Elsevier

Catalysis Communications

Volume 89, 10 January 2017, Pages 56-59
Catalysis Communications

Short communication
Liquid phase aerobic oxidation of benzyl alcohol by using manganese ferrite supported-manganese oxide nanocomposite catalyst

https://doi.org/10.1016/j.catcom.2016.10.027Get rights and content

Highlights

  • Magnetic core-MnOx shell nanocomposite catalyst for benzyl alcohol oxidation

  • Oxidation reaction with the air present inside the reactor

  • Moderate conversion to benzaldehyde with 100% selectivity

  • Synergistic effect of the magnetic core on catalytic activity

Abstract

This study illustrates for the first time the performance of nano-manganese ferrite supported-manganese oxide catalyst in benzyl alcohol (BzOH) oxidation without employing any oxidizing agent other than the air present in the reactor. The magnetic catalyst displayed moderate activity but 100% selectivity in conversion to benzaldehyde (BzH) under mild conditions. Compared to the other heterogeneous MnOx-based systems, the catalyst deserves attention in that an enhancement of the activity can be achieved by tuning the core-shell composition which plays a synergistic role in the catalytic reaction.

Introduction

The oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) is an industrially important reaction because of the wide applications of the product [1]. The reaction is also of environmental importance for the removal of substrate from industrial wastes. Liquid phase catalytic oxidation method is more common for this process than the gas phase oxidation whereas the control in selectivity is generally a problem for the latter. Both homogeneous and heterogeneous transition metal based catalysts have been developed for aerobic alcohol oxidations [2]. Among them, Mn(IV) [3] and Fe(II,III) oxides [4] are regarded as inexpensive and eco-friendly heterogeneous catalysts when compared with the other transition metal compounds.

Conventional manganese dioxide is known to oxidize BzOH to the corresponding aldehyde since 1950s [5]. Many research efforts have been devoted to the preparation of novel and efficient manganese oxide materials for catalytic benzyl alcohol oxidation reactions. Suib and co-workers reported that manganese containing octahedral molecular sieves with tunnel or layer structures selectively oxidize BzOH to BzH in the presence of oxygen [6], [7]. Nanolayered birnessite-type (δ-MnO2) manganese oxides intercalated with potassium ions [8], calcium ions [9] and doped with cobalt ions [10] were reported to be more efficient oxidation catalysts than activated MnO2. Promotion of nanostructured MnOx materials with Ce and Fe resulted in increased reactivity of the active MnOx phase but with no significant contribution to the functionality of the catalyst [11]. Yang et al. demonstrated that when confined in carbon nanotubes K-birnessite shows excellent selectivity (above 99%) towards BzH [12].

In recent years, magnetic nanocatalysts have received great attention in nanoscale heterogeneous catalysis with the advantage of practical recovery from the medium by application of an external magnetic field. They also offer higher surface areas, low-coordinated sites and surface vacancies compared to bulk heterogeneous systems. Free- (unsupported) [13], [14], [15], [16] and mesoporous silica-supported [17] nano-iron oxide systems have been shown to be active, stable and selective catalysts in benzyl alcohol oxidation in the presence of hydrogen peroxide. Recently, Saikia et al. prepared a magnetic nanocomposite catalyst by supporting magnetite on a MOF (Fe3O4@MIL-101-Cr) for the solvent free oxidation of BzOH in the presence of TBHP [18].

In this study, we present the catalytic activity of manganese ferrite (MnFe2O4, MF)-supported birnessite-type manganese oxide, a core-shell structured magnetic nanocomposite catalyst (MnO2@MF), combining the functions of Mn- and Fe-oxides [19], for the oxidation of benzyl alcohol in hexane with air present in the reaction system as the oxidant.

Section snippets

Experimental

The catalyst was prepared as previously described by Elmaci et al. [19], [20]. For details of materials, synthesis, characterization and catalytic reaction, see Supplementary data.

The catalytic oxidation was carried out in a batch type reactor placed in an oil bath and equipped with a reflux condenser and magnetic stirrer. The mixture was stirred at defined reaction temperatures and air inside the reactor was used as the oxidant. After each reaction period, the final mixture was centrifuged to

Characterization of the catalyst

Proposed chemical composition of the nanocomposite catalyst is 1.95{K0·1MnO0.23 H2O}@MnFe2O4 (Table S1, Supplementary data). We set the average Mn-oxidation state to + 3.7 because the XRD analysis (see Fig. S1 in Supplementary data) confirmed the formation of birnessite-type layered MnO2 with interlayer K+ ions [21], [22].

The morphology of the catalyst was examined by SEM and TEM imaging (see Fig. S2 in Supplementary data). The core-shell structure of the catalyst was clearly seen where MF

Conclusions

The catalyzed oxidation of benzyl alcohol was investigated in hexane solvent over a temperature range of 60–90 °C with the core-shell structured manganese ferrite supported-manganese oxide nanocomposite catalyst. Reaction time, temperature, and catalyst amount were identified as important parameters in the formation of the pure BzH. A BzH yield of 40% and selectivity of 100% were achieved in 4 h at 70 °C. The reaction proceeds with moderate conversion but with high selectivity even without using

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