Competing Occupation of Guest Molecules in Hydroquinone

Mar 17, 2014 - When reacted with pure ethylene (C2H4) and pure methane (CH4) at 2.0 and 4.0 MPa, respectively, pure hydroquinone (HQ) was converted in...
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Competing Occupation of Guest Molecules in Hydroquinone Clathrates Formed from Binary C2H4 and CH4 Gas Mixtures Jong-Won Lee,† Seong-Pil Kang,*,‡ and Ji-Ho Yoon*,§ †

Department of Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea ‡ Climate Change Research Division, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea § Department of Energy and Resources Engineering, Korea Maritime and Ocean University, Busan 606-791, Republic of Korea S Supporting Information *

ABSTRACT: When reacted with pure ethylene (C2H4) and pure methane (CH4) at 2.0 and 4.0 MPa, respectively, pure hydroquinone (HQ) was converted into β-form clathrate compounds. Experimental solid-state 13C NMR spectra and powder X-ray diffraction patterns provided direct evidence of C2H4 and CH4 enclathration in the β-form HQ clathrates. On the basis of cage occupancy from the solid-state 13C NMR spectra, C2H4 (cage occupancies of 0.81−0.88) molecules are more likely to occupy the clathrate cages than CH4 molecules (cage occupancies of 0.38−0.39). The selective occupation by C2H4 was also observed for HQ clathrates formed from C2H4 and CH4 gas mixtures of 10, 30, 50, 70, and 90 mol % concentrations of C2H4. The experimental results from this study could be applied to a clathrate-based process for separating and concentrating C2H4 from gas mixtures.

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INTRODUCTION Olefin compounds such as ethylene (C2H4) and propylene (C3H6) are produced in great quantities as starting materials for synthesis of a wide variety of products in the petrochemical industry.1 Pyrolysis is the major process used to produce olefin compounds from mixtures of hydrocarbons.2 However, the separation and recovery processes required after pyrolysis are very energy intensive and expensive because they use a lowtemperature distillation method with products that have very similar volatilities.1,2 Moreover, additional energy can be required when some pretreatment processes, including dehydration, are necessary. To overcome such costs, some researchers evaluated a process using chemical and physical sorption and polymeric membrane technologies, to achieve lower costs for energy and equipment.2,3 However, such alternatives still do not meet the requirements for either separation efficiency or stability and have not yet been commercialized. In addition, the conversion of natural gas to an easily transportable liquid fuel is becoming an important challenge.4 Some researchers have reported the conversion of methane into heavier hydrocarbons by oxidative coupling of methane directly or catalytically.5 Because methane (CH4), ethane (C2H6), and C2H4 are also components of natural gas, these species are present in both the input and the output streams of the oxidative synthesis process.6 Therefore, separation/recovery of a specific component from the stream is also important and has led to proposals for novel processes using various adsorbents.6,7 In particular, Triebe et al.7 investigated adsorption of CH4, C2H6, and C2H4 on molecular sieve zeolites. They also reported the temperature dependence © 2014 American Chemical Society

of the adsorption behaviors because natural light gases are often found above room temperature. In addition, Pereira et al.8 proposed conversion of CH4 into C2H4 and C2H6 over a catalyst with almost complete selectivity below 883 K. Although the application of adsorbents and catalysts for the separation/recovery of a specific component has a long history in the petrochemical industry, the use of clathrate (or inclusion) compounds has been also suggested as a storage/ separation technology for gaseous materials. A clathrate compound is a crystalline compound formed by interaction between host and guest molecules.9 It has potential applications to energy storage or CO2 separation because such materials can hold huge amounts of gas in a unit volume.10 Water and organic chemicals are known to form the host molecules of clathrate compounds, while various gases, including hydrocarbons (e.g., CH4, C2H6, C2H4, propane (C3H8), and C3H6), CO2, H2, and N2, can act as guest species.11 In particular, organic chemicals that are solid at room temperature are the most promising clathrate formers (for storage) because cooling to near the freezing point of water is not necessary and because they are convenient to handle, and their reactions easy to control. Hydroquinone (HQ), one such organic chemical, has three distinct crystal structures, designated as α-, β-, or γ-form. When a guest molecule interacts with pure α-form HQ, the stable form at room temperature and atmospheric pressure, it changes Received: January 21, 2014 Revised: February 24, 2014 Published: March 17, 2014 7705

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to the well-arranged β-form.12,13 The γ-form is known to form by sublimation or rapid evaporation of HQ dissolved in ether.14 Since Palin and Powell reported β-form HQ clathrate compounds with HCl, HCN, and SO2 as guests,12,13 a wide range of investigations on clathrate compounds were performed. These included thermodynamic properties and molecular motions for guest molecules of Ar, Kr, Xe, H2, and N2.15 In addition, Ripmeester16 and Burgiel et al.17 adapted solid-state NMR and far-IR spectroscopic methods to analyze the behavior of guest molecules in the clathrate cages. However, conventional formation of β-form HQ requires recrystallization from solvents, or evaporation, which limits applications for synthesis of HQ clathrates.14,15 Recently, Lee et al.18 reported the gas-phase synthesis of CH4-loaded HQ clathrates and characterized guest enclathration using the solid-state 13C NMR method. Lee et al.19 subsequently identified the HQ clathrates with CO2, N2, and CH4 guest molecules and suggested that conversion rate from the α- to β-forms is not dependent only on formation conditions, such as pressure or reaction time, but also on physical properties of the guest molecules (e.g., molecular size, electrostatic properties). Although many investigations were performed on β-form HQ with various guest species, those investigations were mainly focused on the crystal structures of the clathrates formed, or on the types of guest molecules to be captured. In addition, because gas-phase synthesis of β-form HQ was reported recently, applications of this new synthesis method to more complex (binary or ternary) systems have not been explored sufficiently. For this purpose, CH4 + C2H4 gas mixtures of various ratios were used to create β-form HQ. The HQ compounds formed were analyzed by means of solid-state 13C NMR and powder XRD methods in order to identify crystal structures and guest behaviors within the compounds. Moreover, two different formation pressures were used to investigate differences in guest behavior. The enclathration behaviors of CH4 and C2H4, in relation to the composition of the mixed gas, can be applied to separation-recovery of a specific component from a range of CH4 + C2H4 mixtures, or to purification of natural gas by removing specific components.

After the reactions, samples were collected and analyzed using solid-state 13C NMR and powder XRD measurements. For the X-ray diffraction (XRD) measurements, a highresolution XRD beamline (9B) at Pohang Accelerator Laboratory (PAL) in Korea was used to identify the structures formed in the HQ samples. The incident X-rays were vertically collimated by a mirror and monochromatized to a wavelength of 1.5490 Å using a double-crystal Si(1 1 1) monochromator. The detector arm of the vertical-scan diffractometer is composed of seven sets of Soller slits, flat Ge (1 1 1) crystal analyzers, antiscatter baffles, and scintillation detectors with each set separated by 20°. The HQ sample, of approximately 0.2 g, was prepared on a flat plate holder and a step scan performed at room temperature from 8.00° in 2θ with 0.01° increments and 1.00° overlap to the next detector bank up to 129.00° in 2θ (a step time of 2 s). For the solid-state 13C crosspolarization, magic-angle spinning (CP/MAS) NMR measurements, an NMR instrument (Bruker DSX400) in the National Instrumentation Center for Environmental Management (NICEM) at Seoul National University was used. All 13C NMR spectra were obtained at ambient pressure and temperature conditions, and at a Larmor frequency of 100.6 MHz with a spinning rate of 9 kHz. A pulse length of 2 μs and a pulse repetition delay of 10 s with proton decoupling were employed when radio frequency field strengths of 50 kHz corresponding to 5 μs 90° pulses were used. The downfield signal of adamantane appearing at 38.3 ppm at room temperature (300 K) was used as an external reference for the experimental chemical shifts.

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RESULTS AND DISCUSSION Figure 1 shows solid-state 13C NMR spectra for pure α-form HQ and β-form HQ clathrates prepared with pure C2H4 and pure CH4 gases at 2.0 and 4.0 MPa. Two groups of signals (quintet at about 118 ppm and triplet at about 150 ppm) were observed for pure α-form HQ, indicating two equivalent (hydroxyl-substituted and nonsubstituted) carbon atoms in the HQ molecule.16 However, two groups of signals were converted into three distinct signals representing three centrosymmetric inequivalent carbons after reaction with both C2H4 and CH4.19 These spectra indicate that the crystal structure of α-form HQ was changed to β-form HQ clathrates when guest molecules were captured. In Figure 1a, an additional peak at 124.3 ppm, representing one equivalent carbon atom (−CH2) in the C2H4, is direct evidence of C2H4 enclathration in the HQ clathrate framework.20 We note that a complete formation of β-form HQ clathrate by enclathration of C2H4 was observed at both 2.0 and 4.0 MPa. Similar findings were obtained after reaction with pure CH4 gas (Figure 1b). An atomic signal from one equivalent carbon in the CH4 molecule was observed at −4.4 ppm, which agrees well with the previously reported value.18 However, marginal signals of the αform HQ (two shoulders around the signal at about 150 ppm) still exist for CH4-reacted HQ. Such incomplete conversion into the β-form HQ suggests that the kinetic reaction rate of HQ with CH4 may be slower than that with C2H4. From powder XRD patterns for the same HQ samples (Figure S1, Supporting Information), the lattice parameters for β-form HQ clathrates with C2H4 guest were found to be a = 16.7579 Å and c = 5.5929 Å at 2.0 MPa, and a = 16.7290 Å and c = 5.5856 Å at 4.0 MPa, while those with CH4 guest were a = 16.6337 Å and c = 5.5398 Å at 2.0 MPa, and a = 16.6664 Å and c = 5.5543 Å at 4.0 MPa. The lattice parameters of the C2H4-loaded HQ

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EXPERIMENTAL METHODS Pure (α-form) HQ with a nominal purity of 99 mol % was supplied by Sigma-Aldrich Chemicals Co. Pure C2H4 and pure CH4 gases with minimum purities of 99 and 99.995 mol %, respectively, were purchased from Daemyeong Special Gas Co. (Korea). In addition, C2H4 + CH4 gas mixtures with nominal compositions of 10, 30, 50, 70, and 90 mol % (analyzed compositions of 10.0, 30.1, 50.1, 70.0, and 90.1 mol %, respectively) were also purchased from Daemyeong Special Gas Co. These materials were used without further purification or treatment. A high-pressure reactor (internal volume approximately 50 cm3) made of 316 stainless steel was used to prepare the samples. There was no mechanical stirrer in the reactor. Therefore, pure HQ was ground and filtered into fine powder particles less than 45 μm to promote reaction with the gases. After loading about 7.0 g of pure HQ powder, the reactor was purged with reacting (pure or mixed) gases at least four times to remove any residual air. Then, after pure or mixed gas was introduced into the reactor, and it was brought up to experimental pressure, the content was allowed to react at room temperature for 14 days. The experimental procedures for forming HQ clathrate samples were the same with those in our previous reports.18,19 7706

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Figure 2. Solid-state 13C NMR spectra for HQ samples prepared with (a) 10 mol % C2H4 + 90 mol % CH4, (b) 30 mol % C2H4 + 70 mol % CH4, (c) 50 mol % C2H4 + 50 mol % CH4, (d) 70 mol % C2H4 + 30 mol % CH4, and (e) 90 mol % C2H4 + 10 mol % CH4 gas mixtures at 4.0 MPa. Dotted blue and red lines are used to indicate the chemical shift for atomic signals from C2H4 (124.3 ppm) and CH4 (−4.4 ppm) molecules, respectively. All of the spectra were collected at room temperature and ambient pressure.

from the NMR spectra. However, the signal of C2H4 molecules in the solid-state 13C NMR spectra was detectable even at the lowest C2H4 concentration (10 mol %). To investigate the crystal structure of HQ clathrates formed from the gas mixtures, powder XRD patterns for the HQ samples were also observed (Figure 3 and Figure S3, Supporting Information). All dominant peaks of the HQ samples prepared with (C2H4 + CH4) gas mixtures can be indexed for the β-form HQ clathrates, although there were some marginal peaks for unreacted α-form HQ, particularly for β-form HQ clathrates formed from the gas mixtures with low C2H4 concentrations. The lattice parameters calculated from the experimental XRD patterns generally increased in accordance with increasing C2H4 concentrations in the gas mixtures: For the HQ clathrates formed at 2.0 MPa, a = 16.6179 Å and c = 5.5287 Å (10 mol % C2H4 + 90 mol % CH4); a = 16.6648 Å and c = 5.5745 Å (30 mol % C2H4 + 70 mol % CH4); a = 16.6742 Å and c = 5.5730 Å (50 mol % C2H4 + 50 mol % CH4); a = 16.6517 Å and c = 5.5864 Å (70 mol % C2H4 + 30 mol % CH4); a = 16.7016 Å and c = 5.5933 Å (90 mol % C2H4 + 10 mol % CH4), and for the HQ clathrates formed at 4.0 MPa, a = 16.6497 Å and c = 5.5473 Å (10 mol % C2H4 + 90 mol % CH4); a = 16.6682 Å and c = 5.5844 Å (30 mol % C2H4 + 70 mol % CH4); a = 16.6753 Å and c = 5.5706 Å (50 mol % C2H4 + 50 mol % CH4); a = 16.6784 Å and c = 5.5780 Å (70 mol % C2H4 + 30 mol % CH4); a = 16.7026 Å and c = 5.5949 Å (90 mol % C2H4 + 10 mol % CH4). As the peak intensity in the 13C NMR spectra is proportional to relative amounts of the corresponding atomic signals,18 the peak areas were used to calculate cage occupancies of gas

Figure 1. Solid-state 13C NMR spectra for pure HQ and HQ clathrate samples prepared with (a) pure C2H4 and (b) pure CH4 gases at 2.0 and 4.0 MPa. Dotted lines in the figure indicate the chemical shift for carbon atoms in C2H4 (124.3 ppm) and CH4 (−4.4 ppm) molecules. All of the spectra were collected at room temperature and ambient pressure.

clathrates were slightly larger than those of the CH4-loaded HQ clathrates. We note that the molecular diameter of C2H4 (∼4.82 Å) is larger than that of CH4 (∼4.25 Å) and the average diameter of the cages in β-form HQ clathrates is about 4.8 Å.11,20 After verifying the formation of β-form HQ with C2H4 and CH4 guests, solid-state 13C NMR spectra for HQ samples reacted with C2H4 + CH4 gas mixtures were observed (Figure 2 and Figure S2, Supporting Information). As shown in these figures, the crystal structure of pure α-form HQ was converted into the β-form HQ clathrates by reacting with C2H4 + CH4 gas mixtures. In addition, the chemical shifts for C2H4 and CH4 molecules were observed at 124.3 and −4.4 ppm, respectively, indicating that both components were captured in the β-form clathrate framework. The amount of CH4 stored in the clathrate compounds decreases with decreasing CH4 concentrations in the gas mixtures. For the β-form HQ formed from a 90 mol % C2H4 + 10 mol % CH4 gas mixture, the amount of CH4 molecules in the clathrate cage was too small to be detected 7707

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4.0 MPa, respectively, whereas that of CH4 molecules for pure CH4 gas was 0.38 and 0.39 at 2.0 and 4.0 MPa, respectively. As expected, the cage occupancies of C2H4 molecules were found to increase according to the increase of C2H4 concentration in the mixture gases. At the same time, the cage occupancy of CH4 molecules gradually decreases from 0.37−0.39 for 10 mol % C2H4 to ∼0.03 for 90 mol % C2H4. Therefore, it should be noted that the overall occupancy increases with increasing the C2H4 concentration, indicating preferential occupation by C2H4 molecules, rather than CH4 molecules. The preferential occupation of C2H4 might closely be related to the ratio of the cage space to guest molecular size. We note that the molecular size of C2H4 is slightly larger than that of CH4. This indicates that the molecular size and shape of C2H4 would be more suitable to be captured in the flexible channel cage of HQ clathrate framework, as reported in the literature.21 The preferential occupancy of C2H4 molecules for HQ clathrates formed from the gas mixtures was also confirmed from compositional changes in the gas and the solid (clathrate) phases (Figure 5). On the basis of the HQ-free concentration, Figure 3. Powder X-ray diffraction patterns for HQ samples prepared with (a) 10 mol % C2H4 + 90 mol % CH4, (b) 30 mol % C2H4 + 70 mol % CH4, (c) 50 mol % C2H4 + 50 mol % CH4, (d) 70 mol % C2H4 + 30 mol % CH4, and (e) 90 mol % C2H4 + 10 mol % CH4 gas mixtures at 4.0 MPa. All of the diffraction patterns were collected at room temperature and ambient pressure.

species in combination with the known chemical formula for HQ clathrates (3HQ·1Gas).9 Figure 4 represents the cage

Figure 5. Compositional changes of C2H4 in the gas and the solid phases. Blue and red colors are used to indicate cage occupancy values at 2.0 and 4.0 MPa, respectively.

the concentrations of C2H4 in the hydrate phase are always higher than those in the gas phase. This indicates that the C2H4 molecules are more likely to occupy the cages of HQ clathrates than the CH4 molecules. Therefore, this phenomenon can be applied to C2H4 separation and concentration from C2H4 + CH4 gas mixtures.

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Figure 4. Cage occupancies for C2H4 (□) and CH4 (Δ) molecules, and the overall (○) cage occupancies (C2H4 occupancies + CH4 occupancies) depending on the C2H4 concentration of the gas mixture. Blue and red colors are used to indicate cage occupancy values at 2.0 and 4.0 MPa, respectively. The cage occupancies were calculated using the experimental peak areas in 13C NMR spectra and the known chemical formula for the HQ clathrate.9

CONCLUSIONS When pure HQ reacts with pure C2H4 and pure CH4 at 2.0 and 4.0 MPa, the crystal structure of HQ is converted into β-form clathrate compounds. Experimental NMR spectra and powder XRD patterns provide direct evidence of C2H4 and CH4 enclathration in the β-form HQ clathrates. In addition, the lattice parameters of C2H4 clathrates are slightly larger than those of CH4 clathrates. When pure HQ reacts with C2H4 + CH4 gas mixtures, the HQ clathrate compounds enclose both C 2 H 4 and CH 4 guest molecules at all compositions. Considering the cage occupancies of guest species in relation to the concentration of C2H4 in the gas mixtures, C2H4 molecules are more likely to be enclathrated into the clathrate cages than CH4 molecules. Although the separation is not so

occupancies of C2H4 and CH4 molecules, and the overall cage occupancies (C2H4 occupancies + CH4 occupancies) depending on the C2H4 concentration of the gas mixture. As can be seen in the figure, there was no significant difference in the cage occupancies for the HQ clathrates formed at two different pressure conditions of 2.0 and 4.0 MPa. The cage occupancy of C2H4 molecules for pure C2H4 gas was 0.81 and 0.88 at 2.0 and 7708

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(10) Kang, S.-P.; Lee, H. Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements. Environ. Sci. Technol. 2000, 34, 4397−4400. (11) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008. (12) Palin, D. E.; Powell, H. M. The Structure of Molecular Compounds. Part III. Crystal Structure of Addition Complexes of Quinol with Certain Volatile Compounds. J. Chem. Soc. 1947, 1, 208− 221. (13) Palin, D. E.; Powell, H. M. The Structure of Molecular Compounds. Part VI. The β-Type Clathrate Compounds of Quinol. J. Chem. Soc. 1948, 1, 815−821. (14) Maartmann-Moe, K. The Crystal Structure of γ-Hydroquinone. Acta Crystallogr. 1966, 21, 979−982. (15) Sixou, P.; Dansas, P. Motion of Guest Molecules in Clathrates. Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 364−388. (16) Ripmeester, J. A. Application of Solid State 13C NMR to the Study of Polymorphs, Clathrates and Complexes. Chem. Phys. Lett. 1980, 74, 536−538. (17) Burgiel, J. C.; Meyer, H.; Richards, P. L. Far-Infrared Spectra of Gas Molecules Trapped in β-Quinol Clathrates. J. Chem. Phys. 1965, 43, 4291−4299. (18) Lee, J.-W.; Lee, Y.; Takeya, S.; Kawamura, T.; Yamamoto, Y.; Lee, Y.-J.; Yoon, J.-H. Gas-Phase Synthesis and Characterization of CH4-Loaded Hydroquinone Clathrates. J. Phys. Chem. B 2010, 114, 3254−3258. (19) Lee, J.-W.; Choi, K. J.; Lee, Y.; Yoon, J.-H. Spectroscopic Identification and Conversion Rate of Gaseous Guest-Loaded Hydroquinone Clathrates. Chem. Phys. Lett. 2012, 528, 34−38. (20) Lee, J.-W.; Pratik, D.; Kang, S.-P. Spectroscopic Analysis on Hydroquinone Clathrates for an Application to Storage/Recovery of Olefin Compounds. Trans. Korean Hydrogen New Energy Soc. 2013, 24, 437−443. (21) Kang, S.-P.; Lee, J.-W. Hydrate-Phase Equilibria and 13C NMR Studies of Binary (CH4 + C2H4) and (C2H6 + C2H4) Hydrates. Ind. Eng. Chem. Res. 2013, 52, 303−308.

high, higher efficiency can be achieved by a multiple HQ clathrate formation process. In addition, the proposed separation can have strong potential in terms of energy consumption and capital cost. To apply the favored selection of C2H4 to the clathrate-based process, further investigations are required.

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ASSOCIATED CONTENT

S Supporting Information *

Powder X-ray diffraction patterns for pure C2H4 and pure CH4 HQ clathrate samples, and solid-state 13C NMR spectra and powder X-ray diffraction patterns for mixed (C2H4 + CH4) HQ samples prepared at lower pressure (2.0 MPa). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +82-42-860-3475. Fax: +8242-860-3097 (S.-P.K.). *E-mail: [email protected]. Tel: +82-51-410-4684. Fax: +8251-403-4680 (J.-H.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF2012R1A1A2005206), and by the Human Resources Development program (No. 20134030200230) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy.

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REFERENCES

(1) Teramoto, M.; Shimizu, S.; Matsuyama, H.; Matsumiya, N. Ethylene/Ethane Separation and Concentration by Hollow Fiber Facilitated Transport Membrane Module with Permeation of Silver Nitrate Solution. Sep. Purif. Technol. 2005, 44, 19−29. (2) Shi, M.; Lin, C. C. H.; Kuznicki, T. M.; Hashisho, Z.; Kuznicki, S. M. Separation of a Binary Mixture of Ethylene and Ethane by Adsorption on Na-ETS-10. Chem. Eng. Sci. 2010, 65, 3494−3498. (3) Rungta, M.; Zhang, C.; Koros, W. J.; Xu, L. Membrane-Based Ethylene/Ethane Separation: The Upper Bound and beyond. AIChE J. 2013, 59, 3475−3489. (4) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. The Oxidative Conversion of Methane to Higher Hydrocarbons. J. Catal. 1987, 103, 302−310. (5) Asami, K.; Kusakabe, K.-i.; Ashi, N.; Ohtsuka, Y. Synthesis of Ethane and Ethylene from Methane and Carbon Dioxide over Praseodymium Oxide Catalysts. Appl. Catal., A 1997, 156, 43−56. (6) Choudhary, V. R.; Mayadevi, S. Adsorption of Methane, Ethane, Ethylene and Carbon Dioxide on Silicalite-I. Zeolites 1996, 17, 501− 507. (7) Triebe, R. W.; Tezel, F. H.; Khulbe, K. C. Adsorption of Methane, Ethane and Ethylene on Molecular Sieve Zeolites. Gas. Sep. Purif. 1996, 10, 81−84. (8) Pereira, P.; Lee, S. H.; Somorjai, G. A.; Heinemann, H. The Conversion of Methane to Ethylene and Ethane with Near Total Selectivity by Low Temperature (