Catalytic Hydrocracking—Mechanisms and Versatility of the Process

Abstract

Hydrocracking of saturated hydrocarbons can proceed by means of four distinctly different mechanisms. On bifunctional catalysts comprising hydrogenation/dehydrogenation and Brønsted 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 Catalyst Selectivity Index (CSI): A Framework and Metric to Assess the Impact of Catalyst Efficiency Enhancements upon Energy and CO2 Footprints

“…To avoid significant carbon deposition and present more desirable products from the feed, the process of hydrocracking (HCR) is now becoming an attractive alternative to FCC as the modern refinery conversion process choice [53][54][55][56][57][58]. This also stems from the fact that ''dieselization'' is rapidly developing in many parts of the world, and HCR processing makes it relatively easy to manipulate the socalled product slate to meet rapidly changing market demands.…”

“…Finally, the HCR process can be significantly improved with an increase in the hydrogen partial pressure, but, one notes of course, at the expense of significant hydrogen consumption in this process, with the attendant, underlying large carbon footprint (since hydrogen is derived industrially from the Steam Methane Reforming) [53,54,[58][59][60]. The performance of hydrocracking units is fundamentally and critically dependent on the zeolite catalyst used to break down the heavier oil molecules, and hydrotreatment catalysts, which are in fact designed for the combination of the cracking and hydrotreatment dual functions.…”

Research of diesel fuels dewaxing process via mathematical model

“…1). Main production stages involve hydrotreating and dewaxing of the feedstock (mixture of atmospheric gas oil, straight-run diesel fractions, and visbreaking gasoline), stabilization of the product, and distillation with diesel fractions and stable naphtha production [3,4].…”

Effect of metal loading on activity, selectivity and deactivation behavior of Pd/silica–alumina catalysts in the hydroconversion of n-hexadecane

“…Catalysts comprised of a strong hydrogenating/ dehydrogenating function and a mild acid function were found to satisfy both the requirements of controlled hydrocracking and high isomerization activity [20,21]. The mechanism of alkane hydrocracking has been thoroughly studied [22][23][24][25][26][27][28]. The alkane is dehydrogenated on a metal site, producing the corresponding olefin.…”

“…when the dehydrogenation of the feed molecule and the hydrogenation of the olefinic intermediates are at quasi-equilibrium while the acid catalyzed isomerization and cracking reactions via carbenium ion chemistry are rate determining steps. This theory has been questioned, especially with regards to the importance of the "intimacy" of the two functions and to the role of the metal as the alkane activation function [26]. Alternative theories imply that the alkane can be activated on the acid sites and that the primary role of the metal is to hydrogenate olefinic intermediates to maintain a low steady-state concentration which avoids deactivation by coking [29][30][31].…”