Aluminosilicate gels, alkali

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... 

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. ... 

The method developed by Milton in the late 1940s, involves the hydrothermal crystallization of reactive alkali metal aluminosilicate gels at high pH and low temperatures and pressures, typically 100°C and ambient pressure. Milton, Breck and coworkers synthesis work led to over 20 zeolitic materials with low to intermediate Si/Al ratios (1-5) [86]. Chapter 3 and references [1] and [25] provide more detailed discussion of synthesis. 

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. 

Tfhe most open zeolites crystallize at relatively low temperatures from A highly reactive alkali aluminosilicate gels. These zeolitic phases are metastable, and usually several phases are formed in succession from a particular reaction mixture. This paper describes crystallization sequences observed in mixed Li,Na-ahiminosilicate gels at 100 °C. The details of our investigation of this system at 100°C are reported elsewhere (8, 9). 

Heating Conditions The heating conditions for the precursor gel tend to mimic those which form zeolites in nature. Naturally occurring zeolites are often found in lava flows or volcanic sediment, and so temperatures over 200°C and high pressures (>100 bar) were typically used in early synthesis. With the use of reactive alkali-metal aluminosilicate gels, however, lower temperatures and pressures can be used enabling synthesis to take place at around 100°C and under autogenous pressure. 

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)). ... 

In these, any additional alkali and water were added to the aluminate solution, so that the initial silicate solution was the same through any series some gel time experiments for these aluminosilicate solutions have already been described M 8-201. 

Figure 2 is a three-dimensional representation of the results and shows the dependence of the gel time on composition for potassium aluminosilicate solutions. Gel times do not depend simply on the concentration of any one component. Rather, a valley of shortest gel times extends almost diagonally across the plot from low-silica, low-alkali, to high-silica, high-alkali compositions. Values to either side of the "valley" are higher. The longest gel times - up to two weeks - occur in solutions of high-silica, low-alkali content. 

Figure 3 Log(gel time) of potassium aluminosilicate solutions 1e - 8e prepared by Method 2 of mixing. = normal mixing as in Method 1, = 22% of total alkali added to silicate solution before mixing with aluminate, 44% of total alkali, = 66% of total alkali and 0= 88% of total alkali.

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,... 

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.). ... 

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