Sodium Dodecyl Sulfate Preferentially Promotes Enclathration of Methane in Mixed Methane-Tetrahydrofuran Hydrates

摘要

class="head no_bottom_margin" id="sec1title">IntroductionClathrate or gas hydrates are crystalline non-stoichiometric compounds formed by guest molecules, such as CH4, THF, etc., and host water molecules at a suitable pressure/temperature condition. Depending on the size, composition, and chemical properties of the guest molecules, clathrate hydrates typically crystallize in three different structures, cubic structures I and II (sI and sII) and hexagonal sH. sI and sII hydrates consist of two different types of cages, called small (pentagonal dodecahedron, 5¹²) and large (tetrakaidecahedron, 5¹²6² or 5¹²6⁴) cages. A unit cell of sI hydrate consists of six large 5¹²6² cages and two small 5¹² cages formed by 46 water molecules (pure CH4 hydrate); however, sII hydrate consists of 8 large (5¹²6⁴) cages and 16 small (5¹²) cages formed by 136 water molecules (for example, mixed CH4-THF hydrate) (, , , ). Recently, we demonstrated that mixed CH4-THF hydrates (sII) offer a promising approach to scale up the solidified natural gas (SNG) technology by shifting the methane hydrate storage conditions to the milder side while still maintaining very fast hydrate formation kinetics (, , ). SNG technology provides an excellent opportunity to store natural gas or methane on a large scale (, , ). Furthermore, to understand the rapid kinetics of mixed hydrate formation, Kumar et al., elucidated the mechanism of mixed CH4-THF hydrate formation employing high-pressure differential scanning calorimetry (HP μ-DSC); they reported the formation of two types of sII hydrates (pure THF and mixed CH4-THF hydrates) with a stoichiometric amount of THF (5.56 mol %) in water (). For pure THF hydrates (THF.17H2O) the dissociation temperature is 277.5 K at 1.0 bar (THF occupies the large cages and all the small cages are empty), which aids the shifting of mixed CH4-THF hydrates toward ambient conditions. Furthermore, literature suggests that mixed hydrates of CH4 and THF (with stoichiometric amount, 5.56 mol % THF) form sII hydrates where all the large cages are occupied by THF and small cages by CH4 molecules (, , ). Seo et al. reported a 36.84% occupancy of small cages by CH4 and more than 99% occupancy of large cages by THF molecules for mixed CH4-THF hydrates (). Prasad et al. have also reported that methane did not occupy the large cage for 5.88 mol % THF. At a stoichiometric amount of THF, no C-H stretch from methane was observed in the large cages (href="#bib9" rid="bib9" class=" bibr popnode">Prasad et al., 2009). Thus the relevant literature on CH4-THF hydrates shows that methane neither occupies the large cages of sII nor co-exists in the resultant hydrate as a mixture of sI and sII hydrates. Higher methane occupancy in small and large cages of sII hydrate (in the presence of THF) would result in a higher methane storage capacity at milder formation conditions. Seo et al. have reported methane occupancy in both the cages (small and large) of sII hydrate, when powdered THF·17H2O (formed at 263 K) was exposed to CH4 gas at a pressure of 5.0 MPa and a temperature of 274 K. They suggested that the occupancy of methane in large cages is kinetically controlled and that it takes a longer time to occupy the large cages (href="#bib12" rid="bib12" class=" bibr popnode">Seo et al., 2009). Thus the presence of kinetic promoters (like sodium dodecyl sulfate [SDS]) and suitable experimental conditions may allow methane to share the large cages with THF and also enhance the small cage occupancy, resulting in a thermodynamically more stable mixed CH4-THF hydrate. Thus the focus of the present work is to understand the influence of SDS on mixed CH₄-THF hydrate formation at the molecular level and to distinguish the various types of hydrates (pure methane, pure THF, and mixed methane-THF) formed in different thermodynamic or kinetically controlled conditions. In this work, we employ HP μ-DSC and in situ Raman spectroscopy to characterize the mixed CH₄-THF hydrates in the presence and absence of SDS. Change in heat flow during hydrate formation and dissociation process in the presence of 5.56 mol % THF (stoichiometric composition) with and without a kinetic promoter (SDS, 100 and 1,000 ppm) was monitored employing HP μ-DSC. The formed hydrates were distinguished using in situ Raman spectra in real time. Refer to the href="#mmc1" rid="mmc1" class=" supplementary-material">Supplemental Information for experimental method and procedures (href="#mmc1" rid="mmc1" class=" supplementary-material">Figures S1–S3).