High adsorption performance polymers modified by small molecules containing functional groups for CO₂ separation

Abstract

We proposed the novel idea of using small molecules with functional groups to modify the functional groups and molecular aggregation of a polymer simultaneously. Small molecular amine-modified PVAm samples were prepared as novel adsorbents that have high CO₂ adsorption performance and good stability.

---

Global energy and environmental problems create an urgent need for new functional materials to be used in areas such as energy storage and conversion, catalysis and gas separation.^1,2 Much attention has been paid to CO₂ separation processes, such as CO₂ separation from flue gas (mainly N₂) for environmental remediation^2–5 and CO₂ separation from natural gas (mainly CH₄) and synthesis gas (mainly H₂) for clean energy supply.⁶ In processes such as pressure swing adsorption (PSA) for CO₂ separation, adsorbents such as metal–organic frameworks and molecular sieves have been greatly developed.^7–10 These adsorbents are adequate but difficult to regenerate and be desorbed without significant heating, entailing low productivity and high costs.⁸ And thus the CO₂ adsorption capacity of the adsorbents decreases with time, which will result in a large decrease of the CO₂ selectivity. For CO₂ separation, polymers are one of the most attractive materials, exhibiting fast CO₂ adsorption and desorption and low energy requirements for recycling.^7,11–16 During the past ten years, polyvinylamine (PVAm) has been developed greatly as a potential polymer for CO₂ separation.^17–20 CO₂ separation is estimated to consume more than 30% of the power of a plant using the presently available polymer adsorption technologies, far above the theoretical minimum work of separation because the CO₂ selectivity and adsorption capacity of the polymer is not high.^14,15 The CO₂ adsorption performance of the polymer depends on two aspects of the polymer, namely functional groups and molecular aggregation. Functional groups such as amine groups¹⁹ and carboxylate groups²¹ can make a chemical interaction with CO₂ and metal ions can make a physical interaction with CO₂,²² which are beneficial for CO₂ adsorption. However, a large increase in the amount of functional groups will result in a great increase in the interactions between the functional groups, which will result in a decrease of the CO₂ adsorption capacity and selectivity. The densified molecular aggregation of the polymer will increase the CO₂ selectivity due to CO₂ having a low kinetic diameter and high condensability, which also results in a decrease of the CO₂ adsorption capacity of the polymer. In recent studies, to increase the CO₂ selectivity of the polymer, only the molecular aggregation of the polymer was densified by heat treatment,²³ chemical cross-linking²⁴ and hydrogen bond cross-linking.²⁵ However, densified molecular aggregation of the polymer will decrease CO₂ adsorption capacity.^23–25 Furthermore, for some polymers, the methods used to densify the polymer will decrease the concentration of original functional groups such as amine groups used to adsorb the CO₂, which will make the CO₂ adsorption capacity of the polymer decrease significantly.^23,25 To greatly increase the CO₂ adsorption capacity and selectivity of the polymer simultaneously, we proposed the novel idea of using small molecules with functional groups to modify the functional groups and molecular aggregation of the polymer simultaneously. To confirm this idea, a typical polymer material PVAm was modified by small molecules with amine groups (ethanediamine (EDA), piperazine (PIP), monoethanolamine (MEA) and methylcarbamate (MC)).

The specific chemical materials of the experiments are described in the Supplementary Information S1, ESI.† Small molecular amine (SMA)-modified PVAm was prepared by mixing 3 wt% PVAm aqueous solutions and SMA, wherein the molar ratio of amine groups contained in SMA to that contained in PVAm is 3. Then the mixtures were stirred for 12 h and stood for 24 h at room temperature (22 °C).

The pure PVAm or SMA-modified PVAm samples were obtained by dropping the aqueous solutions of the pure PVAm or SMA-modified PVAm on silicone rubber substrate, then drying for at least 24 h at 30 °C and 40% relative humidity in an artificial climate chamber (climacell 222R, Germany), and finally peeling from the silicone rubber substrate.

The pure PVAm or SMA-modified PVAm samples were characterized by an attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectrometer (FTS-6000, Bio-Rad of America), volume measurement, mass stability measurement, an elemental analyzer (Carlo Erba EA1110, Italy), X-ray diffractometer (XRD) (D/MAX—2500) and Brunauer–Emmett–Teller measurement.

The chemical characterization of the SMA-modified PVAm samples was accomplished by ATR-FTIR. According to analysis of the ATR-FTIR spectra (see the Supplementary Information S2, ESI†), it can be concluded that intermolecular hydrogen bonds formed between SMA and PVAm in the SMA-modified PVAm samples. The intermolecular hydrogen bonds can be summarized as in the schematic diagram Fig. 1.

Fig. 1 Schematic diagram of the intermolecular hydrogen bonds in the SMA-modified PVAm samples: (1) EDA-modified PVAm; (2) PIP-modified PVAm; (3) MEA-modified PVAm; (4) MC-modified PVAm.

With the same amount of PVAm, the volume of the pure PVAm and SMA-modified PVAm samples tested is almost the same due to SMA having low molecular weights and intermolecular hydrogen bonds between SMA and PVAm. Therefore, compared with the pure PVAm sample, the molecular aggregation of the SMA-modified PVAm sample was densified.

In the PSA process, the adsorbents may be lost under pressure alteration during the adsorption and desorption processes.²⁶ For investigating the mass stability of the SMA-modified PVAm, the mass of the SMA-modified PVAm samples treated under different pressure alterations was tested at 50 °C (the specific treatment of the sample is described in the supplementary information S3, ESI†).

The mass of the SMA-modified PVAm samples under various pressure alterations can be considered as the same in the error range of the analytical balance measurement. After treatment under pressure, the SMA-modified PVAm samples were investigated by ATR-FTIR again. According to a comparison of the ATR-FTIR spectra before and after treatment, it was found that the intermolecular hydrogen bonds formed between SMA and PVAm had not changed. Therefore, it can be deduced that under pressure almost all the molecules of the SMAs were fixed stably in the SMA-modified PVAm by intermolecular hydrogen bonds, which will result in a great increase of CO₂ selectivity.

The mass ratio of C : N in the pure PVAm and SMA-modified PVAm samples was measured by an elemental analyzer. Table 1 shows the amine group concentration of the pure PVAm sample, which was calculated according to the molar mass of PVAm and the mass ratio of C : N in the pure PVAm sample. The molar ratio of SMA : PVAm (m_SMA : m_PVAm) was calculated according to the mass ratio of C : N. Table 1 also shows the amine group concentration of the SMA-modified PVAm, which was calculated according to the m_SMA : m_PVAm in the SMA-modified PVAm samples, the molar mass of SMA and the amine group concentration of the pure PVAm sample. The primary or secondary amine groups contained in the SMA-modified PVAm samples can react with CO₂ as shown in the following formulas:^27,28

CO₂ + RR′NH ⇌ RR′NH⁺COO⁻ (1)

RR′NH⁺COO⁻ + RR′NH ⇌ RR′NCOO⁻ + RR′NH₂⁺ (2)

RR′NH⁺COO⁻ + H₂O ⇌ RR′NCOO⁻ + H₃O⁺ (3)

RR′NCOO⁻ + H₂O ⇌ RR′NH + HCO₃⁻ (4)

Table 1 The amine group concentration and crystallinity of the pure PVAm and SMA-modified PVAm samples

	The amine group concentration (10^−2 mol carriers/g PVAm)	Crystallinity
Pure PVAm	1.110	0.380
EDA-modified PVAm	1.721	0.451
PIP-modified PVAm	4.200	0.379
MEA-modified PVAm	4.230	0.378
MC-modified PVAm	4.330	0.378

---

Where R′ may be an H or another group. The reaction between CO₂ and amine carriers is defined by the zwitterion mechanism in formulas (1) to (4). Firstly, CO₂ reacts with primary or secondary amines to form zwitterions as an intermediate. Then, the zwitterions are deprotonated by amine or H₂O to form the carbamate ions. The carbamate ions of the amine carrier are unstable and could react with H₂O to form bicarbonate ions. From the formulas of (1) to (4), the amine carrier can react with CO₂ to form zwitterions, protonated-amines, carbamate ions and bicarbonate ions. The amine carriers can adsorb CO₂ molecules by the CO₂-carrier complex forms of carbamate and bicarbonate. And the reactions between amine groups and CO₂ are reversible.

Compared with the pure PVAm sample, the concentration of amine groups in the SMA-modified PVAm samples has been greatly improved, which will result in great increases in the CO₂ adsorption capacity and selectivity simultaneously. The amine group concentrations of the PIP-, MEA- or MC-modified PVAm samples are higher than that of the EDA-modified PVAm sample due to EDA having a volatile structure.

The crystallinity has an effect on the selectivity and adsorption capacity of CO₂ because the crystalline region is not beneficial for CO₂ adsorption.^29,30 The crystallinity of the pure PVAm and SMA-modified PVAm samples was investigated by using an XRD in reflection mode with 2θ scanned between 5° and 80° under an 8 kW power. Table 1 shows the crystallinity of the SMA-modified PVAm samples. As shown in Table 1, the EDA-modified PVAm sample shows the highest crystallinity due to EDA having a regular and symmetric structure. Compared with the pure PVAm sample, the crystallinity of the PIP-, MEA- or MC-modified PVAm samples does not increase due to PIP, MEA or MC having an asymmetric structure.

In addition, though the amine group concentration of the PIP-, MEA- or MC-modified PVAm samples is almost the same, the reaction ability of the amine groups in the PIP-, MEA- or MC-modified PVAm samples is different. The reaction of the amine group with CO₂ is a reaction gaining protons. The ability to gain protons depends on the electronegativity of the nitrogen atom of the amine groups.³¹ The electronegativity of the nitrogen atom included in MEA or MC with polar groups is higher than that of the nitrogen included in PIP. Therefore, the reaction activity of amine groups in the MEA- or MC-modified PVAm with CO₂ is higher than that in the PIP-modified PVAm.

The CO₂, N₂, CH₄ and H₂ adsorption capacities of the pure PVAm and SMA-modified PVAm samples were characterized by Brunauer–Emmett–Teller measurement using the single component of CO₂, N₂, CH₄ and H₂ gases at low pressure region (below 1 bar). The ideal adsorption solution theory (IAST) of Myers and Prausnitz has been reported to predict binary gas mixture adsorption.^32–34 To judge the merit of the SMA-modified PVAm samples for CO₂/N₂, CO₂/CH₄ and CO₂/H₂ separation, the CO₂/N₂, CO₂/CH₄ and CO₂/H₂ selectivities were predicted by IAST. The adsorption selectivity [defined as S_ads = (q₁/q₂)/(P₁/P₂), where q_i is the amount of i adsorbed and P_i is the partial pressure of i in the mixture] of the pure PVAm and SMA-modified PVAm samples for CO₂ over N₂ in flue gas (typically 15% CO₂ and 85% N₂), CO₂ over CH₄ in natural gas (typically 10% CO₂ and 90% CH₄) and CO₂ over H₂ in synthesis gas (typically 40% CO₂ and 60% H₂) were estimated from the experimental single-component isotherms. As shown in Fig. 2 and 3, the SMA-modified PVAm samples not only displayed high CO₂ adsorption capacities, but also displayed high CO₂/N₂, CO₂/CH₄ and CO₂/H₂ selectivity. Because CO₂ can react with amine groups contained in the SMA-modified PVAm samples, the CO₂ adsorption capacity is much higher than the N₂, CH₄ and H₂ adsorption capacities. As shown in Fig. 3, compared with the pure PVAm sample, the SMA-modified PVAm samples showed higher CO₂ adsorption capacity and selectivity at various pressures. Compared with EDA- or PIP-modified PVAm samples, MEA- or MC-modified PVAm samples showed much higher CO₂ adsorption capacity and selectivity due to MEA or MC having nonvolatile and asymmetric structures and containing amine groups with strong electronegativity. And thus SMAs such as MEA and MC are the most suitable to modify PVAm. The CO₂ adsorption performance of the MC-modified PVAm sample is slightly higher than that of the MEA-modified PVAm sample due to the MC-modified PVAm sample having a higher amine group concentration.

Fig. 2 CO₂ adsorption–desorption experiments at 50 °C. Filled and open symbols represent adsorption and desorption branches, respectively.

Fig. 3 IAST-predicted CO₂ adsorption selectivity of the pure PVAm and SMA-modified PVAm samples, (a) CO₂/N₂, (2) CO₂/CH₄, (3) CO₂/H₂.

Furthermore, because the MC-modified PVAm sample shows the highest CO₂ adsorption selectivity, according to the literature,^35,36 the recovery and purity of the product was calculated. This could be achieved by using the MC-modified PVAm sample in the two-stage adsorption process with a recycle stream for the syngas and natural gas purification and the CO₂ capture from flue gas. The separation targets of H₂ recovery of 99.1% with H₂ purity of 99.9% in the syngas purification, CH₄ recovery of 99.8% with CH₄ purity of 98% in the natural gas purification and CO₂ recovery of 95% with CO₂ purity of 96.3% in the flue gas CO₂ capture can be achieved. The results indicate that the MC-modified PVAm sample shows promising application in CO₂ separation processes.

Conclusions

It is suggested that polymers modified by small molecules with other functional groups besides amine groups, such as carboxylate groups and metal ions, also can increase the functional group concentration and densify the molecular aggregation of the polymer simultaneously. In conclusion, it has been demonstrated that using small molecules with functional groups to modify the polymer can greatly increase CO₂ adsorption capacity and selectivity. Also, the SMA-modified PVAm has good stability. Polymers modified by small molecules with functional groups can be applied as new functional materials in CO₂ separation. Moreover, like CO₂, H₂S and SO₂ are both acidic gases, which can make interaction with functional groups contained in small molecules used to modify polymers. Therefore, to get high H₂S and SO₂ adsorption capacity and selectivity, polymers modified by small molecules with functional groups could be applied in the adsorption process for H₂S and SO₂ separation from gas mixtures with N₂, CH₄ and H₂. In addition, small molecules with functional groups might be used to modify other gas separation materials such as metal–organic frameworks and molecular sieves to increase the gas adsorption capacity and selectivity.

Acknowledgements

This research is supported by the Natural Science Foundation of China (No. 20836006), the National Basic Research Program (No. 2009CB623405), the National High-tech Research and Development Project (No. 2012AA03A611), the Science & Technology Pillar Program of Tianjin (No. 10ZCKFSH01700), the Programme of Introducing Talents of Discipline to Universities (No. B06006), and the Cheung Kong Scholar Program for Innovative Teams of the Ministry of Education (No. IRT0641).

References

1. (a) R. E. Morris and P. S. Wheatley, Angew. Chem., Int. Ed., 2008, 47, 4966–4981; (b) C. Marichy, M. Bechelany and N. Pinna, Adv. Mater., 2012, 24, 1017–1032.
2. F. M. Orr, Energy Environ. Sci., 2009, 2, 449–458.
3. G. P. Hao, W. C. Li, D. Qian and A. H. Lu, Adv. Mater., 2010, 22, 853–857.
4. Q. Wang, J. Z. Luo, Z. Y. Zhong and A. Borgna, Energy Environ. Sci., 2011, 4, 42–55.
5. N. MacDowell, N. Florin, A. Buchard, J. Hallett, A. Galindo, G. Jackson, C. S. Adjiman, C. K. Willians, N. Shah and P. Fennell, Energy Environ. Sci., 2010, 3, 1645–1669.
6. (a) N. P. Patel, A. C. Miller and R. J. Spontak, Adv. Mater., 2003, 15, 729–733; (b) S. Li, J. L. Falconer and R. D. Noble, Adv. Mater., 2006, 18, 2601–2603; (c) M. A. Carreon, S. Li, J. L. Falconer and R. D. Noble, Adv. Mater., 2008, 20, 729–732.
7. R. Banerjee, H. Furukawa, D. Britt, C. Knobler, M. O'Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 3875–3877.
8. J. An and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 5578–5579.
9. Z. R. Herm, J. A. Swisher, B. Smit, R. Krishna and J. R. Long, J. Am. Chem. Soc., 2011, 133, 5664–5667.
10. S. Couck, J. F. M. Denayer, G. A. Baron, T. Remy, J. Gascon and F. Kapteijn, J. Am. Chem. Soc., 2009, 131, 6326–6327.
11. B. S. Ghanem, N. B. Mckeown, P. M. Budd, J. D. Selbie and D. Fritsch, Adv. Mater., 2008, 20, 2766–2771.
12. H. P. Brack, C. Padeste, M. Slaski, S. Alkan and H. H. Solak, J. Am. Chem. Soc., 2004, 126, 1004–1005.
13. E. D. Bates, R. D. Mayton, I. Nati, Jr and J. H. Davis, J. Am. Chem. Soc., 2002, 124, 926–927.
14. C. M. White, B. R. Strazisar, E. J. Granite, J. S. Hoffman and H. W. Pennline, J. Air Waste Manage. Assoc., 2003, 53, 645–715.
15. D. M. D. Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 6058–6082.
16. H. S. Choi and M. P. Suh, Angew. Chem., Int. Ed., 2009, 48, 6865–6869.
17. P. Bernardo, E. Drioli and G. Golemme, Ind. Eng. Chem. Res., 2009, 48, 4638–4663.
18. T. J. Kim, B. Li and M. B. Hägg, J. Polym. Sci., Part B: Polym. Phys., 2004, 43(23), 4326.
19. Z. Qizo, Z. Wang, C. Zhang, S. Yuan, Y. Zhu, J. Wang and S. Wang, AIChE Journal, 2012 DOI:10.1002/aic.13781.
20. http://www.kema.com/news/articles/2011/inauguration-nanoglowa.aspx.
21. M. Wang, Z. Wang, J. Wang, Y. Zhu and S. Wang, Energy Environ. Sci., 2011, 4, 3955–3959.
22. K. Yao, Z. Wang, J. Wang and S. Wang, Chem. Commun., 2012, 48, 1766–1768.
23. I. Mochida, S. Yatsunami, Y. Kawabachi and Y. Nakayama, Carbon, 1995, 33, 1611–1619.
24. W. J. Koros and R. Mahajan, J. Membr. Sci., 2000, 175, 181–196.
25. N. B. MaKeown and P. M. Budd, Macromolecules, 2010, 43, 5163–5176.
26. P. J. E. Harlick and A. Sayari, Ind. Eng. Chem. Res., 2006, 45, 3248–3255.
27. Y. Cai, Z. Wang, C. H. Yi, Y. H. Bai, J. X. Wang and S. C. Wang, J. Membr. Sci., 2008, 310, 184–196.
28. H. Matsuyama, A. Terada and T. Nakagawara, J. Membr. Sci., 1999, 163, 221–227.
29. K. Binder, J. Baschnagle, M. Muller, W. Paul and F. Rampf, Macromol. Symp., 2006, 237, 128–138.
30. Y. H. Jhon, M. Cho, H. R. Jeon, I. Park, R. Chang, J. L. C. Rowsell and J. Kim, J. Phys. Chem. C, 2007, 111, 16618–16625.
31. (a) F. A. Carey, Organic Chemistry (4th edition). McGraw-Hill Higher Education, New York: 2000, 86[4]; (b) F. A. Carey, Organic Chemistry (4th edition). McGraw-Hill Higher Education, New York: 2000, 863–864.
32. W. G. Lu, D. Q. Yuan, J. Sculley, D. Zhao, R. Krishna and H. C. Zhou, J. Am. Chem. Soc., 2011, 133, 18126–18129.
33. W. G. Lu, J. Sculley, D. Q. Yuan, R. Krishna, Z. G. Wei and H. C. Zhou, Angew. Chem., Int. Ed., 2012, 51, 7480–7484.
34. J. A. Mason, K. Sumida, Z. R. Herm, R. Krishna and J. R. Long, Energy Environ. Sci., 2011, 4, 3030–3040.
35. D. X. Yang, Z. Wang, J. X. Wang and S. C. Wang, Ind. Eng. Chem. Res., 2009, 48, 9013–9022.
36. L. Zhao, R. Menzer, E. Riensche, L. Blum and D. Stolten, Energy Procedia. GHGT 9., 2009, 1, 269–278.

---

Footnote

† Electronic supplementary information (ESI) available: Supplementary Information including the S1 (the intermolecular hydrogen bonds in the SMA-modified PVAm). See DOI: 10.1039/c2ra21769d

---