Chemical Engineering Science 62 (2007) 3930 – 3939 www.elsevier.com/locate/ces Characterization of gas hydrates with PXRD, DSC, NMR, and Raman spectroscopy Robin Susilo a , b , John A. Ripmeester a , ∗ , Peter Englezos b a Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ont., Canada b Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, Canada Received 15 October 2006; received in revised form 23 March 2007; accepted 30 March 2007 Available online 19 April 2007 Abstract Structure I (sI) and H (sH) hydrates containing methane were synthesized and characterized with PXRD, DSC, NMR, and Raman spectroscopy. Three well-known large molecule guest substances (LMGSs) were selected as sH hydrate formers: 2,2-dimetylbutane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The solid phase analysis confirmed the presence of sH hydrate whenever a LMGS was present. The presence of a non-hydrate former (n-heptane) did not affect the methane hydrate structure or cage occupancies. Ice to hydrate conversion was limited when the LMGS amount was less than stoichiometric and synthesized at low methane pressure, but nearly complete conversion was achieved with temperature ramping and excess LMGS. The methane occupancies were found to depend on the type of LMGS and increased with pressure. The hydrate with TBME was found to have the smallest methane content followed by the hydrates with NH and MCH. Both NMR and Raman identified methane and LMGS signals from the hydrate phase, however, the cage occupancy values of sH hydrate can only be obtained from NMR spectroscopy. The hydrate structures, ice to hydrate conversion, gas content in hydrate and cage occupancy from the various measurements are consistent with each other.  2007 Published by Elsevier Ltd. Keywords: Structure H hydrate; Methane; LMGS; Gas storage 1. Introduction Gas hydrates are non-stoichiometric crystals composed of water and guest molecules that generally form under high pres- sure and low temperature conditions. The water molecules serve as the crystal host framework that is stabilized by the inclusion of suitably sized guest molecules. There are three well-known hydrate structures: cubic structure I (sI), cubic structure II (sII) and hexagonal structure H (sH). Studies on sI and sII hydrate have been carried out for more than 50 years but not for sH hydrate, which was just discovered in 1987 at the National Re- search Council of Canada (NRC) (Ripmeester et al., 1987). In recent years, the study of sH hydrates has emerged rapidly be- cause of their unique properties. Firstly, two guest molecules of different sizes are required for sH hydrate, unlike sI and sII where one guest molecule is enough to stabilize the crystal. ∗ Corresponding author. Tel.: +1 613 993 2011; fax: +1 613 998 7833. E-mail address: John.Ripmeester@nrc-cnrc.gc.ca (J.A. Ripmeester). 0009-2509/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.ces.2007.03.045 Secondly, sH hydrate has the largest cage among all the known hydrate structures hence larger molecules like methylcyclohex- ane can fit into the cavity. The addition of a large molecule guest substance (LMGS) may also reduce the equilibrium pres- sure while maintaining high gas storage capacity due to the ac- cessibility of both small and medium cages. Hence sH hydrate is seen as a valid potential and attractive opportunity for gas storage application (Englezos and Lee, 2005; Khokhar et al., 1998; Tsuji et al., 2005; Mori, 2003). However, a solid foun- dation on which to base sH hydrate characterization is vital before technological developments can be pursued. The study on a molecular scale is essential to make sure that the hydrate properties are known prior to advancing to a larger synthetic scale. Powder X-Ray diffraction (PXRD), Raman and Nuclear Magnetic Resonance (NMR) spectroscopy are the well-known tools used for solid structural analysis, including that of gas hydrate (Ripmeester and Ratcliffe, 1988, 1999; Sloan, 2003; Tulk et al., 2000). Such techniques are required to obtain information on crystal structure, crystal