Porous amorphous inorganic materials

Extensively studied is oxygen permeation through dense ceramic membranes (e.g. perovskhes). Temperatures > 600 °C are applied. Here, oxygen splits at the surface and is transited as 0 . Porous membranes include porous polymer films (cellulosics, polyamides) as well as amorphous inorganic materials (alumina, silica). 

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

Organic zeolite analogues are commonly referred to as porous solids. These materials promise a new range of applications, e.g., in pharmaceutical manufacture and in molecular sieves, sensors, and devices. They are crystalline or amorphous materials that permit the reversible passage of molecules through holes on their surface. Porous solids are classified according to pore diameter nanoporous or microporous (< 15 A), mesoporous (15—500 A) and macroporous (>500 A). The natural and synthetic inorganic zeolites with uniform pore sizes of 10-20 A are the classical examples of microporous materials with widespread use in industry. 

Once the multi-step reaction sequence is properly chosen, the bifunctional catalytic system has to be defined and prepared. The most widely diffused heterogeneous bifunctional catalysts are obtained by associating redox sites with acid-base sites. However, in some cases, a unique site may catalyse both redox and acid successive reaction steps. It is worth noting that the number of examples of bifunctional catalysis carried out on microporous or mesoporous molecular sieves is not so large in the open and patent literature. Indeed, whenever it is possible and mainly in industrial patents, amorphous porous inorganic oxides (e.g. j -AEOi, SiC>2 gels or mixed oxides) are preferred to zeolite or zeotype materials because of their better commercial availability, their lower cost (especially with respect to ordered mesoporous materials) and their better accessibility to bulky reactant fine chemicals (especially when zeolitic materials are used). Nevertheless, in some cases, as it will be shown, the use of ordered and well-structured molecular sieves leads to unique performances. 

Amorphous silica-alumina materials represent an important class of porous inorganic solids which have not long-range order and usually have a wide distribution of the pore size, in the micro and mesopore region. They show outstanding catalytic behaviours in several acid catalysed reactions (5, 6). 

Monolithic zirconia networks can also be formed using a similar procedure giving porous 2xQ>2 structures [9]. As the titania and zirconia precursors are miscible, binary inorganic networks of various Ti Zr ratios could be produced [9]. The crystallinity and photocatalytic properties of the mixed material were studied X-ray amorphous materials were produced for Ti Zr ratios of 2 8 to 7 3, and the binary material containing 10% zirconia (the presence of which inhibited crystal transformation to the rutile phase) showed the highest photocatalytic activity for the photodecomposition of sahcylic acid and 2-chlorophenol [9]. 

Materials used as solid support catalysts are generally inorganic oxides that are insoluble in the majority of solvents. They are mostly silicas, aluminas or aluminosilicates but magnesium oxides and aluminophosphate materials may also be considered. The many types of solid fall into three broad groups, classification being made according to the structures of the solids [4] rather than their chemical properties. The groups are amorphous solids, lamella solids and porous solids. In a subjective summary such as... 

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