Abstract
The reason that the stoichiometry of gas to water in artificial gas hydrates formed on porous materials is much higher than that in nature is still ambiguous. Fortunately, based on our experimental thermodynamic and kinetic study on the gas hydrate formation behavior with classic ordered mesoporous carbon CMK-3 and irregular porous activated carbon combined with density functional theory calculations, we discover a microscopic pathway of the gas hydrate formation on porous materials. Two interesting processes including (I) the replacement of water adsorbed on the carbon surface by gas and (II) further replacement of water in the pore by gas accompanied with the gas condensation in the pore and growth of gas hydrate crystals out of the pore were deduced. As a result, a great enhancement of the selectivity and regeneration for gas separation was achieved by controlling the gas hydrate formation behavior accurately.
Highlights
Gas hydrates are a type of ice-like clathrate compounds that existed in nature, which have attracted considerable attention because of their unique properties [1,2,3,4,5] and various applications
Based on the thermodynamic and kinetic study on CO2 adsorption combined with density functional theory (DFT) calculations, we have proposed a new mechanism of the gas hydrate formation on porous materials, which is quite different to previous hypothesis
We have discovered that the gas hydrates could not form and stay in the nanopore, and the excess gas observed in artificial gas hydrates resulted from the gas adsorption and condensation in the nanospace when water is driven out to form gas hydrates out of the pore
Summary
Gas hydrates are a type of ice-like clathrate compounds that existed in nature, which have attracted considerable attention because of their unique properties [1,2,3,4,5] and various applications. Due to lack of convincing characterization techniques, the formation of gas hydrates in the confined space has not been experimentally proven yet In comparison with this classic mechanism, we discovered a different process (I-IV in Figure 1) for the gas hydrate formation based on the investigation of CO2 hydrate formation on porous carbon. The excess gas observed in artificial gas hydrates resulted from the gas adsorption and condensation in the nanoconfined space when water is driven to form gas hydrates outside the pore To confirm this assumption, we designed a rigorous experiment to investigate the gas hydrate formation behavior in the nanoconfined space and achieved a novel understanding about the formation behavior of gas hydrates on porous carbon materials. A great enhancement of the selectivity and regeneration for the gas separation on porous materials was achieved based on the mechanism we proposed
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