The Role of the Oxygen Vacancies in the Synthesis of 1, 3-Butadiene from Ethanol

The t-ZrO2 doped with Zn catalyst and t-ZrO2 as reference were employed in the butadiene synthesis from ethanol. Both catalysts were characterized by NH3-TPD, CO2-TPD, TPSR, the MPV model reaction, ICP, BET and EPR. Adding 0.2 wt% Zn to t-ZrO2, the selectivity to butadiene increases three fold whereas the one to ethylene decreases. When ZrO2 is doped, the number of basic sites increases and the number of acid sites decreases. The TPSR spectra indicate that the acetaldehyde generation is the rate limiting step of the butadiene synthesis. The slowest step of the acetaldehyde generation is the H abstraction by a strong basic site. The EPR spectra show the replacement of Zr4+ by Zn2+ in the lattice of the t-ZrO2 oxide(.) This phenomenon forms pairs of oxygen vacancies and coordinatively unsaturated Zr4+ ions (cus), which are strong basic sites and acid sites, respectively. Doping ZrO2 with Zn, the ethanol dehydrogenation and the butadiene synthesis are promoted not only due to the changes in the acidity and basicity of the catalyst but mainly because of the generation of oxygen vacancies and cus pairs during the reaction. These oxygen vacancies seem to behave as strong Bronsted basic sites.