Rational Design of the Polymeric Amines in Solid Adsorbents for

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Rational Design of the Polymeric Amines in Solid Adsorbents for Post-Combustion Carbon Dioxide Capture Kyungmin Min, Woosung Choi, Chaehoon Kim, and Minkee Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05988 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Applied Materials & Interfaces

Rational Design of the Polymeric Amines in Solid Adsorbents for Post-Combustion Carbon Dioxide Capture Kyungmin Min,‡ Woosung Choi,‡ Chaehoon Kim, and Minkee Choi* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea KEYWORDS CO2 capture, amine-based solid adsorbent, polyethyleneimine, functionalization, regeneration heat, stability

ABSTRACT

Substantial efforts have been made to increase the CO2 working capacity of amine adsorbents for efficient CO2 capture. However, the more important metric for assessing adsorbents is the regeneration heat required for capturing a fixed amount of CO2. In this work, we synthesized PEI/SiO2 adsorbents functionalized with various epoxides. This provided adsorbents with six different amine structures showing various CO2/H2O adsorption properties. Our studies revealed that the CO2 working capacity was not a decisive factor in determining the regeneration heat required for CO2 capture. This is because the benefit of large CO2 working capacity was cancelled out by the difficulty of CO2 desorption. Instead, the suppression of H2O co-adsorption

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was critical for reducing regeneration heat, because substantial latent heat is required for H2O desorption. Consequently, the PEI/SiO2 functionalized with 1,2-epoxybutane required much lower regeneration heat (2.66 GJ tCO2-1) than the conventional PEI/SiO2 (4.03 GJ tCO2-1), because of suppressed H2O co-adsorption as well as moderately high CO2 working capacity.

INTRODUCTION The atmospheric concentration of CO2 has exceeded 400 ppm and is expected to reach 550 ppm by 2050.1 This calls for the urgent development of carbon capture and storage (CCS) technologies.2,3 Among various CO2 capture strategies, post-combustion CO2 capture from large point emission sources has attracted extensive scientific attention due to the possibility of retrofitting into existing power plants and industrial plants.2 Chemisorption of CO2 using aqueous solutions of amines such as monoethanolamine (MEA) is currently considered to be a benchmark technology.4 However, this too has inherent limitations such as volatile amine loss, reactor corrosion, and large regeneration heat due to the high heat capacity and vaporization heat of water.2,5,6 To overcome these limitations, solid adsorbents have been proposed as promising alternatives.2,7–9 Among various solid adsorbents, amine-containing porous solids have been most widely investigated due to their large CO2 capacity, high CO2 selectivity over N2, and ease of material preparation.2,7–9 Such adsorbents can be prepared by heterogenization of amines in porous solids via impregnation of polymeric amines such as polyethyleneimine (PEI),10–23 surface-grafting of aminosilanes,13–15,24–28 and polymerization of amine monomers within the support pores.15,28–30 Due to strong CO2 chemisorption on amines and significant electrical energy required for compressing or evacuating a large volume of flue gas, temperature swing adsorption (TSA) has


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generally been proposed as a more suitable process than pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) for amine-containing adsorbents.7,8,31–33 To produce high purity CO2 suitable for further compression (>95%),34,35 the most straightforward TSA condition is to desorb CO2 from saturated adsorbents under a CO2-rich atmosphere primarily using a thermal driving force (i.e., the separated CO2 can be recycled as a purge gas) with13,16,28 and without the aid of steam.18,26,27,36–38 However, many of previous studies measured the CO2 working capacities of adsorbents under impractical adsorbent regeneration conditions that combined an inert gas purge (e.g., N2) and elevated temperatures, which could maximize the CO2 working capacities of adsorbents. It should be noted that such regeneration conditions ultimately do not result in the separation of high purity CO2. Earlier studies on amine-containing adsorbents mainly focused on increasing the CO2 working capacities of adsorbents. However, more appropriate metric for assessing adsorbents is not the CO2 working capacity but the energy required for capturing a fixed amount of CO2 when using the adsorbents (mainly the heat required for adsorbent regeneration). It is noteworthy that high-temperature waste heat suitable for adsorbent regeneration is not actually available in a power plant. This means that adsorbents should be designed not to maximize the CO2 working capacity but to minimize the overall heat required for adsorbent regeneration. The regeneration heat in a conventional TSA process consists of three major contributions: i) sensible heat required for heating the adsorbents from adsorption temperature to desorption temperature, ii) latent heat of CO2 desorption needed to overcome the chemical bonding strength between CO2 and the adsorbent, and iii) latent heat of H2O desorption for removing co-adsorbed H2O from the flue gas.39,40 It is notable that the large CO2 working capacity of adsorbents leads to a small number of TSA cycles for capturing a fixed amount of CO2, which mainly results in the


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reduction of sensible heat. Therefore, an accurate assessment of the adsorbents is possible only after a rigorous determination of all three individual heat contributions. Another important factor determining the heat requirement of a TSA process is the long-term stability of the adsorbents. As adsorbents degrade over the course of repeated TSA cycles, the heat requirement will gradually increase unless the deteriorated adsorbents are continuously replaced with fresh ones. Conventional amine-containing adsorbents are known to deactivate under TSA conditions due to CO2-induced urea formation12–14,16,18,24–26 and oxidative degradation of amines.15,16,23,25 In this regard, the design of amine structures that minimize the overall heat requirement for CO2 capture and suppress amine degradations under harsh TSA conditions is crucial for the development of an advanced CO2 capture process. In the present work, we synthesized a series of PEI/SiO2 adsorbents functionalized with epoxides containing different side chains (Scheme 1). This provided the adsorbents with different polymeric amine structures, which could show systematically altered CO2/H2O adsorption characteristics and chemical stabilities. Specific heat capacities, heat of adsorptions and working capacities of CO2 and H2O, and chemical stabilities were rigorously investigated for all adsorbents. These data were used to analyze the heat required for adsorbent regeneration and thereby to determine the important design principles of adsorbents for minimizing the energy cost of CO2 capture. We demonstrate that the CO2 working capacity of adsorbents is not a decisive factor in determining the heat requirement. The suppression of H2O co-adsorption and adsorbent stability are the more critical factors.

RESULTS AND DISCUSSION


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Synthesis of PEI/SiO2 adsorbents functionalized with various epoxides. The functionalization of PEI (Nippon Shokubai, MW 1200, nitrogen content: 22 mmolN g-1) with epoxides having different side chains (Scheme 1) was carried out by a single-step addition reaction (see details in Experimental Section). The molar ratio between the epoxides and the nitrogen content of PEI was fixed at 0.37. The functionalized PEIs were denoted as ‘R’-E-PEI, where R indicates the type of side chains in epoxides (from H to C4 groups). According to the elemental analysis, all functionalized PEIs showed O/N elemental ratios of 0.37–0.39, which were very close to the value predicted from the stoichiometry of the initial reactants (0.37). The results indicated that the addition reaction proceeded almost completely due to the high ringopening activity of epoxides in the presence of nucleophilic amines.

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C NMR spectra also

confirmed the successful alkylation of amines groups using various epoxides (Figures 1a-f). The amine state distributions in PEI and R-E-PEIs could be quantified using the

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C NMR spectra.

The pristine PEI exhibited a primary (1°): secondary (2°): tertiary (3°) amine ratio of 36:37:27 (bars in Figure 1g). On the other hand, the R-E-PEIs showed significant increases in the 2° and 3° amine portions at the expense of the 1° amine portion (1°: 9–12%, 2°: 50–56%, 3°: 34–38%). The increase of the 2° amine portion was more pronounced than that of the 3° amine portion, which indicated that majority of epoxides were used for the alkylation of 1° amines than of 2° amines. Such a preferred conversion of 1° amines to 2° amines is desirable for the preparation of CO2 adsorbents because 3° amines are less efficient than other types of amines in CO2 adsorption.2,7,8 As expected, the nitrogen content per gram of polymer gradually decreased as the side chains (R) became larger (solid line in Figure 1g). Amine-containing CO2 adsorbents were prepared by impregnating methanolic solutions of PEI and R-E-PEIs into a pre-prepared macroporous silica support containing a small amount of


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trisodium phosphate (TSP) as an oxidation inhibitor for amines.23 The silica is essentially macroporous (pore diameter centered at 56 nm) with large pore volume (1.73 cm3 g-1) (Figure S1). The polymer loading was fixed at 50 wt% of the composite adsorbents (in the case of R-EPEIs, the loading is based on the weight of the final polymer, not based on the weight of the PEI used for functionalization). This means that the total nitrogen content of the final adsorbents should be half of those of the polymers shown in Figure 1g. The silica support was synthesized by spray-drying a water slurry containing a commercial fumed silica followed by calcination at 600 °C.17,18 We reported that the macroporous silica had a spherical particle morphology (75– 200 µm in diameter) suitable for fluidized bed operation. It also showed high CO2 accessibility and excellent hydrothermal stability due to its large pore diameter (56 nm on average) and thick framework (10–15 nm).17,18 With the resultant adsorbents, the heat of CO2 and H2O adsorptions were measured using thermogravimetry-differential scanning calorimetry (TG-DSC) at 60 °C under 15% CO2/N2 and 10% H2O/N2, respectively (Figures S2 and S3). As shown in Figure 2a, PEI/SiO2 showed the largest heat of adsorption for both CO2 (80.5 kJ mol-1) and H2O (47.1 kJ mol-1) among the adsorbents. The R-E-PEI/SiO2 samples showed substantially lower heat of CO2 adsorption, which decreased gradually as the side chains (R) became bulkier. This can be attributed to the fact that the epoxide functionalization generates 2-hydroxyalkyl groups tethered to the amines (Scheme 1); these are electron-withdrawing groups and thus lower the basicity of the amines.18 Besides, the bulkier side chains (R) provide larger steric hindrance near the amine sites, further weakening the interaction between the amines and CO2.41 The heat of H2O adsorption similarly decreased after epoxide functionalization, and the degree of reduction was more pronounced as


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the side chain became bulkier (Figure 2a). This can be attributed to the increased hydrophobicity of the functionalized PEIs. The specific heat capacities of the adsorbents were measured using DSC and the results are summarized in Figure 2b. PEI/SiO2 showed the largest specific heat capacity of 1.41 J g-1 °C-1, which is very close to the theoretical value (1.37 J g-1 °C-1) predicted by the average of the specific heat capacities of PEI (2.0 J g-1 °C-1) and silica (0.74 J g-1 °C-1). The R-E-PEI/SiO2 samples showed slightly lower specific heat capacities in the range of 1.30–1.35 J g-1 °C-1, which gradually decreased with increasing bulkiness of the side chains (R). This can be attributed to the reduced density of hydrogen bonds in the polymeric structures of R-E-PEIs, especially in the presence of bulky side chains. It is worth remembering that liquid H2O has an anomalously large specific heat capacity of 4.18 J g-1 °C-1 due to the presence of high-density hydrogen bonds in which heat can be efficiently stored as vibrational energy.42 CO2/H2O adsorption–desorption properties in a humid flue gas. The CO2 and H2O adsorption–desorption profiles of the adsorbents were investigated under practically meaningful TSA conditions; adsorption was carried out under simulated flue gas (15% CO2, 3% O2, 10% H2O, 2% Ar in N2 balance) at 60 °C and adsorbent regeneration was carried out under pure CO2 at 110 °C. It is notable that higher regeneration temperatures (≥120 °C) have often been used in literature, including in our own studies, to completely desorb CO2 from the saturated adsorbents and thereby achieve maximum CO2 working capacities.12,15,18,24,26,27,29 According to previous studies, however, the amine-containing adsorbents can degrade rapidly at elevated temperatures due to the CO2-induced formation of urea12–14,16,18,24–26 and/or oxidative degradation of amines.15,16,23,25 Considering that adsorbents should remain stable for several months in a commercial TSA process, it is difficult to justify the use of such high regeneration temperatures.


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As will be shown later, the conventional PEI/SiO2 exhibited substantial deactivation even under the present TSA conditions using relatively mild adsorbent regeneration at 110 °C. The CO2 and H2O adsorption–desorption profiles of the fresh amine/silica composite adsorbents are shown in Figure 3. The corresponding working capacities of CO2 and H2O are also summarized in Figure 4. PEI/SiO2 exhibited the largest CO2 uptake (4.05 mmol g-1) during adsorption while the R-E-PEI/SiO2 samples showed substantially lower CO2 uptakes (1.45–2.30 mmol g-1). The decrease in CO2 uptake was more pronounced as the size of the side chains increased (Figure 3). This monotonic decrease can be attributed to the decrease in nitrogen content and the slight increase in 3° amine portions after the epoxide functionalization (Figure 1g). However, in terms of CO2 working capacities (“desorbable” CO2 uptakes), the differences between the adsorbents were relatively insignificant (1.30–1.98 mmol g-1) (Figures 3 and 4). This is because the PEI/SiO2 sample showing the largest CO2 uptake exhibited the biggest difficulty in CO2 desorption during adsorbent regeneration. In contrast, the R-E-PEI/SiO2 samples showed more efficient CO2 desorption especially as the size of the side chains (R) increased (Figure 3). This is consistent with the heat of CO2 adsorption of the adsorbents (Figure 2a); lower heat of CO2 adsorption resulted in more efficient adsorbent regeneration. It is notable that the CO2 working capacity of H-E-PEI was slightly smaller than that of C1-E-PEI (Figure 4). If we compare their CO2 uptakes in the adsorption conditions (Figure 3), H-E-PEI/SiO2 showed a larger CO2 uptake (2.3 mmol g-1) than C1-E-PEI/SiO2 (2.1 mmol g-1). However, in terms of “working” capacity, the trend was reversed, because H-E-PEI/SiO2 showed less efficient regeneration than C1-E-PEI/SiO2 in the desorption conditions (Figure 3). These results indicated that not only the CO2 uptake but also the regenerability of adsorbents are important in determining CO2 working capacity.


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For all adsorbents, the adsorbed H2O could be fully desorbed during adsorbent regeneration (Figure 3). This can be attributed to the substantially lower heat of adsorption for H2O (37.3– 47.1 kJ mol-1) than that of CO2 (64.9–80.5 kJ mol-1) (Figure 2a). Unlike CO2 working capacities that were relatively similar for all adsorbents, the H2O working capacities were significantly different depending on the type of adsorbents. PEI/SiO2 showed the largest H2O working capacity (2.61 mmol g-1), whereas the R-E-PEI/SiO2 samples exhibited significantly lower H2O working capacity as the size of the side chains (R) increased (Figure 4). Especially, the H2O working capacity decreased significantly and remained steady (

Rational Design of the Polymeric Amines in Solid Adsorbents for

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