Unraveling the Surface Reactions during Liquid-Phase Oxidation of Benzyl Alcohol on Pd/Al₂O₃:  an in Situ ATR−IR Study

Abstract

The palladium-catalyzed liquid-phase reaction of benzyl alcohol to benzaldehyde was investigated in the presence and absence of oxygen by attenuated total reflection infrared (ATR-IR) spectroscopy. The 5 wt % Pd/Al2O3 catalyst was fixed in a flow-through ATR-IR cell serving as a continuous-flow reactor. The reaction conditions (cyclohexane solvent, 323 K, 1 bar) were set in the range commonly applied in the heterogeneous catalytic aerobic oxidation of alcohols. The in situ ATR-IR study of the solid-liquid interface revealed a complex reaction network, including dehydrogenation of benzyl alcohol to benzaldehyde, decarbonylation of benzaldehyde, oxidation of hydrogen and CO on Pd, and formation of benzoic acid catalyzed by both Pd and Al2O3. Continuous formation of CO and its oxidative removal by air resulted in significant steady-state CO coverage of Pd during oxidation of benzyl alcohol. Unexpectedly, benzoic acid formed already in the early stage of the reaction and adsorbed strongly (irreversibly) on the basic sites of Al2O3 and thus remained undetectable in the effluent. This observation questions the reliability of product distributions conventionally determined from the liquid phase. The occurrence of the hydrogenolysis of the C-O bond of benzyl alcohol and formation of toluene indicates that Pd was present in a reduced state (Pd0) even in the presence of oxygen, in agreement with the dehydrogenation mechanism of alcohol oxidation.

Selective Alcohol Dehydrogenation and Hydrogenolysis with Semiconductor-Metal Photocatalysts: Toward Solar-to-Chemical Energy Conversion of Biomass-Relevant Substrates

“…31 Interestingly, 1 H NMR and GC-MS also showed formation of toluene, PhCH 3 , as a byproduct. 32,33 Relative PhCHO, H 2 , and PhCH 3 amounts produced are strongly affected by the structure and composition of the SC-M photocatalyst (Table 1, Figure 1). After 2 days under sunlight, the main product was H 2 when CdS-Pt was used and toluene when CdS-Pd was used (entries 1−2, Table 1, Figure 2).…”

Recent Trends in Operando and In Situ Characterization: Techniques for Rational Design of Catalysts

“…This mechanism for the transformation of benzyl alcohol to benzaldehyde has been proposed on the basis of many kinetic, electrochemical, and other studies [89,90]. However, the mechanisms for other consecutive and side reactions, which lead to the formation of undesired by-products such as benzene, toluene, benzoic acid, and benzylbenzoate, have been a matter of debate for a long time [91][92][93][94]. It is crucial to understand these mechanisms to fine-tune or develop new catalytic systems that do not promote these reactions.…”

Oxidation of Alcohols with Molecular Oxygen

“…Additionally, oxygen is necessary for the oxidative removal of strongly adsorbed species originating from various side reactions [93,94]. An important side reaction is decarbonylation of aldehydes [95][96][97]: (4) and the products CO ( Fig. 1) and hydrocarbon fragment adsorb strongly on the Pt-group metal.…”

“…In-situ ATR-IR spectra recorded during oxidation of benzyl alcohol to benzaldehyde (and benzoic acid) on a 5 wt.% Pd/Al 2 O 3 powder catalyst (air, 10 5 Pa, 50 • C, in cyclohexane)[96].…”