Benefits of active site proximity in Cu@UiO-66 catalysts for efficient upgrading of ethanol to n-butanol

Jian Zhou,   Yaohui He,   Bing Xue,   Yunhui Cheng,   Danfeng Zhou,   Dong Wang,   Yajun He,   Weixin Guan,   Kegong Fang,   Lijun Zhang,   Jun Ni  and  Xiaonian Li 

https://pubs.rsc.org/en/content/articlelanding/2021/se/d1se01002f/unauth#!divAbstract

Abstract

The upgrading of ethanol to n-butanol converts abundant bioethanol to a better gasoline replacement, however, as a representative tandem reaction it requires the presence of multiple types of active sites in one catalyst, which imposes a great challenge on the synthesis of catalysts. Although a variety of heterogeneous catalysts have been explored in this reaction, the development of efficient catalysts remains sluggish due to the limited knowledge of active sites. Herein, we demonstrated that by using Cu@UiO-66 catalysts with distinctive structures, two kinds of sites were identified: internal Cu metals and Lewis acid-oxygen vacancy pair (Zr₃-□) of Zr nodes in the pores of UiO-66 are the active sites, whereas Cu metals and Zr nodes on the external surface of UiO-66 are less involved in the upgrading of ethanol to n-butanol. And the intimate contact of internal Cu metals and Zr₃-□ dictates the key step reactions, namely ethanol dehydrogenation, acetaldehyde condensation, and crotonaldehyde hydrogenation. Benefiting from this proximity, a high yield of n-butanol (22.2%) with negligible amounts of gaseous products could be obtained, which ranks the Cu@UiO-66 catalysts among the best of state-of-the-art Cu-based catalysts and thus has great potential applications in industry.