Structural Characterization of Pyrrolidine–Including Structure II Clathrate Hydrates

Abstract

Clathrate hydrates are host–guest crystalline compounds which can capture both small gases such as H2, CH4, and CO2 and larger guest molecules which are in the liquid state. Large guests (LGs) such as tetrahydrofuran (THF) can strongly stabilize the structure II clathrate hydrate under moderate conditions and provide a substantial number of small cages for the encapsulation of smaller guest gases. Most hydrocarbon LGs are volatile and immiscible in water, and even the water-miscible LGs such as THF are still volatile. The use of these LGs requires a recovery process in applications based on hydrates. Amines are one chemical group of alternative LGs which have low vapor pressure compared to hydrocarbon LGs. In this study, we report structural characterization for clathrate hydrates formed with the cyclic amine pyrrolidine (PL) and the structurally related THF. Crystal samples were formed from aqueous solutions of PL and THF with several ratios. Lattice constants determined by the single-crystal X-ray diffraction increased with the increase of PL content in the hydrate. Inspection of the cage size found that 51264 cages expand due to PL inclusion, while the empty 512 cages do not expand to the same degree. The refined X-ray diffraction structure suggests that the PL is incorporated in the large cage without forming hydrogen bonds with the cage water molecules. Molecular dynamics simulations also support weak hydrogen bond interactions between PL and water molecules.

Kinetic Hydrate Inhibition from Thermoresponsive Poly(2-amino-2-oxazoline)s: Size and Shape of the Hydrophobic Groups Are Critical for Performance

“…This is also the reason why diethylamine is only known to make semiclathrate hydrates and not a sII hydrate like pyrrolidine . The pyrrolidine ring has a smaller Tolman angle from the N atom, and, therefore, is able to penetrate a 5 12 6 4 hydrate cavity. − …”