Article pubs.acs.org/JPCA
Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF3 Gas and Ice Particles Published as part of The Journal of Physical Chemistry virtual special issue “W. Lester S. Andrews Festschrift”. Jaruwan Amtawong,a Suvrajit Sengupta, Michael T. Nguyen,b Nicole C. Carrejo,c Jin Guo, Everly B. Fleischer, Rachel W. Martin, and Kenneth C. Janda* Department of Chemistry, University of California, Irvine, California 92697, United States S Supporting Information *
ABSTRACT: We report the formation kinetics of trifluoromethane clathrate hydrate (CH) from less than 75 μm diameter ice particles and CHF3 gas. As previously observed for difluoromethane and propane hydrate formation, the initial stages of the reaction exhibit a strong negative correlation of the reaction rate with temperature, consistent with a negative activation energy of formation. The values obtained for trifluoromethane, ca. −6 kJ/mol (H2O), are similar to those for difluoromethane, even though the two molecules have different intermolecular interactions and sizes. The activation energy is lesser per mole of H2O, but greater per mole of guest molecule, than for propane hydrate, which has a different crystal structure. We propose a possible explanation for the negative activation barrier based on the stabilization of metastable structures at low temperature. A pronounced dependence of the formation kinetics on the gas flow rate into the cell is observed. At 253 K and a flow rate of 15 mmol/h, the stage II enclathration of trifluoromethane proceeds so quickly that the overpressure, the difference between the gas cell pressure and the hydrate vapor pressure, is only 0.06 MPa.
I. INTRODUCTION Clathrate hydrates (CHs) are crystalline solid compounds of water and guest molecules formed under high pressure and low temperature.1 Often, clathrate hydrates are stabilized by the inclusion of hydrophobic guest molecules into a periodic array of cages formed by hydrogen-bonded water molecules. However, both water-miscible and water-immiscible guests can form hydrates, and the net stability is determined by the balance between hydrophobic and hydrophilic moieties of the guest−host interactions.2 Guest gases of varying molecular sizes incorporate into appropriately sized water cages that combine to form different clathrate hydrate crystal structures. The most common clathrate hydrates are categorized into two crystal families, cubic structure I (sI) and cubic structure II (sII). The unit cell of sI consists of two small dodecahedral (512) and six large tetrakaidecahedral (51262) cages.3 The sII unit cell consists of 16 dodecahedral and 8 large hexakaidecahedral (51264) cages.4 Hydrates have been a subject of intense study and discussion, not only in the context of their deleterious role in oil and gas pipeline blockages5,6 but also as gas storage materials,7 desalination agents, 8,9 and cold storage media in air conditioning systems.10 However, characterizing the formation kinetics of clathrate hydrates is still challenging,11 and critical issues including irreproducibility and slow production remain.12 Fast production is necessary for the use of hydrates for the © 2017 American Chemical Society
purpose of safe shipping and other applications mentioned above. As an example of the new ideas being proposed to accelerate hydrate formation, electronucleation was recently explored as a method to promote hydrate nucleation.13 Our group has been studying the formation of clathrate hydrates from ice particles and gases. To observe the onset of nucleation, we slowly add the gas to the ice particles and observe the reaction initiation via a drop in cell pressure as clathrate formation consumes the gas. We first studied propane hydrate formation not only because of the practical application of propane storage but also because this hydrate is notoriously difficult to produce in high percent yield.14 To our surprise, the propane hydrate formation reaction is faster for lower temperatures,15 and methanol doping of ice particles down to one part per 10 000 water molecules catalyzes the reaction.16 Unlike previous studies, we were able to achieve over 60% yield in only an hour.16 These unexpected results stimulated us to study a series of guest gases with varying molecular properties to try to isolate other characteristics that may facilitate the faster formation of hydrates. Here we report results for the formation of trifluoromethane hydrate, which is the least-studied member of the fluoromethane series; to the best of our knowledge there are no Received: September 1, 2017 Published: September 11, 2017 7089
DOI: 10.1021/acs.jpca.7b08730 J. Phys. Chem. A 2017, 121, 7089−7098
Article
The Journal of Physical Chemistry A
sheathed, ungrounded, T-type, thermocouples. The pressure transducer and thermocouple were interfaced to a computer for data acquisition through an Omega model OMB DAQ56 data acquisition module. Nanopure water is dripped into liquid nitrogen, and the resulting ice pellets are ground with a coffee grinder operated in a 253 K freezer and sieved through a 75 μm sieve under liquid nitrogen. The ice particles (
