Aminopolymer-Impregnated Hierarchical Silica Structures

Article Cite This: Chem. Mater. 2019, 31, 5229−5237

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Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO2 Uptake under Simulated Air Capture and Flue Gas Capture Conditions Hyuk Taek Kwon,†,‡ Miles A. Sakwa-Novak,§ Simon H. Pang,† Achintya R. Sujan,† Eric W. Ping,§ and Christopher W. Jones*,†

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†

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States ‡ Department of Chemical Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 48513, Republic of Korea § Global Thermostat LLC, 311 Ferst Drive, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Poly(ethyleneimine)-impregnated sorbents are prepared using a hierarchical silica support with bimodal meso-/macroporosity. The sorbents behave unexpectedly during CO2 adsorption from simulated air and flue gases (400 ppm and 10% CO2) at a fixed temperature, as compared to systems built on commonly studied mesoporous materials. The results demonstrate that (i) impregnation methods influence the efficacy of sorption performance and (ii) the sorbents show almost similar uptake capacities under 400 ppm and 10% dry CO2 at 30 °C, exhibiting step-like CO2 adsorption isotherms. These unusual observations are rationalized via control experiments and a hypothesized sorption mechanism. While the sorption performance near room temperature is unexpectedly identical under 400 ppm and 10% CO2 conditions, there is an optimal temperature at each gas concentration where the uptake is maximized. The maximum sorption capacities are 2.6 and 4.1 mmol CO2/g sorbent at the optimized sorption temperatures using 400 ppm and 10% dry CO2, respectively. The presence of water vapor under 400 ppm CO2 conditions further improves the sorption capacity to 3.4 mmol/g sorbent, which is the highest capacity under direct air capture conditions among known amine sorbents impregnated with a similar polymer, to the best of our knowledge.

1. INTRODUCTION Carbon dioxide is one of the heat-trapping gases that is considered to be a key cause of global climate change.1,2 Thus, there have been significant efforts to slow the rapid increase in atmospheric CO 2 concentration by capturing it from anthropogenic sources3,4 and atmospheric air directly (direct air capture, DAC).5,6 An absorption-based amine scrubbing process is currently the most mature large-scale CO2 capture technology.7 However, the technique has disadvantages such as low energy efficiency (e.g., high regeneration cost), low sorbent stabilities (e.g., evaporation, and thermal/oxidative degradation), equipment corrosion, and need for large absorber volumes in the most well-studied postcombustion CO2 capture applications.8 As a result, researchers have sought more energy-efficient processes for large-scale CO2 separations for many years. Processes employing solid material-based adsorptive separations are one of the alternatives. Among many solid sorbents tested for CO2 capture,3,9 amine-based sorbents are among the most intensively studied.10−18 The amine-based sorbents, which are prepared by introducing amines into porous support materials, have © 2019 American Chemical Society

been previously classified into three categories based on how amines are confined inside porous supports.11,19 While amines are covalently tethered to pore walls in class 2 and class 3 sorbents, class 1 sorbents contain amines inside pores contained only through physical interactions with the support surface. Although the class 1 sorbents possess an intrinsic physical stability issue (e.g., potential for amine leaching) that must be overcome, key advantages such as easy preparation and high amine loading compared to other classes make this class a promising candidate for commercial-scale applications. A wide variety of porous materials (e.g., silica,20 alumina,21,22 carbon,23,24 and metal organic frameworks25−27) have been used as substrates for class 1 sorbents. Among these, the mesoporous silica materials have been most commonly employed primarily because of relatively easy control of their structural and surface properties, making them ideal model structures to construct substrate structure-CO2 capture Received: April 14, 2019 Revised: June 18, 2019 Published: June 18, 2019 5229

DOI: 10.1021/acs.chemmater.9b01474 Chem. Mater. 2019, 31, 5229−5237

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Chemistry of Materials performance correlations.20 However, current mesoporous material-based class 1 amine sorbents present several limitations. First, limited mesopore space puts a restriction on amine loading and in turn, adsorption capacity. As amines fill the substrate pores, gas transport to sorption sites can become restricted, leading to diffusional limitations that impede CO2 uptake capacities and kinetics. In particular, the small mesochannels with a lack of connectivity can easily become clogged and filled with impregnated amines. Additionally, the sorbents are typically evaluated in powder form, which can potentially create handling issues and huge pressure drops in sorption columns, which can prevent practical deployment on a commercial scale. However, the use of structured contactors offers many advantages for gas−solid processing with low-pressure drops,28,29 and materials that can be engineered into such contactors offer key advantages. To these ends, new materials innovations can offer potential to overcome some or all of these challenges. Hierarchical materials with bimodal pore systems (e.g., meso-/macropore) are one of the potential candidate classes of materials that could address these issues. They combine the benefits of each pore size regime, providing enhanced site accessibility by facilitated mass transfer through three-dimensionally connected macropores.30 Furthermore, a significant macropore volume can provide extra pore space for hosting higher loadings of amines. Despite these potential advantages, there are only a few reports employing meso-/macropore hierarchical structures as substrates for amine sorbents (e.g., class 1,31−34 class 2,35 and class 336). Moreover, among those reports, the class 1, amine-impregnated sorbents were primarily tested at CO2 concentrations (e.g., 5 and 100%) not directly relevant to the two largest scale applications, coal-fired postcombustion CO2 capture (10−15%) and DAC (400 ppm). Here, we synthesize a hierarchical silica structure by a dual template method and examine its potential as a substrate for amine-impregnated, class 1 sorbents by evaluating their CO2 sorption characteristics under simulated air and flue gas conditions. The effect of key parameters (e.g., impregnation method, aminopolymer loading, sorbent size, sorption temperature, and humidity) on sorption performance is explored in detail to understand the unusual adsorption behavior of the new composite materials, which have exceptionally high CO2 uptake capacities. The results demonstrate that supports with hierarchical meso-/macroporosity offer significant advantages for practical extraction of CO2 from ambient air.

Figure 1. (a) Schematic illustration of a hierarchical silica structure synthesis, (b) SEM image, (c) transmission electron microscopy (TEM) image, (d) magnified TEM image of (c), and (e) table of meso-/macropore porosity of the hierarchical structure. The mesoporosity was characterized by a N2 physisorption. The macropore size and volume were obtained from a TEM image analysis and Hg porosimetry, respectively. The macropore surface area was back-calculated by considering the macropore size and volume.

capture leads to improved sorption performance via enhancing sorption kinetics.36,37 The textural properties of the H-SiO2 were further characterized by N2 physisorption and Hg porosimetry measurements. Type IV nitrogen isotherm curves (Figure S2) with a hysteresis loop in combination with X-ray diffraction (XRD) patterns (Figure S3) confirmed the ordered mesoporous nature of the silica walls. The estimated mesopore volume and size from the N2 physisorption are ∼0.5 mL/g and ∼4.9 nm (Figure 1e), which are in fairly good agreement with the image analysis (Figure 1d). A Hg intrusion profile revealed that the macropore volume amounts to 4.1 mL/g (Figures 1e and S4), which is significantly higher than that of typical mesoporous substrate materials (e.g., SBA-15, ∼1 mL/g).37 Such a high pore volume is expected to be beneficial for composite materials where accommodating guest species is limited by their pore space. Furthermore, the H-SiO2 exhibits a macropore size distribution ranging from 70 to 700 nm (Figure S4). Given that Hg porosimetry measures the size of pore entrance rather than pore maximum diameter,38 the distribution of macropore sizes could originate from the connecting windows and/or structural imperfections (e.g., local nonspherical voids and cracks) created between the macropores. It is likely that the forced template assembly by centrifugation yields inhomogeneity in macropore template packing, leading the structural imperfections. Motivated by the described structural features of the H-SiO2 (e.g., three-dimensional connectivity and exceptionally high pore volume), we tested its potential as a substrate for class 1 amine-impregnated sorbents for CO2 capture from dilute and ultradilute gases. It is noteworthy that unlike most of the literature whereby the substrates are deployed as fine powders, the materials deployed here had particle sizes of 500−850 μm, unless stated otherwise, which is more practical from the application point of view, as less pressure drop and fewer handling issues might be expected.

2. RESULTS AND DISCUSSION 2.1. Textural Properties. A hierarchical meso-/macroporous silica structure (hereafter, H-SiO2) was prepared using P123 and poly(styrene) beads (ca. 1 μm diameter, Figure S1) as sacrificial templates for the meso- and macropores, respectively. Briefly, a homogeneous mixture of the templates and a silica source (TEOS) in ethanol was concentrated by natural drying under stirring, followed by centrifugation for template assembly. Then, the precipitate was dried and calcined to remove the templates as illustrated in Figure 1a (see the experimental section for details). The resulting structure possessed spherical macropores (∼750 nm, Figure 1b,c) separated by silica walls with monodisperse, relatively ordered mesochannels (∼5 nm, Figure 1d). In addition, the presence of windows on the silica walls enabled threedimensional interconnection between the macropores. Typically, three-dimensional connectivity in solid sorbents for CO2 5230


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Chemistry of Materials 2.2. Effect of Impregnation Methods. Early in the work, it was determined that specific amine impregnation methods significantly influence the sorption performance of aminopolymer-impregnated H-SiO2 materials (hereafter, PEI_HSiO2), with the most commonly employed approach in the literature leading to less effective sorbents than an alternate method favored here. Figures 2 and S5 display comparisons of the sorption performances, which were collected in the presence of 400 ppm dry CO2 at 30 °C, using the PEI_H-

SiO2 materials prepared by two different impregnation methods. The samples made using the first method, R_LPEI_HV, employ the conventional method previously used by our group39−41 and others13,24,42 in which bare substrates are immersed in a diluted aminopolymer solution, and then, the solvent is removed by rotovap, followed by drying under high vacuum (13 mTorr). The R, L-PEI, and HV sample designations stand for rotavapping, low concentration PEI, and high vacuum drying, respectively. The second method is S_H-PEI_LV in which bare substrates are submerged in a concentrated aminopolymer solution, equilibrated overnight, and then decanted, followed by drying in an oven under low vacuum (25 × 103 mTorr). S, H-PEI, and LV sample designations stand for submerging, high concentration PEI, and low vacuum drying, respectively. For clarification, the R_L-PEI_HV method used diluted aminopolymer solutions to achieve target aminopolymer loadings, in which all of the aminopolymers in the solution are impregnated into pores during rotavapping. On the other hand, the S_H-PEI_LV method used highly concentrated aminopolymer solutions in which only some of the aminopolymer infiltrated into pores during the submerging step, which contributes to final loadings. As shown in Figures 2 and S5, the latter method produced more effective sorbents than the former at given aminopolymer loadings, with improved sorption capacities, amine efficiencies, and uptake kinetics. Furthermore, the S_HPEI_LV method produced a volcano amine efficiency (hereafter, AE) curve, which is typically observed in the oftstudied supported class 1 amine sorbents based on mesoporous supports,17,31,32,41,43 while the R_L-PEI_HV method generated an AE curve with an increasing trend with two different slopes depending on the aminopolymer loadings. We implemented control experiments to better understand these observations. The first control sample was prepared by rotavapping and drying under the low vacuum (control 1, R_L-PEI_LV), which only differs in vacuum level for drying as compared to the R_L-PEI_HV method. This control sample performed comparably with the one from the S_H-PEI_LV method at a similar amine loading, implying that the vacuum level used for drying caused the observed deviation for the improved S_H-PEI_LV samples. It is worth noting that further increasing the vacuum pressure for drying (control 2, R_LPEI_Ar, rotavapping + drying under atmospheric pressure in the presence of Ar) did not yield any additional enhancement. Also, when a sorbent was obtained from the S_H-PEI_LV method except with drying under the high vacuum (control 3, S_H-PEI_HV), the sorbent underperformed relative to the one expected from the loading dependency curve of the PEI_H-SiO2s prepared by the S_H-PEI_LV method. Based on these controls, we surmised that aminopolymer is mobile in pores of the PEI_H-SiO2 materials under the high vacuum, leading to unfavorable aminopolymer distributions (e.g., window blockages and uneven aminopolymer film coatings on the walls) resulting in the reduced sorption performance. Because the vacuum pressures used for drying barely influenced the sorption performance of a traditional mesoporous substrate (SBA-15)-based sorbent (Figure S6), the aminopolymer migration likely occurs preferentially in macropores. This is likely related to the relatively smaller amount of macropore surface area, limiting polymer interaction with the macropore walls, as compared to the purely mesoporous counterpart (Figure 1e). For further CO2 sorption tests and

Figure 2. Comparison of aminopolymer loading dependencies of (a) CO2 adsorption capacities and (b) amine efficiencies of PEI_H-SiO2 prepared by two different impregnation methods; (c) CO2 uptake curves of the PEI_H-SiO2 at a similar loading (2.51−2.62 g PEI/g SiO2) prepared by two different impregnation methods. The measurements were implemented at 30 °C under 400 ppm dry CO2 using TGA. S, R, H-PEI, L-PEI, LV, HV, and Ar designations stand for submersing, rotavapping, high concentration PEI, low concentration PEI, low vacuum drying, high vacuum drying, and drying under Ar. 5231


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Figure 3. Comparisons of (a) CO2 adsorption capacities and (b) amine efficiencies of PEI_H-SiO2s in the presence of 400 ppm and 10% dry CO2, respectively; (c) CO2 adsorption isotherms of PEI_H-SiO2s at three different aminopolymer loadings; comparisons of normalized CO2 uptake profiles of PEI_H-SiO2s collected in the presence of 400 ppm and 10% dry CO2 at specific aminopolymer loadings of (d) ∼0.7, (e) ∼1.6, and (f) ∼3.3 g PEI/g SiO2. All measurements were implemented at 30 °C.

Under the assumptions that the impregnated aminopolymer forms a uniform film and the substrate structure is homogeneous, the film thicknesses coated on the macropore pore walls were estimated by considering the textural properties of the H-SiO2 and the aminopolymer loadings (Figure 3 and Table S1). The film thicknesses were estimated to be in the range of 6−160 nm. The thicknesses were mostly larger than the pore sizes of mesoporous substrates frequently used for class 1, amine-impregnated sorbents (e.g., SBA-15, ∼10 nm)37 owing to low macropore surface area. From this analysis, it appears that ∼40 nm is a critical PEI film thickness where the maxima appear in the AE curves. Surface scanning electron microscopy (SEM) images of the fractured sorbents showed fairly smooth surfaces with a trace of aggregation (Figure S9). In the future, detailed three-dimensional imaging of the aminopolymer deposits on the macropore walls by techniques such as X-ray tomography may prove useful in directly determining aminopolymer domain sizes. One highly unexpected observation from these gravimetric adsorption experiments was that the sorbents performed similarly in total capacity and AE for a given sorption time and aminopolymer loading at 30 °C when employing substantially different CO2 concentrations that differed by a factor of 250 (400 ppm vs 10%; Figure 3a,b). The CO2

structural characterizations, PEI_H-SiO2s prepared by the S_H-PEI_LV method were used. 2.3. Effect of Sorbate Concentration: 400 ppm Versus 10% CO2. Figures 3 and S7 exhibit comparisons of CO2 sorption performance of PEI_H-SiO2 materials with varying aminopolymer loadings in the presence of simulated dry air (400 ppm CO2) and simulated dry flue gas (10% CO2) at 30 °C. A few key observations can be made. First, there are maxima in the AE curves (Figure 3b). In general, volcanoshaped AE curves have been ascribed to kinetic limitations that are encountered as aminopolymer loadings increase. The kinetic limitations are believed to be the consequence of pore blockages by impregnated aminopolymer or bulky aminopolymer deposits on pore walls,41 which are difficult to distinguish because of the lack of effective imaging techniques for polymers in porous structures.41 However, because substrates of 500−850 μm size were employed in this study, being large enough to break into smaller pieces, one could indirectly test which effect likely contributes dominantly by comparing sorption performance of different-sized samples. As shown in Figure S8, two different-sized sorbents (125−300 μm vs 500−850 μm) performed very similarly, indicative of the fact that it is likely not pore (window) blockages but rather bulky aminopolymer deposits that cause the volcano curve. 5232


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Figure 4. CO2 uptake capacities at (a) 400 ppm and (b) 10% dry CO2 as a function of adsorption temperature. The PEI_H-SiO2 at the aminopolymer loading of 2.6 g PEI/g sorbent, which showed the maximum uptake capacity in the unit of mmol CO2/g sorbent at 30 °C, was used for the measurements; comparisons of aminopolymer loading dependency curves of amine efficiencies collected at two different temperatures at (c) 400 ppm and (d) 10% dry CO2.

adsorption sites deeper within the aminopolymer film, resulting in a reduction of the uptake rate. Based on this, it is likely that a majority of CO2 adsorption takes place mainly at the interface between the gas phase and aminopolymer film when using 10% CO2. On the other hand, the lower driving force for adsorption at 400 ppm CO2 would result in slower uptake and cross-linking of the aminopolymer surface, allowing CO2 to diffuse relatively deeper into the aminopolymer film and adsorb until nearer to full saturation. This is reflected in the shape of the adsorption curves at 400 ppm CO2 with slow but maintained initial adsorption rates before reaching 70− 80% of the total uptake capacities. Hypothetical cross-linking density maps for these two different CO2 concentrations (400 ppm vs 10%) at saturation are schematically illustrated in Figure S11 in which 400 ppm CO2 produces deep penetration with thin high-density surface barrier, whereas 10% CO2 causes relatively shallow penetration with thick high-density surface barrier. We suggest that the more rapid rate of formation of a thicker dense surface barrier at 10% CO2 than at 400 ppm CO2 caused the PEI_H-SiO2 to perform more poorly than expected, leading to coincidently matched adsorption capacities for the two CO2 concentrations. The PEI_H-SiO2 behaved differently as compared to the purely mesoporous substrate-based sorbent possibly because of the relatively thin aminopolymer deposits that form in mesopores, compared to the film that lines the macropore walls in the H-SiO2. Even though a surface diffusion barrier is formed for both classes of materials, the relatively short diffusion path length for mesoporous materials causes the diffusion barrier to impact adsorption to a lesser extent than for the H-SiO2. If adsorption is kinetically limited by a barrier to diffusion, one can improve the performance by increasing the adsorption temperature. To start, we tested the PEI_H-SiO2 material, which showed the maximum uptake capacity in the units of mmol CO2/g sorbent at 30 °C (Figure S7), at varying

isotherms of selected samples at different loadings further confirmed this observation (Figure 3c). However, their kinetic uptake profiles under two different CO2 concentrations were evidently different (Figure 3d−f). Although the uptake curves at 10% CO2 showed faster initial uptake rates than when using 400 ppm CO2, these higher uptake rates were only maintained for a few minutes during which 30−40% of the total uptake capacities were reached, after which the rates slowed down. On the other hand, 70−80% of the total uptake capacities were achieved during the initial uptake stage at 400 ppm CO2 while showing relatively slower uptake rates than when using 10% CO2. It is typically the case that conventional mesoporous material-supported amine sorbents (class 1) perform better, both thermodynamically and kinetically, under higher sorbate concentrations, as confirmed here (Figure S10) as well as reported elsewhere.21,43,44 It is well known that the dominant CO2 capture mechanism of amine-based materials under dry conditions requires two amine sites close to each other to capture one CO2 molecule, resulting in the formation of alkylammonium carbamates.11,45 Zhang et al. reported a significant increase in viscosity of aminopolymer (e.g., poly(ethylenimine)) upon CO2 adsorption, presumably because of carbamate formation.46 We hypothesized that the formation of alkylammonium carbamate structures upon CO2 adsorption results in the formation of diffusion barriers by ionic cross-linking between aminopolymer chains at the surface. Furthermore, the formation of these cross-links occurs to different extents, depending on the CO2 concentration, during the initial uptake stage. Use of 10% CO2 leads to a higher degree of rapid uptake and cross-linking at the aminopolymer surface compared to 400 ppm CO2 due to higher driving force for adsorption, generating a significant surface diffusion barrier during the initial stage of the adsorption process. Once the CO2-induced dense surface barrier forms, it disturbs further CO2 diffusion to unused 5233


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Chemistry of Materials temperatures at 400 ppm and 10% dry CO2, respectively (Figure 4a,b). Increasing temperature commonly improved uptake capacities at both CO2 concentrations, implying that the adsorption is indeed kinetically limited. By increasing the temperature slightly, the effective diffusivity of CO2 within the aminopolymer film increases, reducing the effect of any formed surface barriers and allowing more adsorption sites to be accessed. The maximum capacities amounted to ∼2.6 mmol CO2/g sorbent at 50 °C at 400 ppm dry CO2 and ∼4.1 mmol CO2/g sorbent at 80 °C at 10% dry CO2. To the best of our knowledge, each capacity is comparable with or better than all those previously reported in the literature under dry air6,13,21,24,40,41,44,47−58 and flue gas18,21,54,57−66 capture conditions, among class 1 amine adsorbents using similar aminopolymers. Interestingly but unexpectedly, the temperature-induced capacity improvement at 400 ppm CO2 was not accompanied by enhanced adsorption kinetics (Figure S12a, with sorption rates modestly decreasing as the temperature increases, as opposed to the case at 10% CO2 (Figures S12b and S13b). The PEI_H-SiO2s with varying loadings were tested at the optimum temperatures at each CO2 concentration (50 °C for dry 400 ppm and 80 °C for dry 10% CO2), and the consequent loading dependency curves were compared with their 30 °C counterparts (Figures 4c,d, and S14). While adsorption transitioned from being thermodynamically to kinetically controlled at 400 ppm CO2 as the aminopolymer loading increased, adsorption was only kinetically controlled using 10% CO2, regardless of the aminopolymer loading (the amount of CO2 adsorbed was always higher at 80 °C). We attempted to understand this observation based on the proposed mechanism: a higher degree of surface diffusion barrier formed using 10% CO2 as compared to 400 ppm CO2 (Figure S11). The observation likely results from different relative contributions of two kinetic limiting factors (thickness of aminopolymer deposit vs surface diffusion barrier) at each CO2 concentration. Because the contribution from the cross-linked surface diffusion barrier is relatively minor at 400 ppm CO2 as compared to one at 10% CO2 (Figure S11), one might presume that the thickness of the aminopolymer deposit is what governs the degree of kinetic limitation. Therefore, when the deposits are thin (at low aminopolymer loadings), increasing temperature decreases the adsorption capacities, consistent with a thermodynamically controlled regime. As the aminopolymer film becomes thicker, adsorption becomes kinetically controlled through the aminopolymer film. On the other hand, for a rapidly forming barrier with a thick, highdensity cross-linked layer at the surface of the PEI film at 10% CO2, it is probable that this surface diffusion barrier is the dominant contributing factor to the total kinetic limitation. In this case, because the formation of the thick surface diffusion barrier is common to all adsorbents irrespective of the aminopolymer loading and aminopolymer film thickness when using 10% CO2, increasing temperature purely serves to reduce the effect of this barrier, and one can see the positive temperature effect across the entire loading range. Typically, one can expect Langmuir (type I) isotherms from chemisorbents because of strong adsorbate−adsorbent chemical interactions. Figure 5 exhibits the CO2 isotherms of the PEI_H-SiO2 sorbents with the aminopolymer loading of 2.62 g PEI/g sorbent as a function of temperature. All appear as Langmuir-type isotherms with enhanced CO2 saturation uptake as increasing adsorption temperature (Figure 5a),

Figure 5. CO2 adsorption isotherms (a) on a linear scale and (b) on a logarithmic scale of the PEI_H-SiO2 at the aminopolymer loading of 2.62 g PEI/g sorbent at three different temperatures.

indicating the adsorption is diffusion-limited. To observe the temperature dependence of the isotherms more clearly, the isotherms were replotted on a logarithmic P/Po scale, which represent the adsorption potential (ΔGads = RT ln P/Po) (Figure 5b). Step-like curves appeared. The step front systematically moved to higher pressure with the maintained step feature without loss in uptake capacity as the adsorption temperature increases, implying controllable uptake pressure by simply optimizing the adsorption temperature. Although a similar step feature was also observed with PEI-impregnated mesoporous sorbents (Figure S15a,b), the step feature was less pronounced than that for PEI_H-SiO2 because of the upward trend of CO2 uptake capacity as uptake pressure increases without reaching a plateau, which is in contrast to that of PEI_H-SiO2. This points out that the CO2 capture characteristics of class 1 materials (polyamine-impregnated materials) can be fine-tuned by varying the physical structures (meso- and macroporosity) of the supports hosting the aminopolymers. The effect of water vapor on the adsorption process was also studied. We tested the PEI_H-SiO2s that showed the maximum adsorption capacity in the units of mmol CO2/g sorbent at 30 °C from thermogravimetric analysis (TGA) measurements (Figure S7) in the presence of a 400 ppm CO2 stream with 19% relative humidity using a fixed bed setup. A comparison of the breakthrough curves between the dry and humid runs at 30 °C showed the synergistic effect of water vapor on the adsorption capacity (Figure S16 and Table S2). The total capacity increased from 2.34 mmol CO2/g sorbent to 3.36 mmol CO2/g sorbent, which is the highest CO2 uptake capacity under simulated air conditions among class 1 amine 5234


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challenging using conventional techniques, is essential to understand the observation.

sorbents impregnated with similar kinds of aminopolymers, to the best of our knowledge.6,13,24,47,52,53,55,56 In general, the copresence of water vapor in a CO2 feed stream is known to enhance the thermodynamic capacities of amine-based adsorbents via allowing access to the carbonate or bicarbonate formation mechanisms.11 Interestingly, the humid CO2 adsorption capacities decreased as the adsorption temperature increased (Table S2), in contrast to the trend under dry conditions (Figure 4a). This implies that the presence of water vapor mitigates kinetic limitations presented in the aminopolymer deposits. Fan et al. made a similar observation with poly(ethyleneimine)-impregnated hollow fiber sorbents under simulated flue gas conditions.67 They suggested several potential mechanisms whereby water vapor can reduce the kinetic limitations. One is via enhancing aminopolymer chain mobility by weakening inter- or intramolecular hydrogen bonds and dipole−dipole interactions, and the other is via preventing aminopolymer chain cross-linking due to suppressed carbamate formation in the presence of water vapor. Finally, humid cyclic tests showed that there was a gradual decrease in total uptakes and amine efficiencies, as shown in Figure 6a. A comparison with cyclic tests under dry

3. CONCLUSIONS A hierarchical meso-/macropore silica structure was utilized as a substrate for class 1 amine sorbents for CO2 capture under simulated air and flue gas conditions. Use of high vacuum during the sorbent drying step after aminopolymer impregnation negatively influenced the adsorption performance of PEI_H-SiO2 materials, which did not occur for mesoporous material-based sorbents. We presume that this is a result of the mobile nature of the aminopolymer in macropores because of the relatively low macropore surface area, as compared to mesoporous materials, causing aminopolymer distribution inhomogeneity during the high vacuum drying. Threedimensionally interconnected macropores through circular windows (100−350 nm) appeared to prevent macropore blockages, assuring full macropore accessibility, regardless of aminopolymer loadings and sorbent sizes in the ranges tested. Estimated aminopolymer film thicknesses on the macropore walls were in the range of 6−160 nm, depending on the aminopolymer loadings calculated based on a thin-film formation assumption. Maximum amine efficiencies appeared at the aminopolymer loading of 1.60 g PEI/g SiO2, which corresponds to ∼40 nm aminopolymer film thickness. Unexpectedly, PEI_H-SiO2s showed similar uptake capacities and thus amine efficiencies at 30 °C, irrespective of aminopolymer loadings, at largely different dry CO2 concentrations (400 ppm vs 10% CO2). We proposed that this might be due to a higher degree of aminopolymer surface crosslinking at 10% CO2 compared to 400 ppm CO2. Increasing temperature was generally effective to enhance the adsorption performances at both 400 ppm and 10%, implying that the adsorption process of the PEI_H-SiO2s is kinetically limited. The maximum dry capacities were 2.6 and 4.1 mmol CO2/g sorbent at each optimized temperature under 400 ppm and 10% CO2, respectively. The PEI_H-SiO2 materials generated step CO2 adsorption isotherms with steps moving to higher pressure as the adsorption temperature increased, which is generally not expected in classical amine-based sorbents. We hypothesized that unfavorable surface amine arrangements on the surface of thick aminopolymer films deposited in macropores are responsible. The presence of water vapor in a 400 ppm CO2 stream further increased the uptake capacity to 3.4 mmol CO2/g sorbent at 30 °C, which is the highest CO2 uptake capacity under simulated air capture conditions among class 1 amine sorbents impregnated with similar kinds of aminopolymers, to the best of our knowledge. While increasing temperature affected uptake capacities positively at 400 ppm CO2 under dry conditions, it was opposite under humid conditions, indicating that the presence of water vapor mitigates kinetic limitations presented in aminopolymer deposits. There was a decrease in uptake capacities during cyclic tests under humid conditions without aminopolymer leaching. This was likely due to aminopolymer arrangement in the pores in the presence of water vapor.

Figure 6. Cyclic adsorption performance at 400 ppm at 30 °C measured by a breakthrough setup under (a) humid and (b) dry conditions. The PEI_H-SiO2 at the aminopolymer loading of 2.62 g PEI/g sorbent, which showed the maximum uptake capacities in the units of mmol CO2/g sorbent at 30 °C from TGA measurements, was used for the measurements.

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conditions using a fixed bed setup (Figure 6b) indicates that the gradual reduction occurs only in the presence of water vapor in the feed stream. Because aminopolymer leaching was almost negligible after the cyclic tests (cycle 1: 67.1 wt % vs cycle 3: 66.8 wt % organic loading), aminopolymer rearrangement in the pores driven by water vapor may be a contributing factor. Further polymer morphology characterization, which is

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.9b01474. 5235


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Experimental details, SEM, N2 physisorption, XRD, Hg intrusion porosity, and CO2 uptake measurements (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christopher W. Jones: 0000-0003-3255-5791 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare the following competing financial interest(s): Jones has a financial interest in Global Thermostat, LLC. Global Thermostat funded this work, and Global Thermostat employees are co-authors.Jones has a conflict-ofinterest management plan in place at Georgia Tech.

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ACKNOWLEDGMENTS This work was funded by Global Thermostat, LLC. REFERENCES

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Aminopolymer-Impregnated Hierarchical Silica Structures

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