Octafluoropropane (C3F8, or R-218) can be sequestered into type-H (sH) clathrate hydrate using methane (CH4) as a help gas. In this study, we examine how well CH4 and other hydrate-forming gases might function as help gases for this hydrate. In our experiments, pure water and C3F8 mixed with known help gases for sH hydrate (CH4, xenon, and carbon dioxide) are pressurized in the vessel below their saturated pressure at about 274 K for micro-Raman spectroscopic measurements. Then we use the Raman peak shift of C3F8 to detect its enclathration. We also examine the possibility of C3F8 enclathration in the type-II structure (sII) hydrates with sII-hydrate formers, specifically propane and CH4-containing mixed gases. Under these conditions, only CH4 functions a help gas for enclathration of C3F8, and only in the sH hydrate structure.

Octafluoropropane (C3F8) is a fluorinated gas (F-gas) known for its high global warming potential. In this study, the role of C3F8 as a novel gaseous structure H (sH) hydrate former was investigated. By employing a comprehensive methodology involving thermodynamic, crystallographic, and spectroscopic analyses, the successful encapsulation of C3F8 into the large cages of sH hydrates was unveiled for the first time. The incorporation of C3F8 into sH hydrates was inferred from a shift in the thermodynamic hydrate phase equilibria, and unequivocal evidence of successful C3F8 encapsulation was obtained using powder X-ray diffraction, 13C nuclear magnetic resonance, and Raman spectroscopy. The findings showed that C3F8 overcame the conventional limitations associated with sH hydrate formers, which are predominantly liquid and solid substances, thereby significantly expanding the scope of potential gaseous sH hydrate formers. This advancement not only enhances the fundamental understanding of gas hydrate formers but also opens up new avenues for hydrate-based technologies, particularly in the separation and recovery of large-size F-gases, which have substantial environmental implications.

Fluorinated gases (F-gases), such as CHF3 and C2F6, which are used in the semiconductor industry and have considerable global warming potential, can be recovered after use through a gas hydrate-based separation method to prevent their release into the atmosphere. In this study, the guest distributions and dissociation enthalpy (ΔHd) of F-gas (CHF3 or C2F6)+N2 hydrates with different F-gas concentrations (CHF3: 20%, 80%, and 100% and C2F6: 20%, 60%, 80%, and 100%) were experimentally investigated using a powder X-ray diffractometer and a high-pressure micro-differential scanning calorimeter, respectively. At high N2 concentrations in the feed gas, the occupancy of N2 in the small (512) cages of the F-gas+N2 hydrates increased significantly. As a result, the F-gas+N2 hydrates exhibited reduced hydration numbers at high N2 concentration. The ΔHd values (in kJ/mol gas) of the F-gas (CHF3 or C2F6)+N2 hydrates decreased with increased N2 concentration. The overall experimental results provide useful insights into the design and operation of gas hydrate-based F-gas separation processes.

No gaseous guests have ever been discovered that can form sH clathrate hydrates. Herein, we show that a large molecular fluorinated gas (octafluorocyclobutane, c-C4F8), can stabilize the crystal structure of sH clathrate hydrate by occupying large (5(12)6(8)) cages. The inclusion of c-C4F8 molecules in sH clathrate hydrate in the presence of CH4 molecules was clearly demonstrated through three-phase (clathrate hydrate (H)-liquid water (LW) -vapor (V)) equilibria, powder X-ray diffraction (PXRD) and C-13 NMR spectroscopy. The discovery of a gaseous sH former, c-C4F8, contributes to broadening clathrate hydrate science and engineering by expanding target components for sH clathrate hydrates, and also offers excellent potential in various applications of sH clathrate hydrates, especially in clathrate hydrate-based gas separation.

F-gases are man-made gases that are utilized mainly in the semiconductor industry and in refrigeration systems. Because F-gases have a high potential of contributing to global warming, various methods, including gas hydrate-based F-gas separation, for separating and recovering F-gases have been widely studied. However, gas hydrate formation with NF3 has not been well studied in spite of its extremely high global warming potential (12,700) and its long atmospheric lifetime (740 years). In this study, the enclathration of NF3 in gas hydrate lattices was investigated, with a focus on phase equilibria and guest distribution. The three-phase (gas hydrate (H) liquid water (Lw) vapor (V)) equilibria of NF3 hydrate were measured to observe the stability conditions for NF3 hydrate. The crystal structure of NF3 hydrate was identified as cubic sl (Pm3n) with a lattice parameter of 11.87 angstrom through powder X-ray diffraction (PXRD). In addition, the cage-filling behavior of NF3 hydrate was examined through both PXRD and in situ Raman spectroscopy. This revealed that the large (5(12)6(2))cages were fully occupied by NF3 molecules whereas the small (5(12)) cages were less populated. The results obtained in this study would be helpful for understanding the cage-specific occupation of F-gas molecules in sl hydrate and devising possible gas hydrate-based F-gas separation methods. (C) 2018 Elsevier B.V. All rights reserved.

In this study, the feasibility of gas hydrate-based greenhouse gas (CHF3) separation was investigated with a primary focus on thermodynamic, structural, and cage-filling characteristics of CHF3 + N-2 hydrates. The three-phase (hydrate (H)-liquid water (L-w)-vapor (V)) equilibria of CHF3 (10%, 20%, 40%, 60%, and 80%) + N-2 + water systems provided the thermodynamic stability conditions of CHF3 + N-2 hydrates. Powder X-ray diffraction revealed that the structure of the CHF3 + N-2 hydrates was identified as sI (Pm3n) for all the CHF3 concentration ranges considered in this study. A pressure composition diagram obtained at two different temperature conditions (279.15 and 283.15 K) demonstrated that 40% CHF3 could be enriched to 88% CHF3 by only one step of hydrate formation and that separation efficiency was higher at the lower temperature. Furthermore, Raman spectroscopy revealed that CHF3 molecules preferentially occupy large (5(12)6(2)) cages of the structure I (sI) hydrate during CHF3 + N-2 hydrate formation. The overall experimental results clearly demonstrated that the hydrate-based separation process can offer highly concentrated CHF3 and would be more effective for recovering CHF3 from exhaust gas when it constitutes a hybrid system with existing separation methods.

C2F6 (hexafluoroethane, R116) is a fluorinated gas (F-gas) widely used in semiconductor industries, which also has a high global warming potential and a long atmospheric lifetime. In this study, the thermodynamic and structural characteristics of the C2F6 + N-2 gas hydrates were investigated for gas hydratebased C2F6 separation from emission sources. This experiment measured the three-phase (hydrate, liquid water, and vapor [H-L-W-V]) equilibria of ternary C2F6 (10, 20, 40, 60, and 80%) + N-2 + H2O systems and indicated the possible existence of hydrate azeotropes at certain temperature ranges. Powder X-ray diffraction (PXRD) revealed that the ternary C2F6 + N-2 + H2O systems form structure II (sII) hydrates (Fd3m) for all C2F6 concentrations considered in this study. The pressure-composition diagram obtained at two different temperatures (275.15 K and 279.15 K) demonstrated that C2F6 is highly enriched in the hydrate phase at 275.15 K, whereas at 279.15 K, the C2F6 + N-2 + H2O systems have a hydrate azeotrope where the composition of the hydrate phase is the same as the composition of the vapor phase. The overall experimental results clearly indicate that hydrate-based C2F6 separation is thermodynamically feasible and the higher separation efficiency is achievable at lower temperature ranges. (C) 2017 Elsevier Ltd.

In this study, gas hydrate-based fluorinated gas (F-gas) separation is proposed as a novel method to capture F-gases. This study investigates the thermodynamic, structural, and cage filling characteristics of the gas hydrates formed by two representative F-gases (CHF3 and C2F6) in order to verify the feasibility of the F-gas separation using gas hydrate formation. The three-phase (gas hydrate (H) – liquid water (LW) – vapor (V)) equilibria of the pure CHF3 and C2F6 hydrates are measured in order to examine the hydrate formation conditions. The PXRD patterns reveal the structure of the CHF3 hydrate and the C2F6 hydrate as a cubic structure I (sI) and structure II (sII), respectively. The enclathration of CHF3 and C2F6 molecules in each pure CHF3 and C2F6 hydrate is confirmed through 13C and 19F NMR analyses. In-situ Raman measurements are used to monitor the growth process of pure CHF3 hydrates, and they reveal the CHF3 molecules trapped in the sI large (51262) cages as well as in the sI small (512) cages. The computational study also demonstrates that CHF3 is encaged in both small (512) and large (51262) cages of the sI hydrate, whereas C2F6 only occupies the large (51264) cages of the sII hydrate.

Dual (thermodynamic and kinetic) inhibition effects of piperazine (PZ) and hydrazine (HZ), two representative diamines, on the formation of CH4 hydrate were investigated, primarily focusing on thermodynamic, spectroscopic, and computational analyses for their potential application in natural gas production and transportation. The phase behavior demonstrated that the addition of PZ and HZ shifts the CH4 hydrate equilibrium lines to higher pressure or lower temperature regions depending on diamine concentrations. The C-13 NMR and Raman spectra of the CH4+ diamine hydrates revealed that PZ and HZ are not enclathrated in the cages of CH4 hydrate and do not affect the hydrate structure. The time-dependent growth patterns and the induction time of CH4 hydrate in the presence of diamines were observed via in-situ Raman spectroscopy. HZ showed a more significant thermodynamic inhibition effect, slower hydrate growth, and longer induction time than PZ did. Interaction energy calculation using density functional theory (DFT) indicated that diamine molecules could disrupt the hydrogen bonding networks of hydrate cages, leading to destabilization of gas hydrate and retardation of hydrate nucleation and growth. Experimental and computational results demonstrated that both PZ and HZ can function as both kinetic and thermodynamic hydrate inhibitors for CH4 hydrate. (C) 2016 Elsevier Ltd. All rights reserved.

The inclusion of tert-butyl alcohol (tBA) as a co-guest of clathrate hydrates in the presence of CH4, CO2, and N-2 was investigated for its potential role in gas storage and CO2 sequestration. The C-13 NMR, Raman spectroscopy, and powder X-ray diffraction revealed that the guest gas (CH4, CO2, and N-2) + tBA + water systems form sII hydrates. The enclathration of tBA molecules in the sII large (5(12)6(4)) cages resulted in significant thermodynamic stabilization of the CH4 + tBA and N-2 + tBA hydrates. However, the hydrate phase equilibrium curves of the CO2 + tBA hydrates were shifted to inhibited regions despite the participation of tBA molecules as a co-guest in the sII hydrate lattices. tBA was found to function as a thermodynamic promoter for both CH4 and N-2 hydrates, whereas it functioned as a thermodynamic inhibitor for CO2 hydrate. The overall experimental results provide a better understanding of the thermodynamic behaviors, structural transitions, and guest distributions of the guest gas (CH4, CO2, and N-2) + tBA hydrates for the potential use of tBA in gas storage and CO2 sequestration. (C) 2015 Elsevier Ltd. All rights reserved.

In this study, the influences of large molecular alcohols (LMAs) including pinacolyl alcohol (PCA) and tert-amyl alcohol (tAA) on thermodynamic phase behaviors and structural characteristics of CH4 and CO2 hydrates were investigated for their potential use in gas storage and CO2 sequestration. The experimentally measured hydrate phase equilibria demonstrated that CH4 hydrates were stabilized in the presence of PCA and tAA. 13C NMR and Raman spectroscopy confirmed sH hydrate formation from both CH4+PCA+water and CH4+tAA+water systems, resulting from the enclathration of LMAs in the large 51268 cages. The sH hydrate formation of the CH4+PCA+water system was also confirmed by an endothermic dissociation thermogram from a differential scanning calorimeter (DSC). In contrast with CH4 hydrates, the addition of both PCA and tAA to CO2 hydrates resulted in thermodynamic inhibition. Through Raman and powder X-ray diffraction (PXRD) analyses, both CO2+PCA and CO2+tAA hydrates were characterized as sI hydrates, indicating that LMAs simply inhibit the formation of CO2 hydrates without being captured in the hydrate lattices. Therefore, PCA and tAA are expected to function as thermodynamic promoters which reduce the hydrate forming pressure in natural gas storage applications, while they can serve as thermodynamic inhibitors which prevent CO2 hydrate formation in pipelines for CO2 transportation to sequestration sites.

The influences of large molecular alcohols (LMAs) (pinacolyl alcohol (PCA) and tert-amyl alcohol (tAA)) on N-2 hydrate were examined with a primary focus on the hydrate phase equilibria and structural transition. The four-phase (H-L-w-L-LMA-V) equilibria of the N-2 + PCA + water and N-2 + tAA + water systems were experimentally measured in order to determine the thermodynamic stability conditions of the N-2 hydrates with LMAs. The H-L-w-L-LMA-V curves of both N-2 + PCA and N-2 + tAA hydrates were significantly shifted to more thermodynamically stable regions. Powder X-ray diffraction (PXRD) and Raman analyses verified that the structure of N-2 hydrates with LMAs was transformed from the original structure of sII to sH (P6/mmm) as a result of the inclusion of LMAs. In addition, the formation of sH hydrate from the N-2 + PCA + water system was further verified using an endothermic dissociation thermogram from a high pressure micro-differential scanning calorimeter (HP mu-DSC) by confirming the appearance of three distinct peaks that correspond to ice, sII hydrate, and sH hydrate. Therefore, the experimental results that were obtained in this study are expected to be informative for understanding the roles of LMAs in affecting the macroscopic hydrate phase behavior and microscopic hydrate structure. (C) 2015 Elsevier B.V. All rights reserved.