Aldo F. Combariza | Universidad de Sucre (Sincelejo. Sucre) (original) (raw)

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Papers by Aldo F. Combariza

Despite the sustained use of forcefield methodologies to study SiO2 polymorphs few reviews on the... more Despite the sustained use of forcefield methodologies to study SiO2 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.

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Physical Chemistry Chemical …, Jan 1, 2012

The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic ... more 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.

Chemical Society Reviews, Jan 1, 2012

Despite the sustained use of forcefield methodologies to study SiO 2 polymorphs few reviews on th... more 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.

The European Physical Journal- …, Jan 1, 2007

We have simulated the non-equilibrium dynamics of methanol adsorbed in FAU zeolite driven by exte... more We have simulated the non-equilibrium dynamics of methanol adsorbed in FAU zeolite driven by external microwave (MW) radiation. We have modelled steady states produced by augmenting this MW-driven system with a thermostat that acts as a balancing heat sink. We have compared results from an implicit thermostat (Andersen velocity replacement) and an explicit thermostat (helium atoms subjected to Andersen velocity replacement). We find very good agreement between the implicit and explicit thermostats for energy distributions and diffusion coefficients produced under MW-heated steady-state conditions. This augurs well for the continued use of implicit thermostats, which are computationally more efficient.

The Journal of Physical Chemistry C, Jan 1, 2011

The Journal of …, Jan 1, 2005

We have performed equilibrium and nonequilibrium molecular dynamics simulations to study how micr... more 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 MWheated 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.

Physical Chemistry Chemical …, Jan 1, 2009

High concentrations of molecular hydrogen adsorption on MOF-5 were evaluated at the semiempirical... more 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 À0.14 kcal mol À1 . 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 À0.33 and À0.57 kcal mol À1 , the latter in reasonable agreement with a recent experimental estimation of À1.0 kcal mol À1 . 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.

The Journal of Physical …, Jan 1, 2010

Molecular dynamics (MD) simulations were performed to study the microscopic motion of methane, et... more Molecular dynamics (MD) simulations were performed to study the microscopic motion of methane, ethane, propene, and propane adsorbed in three pure silica zeolites with windows made of 8 SiO 4 tetrahedral units: Si-LTA, Si-IHW, and Si-ITE. The zeolite framework and guest structures have been simulated allowing full flexibility, using the well-known BKS model and the potential of Oie et al., respectively. The MD approach followed allows us to calculate the intra-and intercage dynamics of the smaller adsorbates, that is, methane and ethane, in statistically meaningful time and length scales in the temperature range studied, whereas for the larger size guest molecules the analysis of intercage motion is limited to higher temperatures. Calculated self-diffusion coefficients for methane, ethane, and propene show a decreasing trend correlated with increasing guest kinetic diameter sizes and decreasing critical window size, confirming experimental measurements on the molecular sieving properties of 8-ring zeolite frameworks. The microscopic motion of propane diffusing in Si-ITE suggests a somewhat anomalous diffusion process, which can be related to the levitation effect. Thus, guest diffusion translational motion is shown to be highly influenced by the topological features of the framework, with the dimensionality of the diffusion path exerting the most noticeable influence. The microscopic understanding of the host-guest dynamics can be used to highlight the trade-off between propane/propylene selectivity and diffusional differentiation in these materials. † Part of the "Alfons Baiker Festschrift".

The Journal of Physical …, Jan 1, 2009

Pure silica zeolites with narrow eight-membered (8-MR) oxygen rings are good candidates for separ... more Pure silica zeolites with narrow eight-membered (8-MR) oxygen rings are good candidates for separations of olefin/paraffin mixtures based on molecular sieving effects. We have used a molecular dynamics technique to study the structural features of two pure silica 8-MR frameworks: Si-CHA and Si-SAS. CHA and SAS are framework types with 3-and 1-dimensional porous systems, respectively, with cages connected through 8-MR windows. Window size temperature fluctuations were studied from equilibrium molecular dynamics simulations by allowing full flexibility of the framework. The thermal influence on diffusional features of propane/propylene molecules adsorbed in Si-CHA and Si-SAS zeolites is studied using classical molecular dynamics simulations. Potential energy surface effects are taken into account by comparing the results from the well-known van