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.