Synergistic effects on Cu, Zn and Al-based catalyst: tracking the change of active sites during glycerol dehydration

Abstract

The highly selective reaction concerning catalytic glycerol dehydration to acetol was studied using Zn, Al and Cu oxide catalysts. The diffractograms and Raman spectroscopy revealed the presence of Al₂O₃, ZnO, CuO, ZnAl₂O₄ and CuAl₂O₄ phases with crystallite nanometer size (8–22 nm). ^AlNMR profiles showed the octahedral, pentacoordinate and tetrahedral coordination of the Al species The redox properties obtained by TPR indicated that at 250 °C, due to SMSI effects, the copper phase is reduced and ZnO is more resistant to reduction while alumina is metastable. The N₂ adsorption/desorption isotherms exhibited the formation of materials in the micro-mesopore range with specific surface area between 90 and 224 m² g⁻¹. The SEM micrographs showed a sponge-like morphology with cavity sizes between 60 and 70 nm. The best catalytic performance occurred with average yield and selectivity to acetol of 26% and 97%, respectively. The catalyst was quite selective to acetol during reuse tests and was almost completely reactivated after regeneration. The ex-situ analyzes investigated the changes that occurred in the Cuⁿ⁺ sites during the reaction, which confirmed the sintering of the copper species by increasing the crystallite size from 25.3 to 36.3 nm. The simple computational theoretical study identified the most exposed sites in planes (hkl), supporting the proposed mechanism. Considering that they are little explored, a brief discussion on the mechanisms involved in the catalyst deactivation by coke was also proposed. Thus, the presence of Cu⁰ and Cu⁺ sites combined with Zn–Al species and their synergy enhances the high selectivity and yield to acetol, while unreduced Cu²⁺ has inferior catalytic performance.