CO2 Capture by As-Synthesized Amine-Functionalized MCM-41

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CO2 Capture by As-synthesized Amine-Functionalized MCM-41 Prepared through Direct Synthesis under Basic Condition Worasaung Klinthong, Kuei-Jung Chao, and Chung-Sung Tan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400865n • Publication Date (Web): 13 Jun 2013 Downloaded from http://pubs.acs.org on June 19, 2013

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Industrial & Engineering Chemistry Research

CO2 Capture by As-synthesized AmineFunctionalized MCM-41 Prepared through Direct Synthesis under Basic Condition Worasaung Klinthong,1 Kuei-Jung Chao,2 Chung-Sung Tan1* 1

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC

2

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC

* Corresponding author: Tel.: +886-3-5721189; Fax: +886-3-5721684; Email: [email protected]

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Abstract The as-synthesized amine-functionalized MCM-41 material was prepared through the direct synthesis by co-condensation of tetraethyl orthosilicate (TEOS) with 3-aminopropyl triethoxysilane (APS) at different molar ratios and a pH of approximately 13 for CO2 capture under various CO2 concentrations, temperatures and moistures. The prepared as-synthesized APS-functionalized MCM-41 (as-APS/MCM) possessed nitrogen content up to 3.46 mmol N/g and CO2 adsorption capacities up to 1.18 mmol/g under 15% CO2 in N2 at 35 oC and 1.74 mmol/g under pure CO2 at 25 °C. The CO2 adsorption capacity was 73% higher than the APSgrafted calcined MCM-41 prepared by post-modification. Because the CO2 adsorption capacity of the as-APS/MCM was found to come mainly from the coated APS rather than the incorporated APS, the prehydrolysis of TEOS and post-treatment including template removal and APS neutralization were not required. Dynamic adsorption-desorption cycles revealed that the as-APS/MCM possessed high thermal stability for CO2 capture.

Keywords: CO2 capture, Amine-functionalized MCM-41, 3-Aminopropyl triethoxysilane, Direct synthesis, Post-treatment, Adsorption capacity.

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1. Introduction The capture and storage of carbon dioxide have recently become important issues worldwide.1-4 Several technologies are available to capture CO2, including absorption by aqueous amine solutions, membrane separation, and adsorption onto solid adsorbents. Although amine absorption is applied widely at present time, it has disadvantages including corrosion and degradation of the amines.5-7 In order to overcome these problems, solid adsorbents containing amines have been proposed.8 Among the developed solid adsorbents, mesoporous silica materials incorporating amines have potential for use in practical adsorption processes.9 Mesoporous silicas can possess one-dimensional (1D), two-dimensional (2D), and threedimensional (3D) mesochannels with pore openings of nanometer size and high surface areas. Accordingly, several mesoporous silicas, including MCM-41 and pore-expanded MCM-41,10-12 MCM-48,13,14 and SBA-15,15,16 are prepared with high amine loadings to provide high CO2 adsorption capacities.17 Two methods have been developed for the preparation of mesoporous silicates incorporating amines: (1) post-modification, in which mesoporous silica supports prepared

with

or

without

organic

templates

are

impregnated

with

solutions

of

aminosilanes,11,12,15,16 amine polymers,10,18-20 or oligoamines21,22 to produce amine-functionalized mesoporous silica adsorbents, the CO2 adsorption capacities of which depend on the nature and quantity of amine loading; (2) direct synthesis, in which co-condensation of a siloxane and a aminosilane is performed in the presence of an organic template in a one-pot reaction, providing the potential benefits of a homogeneous surface coverage of the amine and low levels of pore blocking, but at the cost of the generation of a disordered mesostructure. The formation of 1D hexagonal MCM-41 occurs through the assembly of cationic quaternary ammonium surfactants (S+) and silica sources (I–; e.g., Tetraethyl orthosilicate (TEOS), sodium

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silicate) through electrostatic charge-matching pathways (S+I–).23,24 In general, aminosilanes are covalently grafted onto the intrachannel surface of calcined MCM-41 through silylation to enhance adsorption capacity. The adsorption capacities of 0.83 mmol/g under pure CO2 at 25 oC, 0.57 mmol/g under 10% CO2 in N2 at 25 oC, 0.39 mmol/g under 5% CO2 in N2 at 30 oC, and 0.70 to 1.20 mmol/g under pure CO2 at 30 oC for 3-aminopropyl triethoxysilane (APS)-grafted calcined MCM-41 have been reported by Kim et al.,25 Zeleňák et al.,26 Chang et al.,16 and Ioganathan et al.,27 respectively. When the aminosilane is added directly as part of the silica source, the obtained product, amine-functionalized MCM-41, features a homogeneous distribution of the amino-organic moieties

on

the

silica

walls.

Yokoi

et

al.28

used

the

cationic

surfactant

hexadecyltrimethylammonium bromide (CTAB), TEOS and APS to directly synthesize APSfunctionalized MCM-41. The template-removed samples exhibited reductions in both the ordering of the mesostructure and the pore size occurred after increasing the APS concentration in the reaction mixture. Although Yokoi et al.28 used their obtained template-removed products for cationic adsorption, they did not apply them for CO2 capture. Recently, Osei-Prempeh et al.29 used a cationic fluorinated surfactant as a template to directly synthesize APS-functionalized MCM-41. The APS loading on this template-removed APS-functionalized MCM-41 was 1.20 mmol N/g; it possessed a CO2 adsorption capacity of 0.49 mmol/g under pure CO2 at 50 °C. Although there have been several studies of amine-functionalized MCM-41 prepared through direct synthesis, only a few of these materials have been further tested for CO2 capture. Among them, post-treatment including template removal and neutralization of the amine has generally been used before CO2 capture. To the best of our knowledge, the CO2 adsorption abilities including adsorption capacity and amine efficiency of as-synthesized materials have rarely been

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investigated. In this study, a series of as-synthesized APS-functionalized MCM-41 was prepared through direct synthesis under basic condition and the as-synthesized materials were tested for their CO2 adsorption abilities. The effects of prehydrolysis of TEOS and post-treatment conditions on the amine location and the CO2 adsorption abilities of the APS-functionalized MCM-41 were investigated as well. To verify the applicability of the as-synthesized APSfunctionalized MCM-41 prepared through direct synthesis, the effects of operating conditions on CO2 adsorption at various CO2 partial pressures, temperatures and ambient moisture conditions, as well as the cyclic CO2 adsorption-desorption stability, were also investigated.

2. Experimental 2.1 As-synthesized APS-functionalized MCM-41 prepared through direct synthesis As-synthesized APS-functionalized MCM-41 was prepared using the cationic surfactant CTAB (Sigma-Aldrich). CTAB (3.79 g) was dissolved in deionized water (200 g) at room temperature for 2 h. The pH was increased to 13 through the addition of tetramethylammonium hydroxide (TMAOH, Alfa Aesar; 5.65 g), followed by the addition of silica sources, including TEOS (Acros) and APS (Sigma-Aldrich). The molar ratio of APS to the sum of TEOS and APS, denoted as y, varied from 0.3 to 0.5. It should be noted here that when the proportion y was over 0.5, the precipitation could not be observed. The molar composition of mixture was (1 – y) TEOS:y APS:0.12 CTAB:0.36 TMAOH:130 H2O. To study the effect of prehydrolysis, TEOS was added into the solution of CTAB and TMAOH and stirred for 3 h; APS was then added to this mixture. The solution was stirred vigorously for 2 h at ambient temperature and then kept statically in a Teflon autoclave at 100 °C for 4 days. The obtained sample was filtered, washed with 3 L of deionized water and 500 mL of C2H5OH, and then dried at 70 °C. The obtained as-

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synthesized sample was named as-APS/MCM-y. The prehydrolyzed sample denoted as asAPS/MCM-y-P representing as-APS/MCM-y with 3 h of prehydrolysis.

2.2 Post-treatment The cationic surfactant CTAB in as-APS/MCM-0.3, as-APS/MCM-0.5 and as-APS/MCM0.3-P was removed using a solution of HCl and C2H5OH; each sample (1.0 g) was stirred in a HCl/C2H5OH solution prepared from 35 wt% HCl (10.42 g) in anhydrous C2H5OH (100 mL) at room temperature for 4 h. The template-removed samples were filtered off, washed several times with deionized water, and then dried at 70 °C. Each sample (1 g) was then stirred in 0.2 M ammonium hydroxide (NH4OH) in anhydrous C2H5OH (100 mL) at room temperature for 1 h and before being filtered off, washed with deionized water, and dried at 70 °C. The samples with post-treatment were named ex-APS/MCM-0.3, ex-APS/MCM-0.5 and ex-APS/MCM-0.3-P.

2.3 APS-grafted calcined MCM-41 prepared by post-modification The APS-grafted calcined MCM-41 material was prepared following the reported postmodification method.16 Calcined MCM-41 (cal-MCM, Sigma-Aldrich; 1 g) was dispersed in anhydrous toluene (50 mL) and stirred for 30 min at room temperature. APS (10 mL) was added and the resulting mixture heated under reflux at 100 °C for 16 h. The suspended solid product was filtered off, washed with anhydrous C2H5OH (500 mL), and then dried at 70 °C overnight in an open air to give APS/cal-MCM. For comparison, the calcined MCM-41 was prepared through the direct-synthesis route without addition of APS (as shown in supporting information), named as cal-MCM-D. The APS-grafted cal-MCM-D was denoted as APS/cal-MCM-D. The results and

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discussion of characterization and CO2 adsorption test for cal-MCM-D and APS/cal-MCM-D are given in supporting information.

2.4 Characterization The structures of the samples were identified through low-angle X-ray diffraction using a MAC Science MXP18AHFXRD apparatus, a CuK radiation source ( = 1.542 Å), and a twocircle powder diffractometer. The N2 physical adsorption/desorption isotherms of the samples were measured at –196 °C using a Micromeritics ASAP 2010 analyzer; dehydration was performed by evacuating the samples at 150 °C overnight. The pore size distribution of the samples was determined through Derjaguin-Broekhoff-de Boer (DBdB) analysis.30 The morphologies and pore sizes of the samples were characterized using a JSM-5600 scanning electron microscope and a JEM-2100 transmission electron microscope. For scanning electron microscopy (SEM), the sample was adhered to a carbon tape and coated with a thin layer of gold prior to measurement. For transmission electron microscopy (TEM), the sample was diluted with C2H5OH and ultrasonicated for 5 min; droplets of the prepared solution were placed on a Cu grid and dried in a vacuum oven at 100 °C overnight. Elemental analysis (EA) of each sample was performed using a Heraeus Vario EL III-CHNS instrument. The sample was dried at 120 oC for 2 h before taking the IR measurement. The IR spectra of the samples were obtained using a PerkinElmer RX1 spectrometer. Spectra were acquired at 1 cm−1 resolution and over 32 scans. Thermal analysis (thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC)) was performed using an SDT Q600 (TGA/DSC) apparatus; each sample was heated under an air flow of 50 mL/min from room temperature to 100 °C at a heating rate of 10 °C/min, and then the

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temperature was maintained at 100 °C for 2 h; finally, the sample was heated from 100 to 800 °C at a heating rate of 10 °C/min.

2.5 Carbon dioxide adsorption The TGA/DSC analyzer was employed to measure the adsorption of CO2 on the aminefunctionalized samples. He, N2, and CO2 (purity: 99.99%) and the premixed gas containing 5 to 80 vol.% CO2 in N2 were purchased from Boclh Industrial Gases (Taiwan). The sample (10 mg) was pretreated and dehydrated by heating from room temperature to 120 °C at a rate of 10 °C/min and then maintained at 120 °C for 6 h under a He flow. CO2 adsorption was performed at 35 °C by purging with a mixture of 15% CO2 in N2 or at 25, 35, or 50 °C by purging with pure CO2 for 7 h. The CO2 adsorption isotherms of the samples were also measured by TGA/DSC analyzer under various CO2 concentrations (5% to pure CO2) at 35 oC. The adsorption of CO2 in a packed bed adsorber was also carried out in this study. The experimental apparatus is presented in Figure 1. Typically, as-APS/MCM-0.5 or APS/cal-MCM (100 mg) was loaded into a 3-mL Pyrex-glass adsorber that was wound with heating tape. Three thermal couples, located at the inlet, outlet, and inside the cell, were used to monitor the temperatures during operation. The temperature could be controlled to within ±0.3 °C. The flow rates of the premixed and pure N2 gases were controlled using a mass flow controller (Brooks Instruments, 5850E). The CO2 concentration in the premixed gas was identified using a NDIR CO2 analyzer (Drager, Polytron Transmitter IR CO2), letting the gas bypass the adsorber in each run. The CO2 concentration in the effluent gas stream was also measured using the NDIR CO2 analyzer. The reliable measurement range of the analyzer was up to 30% CO2, with a resolution of 0.01%. Prior to the CO2 mixed gas entering the adsorber, the loaded sample was dehydrated

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under N2 at a flow rate of 30 cm3/min at 120 °C for 12 h. Because only 100 mg of asAPS/MCM-0.5 was used to capture CO2, no temperature variation was observed between the inlet and exit of the adsorber. After the adsorption mixture had reached equilibrium, thermal regeneration of the sample was performed by purging the adsorber with N2 at 120 °C for 2 h. Using this approach, the loaded sample was completely regenerated. To study the effect of water vapor on CO2 adsorption, the sample was treated with a humid N2 stream prior to adsorption of CO2. The humid N2 gas was prepared by passing N2 through a water saturator that had been placed in a constant-temperature oven. After the humidity of the gas stream, as detected using a hygrometer (Lutron Electronic Enterprise, HT-3009), had plateaued, the premixed humid CO2 gas was passed into the adsorber until the CO2 concentration in the effluent stream remained unchanged. Afterward, thermal regeneration was performed under humid N2 at 120 °C for 2 h.

2.6 CO2 adsorption-desorption cycling test To assess the thermal stability of the prepared adsorbents, the CO2 adsorption-desorption cycling test was performed in TGA/DSC for the as-APS/MCM-y and APS/cal-MCM using temperature swing operation. CO2 was adsorbed from anhydrous N2 gas containing 15% CO2 at 35 °C for 1 h and desorbed by He at 120 °C for 1 h. The CO2 adsorption capacity was determined from the weight gain of the sample after exposure to CO2. Eight CO2 adsorption-desorption cycles were performed in this study.

3. Results and discussion 3.1 Characterization

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Figure 2 displays powder X-ray diffraction (PXRD) patterns of as-APS/MCM-y (y = 0.3, 0.4 and 0.5). The patterns exhibited a single broad peak in the 2 range between 2° and 3°, indicating that the as-APS/MCM-y samples were mesostructured silicas featuring a lack of longrange structural order. An increase in the APS concentration in the gel mixture led to decreased peak intensities and broader peaks, indicating more-disordering mesochannel arrays. The presence of the amino (NH2) groups in the mixture meant that the APS chains could adopt a random orientation, even becoming part of the silica matrix, leading to greater disordering of the mesostructure.29 An increase in the disorder of directly synthesized APS-functionalized MCM41 upon increasing the APS content in the gel had also been observed by Yokoi et al. when using the cationic surfactant CTAB as the template.28 Figure 3 displays the N2 adsorption/desorption isotherms and pore size distributions of the directly synthesized samples; Table 1 lists their pore characteristics. A low adsorption volume and broad pore size distribution were observed for the as-APS/MCM-y sample, shown in Figure 3a,c. The low surface areas (20.0 - 27.1 m2/g) and pore volumes (0.01 - 0.03 cm3/g) of the asAPS/MCM-y were resulted from the surfactant CTAB occluded in the intrachannel structure. After as-APS/MCM-0.3 had been post-treated to generate ex-APS/MCM-0.3 (Figure 3b,d), the surface area and pore volume increased significantly (Table 1) - from 20 to 213.2 m2/g and from 0.03 to 0.10 cm3/g, respectively - while the mesopore diameter reached 2.0 nm. The increases in pore surface area and pore volume and the presence of intrachannels for as-APS/MCM-0.3 after post-treatment were due to the removal of CTAB. Commercial cal-MCM with a surface area of 1214 m2/g, a pore volume of 1.14 cm3/g, and a pore diameter of 3.7 nm was grafted with APS through the post-modification. After APS grafting, the surface area, pore volume, and pore diameter of APS/cal-MCM all deceased to 234 m2/g,

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0.32 cm3/g, and 2.5 nm, respectively, shown in Table 1, suggesting that the APS had been grafted on the intrachannel surfaces of cal-MCM. Figure 4 displays SEM and TEM images of exAPS/MCM-0.3. The silica spheres were observed in the SEM image with an average particle size around 4 µm. No apparent ordering but only wormhole-like pore structure is shown in the TEM image. Table 2 lists EA data for as-APS/MCM-y and APS/cal-MCM. Notably, one mole APS corresponds to one mole N. The N contents of as-APS/MCM-0.3, as-APS/MCM-0.4, and asAPS/MCM-0.5 were 2.46, 3.22, and 3.46 mmol N/g, respectively. Thus, the N contents of the asAPS/MCM-y system increased upon increasing the amount of added APS in the gel composition. However, the N content of the as-APS/MCM-y species determined through EA came not only from APS but also from CTAB. Therefore, the actual contents of APS were presumably lower than the measured values. The N content of APS/cal-MCM prepared by post-modification through the reaction between APS and the silanol groups on the surface of cal-MCM was 2.42 mmol N/g comparable to that of as-APS/MCM-0.3. Table 2 reveals the effect of post-treatment on the N content and the C/N atomic ratio of asAPS/MCM-0.3 and as-APS/MCM-0.5. The N content and C/N atomic ratio of as-APS/MCM-0.5 after post-treatment both decreased from 3.46 to 1.98 mmol N/g (ca. 42.6%) and from 9.9 to 3.2, respectively. The N content of as-APS/MCM-0.3 decreased by approximately 33.3% after posttreatment (from 2.46 to 1.64 mmol N/g). The decreases in the N contents of these two samples were due to the removal of the N sources, namely CTAB and APS. Because of the possibility that some of the APS units were coated, but not covalently incorporated, onto the support, they could be removed after post-treatment. The loss of this APS was confirmed by observing a

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decrease in the CO2 adsorption capacity after post-treatment (Table 2), which will be described later. The IR spectra of as-APS/MCM-y and APS/cal-MCM are shown in Figure 5. Both adsorbents exhibited similar adsorption peaks and bands. A broad band in a range of 3200 to 3700 cm-1 corresponded to silanol groups and O-H stretching from free water. In a similar range of 3280 to 3439 cm-1, the absorption band was assigned to symmetric and asymmetric N-H stretching absorption peaks. The absorption peaks in the range of 1000 to 1200 cm-1 were mainly for the siliceous (Si-O-Si) and siloxane bonds (Si-O-C). Besides these, absorption bands corresponding to C-H stretch (2946 and 2837 cm-1), physical adsorbed water remained (1735 cm-1), N-H bending vibration (1550 cm-1), C-H scissor vibration (1470 cm-1), H-C-H deformation (1385 cm1

), H-N-H deformation (1260 cm-1) and C-N primary amine bonds (800 cm-1) were also observed

in the spectra. These results were in agreement with those in the literature,31-33 indicating that asAPS/MCM-y prepared through direct synthesis exhibited the same functional groups as APS/calMCM prepared by post-modification. Moreover, the peak of ammonium ion (R-N+(CH3)3) at 908 and 1472 cm-1 of CTAB was observed only in as-APS/MCM-y due to CTAB was occluded in the intrachannels. It has been reported that prehydrolysis of TEOS could improve the order structure of aminefunctionalized mesoporous silica since TEOS hydrolyses at a slower rate than APS.29,34 However, the effect of the prehydrolysis on CO2 adsorption has not been discussed yet. In this study, we tried to see the effect of prehydrolysis of TEOS prior to adding APS on CO2 adsorption. PXRD in Figure 2 indicated that the intensity and sharpness of the diffraction peak of as-APS/MCM0.3-P both increased relative to those of as-APS/MCM-0.3, indicating a decrease in the disorder of the structure. The surface area and pore volume of ex-APS/MCM-0.3-P were 308.3 m2/g and

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0.15 cm3/g, respectively, larger than those of ex-APS/MCM-0.3, while the pore diameter remained similar, seen in Table 1. The prehydrolyzed sample of as-APS/MCM-0.3-P exhibited a loss of N content of approximately 17.4% (from 2.47 to 2.04 mmol/g) after post-treatment (Table 2). Though as-APS/MCM-0.3 underwent higher loss of N content than that of as-APS/MCM0.3-P after post-treatment, the CO2 adsorption capacity of as-APS/MCM-0.3 was found to be higher than that of as-APS/MCM-0.3-P (Table 2).

3.2 Thermal property The weight losses of cal-MCM, as-APS/MCM-0.3, ex-APS/MCM-0.3 and APS/cal-MCM were analyzed by TGA/DSC and are shown in Figure 6 and Table S1. The desorption of H2O from cal-MCM included 5.4 wt% of physically adsorbed H2O at temperatures below 100 °C, 0.6 wt% of hydrogen-bonded H2O at in a temperature range from 100 to 350 °C, and 1.1 wt% of H2O produced through the condensation of silanol groups to form siloxane at temperatures above 350 °C. For APS/cal-MCM, the decomposition and desorption of APS was shown by the exothermic peak at 300 oC and 16.0% of weight loss at temperatures above 100 oC. The combined decomposition of CTAB and APS resulted in a broad exothermic peak for asAPS/MCM-0.3 at 291 °C. After treating as-APS/MCM-0.3 with the HCl/C2H5OH and NH4OH/C2H5OH solutions to generate ex-APS/MCM-0.3, however, the peak of the decomposition temperature shifted to 334 °C and the weight loss in the range 100 - 350 °C decreased from 34.9 to 10.6% (Table S1). One explanation for these phenomena is the decomposition and desorption of incorporated APS.

3.3 Amine locations

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The elemental analysis (EA) data (Table 2) revealed that there were two APS-presenting regions in the as-APS/MCM-y: the APS noncovalently coated and APS covalently incorporated on the as-APS/MCM-y. The post-treatment appeared to eliminate CTAB and the coated APS units (Figure 7), which could be observed by the large reduction of N content. The prehydrolysis of TEOS decreased the amount of coated APS and induced more incorporated APS on the asAPS/MCM-y, as evidenced by the lower loss of N content after post-treatment (Table 2).

3.4 CO2 adsorption ability Table 2 shows CO2 adsorption capacities and amine efficiencies (CO2/N mole ratio) of the asAPS/MCM-y and APS/cal-MCM. The CO2 adsorption capacities of as-APS/MCM-0.3, asAPS/MCM-0.4, and as-APS/MCM-0.5 under 15% CO2 in N2 at 35 °C were 0.90, 1.16, and 1.18 mmol/g, respectively; their CO2/N mole ratios varied between 0.34 and 0.37. Although the asAPS/MCM-y possessed only 20 - 27 m2/g surface area, the presence of micropore spaces (Figure 3c) provided surface amine groups and micro channel for CO2 penetration to interact with APS, leading to a high CO2 adsorption capacity. The CO2 adsorption capacities of as-APS/MCM-y increased upon increasing the amount of APS on the support. Because the N atoms of the asAPS/MCM-y species derived from CTAB and APS, the CO2/N mole ratio based on APS units alone would be greater than the experimental value in Table 2. When prehydrolysis of TEOS for 3 h was performed, the CO2 adsorption capacity and CO2/N mole ratio of as-APS/MCM-0.3-P (0.74 mmol/g and 0.30, respectively) were lower than those of as-APS/MCM-0.3 (0.90 mmol/g and 0.37, respectively) due to smaller amounts of the coated APS that dominates CO2 adsorption. The CO2 adsorption capacity and CO2/N mole ratio of APS/cal-MCM were 0.88 mmol/g and 0.39, respectively. The CO2 adsorption capacities of the as-APS/MCM-0.3 sample was

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comparable with that of APS/cal-MCM, while those of as-APS/MCM-y (y = 0.4 and 0.5) samples were greater than that of APS/cal-MCM, indicating their higher loadings of APS. Table 2 reveals the effect of post-treatment on the CO2 adsorption capacities and CO2/N mole ratios of as-APS/MCM-0.3, as-APS/MCM-0.5 and as-APS/MCM-0.3-P. For as-APS/MCM-0.3, post-treatment resulted in large decreases in the CO2 adsorption capacity and CO2/N mole ratio, from 0.90 to 0.25 mmol/g and from 0.37 to 0.15, respectively. Similarly, the CO2 adsorption capacity and CO2/N mole ratio of as-APS/MCM-0.5 and as-APS/MCM-0.3-P also decreased after post-treatment. As a result, post-treatment was disadvantageous because it caused the removal of the coated APS, leading to a reduction of CO2 adsorption capacity. The CO2 adsorption isotherms on as-APS/MCM-0.5, ex-APS/MCM-0.5 and APS/cal-MCM at 35 oC are depicted in Figure 8. The CO2 adsorption capacities of as-APS/MCM-0.5 were higher than both ex-APS/MCM-0.5 and APS/cal-MCM at all CO2 concentrations. The CO2 adsorption capacities of these three samples increased nonlinearly with the increase of CO2 concentration in the gas mixtures. The isotherms displayed a steep increase at CO2 concentration less than 15% and a small increase over 15%. The steep curve of the CO2 adsorption isotherms at low CO2 concentration was caused by the quite strong interaction between CO2 and the amine groups. The further small increase was ascribed to the physical adsorption of CO2. The CO2 adsorption isotherm data of these three samples were aptly correlated by Langmuir adsorption isotherm with r2 > 0.9668. The constants in Langmuir equation (qm and b) are shown in Table 3. The constant b of as-APS/MCM-0.5 and APS/cal-MCM were 0.2979 and 0.3672 L/mmol, respectively, indicating a lower heat of adsorption of as-APS/MCM-0.5 over APS/calMCM.

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Page 16 of 38

In addition to as-APS/MCM-0.5, which had the highest CO2 adsorption capacity among the directly synthesized samples, APS/cal-MCM was also tested for its CO2 adsorption capacity under anhydrous and humid 15% CO2 in N2 (relative humidity: 74%) at 35 oC in a packed bed adsorber. The CO2 adsorption capacities of as-APS/MCM-0.5 and APS/cal-MCM under humid 15% CO2 in N2 were 1.30 and 1.05 mmol/g, respectively, while they were 1.14 and 0.83 mmol/g, respectively, under anhydrous 15% CO2 in N2 (Table S2). Thus, the CO2 adsorption capacities under anhydrous 15% CO2 in N2 were lower than those under humid 15% CO2 in N2. The adsorption of CO2 gas on amine-supported silica is known to follow an acid/base mechanism: RNH2 + CO2  RNH2+COO–

(1)

RNH2+COO– + RNH2  RNHCOO– + RNH3+

(2)

RNH2+COO– + H2O  RNHCOO– + H3O+

(3)

The RNHCOOH species and RNHCOO–RNH3+ ion pair are a carbamic acid and an ammonium carbamate salt, respectively. In the presence of water, CO2 can be hydrolyzed to form HCO3– and CO32–, which further interacts with the protonated amine to form the salts ammonium bicarbonate (RNH3+HCO3–) and ammonium carbonate ((RNH3+)2CO32–).35 Typically, two proximate amine molecules react with one molecule of CO2 under anhydrous conditions, while one molecule of amine reacts with one molecule of CO2 in the presence of water. The influence of moisture on the CO2 adsorption capacity of amine-functionalized silicas has been investigated previously. Leal et al.36 reported a large positive effect on the CO2 adsorption capacity of APS/silica gel under pure CO2 at a relative humidity of 98%. In addition, water had a slightly positive or negative effect on CO2 adsorption on the amine-supported mesoporous silicas SBA15, MCM-41, and pore-expanded MCM-41.12,15,16 The effect of water on CO2 adsorption capacity is dependent on the structure of the amine as well as the nature of the porous support.12

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Table 4 lists the CO2 adsorption capacities of as-APS/MCM-y and APS/cal-MCM under pure CO2 at 25, 35, and 50 °C in TGA. The CO2 adsorption capacities of as-APS/MCM-y under pure CO2 were in ranges of 1.35 - 1.74 mmol/g at 25 °C, 1.27 - 1.62 mmol/g at 35 °C, and 1.06 - 1.48 at 50 °C, and those of APS/cal-MCM at 25, 35, and 50 °C were 1.24, 1.08, and 0.89 mmol/g, respectively. In each case the CO2 adsorption capacity decreased upon increasing the adsorption temperature. Based on exothermic adsorption nature, more CO2 could be adsorbed at lower temperatures. The CO2 adsorption capacity of as-APS/MCM-0.5 in pure CO2 (1.74 mmol/g) was slightly higher than those of ex-APS/MCM (0.49 mmol/g),29 APS/cal-MCM (0.83 mmol/g and 1.20 mmol/g),25,27 APS/cal-MCM-48 (0.80 mmol/g),14 and APS/cal-SBA-15 (1.54 mmol/g and 0.97 mmol/g at 25 and 45 oC, respectively),37,38 and comparable with that of PEI/KIT-6 (1.79 mmol/g).25 Figure 9 presents the CO2 adsorption capacities of as-APS/MCM-y and APS/cal-MCM in each adsorption-desorption cycle. The thermal swing was repeated eight times within a total duration of 1400 min (adsorption under 15% CO2 in N2 at 35 °C; desorption at 120 °C). The stable adsorption and desorption behaviors were observed. The CO2 adsorption capacities of asAPS/MCM-y and APS/cal-MCM during the eight cycles appeared to be invariant, indicating the high thermal stability of APS in each directly synthesized sample as well as in APS/cal-MCM. For energy aspect, the as-APS/MCM-y with 0.90 - 1.18 mmol/g CO2 adsorption capacity requires regeneration energy approximately 2,200 - 3,400 kJ/kg CO2, which is lower than the current state-of-the-art MEA CO2 capture technology of 4,530 kJ/kg CO2.39 As a result, the asAPS/MCM-y with high capacity and stability as well as low regeneration energy show their potential for CO2 capture via thermal swing adsorption.

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Page 18 of 38

4. Conclusions The as-APS/MCM-y synthesized directly and readily within a short period of time and with low chemical reagent consumption at a pH of approximately 13 were observed to be effective adsorbents for CO2 capture. Their N contents could be easily adjusted by altering the amount of APS in gel composition during synthesis. The prepared as-APS/MCM-0.5 possessed N content up to 3.46 mmol N/g and CO2 adsorption capacities up to 1.18 mmol/g under 15% CO2 in N2 at 35 oC and 1.74 mmol/g under pure CO2 at 25 °C. The CO2 adsorption capacity of the asAPS/MCM-0.5 was higher than that of the APS/cal-MCM for adsorption under 15% CO2 in N2 and was also higher than the APS-functionalized mesoporous silicas reported in literature for adsorption under pure CO2. Two locations of APS on the as-APS/MCM-y, the coated APS and the incorporated APS, are conjectured to exist. The coated APS has a higher affinity toward CO2 than the incorporated APS. Thus, the prehydrolysis of TEOS during the direct synthesis of the as-APS/MCM-y is not recommended. The post-treatment including template removal and APS neutralization was also proved to be unnecessary for the as-APS/MCM-y due to the loss of the coated APS. Dynamic adsorption-desorption cycles revealed that the as-APS/MCM-y possessed high thermal stability. The as-APS/MCM-y samples are therefore regarded as appropriate adsorbents for CO2 capture when a temperature swing adsorption technology is applied.

Associate content Supporting Information Thermogravimetric analysis data and CO2 adsorption capacity under anhydrous and humid 15% CO2 at 35 oC of the prepared samples are demonstrated in Table S1 and S2, respectively.

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The experimental results (PXRD, pore properties, TEM image, N content and CO2 adsorption capacity) and discussion of cal-MCM-D and APS/cal-MCM-D are presented in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment The authors wish to thank the National Science Council (grant number NSC 101-3113-E-007005) for financial support and National Tsing Hua University, Taiwan, ROC for the scholarship to W. Klinthong.

Nomenclature as

As-synthesized

b

Langmuir constant

cal

Calcined

ex

Post-treated

q

CO2 adsorption capacity

qm

Langmuir constant

y

Molar ratio of APS to sum of TEOS and APS varied from 0.3 - 0.5

Ce

CO2 concentration in feed

P

Prehydrolysis time of 3 h

D

Pore diameter calculated by Derjaguin-Broekhoff-de Boer method

SBET

Surface area calculated by Brunauer–Emmet–Teller method

Vtotal

Pore volume measured by N2 desorption at –196 oC

Nas

N contents of as-synthesized sample

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Nex

N contents of post-treated sample

TEOS

Tetraethyl orthosilicate

APS

3-aminopropyl triethoxysilane

CTAB

Hexadecyltrimethylammonium bromide

TMAOH

Tetramethylammonium hydroxide

cal-MCM

Commercially calcined MCM-41

cal-MCM-D

Calcined MCM-41 synthesized through direct-synthesis route without

Page 20 of 38

addition of APS APS/MCM

APS-functionalized MCM-41 prepared through direct synthesis

APS/cal-MCM

APS-grafted calcined MCM-41 prepared by post-modification

APS/cal-MCM-D

APS-grafted calcined MCM-D prepared by post-modification

APS/cal-SBA-15

APS-grafted calcined SBA-15 prepared by post-modification

APS/cal-MCM-48

APS-grafted calcined MCM-48 prepared by post-modification

PEI/KIT-6

Polyethyleneimine-impregnated KIT-6

References (1) Global CCS Institute, The global status of CCS: 2011, Canberra, Australia, 2011. (2) IEA, Clean energy progress report, Paris, France, 2011. (3) Tan, C. S.; Chen, J. E. Absorption of carbon dioxide with piperazine and its mixtures in a rotating packed bed. Sep. Purif. Technol. 2006, 49, 174. (4) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-combustion CO2 capture using solid sorbents: A review. Ind. Eng. Chem. Res. 2012, 51, 1438. (5) Veaweb, A.; Tontiwachwuthikul, P.; Chakma, A. Corrosion behavior of carbon steel in the

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CO2 absorption process using aqueous amine solutions. Ind. Eng. Chem. Res. 1999, 38, 3917. (6) Lepaumier, H.; Picq, D.; Carrette, P. L. New amines for CO2 capture. I. Mechanism of amine degradation in the presence of CO2. Ind. Eng. Chem. Res. 2009, 48, 9061. (7) Lepaumier, H.; Martin, S.; Picq, D.; Delfort, B.; Carrette, P. L. New amines for CO2 capture. III. Effect of alkyl chain length between amine functions on polyamines degradation. Ind. Eng. Chem. Res. 2010, 49, 4553. (8) Zhang, X.; Zheng, X.; Zhang, S.; Zhao, B.; Wu, W. AM-TEPA impregnated disordered mesoporous silica as CO2 capture adsorbent for balanced adsorption−desorption properties. Ind. Eng. Chem. Res. 2012, 51, 15163. (9) Yu, C. H.; Huang, C. H.; Tan, C. S. A review of CO2 capture by absorption and adsorption, Aerosol Air Qual. Res. 2012, 12, 745. (10) Xu, X.; Song, C.; Miller, B. G.; Scaroni, A.W. Influence of moisture on CO2 separation from gas mixture by a nanoporous adsorbent based on polyethylenimine-modified molecular sieve MCM-41. Ind. Eng. Chem. Res. 2005, 44, 8113. (11) Belmabkhout, Y.; Weireld, G. D.; Sayari, A. Amine-bearing mesoporous silica for CO2 and H2S removal from natural gas and biogas. Langmuir 2009, 25, 13275. (12) Harlick, P. J. E.; Sayari, A. Applications of pore-expanded mesoporous silica. 5. Triamine grafted material with exceptional CO2 dynamic and equilibrium adsorption Performance. Ind. Eng. Chem. Res. 2007, 46, 446. (13) Sharma, P.; Seong, J. K.; Jung, Y. H.; Park, S. D.; Yoon, Y. I.; Baek, I. H. Amine modified and pelletized mesoporous materials: Synthesis, textural-mechanical characterization and application in adsorptive separation of carbon dioxide. Powder Technol. 2012, 219, 86.

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Page 22 of 38

(14) Kim, S.; Ida, J.; Guliants, V. V.; Lin, J. Y. S. Tailoring pore properties of MCM-48 silica for selective adsorption of CO2. J. Phys. Chem. B 2005, 109, 6287. (15) Hiyoshi, N.; Yogo, K.; Yashima, T. Adsorption of carbon dioxide on modified mesoporous materials in the presence of water vapour. Stud. Surf. Sci. Catal. 2004, 154, 2995. (16) Chang, F. Y.; Chao, K. J.; Cheng, H.H.; Tan, C. S.; Adsorption of CO2 onto amine-grafted mesoporous silicas. Sep. Purif. Technol. 2009, 70, 87. (17) D’Alessandro, D. M.; Smit, B.; Long, J. R.; Carbon dioxide capture: Prospects for new materials. Angew. Chem., Int. Ed. 2010, 49, 6058. (18) 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. (19) Wang, X.; Schwartz, V.; Clark, J. C.; Ma, X.; Overbury, S. H.; Xu, X.; Song, C. Infrared study of CO2 sorption over “molecular basket” sorbent consisting of polyethyleniminemodified mesoporous molecular sieve. J. Phys. Chem. C 2009, 113, 7260. (20) Kuwahara, Y.; Kang, D. Y.; Copeland, J. R.; Brunelli, N. A.; Didas, S. A.; Bollini, P.; Sievers, C.; Kamegawa, T.; Yamashita, H.; Jones, C. W. Dramatic enhancement of CO2 uptake by poly(ethyleneimine) using zirconosilicate supports. J. Am. Chem. Soc. 2012, 134, 10757. (21) Yue, M. B.; Sun, L. B.; Cao, Y.; Wang, Y.; Wang, Z. J.; Zhu, J. H. Efficient CO capturer derived from as-synthesized MCM-41 modified with amine, Chem. Eur. J. 2008, 14, 3442. (22) Fisher II, J. C.; Tanthana, J.; Chuang, S. S. C. Oxide-supported tetraethylenepentamine for CO2 capture. Environ. Prog. Sustainable Energy 2009, 28, 589. (23) Huo, Q.; Margolese, D. I.; Ciesia, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Generalized synthesis of periodic surfactant/inorganic

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composite materials. Nature 1994, 368, 317. (24) Yang, C. M.; Chao, K. J. Functionalization of molecularly template mesoporous silica, J. Chin. Chem. Soc. 2002, 49, 883. (25) Kim, S. N.; Son, W. J.; Choi, J. S.; Ahn, W. S. CO2 Adsorption using amine-functionalized mesoporous silica prepared via anionic surfactant-mediated synthesis, Microporous Mesoporous Mater. 2008, 115, 497. (26) Zeleňák, V.; Badaničová, M.; Halamová, D.; Čejka, J.; Zukal, A.; Murafa, N.; Goerigk, G. Amine-modified ordered mesoporous silica: effect of pore size on carbon dioxide capture. Chem. Eng. J. 2008, 144, 336. (27) Ioganathan, S.; Tikmani, M.; Ghoshal, A. K. A novel pore expanded MCM-41 for CO2 capture: synthesis and characterization. Langmuir 2013, 29, 3491. (28) Yokoi, T.; Yoshitake, H.; Tatsumi, T. Synthesis of amino-functionalized MCM-41 via direct co-condensation and post-synthesis grafting methods using mono-, di- and triamino-organoalkoxysilanes. J. Mater. Chem. 2004, 14, 951. (29) Osei-Prempeh, G.; Lehmerm, H. J.; Rankin, S. E.; Knutson, B. L. Direct synthesis and accessibility of amine-functionalized mesoporous silica template using fluorinated Surfactants. Ind. Eng. Chem. Res. 2011, 50, 5510. (30) Chiu, C. Y.; Chiang, A. S. T.; Chao, K. J. Mesoporous silica powders and films-pore size characterization by krypton adsorption. Microporous Mesoporous Mater. 2006, 2, 244. (31) Chang, A. C. C.; Chuang, S. C. C.; Gray, M.; Soong, Y. In-Situ infrared study of CO2 adsorption on SBA-15 grafted with γ-(aminopropyl)triethoxysilane. Energy Fuels 2003, 17, 468. (32) Hiyoshi, N.; Yogo, K.; Yashima, T. Adsorption characteristics of carbon dioxide on

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organically functionalized SBA-15. Microporous Mesoporous Mater. 2005, 84, 357. (33) Calleja, G.; Sanz, R.; Arencibia, A.; Sanz-Pérez, E. S. Influence of drying conditions on amine-functionalized SBA-15 as adsorbent of CO2. Top. Catal. 2011, 54, 135. (34) Hartono, S. B.; Qiao, S. Z.; Jack, K.; Ladewig, B. P.; Hao, Z.; Lu, G. Q. Improving adsorbent properties of cage-like ordered amine functionalized mesoporous silica with very large pore for bioadsorption, Langmuir 2009, 25, 6413. (35) Littel, R. J.; Versteeg, G. F.; Van Swaaij, W. P. M. Kinetics of CO2 with primary and secondary amines in aqueous solutions. I. Zwitterion deprotonation kinetics for DEA and DIPA in aqueous blend of alkanolamines. Chem. Eng. Sci. 1992, 47, 2037. (36) Leal, O.; Bolivar, C.; Garcia, J. J.; Espidel, Y. Reversible adsorption of carbon dioxide on amine surface-bonded silica gel. Inorg. Chim. Acta 1995, 240, 183. (37) Wang, L.; Yang, R. T. Increasing selective CO2 adsorption on amine-grafted SBA-15 by increasing silanol density. J. Phys. Chem. C 2011, 115, 21264. (38) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Pérez, E. S. Amino functionalized mesostructured SBA-15 silica for CO2 capture: Exploring the relation between the adsorption capacity and the distribution of amino groups by TEM. Microporous Mesoporous Mater. 2012, 158, 309. (39) Sjostrom, S.; Krutka, H. Evaluation of solid sorbents as a retrofit technology for CO2 capture. Fuel 2010, 89, 1298.

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Table 1. Pore characteristics of the prepared samples. Sample

SBETa (m2/g)

Vtotalb (cm3/g)

Dc (nm)

as-APS/MCM-0.5

27.1

0.02

-

as-APS/MCM-0.4

24.0

0.01

-

as-APS/MCM-0.3

20.0

0.03

-

ex-APS/MCM-0.3

213.2

0.10

2.0

ex-APS/MCM-0.3-P

308.3

0.15

2.1

cal-MCM

1214.0

1.14

3.7

APS/cal-MCM

234.0

0.32

2.5

a

SBET: surface area calculated by BET method.

b

Vtotal: pore volume measured by N2 desorption at –196 oC.

c

D: pore diameter calculated by DBdB method.

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Page 26 of 38

Table 2. Nitrogen contents, CO2 adsorption capacities and CO2/N of the prepared samples. N contenta Sample

C/Na

CO2 adsorption capacityb

(mmol N/g)

CO2/N

(mmol/g)

APS/cal-MCM

2.42

3.1

0.88

0.39

as-APS/MCM-0.5

3.46

9.9

1.18

0.34

as-APS/MCM-0.4

3.22

10.1

1.16

0.36

as-APS/MCM-0.3

2.46

10.3

0.90

0.37

as-APS/MCM-0.3-P

2.47

9.9

0.74

0.30

ex-APS/MCM-0.5

1.98 (42.6%)c

3.2

0.20

0.10

ex-APS/MCM-0.3

1.64 (33.3%)c

2.3

0.25

0.15

ex-APS/MCM-0.3-P

2.04 (17.4%)c

2.9

0.54

0.26

a

Determined through elemental analysis.

b

Measured under 15% CO2 in N2 at 35 °C by TGA.

c

The value in parenthesis displays N loss (%) after post-treatment which was calculated by N loss (%) = (Nas – Nex)/Nas  100%, where Nas and Nex represent the N contents of asAPS/MCM-y and ex-APS/MCM-y, respectively.

26


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Table 3. Constants of Langmuir equationa of prepared samples. qm (mmol/g)

b (L/mmol)

r2

as-APS/MCM-0.5

1.77

0.2979

0.9980

ex-APS/MCM-0.5

0.44

0.1126

0.9668

APS/cal-MCM

1.13

0.3672

0.9978

Sample

a

Langmuir equation: q = qmbCe/(1+bCe).

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Table 4. CO2 adsorption capacity of the prepared samples measured under pure CO2 by TGA. Sample

Adsorption temperature (°C)

CO2 adsorption capacity (mmol/g)

as-APS/MCM-0.5

25

1.74

35

1.62

50

1.48

25

1.52

35

1.49

50

1.14

25

1.35

35

1.27

50

1.06

25

1.24

35

1.08

50

0.89

as-APS/MCM-0.4

as-APS/MCM-0.3

APS/cal-MCM

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Figure captions Figure 1. Experimental apparatus for adsorption of CO2 in a continuous operation mode. Figure 2. PXRD patterns of the prepared samples. Figure 3. (a,b) N2 adsorption/desorption isotherms and (c,d) pore size distributions of the prepared samples. Figure 4. (a) SEM and (b) TEM images of ex-APS/MCM-0.3. Figure 5. IR spectra of the prepared samples. Figure 6. TGA/DSC profiles of the prepared samples. Figure 7. Location of APS units. Figure 8. CO2 adsorption isotherms of prepared samples at 35 oC measured using TGA. Figure 9. Eight adsorption-desorption cycles of the prepared samples under 15% CO2 in N2 at 35 °C as measured using TGA.

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Page 30 of 38

Figure 1. Experimental apparatus for adsorption of CO2 in a continuous operation mode.

30


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as-APS/MCM-0.5

as-APS/MCM-0.4

Intensity

Page 31 of 38

as-APS/MCM-0.3

as-APS/MCM-0.3-P

2

4

2 Theta (degree) Figure 2. PXRD patterns of the prepared samples.

31


60

(a)

as-APS/MCM-0.3 as-APS/MCM-0.4 as-APS/MCM-0.5

dV/dD

40

Page 32 of 38

(c)

as-APS/MCM-0.3 as-APS/MCM-0.4 as-APS/MCM-0.5

0.008

0.004

20

0 0.0

0.000 0.2

0.4

0.6

0.8

2

1.0

4

150

8

10

(d)

ex-APS/MCM-0.3 ex-APS/MCM-0.3-P

(b)

ex-APS/MCM-0.3 ex-APS/MCM-0.3-P

6

Pore diameter (nm)

Relative pressure (P/P0)

0.04 100

dV/dD

Volume adsorbed (cm3/g STP)

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

Volume adsorbed (cm3/g STP)


0.02

50

0 0.0

0.00 0.2

0.4

0.6

0.8

1.0

2

Relative pressure (P/P0)

4

6

8

10

Pore diameter (nm)

Figure 3. (a,b) N2 adsorption/desorption isotherms and (c,d) pore size distributions of the prepared samples. .

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APS/MCM--0.3. Fiigure 4. (a) SEM and (bb) TEM imaages of ex-A

33


as-APS/MCM-0.5

Page 34 of 38

800-CN primary amine bonds

Si-O-Si Si-O-C

1260-HNH deformation

1735-physisorbed water

1550-NH vibration 1470-CH, scissor vibration 1385-HCH deformation

2946-CH, symmetric

2837-CH, asymmetric

3280-NH, asymmetric

3439-NH, symmetric

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

Intensity (A.U.)


as-APS/MCM-0.4

as-APS/MCM-0.3 APS/cal-MCM

4000

3000

2000

-1

Wavenumber (cm )

1000

Figure 5. IR spectra of the prepared samples. .

34


cal-MCM

99

7

96

0

93

-7 o

300 C

96

APS/cal-MCM 7

88

0

80

-7

96

o

291 C

80

as-APS/MCM-0.3 7 0

64

Heat flow (W/g)

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


Weight (%)

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-7

48 99

ex-APS/MCM-0.3 7 o 334 C 0

88 77

-7

66 0

200 400 600 o Temperature ( C)

800

Figure 6. TGA/DSC profiles of the prepared samples.

35



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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Figure 7. Location of APS units. .

36


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capacity (q, mmol/g)

2.0

CO2 adsorption

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


as-APS/MCM-0.5 ex-APS/MCM-0.5 APS/cal-MCM Langmuir

1.5

1.0

0.5

0.0 0

20

40

60

80

100

CO2 concentration (%)

Figure 8. CO2 adsorption isotherms of prepared samples at 35 oC measured using TGA.

37


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

CO2 adsorption capacity (mmol/g)


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1.2

0.8

0.4

as-APS/MCM-0.5 as-APS/MCM-0.4 as-APS/MCM-0.3 APS/cal-MCM

0.0 2

4

6

8

Cycle

Figure 9. Eight adsorption-desorption cycles of the prepared samples under 15% CO2 in N2 at 35 °C as measured using TGA.

38


CO2 Capture by As-Synthesized Amine-Functionalized MCM-41

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