Double-Layer Structured CO2 Adsorbent Functionalized with Modified

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Double-Layer Structured CO2 Adsorbent Functionalized with Modified Polyethyleneimine for High Physical and Chemical Stability Sunbin Jeon,†,§ Hyunchul Jung,‡,§ Sung Hyun Kim,*,† and Ki Bong Lee*,† †

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Carbon Resources Institute, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea

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S Supporting Information *

ABSTRACT: CO2 capture using polyethyleneimine (PEI)-impregnated silica adsorbents has been receiving a lot of attention. However, the absence of physical stability (evaporation and leaching of amine) and chemical stability (urea formation) of the PEI-impregnated silica adsorbent has been generally established. Therefore, in this study, a double-layer impregnated structure, developed using modified PEI, is newly proposed to enhance the physical and chemical stabilities of the adsorbent. Epoxy-modified PEI and diepoxide-crosslinked PEI were impregnated via a dry impregnation method in the first and second layers, respectively. The physical stability of the double-layer structured adsorbent was noticeably enhanced when compared to the conventional adsorbents with a single layer. In addition to the enhanced physical stability, the result of simulated temperature swing adsorption cycles revealed that the double-layer structured adsorbent presented a high potential working capacity (3.5 mmol/g) and less urea formation under CO2-rich regeneration conditions. The enhanced physical and chemical stabilities as well as the high CO2 working capacity of the double-layer structured adsorbent were mainly attributed to the second layer consisting of diepoxide-cross-linked PEI. KEYWORDS: CO2 capture, polyethyleneimine, amine modification, double-layer structure, stability

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mmol g−1 and good CO2/N2 selectivity at low temperature and low CO2 pressure.18,19 Though PEI-incorporated MOFs are promising CO2 adsorbents, currently there are limitations for commercialization because MOFs are relatively expensive and difficult to be manufactured on a large scale. Therefore, studies for producing large amounts of MOFs at low cost are being carried out. Also, there are studies trying to develop adsorbents using mass-producible and inexpensive materials such as silica. To prepare amine-functionalized adsorbents, two methods of amine-functionalization have been generally carried out: physical impregnation and chemical grafting. In the former, the functionalization of amines is conducted with physical loading, which causes polymeric amines (i.e., PEI) to be incorporated on the surface of porous supports.25−36,40,47 In the latter, the covalent linkages between aminosilanes and the porous surface of supports are used for functionalization of amines.35,37−44 Moreover, in situ polymerization using amine monomers, which results in covalently linked amines on the supports, was suggested as a new method for preparing aminefunctionalized adsorbents.36,40,45 The amine-functionalized adsorbents should be capable of discharging high concentrations of CO2 during repeated

INTRODUCTION The increase of CO2 emissions from coal-fired power plants has become a critical issue among the international community as a major cause of global warming.1−5 The necessity of carbon capture and storage (CCS) technology to capture and store CO2 generated by fossil fuel combustion has been steadily increasing.2,3 In the CCS technology, CO2 capture is the most expensive.4,5 To reduce the ultimate cost of CCS technologies, it is essential to reduce the capture cost; plenty of studies have been carried out widely for CO2 capture. Amine-scrubbing technology using aqueous amine solution is currently being used for CO2 capture.6,7 However, amine-scrubbing technique exhibits technical drawbacks during regeneration, such as device corrosion, evaporation of amines, and large energy consumption.2,8,9 As an alternative to overcoming the drawbacks of amine-scrubbing, there has been an increased interest in the development of solid adsorbents, which are likely to exhibit less device corrosion and low energy consumption.10−12 Among the various solid adsorbents, amine-functionalized porous materials based on carbons,13,14 zeolites,15−17 metal− organic frameworks (MOFs),18−24 and silica8,25−47 have been investigated extensively because these materials exhibit adequate affinity for CO2 even under flue gas condition, which is characterized by moisture and low concentration of CO2.10,11 Recently, polyethyleneimine (PEI)-incorporated MOFs were reported to have a high CO2 uptake of 4.2 © XXXX American Chemical Society

Received: January 30, 2018 Accepted: June 1, 2018

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DOI: 10.1021/acsami.8b01749 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Illustration for Manufacturing Double-Layer Structured Amine-Impregnated Adsorbents via Dry Impregnation (left). Graphical Presentation of Conventional Single-Layer Adsorbent and Double-Layer Structured Adsorbent (right)

the adsorbents and evaluation of their hydrothermal stability were insufficient. In this study, the following two approaches are adopted to ensure that the amine-functionalized silica adsorbents exhibit chemical stability (inhibition of urea formation) as well as physical stability (inhibition of evaporation and leaching of amine and securing hydrothermal stability). The first is the utilization of the modified amine to efficiently convert primary amine groups to secondary amine groups, and the second is the construction of an efficient structure to enhance the hydrothermal stability and prevent the evaporation and leaching of the impregnated amines. Accordingly, this study newly proposes double-layer impregnated structure adsorbents that exhibit both physical and chemical stabilities, by using epoxymodified PEI and diepoxide-cross-linked PEI. Hydrothermal treatment was used to verify the leaching and evaporation of amines and the hydrothermal stability of the prepared adsorbents. Thermogravimetric analysis (TGA) pyrolysis and N2 physisorption were used to analyze the organic contents and textural properties of the adsorbents before and after the hydrothermal treatment. A clearer basis for leaching of amines was verified by the washing treatment of the adsorbents and analyzed using a relatively diverse approach for assessing physical stability. To establish the inhibition of urea formation, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used.

regeneration process. Wang’s group recently suggested an interesting concept and process based on light-triggered CO2 capture to reduce regeneration cost and secondary CO2 emissions.48,49 However, additional assessment is required for practical continuous operation in fossil fuel plants. Temperature swing adsorption (TSA) processes using high-temperature and CO2-rich atmospheric regeneration conditions can be applied to practical operation without consuming additional purge gas, by utilizing the highly pure CO2 stream obtained in the adsorption step.27,46 However, the amine-functionalized adsorbents appeared to lack chemical stability under hightemperature and CO2-rich atmospheric regeneration conditions because of the formation of open chain urea and cyclic urea. Several studies have been conducted to verify the formation of urea, demonstrating similar phenomena through the results including 13C nuclear magnetic resonance (NMR) spectra, Fourier transform infrared (FT-IR) spectra, and weight changes during TSA cycles.30,37,39 To suppress the formation of urea under the regeneration condition mentioned above, Li et al. proposed an alternative regeneration method by utilizing hightemperature steam.36 However, in the high-temperature steam condition, the amine-functionalized adsorbents should have hydrothermal stability, and recent studies reported that the amine-functionalized adsorbents exhibited a structural collapse of the support under the high-temperature steam condition.32,40 The structural collapse of the support because of the absence of physical stability of the amine-functionalized adsorbents can deteriorate the CO2 capture performance. In addition, the lack of physical stability of amine-functionalized adsorbents could be verified by the leaching and evaporation of the functionalized amines. Several previous studies demonstrated the evaporation of amines through the weight reduction of the adsorbents under high-temperature conditions.30,31 Amine modification, which efficiently converts primary amine groups to secondary amine groups, can be a solution for inhibiting the formation of urea under high-temperature and CO2-rich atmospheric conditions. Recently, Choi et al. suggested epoxide functionalization of PEI,27 and our group suggested epoxy cross-linking of PEI for converting the primary amine groups of PEI to secondary amine groups;33 both reported that the formation of urea was inhibited under hightemperature and CO2-rich atmospheric regeneration conditions. However, both the descriptions of the physical stability of

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EXPERIMENTAL SECTION

Materials. PEI (MW: 1200 Da) was purchased from Nippon Shokubai Co. Large pore mesoporous silica gel (LPSG, PQ-2129) provided by PQ Corporation was used as a silica support for impregnation with PEI, modified PEI, and cross-linked PEI. 1,2Butylene oxide (BO, ≥ 99%) and 1,3-butadiene diepoxide (BDDE, 97%) were purchased from Sigma-Aldrich and used as a modifier and a cross-linker, respectively. Methanol (≥99.9%) was supplied by Daejung Chemicals & Metals and used as a solvent for dissolving PEI, BO, and BDDE. PEI Modification and Synthesis of Adsorbents. The modification of PEI with BO was conducted before synthesizing the adsorbents.27 PEI (10 g) and 2.42 g of BO were dissolved in 50 mL of methanol solvent, and the solution was sonicated for 30 min. The epoxy-modification was carried out at room temperature for 24 h with stirring, and the PEI modified with BO (BOPEI) was obtained by heating at 80 °C for 12 h. For all adsorbents, a dry impregnation method was used to add PEI, modified PEI, and cross-linked PEI into B


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ACS Applied Materials & Interfaces the pores of the silica support. For preparing conventional single-layer adsorbents, 1.2 g of PEI or BOPEI was dissolved in 3.11 mL of methanol solvent to ensure its volume is equal to that of the pore in 1.8 g of LPSG. The amine-methanol solution was added to LPSG dropwise, and the mixture was continuously stirred with a stainless steel spatula. Then, the methanol solvent was evaporated at 80 °C for 8 h. The two-step dry impregnation for the manufacture of double-layer structured adsorbents is illustrated in Scheme 1. For constructing the first layer, 0.9 g of PEI or BOPEI was added using the same method as that for preparing single-layer adsorbents. For constructing the second layer, 0.186 g of PEI and 0.114 g of the cross-linker BDDE were dissolved in 3.13 mL of methanol solvent, and the solution was added to LPSG using the dry impregnation method; the amount of methanol solvent was controlled to make the volume of amine-methanol solution equal to the pore volume of the silica support. For sufficient cross-linking between PEI and BDDE, the impregnated samples were dried at 45 °C for 12 h.33 Finally, the methanol solvent was dried at 80 °C for 8 h. The prepared samples were marked in the following order: LPSG_X_Y. Here, LPSG is the silica support, and X and Y represent the species of amines impregnated in the first layer and the species of cross-linked amines impregnated in the second layer, respectively. The samples not marked with Y (LPSG_X) are conventional single-layer amine-impregnated adsorbents. Each adsorbent in this study was prepared to impregnate with species of amines up to approximately 40 wt % of the total adsorbent mass. The molar ratio of BO and PEI in the modified PEI was fixed at 5:1; here, half of the primary amine groups of PEI could be converted to secondary amine groups stoichiometrically. In the case of cross-linked PEI (BDDEPEI), the molar ratio of BDDE to PEI was 8.55:1 for the same reason as that for the modified PEI. Physical Stability Test. To evaluate the physical stability and amine leaching of the prepared adsorbents, hydrothermal and washing treatments were carried out. The schematic diagrams are shown in Schemes 2 and 3, respectively. In the hydrothermal treatment, 1 g of

adsorbent was packed in a stainless steel column having an outer diameter of 1.27 cm, and distilled water was flowed at 1 mL/min using a high-pressure liquid chromatography pump. The distilled water was vaporized through a preheater, and N2 flowing at 50 mL/min was used as a carrier gas to convey the vaporized water into the packed bed column. The temperature of both the preheater and the packed bed was maintained at 130 °C, and the hydrothermal treatment was performed for 12 h. After the hydrothermal treatment, the drying process was performed with N2 flow at 50 mL/min and 130 °C for 1 h to remove the residual water vapor. The thermal decomposition temperature and organic contents of the adsorbents obtained after the hydrothermal treatment were examined by TGA (Q50 TGA, TA Instruments). The residual water and adsorbates were removed under N2 gas conditions at 100 °C for 1 h, and the heating rate was maintained at 1 °C/min to attain 800 °C. After reaching 800 °C, the adsorbents were kept at this temperature for 10 min with blowing air for removing the deposited carbon. In addition, N2 physisorption analysis was performed using a ASAP 2020 physisorption analyzer (Micromeritics) to compare the textural properties of the adsorbents before and after the hydrothermal treatment. Washing treatment was carried out in a water bath to verify the inhibition of amine leaching in the double-layer structure. Half a gram of the adsorbent was added to a vial containing 5 mL of distilled water, and the mixture was stirred at 160 rpm for 8 h in a water bath at 25 °C. Subsequently, the washed samples were treated by filtration at room temperature and dried for 8 h at 80 °C in a vacuum oven. To compare the organic contents between the washed and pristine samples, TGA analysis was conducted. The procedure of the TGA analysis was identical to that mentioned above for comparing the samples before and after the hydrothermal treatment, except for the heating rate, which was 10 °C/min rather than 1 °C/min. CO2 Adsorption and Long-Term Stability Test. To compare the adsorption performances and weight changes of the double-layer structured adsorbents and the conventional single-layer adsorbent, 15 consecutive TSA cycles were tested using TGA. Two types of doublelayer structured adsorbents, which contain pristine PEI or BOPEI in the first layer and BDDEPEI in the second layer (LPSG_PEI_BDDEPEI and LPSG_BOPEI_BDDEPEI), and three types of single-layer structured adsorbents (LPSG_PEI, LPSG_BOPEI, and LPSG_Mix, which contain a mixture of BOPEI, BDDE, and PEI) were used. Prior to the TSA cycle test, pretreatment was conducted at 100 °C for 1 h under N2 gas conditions. In the TSA cycle, adsorption was carried out at 40 °C for 0.5 h with a specified gas composition (15% CO2, 4 vol % H2O, balanced N2), which is similar to the actual coal-fired power plant flue gas composition. Regeneration was conducted at 130 °C for 0.5 h with 100% CO2, which is applicable to the actual CO2 capture processes. Chemical Stability Test and Adsorbent Characterization. FTIR spectroscopy (Nicolet iS 10, Thermo Fisher Scientific) was used to obtain in situ infrared and FT-IR spectra. DRIFTS was used to evaluate the chemical stability of the prepared adsorbents, and the DRIFTS conditions and the physical meaning of each state are shown in Figure 1. The adsorption and regeneration temperature conditions were set identical to those for the 15 consecutive TSA cycles. To verify the structural change of the adsorbents during the urea formation conditions, several analysis points [(III), (IV), and (V)] were set in the temperature increasing process.27,39 On the basis of the physical meaning of each point, the spectra subtraction of point (I) from each

Scheme 2. Schematic Diagram for Hydrothermal Treatment Equipment

Scheme 3. Schematic Diagram for Washing Treatment Procedure

C


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Figure 1. Temperature−time profile of DRIFTS and gas condition. Physical significance of each point: (I) after pretreatment, (II) after CO2 adsorption, (III) at 100 °C, (IV) at 110 °C, (V) at 120 °C, (VI) at 130 °C, (VII) after 3 min at 130 °C, (VIII) after 10 min at 130 °C, and (IX) after 25 min at 130 °C. point was conducted to observe the urea formation. Attenuated total reflectance (ATR) spectroscopy was used to observe the changes in the functional groups between the double-layer structured adsorbent and the single-layer adsorbent after CO2 adsorption under humidified conditions. The samples for the ATR spectrum analysis were obtained from a TGA procedure in which 4% H2O, 15% CO2, and balanced N2 gas was flowed for adsorption at 40 °C for 3 h, following the pretreatment at 100 °C for 1 h.

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RESULTS AND DISCUSSION Physical Stability and Resistance to Leaching of Amine. Figure S1 shows the scanning electron microscopy (SEM) images of silica support and prepared adsorbents. All samples showed an irregular shape and a relatively smooth surface. On the basis of the SEM images, no clear difference can be found in the morphology and structure among samples, implying that the amine impregnation method did not change the morphology and structure of the silica support because liquid amine just flowed into the pores or was partially distributed on the surface of the silica support. Figure 2 describes the results of the TGA pyrolysis for samples before and after the hydrothermal treatment: The weight change profiles and temperature versus time plot of the adsorbents are shown for LPSG_PEI, LPSG_BOPEI, and LPSG_BOPEI_BDDEPEI. The basis wt % was set to 100% at the end of the pretreatment at 100 °C for 1 h. The inset graph shows the derivative of weight versus temperature plot. This derivative of weight profiles was used as an indirect indicator for the leaching of amine in the silica support. The intermolecular interaction among amines is considered to be stronger in the amines distributed inside the pores than in the amines distributed outside the pores and is likely to be more stable at high temperatures. Hence, the peak change of weight derivatives before and after the hydrothermal treatment can be interpreted as a progress in the change of amine distributions. In the case of LPSG_PEI (Figure 2a), it was verified that there was a loss of PEI of approximately 5 wt % of organic content after the hydrothermal treatment. This means that the evaporation of the impregnated PEI was conducted during the hydrothermal treatment. In the case of LPSG_BOPEI (Figure 2b), there was a negligible decrease in the organic content compared to LPSG_PEI. It implies that when BOPEI is impregnated to the silica support, the resistance to evaporation in the hydrothermal treatment is improved. However, after the hydrothermal treatment, the derivative of

Figure 2. Weight change profile and derivative of weight versus temperature of adsorbents (inset). (a) LPSG_PEI, (b) LPSG_BOPEI, and (c) LPSG_BOPEI_BDDEPEI.

the weight profile shifted marginally to the left. The shifted profile indicates that the distribution of the impregnated amine was changed due to amine leaching. Once amine leaches out from the inside to the outside of the pores, the amount of amines present on the outside surface of silica increases. Moreover, the amines present outside the pores are likely to be relatively vulnerable to heat compared to the amines present inside the pores. Hence, the shift of the derivative of weight profile to lower temperature after the hydrothermal treatment is a consequence of amine leaching. Also, the derivative of the weight profile of both LPSG_PEI and LPSG_BOPEI showed a bimodal curve before the hydrothermal treatment but it changed to a unimodal curve after the hydrothermal treatment. The two weight-loss stages of PEI-silica adsorbents were reported previously, and the physical meaning of former and latter peaks in the bimodal derivative of the weight curve is the D


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Figure 3. Profile of pore size distributions (a,c,e) and N2 physisorption isotherms (b,d,f) of adsorbents. (a,b) LPSG_PEI, (c,d) LPSG_BOPEI, and (e,f) LPSG_BOPEI_BDDEPEI.

collapsed owing to the hydrothermal treatment. Li et al. reported similar results; the amine-silica adsorbent was vulnerable to hydrothermal treatment, and the structure of the adsorbent collapsed as the silica support wall collapsed.40 The results of this analysis are linked to the TGA pyrolysis analysis described above. As a result of the hydrothermal treatment, the structural collapse of the adsorbent occurred, and the impregnated PEI flowed out and was exposed to conditions that could result in evaporation.32 The results for LPSG_BOPEI reveal that the pore size distribution marginally shifted rightward after the hydrothermal treatment (Figure 3c). Moreover, the pore volume was marginally decreased (Figure 3d). The rightward shift of the pore size distribution has a significance similar to that of the TGA pyrolysis analysis mentioned above. The average pore size increased owing to the amine leaching from the inside to the outside of the pores. However, the pore volume exhibited a tendency to decrease marginally after the hydrothermal treatment; this is contrary to the prediction that the pore volume would increase as the amine leached out. From these results, it is expected that the structural collapse and leaching of amine progressed simultaneously during the hydrothermal treatment in LPSG_BOPEI.32 Moreover, it is likely that the structural collapse was not a dominant effect compared to that in LPSG_PEI. However, in the case of LPSG_BOPEI_BDDEPEI, it is noteworthy that the pore size distribution (Figure 3e) and the pore volume (Figure 3f) before and after the hydrothermal treatment remained almost unchanged. It implies that the cross-linked PEI in the second layer inhibited the amine leaching and the structural

decomposition of amine located in the shallower and deeper parts of pores, respectively.47 In LPSG_PEI and LPSG_BOPEI, it was thought that the impregnated PEI was originally located in both shallower and deeper parts of pores, but more PEI moved to shallower parts by leaching during the hydrothermal treatment. On the contrary, the plot for LPSG_BOPEI_BDDEPEI (Figure 2c) reveals that there is almost no decrease in the organic content after the hydrothermal treatment, and the derivative of the weight profile is also unchanged. In contrast to the other two single-layer adsorbents in terms of the derivative of the weight profile, a peak is additionally generated in the high-temperature region at ∼600 °C, which implies that the cross-linked PEI (BDDEPEI) was formed adequately in the second layer. As a consequence, the evaporation of amines and the distribution change of amines owing to amine leaching were suppressed by the cross-linked PEI present in the second layer. For the additional investigation of physical stabilities, the pore size distributions and N2 physisorption isotherms of the adsorbents are compared before and after the hydrothermal treatment, as shown in Figure 3. The black square dot line and the red diamond dot line present the results before and after the hydrothermal treatment, respectively. In the case of LPSG_PEI, a drastic change of pore size distribution after the hydrothermal treatment is apparent. Overall, the average pore size decreased, and the profile of pore size distribution also reduced significantly (Figure 3a). In addition, the pore volume, which was estimated from the N2 physisorption isotherm, decreased significantly (Figure 3b). The reason for this phenomenon is that the pore structure of the silica support E


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the loss ratio was remarkably reduced to 24.33%. In brief, it is noteworthy that the conventional amine-silica adsorbents do not guarantee physical stability against phenomena such as amine evaporation and leaching. However, the new structured adsorbent, which utilizes modified PEI and cross-linked PEI to form a double-layer structure, ensured physical stability by efficiently inhibiting amine leaching and evaporation. In addition, the effects of duration and repeated cycles of washing treatment on the change of organic content were tested for LPSG_PEI and LPSG_BOPEI_BDDEPEI, and the results are presented in Figures S3 and S4, respectively. There was no noticeable change in the organic content with increasing duration of treatment beyond 4 h and additional washing treatment cycles. However, the results clearly confirm the inhibition of amine leaching in the double-layer adsorbent during washing treatment. Long-Term Stability and Working Capacity. In the previous section, it was described that the double-layer adsorbent has physical stability owing to the presence of cross-linked PEI in the second layer. In addition to assessing the physical stability, new control groups containing crosslinked PEI were formed to compare the chemical stability, longterm stability, and working capacity. The amounts of PEI, modifier, and cross-linker used in the preparation of new control groups are shown in Table 3. All control groups were impregnated with an identical amount of the cross-linker. The first control group used PEI in the first layer, and its long-term stability and chemical stability were compared with those of the target adsorbent (LPSG_BOPEI_BDDEPEI). The second control group was impregnated with an identical composition and amount of components (PEI, BO, and BDDE) as used in the target adsorbent. However, the components were impregnated together to form a single-layer structure in the second control group, and its working capacity was compared with that of the target adsorbent. Fifteen consecutive TSA cycles were carried out for each adsorbent using TGA. The conditions of the TSA cycles and the results are shown in Figure 4. The weight change profiles in the TSA cycles are shown in Figure 4a, and the CO2 uptake of the adsorbents per cycle is shown in Figure 4b. In the weight change profile, LPSG_PEI_BDDEPEI (black line) signifies a continuous increase of weight under regeneration conditions compared to the other two adsorbents, LPSG_BOPEI_BDDEPEI (blue line) and LPSG_Mix (red line). To observe this tendency more clearly, the results in the second cycle are depicted in Figure 5 on a larger scale. When the temperature reached the regeneration condition of 130 °C and the gas was converted to 100% CO 2 , the weight noticeably increased in LPSG_PEI_BDDEPEI. This is consistent with the results of the previous studies conducted by Sayari et al. and Choi et al.,

collapse of the adsorbent appropriately. Table 1 presents the textural properties of each adsorbent before and after the Table 1. Textural Properties of Adsorbents before and after Hydrothermal Treatment sample

average pore size (nm)

LPSG_PEI LPSG_PEI_H LPSG_BOPEI LPSG_BOPEI_H LPSG_BOPEI_BDDEPEI LPSG_BOPEI_BDDEPEI_H

24.6 22.1 25.3 26.5 23.9 24.0

VBJH totala (cm3g−1) 0.93 0.56 1.06 1.02 0.93 0.94

SBET (m2 g−1) 151.2 102.2 167.0 154.4 155.0 156.3

a The total pore volume was calculated by the Barrett−Joyner− Halenda (BJH) method in the P/P0 of 0.984.

hydrothermal treatment. The notation H, at the end of the adsorbent names, signifies adsorbents after the hydrothermal treatment. LPSG_PEI exhibited a very large decrease in the pore volume and Brunauer−Emmett−Teller (BET) surface area owing to structural collapse, and the average pore size was also decreased. LPSG_BOPEI exhibited a marginal decrease in the pore volume and BET surface area; however, the pore size was increased, which can be interpreted as a result of the simultaneous occurrence of marginal structural collapse and amine leaching. In the case of LPSG_BOPEI_BDDEPEI formed with the double layer, the pore size, pore volume, and BET surface area before and after the hydrothermal treatment showed no noticeable changes compared to the other single-layer adsorbents. A different duration of hydrothermal treatment was also tested, and the change of pore size distribution was compared for LPSG_PEI and LPSG_BOPEI_BDDEPEI (Figure S2). When the duration of the hydrothermal treatment was increased from 12 to 24 h, the pore size distribution of the conventional single-layer adsorbent (LPSG_PEI) continuously changed, implying structural instability. However, the pore size distribution of the double-layer adsorbent (LPSG_BOPEI_BDDEPEI) did not change even with increasing duration of hydrothermal treatment, confirming its structural stability. A washing test was conducted to observe the change in the organic contents before and after the treatment, and the decisive evidence for the leaching of amines from the pores was obtained and is presented in Table 2. For the five adsorbents, the ratio of loss was calculated via dividing the difference in the organic content before and after the treatment by the organic content before the treatment. The loss ratio was significant in both LPSG_PEI and LPSG_BOPEI, which was 70.09 and 68.78%, respectively. However, in LPSG_BOPEI_BDDEPEI,

Table 2. Organic Content of Adsorbents before and after Washing Treatmenta (40) sample

silica

first layer

second layer

(60) LPSG_PEI LPSG_BOPEI LPSG_PEI_BDDEPEI LPSG_BOPEI_BDDEPEI LPSG_Mix a

LPSG LPSG LPSG LPSG LPSG

pristine PEI BOPEI PEI (30) BOPEI (30) BOPEI, PEI, BDDE

BDDEPEI (10) BDDEPEI (10)

organic wt % before washing

organic wt % after washing

ratio% of loss

(y)

(y′)

(y−y′)/y

39.74 38.79 39.83 38.23 39.55

11.55 12.11 24.34 28.93 27.88

70.09 68.78 38.89 24.33 29.51

The numbers in the parenthesis indicate the percentage of the component. F


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ACS Applied Materials & Interfaces Table 3. Amounts of Components Used in Adsorbent Preparation (40) sample

silica (60)

first layer

second layer

LPSG_PEI_BDDEPEIa LPSG_Mixb LPSG_BOPEI_BDDEPEIc

LPSG LPSG LPSG

pristine PEI (30) BOPEI, BDDEPEI (40) BOPEI (30)

BDDEPEI (10) BDDEPEI (10)

LPSG (g) 1.797 1.801 1.799

pristine PEI (g) 1.086 0.878 0.878

BO (g) 0.208 0.208

BDDE (g) 0.114 0.114 0.114

First control group: it has the same structure as the target adsorbent of this study, however, the first layer was impregnated with pristine PEI instead of BOPEI. bSecond control group: it has an identical composition as the target adsorbent, however, it has no double-layer structure. cThe target adsorbent of this study. a

Figure 4. Fifteen consecutive TSA cycles. (a) Weight change profiles of adsorbents and (b) cyclic CO2 uptake of adsorbents.

because, as the modified PEI and cross-linker are impregnated, the conversion of the primary amine groups to secondary amine groups as well as the conversion of the secondary amine groups to tertiary amine groups are likely to occur, as shown in Figure S5. It is established that tertiary amines undergo little CO2 adsorption under dry conditions, and the CO2 adsorption performance of tertiary amines is lower than that of primary and secondary amines because tertiary amines contribute only partially to CO2 adsorption in the presence of water vapor. In the case of LPSG_BOPEI_BDDEPEI, the primary amine groups were efficiently converted into secondary amine groups by using epoxy and diepoxide in the first layer and second layer, respectively; thus, the chemical stability (inhibition of urea formation) was secured without loss of CO2 adsorption sites. There was a little increase of CO2 uptake for LPSG_Mix and LPSG_BOPEI_BDDEPEI in the initial cycles. Because there was no additional activation process for the amine-impregnated silica adsorbent, there could be an effect of increasing temperature on the adsorbent in the initial TSA cycles. The rearrangement of impregnated amines might occur at a high regeneration temperature in the initial cycles. Long-term stability tests were also carried out for the single-layer adsorbents of LPSG_PEI and LPSG_BOPEI, and the results are compared with the previous three adsorbents, as shown in Figure 6S. The single-layer adsorbents had poor cyclic stability and a relatively low working capacity. Functional Groups of Adsorbents. DRIFTS and ATR spectrum analysis were used to verify the functional groups of the adsorbents. The DRIFTS and gas conditions are shown in Figure 1, and the physical significance of each state is as follows: (I) after treatment, (II) after CO2 adsorption, (III) at 100 °C, (IV) at 110 °C, (V) at 120 °C, (VI) at 130 °C, (VII) after 3 min at 130 °C, (VIII) after 10 min at 130 °C, and (IX) after 25 min at 130 °C. Analysis was conducted to verify whether the functional groups changed when temperature was increased from 100 to 130 °C and kept at 130 °C. Figure 6 shows the spectral profiles of LPSG_PEI_BDDEPEI, LPSG_Mix, and LPSG_BOPEI_BDDEPEI, obtained by subtracting the spec-

Figure 5. Weight change profiles of the second cycle from 15 consecutive TSA cycles.

wherein the primary amine reacted with CO2 to form urea at high temperature under the CO2-rich regeneration condition; therefore, the increase of weight occurred during regeneration.27,39 Urea formed in the primary amine groups in the first layer of LPSG_PEI_BDDEPEI, and it caused a continuous decrease of CO2 uptake. The initial CO2 uptake of the TSA cycles was the largest in LPSG_PEI_BDDEPEI; however, the uptake became lower than that of LPSG_BOPEI_BDDEPEI from the 2nd cycle and that of LPSG_Mix from the 13th cycle. In the epoxy-modified PEI, primary amine groups of PEI can be converted to secondary amine groups, resulting in efficient inhibition of urea formation. Therefore, it was verified that the adsorbents prepared with modified PEI (LPSG_BOPEI_BDDEPEI and LPSG_Mix) exhibited long-term stability. In terms of working capacity, the working capacity of LPSG_BOPEI_BDDEPEI was higher than that of LPSG_Mix (Figure 4b). This is a noteworthy result that can demonstrate the advantage of the double-layer structure. Although the two adsorbents had the same organic composition, the formation of a double-layer structure provided an advantage in terms of the working capacity compared to the single-layer structure. This is G


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LPSG_PEI_BDDEPEI after regeneration, did not appear in the spectra of LPSG_Mix and LPSG_BOPEI_BDDEPEI; this result indicates that the urea formed in the high-temperature CO2 atmosphere is mainly because of the primary amine groups. 27,33,39 Moreover, as shown in the results of LPSG_PEI_BDDEPEI, the peaks of the adsorbed species were recovered with the temperature increase and appeared again over time with the urea peak at the regeneration temperature of 130 °C. Thus, it is verified that if adequate energy is supplied, an adsorption mechanism favorable for urea formation occurs at the regeneration temperature. This phenomenon can be obtained from the spectra, wherein the adsorption mechanism peak did not appear up to 10 min after reaching 130 °C; however, the adsorption mechanism peak appeared after 25 min at 130 °C. To further clarify the trends of the peaks in Figure 6, only the profiles obtained by subtracting (I) from (IX) are summarized and presented in Figure 7. As previously mentioned, the −NH2

Figure 7. DRIFTS spectra of adsorbents after 25 min at 130 °C.

stretching and deformation vibrations (3393, 3324, and 1624 cm−1),39 which appeared after sufficient regeneration time, was observed only for LPSG_PEI_BDDEPEI. CO stretching vibration peak at 1687 cm−1 of urea and the carbamate ammonium ion peaks, which are related to the adsorption mechanism, were also observed in the form of distinct peaks only in LPSG_PEI_BDDEPEI. These results indicate that the improvement of chemical stability (inhibition of urea formation) was achieved by using epoxy-modified PEI, as shown in LPSG_BOEPI_BDDEPEI and LPSG_Mix. Although LPSG_Mix exhibited chemical stability similar to LPSG_BOPEI_BDDEPEI, its working capacity was lower than that of LPSG_BOPEI_BDDEPEI. The ATR spectra analysis results to verify the cause is shown in Figure 8. The profiles shown in Figure 8 were obtained by subtracting the spectrum after pretreatment from that after adsorption. Pretreatment and adsorption were performed using TGA, and ATR analysis was performed for the samples after the TGA experiment. The pretreatment was carried out at 100 °C for 1 h, and the adsorption condition was maintained for 3 h after the pretreatment to ensure sufficient adsorption at the gas condition (4% H2O, 15% CO2, balanced N2) similar to the TSA cycles. The peaks common to both the samples are carbamate (1558 cm−1 of −COO− stretching and 1480 cm−1 of COO− stretching) and bicarbonate (1369 cm−1 of −COO− stretching, 1290 cm−1 of C−OH bonding, and 1001 cm−1 of C−OH stretching) peaks.7,34 However, the most notable difference is the peaks of ammonium ions, which pair with carbamate or bicarbonate. The peak of the primary ammonium

Figure 6. DRIFTS spectra of adsorbents. (a) LPSG_PEI_BDDEPEI, (b) LPSG_Mix, and (c) LPSG_BOPEI_BDDEPEI.

trum of state (I) from the spectra of states (II)−(IX).33 In the case of LPSG_PEI_BDDEPEI (Figure 6a), the typical peaks of carbamic acid and carbamate ammonium ion after CO2 adsorption are CO stretching at 1407 cm−1, NCOO− skeletal vibration at 1322 cm−1, −NH3+ stretching at 3060 cm−1, and −NH3+ deformation vibration at 1530 cm−1.26,33,34 As the temperature increased, the peaks of the adsorbed species gradually vanished and relocated to the origin. However, it is noteworthy that the vanished peaks of the adsorbed species began to appear again after 25 min at 130 °C, and new CO stretching vibration of urea formed at 1687 cm−1.27,33,39 In the case of LPSG_Mix and LPSG_BOPEI_BDDEPEI (Figure 6b,c, respectively), the peaks of the adsorbed species observed in the case of LPSG_PEI_BDDEPEI (1675, 1499, 1407, and 1322 cm−1 of carbamic acid and carbamate ion and 3060 and 1530 cm−1 of ammonium ion) also appeared; however, the peaks vanished as the temperature was increased to 130 °C. The peaks of the adsorbed species did not appear even after 25 min at 130 °C, and the peak of urea CO stretching (1687 cm−1) was almost not evident compared to that for LPSG_PEI_BDDEPEI. In addition, the −NH2 stretching vibration of 3393 and 3324 cm −1 , which was evident in the spectra of H


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cross-linker, and PEI was impregnated, the construction of a double layer conserved CO2 adsorption sites. It is rationally expected that the double-layer structured adsorbent using modified PEI in the first layer and cross-linked PEI in the second layer will secure higher physical and chemical stability as well as higher working capacity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01749. SEM images, change of pore size distribution of the adsorbent after hydrothermal treatment for 12 and 24 h, change of organic content of the adsorbent after washing treatment for 4, 8, and 12 h, change of organic content of the adsorbent after cyclic washing treatment, 13C NMR spectra, and CO2 uptake of adsorbents during fifteen simulated TSA processes (PDF)

Figure 8. ATR subtracted spectra of LPSG_BOPEI_BDDEPEI and LPSG_Mix after CO2 adsorption (adsorption condition: 40 °C, 4% H2O, 15% CO2, balanced N2 for 3 h).

ion shows up in LPSG_Mix and the peak of the secondary ammonium ion appears in LPSG_BOPEI_BDDEPEI. It can be mainly distinguished from the C−N stretching vibration. The primary ammonium ion exhibits C−N stretching vibration in the 1124 cm−1 region. However, in the case of the secondary ammonium ion, C−N stretching vibration peak is observed in the 1090 cm−1 region.7,50 The differences in the functional groups involved in the CO2 adsorption of LPSG_Mix and LPSG_BOPEI_BDDEPEI provide an effective basis for explaining the difference in the working capacity measured in TGA. In addition to bicarbonate, carbamate also formed under CO2 adsorption in the presence of water vapor. As expected, the LPSG_Mix has tertiary amine groups converted from secondary amine groups by a cross-linker; consequently, the reduction of adsorption sites owing to the formation of tertiary amine groups affected the decrease of the working capacity. Thus, it is verified again that the double-layer structured adsorbent, which mainly has secondary amine groups by modification and cross-linking, exhibits chemical stability as well as a relatively high working capacity.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.K.). *E-mail: [email protected] (K.B.L.). ORCID

Sunbin Jeon: 0000-0002-8425-3528 Hyunchul Jung: 0000-0002-0554-0108 Ki Bong Lee: 0000-0001-9020-8646 Author Contributions §

S.J., and H.J. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the grants from the Super Ultra Low Energy and Emission Vehicle Engineering Research Center (NRF-2016R1A5A1009592) of the National Research Foundation of Korea (NRF) funded by the Korean Government (Ministry of Science, ICT and Future Planning) and the Korea Institute of Energy Research (B7-2424).

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CONCLUSIONS A new adsorbent structure, which is constructed with a double layer based on modified PEI via a dry impregnation method, was proposed to improve physical and chemical stability and CO2 adsorption performance. The double-layer structured adsorbent (LPSG_BOPEI_BDDEPEI) exhibited remarkable physical stability in terms of phenomena such as leaching and evaporation of amine compared to conventional single-layer amine-impregnated adsorbents (LPSG_PEI and LPSG_BOPEI). Furthermore, the structural collapse caused by hydrothermal treatment was prevented in the double-layer structured adsorbent. The improvements in the double-layer structured adsorbent are owing to the cross-linked PEI present in the second layer. In terms of chemical stability, a meaningful result was obtained in the comparison of the double-layer structure adsorbent and the single-layer structure adsorbent (LPSG_Mix) containing the same composite of epoxymodified PEI and cross-linked PEI. The epoxy-modified PEI prevented urea formation under high-temperature and CO2rich regeneration conditions because the primary amine groups of PEI were converted to secondary amine groups. Also, the working capacity of the double-layer adsorbent was higher than that of the single-layer adsorbent owing to the tertiary amine groups formed in the single-layer adsorbent. It is noteworthy evidence that although a similar composition of the modifier,

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Double-Layer Structured CO2 Adsorbent Functionalized with Modified

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