J. Phys. Chem. A 2010, 114, 13129–13133
13129
Instant Conversion of Air to a Clathrate Hydrate: CO2 Hydrates Directly from Moist Air and Moist CO2(g) J. Paul Devlin* and I. Abrrey Monreal Department of Chemistry, Oklahoma State UniVersity, Stillwater, Oklahoma 74078, United States ReceiVed: NoVember 5, 2010
The rapid conversion of vapor mixtures containing the gases CO2, H2S, and HCN to clathrate hydrates was reported recently. The novel method is based on the pulsing of warm vapor mixtures, including a carrier gas, into a cold condensation chamber. With cooling, the vapors, which also include ∼1% water and either tetrahydrofuran or trimethylene oxide as a catalyst, nucleate aqueous solution nanodroplets that, on a millisecond time scale, crystallize as hydrate nanoparticles that consume 100% of the water. Humid air approximates the content of mixtures used successfully in the vapor-to-hydrate conversions. FTIR spectra are examined for gas hydrates formed directly from air and air enriched with CO2, as well as hydrate particles for which CO2(g) serves as both guest and aerosol medium. In each instance all of the water in the condensed phase converts to a clathrate hydrate. The subsecond ether-catalyzed formation of the hydrates near 230 K requires only a few percent of the CO2 pressure used in conventional processes that yield fractional amounts of gas hydrates on an hour time scale in the same temperature range. Introduction For a number of years intense research efforts have been directed to understanding the formation, transformation, and decomposition dynamics of gas hydrates, so numerous reports are available (see, for example, ref 1-8). Interest in gas hydrates, formed by “tetrahedrally” hydrogen-bonded water lattices that define cages suited in size to the entrapment of gas molecules, has surged primarily because of an increasing need for improved management of methane, hydrogen, and carbon dioxide as well as other low molecular weight hydrocarbons. These gases are important commercially at increasingly high volumes: methane, with potential as a primary fuel based on huge untapped natural deposits4 and as a pollutant;9 hydrogen, as a promising secondary source of energy;6 carbon dioxide, as a waste product with many sources that, combined, threaten the global climate.10 The management of each of these gases is difficult because of high condensation pressures at normal commercial temperatures. However, equilibrium pressures are markedly lower for the same gases confined in cages of clathrate hydrates.4 Despite the growth in research devoted to improved management of gas hydrates, the necessary technology is still lacking.11,12 A serious basic problem stems from the difficulty of rapid conversion of large fractions of a supply of ice or liquid water to a gas hydrate and management of the eventual release of the encaged gas on demand.5 The slow conversion of ice arises from a need for continuous access of potential guest molecules to fresh ice.2,3 A crust of hydrate product, through which guest transport is typically very slow, coats the ice hindering the molecular access. As a direct result, the conversion of even finely divided ice to a gas hydrate requires hours at moderate temperatures and high pressures,2,3 unless a guest-transport catalyst is present.7,8 Attempts at rapid gas hydrate formation from water microdroplets have encountered a similar limitation in the form of a hydrate film that encapsulates the droplets.11,12 * To whom correspondence should be addressed. E-mail: devlin@ okstate.edu.
We have demonstrated that it is not necessary to use finely divided liquid water or ice for the most rapid formation of a gas hydrate.13 Rather it is advantageous to start with an all vapor system, namely, water vapor mixed with the permanent gas plus an ether catalyst all in a warm carrier gas such as nitrogen. As we will show below, the carrier gas can also be one of the guests, e.g., CO2 or methane. Infrared spectra indicate that, milliseconds after the warm vapor mixture is pulsed into a cold chamber, 100% of the water vapor has converted to nanocrystals of the gas hydrate. Our earlier study focused on hydrate formation with cold-chamber temperatures of
