Stability, Adsorption, and Diffusion ofCH4,CO2, andH2in Clathrate Hydrat

Abstract

We present a study of the adsorption and diffusion of CH4, CO2 and H2 molecules in clathrate hydrates using ab initio van der Waals density functional formalism [Dion et al. Phys. Rev. Lett. 92, 246401 (2004)]. We find that the adsorption energy is dominated by van der Waals interactions and that, without them, gas hydrates would not be stable. We calculate the maximum adsorption capacity as well as the maximum hydrocarbon size that can be adsorbed.The relaxation of the host lattice is essential for a good description of the diffusion activation energies, which are estimated to be of the order of 0.2, 0.4, and 1.0 eV for H2, CO2, and CH4, respectively.PACS numbers: 91.50He, 64.70kt, 84.60VeThe existence of complex crystalline structures, made of water molecules hosting in their cavities hydrocarbons and other molecules, that stabilize the otherwise unstable network, has been known for many years [1][2][3]. These gas hydrates, or clathrates, are stable at high pressures and low temperatures. They are very abundant in the Earth's permafrost and marine sediments [4][5][6], and they have been detected in other planetary bodies like Mars and some moons of Saturn [7,8]. They can be prepared in the laboratory under appropriate conditions, and different structural and spectroscopic measurements, like Xray and neutron diffraction, nuclear magnetic resonance (NMR), Raman and infrared spectroscopy, and others, have been performed to characterize their composition and crystal structure (see [2] and references therein).These compounds are important for many practical reasons. Historically, it was soon realized that their formation, in extraction and transportation pipes of hydrocarbons, had to be controlled to avoid their clogging. On a more global perspective, they could potentially be a huge source of hydrocarbons, with reserves much larger than those of oil and natural gas together [1]. However, their destabilization and release to the atmosphere, due to the temperature increase associated to global warming, constitutes a very serious environmental threat. Another source of interest is in their potential use for H 2 storage and also for CO 2 sequestration. On this respect, the extraction of CH 4 from natural hydrates, and its simultaneous substitution by CO 2 , preserving their structure and stability, would be an ideal operation. The viability of such an operation requires, however, a precise knowledge of several magnitudes, like the relative stability of CH 4 and CO 2 hydrates, and the diffusion barriers of these molecules in the networks. We address these magnitudes in this work.The crystalline structure of the clathrates is made up of H-bonded water molecules forming a network with cages of different shapes and sizes. Out of the various crystalline structures, we will address here the so called Structure I and Structure H (see Figure 1). For more details see for instance the Sloan's reviews [1][2][3] From a theoretical point of view, the study of these compounds represents a challenging task: it is necessary to deal...

Ab initioenergetics and kinetics study of H2and CH4in the SI clathrate hydrate

“…5 In this brief report, we present results that elucidate this crucial guest-molecule/hostframework interaction and complement a recent paper by some of the authors. 6 We show additional results of the energy landscape of H 2 and CH 4 in the various cages of the host material, and we show further results for energy barriers for all possible diffusion paths of H 2 and CH 4 through the water framework. We also report structural data of the phases SI, SII, and SH.…”

“…6 We have calculated the optimized lattice parameters for the SI, SII, and SH structures and the results are collected in Table I. We have also calculated the structures when filled with methane (one methane molecule per cage) and filled with hydrogen (up to four H 2 per cage), but the lattice parameters expand less than 0.1% upon filling, such that we have used the parameters for the empty cages henceforth.…”

C–C Stretching Raman Spectra and Stabilities of Hydrocarbon Molecules in Natural Gas Hydrates: A Quantum Chemical Study

“…39 Roman-Perez et al calculated the adsorption energies of different guest molecules captured in the water cavities from clathrate hydrates based on DFT. 43 Li et al studied the barriers of H2 and CH4 diffusion through the water framework in hydrates using dispersion-corrected DFT. 42 The ice-methane clathrate phase transition was studied by periodic hybrid ab initio-DFT computations with force-field corrections by Lenz and Ojamäe.…”

“…[28][29][30][31] The nucleation, growth, and dissociation process has been investigated by MD simulations in the last few years. [32][33][34][35][36][37][38] In the quantum-chemical investigations most focus has been on the interaction between host and guest molecules, [39][40] stability and diffusion of guest molecules in water cavities of clathrate hydrates, [41][42][43][44] and phase transitions. [45][46] Liu et al evaluated the performance of twenty density functional theory (DFT) methods for the description of the intermolecular interaction in methane hydrates.…”

Effect of Pore Geometry on Gas Adsorption: Grand Canonical Monte Carlo Simulation Studies

“…One issue is that the differences in physical properties of gases that should be separated (CO 2 , H 2 , N 2 , and CH 4 ) are relatively small as can be expected in their kinetic diameters. 17,18 Another issue to overcome is that the selectivity of the gas adsorption process is controlled by both solubility (or absorption) and diffusivity, which are inversely correlated. 19 Other issues include reversible adsorption and desorption control, thermal and high pressure stabilities, and so on.…”