Oxabicyclic Guest Compounds as sII Promoters: Spectroscopic Investigation and Equilibrium Measurements.

Jiwoong Seol,Woongchul Shin,Juwoon Park

doi:10.3389/fchem.2020.00614

Abstract

In this study, we investigate three oxabicyclic compounds, 3,6-dioxabicyclo[3. 1.0]hexane (C4H6O2, ETHF), 7-oxabicyclo[2.2.1]heptane (C6H10O, 14ECH), and 7-oxabicyclo[4.1.0]heptane (C6H10O, 12ECH) as novel promoters for gas hydrates. According to the outcomes of powder X-ray diffraction (PXRD) and synchrotron high-resolution powder diffraction (HRPD), all CH4 hydrates formed with ETHF, 14ECH, and 12ECH were identified to be sII (Fd-3m) hydrates with corresponding lattice parameters of 17.195, 17.330, and 17.382 Å, respectively. It was also clearly demonstrated that CH4 molecules are accommodated in the sII-S cages through solid-state 13C NMR and Raman spectra. Consequently, we clarified that the three compounds observed are large guest molecules (LGMs) that occupy the sII-L cages. Moreover, the thermodynamic stability of each LGM + CH4 (and N2) hydrate system was remarkably improved compared to that of the simple CH4 (and N2) hydrate. In particular, 14ECH manifested several unique features compared to the other two promoters. First, the 14ECH + CH4 hydrate did not dissociate up to room temperature (298 K), even at a moderate pressure of approximately 60 bar. At a given pressure, 14ECH increased the dissociation temperature of the CH4 hydrate by ~18 K, indicating that its promotion capability is as strong as that of tetrahydrofuran (THF), currently considered to be the most powerful promoter. Second, more interestingly, it was revealed by further PXRD, NMR, and Raman analyses that 14ECH forms a simple sII hydrate in the absence of help gases. According to differential scanning calorimetry (DSC) outcomes, we revealed that the simple 14ECH hydrate dissociates at 270~278 K under ambient pressure. In addition to the thermodynamic stability, we also note that the 14ECH + CH4 hydrate presented a sufficiently high temperature of formation, requiring little additional cooling. Given these promising features, we expect that the 14ECH hydrate system can be adopted to realize hydrate-based technologies. We also believe that the LGMs introduced here have considerable potential to serve as alternates to conventional promoters and that they can be widely utilized in both engineering and scientific research fields.

Highlights

Clathrate hydrate, consisting of a hydrogen-bonded water framework and guest molecules, is a well-known type of inclusion compound
According to the crystallographic and spectroscopic outcomes, all hydrates were identified to be sII (Fd-3m) double hydrates, of which the large and small cages were predominantly occupied by large guest molecules (LGMs) and gaseous molecules, respectively
We believe that the 14ECH hydrate system can be employed to facilitate hydrate-based technologies

Summary

INTRODUCTION

Clathrate hydrate, consisting of a hydrogen-bonded water framework and guest molecules, is a well-known type of inclusion compound. Gas hydrates bearing simple sII formers exhibit considerably higher thermodynamic stability levels compared to those formed with ordinary LGMs. For example, under pressure of approximately 40 bar, the dissociation temperature of furan + CH4 (∼295 K, Pahlavanzadeh et al, 2016) or tetrahydrofuran (THF) + CH4 (∼297 K, Lee et al, 2012) is considerably higher than that of the pyrrolidine + CH4 hydrate (∼287 K, Shin et al, 2012). We measured the equilibrium P-T conditions of the CH4 (or N2) hydrate systems containing the three LGMs. Each liquid mixture (total mass ∼7 g) consists of a stoichiometric composition of LGM and balanced water was charged into a high-pressure resistance cell (V ∼ 100 ml) and pressurized at various CH4 or N2 pressures at ambient temperature. We note that our measurements demonstrated very good agreement with the reported values (Adisasmito et al, 1991; Lee et al, 2012)

RESULTS AND DISCUSSION

CONCLUSIONS

DATA AVAILABILITY STATEMENT