Propane Clathrate Hydrate Formation Accelerated by Xenon Joanne A. Abbondondola, Everly B. Fleischer, and Kenneth C. Janda* Department of Chemistry, UniVersity of California at IrVine, IrVine, California 92697 ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: December 9, 2008 Experiments are reported that show propane is incorporated into clathrate hydrate cages much more rapidly using propane-xenon mixtures than for pure propane gas. Uptake rates for pure propane type II clathrate hydrate, pure xenon type I clathrate hydrate, and propane and xenon binary type II clathrate were studied for several different synthesis procedures. Upon adding a 0.92 xenon:propane ratio gas mixture to ice particles, the time required for achieving 62% of the theoretical yield of propane enclathration is 20 min, versus 3 days for pure propane. Although the acceleration of clathrate formation decreases as xenon is depleted, enhancement continues even after the composition falls below 3% Xe. It appears that xenon serves to nucleate the dodecahedral 5 12 cages while propane nucleates the larger 5 12 6 4 cages. The type II xenon-propane structure is not only more thermodynamically stable than either pure hydrate; it is also formed much more quickly than propane clathrate, nearly as fast as type I xenon clathrate. Introduction The study of gas clathrate hydrates has recently accelerated due to renewed interest in harvesting natural deposits and using clathrate hydrates as a storage and transportation medium for hydrogen and hydrocarbon gases. 1 The latter application requires the rapid synthesis of gas clathrate hydrates on a large scale. Even on a smaller scale, synthesis can be quite slow. 2-4 Neither hydrogen nor methane nor propane is miscible in water, therefore, simply freezing a mixture is not an option. Often, exposing liquid water to the gas results in the formation of a thin crust on the liquid surface. 5-8 Stern et al. have partially surmounted this problem by exposing finely powdered ice to the gas, but synthesis is still slow and extended cycling about the ice freezing point is required to drive the synthesis to completion. 9,10 In general, it is difficult to perform quantitative formation kinetics studies for these systems because the surfaces of ice grain particles are both rough and constantly changing. Although Staykova et al. have studied hydrate growth from well-defined ice spheres, only small numbers of such spheres were employed so that only small amounts of gas were enclathrated. 11 We are investigating techniques for producing macroscopic samples more quickly. 12 Our goal is to rapidly produce large quantities of binary clathrate product through the use of helper gases or promoters. 13-16 Gulluru and Devlin have shown that a moderately good proton- acceptor molecule, such as ethylene oxide, greatly accelerates hydrate formation at temperatures as low as 120 K. 12 Here, we report results using xenon as the promoter gas. We have chosen propane clathrate hydrate for this study because its type II structure is stable near ambient conditions so that neither cryogenic temperatures nor high pressures are necessary. The type II structure has a unit cell made up of 136 water molecules forming 16 dodecahedral 5 12 cages and 8 hexakaidecahedral 5 12 6 4 cages. Propane only fits into the 5 12 6 4 cages leaving the 5 12 cages available to store another gas. (5 12 dodecahedral cages are made of 20 water molecules forming 12 pentagonal faces; 5 12 6 4 cages are made from 28 water molecules forming 12 pentagonal faces and 4 hexagonal faces.) If an efficient formation mechanism can be found for a mixed methane/propane hydrate, two methane molecules would be contained for each propane molecule, resulting in efficient energy storage at near ambient temperature and pressure. Stern et al. developed a synthesis technique that involves bathing small ice particles in liquid propane and cycling the cell temperature about the ice melting point, 9,10 To achieve 100% propane hydrate formation typically took 17-21 days. At the end of this procedure, the consistency of the sample has changed from that of a powder to a fused, porous solid cylinder. The original ice particles have grown together via clathrate growth at the interface between particles. 11,17 In this paper, we report that the rate of propane enclathration is accelerated by over an order of magnitude if xenon is added to the reaction mixture. We have made quantitative measure- ments of the gas uptake for several different experimental procedures. Although the propane in propane/xenon mixtures is enclathrated much faster than pure propane, the xenon is not acting simply as a catalyst because it is also being enclathrated. The presence of xenon in the mixture results in propane enclathration in a type II structure almost as fast as pure xenon forms its preferred type I structure. Adding a 0.92:1 Xe:C 3 H 8 gas mixture to ice particles yields 62% of the theoretical yield of propane enclathration in 20 min. The same ice particles exposed to pure propane gas require 3 days to achieve 67% theoretical yield. Here “theoretical yield” is defined relative to complete conversion of ice to type II clathrate with a propane molecule in every large cage, i.e., a 1:17 propane to H 2 O ratio. Experimental Section A typical experiment was performed as follows. First, ice pellets were formed by dripping nanopure water into liquid nitrogen. The ice pellets were then ground in a cold coffee grinder. After the powder was sieved through a 250 μm mesh screen, a 10 g sample was placed into a 260 cm 3 cell, warmed to 272 K, and exposed to the absorbate gases. The gas was delivered to the cell through 6.4 mm tubing that is coiled and submerged in the cooling bath to avoid heating of the ice particles during gas addition. The cell pressure and temperature * To whom correspondence should be addressed. E-mail: kcjanda@uci.edu. J. Phys. Chem. C 2009, 113, 4717–4720 4717 10.1021/jp804515h CCC: $40.75  2009 American Chemical Society Published on Web 02/20/2009