NMR/MRI Study of Clathrate Hydrate Mechanisms Shuqiang Gao, ² Waylon House,* ,‡ and Walter G. Chapman* ,² Chemical Engineering Department, Rice UniVersity, Houston, Texas 77251, and Petroleum Engineering Department, Texas Tech UniVersity, Lubbock, Texas 79406 ReceiVed: April 21, 2005; In Final Form: August 12, 2005 Clathrate hydrates are of great importance in many aspects. However, hydrate formation and dissociation mechanisms, essential to all hydrate applications, are still not well understood due to the limitations of experimental techniques capable of providing dynamic and structural information on a molecular level. NMR has been shown to be a powerful tool to noninvasively measure molecular level dynamic information. In this work, we measured nuclear magnetic resonance (NMR) spin lattice relaxation times (T 1 ’s) of tetrahydrofuran (THF) in liquid deuterium oxide (D 2 O) during THF hydrate formation and dissociation. At the same time, we also used magnetic resonance imaging (MRI) to monitor hydrate formation and dissociation patterns. The results showed that solid hydrate significantly influences coexisting fluid structure. Molecular evidence of residual structure was identified. Hydrate formation and dissociation mechanisms were proposed based on the NMR/MRI observations. Introduction Gas hydrates are icelike structures in which water molecules, under pressure, form structures composed of polyhedral cages surrounding gas molecule “guests” such as methane and ethane. Rarely encountered in everyday life, they occur in staggering abundance under the sea floor and permafrost environments where (P, T) conditions ensure their stability. The natural gas trapped in these deposits represents a potential source of energy many times greater than all known natural gas reserves. Hydrates can form as well in undersea piping and above ground gas pipelines where they pose a major problem for gas/oil producers. Detailed understanding of hydrate melting and formation mechanisms on a molecular level is important for successfully tackling all hydrate challenges with accuracy and confidence. 1 However, hydrate growth and dissociation mechanisms still remain unclear because very few experimental techniques can provide in situ dynamic information on a molecular scale. Liquid water structure coexisting with the hydrate phase, especially near the water/hydrate interface, is very important in under- standing the hydrate formation and dissociation processes. The imminent state before guest molecules solidify into the hydrate phase and the fluid structure immediately after clathrate hydrate dissolves into the liquid state may hold the key to unlock the secrets of hydrate mechanisms. NMR has been shown to be a powerful tool to noninvasively measure molecular level dynamic information. T 1 is an indicator of local molecular order around the spin-bearing molecules. 2 T 1 measurement is an effective method to monitor microscopic fluid structure change. In this work, NMR T 1 measurements of THF in D 2 O solution were employed to probe the change of water structure around THF during THF hydrate formation and dissociation to understand the role of the water and hydrate interface. Proton MRI 3 was also utilized to observe the hydrate formation and dissociation patterns. THF molecules become invisible to liquid-state NMR as they are incorporated into the solid hydrate phase; thus, the T 1 ’s of THF in the liquid phase can be measured independently of the THF hydrate. D 2 O is invisible to proton NMR under all conditions, so only THF in the liquid phase is visible to MRI. Results showed that the presence of solid hydrate significantly influences the fluid structure. T 1 measurements also indicated the existence of residual effects after hydrate dissociation. Experimental Details The schematic of the experimental setup is shown in Figure 1. T 1 measurements of THF (Aldrich, 99+%) in D 2 O (Cam- bridge Isotope Laboratories, D 99.9%) and MRI imaging experiments were performed on an 85 MHz Oxford horizontal 31 cm wide bore NMR with imaging capability, using a LITZ RF volume coil (with 14 cm internal diameter) from Doty Scientific, Inc. Data were acquired and processed using Varian VNMR software and INOVA hardware systems. T 1 ’s were measured using the inversion and recovery technique. VNMR software, given inputs of possible minimum and maximum T 1 values, automatically generates standard 180°-τ-90° pulse sequences with various values of delay time τ. It took 4 to ∼6 min to take a T 1 data point and about 1 h to take an MRI image. An air-jet temperature controller supplied dry and cold air to control the sample temperature. It is capable of controlling temperature from -40 to 100 °C with 0.1 °C stability. A glass * Corresponding authors. E-mail: Waylon.House@ttu.edu; wgchap@ rice.edu. ² Rice University. ‡ Texas Tech University. Figure 1. Experimental schematic. 19090 J. Phys. Chem. B 2005, 109, 19090-19093 10.1021/jp052071w CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005