Bifunctional catalysts acidic component

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 trimerization of cyclopentadiene (6) is catalyzed by a homogeneous bifunctional palladium-acid catalyst system.7 The reaction gives trimers 7 and 8 as a 1 1 mixture in 70% yield with bis(acetylacetonato)palladium(II) [Pd(acac)2] or with bis(benzylideneacetone)-palladium(O) as the palladium component of the catalyst. As the phosphorus component, phosphanes like trimethyl-, triethyl-, or triphenylphosphane, and triisopropylphosphite or tris(2-methylphcnyl)phosphite, are suitable. A third component, an organic acid with 3 < pK < 5, is necessary in at least equimolar amounts, in the reaction with cyclopentadiene (6), as catalytic amounts are insufficient. Acids that can be used are acetic acid, chloroacetic acid, benzoic acid, and 2,2-dimethylpropanoic acid. Stronger acids, e.g. trichloroacetic acid, result in the formation of poly(cyclopentadiene). The new catalyst system is able to almost completely suppress the competing Diels-Alder reaction, thus preventing the formation of dimeric cyclopentadiene, even at reaction temperatures between 100 and 130°C. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

As stated above, the aromatization of short alkanes is carried out in presence of bifunctional catalysts, in where the dehydrogenating function is given by the metal component (Ga, Zn, Pt) and the H-ZSM-5 zeolite carries the acid sites. Although there is still some uncertainty concerning the initial activation of the alkane, probably both the metal and the zeolite acid sites are involved in this step. Metal sites can dehydrogenate the alkane to give the corresponding alkene, which can then be protonated on the Bronsted acid sites of the H-ZSM-5 zeolite to produce the carbocation. 

The current theory of bifunctional catalysis assumes that paraffin isomerization is induced by olefin formation at the metal surface, followed by a typical acid-catalyzed reaction of the olefin at the active centers of the acidic component. Consequently, similar skeletal conversions must be found with olefins and an acid catalyst, and paraffins and a bifunctional catalyst. Our findings substantiate this theory. If these results (Figs. 2 and 3) are put together and compared to the predictions of the carbonium mechanism (Fig. 4), one can see that all the expected structures have been obtained in our experiments. 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

ATO and AFO type structures. At present the industrial process of lube oil dewaxing (ChevronTexaco) is realized on bifunctional catalyst with acidic SAPO-11 (AEL) component. Few examples in the literature devoted to comparative study of AEL-, ATO- and AFO-SAPO materials in hydroisomerization reaction are based on a single specimen of each catalyst, sometimes not phase-pure and often prepared by exotic or undefined method. Recently the authors found a new method for selective and reproducible synthesis of SAPO-31 (ATO type structure) materials in the presence of di-n-pentylamine and showed hydroisomerization efficiency of catalysts based on these systems [3,4]. 

The findings of Swain and Brown 55) support the ternary mechanism proposed by Lowry for the mutarotation of tetra-O-methylglucose. It was found that the mutarotation of tetra-O-methylglucose in benzene in the presence of both an acid (phenol) and a base (pyridine) followed third-order kinetics but was first-order with respect to each component tetra-O-methyl-glucose, pyridine, and phenol 55). 2-Hydroxypyridine was found to be a very effective bifunctional catalyst, and since both acid and base functions were in the same molecule, the mutarotation followed second-order kinetics. Its catalytic action was essentially independent of the other acid and base species present. Although it is a much weaker acid or base than either phenol or pyridine, its catalysis of the mutarotation of tetra-O-methylglucose in benzene was much greater than that of either pyridine or phenol, or a mixture of both 55). 

The mechanism of paraffin hydrocracking over bifunctional catalysts is, essentially, the carbenium ion chemistry of acid cracking coupled with metal-centered dehydrogenation/hydrogenation reactions. The presence of excess hydrogen and the hydrogenation component of the catalyst result in hydrogenated products and inhibition of some of the secondary reactions and coke formation. 

Hydroisomerization reactions are generally intimately associated with hydrocracking reactions. The overall scheme is rather complex. It involves the independent action of both types of catalytic sites and the existence of a transport mechanism for olefins between these sites. Therefore, the catalyst must be designed according to this bifunctional mechanism. The relative strength of the hydro-dehydrogenation and acidic components must be adjusted for the desired operation. 

The formation of 2,3-dihydro-3,4-dihydroxy-5-acetyIfuran (48), a flavour component in baking, in the reaction between D-fructose and p-alanine has been reported." Anionic ruthenium iodocaibonyl complexes acted as dehydroxylation catalysts of C3 - Cs polyols and C sugars in aqueous solution, due to their bifunctional nature (acidity and hydrogenation ability). By exposure to [Ru(CO)3l3] in the presence of CO and Hj, glucose, fructose, and xylitol have been transformed to yvalerolactone (49), in up to 40% yield, via levulinic acid (fcumed well known acid-catalysed dehydration and internal oxidation-reduction reactions)."... 

Abstract The concept of bifunctional acid catalysis is very helpful for inventing new catalytic asymmetric reactions. Compared with single functional acid catalysts, cooperative effect of two acid components has the potential to fine tune the reactivity as well as the selectivity of desired reaction pathways. This chapter focuses on some representative examples on the recent developments of bifunctional acid catalysis, including combined acid catalysis and other cooperative acid catalysis. 

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