Preferential Occupation of CO2 Molecules in Hydroquinone Clathrates

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Preferential Occupation of CO2 Molecules in Hydroquinone Clathrates Formed from CO2/N2 Gas Mixtures Jong-Won Lee† and Ji-Ho Yoon*,‡ †

Department of Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Republic of Korea ‡ Department of Energy and Resources Engineering, Korea Maritime University, 1 Dongsam-dong, Yeongdo-gu, Busan 606-791, Republic of Korea ABSTRACT: Organic hydroquinone (HQ) clathrates were synthesized by a gas-phase reaction between HQ and CO2/N2 gas mixtures under high-pressure conditions. The occupation behavior of guests and the gas storage capacity of the HQ clathrates were evaluated using solid-state 13 C NMR, Raman spectroscopy, and elemental analyses. An in-depth examination of the solid-state 13C NMR and Raman results shows that CO2 molecules are captured within the HQ clathrate frameworks more easily than N2 molecules. Selectivity analysis indicates that the efficient separation of CO2 from the CO2/N2 mixtures up to 98 mol % can be attained by the formation of the HQ clathrates, depending on the gas compositions. Complete conversion of pure α-form HQ to the HQ clathrates results in a gas storage capacity of 1.98 2.82 mol/kg.

’ INTRODUCTION Since the initial discovery and exploitation of fossil fuels, they have assumed a vital role in the production of energy for human activities. The increasing concentration of atmospheric carbon dioxide (CO2) caused by oxidation of fossil fuels has also been a crucial concern, because CO2 is a notorious global warming gas for climate change. To stabilize or preferentially reduce atmospheric CO2 concentrations, there is an urgent need to develop CO2 recovery/separation or sequestration techniques.1 The flue gas from thermal power plants is known to be a major source of CO2, and, therefore, many researchers have studied the development of CO2 recovery/separation processes using a variety of adsorbents or membranes.2 Although those techniques have proved successful for selective removal of CO2 from multicomponent gaseous streams, they still have some critical limitations such as large energy consumption, corrosion, foaminess, and low capacity.2,3 In this regard, gas hydrates have recently been suggested as potential gas storage or recovery media because it can hold a large amount of gas per unit volume of the solid structure (as much as 170 volumes of gases under STP conditions). In addition, the selectivity for a specific gas component from gas mixtures has also been demonstrated using formation of the gas hydrate.4 Seo and Lee5 reported changes in the gas-phase composition after hydrate formation, suggesting the unbalanced occupation of guest components in the hydrate cages. Yoon et al.6 investigated the highly selective encaging of CO2 molecules from CO2/N2 mixtures and provided a theoretical concept to support the experimental results at various temperatures and compositions. In addition, a hydrate-based gas separation process especially for r 2011 American Chemical Society

recovering CO2 from flue gas has been suggested.3,7 Recently, it has also been reported that the capture of CO2 from CO2/H2, CO2/H2/C3H8, and CO2/N2 mixtures could be achieved by hydrate formation.8,9 Although the hydrate-based process is environmentally friendly (it only requires water and gas for hydrate formation), there may be an additional energy requirement because the gas hydrate can only be formed and only exists in the low temperature range near the melting point of ice. To overcome this problem, organic clathrates (in particular, hydroquinone (HQ) clathrates) are considered to be an alternative for gas storage because they exist in the solid phase and open form stable inclusion compounds with a variety of gases at room temperature.10 The α-form HQ, which is stable at room temperature, can be transformed into guest-loaded β-form HQ clathrates by the formation of spherical cages where a variety of guest species are accommodated.11 13 Recently, Lee et al. reported the gasphase synthesis and spectroscopic characterization of CH4loaded HQ clathrates as potential media for use in gas storage.14 Lee et al. investigated the high-pressure compression behavior of the α- and β-forms of HQ compounds using in situ synchrotron X-ray powder diffraction and Raman spectroscopy up to 7 GPa.15 On the basis of these studies, there has been considerable knowledge concerning HQ clathrate formation with a guest component. It is well-known, however, that there is only limited Received: August 16, 2011 Revised: September 25, 2011 Published: October 12, 2011 22647

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Figure 1. Solid-state 13C CP/MAS NMR spectra of HQ clathrate samples prepared from CO2/N2 gas mixtures at 298 K and (a) 1.0 MPa and (b) 3.0 MPa for 5 days. Dotted line at 124.4 ppm indicates the carbon signal from captured CO2 molecules.

information on the competing behavior for best occupancy between different guest species. In this study, we have investigated the HQ clathrates formed from CO2/N2 gas mixtures over a wide range of compositions to evaluate preferential occupation by a gaseous component. Crystal structures and occupation behaviors of guests in the clathrate frameworks were investigated using a series of spectroscopic methods. Elemental analysis was also used for quantitative determination of the gas storage capacity and the selectivity of CO2. The experimental results obtained can provide useful information on guest behavior and fundamental data for industrial applications such as selective recovery from mixed gases or storage.

’ EXPERIMENTAL METHODS Pure α-form HQ with a minimum purity of 99 mol % was supplied by Sigma-Aldrich Chemicals Co. Pure CO2 and N2 gases with a nominal purity of 99.9 and 99.5 mol %, respectively, were purchased from Daemyoung Special Gas Co. The CO2/N2 gas mixtures were also supplied from Daemyoung Special Gas Co. The nominal compositions of the gas mixtures are 19.69 mol % CO2, 39.93 mol % CO2, 59.92 mol % CO2, and 80.08 mol % CO2 for the samples hereafter referred to as 20 mol % CO2, 40 mol % CO2, 60 mol % CO2, and 80 mol % CO2, respectively, which are all balanced with N2. These materials were used without further purification or treatment. The β-form HQ clathrates were prepared by charging the pure α-form HQ in a high-pressure cell and allowing it to react with pure or mixture gases. The cell was made of 316 grade stainless steel with an internal volume of approximately 200 cm3. The HQ powders

were finely ground to give powders with a particle size of less than 45 μm to promote the gas-phase reaction. After loading the HQ powders, the cell was purged with the mixed gas at least five times to remove residual air and subsequently pressurized up to a desired pressure. During the reaction, a digital pressure indicator (Heise, ST-2H) was used to monitor the pressure in the cell. An external circulator (Jeio Tech, RW-2040G) was used to control the reaction temperature. When the clathrate formation reached a steady-state (no pressure drop) or after a desired time lapse, the product was collected by slowly releasing the gas in the cell. A Bruker DSX400 NMR spectrometer in the National Instrumentation Center for Environmental Management (NICEM) of Seoul National University was used for the solid-state 13C crosspolarization/magic angle spinning (CP/MAS) NMR measurements. All of the spectra were collected at ambient temperature by placing samples within a 4 mm o.d. zirconia rotor. All 13C NMR spectra were recorded at a Larmor frequency of 100.6 MHz with a spinning rate of 9 kHz. A pulse length of 2 μs and pulse repetition delay of 10 s under proton decoupling were employed when the radio frequency field strengths of 50 kHz corresponding to a 5 μs, 90° pulse were used. The downfield carbon resonance peak of adamantine with a chemical shift of 38.3 ppm at 300 K was used as an external chemical shift reference. Raman spectra were recorded with a dispersive Raman spectrometer (Horiba Jobin Yvon, LabRAM HR model) equipped with a multichannel thermal electric cooled CCD detector in a laser diode back-illumination system. The spectral resolution was 1 cm 1. The 514 nm line of a 40 mW Ar ion laser was used for spectral excitation. All Raman spectra were measured at ambient temperature. The C, H, N, and O contents of the clathrate samples were determined with an automatic elemental 22648

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Figure 2. Raman spectra for HQ clathrate samples prepared from CO2/N2 gas mixtures at 298 K and (a) 1.0 MPa and (b) 3.0 MPa for 5 days. Dotted lines at 1382 and 2326 cm 1 represent the C O vibration mode of CO2 molecules and the N N vibration mode of N2 molecules encaged in HQ clathrates, respectively.

analyzer (CE Instrument, Flash EA1112) in the National Center for Inter-University Research Facility of Seoul National University. Approximately 3.0 mg of the samples was analyzed by the dynamic flash combustion method to determine the oxidized substance with a TCD detector. To confirm the reliability and reproducibility of the measurements, the elemental composition of pure α-form HQ powders was tested and compared to the theoretical values. The experimental results showed good agreement with the theoretical values with deviations of less than 2%. We also note that only a small amount of HQ powders was used to measure the selectivity of CO2 over N2. This indicates that there is no change in the concentration of the gas phase after reaction.

’ RESULTS AND DISCUSSION Figure 1 shows the solid-state 13C NMR spectra of the HQ powders after reaction with the CO2/N2 gas mixtures at 1.0 and 3.0 MPa for 5 days. As can be seen in the figure, the NMR spectra of pure α-form HQ comprise two groups of split peaks corresponding to the hydroxyl-substituted carbon atoms and the 12 inequivalent carbon atoms. Transformation of the crystal structure of the pure α-form HQ to the β-form HQ clathrates by

enclathration of gaseous guests results in resolution of the partially resolved peaks into three distinct signals for the arranged carbon atoms of the centro-symmetric structure.16 The chemical shift of the carbon atoms attached to the hydroxyl group is obtained at 148.8 ppm, whereas the signals from the hydroxyl nonsubstituted carbon atoms are obtained at 118.3 and 116.7 ppm, respectively.14 It should be noted that a small peak at 124.4 ppm is assignable to the carbon signal of CO2 molecules captured in the cages of the HQ clathrate framework. It can also be deduced from Figure 1 that only a slight transformation of the HQ compounds prepared from the gas mixtures containing low CO2 concentrations (less than 40 mol %) under a pressure of 1.0 MPa to the HQ clathrates was achieved. In contrast, at 3.0 MPa, all of the HQ compounds could be transformed to the HQ clathrates, although somewhat incomplete conversion of the HQ sample prepared with the CO2/N2 gas mixture of 20 mol % CO2 to the HQ clathrate was observed. This indicates that complete transformation of the pure α-form HQ to the HQ clathrates from the gas mixtures containing low CO2 concentrations (less than 20 mol %) requires higher pressure than 3 MPa, although CO2 molecules can be enclathrated in the cages of the HQ clathrate framework even at low pressures around 1.0 MPa. 22649

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Figure 3. Formation kinetics of HQ clathrates with CO2 (black lines) and N2 (blue lines) as a function of pressure drop, at temperatures of 268 K (dotted lines), 273 K (dashed lines), and 293 K (solid lines), with a formation pressure of 3.0 MPa.

Raman spectroscopy can provide information on the stability and dynamics of guests occupying the cages of clathrate compounds as well as on the crystal structure of the formed clathrates. Figure 2 presents the Raman spectra of the HQ compounds after reaction with the CO2/N2 gas mixtures. Both the pure α-form HQ and the β-form HQ clathrates show a group of three unresolved peaks at 1601, 1611, and 1625 cm 1, which is attributed to the C C stretching vibrational motion of the HQ molecules.17 However, there is a clear difference in the shape of the Raman spectra of the two structural variations of the HQ compounds. For pure α-form HQ, the Raman peak at 1611 cm 1 (middle peak) is the strongest of the three peaks for the C C stretching vibration mode of the HQ molecules. However, complete transformation of the pure α-form HQ to the guest-loaded HQ clathrates results in a change in the relative intensity of this Raman peak so that it becomes the smallest of the three peaks of the C C stretching vibration mode, as shown in Figure 2. This change in relative intensity may be due to the change in the C C stretching vibrational mode of the HQ molecules upon forming the hydrogen-bonded organic framework (HOF) of the HQ clathrates. Kubinyi et al.17 reported the weakening of a vibrational Raman band of the α-form HQ in CH3CN upon formation of the solid β-modification of HQ. Therefore, the intensity ratio of the peaks of the C C stretching vibrational mode can be used as a qualitative measure of the structural transformation of HQ compounds, that is, conversion to the β-form HQ clathrate. The Raman spectra show that all of the HQ clathrates transformed from the pure α-form HQ exhibit the C O vibrational mode of CO2 molecules caged in the HQ clathrate framework at 1382 cm 1.18 However, only the HQ clathrates prepared from the CO2/N2 gas mixtures containing more than 60 mol % N2 under a pressure of 3.0 MPa show the N N stretching vibrational mode of N2 molecules caged in the HQ clathrate framework at 2326 cm 1.19 As the composition of CO2 in the gas mixture increases, there is no indication of the trapped N2 molecules in the HQ clathrate framework. This implies that the structural transformation of pure α-form HQ into the HQ clathrates is mainly induced by CO2 molecules in the CO2/N2 gas mixture. Therefore, CO2 molecules may preferentially occupy the cages of the HQ clathrates by competition with N2 molecules for the best occupancy.

Figure 4. Amount of gases stored in HQ clathrates prepared with various CO2/N2 gas mixtures at 298 K and 1.0, 2.0, and 3.0 MPa. Gray bars indicate the number of moles of CO2 molecules stored in 1 mol of HQ, while black bars indicate that of N2. Percentages above black bars represent the composition of N2 in gases (CO2 and N2) stored in HQ clathrates.

An in-depth examination of the solid-state 13C CP/MAS NMR and Raman spectroscopic measurements indicates that CO2 molecules are captured in the HQ clathrate structure more easily than N2 molecules. To elucidate the effect of temperature on the preferential occupancy, the formation kinetics of the HQ clathrates were investigated using both pure CO2 and pure N2. Figure 3 shows the formation kinetics of the HQ clathrates as determined in terms of the pressure drop of the gas phase at various temperatures. It is clear that the pressure drop of the gas phase can be attributed to the incorporation of gaseous guests (CO2 and N2) into the cages of the HQ clathrate. As shown in Figure 3, the pressure drops obtained with pure CO2 are larger than those with pure N2 at all temperatures, suggesting the preferential occupation of CO2 molecules in the cages of the HQ clathrate. It is also interesting to note that the initial rate of formation of the HQ clathrate with pure CO2 increases with increasing temperature. As observed for the gas hydrate system, preferential occupation of CO2 molecules can be explained by two factors: molecular size and electrostatic nature. The molecular diameter of N2 is too small to stabilize the β-form HQ, 22650

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The Journal of Physical Chemistry C whereas CO2 having a larger diameter can fit into the clathrate cavities. In addition, CO2 molecules tend to be attracted into the clathrate cavities due to the partial charge on the atoms (positively charged atoms are attracted into the cage center and negatively charged atoms into the top and bottom of the cavity) as compared to N2 molecules.20 The gas storage capacity and the selectivity of the HQ clathrates for CO2 were also estimated. Figure 4 shows the amount of gaseous guests captured in the HQ clathrates when they are prepared with various compositions of CO2/N2 gas mixtures at 1.0, 2.0, and 3.0 MPa, and ambient temperature. At all three pressures considered in this study, the amount of gases captured increases gradually as the composition of CO2 in the gas mixture increases. In addition, as the formation pressure increases, the amount of gases captured becomes larger. Formation of the HQ clathrate with the CO2/N2 (20:80 v/v) gas mixture at 1.0 MPa results in the capture of 0.067 mol of gases per mole of HQ, within the cages. At 3.0 MPa, we can expect 0.312 mol of gases (per 1 mol HQ) to be stored in the HQ clathrate framework using the CO2/N2 (80:20 v/v) gas mixture. The maximum stoichiometry for the HQ clathrate must be 1 gas molecule per 3 HQ molecules if we assume a single occupancy of gas molecules per cage.12 Therefore, 93% of the cages in the HQ clathrate prepared with the CO2/N2 (80:20 v/v) gas mixture at 3.0 MPa is filled with gas molecules, leading to the HQ clathrate with the formula 0.91CO2 3 0.02N2 3 3HQ. In contrast, for the HQ clathrate prepared with the CO2/N2 (20:80 v/v) gas mixture at 1.0 MPa, only 20% of the cages are filled by gas molecules. This is attributed to the incomplete conversion of pure α-form HQ to the HQ clathrate, which is consistent with the solid-state NMR and Raman results. On the basis of the experimental results, the ratio of gas to host molecules corresponds to 3.22 4.58 when the composition of CO2 in the gas mixtures at 2.0 and 3.0 MPa is higher than 40 mol %. As compared to the ideal gas to host ratio of 5.75 for the gas hydrate, the amount of HQ required to store one mole unit of gases is less than the amount of water molecules needed for the gas hydrate. Moreover, this also corresponds to a gas storage capacity of HQ of 1.98 2.82 mol/kg, which is comparable to the CO2 sorption capacity of 0.65 and 2.5 mol/ kg reported in the previous studies.2,21 From the point of view of selectivity, formation of the HQ clathrate from the CO2/N2 (40:60 v/v) gas mixture at 3.0 MPa is accompanied by high selectivity for CO2 over N2 (8 CO2/N2). For the CO2/N2 (80:20 v/v) gas mixture at 3.0 MPa, the selectivity of the HQ clathrate toward CO2 is increased to 48 CO2/N2. This indicates that efficient separation of CO2 from the CO2/N2 mixtures up to 98 mol % may be attained with the HQ clathrates in practical engineering applications.

’ CONCLUSIONS We have presented the preferential incorporation of CO2 molecules in the cages of HQ clathrates formed from CO2/N2 mixture streams with a variety of gas compositions. The solidstate 13C CP/MAS NMR and Raman spectroscopic measurements indicate that CO2 molecules are captured in the HQ clathrate structure more easily than N2 molecules. The gas storage capacity of HQ for CO2/N2 gas mixtures was found to be 1.98 2.82 mol/kg. The experimental results from this study can provide fundamental insight into the thermodynamic stability of clathrate compounds and the interaction between host and guest in the HOF. We also present elementary data required for developing a clathrate-based recovery or separation process of

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CO2 from gas mixtures. Furthermore, the present study provides a useful guide for the tailoring of HOFs for use in gas separation and storage.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +82-51-410-4684. Fax: +82-51-403-4680. E-mail: jhyoon@ hhu.ac.kr.

’ ACKNOWLEDGMENT This research was supported by the Midcareer Researcher Program through an NRF grant funded by MEST (No. 20080061974) and by the National Research Foundation of Korea Grant funded by the Korean Government (331-2008-1-D00112; 2010-0007026). This work was also supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Grant No. 2009T100100988). ’ REFERENCES (1) Soon, W.; Baliunas, S. L.; Robinson, A. B.; Robinson, Z. W. Clin. Res. 1999, 13, 149. (2) Ida, J.-I.; Lin, Y. S. Environ. Sci. Technol. 2003, 37, 1999. (3) Kang, S.-P.; Lee, H. Environ. Sci. Technol. 2000, 34, 4397. (4) Kim, D. Y.; Lee, H. J. Am. Chem. Soc. 2005, 127, 9996. (5) Seo, Y. T.; Lee, H. J. Phys. Chem. B 2004, 108, 530. (6) Yoon, J.-H.; Kawamura, T.; Ohtake, M.; Takeya, S.; Komai, T.; Yamamoto, Y.; Emi, H.; Kohara, M.; Tanaka, S.; Takano, O.; Uchida, K. J. Phys. Chem. B 2006, 110, 17595. (7) Lee, S.-Y.; Holder, G. D. AIChE J. 2002, 48, 161. (8) Kumar, R.; Englezos, P.; Moudrakovskr, I. L.; Ripmeester, J. A. AIChE J. 2009, 55, 1584. (9) Linga, P.; Kumar, R.; Englezos, P. Chem. Eng. Sci. 2007, 62, 4268. (10) Sixou, P.; Dansas, P. Ber Bunsen-Ges. Phys. Chem. 1976, 80, 364. (11) Palin, D. E.; Powell, H. M. J. Chem. Soc. 1947, 208. (12) Palin, D. E.; Powell, H. M. J. Chem. Soc. 1948, 815. (13) Wallwork, S. C.; Powell, H. M. J. Chem. Soc., Perkin Trans. 2 1980, 4, 641. (14) Lee, J.-W.; Lee, Y.; Takeya, S.; Kawamura, T.; Yamamoto, Y.; Lee, Y.-J.; Yoon, J.-H. J. Phys. Chem. B 2010, 114, 3254. (15) Lee, Y.; Lee, J.-W.; Lee, H.-H.; Lee, D. R.; Kao, C.-C.; Kawamura, T.; Yamamoto, Y.; Yoon, J.-H. J. Chem. Phys. 2009, 130, 124511. (16) Ripmeester, J. A. Chem. Phys. Lett. 1980, 74, 536. (17) Kubinyi, M.; Billes, F.; Grofcsik, A.; Keresztury, G. J. Mol. Struct. 1992, 266, 339. (18) Uchida, T.; Takagi, A.; Kawabata, J.; Mae, S.; Hondoh, T. Energy Convers. Manage. 1995, 36, 547. (19) Musso, M.; Matthai, F.; Keutel, D.; Oehme, K.-L. Pure Appl. Chem. 2004, 76, 147. (20) Zubkus, V. E.; Shamovsky, I. L. Chem. Phys. Lett. 1992, 195, 135. (21) Ding, Y.; Alpay, E. Chem. Eng. Sci. 2000, 55, 3461.

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