Structure transition and tuning pattern in the double (tetramethylammonium hydroxide + gaseous guests) clathrate hydrates.
In this study, we present an extraordinary structural transition accompanying the occurrence of more than two coexisting clathrate hydrate phases in the double (CH4 + tetramethylammonium hydroxide (Me(4)NOH)) and (H2 + Me(4)NOH) ionic clathrate hydrates using solid-state NMR spectroscopy (high-powered decoupling and CP/MAS) and powder X-ray diffraction. It was confirmed that structure-I (sI) and structure-II (sII) hydrates coexist as the water concentration increases. In the Me(4)NOH-depleted region, the unique tuning phenomenon was first observed at a chemical shift of -8.4 ppm where relatively small gaseous CH4 molecules partly occupy the sII large cages (sII-L), pulling out large cationic Me(4)N+ that is considered to be strongly bound with the surrounding host lattices. Moreover, we note that, while pure Me(4)NOH.16H(2)O clathrate hydrates melted at 249 K under atmospheric pressure conditions, the double (CH4 + Me(4)NOH) clathrate hydrate maintained a solid state up to approximately 283 K under 120 bar of CH4 with a conductivity of 0.065 S cm(-1), suggesting its potential use as a solid electrolyte. The present results indicate that ionic contributions must be taken into account for ionic clathrate hydrate systems because of their distinctive guest dynamic behavior and structural patterns. In particular, microscopic analyses of ionic clathrate hydrates for identifying physicochemical characteristics are expected to provide new insights into inclusion chemistry.
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong yuseong-gu, Daejeon 305-701, Republic of Korea.
Sukjeong
S
Choi
Kyuchul
K
Shin
Huen
H
Lee
D005740:Gases / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008697:Methane / Q000737:chemistry*; D015394:Molecular Structure; D044367:Phase Transition; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D014961:X-Ray Diffraction
D005740:Gases; D000644:Quaternary Ammonium Compounds; D006859:Hydrogen; C027917:tetramethylammonium; D008697:Methane