Hydrogen Utilization in Chemical Processes - Hydrogen Generation, Storage, and Utilization - page 176

FIGURE 10.2 Classical mechanism of isomerization and hydrocracking of an alkane on a bifunctional
catalyst comprising metal sites for dehydrogenation/hydrogenation and cracking sites. Source : Adapted
with permission from Weitkamp [3].
FIGURE 10.3 A proposed mechanism of hydrogenolysis of chlorobenzene on nickel-chromium metal.
Z H and Z R indicate hydrogen absorption sites and reactant absorption sites, respectively [4].
The third example is Haag-Dessau hydrocracking, which is a nonclassical
monomolecular cracking via carbonium ions [3]. This process usually
happens on monofunctional acidic catalysts (such as acidic chabazite [5] and
zeolite [6] ). Figure 10.4 illustrates the mechanism of Haag-Dessau hydro-
cracking. The initial reaction involves the protonation of alkane molecules
(RH) to form carbonium ions ( RH 2 + ) on the surface of acidic catalyst. Car-
bonium ions then collapse to give short alkanes (R 1 H, usually methane and
ethane) and carbenium ions ( R 2 + ). Carbenium ions then give back protons
and desorb from catalyst to form alkenes, followed by hydrogenated by
hydrogen.