Hydrocracking catalysts reaction mechanisms

The pure compound rate constants were measured with 20-28 mesh catalyst particles and reflect intrinsic rates (—i.e., rates free from diffusion effects). Estimated pore diffusion thresholds are shown for 1/8-inch and 1/16-inch catalyst sizes. These curves show the approximate reaction rate constants above which pore diffusion effects may be observed for these two catalyst sizes. These thresholds were calculated using pore diffusion theory for first-order reactions (18). Effective diffusivities were estimated using the Wilke-Chang correlation (19) and applying a tortuosity of 4.0. The pure compound data were obtained by G. E. Langlois and co-workers in our laboratories. Product yields and suggested reaction mechanisms for hydrocracking many of these compounds have been published elsewhere (20-25). 

Activity of these catalysts depends on the balance between the hydrogenation and acidic functions. For example, it was found that HZSM-5 was effective for the hydrocracking of HDPE and plastic waste [24]. But the liquid product contained much less n-paraffins and a greater amounts of aromatics (34%) and naphthenes (21.7%) because of a lack of sufficient hydrogenation function. The reaction mechanism over HZSM-5 can be considered as follows ... 

Paraffin isomerization over dual function catalysts based on zeolite Y and mordenite has been reviewedand a reaction mechanism was proposed in which olefin-paraffin equilibrium is established and carbonium ions are formed from both paraffins and olefins. The isomerization of n-hexane and hydrocrack-... 

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. ... 

It should be noted that many practically important catalytic transformations (such as isomerization of or hydrocracking of paraffins), which are presumed to proceed via consecutive mechanisms, are performed on multifunctional catalysts, with which the coupling of reactions in the sense just discussed may not necessarily occur. The problem of the selectivity of some models of polystep reactions on these catalysts has been discussed in detail by Weisz (56). 

At 873K, under 2.0MPa and H2/Toluene=4mol/mol the toluene transformation over H-ZSM-5 was studied. The initial reactions rates of dealkylation, hydrocracking and condensation of toluene were correlated with the acidity of the zeolite catalysts. Schematic route for the transformation of toluene is presented and the mechanism of the particular reactions are discussed. 

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