Rate-Enhancing Role of Water in H‑BEA and Sn-BEA for Keto–Enol Tautomerization of Acetone: A DFT Study

Aldol
condensation of furfural with acetone is an important biofuel
upgrading reaction. Herein, we investigated the rate-controlling step
of aldol condensation, i.e., the keto–enol tautomerization
of acetone, in Sn-doped BEA (Sn-BEA) and hydrogen-compensated Al-doped
BEA (H-BEA) zeolites. We also investigated the rate-enhancing role
of molecular water at 373 K. In dehydrated H-BEA, we find that the
reaction takes place concertedly with a free-energy barrier, which
is ∼160 kJ/mol lower than the uncatalyzed gas-phase reaction.
The barrier is further reduced in the presence of water, which provides
an alternate path for proton transfer. Further, we find that defect-free
Sn-BEA under dehydrated conditions is catalytically inactive for the
tautomerization of acetone. In the presence of H2O, we
modeled two different active sites: monohydrated closed Sn-site and
an open Sn-site formed by partial hydrolysis of a closed Sn-site.
We find that a monohydrated closed Sn-site is thermodynamically more
stable than an open Sn-site. Our calculations suggest that both configurations
have comparable activity toward keto–enol tautomerization of
acetone and are much higher than a dehydrated closed Sn-site. Overall,
our calculations suggest that the activity of BEA zeolite is enhanced
in the presence of low concentrations of water, which acts as a cocatalyst.
This is true for both Sn-doped and Al-doped BEA, and the effect is
more dramatic in the Sn-doped BEA.