Hydrocracking catalysts active sites

From the general inaccessibility of both the sodium and TMA ions, we postulate that most of the acidic sites generated by thermal treatment of the derived NH4+/TMA+ zeolite will also be inaccessible to reactant molecules. Likewise, catalytically active metals such as Pt and Pd introduced by ion exchange are expected to be located in or near these same inaccessible sites. This may explain the poor approach to equilibrium observed with the isomerization catalysts, and the poor hydrogenation activity of the hydrocracking catalyst indicated by excessive coking and catalyst decline, even in the presence of a massive 3.1 wt % palladium. 

Poulet et al (79) reported that the isomerization activity for transforming irans- into dv-pentadiene on MoP/Al catalysts decreases as a result of phosphorus addition. Gishti et al (59) noted that the activity for skeletal isomerization of cyclohexene into methylcyclopentenes on reduced MoP/Al was reduced as a result of phosphorus addition. Iwamoto and Grimblot (85) reported, however, that cyclohexene isomerization on sulfided MoP/Al sol-gel catalysts increases with increasing phosphorus content. Since the active sites for hydrocracking and isomerization reactions are predominantly associated with the catalyst acidity, the results should depend strongly on both the nature and surface properties of the catalysts and on the reactants being converted. 

Larson, Maclver, Tobin, and Flinn (35) studied the relationship between hydrogenation activity of supported platinum hydrocracking catalysts and catalyst acidity to determine optimum composition. They observed that with increasing platinum content a selective adsorption of platinum by acid sites causes a reduction in catalyst acidity. 

The first point in the list above does not mean that carriers, binders, or formulation aids must be nonreactive. Rather, there are appKcations in which a self-activity of these materials is desired. For instance, in hydrocracking and fluidized catalytic cracking, mesoporous matrices with acidic properties are employed to promote bottom cracking of heavy feed components that cannot penetrate the small pores of the embedded zeolite. It is important to find out whether the chemical nature of additives and materials used to shape active sites is beneficial or detrimental for the overall performance of the final catalyst body at the conditions of the catalyzed process. 

The ammonia inhibition reduces not only the cracking activity but also the iso-to-normal ratio in the product paraffins because of its partial neutralization of the acid sites on the hydrocracking catalyst. 

The dual mechanism for paraffin hydrocracking includes the steps shown in Figure 9. Dual means that catalyst has two kinds of active sites -acid-based and metal-based. Section 3.7 lists the acids and metals that are used to make these catalysts. 

Both CO and CO2 have similar effects on the Hydrocracking catalyst they are converted on the active sites of the catalyst in the presence of hydrogen to methane and water. This methanation of CO and CO2 competes with the normal hydrocarbon reactants for the catalyst. Therefore, if CO + CO2 is allowed to build up, higher eatalyst temperatures will be required. In an extreme case where a large quantity of CO or CO2 would be introduced to the Hydrocracker in a short period of time, it is theoretically possible that a temperature excursion would result since the methanation reaction is highly exothermic. 

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