molecules Article Carbon Isotope Fractionation during the Formation of CO2 12 Hydrate and Equilibrium Pressures of CO2 and 13 CO2 Hydrates Hiromi Kimura 1, Go Fuseya 1 , Satoshi Takeya 2 and Akihiro Hachikubo 3,* 1 Kitami Institute of Technology, Graduate School of Engineering, 165 Koen-cho, Kitami 090-8507, Japan; [email protected] (H.K.); [email protected] (G.F.) 2 National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan; [email protected] 3 Environmental and Energy Resources Research Center, Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan * Correspondence: [email protected] Abstract: Knowledge of carbon isotope fractionation is needed in order to discuss the formation and dissociation of naturally occurring CO2 hydrates. We investigated carbon isotope fractionation during 12 13 CO2 hydrate formation and measured the three-phase equilibria of CO2–H2O and CO2–H2O systems. From a crystal structure viewpoint, the difference in the Raman spectra of hydrate-bound 12 13 13 CO2 and CO2 was revealed, although their unit cell size was similar. The δ C of hydrate-bound CO2 was lower than that of the residual CO2 (1.0–1.5‰) in a formation temperature ranging between 226 K and 278 K. The results show that the small difference between equilibrium pressures of Citation: Kimura, H.; Fuseya, G.; 12 13 ~0.01 MPa in CO2 and CO2 hydrates causes carbon isotope fractionation of ~1‰. However, the Takeya, S.; Hachikubo, A. Carbon 12 13 difference between equilibrium pressures in the CO2–H2O and CO2–H2O systems was smaller Isotope Fractionation during the than the standard uncertainties of measurement; more accurate pressure measurement is required Formation of CO2 Hydrate and 12 for quantitative discussion. Equilibrium Pressures of CO2 and 13 CO2 Hydrates. Molecules 2021, 26, 4215. https://doi.org/10.3390/ Keywords: CO2 hydrate; carbon isotope; isotopic fractionation; phase equilibrium; Raman spectra molecules26144215 Academic Editors: Nobuo Maeda, Zhiyuan Wang and Xiaodong Shen 1. Introduction Gas hydrates are crystalline clathrate compounds that have guest gas molecules Received: 11 June 2021 encapsulated in hydrogen-bonded water cages and can be thermodynamically stable Accepted: 7 July 2021 at high pressures and low temperatures. The phase equilibrium pressure-temperature Published: 11 July 2021 conditions vary depending on the guest molecule type [1]. Gas hydrates with encapsulated natural gases exist in sub-marine/sublacustrine sediments and below the permafrost on Publisher’s Note: MDPI stays neutral Earth. Because they contain copious amounts of greenhouse gases, such as methane, with regard to jurisdictional claims in concerns arose that their dissociation could add to global warming [2–4]. Hydrate-bound published maps and institutional affil- gas contains both hydrocarbons and CO2 [5–7], and natural gas hydrates encapsulating iations. CO2 have been observed at the venting sites of liquid CO2 [8,9]. On the other hand, the possibility of existing CO2 hydrates on Mars was proposed [10,11] and the conditions for CO2 hydrate formation on Mars have been discussed [12]. As above, CO2 hydrates are critical in both geochemistry and space science and require better understanding. Copyright: © 2021 by the authors. Since CO2 comprises carbon and oxygen atoms, several isotopic species of CO2 Licensee MDPI, Basel, Switzerland. exist, depending on the combination of stable isotopes (isotopologues: 12C, 13C, 16O, This article is an open access article 17 18 13 O, and O). For example, the abundance ratio of CO2 is ~1.1% of the total CO2 and distributed under the terms and 12 the rest is almost CO2. Stable isotope fractionation of the guest gas during the gas conditions of the Creative Commons hydrate formation provides information for discussing the formation, maintenance, and Attribution (CC BY) license (https:// decomposition processes of gas hydrates. For example, the trend of hydrogen and carbon creativecommons.org/licenses/by/ isotope fractionation during the formation of synthetic methane and ethane hydrates was 4.0/). Molecules 2021, 26, 4215. https://doi.org/10.3390/molecules26144215 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 4215 2 of 8 investigated [13]. Moreover, the results were applied to estimate the formation process of natural gas hydrates [14–16]. Carbon isotope fractionation during the formation of 13 CO2 hydrates was reported [17], revealing that the CO2 δ C in the hydrate phase was 12 0.9‰ lower than that in the gas phase at 268 K [17]. Therefore, CO2 is more easily 13 encapsulated in the hydrate phase than CO2. Because the temperature of existing natural gas hydrates at sea/lake bottom sediments is above 273 K, information about carbon isotope fractionation above the freezing point of water is needed to discuss the formation and dissociation of naturally occurring CO2 hydrates. Furthermore, CO2 hydrates have been suggested outside of Earth. Therefore, it is also necessary to confirm isotope fractionation in a wider temperature range. In studies on methane hydrates, the equilibrium pressures of CH3D and CD4 hydrates were ~0.04 MPa and ~0.14 MPa higher than those of the CH4 hydrate, respectively [18]. Because guest molecules, which have lower equilibrium pressure, are preferentially en- capsulated in the gas hydrate cages, the difference in equilibrium pressures of CH3D and CH4 hydrates can explain the hydrogen isotope fractionation in methane during methane hydrate formation [18]. Thus, comparing the phase equilibrium p–T conditions of each gas hydrate encapsulating isotopologues can explain the trend of isotopic fractionation of 13 guest gases during gas hydrate formation. However, the equilibrium pressure of CO2 hydrates has not been reported. In this study, we synthesized gas hydrate samples, encapsulated CO2 isotopologues 12 13 ( CO2 and CO2), and characterized their crystallographic properties using powder X-ray diffraction (PXRD) and Raman spectroscopy. We also measured the equilibrium 12 13 pressures of CO2 and CO2 hydrates at a temperature range between 269 K and 278 K. We investigated carbon isotope fractionation between hydrate-bound gas and residual gas in a pressure cell in the temperature range of 226 K to 278 K. 2. Results and Discussion 12 13 We confirmed the crystallographic structures of CO2 and CO2 hydrates and ob- tained their lattice constants using the PXRD method. The diffraction patterns of cubic structure I (sI) hydrates were observed from these hydrates (Figure S1). The lattice constants 12 13 of CO2 and CO2 hydrates were similar at 11.8352(6) Å and 11.8323(5) Å, respectively. 12 13 Figure1 shows the Raman spectra of CO2 and CO2 hydrates and Table1 summa- rizes the observed Raman shifts of hydrate-bound isotopologue CO2 and their assignments to the vibrational modes. Two distinct peaks corresponding to the Fermi dyad of CO2 −1 −1 12 in the hydrate cages were observed at 1278.2 cm and 1381.9 cm for CO2 hydrates, corresponding to [19–21]. These peaks shifted to 1256.4 cm−1 and 1365.8 cm−1 for the 13 −1 CO2 hydrate. Qin and Kuhs [21] observed 1366.6 ± 4.0 cm for the upper Fermi dyad of 13 hydrate-bound CO2, and our data were consistent with their results. Definite differences (22 cm−1 and 16 cm−1 for lower and upper peaks, respectively) were found between these 12 13 Raman peaks caused by the encapsulated CO2 and CO2 molecules. Table 1. Observed Raman shifts of hydrate-bound CO2 and assignments to the vibrational modes. Raman Shift Raman Shift Guest Molecule Assign Vibrational Mode Gas/cm−1 Hydrate/cm−1 a 12 1285.40 [22] 1278.2 , 1277 [19], 1278 [20], 1275.5 ± 0.8 [21] CO2 a 1388.15 [22] 1381.9 , 1381 [19], 1382 [20], 1379.4 ± 0.8 [21] b CO s-stretch υ1 + 2υ2 b 1266.03 [22] 1256.4 a + bend 13CO 2 1369.90 [22] 1365.8 a, 1366.6 ± 4.0 [21] a This work, uncertainty is <1.1 cm−1. b Fermi resonance. MoleculesMolecules2021 2021,,26 26,, 4215x FOR PEER REVIEW 33 of 88 12 CO2 hydrate Intensity /a.u. Intensity 13 CO2 hydrate 1200 1300 1400 1500 Raman Shift /cm−1 1212 13 FigureFigure 1.1. Raman spectraspectra ofof CO2 and COCO22 hydrateshydrates in in the the CO CO stretching stretching vibration mode region ofof 2 COCO2. The The spectra spectra were were recorded at atmospheric pressure and 140140 K.K. (a.u.,(a.u., arbitraryarbitrary units).units). Table2 2 lists lists and and Figure Figure2 2plots plots the thep p––TT datadata forfor thethe three-phasethree-phaseequilibrium equilibrium (ice/water (ice/water 1212 1313 ++ hydratehydrate ++ vapor)vapor) forfor thethe COCO22–H–H22O and CO22–H–H22OO systems. systems. The The phase phase in in the equilib- 12 riumrium ofof eacheach pointpoint inin TableTable2 2was was determined determined using using the the quadruple quadruple point point of of the the 12COCO22–H–H22OO systemsystem (1.04(1.04 MPa MPa and and 271.6 271.6 K) K) reported reported by [by23 ].[2 To3]. ensureTo ensure the accuracy the accuracy of the of experimental the experi- apparatusmental apparatus described described in the previous in the previous section, section, we evaluated we evalua the equilibriumted the equilibriump–T data p– ofT 12 dataCO 2ofhydrates. 12CO2 hydrates. From Figure From2 Figure, they correlate 2, they correl well withate well the with literature the literature [ 23–26]. [23–26]. The equilib- The 13 riumequilibrium pressures pressures in the inCO the2–H 13CO2O2–H system2O system are higher are higher by 0.007–0.012 by 0.007–0.012 MPa comparedMPa compared with 12 thewith corresponding the corresponding values values in the in theCO 212–HCO22O–H system2O system between between 269 K269 and K and 278 K.278 However, K.