Direct Capture of Low-Concentration CO2 on Mesoporous Carbon

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Direct Capture of Low-Concentration CO2 on Mesoporous CarbonSupported Solid Amine Adsorbents at Ambient Temperature Jitong Wang, Haihong Huang, Mei Wang, Liwen Yao, Wenming Qiao, Donghui Long, and Licheng Ling Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01060 • Publication Date (Web): 29 Apr 2015 Downloaded from http://pubs.acs.org on May 5, 2015

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Industrial & Engineering Chemistry Research

Direct

Capture

of

Low-Concentration

CO2

on

Mesoporous

Carbon-Supported Solid Amine Adsorbents at Ambient Temperature Jitong Wang,

†, ‡

Haihong Huang, † Mei Wang, † Liwen Yao, † Wenming Qiao, †, § Donghui

Long,*, † and Licheng Ling †, §

†

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai 200237, China ‡

Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of

Sciences, Taiyuan 030001, China §

Key Laboratory of Specially Functional Polymeric Materials and Related Technology, East

China University of Science and Technology, Shanghai 200237, China * Corresponding authors Dr. Donghui Long Fax: (86)-21 64252914 Tel: (86)-21 64252924 Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT The CO2 dynamic adsorption behaviors of the mesoporous carbon-supported solid amine adsorbents and their viability for low-concentration CO2 capture were investigated in a fixed bed. The CO2 diffusion, playing a dominate role on the CO2 reaction-adsorption process, was addressed from the following two strategies: improving the support and facilitating the kinetic diffusion. The well-developed mesoporous carbon framework could accommodate high content of polymer amine polyethylenimine (PEI) while maintaining considerable residual channels for CO2 inner pore diffusion. Moreover, the kinetic limitation to CO2 diffusion within the amine films could be mitigated by the employment of diffusion additive, which could facilitate the diffusion of CO2 into the internal PEI films. The as-prepared MC-based solid amine adsorbents exhibit remarkable adsorption capacities of 3.34 mmol·g-1 for 5000 ppm CO2 and 2.25 mmol·g-1 for 400 ppm CO2. The CO2 adsorption capacity of the adsorbent increases significantly in the presence of moisture. The adsorbent also shows excellent stability for low concentration CO2 capture during the temperature swing adsorption/desorption cycling operation. The experimental breakthrough curves under various conditions were analyzed successfully using the deactivation model. The first-order kinetic deactivation model can be applied extensively to analyze the effect of CO2 diffusion on the adsorption kinetics and has very good prediction ability for low-concentration CO2 adsorption on solid amine adsorbents. KEYWORDS: Mesoporous carbon, Solid amine adsorbent, CO2 dynamic adsorption, Kinetic diffusion, Low-concentration CO2


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INTRODUCTION CO2 capture from atmospheric air, usually referred to as direct air capture (DAC), is a plausible alternate approach to CO2 capture from concentrated industrial sources that, if economically practical, can reduce the atmospheric CO2 concentration.1-4 It can be dissociated in space and time from the existing power plant infrastructure and be strategically placed anywhere, preferably where it has the least impact on the environment and human activities or close to CO2 recycling centers.5-8 In the long term, air capture may become indispensable for stabilizing the global CO2 concentration in the atmosphere in view of continuously increasing emissions above 390 ppm.9 From a technology standpoint, DAC presents an even greater challenge because of the extremely dilute CO2 concentrations in ambient air. Practical applications have been implemented for the essential removal of CO2 in personal closed-circuit breathing systems such as submarines and spacecrafts, and in medical applications such as anaesthesia machines, where CO2 concentrations are typically below 5000 ppm.10-13 However, the removal of CO2 from ambient air on a larger scale is still in the initial stage and has gained increased attention recently. The predominant approaches use metal hydroxides or oxides to scrub CO2 from air and convert it to metal carbonates. The concentrated CO2 stream is released via a calcination step by heating to high temperature which has proven to be quite energy intensive making this process unfeasible both technically and economically.

14-16

Another state of the art

technology relies on the solid adsorbents such as zeolites, activated carbon and alumina through physisorption originated from either ion-quadrupole interaction or van der Waals forces. However, the main disadvantages of the physisorption system are the low adsorption



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heat, making low CO2 adsorption capacity, particularly from gas streams with very dilute CO2 concentrations. In addition, the presence of water vapor will adversely affect the selectivity and adsorption capacity for CO2.17-21 To address the drawbacks, an alternate approach, capturing CO2 in solid amine adsorbents via the chemisorption mechanism, have been proposed and developed. These adsorbents can be obtained by chemically bonding or physically immobilizing monomeric or polymeric amine species to a porous support such as silica materials,22-26 polymers,27,

28

carbon nanotubes,29, 30 PMMA-based resins31 and nanofibrillated cellulose.32, 33 Solid amine adsorbents present a number of significant potential advantages including high selectivity for CO2, no or less corrosion problem, high adsorption/desorption rate because of high gas-sorbent interface area, positive effect of moisture on the adsorption performance, and lower energy consumption during regeneration. 34, 35 Thus far, considerable progress has been obtained in the direct capture of low concentration CO2, focusing on developing the support and/or the amine. However, some limited studies have been carried out on the dynamic performance using solid amine adsorbents for low concentration CO2 capture. 36-38 Except for adsorption capacity, kinetic performance of a given solid amine adsorbent is also of great significance for real application. Here in this paper, our research is concerned with the CO2 dynamic adsorption behaviors of the mesoporous carbon-supported solid amine adsorbents and their viability for low concentration CO2 capture. In the case of low concentration CO2 with the low concentration gradient, the diffusion should play a dominate role on the CO2 reaction-adsorption process. On the premise of adsorption capacity guarantee, the diffusion resistance should be decreased as far as possible. Here our strategies to address


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the diffusion issue involved two aspects from pore diffusion and amine diffusion. On one hand, we choose high-pore-volume mesoporous carbons as the support, which could accommodate high content of polymer amine polyethylenimine (PEI) while maintaining considerable residual channels, thus decreasing the surface and inner pore diffusion barriers for adsorption/desorption.39 On the other hand, diffusion additive is used to break the PEI film from a compact entirety into a dual interpenetrating polymer network, alleviating the kinetic inhibition to CO2 internal diffusion within the PEI films. Thus, our mesoporous carbon-supported solid amine adsorbents show very excellent dynamical performance for low concentration CO2 capture. Furthermore, a deactivation model has been employed to predict the breakthrough curve, assuming that the formation of a dense product layer on the surface of the adsorbent would change the number of active sites and create an additional diffusion resistance leading to a drop in the adsorption rate.30, 40-46 This mode could provide a deeper insight into the dynamic adsorption processes of solid amine adsorbents in a fixed-bed column. EXPERIMENTAL SECTION Preparation of Adsorbents Colloidal silica sol (LUDOX SM-30 colloidal silica, 30 wt %) was obtained from DuPont Co., Ltd. Polyethylenimine (PEI) with Mw of 600 and a density of 1.03 g/cm3 (branched type) was purchased from Sigma-Aldrich. All other used chemicals were purchased from Adamas-beta Chemical Co., Ltd. and were used without further treatment. The mesoporous carbon (MC) support was synthesized via a combined hard templating and sol-gel method using resorcinol/formaldehyde polymer as carbon source and colloidal



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silica sol as the hard template, followed by ambient pressure drying and carbonization. Detailed synthesis are given elsewhere.39,

47

In this work, the initial concentration of

resorcinol/formaldehyde (1:2 in molar ratio) polymer was fixed at 15 g/100 mL water, and the mass ratio of silica to resorcinol/formaldehyde polymer was 3/2. PEI-impregnated mesoporous carbon adsorbents were prepared by a conventional wet impregnation method using Span80 as diffusion additive.48 Typically, the desired amounts of PEI and Span80 were dissolved in 10 g of methanol at 40 °C under stirring for 30 min, and then 2 g of the mesoporous support MC was added to the above solution and further stirred for 8 h at 40 °C. The slurry was then transferred to a rotary vacuum evaporator and dried at 40 °C under vacuum conditions to remove methanol. The as-prepared adsorbent was denoted as MC-a-b, where a and b represent the weight percentage of PEI and Span80 in the adsorbents (PEI + MC + Span80), respectively. Characterization of Samples The morphologies and microstructures were observed under a scanning electron microscopy (FEI, Qunta 300) and a transmission electron microscopy (TEM, JEOL 2100F). Nitrogen adsorption/desorption isotherms were measured at 77 K on a Quantachrome Quadrasorb SI gas sorption analyzer. Before the measurements, the samples were degassed in vacuum at 100 °C for 12 h. The specific surface area and pore size distribution were derived from the Brunauer-Emmett-Teller (BET) equation and Barrett-Joiner-Halenda (BJH) method, respectively. CO2 Adsorption Measurement To obtain the dynamic adsorption capacity, the CO2 adsorption experiments were


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performed in a fixed-bed flow adsorber operated at specified temperature.49 The fixed-bed flow adsorber is 20 cm in length with an inner diameter of 1 cm. Inlet gas was a mixture of CO2 (99.99%) and N2 (99.99%) with a certain CO2 mole fraction at atmospheric pressure. Mass flow controllers were used to control the flow rate of the gases. The CO2 concentration of the inlet and outlet gas streams were analyzed with an online gas chromatogram (Shimadzu GC-2014) equipped with a thermal conductivity detector (TCD). In a typical adsorption process, about 0.5 g of the adsorbent was packed in the middle of the adsorber supported with quartz wool. The adsorbents were first treated under a nitrogen flow of 50 mL/min at 110 oC for 2 h, and then cooled to the adsorption temperature. Then, the dry or moist CO2/N2 gas mixture was introduced at a flow rate of 50 mL/min. The N2 flow was adjusted by two electronic mass flow controllers. One of the N2 streams was passed through a bubbler-type humidifier immersed in a heat bath at constant temperature and then mixed with the other dry N2 stream. The relative humidity (RH) of the gas mixture was thus controlled by adjusting the ratio of the two streams. RH was measured at the inlet and outlet of the adsorber with electronic humidity sensor. The adsorption capacity was determined by integration of the area between the breakthrough curve and a line at the inlet CO2 concentration, and from the flow rate, breakthrough and saturation time, and mass of the adsorbent. The controlled experiment was performed to determine the CO2 adsorption capacity of the support and the adsorbent with only 5% diffusion additive, which showed that the support MC and the MC with 5 wt% Span80 alone had negligible CO2 adsorption capacity below 0.05 mmol·g-1 and 0.07 mmol·g-1, respectively. The change of temperature for low concentration CO2 adsorption was small and could be neglected. Therefore, the adsorption process could be considered as



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an isothermal process. During CO2 desorption, the sample was heated up to 110 oC at a rate of 10 oC·min-1 under a N2 flow of 50 mL/min and kept at that temperature until no CO2 could be detected in the outlet. The desorption capacity were determined by integration of the area under the desorption curve, and from the flow rate and mass of the adsorbent. The average concentration of CO2 in the outlet was calculated from integration of the area under the desorption curve divided by the desorption time. In the adsorbent regenerability test, the adsorption/desorption procedure was conducted for 10 cycles to test for the cyclic stability. RESULTS AND DISCUSSION Characterization of the adsorbents The MC support was prepared by a colloidal silica nanocasting approach in which the mesopores are reversely replicated by colloidal silica nanoparticles.47 The typical SEM and TEM images of the MC support are given in Figure 1, which shows that the MC is composed of interconnected amorphous carbon frameworks with a crosslinking three-dimensional (3-D) open structure replicated from the silica nanoparticles. The pore structures of the MC together with their PEI loaded samples were analyzed by N2 adsorption. As shown in Figure 1(c), all samples exhibit similar type IV isotherms with capillary condensation steps occurring over a relatively wide pressure range of 0.7-1.0, indicating the mesoporous characteristic of the materials. The detailed porosity parameters are summarized in Table 1. The pristine MC possesses a high BET surface area of 999 m2/g, a large pore volume of 3.10 cm3/g and an average pore size of 18.9 nm. After impregnation, the porosity of the adsorbent decreases because the pores were filled by PEI molecules. Still, they have considerable porosity (0.34


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cm3/g) until the PEI loading is up to 70%. The residual channels should effectively decrease the pore diffusion resistance for promoting the adsorption. Screening the adsorbent The breakthrough and saturation adsorption capacities of these adsorbents with different PEI contents were evaluated using 5000 ppm CO2 at 25 oC in the fixed bed (Figure 2). Both breakthrough and saturation capacities exhibit a maximum of 2.17 and 2.82 mmol·g-1, respectively, at the PEI loading of 60%. Further increasing the PEI loading to 70%, the capacities drop greatly due to the pore diffusion limitation. Thus, there is an optimized PEI loading of 60% for the MC support, based on the compromise between the residual channels and the total available amines. When exposed to CO2, a dense film of ammonium carbamate could form from the chemical reaction between amines and CO2, thus impeding the CO2 diffusion from the surface into the inner PEI films. To decrease the internal diffusion resistance, a diffusion additive, Span 80 was mixed with PEI, which is hoped to break the film from a compact entirety into a dual interpenetrating network, facilitating the diffusion of CO2 into the internal PEI films, as reported in our previous work.50 At an optimized loading of 60 wt. %, the adsorption capacities for different weight ratios of PEI and Span 80 were investigated and shown in Figure 3. The amine utilization ratio, based on assuming 1 mol of amine could react with 0.5 mol of CO2 under anhydrous condition, is determined and also shown in Figure 3. The CO2 adsorption capacities exhibit a maximum saturation capacity of 3.34 mmol·g-1 for MC-55-5, which is 30.5% greater than the additive-free MC-55 and 20% greater than MC-60. Further increasing the Span80 content, the capacity decreases slightly due to the decrease of



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the amines for CO2 adsorption, but the ratio of breakthrough capacity to saturation capacity and the amine utilization increase, which further suggests the promoted kinetic property by the diffusion additive. The adsorption behavior of the screened adsorbent After screening a suitable adsorbent MC-55-5, the general dynamic adsorption performances were investigated. Figure 4(a) shows the adsorption isotherm of the MC-55-5 at 25 oC. The CO2 partial pressure was changed from 0.04 kPa to 50 kPa, corresponding to the CO2 concentration from 400 ppm to 50%. The isotherm is characterized by a sharp initial rise at lower CO2 partial pressure and slowly increases and reaches a plateau at higher CO2 partial pressure. This can be attributed to the extremely strong affinity for CO2 of the adsorbent, improving the CO2 adsorption at low relative pressure through the chemical reaction between amine and CO2. The isotherm basically obeys a Langmuir-type isotherm as the Langmuir isotherm fit line shown in Figure 4(a). It should be noted that the adsorption capacity of the adsorbent varies from 5.48 mmol/g to 2.25 mmol/g when the CO2 partial pressure was changed from 50 kPa to 0.04 kPa. This corresponds to the decrease of the adsorption capacity by a factor of 2.4 with a dilution of the CO2 partial pressure by a factor of 1250. This result indicates that the adsorbent MC-55-5 can capture considerable amounts of CO2 from ultradilute CO2 sources, specifically at a partial pressure similar to those found in the ambient air. Some solid amine adsorbents used for CO2 adsorption at a relatively low CO2 pressure under room temperature reported in the literature are summarized in Figure 4(b). Under similar conditions, MC-55-5 could provide higher air capture capacity than other adsorbents.


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The high adsorption capacity highlights the potential of this adsorbent for trace CO2 removal in personal closed-circuit breathing systems. Effects of CO2 capture temperature on the MC-55-5 for CO2 concentration of 400 ppm and 5000 ppm are presented in Figure 5. The performance of an adsorbent with respect to the temperature gives information on its operating and regeneration system. The CO2 adsorption capacity of MC-55-5 is slightly improved with the increasing temperature from 25 to 75 oC, and decreases sharply from 75 to 110 oC. It is already known that the adsorption of CO2 over PEI-loaded adsorbents is a kinetic controlled process.54 The increase in temperature facilitates the transfer of the adsorbed CO2 molecules from the surface into the bulk of PEI by overcoming the kinetic barrier. Thus, at medium temperature of 75 °C, the highest adsorption capacities are achieved based on the compromise of thermodynamical adsorption and kinetic diffusion. It should be further noted that based on the advantage of promoted diffusion by the additive, the adsorption performance at low temperature is improved more significantly compared to the additive-free MC-55, due to the low temperature that favors the thermodynamic adsorption. This result further indicates that the addition of a small amount of diffusion additive could greatly improve the low-concentration CO2 capture performance at low temperature. Water vapor is inevitable in the ambient air, which plays a significant role in the CO2 adsorption reaction. In the absence of water, 1 moles of amine groups could react with 0.5 mole of CO2 to form ammonium carbamate. Instead, 1 mole of amine groups is needed to remove 1 mole of CO2, forming ammonium bicarbonate and carbonate species in the presence of water. 55, 56 The effect of moisture on the CO2 adsorption capacity was studied



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using 400 and 5000 ppm CO2/N2 gas mixtures, as shown in Figure 6. When a moist gas mixture of 80% relative humidity was used, the CO2 adsorption capacities increase to 4.05 mmol/g and 2.58 mmol/g for CO2 concentration of 5000 ppm and 400 ppm, respectively, revealing that moisture had a promoting effect on the adsorption of CO2 over the adsorbent. However, the improvement of adsorption capacity for the 400 ppm CO2 is obviously less than that for the 5000 ppm CO2, which is attributed to the competitive adsorption between CO2 and redundant H2O. Desorption behavior and regeneration performance To study the CO2 desorption behavior, MC-55-5 was regenerated by temperature swing process under a N2 flow at 110 oC after the saturation at 5000 ppm CO2 and 400 ppm CO2 flow. The CO2 concentrations of the outlet gas as a function of desorption time are shown in Figure 7. The adsorbent could be regenerated efficiently with 99% of the adsorbed CO2 released in 34 min for 5000 ppm CO2 and 45 min for 400 ppm CO2, respectively, which is attributed to the good thermal conductivity of mesoporous carbon, facilitating the rapid heating desorption process. Furthermore, CO2 is concentrated from 5000 ppm in the inlet to the average concentration of 15% in the outlet and from 400 ppm in the inlet to the average concentration of 11% in the outlet, respectively, during the temperature swing process. Adsorbent reusability is an excellent quality for economic viability of adsorption processes as it aids in efficient treatment of effluents. Ten consecutive cycles of adsorption/desorption were conducted for CO2 concentration of 5000 ppm and 400 ppm and the results are shown in Figure 8. The cyclic data reveals that the adsorption performance of MC-55-5 is relatively stable, with only 2% and 3% drop in adsorption capacity at CO2


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concentration of 5000 ppm and 400 ppm, respectively, after 10 adsorption/desorption cycles. These results suggest the MC-55-5 is a recyclable and moisture stable adsorbent for the removal of low-concentration CO2. Deactivation Model for Breakthrough Analysis The formation of salt bridges and/or hydrogen bonded networks of amine-CO2 zwitterions within the PEI films could create an additional diffusion resistance, resulting in the significant decrease in the dynamic adsorption rate and reactivity of the amine. In addition, change in the accessible pore volume, active surface area and active site distribution during the adsorption are expected to cause significant decrease in the activity of adsorbents with time. To better understand the dynamic adsorption behaviors in the fixed-bed column, the adsorption breakthrough curves of the adsorbent were simulated by the deactivation kinetic model. This model with two rate constants is appropriate to predict such gas-solid reactions in packed adsorption columns, 30, 40-46 which combined the effects of the overall factors on the diminishing rate of CO2 capture in a deactivation rate term. With the assumptions of the pseudo-steady-state and isothermal species，the deactivation rate of the solid amine adsorbent is expressed as −

= (1)

where K0 is the initial adsorption rate constant (mL·min-1·g-1), W is the mass of adsorbent (g), Q is the gas flow rate (mL·min-1) and α is the activity of the solid reactants. The deactivation rate of the solid adsorbent is expressed as −

= (2)



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where Kd is the deactivation rate constant (min-1). The effects of the textural variation (pore structure, active surface area, and activity per unit area) of the solid adsorbent and the formation of a salt bridges or hydrogen bonded networks over adsorbent on the activity of the solid adsorbent were expressed in terms of deactivation rate constant. The following approximate expression was then derived for the CO2 breakthrough curves. (1 − exp(− )))] / [1 − exp(− )] exp(− )} (3) = exp{[1 − exp( The kinetic rate constants K0 and Kd can be obtained from the given W, Q, and C by the regression analysis of the CO2 breakthrough curve using model eq 3. Regression analysis of the experimental breakthrough data was also determined from eq 3. The breakthrough curves of the CO2 adsorption on the additive-free and additive-promoted adsorbents were measured to investigate the effect of diffusion additive on the adsorption kinetics and the results are shown in Figure 9. Once the adsorbents are exposed to the CO2 containing gas mixture, complete adsorption of CO2 is observed with a CO2 concentration close to 0 ppm. After that, CO2 begins to break through and the adsorbent gradually approaches to the saturation, which could be a result of CO2 diffusion resistance through the amine films. The breakthrough curves calculated using the kinetic deactivation model equation is also shown in Figure 9, which is almost identical with the experimental results. Moreover, the correlated coefficients R2 for the regression analysis are 0.993 or higher. Hence, the kinetic deactivation model as expressed by eqs 3 gives an appropriate representation of the experimental CO2 uptakes. The parameters of K0 and Kd in the model estimated by analysis of the experimental breakthrough data and deactivation model are presented in Table 2. The kinetic rate constants (K0 and Kd) correlate closely with the


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diffusion resistance of CO2 in the adsorbents because the effects of structural factor in the adsorbents are included in the rate constants. It is found that the kinetic rate constants K0 and Kd for MC-60 is lower than that for the MC-55, which could be a result of the long diffusion distance of CO2 from the surface into the bulk of PEI films. After introducing the diffusion additive, the value of K0 for MC-55-5 increases significantly compared with MC-60 due to the increasing initial adsorption rate, while the Kd value increases slightly due to the increasing adsorption rate and loss of activity of the amine. These results indicate that diffusion additive could facilitate the CO2 diffusion within the PEI film, leading to the improved dynamic adsorption capacity notably. To investigate the effect of temperature on the dynamic adsorption behavior, the experimental data of MC-55-5 in the range of temperature from 25-90 oC were correlated according to the deactivation model equation and shown in Figure 10. A good fitting of deactivation model predictions to the experimental data can be seen and the corresponding kinetic parameters are summarized in Table 3. It is not surprising to find that the initial adsorption and deactivation rate constants K0 and Kd increase with the increasing temperature, indicating that the CO2 diffusion and adsorption processes of MC-55-5 are enhanced. At a high temperature of 90 oC, the deactivation rate constant increases dramatically, which could be contributed to the stronger desorption than the adsorption process. To determine the dependence of the dynamic adsorption parameters on the CO2 concentration, the CO2 breakthrough curves were simulated to the deactivation model in the CO2 concentration range from 400 ppm to 15%. In general, there is good agreement between calculated and experimental results for all of these tested concentrations, as shown in Figure



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11. The kinetic rate constants K0 and Kd shown in Table 4 increase with the increasing concentration of CO2 due to the acceleration of the CO2 breakthrough, indicating that internal diffusion is the rate-limiting step in the CO2 adsorption kinetics. However, even at 400 ppm CO2, the initial rate constants K0 is not much lower than that for higher concentrations, which could be ascribed to the promotion of the diffusion additive. As seen from the results, the deactivation models used could well describe the CO2 adsorption data as far as the R2 values are concerned. The good fit of the experimental data with the deactivation model suggests a significant decrease of activity of the adsorbent with time with respect to the probable changes in the pore structure, the active surface area, and the active site distribution of the adsorbent. The kinetic deactivation model established can be applied extensively to analyze the effect of CO2 diffusion on the adsorption kinetics in the fixed bed without the requirement of structural property for the adsorbents and has very good prediction ability for CO2 adsorption on the solid amine adsorbent. CONCLUSION This work attempts to study the dynamic adsorption performance of mesoporous carbon-supported solid amine adsorbents for direct capture of low-concentration CO2 in a fixed bed at ambient temperature. The diffusion should play a dominate role on the CO2 reaction-adsorption process for low concentration CO2 capture. Our strategy to alleviate the diffusion resistance involved two aspects. On one hand, the adsorbent was prepared from mesoporous carbon with adequate pore volume, proper pore size and interconnected 3-D pore framework, allowing the easy dispersion and immobilization of PEI and maintaining the residual channels for CO2 inner diffusion. On the other hand, the CO2 diffusion within the


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PEI films could be further improved by the introduction of diffusion additive, offering diffusion path within the PEI films for CO2 capture. Owing to the advanced support and facilitated kinetic diffusion, the optimized adsorbent exhibited outstanding adsorption capacity with CO2 concentration ranging from 400 ppm to 15% at ambient temperature. Even at low and trace CO2 concentrations of 5000 and 400 ppm, the adsorbent could directly capture 3.34 mmol·g-1 and 2.25 mmol·g-1 CO2, respectively. In addition, the MC-based solid amine adsorbent could be easily regenerated under a mild temperature and exhibited a relatively stable CO2 regeneration performance. The deactivation model was successfully applied to predict the uptake of CO2 on the adsorbent under different conditions. The adsorption rate constant and first-order deactivation rate constant were obtained from the model, which should be great significance to provide basic engineering data for the design of adsorption separation process. AUTHOR INFORMATION Corresponding Author *Tel.: +86 21 64252924. Fax: +86 21 64252914. E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was partly supported by National Science Foundation of China (No.



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FIGURE CAPTIONS SEM image (a) and TEM image (b) of the mesoporous carbon. Nitrogen

Figure 1.

adsorption-desorption isotherms (c) and the BJH pore size distributions (d) of the mesoporous carbon and adsorbents with different PEI loading. Figure 2.

The effect of PEI loading on the CO2 saturated capacity and the breakthrough

capacity at 25 oC and 5000 ppm. Qs and Qb denote saturated and breakthrough capacity, respectively. Figure 3.

The effect of diffusion additive amount on the CO2 saturated capacity and the

breakthrough capacity and utilization ratio of the amine compound at 25 oC and 5000 ppm. Qs and Qb denote saturated and breakthrough capacity, respectively. Figure 4.

Effect of the CO2 concentration on the CO2 saturated capacity of MC-55-5 at 25

o

C (a) and comparison of MC-55-5 with other amine-modified adsorbents (b). Effect of the adsorption temperature on the CO2 saturated adsorption

Figure 5.

performance of the adsorbents at 5000 ppm. Figure 6.

Comparison of the saturated adsorption capacity of CO2 from dry and moist gas

on MC-55-5. Figure 7.

CO2 desorption as a function of time under a flow of N2 at 110 oC.

Figure 8.

Regeneration performance of the adsorbents at 25 oC.

Figure 9.

The effect of diffusion additive on the deactivation model for 5000 ppm CO2

adsorption at 25 oC. Figure 10.

Comparison of experimental results to the deactivation model for 5000 ppm

CO2 adsorption at different temperatures.



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Figure 11.

Comparison of experimental results to the deactivation model for CO2

adsorption with different CO2 concentrations at 25 oC.


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TABLE CAPTIONS Table 1.

Structural parameters of the mesoporous carbon support and the PEI loading

adsorbents. Table 2.

The initial adsorption rate constant and deactivation rate constant of the

deactivation model for 5000 ppm CO2 adsorption on the adsorbents. Table 3.

The initial adsorption rate constant and deactivation rate constant for 5000

ppm CO2 adsorption on the adsorbent at different temperatures. Table 4.

The initial adsorption rate constant and deactivation rate constant for CO2

adsorption on the adsorbent with different CO2 concentrations at 25 oC.



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TABLES Table 1. Structural parameters of the mesoporous carbon support and the PEI loading adsorbents. SBETa

Sextb

Vtc

Dpd

m2/g

m2/g

cm3/g

nm

MC

999

789

3.10

18.9

MC-55

153

132

1.11

19.0

MC-60

104

98

0.69

18.8

MC-65

71

69

0.46

18.6

MC-70

46

46

0.34

18.5

MC-57-3

107

103

0.71

18.8

MC-55-5

109

107

0.72

18.9

MC-53-7

110

108

0.72

18.9

MC-50-10

112

109

0.73

18.9

Samples

a

BET surface area;

b

External surface area; c total pore volume;

average pore diameter.


d

BJH desorption

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Table 2. The initial adsorption rate constant and deactivation rate constant of the deactivation model for 5000 ppm CO2 adsorption on the adsorbents. K0

Kd R2

Sample (mL·min-1·g-1)

(min-1)

MC-55

577.67

0.0712

0.993

MC-60

553.54

0.0559

0.999

MC-55-5

883.27

0.0692

0.999



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Table 3. The initial adsorption rate constant and deactivation rate constant for 5000 ppm CO2 adsorption on the adsorbent at different temperatures. Temperature

K0

Kd R2

(oC)

(mL·min-1·g-1)

(min-1)

25

883.27

0.0692

0.999

50

982.18

0.0751

0.993

75

1203.28

0.0881

0.994

90

1746.63

0.201

0.998


Page 31 of 43

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Table 4.

The initial adsorption rate constant and deactivation rate constant for CO2

adsorption on the adsorbent with different CO2 concentrations at 25 oC. K0

Kd R2

Concentration (mL·min-1·g-1)

(min-1)

400 ppm

848.26

0.0165

0.993

5000 ppm

883.27

0.0692

0.999

1%

887.58

0.100

0.997

5%

1249.51

0.196

0.995

15%

1275.31

0.595

0.994



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Figure 1. SEM image (a) and TEM image (b) of the mesoporous carbon. Nitrogen adsorption-desorption isotherms (c) and the BJH pore size distributions (d) of the mesoporous carbon and adsorbents with different PEI loading.


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The effect of PEI loading on the CO2 saturated capacity and the breakthrough capacity at 25 oC and 5000 ppm. Qs and Qb denote saturated and breakthrough capacity, respectively.



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The effect of diffusion additive amount on the CO2 saturated capacity and the breakthrough capacity and utilization ratio of the amine compound at 25 oC and 5000 ppm. Qs and Qb denote saturated and breakthrough capacity, respectively. 99x73mm (300 x 300 DPI)


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Figure 4. Effect of the CO2 concentration on the CO2 saturated capacity of MC-55-5 at 25 oC (a) and comparison of MC-55-5 with other solid amine adsorbents (b). 99x150mm (300 x 300 DPI)



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Figure 5. Effect of the adsorption temperature on the CO2 saturated adsorption performance of the adsorbents at 5000 ppm.


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Comparison of the saturated adsorption capacity of CO2 from dry and moist gas on MC-55-5.



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Figure 7. CO2 desorption as a function of time under a flow of N2 at 110 oC.


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Figure 8. Regeneration performance of the adsorbents at 25 oC.



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Figure 9. The effect of diffusion additive on the deactivation model for 5000 ppm CO2 adsorption at 25 oC.


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Figure 10. Comparison of experimental results to the deactivation model for 5000 ppm CO2 adsorption at different temperatures.



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Figure 11. Comparison of experimental results to the deactivation model for CO2 adsorption with different CO2 concentrations at 25 oC.


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Table of Contents Graphic


Direct Capture of Low-Concentration CO2 on Mesoporous Carbon

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