SELECTIVE OXIDATION OF ALCOHOLS BY SUPPORTED PALLADIUM NANOCATALYSTS

Abstract

Selective oxidation of alcohols is a crucial reaction in either petroleum refinery or biorefinery and biomass processing, because it represents an elegant class of atom-efficient molecular transformations for chemical valorization. This reaction can be catalyzed by a range of platinum group and other noble and/or transition metals. The alcohol oxidation products such as aldehyde, ketone, ester and acid products are valuable intermediates for the fine chemical, pharmaceutical and agrochemical sectors. In addition, alcohols such as benzyl alcohol (BnOH) derivatives are used to design catalytic systems for lignin depolymerization. Lignin is the most important renewable source of aromatics on the planet, and depolymerizing the lignin can result in valuable aromatic compounds currently produced from petroleum. In the present study, the catalytic activity of palladium supported on ceria nanorod and mesoporous silica for the aerobic oxidation of BnOH derivatives and 4-(benzyloxy)phenol is investigated. Different Pd/ceria catalysts were prepared, varying the oxidation state of palladium by changing the preparation method and palladium precursor. A selected number of samples were studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), nitrogen adsorption-desorption analysis, and X-ray photoelectron spectroscopy (XPS) techniques. Catalytic activity of palladium oxide supported on ceria nanorod (PdOx/CeO2-NR) for aerobic selective oxidation of BnOH to benzaldehyde (PhCHO) was evaluated. The ceria nanorod was synthesized hydrothermally and the Pd(NO3)2 was deposited by a wet impregnation method, followed by calcination to acquire PdOx/CeO2-NR. In addition, PdOx/CeO2-NR was reduced (PdOx/CeO2-NR-Red) by H2 using TPR equipment, and tested for BnOH oxidation to assess the effect of palladium oxidation state on the catalytic activity. Furthermore, the effects of solvent and temperature on the catalytic activity of aforementioned catalysts were studied. The PdOx/CeO2-NR showed remarkable activity in ethanol (EtOH): 93% BnOH conversion along with 96% PhCHO selectivity. A mechanistic hypothesis for BnOH oxidation with PdOx/CeO2-NR in EtOH is presented. Pd/ceria catalysts with different preparation methods or different palladium precursor were prepared and tested for selective BnOH oxidation. The results showed that for all the reduced samples, using toluene as solvent for BnOH oxidation resulted in higher conversions compared to utilizing EtOH. In addition, changing the palladium precursor enormously influenced the catalytic activity, and probably changed the mechanism of the reaction. Furthermore, solvent-free aerobic oxidation of BnOH derivatives was performed using palladium oxide supported on ceria nanorod (2 wt.% Pd), leading to 34% BnOH conversion, with 99% PhCHO selectivity. The solvent-free oxidation on BnOH derivatives showed that when electron-donating groups (EDG) were substituted on BnOH (e.g. 4-methoxybenzyl alcohol) the conversion increased, but decrease in selectivity occurred; while electron-withdrawing groups (EWG) eliminated the catalytic activity to zero conversion (e.g. for 4-nitrobenzyl alcohol). Pd-P alloy supported on functionalized mesoporous silica was tested for BnOH to compare the effect of support on the catalytic activity. This catalyst was almost inactive in EtOH, but active in toluene. Unlike other tested catalysts, utilizing supported Pd-P at temperatures above 100 °C led to decrease in selectivity of PhCHO.