(175e) Ethanol Conversion to Butadiene over Isolated Zinc and Yttrium Sites in Dealuminated Beta Zeolite

Isolated Zn and Y sites were introduced into dealuminated BEA zeolite (DeAlBEA) and then investigated for the conversion of ethanol to butadiene. This reaction occurs in three steps – ethanol dehydrogenation to acetaldehyde, reaction of acetaldehyde with ethanol to form crotyl alcohol, and crotyl alcohol dehydration to butadiene. Zn-DeAlDEA and Y-DeAlBEA are particularly effective for one part of the overall reaction. Zn-DeAlBEA exhibits high activity for ethanol dehydrogenation and low activity for to acetaldehyde C-C bonds formation, whereas Y-DeAlBEA exhibits low activity for ethanol dehydrogenation and high activity for crotyl alcohol formation. Crotyl alcohol dehydration is catalyzed by the slightly Bronsted acidic Si-OH groups present in silanol nests of DeAlBEA. Investigations of the mechanism and kinetics of C-C bond formation over Zn-DeAlBEA and Y-DeAlBEA indicate that this process does not proceed via aldol condensation of acetaldehyde but, instead, involves the reaction of acetaldehyde with ethanol. It is also notable that the rate of C-C bond formation over single Y sites in Y-DeAlBEA is 80 time higher than that over isolated silica-supported Y sites. Based on these findings, we propose that the C-C bond coupling over both Zn and Y site present in DeAlBEA require the assistance of the adjacent hydroxyl group. The Si-O-Y(OH)-O-Si (Si-O-Zn-O-Si) together with the adjacent hydroxyl group constitute the active center for crotyl alcohol formation from ethanol and acetaldehyde. Very good EtOH-to-1,3-BD performance was obtained over both the 10Zn-15Y-DeABEA and 25Y-DeABEA catalysts for one-step and two-step processes. When the reaction was performed at 400 ℃, the 1,3-BD productivity is 3.6 g/g_cat/h with a C₄ olefin selectivity of 62%. Over 25Y-DeABEA catalyst at 400 ℃, it was also encouraging to find that the 1,3-BD productivity could reach 6.6 g/g_cat/h with a C₄ olefins selectivity of 77.2%.