Zeolite phase transformation, effect

Figure 6. Effect of reaction time on zeolite phase transformation. Keys to symbols indicate species detected after 1 hour.

Figure 3.8 Liquid phase transformation of phenyl acetate (2.2 mol l-1 in sulfolane solvent) at 433 K. (a) Yield in o-hydroxyacetophenone, o-HAP ( ) and p-hydroxyacetophenone, p-HAP (X) versus reaction time, (b) Effect of the addition of phenol (P) on the p-HAP yield. [P] =0 mol l-1 (x) and [P] =0.6 mol l-1 ( ). Reprinted from Catalysis Letters, Vol. 41, Jayat et al., Solvent effects in liquid phase Fries rearrangement of phenyl acetate using a HBEA zeolite, pp. 181-187, copyright (1996), Kluwer Academic Publishers, with kind permission of Springer Science and Business Media...

The synthesis of zeolite A, mixtures of A and X, and zeolite X using batch compositions not previously reported are described. The synthesis regions defined by triangular coordinates demonstrate that any of these materials may be made in the same area. The results are described in terms of the time required to initiate crystallization at a given reaction temperature. Control of the factors which can influence the crystallization time are discussed in terms of "time table selectors" and "species selectors . Once a metastable species has preferentially crystallized, it can transform to a more stable phase. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are described mathematically. 

The effect of aluminum ions on the transformation of C-S-H into 1.1 nm tobermorite was investigated by Mitsuda and Taylor [184], The solid solution of aluminum ions in tobermorite enlarges also the stability field of this phase in relation to xonothte [185], Diamond [186] found the maximum Al/(Si+Al) ratio in tobermorite to be 0.15 above this value the hydrogamet phase is appearing. Aluminum is tetrahedrally coordinated and accelerates the formation of 1.1 nm tobermorite. Finally, Mitsuda and Taylor [184] revealed that alkalis (l%Na20+1.7%K20) are activating the tobermorite formation from zeolite and calcium hydroxide. 

De Vos DE, Jacobs PA. Zeolite effects in liquid phase organic transformations. Microporous Mesoporous Mater 2005 82 293-304. 

The authors [13] also proposed a multiphase mass transport model for toluene nitration, the details of, which are given in Fig. 2.4. It is based on the formation of a thin aqueous film aroimd the hydrophilic catalyst particles, which are dispersed in toluene medium. The model also accounted for the existence of vapor phase over the liquid-liquid-solid reaction medium. The major mass transfer resistances are offered by the liquid film around the catalyst particles and in the catalyst pores. The aqueous film and the liquid in the pores constitute the micro environment necessary to facilitate the desired level of lattice transformation in the catalyst particles. Figure 2.4 also shows the concept of the microenvironment within and around the catalyst particle. These studies have demonstrated that shape selectively effect of zeolite Beta catalyst is significantly enhanced by the specific microenvironment created within and around the catalyst particles. This has significantly enhanced the para-selectivity from 0.7 to 1.5. The microenvironment has also improved the accessibility of reactant molecules to the catalyst active sites. 

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