Hydrocracking Hydrogenolysis

In petroleum refineries, hydrocracking processes are used to upgrade low value distillate feedstocks. The hydrocracking processes are classified into three groups according to purpose. The purposes and catalysts are as follows  

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. 

There is a report which stresses the importance of the basic properties of the support for M0O3 used for hydrodesulfurization. The number of effectively supported M0O3 molecules increases by increasing the surface basic OH groups, because molybdate anions exchange with the basic OH groups on the surface as shown below. 

The basic OH groups on AI2O3 increase on addition of MgO to AI2O3. The resulting catalyst shows higher activity for hydrodesulfurization of thiophene than MgO free catalyst. 

Mitchel, in Catalysis, Vol. 4, The Royal Soc. Chem., Burlington House, London, Chapter 7, 

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Hydrocracking. Hydrogenolysis of a carbon—carbon bond in an organic molecule produces smaller, hydrogen-saturated molecules and is termed hydrocracking when 50% or more of the feedstock is converted into smaller molecules. 

Other reactions that occur during hydrocracking are the fragmentation followed by hydrogenation (hydrogenolysis) of the complex asphaltenes and heterocyclic compounds normally present in the feeds. 

Starting from C5 molecules, dehydrocyclization (into cyclopentane and derivatives of cyclopentane) is also possible. From C6 on up, aromatization also occurs. These two reactions comprising a dehydrogenation step are only observable at temperatures which on most metals are higher than the region where hydrogenolysis (hydrocracking) is first observed. 

Most multipromoted catalysts have been described for the catalytic reforming of petroleum. For this process it is typical, that several reactions take place simultaneously dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes and dehydrocyclization of alkanes. Isomerization, hydrogenolysis, and hydrocracking are also involved in the process. 

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. 

Figure 5 shows that coke deactivates a HDN activity more than a HG activity. If acidic sites favor hydrogenolysis of a C-N bond like a hydrocracking reaction, loss of the acidic sites of an alumina support by coke deposition will decrease the HDN activity. Therefore, this result may support preferential deposition of coke on an alumina support. 

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