Silica simulation studies

V. V. Murashov, I. M. Svishchev. Quartz family of silica polymorphs comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations. Phys Rev B 57 5639, 1998. 

In a quantum-chemical MNDO-PM3 level study of the hydration of the Mg2+ cation located in a ditrigonal cavity of the basal surface of clays [99], the most favorable area of Mg2+ cation location was predicted to be in the vicinity of the AIO4 tetrahedron formed by the isomorphic substitution of Si for Al in the silica-oxygen sheet. The authors have showed the important role of the hydrogen bond formation between the water molecules and the oxygen atoms of the silica-oxygen sheet in the Mg-ion hydration. This was confirmed in several MC simulation studies [65, 66]. 

Hwang, G.J. and Onuki, K., Simulation study ou the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water splitting IS process. Journal of Membrane Science, 194, 207, 2001. 

So many membrane reactors were reported in the literature for the WGS reaction. Along with Pd membranes, studies on silica membranes, zeolite membranes, hollow membrane reactors, electrochenucal WGS reactor are also available in the literature. However, Pd-based membranes were investigated extensively for the WGS reaction. In the following sections we have given detailed description about various membrane reactors investigated for the WGS reaction. A detailed description about the theoretical and simulation studies of the membrane reactors for the WGS reaction was also discussed. 

In present-day chemistry, and especially in biological chemistry, there is a move toward replacing preindustrial in vivo experiments (expensive in materials and time) and in vitro experiments (expensive in equipment costs and reproducibility efforts across labs) with in cerebro and in silica experiments (i.e., evaluation and computation/simulation studies) [1]. The latter experiment types are reasonably inexpensive because they [2]... 

Simulation of alkali ion adsorption on glass surfaces and their diffusion into subsurface layers is of importance because it helps us understand the effects of contaminants on the properties of optical fibers. Garofalini and Zirl,2oi for example, studied the adsorption and diffusion of K and Li on vitreous silica. These simulation studies revealed that potassium ions diffuse over the surfiice and adsorb preferentially on the nonbridging oxygen, whereas... 

J. O.Titiloye, S. C. Parkei F. S. Stone, and C. R. A. Catlow,/. Phys. Chem., 95,4038 (1991). Simulation Studies of the Structure and Energetics of Sorbed Molecules in High-Silica Zeolites. 1. Hydrocarbons. 

Bhattacharya S, Kieffer J (2008) Molecular dynamics simulation study of growth regimes during polycondensation of silicic acid from silica nanoparticles to porous gels. J Phys Chem C 112 1764-1771. 

Multicomponent systems that present polyamorphism have also been reported in computer simulation studies. For example, in Ref. [35], it is found that silica has a LLCP at very low temperature. Silica is also a tetrahedral liquid and it shares many of the thermodynamic properties observed in water. In Ref. [35], two silica models were considered. In both models, the interactions among O and Si atoms are isotropic, due to single point charges and short-range interacting sites located on each atom. Both models considered in Ref. [35] are characterized by a LLCP at very low temperature and coexistence between two liquids is observed in out of equilibrium simulations close to one of the spinodal lines (see Fig. 2b). The location of the LLCP was estimated to be below the glass transition in real silica and hence, unaccessible in experiments. We note that polyamorphism in the glass state is indeed observed in compression experiments on amorphous silica [14], and is qualitatively reproduced in computer simulations [89]. Other examples of multicomponent systems that show LLPT in simulations are presented in Refs [65,90]. In these cases, a substance that already shows polymorphism is mixed with a second component. 

Here, we review the findings of simulation studies of silica related to the phenomena of polyamorphism and LLPTs. As we discuss below, these simulation studies provide ample evidence that the thermodynamic and dynamical behavior of liquid silica follow the pattern of anomalies that are well known in the case of water. However, simulations have yet to provide unequivocal evidence for an LLPT in silica. Hence, there remains important work to be carried out on this system. 

The thermodynamic behavior of both WAC and BKS silica have been examined in the region of the compressibility miiximum, and in both cases, the pattern of thermodynamic anomalies that occur have been found to be the same, and analogous to those found in simulation studies of water. An example is shown in Figure 2, that compares the equation of state features of ST2 water [7] with WAG silica [9] in the P-T plane. In both cases, a retracing line of density maxima occurs above a monotonic spinodal boundary. The qualitative similarity of these features, despite the widely different T and P scales, is striking, and certainly suggests that a search for an LLPT in these silica models, of the kind found in water simulations, is justified. This is discussed in the next section. 

Modeling Triblock Surfactant Templated Mesoporous Silicas (MCF and SBA-15) A Mimetic Simulation Study... 

This molecular simulation study focuses on the adsorption of water in mesoporous Vycor-like silica glasses, which are of great interest for geophysics, pharmaceutics, industry, or environment. The adsorbent was previously constructed by off-lattice methods to mimic the micro- and meso-textural properties of real Vycor. Whereas most of the previous studies focused on the dynamical or structural properties of confined water this one focuses on the thermodynamic properties which are actually essential prior to a molecular dynamics study for instance (to equilibrate the initial configuration). The Grand Canonical Monte Carlo (GCMC) method has... 

K. Vollmayr, W. Kob, K. Binder, CooUng-rate effects in amorphous silica a computer-simulation study. Phys. Rev. B 54(22), 15808-15827 (1996)... 

J. Puibasset, R. J.-M. Pellenq, Grand canonical Monte Carlo simulation study of water structure on hydrophilic mesoporous and plane silica substrates, J. Chem. Phys. 119 (2003) 9226-9232. 

Maseras E, Morokuma K (1995) IMOMM a new integrated ab initio -H molecular mechanics geometry optimization scheme of equilibrium structures and transition states. J Comput Chem 16(9) 1170 Mischler C, Horbach J, Kob W, Binder K (2005) Water adsorption on amorphous silica surfaces a Car-Parrinello simulation study. J Phys Condens Matter 17(26) 4005... 

A silica membrane reactor will be efficient and effective therefore, when the membrane, working at full capacity, is able to process all the H2 produced by the reaction. This situation arises when the DaPe = 1 and simulations for silica membrane reactors for the water gas shift (WGS) reaction have indeed demonstrated that maximum CO conversion was achieved at DaPe close to 1 (Battersby et al., 2006 Ikuhara et al, 2007). Thus the DaPe number is a valuable metric to evaluate the potential performance of a membrane reactor and a valuable, yet simple, design tool to ensure that both the reactor and membrane components work together for maximum efficacy. However, the DaPe number does not take into account the selectivity of the membrane which obviously does affect the membrane reactor performance. Both experimental and simulation studies have shown that higher permeation results in higher conversion and product yield enhancements (Battersby et al., 2006 Boutikos and Nikolakis, 2010 Lim et al., 2010).That is not to say that a membrane with a low selectivity cannot be successfully utilized in a membrane reactor set-up. Provided the membrane has nominal selectivity for the desired products over reactants, the conversion of equilibrium-limited reactions will be enhanced in a membrane reactor system. However, the product purity will remain dilute and thus additional operational and capital expenditure will be required for further downstream processing. If the membrane is unable to separate gases then the system behaves as a packed bed reactor. 

Adsorption of hard sphere fluid mixtures in disordered hard sphere matrices has not been studied profoundly and the accuracy of the ROZ-type theory in the description of the structure and thermodynamics of simple mixtures is difficult to discuss. Adsorption of mixtures consisting of argon with ethane and methane in a matrix mimicking silica xerogel has been simulated by Kaminsky and Monson [42,43] in the framework of the Lennard-Jones model. A comparison with experimentally measured properties has also been performed. However, we are not aware of similar studies for simpler hard sphere mixtures, but the work from our laboratory has focused on a two-dimensional partly quenched model of hard discs [44]. That makes it impossible to judge the accuracy of theoretical approaches even for simple binary mixtures in disordered microporous media. 

This comprehensive article supplies details of a new catalytic process for the degradation of municipal waste plastics in a glass reactor. The degradation of plastics was carried out at atmospheric pressure and 410 degrees C in batch and continuous feed operation. The waste plastics and simulated mixed plastics are composed of polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile butadiene styrene, and polyethylene terephthalate. In the study, the degradation rate and yield of fuel oil recovery promoted by the use of silica alumina catalysts are compared with the non-catalytic thermal degradation. 9 refs. lAPAN... 

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