Organic templates, zeolite synthesis

In 2012, Mintova and coworkers captured ultra-small EMT zeolite of 6-15 nm from the organic-template-free system at low temperature [30]. Traditionally, pure EMT could only be synthesized with an expensive organic template (18-crown-6 ether) [31]. The novel organic-template-free synthesis of EMT zeolite is from Na-rich precursor suspensions. 

Recently, Okubo and coworkers built a working hypothesis in seed-assisted synthesis without organic templates, which may provide a new idea for synthesis and broaden framework types of zeolites [40]. By analyzing the examples of the seed-assisted and organic-template-free synthesis of zeolites in detail, they concluded that the composite building units that both the frameworks of zeolite seeds and the crystallized zeolites contained were the key to the organic-template-free synthesis. 

Wu Z, Song J, Ji Y, Ren L, Xiao F-S. Organic template-free synthesis of ZSM-34 zeolite from an assistance of zeolite L seeds solution. Chem Mater 2008 20 357-9. 

In zeolite synthesis (ref. 2) an aqueous mixture containing a silicon source, an aluminum source, an alkali source (usually NaOH) is autoclaved and subjected to hydrothermal treatment. Hydrated Na-ions are then filling the pore system in the as-synthesized zeolite. In the case of relatively high Si/Al zeolites an organic template is required which is usually a tetraalkylammonium compound, applied as the bromide or the hydroxide. 

Since then, organic amines, quaternary-ammonium bases, metal complexes, and other compounds have been extensively used in zeolite synthesis, acting as space fillers with low specificity, structure-directing agent, or true templates ... 

Synthesis of zeolite Y in the presence of Gd(III) complexes of 18-crown-6 resulted not only in the encapsulation of the complex but the complex also served as a template for EMT polytype zeolite Y (Fig. 22b) (86). Feijen et al. described how the two different polytypes (the cubic FAU and the hexagonal EMT) can be formed (87). In the absence of an organic template, the FAU structure will form. If Na" "-18-crown-6 is present, it can be absorbed on the surface of the growing zeolite layer. This will influence the interconnection of the layers and, therefore, in the presence of this crown ether, the formation of the EMT framework may be favored. The difference between the pore window sizes is that in the EMT there are two different types 7.3 x 7.3 A in the [001] direction and 7.5 x 6.5 A perpendicular to the [001] direction. (The FAU has pore windows with 7.4 x 7.4 A in the [111] direction.)... 

The templating theory is based on a stereospecificity which cannot be separated from the chemistry of the cation. Zeolites are crystallized in alkaline solutions, most readily at a pH greater than 11, limiting the cations used in zeolite synthesis to alkali, some alkaline earths, and organic cations... 

Structures originated by molecular self-assembly are usually larger (on the order of several nanometers, yielding mesoporous materials, Figure 3.4) than those obtained from organic templates (typically microporous, pore size <2nm) [5], The large size of the mesopore (2-50 nm) facilitates the access of reactants to the interior of the solid. This allows for processing of bulky molecules that cannot access the narrower porosity of microporous materials, like zeolites. Control of the synthesis... 

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. 

The favorable influence of organic molecules in the synthesis of MFI-type zeolites is widely recognized. For example. aluminium-containing ZSM-5 can be obtained also in the absence of an organic template ( LJJK but the synthesis of silicate I seems to require the presence of an organic molecule in the reaction mixture ( 3K... 

Synthesis of Pentasil Zeolites With and Without Organic Templates... 

Amine Additives. It is precisely in this area, suppressing an undesired product in favor of the desired one, that additives can be most useful. High silica zeolites have been formed in the presence of amine additives. Vaughan (16) has prepared faujasite with 7.0 SiC>2/Al2(>3 (24.52 A0) by addition of bis-(2-hydroxyethyl) dimethyl ammonium chloride in a slurry composition whose cation composition is 69% Na and 31% organic template (T). To scale this product up to commercial synthesis would require almost total recovery of the organic template. But its silica content makes it an interesting candidate for catalytic testing. 

ZSM-34 preparation. Preparation of ZSM-34 zeolite was based on the patent literature (j>). However choline was used instead of choline chloride as the organic template, on the basis that zeolites prepared by using choline have better performance for olefin synthesis from methanol than zeolites prepared by using choline chloride (7). 

Suzuki et al. (14) have shown that upon variation of the gel composition as many as five different zeolite structures (ZSM-5, ZSM-35, ZSM-39, ZSM-48 and KZ-1) can be synthesized in the presence of pyrrolidine (14). Hence, not only the nature of the organic template but also tFie gel chemistry and synthesis conditions have a marked influence on the structure of the crystalline product. In a recent systematic study it was pointed out that especially pH and Na+-content have to be carefully adjusted in order to avoid the formation of phases other than ZSM-23 (4). 

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