Isidoro García-Cruz

Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT hydrodesulfurization

Abstract NiMo catalysts supported on SBA-15 were prepared by coimpregnation and successive impregnation methods with the addition of citric acid in the impregnation solutions. Addition of citric acid resulted in an increase in both catalysts’ activity in dibenzothiophene HDS and selectivity towards the direct desulfurization route, which was due to an increase in the MoS2 dispersion and in the amount of Ni-Mo-S species.

Electronic structure in different environments for vanadyl porphyrinate molecules present in crude oil

The role of lateral chains in vanadyl porphyrinate is analyzed from a theoretical point of view. Three different vanadyl porphyrinate molecules are studied in different solutions, one with aliphatic substituents another with aromatic substituents and the molecule without substituents for comparison. The analysis of the results is mainly based on the behavior in different solvents, aromaticity measures, frontier molecular orbital changes and charge behavior. The isolated molecule is compared with the same molecule immersed in three different solvents, with low, medium and high values of dipole moment, tetrachloromethane, pyridine and acetone, respectively. The reactivity of the complexes is improved with the presence of the substituents without relevant aliphatic or aromatic nature, therefore, it is expected that the substituted molecules will be more pollutant to the catalysts than the molecule without substituents.

Docking of An Asphaltene Molecular Model on A Fe2O3 Surface, An Ab Initio Simulated Annealing

Abstract The docking of an asphaltene molecular model over the (1000) hematite surface is analyzed through the use of ab initio molecular dynamics simulation techniques where the Density Functional Theory (DFT) forces were obtained using the Harris functional approach. The (1000) hematite surface is modeled by a supercell slab lattice of 225 atoms with a = b lengths of 25.17 Å; on the c -direction an empty gap space and Fe-O-Fe-Fe-O atomic sequence was built. This structure was thermally stabilized at 400K. The asphaltene-hematite surface interaction was simulated with an annealing process at 400K during 555 fs, and then a geometry optimization process was carried out on the final annealed configuration in order to stabilize the asphaltene-surface system. We find an attractive interaction between the asphaltene and the Fe2O3 which is located mainly at the asphaltene aromatic region. Finally, we have obtained cohesion energy of −417 kcal/mol, indicative of quimisorption energy on the basis of a Harris functional treatment.

Electronic Structure and Mesoscopic Simulations of Nonylphenol Ethoxylate Surfactants. A Combined DFT and DPD Study

The aim of this work was to gain insight into the effect of ethylene oxide (EO) chains on the properties of a series of nonylphenol ethoxylate (NPE) surfactants. We performed a theoretical study of NPE surfactants by means of density functional theory (DFT) and dissipative particle dynamics (DPD). Both approximations were used separately to obtain different properties. Four NPEs were selected for this purpose (EO = 4, 7, 11 and 15 length chains). DFT methods provided some electronic properties that are related to the EO units. One of them is the solvation Gibbs energy, which exhibited a linear trend with EO chain length. DPD calculations allow us to observe the dynamic behavior in water of the NPE surfactants. We propose a coarse-grained model which properly simulates the mesophases of each surfactant. This model can be used in other NPEs applications.

Methodology for Predicting the Phase Envelope of a Heavy Crude Oil and Its Asphaltene Deposition Onset

Abstract We have presented general ideas to develop a new theoretical methodology, based on molecular simulation and equations of state, for obtaining the phase envelope and to predict pressure, volume, temperature (PVT) conditions of asphaltene precipitation by only taking into account the composition of the heavy crude oil and an asphaltene average molecular structure. Those results show that asphaltene precipitation is a reversible thermodynamic process. The precipitated phase is a liquid phase which consists of mainly asphaltene and some heavy fractions from the crude. This methodology can be applied to find complete phase diagrams of different crude oils based on an asphaltene average molecular structure and the composition of crude oil.

Topological and Electronic Structure of Heterocyclic Compounds Adsorbed on Hydrotreating Catalysts

We studied the electronic structure of the adsorption of S- and N-containing aromatic compounds present in crude oils on MoS2 and WS2 clusters by means of all-electron DFT methods. The aim of this work is to understand results related to the hydrotreating catalyst poisoning by quinoline. We studied the adsorption of the organic compounds by flat (π) and perpendicular (σ) adsorption on each cluster catalyst. The calculated adsorption energies indicated that π-adsorption was more favorable over σ-adsorption. In the σ mode, quinoline presented the largest adsorption energy, which led to understand the poisoning of the catalysts. We performed electron localization function (ELF) studies on the molecules adsorbed on a perpendicular orientation. We showed methyl-substituted compounds had a weaker S-{Mo,W} bond due to steric hindrance. Furthermore, atoms-in-molecules (AIM) calculations at the critical points (i.e. {S,N}-{Mo,W} interfaces) revealed a correlation between electron density and Laplacian of the electron density at this region and the adsorption energy. Ellipticity (ε) studies revealed structural information of binding at these sites, as well as the competition between S- and N-containing compounds. Similarly, ε showed that methyl-containing compounds had a very distinct character than non-substituted ones, thus revealing the importance of steric effects. Analytic tools such as ELF and AIM provide correlations between the experimental observations and properties. We find these studies can be further used to understand other catalytic phenomena.Graphical Abstract.

DFT Molecular Dynamics Study of Pyrene Biradical Species

Abstract The pyrene molecule shows its aromaticity localized in specific regions in the molecular framework as it is indicated by the calculated Nuclear Independent Chemical Shift (NICS) values, on previous electronic structure studies (García-Cruz et al. (2004) J. Phys. Chem. A 108, 5111 and Hernández-Trujillo et al. (2005) Chem. Phys. 308, 181), which is in full agreement with Clars theory. The biradical formed at 400 K follows a simple mechanism, which yields an aromatic species as it is demonstrated by the NICS value. The analysis for testing the homolytic fragmentation and the unusual stabilization are also investigated. A possible implication of the reactivity of pyrene species in asphaltene formation is discussed.

A Theoretical Study of the OH Radical Addition to the Xylenes

The initial reaction in the atmospheric oxidation of xylenes is the attack by OH radicals, which occurs mainly by addition of OH to the aromatic cycle. In this work, this reaction has been modeled for o-, m- and p- xylenes by means of ab-initio molecular orbital calculations, using unrestricted MP2 method. The xylene + OH reaction is studied in detail with the 6-31G* basis, by optimizing the geometry of the possible products and of the corresponding transition states. OH-xylene adducts formed by addition of the OH radical to the carbon atom attached to a methyl radical are found to be very stable, and the energy barriers for their formation are comparable to the ones for the addition at the other positions. As far as we know, except for o- xylene, these adducts have not been considered in previous experimental work. Results of B3LYP calculations at the MP2 optimized geometries are also reported. Trends with the two methods agree well.