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

Abstract

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 H 2 O, 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.

Investigating Closed and Open Site Stability of Sn-, Ti-, Zr-, and Hf-Beta Zeolites: A Comprehensive Periodic DFT Study

“…The dissociative adsorption modes, either in the Syn configuration where both hydroxyl groups are located on the same half-space with respect to the Sn atom (Δ r E ads (H 2 O) = −41 kJ mol –1 ) or in the Anti configuration [both hydroxyl groups in opposite half-spaces with respect to the Sn atom, characterized by the Si–O–Sn–OH arrangement, with Δ r E ads (H 2 O) = −46 kJ mol –1 ], are significantly less stable. The fact that the Anti configuration is more stable than the Syn configuration is in line with previous results from the literature. ,,, …”