Selective conversion of aromatic alcohols in almond-scented aldehydes on inexpensive ZnO/MnCO3 catalysts

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

Congreso; 11th World congress of chemical engineering (WCCE11); 2023

Oxidation of alcohols to the corresponding carbonyl compound is a key reaction in organic synthesis for the synthesis of fine chemicals and intermediates. Aldehydes have been widely used as intermediates of synthesis of pharmaceuticals, vitamins, fragrances and agrochemicals. Especially, the oxidation of benzyl alcohol (BA) to the most valuable chemical benzaldehyde (B) has received increasing attention due to the large number of applications of this product.In this work, the synthesis of B from BA was studied at 373 K using Zn and Mn-based catalysts and O2 as oxidant. An inexpensive commercial MnCO3 sample was impregnated with aqueous solutions containing different concentration of Zn2+ to obtain a series of xZnMn solids with x = 1.0, 2.0, 4.3, 8.7, 15.4 and 22.6 wt.% Zn. Changes in structural phases and surface species of xZnMn solids were analyzed by XRD, XPS and through temperature-programmed decomposition experiments in flowing N2 (TPN2). XRD and TPN2 characterization showed that solids investigated consist of different crystalline phases containing Mn and/or Zn species. In these phases Mn oxidation state changes from 2+ toward higher values (3+ and 4+) as Zn content increases entailing the subsequent transformation of MnCO3 to ZnMn2O4 spinel and MnOx oxides. XPS analysis confirmed the transformation Mn2+ species into more oxidized Mn species as Zn loading increased.Benzaldehyde was selectively formed during liquid phase oxidation of BA but the catalytic performance of xZnMn materials depended on Zn loading (Fig. 1). The maximum initial reaction rate value was obtained on the most promising catalyst (15.4 wt.% Zn) and a maximum of 70% B yield was achieved on this catalyst. This final B yield was optimized toward 88% changing experimental conditions. Catalytic activity of xZnMn catalysts could be assigned to a synergistic contribution of Zn and Mn species. While the main role of ZnO would be related to its capacity to chemisorb the alcohol, the role of highly oxidized Mn surface species (Mn4+) is to participate in de redox mechanism involved in BA oxidation toward B.The effect of chemical nature of polar and apolar aprotic solvents was investigated during the aerobic oxidation over 15.4ZnMn. Apolar solvents resulted the most suitable for the reaction. The regeneration and reuse of catalysts were also studied. A mild thermal treatment at 493K in flowing air was enough to fully recover the activity of the most promising catalyst (15.4ZnMn).