Diffusion of Methane and Carbon Dioxide in Carbon Molecular Sieve

Jun 13, 2012 - Carbon molecular sieve (CMS) membranes are promising materials for energy efficient separations of light gases. In this work, we report...
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Diffusion of Methane and Carbon Dioxide in Carbon Molecular Sieve Membranes by Multinuclear Pulsed Field Gradient NMR Robert Mueller,† Rohit Kanungo,† Mayumi Kiyono-Shimobe,‡ William J. Koros,‡ and Sergey Vasenkov*,† †

Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

‡

ABSTRACT: Carbon molecular sieve (CMS) membranes are promising materials for energy efficient separations of light gases. In this work, we report a detailed microscopic study of carbon dioxide and methane self-diffusion in three CMS membrane derived from 6FDA/BPDA(1:1)-DAM and Matrimid polymers. In addition to diffusion of one-component sorbates, diffusion of a carbon dioxide/methane mixture was investigated. Self-diffusion studies were performed by the multinuclear (i.e., 1H and 13C) pulsed field gradient (PFG) NMR technique which combines the advantages of high field (17.6 T) NMR and high magnetic field gradients (up to 30 T/m). Diffusion measurements were carried out at different temperatures and for a broad range of the root-meansquare displacements of gas molecules inside the membranes. The diffusion data obtained from PFG NMR are compared with the corresponding results of membrane permeation measurements reported previously for the same membrane types. The observed differences between the transport diffusivities and self-diffusion coefficients of carbon dioxide and methane are discussed.

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permselectivity of ∼30 for both membranes.7 In addition, the selectivity of CMS membranes is not reduced due to plasticization as commonly found in polymeric membranes with CO2 streams at moderate pressure.2,4,11 In fact, the achievable selectivity of CMS membranes for difficult separations such as CO2/CH4 approaches those of zeolite and metal−organic framework (MOF) membranes.3,12 At the same time, costs of large-scale fabrication of defect-free CMS membranes are expected to be much lower than those of zeolite and MOF membranes. CMS membranes are also less brittle than most zeolite and MOF membranes. Recent breakthroughs in CMS synthesis have demonstrated that pore size, and hence CMS permselectivity and permeability, may be fine-tuned by carefully controlling pyrolysis temperature and soak time5,6,8,9 in addition to the oxygen concentration in the pyrolysis atmosphere.1,13,14 Despite the intense efforts devoted to synthesis and application of CMS membranes, it appears that until now there have been no published reports from other groups on detailed studies of diffusion of light gases in such membranes by microscopic techniques on the submicrometer and micrometer length scales of molecular displacements. Previous investigations of gas transport in CMS membranes3,6−9,15,16 have been conducted using nonequilibrium macroscopic methods under conditions of a macroscopic gradient of gas concentration across a membrane. One widely used macroscopic approach is based on measurement of sorbate permeation (i.e., mass flux)

INTRODUCTION Refinement of raw natural gas streams and biogas streams is a growing challenge using today’s gas separation infrastructure which heavily relies upon energy-intensive processes (e.g., cryogenics and absorption).1−3 Natural gas streams usually contain a high abundance of impurities which include N2, CO2, SO2, and H2S. These impurities must be removed for end use. One of the most important natural gas separations is the separation of CO2 from CH4. The former gas typically represents the most abundant impurity, while the latter is the revenue product. Membrane technology is expected to play an important role in the development of energy-efficient strategies for separations of light gas mixtures including CO2/CH4.2−4 In particular, carbon molecular sieve (CMS) membranes represent a promising class of materials for energy-efficient separations of gas mixtures consisting of similarly sized molecules such as CO2 and CH4. CMS membranes are amorphous, microporous carbon materials formed by pyrolysis of polymer precursors.1,4−6 The pore system of CMS membranes consists of selective ultramicropores ( Ds. Using the sorption isotherms for the 6FDA/ BPDA sample published in ref 7, θ at 345 kPa is estimated to be about 0.8 and 0.5 for carbon dioxide and methane, respectively. At the permeate side of the membrane which is maintained at

Figure 6. Methane self-diffusivities in 6FDA/BPDA as a function of sorption equilibrium pressures of methane in the gas phase. Selfdiffusivities were measured using 1H PFG NMR at T = 297 K. 10301

dx.doi.org/10.1021/la301674k | Langmuir 2012, 28, 10296−10303

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pressure decreases from 51 to 9 kPa. Such behavior can be explained by the existence of the heterogeneity over pore sizes in the studied membranes in the same way as discussed in ref 35. In the case of low loadings, a majority of sorbate molecules are likely to frequently encounter ultramicropores/micropores in the membrane which correspond to the lowest potential energy (and lowest mobility) sites visited on the way through the membrane. At more moderate loadings, the frequency of sorbate molecules encountering such low potential energy sites is expected to be greatly reduced because these pores are already occupied by other sorbate molecules. Once sorbate loading reaches a level where most of the low energy sites are occupied, no additional significant change in diffusivity is expected for increased loadings; this would explain the asymptotic nature of the data in Figure 6. Assuming that the number of such low potential sites is approximately equal to the number of methane molecules at an equilibrium pressure of 9 kPa, it appears that these sites account for around 6−7% of all sorption sites accessible for methane in the 6FDA/BPDA membrane sample.7 Since the transport diffusivities obtained from the permeation measurements were conducted with a large loading gradient, the transport diffusivity through the entire membrane may be similar to the rate through region of greatest resistance (e.g., the permeate side). This explains why the self-diffusivity of methane measured at a sorption equilibrium pressure of 51 kPa may be larger than the overall transport diffusivity obtained through the permeation measurements. Because of the smaller effective size of carbon dioxide molecules, this effect is expected to be less pronounced for CO2 diffusion because passing through the small ultramicropores can occur more easily for carbon dioxide molecules than for larger methane molecules. It is seen in Figure 4 that the difference between the methane diffusivities obtained by PFG NMR and permeation measurements is larger for 6FDA/BPDA than for Matrimid. This observation can be explained by noting that 6FDA/BPDA is expected to have a much broader pore size distribution than Matrimid.7 As a result, the discussed above effect of increased probability of finding sorbate molecules in smaller (and lower potential energy) pores with decreasing loading should have a more pronounced influence on diffusion in 6FDA/BPDA than in Matrimid. This consideration provides an explanation for the larger difference between the transport diffusivity and selfdiffusivity for methane in 6FDA/BPDA than in Matrimid.

all samples. This result points out at a higher relative abundance of sufficiently large pores in 6FDA/BPDA than in Matrimid. The methane self-diffusivity in 6FDA/BPDA loaded with the CH4/CO2 mixture was found to be approximately a factor of 2 lower than that in the corresponding sample loaded with methane only. This diffusivity reduction was attributed to an increased degree of mutual hindrance of sorbate molecules to diffusion process in the sample with the mixture (higher total loading) in comparison to the sample with methane (smaller loading). At the same time, the total loading of the sample with the CH4/CO2 mixture was similar to that of the sample loaded only with CO2, which explained a lack of a significant difference between the carbon dioxide diffusivities in the two samples. PFG NMR self-diffusion data were compared with the previously reported transport diffusivities measured for the same types of sorbates in the same membrane samples. The observed differences between the transport diffusivities and selfdiffusivities were explained by the interplay of the dependencies of the thermodynamic factor and of the corrected transport diffusivity on the loading of sorbate molecules in the membrane samples.

CONCLUSION Detailed microscopic self-diffusion studies performed using multinuclear PFG NMR have been reported for carbon dioxide and methane diffusion in three CMS membrane samples derived from 6FDA/BPDA(1:1)-DAM and Matrimid polymers. PFG NMR studies were performed for single component sorbate, i.e., carbon dioxide or methane, as well as for a mixture of carbon dioxide and methane in a membrane. It was observed that in all cases the measured diffusivity remained independent of the length scale of molecular displacements during the diffusion measurements. This observation allows us to conclude that the transport properties of the studied membranes remain homogeneous over the broad range of length scales between a fraction of a micrometer and those comparable with the membrane thickness (around 60 μm). It was found that methane and carbon dioxide self-diffusivities in 6FDA/BPDA are much larger than those in the Matrimid samples, while the corresponding activation energies of diffusion were the same in

(1) Kiyono, M.; Williams, P. J.; Koros, W. J. Effect of pyrolysis atmosphere on separation performance of carbon molecular sieve membranes. J. Membr. Sci. 2010, 359 (1−2), 2−10. (2) Baker, R. W. Future Directions of Membrane Gas Separation Technology. Ind. Eng. Chem. Res. 2002, 41 (6), 1393−1411. (3) Bernardo, P.; Drioli, E.; Golemme, G. Membrane Gas Separation: A Review/State of the Art. Ind. Eng. Chem. Res. 2009, 48 (10), 4638− 4663. (4) Ismail, A. F.; David, L. I. B. A review on the latest development of carbon membranes for gas separation. J. Membr. Sci. 2001, 193 (1), 1− 18. (5) Steel, K. M.; Koros, W. J. Investigation of porosity of carbon materials and related effects on gas separation properties. Carbon 2003, 41 (2), 253−266. (6) Suda, H.; Haraya, K. Gas Permeation through Micropores of Carbon Molecular Sieve Membranes Derived from Kapton Polyimide. J. Phys. Chem. B 1997, 101 (20), 3988−3994. (7) Kiyono, M.; Williams, P. J.; Koros, W. J. Effect of polymer precursors on carbon molecular sieve structure and separation performance properties. Carbon 2010, 48 (15), 4432−4441.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]fl.edu; Tel +1- 352-392-0315; Fax +1352-392-9513. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of this work by the NSF CAREER award (CBET No. 0951812). NMR data were obtained at the Advanced Magnetic Resonance Imaging and Spectroscopy (AMRIS) facility in the McKnight Brain Institute of the University of Florida. The NMR measurement time provided in the framework of the external user program of the National High Magnetic Field laboratory, AMRIS, is gratefully acknowledged. Members of the Vasenkov Group would especially like to thank Dan Plant at AMRIS for his help with NMR measurements.

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