Amorphous materials silica gels

Adsorbents are natural or synthetic materials of amorphous or microcrystalhne structure. Those used on a large scale, in order of sales volume, are activated carbon, molecular sieves, silica gel, and activated alumina [Keller et al., gen. refs.]. 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Silica gel. Silica gel is a porous amorphous form of silica (Si02) and is manufactured by acid treatment of sodium silicate solution and then dried. The silica gel surface has an affinity for water and organic material. It is primarily used to dehydrate gases and liquids. 

Figure 1.16 Schematic and comparative illustrative of grafting (a) an amorphous silica gel and (b) an ordered MCM-41 silica materials. Entrapment of oxidation catalyst perruthenate inside the channels of MCM-41 silica (below) results in a shape-selective aerobic catalyst, which may not be desirable from the viewpoint of catalyst versatality (Reproduced from ref. 39, with permission.)...

Figure 9.1 Examples of texture of the materials formed on a short range from amorphous Si02 (a) silica gel (b) hydrothermally treated silica gel (c) porous glass (d) mesoporous mesophases type of MCM-41 and (e) opal.

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

The same authors compared catalysts prepared from these precursors and [Ru(BINAP)Cl2]2 adsorbed on MCM-41 (with 26 and 37 A pores) and an amorphous mesoporous silica (with 68 A pores) all treated with combinations of SiPh2Cl2 and Si(CH2)3X (X = NH2, CO2H). Catalysts were also prepared in which the organometallic precursors were immobilized by entrapment into silica (using sol-gel techniques). This is one of the few studies in which the performance of chiral phosphine catalysts immobilized by covalent and noncovalent procedures are compared directly. The materials were examined as catalysts for the hydrogenation of sodium a-acetamidocinnamate and of a-acetamidocinnamic acid under similar conditions to those used for the catalysts on unmodified MCM-41. The catalysts... 

Conformations and aggregate structures of sorbed natural organic matter on muscovite and hematite. Geochim. Cosmo-chim. Ada 65 1047-1057 Nanzyo, M. Watanabe,Y. (1981) Material balance associated with phosphate sorption by amorphous clays, silica-alumina gel and synthetic goethite. Soil Sd. Plant Nutr. 27 329-337... 

Amorphous silica is used as a pigment and filler in paints and coatings. It also is used as an abrasive, absorbent and catalyst support. Silica gel is a common desiccant and adsorbent. It is used in analytical chemistry as a packing material in chromatography columns and in clean-up of organic extracts to remove interference in trace analysis of organic pollutants. 

Packed columns contain an inert and stable porous support on which the stationary phase can be impregnated or bound (varying between 3 to 25%). The solid support is made of spheres of approximately 0.2 mm in diameter, obtained from diatomites, silicate fossils (such as kieselguhr, tripoli) whose skeleton is chemically comparable to amorphous silica. These materials, which have a specific surface area ranging from 2 to 8 m2/g, have been commercialised by several companies such as Johns Manville, under the name of Chromosorb , and are used universally. Other synthetic materials have been developed such as Spherosil , made of small silica beads. All of these supports have a chemical reactivity comparable to silica gel because of the presence of silanol groups. 

A unique way of identifying acid sites in amorphous silica-alumina was tried by Bourne et al. (128). These authors decided to synthesize, then characterize, two extreme types of acid site structures that they felt existed in commercial silica-aluminas. The two catalyst types consisted of low concentrations (<1.4% wt) of aluminum atoms incorporated (a) on the surface of silica gel (termed aluminum-on-silica) and (b) within the silica lattice (termed aluminum-in-silica). From infrared measurements of pyridine chemisorbed on the two materials, they conclude that dehydrated aluminum-on-silica contains only Lewis acid sites and that dehy-... 

The silica gel is amorphous. Using high-resolution electron microscopy, it is known that its amorphous framework is made up of small globular (primary) particles having sizes of 10 to 20 A (Rouquerol, Rouquerol and Sing, 1999). An alternative route involves reactions of silicon alkoxides with water, and a wide variety of materials can be made this way (Jones 1989 Brinker and Sherer, 1990). The processes based on this route are referred to as sol-gel processing, and they offer many promising possibilities. For silica gel, the reaction is... 

Silica is one of the most abundant chemical substances on earth. It can be both crystalline or amorphous. The crystalline forms of silica are quartz, cristobalite, and tridymite [51,52]. The amorphous forms, which are normally porous [149] are precipitated silica, silica gel, colloidal silica sols, and pyrogenic silica [150-156], According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be classified as follows microporous materials are those with pore diameters from 3 to 20 A mesoporous materials are those that have pore diameters between 20 and 500 A and macroporous materials are those with pores bigger than 500 A [149],... 

In addition to zeolites and MMS which are crystalline, ordered nanoporous materials and amorphous adsorbents, such as silica and active carbon, have been used in gas cleaning. The silica gel surface is generally terminated with OH groups bonded with a silicon atom, SiOH units, that is, silanols (see Figure 2.30). The concentration of OH groups at the surface is... 

Amorphous silica, silica gel, can be made by hydrolysis of alkoxides such as Si(OEt)4 it is used, when dehydrated, as a drying agent, and chromatographic and catalyst support material. It appears to contain Si(OSi=)4, Si(OSi=)3OH, and Si(OSi=)2(OH)2 groups. The nmr studies on MSi indicate that silica found in plants, flagellates, and other biological systems has the same type of structure as silica gel. 

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