Rates and mechanisms of conversion of ice nanocrystals to ether clathrate hydrates: Guest-molecule catalytic effects at ∼120 K

Abstract

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 (N 2, CO 2, CH 4, CO, and N 2O) 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. © 2006 American Chemical Society.