Hydrothermal rearrangement

In order to understand the mechanism of the pore size enlargement by hydrothermal treatment, transmission electron microscopy (TEM) was applied. Both treated materials in Fig. 5 consist of primary particles of the same dimension. The difference in contrast indicate that the pore size enlargement is accompanied by partial dissolution and rearrangement of those spheres. 

In a word, in the solid-phase mechanism, it is believed that neither the dissolution of solid gel nor the direct involvement of the liquid phase happened for the nucleation and growth of zeolite crystals during the crystallization process of zeolites. The nucleation and growth of zeolite crystals came from the structural rearrangement of the framework of solid aluminosilicate gel under hydrothermal crystallization conditions. 

Once those parameters which have more influence on mesophase stability are known, the new synthetic pathways utilizing structural rearrangements to create new materials will become a reality. An investigation of phase transformation11581 under hydrothermal conditions indicated the most mildly basic conditions utilized (pH 9), which favor silica condensation, best inhibit the phase transformation, and thus produce the most kinetically... 

Hydrothermal treatment can however be used constructively to modify the properties of an adsorbent. Perhaps the best example is the formation of ultrastable Y by hydrothermal treatment of sodium ammonium Y zeolite. The change is accompanied by a contraction in the unit cell parameter and an increase in the Si/Al ratio due to elimination of aluminum from the lattice. The resulting product shows greater thermal stability than Y zeolites of similar composition which have not been subjected to the hydrothermal treatment. However, with X zeolite the usual result is a loss of crystallinity with attendant deterioration of the adsorptive properties while with A zeolite a more subtle effect referred to as pore closure occurs. Hydrothermally treated 4A zeolite behaves as if the window aperture is somewhat smaller than in normal 4A sieve. This effect can be useful since a pore-closed 4A does not admit chlorinated hydrocarbons and is therefore useful for drying freon refrigerants. If a wider pore sieve is used for this purpose premature breakdown and loss of capacity may occur due to formation of HF and/or HCl by hydrolysis. The precise mechanism of pore closure has not yet been established and it remains uncertain whether it involves a true modification of the ciystal structure or merely a rearrangement of the surface layers. 

Microamorphous silica is not easily crystallized. When an ionic material such as a salt is rapidly precipitated from a highly supersaturated solution, it may initially be amorphous, but it rapidly rearranges to the ordered crystalline state. However, in the case oF silica in which bonds are largely covalent, such rearrangement can occur only at elevated temperature or in the presence of a solvent such as water under hydrothermal conditions. Silica, in effect, is a polymeric material. Walton (106) has pointed out why an intermediate amorphous phase is likely to be precipitated if the material is of high molecular weight or polymeric. 

The hydrothermal decomposition of glycerin 12 with an alkali was studied by Kishida et al He showed that glycerin is first decomposed to pyruvaldehyde 13 with elimination of hydrogen by a H shift to the adjacent hydrogen. Pyruvaldehyde formed is then converted into lactate ion 14 by the benzilic acid rearrangement. 

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