Bifunctional metal/acid catalysis

Acid and Bifunctional Metal/Acid Catalysis Active Sites... 

Noble metal zeolite catalysts are used in various processes, most of them occurring through bifunctional hydrogenating/acid catalysis. One exception, however, is the selective aromatization of n-alkanes (e.g. n-hexane into benzene) proceeding through monofunctional metal catalysis. Indeed the PtLTL catalyst used commercially does not present any protonic sites. 

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... 

Under hydrogen flow, various reactions can be observed during ethylbenzene transformation over bifunctional Pt/acid catalysts. Some of them occur through bifunctional catalysis (reactions 1, 2, 6), the other through acid (reactions 3,4) or metal catalysis (reaction 5). 

The contribution of acid sites in the metal-acid bifunctional catalysis occurring in dewaxing and Selectforming is obvious, and essentially the same as those for cracking and reforming processes. Various contributions of acid and base sites in hydrocracking in the broad sense are described below. 

Beyond the concept of combined acid catalysis, there are also many other bifunctional acid catalysts interacting with nucleophiles and electrophiles simultaneously, and thus benefiting through such cooperative effect. Selected examples on Lewis acid/hydrogen bonding cooperative catalysis and Lewis acid/transition-metal... 

The remaining part of this chapter will highlight some recent examples in which bifunctional acid catalysis does not involve combined acid catalysis, namely, two acid components (either Lewis acid, hydrogen bonding, or transition metal) are... 

Despite the prevalence of amine/metal or Brpnsted acid/metal binary catalysis, bifunctional (thio)urea/metal binary catalysis finds only limited applications in in cascade or tandem reactions and is much less well developed. This may be due partially to deactivation of the Lewis acidic metal catalysts by coordinative (thio)urea catalysts. The examples reported often adopted the strategy of sequential additions of catalysts and substrates to solve the problans mentioned above. 

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... 

The rich variety of active sites that can be present in zeolites (i) protonic acidic sites, which catalyze acid reactions (ii) Lewis-acid sites, which often act in association with basic sites (acid-base catalysis) (iii) basic sites (iv) redox sites, incorporated either in the zeolite framework (e.g., Ti of titanosHicates) or in the channels or cages (e.g., Pt clusters, metal complexes). Moreover, redox and acidic or basic sites can act in a concerted way for catalyzing bifunctional processes. 

Dual-function catalysts possessing both metallic and acidic sites bring about more complex transformations. Carbocationic cyclization and isomerization as well as reactions characteristic of metals occurring in parallel or in subsequent steps offer new reaction pathways. Alternative reactions may result in the formation of the same products in various multistep pathways. Mechanical mixtures of acidic supports (silica-alumina) and platinum gave results similar to those of platinum supported on acidic alumina.214,215 This indicates that proximity of the active sites is not a requirement for bifunctional catalysis, that is, that the two different functions seem to operate independently. 

When metal centers act in conjunction with acid sites on the zeolite, bifunctional catalysis can occur (e.g., Pd/HY). This type of catalysis is used mainly for the hydrocracking and isomerization of long-chain n-alkanes. For example, the rates of formation of 2- and 5-methylnonane isomers obtained from n-decane isomerization over bifunctional zeolite catalysts depend on the size and structure of the zeolites used. This reaction has been developed as a test reaction to characterize zeolite structures (17-19). 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

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