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One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending
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One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending Yun-Ho Ahn a, Seokyoon Moon b, Dong-Yeun Koh a, Sujin Hong b, Huen Lee a, Jae W. Lee a, *, Youngjune Park b, ** a Department of Chemical and Biomolecular Engineering (BK21þ Program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea b School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Clathrate hydrates Inclusion compounds Gas storage Gas-phase promoter Hydrogen-natural gas blends
Harvesting energy gases in the nanoporous water-frameworks of clathrate hydrate enables the widespread of hydrogen-based fuels converted from excess solar or wind energy sources (i.e., Power-to-Gas). However, there has always been a critical trade-off between mild formation condition and maximum working capacity. Here we demonstrate the ‘natural gas modulator’ based synthesis that leads to significantly reduced synthesis pressure (PH2 3 MPa) simultaneously with the formation of hydrogen clusters (up to 3 molecules) in the confined nanoporous cages of clathrate hydrates. Instead of employing hazardous liquid chemicals, clean energy gas of natural gas is used for the first time to multiply load H2 in all cages (512, 51262, and 51264 cages) of hydrogennatural gas hydrates without any postsynthetic modification (e.g., guest-exchange reaction). This approach minimizes the environmental impact and reduces operation cost since clathrate hydrates do not generate any chemical waste in both synthesis and decomposition process, and hydrogen-natural gas mixture can be also utilized as an energy resource as itself.
1. Introduction Integration of renewable energy sources with carbonaceous fuels could revolutionize the energy infrastructure by enabling new synergistic energy-efficient technologies [1–3]. In the context of lessening the current dependence on traditional fossil fuels and shifting the energy paradigm from non-renewable to sustainable energy sources, hydrogen-natural gas blends (hereafter HNGB), also known as ‘Hythans®’ (hydrogen þ methane or ethane), have been explored as either energy carriers or alternative fuels [4,5]. Hydrogen can be blended with natural gas by injecting it into existing natural gas pipeline networks coupled with the Power-to-Gas (P2G) scheme (Fig. S1A), which involves the transformation of excess electricity produced from renewable energy sources into hydrogen via electrolysis [6,7]. Subsequently, the HNGB can be delivered to end-user devices, which utilize the hybrid energy, or near-pure hydrogen can be extracted from the HNGB via conventional separation processes (e.g., pressure swing adsorption) for wide applications [4–9].
For the efficient storage and transportation of hydrogen, numerous materials have been investigated, including metal organic frameworks [10–12], liquid organics [13], metal hydrides [14], and chemical hydrogens [15,16], etc. Among the possible options, clathrate hydrates offer distinct benefits over other contenders, specifically eliminating the concern for complex material synthesis because they are mainly composed of cheap water molecules [17–21]. Clathrate hydrates are nanoporous inclusion compounds composed of the 3D network of polyhedral cages made of hydrogen-bonded ‘host’ water molecules and captured ‘guest’ gas or liquid molecules [18]. Earlier reports revealed that hydrogen can be stored in clathrate hydrates only at extreme conditions (i.e., 220 MPa at 249 K) [17–21]. Forming binary clathrate hydrates by introducing organic liquid thermodynamic promoters (e.g., tetrahydrofuran or cyclopentane) can significantly lower the formation pressure conditions (i.e., 5 MPa at 279.6 K) [22,23]. However, this introduces a trade-off between mild formation condition and hydrogen storage capacity since the promoter molecule inevitably occupies the sorption sites, where the molecular hydrogens can be accommodated.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.W. Lee), [email protected] (Y. Park). https://doi.org/10.1016/j.ensm.2019.06.007 Received 14 March 2019; Received in revised form 5 June 2019; Accepted 5 June 2019 Available online xxxx 2405-8297/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-H. Ahn et al., One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.06.007
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2(512) ∙ 46H2O) hydrate, the use of CH4 þ C2H6 binary mixtures can create a variety of structures, sI, structure II (sII, 8(51264) ∙ 16(512) ∙ 136H2O), or mixed sI and sII, depending on the feed gas composition, pressure, and temperature [18,37]. C2H6 preferentially occupies large cages of sI (51262) or sII (51264) hydrate, whereas CH4 and H2 can be entrapped in both small (512) and large cages [37]. Since the gas-phase mixtures of CH4 and C2H6 form sII hydrate with C2H6 compositions ranging from 1 to 25 mol% [18,37,38], we selected gas-phase mixtures of CH4 (70.0 mol%) þ C2H6 (30.0 mol%) and CH4 (90.0 mol%) þ C2H6 (10.0 mol%) as feed gases to form sI and sII CH4 þ C2H6 hydrates, respectively. In order to encage H2 into the sI or sII hydrates, we designed sample synthesis strategies with different pathways as shown in the schematic illustrations (Fig. 1), and the detailed procedures are explained in the experimental procedures section. The guest compositions of the hydrate samples synthesized via different pathways were analysed by gas chromatography and tabulated in Table 1 with the compositions of feed gas at each step. Considering the three-phase (hydrates (H) – liquid water (Lw) – vapor (V)) P – T equilibrium conditions of the CH4 þ C2H6 þ H2 system measured in this study, the inclusion of H2 in the hydrate framework could be achieved under the moderate pressure and temperature condition (i.e., 9 MPa at 263.15 K) (Fig. S2). To identify the crystal structures of all synthesized hydrate samples, powder diffraction patterns were obtained from synchrotron XRD measurements and analysed via Le Bail fitting method [39]. The synchrotron XRD patterns of the mixed hydrates, which were synthesized under the feed gas having CH4/C2H6 ratios of 7/3 and 9/1, with and without H2, corresponded to the unique patterns of cubic Pm3n and Fd3m structures implying the formation of sI and sII hydrate, respectively (Fig. 2 and S3). Here, there was only a small fraction of ice impurities in the samples synthesized without H2 (Fig. S3), and the formation of pure H2 hydrate generally requires extremely high pressure and low temperature conditions [19,20]. Therefore, it is reasonable to speculate that the inclusion of H2 is caused only by guest replacement mechanisms in the pre-synthesized hydrate media, not by the new formation of pure H2
Tuning binary clathrate hydrates by using less concentrated promoter than the stoichiometric concentration (5.56 mol%) can allow more hydrogen to be captured under mild formation conditions [24–27]. Also, harvesting multiple hydrogen molecules per single cage has been tried to enhance the hydrogen storage capacity [24,28–33]. However, there is still a limit to the use of liquid thermodynamic promoter that does not contribute to any energy utilization with the reduced hydrogen storage capacity. Therefore, we see this potential sorption site (or cages) occupied by promoter as a ‘hidden niche’ for other smaller energy gas molecules. In this study, we propose a practical ‘gas-phase modulator’ based synthesis of hydrogen-natural gas hydrates (Fig. S1B) that does not generate undesirable chemical waste after dissociation for the immediate service (Fig. S1C). We compare several hydrogen-natural gas hydrates synthesized via different pathways and reveal the pathway-dependent hydrogen clustering effect associated with mild formation conditions; hydrogen clusters can only be achieved in the hydrate directly evolved from ice powder. Some light hydrocarbon promoters have been tried, however, H2 cannot be multiply loaded in a single cage of presynthesized hydrocarbon hydrate [34] and no information about H2 multiple occupancy was given in the simple equilibrium measurements [35,36]. In contrast to our direct synthesis approach, N2 hydrate has also been reported to induce the hydrogen clusters successfully via additional postsynthetic guest-exchange reaction [28]. However, this approach requires additional N2 separation process since N2 remaining in hydrate cannot be utilized as an energy resource. Our approach enables the simultaneous storage of both light hydrocarbon and clustered hydrogen, they can be utilized as a low-cost and low-emission fuel-mixture. 2. Results and discussion 2.1. Structure identification and guest population in the mixed hydrates While pure CH4 or C2H6 preferentially forms structure I (sI, 6(51262) ∙
Fig. 1. Two synthetic pathways of sI and sII hydrates. Synthetic pathways of various sI and sII hydrates are shown with the corresponding guest-distributions. H2, CH4, and C2H6 molecules are shown as green, blue, and red balls, respectively. The notation under the hydrate structure refers to the synthetic pathway and its corresponding hydrogen occupancy for each case (i.e., ‘Ice – sII – sII (1S)’: sII hydrate was firstly formed by CH4 and C2H6, and then H2 was ‘singly’ introduced to small cage (1S) via guest exchange method; Ice – sII (1S2S2L3L): All gas-phase guests of CH4, C2H6, and H2 formed sII hydrates directly from ice. Simultaneously, one or two H2 molecules were captured in small cages (1S2S) and two or three H2 molecules were captured in large cages (2L3L)). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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% and 15.5 mol%, respectively. When CH4 and C2H6 existed with a ratio of 7/3 in the feed gas (Case I), the C2H6 composition in the hydrate media was almost identical to that of the hydrate sample before H2 loading (57.1–56.5 mol%). Interestingly, however, increase in the amount of H2 (0–11.0 mol%) in the hydrate media was almost equal to the decrease in the amount of CH4 (42.9–32.5 mol%), and this implies that most enclathrated H2 molecules replaced CH4, which was mainly captured in 512 cages, driven by the difference in the chemical potential. The mechanisms of the replacement reaction of CH4 hydrate by CO2 molecules [40,41] or N2 hydrate by H2 molecules [28,31] have already been reported in the literature. Surprisingly, when the ternary gas mixtures of CH4 þ C2H6 þ H2 were directly contacted with the ice powder (Cases II and IV), the amount of H2 was dramatically increased compared to that of H2 in the guestexchanged hydrates (Cases I and III). The H2 compositions in the hydrate were 21.1 mol% and 22.4 mol% in Cases II and IV, respectively, even with the same ratio of CH4 to C2H6 in Cases I (7/3) and III (9/1). In view of these observations, we postulate that CH4 and C2H6 act as thermodynamic promoters for H2 storage in hydrate, and the amount of enclathrated H2 depends on the ‘synthetic pathway’ even with the same feed gas compositions. At this stage, it is necessary to investigate the guest-distribution of all mixed hydrates synthesized in various pathways to shed light on this peculiar H2 inclusion phenomenon.
Table 1 The compositions of feed gas and mixed hydrate for each sample. Synthetic pathways of mixed hydrate samples and corresponding feed gas compositions (mol%) at each step Ice → CH4 (70.0%) þ C2H6 (30.0%) Ice → CH4 (70.0%) þ C2H6 (30.0%) → CH4 (46.7%) þ C2H6 (20.0%) þ H2 (33.3%) Ice → CH4 (46.7%) þ C2H6 (20.0%) þ H2 (33.3%) Ice → CH4 (90.0%) þ C2H6 (10.0%) Ice → CH4 (90.0%) þ C2H6 (10.0%) → CH4 (60.0%) þ C2H6 (6.7%) þ H2 (33.3%) Ice → CH4 (60.0%) þ C2H6 (6.7%) þ H2 (33.3%)
Hydrate composition (mol%)
H2 Occupancya
CH4
C2H6
H2
42.9
57.1
0
Case I
32.5
56.5
11.0
1S
Case II
37.2
41.7
21.1
1S2S
69.4
30.6
0
Case III
64.3
20.2
15.5
1S
Case IV
60.4
17.2
22.4
1S2S2L3L
Structure
sI
sII
a 1S, one H2 molecule in small cages; 2S, two H2 molecules in small cages; 2L, two H2 molecules in large cages; 3L, three H2 molecules in large cages.
2.2. Synthetic pathway-dependent H2 occupation behavior per cage When the ternary gas mixture directly forms sI hydrate (Case II), the corresponding Raman peaks showed single H2 molecules (1S; 4120–4125 cm1) as well as double H2 molecules (2S; 4155–4160 cm1) in 512 cages (Fig. 3A). In previous studies, double H2 occupancy in small cages or multiple H2 occupancy in large cages was mainly reported in ‘sII’ hydrates since several sII hydrate formers (e.g., tetrahydrofuran [24], propane [30], pyrrolidine [31]) were used as thermodynamic promoter to tune the cage dimension under moderate conditions. The double H2 occupancy in a sI CO2 þ H2 hydrate was reported before [32,33], however, the NMR signal was unclearly assigned to two H2 molecules in 512
hydrate (Cases I and III). Considering the structural consistency of the samples, the ratio of CH4 and C2H6 in the feed gas, starting with or without H2, can play a critical role in determining the structure of the mixed hydrate. Even with the same feed gas compositions, the synthetic pathways of mixed (CH4 þ C2H6 þ H2) hydrates affect the composition of enclathrated guests resulting in different lattice parameters. The guest exchange method (Cases I and III) successfully induces enclathration of H2 in the pre-synthesized sI and sII hydrates with the H2 compositions of 11.0 mol
Fig. 2. Structural characterizations of mixed gas hydrates. Synchrotron XRD patterns of the mixed gas hydrates composed of (A) CH4 (32.5 mol%) þ C2H6 (56.5 mol%) þ H2 (11.0 mol%), (B) CH4 (37.2 mol%) þ C2H6 (41.7 mol%) þ H2 (21.1 mol%), (C) CH4 (64.3 mol%) þ C2H6 (20.2 mol%) þ H2 (15.5 mol%), and (D) CH4 (60.4 mol%) þ C2H6 (17.2 mol%) þ H2 (22.4 mol%) with their corresponding space groups and lattice parameters. Red circles, observed XRD pattern; black solid line, calculated XRD pattern; tick marks, cubic Pm3n sI hydrates (magenta), cubic Fd3m sII hydrates (green), and hexagonal P63/mmc ice (blue), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. Different guest-distributions of mixed gas hydrates. H–H vibron region in Raman spectra of the (A) sI hydrates (Cases I and II) and (B) sII hydrates (Cases III and IV) showing the direct evidence of multiple H2 occupancies. (C) C–H stretching mode in Raman spectra of the mixed sI hydrates composed of CH4 (42.9 mol%) þ C2H6 (57.1 mol%) (black), CH4 (32.5 mol%) þ C2H6 (56.5 mol%) þ H2 (11.0 mol%) (blue, Case I), and CH4 (37.2 mol%) þ C2H6 (41.7 mol%) þ H2 (21.1 mol%) (red, Case II). (D) C–H stretching mode in Raman spectra of the mixed sII hydrates composed of CH4 (69.4 mol%) þ C2H6 (30.6 mol%) (black), CH4 (64.3 mol%) þ C2H6 (20.2 mol%) þ H2 (15.5 mol%) (blue, Case III), and CH4 (60.4 mol%) þ C2H6 (17.2 mol%) þ H2 (22.4 mol%) (red, Case IV). All spectra in (C) and (D) were normalized with the O–H stretching mode of water frameworks for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
cages [42]. In this study, both single and double H2 occupancy in 512 cages of ‘sI’ hydrate are clearly observed for the first time with a large amount of entrapped H2 (21.1 mol%). According to the density functional theory (DFT) calculations results, the H–H vibrons of doubly occupied 512 cages of ‘sII’ hydrate are known to be around or above 4155 cm1 [43]. Although our experimental results were obtained from the sI hydrate, the Raman signals matched well with previous observations of the sII hydrate since both sI and sII hydrates are composed of the same 512 cages [29–31,43]. Thus, it is reasonable to assign the Raman peaks at around 4155–4160 cm1 to doubly occupied H2 in 512 cages of sI hydrate. However, when H2 molecules were introduced to pre-synthesized sI hydrate (Case I, Fig. 3A), the Raman peaks of double H2 molecules did not appear. Only Raman signals of the singly occupied H2 from the two spin isomers of ortho- and parahydrogen were observed [44]. Raman spectra of H2-enclathrated sI hydrates strongly indicate that the double H2 occupancy in 512 cages can be achieved by forming the mixed sI hydrate directly from ice powders, and the guest exchange reaction does not allow two H2 molecules to be captured in one 512 cage of the pre-synthesized sI hydrate. Also, due to the double H2 occupancy in 512 cages, the hydrogen storage capacity (21.1 mol%) of the tuned hydrate can be maximized to around two times that of the mixed hydrate without double occupancy (11.0 mol%). For a qualitative analysis of the guest-distribution, we normalized three Raman spectra with O–H stretching modes of the host water framework (Fig. 3C). The intensities of the C–H stretching mode of C2H6 in large cages (2884 and 2940 cm1) and that of the C–H stretching mode of CH4 in large (2900 cm1) and small cages (2911 cm1) decreased after the guest exchange reaction (black to blue solid line). In the mixed hydrate with double H2 occupancy in 512 cages, the amount of enclathrated CH4 and C2H6 decreased more as the amount of H2 was maximized (Case II), and this tendency was in line with the GC results (Table 1). However, with these Raman results, there is a limit to quantify and compare the exact guest composition in each small and large cage of three different mixed hydrates. When the ternary gas mixture directly forms sII hydrate (Case IV), we
observed double and multiple H2 occupancy in 512 and 51264 cages, respectively (Fig. 3B). The Raman peaks from 4128 to 4142 cm1 corresponded to two to three H2 molecules (2L – 3L) in 51264 cages, in accordance with previous theoretical and experimental results of sII hydrates [29–31,43]. At the initial stage of forming the host water framework, gas-phase co-guest molecules of CH4 and C2H6 can tune the hydrate dimension, resulting in both double H2 occupancy in 512 cages and multiple H2 occupancy in 51264 cages. Hydrogen clusters (two to four molecules) in 51264 cages have been reported in the literature, however, they were commonly achieved under high pressure of H2 ranging from 35 MPa to 58 MPa at low temperature of around 240 K [29–31]. By using our approach, H2 can be multiply enclathrated showing high contents (22.4 mol%) even though the total pressure of the system was 9 MPa at 263.15 K, and the partial pressure of H2 was only around 3 MPa. It is the first time that the energy gas molecules of CH4 and C2H6 successfully derive the multiple H2 occupation in both sI and sII hydrates via direct evolution from ice particles under the moderate formation condition (Table S1 for comparison). On the contrary, as shown in Fig. 3B, hydrogen cluster in any 512 or 51264 cage was not observed in the guest-exchanged sII hydrate (Case III) similar to the guest-exchanged sI hydrate (Case I). In Fig. 3D, we observed that the Raman signal intensities of C–H stretching modes of CH4 and C2H6 in sII hydrate were almost identical between the Case III and Case IV. It appears that, due to the multiple H2 occupancies in both 512 and 51264 cages, the amount of enclathrated H2 increased while that of CH4 and C2H6 did not change significantly. 2.3. The expanded lattice: a niche for H2 clusters These hydrogen inclusion phenomena with the gas-phase modulator can be interpreted with the expanded lattice of the hydrates. According to the previous studies, lattice parameter of hydrate has been known as one of critical factors in determining the multiple H2 occupancy [30,31]. As the amount of H2 increased with the double and multiple H2 occupancy, lattice parameters of sI and sII hydrates increased by 0.471% 4
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(a ¼ 11.879(1) Å for the Case I and a ¼ 11.935(2) Å for the Case II) and 0.041% (a ¼ 17.148(4) Å for Case III and a ¼ 17.156(3) Å for Case IV), respectively, implying that the expanded lattices have a chance to provide enough room for storing hydrogen clusters in a single cage [30,31]. At this stage, we have to consider the procedure of hydrate lattice formation in the two different synthetic pathways. The ‘Direct evolution from ice’ method (Cases II and IV) involves only one crystallization step with the feed gas of CH4, C2H6, and H2. When H2 participates in the initial stage of forming the host water framework, hydrogen clusters can be successfully stored, and the expanded hydrate lattice (a ¼ 11.935(2) Å for the Case II and a ¼ 17.156(3) Å for Case IV) can be stabilized by CH4 and C2H6. However, in the case of the ‘Gas-phase exchange’ method (Cases I and III), even though the exact replacement reaction mechanism has not been revealed yet, we can imagine that there must be additional steps such as local dissociation of structures and diffusion of guest molecules in the pre-synthesized hydrate lattices constructed by CH4 and C2H6. During these procedures, storing hydrogen clusters in a single cage might be difficult because of kinetic issues. In other tuned hydrate systems, as the gas-phase exchange reaction enables loading of hydrogen clusters in the pre-synthesized hydrates, we can guess the guest occupancy of each guest [28,30]. However, in this work, as three types of guests are injected simultaneously to the hydrate where hydrogen clusters are observed, it is somewhat difficult to clarify the tuning mechanism. By the way, the previous molecular dynamics (MD) calculation results reveals that the change in the hydrate lattice highly depends on the guest occupation in tight small cages rather than in loose large cages [45]. Since the double H2 occupancy in sI 512 cages and multiple H2 occupancy in sII 51264 cages dominantly occur, therefore, the degree of peak shift for tuned sI hydrate is somewhat larger than that of sII hydrate (Fig. S4). In order to investigate the synthetic pathway-dependent guest-distribution of mixed hydrates further, we analysed sII hydrate, obtained by structural transition from pre-synthesized sI hydrate (See Supplementary Material Text). It was confirmed that H2 molecule can singly occupy the newly synthesized small cages of sII hydrate during the structural transition. However, multiple H2 occupancy in any cage was not observed. Also, we obtained the hydrogen-natural gas storage capacities of Case I – IV hydrates as well as the weight fractions of hydrogen in the enclathrated gas mixtures with some assumptions (Detailed procedures and results are in the Supplementary Material Text and Fig. S8). Since we used three types of gases in the feed, the hydrogen-natural gas storage capacities cannot be precisely obtained, but, the capacities were between 15 and 22 wt%. From these findings, the clean energy platform of mixed CH4 þ C2H6 þ H2 hydrates must be explored further to optimize and control the gas composition taking into account practical transportation and storage application.
4. Experimental section 4.1. Materials and sample preparations Deionized water of ultrahigh purity was supplied from a Merck Millipore purification unit (MILLI-Q Direct System, Merck KGaA, USA). Gas mixtures of CH4 þ C2H6 with or without H2 were purchased from Special Gas Co. (Republic of Korea). The compositions of the gas mixtures used in the experiments are as follows: CH4 (70.0 mol%) þ C2H6 (30.0 mol%), CH4 (90.0 mol%) þ C2H6 (10.0 mol%), CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol%), and CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%). Gas hydrate samples were synthesized by using finely ground ice powder (<200 μm) under pressure conditions of less than 9 MPa at 263.15 K for each gas mixture. In order to provide higher driving force to obtain higher conversion yield and maintain the temperature and pressure condition above the phase equilibrium line (Fig. S2), we used higher pressure and lower temperature than the phase equilibrium condition. All samples were prepared via three different pathways to examine the different guest occupation behavior depending on the H2 loading sequences, and the detailed procedures are explained below. Before the injection of H2, precursor frameworks, binary gas hydrates of CH4 and C2H6 were synthesized by exposing the ice powders to the feed gases of CH4 þ C2H6 mixtures. The ice powders were loaded in a pressure vessel, and pressurized up to 6 MPa at 263.15 K with the gas mixtures. The reactor was kept in a refrigerated water-ethanol circulator (RW-2025G, Jeio Tech Co., Ltd., Republic of Korea) for 3 days to obtain fully converted CH4 þ C2H6 hydrates. After confirming there was no ice phase in the gas hydrates via synchrotron X-ray diffraction (synchrotron XRD), all the samples were reloaded in the reactor and re-pressurized to 9 MPa at 263.15 K for one week with gas mixtures of H2 to load H2 into the lattices of the mixed gas hydrates: the mixed gas hydrate formed with the CH4 (70.0 mol%) þ C2H6 (30.0 mol%) mixture was re-pressurized with CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol%) (Case I in Table 1), and the mixed gas hydrate formed with the CH4 (90.0 mol%) þ C2H6 (10.0 mol%) mixture was re-pressurized with CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%) (Case III in Table 1); these compositions were aimed at maintaining the CH4 to C2H6 ratio as 7 to 3 and 9 to 1, respectively. The reactor was then quenched in liquid nitrogen and the mixed gas hydrates were recovered for analyses. Ternary mixtures of CH4 þ C2H6 þ H2 were used as a feed gas at the initial step to form mixed gas hydrates. The ice powders were loaded in each pressure vessel and pressurized up to 9 MPa at 263.15 K with feed gases containing CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol %) (Case II in Table 1) and CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%) (Case IV in Table 1), respectively. In order to increase the conversion yield of the mixed gas hydrates, re-grinding and repressurization procedures were performed following the protocols in the previous study. [46] First, 200 μm sized ice particle was exposed to the aforementioned feed gas. After a week, the reactor was quenched in liquid nitrogen for a few seconds, and then the mixed gas hydrate particles were recovered and quenched in liquid nitrogen so that the hydrate particles might not change into ice. The particles were ground to 200 μm under liquid nitrogen and exposed to the feed gas again. By repeating this procedure, more gas hydrates can be formed from the unreacted ice as it can contact gas molecules through the re-grinding step. Lastly, ice powders were exposed to a CH4 (70.0 mol%) þ C2H6 (30.0 mol%) mixture at 6 MPa and 263.15 K, and sI type gas hydrate was initially formed. After confirming there was no ice phase in the gas hydrates, the feed gas was changed from CH4 (70.0 mol%) þ C2H6 (30.0 mol%) to CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%), resulting in a structural transition from sI to sII hydrate. After changing the gas mixtures, we performed the same re-grinding procedure and recovered some parts of samples once in every week. These mixed gas hydrate samples synthesized in different pathways were used for spectroscopic analyses including low-temperature
3. Conclusion We provide a practical concept for storing H2 in clathrate hydrates by introducing co-guest species of natural gas (CH4 and C2H6) instead of eco-unfriendly liquid organic promoter. The three distinct approaches to form mixed CH4 þ C2H6 þ H2 hydrate adopted here enable H2 enclathration under mild formation conditions (Ptotal ¼ 9 MPa and PH2 3 MPaat 263.15 K). Among the three different synthetic pathways, the direct evolution method can tune the hydrate cages allowing hydrogen clusters in both sI and sII hydrate. Even though we used the two specific feed gas compositions to analyze the synthetic pathway-dependent guest occupation behavior in both sI and sII hydrates, various feed gas compositions can be adopted to find the optimal synthetic condition to provide actual fuel-mixtures with demanded compositions. We expect our findings can be applied to not only an energy-efficient gas storage material, but also a smart platform to utilize HNGB (e.g., Hythane®, HCNG (hydrogen and CNG blends)), which can serve as a new alternative energy source with targeted hydrogen content by designing synthetic pathways of mixed gas hydrates. 5
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pressurized up to the desired pressures by using a syringe pump (500D, Teledyne Isco Inc., USA). After achieving stabilization, the temperature of the reactor was decreased at a rate of 0.2–1.0 K/h to form a mixed gas hydrate. After a sudden pressure drop, the reactor was then slowly heated at a rate of 0.1 K/h until the pressure reached the initial pressure. The pressure and temperature condition of the fully dissociated mixed gas hydrate was considered as an equilibrium point. Several equilibrium points were obtained with variation of the initial pressure condition. During the formation-dissociation process of the mixed gas hydrates, the pressure and the temperature of the reactor were monitored via a pressure transducer (PSHB250BCPG, Sensor System Technology Co., Ltd., Republic of Korea), which has an accuracy of 0.15%, and a four-wire type Pt-100Ω probe, which has a full-scale accuracy of 0.05%, respectively. They were automatically recorded by a data acquisition system.
synchrotron XRD, powder X-ray diffraction (PXRD), and dispersive Raman spectroscopy. Also, in order to measure the compositions of the guest molecules in the synthesized gas hydrates, a gas chromatography (GC) analysis was performed. 4.2. Structure analysis using synchrotron XRD and PXRD In order to characterize crystal structures of the mixed gas hydrates, synchrotron XRD and PXRD patterns were obtained. The synchrotron XRD patterns were obtained at the High Resolution Powder Diffraction (HRPD) beamline (9B) facility of the Pohang Accelerator Laboratory (PAL) in the Republic of Korea. During the measurements, a θ/2θ scan mode with a fixed time of 2 s, a step size of 0.005 for 2θ ¼ 5–125 , and the beamline with a wavelength of 1.497 Å were used. The mixed hydrate powder stored in a liquid nitrogen was quickly transferred to a pre-cooled sample stage at 80 K in air, and the experiment was carried out at around 93 K to minimize possible sample damage. The PXRD patterns were measured using a Rigaku D/max-IIIC diffractometer with CuKα as a light source (λ ¼ 1.5406 Å) at a generator voltage of 40 kV and a generator current of 300 mA. During the measurements, the θ/2θ scan mode with a fixed time of 1 s, and a step size of 0.01 for 2θ ¼ 5–55 was used. A low-temperature stage attached to the PXRD unit maintained the working temperature at 93 K to minimize possible sample damage. The gas hydrate samples stored in liquid nitrogen were quickly transferred to the pre-cooled sample stage, and the measurements were carried out immediately. In order to identify the crystal structures and calculate the lattice parameters of the samples, the obtained patterns were analysed by the Le Bail fitting method using profile matching of the FullProf program [39].
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A1A02061741 and NRF2017R1A2B4003586). The X-ray diffraction experiments were carried out at 9B High Resolution Powder Diffraction (HRPD) beamline of Pohang Accelerator Laboratory, which is supported by POSTECH. Appendix A. Supplementary data
4.3. Guest-distribution analysis using Raman spectroscopy Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.06.007.
The Raman spectra of the mixed gas hydrates were obtained by employing a dispersive Raman spectrometer (ARAMIS, Horiba Jobin Yvon Inc., France). We used a focused 514.53 nm line of an Ar ion laser as an excitation source, and the typical intensity was 30 mW. The scattered light was dispersed using a 1,800 grating of the spectrometer and detected by a charge coupled device (CCD) with electric cooling (at 203 K). For the low-temperature experiment, the sample holder made of stainless steel was merged in a liquid nitrogen to maintain temperature of the samples at around 77 K to minimize sample damage during measurements. Small liquid nitrogen bath was an open system for venting the evaporated N2 gas.
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4.4. Gas composition analysis The guest compositions of the mixed gas hydrates were measured by using gas chromatography (GC) after dissociation of the hydrate structure. First, the mixed gas hydrates were loaded in the pre-cooled pressure vessel. The vessel was then evacuated quickly to remove the existing air inside the vessel and subsequently heated to room temperature to fully dissociate the gas hydrates. By employing a direct injection unit (WBI2010 plus, SHIMADZU Co., Japan), the gas released from the dissociated gas hydrate was injected to a GC (gas chromatograph-2010 plus, SHIMADZU Co., Japan) equipped with a thermal conductivity detector (TCD) for a quantitative analysis. The experimental composition deviations were found to be within 0.1% 4.5. Phase stability analysis The phase equilibria of the mixed gas hydrates composed of CH4 þ C2H6 with or without H2 were measured by employing the conventional isochoric method [47]. 25 ml of deionized water was loaded into a mechanically stirred high-pressure reactor with an internal volume of 100 ml, and the reactor was placed in a water-ethanol circulator. The reactor was flushed out with feed gas mixtures to remove residual air and 6
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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm
One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending Yun-Ho Ahn a, Seokyoon Moon b, Dong-Yeun Koh a, Sujin Hong b, Huen Lee a, Jae W. Lee a, *, Youngjune Park b, ** a Department of Chemical and Biomolecular Engineering (BK21þ Program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea b School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Clathrate hydrates Inclusion compounds Gas storage Gas-phase promoter Hydrogen-natural gas blends
Harvesting energy gases in the nanoporous water-frameworks of clathrate hydrate enables the widespread of hydrogen-based fuels converted from excess solar or wind energy sources (i.e., Power-to-Gas). However, there has always been a critical trade-off between mild formation condition and maximum working capacity. Here we demonstrate the ‘natural gas modulator’ based synthesis that leads to significantly reduced synthesis pressure (PH2 3 MPa) simultaneously with the formation of hydrogen clusters (up to 3 molecules) in the confined nanoporous cages of clathrate hydrates. Instead of employing hazardous liquid chemicals, clean energy gas of natural gas is used for the first time to multiply load H2 in all cages (512, 51262, and 51264 cages) of hydrogennatural gas hydrates without any postsynthetic modification (e.g., guest-exchange reaction). This approach minimizes the environmental impact and reduces operation cost since clathrate hydrates do not generate any chemical waste in both synthesis and decomposition process, and hydrogen-natural gas mixture can be also utilized as an energy resource as itself.
1. Introduction Integration of renewable energy sources with carbonaceous fuels could revolutionize the energy infrastructure by enabling new synergistic energy-efficient technologies [1–3]. In the context of lessening the current dependence on traditional fossil fuels and shifting the energy paradigm from non-renewable to sustainable energy sources, hydrogen-natural gas blends (hereafter HNGB), also known as ‘Hythans®’ (hydrogen þ methane or ethane), have been explored as either energy carriers or alternative fuels [4,5]. Hydrogen can be blended with natural gas by injecting it into existing natural gas pipeline networks coupled with the Power-to-Gas (P2G) scheme (Fig. S1A), which involves the transformation of excess electricity produced from renewable energy sources into hydrogen via electrolysis [6,7]. Subsequently, the HNGB can be delivered to end-user devices, which utilize the hybrid energy, or near-pure hydrogen can be extracted from the HNGB via conventional separation processes (e.g., pressure swing adsorption) for wide applications [4–9].
For the efficient storage and transportation of hydrogen, numerous materials have been investigated, including metal organic frameworks [10–12], liquid organics [13], metal hydrides [14], and chemical hydrogens [15,16], etc. Among the possible options, clathrate hydrates offer distinct benefits over other contenders, specifically eliminating the concern for complex material synthesis because they are mainly composed of cheap water molecules [17–21]. Clathrate hydrates are nanoporous inclusion compounds composed of the 3D network of polyhedral cages made of hydrogen-bonded ‘host’ water molecules and captured ‘guest’ gas or liquid molecules [18]. Earlier reports revealed that hydrogen can be stored in clathrate hydrates only at extreme conditions (i.e., 220 MPa at 249 K) [17–21]. Forming binary clathrate hydrates by introducing organic liquid thermodynamic promoters (e.g., tetrahydrofuran or cyclopentane) can significantly lower the formation pressure conditions (i.e., 5 MPa at 279.6 K) [22,23]. However, this introduces a trade-off between mild formation condition and hydrogen storage capacity since the promoter molecule inevitably occupies the sorption sites, where the molecular hydrogens can be accommodated.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.W. Lee), [email protected] (Y. Park). https://doi.org/10.1016/j.ensm.2019.06.007 Received 14 March 2019; Received in revised form 5 June 2019; Accepted 5 June 2019 Available online xxxx 2405-8297/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y.-H. Ahn et al., One-step formation of hydrogen clusters in clathrate hydrates stabilized via natural gas blending, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.06.007
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2(512) ∙ 46H2O) hydrate, the use of CH4 þ C2H6 binary mixtures can create a variety of structures, sI, structure II (sII, 8(51264) ∙ 16(512) ∙ 136H2O), or mixed sI and sII, depending on the feed gas composition, pressure, and temperature [18,37]. C2H6 preferentially occupies large cages of sI (51262) or sII (51264) hydrate, whereas CH4 and H2 can be entrapped in both small (512) and large cages [37]. Since the gas-phase mixtures of CH4 and C2H6 form sII hydrate with C2H6 compositions ranging from 1 to 25 mol% [18,37,38], we selected gas-phase mixtures of CH4 (70.0 mol%) þ C2H6 (30.0 mol%) and CH4 (90.0 mol%) þ C2H6 (10.0 mol%) as feed gases to form sI and sII CH4 þ C2H6 hydrates, respectively. In order to encage H2 into the sI or sII hydrates, we designed sample synthesis strategies with different pathways as shown in the schematic illustrations (Fig. 1), and the detailed procedures are explained in the experimental procedures section. The guest compositions of the hydrate samples synthesized via different pathways were analysed by gas chromatography and tabulated in Table 1 with the compositions of feed gas at each step. Considering the three-phase (hydrates (H) – liquid water (Lw) – vapor (V)) P – T equilibrium conditions of the CH4 þ C2H6 þ H2 system measured in this study, the inclusion of H2 in the hydrate framework could be achieved under the moderate pressure and temperature condition (i.e., 9 MPa at 263.15 K) (Fig. S2). To identify the crystal structures of all synthesized hydrate samples, powder diffraction patterns were obtained from synchrotron XRD measurements and analysed via Le Bail fitting method [39]. The synchrotron XRD patterns of the mixed hydrates, which were synthesized under the feed gas having CH4/C2H6 ratios of 7/3 and 9/1, with and without H2, corresponded to the unique patterns of cubic Pm3n and Fd3m structures implying the formation of sI and sII hydrate, respectively (Fig. 2 and S3). Here, there was only a small fraction of ice impurities in the samples synthesized without H2 (Fig. S3), and the formation of pure H2 hydrate generally requires extremely high pressure and low temperature conditions [19,20]. Therefore, it is reasonable to speculate that the inclusion of H2 is caused only by guest replacement mechanisms in the pre-synthesized hydrate media, not by the new formation of pure H2
Tuning binary clathrate hydrates by using less concentrated promoter than the stoichiometric concentration (5.56 mol%) can allow more hydrogen to be captured under mild formation conditions [24–27]. Also, harvesting multiple hydrogen molecules per single cage has been tried to enhance the hydrogen storage capacity [24,28–33]. However, there is still a limit to the use of liquid thermodynamic promoter that does not contribute to any energy utilization with the reduced hydrogen storage capacity. Therefore, we see this potential sorption site (or cages) occupied by promoter as a ‘hidden niche’ for other smaller energy gas molecules. In this study, we propose a practical ‘gas-phase modulator’ based synthesis of hydrogen-natural gas hydrates (Fig. S1B) that does not generate undesirable chemical waste after dissociation for the immediate service (Fig. S1C). We compare several hydrogen-natural gas hydrates synthesized via different pathways and reveal the pathway-dependent hydrogen clustering effect associated with mild formation conditions; hydrogen clusters can only be achieved in the hydrate directly evolved from ice powder. Some light hydrocarbon promoters have been tried, however, H2 cannot be multiply loaded in a single cage of presynthesized hydrocarbon hydrate [34] and no information about H2 multiple occupancy was given in the simple equilibrium measurements [35,36]. In contrast to our direct synthesis approach, N2 hydrate has also been reported to induce the hydrogen clusters successfully via additional postsynthetic guest-exchange reaction [28]. However, this approach requires additional N2 separation process since N2 remaining in hydrate cannot be utilized as an energy resource. Our approach enables the simultaneous storage of both light hydrocarbon and clustered hydrogen, they can be utilized as a low-cost and low-emission fuel-mixture. 2. Results and discussion 2.1. Structure identification and guest population in the mixed hydrates While pure CH4 or C2H6 preferentially forms structure I (sI, 6(51262) ∙
Fig. 1. Two synthetic pathways of sI and sII hydrates. Synthetic pathways of various sI and sII hydrates are shown with the corresponding guest-distributions. H2, CH4, and C2H6 molecules are shown as green, blue, and red balls, respectively. The notation under the hydrate structure refers to the synthetic pathway and its corresponding hydrogen occupancy for each case (i.e., ‘Ice – sII – sII (1S)’: sII hydrate was firstly formed by CH4 and C2H6, and then H2 was ‘singly’ introduced to small cage (1S) via guest exchange method; Ice – sII (1S2S2L3L): All gas-phase guests of CH4, C2H6, and H2 formed sII hydrates directly from ice. Simultaneously, one or two H2 molecules were captured in small cages (1S2S) and two or three H2 molecules were captured in large cages (2L3L)). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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% and 15.5 mol%, respectively. When CH4 and C2H6 existed with a ratio of 7/3 in the feed gas (Case I), the C2H6 composition in the hydrate media was almost identical to that of the hydrate sample before H2 loading (57.1–56.5 mol%). Interestingly, however, increase in the amount of H2 (0–11.0 mol%) in the hydrate media was almost equal to the decrease in the amount of CH4 (42.9–32.5 mol%), and this implies that most enclathrated H2 molecules replaced CH4, which was mainly captured in 512 cages, driven by the difference in the chemical potential. The mechanisms of the replacement reaction of CH4 hydrate by CO2 molecules [40,41] or N2 hydrate by H2 molecules [28,31] have already been reported in the literature. Surprisingly, when the ternary gas mixtures of CH4 þ C2H6 þ H2 were directly contacted with the ice powder (Cases II and IV), the amount of H2 was dramatically increased compared to that of H2 in the guestexchanged hydrates (Cases I and III). The H2 compositions in the hydrate were 21.1 mol% and 22.4 mol% in Cases II and IV, respectively, even with the same ratio of CH4 to C2H6 in Cases I (7/3) and III (9/1). In view of these observations, we postulate that CH4 and C2H6 act as thermodynamic promoters for H2 storage in hydrate, and the amount of enclathrated H2 depends on the ‘synthetic pathway’ even with the same feed gas compositions. At this stage, it is necessary to investigate the guest-distribution of all mixed hydrates synthesized in various pathways to shed light on this peculiar H2 inclusion phenomenon.
Table 1 The compositions of feed gas and mixed hydrate for each sample. Synthetic pathways of mixed hydrate samples and corresponding feed gas compositions (mol%) at each step Ice → CH4 (70.0%) þ C2H6 (30.0%) Ice → CH4 (70.0%) þ C2H6 (30.0%) → CH4 (46.7%) þ C2H6 (20.0%) þ H2 (33.3%) Ice → CH4 (46.7%) þ C2H6 (20.0%) þ H2 (33.3%) Ice → CH4 (90.0%) þ C2H6 (10.0%) Ice → CH4 (90.0%) þ C2H6 (10.0%) → CH4 (60.0%) þ C2H6 (6.7%) þ H2 (33.3%) Ice → CH4 (60.0%) þ C2H6 (6.7%) þ H2 (33.3%)
Hydrate composition (mol%)
H2 Occupancya
CH4
C2H6
H2
42.9
57.1
0
Case I
32.5
56.5
11.0
1S
Case II
37.2
41.7
21.1
1S2S
69.4
30.6
0
Case III
64.3
20.2
15.5
1S
Case IV
60.4
17.2
22.4
1S2S2L3L
Structure
sI
sII
a 1S, one H2 molecule in small cages; 2S, two H2 molecules in small cages; 2L, two H2 molecules in large cages; 3L, three H2 molecules in large cages.
2.2. Synthetic pathway-dependent H2 occupation behavior per cage When the ternary gas mixture directly forms sI hydrate (Case II), the corresponding Raman peaks showed single H2 molecules (1S; 4120–4125 cm1) as well as double H2 molecules (2S; 4155–4160 cm1) in 512 cages (Fig. 3A). In previous studies, double H2 occupancy in small cages or multiple H2 occupancy in large cages was mainly reported in ‘sII’ hydrates since several sII hydrate formers (e.g., tetrahydrofuran [24], propane [30], pyrrolidine [31]) were used as thermodynamic promoter to tune the cage dimension under moderate conditions. The double H2 occupancy in a sI CO2 þ H2 hydrate was reported before [32,33], however, the NMR signal was unclearly assigned to two H2 molecules in 512
hydrate (Cases I and III). Considering the structural consistency of the samples, the ratio of CH4 and C2H6 in the feed gas, starting with or without H2, can play a critical role in determining the structure of the mixed hydrate. Even with the same feed gas compositions, the synthetic pathways of mixed (CH4 þ C2H6 þ H2) hydrates affect the composition of enclathrated guests resulting in different lattice parameters. The guest exchange method (Cases I and III) successfully induces enclathration of H2 in the pre-synthesized sI and sII hydrates with the H2 compositions of 11.0 mol
Fig. 2. Structural characterizations of mixed gas hydrates. Synchrotron XRD patterns of the mixed gas hydrates composed of (A) CH4 (32.5 mol%) þ C2H6 (56.5 mol%) þ H2 (11.0 mol%), (B) CH4 (37.2 mol%) þ C2H6 (41.7 mol%) þ H2 (21.1 mol%), (C) CH4 (64.3 mol%) þ C2H6 (20.2 mol%) þ H2 (15.5 mol%), and (D) CH4 (60.4 mol%) þ C2H6 (17.2 mol%) þ H2 (22.4 mol%) with their corresponding space groups and lattice parameters. Red circles, observed XRD pattern; black solid line, calculated XRD pattern; tick marks, cubic Pm3n sI hydrates (magenta), cubic Fd3m sII hydrates (green), and hexagonal P63/mmc ice (blue), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 3. Different guest-distributions of mixed gas hydrates. H–H vibron region in Raman spectra of the (A) sI hydrates (Cases I and II) and (B) sII hydrates (Cases III and IV) showing the direct evidence of multiple H2 occupancies. (C) C–H stretching mode in Raman spectra of the mixed sI hydrates composed of CH4 (42.9 mol%) þ C2H6 (57.1 mol%) (black), CH4 (32.5 mol%) þ C2H6 (56.5 mol%) þ H2 (11.0 mol%) (blue, Case I), and CH4 (37.2 mol%) þ C2H6 (41.7 mol%) þ H2 (21.1 mol%) (red, Case II). (D) C–H stretching mode in Raman spectra of the mixed sII hydrates composed of CH4 (69.4 mol%) þ C2H6 (30.6 mol%) (black), CH4 (64.3 mol%) þ C2H6 (20.2 mol%) þ H2 (15.5 mol%) (blue, Case III), and CH4 (60.4 mol%) þ C2H6 (17.2 mol%) þ H2 (22.4 mol%) (red, Case IV). All spectra in (C) and (D) were normalized with the O–H stretching mode of water frameworks for comparison. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
cages [42]. In this study, both single and double H2 occupancy in 512 cages of ‘sI’ hydrate are clearly observed for the first time with a large amount of entrapped H2 (21.1 mol%). According to the density functional theory (DFT) calculations results, the H–H vibrons of doubly occupied 512 cages of ‘sII’ hydrate are known to be around or above 4155 cm1 [43]. Although our experimental results were obtained from the sI hydrate, the Raman signals matched well with previous observations of the sII hydrate since both sI and sII hydrates are composed of the same 512 cages [29–31,43]. Thus, it is reasonable to assign the Raman peaks at around 4155–4160 cm1 to doubly occupied H2 in 512 cages of sI hydrate. However, when H2 molecules were introduced to pre-synthesized sI hydrate (Case I, Fig. 3A), the Raman peaks of double H2 molecules did not appear. Only Raman signals of the singly occupied H2 from the two spin isomers of ortho- and parahydrogen were observed [44]. Raman spectra of H2-enclathrated sI hydrates strongly indicate that the double H2 occupancy in 512 cages can be achieved by forming the mixed sI hydrate directly from ice powders, and the guest exchange reaction does not allow two H2 molecules to be captured in one 512 cage of the pre-synthesized sI hydrate. Also, due to the double H2 occupancy in 512 cages, the hydrogen storage capacity (21.1 mol%) of the tuned hydrate can be maximized to around two times that of the mixed hydrate without double occupancy (11.0 mol%). For a qualitative analysis of the guest-distribution, we normalized three Raman spectra with O–H stretching modes of the host water framework (Fig. 3C). The intensities of the C–H stretching mode of C2H6 in large cages (2884 and 2940 cm1) and that of the C–H stretching mode of CH4 in large (2900 cm1) and small cages (2911 cm1) decreased after the guest exchange reaction (black to blue solid line). In the mixed hydrate with double H2 occupancy in 512 cages, the amount of enclathrated CH4 and C2H6 decreased more as the amount of H2 was maximized (Case II), and this tendency was in line with the GC results (Table 1). However, with these Raman results, there is a limit to quantify and compare the exact guest composition in each small and large cage of three different mixed hydrates. When the ternary gas mixture directly forms sII hydrate (Case IV), we
observed double and multiple H2 occupancy in 512 and 51264 cages, respectively (Fig. 3B). The Raman peaks from 4128 to 4142 cm1 corresponded to two to three H2 molecules (2L – 3L) in 51264 cages, in accordance with previous theoretical and experimental results of sII hydrates [29–31,43]. At the initial stage of forming the host water framework, gas-phase co-guest molecules of CH4 and C2H6 can tune the hydrate dimension, resulting in both double H2 occupancy in 512 cages and multiple H2 occupancy in 51264 cages. Hydrogen clusters (two to four molecules) in 51264 cages have been reported in the literature, however, they were commonly achieved under high pressure of H2 ranging from 35 MPa to 58 MPa at low temperature of around 240 K [29–31]. By using our approach, H2 can be multiply enclathrated showing high contents (22.4 mol%) even though the total pressure of the system was 9 MPa at 263.15 K, and the partial pressure of H2 was only around 3 MPa. It is the first time that the energy gas molecules of CH4 and C2H6 successfully derive the multiple H2 occupation in both sI and sII hydrates via direct evolution from ice particles under the moderate formation condition (Table S1 for comparison). On the contrary, as shown in Fig. 3B, hydrogen cluster in any 512 or 51264 cage was not observed in the guest-exchanged sII hydrate (Case III) similar to the guest-exchanged sI hydrate (Case I). In Fig. 3D, we observed that the Raman signal intensities of C–H stretching modes of CH4 and C2H6 in sII hydrate were almost identical between the Case III and Case IV. It appears that, due to the multiple H2 occupancies in both 512 and 51264 cages, the amount of enclathrated H2 increased while that of CH4 and C2H6 did not change significantly. 2.3. The expanded lattice: a niche for H2 clusters These hydrogen inclusion phenomena with the gas-phase modulator can be interpreted with the expanded lattice of the hydrates. According to the previous studies, lattice parameter of hydrate has been known as one of critical factors in determining the multiple H2 occupancy [30,31]. As the amount of H2 increased with the double and multiple H2 occupancy, lattice parameters of sI and sII hydrates increased by 0.471% 4
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(a ¼ 11.879(1) Å for the Case I and a ¼ 11.935(2) Å for the Case II) and 0.041% (a ¼ 17.148(4) Å for Case III and a ¼ 17.156(3) Å for Case IV), respectively, implying that the expanded lattices have a chance to provide enough room for storing hydrogen clusters in a single cage [30,31]. At this stage, we have to consider the procedure of hydrate lattice formation in the two different synthetic pathways. The ‘Direct evolution from ice’ method (Cases II and IV) involves only one crystallization step with the feed gas of CH4, C2H6, and H2. When H2 participates in the initial stage of forming the host water framework, hydrogen clusters can be successfully stored, and the expanded hydrate lattice (a ¼ 11.935(2) Å for the Case II and a ¼ 17.156(3) Å for Case IV) can be stabilized by CH4 and C2H6. However, in the case of the ‘Gas-phase exchange’ method (Cases I and III), even though the exact replacement reaction mechanism has not been revealed yet, we can imagine that there must be additional steps such as local dissociation of structures and diffusion of guest molecules in the pre-synthesized hydrate lattices constructed by CH4 and C2H6. During these procedures, storing hydrogen clusters in a single cage might be difficult because of kinetic issues. In other tuned hydrate systems, as the gas-phase exchange reaction enables loading of hydrogen clusters in the pre-synthesized hydrates, we can guess the guest occupancy of each guest [28,30]. However, in this work, as three types of guests are injected simultaneously to the hydrate where hydrogen clusters are observed, it is somewhat difficult to clarify the tuning mechanism. By the way, the previous molecular dynamics (MD) calculation results reveals that the change in the hydrate lattice highly depends on the guest occupation in tight small cages rather than in loose large cages [45]. Since the double H2 occupancy in sI 512 cages and multiple H2 occupancy in sII 51264 cages dominantly occur, therefore, the degree of peak shift for tuned sI hydrate is somewhat larger than that of sII hydrate (Fig. S4). In order to investigate the synthetic pathway-dependent guest-distribution of mixed hydrates further, we analysed sII hydrate, obtained by structural transition from pre-synthesized sI hydrate (See Supplementary Material Text). It was confirmed that H2 molecule can singly occupy the newly synthesized small cages of sII hydrate during the structural transition. However, multiple H2 occupancy in any cage was not observed. Also, we obtained the hydrogen-natural gas storage capacities of Case I – IV hydrates as well as the weight fractions of hydrogen in the enclathrated gas mixtures with some assumptions (Detailed procedures and results are in the Supplementary Material Text and Fig. S8). Since we used three types of gases in the feed, the hydrogen-natural gas storage capacities cannot be precisely obtained, but, the capacities were between 15 and 22 wt%. From these findings, the clean energy platform of mixed CH4 þ C2H6 þ H2 hydrates must be explored further to optimize and control the gas composition taking into account practical transportation and storage application.
4. Experimental section 4.1. Materials and sample preparations Deionized water of ultrahigh purity was supplied from a Merck Millipore purification unit (MILLI-Q Direct System, Merck KGaA, USA). Gas mixtures of CH4 þ C2H6 with or without H2 were purchased from Special Gas Co. (Republic of Korea). The compositions of the gas mixtures used in the experiments are as follows: CH4 (70.0 mol%) þ C2H6 (30.0 mol%), CH4 (90.0 mol%) þ C2H6 (10.0 mol%), CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol%), and CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%). Gas hydrate samples were synthesized by using finely ground ice powder (<200 μm) under pressure conditions of less than 9 MPa at 263.15 K for each gas mixture. In order to provide higher driving force to obtain higher conversion yield and maintain the temperature and pressure condition above the phase equilibrium line (Fig. S2), we used higher pressure and lower temperature than the phase equilibrium condition. All samples were prepared via three different pathways to examine the different guest occupation behavior depending on the H2 loading sequences, and the detailed procedures are explained below. Before the injection of H2, precursor frameworks, binary gas hydrates of CH4 and C2H6 were synthesized by exposing the ice powders to the feed gases of CH4 þ C2H6 mixtures. The ice powders were loaded in a pressure vessel, and pressurized up to 6 MPa at 263.15 K with the gas mixtures. The reactor was kept in a refrigerated water-ethanol circulator (RW-2025G, Jeio Tech Co., Ltd., Republic of Korea) for 3 days to obtain fully converted CH4 þ C2H6 hydrates. After confirming there was no ice phase in the gas hydrates via synchrotron X-ray diffraction (synchrotron XRD), all the samples were reloaded in the reactor and re-pressurized to 9 MPa at 263.15 K for one week with gas mixtures of H2 to load H2 into the lattices of the mixed gas hydrates: the mixed gas hydrate formed with the CH4 (70.0 mol%) þ C2H6 (30.0 mol%) mixture was re-pressurized with CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol%) (Case I in Table 1), and the mixed gas hydrate formed with the CH4 (90.0 mol%) þ C2H6 (10.0 mol%) mixture was re-pressurized with CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%) (Case III in Table 1); these compositions were aimed at maintaining the CH4 to C2H6 ratio as 7 to 3 and 9 to 1, respectively. The reactor was then quenched in liquid nitrogen and the mixed gas hydrates were recovered for analyses. Ternary mixtures of CH4 þ C2H6 þ H2 were used as a feed gas at the initial step to form mixed gas hydrates. The ice powders were loaded in each pressure vessel and pressurized up to 9 MPa at 263.15 K with feed gases containing CH4 (46.7 mol%) þ C2H6 (20.0 mol%) þ H2 (33.3 mol %) (Case II in Table 1) and CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%) (Case IV in Table 1), respectively. In order to increase the conversion yield of the mixed gas hydrates, re-grinding and repressurization procedures were performed following the protocols in the previous study. [46] First, 200 μm sized ice particle was exposed to the aforementioned feed gas. After a week, the reactor was quenched in liquid nitrogen for a few seconds, and then the mixed gas hydrate particles were recovered and quenched in liquid nitrogen so that the hydrate particles might not change into ice. The particles were ground to 200 μm under liquid nitrogen and exposed to the feed gas again. By repeating this procedure, more gas hydrates can be formed from the unreacted ice as it can contact gas molecules through the re-grinding step. Lastly, ice powders were exposed to a CH4 (70.0 mol%) þ C2H6 (30.0 mol%) mixture at 6 MPa and 263.15 K, and sI type gas hydrate was initially formed. After confirming there was no ice phase in the gas hydrates, the feed gas was changed from CH4 (70.0 mol%) þ C2H6 (30.0 mol%) to CH4 (60.0 mol%) þ C2H6 (6.7 mol%) þ H2 (33.3 mol%), resulting in a structural transition from sI to sII hydrate. After changing the gas mixtures, we performed the same re-grinding procedure and recovered some parts of samples once in every week. These mixed gas hydrate samples synthesized in different pathways were used for spectroscopic analyses including low-temperature
3. Conclusion We provide a practical concept for storing H2 in clathrate hydrates by introducing co-guest species of natural gas (CH4 and C2H6) instead of eco-unfriendly liquid organic promoter. The three distinct approaches to form mixed CH4 þ C2H6 þ H2 hydrate adopted here enable H2 enclathration under mild formation conditions (Ptotal ¼ 9 MPa and PH2 3 MPaat 263.15 K). Among the three different synthetic pathways, the direct evolution method can tune the hydrate cages allowing hydrogen clusters in both sI and sII hydrate. Even though we used the two specific feed gas compositions to analyze the synthetic pathway-dependent guest occupation behavior in both sI and sII hydrates, various feed gas compositions can be adopted to find the optimal synthetic condition to provide actual fuel-mixtures with demanded compositions. We expect our findings can be applied to not only an energy-efficient gas storage material, but also a smart platform to utilize HNGB (e.g., Hythane®, HCNG (hydrogen and CNG blends)), which can serve as a new alternative energy source with targeted hydrogen content by designing synthetic pathways of mixed gas hydrates. 5
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pressurized up to the desired pressures by using a syringe pump (500D, Teledyne Isco Inc., USA). After achieving stabilization, the temperature of the reactor was decreased at a rate of 0.2–1.0 K/h to form a mixed gas hydrate. After a sudden pressure drop, the reactor was then slowly heated at a rate of 0.1 K/h until the pressure reached the initial pressure. The pressure and temperature condition of the fully dissociated mixed gas hydrate was considered as an equilibrium point. Several equilibrium points were obtained with variation of the initial pressure condition. During the formation-dissociation process of the mixed gas hydrates, the pressure and the temperature of the reactor were monitored via a pressure transducer (PSHB250BCPG, Sensor System Technology Co., Ltd., Republic of Korea), which has an accuracy of 0.15%, and a four-wire type Pt-100Ω probe, which has a full-scale accuracy of 0.05%, respectively. They were automatically recorded by a data acquisition system.
synchrotron XRD, powder X-ray diffraction (PXRD), and dispersive Raman spectroscopy. Also, in order to measure the compositions of the guest molecules in the synthesized gas hydrates, a gas chromatography (GC) analysis was performed. 4.2. Structure analysis using synchrotron XRD and PXRD In order to characterize crystal structures of the mixed gas hydrates, synchrotron XRD and PXRD patterns were obtained. The synchrotron XRD patterns were obtained at the High Resolution Powder Diffraction (HRPD) beamline (9B) facility of the Pohang Accelerator Laboratory (PAL) in the Republic of Korea. During the measurements, a θ/2θ scan mode with a fixed time of 2 s, a step size of 0.005 for 2θ ¼ 5–125 , and the beamline with a wavelength of 1.497 Å were used. The mixed hydrate powder stored in a liquid nitrogen was quickly transferred to a pre-cooled sample stage at 80 K in air, and the experiment was carried out at around 93 K to minimize possible sample damage. The PXRD patterns were measured using a Rigaku D/max-IIIC diffractometer with CuKα as a light source (λ ¼ 1.5406 Å) at a generator voltage of 40 kV and a generator current of 300 mA. During the measurements, the θ/2θ scan mode with a fixed time of 1 s, and a step size of 0.01 for 2θ ¼ 5–55 was used. A low-temperature stage attached to the PXRD unit maintained the working temperature at 93 K to minimize possible sample damage. The gas hydrate samples stored in liquid nitrogen were quickly transferred to the pre-cooled sample stage, and the measurements were carried out immediately. In order to identify the crystal structures and calculate the lattice parameters of the samples, the obtained patterns were analysed by the Le Bail fitting method using profile matching of the FullProf program [39].
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A1A02061741 and NRF2017R1A2B4003586). The X-ray diffraction experiments were carried out at 9B High Resolution Powder Diffraction (HRPD) beamline of Pohang Accelerator Laboratory, which is supported by POSTECH. Appendix A. Supplementary data
4.3. Guest-distribution analysis using Raman spectroscopy Supplementary data to this article can be found online at https://do i.org/10.1016/j.ensm.2019.06.007.
The Raman spectra of the mixed gas hydrates were obtained by employing a dispersive Raman spectrometer (ARAMIS, Horiba Jobin Yvon Inc., France). We used a focused 514.53 nm line of an Ar ion laser as an excitation source, and the typical intensity was 30 mW. The scattered light was dispersed using a 1,800 grating of the spectrometer and detected by a charge coupled device (CCD) with electric cooling (at 203 K). For the low-temperature experiment, the sample holder made of stainless steel was merged in a liquid nitrogen to maintain temperature of the samples at around 77 K to minimize sample damage during measurements. Small liquid nitrogen bath was an open system for venting the evaporated N2 gas.
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