Microporous silicas, crystalline

The insertion of catalytically active guests, such as transition metal ions, is an example of the potentialities of zeolite membranes for applications in catalytic membrane reactors. The well-known catalytic properties of supported vanadium oxides for selective oxidations have recently prompted a number of studies on the possibility of inserting vanadium in the framework of crystalline microporous silica and aluminosilicate powders. " ... 

Crystalline microporous silicas, the porosils, are a family of materials based on [TO4] units with tetrahedral densities below 21 T-atoms per 1000 A, which are synthesized in the presence of teinplating guest molecules. Their silica host frameworks are three-dimensionally four-connected, and in their calcined form, they belong to the large family of silica polymorphs. Porosils with pore openings too small to let the occluded guest molecules out are called clathrasils porosils in which the guests can be removed are called zeosils. 

Experiments employing model systems show that nanoblocks of 10 A organize to macroscopic crystals in a second stage. However, there is still no deep understanding of the synthesis of crystalline microporous silicas. 

The porosils comprise all crystalline microporous silicas with framework densities o/less than 21. In all three-dimensional silica frameworks, the [Si04] tetrahedral building units are four-connected. leading to electroneu-tral three-dimensional host framework structures. The calcined crystals are colorless and hydrophobic. The hardnesses of the microporous silicas are similar to that of quartz. The refractive indices o/the different crystalline phases are directly coiTelated with the framework d mxty, as expected from the general refractivity equation.The framework density varies between 20.9 and 15.0 T-atoms/... 

Channel Inclusion Compounds, p. 223 Crystal Growth Mechanisms, p. 364 Crystalline Microporous Silicas, p. 380 Gels, p. 586... 

Chemical Topology, p. 229 Concepts in Crystal Engineering, p. 2 9 Crystalline Microporous Silicas, p. 380 Hofmann-Type Clathrates, p. 645 Interpenetration, p. 135... 

Gies H, Marler B (1996) Crystalline microporous silicas as host-guest systems. In Mac-Nicol DD, Toda F, Bishop R (eds) Supramolecular chemistry, vol 6. Elsevier Science, Oxford, Chap 26,851 pp... 

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. 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

A new family of crystalline molecular sieves, 2) having aluminophosphate frameworks was synthesized. Strict alternation of A1 and P on the tetrahedral nodes yields neutral oxygens in contrast to the aluminosilicate zeolites, and non-framework cations are not needed for charge balance. Whereas a microporous silica (silicalite, 3 ) with neutral oxygens is hydrophobic, the aluminophosphate sieves are moderatley hydrophilic. 

The first iron-containing silsesquioxanes which appeared in the literature were compounds containing ferrocenyl units as side-groups.102 104 However, these are not within the scope of this review as iron is not part of the metallasilsesquioxane skeleton. Meanwhile, several ferrasilsesquioxane complexes have been synthesized. The first iron(III) compound of this type was prepared in our laboratory according to Scheme 56.105 In 161, the coordination sphere of iron is completed by TMEDA (NjNjN N -tetramethylethylenediamine) as a chelating amine ligand. Pale yellow, crystalline 161 was isolated in 80% yield and structurally characterized by X-ray diffraction. This compound was later used by Maxim et al.106 107 to prepare iron particles dispersed on microporous silica via controlled calcination of the ferrasilsesquioxane precursor as depicted in Scheme 56. 

In other respects, we can consider zeohte membranes as pertaining to the ceramic material category. Indeed zeolites are classified for the most part as microporous, crystalline silico-aluminate stmctures with different alumininum/silicon ratios. Thus, the chemical compositions are close to those of ceramic oxide membranes, in particular of microporous silica and alumina membranes. On the other hand, zeohtes are crystalline materials and they have a structural porosity very different from microporous amorphous silica [124]. Zeohte membranes are well adapted to the separation of gases, in particular H2 from hydrocarbons, but these membranes are not very selective for the separation of mixtures of noncondensable gases. 

Before the distinctive adsorptive properties of porous silica can be described, the different ranges of pore size that are of special importance to the mechanisms of physisorption must be identified. Micropores are the pores of the smallest width (d < 2 nm) mesopores are of intermediate size (id 2-50 nm) macropores are the widest pores (d > 50 nm) (5). Amorphous silica gels tend to be mesoporous or microporous, whereas the crystalline zeolitic silicas possess intracrystalline microporosity. The precipitated silicas are macroporous and also, to a small extent, microporous. These and other aspects of the microstructures will be discussed in the following sections. 

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. 

Microporous solids with surface areas in the range of 400 m /g or larger can be prepared in several ways. Controlled thermal decomposition of silica gel or aluminum hydroxides to AIO(OH) and then to the 7- or r/-phases of alumina produces such high-surface-area materials. The so-called sol-gel method, which uses both aluminate and silicate ions or aluminum and silicon alkyl compounds at a well-controlled hydrogen ion concentration, produces crystalline microporous solids that... 

Microporous silicas with pore size of 12.2 A were prepared by either S XT or S " pathways at room temperature using dodecyidimethylbenzylammonium chloride (DDBAC) surfactant. The pore size of the synthesized materials could be tuned from microporous to mesoporous range by the crystalline temperature or using mixed surfactants of DDBAC and cetyl-pyridinium chloride. The pore structure can transfer from disordered tubular array to ordered hexagonal phase by the increase of the molar ratio of CPCl to DDBAC. 

Nowadays, the term zeolite includes all microporous solids based on silica and exhibiting crystalline walls, as well as materials where a fraction of Si atoms has been substituted by another element, T, such as a trivalent (T = Al, Fe, B, Ga,. ..) or a tctravalent (T = Ti, Ge,...) metal. Crystalline microporous phosphates are known as zeotypes or as related microporous solids (14, 54). At present, there are 179 confirmed zeoHtc framework types. For the structure types, three-letter codes are used, which were adopted from the name of the first material reported with a specific stmcturc. As an example, FAU is given for the structure of faujasite and its synthetic equivalents X and Y, and MFl for the stracture of ZSM-5 or silicalite-1 (105). Figure 9.11 shows prominent examples of zeolite firameworks, for example, FAU, LTA, and MFI types (pentasil). 

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