Acid-base bifunctional heterogeneous catalysts

Bass JD, Solovyov A, Pascal AJ, Katz A (2006) Acid-base bifunctional and dielectric outer-sphere effects in heterogeneous catalysis A comparative investigation of model primary amine catalysts. J Am Chem Soc 128 3737... 

Selective carbon-carbon bond formation is one of the most important reactions in both industrial and academic fields. Clean carbon-carbon bond formation depends on the development of heterogeneous acid and base catalysts, as described in this part. Mechanistic considerations can also identify new carbon-carbon bond formations. Section 6.2.2.1 focuses on catalysis by montmorilonite clays because various types of such solid acid catalysts have been simply prepared. The following section reviews recent developments in solid base catalysts for such chemistry and the final section of this chapter describes solid acid-base bifunctional catalysts, which are important concepts for one-pot sequential reactions. 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

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. 

Type of Active Sites. - In heterogeneous catalysis the following type of actives sites can be distinguished (i) metallic, (ii) acid-base, (iii) red-ox type, and (iv) anchored metal-complex. The catalytic sites may contain one of the above types of active sites or can include several types of sites. In case of different type of sites the catalysts are bifunctional or multifunctional. For instance, Pt/Al203 and Pt/mordenite are typical bifunctional catalysts containing both metallic and acidic types of active sites. On the other hand, Pt or Pd supported on silicon carbide, nitride, or Pt/L-zeolite are mono-functional catalysts. There are important industrial reactions, such as isomerization and aromatization of linear hydrocarbons, which requires bifunctional catalysts, such as chlorinated... 

Besides doping, co-doping in where two or more elements are present on the G sheet is also interesting from the point of view of using Gs as catalysts. One case that could be of interest is the presence of electron deficient atoms such as B and heteroatoms with excess of electrons such as N. In a certain way the simultaneous presence of B and N could be similar to the introduction of acid and basic Lewis sites. The concept of bifunctional acid-base solid catalyst has found wide application in heterogeneous catalysis of solids such as aluminophosphates containing NH groups (ALPONs) due to the cooperative activity of both sites in the reaction mechanism. 

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