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Development of hybrid amine-functionalized MCM-41 sorbents for CO2 capture
Chemical Engineering Journal 260 (2015) 573–581
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Development of hybrid amine-functionalized MCM-41 sorbents for CO2 capture Xia Wang a,b, Linlin Chen a, Qingjie Guo a,⇑ a b
Key Laboratory of Clean Chemical Processing of Shandong Province, College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, Shandong, China Department of Chemistry and Chemical Engineering, Weifang University, Weifang 261061, Shandong, China
h i g h l i g h t s A novel MCM-41 sorbent was synthesized by both grafting and impregnation method. New formed network facilitated the dispersion of TEPA and the diffusion of CO2. Effects of amine loadings, temperature and flow rate on CO2 sorption were studied. The sorbent exhibited excellent cyclic regenerability and thermal stability.
a r t i c l e
i n f o
Article history: Received 28 June 2014 Received in revised form 28 August 2014 Accepted 30 August 2014 Available online 19 September 2014 Keywords: Two-step method CO2 adsorption Hybrid amines Cyclic regenerability Adsorption kinetics
a b s t r a c t In consideration of the increasing warming trends worldwide, the development of highly efficient CO2 adsorbents becomes of vital importance. In this paper, a novel hybrid 3-aminopropyltrimethoxysilane (APTS) and tetraethylenepentamine (TEPA) modified MCM-41 sorbent was synthesized by a two-step method. APTS was first grafted onto the MCM-41, and then TEPA was impregnated. The as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and N2 adsorption–desorption techniques. CO2 adsorption–desorption experiments were performed in a self-assembled fixed bed reactor under ambient pressure. The effects of hybrid amine loadings, adsorption temperatures, and influent velocities on CO2 sorption were investigated. Among the composites synthesized, MCM-41 modified with 30 wt.% APTS and 40 wt.% TEPA (MCM-41-APTS30%–TEPA40%) showed maximum adsorption capacity of 3.50 mmol-CO2/g-sorbent. The MCM-41-APTS30%–TEPA40% sorbent exhibited well cyclic regenerability after ten adsorption–desorption cycles. Further, the adsorption kinetics of MCM-41 and MCM-41-APTS30%–TEPA40% were studied. CO2 adsorption was a two-stage process, and the Avrami model could fit well with the experimental data of MCM-41-APTS30%–TEPA40%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The swift growth of anthropogenic CO2 emissions in the atmosphere, especially the CO2 emissions caused by the combustion of fossil fuels has drawn great attention because of the increasing greenhouse effect [1–4]. The atmospheric concentration of CO2 is expected to reach 550 ppm by 2050 from its value of ca. 384 ppm in 2007, assuming that the CO2 emissions would be at a stable level in 2050 [5]. The development of more efficient carbon capture and storage (CCS) technology is therefore extremely urgent. To date, a variety of CO2 capturing technologies, including
⇑ Corresponding author. Tel.: +86 532 84022757. E-mail address: [email protected] (Q. Guo). http://dx.doi.org/10.1016/j.cej.2014.08.107 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
absorption, adsorption, membrane separation technology and cryogenic distillation methods, have been developed [6,7]. Currently, the traditional and commercially adopted CO2 capture technology in large scale industry is liquid amine absorption, which selectively absorbs CO2 from a gas stream over liquid amine solvents such as the most commonly used primary amine (monoethanolamine, MEA) and secondary amine (diethanolamine, DEA). The solubility of CO2 in MEA can reach 0.35 mol-CO2/mol-MEA at atmospheric pressure. Amine solvents react selectively with CO2 to form carbaminate, and the carbaminate that is formed can be regenerated reversibly [8]. However, the regeneration of amine solvents consumes large amounts of energy, the primary and secondary amines volatilize extensively during regeneration, and the alkaline amine can cause serious equipment corrosion problems [9]. Compared with volatile organic solvents, ionic liquids (ILs)
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are non-volatile under ambient conditions and pose a lower exposure risk [10]. Chen et al. researched the adsorption capacity of 1-butyl-3-methylimidazolium acetate using TGA at atmospheric pressure. The adsorption capacity of 0.95 mol-CO2/mol-ILs was determined at 50 °C [11]. However, the relatively high viscosity and the lack of detailed physical property data are the main barriers for the utilization of ILs [10]. The above-mentioned challenges have led researchers to design more energy-efficient, less toxic and corrosive, and more easily handled adsorbents [12,13]. Thus, solid amine-modified sorbents came into being. In the past few years, various amine-modified solid sorbents have been developed worldwide for CO2 capture and separation [14–18], and many well-written reviews have comprehensively depicted the progress of the research [10,19–21]. Solid aminemodified sorbents are composed primarily of supports with highly developed porosity and active components with sites having an affinity for CO2. Two technologies are mainly employed to prepare amine-modified solid sorbents. One technology is the wet impregnation method, where liquid amine molecules are impregnated into the porous supports via van der Waals forces. The other technology is the grafting method, where alkanolamine molecules are grafted onto the inner surfaces of porous supports by reacting with the silicon hydroxyl groups. The grafting method usually has the merits of a quicker adsorption rate and higher amine efficiency, while the impregnation method is more convenient and easier to operate, consumes less energy, and can load more active CO2-philic sites [22–24]. In the previous studies, progress has been made by immobilizing liquid amines (such as TEPA, APTS, MEA, DEA, etc.) in various solid supporting materials such as the metal organic framework (MOF) [25], montmorillonite [26], fly ash [27], mesoporous carbon [28], activated carbon [29], mesocellular silica foams [30], SBA-15 [31–33], MCM-22, MCM-36, ITQ-2 [34], and MCM-41 [24,35–36]. Kamarudin et al. found that MCM-41 with a BET surface area of 750 m2/g modified with 25 wt.% MEA showed a high degree of CO2 removal [35]. Chen et al. studied the adsorption capacity of an ethylenediamine grafted zeolite-like metal organic framework with a sod topology (sod-ZMOF). The optimum adsorption capacity of 69 mg-CO2/g-sorbent at 298 K and a CO2 relative pressure of 1.0 was reached [25]. Liu et al. studied the adsorption capacity of MCM-41 modified with different types of amines. The studies of Liu et al. showed that the CO2 adsorption capacity for MCM-41 modified with 40 wt.% TEPA could reach up to 2.70 mmol-CO2/g-sorbent [24]. Jing et al. researched the molecular simulation of MCM-41, which provided guidance for the further modification of the support [36]. To date, most researchers have focused on immobilizing the liquid amine in the support by the impregnation or grafting method, but very few investigators combined the two methods. In this paper, mesoporous sieve MCM-41 was functionalized by a two-step method: first, grafted with APTS and then, impregnated with TEPA. The textural properties and thermal stability of the hybrid composites were characterized, and their CO2 adsorption performance was investigated. In addition, three kinetic models were used to fit the adsorption process.
2. Experimental section 2.1. Materials MCM-41 was purchased from the catalyst plant of NanKai University, Tianjin, China. TEPA (90%) was provided by Tianjin BASF Chemical Co., Ltd, Tianjin, China. Anhydrous ethanol solvent and APTS were produced by Sinopharm Chemical Reagent Co., Ltd, and Sigma–Aldrich, respectively. Highly pure N2 and a mixed
gas stream (15 vol.% CO2/85 vol.% N2) were prepared by the Section 2.1 at Qingdao University of Science & Technology. 2.2. Preparation of hybrid amine-functionalized sorbents 2.2.1. APTS grafted onto MCM-41 APTS was chosen as the grafting agent. A conventional grafting technology was adopted to graft APTS onto the MCM-41 support [37–39]. In a typical procedure, 6 g of MCM-41 was added to a round-bottom flask with 350 mL anhydrous ethanol solvent. The solution was then stirred vigorously at room temperature until the suspension was well-distributed. Then, 1 mL of distilled water was added to the mixture, and the mixture was stirred for another 30 min. The temperature was then raised rapidly to 70 °C, and different amounts of APTS were added dropwise. The whole system was stirred and refluxed continuously for 10 h. The APTS modified MCM-41 that was obtained was washed repeatedly with anhydrous ethanol and then dried in an oven at 85 °C for 12 h. The addition of distilled water was to promote the dispersion and hydrolysis of APTS by improving its polarity. As-prepared samples were labeled as MCM-41-APTSx, with x representing the weight loadings of APTS on as-prepared sorbent. In this study, x was set as 20%, 30%, 40% and 50%. Eq. (1) shows the calculation equation of x.
x¼
mAPTS 100% mAPTS þ mMCM-41
ð1Þ
where mAPTS is the weight of APTS added, g; mMCM-41 is the weight of the MCM-41 support, g. 2.2.2. TEPA impregnated into MCM-41-APTSx The MCM-41-APTSx sample that was obtained was further modified by a wet impregnation method as described previously [40]. A desirable amount of TEPA was dissolved in 30 mL of anhydrous ethanol, and the mixture was subjected to sonication for 30 min in an ultrasound bath at room temperature to promote the complete dissolution of TEPA. Then, 1.5 g of MCM-41-APTSx was added to the solution, and the system was sonicated for another 4 h. The slurry was then placed in the oven at 85 °C for 16 h to obtain dry solid power. The as-prepared samples were marked as MCM-41-APTSx–TEPAy, where y represents different weight loadings of TEPA onto the sorbent. Here, y ranges from 10% to 50%. Eq. (2) shows the calculation of y.
y¼
mTEPA 100% mTEPA þ mMCM-41-APTSx
ð2Þ
where mTEPA is the weight of TEPA added, g; mMCM-41-APTS is the weight of APTS modified MCM-41 sample, g. The amount of APTS and TEPA in MCM-41-APTSx–TEPAy are listed in Table 2S (in Supplementary material). 2.3. Characterization (1) N2 adsorption–desorption. N2 adsorption–desorption utilizes the principle of physical adsorption of N2 at 77 K to obtain the adsorption–desorption isotherm. The analysis was carried out using a Quadrasorb SI analyzer (Quantachrome, USA). Prior to each measurement, the sample was degassed at 353 K in a high vacuum for 12 h. The specific surface area was calculated by using the conventional Brunauer–Emmett–Teller (BET) method. The total pore volume was obtained from the adsorbed amount after finishing the capillary condensation at a relative pressure of P/P0 = 0.996. The pore size was determined using the Barrett–Joyner–Halenda (BJH) model from the desorption branch. (2) Fourier transform infrared spectroscopy (FT-IR). The characteristic functionalities of prepared samples were derived from Fourier transform infrared
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spectroscopy (model of TENSOR-27, Bruker). The sample was first ground and then diluted with KBr, and the spectra were recorded in the range of 4000–400 cm1. (3) X-ray diffraction (XRD). The structures and fingerprint characterization of blank and modified MCM-41 were derived from the XRD patterns. In this work, XRD tests were carried out using the ASAP 2020 V4.01 X-ray diffractometer with Cu Ka radiation (k = 0.154 nm) in the range of 2h = 1–10 °C. (4) Thermogravimetric analysis (TGA). TGA was used to characterize the thermal stability of the sorbents. The tests were conducted using NETZSCH STA 409PC. Thermogravimetric analysis was performed in a highly pure N2 atmosphere at a flow rate of 30 mL/min. In each measurement, 10 mg of sample was ground properly and then placed in a platinum pan. The temperature was heated from 25 °C to 625 °C at a rate of 10 °C/min. 2.4. CO2 adsorption–desorption measurement The CO2 adsorption–desorption performance was assessed at atmospheric pressure in a self-assembled fixed-bed reactor (the stainless tube with inner diameter of 10 mm, length of 200 mm) as shown in Fig. 1. In a typical adsorption–desorption process, 1 g of sample was placed in the reactor supported by quartz sand. Prior to each measurement, the sample was heated to 100 °C in a highly pure N2 stream at a flow rate of 30 mL/min for 60 min to eliminate physically adsorbed H2O and CO2, and then the sample was cooled to the desired adsorption temperature. The gas stream was quickly switched to highly pure 15 vol.% CO2/85 vol.% N2 at the desired flow rates for 40 min. The flow rates of the gas stream were controlled by the glass flowmeters. The CO2 concentration in the influent and effluent gas stream was analyzed by gas chromatography (GC). The CO2 adsorption capacity was calculated by the integration of the breakthrough curves. After adsorption, the sample was heated to 100 °C in highly pure N2 for 60 min to fully release adsorbed CO2. Ten adsorption–desorption cycles were conducted on the same sample to test the cyclic regenerability and stability of CO2 sorbent. The integral equation of the breakthrough curve is displayed in Eq. (3):
Qs ¼
F
Rt 0
ðC 0 CÞdt m
ð3Þ
where Qs is the saturated adsorption capacity of CO2 over sorbents, mmol g1; F represents the influent velocity of CO2, mL min1; m is the weight of sorbents, g; C0, C represents the concentration of influent and effluent CO2, respectively, mmol L1; t is the adsorption time, min. Qs is defined as the adsorption capacity of CO2 when C is equal to C0, namely, C/C0 is equal to 1.0, while breakthrough
Fig. 1. The schematic diagram of self-assembled fixed bed reactor.
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adsorption capacity (Qb) is defined as the adsorption capacity of CO2 when C is equal to five percent of C0, namely, C/C0 is equal to 0.05; the time corresponding to the breakthrough adsorption capacity is called the breakthrough time. 3. Results and discussions 3.1. Characterization of MCM-41-APTSx–TEPAy The FT-IR spectra of MCM-41-APTSx–TEPAy sorbents are shown in Fig. 2. A broad band at 3425 cm1 is assigned to the stretching vibration of Si–OH and physically adsorbed H2O [41], three peaks at 461 cm1, 791 cm1, and 1070 cm1 are due to the bending vibration, symmetric stretching vibration and asymmetric stretching vibration of Si–O–Si, indicating that samples prepared retained the framework of MCM-41 after modification. The bands at 1231 cm1, 1666 cm1 and 1570 cm1 are associated with the stretching vibration of C–N, the bending vibration of N–H from the secondary amine (N(R)H) and primary amine (NH2) in TEPA, respectively [28], which demonstrates that TEPA was successfully impregnated into the support. Compared with MCM-41 (Fig. 1S), the new bands for MCM-41-APTS30% at 1475 cm1 and 2941 cm1 are caused by the bending vibration and stretching vibration of CH2 in APTS, and the new band at 1570 cm1 is caused by the bending vibration of N–H from the primary amine (NH2) in APTS [42], suggesting that APTS was successfully grafted onto MCM-41. The results above indicated the successful preparation of designed composite sorbents. N2 adsorption–desorption isotherms for MCM-41-APTSx–TEPAy composites are depicted in Fig. 3. As shown in Fig. 3, before and after single or hybrid amine functionalization, MCM-41 exhibited a typical type-IV adsorption isotherm with a H4 hysteresis loop over a wide relative pressure range from 0.42 to 1.0, and a sharp pore filling step below the relative pressure of 0.42. The steep rise was reversible and might indicate the uniform mesoporous structure of samples. For MCM-41-APTS30%–TEPA30% and MCM-41APTS30%–TEPA40%, the hysteresis loop could still be observed. However, for MCM-41-TEPA60%, the hysteresis loop almost disappeared. When more and more active components were incorporated into the support, more and more pores of MCM-41 were filled, which was the main reason for the gradual disappearance of hysteresis loop. Table 1 lists the textural properties of
Fig. 2. FT-IR spectra of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-TEPA60%, (d) MCM-41-APTS30%–TEPA20%, (e) MCM-41-APTS30%–TEPA30%, and (f) MCM-41APTS30%–TEPA40%.
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Fig. 3. N2 adsorption–desorption isotherm of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-APTS30%–TEPA20%, (d) MCM-41-APTS30%–TEPA30%, (e) MCM-41APTS30%–TEPA40%, and (f) MCM-41-TEPA60%.
Fig. 4. XRD patterns of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-APTS30%– TEPA20%, (d) MCM-41-APTS30%–TEPA30%, (e): MCM-41-APTS30%–TEPA40%, and (f) MCM-41-TEPA60%.
Table 1 Textural properties of MCM-41-APTSx–TEPAy samples. Samples
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
MCM-41 MCM-41-APTS30% MCM-41-APTS30%–TEPA20% MCM-41-APTS30%–TEPA30% MCM-41-APTS30%–TEPA40% MCM-41-TEPA60%
997.21 958.11 801.26 47.06 33.77 5.32
0.90 0.51 0.43 0.06 0.05 0.03
2.87 2.34 2.35 6.62 7.06 -----
- - - - -: the data were not available.
MCM-41 before and after modification. For blank MCM-41, the specific surface area, total pore volume and pore size is 997.21 m2/g, 0.90 cm3/g and 2.87 nm, and these parameters decreased with the increase of active component loadings. For MCM-41-APTS30%–TEPA30% and MCM-41-APTS30%–TEPA40%, the surface areas (47.06 m2/g, 33.77 m2/g) and pore volumes (0.06 cm3/g, 0.05 cm3/g) could still be obtained, and the pore size of 6.62 nm and 7.06 nm were obtained (small pores were filled while large pores remained, so the pore size increased). For MCM-41-TEPA60%, the specific surface area and pore volume was only 5.32 m2/g and 0.03 cm3/g, but the pore size was not available, which indicated that most of the pores were filled. Fig. 4 introduces the XRD patterns of MCM-41-APTSx–TEPAy composites. For MCM-41, there was a broad peak at 2h = 2.3°, which was in accordance with the (1 1 0) face of the p6mm structure of the MCM-41 framework [35]. After modification with single or hybrid amine, all composites retained the character of MCM-41 but with a little shift at 2h = 2.3°, suggesting that single or hybrid amines were successfully incorporated into the support, and the interaction between active components and support caused the little shift of peak position. With the increase of amine loadings, diffraction intensity decreased gradually or even disappeared, which could be explained. With the increase of amine loadings, the diffraction between the walls and pores of the support becomes weaker, and the pore filling effect causes more and more pores to collapse [43]. The trends agreed well with the change of specific surface area and pore volume data listed in Table 1. Fig. 5 demonstrates the thermal stability of MCM-41 before and after single or hybrid amine modification. For MCM-41, the weight loss was almost negligible before 600 °C, suggesting the thermal
Fig. 5. Thermal stability of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41APTS30%–TEPA20%, (d) MCM-41-TEPA60%, (e) MCM-41-APTS30%–TEPA30%, and (f) MCM-41-APTS30%–TEPA40%.
stability of MCM-41 in itself. All amine-modified MCM-41 composites showed their first weight loss peaks before 100 °C, which were due to the volatilization of physically adsorbed H2O, CO2 and remaining solvent. After 100 °C, amine-modified MCM-41 composites showed their second weight loss peaks, suggesting the decomposition of loaded amines. For MCM-41-TEPA60% and MCM-41-APTS30%, their second weight loss peaks appeared at 135.72 °C and 210.42 °C. However, for MCM-41-APTS30%– TEPA20%, MCM-41-APTS30%–TEPA30% and MCM-41-APTS30%– TEPA40%, the beginning decomposition temperatures were 162.17 °C, 141.71 °C and 146.10 °C, respectively, ranging between MCM-41-TEPA60% and MCM-41-APTS30%. The hydrogen-bonding interactions between APTS and TEPA may make the interaction between TEPA and the support stronger, and the starting decomposition temperatures are higher than MCM-41-TEPA60%. Moreover, all samples remained stable below 135 °C, suitable for the adsorption (70 °C) and desorption (100 °C) process. However, according to the mass loss data in Fig. 5 and amine loading amount in Table 2S, both APTS and TEPA were not loaded on MCM-41 completely, possibly because of the loss of amines
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during the preparation of sorbents. TEPA may volatilize while drying in the oven; partially viscous APTS and TEPA that are not well dispersed in the solvent may aggregate on the surface of the support and dissolve during repeatedly washing with anhydrous ethanol.
of MCM-41, which hindered the diffusion of CO2 to react with active sites in the pores. Among the prepared single aminefunctionalized sorbents, MCM-41-TEPA60% showed optimal breakthrough time and saturated adsorption capacity, and the value was 10 min with 2.45 mmol-CO2/g-sorbent. The maximum value for MCM-41-APTS30% was 8 min with 1.88 mmol-CO2/g-sorbent. After modification with hybrid APTS and TEPA, the breakthrough time and adsorption capacity of some composite sorbents obviously increased, such as MCM-41-APTS30%–TEPA20%, MCM41-APTS30%–TEPA30%, MCM-41-APTS30%–TEPA40%, MCM-41APTS30%–TEPA50%, and MCM-41-APTS40%–TEPA30%. Especially for MCM-41-APTS30%–TEPA40%, the breakthrough time, breakthrough adsorption capacity and saturated adsorption capacity increased to 14 min, 2.82 mmol-CO2/g-sorbent and 3.45 mmolCO2/g-sorbent, separately improved 40%, 40% and 41% compared with the optimal value of MCM-41-TEPA60%. The improvement can be attributed to the synergistic effect of APTS and TEPA on MCM-41. APTS is an alkanolamine, which was grafted onto the inner surface of MCM-41, and active NH2 sites were exposed to the channel cavity. When TEPA was introduced, the TEPA connected with the APTS and MCM-41 inner surface through hydrogen bonds. New intertwined network structure was formed, which facilitated the better dispersion of TEPA and reduced the diffusion resistance of CO2 [14], and more CO2 molecules could diffuse to CO2-philic sites to form carbaminate. Therefore, the increase of adsorption capacity for MCM-41-APTS30%–TEPA40% was the result of the synergistic effect of APTS and TEPA in MCM-41. The FT-IR spectra of MCM-41-TEPA60% (before and after CO2 adsorption) and MCM-41-APTS30%–TEPA40% (before and after CO2 adsorption) were further collected to investigate the CO2 adsorptive reaction mechanism. As displayed in Fig. 6, when CO2 molecules were adsorbed by the sorbents, three absorption bands at 1650 cm1 (the peak intensity strengthened), 1520 cm1 and 1420 cm1 are assigned to the N–H deformation in R-NH+3, the
3.2. Dynamic CO2 adsorption performance of the sorbents
3.2.1. Effect of APTS and TEPA loadings Different ratios of APTS and TEPA were immobilized on MCM-41 to investigate the effect of hybrid loadings on the performance of CO2 adsorption. To validate the improved adsorption performance of hybrid amine-functionalized MCM-41, single TEPA or APTS modified MCM-41 were also tested for comparison. The operating conditions were as follows: adsorption temperature, 70 °C; influent velocity, 30 mL/min; CO2/N2 (v/v) = 15%/85%; at atmospheric pressure. The CO2 adsorption breakthrough curves and adsorption capacities (both breakthrough and saturated adsorption capacity) with different amine loadings are presented in Fig. 2S and Table 2. Each sample reaches adsorption equilibrium in a short time after achieving breakthrough adsorption, which can be attributed to the fast kinetics of the sorption process and small mass transfer resistance of the mixed gas stream in the sorbent. Blank MCM-41 showed no breakthrough and low adsorption capacity. After modification with single APTS, TEPA and hybrid amines, the breakthrough time and adsorption capacity increased significantly. Under only TEPA or APTS modification, the adsorption capacity increased with the increase of TEPA or APTS loading. For MCM-41-APTSx, when x exceeded 30%, the breakthrough time and adsorption capacity decreased, explained by the introduction of more amine into the MCM-41 support providing more affinity sites for CO2 sorption. However, with a further increase in amine loadings, APTS aggregated in the channel or coated on the surface
Table 2 The breakthrough time and CO2 adsorption capacity of as-prepared MCM-41-APTSx–TEPAy. Sorbent MCM-41
Breakthrough time (min) 0
Breakthrough adsorption capacity (mmol-CO2/g-sorbent)
Saturated adsorption capacity (mmol-CO2/g-sorbent)
0
0.54
MCM-41-TEPA20% MCM-41-TEPA30% MCM-41-TEPA40% MCM-41-TEPA50% MCM-41-TEPA60%
6 8 8 8 10
1.20 1.61 1.61 1.61 2.01
1.83 2.01 2.13 2.25 2.45
MCM-41-APTS20% MCM-41-APTS20%–TEPA20% MCM-41-APTS20%–TEPA30% MCM-41-APTS20%-TEPA40% MCM-41-APTS20%–TEPA50%
6 8 8 8 10
1.20 1.61 1.61 1.61 2.01
1.71 1.92 2.10 2.21 2.24
MCM-41-APTS30% MCM-41-APTS30%–TEPA20% MCM-41-APTS30%–TEPA30% MCM-41-APTS30%–TEPA40% MCM-41-APTS30%–TEPA50%
8 10 12 14 12
1.61 2.01 2.42 2.82 2.42
1.88 2.55 2.73 3.45 3.32
MCM-41-APTS40% MCM-41-APTS40%–TEPA20% MCM-41-APTS40%–TEPA30% MCM-41-APTS40%–TEPA40% MCM-41-APTS40%–TEPA50%
6 8 12 10 6
1.20 1.61 2.42 2.01 1.20
1.84 2.13 2.88 2.45 1.67
MCM-41-APTS50% MCM-41-APTS50%–TEPA10% MCM-41-APTS50%–TEPA20% MCM-41-APTS50%–TEPA30% MCM-41-APTS50%–TEPA40%
6 8 10 8 4
1.20 1.61 2.01 1.61 0.81
1.74 2.00 2.24 2.07 1.50
Operating conditions: adsorption temperature, 70 °C; influent velocity, 30 ml/min; CO2/N2 (v/v) = 15/85; 1 atm.
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Fig. 6. FT-IR spectra of (a) MCM-41-TEPA60%, (b) MCM-41-TEPA60% after adsorption, (c) MCM-41-APTS30%–TEPA40%, and (d) MCM-41-APTS30%–TEPA40% after adsorption.
R1R2-NH+2 stretching vibration, and the NCOO skeletal vibration [16], indicating the formation of carbaminate. The chemically adsorptive mechanism can be expressed as follows.
CO2 þ 2RNH2 ! RNHCOO þ RNHþ3
ð4Þ
CO2 þ 2R1 R2 NH ! R1 NCOO þ R2 NHþ2
ð5Þ
CO2 þ R1 NH2 þ R2 NH ! R1 NHþ3 þ R2 NCOO
ð6Þ
is synonymous with mass transfer [44]. Fig. 8(a) and Fig. 8(b) show the influence of different influent velocities on CO2 adsorption breakthrough curves and adsorption capacity, respectively. As seen from the figures, with the increase of influent velocity from 30 mL/ min, 40 mL/min and 50 mL/min to 60 mL/min, the breakthrough time and breakthrough adsorption capacity decreased significantly. The higher flow rate corresponds to shorter residence time and smaller mass transfer resistance of CO2 in the sorbent, so the increase of influent velocity resulted in decrease of breakthrough time, saturation time and breakthrough adsorption capacity. However, a higher feed flow rate broadens the contact area between CO2 molecules and sorbent. Therefore, the synergistic effect of shorter residence time and larger contact area made the saturated adsorption capacity of MCM-41-APTS30%–TEPA40% reach to 3.50 mmol-CO2/g-sorbent at the flow rate of 40 mL/min. With a further increase of the flow rate, the adsorption capacity decreased. The decrease can be explained because with a further increase of the flow rate, the residence time of CO2 through the sorbent becomes much shorter compared with the residence time of CO2 at 40 mL/min as shown in Fig. 8(a). Though the contact area between CO2 and the sorbent may become larger, here the much shorter residence time has become the decisive factor. Accordingly, the adsorption capacity decreased with the further increase of the flow rate. Table 3 shows the comparison of adsorption capacity of some amine-modified MCM-41 sorbents both in the document and
3.2.2. Effect of adsorption temperature Adsorption temperature is an important factor for CO2 sorption kinetics. In this paper, 40 °C, 55 °C, 70 °C, and 85 °C were investigated to explore the effect of temperature on the MCM41-APTS30%–TEPA40% sorbent at a feed flow rate of 30 mL/min and atmospheric pressure. Fig. 7(a) and (b) depict the effect of adsorption temperatures on CO2 adsorption breakthrough curves and adsorption capacity, respectively. The breakthrough time and adsorption capacity increased with the increase of adsorption temperatures and reached their optimal at 70 °C. With the further increase of temperature from 70 °C to 85 °C, the value of breakthrough time and adsorption capacity obviously decreased. The trend was well in accordance with the results reported previously [26,28]. The CO2 adsorption process is the combination of kinetics and thermodynamics. The adsorption process is exothermic in itself, and a low temperature is favorable. Nevertheless, TEPA is highly viscous, and its reactivity is also of vital importance. When the temperature was increased from 40 °C to 70 °C, TEPA became more flexible and could disperse more evenly. Moreover, the higher temperature could facilitate the diffusion of CO2 to react with active sites in MCM-41. Therefore, the adsorptive reaction was dominated by dynamics rather than thermodynamics below 70 °C. Once most active sites were occupied by CO2, the pores were filled and the diffusion resistance was enhanced, so when the temperature was increased to 85 °C, the adsorption capacity decreased and the reaction switched to determination by thermodynamics. The temperature of 70 °C was chosen as the optimal adsorption temperature to further explore the influence of feed flow rates. 3.2.3. Effect of influent velocities Influent velocity is an important factor in influencing the adsorption performance of the sorbent because influent velocity
Fig. 7. The influence of adsorption temperatures on (a) breakthrough curves and (b) saturated adsorption capacity of MCM-41-APTS30%–TEPA40%. C0, C represents the concentration of influent and effluent CO2, respectively.
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Fig. 9. The cyclic CO2 adsorption capacity of MCM-41-APTS30%–TEPA40%.
the MCM-41-APTS30%–TEPA40% sorbent exhibits excellent regenerability, much better than the regenerability of only TEPA modified MCM-41, where the adsorption capacity decreased by 7.4% after ten cycles [24]. TEPA was immobilized much more strongly by a hydrogen-bonding connection with APTS in MCM-41, which made less TEPA volatize during the adsorption–desorption process. Therefore, after hybrid amine modification, MCM-41 showed better regenerability. 3.4. Adsorption kinetics
Fig. 8. The effects of influent velocities on (a) breakthrough curves and (b) saturated adsorption capacity of MCM-41-APTS30%–TEPA40%. C0, C represents the concentration of influent and effluent CO2, respectively.
present work. The feed flow rate of 40 mL/min was used for investigating regeneration performance.
Among the properties expected in an excellent sorbent, fast adsorption kinetics are also of vital importance. The kinetics of CO2 adsorption over MCM-41 and MCM-41-APTS30%–TEPA40% were studied at 70 °C and 1 bar. In this work, three types of models were used to fit experimental data, pseudo-first-order, pseudosecond-order and Avrami models. Their kinetic equations are given as follows. The pseudo-first-order kinetic equation:
Q t ¼ Q e ½ð1 expðk1 tÞ
Qt ¼
3.3. Regenerability In real industrial application, not only long breakthrough time and high adsorption capacity but also excellent regenerability of the sorbent in the cyclic adsorption–desorption mode is crucial. In this work, ten cycles were carried out to test the regeneration ability of the MCM-41-APTS30%–TEPA40% sorbent. The adsorption and desorption temperatures were designed as 70 °C and 100 °C, respectively. The calculated saturated adsorption capacity after each cycle is shown in Fig. 9. After ten cycles, the adsorption capacity decreased from 3.50 mmol-CO2/g-sorbent to 3.38 mmolCO2/g-sorbent, dropping by only 3.43%. This result suggests that
ð7Þ
The pseudo-second-order kinetic equation:
k2 Q 2e t 1 þ k2 Q e t
ð8Þ
Avrami: n
Q t ¼ Q e ½1 expððka tÞ a Þ
ð9Þ
k1 (1/min), k2 (g/mmol min) and ka (1/min) are rate constants, Qt (mmol-CO2/g-sorbent) and Qe (mmol-CO2/g-sorbent) represent the adsorption capacity at a given time t and equilibrium time, and na is the order of kinetic equation. The plots of CO2 adsorption capacity versus time are shown in Fig. 10. CO2 capture appears to be a two-stage process. Fast adsorption kinetics were observed on both samples in the initial period of adsorption (breakthrough
Table 3 Comparison of adsorption capacity of MCM-41 based sorbents for CO2 capture. Adsorbents
Test conditions
Adsorptive method
CO2 adsorption capacity (mmol-CO2/g-sorbent)
Reference
MCM-41-PEI50% MCM-41-PEI40% MCM-41-TEPA40% MCM-41-TEPA60% MCM-41-APTS30%–TEPA40%
15 10 10 15 15
Fixed-bed reactor TGA TGA Fixed-bed reactor Fixed-bed reactor
2.00 1.60 2.70 2.45 3.50
[45] [24] [24] Present work Present work
vol.% vol.% vol.% vol.% vol.%
CO2, CO2, CO2, CO2, CO2,
75 °C 35 °C 35 °C 70 °C 70 °C
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4. Conclusions A regenerable hybrid APTS and TEPA functionalized MCM-41 sorbent was successfully developed by a two-step method. APTS was grafted onto the inner surface of the MCM-41, and then TEPA was impregnated into the APTS grafted support. By the combination of APTS and TEPA in MCM-41, an extra net-waved structure for facilitating the dispersion of CO2-philic sites and the diffusion of CO2 emerged. Under the same operating conditions, MCM-41APTS30%–TEPA40% exhibited a breakthrough time and saturated adsorption capacity of 14 min and 3.45 mmol-CO2/g-sorbent, respectively, much higher than the 10 min and 2.45 mmol-CO2/ g-sorbent values obtained with the optimal MCM-41-TEPA60%. Ten adsorption–desorption cycles showed that the MCM-41APTS30%–TEPA40% sorbent is characterized by excellent regenerability at the temperature ranging from 70 °C (adsorption) to 100 °C (desorption). The adsorption kinetics showed a two-stage process, and the Avrami model could fit well with the experimental data obtained with MCM-41-APTS30%–TEPA40%. However, practical flue gas components still contain O2, H2O and small amounts of SOx and NOx, and their effect on CO2 adsorption performance will be studied in the future investigation. Acknowledgments The financial support from the Natural Science Foundation of China (21276129, 20876079) and Natural Science Funds for Distinguished Young Scholar in Shandong Province (JQ200904) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.08.107. Fig. 10. Experimental adsorption capacity of (a) MCM-41 and (b) MCM-41APTS30%–TEPA40% along with corresponding fit to kinetic models.
process) during which more than 80% of the saturated adsorption capacity was reached, and then followed by a slower second stage. Similarly, a two-stage adsorption process was also observed on other amine modified sorbents [16]. Table 1S lists the kinetic constants of sorbents explored in this work. The pseudo-first-order model can precisely describe the physical adsorption mechanism of MCM-41 [46]. For MCM-41-APTS30%–TEPA40%, both pseudo-first-order and pseudo-second-order models overestimated the adsorption kinetics in the initial rapid period and underestimated the adsorption process near equilibrium, then overestimated the saturated adsorption capacity at the equilibrium stage. However, the Avrami model could fit well with the whole CO2 capture process. The phenomenon illustrated that the adsorption mechanism might change from simply physical adsorption to both chemical and physical adsorption after modification [47]. In the initial period, CO2 diffused to fresh amine modified sorbent to react rapidly with welldistributed active sites, which might be dominated by chemical reaction. As more exposed active sites were occupied, the viscosity of TEPA and APTS increased, and the diffusion resistance increased significantly, so the adsorption process switched to a slower second stage. However, more than 80% of the saturated adsorption capacity could be achieved in the first rapid stage, which is key for practical industry application.
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Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Development of hybrid amine-functionalized MCM-41 sorbents for CO2 capture Xia Wang a,b, Linlin Chen a, Qingjie Guo a,⇑ a b
Key Laboratory of Clean Chemical Processing of Shandong Province, College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, Shandong, China Department of Chemistry and Chemical Engineering, Weifang University, Weifang 261061, Shandong, China
h i g h l i g h t s A novel MCM-41 sorbent was synthesized by both grafting and impregnation method. New formed network facilitated the dispersion of TEPA and the diffusion of CO2. Effects of amine loadings, temperature and flow rate on CO2 sorption were studied. The sorbent exhibited excellent cyclic regenerability and thermal stability.
a r t i c l e
i n f o
Article history: Received 28 June 2014 Received in revised form 28 August 2014 Accepted 30 August 2014 Available online 19 September 2014 Keywords: Two-step method CO2 adsorption Hybrid amines Cyclic regenerability Adsorption kinetics
a b s t r a c t In consideration of the increasing warming trends worldwide, the development of highly efficient CO2 adsorbents becomes of vital importance. In this paper, a novel hybrid 3-aminopropyltrimethoxysilane (APTS) and tetraethylenepentamine (TEPA) modified MCM-41 sorbent was synthesized by a two-step method. APTS was first grafted onto the MCM-41, and then TEPA was impregnated. The as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and N2 adsorption–desorption techniques. CO2 adsorption–desorption experiments were performed in a self-assembled fixed bed reactor under ambient pressure. The effects of hybrid amine loadings, adsorption temperatures, and influent velocities on CO2 sorption were investigated. Among the composites synthesized, MCM-41 modified with 30 wt.% APTS and 40 wt.% TEPA (MCM-41-APTS30%–TEPA40%) showed maximum adsorption capacity of 3.50 mmol-CO2/g-sorbent. The MCM-41-APTS30%–TEPA40% sorbent exhibited well cyclic regenerability after ten adsorption–desorption cycles. Further, the adsorption kinetics of MCM-41 and MCM-41-APTS30%–TEPA40% were studied. CO2 adsorption was a two-stage process, and the Avrami model could fit well with the experimental data of MCM-41-APTS30%–TEPA40%. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The swift growth of anthropogenic CO2 emissions in the atmosphere, especially the CO2 emissions caused by the combustion of fossil fuels has drawn great attention because of the increasing greenhouse effect [1–4]. The atmospheric concentration of CO2 is expected to reach 550 ppm by 2050 from its value of ca. 384 ppm in 2007, assuming that the CO2 emissions would be at a stable level in 2050 [5]. The development of more efficient carbon capture and storage (CCS) technology is therefore extremely urgent. To date, a variety of CO2 capturing technologies, including
⇑ Corresponding author. Tel.: +86 532 84022757. E-mail address: [email protected] (Q. Guo). http://dx.doi.org/10.1016/j.cej.2014.08.107 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
absorption, adsorption, membrane separation technology and cryogenic distillation methods, have been developed [6,7]. Currently, the traditional and commercially adopted CO2 capture technology in large scale industry is liquid amine absorption, which selectively absorbs CO2 from a gas stream over liquid amine solvents such as the most commonly used primary amine (monoethanolamine, MEA) and secondary amine (diethanolamine, DEA). The solubility of CO2 in MEA can reach 0.35 mol-CO2/mol-MEA at atmospheric pressure. Amine solvents react selectively with CO2 to form carbaminate, and the carbaminate that is formed can be regenerated reversibly [8]. However, the regeneration of amine solvents consumes large amounts of energy, the primary and secondary amines volatilize extensively during regeneration, and the alkaline amine can cause serious equipment corrosion problems [9]. Compared with volatile organic solvents, ionic liquids (ILs)
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are non-volatile under ambient conditions and pose a lower exposure risk [10]. Chen et al. researched the adsorption capacity of 1-butyl-3-methylimidazolium acetate using TGA at atmospheric pressure. The adsorption capacity of 0.95 mol-CO2/mol-ILs was determined at 50 °C [11]. However, the relatively high viscosity and the lack of detailed physical property data are the main barriers for the utilization of ILs [10]. The above-mentioned challenges have led researchers to design more energy-efficient, less toxic and corrosive, and more easily handled adsorbents [12,13]. Thus, solid amine-modified sorbents came into being. In the past few years, various amine-modified solid sorbents have been developed worldwide for CO2 capture and separation [14–18], and many well-written reviews have comprehensively depicted the progress of the research [10,19–21]. Solid aminemodified sorbents are composed primarily of supports with highly developed porosity and active components with sites having an affinity for CO2. Two technologies are mainly employed to prepare amine-modified solid sorbents. One technology is the wet impregnation method, where liquid amine molecules are impregnated into the porous supports via van der Waals forces. The other technology is the grafting method, where alkanolamine molecules are grafted onto the inner surfaces of porous supports by reacting with the silicon hydroxyl groups. The grafting method usually has the merits of a quicker adsorption rate and higher amine efficiency, while the impregnation method is more convenient and easier to operate, consumes less energy, and can load more active CO2-philic sites [22–24]. In the previous studies, progress has been made by immobilizing liquid amines (such as TEPA, APTS, MEA, DEA, etc.) in various solid supporting materials such as the metal organic framework (MOF) [25], montmorillonite [26], fly ash [27], mesoporous carbon [28], activated carbon [29], mesocellular silica foams [30], SBA-15 [31–33], MCM-22, MCM-36, ITQ-2 [34], and MCM-41 [24,35–36]. Kamarudin et al. found that MCM-41 with a BET surface area of 750 m2/g modified with 25 wt.% MEA showed a high degree of CO2 removal [35]. Chen et al. studied the adsorption capacity of an ethylenediamine grafted zeolite-like metal organic framework with a sod topology (sod-ZMOF). The optimum adsorption capacity of 69 mg-CO2/g-sorbent at 298 K and a CO2 relative pressure of 1.0 was reached [25]. Liu et al. studied the adsorption capacity of MCM-41 modified with different types of amines. The studies of Liu et al. showed that the CO2 adsorption capacity for MCM-41 modified with 40 wt.% TEPA could reach up to 2.70 mmol-CO2/g-sorbent [24]. Jing et al. researched the molecular simulation of MCM-41, which provided guidance for the further modification of the support [36]. To date, most researchers have focused on immobilizing the liquid amine in the support by the impregnation or grafting method, but very few investigators combined the two methods. In this paper, mesoporous sieve MCM-41 was functionalized by a two-step method: first, grafted with APTS and then, impregnated with TEPA. The textural properties and thermal stability of the hybrid composites were characterized, and their CO2 adsorption performance was investigated. In addition, three kinetic models were used to fit the adsorption process.
2. Experimental section 2.1. Materials MCM-41 was purchased from the catalyst plant of NanKai University, Tianjin, China. TEPA (90%) was provided by Tianjin BASF Chemical Co., Ltd, Tianjin, China. Anhydrous ethanol solvent and APTS were produced by Sinopharm Chemical Reagent Co., Ltd, and Sigma–Aldrich, respectively. Highly pure N2 and a mixed
gas stream (15 vol.% CO2/85 vol.% N2) were prepared by the Section 2.1 at Qingdao University of Science & Technology. 2.2. Preparation of hybrid amine-functionalized sorbents 2.2.1. APTS grafted onto MCM-41 APTS was chosen as the grafting agent. A conventional grafting technology was adopted to graft APTS onto the MCM-41 support [37–39]. In a typical procedure, 6 g of MCM-41 was added to a round-bottom flask with 350 mL anhydrous ethanol solvent. The solution was then stirred vigorously at room temperature until the suspension was well-distributed. Then, 1 mL of distilled water was added to the mixture, and the mixture was stirred for another 30 min. The temperature was then raised rapidly to 70 °C, and different amounts of APTS were added dropwise. The whole system was stirred and refluxed continuously for 10 h. The APTS modified MCM-41 that was obtained was washed repeatedly with anhydrous ethanol and then dried in an oven at 85 °C for 12 h. The addition of distilled water was to promote the dispersion and hydrolysis of APTS by improving its polarity. As-prepared samples were labeled as MCM-41-APTSx, with x representing the weight loadings of APTS on as-prepared sorbent. In this study, x was set as 20%, 30%, 40% and 50%. Eq. (1) shows the calculation equation of x.
x¼
mAPTS 100% mAPTS þ mMCM-41
ð1Þ
where mAPTS is the weight of APTS added, g; mMCM-41 is the weight of the MCM-41 support, g. 2.2.2. TEPA impregnated into MCM-41-APTSx The MCM-41-APTSx sample that was obtained was further modified by a wet impregnation method as described previously [40]. A desirable amount of TEPA was dissolved in 30 mL of anhydrous ethanol, and the mixture was subjected to sonication for 30 min in an ultrasound bath at room temperature to promote the complete dissolution of TEPA. Then, 1.5 g of MCM-41-APTSx was added to the solution, and the system was sonicated for another 4 h. The slurry was then placed in the oven at 85 °C for 16 h to obtain dry solid power. The as-prepared samples were marked as MCM-41-APTSx–TEPAy, where y represents different weight loadings of TEPA onto the sorbent. Here, y ranges from 10% to 50%. Eq. (2) shows the calculation of y.
y¼
mTEPA 100% mTEPA þ mMCM-41-APTSx
ð2Þ
where mTEPA is the weight of TEPA added, g; mMCM-41-APTS is the weight of APTS modified MCM-41 sample, g. The amount of APTS and TEPA in MCM-41-APTSx–TEPAy are listed in Table 2S (in Supplementary material). 2.3. Characterization (1) N2 adsorption–desorption. N2 adsorption–desorption utilizes the principle of physical adsorption of N2 at 77 K to obtain the adsorption–desorption isotherm. The analysis was carried out using a Quadrasorb SI analyzer (Quantachrome, USA). Prior to each measurement, the sample was degassed at 353 K in a high vacuum for 12 h. The specific surface area was calculated by using the conventional Brunauer–Emmett–Teller (BET) method. The total pore volume was obtained from the adsorbed amount after finishing the capillary condensation at a relative pressure of P/P0 = 0.996. The pore size was determined using the Barrett–Joyner–Halenda (BJH) model from the desorption branch. (2) Fourier transform infrared spectroscopy (FT-IR). The characteristic functionalities of prepared samples were derived from Fourier transform infrared
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spectroscopy (model of TENSOR-27, Bruker). The sample was first ground and then diluted with KBr, and the spectra were recorded in the range of 4000–400 cm1. (3) X-ray diffraction (XRD). The structures and fingerprint characterization of blank and modified MCM-41 were derived from the XRD patterns. In this work, XRD tests were carried out using the ASAP 2020 V4.01 X-ray diffractometer with Cu Ka radiation (k = 0.154 nm) in the range of 2h = 1–10 °C. (4) Thermogravimetric analysis (TGA). TGA was used to characterize the thermal stability of the sorbents. The tests were conducted using NETZSCH STA 409PC. Thermogravimetric analysis was performed in a highly pure N2 atmosphere at a flow rate of 30 mL/min. In each measurement, 10 mg of sample was ground properly and then placed in a platinum pan. The temperature was heated from 25 °C to 625 °C at a rate of 10 °C/min. 2.4. CO2 adsorption–desorption measurement The CO2 adsorption–desorption performance was assessed at atmospheric pressure in a self-assembled fixed-bed reactor (the stainless tube with inner diameter of 10 mm, length of 200 mm) as shown in Fig. 1. In a typical adsorption–desorption process, 1 g of sample was placed in the reactor supported by quartz sand. Prior to each measurement, the sample was heated to 100 °C in a highly pure N2 stream at a flow rate of 30 mL/min for 60 min to eliminate physically adsorbed H2O and CO2, and then the sample was cooled to the desired adsorption temperature. The gas stream was quickly switched to highly pure 15 vol.% CO2/85 vol.% N2 at the desired flow rates for 40 min. The flow rates of the gas stream were controlled by the glass flowmeters. The CO2 concentration in the influent and effluent gas stream was analyzed by gas chromatography (GC). The CO2 adsorption capacity was calculated by the integration of the breakthrough curves. After adsorption, the sample was heated to 100 °C in highly pure N2 for 60 min to fully release adsorbed CO2. Ten adsorption–desorption cycles were conducted on the same sample to test the cyclic regenerability and stability of CO2 sorbent. The integral equation of the breakthrough curve is displayed in Eq. (3):
Qs ¼
F
Rt 0
ðC 0 CÞdt m
ð3Þ
where Qs is the saturated adsorption capacity of CO2 over sorbents, mmol g1; F represents the influent velocity of CO2, mL min1; m is the weight of sorbents, g; C0, C represents the concentration of influent and effluent CO2, respectively, mmol L1; t is the adsorption time, min. Qs is defined as the adsorption capacity of CO2 when C is equal to C0, namely, C/C0 is equal to 1.0, while breakthrough
Fig. 1. The schematic diagram of self-assembled fixed bed reactor.
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adsorption capacity (Qb) is defined as the adsorption capacity of CO2 when C is equal to five percent of C0, namely, C/C0 is equal to 0.05; the time corresponding to the breakthrough adsorption capacity is called the breakthrough time. 3. Results and discussions 3.1. Characterization of MCM-41-APTSx–TEPAy The FT-IR spectra of MCM-41-APTSx–TEPAy sorbents are shown in Fig. 2. A broad band at 3425 cm1 is assigned to the stretching vibration of Si–OH and physically adsorbed H2O [41], three peaks at 461 cm1, 791 cm1, and 1070 cm1 are due to the bending vibration, symmetric stretching vibration and asymmetric stretching vibration of Si–O–Si, indicating that samples prepared retained the framework of MCM-41 after modification. The bands at 1231 cm1, 1666 cm1 and 1570 cm1 are associated with the stretching vibration of C–N, the bending vibration of N–H from the secondary amine (N(R)H) and primary amine (NH2) in TEPA, respectively [28], which demonstrates that TEPA was successfully impregnated into the support. Compared with MCM-41 (Fig. 1S), the new bands for MCM-41-APTS30% at 1475 cm1 and 2941 cm1 are caused by the bending vibration and stretching vibration of CH2 in APTS, and the new band at 1570 cm1 is caused by the bending vibration of N–H from the primary amine (NH2) in APTS [42], suggesting that APTS was successfully grafted onto MCM-41. The results above indicated the successful preparation of designed composite sorbents. N2 adsorption–desorption isotherms for MCM-41-APTSx–TEPAy composites are depicted in Fig. 3. As shown in Fig. 3, before and after single or hybrid amine functionalization, MCM-41 exhibited a typical type-IV adsorption isotherm with a H4 hysteresis loop over a wide relative pressure range from 0.42 to 1.0, and a sharp pore filling step below the relative pressure of 0.42. The steep rise was reversible and might indicate the uniform mesoporous structure of samples. For MCM-41-APTS30%–TEPA30% and MCM-41APTS30%–TEPA40%, the hysteresis loop could still be observed. However, for MCM-41-TEPA60%, the hysteresis loop almost disappeared. When more and more active components were incorporated into the support, more and more pores of MCM-41 were filled, which was the main reason for the gradual disappearance of hysteresis loop. Table 1 lists the textural properties of
Fig. 2. FT-IR spectra of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-TEPA60%, (d) MCM-41-APTS30%–TEPA20%, (e) MCM-41-APTS30%–TEPA30%, and (f) MCM-41APTS30%–TEPA40%.
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Fig. 3. N2 adsorption–desorption isotherm of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-APTS30%–TEPA20%, (d) MCM-41-APTS30%–TEPA30%, (e) MCM-41APTS30%–TEPA40%, and (f) MCM-41-TEPA60%.
Fig. 4. XRD patterns of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41-APTS30%– TEPA20%, (d) MCM-41-APTS30%–TEPA30%, (e): MCM-41-APTS30%–TEPA40%, and (f) MCM-41-TEPA60%.
Table 1 Textural properties of MCM-41-APTSx–TEPAy samples. Samples
BET surface area (m2/g)
Pore volume (cm3/g)
Pore size (nm)
MCM-41 MCM-41-APTS30% MCM-41-APTS30%–TEPA20% MCM-41-APTS30%–TEPA30% MCM-41-APTS30%–TEPA40% MCM-41-TEPA60%
997.21 958.11 801.26 47.06 33.77 5.32
0.90 0.51 0.43 0.06 0.05 0.03
2.87 2.34 2.35 6.62 7.06 -----
- - - - -: the data were not available.
MCM-41 before and after modification. For blank MCM-41, the specific surface area, total pore volume and pore size is 997.21 m2/g, 0.90 cm3/g and 2.87 nm, and these parameters decreased with the increase of active component loadings. For MCM-41-APTS30%–TEPA30% and MCM-41-APTS30%–TEPA40%, the surface areas (47.06 m2/g, 33.77 m2/g) and pore volumes (0.06 cm3/g, 0.05 cm3/g) could still be obtained, and the pore size of 6.62 nm and 7.06 nm were obtained (small pores were filled while large pores remained, so the pore size increased). For MCM-41-TEPA60%, the specific surface area and pore volume was only 5.32 m2/g and 0.03 cm3/g, but the pore size was not available, which indicated that most of the pores were filled. Fig. 4 introduces the XRD patterns of MCM-41-APTSx–TEPAy composites. For MCM-41, there was a broad peak at 2h = 2.3°, which was in accordance with the (1 1 0) face of the p6mm structure of the MCM-41 framework [35]. After modification with single or hybrid amine, all composites retained the character of MCM-41 but with a little shift at 2h = 2.3°, suggesting that single or hybrid amines were successfully incorporated into the support, and the interaction between active components and support caused the little shift of peak position. With the increase of amine loadings, diffraction intensity decreased gradually or even disappeared, which could be explained. With the increase of amine loadings, the diffraction between the walls and pores of the support becomes weaker, and the pore filling effect causes more and more pores to collapse [43]. The trends agreed well with the change of specific surface area and pore volume data listed in Table 1. Fig. 5 demonstrates the thermal stability of MCM-41 before and after single or hybrid amine modification. For MCM-41, the weight loss was almost negligible before 600 °C, suggesting the thermal
Fig. 5. Thermal stability of (a) MCM-41, (b) MCM-41-APTS30%, (c) MCM-41APTS30%–TEPA20%, (d) MCM-41-TEPA60%, (e) MCM-41-APTS30%–TEPA30%, and (f) MCM-41-APTS30%–TEPA40%.
stability of MCM-41 in itself. All amine-modified MCM-41 composites showed their first weight loss peaks before 100 °C, which were due to the volatilization of physically adsorbed H2O, CO2 and remaining solvent. After 100 °C, amine-modified MCM-41 composites showed their second weight loss peaks, suggesting the decomposition of loaded amines. For MCM-41-TEPA60% and MCM-41-APTS30%, their second weight loss peaks appeared at 135.72 °C and 210.42 °C. However, for MCM-41-APTS30%– TEPA20%, MCM-41-APTS30%–TEPA30% and MCM-41-APTS30%– TEPA40%, the beginning decomposition temperatures were 162.17 °C, 141.71 °C and 146.10 °C, respectively, ranging between MCM-41-TEPA60% and MCM-41-APTS30%. The hydrogen-bonding interactions between APTS and TEPA may make the interaction between TEPA and the support stronger, and the starting decomposition temperatures are higher than MCM-41-TEPA60%. Moreover, all samples remained stable below 135 °C, suitable for the adsorption (70 °C) and desorption (100 °C) process. However, according to the mass loss data in Fig. 5 and amine loading amount in Table 2S, both APTS and TEPA were not loaded on MCM-41 completely, possibly because of the loss of amines
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during the preparation of sorbents. TEPA may volatilize while drying in the oven; partially viscous APTS and TEPA that are not well dispersed in the solvent may aggregate on the surface of the support and dissolve during repeatedly washing with anhydrous ethanol.
of MCM-41, which hindered the diffusion of CO2 to react with active sites in the pores. Among the prepared single aminefunctionalized sorbents, MCM-41-TEPA60% showed optimal breakthrough time and saturated adsorption capacity, and the value was 10 min with 2.45 mmol-CO2/g-sorbent. The maximum value for MCM-41-APTS30% was 8 min with 1.88 mmol-CO2/g-sorbent. After modification with hybrid APTS and TEPA, the breakthrough time and adsorption capacity of some composite sorbents obviously increased, such as MCM-41-APTS30%–TEPA20%, MCM41-APTS30%–TEPA30%, MCM-41-APTS30%–TEPA40%, MCM-41APTS30%–TEPA50%, and MCM-41-APTS40%–TEPA30%. Especially for MCM-41-APTS30%–TEPA40%, the breakthrough time, breakthrough adsorption capacity and saturated adsorption capacity increased to 14 min, 2.82 mmol-CO2/g-sorbent and 3.45 mmolCO2/g-sorbent, separately improved 40%, 40% and 41% compared with the optimal value of MCM-41-TEPA60%. The improvement can be attributed to the synergistic effect of APTS and TEPA on MCM-41. APTS is an alkanolamine, which was grafted onto the inner surface of MCM-41, and active NH2 sites were exposed to the channel cavity. When TEPA was introduced, the TEPA connected with the APTS and MCM-41 inner surface through hydrogen bonds. New intertwined network structure was formed, which facilitated the better dispersion of TEPA and reduced the diffusion resistance of CO2 [14], and more CO2 molecules could diffuse to CO2-philic sites to form carbaminate. Therefore, the increase of adsorption capacity for MCM-41-APTS30%–TEPA40% was the result of the synergistic effect of APTS and TEPA in MCM-41. The FT-IR spectra of MCM-41-TEPA60% (before and after CO2 adsorption) and MCM-41-APTS30%–TEPA40% (before and after CO2 adsorption) were further collected to investigate the CO2 adsorptive reaction mechanism. As displayed in Fig. 6, when CO2 molecules were adsorbed by the sorbents, three absorption bands at 1650 cm1 (the peak intensity strengthened), 1520 cm1 and 1420 cm1 are assigned to the N–H deformation in R-NH+3, the
3.2. Dynamic CO2 adsorption performance of the sorbents
3.2.1. Effect of APTS and TEPA loadings Different ratios of APTS and TEPA were immobilized on MCM-41 to investigate the effect of hybrid loadings on the performance of CO2 adsorption. To validate the improved adsorption performance of hybrid amine-functionalized MCM-41, single TEPA or APTS modified MCM-41 were also tested for comparison. The operating conditions were as follows: adsorption temperature, 70 °C; influent velocity, 30 mL/min; CO2/N2 (v/v) = 15%/85%; at atmospheric pressure. The CO2 adsorption breakthrough curves and adsorption capacities (both breakthrough and saturated adsorption capacity) with different amine loadings are presented in Fig. 2S and Table 2. Each sample reaches adsorption equilibrium in a short time after achieving breakthrough adsorption, which can be attributed to the fast kinetics of the sorption process and small mass transfer resistance of the mixed gas stream in the sorbent. Blank MCM-41 showed no breakthrough and low adsorption capacity. After modification with single APTS, TEPA and hybrid amines, the breakthrough time and adsorption capacity increased significantly. Under only TEPA or APTS modification, the adsorption capacity increased with the increase of TEPA or APTS loading. For MCM-41-APTSx, when x exceeded 30%, the breakthrough time and adsorption capacity decreased, explained by the introduction of more amine into the MCM-41 support providing more affinity sites for CO2 sorption. However, with a further increase in amine loadings, APTS aggregated in the channel or coated on the surface
Table 2 The breakthrough time and CO2 adsorption capacity of as-prepared MCM-41-APTSx–TEPAy. Sorbent MCM-41
Breakthrough time (min) 0
Breakthrough adsorption capacity (mmol-CO2/g-sorbent)
Saturated adsorption capacity (mmol-CO2/g-sorbent)
0
0.54
MCM-41-TEPA20% MCM-41-TEPA30% MCM-41-TEPA40% MCM-41-TEPA50% MCM-41-TEPA60%
6 8 8 8 10
1.20 1.61 1.61 1.61 2.01
1.83 2.01 2.13 2.25 2.45
MCM-41-APTS20% MCM-41-APTS20%–TEPA20% MCM-41-APTS20%–TEPA30% MCM-41-APTS20%-TEPA40% MCM-41-APTS20%–TEPA50%
6 8 8 8 10
1.20 1.61 1.61 1.61 2.01
1.71 1.92 2.10 2.21 2.24
MCM-41-APTS30% MCM-41-APTS30%–TEPA20% MCM-41-APTS30%–TEPA30% MCM-41-APTS30%–TEPA40% MCM-41-APTS30%–TEPA50%
8 10 12 14 12
1.61 2.01 2.42 2.82 2.42
1.88 2.55 2.73 3.45 3.32
MCM-41-APTS40% MCM-41-APTS40%–TEPA20% MCM-41-APTS40%–TEPA30% MCM-41-APTS40%–TEPA40% MCM-41-APTS40%–TEPA50%
6 8 12 10 6
1.20 1.61 2.42 2.01 1.20
1.84 2.13 2.88 2.45 1.67
MCM-41-APTS50% MCM-41-APTS50%–TEPA10% MCM-41-APTS50%–TEPA20% MCM-41-APTS50%–TEPA30% MCM-41-APTS50%–TEPA40%
6 8 10 8 4
1.20 1.61 2.01 1.61 0.81
1.74 2.00 2.24 2.07 1.50
Operating conditions: adsorption temperature, 70 °C; influent velocity, 30 ml/min; CO2/N2 (v/v) = 15/85; 1 atm.
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Fig. 6. FT-IR spectra of (a) MCM-41-TEPA60%, (b) MCM-41-TEPA60% after adsorption, (c) MCM-41-APTS30%–TEPA40%, and (d) MCM-41-APTS30%–TEPA40% after adsorption.
R1R2-NH+2 stretching vibration, and the NCOO skeletal vibration [16], indicating the formation of carbaminate. The chemically adsorptive mechanism can be expressed as follows.
CO2 þ 2RNH2 ! RNHCOO þ RNHþ3
ð4Þ
CO2 þ 2R1 R2 NH ! R1 NCOO þ R2 NHþ2
ð5Þ
CO2 þ R1 NH2 þ R2 NH ! R1 NHþ3 þ R2 NCOO
ð6Þ
is synonymous with mass transfer [44]. Fig. 8(a) and Fig. 8(b) show the influence of different influent velocities on CO2 adsorption breakthrough curves and adsorption capacity, respectively. As seen from the figures, with the increase of influent velocity from 30 mL/ min, 40 mL/min and 50 mL/min to 60 mL/min, the breakthrough time and breakthrough adsorption capacity decreased significantly. The higher flow rate corresponds to shorter residence time and smaller mass transfer resistance of CO2 in the sorbent, so the increase of influent velocity resulted in decrease of breakthrough time, saturation time and breakthrough adsorption capacity. However, a higher feed flow rate broadens the contact area between CO2 molecules and sorbent. Therefore, the synergistic effect of shorter residence time and larger contact area made the saturated adsorption capacity of MCM-41-APTS30%–TEPA40% reach to 3.50 mmol-CO2/g-sorbent at the flow rate of 40 mL/min. With a further increase of the flow rate, the adsorption capacity decreased. The decrease can be explained because with a further increase of the flow rate, the residence time of CO2 through the sorbent becomes much shorter compared with the residence time of CO2 at 40 mL/min as shown in Fig. 8(a). Though the contact area between CO2 and the sorbent may become larger, here the much shorter residence time has become the decisive factor. Accordingly, the adsorption capacity decreased with the further increase of the flow rate. Table 3 shows the comparison of adsorption capacity of some amine-modified MCM-41 sorbents both in the document and
3.2.2. Effect of adsorption temperature Adsorption temperature is an important factor for CO2 sorption kinetics. In this paper, 40 °C, 55 °C, 70 °C, and 85 °C were investigated to explore the effect of temperature on the MCM41-APTS30%–TEPA40% sorbent at a feed flow rate of 30 mL/min and atmospheric pressure. Fig. 7(a) and (b) depict the effect of adsorption temperatures on CO2 adsorption breakthrough curves and adsorption capacity, respectively. The breakthrough time and adsorption capacity increased with the increase of adsorption temperatures and reached their optimal at 70 °C. With the further increase of temperature from 70 °C to 85 °C, the value of breakthrough time and adsorption capacity obviously decreased. The trend was well in accordance with the results reported previously [26,28]. The CO2 adsorption process is the combination of kinetics and thermodynamics. The adsorption process is exothermic in itself, and a low temperature is favorable. Nevertheless, TEPA is highly viscous, and its reactivity is also of vital importance. When the temperature was increased from 40 °C to 70 °C, TEPA became more flexible and could disperse more evenly. Moreover, the higher temperature could facilitate the diffusion of CO2 to react with active sites in MCM-41. Therefore, the adsorptive reaction was dominated by dynamics rather than thermodynamics below 70 °C. Once most active sites were occupied by CO2, the pores were filled and the diffusion resistance was enhanced, so when the temperature was increased to 85 °C, the adsorption capacity decreased and the reaction switched to determination by thermodynamics. The temperature of 70 °C was chosen as the optimal adsorption temperature to further explore the influence of feed flow rates. 3.2.3. Effect of influent velocities Influent velocity is an important factor in influencing the adsorption performance of the sorbent because influent velocity
Fig. 7. The influence of adsorption temperatures on (a) breakthrough curves and (b) saturated adsorption capacity of MCM-41-APTS30%–TEPA40%. C0, C represents the concentration of influent and effluent CO2, respectively.
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Fig. 9. The cyclic CO2 adsorption capacity of MCM-41-APTS30%–TEPA40%.
the MCM-41-APTS30%–TEPA40% sorbent exhibits excellent regenerability, much better than the regenerability of only TEPA modified MCM-41, where the adsorption capacity decreased by 7.4% after ten cycles [24]. TEPA was immobilized much more strongly by a hydrogen-bonding connection with APTS in MCM-41, which made less TEPA volatize during the adsorption–desorption process. Therefore, after hybrid amine modification, MCM-41 showed better regenerability. 3.4. Adsorption kinetics
Fig. 8. The effects of influent velocities on (a) breakthrough curves and (b) saturated adsorption capacity of MCM-41-APTS30%–TEPA40%. C0, C represents the concentration of influent and effluent CO2, respectively.
present work. The feed flow rate of 40 mL/min was used for investigating regeneration performance.
Among the properties expected in an excellent sorbent, fast adsorption kinetics are also of vital importance. The kinetics of CO2 adsorption over MCM-41 and MCM-41-APTS30%–TEPA40% were studied at 70 °C and 1 bar. In this work, three types of models were used to fit experimental data, pseudo-first-order, pseudosecond-order and Avrami models. Their kinetic equations are given as follows. The pseudo-first-order kinetic equation:
Q t ¼ Q e ½ð1 expðk1 tÞ
Qt ¼
3.3. Regenerability In real industrial application, not only long breakthrough time and high adsorption capacity but also excellent regenerability of the sorbent in the cyclic adsorption–desorption mode is crucial. In this work, ten cycles were carried out to test the regeneration ability of the MCM-41-APTS30%–TEPA40% sorbent. The adsorption and desorption temperatures were designed as 70 °C and 100 °C, respectively. The calculated saturated adsorption capacity after each cycle is shown in Fig. 9. After ten cycles, the adsorption capacity decreased from 3.50 mmol-CO2/g-sorbent to 3.38 mmolCO2/g-sorbent, dropping by only 3.43%. This result suggests that
ð7Þ
The pseudo-second-order kinetic equation:
k2 Q 2e t 1 þ k2 Q e t
ð8Þ
Avrami: n
Q t ¼ Q e ½1 expððka tÞ a Þ
ð9Þ
k1 (1/min), k2 (g/mmol min) and ka (1/min) are rate constants, Qt (mmol-CO2/g-sorbent) and Qe (mmol-CO2/g-sorbent) represent the adsorption capacity at a given time t and equilibrium time, and na is the order of kinetic equation. The plots of CO2 adsorption capacity versus time are shown in Fig. 10. CO2 capture appears to be a two-stage process. Fast adsorption kinetics were observed on both samples in the initial period of adsorption (breakthrough
Table 3 Comparison of adsorption capacity of MCM-41 based sorbents for CO2 capture. Adsorbents
Test conditions
Adsorptive method
CO2 adsorption capacity (mmol-CO2/g-sorbent)
Reference
MCM-41-PEI50% MCM-41-PEI40% MCM-41-TEPA40% MCM-41-TEPA60% MCM-41-APTS30%–TEPA40%
15 10 10 15 15
Fixed-bed reactor TGA TGA Fixed-bed reactor Fixed-bed reactor
2.00 1.60 2.70 2.45 3.50
[45] [24] [24] Present work Present work
vol.% vol.% vol.% vol.% vol.%
CO2, CO2, CO2, CO2, CO2,
75 °C 35 °C 35 °C 70 °C 70 °C
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4. Conclusions A regenerable hybrid APTS and TEPA functionalized MCM-41 sorbent was successfully developed by a two-step method. APTS was grafted onto the inner surface of the MCM-41, and then TEPA was impregnated into the APTS grafted support. By the combination of APTS and TEPA in MCM-41, an extra net-waved structure for facilitating the dispersion of CO2-philic sites and the diffusion of CO2 emerged. Under the same operating conditions, MCM-41APTS30%–TEPA40% exhibited a breakthrough time and saturated adsorption capacity of 14 min and 3.45 mmol-CO2/g-sorbent, respectively, much higher than the 10 min and 2.45 mmol-CO2/ g-sorbent values obtained with the optimal MCM-41-TEPA60%. Ten adsorption–desorption cycles showed that the MCM-41APTS30%–TEPA40% sorbent is characterized by excellent regenerability at the temperature ranging from 70 °C (adsorption) to 100 °C (desorption). The adsorption kinetics showed a two-stage process, and the Avrami model could fit well with the experimental data obtained with MCM-41-APTS30%–TEPA40%. However, practical flue gas components still contain O2, H2O and small amounts of SOx and NOx, and their effect on CO2 adsorption performance will be studied in the future investigation. Acknowledgments The financial support from the Natural Science Foundation of China (21276129, 20876079) and Natural Science Funds for Distinguished Young Scholar in Shandong Province (JQ200904) are gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.08.107. Fig. 10. Experimental adsorption capacity of (a) MCM-41 and (b) MCM-41APTS30%–TEPA40% along with corresponding fit to kinetic models.
process) during which more than 80% of the saturated adsorption capacity was reached, and then followed by a slower second stage. Similarly, a two-stage adsorption process was also observed on other amine modified sorbents [16]. Table 1S lists the kinetic constants of sorbents explored in this work. The pseudo-first-order model can precisely describe the physical adsorption mechanism of MCM-41 [46]. For MCM-41-APTS30%–TEPA40%, both pseudo-first-order and pseudo-second-order models overestimated the adsorption kinetics in the initial rapid period and underestimated the adsorption process near equilibrium, then overestimated the saturated adsorption capacity at the equilibrium stage. However, the Avrami model could fit well with the whole CO2 capture process. The phenomenon illustrated that the adsorption mechanism might change from simply physical adsorption to both chemical and physical adsorption after modification [47]. In the initial period, CO2 diffused to fresh amine modified sorbent to react rapidly with welldistributed active sites, which might be dominated by chemical reaction. As more exposed active sites were occupied, the viscosity of TEPA and APTS increased, and the diffusion resistance increased significantly, so the adsorption process switched to a slower second stage. However, more than 80% of the saturated adsorption capacity could be achieved in the first rapid stage, which is key for practical industry application.
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