Zeolite synthesis silicates

IR and Raman spectroscopy can fulfill these requirements and they are also robust enough for in situ silicate analysis in plant reactors [7]. Both of these techniques have been used for identifying the symmetric (s) and asymmetric (as), stretching (va, vas) and bending (5a, 5as) O-Si-O vibrations in aqueous alkaline silicate solutions which are the cheapest hence most frequently used ingredients for zeolite synthesis [8, 9 and references herein]. However, this information has to be "translated" into siloxane ring... 

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... 

The most important factor in zeolite synthesis in the laboratory, or factory, is the rate of crystallization. Composition and concentration of the liquid solution acting on the solids is important to the process as is the absolute necessity of maximum disorder of the Si-O-Al bonds in the initial solids reacted (Zhdanov, 1970). It is thus evident that not only bulk chemical (equilibrium) factors are important in the initial crystallization of zeolites but also the. relative free energies of the reactants. It is apparent that zeolite equilibria are essentially aqueous i.e., that silicate equilibrium or approach to it is attained through reaction with solutions, and thus the solubilities of the solids present are of primary importance. If materials are slow to enter into solution they are essentially bypassed in the rapid crystallization sequence (Schwochow and Heinze, 1970 Aiello, et al , 1970). In most studies the zeolites precipitated from solution appear to respond to the laws concerning chemical activity of solutions (Zhdanov, 1970). 

While silicate and aluminate solutions have been extensively studied using 29Si and 27A1 NMR (see Sections III,B and III,I, respectively), there is only a handful of publications that investigate mixed (Si,Al)-bearing solutions by NMR spectroscopy with a view to elucidating the mechanism of zeolite synthesis. There is thus a considerable scope for further work in this important area. 

Control of pH is critical in the determination of the Si/Al ratio. As the pH increases, the ability of the silicate to condense decreases because of a decrease in the amount of Si-CCspecies relative to Si-OH. The anionic form is necessary in order for the initial nucleophilic attack to take place. In contrast, the condensation rate of Al(OH) 4 remains constant and so aluminium-rich zeolites crystallise preferentially at high pH and vice versa. Zeolite synthesis also depends on a wide range of experimental parameters, including concentrations and degree of supersaturation, the source of the framework materials, solvent... 

In the high pH range needed for zeolite synthesis aluminate solutions are relatively simple in that the anions present are almost exclusively A1(0H). on the other hand in silicate solutions around room... 

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. 

At the concentration levels characteristic of zeolite synthesis, highly viscous gels are formed upon mixing silicate and aluminate solutions. The use of liquid-line NMR techniques to observe such gels results in very hroad spectral features due to the fact... 

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 the Structure of Organic Quaternary Ammonium Ions. The tetramethylammonium ion (N C J ), first introduced in zeolite synthesis by Barrer and Denny (30), and Kerr and Kokotailo (21) is effective in forming the cubic octameric silicate anion (Sig02Q°, cubic octamer) (2-16). In the tetramethylammonium silicate aqueous solutions at higher S3.O2 concentrations or cation-to-silica molar ratios (abbreviated to the N/Si ratios), the cubic octamer is singularly formed. 

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). 

The conversion of Cubic P to organozeolites is observed by following the rise in pH (measured at room temperature) (J ). X-ray diffraction was used to measure extent of conversion. Zeolite synthesis reactions were carried out at 135 C with 30 rpm tumbling in Parr 4745 reactors with Teflon cups. Unless otherwise stated, the synthesis mix consisted of 0.50 g Cubic P, 2.5 millimols quaternary ammonium halide, 5.0 g Banco "N" silicate solution (38.5% solids, Si02/Na20 = 3.22), and 12 mL water. Cubic P was prepared by reacting "N" silicate, water, Al2(SO )3, at 140 C for six... 

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