Search

The dynamics of adsorbed CO 2 in the metal-organic framework Zn 2 (bdc) 2 dabco (DMOF-1) was investigated using molecular dynamics (MD) simulations and 13 C NMR spectroscopy. The statistical analysis of the MD trajectories suggest a preferred localization of the CO 2 molecules in the Zn 2 (bdc) 4 corners of the DMOF-1 lattice. The adsorbed molecules retain a high but anisotropic rotational and translational mobility in the channel system. Based on these MD-results, the residual chemical shift anisotropy 〈Δ δ 〉 MD = −114 ppm and the diffusion anisotropy ( D ∥ / D ⊥ ) MD = 9.8 ± 0.5 were calculated. They are found to be in reasonable agreement with the experimental NMR data of 〈Δ δ 〉 NMR =−(55 ± 2) ppm and ( D ∥ / D ⊥ ) NMR = 3 respectively.

We reconstructed pore structures of three porous solids that differ from each other in morphology and topology of pore space. To achieve this, we used a stochastic method based on simulated annealing and X-ray computed microtomography. Simulated annealing was constrained by the following microstructural descriptors sampled along the principal and diagonal directions: the two-point probability function for the void phase and the lineal-path functions for both void and solid phases. The stochastic method also assumed the isotropic pore structures in accordance with a recent paper (Capek et al. in Transp Porous Media 88(1): 87–106 (2011)). With the exception of the solid with the widest pores, we made tomographic volume images in high and low resolution, which enabled us to study the effect of resolution on microstructural descriptors and effective transport properties. A comparison of the two-point probability function and the lineal-path function sampled in the principal directions revealed that the pore structures derived from the tomographic volume images were slightly anisotropic, in opposition to the assumption of the stochastic method. Besides the anisotropy, other microstructural descriptors including the pore-size function and the total fraction of percolating cells indicated that the morphological and topological characteristics of the pore structures depended on the reconstruction method and its parameters. Particularly, the pore structures reproduced using the stochastic method contained wider pores than those obtained using X-ray tomography. Deviations between the pore structures derived from low- and high-resolution tomographic volume images were also observed and imputed to partial volume artefacts. Then, viscous flow of incompressible liquid, ordinary diffusion, Knudsen flow and self-diffusion of water in the reconstructed pore spaces were simulated. As counterparts, experimental data were measured by means of permeation and Wicke–Kallenbach cells and pulsed field gradient NMR. Deviations between the simulated quantities on the one hand and experimental data on the other hand were generally acceptable, which corroborated the pore-space models. As expected, the predictions based on the tomographic models of pore space were more successful than those derived from the stochastic models. The stationary effective transport properties, i.e. the effective permeability, the effective pore size and the geometric factor, were sensitive to a bias in long-range pore connectivity. Furthermore, the time-dependent effective diffusivity was found to be especially sensitive to relatively small morphological deviations between the real and reconstructed pore structures. It is concluded that the combined predictions of the effective permeability, the effective pore size, the geometric factor and time-dependent effective self-diffusivity of water are needed for the reliable evaluation of pore-space reconstruction.

DMOF-1 (Zn 2 (bdc) 2 dabco) is a metal–organic framework with tetragonal symmetry. It contains parallel one-dimensional channels with a diameter of about 0.75 nm interconnected by smaller windows. For carbon dioxide adsorbed in the nanochannels the chemical shift tensor and the diffusion tensor were investigated by 13 C NMR. The spectra acquired under static conditions reveal a powder pattern with a residual chemical shift anisotropy of 〈 Δ δ 〉 = - 55 ppm. The collinearity of the axisymmetrical residual chemical shift and the diffusion tensors was utilized to assess the diffusion anisotropy via the diffusion attenuation of the 13 C PFG NMR powder pattern. The results show that carbon dioxide is highly mobile in DMOF-1. The apparent diffusion coefficient (the trace of diffusion tensor) is ( 6.2 ± 1.0 ) × 10 - 9 m 2 s - 1 at 298 K and the corresponding anisotropy expressed as D ‖ / D ⊥ is approximately equal to 3.

The microscopic structure of ethanol in the liquid state is characterized as a dynamic equilibrium of hydrogen-bonded clusters of different sizes and topologies. We have developed a novel method for determination of the average size of the clusters that combines the measurement of diffusion coefficient by means of NMR diffusometry technique and hydrodynamic simulations. The approach includes the use of HydroNMR [J. Garcı̀a de la Torre, M. L. Huertas, and B. Carrasco, J. Magn. Reson. 147, 2000, 138] for small molecules, which is attained here by the calibration procedure using a dilute solution of tetramethylsilane. It is thus possible to correlate the experimentally determined diffusion coefficient of ethanol with calculated diffusion coefficients of the modeled clusters of different sizes. We found that average size of the clusters in 0.16 M solution of ethanol in n-hexane corresponds to the monomer above 300 K and to the pentamer/hexamer below 240 K. The clusters in the case of 0.44 M solution are generally slightly larger, from the average size corresponding to the dimer at 320 K and the hexamer at 210 K.