Microporous silicas, crystalline structural properties

The first discovered member of the group of crystalline microporous materials made of oxides of titanium and silicon is titanium silicalite-1 (TS-1). TS-1 has attracted much interest for its unique catalytic properties it is also of interest by virtue of the proposal that Tiiv assumes tetrahedral coordination in substituting for SiIV in framework positions of crystalline silica, as stated above. To clarify this point, many detailed studies of the TS-1 structure have been carried out. An outcome of the work was the discovery of new crystalline microporous titanium silicates. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

To achieve a significant adsorptive capacity an adsorbent must have a high specific area, which implies a highly porous structure with very small micropores. Such microporous solids can be produced in several different ways. Adsorbents such as silica gel and activated alumina are made by precipitation of colloidal particles, followed by dehydration. Carbon adsorbents are prepared by controlled burn-out of carbonaceous materials such as coal, lignite, and coconut shells. The crystalline adsorbents (zeolite and zeolite analogues are different in that the dimensions of the micropores are determined by the crystal structure and there is therefore virtually no distribution of micropore size. Although structurally very different from the crystalline adsorbents, carbon molecular sieves also have a very narrow distribution of pore size. The adsorptive properties depend on the pore size and the pore size distribution as well as on the nature of the solid surface. 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Silicalite is a microporous crystalline silica, i.e., an aluminium-free zeolite, belonging to the MFI-type structure and being the Si/Al oo limit of the ZSM-5 zeohte. The nature, population, and acidic properties of the hy-droxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicahtes have been characterized by adsorption of NH3 at room temperature, monitored through the combined use of microcalorimetry and IR spectroscopy [196]. It was foimd that a perfect sihcahte sample eidiibits a very low activity towards NH3, confirming the almost complete absence of defects. The energetics of the interaction indicated that the (mild) acidity of silanols increases as far as the extension of the silanol patches increases. 

Microporous crystalline solids in which transition metals are tetrahedrally substituted via template-mediated hydrothermal synthesis have remarkable properties in selective oxidation reactions [66]. Unfortunately, the microporous structure and the rigidity of the crystalline frameworks limit the substitution degree, variety of substituted metal, and their general applicability. For this reason, the amorphous microporous mixed oxides (AMMs) with uniform microporosity and wide compositional variability are devoid of Bronsted acidity, but are not associated with the redox-active elements. However, metals such as Ti, V, and Mo incorporated in amorphous silica were good catalysts in allylic oxidation or epoxidation of olefins [67]. 

---