Concluding Remarks | AIChE

Concluding Remarks

C-H
Bond Activation and Light Alkane Dehydrogenation Catalysis: Oxidative and
Non-Oxidative Routes and Equilibrium Shifting via Tandem H2
Oxidation Reactions

Enrique Iglesia

Department of Chemical Engineering

University of California at Berkeley

Berkeley, CA 94720

iglesia@berkeley.edu

This presentation will address
mechanistic details of the activation of C-H bonds in light alkanes in the
absence or presence of O2 co-reactants on catalytic surfaces of oxides,
suboxides, and metals and their ubiquitous consequences for reactivity and
selectivity in the synthesis of alkenes via dehydrogenation of alkane
feedstocks. In aerobic routes,
dehydrogenation involves kinetically-relevant C-H bond activation and the
abstraction of a H-atom from organic reactants by lattice O-atoms at surfaces
of stoichiometric oxides. The late
transition states in these steps consist of diradical pairs; their formation
free energies depend on the dissociation energy of the cleaved C-H bond and on
the strength of the O-H bond formed at the oxide surface, but also on the
interactions between the two radicals, which depend, in turn, on the extent of electron
delocalization within the organic moieties at the transition states. The consequent destabilization of delocalized
allylic radicals in alkenes leads to C-H bond activation rates much lower than
expected from their weak C-H bonds, thus leading to higher alkene yields than predicted
from bond energy considerations without considering di-radical
interactions. Non-oxidative
dehydrogenation routes require high temperatures to circumvent the
thermodynamic barriers removed by H2O formation in oxidative
dehydrogenation. Refractory oxide
structures that form stable sub-oxides, but which do not reduce to the
respective zero-valent metals or form stable carbides during dehydrogenation
reactions form alkenes with very high selectivity and remarkable catalyst
stability even in very severe reaction environments. These anaerobic reactions also involve C-H
activation kinetically-relevant steps and the subsequent desorption of H2,
but do not require the redox cycles that mediate oxidative dehydrogenation
routes. The selective scavenging of such H2 molecules by O2
brings together the high selectivity of non-oxidative routes with the
equilibrium-shifting features of oxidative dehydrogenations. In this work, high selectivities are achieved
in such systems by exploiting the very high reactivity of H2
(relative to alkanes and alkenes) in homogeneous and metal-catalyzed reactions
with O2 and also by encapsulating the active function for H2-O2
reactions within small-pore zeolites, which allow the unhindered diffusion of H2
and O2, but not of the alkane reactants or the primary alkene
products.