Highly Dispersed MnO x Nanoparticles on Shape-Controlled SiO 2 Spheres for Ecofriendly Selective Allylic Oxidation of Cy
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Highly Dispersed MnOx Nanoparticles on Shape‑Controlled SiO2 Spheres for Ecofriendly Selective Allylic Oxidation of Cyclohexene Bolla Govinda Rao1 · Putla Sudarsanam2,3 · Tumula Venkateshwar Rao1 · Mohamad Hassan Amin2 · Suresh K. Bhargava2 · Benjaram M. Reddy1 Received: 18 January 2020 / Accepted: 1 April 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Shape-controlled metal nanomaterials are considered as a unique class of catalysts because of their synergistic size- and shape-dependent catalytic properties. This work reports the synthesis of a novel size- and shape-controlled catalyst, consisting of highly dispersed MnOx nanoparticles (average particle size of 4.5 nm) on shape-controlled SiO2 nanospheres (250–300 nm) for selective cyclohexene oxidation using air as the oxidant under solvent- and base-free conditions. The M nOx/SiO2 catalyst exhibited an excellent cyclohexene conversion (~ 92%) with a high selectivity (~ 96%) to the allylic products (2-cyclohexeneol and 2-cyclohexene-one) under mild conditions, outperforming various SiO2 supported CoOx, FeOx, and CuOx catalysts. The better performance of shape-controlled MnOx/SiO2 nanocatalyst is due to high redox nature of Mn, uniform dispersion of smaller sized M nOx nanoparticles, and synergetic interaction between M nOx and S iO2 spheres, as evidenced by XPS and TEM studies. Further, the MnOx/SiO2 catalyst could be reused at least 5 times for selective cyclohexene oxidation with a negligible loss in its catalytic performance, indicating the excellent stability of shape-controlled metal nanocatalysts in organic synthesis under economically viable and mild conditions. Graphic Abstract Shape-controlled MnOx/SiO2 nanocatalyst shows an excellent catalytic activity and a high selectivity to allylic products in the oxidation of cyclohexene under mild conditions
Keywords SiO2 nanospheres · MnOx nanoparticles · Synergetic interaction · Shape-controlled catalyst · Cyclohexene oxidation · Structure-performance relationship Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-020-03205-z) contains supplementary material, which is available to authorized users. Extended author information available on the last page of the article
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1 Introduction Over the last two decades, several innovative strategies have been developed for the synthesis of versatile heterogeneous catalysts for various energy and environmental applications [1–4]. Controlling catalyst’s particle shape at the nanoscale range is a potential restructuring route for obtaining novel heterogeneous catalysts, with synergistic size- and shape-dependent catalytic properties that will exhibit remarkable catalytic performance in organic synthesis and other important applications [5–7]. On one hand, the specific surface area and the concentration of coordinatively unsaturated metal sites can be improved by reducing the catalyst’s particle size to nanoscale range. On the other hand,
