Ethanol Conversion to Ethylene and Acetaldehyde over Rhodium(I) Exchanged Faujasite Zeolite. A QM/MM and Microkinetic Study

In a computational study, we examined
the conversion of ethanol
to ethylene or acetaldehyde over Rh­(I)-exchanged zeolite (faujasite)
using QM/MM DFT calculations and microkinetic modeling. To elucidate
how the composition of the active site affects the reactivity in this
model system, the dehydration and dehydrogenation reaction mechanisms
at the Rh center were modeled by varying the number n (n = 1–3) of preadsorbed ethanol molecules.
For a coverage of one ethanol, ethylene formation was determined to
proceed via a two-step mechanism, whereas concerted C­(β)-H and
C–O bond cleavage occurs for higher coverage at the metal center.
In contrast, the dehydrogenation mechanism of ethanol does not vary
with coverage. A crucial step in both transformations is the expulsion
of the products which was found to be more facile at higher ethanol
coverage, n = 2, 3. However, at n = 1, we calculated much lower barriers for the corresponding bond
scission steps. Microkinetic modeling, based on the QM/MM results,
revealed a strong temperature dependence of the activity of the catalytic
system. Little to no reactivity is predicted at lower temperatures,
whereas both types of reactions appear likely at temperatures above
500 K. Up to 700 K, the accumulated amount of acetaldehyde is higher
than that of ethylene. At even higher temperatures ethylene is predicted
as the preferred product.