Effects of Support on the Formation and Activity of Gold Catalysts for Ethanol Conversion to Butanol

Using a combination of physicochemical methods, such as TEM, SEM, EDS, XPS, NH3–TPD, and N2 adsorption, the study investigates the structure of a number of supports (Al2O3, SiO2, TiO2, ZrO2, and C) and of Au/support catalyst samples (Au = 0.5%). The concentration of highly active 2–4 nm gold particles in Au catalysts is influenced by the support’s texture; this concentration increases in the following order: Au/TiO2 < Au/ZrO2 < Au/C < Au/SiO2 << Au/Al2O3. The acidity of Au catalysts is influenced by the support’s nature; this acidity decreases in the following order: Al2O3 > TiO2 > ZrO2 > SiO2 >> Au/C. At 275°C, a carbon support is inactive in ethanol conversion to butanol. In the presence of oxide supports, the target reaction occurs at a relatively low rate by a bimolecular condensation mechanism. Over Au/Al2O3, Au/SiO2, Au/TiO2, or Au/ZrO2, the reaction occurs more rapidly by an aldol condensation mechanism. At an ethanol conversion of 14–18%, the butanol selectivity increases in the following order: Au/C(0) << Au/SiO2 (0.4%) < Au/ZrO2 (1.5%) < Au/TiO2 (2%) << Au/Al2O3 (78%). The high efficiency of Au/Al2O3 stems from the high density of the Aln+–O2– sites located on the support’s surface, and of the coordination-unsaturated Au0(KH) atoms located on the surface of 2–4 nm gold particles.