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ABSTRACT:We carried out a theoretical study of the HZSM-5 zeolite, for different SiO 2 /Al 2 O 3 ratios, that interacts with the n-heptane molecule. The study was performed using a QM/MM (quantum mechanics/molecular mechanics) methodology. For the QM part, we have chosen a hybrid Hartree-Fock density functional theory (DFT). The hybrid ACM/DZP approach, as implemented in Turbomole, was used for the treatment of the QM cluster containing 84 atoms that represents a ring structure model of the zeolite-n-heptane interacting system. The MM part was represented by means of an electrostatic forcefield (ESFF), which assesses the electronic embedding. The chosen QM/MM silicalite base model contains 3862 atoms. The studied SiO 2 /Al 2 O 3 ratios were 2300, 573.5, 287.7, and 189.83, containing 1, 4, 8, and 12 Al atoms, respectively. For the first ratio, the site for the substitution of Al for Si was that of minimum QM total energy value, because this replacement was done in the QM region. For the other SiO 2 / Al 2 O 3 ratios, the Al atoms were randomly spread through the MM region in accordance with the Lowenstein substitution rule. These results show the importance of the environment on the electronic properties in the QM region, where the active site lies, and their effects on the earlier steps on the activation experienced by the n-heptane

A theoretical study of poly(vinylidene flouride-trifluoroethylene) and poly(vinylidene fluoride-chlorotrifluoroethylene, is presented. By density functional theory calculations, some of the properties of these materials have been obtained. Among such properties, the dipolar moment and the energies associated to the structural changes. The B3LYP functional and 6311þG(d,p) bases set were used with Gaussian program. Calculations associated to different conformations were carried out to get insight about the involved phase changes. The energetic, charges, and dipole moment were calculated. The conformations, namely, I ¼ T p , II ¼ TG a , and III ¼ TG p , where T means trans and G means gauche, for the two polymers aforementioned were compared with the poly(vinilydene fluoride) studies previously obtained.

Adsorption of small molecules like CH 4 , CO and NH 3 into the acid sites of zeolites is analysed as an interaction between an electrophile and a nucleophile. Global reactivity descriptors like softness and electrophilicity, and local reactivity descriptors like the Fukui function, local softness and local philicity are calculated within density functional as well as Hartree-Fock frameworks using both Mulliken and Hirshfeld population analysis schemes. The HSAB principle and the best electrophilenucleophile combination suggest that the reaction between the NH 3 and Brönsted acid site of the zeolite is the strongest. Interaction between the zeolite and a small probe molecule takes place through the most electrophilic atom of one with the most nucleophilic atom of the other. This result is in conformity with those provided by the frontier orbital theory and the local HSAB principle.

The effect of microwave radiation on the hydrothermal synthesis of the double perovskite Sr2FeMoO6 has been studied based on a comparison of the particle size and structural characteristics of products from both methods. A temperature, pressure, and pH condition screening was performed, and the most representative results of these are herein presented and discussed. Radiation of microwaves in the hydrothermal synthesis method led to a decrease in crystallite size, which is an effect from the reaction temperature. The particle size ranged from 378 to 318 nm when pH was 4.5 and pressure was kept under 40 bars. According to X-ray diffraction (XRD) results coupled with the size-strain plot method, the product obtained by both synthesis methods (with and without microwave radiation) have similar crystal purity. The Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) techniques showed that the morphology and the distribution of metal ions are uniform. The Curie temperature obtained by thermogravimetric analysis indicates that, in the presence of microwaves, the value was higher with respect to traditional synthesis from 335 K to 342.5 K. Consequently, microwave radiation enhances the diffusion and nucleation process of ionic precursors during the synthesis, which promotes a uniform heating in the reaction mixture leading to a reduction in the particle size, but keeping good crystallinity of the double perovskite. Precursor phases and the final purity of the Sr2FeMoO6 powder can be controlled via hydrothermal microwave heating on the first stages of the Sol-Gel method.

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