UC Berkeley

Zinc and Yttrium single sites were introduced into the silanol nests of dealuminated BEA zeolite to produce Zn-DeAlBEA and Y-DeAlBEA. These materials were then investigated for the conversion of ethanol to 1,3-butadiene. Zn-DeAlBEA was found to be highly active for ethanol dehydrogenation to acetaldehyde and exhibited low activity for 1,3-butadiene generation. By contrast, Y-DeAlBEA was highly active for 1,3-butadiene formation but exhibited no activity for ethanol dehydrogenation. The formation of 1,3-butadine over Y-DeAlBEA and Zn-DeAlBEA does not occur via aldol condensation of acetaldehyde but, rather, by concerted reaction of coadsorbed acetaldehyde and ethanol. The active centers for this process are ≡Si-O-Y(OH)-O-Si≡ or ≡Si-O-Zn-O-Si-O≡ groups closely associated with adjacent silanol groups. The active sites in Y-DeAlBEA are 70 times more active than the Y sites supported on silica, for which the Y site is similar to that in Y-SiO₂ but which lacks adjacent hydroxyl groups, and are 7 times more active than the active sites in Zn-DeAlBEA. We propose that C-C bond coupling in Y-DeAlBEA proceeds via the reaction of coadsorbed acetaldehyde and ethanol to form crotyl alcohol and water. The dehydration of crotyl alcohol to 1,3-butadiene is facile and occurs over the mildly Brønsted acidic ≡Si-OH groups present in the silanol nest of DeAlBEA. The catalysts reported here are notably more active than those previously reported for both the direct conversion of ethanol to 1,3-butadiene or the formation of this product by the reaction of ethanol and acetaldehyde.