Computational
Study of <i>p</i>‑Xylene
Synthesis from Ethylene and 2,5-Dimethylfuran Catalyzed by H‑BEA
- Publication date
- Publisher
Abstract
Detailed mechanisms for the synthesis
of <i>p</i>-xylene
as well as the primary byproducts observed experimentally, 2,5-hexadione
and 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan,
from ethylene and 2,5-dimethylfuran (DMF) mediated by H-BEA are obtained
using an extended QM/MM model containing 208 tetrahedral atoms. The
formation of <i>p</i>-xylene proceeds via Diels–Alder
cycloaddition of ethylene and DMF, which is rate-limiting, followed
by Brønsted acid-catalyzed dehydration. Secondary addition of
DMF to the substrate following the Diels–Alder reaction leads
to 2,5-dimethyl-3-[(4-methyl-1,3-cyclohexadien-1-yl)methyl]furan.
The analysis of the free energies associated with the mechanisms suggests
that the secondary addition can be eliminated by introducing <i>n</i>-heptane as an inert solvent to decrease the loading of
DMF in the zeolite or by using a weak Brønsted acid site to facilitate
the dehydration of the Diels–Alder product, for which the rate
is determined by the deprotonation via the conjugate base of the active
site. Water formed in the dehydration process can react directly with
DMF to form 2,5-hexadione, thereby decreasing the yield of <i>p</i>-xylene. However, the free-energy barriers for the formation
of 2,5-heaxdione compared to the Diels–Alder reaction indicate
that DMF and 2,5-hexadione will be equilibrated. Therefore, the 2,5-hexadione
yield can be minimized by operating at a high conversion of DMF