Octahedral molecular sieves of the type K-OMS-2 with different particle sizes and morphologies: Impact on the catalytic properties in the aerobic partial oxidation of benzyl alcohol

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Abstract

Manganese-based octahedral molecular sieves of the type K-OMS-2 (cryptomelane structure) with different morphologies and specific surface areas in the range of 20–135 m2 g−1 were prepared via synproportionation of KMnO4 and Mn2+ salts, either in acidic aqueous suspension (reflux method) or by a solid-state reaction, and via oxidation of MnSO4 either by K2Cr2O7 or by molecular oxygen in aqueous solution. For the reflux method, the influence of K+ cations for the formation of the OMS-2 structure was proven. When KMnO4 is replaced by Ba(MnO4)2, the anion in the Mn2+ salt exerts a strong influence on the synthesis product. All materials were characterized by elemental analysis (ICP-OES), XRD, nitrogen sorption, scanning electron microscopy and TGA.

For the oxidation of benzyl alcohol with molecular oxygen in liquid toluene at 110 °C, the catalytic activity of the K-OMS-2 materials is directly correlated to their specific surface area ABET. Since ABET increases with decreasing average crystallite diameter, the catalytic conversion presumably occurs at the outer crystallite surface. These relations are independent of morphology or synthesis procedure. The K-OMS-2 materials are more active than crystalline manganese oxides (MnO, Mn2O3, Mn3O4, β-MnO2), but less active than amorphous MnO2. Regeneration of deactivated K-OMS-2 catalysts can be achieved by calcining at 300 °C in air, partly due to the reversible desorption of water.

Graphical abstract

Crystalline, manganese-oxide-based octahedral molecular sieves of the type K-OMS-2 with different particle size and morphology were studied as catalysts in the selective liquid-phase oxidation of benzyl alcohol. A systematic comparison of these materials with both crystalline and amorphous manganese oxides proofs the superior activity of the K-OMS-2 catalysts. The catalytic conversion is shown to occur predominantly on the outer surface of the crystallites.

Introduction

Porous manganese oxides possess a wealth of different structures. Among them are the octahedral molecular sieves (OMS) which are crystalline mixed-valent manganese oxides with a defined microporous tunnel structure consisting of egde- and corner-shared [MnO6]-octahedra [1], [2]. The diameters of the tunnel cross sections are in the range of that typical for zeolite pores. In the case of K-OMS-2 (cryptomelane structure) with the chemical composition KMn8O16, for instance, the square-shaped tunnels have a size of 0.46 nm × 0.46 nm [1]. OMS-2-type materials can be prepared starting from Mn2+-precursors, either in aqueous solution or in the solid state, using oxidants like KMnO4 [1], [3], [4], O2 [1], [5], K2Cr2O7 [6], KClO3 [7], K2S2O8 [8] or maleic acid [9], [10]. Thus, a broad variety of morphologies may be obtained. A recent review of the structural and textural variety, the particular properties and potential applications of OMS-type materials is given by Suib [11].

Due to the presence of manganese in different oxidation states within the framework of OMS materials [1], [12], due to their ability to transfer oxygen [3] and due to the possibility to incorporate other transition metals as dopants [11], these materials were widely investigated as catalysts for partial and total oxidation reactions. These include, e.g., alkane oxidations [13] or olefin epoxidations with tert-butyl hydroperoxide (TBHP) [14], [15], the oxidative dehydrogenation of ethylbenzene [16], side-chain oxidation of alkyl aromatics [17], oxidation of alcohols [3], [4], [12], [17], [18], [19], [20], [21], [22] and the total oxidation of VOC model compounds [23], [24]. For these conversions, OMS-type catalysts offer an environmentally benign and cost-effective alternative to conventional catalyst such as supported noble metals. It is noteworthy, that the alcohol oxidation over K- or H,K-OMS-2 catalysts occurs at mild conditions with high selectivity to the ketone or the aldehyde and with molecular oxygen as the terminal oxidant [20]. In particular, this conversion was discussed as one of the few examples of a liquid-phase oxidation the kinetics of which are consistent with the Mars-van Krevelen mechanism [3].

For several alcohol oxidation reactions, the activity of the OMS-2-type catalyst was shown to be related to its specific surface area. A very small fraction of this surface area was, however, due to the micropores of the OMS catalysts. Examples for a correlation of catalytic activity and specific surface area are the cyclohexanol oxidation over OMS-2 catalysts prepared via different alkali metal cations [21] or the oxidation of cyclohexanol, 2-thiophenemethanol or furfuryl alcohol over OMS-2 synthesized via a solvent-free, solid-state route [4]. Also, a recent study on the styrene epoxidation over K-OMS-2 catalysts with different morphologies supports the relation of specific surface area and catalytic activity [15]. However, the acid–base-properties of the surface sites play an important role for the catalytic behavior of the catalysts, too. In contrast, a correlation of the catalytic conversion and the specific surface area was not found for the alcohol oxidation over K-OMS-2 catalysts with specific surface areas in the range of 20–250 m2 g−1 [20].

The goal of the present study was, therefore, to systematically investigate whether the activity of K-OMS-2 catalysts in the aerobic oxidation of alcohols in the liquid phase is directly dependent on the specific surface area and the morphology of the catalysts. For the benzyl alcohol conversion with oxygen as a model reaction, a variety of K-OMS-2 materials with different specific surface areas and morphologies as obtained from different preparation routes was used as catalysts. It was an additional aim to examine whether – besides the specific surface area – the average crystallite size as accessible from line-broadening analysis of powder X-ray diffraction reflections is correlated to the catalytic activity. Moreover, the question whether K-OMS-2-type materials as crystalline, microporous mixed-valent manganese-oxide-based catalysts possess particular properties was of interest. Therefore, a direct comparison of the catalytic properties of the K-OMS-2 materials with that of the crystalline manganese oxides MnO, Mn2O3, Mn3O4, MnO2 (β-phase) as well as an amorphous MnO2 was included into this study.

Section snippets

Preparation of K-OMS-2 by a reflux method after [1]

In a typical preparation, 9.90 g MnSO4·H2O (0.066 mol; Fluka, p.a.; ≥99.0%) were dissolved in 34 cm3 demineralized water at room temperature. To this solution, 3.40 cm3 concentrated nitric acid (65 wt.% HNO3, Fluka, p.a.; ≥99.5%) were added. A second solution consisting of 6.65 g KMnO4 (0.042 mol, Fluka, p.a.; ≥99.0%) in 112 cm3 demineralized water was added under continuous stirring. The resulting mixture was refluxed at 110 °C for 24 h. The solid synthesis product was removed by filtration, washed with

Preparation of K-OMS-2 materials with different particle sizes and morphologies

In order to obtain K-OMS-2 materials with different particle sizes and morphologies for catalytic experiments different synthesis routes as described in the literature were followed. These include refluxing of a solid “precursor” obtained from synproportionation of KMnO4 and MnSO4 in an acidic aqueous suspension (reflux method) [1], the synproportionation of KMnO4 and Mn(Ac)2 in a solid-state reaction [4], and the oxidation of MnSO4 either by K2Cr2O7 [6] or by molecular oxygen [1] in aqueous

Conclusions

The role of K+ cations for the formation of K-OMS-2 materials by synproportionation of KMnO4 with Mn2+ salts in aqueous acidic solution was confirmed. Larger crystallites of K-OMS-2 are formed upon addition of a cation-complexing agent like 18-crown-6 to the synthesis mixture. Nevertheless, the cryptomelane structure of OMS-2 can also be obtained in the absence of alkali-metal cations, if, for instance, Ba(MnO4)2 is used instead of KMnO4. In that case, an anion influence on the synthesis

Acknowledgements

This work was performed within the Collaborative Research Centre SFB 706 (Selective Catalytic Oxidations Using Molecular Oxygen; Stuttgart) and funded by the German Research Foundation.

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    Present address: Laboratory of Engineering Thermodynamics, University of Kaiserslautern, Erwin-Schrödinger-Straße 44, 67663 Kaiserslautern, Germany.

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