Amine-Functionalized Metal Organic Framework as a Highly Selective

An amine-functionalized metal organic framework, TEPA-MIL-101, was prepared by grafting tetraethylenepentamine (TEPA) on the coordinatively unsaturate...
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Amine-Functionalized Metal Organic Framework as a Highly Selective Adsorbent for CO2 over CO Xingrui Wang,†,‡ Huiquan Li,*,† and Xin-Juan Hou† †

Key Laboratory of Green Process and Engineering, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: An amine-functionalized metal organic framework, TEPA-MIL-101, was prepared by grafting tetraethylenepentamine (TEPA) on the coordinatively unsaturated Cr(III) sites of MIL-101 for the selective adsorption of CO2 over CO. The adsorbents were characterized using various techniques. The results indicate that the TEPA molecule was successfully grafted on Cr(III) without destroying the intrinsic structure of MIL-101. Isotherms for CO2 and CO adsorption on MIL-101 and TEPA-MIL-101 were obtained to determine the effects of the grafted TEPA on the CO2 adsorption capacity and selectivity. The results show that the CO2 capacity on TEPA-MIL-101 was higher than that on MIL-101 at lower pressures, whereas the CO capacity sharply decreased. The selectivity for CO2 over CO was clearly improved from 1.77 to 70.2 at 298 K and total pressure 40 kPa. The density functional theory calculation for the adsorption of CO2 and CO on TEPA indicates that the bonding energy of CO2 is obviously higher than that of CO. Analysis of the cyclic adsorption performance reveals the high stability of the adsorbent. On the basis of the experimental and simulation results, the grafting of amines on coordinatively unsaturated sites of metal organic frameworks is an effective method of achieving selective adsorption of CO2 over CO.

1. INTRODUCTION The rapidly increasing CO2 concentration in the atmosphere is the major contributor to global warming. CO2 capture and sequestration (CCS) can be the most practical method of reducing CO2 emission in the near future. However, the high cost of CO2 capture, which accounts for approximately 75% of the total cost of CCS, limits the development of this technology.1 Therefore, the research and development of lowcost capture technologies is essential for CCS. Blast furnace gas (CO 22%, CO2 16%−20%, N2 54%−60%) from the iron and steel industry is one of the major CO2 emission sources. A costeffective CO2 separation approach is the key to the effective utilization of blast furnace gas. Several different approaches have been proposed to separate CO2 from other light gases such as N2 and CO.2 Adsorption on solid materials is now considered a potential alternative to amine-based processes because of its lower energy cost and absence of corrosion. This approach depends on the development of an effective adsorbent with high stability as well as CO2 adsorption capacity and selectivity. Traditional carbon-based materials3−5 and zeolites6−9 have large adsorption capacities at low temperatures and high pressures because of their physical adsorption properties. They have been successfully used in the pressure swing adsorption (PSA) process for CO2/N2 or CO2/ H2 separation. However, as for blast furnace gas, CO is a polar © 2012 American Chemical Society

molecule similar to CO2. Both gases tend to interact with the adsorbent, which results in the poor adsorption selectivity for CO2 over CO.10,11 In recent years, a new type of porous material, metal−organic frameworks (MOFs), has been extensively investigated for the selective adsorption of CO2.12−19 Chromium(III) terephthalate (MIL-101), one of the most porous materials to date, is a very prominent example of MOFs that possess unsaturated metal ions.20 This material exhibits high CO2 adsorption capacity, approximately 40 mmol/g at 298 K and 5 MPa, and thus has potential application for CO2 separation and storage. However, MIL101 can capture CO and CO2 synchronously through a Lewis acid−base interaction between the coordinatively unsaturated sites, Cr(III) and O atoms in CO or CO2 molecule.21,22 Hence, MIL-101 is unsuitable for CO2 separation from blast furnace gas because of its low selectivity. Finding a method to improve the selectivity for CO2 over CO is therefore important for the industrial application of the MOFs possessing unsaturated metal ions such as MIL-101 (Table 1). The introduction of amine groups onto the surface of MOFs is considered an effective method of increasing the CO2 Received: May 31, 2012 Revised: August 22, 2012 Published: August 27, 2012 19814

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unsaturated sites. The resulting MIL-101 was then mixed with 240 mL of TEPA/toluene solution (0.1 mol of TEPA/L of toluene). The solution was stirred at room temperature for 2 h to facilitate the TEPA molecules to diffuse into the MIL-101 pores. Afterward, the reaction system was heated to 383 K and refluxed for 12 h to allow TEPA grafting. The entire preparation process was conducted in a nitrogen atmosphere, and the velocity is 150 mL/min. The resulting green precipitate was filtered, washed with ethanol three times, and dried at 333 K in a vacuum for 4 h. 2.2. Characterization. The X-ray diffraction (XRD) patterns of adsorbents were recorded on an X’pert Pro MPD X-ray diffractometer (PANalytical) with Cu Kα radiation (40 kV, 30 mA) in the 0.5−15 2θ range. N2 adsorption isotherms at 77 K were obtained on a Quantachrom Autosorb-1-C-TCD analyzer. All samples were outgassed for 12 h using the outgas station of the apparatus at 423 K prior to the measurements. The Brunauer−Emmett−Teller (BET) surface area derived from the adsorption data measured at a relative pressure range of 0.05−0.30; the total pore volume was determined from the data at a relative pressure of 0.995. The pore diameter distribution curves were obtained from the adsorption branch of the isotherm via the Barrett−Joyner−Halenda (BJH) method. The Fourier transform infrared (FTIR) spectra were recorded on a Bruker TENSOR 27 spectrometer using the KBr wafer technique. Diffuse reflectance spectra were measured over 200−1000 nm on a LAMBDA 750 UV−vis−NIR spectrophotometer. Elemental analysis (C, N, and O) was conducted on a Flash EA 1112 elemental analyzer. For FTIR and elemental analysis, the samples were outgassed in high vacuum at 423 K for 12 h. 2.3. CO2 and CO Adsorption. CO2 and CO adsorption isotherms were obtained on a Quantachrom Autosorb-1-CTCD analyzer via the static volume method. The samples were heated to 423 K and kept under high vacuum for 12 h to remove all adsorbed water and CO2. The CO2 or CO adsorption volumes for MIL-101 and TEPA-MIL-101 at different temperatures (298, 323, 348, 373, and 398 K) were then measured. Adsorbent regeneration was performed at 373 K for 1 h under high vacuum. 2.4. Isosteric Heat of Adsorption (Qst) Calculations. The Freundlich model [eq 1] was used to fit the CO2 and CO isotherms of MIL-101 and TEPA-MIL-10132 as follows:

Table 1. Physical Properties of MIL-101 and TEPA-MIL-101 sample

BET surface (m2/g)

pore volume (cm3/g)

MIL-101 TEPA-MIL-101

2941 1553

1.79 1.03

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 aminoMOFs. For example, Couck et al.23 synthesized aminefunctionalized MIL-53(Al) by using 2-aminoterephthalic acid as a linker for the selective adsorption of CO2 over CH4. Functionalization with amine groups resulted in an increase in selectivity from ∼5 to 60, as well as an increase in the adsorption enthalpy from 20.1 to 38.4 kJ mol−1. Arstad et al.24 reported three new types of MOFs in two versions: with and without amine substituent on the organic linker. The aminefunctionalized adsorbent achieved a CO2 capacity of approximately 14% at 298 K and 100 kPa CO2 pressure, and the adsorption enthalpy increased from approximately 30 to 50 kJ mol−1. The other approach involves the grafting of amine groups onto the coordinatively unsaturated MOF sites via postsynthetic modification; this method has been used in related investigations for various objectives. Ferey et al.25,26 grafted ethylenediamine, diethylenetriamine, and 3-aminopropyltrialkoxysilane on the coordinatively unsaturated Cr(III) sites of MIL-101 and then investigated the catalytic condensation of benzaldehyde with cyanoethylacetate. Long et al.27 grafted ethylenediamine on the coordinatively unsaturated Cu2+ sites of an MOF, H3[(Cu4Cl)3-(BTTri)8], achieving a higher CO2 adsorption capacity at pressures below 10 kPa; however, the adsorption capacity was approximately 70% lower than that of the parent MOF at 100 kPa. Most studies focused on the selective adsorption of aminefunctionalized MOFs for CO2 over other nonpolar molecules such as N2 and CH4. Research related to the effect of amine groups on the selective adsorption for CO2 over other polar molecules such as CO remains lacking. In this study, a long aliphatic amine, tetraethylenepentamine (TEPA), which has been used as an active component in CO2 adsorbents,28−31 was grafted on the coordinatively unsaturated Cr(III) sites of MIL101 for the selective CO2 adsorption over CO. The amine groups in TEPA can retain the high CO2 adsorption capacity of MIL-101, and the coverage of coordinatively unsaturated Cr(III) sites strongly prevents CO adsorption. Thus, high CO2 adsorption capacity and selectivity for CO2 over CO can be achieved.

q = Kp1/ n

(1)

where q is the adsorption capacity expressed in mmol·g−1, P is the pressure expressed in kPa, and K and n are coefficients of the Freundlich equation. The selectivity of CO2 over CO is calculated using the Ideal Adsorption Solution Theory (IAST).33,34 The specific calculation process is shown in the Supporting Information. The isosteric heat of adsorption for CO2 and CO on MIL101 before and after TEPA grafting was calculated according to eq 3, which is derived from the combination of the Clausius− Clapeyron equation [eq 2] and Freundlich equation [eq 1] as follows:

2. EXPERIMENTAL SECTION 2.1. Adsorbent Preparation. Chromium(III) nitrate nonahydrate [Cr(NO3)3·9H2O, ≥99.0%; Sinopharm Chemical Reagent Co., Ltd.], terephthalic acid (HOOC-C6H4−COOH, ≥99.0%; Sinopharm Chemical Reagent Co., Ltd.), hydrofluoric acid (HF, ≥40.0%; Beijing Chemical Works), tetraethylenepentamine (≥90%; Sinopharm Chemical Reagent Co., Ltd.), and toluene (≥99.5%; Beijing Chemical Works) were used as received and without further purification, except for the removal of trace H2O in toluene using a 4A molecular sieve. The synthesis and purification of MIL-101 were conducted following a previously reported method.20 TEPA-MIL-101 was prepared through the following route: first, 1.1 g of MIL-101 (purified) was outgassed at 423 K under high vacuum for 12 h23,24 to remove water and generate the coordinatively

⎛ ⎞ Q st ⎜ ∂(ln P) ⎟ = − ⎜ ⎟ 1 R ⎝ ∂ T ⎠q

()

19815

(2)

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The Journal of Physical Chemistry C ⎛ ∂ nln ⎜ Q st = −R ⎜ 1 ⎝ ∂ T

(

q K

()

Article

) ⎞⎟

⎟ ⎠q

(3)

where T is the temperature expressed in K, R is the universal gas constant, and Qst is the isosteric heat of adsorption at a specific loading, expressed in kJ/mol. Qst was subsequently calculated from the slope of the (ln p)q plots as a function of 1/ T. 2.5. Theoretical Calculations. Theoretical calculations were performed for the adsorption of CO2 and CO molecules on TEPA at different positions to determine the CO2 and CO adsorption sites and the adsorption ability of TEPA-MIL-101. For TEPA, gas molecule and TEPA coordination with gas molecules (TEPA···CO2/CO), full optimization, and frequency analysis were performed at the PW91LYP/6-311G(d,p) level. On the basis of the optimized PW91LYP/6-311G(d,p) geometry, the binding energies (BE) between the gas molecules and TEPA were calculated by applying the Møller−Plesset second-order perturbation theory (MP2) using the 6-311G** basis set. The BE of gas molecules with TEPA was calculated using the following formula:

Figure 2. N2 adsorption isotherms at 77 K of MIL-101 and TEPAMIL-101.

BE = E(TEPA···CO2 /CO) − E(TEPA) − E(CO2 /CO)

where BE is the energy difference between the isolated TEPA with CO2/CO and TEPA···CO2/CO. All calculations were performed using the Gaussian 03 software.35

3. RESULTS AND DISCUSSION XRD analysis was performed to confirm the structure of the adsorbents. For MIL-101, the locations and comparative

Figure 3. Pore diameter distribution of MIL-101 and TEPA-MIL-101.

Figure 1. XRD patterns of MIL-101 and TEPA-MIL-101.

intensities of the diffraction peaks (Figure 1) show good agreement with the experimental and calculated patterns reported on MIL-101,20,25,26,36,37 thus suggesting that the synthesized product exhibits the MIL-101 structure. After TEPA grafting, the peak locations do not change, indicating that the MIL-101 structure is retained during TEPA grafting. However, the peak intensity at 3.3°, as well as in the 4−6° range, dramatically decreases. These results may have resulted from the partial occupation of the mesocage windows of MIL-

Figure 4. FTIR patterns of MIL-101 and TEPA-MIL-101.

101 by TEPA. Therefore, TEPA has been successfully grafted on MIL-101. 19816

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Figure 5. DR UV−vis patterns of MIL-101 and TEPA-MIL-101.

Table 2. Element Analysis of MIL-101 and TEPA-MIL-101 element MIL-101 TEPA-MIL-101

C

H

N

31.86 40.53

2.51 4.7

10.07

The N2 adsorption isotherms of MIL-101 and TEPA-MIL101 are shown in Figure 2. MIL-101 exhibits a type VI isotherm according to IUPAC nomenclature, which indicates that it has a mesoporous structure.22 The surface area and total pore volume are 2941 m2/g and 1.79 cm3/g, respectively, which are comparable to the results reported by Jhung38 and Liu.39 After TEPA grafting, the surface area and pore volume of TEPA-MIL-101 dramatically decrease to 1553 m2/g and 1.03 cm3/g, respectively. This result may be attributed to TEPA grafting, which obstructs N2 diffusion and adsorption and results in the partial occupation of the space inside the pores. Meanwhile, the pore diameter distribution (Figure 3) shows two peaks at 1.7 and 2.1 nm. This finding is consistent with the results reported by Stock.40 The pore diameter distribution curve of TEPA-MIL-101 also exhibits two peaks. The pore diameters of TEPA-MIL-101, 1.4 and 1.9 nm, are both smaller than those of MIL-101, indicating the successful grafting of TEPA. These findings are consistent with the XRD results. FTIR characterization was conducted to detect the MIL-101 functional groups and confirm the grafting of TEPA; the patterns are shown in Figure 4. For MIL-101, the band at 1705 cm−1 is attributed to −COOH vibration, which indicates the presence of unreacted terephthalic acid residues in the pores.41 Another band at 1622 cm−1 indicates the presence of adsorbed water. The bands at 1550 and 1400 cm−1 correspond to the νas (O−C−O) and νs (O−C−O) vibrations, respectively, implying the presence of dicarboxylate within the MIL-101 framework.42,43 The other bands between 600 and 1600 cm−1 are attributed to benzene, including ν (CC) at 1507 cm−1, and δ (C−H) at 1159, 1017, 882, and 750 cm−1. As compared to MIL-101, TEPA-MIL-101 exhibits few additional absorption bands at 2958, 2837, 1593, 1053, and 652 cm−1, which correspond to νas (C−H), νs (C−H), δ (N−H), ν (C−N), and δ (N−H), respectively. These additional absorption bands confirm the presence of amine and methylene groups in the sample. Interestingly, the absorption band at 1705 cm−1

Figure 6. CO2 (a) and CO (b) adsorption isotherms on MIL-101 at different temperatures.

disappears after TEPA grafting, indicating that the dissociative −COOH is neutralized by the amine groups. UV−vis analysis was conducted to determine the change in Cr(III) coordination before and after TEPA grafting, and the spectra are shown in Figure 5. For MIL-101, the two broad adsorption peaks at 440 and 600 nm correspond to the d−d transition of Cr(III), 4A2g→4T1g and 4A2g→4T2g, respectively. After TEPA grafting, the two peaks shift to 408 and 587 nm, which indicates increases in the electron transition energy. According to the crystal field theory, the amine group provides a higher splitting energy than H2O. Therefore, the UV−vis results confirm the successful grafting of TEPA on the unsaturated Cr(III) in MIL-101. The elemental analysis results are listed in Table 2. According to the formula, Cr3OF(BDC)3 (BDC = benzenedicarboxylate), approximately 3 mmol g−1 coordinatively unsaturated Cr(III) sites are present in MIL-101 after the removal of terminal water (there are two of three Cr ions coordinatively unsaturated, and the other one is coordinated by F atom). The corresponding N content is 13.20 wt % if all coordinatively unsaturated Cr(III) sites are occupied by TEPA. Given that the sample was outgassed under high vacuum at 423 19817

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Figure 9. CO2 adsorption isotherms on MIL-101 and TEPA-MIL-101 at 298 K.

Figure 10. Isosteric heats of adsorption for CO2 and CO on MIL-101 and TEPA-MIL-101.

Figure 7. CO2 (a) and CO (b) adsorption isotherms on TEPA-MIL101 at different temperatures.

shows that the N content of TEPA-MIL-101 is 10.07 wt %, indicating that most of the coordinatively unsaturated Cr(III) sites are occupied by amine groups. This finding agrees with the XRD, N2 adsorption, FTIR, and UV−vis characterization results. It also illustrates the stability of TEPA at 423 K. Figure 6 shows the CO2 and CO adsorption data and Freundlich isotherms on MIL-101 at different temperatures. The CO2 adsorption capacity of MIL-101 dramatically decreases from 1.60 to 0.29 mmol/g at 20 kPa as the temperature increases from 298 to 398 K. This result is attributed to the physical adsorption behavior. The CO isotherms on MIL-101 show the same trend as that of CO2 but with a smaller adsorption capacity, which results from the Lewis acid−base interaction between the unsaturated Cr(III) and the O atom of CO.21,22 After TEPA grafting, the CO2 adsorption capacity of TEPAMIL-101 is retained [Figure 7a]. Despite the coverage of the coordinatively unsaturated Cr(III) by TEPA, the presence of amine groups in TEPA can capture CO2 through the Lewis acid−base interaction. Meanwhile, CO adsorption [Figure 7b] is highly restrained. The CO adsorption capacity dramatically decreases from 0.89 to 0.05 mmol/g at 298 K and 20 kPa as a

Figure 8. The selectivity of CO2 over CO on MIL-101 and TEPAMIL-101 at 298 K.

K for 12 h prior to characterization, the supported TEPA (on the MIL-101 surface but not grafted) was removed. Table 2 19818

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Figure 11. The favorite adsorption sites of CO2 and CO molecules in different positions of TEPA optimized at the PW91LYP/6-311G** level. The measured distances and BE are presented in Å and kJ mol−1, respectively.

result of the occupation of the coordinatively unsaturated Cr(III) by TEPA molecules. The selectivity for CO2 over CO based on IAST at 298 K is shown in Figure 8. After TEPA grafted, the selectivity increases dramatically from 1.77 to 70.2 at 40 kPa. Furthermore, the selectivity decreases sharply as the total pressure increases, which results from the smaller slope of CO2 adsorption isotherm at higher pressure. This indicates that the TEPA-modified MIL-101 is suitable for CO2 separation at low CO2 partial pressure without pressing the mixed gas. The selectivity at other temperatures is shown in the Supporting Information. A comparison of CO2 adsorption on MIL-101 and TEPAMIL-101 at different temperatures was conducted to investigate the effects of TEPA further. At 298 K, TEPA-MIL-101 achieves a higher capacity at pressures below 10 kPa (Figure 9). However, as the pressure further increases, the increase in the adsorption capacity slows, and the capacity is lower than that of MIL-101. This finding is similar to the results reported by Long et al.27 The CO2 molecule is captured mainly on the TEPAMIL-101 surface initially at low pressure, at which TEPA improves its CO2 adsorption capacity as a result of the interaction between CO2 and amines. Further increase in pressure promotes the saturation of the surface adsorption sites, thus forcing the CO2 molecules to diffuse into the TEPA-MIL101 pores. However, the grafted TEPA molecules partially occupy the pores and disrupt the CO2 diffusion, which results in the slightly lower adsorption capacity of TEPA-MIL-101 as compared to that of MIL-101. The MIL-101 and TEPA-MIL101 isotherms at other temperatures (Supporting Information, Figure S2) show the same trends, except that the advantage of TEPA-MIL-101 over MIL-101 at lower pressures is more obvious.

Figure 10 shows the calculated isosteric heats of adsorption of CO2 and CO on MIL-101 and grafted TEPA-MIL-101. The isosteric heats of adsorption for CO2 on both MIL-101 and TEPA-MIL-101 decrease as the adsorption capacity increases. In addition, the adsorption enthalpy of TEPA-MIL-101 is close to that of amine-functionalized MOFs24,27,44 and slightly higher (2−3 kJ mol−1) than that of MIL-101, indicating that the CO2 adsorption capacity is retained. The isosteric heat of adsorption for CO dramatically decreases from about 50−60 to approximately 7 kJ mol−1 after TEPA grafting. This decrease indicates a weak interaction between CO and the amine groups in TEPA. The results show that once MIL-101 is grafted with TEPA, the interaction between the coordinatively unsaturated Cr(III) sites and the gas molecules is replaced by the interaction between TEPA and the gas molecules. The obvious difference between the interaction of CO2 and CO with TEPA leads to the high CO2 selectivity of TEPA-MIL-101. To verify the experimental results, density functional and MP2 calculations were used to investigate the adsorption of CO2 and CO on TEPA. Here, the BEs of CO2 and CO with TEPA at different adsorption sites were calculated (Figure 11). Three possible adsorption sites for CO2 and CO are present. In the most stable structures, (c) and (e) of the TEPA CO2 complex, the CO2 molecule is near the amino group, at a CCO2 and N distance of 3.0 Å, whereas the two O atoms of CO2 form weak hydrogen bonds with two nearby methylene groups. Namely, the Lewis acid−base interaction and weak hydrogen bonds are involved in the interaction between CO2 and TEPA. Although the adsorption modes of CO on TEPA are similar to those of adsorbed CO2, its Lewis acid−base interaction and weak hydrogen bonding are considerably weaker than those of CO2. On the basis of the CO2 or CO adsorption on TEPA, the BE of 19819

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CO2 on TEPA-MIL-101 should be higher than that of CO. Therefore, the bond energy difference between CO2 and CO on TEPA-MIL-101 leads to the high selectivity of MIL-101 for CO2 over CO. For the evaluation of the cyclic performance, the adsorbent TEPA-MIL-101 was generated at 373 K under high vacuum for 1 h. The CO2 adsorption capacity is nearly constant after five cycles, thus illustrating the high adsorption stability of this adsorbent (Supporting Information, Figure S3).

4. CONCLUSIONS TEPA was successfully grafted on the coordinatively unsaturated Cr(III) sites of MIL-101 and was thermally stable at 423 K. The grafted TEPA-MIL-101 prevents the CO adsorption without decreasing the CO2 adsorption capacity at low pressures, which results in an excellent CO2 selectivity over CO. The adsorption capacity is maintained after several adsorption−desorption cycles. The theoretical study for the adsorption of CO2 and CO on TEPA gives the clue on understanding the high CO2 selectivity of TEPA-MIL-101. TEPA-MIL-101 is a potential adsorbent for the selective adsorption of CO2 from blast furnace gas via the thermal swing adsorption process. On the basis of the experiment results and theoretical calculations, this study shows that grafting amines on the open metal sites of MOFs is an effective method of achieving selective adsorption of CO2 over CO.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-6262-1355. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 20903099 and 20906084). REFERENCES

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