Chlorocarbon adsorption

In this chapter, we have discussed the application of metal oxides as catalysts. Metal oxides display a wide range of properties, from metallic to semiconductor to insulator. Because of the compositional variability and more localized electronic structures than metals, the presence of defects (such as comers, kinks, steps, and coordinatively unsaturated sites) play a very important role in oxide surface chemistry and hence in catalysis. As described, the catalytic reactions also depend on the surface crystallographic structure. The catalytic properties of the oxide surfaces can be explained in terms of Lewis acidity and basicity. The electronegative oxygen atoms accumulate electrons and act as Lewis bases while the metal cations act as Lewis acids. The important applications of metal oxides as catalysts are in processes such as selective oxidation, hydrogenation, oxidative dehydrogenation, and dehydrochlorination and destructive adsorption of chlorocarbons. 

Interestingly, the interactions between zeolites and unsaturated chlorocarbons like trichloroethylene (TCE) are found to be strikingly different from those between zeolites and unsaturated hydrocarbons (i.e. ethylene and benzene). Both our simulations and our spectroscopic results on the adsorption of TCE in faujasites show that interactions between the n electrons and the cations, which dominate in the case of hydrocarbons, are replaced by interactions between the chlorine atoms and the cations [18]. Figure 3 shows typical positions of TCE in NaY zeolite as predicted by energy minimizations. This is a consequence of the different charge distribution in hydrocarbons and halocarbons. 

As discussed in a previous section, metal oxides represent an important class of materials exhibiting a broad range of properties from insulators to semiconductors and conductors and have found applications as diverse as electronics, cosmetics and catalysts. Metal oxides have been widely used in many valuable heterogeneous catalytic reactions. Typical metal oxide-catalyzed reactions, including alkane oxidation, biodiesel production, methanol adsorption and decomposition, destructive adsorption of chlorocarbons and warfare agents, olefin metathesis and the Claisen-Schmidt condensation will be briefly discussed as examples of metal oxide-catalyzed reactions. 

Beside -alkanes, MN-100 also shows high affinity to halogenated hydrocarbons (Table 10.6). Chlorocarbons are retained somewhat longer than the corresponding -alkanes having approximately the same polarizability. The especially strong retention of trichloroethylene is peculiar, and is most likely caused by an additional contribution of specific interactions between the 7t-system of the adsorbate s double bond and the aromatic polymer to the total adsorption energy. In contrast, fluorocarbons are more weakly retained than -alkanes with similar polarizability most likely, the dispersion interactions of fluorocarbons with aromatic polymers are weaker. 

Figure 6 Reversible adsorption of CHjCl, CHCI3, and CCI4 based on H20C[Cd(L,2)2(C104)2] [5], and their single-crystal structures. Single-crystal analysis indicates that the encapsulated chlorocarbon species are disordered in the pores, so only major disordered components are shown in the figure. The different chlorocarbon guests are shown in different colors for clarity...

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