Molecular insights into CH4/H2O transport and hydrate formation in hydrophobic metal-organic frameworks ZIF-8: Implication for CH4 storage by adsorption-hydration hybrid method

J Duan and XY Jiang and Y Fu and SJ Chen and MC Zi, FUEL, 337, 126851 (2023).

DOI: 10.1016/j.fuel.2022.126851

The method of adsorption-hydration hybrid has been considered to have great application potential for future CH4 storage. However, the mechanism of CH4 /H2O transport and hydrate formation in metal-organic frame-works (MOFs) systems is not clear. In this work, the hydrophobicity of ZIF-8 was demonstrated by measuring the equilibrium contact angle (115.8 +/- 1.2 degrees) of the substrate at molecular level. The interaction between H2O molecules promotes the hydrogen bonds network formation on ZIF-8 surface. Because of the strong physisorption of CH4 by ZIF-8 cavities, excess CH4 accelerates migration and forms larger flat nanobubbles on ZIF-8 surface to cause separation of gas and liquid. On the basis of CH4 /H2O transport results, the whole process of hydrate nucleation and growth in ZIF-8 system was first investigated to reveal the microscopic mechanism of promoting CH4 storage by adsorption- hydration hybrid method. For one thing, a preferential direction may exist during the hydrate nucleation. For another, the migration of intergranular CH4 and H2O is enhanced to promote hydrate nucleation at early stage. Finally, the link between transportation of CH4 /H2O and hydrate formation were discussed in three ZIF-8 systems, elucidating that the larger gas-liquid ratio can accelerate the transportation of intergranular CH4 and H2O at early stage to promote hydrate nucleation and growth. However, because of the competitive adsorption between ZIF-8 inner cavities and hydrate cages, the flat nanobubbles aggregates and the solubility of intergranular CH4 decreases, which accelerate the diffusion of CH4 and H2O to hinder the hydrate continues growth and nucleation, respectively. This study provides crucial microscopic insight into the trans-portation of CH4 /H2O promotes hydrate formation, which facilitates application for natural gas storage in MOFs by adsorption-hydration hybrid method.

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