Zeolites bifunctional catalysis

Under the operating conditions, the reaction intermediates (w-hexenes and i-hexenes in n-hexane isomerization) are thermodynamically very adverse, hence appear only as traces in the products. These intermediates (which are generally olefinic) are highly reactive in acid catalysis, which explains that the rates of bifunctional catalysis transformations are relatively high. The activity, stability, and selectivity of bifunctional zeolite catalysts depend mainly on three parameters the zeolite pore structure, the balance between hydrogenating and acid functions, and their intimacy. In most of the commercial processes, the balance is in favor of the hydrogenation function, that is, the transformations are limited by the acid function. 

The chain length of n-alkanes has a marked influence on reactivities for hydroisomerization, and especially for hydrocracking. To a very small extent a methane and ethane abstracting mechanism, probably hydrogenolysis as predicted in a basic work on bifunctional catalysis (14), is found to be superimposed when lower carbon number feeds (C, Cg, Cg) are used. n-Hexane is excluded from ideal hydrocracking. On the Pt/Ca-Y-zeolite catalyst it is cracked via a mechanism that is mainly hydrogeno-lytic. 

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. 

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). 

Metal incorporation into the zeolite using metal loaded seed materials. The combination of catalyst metal with zeolite catalyst is one of the most intriguing subjects for bifunctional catalysis. The achievement of prominent effect of the seed crystals on the crystallization of ZSM-34 type catalyst induced an idea that the seed material on which a catalyst metal had been supported previously would also be effective for rapid crystallization. 

Each zeolite type can be easily obtained over a wide range of compositions directly by synthesis and/or after various post synthesis treatments. Moreover, various compounds can be introduced or even synthesized within the zeolite pores (ship in a bottle synthesis). This explains why zeolites can be used as acid, base, acid-base, redox and bifunctional catalysts, most of the applications being however in acid and in bifunctional catalysis. 

The incorporation of a second phase in a zeolite catalyst, inside or outside the zeolite lattice, introduces new catalytic properties and the possibility of bifunctional catalysis. The subject is well covered in recent reviews. ... 

The active sites within the zeolite can be either the intrinsic acid sites or others introduced by ion exchange, etc. The function added most often is hydrogenation-dehydrogenation, which then allows bifunctional catalysis, or alternatively hydrogenation/dehydrogenation alone if the acid site is neutralized with base. 

Isomorphous substitution of T element in a molecular sieve material is very interesting in order to modify its acidic or redox catalytic and shape selective properties. Different ways to perform such a substitution are now well established either during synthesis or post synthesis in( luding solid-solid reaction between the zeolite and another oxide. The substituted eliiment may be strongly or weakly bound to the framework i.e. may remain stable or may give rise to well dispersed metallic oxide particles entrapped in the cavities. This results in different catalytic properties and may even lead to bifunctional catalysis as for Ga-ZSM-5 material. 

Zeolites 300-600 refinery processing, bifunctional catalysis, oiganic syntheses... 

In the present study the catalytic site is introduced on the support at the interface between zeolite and support. The diffusion pathway through the framework to the catalytic site on the support must be minimized, while the channels must be oriented in the direction of the support. Advantages offered by these composites are i) combination of a catalytic site which can not be synthesized or stabilized in the zeolite lattice with framework shape selectivity and ii) bifunctional catalysis by addition of framework activity. A schematical drawing of the composite system design is given in Figure 1. 

Zeolites are widely applied in the processing of oil Acid-catalyzed reactions of hydrocarbons proceed through carbenium ion intermediates. Bifunctional catalysis combines the catalytic properties of metal particles and of zeolites. 

A well-known cooperation effect is due to bifunctional catalysis in catalytic reforming, both Pt and alumina carry out part of the catalytic work. Several other types of cooperation have been mentioned in the last years, e.g. between catalysts within zeolitic structures and another component (1-3) or between separate oxide phases in oxidation or related reactions (4-12). Explanations for these cooperations often rest on spill over processes (13-15). 

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. 

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. 

If the epoxide rearrangement (see chapter 15.2.1) of styrene oxide is carried out in the presence of hydrogen and by use of a bifunctional boron-pentasil zeolite catalyst having a hydrogenation component such as Cu, then 2-phenylethanol is obtained in one step. This hydro-isomerization renders high yields (> 85%) at 250 °C under the gas phase conditions. It is an example for multifunctional catalysis in a one pot-reaction, that means simultaneous rearrangement and hydrogenation. 

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