Article pubs.acs.org/JPCC
Highly Selective Enclathration of Ethylene from 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 ABSTRACT: Hydroquinone (HQ) is known to form clathrate compounds with various gases as guests. In this study, pure ethylene (C2H4), pure ethane (C2H6), and mixtures thereof with various compositions were used at 4.0 MPa and ambient temperature to form clathrate compounds. Spectroscopic results for the HQ samples show highly selective enclathration of C2H4 when HQ is converted into the clathrate compounds. The captured amount of C2H4 is found to increase when the partial pressures of C2H4 in the mixed gases are increased, and is estimated to store 44.5−82.3 L (at STP condition) of C2H4 gases per 1 L of the solid HQ compounds. A combined effect of the guest−host interaction and molecular size of guests on the hydrogen-bonded organic framework of the HQ clathrates may be contributed to this selective enclathration of C2H4 by acting as a molecular sieve.
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INTRODUCTION Ethylene (C2H4) is one of the most important and one of the largest volume petrochemicals produced in the world today.1 It has been the principal building block of the petrochemical industry. Ethylene production is currently carried out by thermal cracking to produce a mixture of hydrocarbons, which is subsequently separated with cryogenic distillation. Although this traditional technology is effective and reliable, the separation technology of ethylene from ethane (C2H6) is one of the most energy intensive processes in the petrochemical industry, and requires distillation columns over 100 trays due to the very small differences in the relative volatilities.2 Moreover, additional energy is required when pretreatment processes including a dehydration unit are necessary. In order to reduce such high energy costs, attempts have been made to develop processes with lower energy and equipment costs.1−4 Chemical and physical sorption and polymeric membrane technologies have been investigated. However, these alternatives lack either adequate separation efficiency or stability and have not been commercialized yet. Clathrate compounds recently have been proposed for gas storage/recovery. Clathrate compounds are solid crystalline compounds formed by interactions between host and guest molecules.5 Hydrogen-bonded lattice structures of the host molecules capture gaseous guest species and stabilize the entire structure. Many materials including water and organic chemicals are known to act as a host material, while a variety of gases including methane (CH4), carbon dioxide (CO2), and hydrogen (H2) can be enclathrated into the clathrate compounds. Some researchers have suggested that gas hydrates whose host material is water can be applied to energy storage or CO2 sequestration because they can hold a large volume of gas in a unit volume of the solid phase.6−8 However, large energy is also necessary because the gas hydrate can be formed and exists © 2014 American Chemical Society
stably at low temperature conditions. In this light, hydroquinone (HQ) has been recognized as one of the most promising clathrate formers in recent research.9 HQ has three distinct crystal modifications, α-, β-, and γ-forms. The α-form, which is stable at room temperature, transforms to the β-form when guest molecules are accommodated into the ordered HQ lattice structure.10−12 The γ-form can be obtained from sublimation or rapid evaporation of a HQ solution in ether.13 Since Palin and Powell reported β-form HQ with HCl, HCN, and SO2,10 a wide range of researches have been performed using various guest species including Ar, Kr, Xe, H2, and N2.14,15 Complex solvent evaporation and/or recrystallization were usually required for the formation of β-form HQ clathrates, thus limiting application of the material. However, Lee et al. recently reported dry gas-based formation of a HQ clathrate with CH4 and analyzed the prepared samples by means of spectroscopic measurements.16 Lee et al. later found that CO2, N2, and CH4 can be captured into the HQ clathrate with a dry synthesis method.17 In addition, they also suggested that the conversion of HQ from the α-form to the β-form is strongly dependent not only on physical properties of the guest materials (for example, molecular size), but also on formation conditions (for example, powder size and reaction time). Many investigations have been performed on HQ clathrates using a variety of guest species. However, they mainly focused on crystallographic properties or identification of guests. Moreover, because dry synthesis of the β-form HQ clathrate without using a solvent was only recently reported, few applications of the material have been investigated. In this study, HQ samples using a (C2H4 + C2H6) gas mixture were Received: December 20, 2013 Revised: March 4, 2014 Published: March 10, 2014 6059
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prepared and analyzed by solid-state 13C NMR and powder Xray diffraction (XRD) measurements in order to apply the material to selective separation/recovery of the olefin compound. Experimental results obtained in this study showed excellent separation efficiency for C2H4, and can be used as fundamental data for further evaluation of the clathrate-based separation technology. In addition, differences in the enclathration behaviors of C2H4 and C2H6 also provide useful information on the cavity size of the clathrate compounds and molecular sizes of C2H4 and C2H6 as well.
Article
RESULTS AND DISCUSSION Solid-state 13C NMR spectroscopy is widely used for identifying the crystal structure of clathrate compounds and the molecular behavior of guest species. Figure 1 shows solid-state 13C NMR
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EXPERIMENTAL METHODS HQ (the α-form) with minimal purity of 99 mol % was purchased from Sigma-Aldrich Co. Pure C2H4 and pure C2H6 gases were supplied by Daemyoung Special Gas Co. (Korea) and both had nominal purity of 99 mol %. In addition, C2H4 + C2H6 gas mixtures were manufactured and supplied by Daemyoung Special Gas Co. The nominal compositions of the gas mixtures were 20, 40, 60, and 80 mol % (analyzed compositions of 20.1, 40, 60, and 79.9 mol %, respectively). These gases and the material were used without further purification or treatment. In order to prepare clathrate samples, about 5.0 g of HQ was charged in a high-pressure reactor before introducing pure and mixed gases and allowing for the reaction at room temperature for 7 days. A reactor made from 316 stainless steel (SUS 316) with an internal volume of approximately 50 cm3 was used for the experiments. Before the reaction was started, pure or mixed gas was purged 5 times to remove residual air in the reactor. Because no stirrer was installed in the reactor, HQ was ground into fine powders using sieves with sizes of 100 or 45 μm to promote the reaction. The experimental procedures were the same as those used in our previous reports.16,17 After the reactions, the prepared samples were collected and analyzed using solid-state 13C NMR and powder XRD measurements. For the XRD measurements, a high-resolution X-ray powder diffraction beamline (9B) at Pohang Accelerator Laboratory (PAL) in Korea was used to identify formed structures of 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 the step scan was performed at room temperature from 8.00° in 2θ with 0.01° increment and 1.00° overlaps to the next detector bank up to 129.00° in 2θ (a step time of 2 s). For the solid-state 13 C cross-polarization/magic angle spinning (CP/MAS) NMR measurements, an NMR instrument (Bruker DSX400) in the National Instrumentation Center for Environmental Management (NICEM) of 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.
Figure 1. Solid-state 13C NMR spectra for HQ samples prepared at 4.0 MPa with (a) pure C2H4 and (b) pure C2H6 gases. Red dotted line indicates the chemical shift (124.3 ppm) for carbon atoms in C2H4 molecules. All of the spectra were collected at room temperature and ambient pressure.
spectra of HQ before and after reactions with pure C2H4 and pure C2H6 at 4.0 MPa for 7 days. As shown in the figure, two groups of signals (quintet at about 118 ppm and triplet at about 150 ppm) were observed for pure HQ before the reaction with gases. These signals correspond to two atomic states of carbon atoms in HQ molecules, that is, one for the hydroxylsubstituted carbon atoms and the other for the nonsubstituted carbon atoms.18 However, when the crystal structure of HQ is converted into the β-form with enclathration of C 2 H 4 molecules, partially resolved peaks rearrange to three distinct signals, representing three centrosymmetric inequivalent carbons.16,18 In addition, an additional signal is observed at 6060
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124.3 ppm for the HQ sample after the reaction with C2H4 gas. The signal corresponds to the one atomic state in C2H4 molecules, in close agreement with a previous report.19 On the basis of the spectral change and the atomic signal observation, it can be concluded that HQ is converted into the β-form clathrate after the reaction with C2H4. In addition, the conversion into the β-form clathrate is completed with a powder size of 45 μm, while the remaining α-form signals are still detected when a powder size of 100 μm is used. On the contrary, C2H6 molecules are not found to contribute to the clathrate formation of HQ, because two NMR spectra before and after the reaction show no differences. Moreover, no additional peak other than the shadow peak of the hydroxylsubstituted carbon atoms is identified in the region of around 5−10 ppm, where one atomic signal (−CH3) of C2H6 is expected.20 This distinct difference in the enclathration behaviors between C2H4 and C2H6 may closely be related to the interactions between the host HQ and the guest (C2H4 or C2H6) molecules, which is mainly due to the formed cavity size of the formed HQ clathrate. The cavity size (4.8 Å) of the βform HQ is large enough to accommodate C2H4 molecules (molecular diameter = 4.82 Å), while C2H6 molecules (molecular diameter = 5.08 Å) are too large to fit into the clathrate cavity.5,20 Moreover, steric hindrance of C2H6 molecules might be further contributed to the repulsive interaction between C2H6 and HQ molecules of the lattice structure, while planar C2H4 molecules are more appropriate to fit into the clathrate cavities. On the basis of the full conversion into the β-form observed during the experimental reaction time as described above, all of the samples with gas mixtures were prepared using a powder size of 45 μm. Figure 2 shows solid-state 13C NMR spectra obtained from C2H4 + C2H6 gas mixtures with various compositions at 4.0 MPa and room temperature. As shown in the figure, all of the gas mixtures except for 20 mol % C2H4 are found to form the β-form HQ on the basis of the experimental NMR spectra. In addition, the atomic signal at 124.3 ppm and the absence of a signal in the range of 5−10 ppm except for the shadow peak from HQ molecules provide direct evidence that C2H4 molecules are the only guest in the clathrate cavity. However, it should be noted that the remaining α-form HQ can be identified by partially resolved signal groups at the same pressure as that used for the sample preparation with pure gas. Such incomplete conversion into the β-form is attributed to the lower partial pressure of C2H4. Although the total pressure of 4.0 MPa is the same as the case of the sample preparation with pure gas, partial pressures of the gas mixtures would be decreased to roughly 0.8, 1.6, 2.4, and 3.2 MPa with compositions of 20, 40, 60, and 80 mol % C2H4, respectively. The decreased partial pressures of C2H4 would lead to a smaller driving force to push C2H4 molecules into the clathrate cavity. In addition, with the gas mixture of 20 mol % C2H4, the lowest C2H4 concentration (or the lowest C2H4 partial pressure), there was no detectable conversion of HQ into the β-form. In this regard, higher total pressure or longer reaction time may be required to reach full conversion into the β-form for C2H4 + C2H6 gas mixtures with low C2H4 concentrations. In order to verify the crystal structures of the HQ samples prepared with mixed gases, powder XRD measurements were used for the same samples as used in the NMR experiments. Obtained XRD patterns for the HQ samples prepared from (C2H4 + C2H6) gas mixtures with various C2H4 concentrations are illustrated in Figure 3. As indicated in the figure, there are
Figure 2. Solid-state 13C NMR spectra for HQ samples prepared at 4.0 MPa with (a) 20 mol % C2H4 + 80 mol % C2H6, (b) 40 mol % C2H4 + 60 mol % C2H6, (c) 60 mol % C2H4 + 40 mol % C2H6, and (d) 80 mol % C2H4 + 20 mol % C2H6 gas mixtures. Red dotted line is used to indicate the chemical shift (124.3 ppm) for atomic signals from C2H4 molecules. All of the spectra were collected at room temperature and ambient pressure.
Figure 3. Powder X-ray diffraction patterns for HQ samples prepared at 4.0 MPa with (a) 20 mol % C2H4 + 80 mol % C2H6, (b) 40 mol % C2H4 + 60 mol % C2H6, (c) 60 mol % C2H4 + 40 mol % C2H6, and (d) 80 mol % C2H4 + 20 mol % C2H6 gas mixtures. All of the diffraction patterns were collected at room temperature and ambient pressure.
no significant differences in the obtained XRD patterns except for the 20 mol % C2H4 sample, confirming the identification from the 13C NMR spectra. In other words, the unreacted α6061
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form is obtained for the 20 mol % C2H4 sample, while the other gas mixtures were used to form the β-form HQ with selective enclathration of C2H4 molecules. In addition, although the diffractions from the β-form are dominant in the patterns, some diffractions from the α-form are also observed, indicating remaining unreacted HQ. Pure HQ (the α-form) is known to have a rhombohedral space group (R-3) with 54 molecules per unit cell and hexagonal dimensions of a = b = 38.46 Å and c = 5.650 Å, while the β-form HQ has an R3 space group with lattice parameters of a = b = 16.650 Å and c = 5.453 Å.12,21 Calculated lattice parameters from the powder XRD patterns are listed in Table 1. It should be noted that the lattice parameters for C2H4Table 1. Lattice Parameters of HQ Samples Prepared with Mixed (C2H6 + C2H4) Gases C2H4 concn (mol %)
a = b (Å)
c (Å)
crystal structure
20 40 60 80
38.460 16.645 16.668 16.734
5.650 5.494 5.496 5.494
α-form β-form β-form β-form
Figure 4. Cavity occupancy and amount of gases stored in HQ clathrates prepared with various C2H4 + C2H6 gas mixtures at 4.0 MPa. Blue bars indicate the volume (at STP condition) of gases stored in 1 L of the HQ clathrates and percentages above blue bars represent the composition of C2H4 in gases stored in the HQ clathrates.
formation, it is interesting to note that the cavity dimension of the β-form HQ clathrate can act as a selective molecular sieve for C2H4 by enclathrating it from (C2H4 + C2H6) gas mixtures. Because this enclathrating behavior is found to be dependent on formation conditions (partial pressure of C2H4), further experimental works at various pressures are necessary to verify guest enclathration under various formation conditions, and to apply the material for a clathrate-based separation/recovery process.
loaded HQ clathrates obtained in this study are slightly larger than those for CH4-loaded HQ.16 In addition, the lattice constants, especially a values, increase with increasing the concentration of C2H4 in the gas phase, resulting in the higher occupancy of C2H4 in the cages. This may be attributed to lattice expansion of the HQ clathrate for capturing C2H4 molecules. Because 13C NMR signals are proportional to the amount of corresponding carbon atoms, the obtained NMR spectra can be used to calculate the amount of captured C2H4 in the clathrate compound.17 The captured amounts of C2H4 in the β-form HQ are found to be 0.16, 0.23, and 0.25 mol per 1 mol of HQ for 40, 60, and 80 mol % C2H4 + C2H6 gas mixtures and 0.3 mol for pure C2H4 gas, respectively. In other words, the chemical formula for the prepared HQ (ideal formula of 1Gas·3HQ10) can be expressed as 0.48C2H4·3HQ, 0.69C2H4·3HQ, and 0.75C2H4·3HQ for 40, 60, and 80 mol % C2H4 + C2H6 gas mixtures and 0.9C2H4·3HQ for pure C2H4 gas, respectively. This indicates that 90% of the cavities in the HQ clathrate framework are filled by C2H4 molecules, when pure C2H4 gas is used to form the HQ clathrate. Such increased occupancy per unit structure of the β-form HQ clathrate can be explained by the increased partial pressure of C2H4 in the reacted gases. In addition, as reported by Ripmeester, the chemical shifts of the β-form HQ vary over a narrow range depending on the nature of the guest and cavity occupancy.18 The chemical shifts obtained in this study are about 149.0 ppm for the hydroxylsubstituted carbon atoms and about 118.0 and 116.8 ppm for the nonsubstituted carbon atoms regardless of the reaction gases used. In addition, the difference in the chemical shifts for nonsubstituted carbons is approximately 1.2 ppm in this study, which is smaller than reported values for other gases.18 The calculated density of the β-form HQ is 1.29 g cm−3, which is compared with 1.33 g cm−3 for the α-form HQ.16 On the basis of this value and 13C NMR results, it is estimated that 1 L of the β-form HQ can store 44.5−82.3 L (at STP condition) of gases at ambient temperature (Figure 4). It should also be noted that all the stored gases are pure C2H4, even though the β-form HQ clathrates form with the C 2H4 + C2H6 gas mixtures. Considering no significant cavity distortion during the clathrate
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CONCLUSIONS Pure ethylene (C2H4) is found to form the β-form HQ clathrate at 4.0 MPa, while pure ethane (C2H6) is not captured into the clathrate cavity at the same pressure. In an attempt to apply such selective enclathration of the clathrate compounds, HQ samples were prepared from (C2H4 + C2H6) gas mixtures with various compositions and analyzed by means of solid-state 13 C NMR and powder XRD measurements. Obtained 13C NMR spectra showed that C2H4 molecules can be selectively captured into the clathrate cavity from all of the experimental compositions except for the 20 mol % C2H4 sample. In addition, the captured amount of C2H4 per unit mole of HQ is found to increase in accordance with increased C 2 H 4 concentrations in the gas mixtures. This increase is thought to be caused by increased partial pressures of C2H4 in the mixed gases. However, both the 13C NMR spectra and the XRD patterns also showed that the α-form HQ is not completely converted into the β-form when gas mixtures at 4.0 MPa are used. Such incomplete conversions are thought to be attributable to decreased C2H4 partial pressure relative to pure C 2 H 4 at the same pressure. Therefore, further investigations regarding guest enclathration and structural conversion are required under various experimental conditions.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Telephone: +82-51-410-4684, Fax: +82-51-403-4680 (J.-H.Y.). *E-mail:
[email protected]. Telephone: +82-42-860-3475, Fax: +82-42-860-3097 (S.-P.K.). Notes
The authors declare no competing financial interest. 6062
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(19) 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. (20) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases. 3rd Ed.; CRC Press: Boca Raton, 2008. (21) Boeyens, J. C. A.; Pretorius, J. A. X-ray and Neutron Diffraction Studies of the Hydroquinone Clathrate of Hydrogen Chloride. Acta Crystallogr. B 1977, 33, 2120−2124.
ACKNOWLEDGMENTS This research was supported by 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. In addition, This work was also supported by the research grant of the Kongju National University in 2011.
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