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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.

The kinetics of the selective hydrogenation of pyrolysis gasoline (pygas) over commercial Pd/Al 2 O 3 catalyst particles were investigated using a stirred semi-batch reactor in the absence of transport limitations. The effects of reaction temperature and pressure on the conversion of styrene, cyclopentadiene, cyclopentene and 1-hexene were obtained over ranges of temperature (313-343 K) and total pressure (2-5 MPa). Competitive hydrogenation between monoolefins and diolefins was extensive, and the reaction rates of diolefins were much faster than those of the monoolefins. A Langmuir-Hinshelwood type model was proposed and successfully fitted to the experimental data. The kinetic and adsorption parameters were estimated by using the fourth-order Runge-Kutta method together with the Levenberg-Marquardt algorithm, which minimized the residual sum of squares between the experimental concentrations and the calculated values. The orders of the estimated activation energies and the adsorption parameters were consistent with the order of the reaction rates of monoolefins and diolefins.

A model is developed based on a two-stage hydrogenation of pyrolysis gasoline to obtain a C6–C8 cut suitable for extraction of aromatics. In order to model the hydrogenation reactors, suitable hydrodynamic and reaction submodels should be solved simultaneously. The first stage hydrogenation takes place in a trickle bed reactor. The reaction rates of different di-olefines as well as hydrodynamic parameters of the trickle bed (i.e., catalyst wetting efficiency, pressure drop, mass transfer coefficient and liquid hold-up) have been combined to derive the equations to model this reactor. The second stage hydrogenation takes place in a two compartment fixed bed reactor. Hydrogenation of olefines takes place in the first compartment while sulfur is eliminated from the flow in the second compartment. These reactions occur at relatively higher temperature and pressure compared to the first stage. The key component in this stage is considered to be cyclohexene, of which the hydrogenation was found to be the most difficult of the olefines present in the feed. The Langmuir-Hinshelwood kinetic expression was adopted for the hydrogenation of cyclohexene and its kinetic parameters were determined experimentally in a micro-reactor in the presence of the industrial catalyst. The model was solved for the whole process of hydrogenation, including hydro-desulfurization. The predictions of the model were compared with actual plant data from an industrial scale pyrolysis gasoline hydrogenation unit and satisfactory agreement was found between the model and plant data.

Mit dem Inkrafttreten der zweiten Stufe des Auto-Ol-Programms der EU im Jahr 2005 muss der Aromatengehalt des Benzins erheblich gesenkt werden, was zu einem Aromatenuberschuss fuhren konnte. Die Quellen fur aromatische Kohlenwasserstoffe und deren Verwendung in der Petrochemie werden beschrieben. Moglichkeiten zur Vermeidung eines Aromatenuberschusses werden aufgezeigt, wobei sowohl die Reduzierung der Aromatenproduktion als auch das Wachstumspotential der Folgeprodukte von BTX-Aromaten diskutiert werden. Sodann werden Verfahren zur Ringoffnung von Aromaten beschrieben. Naher eingegangen wird auf ein neues Verfahren zur katalytischen Umwandlung von Aromaten aus dem Pyrolysebenzin von Naphtha-Steamcrackern in einen synthetischen Steamcracker-Einsatz bestehend aus Ethan, Propan und n-Butan. Diese Umsetzung der Aromaten mit Wasserstoff kann einstufig an bifunktionellen Zeolithen oder zweistufig durch Vorhydrierung zu Cycloalkanen mit anschliesender hydrierender Ringoffnung an sauren Zeolithen erreicht werden. Aromatics: From Valuable Base Chemicals to Surplus Components? With the advent of the second stage of the European Auto Oil Programme in the year 2005, the aromatics content of gasoline has to be reduced significantly, which could lead to an oversupply of aromatics. The sources of aromatic hydrocarbons and their petrochemical use are described. Options for avoiding a surplus of aromatics are discussed, i.e., diminishing the production of aromatics and intensifying their conversion into valuable products. Finally, a novel catalytic process for hydrogenative ring opening of aromatics is introduced which allows the conversion of pyrolysis gasoline from naphtha steamcrackers into a high-quality synthetic steamcracker feedstock composed of ethane, propane, and n-butane. There are two process variants, namely the direct conversion of aromatics on bifunctional zeolites or a two-stage process comprising a conventional ring hydrogenation to cycloalkanes followed by ring opening of the latter on acidic zeolites.

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.

A basic process for production of benzene from petroleum, along with catalytic reforming, is processing of liquid pyrolysis products and toluene. The conversion of pyrolysis gasoline and toluene on an iron-chromium oxide catalyst in a medium of steam and hydrogen at atmospheric pressure was investigated. Catalytic conversion of the pyrolysis gasoline was carried out in a medium of steam in a gradientless spherical reactor made of Kh23N18T steel under the following conditions: temperature 750 to 840/sup 0/C; steam pyrolysis gasoline weight ratio 1:1; pyrolysis gasoline feed rate 1 g per g catalyst per hour; experiment time 1 hour; catalyst volume 8 cm/sup 3/. Hydrodealkylation of toluene was also studied with the goal of producing benzene. In contrast to the conversion of pyrolysis gasoline in a medium of steam, hydrodealkylation was accomplished in a medium of steam and hydrogen. The preliminary tests showed that higher selectivity for formation of benzene is achieved in the presence of hydrogen. 11 references, 4 tables.