Selective aerobic oxidation of benzyl alcohol on inexpensive and reusable ZnO/MnCO3 catalysts

HERNÁN DUARTE; PABLO JORGE LUGGREN; JUAN ZELIN; MARÍA EUGENIA SAD; VERÓNICA DIEZ; ISABEL DI CÓSIMO

CATALYSIS TODAY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2022 vol. 394 p. 178 - 189

0920-5861

Theselective synthesis of benzaldehyde (B) by oxidation of benzyl alcohol (BA) wasstudied at 373 K using ZnO-MnCO3 catalysts, O2 as oxidantand mild reaction conditions. An inexpensive commercial MnCO3 samplewas impregnated with aqueous solutions containing different concentration of Zn2+cations to obtain a series of ZnO-MnCO3 catalysts with 1.0-22.6 wt.%Zn. Catalysts were characterized by N2 physisorption, atomicabsorption spectrometry, X-ray diffraction and X-ray photoelectron spectroscopytechniques. Benzaldehyde was selectively formed during liquid phase oxidationof benzyl alcohol but the catalytic performance of the ZnO-MnCO3materials depended on Zn loading.  Themaximum initial reaction rate value was measured on the most promising catalyst(15.4 wt.% Zn) and a  a maximum of 88%benzaldehyde yield was achieved on this catalysts after 6 h of reaction usingtoluene as solvent.     Thecatalytic activity of the ZnO-MnCO3 materials could be assigned to asynergistic contribution of Zn and Mn species. While the main role ofZnO would be related to its capacity to chemisorb the alcohol, the role ofhighly oxidized Mn surface species (mainly Mn4+) generated duringcatalyst preparation, is to participate in de redox mechanism involved inbenzyl alcohol oxidation toward aldehyde.     The effect of chemical nature of polar andapolar aprotic solvents was investigated during the aerobic oxidation over15.4ZnMn. Apolar solvents, in which the oxygen solubility is higher than in thebenzyl alcohol, are more suitable for the reaction. An optimum in solventbasicity requirements (hydrogen-bond-acceptance) in order to optimize thecatalyst activity wasfound.            The regeneration and reuse of themost promising catalyst were also investigated. A mild thermal treatment at493K in flowing air was enough to fully recover the activity of the 15.4ZnO-MnCO3catalyst.