In this work a novel method for enhancing the gas storage capacity of metal–organic frameworks (MOFs), i.e. saturating the MOF with a suitable quantity of water and forming hydrates in it, was proposed. Commercialized ZIF-8 was adopted as it is very stable under an atmosphere of water. The adsorption and hydrate formation behaviors of methane in wet ZIF-8 with five different water contents (0.0%, 16.3%, 27.7%, 30.6%, and 35.1%, mass percentages) were investigated under hydrate formation conditions and the storage capacities of both ZIF-8 frameworks and ZIF-8 particle beds were determined. Our results show that hydrates can form in wet ZIF-8 pores and thus increase the overall storage capacities of both ZIF-8 frameworks and ZIF-8 particle beds remarkably. The contribution of hydrates to the total gas uptake of a ZIF-8 framework can be as high as 45%. Compared with dry ZIF-8 frameworks, the net storage capacity of the wet analogue with a water content of 35.1% increases from 5.954 to 9.304 mmol g⁻¹ at 269.15 K and 2.85 MPa, in other words, raised by more than 56%. The ideal volume storage capacity of the wet ZIF-8 framework can achieve more than 190 V/V at 3.0 MPa or so, 7% higher than the DOE target (180 V/V) for methane storage. In addition, our SEM measurements and XRD analysis demonstrate that ZIF-8 is stable during the saturation and hydrate formation processes, illustrating that it can be used repeatedly.

Gas hydrates provide alternative solutions for gas storage & transportation and gas separation. However, slow formation rate of clathrate hydrate has hindered their commercial development. Here we report a form of porous ice containing an unfrozen solution layer of sodium dodecyl sulfate, here named active ice, which can significantly accelerate gas hydrate formation while generating little heat. It can be readily produced via forming gas hydrates with water containing very low dosage (0.06 wt% or 600 ppm) of surfactant like sodium dodecyl sulfate and dissociating it below the ice point, or by simply mixing ice powder or natural snow with the surfactant. We prove that the active ice can rapidly store gas with high storage capacity up to 185 V_gV_w⁻¹ with heat release of ~18 kJ mol⁻¹ CH₄ and the active ice can be easily regenerated by depressurization below the ice point. The active ice undergoes cyclic ice−hydrate−ice phase changes during gas uptake/release, thus removing most critical drawbacks of hydrate-based technologies. Our work provides a green and economic approach to gas storage and gas separation and paves the way to industrial application of hydrate-based technologies.

The replacement of methane with carbon dioxide in natural gas hydrate-bearing sediments is considered a promising technology for simultaneously recovering natural gas and entrapping CO₂. During the CH₄-CO₂ replacement process, the variations of geophysical property of the hydrate reservoir need to be adequately known. Since the acoustic wave velocity is an important geophysical property, in this work, the variations of P-wave velocity of hydrate-bearing sediments were measured during different CH₄-CO₂ replacement processes using pure gaseous CO₂ and CO₂/N₂ gas mixtures. Our experimental results show that P-wave velocity continually decreased during all replacement processes. Compared with injecting pure gaseous CO₂, injection of CO₂/N₂ mixture can promote the replacement process, however, it is found that the sediment experiences a loss of stiffness during the replacement process, especially when using CO₂/N₂ gas mixtures.