially, the stretching vibration of C-H in phenyl group shift from 3079 to 3066 cm-1 in Fig. 3 (c), which demonstrated the CHBs of [BzAm][Tf2N] destroyed in the process of ammonia absorption. The destroyed CHBs would reformed through ammonia


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desorption for the stretching vibration of C-H in phenyl group reappeared at 3079 cm-1 as well as the [BzAm][Tf2N] changed to solid. The results demonstrate the breakage and reformation of CHBs benefit to NH3 absorption and desorption, and attribute to phase change of [BzAm][Tf2N].

Fig. 4 Rreaction enthalpies of [BzAm] cation and NH3 to proposed complexes calculated by Gaussian process in the level of b3lyp sets.

The acid sites of [BzAm][Tf2N] detected by NH3 temperature programmed desorption (NH3-TPD) in Fig. 5 (a) show three acid sites appear at 61, 68, and 92 oC, respectively, which indicate [BzAm][Tf2N] reacts with NH3 in three different forms. Interestingly, the ammonia could be desorpted sequentially after being triggered at about 52 oC as showed in Fig. 5 (b) and in spite of lowering temperature, it could see obvious NH3 desorption until temperature decrease to 35 oC, and two obvious peaks appeared at 51 and 49 oC. The sample after TPD detection was characterized by IR spectroscopy and showed no NH3 remained in [BzAm][Tf2N] as in Fig. S5. We thought NH3 desorption would be spontaneous after being activated, and the energy consumption for NH3 desorption might acquire from elsewhere besides of heating.


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Fig. 5 NH3-TPD detection of [BzAm][Tf2N]. (a) NH3 desorption with temperature increase from 25 to 200 oC; (b) NH3 desorption with temperature increase from 25 to 52 oC and then natural cooled; dot line, temperature; full line, ammonia signal.

The effect of cooperative hydrogen bonds. The hydrogen bond is energy-intensive and might be destroyed by heating and reformed by cooling. The heat absorption of the breakage of ionic networks was detected by DSC and showed in Fig. 6, there is about 91.56 J/g (36.7 kJ/mol) heat absorbed by [BzAm][Tf2N], which would cause the NH3 absorption enthalpy increase. The breakage of CHBs permits NH3 absorption possible, that why NH3 absorpted by IL with reversible construction of ionic networks existing an introduction period and sigmoidal absorption isotherm. Conversely, the energy consumption for NH3 desorption decrease because the reformation of CHBs is exothermic. The energy


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consumption provided for ammonia desorption measured by DTA-DSC and listed in Table 1, the result showed the desorption energy of NH3 from [BzAm][Tf2N] was just 39.3 kJ/mol NH3, (78.6 kJ for NH3 desorpted from [BzAm][Tf2N]-2NH3 complex), which is much lower than the calculated value of reaction enthalpy (127.26 kJ/mol) as well as the reported desorption consumption of Brønsted agent. To hinder CHBs formation, [EtAm][Tf2N] with methyl instead of phenyl was synthesized for the phenyl group facilitating CHBs formation from the crystal structure of [BzAm][Tf2N]. [EtAm][Tf2N] is a solid with melting point at 42 oC, and the NH3 absorption experiment in Fig. 3 (a) showed its ammonia capacity was similar to [BzAm][Tf2N] as 2.8 mol/mol at 30 oC and 1 bar, however, its absorption isotherm was different from [BzAm][Tf2N], as seen from Fig. 3 (b) that the NH3 capacity increase along with the increase of pressure but without obvious inflection point, which was similar to the reported NH3 absorption by ILs with Langmuir-shaped absorption isotherm 17, 38, 39. The 1H NMR spectra in Fig. S6 (a) showed the H of amidinium shift from 8.86 and 8.34 ppm to 3.87 ppm with taking up of ammonia, furthermore, the proton number increase from 4 to 13.0, which was consistent with the ammonia absorption experiment. However, the desorption of NH3 absorption with [EtAm][Tf2N] in Fig. 2 (a) showed 0.99 mol residual NH3 (about 35% captured NH3) in per mol [EtAm][Tf2N] under 50 oC and 1Kpa for 30 min, and the IR spectra also showed the vibration of amidinium at 3427 cm-1 did not coincide with the fresh IL as showed in Fig. S6 (b). With temperature increasing to 70 oC, the NH3 could be desorpted after 30 min as showed in Fig. S7, the IR spectra of the recovered sample in Fig. S8 shows an obvious sharp peak at 3427 cm-1 for the complete desorption of NH3. Therefore, more energy consumption should be provided for NH3 desorption from [EtAm][Tf2N], which lacks of ionic networks compared with [BzAm][Tf2N].


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Table 1 NH3 absorption and desorption properties of ILs. Heat (kJ/mol)

State of State

NH3

of IL

capacitya

IL

IL-NH3

Provided for

Absorption

complex

desorption b

by ILc

Energy contribution of Total

IL

[BzAm][Tf2N]

Solid

2.8

Liquid

78.6

36.7

115.3

31.8%

[EtAm][Tf2N]

Solid

2.8

Liquid

135.8

6.5

141.3

4.6%

Liquid

2.9

Liquid

112.2

19.4

131.6

14.7%

Solid

3.8

Liquid

93.0

23.6

116.6

20.2%

[MeOBzAm][Tf2N] [2PyH][Tf2N] a,

mol NH3 per mol IL, the absorption was operated at 30 oC, 1 bar. b, the energy consumption for NH3

desorption from 2 mol NH3 saturated IL, which was measured by TGA-DSC with temperature increase by 10 oC/min

and under N2 gas protection. c, the heat was detected by DSC calorimeter with temperature various

from 25 oC to 200 oC by heating ratio of 10 oC/min.

To further investigate the effect of CHBs on NH3 desorption, [MeO-BzAm][Tf2N] with methoxy-group in p-site to hinder CHBs formation was synthesized and it is a liquid. It was applied for NH3 absorption and desorption, the results showed the NH3 capacity was 2.9 mol/mol and the absorption isotherm similar to [EtAm][Tf2N] without obvious inflection point, the ammonia desorption operated under 50 oC and 1Kpa for 30 min showed 0.5 mol residual NH3 per mol IL (about 17% captured NH3) as showed in Fig. 3 (a). The comparation of IR absorption at 3282 and 3378 cm-1 between fresh [MeO-BzAm][Tf2N] and its NH3 desorption sample in Fig. S9 indicate the desortpion sample with some residual ammonia. The stretching vibration of C-H of methyl and phenyl group at 2800-3100 cm-1 without observable shift for NH3 absorption or


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desorption, which is different to [BzAm][Tf2N]. Thus, the existence of CHBs in [BzAm][Tf2N] is in favor of NH3 desorption. The heat absorption of [EtAm][Tf2N] and [MeO-BzAm][Tf2N] along with temperature increase was also detected by DSC and showed in Fig. 6, there was 19.14 J/g (6.5 kJ/mol) and 44.87 J/g (19.4 kJ/mol) heat absorption with [EtAm][Tf2N] and [MeO-BzAm][Tf2N], respectively, which are much lower than [BzAm][Tf2N]. It is worth notice that the [MeOBzAm][Tf2N] is a liquid and has more heat absorption than [EtAm][Tf2N], indicating heat providing for the breakage of hydrogen bonds. The energy consumption provided by heating for ammonia desorption in Table 1 indicated the desorption energies of NH3 from [EtAm][Tf2N]2NH3 and [MeO-BzAm][Tf2N]-2NH3 were 135.8 and 112.2 kJ per/mol, respectively, which are appreciably higher than [BzAm][Tf2N]. Thus, we thought the CHBs played an important role to these ILs as ammonia capture agents. The total energy consumption for NH3 desorption from [BzAm][Tf2N], [EtAm][Tf2N], and [MeO-BzAm][Tf2N] were listed in Table 1 as 115.3 141.3, and 131.6 kJ/mol, and 31.8%, 4.6%, and 14.7% energy were obtained from the reformation of CHBs along with phase change of ILs, respectively.

Fig. 6 DSC plot of [BzAm][Tf2N] (black full line), [MeO-BzAm][Tf2N] (red dash dot line) and [EtAm][Tf2N] (blue dash line) with temperature increase from 30 to 180 oC and increase ratio as 10 oC/min.


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The proposed ammonia capture mechanism of [BzAm][Tf2N] could be speculated as Fig. 7 (a) that the -NH2 moiety in amidinium reacted with NH3 through hydrogen bond interaction along with the breakage of CHBs, the concomitant heat absorption of Q leaded to NH3 absorption with sigmoidal absorption isotherm and enthalpic increase of NH3 absorption. The energy consumption for NH3 desorption decrease to Q2 thanks to reformation of CHBs correspondingly, where Q2=Q1-Q and Q1 represents the heat release from reaction of NH3 and IL. Commonly, enthalpic increase of NH3 absorption goes against to achieve high capacity, because of the well-organized [BzAm][Tf2N] changed to unordered NH3-[BzAm][Tf2N] complex with NH3 absorption, the entropic increase for the breakage of CHBs compensates to the increase of enthalpy and good for NH3 absorption from ΔG=ΔH-TΔS, which leads to high capacity and low energy consumption for NH3 desorption achieved simultaneously. Thus, enthalpy-entropy compensation by taking advantage of ionic networks made Path B described as in Fig. 1 (b) and (c) realized. Based on this opinion, [2PyH][Tf2N] with weaker CHBs than [BzAm][Tf2N] was designed for improved NH3 absorption properties. The hydrogen bonds information of [2PyH][Tf2N] got from XRD results in Fig. S 11were listed in Table S2. Similar to [BzAm][Tf2N], there are ionic networks formed in [2PyH][Tf2N] benefiting from lots of hydrogen bonds but not with less hydrogen bond forms and number than [BzAm][Tf2N]. The heat absorption of the breakage of ionic networks was detected by DSC and showed about 62.8 J/g (23.6 kJ/mol) heat absorbed by [2PyH][Tf2N], which means less energy should be provided for overcoming the hydrogen bonds similar to [BzAm][Tf2N]. Similarly, there’s phase change from solid to liquid along with absorption of NH3 and reversibly, and it’s can be seen from Fig 7(b) that there’s decreased threshold pressure at 0.04 bar and enhanced NH3 capacity as 3.8 mol NH3/mol IL at 1 bar. The


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TGA measurement result in Fig. S11 demonstrated that [2PyH][Tf2N] remains stable when temperature less than 298 oC. Base on the outstanding properties of [2PyH][Tf2N] as NH3 absorption agent, its cycle performance was further investigated without obvious decrease of capacity after 10 cycles as seen from Fig. 7 (b).

Fig. 7 NH3 absorption and desorption properties of IL with ionic networks. (a), Proposed NH3 absorption mechanism; (b), NH3 absorption by [2PyH][Tf2N] at 30 oC varied with ammonia pressure and its recovery at 50 oC in 1kPa vacuum for 30 min (inside).

Conclusions In summary, a strategy for efficient ammonia absorption in the form of sigmoid isotherm by ILs with reversible construction of ionic networks such as [BzAm][Tf2N] and [2PyH][Tf2N] through taking advantage of CHBs and phase change of ILs were reported. Combined with NH3 absorption and desorption experiments, IR and NMR spectroscopies, and Gaussian calculation results, it’s indicated NH3 absorpted through hydrogen bond interaction along with the breakage of ionic networks in IL, which leads to sigmoidal ammonia absorption isotherm for endothermic breakage of CHBs and the phase change from solid to liquid and reversely. Thereinto, [BzAm][Tf2N] showed NH3 absorption with threshold pressure as 0.28 bar and capacity as 2.8 mol NH3/mol IL at 1 bar. Encouragingly, 31.8% of energy provided for NH3 desorption provided


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from CHBs reformation with [BzAm][Tf2N], which lead to energy-saving desorption just through pressure swing, the fixed ammonia also could be striped rapidly at 50 oC and 1kPa within 30 min along with the regeneration of [BzAm][Tf2N]. The sigmoidal ammonia absorption isotherm could be tuned by varying the ionic networks in IL, that [2PyH][Tf2N] with weaker CHBs demonstrating decreased threshold pressure at 0.04 bar and enhanced NH3 capacity of 3.8 mol NH3/mol IL at 1 bar. We believe this highly efficient and reversible process by reversible construction of absorbents can provide a potential alternative for NH3 absorption. The use of sigmoidal absorption isotherm promises to provide a general energy saving strategy in gas separation. Supporting Information. Supporting information as Figure S1-S11 and Table S1-S2 for this article is given via a separated text. Corresponding Author * Jinqing Lin: [email protected]; * Congmin Wang: [email protected]. Acknowledgement We acknowledge the Natural Science Foundation of China (21803021, 21246008), Natural Science of Fujian Educational Foundation (JZ160407), Natural Science Foundation of Fujian Province (2016J01060), Education and scientific research project for middle and young teachers of Fujian (JAT170032). We acknowledge the instrumental analysis center of Huaqiao University. References 1. Wang, Z. X.; Wang, L. W.; Gao, P.; Yu, Y.; Wang, R. Z., Analysis of composite sorbents for ammonia storage to eliminate NOx emission at low


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Table of Contents

Sigmoidal NH3 absorption with high capacity and facile desorption was achieved through reversible construction of ionic networks.


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Reversible Construction of Ionic Networks Through Cooperative

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