Solid hydrogel transformation

Another name for solid hydrogel transformation mechanism is solid-phase mechanism, while solution-mediated transport mechanism is also called liquid-phase mechanism. The main difference in explaining the formation process of zeolites by these two mechanisms lies in whether the liquid component is involved during the crystallization of zeolites. The views of these two mechanisms are opposite to each other and have their own experimental supporting evidence. To date, the liquid-phase mechanism has more experimental support than does the solid-phase mechanism. 

X-ray and sorption analyses of the solid phase revealed that in the case of metasilicate only small amounts of zeolites were formed from the hydrogel after 1 hr of aging with heating after a longer period—e.g., 24 hr, almost quantitative transformations into zeolite X, with a small amount of zeolite A, were observed. In one experiment (V) no zeolite A was found. If sol was used as the silica source, the crystallization was almost complete after 1 hr of aging, but about half of the product was zeolite A. Some samples (KG, Gk, Gz) contained small amounts (up to 20%) of zeolite X in the solid phase after prolonged aging. 

In 1968, Breck and Flanigen for the first time proposed the solid-phase mechanism based on their studies of the crystallization of aluminosilicate. They found that the formation and transformation of amorphous aluminosilicate hydrogel always happened during the crystallization process of zeolites and that the composition of the hydrogel was similar to that of the resultant zeolites. 

Solid phase (X-ray amorphous aluminosilicate) separated from the hydrogel having the batch composition 3.23 Na20 Al203-1.93 Si02-128 H2O, was calcined at different temperatures (100-800 C) for Ih and then transformed into zeolite A by heating in a 2M NaOH solution at SO C. Analysis of both precursors and products (zeolite A) has shown that the calcination of the precusor lowers the concentration of nuclei in the gel matrix and hence influences the particulate properties of the zeolite obtained during the hydrothermal treatment of the precursors... 

The data are presented in Fig. 4 for the solids obtained from hydrosols or hydrogels at pH = 3.3, by the method of thermal blow. It can be seen that the activity of the alumino-silicate catalysts passes through a maximum. The catalyst with the maximum activity was synthesized at a syneresis time of 300 min, namely at the border of the sol into gel transformation. 

In a series of papers, Nicolaon and Teichner (318) have described the methods of preparation, the transformation of hydrogels to alcogels, the mechanical and thermal stability, and the chemical structure of aerogels. An aerogel is probably the lightest (lowest density) coherent solid that can be made. 

Schmuck and coworkers [43] recentiy reported that by attaching the self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterionic moieties to the amino groups of polyethyleneimine (PEI), the polymer can be converted to a pH switchable hydrogel. After functionalization (18, Scheme 6.6), PEI was transformed from a viscous liquid to a solid. Using the characteristic UV absorption of the GCP group, the approximate loading of the zwitterionic units onto the polymer was calculated to be 40 %. This was further confirmed by GPC analysis of the molecular mass of the functionalized polymer. The functionalized PEI polymer 18 was readily soluble in water but produced more viscous samples than that observed with PEI. 

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