Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature.     

Simulations of p-tert-butylcalix[4]arene with multiple occupancies of small guest molecules

Classical molecular dynamics simulations were used to study low-density beta(0)-phase p-tert-butylcalix[4]arene inclusion compounds with multiple calix occupancies of xenon, carbon dioxide, methane, and hydrogen guest molecules with guest-host ratios ranging from 1:4 to 4:1. Custom parameterized force fields were used for the guests and the AMBER force field for the calixarene units was validated in our previous work (Chem. Eur. J. 2006, 12, 5231). The inclusion energy and unit cell volume of the calixarene inclusion compound were determined for various guest occupancies and for occupancies greater than 1:1, strong guest-guest interaction effects are observed. The structure and energetics of the 2:1 CO(2)/beta(0)-phase inclusion compound were compared to those of the low-temperature 2:1 CO(2)/calixarene in which the guest molecules occupy both cage and interstitial sites.     

Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations

Nonspherical cages in inclusion compounds can result in non‐uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. Herein, we develop a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO₂ molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO₂ guests in terms of a polar angle θ and azimuth angle ? and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO₂ sI clathrate. The experimental ¹³C NMR lineshapes of CO₂ guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the ¹³C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.     

NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts

(1)H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host-guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.     

Molecular dynamics study of structure H clathrate hydrates of methane and large guest molecules

Methane storage in structure H (sH) clathrate hydrates is attractive due to the relatively higher stability of sH as compared to structure I methane hydrate. The additional stability is gained without losing a significant amount of gas storage density as happens in the case of structure II (sII) methane clathrate. Our previous work has showed that the selection of a specific large molecule guest substance (LMGS) as the sH hydrate former is critical in obtaining the optimum conditions for crystallization kinetics, hydrate stability, and methane content. In this work, molecular dynamics simulations are employed to provide further insight regarding the dependence of methane occupancy on the type of the LMGS and pressure. Moreover, the preference of methane molecules to occupy the small (5(12)) or medium (4(3)5(6)6(3)) cages and the minimum cage occupancy required to maintain sH clathrate mechanical stability are examined. We found that thermodynamically, methane occupancy depends on pressure but not on the nature of the LMGS. The experimentally observed differences in methane occupancy for different LMGS may be attributed to the differences in crystallization kinetics and/or the nonequilibrium conditions during the formation. It is also predicted that full methane occupancies in both small and medium clathrate cages are preferred at higher pressures but these cages are not fully occupied at lower pressures. It was found that both small and medium cages are equally favored for occupancy by methane guests and at the same methane content, the system suffers a free energy penalty if only one type of cage is occupied. The simulations confirm the instability of the hydrate when the small and medium cages are empty. Hydrate decomposition was observed when less than 40% of the small and medium cages are occupied.     

Simulations of structure II H2 and D2 clathrates: potentials incorporating quantum corrections

Molecular dynamics simulations are used to study the stability of structure II H(2) and D(2) clathrates with different large and small guest occupancies at 160 and 250 K and 2.0 kbars. Simulations are performed with the recently proposed anisotropic site-site potentials of Wang for H2 and D2 [J. Quant. Spectrosc. Radiat. Transf. 76, 23 (2003)] which are parameterized to account for quantum corrections of order variant Planck's over 2pi(2) in the second virial coefficient. Occupancies of 0-2 in the small cages and 2-5 in the large cages are considered. Thermodynamic integration is used to determine the most stable guest occupancy at each temperature. Since lattice free energy and configurational energy differences are small for a number of different combinations of cage occupancies, one must expect that in bulk samples various combinations will indeed be observed. Special attention is given to the differences between H(2) and D(2) guests and implications on the hydrogen storage capacity of the clathrates are discussed.     

Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis

The search for metal‐free organic photocatalysts for H₂ production from water using visible light remains a key challenge. Reported herein is a molecular structural design of pure organic photocatalysts, derived from conjugated polybenzothiadiazoles, for photocatalytic H₂ evolution using visible light. By alternating the substitution position of the electron‐withdrawing benzothiadizole unit on the phenyl unit as a comonomer, various polymers with either one‐ or three‐dimensional structures were synthesized and the effect of the molecular structure on their catalytic activity was investigated. Photocatalytic H₂ evolution efficiencies up to 116 μmol h^?1 were observed by employing the linear polymer based on a phenyl‐benzothiadiazole alternating main chain, with an apparent quantum yield (AQY) of 4.01 % at 420 nm using triethanolamine as the sacrificial agent.     

How much carbon dioxide can be stored in the structure H clathrate hydrates?: a molecular dynamics study

The stability of structure H (sH) carbon dioxide clathrate hydrates at three temperature-pressure conditions are determined by molecular dynamics simulations on a 3x3x3 sH unit cell replica. Simulations are performed at 100 K at ambient pressure, 273 K at 100 bars and also 300 K and 5.0 kbars. The small and medium cages of the sH unit cell are occupied by a single carbon dioxide guest and large cage guest occupancies of 1-5 are considered. Radial distribution functions are given for guests in the large cages and unit cell volumes and configurational energies are studied as a function of large cage CO(2) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at three temperature/pressure conditions stated above. At the low temperature, large cage occupancy of 5 is the most stable while at the higher temperature, the occupancy of 3 is the most favored. Calculations are also performed to show that the CO(2) sH clathrate is more stable than the methane clathrate analog. Implications on CO(2) sequestration by clathrate formation are discussed.     

Numerical simulations of a Cu–water nanofluid-based parabolic-trough solar collector

Journal of Thermal Analysis and Calorimetry - In this study, the thermal and flow characteristics of a parabolic-trough solar collector have been numerically investigated. The turbulent flow inside...     

An Optimal Extension of the Polak–Ribière–Polyak Conjugate Gradient Method

Based on a singular value analysis on an extension of the Polak–Ribière–Polyak method, a nonlinear conjugate gradient method with the following two optimal features is proposed: the condition number of its search direction matrix is minimum and also, the distance of its search direction from the search direction of a descent nonlinear conjugate gradient method proposed by Zhang et al. is minimum. Under proper conditions, global convergence of the method can be achieved. To enhance e?ciency of the proposed method, Powell’s truncation of the conjugate gradient parameters is used. The method is computationally compared with the nonlinear conjugate gradient method proposed by Zhang et al. and a modified Polak–Ribière–Polyak method proposed by Yuan. Results of numerical comparisons show e?ciency of the proposed method in the sense of the Dolan–Moré performance profile.