Isomorphism substitution

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

The table includes structure types only, compounds obtained by isomorphic substitution are not listed. Dark shaded balls Nb light shaded balls Cl black balls 0 semi-shaded balls sites partially occupied by Cl and O. 

Nowadays there is a general consensus that the Ti(IV) atoms are incorporated as isolated centers into the framework and are substituting Si atoms in the tetrahedral positions forming [Ti04] units. The model of isomorphous substitution has been put forward on the basis of several independent characterization techniques, namely X-ray [21-23] or neutron [24-26] diffraction studies, IR (Raman) [52-57], UV-Vis [38,54,58], EXAFS, and XANES [52, 58-62] spectroscopies. 

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Selected OSC values are reported in Table 8.1 for ceria and cerium-zirconium mixed oxides. These results confirm that the isomorphous substitution of Ce4+ by Zr4+ ions clearly improves the catalyst stability. BET (Brunauer, Emmett, Teller) area of ceria treated at 900°C is close to 20m2g 1 while it amounts to 35 15 m2g 1 for most mixed... 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

Prakash, A. M., H. M. Sung-Suh et al. (1998). Electron spin resonance evidence for isomorphous substitution of titanium into titanosilicate TiMCM-41 mesoporous molecular sieve. J. Phys. Chem. B 102 857-864. 

Zinc, Cu and Ni have similar ionic radii and electron configurations (Table 5.6). Due to the similarity of the ionic radii of these three metals with Fe and Mg, Zn, Cu and Ni are capable of isomorphous substitution of Fe2+ and Mg2+ in the layer silicates. Due to differences in the electronegativity, however, isomorphous substitution of Cu2+ in silicates may be limited by the greater Pauling electronegativity of Cu2+ (2.0), whereas Zn2+ (1.6) and Ni2+ (1.8) are relatively more readily substituted for Fe2+ (1.8) or Mg2+ (1.3) (McBride, 1981). The three metals also readily coprecipitate with and form solid solutions in iron oxides (Lindsay, 1979 Table 5.7). 

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

The catalyst samples were prepared in our laboratory. The synthesized Na-ZSM-5 zeolite was modified by conventional or solid state ion-exchange [11] to form H-, Fe-, Cu-, Ni- and Ti-ZSM5 samples, while the mesoporous catalysts (Fe- and Ti-MCM-41) were synthesized by isomorphous substitution [12], as well as the hydrotalcites containing Fe-, Cu-, Cr- or Ca-oxide in the Mg,Al-LDH structure [13]. 

In the present study isomorphic substitution of Al with Sn [11-13] in the silicalite framework was attempted in order to induce catalytic functionalities and these catalysts was studied for Baeyer-Villiger oxidation of 2-adamantanone and norboranone. 

The ZSM5 (Si/Al=40) as base zeolite was prepared by us. Cu was built in the framework by solid phase ion-exchange, Cr by conventional ion-exchange. Ni-samples were obtained by both methods. Ti-ZSM5 was synthesized by isomorphic substitution [8]. 

We have described above the evolution of the magnetic properties of the [Cp2M (dmit)]AsFg salts upon isomorphous Mo/W substitution. Another possibility offered by this attractive series is the isomorphous substitution of the counter ion, that is PFg- vs AsF6 vs Sbl- fi. Electrocrystallization experiments conducted with [Cp2Mo(dmit)] and the three different electrolytes afforded an isomorphous series, with a smooth evolution of the unit cell parameters with the anion size [32], This cell expansion with the anion size leads to decreased intermolecular interactions between the [Cp2Mo(dmit)]+ radical cation, as clearly seen in Table 2 from the decreased Curie-Weiss temperatures and Neel temperatures (associated with the transition they all exhibit to an AF ground state). 

Electron paramagnetic resonance (EPR) spectroscopy is yet another diagnostic tool for the detection of isomorphous substitution of Ti. Its sensitivity is very high, and investigations can be performed with samples even with very low contents of paramagnetic species. The spectra and g parameters are sensitive to the local structure and associated molecular distortions. Hence, it is an ideal tool to characterize Ti in titanosilicates. Ti in the + 4 oxidation state in titanosilicates is diamagnetic and hence EPR-silent. Upon contacting with CO or H2 at elevated... 

Appendix A summarizes what we believe to be the basic fingerprint features for the isomorphous substitution of Ti in silicate-1 lattice. 

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