Silica-based zeolite

The discovery of TS-1 and its unique catalytic properties constitutes a significant contribution to the knowledge of silica-based zeolite-like materials containing elements different from Al(III) and opens new technological possibilities for oxidation processes with H2O2. 

There is also rapid development in the domain of standard silica-based zeolites. Their versatility can be extended by imprinting. For instance, Davis and Katz [15] recently successfully carried out imprinting and obtained a silica framework with pore walls anchoring three aminopropyl groups in cavities. Another achievement was reported by Ramamurthy, Schefer and coworkers [16]. The latter authors were able to obtain 90% diastereomeric excess of a product of the photochemical reaction in a commercially available zeolite containing chiral tropolone ether 433 in its pores. 

Guth J-L and Kessler H 1999 Synthesis of aluminosilicate zeolites and related silica-based materials Catalysis and Zeolites, Fundamentals and Applications ed J Weitkamp and L Puppe (Berlin Springer) pp 1-52... 

Strong acids are able to donate protons to a reactant and to take them back. Into this class fall the common acids, aluminum hahdes, and boron trifluoride. Also acid in nature are silica, alumina, alumi-nosihcates, metal sulfates and phosphates, and sulfonated ion exchange resins. They can transfer protons to hydrocarbons acting as weak bases. Zeolites are dehydrated aluminosilicates with small pores of narrow size distribution, to which is due their highly selective action since only molecules small enough to enter the pores can reacl . 

Presently, the most successful adsorbents arc microporous carbons, but there is considerable interest in other possible adsorbents, mainly porous polymers, silica based xerogels or zeolite type materials. Regardless of the type of material, the above principles still apply to achieving a satisfactory storage capacity. The limiting storage uptake will be directly proportional to the accessible micropore volume per volume of storage capacity. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

Kawase, M., Nomura, K., Nagamori, Y., and Kinishita, J. (2001) High silica content zeolite based catalyst. U.S. Patent 6, 207,605. 

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

Aluminas, silicas, clays, zeolites or graphite can be selected as acidic, basic or neutral supports, depending on the type of reactions. Alumina alone can act as a base towards a acid compounds but if a strong base is necessary, KF on alumina can ionize carbon acids up to pKa = 35. On the other hand, montmorillonites (clays) such as K10 or KSF offer acidities very comparable to strong mineral acids (e.g., nitric and sulfuric acids, respectively). Reactions are simply performed by mixing reagents and a mineral support in the absence of any organic solvent followed by microwave irradiation [52]. 

In the supported systems the catalyst can be coated on the walls of the reactor, supported on a solid substrate or deposited around the case of the light source. Many are the supported materials used in literature, such as glass beads, and tubes [69], silica-based materials [70], hollow beads, membranes [71], optical fibers, zeolites, activated carbon, organic fibers [72], and so on. 

Although micro- and not mesoporous but worth mentioning in any case is a new class of materials in this evolution series of porous silica-based hybrid materials, the so-called ZOLs (zeolites with organic group as lattice). As we will see, many studies in the past focused on the organic functionalization of amorphous silica porous... 

Amorphous Ti/SiCL oxides and crystalline Ti zeolites are two classes of well-studied solid Ti catalysts (11-14). In both classes, a Lewis-acidic Ti atom is anchored to the surrounding siliceous matrix by Si-O-Ti bonds. The oxidant of choice for Ti zeolites such as titanium silicalite 1 (TS-1) and 11-/1 is H2O2, whereas the amorphous, silica-based materials function optimally with organic peroxides such as /-butyl hydroperoxide (/-BuOOH) or ethyl benzene hydroperoxide. However, there are strictly no homogeneous analogues of these materials, and they therefore do not fit within the context of anchoring of homogeneous catalysts. 

Membranes are classified as organic or inorganic, taking into account the material used for their syntheses porous or dense, based on the porosity of the material applied and symmetric and asymmetric for a membrane made of a single porous or dense material or for a membrane made of a porous support and a dense end, respectively [16,64], We are fundamentally interested here in asymmetric inorganic membranes made of a porous end to bring mechanical stability to the membrane and made of alumina, silica, carbon, zeolites, and other materials, and a dense end to give selectivity to the membrane (see Chapter 10). However, we also analyze the performance of porous polymers. 

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. 

It is also possible to prepare crystalline electrides in which a trapped electron acts in effect as the anion. The bnUc of the excess electron density in electrides resides in the X-ray empty cavities and in the intercoimecting chaimels. Stmctures of electri-dides [Li(2,l,l-crypt)]+ e [K(2,2,2-crypt)]+ e , [Rb(2,2,2-crypt)]+ e, [Cs(18-crown-6)2]+ e, [Cs(15-crown-5)2]" e and mixed-sandwich electride [Cs(18-crown-6)(15-crown-5)+e ]6 18-crown-6 are known. Silica-zeolites with pore diameters of vA have been used to prepare silica-based electrides. The potassium species contains weakly bound electron pairs which appear to be delocalized, whereas the cesium species have optical and magnetic properties indicative of electron locahzation in cavities with little interaction between the electrons or between them and the cation. The structural model of the stable cesium electride synthesized by intercalating cesium in zeohte ITQ-4 has been coirfirmed by the atomic pair distribution function (PDF) analysis. The synthetic methods, structures, spectroscopic properties, and magnetic behavior of some electrides have been reviewed. Theoretical study on structural and electronic properties of inorganic electrides has also been addressed recently. ... 

On the other hand, information about the framework of zeolite can be obtained form the Si-MAS-NMR spectrum. In the case of the high silica-type zeolite used here, the Si/metal ratio in the zeolite framework is determined from the relative values of Si(OM) and Si(lM). As is seen in Table IV, this ratio becomes larger in comparison with that of the fresh catalyst for all spent catalysts, thus indicating that some active components in the form of A1 or Ga are being eliminated from the framework by the reaction and regeneration cycles. Based on this finding, the major... 

Even though zeolites and porous glasses are materials that allow coveting two orders of magnitude in pore diameter, they remain silica based. A complete description of the role of the confining material in radiolysis would require the access to alternative porous materials. The development of metallic porous frameworks has been studied for catalysis (Raney Nickel), electrochemistry and heat dissipation. 

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