(81f) C-H Bond Activation and Light Alkane Dehydrogenation Catalysis: Oxidative and Non-Oxidative Routes and Equilibrium Shifting Via Tandem H2 Oxidation Reactions

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.