Isomorphic substitution solids

Figure 4. Heat of adsorption of ammonia versus coverage for the isomorphously substituted solids (IS).

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. 

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). 

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]. 

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]. 

Preparation by gas-solid isomorphous substitution of Ti4+ for Si4+ and hydrothermal crystallization using TPABr as template... 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

The particular combinations of ions and molecules that will form precipitates in a given solution can be predicted from equilibrium thermodynamics. However, this often gives a misleading picture because there are kinetic limitations or there is inhibition, particularly in soil solutions. There may also be isomorphous substitution of one cation for another in the precipitate, resulting in a solid solution with a different solubility to the pure compound. 

The impurities can become incorporated in the solid phase in three modes i) interstitially, i.e. between regular lattice positions, ii) by coprecipitation as a separate insoluble phase or iii) by isomorphous substitution of... 

The existence of these isostructural compounds suggests that solid solutions could be formed between two end members via isomorphous substitution for Fe " by other cations. The likelihood of substitution depends on the similarity of the ionic radii and the valency of the cations (Goldschmidt, 1937). m " is the most suitable cationic species and a radius about 18% higher or lower than that of high-spin Fe " in sixfold coordination can be tolerated. Isomorphous replacement of Fe in Fe oxides by a number of cations has been observed in nature and, more frequently, in the laboratory. As far as is known, however, almost all these solid solutions have broad miscibility gaps, possibly induced by development of structural strain as substitution rises. 

Macedo et al. [227] studied HY zeolites dealuminated by steaming, and found that the strength of intermediate sites decreased with increasing dealumination for Si/Al ratios varying from 8 to greater than 100. For comparison, isomorphously substituted HY, which is free of extra-framework cationic species, possesses more acid sites than conventionally dealuminated solids with a similar framework Si/Al ratio [227], This is because some of the extra-framework aluminum species act as charge-compensating cations and therefore decrease the number of potential acid sites. 

A new microporous solid material has been obtained made of Ti02 and Si02 (TS-1) which has a silicalite-1 structure modified by isomorphous substitution of Si(IV) with Ti(IV), Its synthesis takes place in the presence of tetraalkylammonium bases under carefully controlled conditions,... 

From the preceding sections it is clear that isomorphism of monomeric units in synthetic copolymeric systems is a quite general phenomenon. We wish to recall here that the requirements for the isomorphous substitution in the macromolecular field are similar to those holding for solid solutions in ionic or metallic crystals however, the degree of... 

Post-synthesis gas-solid isomorphous substitution methods are also known [61]. Ti-beta essentially free of trivalent metals can be prepared from boron-beta. However, the gas-phase method is not efficient for Ti incorporation and could have some disadvantages such as the deposition of Ti02 [62],... 

Mixed solid (or solid solution) formation can occur, for example, when secondary minerals, such as carbonates, metal oxides, or clay minerals, precipitate from the soil solution during weathering. These solids often are characterized by a wide range of isomorphic substitutions, in which both cations... 

Solid particle surfaces develop charge in two principal ways either permanently, from isomorphic substitutions of component ions in the bulk structure of the solid, or conditionally, from the reactions of surface functional groups with adsorptive ions in aqueous solution. A surface functional group is a chemically reactive molecular unit bound into the structure of an adsorbent at its periphery, such that the reactive portion of the functional group can be exposed to an aqueous solution contacting the adsorbent [3]. 

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