Insights into the Mechanism and Reactivity of Zeolite-Catalyzed Alkylphenol Dealkylation

In the stride toward the production of low-carbon-footprint
commodity
chemicals, the development of a complete wood biorefinery plays a
pivotal role. The lignin fraction of wood can be depolymerized and
demethoxylated mainly into 4-alkylphenols. These phenolic compounds
can further catalytically be C-dealkylated within the H-ZSM-5 zeolite
at relatively high temperatures and in the presence of steam, producing
phenol and olefins. Experimentally, the dealkylation reaction was
found to have two striking features: first, different reactants possess
very different reactivity. 4-Ethylphenol (4-EP) is somehow less reactive
than 4-n-propylphenol (4-n-PP),
which is in turn much less reactive than 4-isopropylphenol (4-iso-PP). Second, cofeeding of steam in the reaction mixture
was necessary to prevent rapid and reversible catalyst deactivation.
Herein, a combination of static and dynamic density functional theory
(DFT) simulations is used to unravel the molecular and mechanistic
origin of these observations. Free-energy profiles obtained from static
calculations confirm the experimentally observed reactivity sequence,
where our computations show that the secondary nature of the alkyl
carbon involved in 4-iso-PP dealkylation strongly
stabilizes the respective transition states. To investigate the effect
of water on the mobility of the reactive species and their interaction
with the active site, we investigated the diffusion of phenol along
the H-ZSM-5 straight channel in the presence of water loadings from
0 to 3 molecules per zeolite unit cell. We show that water has a strongly
beneficial effect in promoting desorption and diffusion of phenol
away from the Brønsted acid site through competitive adsorption
and by the formation of hydrogen bond chains with the diffusing phenol.
This effect could lead to a shorter residence time inside the zeolite,
preventing active site poisoning and condensation to bulkier biphenylether
moieties.