Effect of SO2 on CO2 Capture Using Liquid-like ... - ACS Publications

May 13, 2013 - ABSTRACT: Liquid-like nanoparticle organic hybrid materials (NOHMs), consisting of silica nanoparticles with a grafted polymeric canopy...
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Effect of SO2 on CO2 Capture Using Liquid-like Nanoparticle Organic Hybrid Materials Kun-Yi Andrew Lin,† Camille Petit, and Ah-Hyung Alissa Park* Department of Earth and Environmental Engineering and Department of Chemical Engineering, Lenfest Center for Sustainable Energy, Columbia University, 500 West 120th Street, New York City, New York 10027, United States ABSTRACT: Liquid-like nanoparticle organic hybrid materials (NOHMs), consisting of silica nanoparticles with a grafted polymeric canopy, were synthesized. Previous work on NOHMs has revealed that CO2 capture behaviors in these hybrid materials can be tuned by modifying the structure of the polymeric canopy. Because SO2, which is another acidic gas found in flue gas, would also interact with NOHMs, this study was designed to investigate its effect on CO2 capture in NOHMs. In particular, CO2 capture capacities as well as swelling and CO2 packing behaviors of NOHMs were analyzed using thermogravimetric analyses and Raman and attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopies before and after exposure of NOHMs to SO2. It was found that the SO2 absorption in NOHMs was only prominent at high SO2 levels (i.e., 3010 ppm; Ptot = 0.4 MPa) far exceeding the typical SO2 concentration in flue gas. As expected, the competitive absorption between SO2 and CO2 for the same absorption sites (i.e., ether and amine groups) resulted in a decreased CO2 capture capacity of NOHMs. The swelling of NOHMs was not notably affected by the presence of SO2 within the given concentration range (Ptot = 0−0.68 MPa). On the other hand, SO2, owing to its Lewis acidic nature, interacted with the ether groups of the polymeric canopy and, thus, changed the CO2 packing behaviors in NOHMs.

1. INTRODUCTION Carbon dioxide (CO2) has become one of the most threatening greenhouse gases (GHGs) with the rapid increase in atmospheric CO2 concentrations, from 280 ppm in preindustrial times to 397 ppm as of March 2013.1 Despite continuous warnings on the detrimental impacts of CO2 emission on the environment, the use of fossil fuels remains the dominant route to obtain energy in the foreseeable future. To prevent the negative consequences of the high atmospheric level of CO2, efficient carbon management technologies must be developed and implemented. A proposed option to mitigate CO2, referred to as carbon capture and storage (CCS), involves the separation of CO2, followed by transportation and storage.2 CO2 capture is mainly envisioned at concentrated point sources, such as coal-fired power plants. Currently, the most mature technology for CO2 capture is amine scrubbing using aqueous solutions of amines [e.g., 15−30 wt % monoethnolamine (MEA)] to chemically react with gaseous CO2 and result in carbamate formation.3−7 The advantages of the amine-based solvent technology include a high CO2 capture capacity and a fast reaction kinetic. However, the amine scrubbing process is practically and economically challenged because of the high volatility and the corrosiveness of the solvent, as well as the high energy demand associated with the solvent regeneration step.8,9 To overcome these issues, the development of environmentally benign, efficient, and economically feasible solvent technologies to capture CO2 is desired. Recently, a new class of hybrid materials, known as liquid-like nanoparticle organic hybrid materials (NOHMs), has been developed.10−12 NOHMs consist of a surface-functionalized inorganic nanoparticle with a layer of ionically or covalently tethered polymeric canopy and act as a single fluid unit (Figure 1). Because the polymer chains are strongly anchored to the surface-functionalized nanoparticles, the materials exhibit high © 2013 American Chemical Society

thermal stability and negligible vapor pressures. NOHMs can also offer versatile chemical and physical tunability, owing to the diversity of nanocores and polymeric canopies that can be assembled to target specific applications. With such unique features, NOHMs can be applied in various fields,11−17 including but not limited to CO2 absorption18−21 and energy storage.22−24 In our prior study, it was found that NOHMs exhibit promising CO2 capture capacity, selectivity (over N2 and O2), and recyclability.21 While these features enable NOHMs to be a suitable candidate for CO2 capture, a number of additional aspects must be addressed to fully investigate the potential of these materials to capture CO2 in a post-combustion setting. Specifically, it is important to study the effects of flue gas components other than N2 and O2 (i.e., water vapor, particulate matter, SO2, and NOx) on CO2 capture. A separate study on the effect of water on the CO2 capture of NOHMs showed that water in flue gas does not impede the performance and chemical stability of NOHMs.25 The concentration of SO2 in the flue gas stream is very low compared to the concentration of CO2 (∼10−15%),26,27 ranging from about 180 to 250 ppm for typical oil burners up to 2000 ppm when burning poorquality coal.26−28 SO2 is known to cause the degradation of amine solvents and, thus, cause a drop in CO2 capture capacity.29−31 Therefore, to comprehensively evaluate NOHMs synthesized with amine functional groups for CO2 capture, the effect of SO2 on their performance must be investigated. Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: March 4, 2013 Revised: May 9, 2013 Published: May 13, 2013 4167

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Figure 1. Schematics of (a) overall configuration of NOHMs, (b) canopy structure of NOHM-I-HPE, and (c) canopy structure of NOHM-C-HPE.

Figure 2. Schematic diagram of the dual-chamber reactor for the gas sorption measurements in NOHMs with controlled temperature and pressure. enhance the overall CO2 capture capacity of NOHMs, for this study, it was important to synthesize NOHMs with minimal task-specific functional groups (i.e., amine). This is because the physical change of the structure of NOHMs that leads to the entropic contribution of CO2 capture studied here is difficult to detect if there is strong enthalpic interaction between CO2 and the amine groups of NOHMs. NOHM-I-HPE was synthesized using previously reported methods.10,18,21 Briefly, 3 wt % silica aqueous suspension (Ludox HS-30, 10−15 nm, Sigma-Aldrich) and 6 wt % 3-(trihydroxysilyl)-1-propane sulfonic acid (Gelest, Inc.) aqueous solution were prepared and mixed with each other. The pH of this mixture was adjusted to about 5 by dropwise adding 1 M NaOH solution. The mixture was then slowly reacted at 70 °C overnight, and then the excess silane was removed by dialysis [3500 molecular weight cut-off (MWCO), Thermo Scientific] against deionized water for 2 days. A cation exchange system (Dowex HCR-W2, Sigma-Aldrich) was employed to remove Na+ ion from the functionalized silica suspension and to protonate the sulfonate group. Finally, polymer chains were grafted onto the functionalized silica nanoparticles by dropwise adding 10 wt % Jeffamine M2070 [molecular weight (MW) of 2000, Huntsman Co.] solution to neutralize all of the sulfonate groups of the linker. The final product is called NOHM-I-HPE, and the NOHM sample was dried under vacuum at 35 °C prior to testing. NOHM-C-HPE was also synthesized according to a previously developed method.21 Again, a 3 wt % silica suspension in water was prepared by diluting a 30 wt % silica colloidal suspension (Ludox HS30, 10−15 nm, Sigma-Aldrich). Separately, 3 wt % Jeffamine M2070 (MW of 2000, Huntsman Co.) in ethanol was prepared, and a molar equivalence of (3-glycidyloxypropyl)trimethoxysilane (Gelest, Inc.) was added to the polyetheramine solution. After stirring for 12 h at 45 °C, the silica suspension was added to the polyetheramine−silane solution. The amount of silica suspension added to this system was controlled in such a way that the final polymeric content in NOHM-CHPE was equal to that measured for NOHM-I-HPE (∼15 wt %). The resulting mixture was stirred for 5 h and dialyzed (3500 MWCO, Thermo Scientific) against deionized water for 48 h to remove the excess of unbound polyetheramine−silane. Finally, the solvents were removed under vacuum at 35 °C to form liquid-like NOHMs as novel

Hence, the present study focused on the effect of SO2 on the CO2 capture in NOHMs. In particular, the SO2 capture capacity of NOHMs, the effect of SO2 on the CO2 capture capacity, and gas-sorption-induced swelling were investigated to provide important information on NOHMs as CO2 capture media and obtain insights into the packing behavior of gas molecules inside NOHMs after exposure to SO2. Both NOHMs with free amine sites as well as NOHMs deprived of task-specific functional groups were synthesized for this study. Overall, the experiments were intended as a first investigation of the effect of SO2 on CO2 capture in NOHMs rather than as a study of an optimal material for CO2 capture. Therefore, the synthesized materials were selected on the basis of the relevant comparison they allowed rather than their potential for CO2 capture. To the best of our knowledge, the effect of SO2 on the performance of NOHMs for CO2 capture had not yet been reported, and therefore, this work was necessary to provide a first insight on the impact of SO2 and lay the groundwork for future studies.

2. EXPERIMENTAL SECTION 2.1. Design and Synthesis of NOHMs. Two types of NOHMs were synthesized by tethering a polyetheramine onto a functionalized silica nanoparticle via ionic and covalent bonding, respectively. They are referred to as NOHM-I-HPE and NOHM-C-HPE, where “I” and “C” stand for ionic and covalent bonding between the functionalized core and the polymeric canopy, respectively, and “HPE” designates the polyetheramine (PE) with a high (H) content of ether groups. Figure 1 shows the schematic structures of these NOHMs. The main difference between NOHM-I-HPE and NOHM-C-HPE was that NOHM-I-HPE only contained a quaternary amine group, whereas a secondary amine group of NOHM-C-HPE was available for chemical CO2 capture (Figure 1c). Therefore, physisorption of CO2 was expected to be the main absorption mechanism in the case of NOHMI-HPE (Figure 1b). While the addition of large amounts of amine functional groups along the polymeric canopy would significantly 4168

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capacities, and discrepancies compared to the results from the highpressure cell setup can be observed. However, this technique was employed throughout our study because it is the only one enabling the simultaneous evaluation of the CO2 capture capacity, gas-sorptioninduced swelling, and CO2 packing behaviors for the synthesized NOHMs. Tests were carried out at 25 °C using a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Inc.) equipped with an ATR cell consisting of a high-pressure cell (Golden Gate supercritical fluids analyzer, Specac, Ltd.). The details of the experimental setup can be found in our prior work.18−21 FTIR spectra were again collected 16 times with a resolution of 4 cm−1, under pressures ranging from 0.13 to 0.68 MPa of pure CO2 or SO2/CO2 mixture ([SO2] = 2000 ppm). The gas-sorption-induced swelling was calculated following a method described in our prior studies.18−20,32 Briefly, the swelling percentage was calculated on the basis of the change in the CH2 stretching band of the polymeric canopy at 2850 cm−1, as expressed in eq 1

anhydrous solvents. Thermogravimetric analyses revealed that the nanoparticle fraction of the synthesized NOHMs was 15 wt %, while the remaining corresponded to the organic portion (i.e., silane + polymer). 2.2. Characterization of NOHMs. The synthesized NOHM samples were characterized using attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy at 25 °C (Nicolet 6700 spectrometer, Thermo Fisher Scientific, Inc.). Spectra were collected 16 times with a resolution of 4 cm−1, and they were used to confirm the reaction between the primary amine groups of the polymer chains and the functional groups of the modified nanoparticle cores (i.e., ionic bonding between the sulfonate and ammonium groups for NOHM-I-HPE and covalent bonding between the glycidyl ether and amine groups for NOHM-C-HPE).21 2.3. Gas Sorption in NOHMs. A high-pressure dual-chamber reactor equipped with a temperature control was built to quantify the capture capacity of various gases in NOHMs (Figure 2). The internal volume of each chamber was 15 mL, and a sample holder was designed to hold about 0.15 g of a thin layer of a NOHM sample. Each chamber was mounted with a high-accuracy pressure transducer (Omega, Inc.). First, the loaded sample holder was placed into one of the chambers. Before each sorption measurement, any air or gas in the loaded reactor was removed by applying vacuum. The pressure was monitored and recorded by the pressure transducers and a data acquisition system (DaqView). A selected gas or mixture of gases (N2, SO2 in N2, or pure CO2) was then introduced into the first chamber at a desired pressure, and then the valve between the two chambers was opened to introduce the gas into the chamber containing the sample. All gases and gas mixtures were ultrapure-grade. Once the gas sorption in NOHMs reached equilibrium, the capture capacity was calculated on the basis of the pressure decay using the ideal gas law. For these gas sorption tests, the concentration of SO2 in the SO2/ N2 mixture was set within the typical to maximum range of SO2 in flue gas (i.e., 200 or 3010 ppm). For the tests with N2 and SO2/N2 mixture with [SO2] = 200 ppm, the total initial pressure of the reactor system was set at 0.2 MPa. In the case of the SO2/N2 mixture with [SO2] = 3010 ppm, the total initial pressure was 0.4 MPa. To investigate the effect of SO2 on the CO2 capture capacity, the NOHM samples were first exposed to a SO2/N2 mixture ([SO2] = 3010 ppm and Ptotal = 0.2 MPa), and then the CO2 capture capacity was determined by exposing the SO2-loaded NOHM samples to pure CO2 (PCO2 = 0.3 MPa). The results from this study were compared to the CO2 capture capacity of fresh NOHMs. In this case, the NOHM samples were first exposed to pure N2 (PN2 = 0.2 MPa) and subsequently to CO2 (PCO2 = 0.3 MPa). Although the SO2 concentrations tested herein reflected well those encountered in flue gases, the pressure range employed was higher than that commonly found in power plants. The higher pressures were used to better highlight the impact of SO2, derive the sorption mechanisms, and better visualize the difference between sorption in NOHM-I-HPE and sorption in NOHM-C-HPE. Lower pressures, although more realistic, might have led to inconclusive trends, owing to the low SO2 sorption. For each NOHM sample, thermogravimetric analyses (TGAs) and Raman spectroscopy were used to investigate the effect of SO2 on the structure of NOHMs. The TGA runs were conducted using a thermal analyzer (TA Q50, TA Instruments, Inc.) under an oxygen environment (flow rate of 40 mL min−1) from 15 to 600 °C with a temperature ramping rate of 5 °C min−1. Tests were conducted on NOHMs before and after exposure to the SO2/N2 mixture gas ([SO2] = 3010 ppm, Ptotal = 0.4 MPa, and T = 25 °C). Raman spectra were collected at room temperature using a LabRAM ARAMIS spectrometer (Horiba JobinYvon) equipped with a microscope and a 50× objective. A solid-state frequency-doubled YAG532 nm laser and 1200 g mm−1 grating were used for this study. The exposure time was set at 20 s, and five scans were collected for each sample to improve the signal-to-noise ratio. 2.4. Investigation of Gas-Sorption-Induced Swelling and CO2 Packing Behaviors in NOHMs. ATR FTIR spectroscopy is generally not the most accurate method of quantifying the gas sorption

⎛ A0 d ⎞ e S (%) = ⎜ − 1⎟ × 100 0 ⎝ A de ⎠ 0

(1)

de0,

A, and de are the absorbance of the CH2 band and the where A , effective path lengths without and with CO2 (or CO2/SO2), respectively. The equation was simplified assuming that the effective thickness does not change with gas absorption. Thus, the swelling percentage, S (%), was calculated only depending upon the change in the FTIR absorbance of the CH2 band during CO2 sorption. Next, the CO2 capture capacity (mCO2) was calculated, taking into account the change in the CO2 asymmetric stretching band of the canopy at 2340 cm−1 and the swelling at a given pressure, as expressed in eq 218−20,32,33 mCO2 (mmol g −1) =

CCO2 (g cm−3) ρM (1 + S)

× 1000 (2)

where S, ρ, M, and CCO2 correspond to the gas-sorption-induced swelling, the density of NOHMs, the CO2 molecular weight, and the CO2 concentration in NOHMs, respectively. The CO2 concentration in NOHMs was estimated using the Beer−Lambert law

A = εCCO2de

(3)

where A is the CO2 band absorbance, ε is the molar absorptivity, CCO2 is the CO2 concentration in NOHMs, and de is the effective path length. The literature value of 1.0 × 106 cm2/mol was used for the molar absorptivity of CO2.34 Here, the effective path length was used in place of the regular infrared path length because the experiments were conducted using the ATR FTIR setup instead of using a transmission mode. As described in the literature,35 de can be calculated as the arithmetical mean between the effective path length for perpendicular (de⊥) and parallel (de∥) polarization. This calculation involved the density of NOHMs, the refractive index (nD = 1.54, determined by a Rudolph Research analytical refractometer), and the angle of incidence of the light (θ = 43°). Throughout the study, the CO2 capture capacity was calculated and reported in millimoles of CO2 per gram of solvent, where the solvent refers to only the organic fraction of the NOHMs. This allowed for direct comparison between the NOHMs and their parent polymers.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized NOHMs. Synthesized NOHM-I-HPE and NOHM-C-HPE samples were characterized using FTIR spectroscopy to confirm the attachment of the polymer chains onto the functionalized silica nanoparticles. The obtained FTIR spectra were the same as those reported in our prior work verifying the ionic or covalent bonding of polymers onto the inorganic cores.21 In the case of NOHM-I-HPE, the grafting process involved an acid−base 4169

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HPE. Slight SO2 absorption was observed for the N2/SO2 mixture containing 200 ppm of SO2, while the extreme SO2 concentration (3010 ppm) resulted in significant sorption of SO2 in NOHM-I-HPE (0.1 mmol of SO2/gsolvent). When these sorption capacity values were translated to the percentage captures, they represented 32 and 53% of the initially introduced amounts of SO2 for 200 and 3010 ppm cases, respectively. As expected, a higher SO2 concentration led to a higher driving force for the SO2 dissolution into NOHMs. The increase from 32 to 53% with an increased SO2 concentration indicates that the SO2 sorption in NOHMs was not limited by the amount of available NOHMs. These results suggest that, even without the task-specific functional groups, SO2 loading in NOHMs can be significant, particularly at high SO2 concentrations. Because the absorption of both CO2 and SO2 in NOHM-I-HPE mainly occurs via physisorption and weak chemisorption, the kinetics and thermodynamics of SO2 and CO2 absorption are likely similar, and therefore, the release of these two species would be achieved under similar regeneration conditions. If any, the presence of residual SO2 in NOHMs could affect the CO2 capture behaviors of these materials, and therefore, this aspect is further discussed in the next section. 3.3. Effect of SO2 Exposure on the CO2 Capture Capacity of NOHMs. Prior to investigating the effect of SO2 on CO2 capture, the effect of N2 was first evaluated. Indeed, because different SO2 concentrations were prepared by diluting SO2 with N2, it was essential to examine whether N2 also competed with CO2 in the CO2 absorption process. Thus, both NOHM-I-HPE and NOHM-C-HPE were first exposed to N2, and then CO2 was introduced to test for their CO2 capture capacity. The results were found to be identical to the CO2 capture capacity of these two NOHMs without pre-exposure to N2, indicating no absorption competition between CO2 and N2 in NOHMs. These results confirmed the findings of a prior study,21 demonstrating the selectivity of NOHMs toward CO2 compared to N2, O2, and N2O. The impact of exposure of NOHMs to SO2 prior to CO2 capture was then investigated, especially for NOHM-I-HPE. Because NOHM-I-HPE was synthesized via the ionic bonding between a primary amine and a sulfonic group, the exposure to SO2 may cause the dissociation of the ionic bonding as a result of a competition between SO2 and the sulfonic group to react with the primary amine of the polyetheramine.36 If this is the case, the polymeric canopy will no longer be anchored to the nanoparticles, resulting in the dissociation and segregation of the polymer chains from the core nanoparticles. As discussed in a prior study, the frustrated structure of the tethered polymer chains provides enhanced CO2 capture in NOHMs compared to the unbound polymers, owing to the decreased entropy and the creation of favorable pathways for CO2 sorption.18,20 Therefore, it was important to verify the grafting of polymers for each NOHM sample before and after exposure to SO2. As shown in Figure 4, NOHM-I-HPE and NOHM-C-HPE that were previously exposed to SO2 showed lower CO2 capture capacities than the fresh NOHM samples. This phenomenon was more pronounced in the case of NOHMC-HPE, which possesses functional groups, i.e., hydroxyl and primary amine groups, that can react with CO2 and SO2. As shown by Heldebrant et al.,37 the reactions of SO2 and CO2 with these two groups may occur. It is suspected that the reaction with the secondary amine group may be dominant, owing to its greater basicity/nucleophilicity. Regardless of the

reaction between the primary amine group of the polyetheramine and the sulfonic group of the surface-functionalized nanoparticle. Thus, in the FTIR spectrum of NOHM-I-HPE, the presence of NH3+ was observed via the bands at 1530 cm−1 [δs(NH3+)] and 1630 cm−1 [δa(NH3+)]. This confirmed the successful protonation of the amine by the sulfonic group. In the case of the synthesis of NOHM-C-HPE, the grafting of the polymeric canopy was accomplished via a ring opening of the glycidyl ether group of the surface-functionalized nanoparticles as a result of its reaction with the primary amine of the polyetheramine. In the FTIR spectrum of NOHM-C-HPE, the formation of the polyether−silane was confirmed by the disappearance of a band at 3056 cm−1 (vC−H epoxy) and the appearance of hydroxyl group at 3400−3600 cm−1.21 3.2. Sorption of N2 and SO2 in NOHMs. Both CO2 and SO2 are acidic gases, and thus, it is expected that solvents that are designed to capture CO2 may capture SO2 as well. In fact, SO2, being a stronger acidic gas, may react faster with taskspecific functional groups that target CO2 (i.e., amine groups). This phenomenon has been observed by many CO2 capture solvents, including MEA, and it is one of the major pathways for the solvent degradation. Therefore, the interaction of NOHMs with SO2 was investigated with an emphasis on its effect on the CO2 capture behavior of NOHMs. Prior to investigating the competitive sorption of SO2 and CO2 in NOHMs, it was important to independently examine the solubility of SO2 in NOHMs. In general, the concentration of SO2 in the flue gas stream is in the range of 180−250 ppm, but in some cases, it can exceed 2000 ppm, depending upon types and sources of combusted fossil fuels.26−28 The major components of the flue gas stream include N2 and CO2.26 Therefore, in this study, SO2 sorption in NOHMs was investigated by exposing the NOHM samples to N2/SO2 mixtures with SO2 concentrations of 200 and 3010 ppm to mimic the average and extreme SO2 concentrations in a flue gas stream of a coal-fired power plant, respectively. N2 sorption in NOHMs was first evaluated to serve as a baseline for subsequent SO2 and CO2 sorption experiments. Figure 3 shows the sorption kinetics of N2 and N2/SO2 mixtures in NOHM-I-HPE for the period of 90 min. As expected, the sorption of N2 was below the detection limit of the pressure transducer, indicating negligible N2 solubility in NOHM-I-

Figure 3. Gas sorption capacities of N2 (Ptotal = 0.2 MPa), SO2/N2 ([SO2] = 200 ppm and Ptotal = 0.2 MPa), and SO2/N2 ([SO2] = 3010 ppm and Ptotal = 0.4 MPa) in NOHM-I-HPE at T = 25 °C. 4170

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Figure 5. TG and DTG curves of NOHM-I-HPE before and after exposure to SO2/N2 mixture ([SO2] = 3010 ppm, Ptotal = 0.4 MPa, and T = 25 °C). The runs were performed in an oxygen environment with a heating ramping rate of 5 °C/min.

Figure 4. CO2 capture capacities of NOHM-I-HPE and NOHM-CHPE with prior exposure to pure N2 (PN2 = 0.2 MPa) or SO2/N2 mixture ([SO2] = 3010 ppm, Ptotal = 0.2 MPa, and T = 25 °C) at 25 °C. For all cases, PCO2 = 0.3 MPa and Ptotal = 0.4 MPa.

dissociated because of the SO2 exposure, the SO2-exposed NOHM-I-HPE should exhibit less conformational order than the pristine NOHM-I-HPE. The spectral indicator selected to investigate this feature was the intensity ratio of the va(CH2) band (asymmetric stretching mode, ∼2850 cm−1) to the vs(CH2) band (symmetric stretching mode, ∼2885 cm−1) in the Raman spectrum of NOHM samples.39 The intensity ratio for NOHM-I-HPE before and after exposure to SO2 was 0.8 (spectra shown in Figure 6), indicating no visible difference

type of functional group involved, it is clear that SO2 would compete with CO2 to react with the same groups, causing the reduced capture capacity in terms of CO2. The degree of capture capacity reduction would be influenced by the capture mechanisms in each solvent. In the case of NOHM-I-HPE, the main mode of CO2 capture was physisorption and weak chemisorption (i.e., with ether groups in HPE), and SO2 would also experience a similar absorption process in NOHM-I-HPE. Thus, the exposure to SO2 likely caused a competition between CO2 and SO2 for absorption sites in NOHM-I-HPE and resulted in slightly reduced CO2 capture capacity. On the other hand, in the case of NOHM-C-HPE, the amine sites likely favored the reaction with SO2 compared to CO2. Therefore, the amine sites were occupied by SO2, and this led to a significant CO2 capture reduction after exposure to SO2. However, the overall CO2 capture capacity of NOHM-C-HPE was still slightly greater and comparable to that of NOHM-I-HPE because the sites of physisorption and weak chemisorption were still available for CO2 capture. In other words, considering the low ratio of SO2/ CO2 often found in coal-fired power plants, the accumulation of SO2 in NOHMs can be kept minimal if the main capture mode is physisorption or weak chemisorption. Another possibility for the CO2 capacity reduction in NOHM-I-HPE is the dissociation of the polymer, resulting in randomly oriented polymer chains, which is less favorable for CO2 capture. Because tethered polymer chains of NOHMs exhibit a higher thermal stability than unbound polymer chains,21 TGA was used to examine the possibility of the dissociation of the polymeric canopy of NOHMs after exposure to SO2. Figure 5 shows the thermogravimetric (TG) curves and their derivatives (DTG) to verify the thermal stability of NOHM-I-HPE before and after SO2 exposure. On the basis of the thermal analysis, it was found that there was no noticeable dissociation of the polymeric canopy caused by SO2. To further support this result, another approach to probe the conformational order of polymer chains in NOHM-I-HPE was employed. As discussed in prior studies, unbound polymer chains exhibit decreased conformational order compared to tethered polymer chains, and this feature can be verified using Raman spectroscopy.18,38,39 If the canopy of NOHM-I-HPE

Figure 6. Raman spectra of NOHM-I-HPE before and after exposure to SO2/N2 mixture ([SO2] = 3010 ppm and Ptotal = 0.4 MPa) at 25 °C.

between the two cases. Therefore, the possibility of canopy dissociation in NOHM-I-HPE because of SO2 absorption was ruled out. The reduction of CO2 capture capacity in NOHM-IHPE after SO2 exposure was thus likely caused by a decrease in the number of available absorption sites in NOHM-I-HPE as a result of prior loading of SO2. While there are no strong enthalpic sites for SO2 capture (e.g., amine groups) in NOHMI-HPE, it is envisioned that, similar to CO2, SO2 could react with the Lewis basic ether groups along the polymer chains of NOHMs. The formation of the Lewis acid−base complex between SO2 and organic basic groups, such as carbonyl groups, was previously evidenced by computational studies.40 As discussed earlier, in the case of NOHM-C-HPE, CO2 capture capacity decreased by about 30% after exposure to SO2. Because NOHM-C-HPE was synthesized via covalent bonding, 4171

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Figure 7. Gas-sorption-induced swelling as a function of the CO2 capture capacity in the absence or presence of SO2 ([SO2]in CO2 = 2000 ppm) at 25 °C: (a) NOHM-I-HPE and (b) NOHM-C-HPE. The total pressures for each data point along the lines were 0, 0.13, 0.26, 0.39, 0.55, and 0.68 MPa from the left to the right.

Figure 8. Area (A) ratios of in-plane (v2,in‑plane) to out-of-plane (v2,out‑of‑plane) bending modes of CO2 absorbed in (a) NOHM-I-HPE and (b) NOHM-C-HPE, at 25 °C, in the absence or presence of SO2 ([SO2]in CO2 = 2000 ppm). The total pressure range was 0.13−0.68 MPa.

the release of the two gases after their absorption should proceed under similar conditions. Considering the results of prior studies,21,41 the regeneration of NOHM-I-HPE could be achieved via pressure swing with no CO2 capture capacity loss (over 10 cycles). In the case of NOHM-C-HPE, heating at about 105−120 °C would be necessary to release the chemisorbed species and fully recycle the materials. Moreoever, thermal stability tests performed at 120 °C showed that NOHMs were stable over 100 thermal cycles. 3.4. Effect of SO2 on Gas-Sorption-Induced Swelling and CO2 Packing Behaviors in NOHMs. Owing to their liquid-like property, NOHMs exhibit a volume change (i.e., swelling) during the absorption of gas molecules, such as CO2.18−20 As discussed in our prior study, the degree of swelling induced by gas sorption in NOHMs provides interesting information on how small gas molecules are adsorbed by NOHMs.18−20 To investigate the effect of SO2 on the gas-sorption-induced swelling, NOHM-I-HPE and NOHM-C-HPE were exposed to a mixture of SO2/CO2 ([SO2] = 2000 ppm) at different pressures (Ptotal = 0.13−

the dissociation of the polymer chains from the surfacefunctionalized nanoparticle could be ruled out. Thus, the decrease in CO2 capture capacity in NOHM-C-HPE after SO2 exposure was most likely due to the unavailability of the secondary amines in NOHM-C-HPE, which would have previously reacted with SO2,36 thereby decreasing the overall CO2 capture capacity in NOHM-C-HPE. As observed in Figure 4, the CO2 capture capacities of NOHM-I-HPE and NOHM-C-HPE remain rather low ( 2000 ppm and Ptot = 0.4 MPa, up to 0.10 mmol of SO2/g of material was absorbed. As a result, a 6−25% decrease in the CO2 capture capacity of NOHMs was observed depending upon the type of NOHMs (i.e., without or with amine sites). This competitive absorption between SO2 and CO2 for the same absorption sites seems to result in minor changes of gas-sorption-induced swelling behavior and CO2 packing behavior. Indeed, the change in the extent of swelling caused by SO2 sorption was less than 1%. It was also confirmed that the exposure of SO2 did not damage the ionic bonding between the amine and sulfonate groups in NOHM-I-HPE and the material remained stable up to about 300 °C.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-212-854-8989. Fax: +1-212-854-7081. E-mail: [email protected]. Present Address †

Kun-Yi Andrew Lin: Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang Road, South District, Taichung City 402, Taiwan, Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication was based on work supported by Award KUSC1-018-02, made by King Abdullah University of Science and Technology (KAUST). The authors are grateful to Dr. Youngjune Park for his help with the deconvolution of FTIR spectra.

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