Zirconia Nanoparticles Uniformly Supported on Mesoporous Alumina–Carbon as a Highly Efficient Catalyst for the Direct Ethanol-to-Butadiene Reaction

The direct ethanol-to-butadiene (ETB) reaction is a complex tandem reaction, and the precise control of multifunctional active sites is the key to improving the production of butadiene. Considering the fact that the synergistic effect of Zr–Al dual active centers and the hydrophobicity of the carbon surface are beneficial for the direct ETB reaction, a multifunctional Zr/Al–C catalyst has been prepared via the initial wet impregnation using zirconium nitrate as the precursor and mesoporous alumina–carbon as the support. The obtained zirconia nanoparticles are uniformly dispersed on mesoporous alumina–carbon with a mean particle size of about 3.5 nm due to the confinement of the mesoporous channels and the strong interaction between alumina and zirconia. The 10Zr/Al–C catalyst with a zirconia loading of about 10 wt % exhibits much higher butadiene selectivity (58 vs 6.2%) and ethanol conversion (95 vs 70%) compared with mesoporous Al–C at 375 °C under a space velocity of 4.74 h–1, due to the formation of a zirconia–alumina solid solution. Moreover, the increase of Zr loading led to the increased medium strong acid active sites and the decreased strong acid active sites due to various oxygen vacancies, which affected the butadiene selectivity. It was found that an appropriate strong acid/medium strong acid ratio of about 0.43 and a strong base/total base ratio of about 0.17 were beneficial for the balance of each tandem step, which was necessary to achieve high butadiene selectivity. The recycling studies showed that the 10Zr/Al–C catalyst could maintain a high ethanol conversion of 89% and a butadiene selectivity of 50% after running for 120 h. The multifunctional Zr/Al–C catalyst provides an efficient method for the direct ethanol-to-butadiene reaction.