Synthesis of methane hydrate at ambient temperature with ultra-rapid formation and high gas storage capacity

Abstract

With the emergence of natural gas (NG) as a crucial transition fuel, NG storage techniques have also become essential components of nations’ energy resilience portfolios. The Solidified Natural Gas (SNG) technology is one emerging technique that promises safe, long-term NG storage under moderate pressure and temperature (P–T) conditions. Herein we investigate 1,3-dioxane (dioxane), a low-toxicity chemical additive, as a potential dual-function (thermodynamic and kinetic) promoter for the SNG technology. By enabling mixed methane (sII) hydrate formation, dioxane significantly moderates the P–T conditions required for SNG synthesis. At 283.2 K temperature and 7.2 MPa initial pressure, mixed CH₄/dioxane (5.56 mol%) sII hydrate growth under unstirred conditions reaches 90% completion within 15 min. At ambient temperature (298.2 K) and an initial pressure of 16.7 MPa, a breakthrough volumetric methane storage capacity of 135.13 (±1.08) v/v (volume of gas at STP/volume of hydrate) is achieved, wherein methane molecules occupy about 34% of the sII-large (5¹²6⁴) cages in addition to all of the sII-small (5¹²) cages demonstrating tunability of methane in the large cages. Finally, a mixed CH₄/dioxane hydrate pellet, stored in a tightly sealed container under near atmospheric pressure of 135.2 kPa (gauge pressure) and a moderate average temperature of 268.3 (±0.2) K for 120 days, exhibits excellent stability throughout the duration of storage. The present study demonstrates that the gas storage capacity of methane in sII hydrate can be tuned to exceed the acknowledged limit of about 115.36 v/v and that sII hydrates can be readily and rapidly synthesized at room temperature, aiding the development of an environmentally and commercially viable SNG technology.