Modification of nano-fibriform silica by dimethyldichlorosilane

The modification of nano-fibriform silica by dimethyldichlorosilane was studied by transmission electron microscopy, X-ray powder diffraction, infrared spectroscopy, Raman spectroscopy, physical N₂ adsorption techniques, differential thermal and thermogravimetric analysis, scanning electron microscopy, and elemental analyzer.The results show that dimethyl silane derivatives have been successfully covalently grafted on nano-fibriform silica. The polarity of the modified product decreases with the substitution of -OH groups by siloxyl groups. Therefore, the modified product can be easily dispersed in organic solvent and its compatibility with organic molecules is improved. After modification the pore volume decreases and the ductility greatly increases, indicating that the modified product is of a higher strength than before. The study demonstrates that the modified product can be used as an ideal additive to reinforce the strength of organic materials.     

High-pressure phase equilibria for chlorosilane + carbon dioxide mixtures

Fluid-phase equilibria, including dew points, bubble points, and critical points were measured for four binary systems composed of a chlorosilane and carbon dioxide. The measurements were carried out in a constant-composition, variable-volume cell equipped with a sapphire window, which allowed visual observation of the phases in the cell. A syringe pump was used to inject the CO₂ into the cell and to control its pressure. Methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, and diethyldichlorosilane up to about 0.14 mol fraction were studied in this apparatus and a total of 243 phase-boundary points were obtained. Displacements in the critical point with respect to pure CO₂ of up to 11.81 MPa and 348.05 K were observed. Modeling of the fluid-phase equilibria for three of the four binary systems was done using the Peng–Robinson equation of state, standard van der Waals mixing rules with two binary interaction parameters, and a φ–φ formulation of the equilibrium. The binary interaction parameters were obtained by fitting the model to the experimental data. The model produced excellent agreement between computed and experimental data. Graphical representations of the modeling results are presented and compared to experimental results. The results indicate that the largest chlorosilane (diethyldichlorosilane) produced the largest shift in critical pressure and critical temperature with respect to pure CO₂.     

Thermodynamic assessment of the copper catalyzed direct synthesis of methylchlorosilanes

The copper catalyzed direct synthesis of methylchlorosilanes is a reaction of enormous complexity. Although, there is a general acceptance about a silylenoide based reaction mechanism, many details of the reaction are not yet fully understood. The present work is a comprehensive thermodynamic study on the reaction system of the direct synthesis. The calculations are based on a broad database containing the elements, all silanes, chlorosilanes, methylsilanes, and methylchlorosilanes with one Si atom, hydrocarbons and chlorinated hydrocarbons as well as other relevant compounds in the system Si-C-H-Cl. A calculation of the reaction between silicon and methylchloride, excluding only SiC as an unlikely reaction product, results in the total decomposition of methylchloride and the formation carbon, methane, hydrogen, trichlorosilane, and silicon tetrachloride. The systematic suppression of certain reaction products from the calculation yields finally into a product distribution close to the experimentally observed ones. The chosen approach to remove certain compounds from the calculation is equivalent to the introduction of unspecific kinetic constraints arising from a hypothetically total and selective blocking of certain reaction pathways. From this, three major kinetically determined reaction pathways are identified: (i) the formation of carbon, hydrocarbons, hydrogen, and hydrogen chloride due to the cleavage of the C-H bond in methyl chloride, (ii) the formation of hydrogen-containing methylchlorosilanes that occurs only in the presence of hydrogen or hydrogen chloride, and (iii) the competition between the thermodynamically favored chlorosilanes and the kinetically favored methylchlorosilanes. The presence of transition metals (regardless whether Cu, Fe, or Ni) during the direct synthesis gives no thermodynamic preference for the formation of methylchlorosilanes. The metals effect is to open a kinetically controlled reaction pathway to the formation of methylchlorosilanes far away from the formation of chlorosilanes or from other thermodynamically more favored compounds. Furthermore, processes related to the induction period, the addition of hydrogen to the direct synthesis, constrained equilibriums between methylchlorosilanes, and the limits of the applied calculation procedure are discussed in detail.     

Mechanism on The Disproportionating Synthesis of Dichlorodimethyl- silane by ZSM-5(5 T)@γ-Al2O3 Series Core-Shell Catalysts

In view of the wide application of organosilicon bifunctional structure in industry, the most effective disproportionation solution is used by the new catalyst to prepare the largest and the most versatile organic silicon monomer dimethyldichlorosilane. However, there are still remaining doubts on the disproportionation mechanism of the catalyst. The Density Functional Theory (DFT) was used to theoretically calculate the disproportionation mechanism of 5 T clusters ZSM-5@γ-Al₂O₃ series catalysts at the B3LYP/6–311++G(3df, 2pd) level. The properties were verified and the catalytic effects of different active sites pre- and post- modified by AlCl₃ were calculated and compared. The active center of HZSM-5@γ-Al₂O₃ was proton, and that of AlCl₃/ZSM-5@γ-Al₂O₃ changed to lewis acidic center. The presence of the lewis acid center enhances the total activity of the catalyst to some extent. The catalytic activity of the 5 T cluster ZSM-5@γ-Al₂O₃ catalyst modified by AlCl₃ was higher, which was the same as the experimental results.