Heterogeneous catalysis bifunctional catalysts

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. 

The topic of catalysis with Nafion has recently been reviewed in detail (56). Apart from using Nafion-H primarily as a solid superacid catalyst, a number of reports have described the use of functionalized Nafion derivatives by metal cation exchange to achieve various types of organic reaction. These include a bifunctional catalyst (acid and cation site), a heterogeneous perfluorosulfonate salt (only cation sites), and a trifunctional... 

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... 

They have outstanding properties which are of great value for catalysis, such as strong Bnansted acidity [8], ability to catalyze reversible redox reactions under mild conditions [9], and high solubility in water [10]. In most applications, they are used as acid, redox, and bifunctional catalysts in homogeneous and heterogeneous systems. 

The classical model of solid acid and bifimctional heterogeneous catalysis basically assumed that the acid catalyzed reactions followed the laws known for liquid acids. It also assumed that catalysis over bifunctional catalysts, exposing both metal sites and acid sites, could be described by assuming simple additivity, reaction intermediates were thought to shuttle fi equently between metal sites and acid sites. More recent research has, however, shown that both assumptions are inadequate. Some reactions which are not catalyzed by liquid acids are fast over solid acids, moreover bifimctional catalysts display substantial deviations fi"om the additivity model. 

Bond (1987) covers the basic principles of catalysis, adsorption on solid surfaces, chemisorption at metal and oxide surfaces, the kinetics of catalyzed reactions, the quantitative aspects of catalysis by metals and the structure, preparation and use of heterogeneous catalysts. The book also discusses the application of catalysts in different fields including energy conservation, production of hydrocarbon feedstocks, bifunctional catalysts in petroleum industry, oxidation catalysts in the petrochemical industry, heavy inorganic industry, hydrogenation of multiple bonds and catalysts used in atmospheric pollution control. 

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... 

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. 

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. 

The direct synthesis of bifunctional zeolitic catalysts creates new opportunities for the application of shape selective materials in heterogeneous catalysis. 

Some heterogeneous catalytic reactions proceed by a sequence of elementary processes certain of which occur at one set of sites while others occur at sites which are of a completely different nature. For example, some of the processes in the reforming reactions of hydrocarbons on platinum/ alumina occur at the surface of platinum, others at acidic sites on the alumina. Such catalytic reactions are said to represent bifunctional catalysis. The two types of sites are ordinarily intermixed on the same primary particles ( 1.3.2) but similar reactions may result even when the catalyst is a mixture of particles each containing but one type of site. These ideas could, of course, be extended to crea te the concept of polyfunctional catalysis. 

The complexity of many heterogeneous systems used in multi-phase reactions, the use of a solid support, the difficulty in analyzing highly dispersed active sites and the bifunctional nature of many solid supported metal catalysts, make a detailed and complete study challenging. The simpler homogeneous systems teach many of the principles of catalysis active sites, reaction mechanisms, reaction kinetics and catalytic cycles, which can often be applied elsewhere. 

Conna, A Garcia, H Leyva, A Primo. A. Basic zeolites containing palladium as bifunctional heterogeneous catalysts for the Heck reaction. Applied Catalysis A General, 2003 247,41 -49. 

Da Silva Rocha, K.A., Robles-Dutenhefner, P.A., Sousa, E.M.B., Kozhevnikova, E.F., Kozhevnikov, I.V., Gusevskaya, E.V. 2007. Pd-heteropoly acid as a bifunctional heterogeneous catalyst for one-pot conversion of citronellal to menthol. Applied Catalysis A General 317 171-174. 

Other publications have also appeared describing condensation kinetics with heterogeneous catalysts by a designed experimental approach to optimize reaction conditions, stannous octanoate condensation catalysis and the role of bifunctional catalysis mechanisms. A bifunctional transition state , shown for compound 8, was postulated to explain the kinetics of heterofunctional condensation reactions catalysed by CI3CCOOH, Et2NOH, MeEtC=NOH or CH3COOH (equation 6). 

Sharma et al. reported on the synthesis and catalytic properties of bifunctional heterogeneous catalysts with basic and organometallic catalysts for efficient catalysis of two-step Sonogashira and Henry reactions [55]. With a solvent-assisted grafting method, two different catalytic groups— an organoamine and a palladium-organodiamine... 

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