Supporting Information

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1 Supporting Information Spontaneous Cooling Absorption of CO 2 by a Polymeric Ionic Liquid for Direct Air Capture Tao Wang, *, Chenglong Hou, Kun Ge, Klaus S. Lackner, *, Xiaoyang Shi, Jun Liu, Mengxiang Fang, and Zhongyang Luo State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou , PR China School of Sustainable Engineering and the Built Environment, Arizona State University, Tempe, Arizona , United States AUTHOR INFORMATION Corresponding Author *Tao Wang.: oatgnaw@zju.edu.cn * Klaus S. Lackner.: Klaus.Lackner@asu.edu S1

2 Experimental Procedures 1.1 Materials. A heterogeneous ion-exchange resin sheet (Excellion TM, I-200) was employed in this study. The resin contains quaternary ammonium functionality which was immobilized on the polystyrene backbone. 1 The original counter Cl - anion was replaced by CO 2-3 through pretreatment. The pretreatment method, charge density and CO 2 absorption capacity were reported by previous studies. 2-3 The structure of the resin from the sheet was studied by SEM (Sirion 200, FEI) and is shown in Figure S1. There s no obvious pore structure observed. The sorbent has a surface area of about 2.0 m 2 /g (NOVA-2000, Quantachrome). 3 The FTIR spectrum of the sorbent sheet is shown in Figure S2 (Nicolet 6700, Themo Fisher Scientific). The gases used (N 2 and CO 2 ) in all experiments are of % purity. Figure S1. SEM picture of resin particles from the sheet. S2

3 Figure S2. FTIR spectra of the sorbent. The spectrum exhibited typical bands for benzene ring (C-H stretch cm -1 ; C=C skeleton cm -1 ), quaternary ammonium (C-N stretch cm -1 ) and the band at cm -1 indicates the - formation of HCO 3 species 1.2 Experimental methods Isothermal equilibrium of CO 2. Wet, fresh samples (CO 2-3 form with blank CO 2 ) were dried in a reaction chamber (1.5 L) with purge gas of bone dry N 2 and the water concentration of the outlet gas was measured by an infrared gas analyzer (IRGA, LI-840A, LI-COR) to determine whether the samples were sufficiently dry. Drying was performed with a gas flow rate of 17 cm 3 /s, and it was stopped when the exhaust gas contained less than 0.2% water vapor. A fixed volume of CO 2 (0.5 cm 3 ) was repeatedly injected into the sample container by syringe, and the humidity was adjusted until the CO 2 absorption stabilized at 400ppm. The temperature of reactor chamber is controlled with a fluctuation of ± 0.2 K. At equilibrium, the CO 2 absorbed can be determined as: S3

4 q = q V C (1) e inj 0 where q inj, V 0, and C are the total amount of injected CO 2, the volume of the sample chamber, and the observed equilibrium CO 2 concentration, respectively. Isothermal equilibrium of water. The hydration/dehydration equilibrium coupled with CO 2 absorption is quantified in the isothermal system. Pieces of sample with total weight of 6 gram were placed in a chamber with volume of 20 L. The temperature of reactor chamber is controlled with a fluctuation of ± 0.2 K. Weight of sample was monitored in real-time by analytical balance (AL204, Mettler). First, the sample (CO 2-3 form with blank CO 2 ) was dried in reaction chamber with purge gas of bone dry N 2 until the sample weight is stable as W c-0 (fluctuation less than 0.2 mg in 30 minutes). N 2 with water vapor was then introduced into the reaction chamber through a dew point generator. The vapor concentration inside the reaction chamber was controlled with an accuracy of ± 0.01% based on the feedback from the IRGA. When the sample weight is stable, W c-h, under a certain humidity, h i, water isotherm data were obtained for the sample in CO 3 2- form. Then, pure CO 2 was charged into the reactor through the dew point generator until the sample had a stable reading from the balance, W b. According to equilibrium constants reported in previous study 3, the 100% CO 2 concentration can ensure similar CO 2 saturation state for - samples in HCO 3 form as the relative humidity changes in this work. The water - isotherm data for the sample in HCO 3 form would then be obtained. The amount of 2- hydration for the sorbent in CO 3 form (sorbent without CO 2 loading), W h, from S4

5 atmosphere of bone dry (W c-0 ) to a certain relative humidity h (W c-h ), can be calculated as: W = W W (2) h c h c 0 The amount of water exchange between interface and atmosphere, W exc, during CO 2 absorption under humidity h, can be calculated as: W = W W W (3) exc b c 0 CO 2 W CO 2 is the equilibrium capacity of CO 2 absorbed under the specific condition. Calorimetric experiments. The enthalpies of absorption of CO 2 and H 2 O were determined by calorimetric experiments. The calorimetric system mainly includes a Calvet-type microcalorimeter (C80, Setaram Sensys), an infrared gas analyzer (LI-840A, LI-COR) and a dew point generator as shown in Figure S3. 4 Figure S3. Layout of the calorimetric measurement system. Sorbent (about 0.5 g) was placed in the sample chamber of the C80 calorimeter whose temperature was controlled with the fluctuation of ± 0.01 K. The wet sample in CO 2-3 form was first flushed by bone dry N 2 with flow rate of 1.5 L/min until water vapor concentration of outlet gas was less than 0.01%. N 2 with water vapor was then fed into the sample chamber with flow rate of 200 ml/min. When the fluctuations of S5

6 water vapor concentration and heat flow data were less than ± 0.1% and ± 0.01 mw, respectively, the system was regarded as at equilibrium. CO 2 with set water vapor concentration was then charged into the chamber with flow rate of 200 ml/min until the heat flow is stable. The enthalpy of H 2 O/CO 2 absorption under different humidity could be calculated by integration of the heat flow curve. As the sample preparation, temperature and humidity equilibrium are time consuming (more than 10 hours), selected RH will be selected for representative conditions. Sorbent temperature measurement. The evolution of sorbent temperature during absorption was characterized under constant humidity and temperature. Mini T-type thermocouples (TT-T-30 SLE, Omega) with diameter of 0.25 mm were embedded between two tightly contacted sample sheets (6 cm 6 cm with a thickness of 0.8 mm). The sample was placed in a cell with volume of 250 cm 3. The reaction gas was pre-heated by a water bath before entering the cell to stable the cell temperature. The sample was first dried with purge of bone dry N 2 and then reacted with CO 2 containing certain water vapor. Temperature of the sorbent was obtained by averaging the data from the four thermocouples which were recorded every second. 1.3 Models H 2 O/CO 2 absorption isotherm models. Water vapor adsorption on solid surface has always been a complicated issue and different isotherm models, e.g. BET model, 5 Dubinin-Serpinsky model, 6 Anti-Langmuir 7 and D Arcy & Watt model 8 have been reported. These models S6

7 consider differences in pore size, interfacial morphology, surface functionality and so on. The D Arcy & Watt model can distinguish strong and weak sorption sites and is widely employed in multilayer adsorption. Table S1. Isotherm models for water adsorption. Model Description BET P/(θ(P 0 -P))=1/C+(C-1)P/CP 0 Dubinin-Serpinsky a/h=c(a 0 +a)(1-ka) Anti-Langmuir q=q e Kp/(1-Kp) D Arcy and Watt a=a ml K L h/((1/p s )+K L h)+a 0 kh/(1-kh) The absorption reaction between CO 2 and hydrated quaternary ammonium sorbent can be described as below 2 : CO (R ) xh O CO 2[HCO R yh O ] ( x 2y 1)H O ( h) ( h) 2 ( g ) + + (4) where R + represents the quaternary ammonium cation in the sorbent, H 2 O (h) represents the hydrated water, H 2 O (g) represents the gaseous water, and x and y represent the number of water molecules around the anions. By coupling the chemical equilibria of H 2 O/CO 2 in the gas-solid system, one can obtain the following relationship 2 : RT ln k = G ( T ) (2 y x) RT ln h (5) a Here n= 2 y x, is the number of water molecules gained by the resin when one CO 2 molecule is absorbed. G a is the standard free energy change of CO 2 sorption on a pseudo anhydrous sorbent: r CO (R ) H O CO 2[HCO R ] ( g ) (6) The basic properties of sorbent and values of isothermal parameters have been S7

8 reported in previous study and are list in Table S2. 2 Table S2. Basic parameters of the sorbent. Temperature ºC Relative humidity % K [a] 10 5 Θ [b] ρ c [c] mol/kg q [d] m 3 /kg G a kj/mol CO [a] Equilibrium constant of absorption. [b] Saturation of the sorbent. [c] Sorbent s charge density. [d] Maximum CO 2 capacity obtained by isotherms. Atomic model based on density functional theory. The finite oligomer method 9-10 was applied for computation of the electronic structure of the quaternary ammonium based sorbent whose repeat unit is illustrated in Figure S4. The quaternary ammonium cation is covalently linked to polystyrene backbone and carbonate anion is connected to the anion by ionic bonds. ONIOM 10 method was employed, considering full repeat unit with backbone. The computational system are separated into three layers as the chemical active portion. Anions, CO 2 and hydrated water is treated as the high layer, the quaternary ammonium cation is regarded as the medium layer and the backbones are tricked as low layer. All calculations were carried out using the Gaussian 09 package. Geometry optimization and vibrational frequency calculations were carried out at ONIOM (B3LYP-D3(BJ)/aug-cc-pVDZ:B3LYP/3-21+g*) level of theory. DFF-D3 was applied in the calculation of high layer as the most accurate method 11. DFT method with the S8

9 Becke s three-parameter functional and the nonlocal correlation of Lee, Yang, and Parr (B3LYP) 12 together with the 3-21+g* basis set 13 was applied in the calculation of medium layer. Single-point energy were corrected at the ONIOM (MP2/ aug-cc-pvtz: B3LYP /3-21+g*) level based on the optimized geometry at the ONIOM(B3LYP-D3(BJ)/aug-cc-pVDZ: B3LYP/3-21+g*) level Energy values were corrected by zero-point vibrational energies (ZPE). All interacting energy values were calculated at the same theoretical level, with basis set superposition error (BSSE) corrected through the counterpoise procedure. Figure S4. Representative of model compounds and ONIOM method employed in this study. The calculation of the standard Gibbs free energy of water hydration The number of hydrated water molecules is statistical average value of all hydration states, which could be a non-integer. However, on a given absorption site, the number of hydrated water molecules must be an integer. To calculate the average number of S9

10 hydrated water molecules, we must obtain the probability of each hydration state. Considering the following elementary hydration reaction: N A ( n 1)H O H O = N A nh O (7) Where N + represents the quaternary ammonium cation and the backbone covalently linked to it; A - denotes the counter anion; n is the number of hydrated water molecules of a certain hydration state; G 0 n is defined as the standard Gibbs free energy change of the elementary hydration reaction, which could be directly obtained from the theoretical calculation. Results and Discussion Figure S5. Fitting of H 2 O absorption isotherm of PIL (CO 3 2- ). The D Arcy and Watt model clearly provides the best fit. S10

11 Figure S6. Fitting of H 2 O absorption isotherm of PIL (HCO - 3 ). The D Arcy and Watt - model clearly provides the best fit. The quantity of hydration of sorbent in HCO 3 form under a certain humidity was determined by distracting the quantity of hydration in CO 2-3 form from the amount of water exchange during absorption calculated with Wang s model. (a) (b) S11

12 (c) (d) Figure S7. Calorimetric curve of the sorbent under 30 ºC and different relative humidity (a) 0%; (b) 15.9%; (c) 36.4%; (d) 48.6%. With the increase of environmental humidity, the CO 2 absorption transits from exothermic reaction to endothermic reaction. Figure S8. Temperature profile of the sorbent at different relative humidity. With the increase of relative humidity, the peak value of temperature change declines from 15.8 K to 7.6 K due to stronger dehydration. A subsequently mild temperature decrease of sorbent is observed and the sorbent temperature is under environment temperature for S12

13 quite a long period. Figure S9. Optimized structures of the model compounds of the sorbent with different anions (carbonate and bicarbonate) and hydrated water molecule numbers (n). To emphasize the ion pairs, only half of the backbone structure is shown for bicarbonate form. In the case of CO 2-3, water molecules located around the anion uniformly, while the water molecules self-associated or formed cluster in the case of HCO - 3. S13

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