Ethanol Conversion to Butadiene over Isolated Zinc and Yttrium Sites Grafted onto Dealuminated Beta Zeolite

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.