Nitrogen-Doped Nanoporous Carbons for Selective Separation of Ar

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Nitrogen-Doped Nanoporous Carbons for Selective Separation of Ar/Kr/Xe/Rn Gases: an Experiment-Based Simulation Study Fei Chen, Xiaofei Zeng, and Dapeng Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04213 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Physical Chemistry

Nitrogen-Doped Nanoporous Carbons for Selective Separation of Ar/Kr/Xe/Rn Gases: An Experiment-Based Simulation Study Fei Chen1,ǂ, Xiaofei Zeng 1,ǂ, Dapeng Cao1, 2,* 1

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China 2

Beijing Advanced Innovation Center for Soft Matter Science and Engineering,

Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

Abstract It is still a challenge to find adsorbents with high efficiency for the separation of noble gases. In this work, we combine the grand canonical Monte Carlo (GCMC) simulation and adsorption integral equation to theoretically characterize the pore size distribution (PSD) of experimentally synthesized nitrogen-doped nanoporous carbon (Carbon-ZX), and further predict the selectivity of Carbon-ZX for Xe/Kr, Xe/Ar and Rn/N2 mixtures. Results indicate that the selectivities of Carbon-ZX for Xe/Kr and Xe/Ar are apparently higher than that of other MOFs in the same conditions, which also is further confirmed by Henry's constant and isosteric adsorption heat. Moreover, the Carbon-ZX for the Rn/N2 binary mixture shows the extremely high selectivity (about 800~ 1200) in the molar fraction XRn Xe (27.82 mol∙kg-1∙bar-1) > Kr (1.83 mol∙kg-1∙bar-1) > Ar (0.43 mol∙kg-1∙bar-1), which means the Crarbon-ZX shows the strongest affinity for Rn. Moreover, this order is in entire agreement with that of the Lennard-Jones force and the polarizability of the gases. In addition, Figure 4b shows the isosteric heats of noble gases calculated by Equal (3). As expected, the Qst values follow the sequence Rn > Xe > Kr > Ar, which is also consistent with the order of preferential adsorption as the uptakes presented in Figure 4a and the Henry’s constants. To explore the effect of temperature on the adsorption behavior of Ar, Kr, Xe and Rn, Figure S4 shows adsorption isotherms of four gases at different temperatures, in which adsorption amounts of four gases significantly increase with the decrease of system temperature, especially for Ar and Kr. In particular, adsorption amounts of Ar and Kr at 1 bar (T = 198 K) increase to ninefold and fivefold, respectively, compared with the case of 11

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normal condition. Correspondingly, with decrease of temperature, the shape of Kr and Ar isotherms is not linear any more. In order to intuitively observe the relationship between the uptakes of pure component and the temperature, the calculated adsorption amounts at 1 bar and five different temperatures are shown in Figure S5. The slopes of Ar, Kr and Xe perform very sharp at cryogenic temperatures. However, with the increase of temperature, the slope of Ar gradually becomes mild. Nevertheless, for Rn, variation tendency of the slope is completely opposite to that of Ar. That is to say, at first stage (approximately 200 ~ 220 K) the slope of Rn has a slight change, but when the temperature exceeds about 220 K, that of Rn sharply gets down. 3.3 Separation of Xe/Kr, Xe/Ar and Rn/N2 mixtures The obvious difference of the adsorption behavior of Xe, Kr and Ar motivates us to further explore the separation performance of the Carbon-ZX for noble gas mixtures. As well known, the concentrations of Xe and Kr are extremely low in the air. Consequently, the selectivity calculation of Xe/Kr (20/80) mixture was performed in extremely low pressure (~20 kPa) at normal temperature, and shown in Figure 5a, in which the selectivity of the MOFs were calculated with the IAST method of Myers and Prausnitz.57 With the increase of pressure, the selectivity of Carbon-ZX for Xe/Kr mixture shows a slight elevation in P< 3k Pa (S = 19.9 at 3 kPa), and then gradually declines to ~14 until 20 kPa. Nevertheless, the selectivity of Carbon-ZX for Xe/Kr mixture remains at a very high level, compared to other MOF materials. The separation performance of Xe/Kr mixture is Carbon-ZX > CoFormate > Ag-NiMOF-74 > NiMOF-74 > CuBTC > IRMOF-1 > FMOFCu. Besides, Figure 5b also shows adsorption amount of Xe in Carbon-ZX for the 12


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20/80 Xe/Kr mixture, in which these of MOF materials from the literature were also presented for comparison 58. Interestingly, the advantage of Carbon-ZX for Xe adsorption is amplified significantly and with the increase of pressure, the adsorption capacity is almost linear growth. The Xe uptake follows a similar order: Carbon-ZX > Ag-NiMOF-74 > CoFormate > NiMOF-74 > CuBTC > IRMOF-1 > FMOFCu. For convenience, Figure 5c presents the selectivity of adsorbents versus Xe uptake at 5kPa total pressure. As well known, an excellent adsorbent possesses not only high Xe uptake but also high selectivity. Obviously, nitrogen-doped porous Carbon-ZX holds the two features mentioned above, which means that the Carbon-ZX performs absolute predominance on both adsorption selectivity and Xe uptake for Xe/Kr (20/80) mixture, compared with the other MOFs materials. Figure 6 shows that the Xe selectivity of different adsorbents for Xe/Ar (10/90) mixture at normal condition, in which the nitrogen-doped porous Carbon-ZX still presents an apparent advantage, compared to other MOFs59. The selectivity follows the order: Carbon-ZX > BiMOF-11 > CuBTC > ZIF-3 > ZIF-10 > IRMOF-1. It is found that the adsorption selectivity is greatly enhanced when Xe is mixed with smaller atoms (Ar), compared to the larger atoms (Kr) (see Figure S6). For example, the selectivity of Carbon-ZX for Xe/Ar can reach ~82.5 at 5 kPa, but it is ~19 for Xe/Kr, which means that the larger deviations in both gas molecule size and the affinity of adsorbent for the gas are more beneficial for separation in the case where the pore size is larger than any components of the chosen molecules; while it is more difficult to separate these gas molecules with almost same size and property. 13



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As mentioned in Introduction, how to find an effective adsorbent to remove Rn is also important issue. Because of the radioactivity of Rn, it is very difficult to adsorb and separate it experimentally.21 Fortunately, in this regard, GCMC can provide a reliable prediction on the separation of Rn. Considering the fact that the concentration of Rn is very low in the air, we consider the following seven Rn/N2 mixture cases, in which the molar fraction of Rn is 0.1, 0.05, 0.01, 0.005, 0.001, 0.0005 and 0.0001. Figure 7a shows the selectivity of Carbon-ZX for Rn/N2 in seven cases mentioned.

The selectivity of

Carbon-ZX for Rn maintains an extremely high level (S = 800~1200) in pressure p40 kPa, the selectivity of Carbon-ZX for Rn is closely related to molar fraction of Rn, and when molar fraction of Rn is less than 0.001, the selectivity of Carbon-ZX for Rn is still in the range of 800~1200. In order to clearly show the influence of concentration of Rn on the separation performance of adsorbents, the selectivity of the Carbon-ZX for Rn/N2 mixtures at 1 bar, as a function of the concentration of Rn in the bulk phase, was shown in Figure 7b. Apparently, the relationship between the molar composition and the selectivity is not linear, but presents an inverted U-shape, which means there exists an optimal value. Briefly, when the molar fraction of Rn is about 0.0005, the Carbon-ZX shows higher selectivity, which means that Carbon-ZX can definitely contribute to purifying air at home and detecting the existence of indoor radioactive Rn. 4. Conclusions In summary, we have used the grand canonical Monte Carlo (GCMC) simulation to calculate adsorption isotherms of noble gases in a series of modeled slit pores, and 14


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combined adsorption integral equation to computationally characterize the pore size distribution (PSD) of experimentally synthesized nitrogen-doped nanoporous Carbon-ZX by minimizing the calculated adsorption capacity and experimental one. On the basis of the PSD, we further predict the selectivity of Carbon-ZX for Xe/Kr, Xe/Ar and Rn/N2 mixtures. Results indicate that the selectivities of Carbon-ZX for Xe/Kr and Xe/Ar are apparently higher than that of other MOFs in the same conditions, which also is further confirmed by Henry's constant and isosteric adsorption heat. Moreover, the Carbon-ZX for the Rn/N2 binary mixture shows the extremely high selectivity (about 800~ 1200) in the molar fraction XRn

Nitrogen-Doped Nanoporous Carbons for Selective Separation of Ar

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