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 similar to 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.