Removal of CO₂ from CH₄ and CO₂ capture in the presence of H₂O vapour in NOTT-401

Abstract

From a binary equimolar gas-mixture of CO₂ and CH₄, NOTT-401 exhibits CO₂ separation from CH₄. By kinetic uptake experiments, NOTT-401 shows a maximum of 1.47 wt% CO₂ capture at 30 °C and a significant 7-fold increase (∼9.90 wt%) in CO₂ capture under 40% relative humidity.

---

Introduction

The use of natural gas as fuel is indeed very advantageous for the environment and is also economically attractive. If natural gas is used as a vehicular fuel, the reductions in CO, SO₂ and CO₂ emissions will be 97, 90 and 24%, respectively, and there will be no release of lead in exhaust gases. In addition, it is cheaper than gasoline or diesel. To date, natural gas supplies one-fourth of the energy needed in homes, factories, business, vehicles, industries and power plants around the world. It is estimated that the consumption of natural gas will grow by 50% over the next 10 years.^1,2 The main constituent of natural gas is methane (CH₄), which has the highest hydrogen to carbon (H/C) ratio among all hydrocarbon fuels.³ However, the quality of natural gas, coming from land fields and biogas plants, is considerably low with impurities like CO₂ (20 to 35%), N₂, H₂O and H₂S.⁴ Then, pre-combustion CO₂ sequestration from natural gas is essential to maximize its energy content (CH₄). In addition, post-combustion CO₂ capture from plant flue gas is also crucial in order to control greenhouse emissions.⁵

CO₂ directly affects our environment by raising global temperature and causing acidification of the oceans.⁶ The main reason for the increasing CO₂ levels is the accelerating global energy demands.⁷ These energy requirements are expanding fast due to rapid growth in world population, increase in standards of living and the development of technologies leading to a doubling in the energy demand over the last three decades.⁸ Thus, CO₂ separation and capture have motivated many governments to invest in the development of new methods for efficiently and effectively capturing CO₂.⁹ Typical absorption in aqueous alkanolamine solutions has been widely used and studied, but it has many major limitations as an adsorbent for industrial CO₂ capture due to its heat instability and corrosion towards vessels and pipelines.¹⁰ Therefore, the use of porous solids for the adsorption of CO₂ is an appropriate research area and the search for materials with a high adsorption capacity, structural stability, fast sorption kinetics and mild regeneration properties, remains a major challenge.

Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) are among the most promising candidates for gas separation, because their sorption selectivity towards small molecule adsorbates is directly tunable as a function of the topology and chemical composition of the micropores.¹¹ Porous metal–organic materials showing high surface area and high pore volume normally show high CO₂ storage capacities at room temperature and relatively high pressures.¹² Although PCPs show high CO₂ capacity and selectivity, many gas separation processes involve the exposure to water vapor.

However, a small number of PCPs have shown good stability to water, and water is most often unfavorable to gas separations.¹³ Along with those few examples, Hong et al.¹⁴ reported a water-stable PCP based on a binuclear [In₂(μ₂-OH)] building block (see Scheme S1, ESI†), InOF-1, constructed from a flexible BPTC⁴⁻ ligand (H₄BPTC = biphenyl-3,3′,5,5′-tetracarboxylic acid) which also showed a high CO₂/N₂ and CO₂/CH₄ selectivities (by using the experimental single-component gas adsorption isotherms). The effect of water on the CO₂ capture has only recently been investigated on PCPs.¹⁵ Matzger and co-workers^15b studied the effect of humidity on the performance of M/DOBDC (M = Zn^II, Ni^II, Co^II or Mg^II) by collecting N₂/CO₂/H₂O breakthrough curves at different relative humidities. LeVan et al.¹⁶ found that a small amount of water did not decrease but may actually increase the CO₂ capacity of PCPs.

Interestingly, Llewellyn and co-workers¹⁷ investigated the CO₂ adsorption in some PCPs under different relative humidities. Indeed, HKUST-1 was shown to degrade in the presence of humidity, and UiO-66 did not show any enhanced CO₂ uptake.¹⁷ In the case of MIL-100(Fe), a remarkable 5-fold increase in CO₂ uptake was observed with increasing relative humidity (RH), 105 mg g⁻¹ at 40% RH. In addition, Yaghi et al.¹⁸ showed that the presence of hydroxyl functional groups increase the affinity of the framework for water. Thus, in the present work we have chosen a material NOTT-401¹⁹ (Fig. 1) based on a binuclear [Sc₂(μ₂-OH)] building block (see Scheme S1, ESI†) which is the same building block as for the water-stable InOF-1¹⁴ and possesses hydroxo functional groups (μ₂-OH) to study the separation of a binary gas mixture (not a single-component gas) of CO₂ and CH₄, and we have successfully performed CO₂ capture in the presence of water vapour.

Fig. 1 Space-filling view of the structure of NOTT-401 along the c-axis (scandium: green; sulphur: yellow; oxygen: red; carbon: grey; hydrogen: small grey).¹⁹

Experimental

Scandium triflate (0.057 g, 0.116 mmol) and thiophene-2,6-dicarboxylic acid, H₂TDA, (0.01 g, 0.058 mmol) were dispersed in THF (4.0 ml), DMF (3.0 ml), H₂O (1.0 ml) and HCl (36.5%, 2 drops) and sealed in a pressure tube. The clear solution was heated at 90 °C in an oil bath for 72 h. The tube was cooled to room temperature over a period of 12 h and the colorless crystalline product was separated by filtration, washed with DMF (5.00 ml) and dried in air. Yield: 71.1% (based on the ligand).

The uncoordinated solvent molecules in the pores of the as-synthesized NOTT-401 were exchanged with acetone and this promotes accessibility to the desolvated framework after activation by heating. Thus, thermogravimetric (see Fig. S1, ESI†) analysis and bulk powder X-ray diffraction patterns (see Fig. S2, ESI†) of the as-synthesised and desolvated NOTT-401 confirmed that the material consistently retains its structural integrity upon solvent removal. N₂ adsorption isotherms for activated NOTT-401¹⁹ at 77 K were used to calculate the BET surface area (0.01 < p/p_o < 0.04) of 1514 m² g⁻¹.

A catalytic reactor system (BEL-REA, BEL Japan; see Fig. S3, ESI†) was employed to evaluate the separation of CO₂ from CH₄. This catalytic reactor system allowed each sample of acetone-exchanged NOTT-401 to be activated (150 °C for 2 h) under a flow of N₂ gas and then directly exposed to adsorbates (CO₂ and CH₄) in situ, and studied by FTIR spectroscopy over many cycles without physical manipulation or exposure to air.

Kinetic uptake experiments were performed by using a thermobalance (Q500 HR, from TA) at different temperatures with a constant CO₂ flow (60 mL min⁻¹). Then, acetone-exchanged samples of NOTT-401 were placed in the thermobalance and activated by heating from room temperature to 150 °C for 2 h and under a flow of N₂ gas. After the activated sample was cooled down, the desired temperature was set and a constant CO₂ flow (60 mL min⁻¹) was started. With a humidity-controlled thermobalance (Q5000 SA, from TA) we carried out kinetic uptake experiments at 30 °C with a constant CO₂ flow (60 mL min⁻¹) on activated samples (150 °C for 2 h and under a flow of N₂ gas) of NOTT-401.

Results

An acetone-exchanged sample of NOTT-401 (40 mg) was packed into the holder sample in the BEL-REA system (see Fig. S3, ESI†) and activated as described before (vide supra). Then, the system was allowed to cool down to room temperature (30 °C) and the activated NOTT-401 sample was exposed to a flow of the binary equimolar (0.13 mmol min⁻¹) gas mixture of CO₂ and CH₄. This mixture corresponds to a more realistic composition in the field of gas-separation processes. Then, after stabilization of the gas flow within the sample, the resulting exhaust gases were analysed by FTIR spectroscopy (see Fig. S3, ESI†). Each FTIR spectrum was recorded every 40 seconds (∼0.66 min), until the detector was saturated, to obtain a total of 10 FTIR spectra (see Fig. S4, ESI†).

The most characteristic FTIR bands for the CO₂ and CH₄ molecules were at 2349 cm⁻¹ and 3016 cm⁻¹, respectively, Fig. 2. In both cases, it is possible to monitor a continuous increase in the characteristic band intensities (Fig. 2), for CO₂ and CH₄ in time. Therefore, from spectrum 1 to spectrum 10 the intensity of the characteristic FTIR band is increasing while the transmittance is decreasing.

Fig. 2 FTIR spectra of the resulting exit exhaust of the binary equimolar (0.13 mmol min⁻¹) gas-mixture of CO₂ and CH₄ of: (top); the characteristic CO₂ band and (bottom); the characteristic CH₄ band.

The intensities of these characteristic CO₂ and CH₄ FTIR bands were different. This suggests that the CO₂ and CH₄ molecules arrive at the FTIR detector at different times. By normalizing the intensities, considering their respective transmittances, it is possible to plot the increase in intensity of each scan for CO₂ and CH₄ simultaneously (Fig. 3, top). The normalized intensity, for each scan, of CH₄ is higher than CO₂, suggesting that the molecules of CH₄ effectively arrive at the FTIR detector before the CO₂ molecules. It is possible to rationalize this result as follows; when the binary gas mixture (CO₂ and CH₄) goes through the activated sample NOTT-401 this material retains CO₂ stronger than CH₄, and therefore, the CH₄ gas molecules flow ‘faster’ inside the material and are detected earlier. To verify this hypothesis, we carried out three more experiments: first, an acetone-exchanged sample of NOTT-401 (40 mg) was packed into the BEL-REA system, activated and stabilized as described (see below) and a flow of only CO₂ (0.13 mmol min⁻¹) was set. Then, the exhaust gas was analyzed by FTIR spectroscopy and 10 scans were collected, until the detector was saturated (see Fig. S5, ESI†).

Fig. 3 Normalized characteristic FTIR intensities of CO₂ and CH₄ as a function of the number of scans. (Top) FTIR intensities from a resulting exit exhaust of the binary equimolar (0.13 mmol min⁻¹) gas-mixture of CO₂ and CH₄; (bottom) FTIR intensities from individual flows of CO₂ and CH₄.

Second, another acetone-exchanged NOTT-401 (40 mg) sample was packed in the BEL-REA system, activated, stabilized and analyzed as described before. Then, the sample was exposed to a flow of only CH₄ gas (0.13 mmol min⁻¹). As in the previous experiment, 10 FTIR spectra were recorded from the exhaust gas until the detector was saturated (see Fig. S6, ESI†). Again, by normalizing the characteristic FTIR intensities it was possible to simultaneously plot the normalized intensity of each scan for CO₂ and CH₄ (Fig. 3, bottom).

The normalized intensities for CO₂ and CH₄ at each scan (from 1 to 10) were basically the same (Fig. 3, bottom) suggesting that when the resulting exhaust of each pure-gas component (not a gas mixture) is analyzed separately by FTIR spectroscopy, the molecules of CO₂ and CH₄ arrive at the same time at the FTIR detector. These results corroborate that NOTT-401 is more selective to CO₂ than CH₄ when a binary equimolar (0.13 mmol min⁻¹) gas-mixture of CO₂ and CH₄ travels within an activated sample. We interpreted this selectivity as the time delay of the CO₂ molecules in reaching the FTIR detector. By polynomial regressions of the normalized intensities on Fig. 3 (top), we estimated this delay to be ∼ 23 s (see Fig. S7 and S8, ESI†).

Finally a third experiment was carried out: in order to confirm that this delay was due to the adsorption selectivity shown by NOTT-401 (a microporous PCP) rather than any other phenomena, a non-porous material (PCM-14²⁰) was packed in the BEL-REA system. PCM-14 is a dense coordination polymer that has shown to be a non-porous material when it is activated between 25 and 150 °C. Then, a sample of PCM-14 (40 mg) was activated at 150 °C for 2 h under a flow of N₂ gas and then directly exposed to binary equimolar (0.13 mmol min⁻¹) CO₂ and CH₄ gas mixture. The resulting exhaust gas was analyzed by FTIR spectroscopy and just 6 scans were recorded, until the detector was saturated. By normalizing the characteristic FTIR intensities, we plotted the normalized intensities of each CO₂ and CH₄ scans (see Fig. S9, ESI†). Interestingly, the normalized intensities for CO₂ and CH₄ at each scan (from 1 to 6) were essentially the same (see Fig. S9, ESI†) corroborating that the time delay is due to the microporosity of NOTT-401.

Dynamic and isothermal CO₂ experiments were carried out on NOTT-401. Fig. 4, top, shows the kinetic uptake experiments from 30 °C to 100 °C. At 30 °C the material exhibited the maximum weight% gain, which represents the maximum amount of CO₂ captured. This amount corresponds to 1.47 wt% which was rapidly reached (constant uptake) after just ∼300 s (5 min) and it was constant until the end of the experiment (3600 s or 60 min). At 40 °C the uptake was estimated to be 1.16 wt% which was also reached after around 300 s (Fig. 4, top). Clearly, while the temperature is increased (from 30 to 100 °C), the CO₂ weight (%) gradually decreases (Fig. 4, top) to 0.09 wt% (at 100 °C) and interestingly, the kinetic uptake experiments did not show any difference in the equilibrium times. These equilibrium times have been previously observed in another microporous material NOTT-400.²¹

Fig. 4 (Top) Kinetic uptake experiments performed at different temperatures (30, 40, 50, 60, 70, 80, 90 and 100 °C) with a CO₂ flow of 60 mL min⁻¹; (bottom) PXRD patterns of each NOTT-401 samples after the kinetic CO₂ isotherms were carried out at different temperatures.

In order to corroborate that this decrease is not due to sample degradation, we have carried out PXRD experiments on each sample after these CO₂ capture experiments. Fig. 4 (bottom) confirms that the crystallinity of the samples after each CO₂ capture experiments is retained.

Encouraged by the promising results that Hong et al. reported,¹⁴ by showing a water stable framework (InOF-1) with the same binuclear [M₂(μ₂-OH)] building block to NOTT-401, we explored the water stability of NOTT-401. Then, acetone-exchanged samples of NOTT-401 were exposed to air and soaked in distilled water. PXRD patterns of these samples (see Fig. S10, ESI†) confirmed structural stability of NOTT-401 in water. In addition, we calculated the BET surface areas (by N₂ adsorption isotherms, 0.01 < p/p_o < 0.04) of samples NOTT-401 exposed to air and NOTT-401 soaked in distilled water of 1510 and 1516 m² g⁻¹, respectively. This demonstrates that NOTT-401 retains its surface area after water exposure.

This water-stability can be attributed to the presence of the hydroxo functional groups (inside the pores of NOTT-401) which has been previously shown¹⁹ and these functional groups, increase the affinity of the material for water. Indeed, Walton et al.²² proposed that the functional groups act as a directing agent for water in the pores, which allows for more efficient packing. Thus, after finding the best CO₂ capture temperature (30 °C, Fig. 4, top), under anhydrous conditions, a kinetic isotherm experiment at 30 °C, with a constant CO₂ flow, and a relative humidity (RH) of 40% was carried out. It was decided to run this experiment with a 40% RH based on the remarkable results that Llewellyn et al.¹⁷ had previously reported (5-fold increase in CO₂ uptake for MIL-100(Fe)). An activated NOTT-401 sample (150 °C for 2 h and under a flow of N₂ gas) was placed in a humidity-controlled thermobalance. After activation of the material, the equipment was stabilized at 40% RH (30 °C) and a constant CO₂ flow (60 mL min⁻¹) was started. Afterwards, we repeated this experimental procedure on a new activated NOTT-401 sample and set a constant N₂ flow (60 mL min⁻¹). Fig. 5 exhibits the kinetic uptake experiments at 30 °C and 40% RH for CO₂ and N₂. For both isotherms, it is clearly observed that the material shows a constant increase in weight (while the experiment is still continuing, see Fig. 5). This increase in weight is due to the contribution of H₂O and CO₂ or H₂O and N₂, respectively.

Fig. 5 Kinetic uptake experiments carried out at 30 °C and 40% RH with CO₂ (green line) and N₂ (blue line) flows of 60 mL min⁻¹, respectively.

In order to find the maximum CO₂ capture under 40% RH conditions, we need to differentiate the contribution of H₂O to the weight increase. By just taking the difference of the two isotherms (CO₂ and N₂) we could obtain the CO₂ capture at 40% RH. This is valid if the material does not capture any N₂ at 30 °C. Consequently, by performing a kinetic uptake experiment on a new activated NOTT-401 sample at 30 °C without the presence of H₂O vapor (0% RH) with a constant N₂ flow (60 mL min⁻¹) we obtained a N₂ capture of approximately 0.01 wt%. This result is consistent with previous reports where the capture capacity of N₂ in PCPs at room temperatures is basically negligible.²³ In Fig. 5 it is shown that the gradual weight increase (for CO₂/H₂O and N₂/H₂O) starts at 0 s and stabilises at ∼7000 s (117 min). In contrast, under anhydrous conditions the CO₂ uptake rapidly reaches stability (5 min, see Fig. 4, top). This equilibrium discrepancy is due to the nature of the vapour adsorption process that in general takes considerably more time to reach stability than the gas adsorption process in microporous materials.²⁴ Then, from 7000 s until approximately 8700 s (145 min) both isotherms seem to reach a plateau where both uptakes are practically constant (Fig. 5, red rectangle). At 8700 s, the maximum amounts of CO₂/H₂O and N₂/H₂O captured are 12.60 wt% and 2.70 wt%, respectively and by simply taking the difference of these two values (since there is no N₂ uptake at 30 °C) the CO₂ capture in the materials is ∼9.90 wt%. Finally, from 8700 s to 11 000 s (Fig. 5) the flow of each gas and the relative humidity were stopped and the decrease in weight represents the gas and water vapour desorption.

Therefore, the CO₂ capture was increased approximately 7-fold with a 40% RH. This increase in CO₂ uptake in the presence of water can be explained by CO₂ confinement effects induced by bulky molecules (H₂O).²⁵ In addition, we decided to carry a CO₂ experiment (60 mL min⁻¹) at 40% RH and 30 °C on an activated PCM-14²⁰ sample (150 °C for 2 h, under a flow of N₂ gas). Since PCM-14 is a non-porous coordination polymer, when activated between 25 and 150 °C, it offered a direct CO₂ capture comparison to NOTT-401 (microporous material). Thus, from 0 s to 11 000 s the maximum CO₂ uptake (under 40% RH) was 0.8 wt% (see Fig. S11, ESI†). This result corroborated that there is no CO₂ sequestration in a non-porous material when the relative humidity is 40% at 30 °C.

Conclusions

In conclusion, NOTT-401, a Sc(III) porous coordination polymer, exhibited CO₂ separation from CH₄, in a more realistic scenario, when this microporous material was exposed to a binary, CO₂/CH₄, equimolar gas-mixture. By running these equimolar (CO₂/CH₄) gas experiments on PCM-14, a non-porous coordination polymer, it was established that the intrinsic microporosity of NOTT-401 is responsible for the gas separation.

By kinetic isotherm experiments, NOTT-401 exhibits a total CO₂ amount of 1.47 wt% at 30 °C, which was rapidly reached after just approximately 300 s. While increasing the temperature of the kinetic isotherm experiments, the CO₂ capture capacity of NOTT-401 decreased considerably to less than 0.1 wt% at 100 °C. Remarkably, NOTT-401 exhibits high stability towards humidity, which was corroborated by PXRD. We attributed this water stability, as previously reported,^18,22 to the presence of hydroxo functional groups within the pores of NOTT-401. Due to this particularly high water stability, NOTT-401 performs CO₂ uptake under relative humidity conditions (40% RH) and 30 °C, displaying a maximum CO₂ capture of approximately 9.9 wt%.

Significantly, this CO₂ capture, under humid conditions, represents a 7-fold increase in comparison to anhydrous conditions. PCM-14 showed non CO₂ capture under RH conditions, suggesting that the microporosity provided by NOTT-401 is fundamental for this capture process. Since PCM-14 is a non-porous coordination polymer, when activated between 25 and 150 °C, the CO₂ confinement effects induced by H₂O²⁵ in porous materials cannot take place unlike NOTT-401 where these effects occur within the micropores.

Acknowledgements

The authors thank Dr A. Tejeda-Cruz (X-ray; IIM-UNAM), CONACyT Mexico (212318), PAPIIT UNAM Mexico (IN100415) for financial support. M. R. G. thanks CONICET and BECAR Argentina for scholarship funding. Thanks to U. Winnberg (ITAM) for scientific discussions.

Notes and references

1. F. W. Lipfert, Air Pollution and Community Health: A Critical Review and Data Source Book, Van Nostrand Reinhold, New York, USA, 1994, p. 556.
2. P. D. Noyes, M. K. McElwee, H. D. Miller, B. W. Clark, L. A. Van Tiem, K. C. Walcott, K. N. Erwin and E. D. Levin, Environ. Int., 2010, 35, 971.
3. T. Burchell and M. Rogers, SAE Tech. Pap. Ser., 2000, 2000.
4. M. Herout, J. Malat’ák, L. Kučera and T. Dlabaja, Res. Agric. Eng., 2011, 57, 137.
5. D. M. D'Alessandro, B. Smit and J. R. Long, Angew. Chem., Int. Ed., 2010, 49, 6058.
6. J. T. Litynski, S. M. Klara, H. G. McIlvried and R. D. Srivastava, Environ. Int., 2006, 32, 128.
7. M. Z. Jacobson, Energy Environ. Sci., 2009, 2, 148.
8. International Energy Agency (IEA), Key World Energy Statistics, OECD/IEA, France, 2013.
9. K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, Z. T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724.
10. (a) G. T. Rochelle, Science, 2009, 325, 1652; (b) F. Karadas, M. Atilhan and S. Aparicio, Energy Fuels, 2010, 24, 5817.
11. (a) S. Yang, G. S. B. Martin, G. J. J. Titman, A. J. Blake, D. R. Allan, N. R. Champness and M. Schröder, Inorg. Chem., 2011, 50, 9374; (b) A. J. Nuñez, L. N. Shear, N. Dahal, I. A. Ibarra, J. W. Yoon, Y. K. Hwang, J.-S. Chang and S. M. Humphrey, Chem. Commun., 2011, 47, 11855.
12. (a) H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. Ö. Yazaydin, R. Q. Snurr, M. O'Keeffe, J. Kim and O. M. Yaghi, Science, 2010, 329, 424; (b) A. M. Bohnsack, I. A. Ibarra, P. W. Hatfield, J. W. Yoon, Y. K. Hwang, J.-S. Chang and S. M. Humphrey, Chem. Commun., 2011, 47, 4899; (c) P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Nature, 2013, 495, 80.
13. (a) S. S. Nagarkar, A. K. Chaudhari and S. K. Ghosh, Inorg. Chem., 2012, 51, 572; (b) H. J. Choi, M. Dincă, A. Daily and J. R. Long, Energy Environ. Sci., 2010, 3, 117.
14. J. Qian, F. Jiang, D. Yuan, M. Wu, S. Zhang, L. Zhang and M. Hong, Chem. Commun., 2012, 48, 9696.
15. (a) J. Liu, A. I. Benin, A. M. B. Furtado, P. Jakubczak, R. R. Willis and M. D. LeVan, Langmuir, 2011, 27, 11451; (b) A. C. Kizzie, A. G. Wong-Foy and A. J. Matzger, Langmuir, 2011, 27, 6368; (c) H. Jasuja, Y.-g. Huang and K. S. Walton, Langmuir, 2012, 28, 16874; (d) H. Jasuja, J. Zang, D. S. Sholl and K. S. Walton, J. Phys. Chem. C, 2012, 116, 23526; (e) J. B. DeCoste, G. W. Peterson, H. Jasuja, T. G. Glover, Y.-g. Huang and K. S. Walton, J. Mater. Chem. A, 2013, 1, 5642.
16. J. Liu, Y. Wang, A. I. Benin, P. Jakubczak, R. R. Willis and M. D. LeVan, Langmuir, 2010, 26, 14301.
17. E. Soubeyrand-Lenoir, C. Vagner, J. W. Yoon, P. Bazin, F. Ragon, Y. K. Hwang, C. Serre, J.-S. Chang and P. L. Llewellyn, J. Am. Chem. Soc., 2012, 134, 10174.
18. H. Furukawa, F. Gándara, Y.-B. Zhang, J. Jiang, W. L. Queen, M. R. Hudson and O. M. Yaghi, J. Am. Chem. Soc., 2014, 136, 4369.
19. I. A. Ibarra, S. Yang, X. Lin, A. J. Blake, P. J. Rizkallan, H. Nowell, D. R. Allan, N. R. Champness, P. Hubberstey and M. Schröder, Chem. Commun., 2011, 47, 8304.
20. I. A. Ibarra, K. E. Tan, V. M. Lynch and S. M. Humphrey, Dalton Trans., 2012, 41, 3920.
21. M. R. Gonzalez, J. H. González-Estefan, H. A. Lara-García, P. Sánchez-Camacho, E. I. Basaldella, H. Pfeiffer and I. A. Ibarra, New J. Chem., 2015, 39, 2400.
22. G. E. Cmarik, M. Kim, S. M. Cohen and K. S. Walton, Langmuir, 2012, 28, 15613.
23. (a) E. Haldoupis, S. Nair and D. S. Sholl, J. Am. Chem. Soc., 2012, 134, 4313; (b) Y.-S. Bae, O. K. Farha, J. T. Hupp and R. Q. Snurr, J. Mater. Chem., 2009, 19, 2131.
24. I. P. O'koye, M. Benham and K. M. Thomas, Langmuir, 1997, 13, 4054.
25. N. L. Ho, F. Porcheron and R. J.-M. Pellenq, Langmuir, 2010, 26, 13287.

---

Footnote

† Electronic supplementary information (ESI) available: TGA data, PXRDP data, FTIR data, polynomial regressions and kinetic uptake experiments. See DOI: 10.1039/c5qi00049a

---