Aluminium isophthalate CAU-10-H is a promising stable adsorbent for application in heat-exchange processes.

ENERGIE:MATERIAUX+EPA:HJOThis study describes the development and application of a new computational methodology for calculating the self-diffusivity of sorbate molecules strongly confined within shape-selective nanoporous materials. An umbrella sampling strategy, employing repulsive walls to confine the sorbate within specific regions of the pore space, is invoked to extract free energy profiles with respect to the sorbate degrees of freedom. Based on these profiles, it is shown how the multidimensional problem of translational diffusion of benzene in flexible silicalite-1 can be reduced first to a six-dimensional problem, then to a three-dimensional (3D) problem and finally, to a 1D problem. A 3D free energy distribution is accumulated as a function of the benzene centre of mass position and ultimately reduced to a set of 1D profiles for the benzene centre of mass along the pore axes. From these profiles, the rate constants for jumps executed by benzene between sorption sites are calculated using transition state theory; from the latter rate constants, the low-occupancy self-diffusivity is obtained using the MESoRReD method [Kolokathis PD, Theodorou DN. On solving the master equation in spatially periodic systems. J Chem Phys. 2012;137:034112]. The activation energy for diffusion and preferred orientations in the various sorption states in silicalite are in very favourable agreement with available experimental measurements

Molecular dynamics computer experiments were conducted to study the thermodynamics and kinetics of the water-sorbed phase within a digitized hybrid (inorganic− organic) iron carboxylate sorbent, the MIL-100(Fe), relying on a synergy of statistical mechanics-based methodology and the time evolution of the system captured by classical mechanics. To achieve sufficient statistics, the entire unit cell of this extremely large host material was utilized, and consequently, the offset of the imposed computational burden became one of the tasks of the presented study. Analysis of sorption thermodynamics reveals that the kind of the MIL-100 terminal species (fluorine and water, bound to iron), as well as their relative position around the cavity joints of the small and large mesopore networks of this material, may tune the sorption phenomena and control the guest population rate within the two pore systems. Computed singlet and pair density distributions along with the transport predictions of water in the host material support the findings of thermodynamics and interpret the diffusivity loading dependence on the basis of the spatial free energy distribution of the guest phase.

Measured, via pulsed field gradient (PFG) NMR, and computed molecular dynamics (MD) were utilized for the study of the phase equilibrium and kinetics of water sorbed in a bed of MIL-100(Al) crystallites. The computations rely on our recent methodology for modeling water equilibria and dynamics in the Fe-homologue MIL-100 crystal; in that sense, the particular NMR technique serves also as a validation tool of the previous simulation work which is adapted to the current system. In addition, a computational scheme for assigning partial charges on the host framework atoms was devised; it involves density functional theory (DFT) combined with electronegativity equalization method (EEM) calculations. The derived this way electronegativity, hardness, and gamma parameters for the specific MIL-100(Al) atoms can be used in EEM calculations of other aluminum metalorganic frameworks (MOF) bearing similar atom types. The thermodynamics predictions obtained via MD, comprising equilibria, enthalpies, adsorbate probability densities, and host's terminal species effects, were compared with data from the real system's phase equilibria measured in this work. The intracrystalline self-diffusivity of the sorbed water was extracted by means of the spin echo curves obtained by PFG NMR for various guest loadings as a function of observation time and a theoretical short-time expansion of the diffusion coefficient of random walkers, assuming spherical particles under reflecting boundary conditions following Mitra et al. The experimental activation energies for diffusion confirmed previous, in MIL-100(Fe), and current modeling results, with respect to the adsorbed water dynamics and singlet probability density distribution.

A specific computational methodology based on transition state theory (Kolokathis, P. D. et al. Mol. Simul., 2014, 40, 80−100) is evolved and applied for calculation of the selfdiffusion coefficients of p-xylene and benzene in silicalite-1 at infinite dilution. In addition, we study the orientational distributions of phenyl rings and methyl stems of p-xylene and benzene sorbed in the zeolite and check for the existence of entropic barriers to translational motion. A new reduction method for the states appearing in the free energy profiles is presented and used for calculation of transition rate constants for elementary jumps. Quasi-elastic neutron scattering measurements are also conducted and compared with the simulation results. A major conclusion from both experiments and simulations is that p-xylene diffuses roughly 100 times faster than benzene when sorbed at low occupancy in silicalite. Benzene encounters strong entropic barriers to translational motion at channel intersections, where it can adopt a variety of orientations. The corresponding barriers for p-xylene are much lower, reflecting the inability of its major axis to reorient within channel intersections.