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

Abstract

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 Brønsted basic sites.

Ethanol-to-butadiene: the reaction and its catalysts

“…The addition of zinc oxide successfully suppressed ethanol dehydration-a phenomenon observed in many catalytic systems for the ethanol-tobutadiene reaction. 127,197,247 Still, the butadiene yield remained below 20%. Only with the addition of ZrO 2 could a yield of 52.5% be achieved, marked by a sharp consumption of acetaldehyde compared to other samples.…”

“…Recent findings concerning Znmodified ZrO 2 catalysts used in the Lebedev process lead to the same conclusion, that zinc-dopants also introduce oxygen vacancies, and that the latter act as the basic sites needed for acetaldehyde formation. 247 The addition of ZnO or hemimorphite has also been shown to introduce new Lewis acid sites in catalysts. 109,125,227,230,248 Some authors have attributed the higher selectivity towards dehydration products compared to Ag or Cu to these acid properties.…”

CO2hydrogenation to methanol and hydrocarbons over bifunctional Zn-doped ZrO2/zeolite catalysts

“…Zn 2+ ) induces the formation of oxygen vacancies (V O ) and, as a direct consequence, generates defects featured by coordinatively unsaturated Zr 4+ sites (cus-Zr 4+ ) which can act as strong basic and acid sites respectively. 34 Carbon mono-and di-oxide activation was reported to be facilitated by the presence of neighbouring cus-Zr 4+ ion sites and V O . 25,[35][36][37] Thus, V O enhance the Brønsted acidity of Zr-OH groups adjacent to cus-Zr 4+ cations.…”

Recent Advances in Catalysts for the Conversion of Ethanol to Butadiene

“…A comparison of acidic ZrO 2 /SiO 2 and basic MgO/SiO 2 catalysts suggested that the strong basic sites in MgO/SiO 2 are more reactive in condensation reactions but suffer fast deactivation, in contrast to the acidic sites of both ZrO 2 /SiO 2 and MgO/SiO 2 . [81] Table 3 summarizes recent ETB acidic catalytic systems with SiO 2 [82][83][84][85][86][87][88][89][90][91][92][93] and ZrO 2 [94][95][96] as support. Some of these systems are used for the ethanol/acetaldehyde to butadiene conversion (step 2 of the two-step process).…”

“…Chagas et al added 0.2 wt% Zn to tetragonal ZrO 2 which increased the number of basic sites whereas the number of acid sites decreased. [93] The substitution of Zn 2 + for Zr 4 + in ZrO 2 forms oxygen vacancies as well as coordinatively unsaturated (cus) Zr 4 + ions. The oxygen vacancies are strong Brønsted basic sites while cus Zr 4 + are Lewis acid sites.…”