CO2 Adsorption Behavior and Kinetics on Amine-Functionalized

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CO2 adsorption behavior and kinetics on amine-functionalized composites silica with trimodal nanoporous structure Peiyu Zhao, Guojie Zhang, Yinghui Sun, and Ying Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02292 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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CO2 Adsorption Behavior and Kinetics on Amine-Functionalized Composites Silica with Trimodal Nanoporous Structure Peiyu Zhao1, Guojie Zhang 1, 2, *, Yinghui Sun1, Ying Xu1

1

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi

Province, Taiyuan University of Technology, Taiyuan 030024, China 2

State Key Laboratory of Coal and Coalbed methane Co-Extraction, Jincheng 048012,

Shanxi, China * Corresponding Author’s E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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Abstract A trimodal porous support with special trimodal pore structure has been prepared by physically mixing the silica gel (HPS) and SBA-15 and then devoted to fabricate TEPA-functionalized adsorbent for CO2 capture. The trimodal multistage mesopores structure can promote the TEPA dispersion and mitigate the mass transfer resistance in the adsorbent and hence improved capture performance compared to the single mesoporous support. The influence of the mass ratios of HPS to SBA-15, amine loaded amount, CO2 concentration, adsorption temperatures, and water vapor were studied. The CO2 saturated adsorption amount of 5.05 mmol/g was obtained at 75 oC in dry N2 flow containing 15 vol. % CO2 when the mass ratio of SBA-15 to HPS was 1:2 with 50 wt. % TEPA loadings. And the CO2 saturated adsorption amount presented a 16% improvement in humid N2 flow containing 15 vol. % CO2 flow at 75 oC. In addition, the S2HPS-TEPA50% also demonstrated good stability after ten cycles of adsorption/desorption. Based on in situ DRIFTS results of CO2 adsorption/desorption process, the reaction mechanism of CO2 with active sites was proposed by analyzing the relationships among variations of intensities of functional groups during the reaction. The intraparticle diffusion model was adapted to study CO2 kinetics and the intraparticle diffusion prediction indicated that boundary layer diffusion was the rate-controlling step in the process of CO2 capture. Overall, these results indicate that S2HPS-TEPA50% is promising for CO2 capture.

Key word: Amine impregnated; adsorption kinetics; CO2 capture; diffusion; regenerability


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1. Introduction Carbon dioxide is considered as a major factor in the rise of greenhouse gas at present, and the high demand of fossil fuels (coal, petroleum and natural gas) is the main contributor for CO2 excessive emission into the atmosphere [1-2]. Hence, the capture of CO2 from factory combustion flue gas is regarded as a promising way to prevent the worsening of “greenhouse effect” [3-4]. Currently, chemical adsorption of amine solution has been widely used as traditional technologies for CO2 capture. Although, this method has the advantage of good selectivity and high CO2 absorption capacity [5-6], the amine solution for CO2 capture still exists many shortcomings, such as the toxic, flammable, corrosive, and the high regeneration energy and so forth. These shortcomings make a bottleneck for their long term application [7-8]. To overcome difficulties of the amine solution adsorption, many nanoporous materials has been developed to capture CO2, such as metal organic frameworks [9-11], activated carbons [12-14], mesoporous silica [15-16], metal oxides [17-18], zeolites [19-21], with the advantages of energy savings and stable performance by physical adsorption. However, these solid adsorbents exhibit poor performance at high temperature due to the strong temperature sensitivity and low CO2 selectivity. Meanwhile, these solid porous materials have the shortcoming of poor water tolerance [22]. Recently, solid amine adsorbents has been proposed and widely investigated for CO2 capture as a promising alternative technology to increase CO2 selectivity and capture amount [23-27]. These solid amine adsorbents have significant advantages of


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lower capital cost, fast adsorption kinetics as well as high capture capacity and high CO2 selectivity by contrast with porous materials mentioned above. In addition, the equipment corrosion problems also can be resolved which observed in aqueous amine absorption process. The solid amnie adsorbents can be prepared either by grafting with different amine species covalently bound to the support [28] or wet impregnation with organic amines dissolved in an organic solvent [29]. After the first report of amine-functionalized MCM-41 [30], various types solid materials with lots of nano-pores have been studied for amine-functionalized adsorbents [31]. Among them, many porous silica materials, such as SBA-15 [32], MCM-41 [33], have been widely used as amine supports for CO2 capture because of its excellent surface textural properties, such as large surface area. For these amine modified solid adsorbents, amines can be loaded into the carrier channel leading to the better dispersion of amines, which increased CO2 adsorption capacity. But, there are some deficiencies on these amine-functionalized adsorbents, especially with respect to the carrier materials. These silica carrier materials have single-sized mesoporous which have a great chance of being blocked by organic amines chains, leading to increase the resistance for CO2 diffusion and reduce the CO2 molecular mass transfer rate in the channel. It should be noted that pore structure of the carrier certainly plays an important role in CO2 capture of the solid amine composite nanoporous material. To overcome the limitations of the single-sized mesoporous, and make full use of the advantages of various sizes of porous, the new composite support with multistage mesopores structures attracts the attention of research [34-35].


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By contrast with single-sized porous materials like MCM-41 and SBA-15, multistage mesoporous structures of support have a great advantage. For example, large pore size of the support can provide easier access for CO2 molecular to the amine active sites and prevent stoppage of the channel; large pore volume of the support can improve the loading capacity of amine species and facilitate dispersion and exposure of amine active sites. The multistage mesoporous structure also can decrease the resistance for CO2 diffusion inside the pore of the adsorbents leading to enhancement of CO2 capture amount [36-37]. Guo et al. successfully prepared a PEI-modified silica monoliths with hierarchical pore structures centered at 12.2 nm and 1 um. The reported adsorption amount for a 60% PEI/monolith reached 2.44 mmol/g at 75 oC in the CO2 partial pressure of 100 kPa [38]. Han et al. synthesized a spherical silica foam support with a hierarchical mesoporous–macroporous structure and modified it with PEI using the wet impregnation method. The CO2 adsorption capacity can reach to 4.28 mmol/g [39]. The excellent adsorption capacities for these sorbents were attributed to the hierarchical mesoporous–macroporous structures of the supports, where the mesopores primarily acted as active sites, and the macropores provided easy accessibility to the active sites. However, the silica material with hierarchical porous structure using as support for CO2 adsorption separation still has some problems, such as complex preparation process, longer preparation cycle. For example, the wet silica gel must be aged for 3 d and the solvent exchange process was conducted every 24 h by replacing the solution with ethanol for three times in the preparation process of silica monoliths prepared by Guo et al [38]. Thus, a support


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layer with a unique trimodal mesopores structure was obtained by simple physically mixing a mesoporous SBA-15 and silica gel (HPS) to solve the above problems. Compared with the preparation method of general silica hierarchical pore support, this kind of physical mixing method is simple and time-saving. In this work, a new type support with a trimodal multistage mesopores was prepared by sample physically mixing SBA-15 and silica gel (HPS). Then the trimodal mesopores silica was utilized as the support to prepare amine-functionalized adsorbents using wet impregnation with TEPA for CO2 adsorption. The textural properties (e.g., pore structure, pore volume) and thermal stability of the before and after modified support were characterized. CO2 adsorption performances of the prepared adsorbents were investigated in a homemade fixed bed reactor under different conditions (e.g., weight ratios of SBA-15 to HPS, adsorption temperatures, the presence of water vapor, CO2 concentration). Besides, the adsorption kinetics and reaction mechanism of CO2 were also studied. 2. Experimental 2.1. Materials Cetyltrimethy-lammonium bromide (CTAB), P123 (PEO20PPO70PEO20, Mn=5800), Tetraethyl orthosilicate (TEOS, AR), ammonia solution (28%), tetraethylenepentamine (TEPA, 90wt. %) and anhydrous ethanol was were provided by Taiyuan Chemical (Taiyuan, China). Carbon dioxide (99.999%) and nitrogen (99.999%) were obtained from Lifeng Chemical Reagent (Taiyuan, China). 2.2 Synthesis of support


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The HPS was synthesized based on the methods reported by Sravanthi et al. [40]. 36 mL PEG was dissolved in 240 mL deionized water and stirred for 10 min to form a mixed solution at 40 oC. Then, 20 mL TEOS was drop wised into the above solution, followed by the addition of 4 g CTAB under stirring. Then the gel mixture was formed after the addition of 2 mL ammonia solution with keep stirring. The gel mixture was sealed to prevent ammonia volatilization. After the gel mixture was stirred at 40 oC for 12 h, the mixture was vacuumed filtered and washed with the ethanol and deionized water, and dried at 100 oC for 24 h. Then, the resulting product was obtained after calcination at 550 oC for 6 h to remove the surfactant. SBA-15 was prepared referring to the following methods [41]. Specifically, 4 g template agent (P123) was dissolved in 144 mL of 1.7 M hydrochloric acid at 40 oC with a magnetic stirrer. After P123 complete dissolution, 9.2 mL silicon source (TEOS) was dropwise in the above solution. The mixture was stirred constantly for 24 h and diverted to a steel autoclave and kept 24 h at 100 oC. Then, mixture was filtered with the distilled water and ethanol, dried at 100 oC for 12 h, and finally calcined at 550 oC for 6 h to remove template agent. The HPS and SBA-15 were vacuum dried 12 h to get rid of the physically adsorption CO2 and H2O. Afterwards, ratios of HPS and SBA-15 were physically mixed to obtained well-distribution mixed carrier. The obtained carriers were named as SHPS, 2SHPS and S2HPS, which stands for the mass ratios of SBA-15 to HPS of 1:1, 2:1 and 1:2, respectively. 2.3. Preparation of amine-functionalized adsorbents


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The TEPA-modified adsorbents were prepared through a typical impregnation technique [42]. The desired amount TEPA was added in 20 mL of methanol with magnetic stirring for 20 min to facilitate the TEPA dispersion. Afterwards, 1 g mixed carrier was introduce to the above solution. The solution was further magnetic stirred forwards the slurry mixture generated. Then, the slurry mixture was dried at 85 oC for 12 h to obtain the adsorbents. The adsorbents was designated as SHPS-TEPA40%, SHPS-TEPA50%, 2SHPS-TEPA40%, 2SHPS-TEPA50%, S2HPS-TEPA40% and S2HPS-TEPA50%, where 40% and 50% stands for the mass loaded percentages of TEPA in adsorbents. 2.4. Characterization The isotherm of adsorption-desorption was obtain at -196 oC by using of the principle of physical adsorption of N2. The adsorbents were vacuum-treated for 4 hours at 100 oC before adsorption of N2. The B.E.T. equation was applied to calculate the specific surface area. The total pore volumes were estimated at P/P0 = 0.99. The distribution of pore structure was obtained from desorption branches of isotherms through the BJH model. Fourier transform infrared (FTIR) was utilized to analysis the functional groups characteristic of the sample. The samples were firstly mixed and milled with KBr, and then spectra were recorded in the 400-4000 cm−1 region. In situ IR DRIFTs (Thermal Scientific) used for CO2 adsorption. The inlet gases were controlled by flow monitor and switched through a 4-port valve. Solid adsorbent was filled in the DRFITS with 80 mg. The IR cells were placed inside of FTIR (BrukerV70). The spectra was recorded in the frequency 400-4000 cm−1 region, the


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adsorption progress of adsorbent was scanned once every 20 s, the desorption progress of adsorbent was scanned once every 30 s. Thermogravimetric analysis was carried

out

to

characterize

the

thermal

stability

of adsorbents

with

a

thermogravimetric analyzer (NETZSCH STA 409PC) from room temperature to 600 oC at a heating rate of 10 oC min-1 with a N2 atmosphere. The C, H, and N elemental contents present in the adsorbents were tested by an elemental analyzer (EA3000, Euro Vector). 2.5. Carbon dioxide adsorption-desorption test. CO2 adsorption/desorption tests were performed in a homemade fixed-bed reactor at atmospheric pressure (Fig. 1). 1.0 g adsorbent was placed in the middle of the reaction tube and supported by quartz cotton. Before each CO2 adsorption test, the adsorbent was heated to 100 oC for 90 min with 100 mL/min N2 stream. After cooling to the desired adsorption temperature, the gaseous mixture containing the desired CO2 concentration with a flow rate of 60 mL/min was introduced and passed through the reaction tube. When the effluent CO2 concentration of reaction tube was equal to the influent concentration of CO2, the adsorption process was done, and then the temperature of adsorbent was risen to 100 oC and kept for 90 min with 100 mL/min pure N2 to release of adsorbed CO2. The mass flow controllers were used to control the flow rates of the gaseous mixture. The outlet CO2 concentration of the fixed-bed reactor was monitored with an online gas analyzer. In order to study the influence of water vapor on CO2 capture, water vapor was introduced into the gas stream by passing N2 through a water bubbler that had been


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placed in a oil bath. The integral equation of the capture amount of CO2 is described in eq. (1):

q =

Q ×

t

∫ (C 0

0

W

− C )dt (1)

where q stand for the capture amount of CO2 on the adsorbents (mmol/g); Q stand for the influent rate of the gaseous (mL/min); t stand for adsorption time (min);W stand for the weight of adsorbent (g); C0 and C stand for the CO2 content of the inlet and outlet of reactor, respectively. Qs is defined as the CO2 saturated capture amount when C is equal to C0, while breakthrough capture amount (Qb) is defined as the CO2 capture amount when C is equal to five percent of C0; the time corresponding to the breakthrough capture amount is described as breakthrough time. 3. Results and discussions 3.1 Characterization of the adsorbents N2 adsorption/desorption isotherm, pore structure distributions of support and adsorbents are presented in Fig. 2, Fig. 3, Fig. 4, Fig. 1S and 2S, respectively. The textural properties parameters of support and adsorbent are summarized in Table 1. As shown in Fig. 2, the S2HPS shows a classic type IV isotherm on the basis of the IUPAC classification. And the isotherm has a rapid increase in N2 volume adsorbed in a 0.6-1.0 relative pressure region and shows a small enhancement of N2 volume adsorbed in a 0.1-0.6 relative pressure region, indicating that the existence of the interparticle larger mesoporous and intraparticle mesoporous in S2HPS support [43]. After 40 wt.% and 50 wt.% TEPA loading, the isotherms still has a hysteresis loops


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but the adsorption capacity of N2 decreases with the increase of TEPA loading, indicating the reduce of the corresponding surface area and pore volume of S2HPS-TEPA40% and S2HPS-TEPA50%. As indicated in Fig. 3, pore size distribution represents two types of pore size in HPS (2.6 nm and 23 nm), and the pore size was centered at 6.1 nm for SBA-15. After the physically mixing the SBA-15 and HPS with a mass ratio of 1:1, the channel centered at 2.6 nm , 6.1 nm and 23 nm still exists. As shown in Fig. 4, when the 40 wt. % TEPA loaded on the S2HPS, the peak of the channel centered at 2.6 nm disappeared for the S2HPS-TEPA40%, and the peak intensity of the channel centered at 6.1 nm and 23 nm declined. When the 50 wt. % TEPA loaded on the S2HPS, the peak intensities of the channel centered at 2.6 nm and 6.1 nm disappeared, and the peak intensity of the channel centered at 23 nm declined. For TEPA impregnated 2SHPS and TEPA impregnated SHPS, the same phenomenon could also be observed, respectively (Fig. 1S and 2S). This phenomenon might be related to the fact that with the enhancement of TEPA loading into the trimodal multistage mesoporous support, the small size channel of support were preferentially filled by the TEPA molecules. That is to say, the relatively small mesoporous shows high selectivity for TEPA, and the relatively large mesoporous can promote the diffusion of CO2 molecules to the active sites, the dispersion of TEPA and the reduced pressure drop across the materials in the hierarchical mesoporous structures support. When the organic amine is loaded on the support, the amine was covered in a multilayer form or even caking form on the support pore surface with the increase of the amine loading. Small mesoporous are


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easy to be blocked, but the larger mesoporous are not easy to be blocked. The addition of 6.1 nm pores of SBA-15 can not only provide a large mesoporous but form a trimodal hierarchical pore structures. The presence of 6.1 nm mesoporous can share the TEPA loading amount of smaller mesoporous to reduce the possibility of smaller mesoporous clogging. More smooth diffusion channel retained in the adsorbents after TEPA modification can greatly minimize diffusion barriers of CO2 to the active sites in the adsorbent. As list in Table 1, for the S2HPS, the Brunauer–Emmett–Teller (BET) surface area was 857.56 m2/g; the pore volume was 1.79 cm3/g. These two parameters both arrayed between the corresponding values for SBA-15 and HPS. When 40 wt. % and 50 wt. % TEPA loaded into the S2HPS, the surface area and pore volume significantly declined, indicating that organic amines were successfully introduced into the carrier. As can see from Table 1, the surface area and pore volume of S2HPS-TEPA50% was 54.24 m2/g and 0.48 cm3/g, respectively. They were much higher than the corresponding values of SBA-15-TEPA50% and HPS-TEPA50%. These results indicate that the addition of 6.1 nm mesopores of SBA-15 increases the residual pore volume and specific surface area in the adsorbent. These larger surface area and pore volume retained in the adsorbents after TEPA modification are able to potentially enhance the distribution of TEPA active sites and greatly minimize diffusion barriers of CO2 to the active sites in the adsorbent pores and reduce the pressure drop across the materials. So the CO2 adsorption performance of S2HPS-TEPA50% was notably improved. The saturated capture amount of S2HPS-TEPA50% (5.05 mmol/g) was


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higher than the corresponding values of HPS-TEPA50% and SBA-15-TEPA50% (4.26 mmol/g and 4.1 mmol/g, respectively). Figure 5 shows the SEM images of the S2HPS. Form Fig. 5, it can be found that the large external pores between the particles were observed, which promoted the CO2 molecules transfer from the gas phase to the surface of the organic amines active sites. In a word, with the trimodal multistage mesopores structure characteristics, the support S2HPS makes it possible to provide easier access to the active sites and reduce diffusion resistance for CO2 in the channel. Thus, the obtained support S2HP with trimodal multistage mesopores structure can be used as perfect support in the process of CO2 adsorption. The TGA curves of S2HPS and adsorbents modified with TEPA are displayed in Fig. 6. For S2HPS, the weight loss was almost negligible with the temperature increase, a little loss of weight is mainly because the dismantlement of physically adsorbed H2O, suggesting that the support has a better thermal stability performance. For the adsorbents with different TEPA loading amount, the differential curve of weight to temperature clearly shows two weight loss stages. A small weight loss stage was observed below the temperature of 135 ℃, this part weight loss could be due to the release of physically adsorbed water, pre-adsorbed CO2 and residual solvent which cannot be fully evaporated in the preparation of adsorbents. A more rapid decrease stage in quality of the adsorbent appeared in the temperature range of 135-600 oC. This rapid weight loss of the adsorbent is mainly caused by the decomposition combustion and volatilization of TEPA molecules impregnated in the mixed support. It confirmed that TEPA was actually loaded into or onto the mixed


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support. Apart from the weight loss of adsorbed H2O and pre-adsorbed CO2 and residual solvent, the total weight loss of S2HPS-TEPA40%, S2HPS-TEPA50% adsorbents is about 38%, 47% in the temperature range of 24 to 600 oC, respectively, which is about equal to the designed TEPA impregnation amounts. These results indicated that almost the same amounts of TEPA were introduced into the mixed support. And this is no significant TEPA mass loss occurred in the process of adsorbent preparation. Moreover, the prepared adsorbents remained stable when the temperature is under 135 oC, which can be apply to the whole process of CO2 adsorption-desorption. Fig. 7 provides the FT-IR characterizations of S2HPS and adsorbents. For the S2HPS, a wave number at 3427 cm-1 is related to the stretching vibration of Si-OH and physically adsorbed H2O [44], three wave numbers at 459 cm-1, 806 cm-1, and 1081 cm-1 are caused by the bending vibration, symmetric stretching vibration and asymmetric stretching vibration of Si-O-Si [45]. For adsorbents modified with different mass fraction TEPA, the wave numbers for the S2HPS were still exist, indicating that the framework structure of S2HPS did not be change due to the introduction of TEPA. Compared with S2HPS, the new peaks for adsorbents with modified 40wt. % and 50 wt. % TEPA at 3383 cm-1 and 1312cm-1 for NH stretching from the secondary amine in TEPA were found [46]. The wave numbers at 2955 and 2845 cm-1 are caused by the CH asymmetric and symmetric stretching modes of the TEPA molecular [47]. The new peak at 1470 cm-1 and 1566 cm-1 was attributed to the symmetric and non-symmetric stretching vibrations of N-H in the primary amine in


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TEPA [46]. These results suggesting that the support was modified successfully by TEPA. Moreover, the intensity of characteristic peaks gradually increased with the increase of TEPA impregnation amount, suggesting that the support accommodated more and more TEPA molecular. This phenomenon mentioned above indicated that the designed composite adsorbents have been successfully prepared. This result was further certified by the elemental analysis (Table 2). It could be found that the support contains small amount nitrogen due to the residual templating agent. On the contrary, compared with support, the nitrogen content of adsorbents increased with the increase of TEPA loading amount, which indicated that TEPA had been introduced into the S2HPS. 3.2 CO2 adsorption performance of the adsorbents 3.2.1 Effect of weight ratios of SBA-15 to HPS and TEPA loadings The SBA-15 and HPS were physically mixed with the mass ratios of 2:1, 1:1 and 1:2, and then functionalized by 40 wt. %, 50 wt. % TEPA. The CO2 capture performances of these adsorbents were investigated at 75 oC with N2 flow containing 15 vol. % CO2 of 60 mL/min. The CO2 capture amount and breakthrough curves of these adsorbents are presented in Fig. 8, Fig. 3S and Table 3, respectively. The saturated capture amount, breakthrough capture amount and breakthrough time for the TEPA-functionalized S2HPS were higher than the corresponding values of the functionalized SHPS and 2SHPS. It showed that S2HPS, with a 1:2 mass ratio of SBA-15 to HPS, was more suitable as the carrier for the CO2 adsorbents. Moreover, breakthrough capture amount and saturated capture amount of 4.09 mmol/g and 5.05


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mmol/g for S2HPS-TEPA50% were higher than the corresponding values of 3.24 mmol/g and 4.26 mmol/g for the TEPA-functionalized HPS as well as TEPA-functionalized SBA-15. As shown in Table 3, the S2HPS-TEPA50% showed optimal breakthrough time and saturated capture amount among the prepared TEPA modified adsorbents, and the value was 10.2 min with 5.05 mmol/g. In addition, the breakthrough adsorption amount of S2HPS-TEPA50% can reach to 81% of the saturated capture amount. And the breakthrough capture amount of above 3.0 mmol/g is applicable in industrial processes. Therefore, it can be realized to capture CO2 quickly from the flue gas by using S2HPS-TEPA50% as adsorbent in industrial application. The CO2 capture amount in this work was compared with those of different amine-functionalized adsorbents reported in the literature in Table 4. It is noted that S2HPS-TEPA50% had excellent performance in terms of the CO2 adsorption capacity. The trimodal multistage mesopores structure existed in the carrier S2HPS may make attributed to the enhancement of the CO2 capture amount. The different pore size distribution range can provide easier access for the CO2 to the amine active sites and reduce diffusion resistance for CO2, facilitate the TEPA dispersion in the channel and reduce pressure drop of the reactor [54]. With the TEPA loaded amount increase from 40 wt. % to 50 wt. %, more amine active sites were exposed in the carrier channel. Moreover, the small mesoporous disappeared and the larger mesoporous still remained.

Because

of

mentioned

above,

the

CO2

S2HPS-TEPA50% was improved.


capture

amount

of

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Because of the fact that the good CO2 capture performance of S2HPS-TEPA50%, this adsorbent was further test to study the influence of capture temperature, CO2 concentration and water vapor on CO2 capture. 3.2.2 Effect of the capture temperature CO2 capture temperature plays an important role in the adsorption performance of solid amine adsorbents that can be optimized to maximize the CO2 capture capacity. To verify the effect of temperature on the CO2 capture process, the CO2 adsorption performance of S2HPA-TEPA50% was investigated at 35 oC, 55 oC, 75 oC and 85 oC with 60 mL/min N2 flow. The results were shown in Fig. 9 and Fig. 4S. With the increase of temperature, the CO2 capture amount increased, and the maximum value of adsorption capacities are 5.05 mmol/g at 75 oC, and then decreased with the temperature further rises. The adsorption capacities of CO2 are 3.96 mmol/g and 4.25 mmol/g at 35 oC and 55 oC, respectively. And the CO2 capture capacity dropped to 4.21 mmol/g at 85 oC. The breakthrough adsorption capacity and breakthrough time have the same rule and all reached their maximum values at 75 oC. The resulting phenomenon gives an agreement with the previously reported by Chen et al. [35]. They also observed the optimum temperature at 75 oC for performance of CO2 capture over the TEPA/MCM-41 from a simulated gas stream via a fix-bed method. It is an exothermic reaction process between CO2 with amine functional groups. And it is beneficial for this reaction in the low temperature. But, the low temperatures are not beneficial for the transfer of carbon dioxide molecules and the dispersed and exposed of active sites. So the phenomenon of temperature-dependent adsorption


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indicates that the CO2 adsorption process is controlled by kinetics process and thermodynamics process together. At low temperature, some of TEPA loaded inside the pore dispersed as bulk nanoparticles, the CO2 molecules only can accessible to external active sites of the organic amine layer [42]. And the CO2 molecules diffusion from the surface to the bulk of TEPA is considered to be the main adsorption resistance. When the temperature increase to 75 oC, the kinetic energy of CO2 molecules increased and the TEPA viscosity is decreased, leading to the increase of the spacing between TEPA molecules and the decrease of internal diffusion resistance from the surface to the bulk of TEPA. Meanwhile, more active sites of organic amine were better dispersed to occupy all the available space in the support channel to reaction with CO2 with the enhancement of TEPA molecules activity. This makes a great contribution for the enhancement of CO2 adsorption amount. Thus, when the temperature increased from 35

o

C to 75

o

C, the adsorption amount of

S2HPS-TEPA50% increased. Although the reaction process between CO2 and TEPA is exothermic, the observed temperature dependence in the 35-75 oC indicated that the CO2 capture process is controlled by the diffusion kinetics rather than thermodynamic factors [29]. With the increase of temperature to 85 oC, although more amine active sites are becoming accessible, CO2 desorption from capture active sites in the pore becomes more preferential and the CO2 capture process was controlled by thermodynamics, resulting in a decrease of the CO2 capture amount. 3.2.3 Effect of the CO2 concentration In practical applications, the solid amine adsorbent should have superior


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adsorption properties over a wide range of CO2 concentrations under prolonged operations. The adsorption properties of CO2 in mixed gases with different CO2 concentrations are shown in Fig. 10. As the CO2 concentration increased from 5 vol. % to 45 vol. %, the breakthrough and saturated capture amount of S2HPS-TEPA50% also increased. When the initial concentration of CO2 is 5 vol. %, the adsorption capacity of solid amine adsorbent is 3.89 mmol/g, which is about 16.5% lower than that of CO2 concentration of 45 vol. %. When the concentration increases from 20 vol. % to 45 vol. %, the adsorption capacity was essentially no longer changed. The result indicated that the dynamic adsorption capacity of S2HPS-TEPA50% is related to the initial concentration of CO2. With the increase of CO2 concentration, the content of CO2 per unit volume increased, suggesting that more and more CO2 molecules could be in touch with organic amines active sites in the support channel, and thereby improve the capture amount of S2HPS-TEPA50%. It is noteworthy that the breakthrough adsorption amount of S2HPS-TEPA50% exceeds 80% of the saturated adsorption amount in the different CO2 concentration. And S2HPS-TEPA50% can maintain high carbon dioxide adsorption capacity even if the carbon dioxide concentration is very low. So this adsorbent can be not only suitable for the efficient CO2 capture for the flue gas but also be fit for the adsorption of low-concentration carbon dioxide. 3.2.4 Effect of the water Water vapor is one of the ingredients of flue gas. Taking the practical application into account, it is necessary to investigate the influence of water vapor on the CO2


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capture amount. The influence of the water on the CO2 capture was investigated under humid condition at 75 oC with 15 vol. % CO2 in N2 flow of 60 mL/min. The water vapor was introduced to the mixed gas by water bubbling. Fig. 11 shows the CO2 capture amounts and breakthrough curves of S2HPS-TEPA50% and S2HPS-TEPA40% in both absence and presence of water vapor. As shown in the Fig. 11, compared with the dry condition, carbon dioxide adsorption amount and breakthrough time both increased significantly in the presence of water vapor for S2HPS-TEPA50% and S2HPS-TEPA40%. CO2 capture amount under dry conditions were lower than those under humid conditions for S2HPS-TEPA50% and S2HPS-TEPA40%. The CO2 capture amount was as high as 5.86 mmol/g for S2HPS-TEPA50% at the presence of water vapor with relative humidity of 60%, which has 16% improvements by contrast with that in the absence of water CO2 adsorption. These results indicated that water vapor had a positive influence on the CO2 capture over the TEPA modified adsorbents. The better performance of water tolerance would wipe out the need for strict control of water vapor content before the CO2 capture. This should be because of the different reactions way taking place in the absence and presence of water vapor. 1 mole CO2 react with 2 moles of amine groups to form a carbamate compound in the absence of water vapor. However, in the presence of water vapor, 1 mole CO2 can directly react with 1 mole of CO2 and H2O to form carbamic acid. So it deserved much higher improvement of CO2 capture amount in the presence of water vapor [46]. Theoretically, according to the chemistry reaction stoichiometries ratio, in the


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Energy & Fuels

presence of water, the CO2 capture amount should be twice as high as that in the dry condition for the TEPA modified adsorbents. However, the experimental value of the CO2 capture amount under humid condition was 1.16 times as much as the value obtained under dry condition. The enhancements were not as good as expected. It is due to a thin layer formed by the introduction of water vapor in the adsorbent channel. This thin layer could prevent the carbon dioxide molecules from contacting with the active sites of the organic amine and increase the diffusion resistance for CO2 to transfer into the TEPA layer [55-56]. 3.2.5 Regenerability of the adsorbent Taking the real industrial application into account, except the large capture amount of CO2, the excellent regeneration performance of adsorbents is also very necessary for adsorbents in a long-time operation process of adsorption-desorption. In present study, ten cycles of CO2 adsorption-desorption were carried out to assess the regeneration performance of S2HPS-TEPA50%. The capture process of CO2 was performed at the temperature of 75 oC from the mixed gas containing 15 vol. % CO2 and 85 vol. % N2. Then the adsorbent was desorption at a temperature of 100 oC, taking pure N2 as a stripping gas with the flow rate of 100mL/min. The CO2 saturated capture amount for each adsorption-desorption cycles is shown in Fig. 12. The result shows that with the increase of the cycle’s number, the CO2 capture amount gradually reduced. The CO2 saturated capture capacity reduce from 5.05 mmol/g to 4.7 mmol/g after ten CO2 adsorption-desorption cycles. The capture capacity only reduced 6.9% by contrast with the first capture amount. The results indicated that the


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S2HPS-TEPA50% has a better regeneration performance by contrast with TEPA-functionalized MCM-41 and silicone gel, in which the values of 7.4% [57] and 8.0% [58] were reported in the literature, respectively. In addition, the elemental content for the fresh and regenerated adsorbent was test in order to further prove the stability of S2HPS-TEPA50%. The results were shown in Table 5. It is very worth noting that the nitrogen content of S2HPS-TEPA50% only has a slight decrease after ten adsorption-desorption cycles by contrast with the fresh one. There are two possible reasons for the decrease of CO2 capture amount during the CO2 adsorption-desorption cyclic process. One of the reasons is the incomplete desorption of pre-adsorbed CO2 during the CO2 adsorption-desorption cyclic process. Another reason is the evaporation of TEPA species from loading layer on the adsorbent during the CO2 adsorption-desorption cyclic process. Compared with single-size minor mesoporous support blocked easily by the TEPA resulting in some TEPA coat the outer surface of carrier as the bulk, the S2HPS-TEPA50% with a trimodal multistage mesopores structure can disperse almost all of the TEPA into the different levels channel, so the TEPA was more stably immobilized than that coated outside. Furthermore, the saturated capture amount for S2HPS-TEPA50% still can reach to 4.7 mmol/g after ten adsorption-desorption cycles, which completely fulfills the actual industrial requirement of 3 mmol/g. Therefore, S2HPS-TEPA50% is applicable for CO2 capture from the flue gas in industrial processes. 3.3 CO2 adsorption reaction mechanism The in situ FT-IR experiments have been carried out on the S2HPS-TEPA50%


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during CO2 capture process to further understand the reaction mechanism between the CO2 and amine group. The in situ FT-IR difference spectra of CO2 adsorption-desorption was shown in Fig. 13 and Fig. 14. Fig. 13 provides an overview of 2D waterfall IR absorbance spectra of CO2 adsorption and desorption cycle during CO2 capture, including CO2 capture at 75 oC from the mixed gas (15 vol.% CO2 and 85 vol.% N2) and CO2 desorption at 100 oC with pure N2 as a stripping gas. In situ FT-IR difference spectra of the CO2 adsorption and desorption process was shown in Fig. 14 (a) and (b), respectively. Table 6 provides the summary of the IR band assignments. The CO2 adsorption process leads to an increase of peak intensity of the many functional groups. These peaks produced by captured CO2 can be clearly observed in the absorbance spectra in Fig. 14 (a), which were obtained by subduction adsorbent itself spectrum from those spectra collected in the CO2 adsorption process. The characteristic of captured CO2 can be revealed by examining relationships among variations of intensities of these group peaks, including the NH band at 3298 cm-1, NH2 at 3365 cm-1 from the TEPA, NH3+ at 1639 cm-1, C=O at 1685 cm-1 from carbamic acid (NCOOH) and O=C=O- at 1494 cm-1 from carbamate (NHCOO-) in the CO2 adsorption and desorption process in Fig. 14 (a) and (b) [59]. During CO2 adsorption process, with the increase of reaction time of CO2 and organic amine groups, the intensity of NH2 and NH decreased, the wavenumbers at 3365 cm-1 and 3298 cm-1 respectively. On the contrary, the intensity of NHCOO- at 1494 cm-1, NCOOH at 1685 cm-1 and NH3+ at 1639 cm-1 increased. The CO2 desorption process


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was provided in Fig. 14 (b), which lead to the restoration of NH2 at 3365 cm-1 and NH at 3298 cm-1, and decrease in the intensities of NCOOH, NH3+ and NHCOO-. The gas phase carbon dioxide at 2360 cm-1 was almost completely removed by the flowing pure N2. According to relationships among variations of intensities of above group peaks and the influence of water vapor on the adsorption capacity, it can be known that the formation of carbamic acid plays a very important role in CO2 adsorption process because of its stoichiometry between CO2 and organic amine is 1:1. This is by contrast to the ammonium carbamate. The formation of ammonium carbamate has a stoichiometric ratio of CO2 to amine of 1:2. Therefore, the increase of carbamic acid formation content on the adsorbent will enhance CO2 adsorption amount. Scheme 1, Scheme 2 and Scheme 3 displays the proposed interaction pathway between CO2 and amines on the S2HPS-TEPA50% adsorbent. The impregnated TEPA only contains primary amines (NH2) and secondary amines (NH). The zwitterion is regarded as an intermediate during the adsorption reaction process between CO2 and amine. Scheme 1 depicts that recreation process between two primary amine (NH2) sites and CO2 under the dry and humid condition. Firstly, a primary amine reacts with CO2 to form the ammonium-carbamate zwitterion intermediates. Then the zwitterion converted to carbamate through transfer of a proton from zwitterion to another primary amine active site. The carbamate continues to react with water vapor to produce carbamic acid and primary amines active site under the humid condition. Scheme 2 depicts that the CO2 adsorption process by reacting with a primary amine and a secondary amine


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active sites. Scheme 3 shows the possible process of CO2 adsorption by two secondary amines active sites. 3.4 Intraparticle diffusion model The process of gases adsorption on the solid adsorbent may be controlled by the mass transportation of gas phase or the intraparticle mass transportation rate. According to Weber-Morris, if process of the gases adsorption is governed by intraparticle diffusion, the capture amount of CO2 (qt) should vary linearly with the square root of time. The intraparticle diffusion model generally consists of three steps [65-66]. The first stage is the gases external diffusion adsorption or boundary layer diffusion of gas phase. The second stage is the gradual adsorption stage where the intraparticle diffusion of gas molecules occurs. The third stage is the adsorption equilibrium. The intraparticle diffusion model is come from Fick’s second law of diffusion. Assumptions: the mass transfer resistance of external diffusion is only important for a very short time at the beginning of diffusion; the axial diffusion is negligible and the direction of diffusion is only radial; the pore diffusivity is constant and does not change with time. The CO2 capture amount is described as the following form proposed to predict the rate-controlling step:

q = k t +C 1/ 2

t

id

Where kid (mmol/g min1/2) represents the intraparticle diffusion rate constant; t (min) represents CO2 adsorption time; C is the boundary layer thickness; qt (mmol/g) represents the CO2 capture amount of S2HPS-TEPA50%. Weber-Morris put forward


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that if different stages appear in the CO2 capture process with the different diffusion rates, multi-linearity can be observed. The slope of the linear part of each stage represents the adsorption rate. And the rate-controlling step can be determined by the lowest slope stage. Fig. 15 shows the Weber-Morris plot for CO2 capture on S2HPS-TEPA50% at 55 oC and 75 oC with the influent rate of 60 mL/min and 15 vol. % CO2/85 vol. % N2 to predict the rate-controlling stage. It is obvious that the plots are not linear over the whole time range. And they presents three mode-linearity indicating that three continuous stages with different rate constants existence. Loganathan et al. observed same situation the on a MCM-41 [67]. The first scope ‘I’ is bound up with CO2 diffusion through the gas phase to the surface of S2HPS-TEPA50% corresponds to the CO2 boundary layer diffusion. The second scope ‘II’ shows the gradual adsorption stage, where the intraparticle diffusion of CO2 molecules occurs. The third scope ‘III’ is bound up with the reaction equilibrium stage. The intraparticle diffusion begins to slow down as a result of adsorbent amines active sites saturation in the equilibrium stage. It can be seen that the first stage show the least slope. This indicates that the CO2 diffusion from bulk gases phase to the surface of carrier or the surface of the adsorbent amines active sites is the slowest and therefore the rate-controlling step for CO2 capture on S2HPS-TEPA50%. Thus, it is very necessary to create appropriate conditions to enhance the boundary layer diffusion for CO2 capture on solid amine adsorbent. Using continuous mixing and microwave oscillation methods to improve the dispersion of the active ingredient in the support pores, adding polymer dispersing


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additives to reserve a few residual pores after amine impregnated, utilizing multistage pores material as support to promote CO2 diffuse to the active ingredient surface are beneficial for facilitating the boundary layer diffusion. 4. Conclusions A new type support with trimodal multistage pore structure has been prepared by physically mixing the SBA-15 and silica gel, and then devoted as the support to fabricate TEPA-functionalized adsorbent for CO2 capture. The trimodal multistage mesopores structure can promote the activated sites dispersion and mitigate the mass transfer resistance in the adsorbent, leading to a dramatically enhancement CO2 capture capacity compared to the single mesoporous support. The CO2 saturated adsorption amount of S2HPS-TEPA50% was 5.05 mmol/g obtained in dry N2 flow containing 15 vol. % CO2 at 75 oC, which was higher than the capture amount for the TEPA-functionalized SBA-15 and HPS. And the CO2 saturated adsorption amount presented a 16% improvement in N2 flow containing 15 vol. % CO2 flow with a relative humidity of 60%, it indicated that the presence of water vapor had an important effect on the CO2 capture amount. Moreover, the reaction mechanism of CO2 with active sites was proposed according to the in situ DRIFTS results by analyzing the relationships among variations of intensities of functional groups. The intraparticle diffusion prediction indicated that boundary layer diffusion was the rate-controlling step in the process of CO2 capture. In addition, the S2HPS-TEPA50% also demonstrated good stability after ten cycles of adsorption-desorption. This new type trimodal multistage pore adsorbents not only has high CO2 capture amount and


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good stability, but also can provide fast diffusion and mass transfer channel. Thus this adsorbent is a promising material for CO2 capture from the flue gas. Supporting Information The pore size distributions and adsorption breakthrough curves of adsorbents. Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 21676174, 21376003 and U1610115), International S&T Cooperation Program of Shanxi province (201703D421038), Shanxi Scholarship Council of China(No. 2017-036) and Joint Fund of Shanxi Provincial Coal Seam Gas (2015012019). References [1] Yaumi, A. L.; Abu Bakar, M. Z.; Hameed, B. H. Recent advances in functionalized composite solid materials for carbon dioxide capture. Energy 2017, 124, 461-480. [2] House, K. Z.; Harvey, C. F.; Aziz, M. J.; Schrag, D. P. The energy penalty of post-combustion CO2 capture & storage and its implications for retrofitting the U.S. installed base. Energy Environ. Sci. 2009, 2, 193-205. [3] Baciocchi, R.; Carnevale, E.; Costa, G.; Lombardi, L.; Olivieri, T.; Paradisi, A.; Zanchi, L.; Zingaretti, D. Pilot-Scale Investigation of an Innovative Process for Biogas Upgrading with CO2 Capture and Storage. Energy Procedia 2013, 37, 6026-6034. [4] MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C.S.; Williams, C.K.; Shah, N.; Fennell, P. An overview of CO2 capture


Page 28 of 62

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technologies. Energy Environ. Sci. 2010, 3, 1645-1669. [5] Wang, J.; Huang, L.; Yang, R.; Zhang, Z.; Wu, J.; Gao, Y.; Wang, Q.; O'Hareb, D.; Zhong, Z. Recent advances in solid sorbents for CO2 capture and new development trends. Energy Environ. Sci. 2014, 7, 3478-3518. [6] Prakash, P.; Venkatnathan, A. Molecular mechanism of CO2 absorption in phosphonium amino acid ionic liquid, RSC Adv. 2016, 6, 55438-55443. [7] Rahman, F. A.; Aziz, M. A.; Saidur, R.; Wan, A.; Hainin, M. R.; Putrajaya, R.; Hassan, N. Pollution to solution: Capture and sequestration of carbon dioxide (CO2) and its utilization as a renewable energy source for a sustainable future. Renewable Sustainable Energy Rev. 2017, 71, 112-126. [8] Wang, S.; Yan, S.; Ma, X.; Gong, J. Recent advances in capture of carbon dioxide using alkali-metal-based oxides. Energy Environ. Sci. 2011, 4, 3805–3819. [9] Yang, D. A.; Cho, H. Y.; Kim, J.; Yang, S. T.; Ahn, W. S. CO2 capture and conversion using Mg-MOF-74 prepared by a sonochemical method, Energy Environ. Sci. 2012, 5, 6465-6473. [10] Lin, Y.; Kong, C.; Chen, L. Amine-functionalized metal-organic frameworks: structure, synthesis and applications. RSC Adv. 2016, 6, 32598-32614. [11] Xiang, H.; Jian, L.; Wei, W.; Xiao, Wei.; Jing, D. Experimental and computational investigation of CO2 capture on amine grafted metal-organic framework NH2-MIL-101. Appl. Surf. Sci. 2016, 371, 307-313. [12] Yoo, H. M.; Lee, S. Y.; Park, S. J. Ordered nanoporous carbon for increasing CO2 capture. J. Solid State Chem. 2013, 197, 361-365.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[13]

Goel,

C.; Bhunia,

nanostructured

carbons

H.; Bajpai, for

CO2

Page 30 of 62

P.K. Resorcinol–formaldehyde adsorption:

kinetics,

isotherm

based and

thermodynamic studies. RSC Adv. 2015, 5, 93563-93578. [14] Ma, X.; Li, Li.; Wang, S.; Lu, Ming.; Li, Hai.; Ma, Wei.; Keener, Tim. C. Ammonia-treated porous carbon derived from ZIF-8 for enhanced CO2 adsorption. Appl. Surf. Sci. 2016, 369, 390-397. [15] Kishor, R.; Ghoshal, A. K. Amine-Modified Mesoporous Silica for CO2 Adsorption: The Role of Structural Parameters. Ind. Eng. Chem. Res. 2017, 56, 6078-6087. [16] Chen，C.;

Kim，S. S.; Cho, W. S.; Ahn, W. S. Polyethylenimine-incorporated

zeolite 13X with mesoporosity for post-combustion CO2 capture, Appl. Surf. Sci. 2015, 332, 167-171. [17] Li, L.; Li, Y.; Wen, X.; Wang, F.; Zhao, N.; Xiao, F. CO2 capture over K2CO3/MgO/Al2O3 dry sorbent in a fluidized bed. Energy Fuels 2011, 25, 3835-3842. [18] Na, Meysam. Adsorption of carbon dioxide (CO2) at S functionalized boron nitride (BN) and aluminum nitride (AlN) nanotubes (9, 0): A quantum chemical investigation. Appl. Surf. Sci. 2016, 384, 380-385. [19] Bastani, D.; Esmaeili, N.; Asadollahi, M. Polymeric mixed matrix membranes containing zeolites as a filler for gas separation applications: A review. J. Ind. Eng. Chem. 2013, 19, 375-396. [20] Cheung, O.; Hedin, N. Zeolites and related sorbents with narrow pores for CO2


Page 31 of 62

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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separation from flue gas. RSC Adv. 2014, 4, 14480-14494. [21] Zhang, Jun.; Clennell, Michael. B.; Liu, Ke.; Pervukhina, Marina.; Chen, Guo.; Dewhurst, David. N. Methane and Carbon Dioxide Adsorption on Illite. Energy Fuels 2016, 30, 10643-10652. [22] Choi, S.;

Drese, J. H.; Jones, C. W. Adsorbent materials for carbon dioxide

capture from large anthropogenic point sources. ChemSusChem 2009, 2, 796-854. [23] Zhao, Y.; Liu, X.; Han, Y. Microporous carbonaceous adsorbents for CO2 separation via selective adsorption. RSC Adv. 2015, 5, 30310-30330. [24] Zhang, G.; Zhao, P.; Xu, Y. Development of amine-functionalized hierarchically porous silica for CO2 capture. J. Ind. Eng. Chem. 2017, 54, 59-68. [25] Chai, S.; Liu, H.; Zhang, X.; Han, Y.; Hu, N.; Wei, L.; Cong, F.; Wei, H.; Wang, L. Synthesis of a novel beta-ketoenamine-linked conjugated microporous polymer with N-H functionalized pore surface for carbon dioxide capture. Appl. Surf. Sci. 2016, 384, 539-543. [26] Yu, J.; Le, Y.; Cheng, B. Fabrication and CO2 adsorption performance of bimodal porous silica hollow spheres with amine-modified surfaces. RSC Adv. 2012, 2, 6784-6791. [27] Le, Y.; Guo, D.; Cheng, B.; Yu, J. Amine-functionalized monodispersed porous silica microspheres with enhanced CO2 adsorption performance and good cyclic stability. J. Colloid Interf. Sci. 2013, 408, 173-180. [28] Sanz-Pérez, E. S.; Dantas, T. C. M.; Arencibia, A.; Calleja, G.; Guedes, A. P. M.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

A.; Araujo, A. S.; Sanz, R. Reuse and recycling of amine-functionalized silica materials for CO2 adsorption. Chem. Eng. J. 2017, 308, 1021-1033. [29] Dao, D. S.; Yamad, H.; Yogo. K. Large-Pore Mesostructured silica impregnated with blended amines for CO2 capture. Ind. Eng. Chem. Res. 2013, 52, 13810-13817. [30] Xu, X.; Song, C.; Andresen, J. M.; And, B. G. M.; Scaroni, A. W. Novel Polyethylenimine-Modified Mesoporous Molecular Sieve of MCM-41 Type as High-Capacity Adsorbent for CO2 Capture. Energy Fuels 2002, 16, 1463-1469. [31] Dutcher, B.; Fan, M.; Russell, A. G. Amine-based CO2 capture technology development from the beginning of 2013-A Review. ACS Appl. Mater. Interfaces 2015, 7, 2137-2148. [32] Bhagiyalakshmi, M.; Lee, J.; Jang, H. Synthesis of mesoporous magnesium oxide: Its application to CO2 chemisorption. Int. J. Greenh. Gas Con. 2010, 4, 51-56. [33] Kamarudin, K. S. N.; Alias, N. Adsorption performance of MCM-41 impregnated with amine for CO2 removal. Fuel Process. Technol. 2013, 106, 332-337. [34] Jiao, J.; Cao, J.; Xia, Y.; Zhao, L. Improvement of adsorbent materials for CO2 capture by amine functionalized mesoporous silica with worm-hole framework structure. Chem. Eng. J. 2016, 306, 9-16. [35] Chen, C.; Yang, S. T.; Ahn W. S.; Ryoo, R. Amine-impregnated silica monolith with a hierarchical pore structure: Enhancement of CO2 capture capacity. Chem. Commun. 2009, 24, 3627-3629.


Page 32 of 62

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Energy & Fuels

[36] Chen, C.; Bhattacharjee, S. Trimodal nanoporous silica as a support for amine-based CO2 adsorbents: Improvement in adsorption capacity and kinetics. Appl. Surf. Sci. 2017, 396, 1515-1519. [37] Chen, C.; Son, W. J.; You, K. S.; Ahn, J. W.; Ahn, W. S. Carbon dioxide capture using amine-impregnated HMS having textural mesoporosity. Chem. Eng. J. 2010, 161, 46-52. [38] Guo, X.; Ding, L.; Kanamori, K.; Nakanishi, K.; Yang, H. Functionalization of hierarchically porous silica monoliths with polyethyleneimine (PEI) for CO2 adsorption. Micropor. Mesopor. Mater. 2017, 245, 51-57. [39] Han, Y.; Hwang, G.; Kim, H.; Haznedaroglu, B.Z.; Lee, B. Amine-impregnated millimeter-sized spherical silica foams with hierarchical mesoporous – macroporous structure for CO2 capture. Chem. Eng. J. 2015, 259, 653-662. [40] Loganathan, S.; Tikmani, M.; Ghoshal, A. K. Novel Pore-Expanded MCM-41 for CO2 Capture: Synthesis and Characterization. Langmuir 2013, 29, 3491-3499. [41] Sakwanovak, M. A.; Tan, C. S.; Jones, C. W. Role of additives in composite PEI/Oxide CO2 adsorbents: enhancement in the amine efficiency of supported PEI by PEG in CO2 capture from simulated ambient air. ACS Appl. Mater. Interfaces 2015, 7, 24748-24759. [42] Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Perez, E.S. CO2 adsorption on branched polyethyleneimine-impregnated mesopoous silica SBA-15. Appl. Surf. Sci. 2010, 256, 5323-5328. [43] Yang, X.; Li, Y.; Tendeloo, G. V.; Xiao, F.; Su, B. L. One-pot synthesis of


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catalytically stable and active nanoreactors: encapsulation of size-controlled nanoparticles within a hierarchically macroporous core@ordered mesoporous shell system. Adv. Mater. 2009, 21, 1368-1372. [44] Wang, X.; Chen, L.; Guo, Q. Development of hybrid amine-functionalized MCM-41 sorbents for CO2 capture. Chem. Eng. J. 2015, 260, 573-581. [45] Li, K.; Jiang, J.; Tian, S.; Yan, F.; Chen, X. Polyethyleneimine–nano silica composites: a lowcost and promising adsorbent for CO2 capture. J. Mater. Chem. 2015, A 3, 2166-2175. [46] Klinthong, W.; Chao, K.; Tan, C. CO2 Capture by As-Synthesized Amine-Functionalized MCM-41 Prepared through Direct Synthesis under Basic Condition. Ind. Eng. Chem. Res. 2013, 52, 9834-9842. [47] Chen, Q.; Fan, F.; Long, D.; Liu, X.; Liang, X.; Qiao, W.; Ling, L. Poly(ethyleneimine)-loaded silica monolith with a hierarchical pore structure for H2S adsorptive removal. Ind. Eng. Chem. Res. 2010, 49, 11408-11414. [48] Wang, X. R.; Li, H. Q.; Liu, H. T.; Hou, X. J. AS-synthesized mesoporous silica MSU-1modified with tetraethylenepentamine for CO2 adsorption. Micropor. Mesopor. Mater. 2011, 142, 564-569. [49] Yan, X. L.; Zhang, L.; Zhang, Y.; Qiao, K.; Yan, Z. F.; Komarneni, S. Amine-modified mesocellular silica foams for CO2 capture. Chem. Eng. J. 2011, 168, 918-924. [50] Ma, X.; Wang, X.; Song, C.

“Molecular Basket” sorbents for separation of CO2

and H2S from various gas streams. J. Am. Chem. Soc. 2009, 131, 5777-5783.


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[51] Son, W. J.; Choi, J. S.; Ahn, W. S. Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials. Micropor. Mesopor. Mater. 2008, 113, 31-40. [52] Liu, Y.; Shi, J.; Chen, J.; Ye, Q.; Pan, H.; Shao, Z.; Shi, Y. Dynamic performance of CO2 adsorption with tetraethylenepentamine-loaded KIT-6. Micropor. Mesopor. Mater. 2010, 134, 16-21. [53] Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Y.; Wang, Z. J.; Zhu, J. H. Efficient CO2 capturer derived from as-synthesized MCM-41 modified with amine. Chem. Eur. 2008, 14, 3442-3451. [54] Chen, H.; Liang, Z.; Yang, X.; Zhang, Z.; Zhang, Z. Experimental Investigation of CO2 Capture Capacity: Exploring Mesoporous Silica SBA-15 Material Impregnated with Monoethanolamine and Diethanolamine. Energy Fuels 2016, 30, 9554-9562. [55] Zhang, L.; Zhan, N.; Jin, Q.; Liu, H.; Hu, J. Impregnation of polyethylenimine in mesoporous multilamellar silica vesicles for CO2 capture: a kinetic study. Ind. Eng. Chem. Res. 2016, 55, 5885-5891. [56] Govar, A.; Ebner, A. D.; Ritter, J. A. Effect of H2O vapor on the adsorption and desorption behavior of CO2 in a solid amine sorbent. Energy Fuels 2016, 30, 10653-10660. [57] Liu, Z. L.; Teng, Y.; Zhang, K.; Cao, Y.; Pan, W. P. CO2 adsorption properties and thermal stability of different amine-impregnated MCM-41 materials. J. Fuel Chem. Technol. 2013, 41, 469-476.


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[58] Chen, L.; Wang, X.; Guo, Q. Study on CO2 adsorption properties of tetraethylenepentamine modified mesoporous silica gel. J. Fuel Chem. Technol. 2015, 1, 108-115. [59] Wilfong, W. C.; Srikanth, C. S.; Chuang, S. S. C. In situ ATR and DRIFTS studies of the nature of adsorbed CO2 on tetraethylenepentamine films. ACS Appl. Mater. Interfaces 2014, 6, 13617-13626. [60] Yu, J.; Chuang, S. S. C. The structure of adsorbed species on immobilized amines in CO2 capture: An in situ IR study. Energy Fuels 2016, 30, 7579-7587. [61] Didas, S. A.; Sakwa-Novak, M. A.; Foo, G. S.; Sievers, C.; Jones, C. W. Effect of amine surface coverage on the co-adsorption of CO2 and water: spectral deconvolution of adsorbed species. J. Phys. Chem. Lett. 2014, 5, 4194-4200. [62] Danon, A.; Stair, P. C.; Weitz, E.

FTIR study of CO2 adsorption on

amine-grafted SBA-15: elucidation of adsorbed species. J. Phys. Chem. C 2011, 115, 11540-11549. [63] Srikanth, C. S.; Chuang, S. S. Infrared study of strongly and weakly adsorbed CO2 on fresh and oxidative degraded amine sorbents. J. Phys. Chem. C 2013, 117, 9196-9205. [64] Zhai, Y.; Chuang, S. S. C. The nature of adsorbed carbon dioxide on immobilized amines during carbon dioxide capture from air and simulated flue gas. Energy Technol. 2017, 5, 510-519. [65] Abramian, L.; Rassy, H. Adsorption kinetics and thermodynamics of azo-dye orange II onto highly porous titania aerogel. Chem. Eng. J. 2009, 150, 403-410.


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[66] Yousef, R. I.; Eswed, B.; Muhtaseb, A. H. Adsorption characteristics of natural zeolites as solid adsorbents for phenol removal from aqueous solutions: kinetics, mechanism, and thermodynamics studies. Chem. Eng. J. 2011, 171, 1143-1149. [67] Loganathan, S.; Tikmani, M.; Edubilli, S.; Mishra, A.;

Ghoshal, A. K. CO2

adsorption kinetics on mesoporous silica under wide range of pressure and temperature. Chem. Eng. J. 2014, 256, 1-8.


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Figure Lists Fig. 1. The schematic diagram of fixed bed reactor for the CO2 capture. Fig. 2. The N2 adsorption-desorption isotherms of S2HPS before and after TEPA modification. Fig. 3. The pore size distributions of SBA-15, HPS and S2HPS. Fig. 4. The pore size distributions of S2HPS before and after TEPA modification. Fig. 5. SEM images of S2HPS. Fig. 6. The thermal stability of S2HPS before and after TEPA modification. Fig. 7. The FT-IR spectra of S2HPS before and after TEPA modification. Fig. 8. The CO2 adsorption amount of 50 wt.% TEPA-modified mixed supports with different weight ratios of SBA-15 to HPS. Fig. 9. The CO2 adsorption capacity of S2HPS-TEPA50% at different temperatures. Fig. 10. The CO2 adsorption capacity of S2HPS-TEPA50% at different CO2 concentration. Fig. 11. The CO2 adsorption breakthrough curves (a) and adsorption amount (b) of different adsorbents under dry or humid conditions. Fig. 12. The CO2 adsorption cyclic performance of S2HPS-TEPA50%. Fig. 13. 2D waterfall IR absorbance spectra of CO2 adsorption (a) and desorption (b) on S2HPS-TEPA50%. Fig. 14.

In situ FT-IR difference spectra of (a) CO2 adsorbed species and (b) CO2 desorbed species on S2HPS-TEPA50% taken from different minutes during CO2 adsorption/desorption process.


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Fig. 15. The prediction of the intraparticle diffusion model for CO2 adsorption over S2HPS-TEPA50% at 55 oC and 75 oC. Scheme List Scheme 1. Proposed a mechanism for the reaction between CO2 and two NH2.

Scheme 2. Proposed a mechanism for the reaction between CO2 and two NH.

Scheme 3. Proposed a mechanism of CO2 reaction with NH2 and NH.


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Table List Table 1 Textural properties of support materials before and after TEPA modification. Table 2 Summary of the elemental analysis of the adsorbents. Table 3 The CO2 adsorption performance of adsorbents with different ratios of SBA-15 to HPS and TEPA loading amount. Table 4 Summary of CO2 adsorption performance using S2HPS as support compared to other TEPA impregnated adsorbent materials. Table 5 Summary of the elemental analysis of the regenerated and fresh samples. Table 6 IR band assignments for adsorbed CO2 species.


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Table 1 Textural properties of support materials before and after TEPA modification. BET surface area

Pore volume

(m2/g)

(cm3/g)

HPS

929.6

2.09

2.6, 23

HPS-TEPA40%

79.28

0.30

23

HPS-TEPA50%

42.15

0.19

23

SBA-15

804

1.47

6.1

SBA-15-TEPA40%

61.72

0.44

6.1

SBA-15-TEPA50%

28.70

0.2

/

SHPS

821.35

1.69

2.6, 6.1, 23

SHPS-TEPA40%

104.37

0.63

6.1, 23

SHPS-TEPA50%

34.62

0.32

23

S2HPS

857.56

1.79

2.6, 6.1, 23

S2HPS-TEPA40%

93.42

0.76

6.1, 23

S2HPS-TEPA50%

54.24

0.48

23

2SHPS

797.05

1.64

2.6, 6.1, 23

2SHPS-TEPA40%

92.95

0.52

6.1, 23

2SHPS-TEPA50%

30.12

0.26

23

Adsorbent

The pore size refers to the position where the peak centered.


Pore size (nm)

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Table 2 Summary of the elemental analysis of the adsorbents. Element content (wt. %) Adsorbent N

C

H

S2HPS

0.89

0.67

2.58

S2HPS-TEPA40%

11.97

19.32

5.39

S2HPS-TEPA50%

14.18

23.17

6.61


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Table 3 The CO2 adsorption performance of adsorbents with different ratios of SBA-15 to HPS and TEPA loading amount. Breakthrough

Saturated adsorption

adsorption capacity (mmol/g)

capacity (mmol/g)

Adsorbent

Breakthrough time (min)

HPS-TEPA40%

6.1

2.21

3.09

HPS-TEPA50%

8.13

3.24

4.26

SBA-15-TEPA40%

5.9

2.07

2.94

SBA-15-TEPA50%

7.87

3.21

4.1

SHPS-TEPA40%

6.7

2.55

3.18

SHPS-TEPA50%

9.1

3.72

4.61

S2HPS-TEPA40%

6.5

2.5

3.27

S2HPS-TEPA50%

10.2

4.09

5.05

2SHPS-TEPA40%

6.3

2.33

3.14

2SHPS-TEPA50%

9.2

3.74

4.63


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Table 4 Summary of CO2 adsorption performance using S2HPS as support compared to other TEPA impregnated adsorbent materials. Dry CO2 Adsorbent

Temp. (oC)

CO2 (vol.%)

Test mode

adsorption capacity (mmol/g)

SBA-15-TEPA50%

75

15

fixed-bed

4.1

HPS-TEPA50%

75

15

fixed-bed

4.26

S2HPS-TEPA50%

75

15

fixed-bed

5.05

MCM-41-TEPA50%

70

15

fixed-bed

2.25

[44]

MSU-1-TEPA50%

75

10

fixed-bed

3.87

[48]

40

20

fixed-bed

5.53

[29]

MCF-PEI50%

75

15

fixed-bed

3.45

[49]

MCM-41-PEI50%

75

15

fixed-bed

2

[50]

SBA-15-PEI50%

75

15

fixed-bed

3.18

[50]

KIT-6-PEI50%

75

100

TGA

3.07

[51]

MCM-41-TEPA60%

75

15

fixed-bed

2.45

[44]

KIT-6-TEPA60%

60

10

GC

3.2

[52]

MCM-41-TEPA60%

75

100

TGA

4

[53]

Ref.

this study

MSU-F-TEPA40%+30 %DEA


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Table 5 Summary of the elemental analysis of the regenerated and fresh samples. Element content (wt.%) Adsorbent N

C

H

Fresh adsorbent

14.18

23.17

6.61

Ten regeneration

14.14

23.04

6.731


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Table 6 IR band assignments for adsorbed CO2 species Wavenumber (cm-1)

assignment

species

Ref.

carbamate

[59]

secondary ammonium ions

[60]

carbamate

[59]

NCOO− skeletal 1327 vibration 1411

NH2+ deformation COO− symmetric

1494 stretching 1639

NH3+ deformation

primary ammonium ions

[61]

1685

COOH stretching

carbamic acid

[62]

2165

N−H combination

primary/secondary [63] ammonium ions 2360

CO2 gas phase

2496

N−H stretching

CO2

[64]

primary/secondary [59] ammonium ions 3046

NH3+ stretching

primary ammonium ions

[63]

primary/secondary amines

[64]

secondary amines

[64]

N-H symmetric 3298 stretching N-H asymmetric 3365 stretching


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Fixed bed

CO

Mass flowcontroller

Temperature contraller

Mass flowcontroller

Adsorbent

N

Water saturator CO analyzer Fig. 1. The schematic diagram of fixed bed reactor for the CO2 capture.


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Fig. 2. The N2 adsorption-desorption isotherms of S2HPS before and after TEPA modification.


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Fig. 3. The pore size distributions of SBA-15, HPS and S2HPS.


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Fig. 4. The pore size distributions of S2HPS before and after TEPA modification.


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Fig. 5. SEM images of S2HPS.


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Fig. 6. The thermal stability of S2HPS before and after TEPA modification.


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Fig. 7. The FT-IR spectra of S2HPS before and after TEPA modification.


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Fig. 8. The CO2 adsorption amount of 50 wt.% TEPA-modified mixed supports with different weight ratios of SBA-15 to HPS.


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Fig. 9. The CO2 adsorption capacity of S2HPS-TEPA50% at different temperatures.


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Fig. 10. The CO2 adsorption capacity of S2HPS-TEPA50% at different CO2 concentration.


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Fig. 11. The CO2 adsorption breakthrough curves (a) and adsorption amount (b) of different adsorbents under dry or humid conditions.


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Fig. 12. The CO2 adsorption cyclic performance of S2HPS-TEPA50%.


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Fig. 13. 2D waterfall IR absorbance spectra of CO2 adsorption (a) and desorption (b) on S2HPS-TEPA50%.


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Fig. 14. In situ FT-IR difference spectra of (a) CO2 adsorbed species and (b) CO2 desorbed species on S2HPS-TEPA50% taken from different minutes during CO2 adsorption/desorption process.


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Fig. 15. The prediction of the intraparticle diffusion model for CO2 adsorption over S2HPS-TEPA50% at 55 oC and 75 oC.

61


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Scheme

Scheme 1. Proposed a mechanism for the reaction between CO2 and two NH2.

Scheme 2. Proposed a mechanism for the reaction between CO2 and two NH.

Scheme 3. Proposed a mechanism of CO2 reaction with NH2 and NH.

62


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CO2 Adsorption Behavior and Kinetics on Amine-Functionalized

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