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Novel PEI-impregnated UiO-66 (PEI@UiO-66) composites with enhanced CO2 adsorption capacity and CO2/CH4 selectivity were synthesized and ...
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Enhanced Dynamic CO2 Adsorption Capacity and CO2/CH4 Selectivity on Polyethylenimine-Impregnated UiO-66 Shikai Xian,† Ying Wu,† Junliang Wu,‡ Xun Wang,‡ and Jing Xiao*,† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China School of Environment and Energy, Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou, 510640, China



S Supporting Information *

ABSTRACT: Novel PEI-impregnated UiO-66 (PEI@UiO-66) composites with enhanced CO2 adsorption capacity and CO2/ CH4 selectivity were synthesized and characterized. The dynamic CO2/CH4 separation performances were evaluated in the fixed bed. CO2 working capacity and CO2/CH4 selectivity of PEI@UiO-66 at 338 K were up to 1.65 mmol/g and 111, respectively, being 12.7 and 58 times greater than those of the parent UiO-66, respectively. In the presence of water vapor in feed stream, the CO2 capacity and CO2/CH4 selectivity of PEI@UiO-66 separately reached values of 2.41 mmol/g and 251, respectively, at a relative humidity (RH) of 55% and 338 K, having increases of 48.8% and 126%, respectively, in comparison with those under dry conditions, which were higher than many MOF-based adsorbents. Density functional theory (DFT) calculations suggested that the presence of water vapor promoted CO2 adsorption, likely because of the formation of bicarbonate with a lower amine/CO2 ratio than that required under dry conditions. The multiple adsorption-regeneration tests suggested that CO2 adsorption capacity of PEI@UiO-66 can be fully recovered after regeneration. MOF-74, a framework with open Mg2+ sites, had a high CO2 storage capacity of 35.2 wt % at 298 K and 1 bar. These MOFs had very high CO2 adsorption capacity, but their low physicochemical stabilities (especially moisture stability) would limit their industrial applications, since water vapor is ubiquitous in realistic situations. In addition, although some MOFs, such as MIL-101(Cr), have high moisture stability and CO2 capacity,17 its high hydrophilicity resulted in a sharp decrease in its CO2 uptake in a humid atmosphere, because of the competitive adsorption of water vapor.18 Therefore, it is crucial to develop novel adsorbents with high CO2 capacity and moisture stability. Cavka et al. first synthesized a thermally and chemically stable zirconium(IV)-based MOF (UiO-66)19,20 using low-cost precursors under low energy consumption. Li et al.21 synthesized the UiO-66(CH3)2 using 2,5-dimethylterephthalic acid as the ligand and reported its CO2 capacity up to 3.6 mmol/g at 293 K, which is higher than that of UiO-66. Lau et al.22 synthesized UiO66(Ti56) and reported its CO2 uptake of 3.5 mmol/g at 273 K. Hong et al.23 modified UiO-66 with various flexible alkanedioic acids (HO2C(CH2)n-CO2H), and reported its CO2 uptake of 2.5 mmol/g at 298 K. Previous works suggested that the stable UiO66 can be tuned for improved CO2 capacity and selectivity of UiO-66, and the development of a new functional UiO-66 adsorbent is worthy of further investigation. Polyethylenimine (PEI) is a polymer functionalized with highly dense amine groups, which can bind with CO2 forming carbamates.24 Therefore, PEI was reported to be grafted or impregnated on conventional porous materials, such as activated carbon,25 SBA-15,26 MCM-41,27 resin,28 and clays,29,30 to

1. INTRODUCTION The world is seeking new energy resources, because of the gradual depletion of petroleum and the rapid increase of energy consumption worldwide. Given this scenario, shale gas, which is mainly comprised of methane, has emerged as a promising and increasingly important natural gas resource, because it is expected to greatly expand the energy supply worldwide.1,2 In shale gas, however, methane is generally mixed with other gases, such as carbon dioxide ( 328 K > 318 K > 308 K. At 338 K, the CO2 working capacity and CO2/CH4 selectivity reached maximum (1.65 mmol/g and 111), having increases of 112% and 1005%, in comparison with those at 308 K, which were ∼12.7 and 58 times of that of UiO-66 at 308 K. The higher CO2 sorption capacity at 338 K can be attributed to the decreased kinetic barrier for the diffusion of CO2 from the surface into the bulk of PEI on UiO-66 at higher temperature,13 resulting in an increased number of accessible sorption sites 11154

DOI: 10.1021/acs.iecr.5b03517 Ind. Eng. Chem. Res. 2015, 54, 11151−11158

Article

Industrial & Engineering Chemistry Research

Figure 7. (a) Breakthrough curves of 30PEI@UiO-66 for CO2/CH4 binary mixture (vCO2/vCH4 of 1/9) under varied relative humidity (RH) values at 308 K. (b) Effect of RH on CO2 working capacity and CO2/CH4 selectivity of 30PEI@UiO-66 at 308 K.

Figure 8. (a) Breakthrough curves of 30PEI@UiO-66 for CO2/CH4 binary mixture (vCO2/vCH4 of 1/9) under varied RH values at 338 K. (b) Effect of RH on CO2 working capacity and CO2/CH4 selectivity of 30PEI@UiO-66 at 338 K.

synergestic effect of the water vapor addition and PEI impregnated on UiO-66 for CO2 adsorption. Moreover, the enhancement followed the order of 55% RH > 80% RH > 30% RH > 0% RH (Figure 7b). At 55% RH, CO2 working capacity and CO2/CH4 selectivity of 30PEI@UiO-66 reached 1.11 mmol/g and 18, respectively, having increases of 50% and 80%, in comparison with those under dry conditions. Breakthrough curves of 30PEI@UiO-66 for CO2/CH4 binary mixture were also investigated at 338 K, as shown in Figure 8a. The CO 2 working adsorption capacities and CO 2 /CH 4 adsorption selectivities of 30PEI@UiO-66 at varied humidities were calculated and plotted in Figure 8b (CO2 adsorption under 80% RH at 338 K was difficult to perform; thus, the result was excluded from the work). A similar trend was noted that the enhancement on CO2 working adsorption capacities and CO2/ CH4 adsorption selectivities of 30PEI@UiO-66 followed the order of 55% RH > 10% RH > 0% RH. At 55% RH, CO2 working capacity and CO2/CH4 selectivity of 30PEI@UiO-66 reached as high as 2.41 mmol/g and 251, respectively, having increases of 48.8% and 126%, in comparison with those under dry conditions, which is much higher than those of the adsorbents such as MOF5,48 HKUST-1,14 MIL-53,42 ZIF-95,43 COP-27-Ni,49 and BPLcarbon50 (see Figure 9). Density functional theory (DFT) was applied to understand the enhanced CO2 adsorption over PEI@UiO-66 by the addition of water vapor. Figure 10 shows the optimized interaction configurations and adsorption energies for CO2 adsorption over tetraethylenepentamine under both dry and wet conditions.

(308−338 K). Above 338 K, it began to decrease gradually with temperature, likely because of the facilitated CO2 desorption from the PEI@UiO-66 surface. At lower adsorption temperature (308−338 K), the CO2 capacity may be governed by the diffusion of CO2 molecules into the 30PEI@UiO-66, whereas, at higher adsorption temperatures (>338 K), the CO2 adsorption− desorption thermodynamics may be the dominant mechanism.25,40 Note that the dynamic CO2/CH4 selectivity (111) of 30PEI@UiO-66 at 338 K was much higher than that of many reported CO2 adsorbents, such as MOF-508b,41 MIL-53,42 and ZIF-95.43 3.2.3. Effect of Humidity. Under realistic situations,51,52 water vapor is often omnipresent in CO2 gas mixtures. In the case of shale gas, the amount of water vapor varied from shale gas source, which can even reach as high as the saturation vapor pressure of water.16,44 Therefore, the effect of water vapor on CO2/CH4 separation is worthy of further investigation. Figure 7a, and Figure S8 in the Supporting Information present the breakthrough curves of 30PEI@UiO-66 and the parent UiO-66 for CO2/CH4 mixture under different relative humidity (RH) values at 308 K. The CO2 working adsorption capacities and CO2/CH4 adsorption selectivities of 30PEI@UiO-66 at varied humidities were calculated and plotted in Figure 7b. For UiO-66 (Figure S8), the addition of water vapor (0−80%) slightly decreased the CH4 and CO2 working capacities, suggesting weak competitive adsorption from water vapor on UiO-66. In distinct contrast, for 30PEI@UiO-66, the addition of water vapor increased the CO2 working capacity and CO2/CH4 selectivity, suggesting a 11155

DOI: 10.1021/acs.iecr.5b03517 Ind. Eng. Chem. Res. 2015, 54, 11151−11158

Article

Industrial & Engineering Chemistry Research

Theoretically, with the addition of water vapor, CO2 adsorption capacity on PEI-based materials could be doubled. However, its impact on PEI@UiO-66 was less significant (48.8% increase in CO2 working capacity at RH = 55% at 338 K), compared to that under dry conditions, similar to that reported for PEI@clay,54 PEI@silica gel,55 etc. This may be attributed to the faster kinetics for carbomate formation than that for bicarbonate formation (with the additional water vapor),56 and the dynamic gas retention time in the adsorption column is shorter than that required for bicarbonate formation.57 Also, it was noticed in Figure 7b that by further increasing RH to 80%, the CO2 working capacity and CO2/CH4 selectivity decreased (to 0.9 and 15.5, respectively). The decreases could be attributed to the competitive adsorption of the excess amount of water (more than that required for bicarbamate formation) with CO2 under high humidity. 3.3. Regeneration. Besides adsorption capacity, selectivity, and adsorption/desorption kinetics, adsorbent regeneration is an important parameter to evaluate the feasibility of the adsorbent for industrial applications. Figure 11 shows the CO2 break-

Figure 9. CO2/CH4 selectivity of PEI@UiO-66 and other reported adsorbents.

Figure 10. Optimized interaction configurations and adsorption energies (BE, in kJ mol−1) for CO2 adsorption over tetraethylenepentamine: (a) under dry conditions and (b) under wet conditions. Color code: gray, C; red, O; blue, N; white, H.

Under anhydrous conditions, CO2 interacts with amine groups in PEI via the following reactions (as shown in Figure 10a):53 CO2 + 2RNH 2 → RNHCOO− + RNH+3

(3)

CO2 + 2R 2NH → R 2NCOO− + R 2NH+2

(4)

Figure 11. CO2 breakthrough curves of the 30PEI-UiO-66 at five consecutive CO2 adsorption−desorption cycles.

through curves of the 30PEI@UiO-66 at five consecutive CO2 adsorption−desorption cycles. It can be seen that negligible decrease on CO2 working capacity was noted after five adsorption−desorption cycles, suggesting CO2 adsorption− desorption on the PEI-UiO-66 was reversible. Moreover, the PEI-UiO-66 can be well-regenerated after cyclic usage at 393 K, suggesting a fairly good stability of PEI-UiO-66 under adsorption/desorption conditions.

In this case, two amine groups were required to react with one CO2 molecule, forming a stable carbamate. The primary amine (BE = −56.69 kJ mol−1) interacts with CO2 more strongly than the secondary amine (BE = −34.18 kJ mol−1). While under humid conditions, as shown in Figure 10b, CO2 interacts with an amine group in PEI through the following reaction:45−47 CO2 + H 2O + R1NHR 2 ↔ R1R 2NH+2 + HCO−3

4. CONCLUSION A series of robust and CO2-selective PEI@UiO-66 was synthesized and characterized. TGA characterizations indicated that PEI@UiO-66 possesses superior thermal stability until 500 K. Although the loading of PEI onto UiO-66 decreased the surface area of UiO-66 dramatically, it greatly enhanced the CO2 adsorption capacities and CO2/CH4 selectivity of PEI@UiO-66, and the optimized PEI loading was 30% with a BET surface area (SBET) of 37 m2/g. The enhancement of sorption temperature on CO2 adsorption capacity of 30PEI@UiO-66 followed the order of 338 K > 348 K > 328 K > 318 K > 308 K. At 338 K, the CO2 working capacity and CO2/CH4 selectivity of PEI@UiO-66 were up to 1.65 mmol/g and 111, respectively, being 12.7 and 58 times greater than those of the parent UiO-66, respectively. Water vapor in the feed stream significantly enhanced the CO2 working

(5)

First, CO2 and H2O may form bicarbonate, and then the resulting bicarbonate adsorbs on an amine group through hydrogen bonding. It was noteworthy that, in this case, only one amine group was required to react with one CO2 molecule, forming a more stable bicarbonate with the adsorption energy of 70−75 kJ mol−1. As a result, the presence of water vapor promoted more CO2 molecules adsorbed on PEI@UiO-66, compared to that in the absence of water vapor. When the RH value increased from 0% at 308 K to 55%, the CO2 working capacity and the CO2/CH4 selectivity of 30PEI@ UiO-66 increased (see Figures 7b and 8b). The enhancement could be attributed to the synergy action of water vapor and PEI on the surfaces of PEI@UiO-66 under wet conditions. 11156

DOI: 10.1021/acs.iecr.5b03517 Ind. Eng. Chem. Res. 2015, 54, 11151−11158

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Industrial & Engineering Chemistry Research

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capacity and CO2/CH4 dynamic selectivity of 30PEI@UiO-66, and the enhancement followed the order of 55% RH > 80% RH > 30% RH > 0% RH. At the RH of 55% and 338 K, the CO2 working capacity and CO2/CH4 selectivity of 30PEI@UiO-66 reached 2.41 mmol/g and 251, respectively, having increases of 48.8% and 126% in comparison with those under dry conditions. The DFT calculation suggested that the enhancement by the addition of water vapor could be attributed to the synergy action of water steam and PEI for bicarbonate formation on PEI@UiO66. The positive impact of water vapor is plausibile as water vapor is always present in realistic situations. In addition, the multiple consecutive adsorption−regeneration tests suggested that the CO2 working capacity of PEI@UiO-66 can be fully recovered after cyclic regeneration. PEI@UiO-66 was demonstrated as an effective and stable MOF-based sorbent for CO 2 /CH 4 separation.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03517. Synthesis of UiO-66 (section S1); fixed-bed adsorption experiments (section S2); calculation of working capacity (section S3); calculation of dynamic CO2/CH4 adsorption selectivity (section S4); the self-assembly fixed-bed experimental setup (Figure S1), FTIR spectra of PEI (Figure S2), PXRD patterns of UiO-66 and PEI@UiO-66s (Figure S3), CO2 adsorption isotherms of PEI@UiO-66s (surface area-basis) with different PEI loading at 298 K (Figure S4), fitted DSL isotherm curves of CO2 on UiO-66 and PEI@UiO-66s (Figure S5), breakthrough curves of CO2/CH4 over 30% PEI@UiO-66 from binary CO2/CH4 mixture (vCO2/vCH4 of 1/9) at 318 K (Figure S6), breakthrough curves of CO2/CH4 over 30% PEI@UiO66 from binary CO2/CH4 mixture (vCO2/vCH4 of 1/9) at 328 K (Figure S7), breakthrough curves of CO2/CH4 binary mixture (vCO2/vCH4 of 1/9) over UiO-66 at 308 K and under different relative humidities (Figure S8); parameters from the fitted DSL isotherm curves of CO2 on UiO-66 and PEI@UiO-66s (Table S1) (PDF)



AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-20-87113501. Fax: +86-20-87113513. E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We gratefully acknowledge the research grants provided by Science and Technology Program of Guangzhou (No. 201510010248), Research Foundation of State Key Lab of Subtropical Building Science of China (No. C713001z), Guangdong Natural Science Foundation (No. 2014A030312007), National Natural Science Foundation of China (No. 21576093), and Fundamental Research Funds for the Central Universities.



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DOI: 10.1021/acs.iecr.5b03517 Ind. Eng. Chem. Res. 2015, 54, 11151−11158

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published October 23, 2015, with an error in the Abstract. The corrected version was reposted October 27, 2015.

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DOI: 10.1021/acs.iecr.5b03517 Ind. Eng. Chem. Res. 2015, 54, 11151−11158