Abstract
Alkane
dehydrogenation is an enabling route to make alkenes useful as chemical
intermediates. This study demonstrates the high reactivity of Lewis
acid–base (LAB) site pairs at ZrO2 powders for dehydrogenation of C2–C4 alkanes and the essential requirement for chemical treatments to remove strongly bound H2O and CO2
titrants to avoid the high temperatures required for their desorption
and the concomitant loss of active sites through sintering and annealing
of ZrO2 crystallites. The energies and free energies of
bound intermediates and transition states from density functional theory
(DFT), taken together with kinetic analysis and isotopic methods,
demonstrated the kinetic relevance and heterolytic character of the
first C–H activation at terminal C-atoms for all alkanes with a modest
activation barrier (84 kJ mol–1) at essentially bare Zr–O LAB
site pairs. β-Hydride elimination from the formed alkyl carbanions lead
to their desorption as alkene products in steps that are favored over
their parallel C–C cleavage reactions (by 100 kJ mol–1),
leading to high dehydrogenation selectivities (>98%) at the
temperatures required for practical yields in such endothermic
dehydrogenation reactions (700–900 K). The facile recombination of bound
proton-hydride pairs then completes a dehydrogenation turnover. These
findings provide compelling evidence for the remarkable reactivity and
selectivity of LAB sites on earth-abundant oxides and for the need to
uncover them through chemical treatments, which combine to give
gravimetric dehydrogenation rates that exceed those on the toxic (Cr) or
costly (Pt) catalysts used in practice.
N.R. Jaegers, V. Danghyan, J. Shangguan, C. Lizandara-Pueyo, P. Deshlahra, E. Iglesia, Heterolytic c–h activation routes in catalytic dehydrogenation of light alkanes on lewis acid–base pairs at zro2 surfaces, Journal of the American Chemical Society, 2024. DOI: 10.1021/jacs.4c07766.
