The Haag–Dessau mechanism of protolytic cracking of alkanes

Abstract

Haag and Dessau's 1984 mechanism of protolytic alkane cracking is a landmark because it links petroleum refining chemistry to Olah's hydrocarbon chemistry in superacids. According to this now widely accepted mechanism, at about 800 K an acidic catalyst such as a zeolite protonates an alkane to give carbonium ion transition states that collapse to give alkanes (or dihydrogen) and carbenium ions, which give back protons to the catalyst to form alkenes. The cracking products include dihydrogen, methane, and ethane (as observed in large-scale petroleum cracking), in contrast to those of classical catalytic cracking. The Haag–Dessau mechanism is favored by medium-pore zeolite catalysts that allow the monomolecular reaction while restricting the bimolecular (hydride transfer) reaction of classical cracking because of the steric limitations in the pores; protolytic cracking is kinetically significant only when alkene concentrations are low, because alkenes are much better proton acceptors than alkanes, and their protonation leads to classical cracking. The Haag–Dessau mechanism is the key to unraveling the competing mechanisms of catalytic cracking (including classical cracking and oligomerization cracking) and to successful quantitative representation of these simultaneous reactions — one of the most successful examples of microkinetics analysis. Understanding of protolytic cracking has helped in the diagnosis of shape selectivity and mass transfer effects in zeolite-catalyzed cracking. Furthermore, protolytic cracking is one of the surface-catalyzed reactions most susceptible to theoretical modeling with density functional methods; the theory shows that the assumption of a carbonium ion transition state is simplified and that bonding of the transition state to the catalyst surface is significant.