Hydrocracking of saturated hydrocarbons can proceed by means of four distinctly different mechanisms. On bifunctional catalysts comprising hydrogenation/dehydrogenation and Bronsted acid sites alkenes and carbocations occur as intermediates. The current mechanistic views of bifunctional hydrocracking of long-chain n-alkanes are discussed in detail with emphasis on the now widely accepted concept of ideal hydrocracking. Other mechanisms are hydrogenolysis and Haag–Dessau hydrocracking which proceed, respectively, on monofunctional metallic and acidic catalysts. Even without a catalyst, thermal hydrocracking occurs in chain reactions via radicals. The chemistry of hydrocracking naphthenes on bifunctional catalysts resembles that of alkanes. A peculiarity, however, is the pronounced reluctance of cyclic carbenium ions to undergo endocyclic β-scissions. The effect manifests itself in the so-called paring reaction, which, in turn, forms the basis for measuring the Spaciousness Index for characterizing the effective pore width of zeolitic catalysts. Hydrocracking on bifunctional catalysts is among the very important processes in modern petroleum refining. It is primarily used for converting heavy oils into diesel and jet fuel. Besides, hydrocracking is appreciated for its pronounced versatility: numerous process variants exist which help to meet specific requirements in refineries or petrochemical plants. Two recent developments are briefly discussed in this review, viz. the conversion of surplus aromatics, e.g., in pyrolysis gasoline, into a synthetic feedstock for steam crackers, and quality enhancement of diesel fuel by selective ring opening of polynuclear aromatics.

Catalytic Hydrocracking—Mechanisms and Versatility of the Process

Current views on the mechanism of catalytic cracking

This review paper presents the significant advances which were made in the last decade in the understanding of the transformation over acid and bifunctional zeolite catalysts of the cheap and readily available C2–C4 alkanes into more valuable products: mechanism of activation, reaction scheme, nature of the active sites. Both the transformations of pure alkanes: n-butane isomerization, C2–C4 alkane aromatization and of alkanes in mixture with alkenes: isobutane–butene alkylation or with aromatic hydrocarbons: benzene alkylation with ethane or propane are considered.

Activation of C2–C4 alkanes over acid and bifunctional zeolite catalysts

The activity and activation energy for cracking of n-hexane were measured over three zeolites: HZSM-5, H-MOR, H-USY (ultrastable Y zeolite), and CDHY (AHFS dealuminated Y zeolite), under conditions where the product selectivities were nearly identical and there was no deactivation of the catalysts. The identical selectivities implied that the same reaction mechanism for cracking was occurring over all three catalysts. Within experimental error, the differences in apparent activation energies could be entirely attributed to differences in heats of n-hexane adsorption, such that the intrinsic activation energies were identical. These results suggest that the kinetics of cracking is insensitive to any possible differences in acid strength among these catalysts, and the large enhancement in activity at lower temperatures observed upon steam dealumination of Y zeolite is due to effects other than changes in acid strength.

Monomolecular cracking of n-hexane on Y, MOR, and ZSM-5 zeolites

The review discusses chemical mechanisms of most important reactions that take place in the course of catalytic cracking of alkenes and alkanes over solid acidic catalysts. The main subject of the review is the mechanism of the principal cracking step, fission of C─C bonds in aliphatic hydrocarbons. The cracking mechanism of alkenes via carbenium ion intermediates (with complications caused by oligomerization reactions) is well established. In contrast, the C─C bond fission mechanism in alkanes is still a controversial subject. The review compares merits and difficulties of different mechanisms proposed in the literature: several carbenium-ion mechanisms, the carbonium-ion mechanism, the chain-reactions mechanism with participation of both carbonium and carbenium ions, and the oxonium-ion mechanism. Reactions accompanying C─C-bond fission in cracking reactions, hydrogen redistribution, formation of cycloalkanes, light aromatic compounds, and coke are also briefly discussed.

Chemical mechanisms of catalytic cracking over solid acidic catalysts: alkanes and alkenes

Relation between properties and performance of zeolites in paraffin cracking

An overview is presented of computer simulations applied to zeolite catalysis research. It is shown that using (numerical) statistical methods one is able to explain features observed in Ni2+ ion exchange. Lattice energy minimisation calculations confirm the validity of the model used. Furthermore, Monte Carlo and molecular dynamics simulations offer opportunities to describe adsorption and mass transport phenomena at a molecular level which is of importance in studies on shape selective catalysis.

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Computer Simulations in Zeolite Catalysis Research

Characterisation of spent fluid catalytic cracking catalysts by nuclear microprobe techniques.

Ion exchange of Ni 2+ and K + ions in siliceous ZSM-5 with widely varying aluminium contents (20 Si/Al 250) has been studied. Whereas regardless of the aluminium content near-stoichiometric exchange can be achieved with the monovalent K + ions, only limited exchange is observed with the divalent Ni 2+ ions. With decreasing aluminium content the degree of Ni 2+ exchange drops markedly. These results are interpreted using a model which relates the position of the extra framework Ni 2+ ions to a specific configuration of the aluminium ions with regard to each other in the lattice.

A.F.H. Wielers

Papers

Relation between properties and performance of zeolites in paraffin cracking

Computer Simulations in Zeolite Catalysis Research

Characterisation of spent fluid catalytic cracking catalysts by nuclear microprobe techniques.

ION-Exchange of Ni2+ IONS in Siliceous ZSM-5