Aluminosilicate gels, alkali cations

Alkanes—Continued reactions—Continued with ozonide ions, 135 with superoxide ions, 134-35 role of oxygen ions in oxidation. 138-41 Alkenes, reactions with oxygen ions, 134 with ozonide ions, 135 with superoxide ions, 134-35 Aluminosilicate gels, alkali cations, 241... 

Role of alkali and NH cations in the crystallization of ZSM-5 Introduced in an aqueous (alumino) silicate gel (sol), the bare alkali cations will behave in various ways firstly, they will interact with water dipoles and increase the (super) saturation of the sol. Secondly, once hydrated, they will interact with the aluminosilicate anions with, as a result, the precipitation of the so formed gel (salting-out effect). Thirdly, if sufficiently small, they also can order the structural subunits precursors to nucleation species of various zeolites (template function-fulfilled by hydrated Na+ in the case of ZSM-5 (11,48)). ... 

The general behaviour of the alkali cations in presence of aluminosilicate gels and their influence on zeolite nucleation and crystallization rates has been discussed in detail elsewhere (26). Their specific role in the formation of the various (M)ZSM-5 zeolites can be depicted on the basis of the above described observations. ... 

A range of aluminosilicate solutions were investigated. The gelation behaviour, the species in solution (as observed by NMR) and the zeolite crystallization products are described. The effect of concentration and type of alkali metal cation present in solution gives information about the formation of aluminosilicate complexes and how they interact, under the influence of the cation, to form an aluminosilicate gel, the precursor to zeolite crystallization. 

As a conclusion, HM++ ions are not exclusive templates for a given structure but will stabilize an aluminosilicate structure that is preliminarily favoured by other variables such as the Al content in the gel. They will act partly as counterions to the negative framework, partly as pore fillers. Obviously, they exclusively act as pore fillers in the Al free gels, in which alkali cations can be present (this work) or absent (2.). ... 

High resolution Si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. NMR has proven useful in identifying the connectivity of A1 to Si,... 

---