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.

We present a new method for solving the master equation for a system evolving on a spatially periodic network of states. The network contains 2(ν) images of a "unit cell" of n states, arranged along one direction with periodic boundary conditions at the ends. We analyze the structure of the symmetrized (2(ν)n) × (2(ν)n) rate constant matrix for this system and derive a recursive scheme for determining its eigenvalues and eigenvectors, and therefore analytically expressing the time-dependent probabilities of all states in the network, based on diagonalizations of n × n matrices formed by consideration of a single unit cell. We apply our new method to the problem of low-temperature, low-occupancy diffusion of xenon in the zeolite silicalite-1 using the states, interstate transitions, and transition state theory-based rate constants previously derived by June et al. [J. Phys. Chem. 95, 8866 (1991)]. The new method yields a diffusion tensor for this system which differs by less than 3% from the values derived previously via kinetic Monte Carlo (KMC) simulations and confirmed by new KMC simulations conducted in the present work. The computational requirements of the new method are compared against those of KMC, numerical solution of the master equation by the Euler method, and direct molecular dynamics. In the problem of diffusion of xenon in silicalite-1, the new method is shown to be faster than these alternative methods by factors of about 3.177 × 10(4), 4.237 × 10(3), and 1.75 × 10(7), respectively. The computational savings and ease of setting up calculations afforded by the new method of master equation solution by recursive reduction of dimensionality in diagonalizing the rate constant matrix make it attractive as a means of predicting long-time dynamical phenomena in spatially periodic systems from atomic-level information.

We present a new method for calculating the diffusion tensor for the systems of sorbates inside nanoporous materials at different loadings by just using transition rate constants. In addition, a userfriendly program with graphical user interface has been developed and is freely provided to be used (https://sourceforge.net/projects/ kobra/). It needs from the user just to provide the values of the unit cell lengths and angles, the transition rate constants for each sorbate, and any spatial constraint between these sorbates. This program is shown to be about 30 times faster than kinetic Monte Carlo method. Application of the method to the problem of diffusion of aromatics in silicalite-1 at different loadings is presented too.

We identified computationally a number of hydration-induced structural phases of the SAPO-34 zeolite, possessing different energetic characteristics as revealed by density functional theory calculations and exhibiting different adsorption thermodynamics, thereby explaining previous experimental findings. The successive transitions between phases A, B, C, D, and E, sorted in terms of increasing stability (decreasing chemical potential), were proved to give rise to hysteresis loops that appeared during the water desorption isotherm for various temperatures. Our sorbate water molecular dynamics simulations are in agreement with previous pulsed-field gradient NMR results and showed that the sorbate diffusivity rises with increasing loading because of a decrease of the free energy barrier for surmounting the sorbent windows, which are being primarily water-populated. Moreover, we found that the phase-dependent water diffusivity decreases for all loadings as a result of the gradual reduction in width of the aforesaid apertures upon transitioning from phase A to E.