Capture of carbon dioxide from flue gas on TEPA ... - ScienceDirect

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JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X

Journal of Environmental Sciences 2013, 25(10) 2081–2087

www.jesc.ac.cn

Capture of carbon dioxide from flue gas on TEPA-grafted metal-organic framework Mg2 (dobdc) Yan Cao1,2 , Fujiao Song1 , Yunxia Zhao1 , Qin Zhong1, ∗ 1. School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China. E-mail: [email protected] 2. School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224000, China. Received 13 December 2012; revised 08 April 2013; accepted 17 April 2013

Abstract Carbon dioxide (CO2 ) adsorption on a standard metal-organic framework Mg2 (dobdc) (Mg/DOBDC or Mg-MOF-74) and a tetraethylenepentamine (TEPA) modified Mg2 (dobdc) (TEPA-Mg/DOBDC) were investigated and compared. The structural information, surface chemistry and thermal behavior of the adsorbent samples were characterized by X-ray powder diffraction (XRD), infrared spectroscopy (IR), thermogravimetric analysis (TGA) and nitrogen adsorption-desorption isotherm analysis. CO2 adsorption capacity was measured by dynamic adsorption experiments with N2 -CO2 mixed gases at 60°C. Results showed that the CO2 adsorption capacity of Mg/DOBDC was significantly improved after amine modification, with an increase from 2.67 to 6.06 mmol CO2 /g adsorbent. Moreover, CO2 adsorption on the TEPA-Mg/DOBDC adsorbent was promoted by water vapor, and the adsorption capacity was enhanced to 8.31 mmol CO2 /g absorbent. The adsorption capacity of the TEPA-Mg/DOBDC adsorbent dropped only 3% after 5 consecutive adsorption/desorption cycles. Therefore, this kind of adsorbent can be considered as a promising material for the capture of CO2 from flue gas. Key words: metal-organic framework; amine modification; carbon dioxide adsorption DOI: 10.1016/S1001-0742(12)60267-8

Introduction Carbon dioxide (CO2 ) capture and storage (CCS) is considered to be the most efficient way to control global warming effects (Richard, 2006). The rapid increase of CO2 concentration in the atmosphere is mainly due to the intensive use of fossil fuels (Knuutila et al., 2009). There are many CCS technologies, such as chemical solvent methods, physical absorption methods, cryogenic methods, membrane systems, and other techniques (Aaron and Tsouris, 2005). Among these methods, the absorption method has been extensively researched and has been rather mature for several decades (Rochelle, 2009). Furthermore, there are many specific examples of industrial applications of absorption at home and abroad (Kevitiyagala, 2009). However, many absorption processes, such as the use of amine baths, have drawbacks in terms of high-energy consumption for solvent regeneration and equipment corrosion (Huang et al., 2001). Adsorption is one of the most promising methods that could be applied to separate CO2 . For industrial purposes, porous materials * Corresponding author. E-mail: [email protected]

(such as zeolites, mesoporous silicas, activated carbons and metal-organic frameworks) with high selectivity, capacity for CO2 uptake, stability under industrial conditions as well as minimal regeneration energy are desired (Chue et al., 1995; Haszeldine, 2009). Metal-organic frameworks (MOFs) are novel adsorbents for gas adsorption with high surface areas, large pore volumes, and facile modification ability. Many kinds of MOFs have been reported as potential adsorbents for CO2 capture. Yaghi and Rowsell (2004) synthesized a three-dimensional framework MOF-5 by connecting metal carboxylate clusters and organic dicarboxylate linkers, resulting in a structure with higher apparent surface area and pore volume than most porous crystalline zeolites. The apparent Langmuir surface area was estimated at 2900 m2 /g and gravimetric CO2 capacities for MOF-5 were 22 mmol/g at ambient temperature and pressures up to 3.5 MPa. Férey et al. (2005) created porous chromium terephthalate, MIL-101, with very large pore sizes and surface area, at about 3 nm and 5900 m2 /g respectively, they reported that the best activated MIL-101 exhibited very high loading of CO2 , with a capacity of 390 cm3 /cm3 at 5 MPa. However, while these MOFs are useful for storage

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or sequestration CO2 at high pressure, the CO2 partial pressures in flue gases are usually around 1.5 × 104 Pa. Therefore many researchers have studied CO2 adsorption in MOFs at low pressures. Yazaydin et al. (2009) used both experiments and a modeling approach to screen a diverse set of 14 MOFs for low pressure CO2 uptake, and found that Mg/DOBDC and Ni/DOBDC had the highest CO2 capacities at 1 × 104 Pa and 1 × 105 Pa, respectively. Liu et al. (2012) found that Ni/DOBDC pellets had a high CO2 capacity of 3.74 mmol/g at 1.5 × 104 Pa and 5.61 mmol/g at 1 × 105 Pa, and a high CO2 /N2 selectivity of 38. Ni/DOBDC has much better performance under dry conditions than the reported metal-organic frameworks and zeolites. LeVan et al. (2010) studied the adsorption equilibria of CO2 , H2 O, and CO2 /H2 O for two MOFs, HKUST-1 and Ni/DOBDC. Considering its less intensive regeneration process compared with the benchmark zeolites as well as its hydrothermal stability, Ni/DOBDC may have a promising future for capturing CO2 from flue gases. Thus the DOBDC series of MOFs with open metal sites (i.e., unsaturated metal centers) are considered to be a family of excellent potential adsorbent materials for CO2 capture from flue gas. The introduction of amine groups onto the surface of MOFs is considered an effective method of increasing the CO2 adsorption capacity and selectivity. Two approaches can be used to introduce amine groups to MOFs. One utilizes organic linkers with amine groups as raw materials to synthesize amino-MOFs; this method has been used in related investigations for various objectives. For example, an amine-functionalized MIL-53(Al) MOF was synthesized by using 2-aminoterephthalic acid as a linker, and its basic properties were tested in Knoevenagel condensation reactions of ethyl cyanoacetate and ethyl acetoacetate with benzaldehyde (Gascon et al., 2009). Arstad et al. (2008) prepared three types of MOFs, with and without uncoordinated amine functionalities inside the pores. The highest CO2 adsorption capacity was up to 34.1 mmol/g (60.0 wt.%) at 2.5 MPa. The other approach involves the grafting of amine groups onto coordinatively unsaturated MOF sites via post-synthesis modification. Long et al. (2012) reported that Mg2 (dobpdc) displayed an exceptional capacity for CO2 adsorption at low pressures after grafting N,N’-dimethylethylenediamine on the coordinatively unsaturated Mg(II) sites of an MOF, with a CO2 adsorption capacity of 2.0 mmol/g (8.1 wt.%) at 39 Pa and 25°C, and 3.14 mmol/g (12.1 wt.%) at 1.5 × 104 Pa and 40°C. Anbia and Hoseini (2012) grafted pentaethylenehexamine on the coordinatively unsaturated Cr(III) sites of MIL-101, and found that CO2 adsorption capacity was significantly improved by about 50% after amine modification at 1 MPa and 25°C. In this article, we developed a promising CO2 adsorbent, referred to as TEPA-Mg/DOBDC, obtained by grafting a long aliphatic amine, tetraethylenepentamine

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(TEPA), on the coordinatively unsaturated Mg(II) sites of Mg/DOBDC. This material removed CO2 from flue gas. We also studied the effects of the moisture on the CO2 adsorption behavior. Additionally, to study the regenerability of the adsorbents, a CO2 cyclical adsorption/desorption experiment was carried out.

1 Experimental 1.1 Synthesis of Mg2 (dobdc) The main synthesis route followed the procedure reported by Herm et al. (2011) with some amendments. Generally, Mg(OAc)2 ·6H2 O (1.758 g, 8.2 mmol) and dihydroxyterephthalic acid (H4 DOBDC) (0.478 g, 2.5 mmol) were dissolved in 200 mL of mixed solvent (15/1/1, V/V/V, DMF/EtOH/H2 O; DMF = N,N-dimethyl rormamide), then transferred into the reaction vessel, where it was reacted for 20 hr in an autoclave at 125°C. After it was cooled to room temperature, the mother liquor was decanted. The product was washed by methanol 4 times over 2 days. After decanting the methanol, the yellowish Mg/DOBDC was dried by heating at 180°C under vacuum for 12 hr. 1.2 Post-synthetic modification of Mg2 (dobdc) with TEPA(TEPA-Mg/DOBDC) A 0.80-g of Mg/DOBDC was placed in 30 mL of anhydrous toluene, then the appropriate amount of TEPA (to yield TEPA/Mg2 (dobdc) with ratios of 30 wt.%, 40 wt.% and 50 wt.% TEPA) was added respectively, and reacted under reflux with vigorous stirring for 12 hr. The product was then filtered and washed with anhydrous toluene, and dried 5 hr at 80°C to collect a dark yellowish powder, referred to as TEPA-Mg/DOBDC-30, TEPA-Mg/DOBDC40 and TEPA-Mg/DOBDC-50. All reagents were used as received (AR grade). 1.3 Samples characterization X-ray diffraction (XRD) measurement was performed on a XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd., China). The tube voltage was 35 kV, and the current was 20 mA. The XRD diffraction patterns were taken in the 2θ range of 5◦ –80◦ at a scan speed of 8/min. Infrared (IR) spectra were recorded on a MB154S-FTIR spectrometer (Bomem, Canada). The samples were measured in the form of KBr pellets. The BET surface areas (S BET ) and pore volumes (Vpore ) of samples were determined with a surface area and porosimetry analyzer (V-Sorbet 2008S, Beijing Jinaipu General Instrument Co., Ltd., China). The sample was kept in vacuum at 120°C for 12 hr prior to measurement, then the N2 adsorption-desorption isotherm of the sample was measured at –196°C up to 0.1 MPa pressure. Simultaneous thermogravimetric analysis (TGA) was conducted with a SDTQ600 thermal analyzer (PerkinElmer Pyris Diamond, USA). Analyses were done under a nitrogen atmosphere (30 mL/min flow rate) at a

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Capture of carbon dioxide from flue gas on TEPA-grafted metal-organic framework Mg2 (dobdc)

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The subsequent processes for CO2 adsorption and TPD experiments followed exactly the same procedures.

heating rate of 10°C/min from room temperature to 800°C. 1.4 CO2 adsorption/desorption measurements The dynamic CO2 /N2 co-adsorption experiments were investigated by use of a homemade breakthrough curve apparatus (Fig. 1). The sample column was loaded with 0.5 g adsorbent, which was heated at 100°C for 1 hr in a stream of N2 (99.999%) flow at a rate of 34 mL/min, and then CO2 /N2 mixtures were passed through it at the adsorption temperature of 60°C. Each of the gases was respectively regulated by mass flow controllers, and the total flows of mixed gases were 40 mL/min with a CO2 mole fraction of 15%. The gas stream at the inlet and outlet of the column was analyzed on-line with a gas chromatograph (GC). The output concentration of CO2 was used to calculate a breakthrough curve. Water vapor was introduced into the gas stream by passing the CO2 /N2 mixture through a flask containing 100 mL water. The adsorption capacity was calculated from the breakthrough curves using Eq. (1): t 1 C Q= FC0 (1 − )dt (1) mVm C0 0

2 Results and discussion 2.1 Characterization of Mg/DOBDC and TEPAMg/DOBDC 2.1.1 XRD analysis XRD patterns of Mg/DOBDC and TEPA-Mg/DOBDC samples with different amounts of TEPA are shown in Fig. 2. As seen in Fig. 2, the locations and comparative intensities of the diffraction peaks match well with the experimental and calculated patterns reported for Mg/DOBDC (Kizzie et al., 2011; Liu et al., 2011), indicating that prepared sample was probably Mg/DOBDC. After TEPA grafting, the peak locations do not change, indicating that the Mg/DOBDC structure is retained during TEPA grafting. However, the specific peak of Mg/DOBDC becomes weaker and leans to the right when Mg/DOBDC is treated by TEPA. The peaks of TEPA-Mg/DOBDC-40 and TEPA-Mg/DOBDC-50 are far weaker than those of TEPA-Mg/DOBDC-30. The larger the amount of grafted TEPA, the greater the changes became. These changes are possibly caused by the pore filling effect of the support channels and TEPA coating on the pore surfaces of the support (Yoshitake et al., 2002).

where, Q (mmol/g) is the adsorption capacity, F (mL/min) is the gas flow rate, m (g) is the weight of adsorbents, Vm is 22.4 mL/mmol, C (vol.%) and C0 (vol.%) are the concentrations of gas at the outlet and inlet of the column, respectively. The CO2 temperature-programmed desorption (TPD) was performed on a Quantachrome Automated chemisorption analyzer (ChemBET Pulsar). First, 0.1 g sorbent that was evacuated at 100°C overnight was swept using flowing helium at 120°C for 1 hr to remove the impurities on the surfaces. After the sample was cooled to 30°C, CO2 flow with a rate of 70 mL/min was introduced to the sample cell for 1 hr to adsorb CO2 . Afterwards the TPD experiment was carried out in flowing helium at 70 mL/min from 30 to 300°C, with a temperature ramp of 5°C/min.

2.1.2 Infrared spectroscopy analysis IR spectra of Mg/DOBDC and TEPA-Mg/DOBDC are shown in Fig. 3. IR characterization was conducted to detect the Mg/DOBDC functional groups and confirm the grafting of TEPA. It can be seen that the stretching vibrations of an OH group of crystalline water or the acidic OH of a carbonyl group at 3396 cm−1 is present in the IR spectrum of Mg/DOBDC. In the spectrum of Mg/DOBDC, sharp peaks with high intensity in the range of 1400–1600 cm−1 indicate the stretching vibrations of C=C bonds in an

9

10

3

3

4 Release into the atmosphere P-3

1

2

7 4 8

5 6

6 Fig. 1 Schematic of the CO2 dynamic adsorption experimental. (1) nitrogen cylinder; (2) CO2 cylinder; (3) gas pressure relief valve; (4) mass flow controller; (5) surge flask; (6) water bath; (7) balloon flask; (8) adsorbent; (9) gas chromatography; (10) workstation.


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Table 1

5500 5000

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Pore textural properties of different materials

Sample

S BET (m2 /g)

S Langmuir (m2 /g)

Vpore (cm3 /g)

Adsorption capacity (mmol/g)

Mg/DOBDC TEPA-Mg/DOBDC-30 TEPA-Mg/DOBDC-40 TEPA-Mg/DOBDC-50

780.46 312.63 132.24 23.54

886.41 410.68 230.50 54.21

0.21 0.19 0.15 0.05

2.67 4.49 6.06 3.48

4500 3500 3000 2500 2000

Mg/DOBDC

1500

TEPA-Mg/DOBDC-30

1000

TEPA-Mg/DOBDC-40

500

TEPA-Mg/DOBDC-50

0

0

10

20

30

40 50 60 70 80 2θθ(degree) Fig. 2 XRD patterns for Mg/DOBDC modified by different amount of TEPA. 100 Mg/DOBDC 80

TEPA-Mg/DOBDC-30

Absorbance (a.u.)

60

TEPA-Mg/DOBDC-40

40 TEPA-Mg/DOBDC-50

20 0 -20 -40 4000

3500

3000 2500 2000 1500 1000 500 Wavenumbers (cm-1) Fig. 3 IR spectra of the adsorbents of Mg/DOBDC, TEPA-Mg/DOBDC30, -40; and -50.

aromatic ring (Anbia and Hoseini, 2012). The presence of aromatic rings indicates that the organic ligand is present in the final product. The broad peak at 580 cm−1 is due to the Mg–O vibration, proving the formation of the metal organic framework. The peak at 2359 cm−1 is attributed to CO2 , which arises from the decomposition of labile functional groups (Zhao et al., 2012). In the IR spectrum of TEPA-Mg/DOBDC, the stretching vibrations of C–H bond are present at 2940 and 2838 cm−1 . The peaks at 1582 and 1420 cm−1 are caused by the bending vibration of N–H bond and stretching vibrations of C–N bond, respectively. After modification with amine, although the N–H stretching vibration peaks (3400–3500 cm−1 ) are masked by the much more intense stretching vibrations of the OH group, the molecular interaction cyclization skeleton vibration peak of TEPA is present at 1669 cm−1 . This confirms that the compound has been functionalized. The larger the amount of grafted TEPA is, the stronger the peak becomes. 2.1.3 Thermo-gravimetric analysis In addition to having a large adsorption capacity, the successful application of adsorbent materials in flue

gas treatment also requires good stability under operational conditions. The thermal stability of Mg/DOBDC and TEPA-Mg/DOBDC materials was investigated using thermo-gravimetric analysis (TGA) (Fig. 4). The small weight loss that occurs below 150°C can be attributed to the removal of pre-adsorbed CO2 (as indicated by the characteristic IR peak of CO2 mentioned above) and volatile species such as moisture. The second weight loss around 200°C is due to the removal of guest molecules from open metal sites on Mg/DOBDC. A significant loss is seen in the temperature range from 250 to 450°C, with the samples showing different mass losses because of the volatilization and decomposition of TEPA at varying loading percentages. This confirms that TEPA was actually loaded into or onto the Mg/DOBDC support. The final weight loss appeared above 450°C due to structural collapse, and all samples degraded to magnesium oxide, which agreed well with previous studies (Millward and Yaghi, 2005). In summary, subtracting the mass loss seen for Mg/DOBDC, the mass changes observed for the modified Mg/DOBDC samples were roughly equivalent to the designed TEPA loading amounts. 2.1.4 BET analysis Table 1 shows the pore textural properties of Mg/DOBDC, TEPA-Mg/DOBDC-30, -40 and -50. The BET and Langmuir surface areas of the Mg/DOBDC prepared were 780.46 and 886.41 m2 /g, respectively, which are lower than those in the literature (Deng et al., 2011; Liu et al., 2012). Different starting materials, concentration, solvent

100 90

Mg/DOBDC

80 Weight loss (%)

Intensityθ(a.u.)

4000

TEPA-Mg/DOBDC-30

70 60

TEPA-Mg/DOBDC-40

50 TEPA-Mg/DOBDC-50

40 30 20 100

200

300

400 500 600 Temperature (°C) Fig. 4 TGA curves of the adsorbents.

700

800

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type, pH value, synthesis temperature, synthesis time, reaction vessel and activating method will result in different properties for Mg/DOBDC material. Whether the solvent is dried or not will also affect the pore textural properties of Mg/DOBDC. Thus, the observation of surface area lower than that reported in the literature is due to a combination of factors. After amine modification, the surface area and total pore volume decreased distinctly. This confirms that TEPA is introduced onto the Mg/DOBDC support, which is consistent with the conclusion from Table 1. The surface area and pore volume of TEPA-Mg/DOBDC-50 are 23.54 m2 /g and 0.05 cm3 /g, respectively, which imply that the support has been almost completely filled with TEPA.

tween CO2 and the amine, and the CO2 adsorption capacity increases with increasing amounts of TEPA for 30 wt.% and 40 wt.% samples. However, after loading with TEPA up to 50 wt.%, the adsorption capacity slightly decreases, which probably occurred because the support had almost been filled with TEPA and there was not enough space to capture more CO2 . Primary and secondary amines readily react with CO2 to generate stable carbamates (Reactions (2) and (3)) (Xu et al., 2005),

2.2 CO2 adsorption properties of Mg/DOBDC and TEPA-Mg/DOBDC

2.2.2 Effect of moisture content in flue gas To simulate the moisture conditions of a real-world flue gas, which commonly contains a nearly saturated amount of water vapor, the synthetic flue gas was humidified by bubbling it through a flask filled with 100 mL water in a water bath at different temperatures. The gas flow of this simulated flue gas stream was maintained at 40 mL/min for CO2 capture tests. Prior to the detection of CO2 concentration, the tail gas went through an ice bath for the condensation of moisture. The effect of the moisture concentration in the simulated flue gas on the CO2 adsorption separation by the TEPA-Mg/DOBDC-40 adsorbent was examined at 60°C. Figure 6 shows the results of the experiments. The CO2 adsorption capacity increases with increasing temperature of the water bath from 60°C to 80°C. The CO2 adsorption capacity was 6.06 mmol/g in dry conditions. When the water bath temperature was 60°C, the CO2 adsorption capacity increased to 7.56 mmol/g, which was 24% higher than that for simulated dry flue gas. When the water bath temperature was 80°C, the CO2 adsorption capacity increased steadily to 8.31 mmol/g, which was 10% higher than when the water bath temperature was 60°C and 37% higher than that for simulated dry flue gas. However, after the temperature of the water bath was raised to 90°C, the adsorption

2.2.1 CO2 adsorption of Mg/DOBDC with different TEPA grafting The adsorption curves of Mg/DOBDC and the three TEPAMg/DOBDC samples (weight about 0.5 g) are shown in Fig. 5. The CO2 adsorption capacity of Mg/DOBDC is 2.67 mmol/g, and the CO2 adsorption capacities of TEPA-Mg/DOBDC-30, -40 are 4.49 and 6.06 mmol/g, respectively, rising with increasing amount of TEPA for the 30 wt.% and 40 wt.% compositions. This shows that the adsorption capacity is enhanced substantially after modification. However, after loading with TEPA at 50 wt.%, the adsorption capacity of TEPA-Mg/DOBDC50 dramatically decreased to 3.48 mmol/g, which can probably be attributed to blockage of the pore entrances caused by excessive amounts of TEPA. This indicates that TEPA-Mg/DOBDC-40 possesses the maximum adsorption capacity. The adsorption of CO2 on Mg/DOBDC derives from physical adsorption. The adsorption of CO2 on TEPA-Mg/DOBDC occurs via the chemical interaction beƵ Ʒ

Mg/DOBDC TEPA-Mg/DOBDC-40

ƽ TEPA-Mg/DOBDC-30 ͩ TEPA-Mg/DOBDC-50

CO2 + 2RNH2 ⇐⇒ NH+4 + R2 NCOO−

(2)

CO2 + 2R2 NH ⇐⇒ R2 NH+2 + R2 NCOO−

(3)

1.0 1.0

■ Absenceθofθmoisture

0.8

θ ● Waterθ bathθatθ60°C ▲ Waterθbathθatθ80°C

0.8

C/C0

C/C0

0.6 0.4

0.6 0.4

0.2 0.2 0.0 0.0 0

2

4

6

8

10 12 14 16 18 20 Time (min) Fig. 5 Breakthrough curves of CO2 on Mg/DOBDC TEPAMg/DOBDC-30, -40 and -50. Conditions: temperature 60◦ ; gas flow rate 40 mL/min; CO2 concentration 15 vol.%.

0

2

4

6

8

10 12 14 16 18 20 22 Timeθ(min) Fig. 6 Breakthrough curves of TEPA-Mg/DOBDC-40 in different moistures. Conditions: temperature 60◦ ; gas flow rate 40 mL/min; CO2 concentration 15 vol.%.


experiment could not be carried on normally because the adsorbents formed a slurry in the presence of the large partial pressure of water vapor and led to a blockage of the U tube. Therefore, an appropriate amount of moisture has a promoting effect on the adsorption separation of CO2 from simulated flue gas by TEPA-Mg/DOBDC-40. In summary, water vapor plays a significant role in the CO2 adsorption reaction. When moisture is added to the flue gas, the carbamate formed in Reactions (1) and (2) will further react with CO2 and H2 O to form bicarbonate as shown in Reaction (4) (Wang et al., 2009). The amine group itself can also directly react with CO2 and H2 O to form bicarbonate, as shown in Reactions (5) and (6) (Xu et al., 2005; Wang et al., 2009). Therefore, in the presence of water, 1 mol of amine groups can adsorb 1 mol of CO2 . However, in the absence of water, 2 mol of amine groups can adsorb 1 mol of CO2 (Reactions (2) and (3)). The increase in the CO2 adsorption capacity with the moist gas mixture can be ascribed to the formation of bicarbonate. CO2 + R2 NCOO− + 2H2 O ⇐⇒ R2 NH+2 + 2HCO−3

CO2 + RNH2 + H2 O ⇐⇒ RNH+3 + HCO−3

CO2 + R2 NH + H2 O ⇐⇒

R2 NH+2

+

HCO−3

(4)

(5)

(6)

2.3 Regeneration and cycle test Figure 7 shows the CO2 -TPD profiles of Mg/DOBDC and TEPA-Mg/DOBDC-40 materials. As discussed earlier, the adsorption of CO2 on TEPA-Mg/DOBDC occurs via the chemical interaction of CO2 molecules with amine groups. Thus, the desorption of CO2 at a temperature higher than 100°C is due to chemically adsorbed CO2 ; as shown in the CO2 -TPD profile of TEPA-Mg/DOBDC40, the desorption peak ranged from 100 to 250°C. In the case of Mg/DOBDC, the CO2 molecules captured by physical interforces can be detached in the temperature range of 80–140°C. That is because physical adsorption is a readily reversible process. This implies that the TEPA-Mg/DOBDC-40 sample, as CO2 sorbent, is readily regenerated at low temperatures below 150°C. The regenerability of adsorbents is another important parameter for flue gas capture applications, as the adsorbent should retain stable CO2 adsorption performance for prolonged cyclic operation. Table 2 shows the dynamic adsorption capacity of Mg/DOBDC and TEPA-Mg/DOBDC-40 materials during repetitive cycles of CO2 adsorption at 60°C and desorption under flowing N2 at 100°C. The initial CO2 adsorption capacity of the Mg/DOBDC is 2.67 mmol/g in the first cycle, but decreases considerably under recurring operations. For instance, the adsorption capacities of the Mg/DOBDC

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250 200 Intensity (a.u.)

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150 100 TEPA-Mg/DOBDC-40 50 Mg/DOBDC

0 50

150 200 250 300 Temperature (°C) Fig. 7 CO2 -TPD profiles of Mg/DOBDC and TEPA-Mg/DOBDC-40 after adsorption.

Table 2

100

Cyclic adsorption/desorption behavior of Mg/DOBDC and TEPA-Mg/DOBDC-40

Cycle number

1

2

3

4

5

QMg/DOBDC (mmol/g) QTEPA-Mg DOBDC-40 (mmol/g)

2.67 6.06

2.40 6.02

2.37 5.98

2.30 5.94

2.26 5.92

Q represents CO2 adsorption capacity.

was lowered by 15% after five cycles, from 2.67 to 2.26 mmol/g. Significant degradation of the adsorption capacity found for Mg/DOBDC can possibly be ascribed to the material stability (Liu et al., 2011; Bao et al., 2011) or the activation and desorption method (Choi et al., 2012). The cyclical data reveals that the new TEPA-Mg/DOBDC-40 material synthesized in this work is a more stable adsorbent compared to the parent Mg/DOBDC, with only 3% drop in the adsorption capacity after 5 adsorption/desorption cycles under mild regeneration conditions.

3 Conclusions In this study, CO2 adsorption on the metal organic framework materials Mg/DOBDC and TEPA-Mg/DOBDC is investigated. After synthesis of Mg/DOBDC by a hydrothermal method, the unsaturated sites of the prepared Mg/DOBDC sample were selectively functionalized with tetraethylenepentamine. The samples were characterized by BET, TGA, XRD and FT-IR methods. After amine modification, the BET surface area and pore volume of TEPA-Mg/DOBDC decreased distinctly with the increasing amine loading. This phenomenon suggests the occupation of the pores by amine. CO2 dynamic adsorption experiments were carried out. It was found that TEPA-Mg/DOBDC-40 adsorbs more CO2 (6.06 mmol/g) at 60°C than the parent Mg/DOBDC (2.67 mmol/g). Moisture was found to promote the CO2 adsorption performance of the adsorbents. The adsorption capacity reached

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as high as 8.31 mmol/g. However, a further increase of the water vapor pressure led to the adsorption experiment not being able to be carried out normally. Following the adsorption step, the sorbents were regenerated by TPD, and the modified Mg/DOBDC samples, as CO2 sorbents, were readily regenerated. The results of multiple adsorption/desorption separation cycles showed that the TEPA-Mg/DOBDC-40 adsorbent is stable in cyclic operations of CO2 adsorption separation from flue gas. Future work will be focused on finding the optimum adsorption conditions such as temperature and gas velocity to show the best adsorption performance.

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