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Carbon dioxide adsorption/desorption performance of single- and blended-amines-impregnated MCM-41 mesoporous silica in post-combustion carbon capture

Carbon dioxide adsorption/desorption performance of single- and blended-amines-impregnated MCM-41... Abstract High-surface-area, hexagonal-structured mesoporous silica, MCM-41, was synthesized and wet impregnated with three different amines of 2-(ethylamino) ethanol (EAE), ethylenediamine (EDA), and tetraethylenepentamine (TEPA) for use as solid adsorbents in post-combustion CO2 capture application. The CO2 adsorption test was performed at 25°C and atmospheric pressure using 15/85 vol% of CO2/N2 at a 20-mL/minute flow rate. Desorption was carried out at 100°C under 20 mL/minute of N2 flow. The results show that the capacity and rate of CO2 adsorption obtained from all the amine-modified adsorbents were significantly increased with increasing amine loading due to carbamate formation. Desorption efficiency and heat duty for regeneration were also affected by the amount of amine loading. The more stable the carbamate produced, the higher the energy was required. They exhibited the highest adsorption–desorption performance at 60 wt% amines used for impregnation. Blended EAE/TEPA at different weight ratios at a total concentration at 60 wt% amines was impregnated on MCM-41 adsorbent. Sorbent impregnated with 50%/10% of EAE/TEPA showed the best performance of 4.25 mmolCO2/g at a high adsorption rate, a low heat duty of 12 kJ/mmolCO2 and with 9.4% reduction of regeneration efficiency after five repeated adsorption–desorption cycles. Open in new tabDownload slide MCM-41, CO2 adsorption, amine-impregnated MCM-41, adsorbent Introduction The emission of carbon dioxide (CO2) in Earth’s atmosphere contributing to global warming and climate change has an immense impact on humankind. According to a US National Oceanic and Atmospheric Agency (NOAA) report, CO2 emission reached almost 415 ppm in 2021 [1]. To maintain a carbon budget that keeps the global warming temperature to within 1.5°C above pre-industrial levels, carbon capture and storage have been necessary. The major source of the CO2 emission comes from fossil-fuel combustion, the largest source of global energy-consumption needs [2, 3], with 76% of the total global warming potential (GWP)-weighted emissions from all emission sources [4]. Due to the greatest potential to be retrofitted to existing power-generation plants, post-combustion CO2 capture is the most promising technology for CO2 removal among other existing CO2 capture technologies. In this process, CO2 gas is removed from the flue gases after the combustion of fossil fuels or other carbonaceous materials [5, 6]. There are various technologies that can be used in post-combustion CO2 capture including chemical absorption [7, 8], cryogenic separation [9, 10], membrane-based separation [10, 11] and adsorption separation processes [12, 13]. Among these technologies, amine-based CO2 capture has been commercially utilized in large-scale CO2 capture plants [14–17] due to the advantages of the feasibility and maturity of the technology to handle a larger amount of exhaust stream, reasonable operation cost and efficiency. It also possesses high CO2 reactivity, low hydrocarbon absorption and regenerability [18–20]. Unfortunately, this process also has some drawbacks in terms of high energy consumption for solvent regeneration accounting for ~70% of the overall operation cost [14, 21–23], a high equipment corrosion rate [22, 24–26], large absorber volume required [22] and degradation of the amine solvent by SO2, NO2, O2 and metal ions in the flue gases, which results in a high solvent makeup rate [24, 27]. Many researchers have attempted to look for alternative technologies to replace conventional ones. Based on the advantages of CO2 adsorption using solid adsorbents in terms of avoiding high energy costs for sorbent regeneration, solvent evaporation and corrosion in the solvent column [28, 29], it has been considered one of the most efficient alternative technologies. In this process, CO2 in the flue gas would be bound onto the solid adsorbent by either physical or chemical interaction forces depending on the active compounds on the adsorbent. Solid adsorbents with a high surface area, low heat capacity and high stability are typically required for a high contact area between the adsorbent and CO2, for regeneration ability and to be capable of operating at high temperature [30]. There are many porous materials that have been utilized in this application such as various types of morphologies of mesoporous silica [31–34]; silica gel [34, 35]; activated carbon derived from diverse precursors such as glucose, cellulose, corn-starch, shrimp shell biowaste and polymers [36–42]; multiwalled carbon nanotubes [34, 43]; zeolites [44, 45]; metal–organic frameworks [46, 47]; porous styrene-divinylbenzene copolymers [48, 49], etc. Typically, these adsorbents are modified with functional groups selective for CO2. The functional groups would be introduced into the adsorbent via either impregnation or grafting methods to provide the chemical interaction to achieve high adsorption capacity. Mesoporous silica is a popular material in CO2 capture applications due to its characteristic ordered pore structure system, high specific surface area and well-defined pore size distribution with a larger pore diameter than zeolite [50]. It can also be synthesized in various morphologies such as spheres, rods, discs, powders, etc. [51]. The diversity of mesoporous silica that is commercially applicable includes Mobil Crystalline Material or MCM (e.g. MCM-41, MCM-48), Santa Barbara Amorphous or SBA (e.g. SBA-12, SBA-15 and SBA-16) and KIT (e.g. KIT-5 and KIT-6) series [50]. They are different in wall thickness, pore size and shape. MCM-41 has a 2D highly uniform hexagonal phase with ordered cylindrical mesopores with a tunable pore size in the range of 15–100 Å, large pore volume (0.7 cm3/g), high surface area (>700 m2/g) and a high number of silanol groups (~40–60%) [52, 53]. The preparation of MCM-41 is simpler than the preparation method of other mesoporous silicas, such as KIT and SBA [54]. In addition, this mesoporous silica is widely used and commercially applicable in various applications. Therefore, it would have benefits in cost and be economical for use in CO2 capture applications as reported in the literature. Zhang et al. prepared a novel double-functionalized mesoporous silica by grafting 3-aminopropyltriethoxysilane and impregnating 70 wt% tetraethylenepentamine on SBA-15. Combining both modification methods led to a high CO2 adsorption capacity of 5.69 mmol/g at 75°C using 20 vol% of CO2. Moreover, its adsorption capability was only reduced by 6.1% after 25 adsorption/desorption cycles [55]. Yan et al. prepared supported HMS-4 mesoporous multimodal pore silica impregnated with TEPA for use as an adsorbent in CO2 capture applications. 75%TEPA on HMS-4 with 54 m2/g of the surface area and 0.05 cm3/g of pore volume showed an adsorption capacity of 4.55 and 6.04 mmolCO2/g at 30°C and 90°C using 15% CO2. They also investigated the regeneration heat of adsorbents at 100°C [56]. In the work of Belmabkhout et al., MCM-41 showed higher volumetric CO2 uptake than activated carbons and 13X zeolite at 25 bar and ambient temperature using a high pure CO2 feed gas [51]. In addition, its surface is also easy for modification by the amine functional group to accomplish high CO2 adsorption capacity. Several studies on CO2 adsorption using amine-modified MCM-41 have been reported in the literature. Wang et al. prepared polyethylene polyamine (PEPA)-loaded MCM-41 and dispersed with methoxypolyethylene glycol (MPEG). The study found that 50wt%PEPA and 5%MPEG-dispersed MCM-41 showed a high adsorption capacity of 2.39 mmol/g with a rapid breakthrough adsorption [57]. Loganathan and Ghoshal prepared mono- and triamine-tethered pore-expanded MCM-41 for use as an adsorbent for CO2 capture and, from the study, it was found that mono- and triamine-tethered pore-expanded MCM-41 displayed 1.2 and 2.1 mmolCO2/g of adsorption capacity, respectively, under a flue gas condition of 0.2 bar and 75°C, indicating that the amine-tethered MCM-41-30 adsorbents were suitable and adequate for CO2 capture [58]. Mukherjee et al. studied the behaviour of different amines impregnated on MCM-41 for post-combustion CO2 capture. Monoethanolamine (MEA), N-(2-aminoethyl) ethanolamine (AEEA) and benzylamine (BZA) were selected to impregnate on the surface of MCM-41. The 40%AEEA, 40%BZA and 50%MEA-impregnated MCM-41 showed different adsorption abilities of 2.34, 0.908 and 1.47 mmolCO2/g, respectively [59]. Liu et al. studied the CO2 adsorption performance of different amine-based MCM-41 adsorbents and the results suggested that amines with a shorter chain exhibited better adsorption/desorption performance due to better amine dispersion and a lower diffusion barrier [60]. From the literature, it was found that amine types played an essential role in the CO2 adsorption performance of the adsorbent. Different types of amines supported on the adsorbent react with CO2 via different mechanisms. Primary and secondary amines form a stable carbamate with a high reaction rate through Equations (1) and (2). First, the amine molecule reacts with CO2 to form zwitterion. This zwitterion reacts with another amine molecule to form carbamate by a proton-transfer reaction. It is indicated that two molecules of amines are consumed for one molecule of CO2 [61–63]: R1R2NH+CO2↔R1R2N+HCOO−(zwitterion)(1) R1R2NH+R1R2NH+COO−↔R1R2N+H2+R1R2NCOO−(carbamate)(2) For tertiary amines, hydrolysis of CO2 with H2O occurs to form a bicarbonate ion, which reacts slowly with the amine to form the bicarbonate via an acid–base catalyst reaction. In this case, one molecule of amine is consumed for each molecule of CO2 as in Equations (3) and (4) [64]: CO2+H2O↔H++HCO3−(3) R1R2R3N+CO2+H2O↔R1R2R3N+H+HCO3−(4) For amines containing more than one amino group in their structures, their reaction mechanisms with the CO2 are pretty complicated compared to mono-amines because polycarbamate species are formed simultaneously [65]. A higher number of amino groups in the amines provides more active sites for reacting with CO2, leading to higher adsorption capacity [66]. This amine type has been mainly utilized to achieve high efficiency for CO2 capture in both absorption and adsorption techniques. In addition, an alkanolamine with a moderate steric hindrance of one hydroxyl group attached to the amine structure exhibited the best performance for CO2 capture [67]. Therefore, an alkanolamine of 2-(ethylamino)ethanol (EAE), which is a secondary alkanolamine containing one ethyl group and one ethanol group attached to the nitrogen atom, and multi-alkylamines of ethylenediamine (EDA) and tetraethylenepentamine (TEPA) containing two and five amino groups, respectively, with the ethylene (–CH2CH2–) group in between, were used in this work. Their chemical structures are shown in Fig. 1. Fig. 1: Open in new tabDownload slide Chemical structure of EAE, EDA and TEPA. Moreover, as described in the works related to CO2 absorption using amine solutions, they also used blended amine to compromise some drawbacks of the other amines in terms of adsorption capacity or regeneration. This inspiration brings about study of the blended solvent system on an alternative adsorption technique. In addition, the optimum ratio between these two amines was also determined and the cyclical test was carried out for practical use. In this work, MCM-41 was synthesized and modified by the wet impregnation method with EAE, EDA and TEPA as adsorbents in CO2 capture applications. The individual- and blended-amine concentrations were also varied to get the optimum amine loading that significantly impacted the CO2 adsorption capacity. The adsorption rate, desorption efficiency and energy for adsorbent regeneration were also investigated. 1 Experimental 1.1 Chemicals Tetraethyl orthosilicate (TEOS, 98%), ammonia solution (30%), aminopropyl triethoxysilane (APTES, 99%) and cetyltrimethylammonium bromide (CTAB, ≥98%) used for MCM-41 synthesis were purchased from Sigma–Aldrich. Ethylenediamine (EDA, ≥98%); 2-(ethylamino) ethanol (EAE, ≥98%) and tetraethyl enepentamine (TEPA, ≥98%) supplied by Sigma–Aldrich were used for wet impregnation. Methanol (Analytical reagents grade) was purchased from RCI Labscan. Gas chromatography (GC)-grade nitrogen (99.99%), ultrapure helium (99.99%), air and premixed gas containing 15 vol% CO2 with nitrogen balance were obtained from Praxair, Thailand. 1.2 Preparation of amine-impregnated MCM-41 Hexagonal mesoporous silica MCM-41 was synthesized by dissolving 2 g of pore-generating agent, CTAB, with 120 mL of deionized water under constant agitation. After the CTAB was completely dissolved, 7.5 mL of ammonia solution was slowly added and the mixture was stirred for 1 hour. Then, 10 mL of TEOS, which was the precursor of silica to obtain the suspension solution, was added to the well-mixed solution. The suspension was stirred at room temperature for 12 hours. Later it was filtered using Whatman® filter paper grade 4, washed with deionized water until neutral and then dried at 100°C in an oven for 12 hours. Synthesized MCM-41 was ground to a fine powder before calcination at 550°C in a furnace with a heating rate of 10°C/minute for 5 hours. The procedure of MCM-41 synthesis was similar to the previous work [68], which was adopted from Loganathan and Ghoshal [58]. For use as adsorbents for CO2 capture, the synthesized MCM-41 was further modified with amines via the wet impregnation method [69]. It was done by mixing a certain amount of amine with 50 mL of methanol. The weight concentration of amines was varied at 30, 50 and 60 wt%. Then, 1 g of synthesized MCM-41 powder was added to the amine–methanol solution. The reaction was refluxed at 70°C for 6 hours and the suspension sample was dried in a rotary evaporator to remove the methanol. The amine-impregnated MCM-41 was kept in a desiccator before use. 1.3 Adsorbent characterization The crystal pattern of the synthesized MCM-41 was confirmed by x-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα radiation at a wavelength of 0.154 nm, tube voltage of 40 kV and current of 40 mA. The XRD diffraction patterns were taken in a small angle 2θ range of 0.5−10° at a scan speed of 0.5°/minute. The morphology and the particle size of the synthesized MCM-41 were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan) and transmission electron microscope (TEM, JEM-1400, USA). Nitrogen adsorption/desorption was performed on a surface area and pore size analyser (Quantachrome’s Autosorb-1 MP, USA) to measure the surface properties of the adsorbents. The sample was dried in a vacuum oven at 60°C overnight prior to testing. A certain amount of MCM-41 sample was loaded into the glass tube and degassed at 100°C under N2 flow for 24 hours before measurement. The Brunauer–Emmett–Teller surface area was calculated over a relative pressure range (P/P0) of 0.05–0.30 and pore volume at 0.99. The Barrett–Joyner–Halenda method of the nitrogen adsorption curve was used to measure the average pore size and pore volume. The impregnation of amine on MCM-41 was confirmed using a Fourier transform infrared spectrophotometer (FTIR, Nexus 670, Bruker, USA). The amount of amine loading on MCM-41 was quantitatively determined by investigating the weight change using a diamond thermogravimetric/differential thermal analyser (TG/DTA) (Perkin Elmer, USA). A certain amount of ample powder was placed in a platinum pan and heated at a heating rate of 10°C/minute under an N2 atmosphere with a 15-mL/minute flow rate. 1.4 CO2adsorption and desorption performance measurement The adsorption of CO2 was carried out at 25°C and atmospheric pressure using 15/85 vol% CO2/N2 as a feed gas representing the concentration of CO2 in the flue gas from the coal-fired power plant. In a typical run, 0.5 g of adsorbent was packed into a stainless-steel tube reactor (12 mm id) supported by glass wool. Before measurement, the adsorbent was pre-treated at 100°C using heating tape (±2°C) under an N2 flow rate of 20 mL/minute for 1 hour to remove physically adsorbed moisture and remaining CO2 in the adsorbent and then it was cooled to room temperature (25°C), which was measured using a thermometer (±2°C). CO2 adsorption was begun by switching to the 15% CO2 at 20 mL/minute. The flow rate of the feed gas was controlled using a mass flow controller (AALBORG, GFC-17 with a range of 0–200 mL/minute) and accurately measured using a volumetric (bubble) flowmeter (Agilent Optiflow Digital Flowmeter) with an accuracy of within ±3%. When 15% CO2 was purged to the reactor, the CO2 concentration at the outlet of the reactor was continuously detected using a gas chromatograph equipped with a thermal conductivity detector (GC-TCD, Agilent, model 7280A, USA) fitted with Porapak-Q column (0.32 mm id × 60 m l dimension) until saturation (i.e. the CO2 concentration in the effluent gas was equal to the feed concentration). The desorption experiment was further carried out after the adsorption was complete by switching the CO2 feed gas to the N2 gas at the same flow rate and applying heat at 100°C. It was performed by continuously detecting the CO2 concentration until no more CO2 peak was observed in the gas chromatogram. The experimental set-up for the CO2 adsorption–desorption is shown in Fig. 2. Fig. 2: Open in new tabDownload slide Experimental set-up for CO2 adsorption/desorption performance. The CO2 adsorption efficiency of the adsorbent was presented as an adsorption capacity (Qads) that was determined by plotting between Cou/Cin versus time to obtain a breakthrough profile. The Qads could be calculated using Equations (5) and (6) [68, 70]: Qads=FCintstM(5) tst=∫0t(1−CouCin)dt(6) where Qads is the adsorption capacity (molCO2/g), F is the total flow rate (mol/minute), Cin is the concentration of CO2 entering the reactor (vol%), Cou is the concentration of CO2 downstream of the reactor (vol%) and t is the time at which the Cou reaches its maximum level (minutes). M is the weight of the adsorbent (g) and tst is the stoichiometric time corresponding to the CO2 stoichiometric adsorption capacity (minutes). The stoichiometric adsorption capacity can be shown to be proportional to the area between the breakthrough curve and a line at Cou/Cin = 1.0. The rate of CO2 adsorption was also determined by observing the slope of the plot of Qads with respect to time. The regeneration performance of the adsorbents could be determined by comparing the Qads of the first and the second adsorption runs of the same adsorbent. The energy requirement for adsorbent regeneration (Qreg, kJ/molCO2) was calculated using Equation (7), which was reported in the previous work [68] and adopted from the work of Singto et al. [71]: Qreg=KAΔt/ΔxCO2 produced(7) where K is the thermal conductivity of the stainless-steel reactor (16.5 W/mK), A is the surface area of the cylindrical sector of the adsorbent in the reactor (m2), Δt is the temperature difference between inside and outside the reactor (K) and Δx is the thickness of the reactor (m). The CO2 produced (mmolCO2/g.s) is obtained from the CO2 adsorption capacity of the second run of the CO2 adsorption experiment. 2 Results and discussion 2.1 Morphology, surface properties and amine loading in the adsorbent The XRD pattern of the synthesized MCM-41 is shown in Fig. 3a and it exhibited a sharp peak at 2θ of 2.6° and two small peaks at 2θ of 4.3° and 5.2° distributed to d100, d110 and d200 faces of the p6mm structure of MCM-41. This XRD pattern was also similar to MCM-41 reported in the work of Beck et al. [44]. From SEM and TEM micrographs (Fig. 3b and c), it is shown that the synthesized MCM-41 had a 2D hexagonal structure with a parallel cylindrical pore with a particle size of ~6 mm. The specific surface area, pore volume and pore size of MCM-41 and amine-modified MCM-41 are shown in Table 1. MCM-41’s surface area was reduced to 867, 772 and 658 m2/g when it was modified with 30, 50 and 60 wt% EAE, respectively. It was reduced to 721, 636 and 473 m2/g for 30, 50 and 60 wt% of EDA impregnation. For TEPA/MCM-41 adsorbents, their surface area was reduced to 373, 123 and 29 m2/g for 30, 50 and 60 wt% of TEPA impregnation, respectively. The pore volume and pore size of the material were found to decrease with the increasing weight concentration of amines because the pore channels of MCM-41 filled with amines. The decrease in the surface area and pore volume of porous supports after impregnation is also observed in the literature [31, 32, 59]. This reduction in the surface properties was also observed in the MCM-41 impregnated with blended amines (see Table 1). Table 1: Specific surface area, pore volume, pore size and amine loading of MCM-41 adsorbents Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Open in new tab Table 1: Specific surface area, pore volume, pore size and amine loading of MCM-41 adsorbents Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Open in new tab Fig. 3: Open in new tabDownload slide XRD pattern (a), SEM (b) and TEM (c) of MCM-41. Fig. 4 shows the FTIR spectra of MCM-41, 60EAE/MCM-41, 60EDA/MCM-41, 60TEPA/MCM-41 and 50EAE-10TEPA/MCM-41. In the spectra of MCM-41, a broad peak at 3500−3000/cm is associated with the OH stretching vibration of silanol (Si-OH) groups and absorbed water molecules [72]. The stretching vibrations of Si–O–Si were observed at ~1070 and 790/cm. In the spectra of amine-impregnated MCM-41, the characteristic peaks of amine structure were found. Bands associated with N–H stretching and bending vibrations at 3500−3300 and 1640−1500/cm, respectively, were observed. The asymmetric and symmetric stretching vibrations of alkyl in amine molecules were presented at ~2900 and ~2800/cm, respectively. The vibration of C–C and C–N stretching vibration was also displayed at ~1400−1500/cm [73]. This confirms the successful impregnation of amines on MCM-41. Fig. 4: Open in new tabDownload slide FTIR spectra of MCM-41, EAE/MCM-41, EDA/MCM-41, TEPA/MCM-41 and EAE-TEPA/MCM-41. Thermogravimetric (TG) profiles of amine-modified MCM-41 are shown in Fig. 5. The amount of amine loading in MCM-41 was determined by observing the weight loss in the TG profile. All samples show the first weight loss at a temperature of <100°C, corresponding to the release of physically adsorbed moisture and gas [69, 74]. The second stage of weight loss of EDA/MCM-41, EAE/MCM-41 and TEPA/MCM-41 was in the temperature range of 120–500, 250–600 and 150–600°C, respectively, contributing to the evaporation and decomposition of amines. This result indicated that EDA/MCM-41, TEPA/MCM-41 and EAE/MCM-41 are stable under temperatures of ~120, ~150 and ~250°C, respectively, suitable for this adsorption/desorption experiment. The overall weight losses of modified MCM-41 adsorbents were 14.8, 17.3 and 19.4% for 30, 50 and 60 wt% EDA, respectively. The overall weight losses of TEPA/MCM41 were 29.04, 48.1 and 54.17% for 30, 50 and 60 wt% TEPA, respectively. For EAE/MCM-41, the overall weight losses were 11.1, 11.3 and 13.9% for 30, 50 and 60 wt% EAE, respectively. The calculated amount of amine loading in MCM-41 is also shown in Table 1. It was observed that the amount of amine loading in MCM-41 increased with an increasing amine concentration used for impregnation. The amount of amine loading was lower than the actual amount of amine added initially because of the incomplete immobilization of the amine on the support [75]. In the blended EAE/TEPA-impregnated MCM-41 adsorbents, the complication of the weight-loss stage in the TG profile was observed due to the combination of two amine types. The first weight loss was also found at <100°C due to the release of physically adsorbed moisture and residual gas in the adsorbent. The second weight loss contributing to the decomposition and volatilization of the EAE and TEPA was observed in the temperature range of 150–800°C with the overall weight loss of 41.45% (2.74 mmol/g) for 10EAE-50TEPA/MCM-41, 38.77% (3.09 mmol/g) for 20EAE-40TEPA/MCM-41, 28.8% (2.62 mmol/g) for 30EAE-30TEPA/MCM-41, 23.91% (2.54 mmol/g) for 40EAE-20TEPA/MCM-41 and 13.28% (1.59 mmol/g) for 50EAE-10TEPA/MCM-41. Fig. 5: Open in new tabDownload slide TG profiles of amine-modified MCM-41 with different weight concentrations. (a) EDA/MCM-41, (b) TEPA/MCM-41, (c) EAE/MCM-41 and (d) EAE-TEPA/MCM-41. 2.2 CO2adsorption/desorption performance of amine-impregnated MCM-41 2.2.1 Single-amine-impregnated MCM-41 Adsorption capacity (Qads) and adsorption rate. Fig. 6 shows the breakthrough curve of MCM-41 and amine-modified MCM-41 adsorbents. The Cou/Cin indicates the continuous increase in the CO2 concentrations of the effluent, which gradually reaches 1.0 when the adsorbent is saturated with CO2. The CO2 adsorption capacity (Qads) of the adsorbents was obtained by integrating the area of the breakthrough profile and further calculated using Equations (5) and (6) [76]. As seen in the breakthrough curve of unimpregnated MCM-41, no breakthrough time was observed and the Qads was 0.63 mmolCO2/g, contributing to the physisorption mechanism. When MCM-41 was impregnated with EAE, EDA and TEPA, their breakthrough times were delayed and the total CO2 uptake was increased. EAE/MCM41 showed breakthrough times of 2.65, 7.72 and 9.85 minutes with Qads of 3.34, 3.84 and 4.52 mmolCO2/g for 30, 50 and 60 wt% EAE. The breakthrough points of EDA/MCM-41 were increased to 0.52, 0.52 and 7.72 minutes for 30, 50 and 60 wt% EDA/MCM-41 with Qads of 0.95, 1.23 and 3.20 mmolCO2/g, respectively. TEPA/MCM-41 showed breakthrough times of 0.52, 5.58 and 5.58 minutes with Qads of 0.99, 2.13 and 3.30 mmolCO2/g for 30, 50 and 60 wt% TEPA/MCM-41, respectively (see Table 2). The Qads was enhanced due to the strong chemical interaction between CO2 and the amine functional group existing in the MCM-41 even though the surface area of the MCM-41 was reduced. It indicated that the strong chemical reaction is more dominant than the physisorption process [32]. The results showed that EAE/MCM-41 had the highest Qads among others. EAE is a secondary alkanolamine with one ethyl group and one ethyl alcohol group attached to the nitrogen atom. The presence of the ethyl group increases the basicity of the amine as a property of an electron-donating group that can provide an electron density to the nitrogen [66]. The amines’ basicity was 10.12 for EAE, which was higher than that of EDA (pKa = 9.99) and TEPA (pKa = 8.91). Multi-alkylamines could form intramolecular hydrogen bonding, decreasing their amine groups’ basicity [77]. In addition, the Qads was also increased with increasing amine loading. The enhancement of Qads as the amine concentration increased was obtained due to the higher affinity of amine sites for CO2. At lower amine loading, the Qads might be obtained from physical and chemical adsorption. When the pore channels of MCM-41 were filled with ~60 wt% loading, the chemisorption became dominant [32]. Table 2: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of single amine-modified MCM-41 adsorbents Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Open in new tab Table 2: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of single amine-modified MCM-41 adsorbents Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Open in new tab Fig. 6: Open in new tabDownload slide CO2 adsorption breakthrough profile of amine-impregnated MCM-41. (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41. Fig. 7 shows the CO2 adsorption profile plotted between the Qads and time. The linear portion at any stage represents the rate of CO2 adsorption. As seen in the CO2 adsorption profiles of all adsorbents, three stages of adsorption were presented, addressed as early, middle and final stages in this work. The fast adsorption rate was observed in the early stage (initial breakthrough period) for all adsorbents, a slower rate at the middle stage followed and then the slowest rate at the final stage, as observed in Fig. 8. This occurred because the CO2 almost entirely occupied MCM-41 pores at the initial period and the CO2 diffusion resistance was increased [75]; thus, the adsorption rate for modified MCM-41 was getting slower at the middle and final stages. Fig. 7: Open in new tabDownload slide CO2 adsorption capacity of (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41 at different wt% of amines. Fig. 8: Open in new tabDownload slide CO2 adsorption rate at early, middle and final stages of (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41. Desorption efficiency and heat duty (Qreg) for adsorbent regeneration. The regeneration ability of the adsorbent is one of the most critical factors that need to be considered for the practical application of CO2 capture. In this work, the desorption efficiency and heat duty (Qreg) for adsorbent regeneration were investigated. The degree of desorption efficiency (%) of each adsorbent was determined by comparing the Qads of the second CO2 adsorption run with the first CO2 adsorption run. The results of desorption efficiency are shown in Table 2. The desorption efficiency of all adsorbents was >95%. It decreased with increasing amine loading due to incomplete bond breaking between CO2 and amine groups due to stable carbamate. The heat duty (Qreg) for the regeneration of all amine-impregnated MCM-41 adsorbents calculated from Equation 7 is also shown in Table 2. 30EAE/MCM41 showed the lowest heat duty of 10.39 kJ/mmolCO2. The Qreg was increased to 12.16 and 30.22 kJ/mmolCO2 for 50EAE/MCM-41 and 60EAE/MCM41, respectively. The increase in the Qreg with increasing amine loading concentration was also found in EDA- and TEPA-impregnated MCM-41s. The Qreg of 30, 50 and 60 wt% TEPA/MCM-41 was 10.23, 51.07 and 65.92 kJ/mmolCO2 and the Qreg of 30, 50 and 60 wt% EDA/MCM-41 was 14.62, 22.03 and 25.11 kJ/mmolCO2, respectively. This could be due to the formation of more stable carbamate, in which case more energy would be required for breaking the bond between amine functional groups and CO2 molecules. 2.2.2 Blended-amine-impregnated MCM-41 The earlier results of the single amine impregnated on MCM-41 indicated that EAE and TEPA showed the best performance in terms of both adsorption/desorption performance and thermal stability for CO2 adsorption application. These two amines were blended and impregnated on the MCM-41 adsorbent. The weight ratios of EAE/TEPA were 10/50, 20/40, 30/30, 40/20 and 50/10. Adsorption capacity (Qads) and adsorption rate. Fig. 9 shows the breakthrough profile of EAE/TEPA-blended-amine-modified MCM-41 adsorbents with different impregnation weight ratios. The breakthrough times of 10/50, 20/40, 30/30, 40/20 and 50/10 of EAE-TEPA/MCM-41s were 7.72, 5.58, 5.58, 2.65 and 7.72 minutes with Qads of 2.98, 2.62, 2.54, 2.46 and 4.25 mmolCO2/g, respectively (see Table 3). The Qads of EAE/TEPA_MCM-41 decreased with increasing the weight ratio of EAE/TEPA up to 40/20 and subsequently increased to 4.25 mmolCO2/g at 50/10 of EAE/TEPA. The results imply that EAE is mainly responsible for capturing CO2 compared to 60 wt% EAE/MCM-41 (4.52 mmolCO2/g of Qads). In addition, the surface area of 50EAE-10TEPA/MCM-41 was 651 m2/g providing a high contact area for the amine–CO2 reaction. Table 3: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of blended EAE/TEPA-impregnated MCM-41 at different concentration ratios Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Open in new tab Table 3: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of blended EAE/TEPA-impregnated MCM-41 at different concentration ratios Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Open in new tab Fig. 9: Open in new tabDownload slide CO2 adsorption breakthrough profile of blended EAE/TEPA-impregnated MCM-41s. The adsorption rate obtained from the adsorption profile of EAE-TEPA/MCM-41 (see Fig. 10) also shows three stages of adsorption as with the single-amine-impregnated MCM-41. The fast adsorption rate was observed at the initial breakthrough period (early stage), then getting slower in the middle and final stages, as seen in Fig. 11. It was due to the pores of the MCM-41 being occupied mainly by CO2 at the initial period and the CO2 diffusion resistance was increased. This result was also reported in the work of Wang et al. [75]. In addition, the adsorption rate tended to increase when increasing EAE in the blended system due to the dominant effect of EAE in terms of adsorption rate. Fig. 10: Open in new tabDownload slide CO2 adsorption profile of blended EAE-TEPA/MCM-41s. Fig. 11: Open in new tabDownload slide CO2 adsorption rate at early, middle and final stages of EAE-TEPA/MCM-41 at different weight ratios. Desorption efficiency and heat duty (Qreg) for adsorbent regeneration. The desorption efficiency and heat duty for EAE-TEPA/MCM-41s are shown in Table 3. The desorption efficiency was increased from 94.98 to 97.18% with the EAE/TEPA weight ratio increased due to the dominance of EAE in terms of desorption efficiency and heat duty for regeneration. The Qreg of EAE-TEPA/MCM-41 adsorbents tended to decrease with an increasing weight ratio of EAE as 39.91, 44.03, 39.62, 32.46 and 12.03 kJ/mmolCO2 for 10/50, 20/40, 30/30, 40/20 and 50/10 of EAE-TEPA/MCM-41, respectively. Based on these results, 50EAE-10TEPA/MCM-41 exhibited the most optimum CO2 adsorption/desorption performance. It showed 4.25 mmolCO2/g of Qads with a high adsorption rate and 97.18% desorption efficiency with 12.03 kJ/mmolCO2 of Qreg. The CO2 adsorption result of the optimum blended EAE/TEPA MCM-41 in this work was also compared with those reported in the literature in Table 4. Their results are comparable to the reported adsorbents and also showed that 50EAE/10TEPA-impregnated MCM-41 exhibited good CO2 adsorption performance. Moreover, five adsorption/desorption cycles were run to investigate its reproducibility, as seen in Fig. 12. It was found that the CO2 adsorption capacity of 50EAE-10TEPA/MCM-41 dropped by 9.4% after five repeated adsorption–desorption cycles, which indicated the incomplete regeneration at 100°C. Table 4: Comparison of CO2 adsorption capacity of amine-impregnated mesoporous silica as reported in the literature Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Open in new tab Table 4: Comparison of CO2 adsorption capacity of amine-impregnated mesoporous silica as reported in the literature Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Open in new tab Fig. 12: Open in new tabDownload slide Breakthrough profiles of CO2 adsorption/desorption cycles of 50EAE-10TEPA/MCM-41. 3 Conclusions In this work, secondary alkanolamine of EAE and multi-alkylamines of EDA and TEPA were used to modify the MCM-41 via a conventional wet impregnation method for use as adsorbents in CO2 capture. The CO2 adsorption capacity and the adsorption rate were increased when increasing the amine concentration due to the more active amines’ affinity to CO2. The performance of adsorption–desorption was found to be dependent on the type and the loading concentration of the amine; 60wt% EAE/MCM-41 exhibited the highest adsorption capacity with a high adsorption rate at all stages of adsorption and also showed good desorption properties of 98% desorption efficiency with an optimum Qreg of 30 kJ/mmolCO2. The blend of EAE /TEPA solution helped enhance the adsorption–desorption performance compared to single-TEPA-modified MCM-41 due to the dominance of EAE on the adsorption–desorption performance. 50EAE-10TEPA/MCM-41 showed outstanding performance with a Qads of 4.25 mmolCO2/g at a high adsorption rate and low heat duty of 12 kJ/mmolCO2 with 9.4% reduction in regeneration efficiency after five repeated adsorption–desorption cycles. However, the selectivity of CO2 over other major components in the flue gas and the adsorption mechanism, derived from either the in situ FTIR or NMR techniques, would be recommended and further investigated in a future study to elucidate and monitor the reaction between CO2 molecules and functional groups on the adsorbent. Funding This work was supported by the Petroleum and Petrochemical College and the Rachadapisek Sompote Fund, Chulalongkorn University for Postdoctoral Fellowship to the first author. Conflict of interest statement None declared. References [1] National Oceanic and Atmospheric Agency. NOAA Research News, Carbon Dioxide Peaks Near 420 Parts Per Million at Mauna Loa Observatory , 2021 . https://research.noaa.gov/article/ArtMID/587/ArticleID/2764/Coronavirus-response-barely-slows-rising-carbon-dioxide ( 20 October 2021 , date last accessed). [2] Figueroa JD , Fout T, Plasynski S, et al. 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Carbon dioxide adsorption/desorption performance of single- and blended-amines-impregnated MCM-41 mesoporous silica in post-combustion carbon capture

Clean Energy , Volume 6 (3): 14 – Jun 1, 2022
14 pages

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Oxford University Press
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Copyright © 2022 National Institute of Clean-and-Low-Carbon Energy
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2515-4230
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2515-396X
DOI
10.1093/ce/zkac020
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Abstract

Abstract High-surface-area, hexagonal-structured mesoporous silica, MCM-41, was synthesized and wet impregnated with three different amines of 2-(ethylamino) ethanol (EAE), ethylenediamine (EDA), and tetraethylenepentamine (TEPA) for use as solid adsorbents in post-combustion CO2 capture application. The CO2 adsorption test was performed at 25°C and atmospheric pressure using 15/85 vol% of CO2/N2 at a 20-mL/minute flow rate. Desorption was carried out at 100°C under 20 mL/minute of N2 flow. The results show that the capacity and rate of CO2 adsorption obtained from all the amine-modified adsorbents were significantly increased with increasing amine loading due to carbamate formation. Desorption efficiency and heat duty for regeneration were also affected by the amount of amine loading. The more stable the carbamate produced, the higher the energy was required. They exhibited the highest adsorption–desorption performance at 60 wt% amines used for impregnation. Blended EAE/TEPA at different weight ratios at a total concentration at 60 wt% amines was impregnated on MCM-41 adsorbent. Sorbent impregnated with 50%/10% of EAE/TEPA showed the best performance of 4.25 mmolCO2/g at a high adsorption rate, a low heat duty of 12 kJ/mmolCO2 and with 9.4% reduction of regeneration efficiency after five repeated adsorption–desorption cycles. Open in new tabDownload slide MCM-41, CO2 adsorption, amine-impregnated MCM-41, adsorbent Introduction The emission of carbon dioxide (CO2) in Earth’s atmosphere contributing to global warming and climate change has an immense impact on humankind. According to a US National Oceanic and Atmospheric Agency (NOAA) report, CO2 emission reached almost 415 ppm in 2021 [1]. To maintain a carbon budget that keeps the global warming temperature to within 1.5°C above pre-industrial levels, carbon capture and storage have been necessary. The major source of the CO2 emission comes from fossil-fuel combustion, the largest source of global energy-consumption needs [2, 3], with 76% of the total global warming potential (GWP)-weighted emissions from all emission sources [4]. Due to the greatest potential to be retrofitted to existing power-generation plants, post-combustion CO2 capture is the most promising technology for CO2 removal among other existing CO2 capture technologies. In this process, CO2 gas is removed from the flue gases after the combustion of fossil fuels or other carbonaceous materials [5, 6]. There are various technologies that can be used in post-combustion CO2 capture including chemical absorption [7, 8], cryogenic separation [9, 10], membrane-based separation [10, 11] and adsorption separation processes [12, 13]. Among these technologies, amine-based CO2 capture has been commercially utilized in large-scale CO2 capture plants [14–17] due to the advantages of the feasibility and maturity of the technology to handle a larger amount of exhaust stream, reasonable operation cost and efficiency. It also possesses high CO2 reactivity, low hydrocarbon absorption and regenerability [18–20]. Unfortunately, this process also has some drawbacks in terms of high energy consumption for solvent regeneration accounting for ~70% of the overall operation cost [14, 21–23], a high equipment corrosion rate [22, 24–26], large absorber volume required [22] and degradation of the amine solvent by SO2, NO2, O2 and metal ions in the flue gases, which results in a high solvent makeup rate [24, 27]. Many researchers have attempted to look for alternative technologies to replace conventional ones. Based on the advantages of CO2 adsorption using solid adsorbents in terms of avoiding high energy costs for sorbent regeneration, solvent evaporation and corrosion in the solvent column [28, 29], it has been considered one of the most efficient alternative technologies. In this process, CO2 in the flue gas would be bound onto the solid adsorbent by either physical or chemical interaction forces depending on the active compounds on the adsorbent. Solid adsorbents with a high surface area, low heat capacity and high stability are typically required for a high contact area between the adsorbent and CO2, for regeneration ability and to be capable of operating at high temperature [30]. There are many porous materials that have been utilized in this application such as various types of morphologies of mesoporous silica [31–34]; silica gel [34, 35]; activated carbon derived from diverse precursors such as glucose, cellulose, corn-starch, shrimp shell biowaste and polymers [36–42]; multiwalled carbon nanotubes [34, 43]; zeolites [44, 45]; metal–organic frameworks [46, 47]; porous styrene-divinylbenzene copolymers [48, 49], etc. Typically, these adsorbents are modified with functional groups selective for CO2. The functional groups would be introduced into the adsorbent via either impregnation or grafting methods to provide the chemical interaction to achieve high adsorption capacity. Mesoporous silica is a popular material in CO2 capture applications due to its characteristic ordered pore structure system, high specific surface area and well-defined pore size distribution with a larger pore diameter than zeolite [50]. It can also be synthesized in various morphologies such as spheres, rods, discs, powders, etc. [51]. The diversity of mesoporous silica that is commercially applicable includes Mobil Crystalline Material or MCM (e.g. MCM-41, MCM-48), Santa Barbara Amorphous or SBA (e.g. SBA-12, SBA-15 and SBA-16) and KIT (e.g. KIT-5 and KIT-6) series [50]. They are different in wall thickness, pore size and shape. MCM-41 has a 2D highly uniform hexagonal phase with ordered cylindrical mesopores with a tunable pore size in the range of 15–100 Å, large pore volume (0.7 cm3/g), high surface area (>700 m2/g) and a high number of silanol groups (~40–60%) [52, 53]. The preparation of MCM-41 is simpler than the preparation method of other mesoporous silicas, such as KIT and SBA [54]. In addition, this mesoporous silica is widely used and commercially applicable in various applications. Therefore, it would have benefits in cost and be economical for use in CO2 capture applications as reported in the literature. Zhang et al. prepared a novel double-functionalized mesoporous silica by grafting 3-aminopropyltriethoxysilane and impregnating 70 wt% tetraethylenepentamine on SBA-15. Combining both modification methods led to a high CO2 adsorption capacity of 5.69 mmol/g at 75°C using 20 vol% of CO2. Moreover, its adsorption capability was only reduced by 6.1% after 25 adsorption/desorption cycles [55]. Yan et al. prepared supported HMS-4 mesoporous multimodal pore silica impregnated with TEPA for use as an adsorbent in CO2 capture applications. 75%TEPA on HMS-4 with 54 m2/g of the surface area and 0.05 cm3/g of pore volume showed an adsorption capacity of 4.55 and 6.04 mmolCO2/g at 30°C and 90°C using 15% CO2. They also investigated the regeneration heat of adsorbents at 100°C [56]. In the work of Belmabkhout et al., MCM-41 showed higher volumetric CO2 uptake than activated carbons and 13X zeolite at 25 bar and ambient temperature using a high pure CO2 feed gas [51]. In addition, its surface is also easy for modification by the amine functional group to accomplish high CO2 adsorption capacity. Several studies on CO2 adsorption using amine-modified MCM-41 have been reported in the literature. Wang et al. prepared polyethylene polyamine (PEPA)-loaded MCM-41 and dispersed with methoxypolyethylene glycol (MPEG). The study found that 50wt%PEPA and 5%MPEG-dispersed MCM-41 showed a high adsorption capacity of 2.39 mmol/g with a rapid breakthrough adsorption [57]. Loganathan and Ghoshal prepared mono- and triamine-tethered pore-expanded MCM-41 for use as an adsorbent for CO2 capture and, from the study, it was found that mono- and triamine-tethered pore-expanded MCM-41 displayed 1.2 and 2.1 mmolCO2/g of adsorption capacity, respectively, under a flue gas condition of 0.2 bar and 75°C, indicating that the amine-tethered MCM-41-30 adsorbents were suitable and adequate for CO2 capture [58]. Mukherjee et al. studied the behaviour of different amines impregnated on MCM-41 for post-combustion CO2 capture. Monoethanolamine (MEA), N-(2-aminoethyl) ethanolamine (AEEA) and benzylamine (BZA) were selected to impregnate on the surface of MCM-41. The 40%AEEA, 40%BZA and 50%MEA-impregnated MCM-41 showed different adsorption abilities of 2.34, 0.908 and 1.47 mmolCO2/g, respectively [59]. Liu et al. studied the CO2 adsorption performance of different amine-based MCM-41 adsorbents and the results suggested that amines with a shorter chain exhibited better adsorption/desorption performance due to better amine dispersion and a lower diffusion barrier [60]. From the literature, it was found that amine types played an essential role in the CO2 adsorption performance of the adsorbent. Different types of amines supported on the adsorbent react with CO2 via different mechanisms. Primary and secondary amines form a stable carbamate with a high reaction rate through Equations (1) and (2). First, the amine molecule reacts with CO2 to form zwitterion. This zwitterion reacts with another amine molecule to form carbamate by a proton-transfer reaction. It is indicated that two molecules of amines are consumed for one molecule of CO2 [61–63]: R1R2NH+CO2↔R1R2N+HCOO−(zwitterion)(1) R1R2NH+R1R2NH+COO−↔R1R2N+H2+R1R2NCOO−(carbamate)(2) For tertiary amines, hydrolysis of CO2 with H2O occurs to form a bicarbonate ion, which reacts slowly with the amine to form the bicarbonate via an acid–base catalyst reaction. In this case, one molecule of amine is consumed for each molecule of CO2 as in Equations (3) and (4) [64]: CO2+H2O↔H++HCO3−(3) R1R2R3N+CO2+H2O↔R1R2R3N+H+HCO3−(4) For amines containing more than one amino group in their structures, their reaction mechanisms with the CO2 are pretty complicated compared to mono-amines because polycarbamate species are formed simultaneously [65]. A higher number of amino groups in the amines provides more active sites for reacting with CO2, leading to higher adsorption capacity [66]. This amine type has been mainly utilized to achieve high efficiency for CO2 capture in both absorption and adsorption techniques. In addition, an alkanolamine with a moderate steric hindrance of one hydroxyl group attached to the amine structure exhibited the best performance for CO2 capture [67]. Therefore, an alkanolamine of 2-(ethylamino)ethanol (EAE), which is a secondary alkanolamine containing one ethyl group and one ethanol group attached to the nitrogen atom, and multi-alkylamines of ethylenediamine (EDA) and tetraethylenepentamine (TEPA) containing two and five amino groups, respectively, with the ethylene (–CH2CH2–) group in between, were used in this work. Their chemical structures are shown in Fig. 1. Fig. 1: Open in new tabDownload slide Chemical structure of EAE, EDA and TEPA. Moreover, as described in the works related to CO2 absorption using amine solutions, they also used blended amine to compromise some drawbacks of the other amines in terms of adsorption capacity or regeneration. This inspiration brings about study of the blended solvent system on an alternative adsorption technique. In addition, the optimum ratio between these two amines was also determined and the cyclical test was carried out for practical use. In this work, MCM-41 was synthesized and modified by the wet impregnation method with EAE, EDA and TEPA as adsorbents in CO2 capture applications. The individual- and blended-amine concentrations were also varied to get the optimum amine loading that significantly impacted the CO2 adsorption capacity. The adsorption rate, desorption efficiency and energy for adsorbent regeneration were also investigated. 1 Experimental 1.1 Chemicals Tetraethyl orthosilicate (TEOS, 98%), ammonia solution (30%), aminopropyl triethoxysilane (APTES, 99%) and cetyltrimethylammonium bromide (CTAB, ≥98%) used for MCM-41 synthesis were purchased from Sigma–Aldrich. Ethylenediamine (EDA, ≥98%); 2-(ethylamino) ethanol (EAE, ≥98%) and tetraethyl enepentamine (TEPA, ≥98%) supplied by Sigma–Aldrich were used for wet impregnation. Methanol (Analytical reagents grade) was purchased from RCI Labscan. Gas chromatography (GC)-grade nitrogen (99.99%), ultrapure helium (99.99%), air and premixed gas containing 15 vol% CO2 with nitrogen balance were obtained from Praxair, Thailand. 1.2 Preparation of amine-impregnated MCM-41 Hexagonal mesoporous silica MCM-41 was synthesized by dissolving 2 g of pore-generating agent, CTAB, with 120 mL of deionized water under constant agitation. After the CTAB was completely dissolved, 7.5 mL of ammonia solution was slowly added and the mixture was stirred for 1 hour. Then, 10 mL of TEOS, which was the precursor of silica to obtain the suspension solution, was added to the well-mixed solution. The suspension was stirred at room temperature for 12 hours. Later it was filtered using Whatman® filter paper grade 4, washed with deionized water until neutral and then dried at 100°C in an oven for 12 hours. Synthesized MCM-41 was ground to a fine powder before calcination at 550°C in a furnace with a heating rate of 10°C/minute for 5 hours. The procedure of MCM-41 synthesis was similar to the previous work [68], which was adopted from Loganathan and Ghoshal [58]. For use as adsorbents for CO2 capture, the synthesized MCM-41 was further modified with amines via the wet impregnation method [69]. It was done by mixing a certain amount of amine with 50 mL of methanol. The weight concentration of amines was varied at 30, 50 and 60 wt%. Then, 1 g of synthesized MCM-41 powder was added to the amine–methanol solution. The reaction was refluxed at 70°C for 6 hours and the suspension sample was dried in a rotary evaporator to remove the methanol. The amine-impregnated MCM-41 was kept in a desiccator before use. 1.3 Adsorbent characterization The crystal pattern of the synthesized MCM-41 was confirmed by x-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα radiation at a wavelength of 0.154 nm, tube voltage of 40 kV and current of 40 mA. The XRD diffraction patterns were taken in a small angle 2θ range of 0.5−10° at a scan speed of 0.5°/minute. The morphology and the particle size of the synthesized MCM-41 were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S-4800, Japan) and transmission electron microscope (TEM, JEM-1400, USA). Nitrogen adsorption/desorption was performed on a surface area and pore size analyser (Quantachrome’s Autosorb-1 MP, USA) to measure the surface properties of the adsorbents. The sample was dried in a vacuum oven at 60°C overnight prior to testing. A certain amount of MCM-41 sample was loaded into the glass tube and degassed at 100°C under N2 flow for 24 hours before measurement. The Brunauer–Emmett–Teller surface area was calculated over a relative pressure range (P/P0) of 0.05–0.30 and pore volume at 0.99. The Barrett–Joyner–Halenda method of the nitrogen adsorption curve was used to measure the average pore size and pore volume. The impregnation of amine on MCM-41 was confirmed using a Fourier transform infrared spectrophotometer (FTIR, Nexus 670, Bruker, USA). The amount of amine loading on MCM-41 was quantitatively determined by investigating the weight change using a diamond thermogravimetric/differential thermal analyser (TG/DTA) (Perkin Elmer, USA). A certain amount of ample powder was placed in a platinum pan and heated at a heating rate of 10°C/minute under an N2 atmosphere with a 15-mL/minute flow rate. 1.4 CO2adsorption and desorption performance measurement The adsorption of CO2 was carried out at 25°C and atmospheric pressure using 15/85 vol% CO2/N2 as a feed gas representing the concentration of CO2 in the flue gas from the coal-fired power plant. In a typical run, 0.5 g of adsorbent was packed into a stainless-steel tube reactor (12 mm id) supported by glass wool. Before measurement, the adsorbent was pre-treated at 100°C using heating tape (±2°C) under an N2 flow rate of 20 mL/minute for 1 hour to remove physically adsorbed moisture and remaining CO2 in the adsorbent and then it was cooled to room temperature (25°C), which was measured using a thermometer (±2°C). CO2 adsorption was begun by switching to the 15% CO2 at 20 mL/minute. The flow rate of the feed gas was controlled using a mass flow controller (AALBORG, GFC-17 with a range of 0–200 mL/minute) and accurately measured using a volumetric (bubble) flowmeter (Agilent Optiflow Digital Flowmeter) with an accuracy of within ±3%. When 15% CO2 was purged to the reactor, the CO2 concentration at the outlet of the reactor was continuously detected using a gas chromatograph equipped with a thermal conductivity detector (GC-TCD, Agilent, model 7280A, USA) fitted with Porapak-Q column (0.32 mm id × 60 m l dimension) until saturation (i.e. the CO2 concentration in the effluent gas was equal to the feed concentration). The desorption experiment was further carried out after the adsorption was complete by switching the CO2 feed gas to the N2 gas at the same flow rate and applying heat at 100°C. It was performed by continuously detecting the CO2 concentration until no more CO2 peak was observed in the gas chromatogram. The experimental set-up for the CO2 adsorption–desorption is shown in Fig. 2. Fig. 2: Open in new tabDownload slide Experimental set-up for CO2 adsorption/desorption performance. The CO2 adsorption efficiency of the adsorbent was presented as an adsorption capacity (Qads) that was determined by plotting between Cou/Cin versus time to obtain a breakthrough profile. The Qads could be calculated using Equations (5) and (6) [68, 70]: Qads=FCintstM(5) tst=∫0t(1−CouCin)dt(6) where Qads is the adsorption capacity (molCO2/g), F is the total flow rate (mol/minute), Cin is the concentration of CO2 entering the reactor (vol%), Cou is the concentration of CO2 downstream of the reactor (vol%) and t is the time at which the Cou reaches its maximum level (minutes). M is the weight of the adsorbent (g) and tst is the stoichiometric time corresponding to the CO2 stoichiometric adsorption capacity (minutes). The stoichiometric adsorption capacity can be shown to be proportional to the area between the breakthrough curve and a line at Cou/Cin = 1.0. The rate of CO2 adsorption was also determined by observing the slope of the plot of Qads with respect to time. The regeneration performance of the adsorbents could be determined by comparing the Qads of the first and the second adsorption runs of the same adsorbent. The energy requirement for adsorbent regeneration (Qreg, kJ/molCO2) was calculated using Equation (7), which was reported in the previous work [68] and adopted from the work of Singto et al. [71]: Qreg=KAΔt/ΔxCO2 produced(7) where K is the thermal conductivity of the stainless-steel reactor (16.5 W/mK), A is the surface area of the cylindrical sector of the adsorbent in the reactor (m2), Δt is the temperature difference between inside and outside the reactor (K) and Δx is the thickness of the reactor (m). The CO2 produced (mmolCO2/g.s) is obtained from the CO2 adsorption capacity of the second run of the CO2 adsorption experiment. 2 Results and discussion 2.1 Morphology, surface properties and amine loading in the adsorbent The XRD pattern of the synthesized MCM-41 is shown in Fig. 3a and it exhibited a sharp peak at 2θ of 2.6° and two small peaks at 2θ of 4.3° and 5.2° distributed to d100, d110 and d200 faces of the p6mm structure of MCM-41. This XRD pattern was also similar to MCM-41 reported in the work of Beck et al. [44]. From SEM and TEM micrographs (Fig. 3b and c), it is shown that the synthesized MCM-41 had a 2D hexagonal structure with a parallel cylindrical pore with a particle size of ~6 mm. The specific surface area, pore volume and pore size of MCM-41 and amine-modified MCM-41 are shown in Table 1. MCM-41’s surface area was reduced to 867, 772 and 658 m2/g when it was modified with 30, 50 and 60 wt% EAE, respectively. It was reduced to 721, 636 and 473 m2/g for 30, 50 and 60 wt% of EDA impregnation. For TEPA/MCM-41 adsorbents, their surface area was reduced to 373, 123 and 29 m2/g for 30, 50 and 60 wt% of TEPA impregnation, respectively. The pore volume and pore size of the material were found to decrease with the increasing weight concentration of amines because the pore channels of MCM-41 filled with amines. The decrease in the surface area and pore volume of porous supports after impregnation is also observed in the literature [31, 32, 59]. This reduction in the surface properties was also observed in the MCM-41 impregnated with blended amines (see Table 1). Table 1: Specific surface area, pore volume, pore size and amine loading of MCM-41 adsorbents Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Open in new tab Table 1: Specific surface area, pore volume, pore size and amine loading of MCM-41 adsorbents Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Samples . Specific surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Amine loading (mmol/g) . MCM-41 1007 0.94 2.91 – 30EAE/MCM-41 867 1.24 2.23 1.48 50EAE/MCM-41 772 0.89 1.98 1.50 60EAE/MCM-41 658 0.57 1.93 1.86 30EDA/MCM-41 721 1.04 2.21 2.46 50EDA/MCM-41 636 0.75 1.97 2.88 60EDA/MCM-41 473 0.46 1.84 3.22 30TEPA/MCM-41 373 0.34 2.16 1.53 50TEPA/MCM-41 123 0.12 1.99 2.54 60TEPA/MCM-41 29 0.10 1.97 2.86 10EAE-50TEPA/MCM-41 143 0.19 1.39 2.74 20EAE-40TEPA/MCM-41 49 0.13 1.96 3.09 30EAE-30TEPA/MCM-41 105 0.23 1.94 2.68 40EAE-20TEPA/MCM-41 117 0.25 1.92 2.54 50EAE-10TEPA/MCM-41 651 0.85 2.19 1.59 Open in new tab Fig. 3: Open in new tabDownload slide XRD pattern (a), SEM (b) and TEM (c) of MCM-41. Fig. 4 shows the FTIR spectra of MCM-41, 60EAE/MCM-41, 60EDA/MCM-41, 60TEPA/MCM-41 and 50EAE-10TEPA/MCM-41. In the spectra of MCM-41, a broad peak at 3500−3000/cm is associated with the OH stretching vibration of silanol (Si-OH) groups and absorbed water molecules [72]. The stretching vibrations of Si–O–Si were observed at ~1070 and 790/cm. In the spectra of amine-impregnated MCM-41, the characteristic peaks of amine structure were found. Bands associated with N–H stretching and bending vibrations at 3500−3300 and 1640−1500/cm, respectively, were observed. The asymmetric and symmetric stretching vibrations of alkyl in amine molecules were presented at ~2900 and ~2800/cm, respectively. The vibration of C–C and C–N stretching vibration was also displayed at ~1400−1500/cm [73]. This confirms the successful impregnation of amines on MCM-41. Fig. 4: Open in new tabDownload slide FTIR spectra of MCM-41, EAE/MCM-41, EDA/MCM-41, TEPA/MCM-41 and EAE-TEPA/MCM-41. Thermogravimetric (TG) profiles of amine-modified MCM-41 are shown in Fig. 5. The amount of amine loading in MCM-41 was determined by observing the weight loss in the TG profile. All samples show the first weight loss at a temperature of <100°C, corresponding to the release of physically adsorbed moisture and gas [69, 74]. The second stage of weight loss of EDA/MCM-41, EAE/MCM-41 and TEPA/MCM-41 was in the temperature range of 120–500, 250–600 and 150–600°C, respectively, contributing to the evaporation and decomposition of amines. This result indicated that EDA/MCM-41, TEPA/MCM-41 and EAE/MCM-41 are stable under temperatures of ~120, ~150 and ~250°C, respectively, suitable for this adsorption/desorption experiment. The overall weight losses of modified MCM-41 adsorbents were 14.8, 17.3 and 19.4% for 30, 50 and 60 wt% EDA, respectively. The overall weight losses of TEPA/MCM41 were 29.04, 48.1 and 54.17% for 30, 50 and 60 wt% TEPA, respectively. For EAE/MCM-41, the overall weight losses were 11.1, 11.3 and 13.9% for 30, 50 and 60 wt% EAE, respectively. The calculated amount of amine loading in MCM-41 is also shown in Table 1. It was observed that the amount of amine loading in MCM-41 increased with an increasing amine concentration used for impregnation. The amount of amine loading was lower than the actual amount of amine added initially because of the incomplete immobilization of the amine on the support [75]. In the blended EAE/TEPA-impregnated MCM-41 adsorbents, the complication of the weight-loss stage in the TG profile was observed due to the combination of two amine types. The first weight loss was also found at <100°C due to the release of physically adsorbed moisture and residual gas in the adsorbent. The second weight loss contributing to the decomposition and volatilization of the EAE and TEPA was observed in the temperature range of 150–800°C with the overall weight loss of 41.45% (2.74 mmol/g) for 10EAE-50TEPA/MCM-41, 38.77% (3.09 mmol/g) for 20EAE-40TEPA/MCM-41, 28.8% (2.62 mmol/g) for 30EAE-30TEPA/MCM-41, 23.91% (2.54 mmol/g) for 40EAE-20TEPA/MCM-41 and 13.28% (1.59 mmol/g) for 50EAE-10TEPA/MCM-41. Fig. 5: Open in new tabDownload slide TG profiles of amine-modified MCM-41 with different weight concentrations. (a) EDA/MCM-41, (b) TEPA/MCM-41, (c) EAE/MCM-41 and (d) EAE-TEPA/MCM-41. 2.2 CO2adsorption/desorption performance of amine-impregnated MCM-41 2.2.1 Single-amine-impregnated MCM-41 Adsorption capacity (Qads) and adsorption rate. Fig. 6 shows the breakthrough curve of MCM-41 and amine-modified MCM-41 adsorbents. The Cou/Cin indicates the continuous increase in the CO2 concentrations of the effluent, which gradually reaches 1.0 when the adsorbent is saturated with CO2. The CO2 adsorption capacity (Qads) of the adsorbents was obtained by integrating the area of the breakthrough profile and further calculated using Equations (5) and (6) [76]. As seen in the breakthrough curve of unimpregnated MCM-41, no breakthrough time was observed and the Qads was 0.63 mmolCO2/g, contributing to the physisorption mechanism. When MCM-41 was impregnated with EAE, EDA and TEPA, their breakthrough times were delayed and the total CO2 uptake was increased. EAE/MCM41 showed breakthrough times of 2.65, 7.72 and 9.85 minutes with Qads of 3.34, 3.84 and 4.52 mmolCO2/g for 30, 50 and 60 wt% EAE. The breakthrough points of EDA/MCM-41 were increased to 0.52, 0.52 and 7.72 minutes for 30, 50 and 60 wt% EDA/MCM-41 with Qads of 0.95, 1.23 and 3.20 mmolCO2/g, respectively. TEPA/MCM-41 showed breakthrough times of 0.52, 5.58 and 5.58 minutes with Qads of 0.99, 2.13 and 3.30 mmolCO2/g for 30, 50 and 60 wt% TEPA/MCM-41, respectively (see Table 2). The Qads was enhanced due to the strong chemical interaction between CO2 and the amine functional group existing in the MCM-41 even though the surface area of the MCM-41 was reduced. It indicated that the strong chemical reaction is more dominant than the physisorption process [32]. The results showed that EAE/MCM-41 had the highest Qads among others. EAE is a secondary alkanolamine with one ethyl group and one ethyl alcohol group attached to the nitrogen atom. The presence of the ethyl group increases the basicity of the amine as a property of an electron-donating group that can provide an electron density to the nitrogen [66]. The amines’ basicity was 10.12 for EAE, which was higher than that of EDA (pKa = 9.99) and TEPA (pKa = 8.91). Multi-alkylamines could form intramolecular hydrogen bonding, decreasing their amine groups’ basicity [77]. In addition, the Qads was also increased with increasing amine loading. The enhancement of Qads as the amine concentration increased was obtained due to the higher affinity of amine sites for CO2. At lower amine loading, the Qads might be obtained from physical and chemical adsorption. When the pore channels of MCM-41 were filled with ~60 wt% loading, the chemisorption became dominant [32]. Table 2: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of single amine-modified MCM-41 adsorbents Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Open in new tab Table 2: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of single amine-modified MCM-41 adsorbents Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . MCM-41 0.63 21.66 99.72 30EAE/MCM-41 3.34 10.39 99.00 50EAE/MCM-41 3.84 12.16 98.50 60EAE/MCM-41 4.52 30.22 98.45 30EDA/MCM-41 0.95 14.62 97.20 50EDA/MCM-41 1.23 22.03 96.00 60EDA/MCM-41 3.20 25.11 95.34 30TEPA/MCM-41 0.99 10.23 95.80 50TEPA/MCM-41 2.13 51.07 95.60 60TEPA/MCM-41 3.30 65.25 94.55 Open in new tab Fig. 6: Open in new tabDownload slide CO2 adsorption breakthrough profile of amine-impregnated MCM-41. (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41. Fig. 7 shows the CO2 adsorption profile plotted between the Qads and time. The linear portion at any stage represents the rate of CO2 adsorption. As seen in the CO2 adsorption profiles of all adsorbents, three stages of adsorption were presented, addressed as early, middle and final stages in this work. The fast adsorption rate was observed in the early stage (initial breakthrough period) for all adsorbents, a slower rate at the middle stage followed and then the slowest rate at the final stage, as observed in Fig. 8. This occurred because the CO2 almost entirely occupied MCM-41 pores at the initial period and the CO2 diffusion resistance was increased [75]; thus, the adsorption rate for modified MCM-41 was getting slower at the middle and final stages. Fig. 7: Open in new tabDownload slide CO2 adsorption capacity of (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41 at different wt% of amines. Fig. 8: Open in new tabDownload slide CO2 adsorption rate at early, middle and final stages of (a) EAE/MCM-41, (b) EDA/MCM-41 and (c) TEPA/MCM-41. Desorption efficiency and heat duty (Qreg) for adsorbent regeneration. The regeneration ability of the adsorbent is one of the most critical factors that need to be considered for the practical application of CO2 capture. In this work, the desorption efficiency and heat duty (Qreg) for adsorbent regeneration were investigated. The degree of desorption efficiency (%) of each adsorbent was determined by comparing the Qads of the second CO2 adsorption run with the first CO2 adsorption run. The results of desorption efficiency are shown in Table 2. The desorption efficiency of all adsorbents was >95%. It decreased with increasing amine loading due to incomplete bond breaking between CO2 and amine groups due to stable carbamate. The heat duty (Qreg) for the regeneration of all amine-impregnated MCM-41 adsorbents calculated from Equation 7 is also shown in Table 2. 30EAE/MCM41 showed the lowest heat duty of 10.39 kJ/mmolCO2. The Qreg was increased to 12.16 and 30.22 kJ/mmolCO2 for 50EAE/MCM-41 and 60EAE/MCM41, respectively. The increase in the Qreg with increasing amine loading concentration was also found in EDA- and TEPA-impregnated MCM-41s. The Qreg of 30, 50 and 60 wt% TEPA/MCM-41 was 10.23, 51.07 and 65.92 kJ/mmolCO2 and the Qreg of 30, 50 and 60 wt% EDA/MCM-41 was 14.62, 22.03 and 25.11 kJ/mmolCO2, respectively. This could be due to the formation of more stable carbamate, in which case more energy would be required for breaking the bond between amine functional groups and CO2 molecules. 2.2.2 Blended-amine-impregnated MCM-41 The earlier results of the single amine impregnated on MCM-41 indicated that EAE and TEPA showed the best performance in terms of both adsorption/desorption performance and thermal stability for CO2 adsorption application. These two amines were blended and impregnated on the MCM-41 adsorbent. The weight ratios of EAE/TEPA were 10/50, 20/40, 30/30, 40/20 and 50/10. Adsorption capacity (Qads) and adsorption rate. Fig. 9 shows the breakthrough profile of EAE/TEPA-blended-amine-modified MCM-41 adsorbents with different impregnation weight ratios. The breakthrough times of 10/50, 20/40, 30/30, 40/20 and 50/10 of EAE-TEPA/MCM-41s were 7.72, 5.58, 5.58, 2.65 and 7.72 minutes with Qads of 2.98, 2.62, 2.54, 2.46 and 4.25 mmolCO2/g, respectively (see Table 3). The Qads of EAE/TEPA_MCM-41 decreased with increasing the weight ratio of EAE/TEPA up to 40/20 and subsequently increased to 4.25 mmolCO2/g at 50/10 of EAE/TEPA. The results imply that EAE is mainly responsible for capturing CO2 compared to 60 wt% EAE/MCM-41 (4.52 mmolCO2/g of Qads). In addition, the surface area of 50EAE-10TEPA/MCM-41 was 651 m2/g providing a high contact area for the amine–CO2 reaction. Table 3: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of blended EAE/TEPA-impregnated MCM-41 at different concentration ratios Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Open in new tab Table 3: Adsorption capacity (Qads), heat duty (Qreg) and desorption efficiency (%) of blended EAE/TEPA-impregnated MCM-41 at different concentration ratios Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Adsorbents . Qads (mmolCO2/g) . Qreg (kJ/mmolCO2) . Desorption efficiency (%) . 10EAE-50TEPA/MCM-41 2.98 39.91 94.98 20EAE-40TEPA/MCM-41 2.62 44.03 95.04 30EAE-30TEPA/MCM-41 2.54 39.62 96.46 40EAE-20TEPA/MCM-41 2.46 32.40 96.75 50EAE-10TEPA/MCM-41 4.25 12.03 97.18 Open in new tab Fig. 9: Open in new tabDownload slide CO2 adsorption breakthrough profile of blended EAE/TEPA-impregnated MCM-41s. The adsorption rate obtained from the adsorption profile of EAE-TEPA/MCM-41 (see Fig. 10) also shows three stages of adsorption as with the single-amine-impregnated MCM-41. The fast adsorption rate was observed at the initial breakthrough period (early stage), then getting slower in the middle and final stages, as seen in Fig. 11. It was due to the pores of the MCM-41 being occupied mainly by CO2 at the initial period and the CO2 diffusion resistance was increased. This result was also reported in the work of Wang et al. [75]. In addition, the adsorption rate tended to increase when increasing EAE in the blended system due to the dominant effect of EAE in terms of adsorption rate. Fig. 10: Open in new tabDownload slide CO2 adsorption profile of blended EAE-TEPA/MCM-41s. Fig. 11: Open in new tabDownload slide CO2 adsorption rate at early, middle and final stages of EAE-TEPA/MCM-41 at different weight ratios. Desorption efficiency and heat duty (Qreg) for adsorbent regeneration. The desorption efficiency and heat duty for EAE-TEPA/MCM-41s are shown in Table 3. The desorption efficiency was increased from 94.98 to 97.18% with the EAE/TEPA weight ratio increased due to the dominance of EAE in terms of desorption efficiency and heat duty for regeneration. The Qreg of EAE-TEPA/MCM-41 adsorbents tended to decrease with an increasing weight ratio of EAE as 39.91, 44.03, 39.62, 32.46 and 12.03 kJ/mmolCO2 for 10/50, 20/40, 30/30, 40/20 and 50/10 of EAE-TEPA/MCM-41, respectively. Based on these results, 50EAE-10TEPA/MCM-41 exhibited the most optimum CO2 adsorption/desorption performance. It showed 4.25 mmolCO2/g of Qads with a high adsorption rate and 97.18% desorption efficiency with 12.03 kJ/mmolCO2 of Qreg. The CO2 adsorption result of the optimum blended EAE/TEPA MCM-41 in this work was also compared with those reported in the literature in Table 4. Their results are comparable to the reported adsorbents and also showed that 50EAE/10TEPA-impregnated MCM-41 exhibited good CO2 adsorption performance. Moreover, five adsorption/desorption cycles were run to investigate its reproducibility, as seen in Fig. 12. It was found that the CO2 adsorption capacity of 50EAE-10TEPA/MCM-41 dropped by 9.4% after five repeated adsorption–desorption cycles, which indicated the incomplete regeneration at 100°C. Table 4: Comparison of CO2 adsorption capacity of amine-impregnated mesoporous silica as reported in the literature Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Open in new tab Table 4: Comparison of CO2 adsorption capacity of amine-impregnated mesoporous silica as reported in the literature Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Adsorbents . Surface area (m2/g) . Pore volume (cm3/g) . Pore size (nm) . Test condition . Adsorption capacity (mmol/g) . Reference . EAE60/MCM-41 658 0.57 1.93 15%CO2, 25°C 4.52 This work EDA60/MCM-41 473 0.46 1.84 15%CO2, 25°C 3.20 This work TEPA60/MCM-41 29 0.10 1.97 15%CO2, 25°C 3.30 This work TEPA10-EAE50/MCM-41 651 0.85 2.19 15%CO2, 25°C 4.25 This work TEPA40-DEA30/MCM-41 0.89 0.0007 n/a 100%CO2, 40°C 3.38 [78] TEPA70/MSU-F 4.26 0.01 n/a 100%CO2, 40°C 4.17 [79] DEA70/MSU-F 5.55 0.019 n/a 100%CO2, 40°C 3.38 TEPA10-DEA60/MSU-F 5.05 0.006 n/a 100%CO2, 40°C 4.20 DEA50/SBA-15 273.01 0.6 n/a 400 p.p.m. CO2, 25°C 0.54 [80] TEPA50/SBA-15 174.1 0.35 n/a 400 p.p.m. CO2, 25°C 1.91 TEPA10-DEA40/SBA-15 78.59 0.19 n/a 400 p.p.m. CO2, 25°C 1.10 TEPA40-DEA10/SBA-15 48.18 0.12 n/a 400 p.p.m., 25°C 1.93 TEPA50/Si-MCM-41 11 0.05 1.8 100%CO2, 25°C 1.24 [81] PEPA50/MCM-41 10.23 0.023 8.91 15% CO2, 60°C 1.66 [57] MEA50/MCM-41 448 0.4 3.1 100% CO2, 25°C 1.47 [59] BZA40/MCM-41 1398 0.9 3.4 100% CO2, 25°C 0.97 AEEA40/MCM-41 210 0.3 3.4 100% CO2, 25°C 2.34 Open in new tab Fig. 12: Open in new tabDownload slide Breakthrough profiles of CO2 adsorption/desorption cycles of 50EAE-10TEPA/MCM-41. 3 Conclusions In this work, secondary alkanolamine of EAE and multi-alkylamines of EDA and TEPA were used to modify the MCM-41 via a conventional wet impregnation method for use as adsorbents in CO2 capture. The CO2 adsorption capacity and the adsorption rate were increased when increasing the amine concentration due to the more active amines’ affinity to CO2. The performance of adsorption–desorption was found to be dependent on the type and the loading concentration of the amine; 60wt% EAE/MCM-41 exhibited the highest adsorption capacity with a high adsorption rate at all stages of adsorption and also showed good desorption properties of 98% desorption efficiency with an optimum Qreg of 30 kJ/mmolCO2. The blend of EAE /TEPA solution helped enhance the adsorption–desorption performance compared to single-TEPA-modified MCM-41 due to the dominance of EAE on the adsorption–desorption performance. 50EAE-10TEPA/MCM-41 showed outstanding performance with a Qads of 4.25 mmolCO2/g at a high adsorption rate and low heat duty of 12 kJ/mmolCO2 with 9.4% reduction in regeneration efficiency after five repeated adsorption–desorption cycles. However, the selectivity of CO2 over other major components in the flue gas and the adsorption mechanism, derived from either the in situ FTIR or NMR techniques, would be recommended and further investigated in a future study to elucidate and monitor the reaction between CO2 molecules and functional groups on the adsorbent. Funding This work was supported by the Petroleum and Petrochemical College and the Rachadapisek Sompote Fund, Chulalongkorn University for Postdoctoral Fellowship to the first author. Conflict of interest statement None declared. References [1] National Oceanic and Atmospheric Agency. NOAA Research News, Carbon Dioxide Peaks Near 420 Parts Per Million at Mauna Loa Observatory , 2021 . https://research.noaa.gov/article/ArtMID/587/ArticleID/2764/Coronavirus-response-barely-slows-rising-carbon-dioxide ( 20 October 2021 , date last accessed). [2] Figueroa JD , Fout T, Plasynski S, et al. 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Journal

Clean EnergyOxford University Press

Published: Jun 1, 2022

References