Mechanism and Kinetics of Ethanol–Acetaldehyde Conversion to 1,3-Butadiene over Isolated Lewis Acid La Sites in Silanol Nests in Dealuminated Beta Zeolite

Biomass-derived ethanol (EtOH) and acetaldehyde (AcH) conversion to 1,3-butadiene (1,3-BD) is an alternative process for 1,3-BD production. The present investigation reports the preparation and characterization of isolated La sites introduced into the silanol nests in DeAlBEA as well as detailed studies of the mechanism and kinetics for the conversion of an EtOH–AcH mixture to 1,3-BD. La sites supported on DeAlBEA are found to be present as (≡SiO)2La–OH groups that are H-bonded with adjacent Si–OH groups, possessing high C–C coupling activity and stability, superior to state-of-the-art Y-DeAlBEA. La sites supported on silica (La-SiO2) with a similar chemical structure but no H-bonding interaction with Si–OH groups were prepared for comparison. Lewis acid La sites promote AcH aldol condensation, and the activity of such sites is nearly identical for both La-DeAlBEA and La-SiO2. The rate of C4 product formation increases by a factor of 4.8 upon addition of EtOH to the feed of AcH over La-DeAlBEA, whereas that over La/SiO2 remains unchanged. Investigation of the mechanism and kinetics of AcH aldol condensation and EtOH–AcH conversion to 1,3-BD revealed two C–C bond forming pathways─AcH aldol condensation by Lewis acid La sites and direct coupling of EtOH–AcH over H-bonded (≡SiO)2La–OH···HO–Si≡ sites. This study provides important information about the role of the local environment of isolated Lewis acid sites and their effects on the direct coupling of EtOH and AcH to form 1,3-BD.