Spectroscopic Observations of Host–Guest Hydrogen Bonding in Binary Cyclopropanemethanol + Methane Hydrate

Abstract

Herein, we report a new structure II (sII) hydrate with cyclopropanemethanol (CPM) in the presence of methane (CH4) gas for the first time and investigate the host–guest hydrogen bonding in the binary CPM + CH4 hydrate via solid-state NMR, Raman spectroscopy, and synchrotron X-ray powder diffraction analysis. The crystal structure and guest distribution of the binary CPM + CH4 hydrate were characterized using solid-state 13C NMR and powder diffraction analyses, which confirmed the formation of the binary sII CPM + CH4 hydrate. The CPM in the large cages of sII hydrate was predicted to be present in the dynamic Anti (Anti I or Anti II) form based on the measured- and calculated- NMR spectra. The inclusion of CH4 molecules in both the large and small cages of the sII hydrate was observed from the 13C NMR and Raman spectra. No free OH signals of the CPM guest were observed in the Raman spectra of the binary CPM + CH4 hydrate, which indicated that the CPM guest participated in host–guest hydrogen bonding with the water framework. The powder diffraction pattern of the binary CPM + CH4 hydrate was refined using Rietveld analysis with the direct space method, and the results supported the hydrogen bonding of the hydroxyl group of the CPM guest to the host water molecules. The phase equilibrium temperature and pressure conditions of the binary CPM + CH4 hydrate were also measured, and the binary CPM + CH4 hydrate was found to be less stable than the binary tetrahydrofuran + CH4 hydrate.

Phase Equilibria and Spectroscopic Identification of Structure II Hydrates with New Hydrate-Forming Agents (Cyclopropylamine and Cyclopentylamine)

“…Therefore, we may consider the possibility of the formation of sII or structure H (sH) hydrate in the presence of help gas from the consideration of molecular size and geometry. 1,2,7,26,31,34 The crystalline hydrates of some amines in the absence of help gas were reported by several researchers, 50−52 and thus we prepared the frozen (CPrA + H 2 O) and (CPeA + H 2 O) samples and analyzed the crystallography patterns of both samples through the Le-Bail refinement (Figure 3). Figure 3 shows the crystallography patterns of (a) the CPrA + H 2 O system (5.56 mol % of CPrA) and (b) the CPeA + H 2 O system (5.56 mol % of CPeA) measured at 100 K. In Figure 3a, the Le-Bail refinement results of the CPrA + H 2 O system (5.56 mol % of CPrA) indicate that the crystalline hydrate of CPrA is isostructural with the cubic structure I (sI) hydrate having a space group of Pm-3n (a= 12.081 Å), in good agreement with the crystalline hydrate of ethylamine (C 2 H 5 NH 2 ).…”

“…As shown in Figure 4b, we can observe the sharp carbon peak at δ = −4.6 ppm, implying the enclathration of CH 4 molecules in the small (sII-S, 5 12 ) cages of sII hydrates. 1,2,7,26,31,34 A relatively weak carbon signal at δ = −8.3 ppm can also be observed, indicating the partitioning of small amounts of CH 4 molecules in the large (sII-L, 5 12 6 4 ) cages of sII hydrates. 1,2,7,26,31,34 Similar to the structural transition of ethylamine crystalline hydrate (cubic sI hydrate with a space group of Pm-3n) into the cubic sII hydrate with help gas, CPrA can form sII hydrate in the presence of CH 4 gas.…”

“…1,2,7,26,31,34 A relatively weak carbon signal at δ = −8.3 ppm can also be observed, indicating the partitioning of small amounts of CH 4 molecules in the large (sII-L, 5 12 6 4 ) cages of sII hydrates. 1,2,7,26,31,34 Similar to the structural transition of ethylamine crystalline hydrate (cubic sI hydrate with a space group of Pm-3n) into the cubic sII hydrate with help gas, CPrA can form sII hydrate in the presence of CH 4 gas. 41 In the absence of CH 4 gas, CPrA molecules is expected to be captured in the large (sI-L, 5 12 6 2 ) cages of sI hydrate (Figure 3); thus, CPrA in the large (sI-L, 5 12 6 2 ) cages of sI hydrate may have strong host−guest interactions via hydrogen bonding.…”

“…The obtained samples of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates were placed in a 4 mm zirconia rotor, and the rotor was placed in the NMR instrument at ∼200 K. The featured conditions of the NMR instruments can be found in our previous studies. 7,26,31,34 The two-dimensional supramolecular crystallography (2D-SMC) beamline at the Pohang Accelerator Laboratory was used to investigate the crystal structures and possible host− guest interactions of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates. The obtained sample of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates was put into a polyimide tube (0.635 mm I.D.…”

“…The sharp inflection point during the hydrate dissociation process was considered the phase equilibrium condition of the binary (CPrA + CH 4 ) and (CPeA + CH 4 ) hydrates, and the detailed phase equilibria measurements can be found in our previous papers. 7,26,31,34 ■…”