Instant Conversion of Air to a Clathrate Hydrate: CO2 Hydrates Directly from Moist Air and Moist CO2(g)

Induced Migration of CO2 from Hydrate Cages to Amorphous Solid Water under Ultrahigh Vacuum and Cryogenic Conditions

Restricted migration of reactive species limits chemical transformations within interstellar and cometary ices. We report the migration of CO₂ from clathrate hydrate (CH) cages to amorphous solid water (ASW) in the presence of tetrahydrofuran (THF) under ultrahigh vacuum (UHV) and cryogenic conditions. Thermal annealing of sequentially deposited CO₂ and H₂O ice, CO₂@H₂O, to 90 K resulted in the partitioning of CO₂ in 5¹² and 5¹²6² CH cages (CO₂@5¹², CO₂@5¹²6²). However, upon preparing a composite ice film composed of CO₂@5¹², CO₂@5¹²6² and THF distributed in the water matrix at 90 K, and annealing the mixture for 6 h at 130 K produced mixed CO₂-THF CH, where THF occupied the 5¹²6⁴ cages (THF@5¹²6⁴) exclusively while CO₂ in 5¹²6² cages (CO₂@5¹²6²) got transferred to the ASW matrix and CO₂ in the 5¹² cages (CO₂@5¹²) remained as is. This cage-matrix exchange may create a more conducive environment for chemical transformations in interstellar environments.

Co-deposition of gas hydrates by pressurized thermal evaporation

Gas hydrates are usually synthesized by bringing together a pressurized gas and liquid or solid water. In both cases, the transport of gas or water to the hydrate growth site is hindered once an initial film of hydrate has grown at the water-gas interface. A seemingly forgotten gas-phase technique overcomes this problem by slowly depositing water vapor on a cold surface in the presence of the pressurized guest gas. Despite being used for the synthesis of low-formation-pressure hydrates, it has not yet been tested for hydrates of CO₂ and CH₄. Moreover, the potential of the technique for the study of hydrate decomposition has not been recognized yet. We employ two advanced implementations of the condensation technique to form hydrates of CO₂ and CH₄ and demonstrate the applicability of the process for the study of hydrate decomposition and the phenomenon of self-preservation. Our results show that CO₂ and CH₄ hydrate samples deposited on graphite at 261-265 K are almost pure hydrates with an ice fraction of less than 8%. Rapid depressurization experiments with thin deposits (approx. 330 μm thickness) of CO₂ hydrate on an aluminum surface at 265 K yield identical dissociation curves when the deposition is done at identical pressure. However, hydrates deposited at 1 MPa almost completely withstand decomposition after rapid depressurization to 0.1 MPa, while samples deposited at 2 MPa decompose 7 times faster. Therefore, this synthesis technique is not only applicable for the study of hydrate decomposition but can also be used for the controlled deposition of a super-preserved hydrate.

NH3 as simple clathrate-hydrate catalyst: Experiment and theory

The catalytic action of NH₃ within the all-vapor approach for instant clathrate hydrate (CH) formation is studied using both FTIR spectroscopy and ab initio molecular dynamics simulations. A unique property of NH₃, namely, the rapid abundant penetration and occupation of the water network, creates defects, particularly Bjerrum D-defects, in the hydrate frame that are generally stabilized by guest NH₃ molecules in the cages. Furthermore, insertion of NH₃ seriously disturbs the hydrate network where the guest NH₃ molecules also make fluxional H-bonds with the host water molecules. These defects strongly facilitate a sub-second formation of the simple NH₃ s-II gas hydrate at 160 K. FTIR spectra of aerosols of the NH₃ s-II CH have been measured, and the displacement of both small and large cage NH₃ guests by CO₂ and tetrahydrofuran is examined.

NH3 as unique non-classical content-former within clathrate hydrates

High quality FTIR spectra of aerosols of NH₃-THF and NH₃-TMO binary clathrate hydrates (CHs) have been measured. Our recently developed all-vapor sub-second approach to clathrate-hydrate formation combined with computational studies has been used to identify vibrational spectroscopic signatures of NH₃ within the gas hydrates. The present study shows that there are three distinct NH₃ types, namely, classical small-cage NH₃, nonclassical small-cage NH₃, and NH₃ within the hydrate network. The network ammonia does not directly trigger the non-classical CH structure. Rather, the ammonia within the network structure perturbs the water bonding, introducing orientational defects that are stabilized by small and/or large cage guest molecules through H-bonding. This unusual behavior of NH₃ within CHs opens a possibility for catalytic action of NH₃ during CH-formation. Furthermore, impacts over time of the small-cage NH₃-replacement molecules CO₂ and CH₄ on the structure and composition of the ternary CHs have been noted.

Molecular Modes and Dynamics of HCl and DCl Guests of Gas Clathrate Hydrates

Recent years have yielded advances in the placement of unusual molecules as guests within clathrate hydrates (CHs) without severe distortion of the classic lattice structures. Reports describing systems for which observable but limited distortion does occur are available for methanol, ammonia, acetone, and small ether molecules. In these particular examples, the large-cage molecules often participate as non-classical guests H-bonded to the cage walls. Here, we expand the list of such components to include HCl/DCl and HBr as small-cage guests. Based on FTIR spectra of nanocrystalline CHs from two distinct preparative methods combined with critical insights derived from on-the-fly molecular dynamics and ab initio computational data, a coherent argument emerges that these strong acids serve as a source of molecular small-cage guests, ions, and orientational defects. Depending on the HCl/DCl content the ions, defects and molecular guests determine the CH structures, some of which form in sub-seconds via an all-vapor preparative method.

The nucleation of gas hydrates near silica surfaces

Understanding the nucleation and crystal growth of gas hydrates near mineral surfaces and in confinement are critical to the methane recovery from gas hydrate reservoirs. In this work, through molecular dynamics simulation studies, we present an exploration of the nucleation behavior of methane hydrates near model hydroxylated silica surfaces. Our simulation results indicate that the nucleation of methane hydrates can initiate from the silica surfaces despite of the structural mismatch of the two solid phases. A layer of intermediate half-cage structures was observed between the gas hydrate and silica surfaces, apparently helping to minimize the free energy penalty. These results have important implications to our understanding of the effects of solid surfaces on hydrate nucleation processes.

Catalytic activity of methanol in all-vapor subsecond clathrate-hydrate formation

Methanol's property as a catalyst in the formation of gas clathrate hydrates has been recognized for several years and was recently employed in a broad ranging study [K. Shin, K. A. Udachin, I. L. Moudrakovski, D. M. Leek, S. Alavi, C. I. Ratcliffe, and J. A. Ripmeester, Proc. Natl. Acad. Sci. U.S.A. 110, 8437 (2013)]. A new measure of that activity is offered here from comparative rates of formation of methanol (MeOH) clathrate hydrates within our all-vapor aerosol methodology for which tetrahydrofuran (THF) and other small ethers have set a standard for catalytic action. We have previously described numerous examples of the complete conversion of warm all-vapor mixtures to aerosols of gas clathrate hydrates on a sub-second time scale, generally with the catalyst confined primarily to the large cage of either structure-I (s-I) or structure-II (s-II) hydrates. THF has proven to be the most versatile catalyst for the complete subsecond conversion of water to s-II hydrate nanocrystals that follows pulsing of appropriate warm vapor mixtures into a cold chamber held in the 140-220 K range. Here, the comparative ability of MeOH to catalyze the formation of s-I hydrates in the presence of a small-cage help-gas, CO2 or acetylene, is examined. The surprising result is that, in the presence of either help gas, CH-formation rates appear largely unchanged by a complete replacement of THF by MeOH in the vapor mixtures for a chamber temperature of 170 K. However, as that temperature is increased, the dependence of effective catalysis by MeOH on the partial pressure of help gases also increases. Nevertheless, added MeOH is shown to markedly accelerate the s-II THF-CO2 CH formation rate at 220 K.

CO2 and C2H2 in cold nanodroplets of oxygenated organic molecules and water

Recent demonstrations of subsecond and microsecond timescales for formation of clathrate hydrate nanocrystals hint at future methods of control of environmental and industrial gases such as CO2 and methane. Combined results from cold-chamber and supersonic-nozzle [A. S. Bhabhe, "Experimental study of condensation and freezing in a supersonic nozzle," Ph.D. thesis (Ohio State University, 2012), Chap. 7] experiments indicate extremely rapid encagement of components of all-vapor pre-mixtures. The extreme rates are derived from (a) the all-vapor premixing of the gas-hydrate components and (b) catalytic activity of certain oxygenated organic large-cage guests. Premixing presents no obvious barrier to large-scale conditions of formation. Further, from sequential efforts of the groups of Trout and Buch, a credible defect-based model of the catalysis mechanism exists for guidance. Since the catalyst-generated defects are both mobile and abundant, it is often unnecessary for a high percentage of the cages to be occupied by a molecular catalyst. Droplets represent the liquid phase that bridges the premixed vapor and clathrate hydrate phases but few data exist for the droplets themselves. Here we describe a focused computational and FTIR spectroscopic effort to characterize the aerosol droplets of the all-vapor cold-chamber methodology. Computational data for CO2 and C2H2, hetero-dimerized with each of the organic catalysts and water, closely match spectroscopic redshift patterns in both magnitude and direction. Though vibrational frequency shifts are an order of magnitude greater for the acetylene stretch mode, both CO2 and C2H2 experience redshift values that increase from that for an 80% water-methanol solvent through the solvent series to approximately doubled values for tetrahydrofuran and trimethylene oxide (TMO) droplets. The TMO solvent properties extend to a 50 mol.% solution of CO2, more than an order of magnitude greater than for the water-methanol solvent mixture. The impressive agreement between heterodimer and experimental shift values throughout the two series encourages speculation concerning local droplet structures while the stable shift patterns appear to be useful indicators of the gas solubilities.

Communication: Fourier-transform infrared probing of remarkable quantities of gas trapped in cold homogeneously nucleated nanodroplets

Studies of catalyzed all-vapor gas-hydrate formation on a sub-second timescale have been extended with a special focus on liquid-droplet compositions at the instant of hydrate crystallization. This focus has been enabled by inclusion of methanol in the all-vapor mixture. This slows droplet to gas-hydrate conversion near 200 K to a time scale suited for standard FTIR sampling. Such droplet data are sought as a guide to ongoing efforts to reduce the amount of guest catalyst required for instant formation of the gas hydrates. For the same reason, all-vapor sampling has also been extended to the generation of long-lived liquid droplets with reduced or no water content. Observations of single-solvent droplets show that surprising quantities of gas molecules are trapped during rapid droplet growth. For example, CO2 is trapped at levels near 50 mol. % in droplets of acetone, tetrahydrofuran, or trimethylene oxide formed under CO2 pressures of several Torr in a cold-chamber at 170 K. Less but significant amounts of gas are trapped at higher temperatures, or in methanol or water-methanol droplets. The droplet metastability appears to commonly lead to formation of bubbles larger than the original nanodroplets. Besides serving as a guide for the all-vapor gas-hydrate studies, the semiquantitative evidence of extensive trapping of gases is expected to have a role in future studies of atmospheric aerosols.

Tracking all-vapor instant gas-hydrate formation and guest molecule populations: a possible probe for molecules trapped in water nanodroplets

Quantitative Fourier-transform infrared spectra for low-temperature (160-200 K) aerosols of clathrate-hydrate nanoparticles that contain large-cage catalysts and small-cage nonpolar guests have been extended to a broad range of vapor compositions and sampling conditions. The data better reveal the stages by which room-temperature vapor mixtures, when cooled below ∼220 K, instantly generate aerosols with particles composed exclusively of the corresponding clathrate hydrates. In particular the quantitative data help relate the nature of the hydrates that form to the composition of the aqueous nanodroplets of the first stages of the rapid transition from the all-vapor mixture. The overall transition from an all-vapor mixture to "gas"-hydrate nanocrystals is a multistage one that has been characterized as homogeneous nucleation and growth of solution nanodroplets (∼240 K) followed by nucleation and growth of the gas-hydrate particles (∼220 K); all occurring within a subsecond that follows pulsing of the warm vapor into a sampling cold chamber. This may serve well as a general description of the instantaneous generation of the gas-hydrate aerosols, but closer consideration of the nature of the sampling method, in context with recent computation-based insights to (a) gas-hydrate nucleation stages∕rates and (b) the lifetimes of trapped small nonpolar molecules in cold aqueous nanodroplets, suggests a more complex multistage transition. The simulated lifetimes and extensive new quantitative infrared data significantly broaden the knowledge base in which the instantaneous transition from vapor to crystalline hydrate particles is viewed. The apparent need for a high occupancy of large-cage catalytic guest molecules currently limits the practical value of the all-vapor method. Only through greater clarity in the molecular-level description of the transition will the ultimate limits be defined.