Catalytic and Thermal Cracking of Pure Hydrocarbons: Mechanisms of Reaction

Abstract

T h e primary cracking of pure hydrocarbons both with and without catalysts has been studied in terms of the distribution by carbon number of the cracked fragments to allow arriving a t a mechanism of molecular disintegration. The secondary reactions of the cracked fragments have been followed by analyses of the product fractions to allow a further definition of the nature of the cracking system. On the basis of this work, cracking systems are assigned to two fundamental classes; each class is described by a set of characteristic reactions covering both the primary cracking and the secendary reactions. Correspondingly, two types of reaction mechanisms are proposed, one a free radical (thermal type) mechanism based on the Rice-Kossialroff theory of cracking, the other a carbonium ion (acid-activated type) mechanism RIOR work on the catalytic cracking of pure hydrocarbons P has led to a general characterization of the rates of cracking and product distributions of the principal classes of petroleum hydrocarbons (10-13). I n addition, a number of secondary reactions of olefins have been investigated and the effects of structural isomerism on the rates of cracking of several types of hydrocarbons were examined (9, 54). Consistent mechanisms of reaction are now proposed, based on the primary hypothesis that any hydrocarbon reacting over this type of catalyst is transformed into a carbonium ion (33, which then cracks or undergoes secondary reactions according to definite rules. This hypothesis is directly coupled with the requirement that the acidic oxide type of cracking catalyst must make available reactive positive hydrogen ions (protons) capable of producing carbonium ions on contact with the hydrocarbon feed. A similar type of approach was proposed independently by Thomas (52).The properties of carbonium ions, which are postulated to represent the reactive form of the hydrocarbon in conventional catalytic cracking, also determine the mechanism of reaction and the type of product in many other acid-catalyzed hydrocarbon reactions, such a8 the isomerization, polymerization, parafKn alkylation, and hydrogen transfer reactions of olefins, the isomerization of paraffins, and the alkylation of aromatics. Funda-derived from the work of Whitmore and others on the properties of carbonium ion systems. Cracking catalysts are available for either type of reaction mechanism; those which accelerate free radical type reactions are nonacidic, and those which accelerate carbonium ion type reactions are acidic. Commercial acid-treated clay and synthetic silica-alumina cracking catalysts belong to the latter class. Activated carbon, a highly active, nonacidic catalyst, gives a unique product distribution which is explained as a quenched free radical type of cracking. Activated pure alumina has weakly acidic properties and produces moderate catalysis of both types of reaction mechanism, the primary cracking corresponding to a free radical mechanism and the secondary reactions of product olefins following a carbonium ion mechanism. me...

Transhydrogenation of propyne with butane over a vanadia/θ-alumina catalyst

“…5.8 % for butane. Such behaviour has been well known for over 60 years [14][15][16] as is the formation of higher molecular weight by-products leading to catalyst coking. Indeed with propyne feed there is a 78 % loss in catalytic activity in 15 min on stream ( Table 1).…”

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…472,476 Consequently, the acid sites on (nonshape selective) amorphous aluminosilicates do not release alkenes until they are hexane isomers or smaller. 538,539 Once long alkanes have been cracked that far, the resulting fragments have a high enough free energy of adsorption to be replaced by the longer, unprocessed alkanes and alkenes in the feed. 538,539 Figure 65 shows a representative product slate for the nonshape selective catalytic cracking of n-C 16 .…”

“…538,539 Once long alkanes have been cracked that far, the resulting fragments have a high enough free energy of adsorption to be replaced by the longer, unprocessed alkanes and alkenes in the feed. 538,539 Figure 65 shows a representative product slate for the nonshape selective catalytic cracking of n-C 16 . 539 It only has a single maximum.…”

“…538,539 Figure 65 shows a representative product slate for the nonshape selective catalytic cracking of n-C 16 . 539 It only has a single maximum. In marked contrast to this type of a product distribution with only a single maximum, 538 the catalytic cracking of long n-alkanes on ERI-type zeolites shows two (see Figure 66) or even four maxima (see Figure 67).…”

Novel Fast Pyrolysis/Catalytic Technology for the Production of Stable Upgraded Liquids

“…Primarily, there are two main types of mechanisms that govern acid catalyzed cracking of hydrocarbons [71,72]. They are namely bimolecular (classical) cracking which involves hydride transfer reactions [73] and monomolecular cracking which occur by formation of carbonium ions [74]. The monomolecular (non-classical) cracking proposed by Haag and Dessau [74] extensively occurs in medium pore shape selective zeolites (ZSM-5) [75][76][77] since bimolecular reaction intermediates cannot be formed in the narrow pores.…”