Zeolites secondary building units

One-step direct synthesis of the highly stable mesoporous silica-based material is also possible. MMS-H[209] has a structure analogous to that of MCM-48 but which contains zeolite building units. A mixture of CTAB and Brij30 was used as template for the mesopores. The use of TPAOH without the assistance of NaOH helps to introduce zeolite secondary building units, as well as the direct formation of acidity after removal of the template. This material was also found to possess superior thermal, hydrothermal, steam, and mechanical stabilities. 

Knight, C.T.G. (1990) Ae zeolite secondary building units really red herrings Zeolites 10, 140-144. 

Figure C2.12.3. Secondary building units in zeolites. Each comer represents a T-atom (Si, Al) while tire connecting lines represent oxygen bridges witli tire oxygen atom in tire middle.

The use of FOSS polyhedra as models for silica surfaces or as secondary building units in inorganic materials such as zeolites or other porous solids is likely to increase rapidly as more is understood about the mechanisms by which the polyhedra may be constructed. It will be of particular interest to see if the larger structures such as TeoHeo or T240H240 or their derivatives (Section VII.C) and analogous to carbon structures such as Cgo or nanotubes, can be prepared. 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

It is convenient to visualize zeolites in terms of secondary building units (SBUs). Several of the SBUs currently recognized by the IZA are depicted in Fig. 3 along with other useful structural units, the sodalite and pentasil cages. The sodalite unit is... 

Recently, mesoporous aluminosilicates with strong acidity and high hydrothermal stability have been synthesized via self-assembly of aluminosilicate nanoclusters with templating micelles. The materials were found to contain both micro- and mesopores, and the pore walls consist of primary and secondary building units, which might be responsible for the acidity and stability (181). These materials were tested in isobutane/n-butene alkylation at 298 K, showing a similar time-on-stream behavior to that of zeolite BEA. No details of the product distribution were given. 

Many structures of zeolites can be described by certain structural units called building units, such as double 4 or 6 membered rings (D4R or D6R), structural sheets and rods. The main requirement for stmcture analysis of zeolites at present is not the refinement of electron charge distribution in the unit cell but the determination of the framework t5q)e stmctures, manner of arrangements of secondary building units or characterizing their lattice defects. 

Framework infrared has also been used to look for the formation of a zeolite during synthesis. Since many of the secondary building units can be detected in the infrared spectrum, it is possible to see zeolite formation at very early stages in the synthesis. In fact, zeolite formation can be detected in the infrared before crystallinity is observed by X-ray diffraction. Most of the reported work has been done by sampling the zeolite synthesis at various stages, isolating the soUds and measuring infrared spectra of the dried samples. 

Approaches to the formation of three new types of micro-porous materials that complement zeolites will be discussed. In each case, whether metal coordination polymers, metal-linked ceramic oxide clusters, or new hybrids containing both coordination and ceramic components, engineering of the Secondary Building Unit (SBU) is of critical importance. Successful examples of these approaches include the first thermally stable 3-D micro-porous coordination polymer with chemical functionalizability [Cu3(TMA)2(H20)3] , as well as a 3-D micro-porous cluster based material [V,2B18O60H8(Cd(en)(H2O) 3]". ... 

Classification of Zeolites According to Secondary Building Units... 

What is the nature of the Al- and Si-bearing species in the mixture Are aluminosilicate ions present Are the secondary building units found in zeolitic frameworks already present in solution ... 

Zeolite Structures These are crystalline, microporous solids that contain cavities and channels of molecular dimensions (3 A to 10A) and sometimes are called molecular sieves. Zeolites are used principally in catalysis, separation, purification, and ion exchange The fundamental building block of a zeolite is a tetrahedron of four oxygen atoms surrounding a central silicon atom (i.e.. (Si04)4-). From the fundamental unit, numerous combinations of secondary building units (polygons) can be formed. The corners of these polyhedra may he Si or A1 atoms.2... 

In most zeolite structures. Ihe primary structural units, telrahedru. are assembled into secondary building units, which may he simple polyhedra such as cubes, hexagonal prisms, or truncated octahedra. The tinal framework structure consists of assemblages of the secondary units. 

Figure 9.7 Zeolite cage structures incorporated as secondary building units.

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

The absence of the IR band at 560 cm- (characteristic for D6R secondary building units of zeolite X) (29) and the presence of the broad band with the maximum at 600 cm-1 (characteristic for D4R secondary building units of zeolite P) (29) even in the IR spectra of the solid phase of the gel aged for 10 d (see Figure 8c), as well as the much lower values N0/Na for zeolite X than the values N0/Na for zeolite Na-Pc (see Table IV.), indicate that the total number of nuclei (nuclei-I + nuclei-II) of zeolite Na-Pc is much greater than the total number of nuclei of zeolite X, for all the gels examined. 

The ionic composition of dilute sodium silicate is a very complex problem involving Na20/SiC>2 ratio, water content, and even trace impurities. Equilibration seems to be very slow at ordinary temperatures. As shown in Figure 5, Harris, et.al, were able to identify a wide variety of structures in potassium silicate solutions which bear a striking resemblence to the secondary building units proposed for zeolite... 

Yang, Y, Waiawalkar, M, G. Pinkas, J, Roesky, H, W. Schmidt, H.-G. Molecular aluminophosphonate Model compound for the isoelectronic doubie-six-ring (D6R) secondary building unit of zeolites, Angew. Chem,. Int. Ed. 1998, 37, 96-9B,... 

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