Zeolite synthesis species

MFI and MTW zeolites with Fe species introduced during zeolite synthesis were investigated. Fe-silicalite and Fe-MTW catalysts were synthesized accordingly to... 

I. Hasegawa and S. Sakka, Silicate Species with Cagelike Structure in Solutions and Rapid Solidification with Organic Quaternary Ammonium Ions, Zeolite Synthesis (ACS Symposium Series 398) Am, Chem. Soc., Washington, DC 1989, p.140/51... 

Figure 2 depicts the compositions of the different solutions. For the polymeric species the absolute amounts are shown for the other, smaller silicates the relative amounts. It is evident that, especially at low OH/Si ratios (i.e., < 0.5, which is a normal value for Si-rich zeolite synthesis mixtures) the larger part of the silicate species present in solution consists of uncharacterized, polymeric silicates. The values obtained in the absence of DMSO (lower part of Table III and Figures 2a and 2b are in good agreement with literature findings (14). 

First, although the use of bulky organic bases clearly shifts the silicate equilibrium to the DnR species, there may be a large amount (up to more than 90%) of polymeric species present in silicate solutions. This is true especially at low OH/Si ratios (<0.5) or high Si concentrations (>2), i.e., normal values for a zeolite synthesis composition. This range of polymeric silicates cannot at present be characterized satisfactorily, and the presence of zeolite precursor species other than DnR silicates in this range cannot be excluded. 

Second, compared to the time scale of a zeolite synthesis, the rate of exchange between all small silicates is very fast. The concentration of possible zeolite precursor species such as the DnR in this silicate range is accordingly expected to be constant throughout a synthesis (vide infra). On this basis, it is very well possible that, for instance, the DnR silicates do play a precursor role however, a conclusive proof for this will be difficult to obtain (labelling, for example, is not possible). 

Silicalite Synthesis from Clear Solution. In order to check whether the above findings on the presence of polymeric species in solution and on the fast rate of exchange between the smaller silicates are also applicable in a real zeolite synthesis mixture, we have studied a silicalite synthesis in more detail. Starting from a clear, filtered solution of molar composition 25 Si02 9 TPAOH ... 

On the basis of the composition and dynamics of silicate solutions and zeolite synthesis mixtures only, a precursor role for the D5R silicates during both stages cannot be excluded since these species are present in relatively high concentrations during the zeolite synthesis process. 

A typical zeolite synthesis involves mixing together silicate and aluminate solutions or sols to form an aluminosilicate gel, usually instantaneously, which is then treated hydrothermally to give the crystalline product. The composition and structure of the aluminosilicate gel are of considerable interest and characterization of the aluminosilicate species present would give insight into the crystallization process. 

It is important to know what species are present at the beginning of the reaction. Silicate and aluminate solutions have been well studied so that one can be reasonably sure what species are present in a given solution of known concentration and pH. Aluminate solutions have been shown to contain only one type of ion at high pH the tetrahedral AI(OH)4 ion (1). It is only when the pH drops towards neutral that other, polymeric ions appear which ultimately give way to AI(H20) 3+ in acid conditions. The tetrahedral aluminate ion is the important species for normal zeolite synthesis. 

NMR spectroscopy is a powerful technique for identifying the structure and concentration of silicate and aluminosilicate anions in gels and solutions used for zeolite synthesis. A review is presented of the types species that have been observed and the dependence of the distribution of these species on pH and the nature of the cations present. 

Synthesis of silica-based materials with controlled skeleton structures, such as zeolites, requires controlling the structure of oligomeric silicate species at the first reaction step. Organic quaternary ammonium ions, which are known as organic templates in zeolite synthesis (1 ), have a role in making up the specific structures of silicate anions, whereas silicate anions randomly polymerize in aqueous solutions containing alkali metal ions, resulting in the presence of silicate anions with different structures. 

Effect of Addition of Sodium Ions to Tetramethylammonium Silicate Aqueous Solution. In zeolite synthesis, alkali metal cations are combined with organic quaternary ammonium ions to produce zeolites with different structures from the one produced with only the organic quaternary ammonium ion (2) It is then expected that other types of silicate species are formed in the silicate solutions when organic quaternary ammonium ions and alkali metal cations coexist. In such silicate aqueous solutions, however, alkali metal cations only act to suppress the ability of the organic quaternary ammonium ions to form selectively silicate species with cage-like structures (13,14,28,29). 

MASNMR), spectroscopy to study molecular sieves has been widely reported (1). MASNMR spectroscopy has been used to elucidate a wealth of structural information about molecular sieves. The technique has also been used to study chemical properties, sorption and numerous chemical processes. Another area that has been exploited is the use of MASNMR spectroscopy to identify species in dilute wet gels and in solids extracted from zeolite synthesis mixtures (2,3,4,5). 

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