Zeolite precursor solution

Xiang and Ma [86] also recognised the problems of treating mesoporous y-alumina membranes with highly alkaline zeolite precursor solutions. They... 

The aqueous chemistry of aluminum(III) above pH 6 differs from that of silicates in that the only important species, other than solid Al(OH)3 at pH 5-8, is Al(OH)4, which, although isoelectronic with Si(OH)4, shows no tendency to catenate. On the other hand, below pH 5 Alm, unlike the poorly soluble Si(OH)4, is freely soluble as Al3+(aq) [actually Al(OH2)63+, Section 13.2], while at intermediate pH hydrolytic A1 species, including the ion Ali304(0H)24(0H2)127+ referred to above, predominate in solution. However, Al(OH)4 units can readily insert themselves into silicate anion species in solution. The result is usually the prompt precipitation of an aluminosilicate gel (a typical zeolite precursor), although over some limited Al, Si, and OH- concentration ranges quite high concentrations of dissolved aluminosilicates can be maintained over many months.9... 

The composition and dynamic properties of basic silicate solutions and the implications derived therefrom as to the possibility of positively identifying zeolite precursor species. 

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

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. 

The order for the nucleation rste (see Figure 6 TEA < orga-nics-free < TPA), and the observation that when X - 5 NaOH + 3 wt % seed ZSM-5, ZSM-5 is formed about as fast as with X - 5 TPAOH (see Table II), do not support a precursor role for D5R silicates in all these synthesis reactions. This is because, on the basis of the D5R concentrations in analogous silicate solutions, the order TEA = TPA > organics-free is expected (cf. Table III) (4). If particular zeolite precursors are responsible for the formation of ZSM-5 then, clearly, TEA has a very retarding effect on their mutual condensation rate. 

A crystalline zeolite can also be studied by a variety of methods including crystallography and NMR but the intermediate phase, the gel, has proved very resistant to any type of study. The aim of this work, therefore, was to delay the formation of the gel long enough to investigate the precursor solution because solutions are generally easier to study than gels. 

The molar ratios in the precursor solution were 100 SiO2 10 Na2O 10 (TPA)20 1600 H2O while hydrothermal treatment was conducted at 130°C for 48 h, followed by an additional treatment at 200°C for 16 h. After washing, drying and calcination at 600°C a zeolite membrane with good gas separation properties resulted. No further characteristics of the layer were given. 

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

Permeation and separation data on well defined, high quality zeolite membranes are only reported for MFI (ZSM-5, silicalite) zeolites grown in situ directly from the precursor solution on top of a substrate. The experimental single gas permeation results could be in a number of cases consistently described using Eqs. (9.43)-(9.48) for the Langmuir and Henry regimes. 

Zeolite Beta seeds were prepared from 0.816 g of aluminum isopropoxide mixed with 40.0 g of silica sol and 53.02 g of tetraethylammonium hydroxide in a polypropylene bottle. The resulting mixture was stirred at room temperature for 1 h. The clear precursor solution was hydrothermally treated in an oven at 100 °C and the crystallization was stopped when a milky solution containing colloidal particles of zeolite Beta was obtained (at least after 24 h). Simultaneously, solution for the synthesis of MCM-48 was prepared from 0.868 g... 

Due to the number of experiments necessary in such studies which take typically times of several hours to several days, there is a strong incentive to work on acceleration of synthesis experiments in zeolite science. On the other hand, there are also major obstacles to overcome in such an endeavour. Zeolite syntheses are often carried out at elevated pressure, reaction media are typically corrosive, precursor solutions and the synthesis gel itself may be highly viscous and difficult to handle, the synthesis is very sensitive to preparation conditions, such as sequence of reagent addition, stirring (difficult for small volumes) may be necessary for some formulations, aging conditions may differ from batch to batch, if automated sequential preparation is chosen, the work-up involving many steps is complex, and the resulting materials are often not phase-pure and difficult to characterize. Nevertheless, in spite of these problems some of the earliest examples for the synthesis of catalytically relevant solids in parallelized and - some points -automated equipment were reported for zeolites [4]. 

Precursor solutions for the phosphates are much easier to handle, since the pH is not extreme, all components are liquid, and typically no two phase systems which are difficult to mix are formed, as could happen with alkoxysilane precursors. The additional problems encountered when doing zeolite synthesis on miniature scale have been discussed by Newsam et al. [4], and include ... 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

The zeolite ZSM-3 was prepared from aluminosilicate hydrogels containing sodium and lithium cations. The crystallization technique consists of first preparing a precursor solution of concentrated sodium aluminosilicate and then mixing it with aqueous sodium silicate and aluminum chloride solutions to form the starting hydrogel slurry. This slurry is filtered to remove excess soluble sodium silicate. Lithium is added to this filter cake as lithium hydroxide solution. This mixture is held at temperatures of 60° to 100 °C until ZSM-3 crystals form. At 60 °C, crystallization requires 5 days while at 100 °C, crystals are formed in 16 hours. In order to obtain the desired SiOo/Al203 ratio in the crystalline product, the aluminum chloride content is varied. 

Most zeolites in sedimentary rocks formed during diagenesis by the reaction of aluminosilicate materials with the pore water. Silicic volcanic glass is the aluminosilicate material that most commonly served as a precursor for the zeolites, although materials such as clay minerals, plagioclase, leucite, and nepheline also have reacted locally to form zeolites (48). Solution of silicic glass by the pore water provided the constituents necessary for the formation of the zeolites. Deffeyes... 

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