Hydrocracking bifunctional

Figure 16.6 Schematic illustration of the elementary steps in bifunctional hydrocracking.

This paper thus describes the construction of various mechanistic models for paraffins ranging from Cig to Cgo. All the models incorporate mechanistic acidic chemistry and pathways-level metal chemistry for the prototypical bifunctional hydrocracking catalyst, which has both a metal function for hydrogenation/dehydrogenation and an acid function for isomerization and cracking. 

These mechanistic features were elucidated in detail in the 1960s. Based on the pioneering work of Mills et al. and Weisz ", a carbenium ion mechanism was proposed, similar to catalytic cracking plus additional hydrogenation and skeletal isomerization. More recent studies of paraffin hydrocracking over noble metal-loaded, zeolite based catalysts have concluded that the reaction mechanism is similar to that proposed earlier for amorphous, bifunctional hydrocracking catalysts. ... 

Table 3. P-scission mechanisms for carbenium ion conversion over bifunctional hydrocracking...

Hydrocracking is a bifunctional process requiring metal as well as acid sites. Saturated reactants are dehydrogenated (Ktjeh,p, o,) on the metal sites yielding dehydrogenated species... 

Thus, under HCK conditions, and under hydrodewaxing (HDW) as well, paraffins are hydroisomerized and hydrocracked by a bifunctional mechanism involving the metallic and the acid sites. This classical mechanism involves ... 

Bifunctional catalysis is one of the most important routes to green (more economical and more environmentally friendly) processes. Indeed, the deactivation of bifunctional catalysts by coking is much slower than that of monofunctional catalysts and their selectivity generally higher (e.g., hydrocracking compared to... 

P. A. (1997) Isomerization and hydrocracking of heptane over bimetallic bifunctional PtPd/H-beta and PtPd/USY zeolite catalysts. . Catal,... 

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. 

The paper deals with some new data concerning the state of the metal after reduction and the catalytic functions of zeolite catalysts containing nickel and platinum. By using the molecular sieve selectivity in the hydrogenation of mesitylene it has been proved that metal (platinum) is contained in the volume of the zeolite crystal. The temperature dependence of the formation of nickel crystals was investigated. The aluminosilicate structure and the zeolite composition influence mainly the formation of the metal surface which determines the catalytic activity. In the hydrocracking of cumene and disproportionation of toluene a bifunctional action of catalysts has been established. Hydrogen retarded the reaction. 

Literature on hydroisomerization of long chain alkanes > is very limited (2, 9,15,16) due to both analytical difficulties and the fact that hydrocracking predominates unless the bifunctional... 

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. 

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). 

In general, bifunctional catalysts are applied in hydrocarbon hydrogenation. In this regard, clinoptilolite [20,21] exchanged with Ni2+, and then thermally reduced, was used for the hydrogenation of ethylbenzene [21] and 1-hexene [24,25], A natural erionite ore exchanged with NH4 and Ni2+ and then calcined was tested for the hydrocracking of n-paraffin the catalyst was tested in a pilot plant and it was demonstrated that a catalytic life of more than 1 year is possible [22,23],... 

Hydrocracking. Conventionally, the term hydrocracking is used to describe a process wherein hydrogen and a distillate hydrocarbon are passed over a bifunctional catalyst which provides both a hydrogenation function and an acidic Evidence of acidic site... 

Bifunctional zeolite catalysts are used in various commercial processes light alkane hydroisomerization (chapter 7), hydrocracking (chapter 6),hydrodewaxing (chapter 8), light alkane aromatization and hydroisomerization of the C8 aromatic cut (chapter 9). The hydrogenation/dehydrogenation components included in zeolite catalysts can be very different and located in different positions ... 

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