ABSTRACT
Despite the sustained use of forcefield methodologies to study SiO(2) polymorphs few reviews on the subject are available in the literature. The present study is an attempt to help fill this gap, focusing on classical forcefields used to reproduce and predict properties of pure silica zeolites (or zeosils) such as cell parameters, SiO distance and especially pore size. Instead of an exhaustive study we have focused on an application where diffusion of hydrocarbons makes important the use of pure silica zeolites. A particular area of interest is small pore zeosils containing 8-rings as the largest window, which are industrially interesting for their ability to perform kinetic separations of mixtures of C3 hydrocarbon molecules whose dimensions are of similar characteristics. A set of forcefields have been selected from the literature to analyze their accuracy and transferability when predicting structural, mechanical and dynamical properties of small pore pure silica zeolites and their performance at selective diffusion of C3 hydrocarbons.
ABSTRACT
The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic frameworks (MOFs) were evaluated and rationalized in terms of the structural properties (cavity types and pore diameters) of PCN-12, HKUST-1, MOF-505, NOTT-103 and NOTT-112. First-principles calculations were employed to identify the strongest adsorption positions at the paddle wheel inorganic building unit (IBU). The adsorption centres due to confinement were located through analysis of 3D occupancy maps obtained from the hydrogen trajectories computed via molecular dynamics simulations. It was found that the confinement enhances the adsorption on the weakest adsorption centres around the IBU in regions close to the narrowest windows and promotes the formation of new adsorption regions into the small cavities. Our results indicate that at low pressure, the high H(2) uptake in these materials is partly due to the presence of small cavities (5.3-8.5 Å) or narrow windows where the long-range contribution to the adsorption becomes important. Conversely, confinement effects in cavities with diameters >12 Å were not observed.
ABSTRACT
High concentrations of molecular hydrogen adsorption on MOF-5 were evaluated at the semiempirical PM6 (periodic and cluster) and ab initio MP2 (cluster) theoretical levels. From the semiempirical calculations, an uptake of 3.9% weight on the inorganic building unit of MOF-5 was estimated, in good agreement with a recent accurate estimation of 4.5-5.2%. Although PM6 allows a correct estimation of the maximum uptake, the adsorption energy was overestimated and hence ab initio calculations, including a correlation treatment at the MP2 level as well as corrections for basis set superposition error, were performed with full optimisation, including the 6-31G basis set, which rendered an adsorption energy (per hydrogen molecule) of -0.14 kcal mol(-1). The crucial role of the quality of the basis set, as well as the importance of simulating high hydrogen loading (resembling experimental measurements), are remarked. Single point calculations (using the 6-31G geometry) with improved basis sets 6-31G(d,p) and 6-31++G(d,p) yielded adsorption energies of -0.33 and -0.57 kcal mol(-1), the latter in reasonable agreement with a recent experimental estimation of -1.0 kcal mol(-1). The role of the intermolecular hydrogen interactions is highlighted in this study, since many previous computational studies were performed at low hydrogen loadings, far from the experimental uptake conditions.
ABSTRACT
We have performed equilibrium and nonequilibrium molecular dynamics simulations to study how microwave (MW)-heated zeolite systems relax to thermal equilibrium. We have simulated the relaxation of both ionic and dipolar phases in FAU-type zeolites, finding biexponential relaxation in all cases studied. Fast-decay times were uniformly below 1 ps, while slow-decay times were found to be as long as 14 ps. Fast-decay times increase with an increase in the initial temperature difference between MW-heated ions/dipoles and the equilibrium system. Slow-decay times were found to be relatively insensitive to the details of the MW-heated nonequilibrium state. Velocity, force, and orientational correlation functions, calculated at equilibrium to explore the natural dynamics of energy transfer, decay well before 1 ps and show little evidence of biexponential decay. In contrast, kinetic energy correlation functions show strong biexponential behavior with slow-decay times as long as 14 ps. We suggest a two-step mechanism involving initial, efficient energy transfer mediated by strongly anharmonic zeolite-guest forces, followed by a slower process mediated by weakly anharmonic couplings among normal modes of the zeolite framework. In addition to elucidating relaxation from MW-heated states, we expect that these studies will shed light on energy transfer in other contexts, such as adsorption and reaction in zeolites, which often involve significant heat release.
