J. Phys. Chem. A 2006, 110, 1901-1906
1901
Rates and Mechanisms of Conversion of Ice Nanocrystals to Ether Clathrate Hydrates: Guest-Molecule Catalytic Effects at ∼120 K Dheeraj B. Gulluru and J. Paul Devlin* Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078 ReceiVed: October 30, 2005; In Final Form: December 8, 2005
A Fourier transform infrared investigation of the rates and energetics of conversion of ice nanocrystals within 3-D arrays to ether clathrate-hydrate (CH) particles at ∼120 K is reported. After an induction period, apparently necessitated by relatively slow nucleation of the CH phase, the well-established shrinking-core model of particle-adsorbate reaction applies to these conversions in the presence of an abundance of adsorbed ether. This implies that the transport of the ether adsorbate through the product crust encasing a reacting particle core (a necessary aspect of a particle reaction mechanism) is the rate-controlling factor. Diffusion moves adsorbed reactant molecules to the reaction zone at the interface of the ice core with the product (CH) crust. The results indicate that ether hydrate formation rates near 120 K resemble rates for gas hydrates measured near 260 K, implying rates greater by many orders of magnitude for comparable temperatures. A surprising secondary enhancement of ether CH-formation rates by the simultaneous incorporation of simple small gas molecules (N2, CO2, CH4, CO, and N2O) has also been quantified in this study. The rapid CH formation at low temperatures is conjectured to derive from defect-facilitated transport of reactants to an interfacial reaction zone, with the defect populations enhanced through transient H bonding of guest-ether proton-acceptor groups with O-H groups of the hydrate cage walls.
1. Introduction Clathrate-hydrate formation/decomposition is of both fundamental and practical interest.1-5 The practical interest stems primarily from the great abundance of CHs on earth (and presumably elsewhere) and the potential commercial value and societal danger represented by their presence. There exist (e.g., on the floor of the oceans) methane deposits adequate to fuel society for a considerable time and sufficient CH4 and CO2 deposits to suggest a threat to civilization should they be released.6 Our more basic interests derive from the ability of moderately good proton-acceptor molecules, such as ethers, formaldehyde,7 and acetone,8 to form crystalline clathrate hydrates at remarkably low temperatures. We showed in the past that the ether CHs can be formed by several methods at ∼120 K: direct vapor deposition of the proper mixtures,9 by warming an appropriate amorphous solid mixture,10 as particles condensed in cold cluster cells11 and by vapor interaction with ice nanocrystals.12 This ability, of the complex crystal structures of the classic structure I and structure II CHs, with unit cells of 46 and 136 water molecules,13 to organize at temperatures well below the minimum temperature at which crystalline ice (of relatively simple structure) can normally be deposited is impressive. Here, we seek increased understanding of this low-temperature formation of CHs of proton-acceptor guest molecules using a different approach: the determination of the transport rates and associated activation energies by which the ether guest molecules move through the condensed phase during conversion of ice nanocrystals to nanocrystals of the CHs. Most CHs, including those of CO2 and CH4, will not form extensively in the absence of high pressures and temperatures > 200 K. These * To whom correspondence should be addressed. E-mail: devlin@ okstate.edu.
are the conditions used recently by Wang et al. in novel kinetic measurements of the formation rates of CHs of CO2 and CH4 as monitored by neutron diffraction.1,14 Their results established that, for gas pressures of ∼1000 psi and temperatures near 270 K, conversion of micrometer-sized ice particles (i.e.,
