Effect on isomorphous substitution

CO clearly reveals the effect of isomorphous substitution on the acidity of the bridging hydroxyls. For example, for MFl zeofites, the acidity decreases in the following sequence H[Al]ZSM-5 >H[Ga]ZSM-5 >H [Fe]ZSM-5 (208,209). Comparison of isostructural samples also indicates that the acidity of the bridging hydroxyls is always higher in zeoHtes than in the corresponding alumophosphates (212). 

TABLE 3a Effect of Isomorphous Substitution on Bridged Hydroxyls in Unit (a) H3Si(OH)TH3... 

Pyrophyllite, muscovite, margarite, beidellite, montmorillonite, illites MM lattice minimization (based on empirical potentials) Geometry, effect of isomorphic substitution 131... 

Brown, G., 1955. The effect of isomorphous substitutions on the intensities of (00/) reflections of mica-and chlorite-type structures. Mineral Mag. 30 657-665. 

Evaluation of the effects of chemistry on volume properties of amphiboles is comphcated by the high number of sites in which isomorphous substitutions take place. Limiting ourselves to the main structural classes C2lm, Pnma, and P2, m, we present the following evaluations. 

In the attempt to synthesize molecular sieves with isomorphous substitutions of A1 and/or Si by the divalent calcium element in the tetrahedral positions, we obtained a new calcium silicate phase by inclusion of heteroatom calcium into silicate sols. The characterization results showed that as-synthesized calcium silicate, named CAS-1 (Calcium silicate No. 1), was a novel zeolite-like crystal material with the cation reversibly exchangeable and selectively adsorptive properties. In this paper, the effects of composition of raw materials, reaction temperature and the different alkali ion on the hydrothermal synthesis of calcosilicate crystal material CAS-1 were investigated and the uptake of different cation on the thermal stability of CAS-1 structure was also examined. The sample was characterized by XRD, TEM, SEM, DT-TGA, BET, AAS and chemical analysis. 

Adsorption experiments were carried out using only the silt fraction because this fraction lends itself to batch experiments and is similar in composition to the sand fraction, consisting primarily of quartz and feldspar. Furthermore, it was felt that the application of double-layer theories of adsorption would be less appropriate on the clay fraction, where the effects of valence deficiencies resulting from isomorphic substitution in actual clays are probably more important. 

D. Mehn, A. Kukovecz, I. Kiricsi, F. Testa, E. Nigro, R. Aiello, G. Daelen, P. Lentz, A. Fonseca, and J.B. Nagy, The Effect of Calcination on the Isomorphously Substituted Microporous Materials Using Ozone. Stud. Surf. Sci. Catal., 2001, 135, 215-251. 

Substituting the 2-position by methyl or bromine has no severe effect on the structure of [6]helicene. The space group remains similar, but the introduction of such substituents leads to a translation of the atoms of the molecule parallel to the C axis the crystals are not isomorphic... 

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