Evaluation of filling rate of methane in methane-propane hydrate by neutron powder diffraction

Abstract

Clathrate hydrates have a polyhedral cage structure (host structure) formed by hydrogen bond-linked water molecules, and various guest molecules can be accommodated in these cages. Methane-propane hydrate is a clathrate hydrate with a crystal structure similar to propane hydrate (structure II, cubic, Fd-3m). The host lattice consists of small cages (S cages) and large cages (L cages). Due to their size, propane molecules (C3H8) are contained only in the L cages, while methane molecules (CH4) can be included in both the S cages and the L cages. In this report, methane-propane hydrate was synthesized at −10 °C using a pressurized gas mixture with a methane-to-propane ratio of 2:1. We estimated the filling rates of the guest molecules using a precise structure analysis including hydrogen atoms. Methane molecules were contained only in the S cages under our synthesis conditions.