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

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 metal sites and Lewis acid–oxygen vacancy pairs (Zr₃–□) of Zr nodes in the pores of UiO-66 are the active sites, whereas Cu metal sites 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 between internal Cu metal sites 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 Cu@UiO-66 catalysts among the best state-of-the-art Cu-based catalysts thus giving them great potential applications in industry.