Catalysts, bifunctional reforming

This review is concerned with a discussion of the reactions of hydrocarbons over bifunctional catalysts, primarily from the viewpoint of mechanism and kinetics. Some discussion will also be given of the structure and properties of typical bifunctional reforming catalysts, since this is somewhat helpful in understanding how the catalyst functions in promoting the various reactions. In addition, at appropriate places in the article, the practical application of the principles of bifunctional catalysis in commercial reforming processes will be considered. 

Role of Individual Functions of Bifunctional Reforming Catalysts Effect of Catalyst Type and Hydrocarbon Structure on Product (M2)°... 

Hydrotreating. In order to successfully reform a naphtha with the conventional bifunctional reforming catalyst, heteroatoms must be reduced to a level such that they will not deactivate the reforming catalyst. Sulfur and nitrogen levels of well below 5 ppm seem to be required for conventional reforming catalysts while levels below 1 ppm appear necessary for the newer bimetallics. The assumption was made that, if sulfur and nitrogen levels could be reduced to specification, the chemical oxygen levels would follow. 

Figure S Reaction mechanism for MCP conversion and coke formation on bifunctional reforming catalysts.

The deactivation of bifunctional reforming catalysts is mainly due to the deposition of coke on the metal and the acid sites. Coking on the metallic function is responsible for the rapid initial deactivation which is also accompanied by changes in the selectivity of products formed on its surface. The acidic function deactivates more gradually with time as a result of the add site fouling by coke coverage. 

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. 

Pt-alumina is a profoundly subtle and possibly unique system for bifunctional reforming catalysis. The Pt crystallite sizes which gamma alumina can stabilise contain high proportions of sites upon which selective catalysis can take place in addition, the acidity of gamma alumina can be promoted and controlled for selective carbenium chemistry in-situ. Further enhancement of Pt selectivity has been achieved by the addition of a second element, notably Re, Ir and Sn. Both ensemble and electronic models have been put forward to explain the effects, and have been vigorously discussed in the open literature. Geometric effects tend to be preferred for Pt-Re and Pt-Sn, but inevitably in such multivariate systems, there is still room for debate. The commercial success of these catalyst systems depends on their ability... 

Scheme 14.1B. Pathways for the reaction of n-hexane on a bifunctional reforming catalyst.

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

Takanabe, K. Aika, K.-I. Inazu, K. T. B. Seshan, K. Lefferts, L., Steam reforming of acetic acid as a biomass derived oxygenate Bifunctional pathway for hydrogen formation over Pt/ZrOz catalysts. Journal of catalysis 2006,243(2), 263-269. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

As seen in Figure 2.2 and from the corresponding discussion, dehydrocyclization is a key reaction in forming aromatic compounds.307 A study comparing dehydrocyclization over mono- and bifiinctional catalysts at atmospheric pressure and high pressure representative of naphtha reforming conditions concludes that primary aromatic products at all pressures are formed by direct six-carbon ring formation.313 Over bifunctional catalysts the acid-catalyzed cyclization is more rapid... 

Some of the typical reforming reactions catalyzed by the bifunctional, metal-acidic oxide catalysts, along with a specific example of each, are listed below ... 

Early reported studies on the application of bifunctional catalysts to the foregoing reactions were those of Haensel and Donaldson (H2), Ciapetta and Hunter (C2, C4, C5), Heinemann and co-workers (H3), and Hettinger and co-workers (H7). These reactions form the heart of catalytic reforming and have been exploited commercially in a number of processes, including the following ... 

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