DOI: 10.1002/chem.201301498 Mesoporous Core–Shell Fenton Nanocatalyst: A Mild, Operationally Simple Approach to the Synthesis of Adipic Acid Astam K. Patra, Arghya Dutta, and Asim Bhaumik* [a] Introduction The development of green and sustainable strategies for ef- ficient synthesis of fine and bulk chemicals is one of the most fascinating areas of research today. [1] In the twenty- first century, considerable efforts have been devoted to the preparation of core–shell nanoparticles to improve their properties in catalysis, [2] separation, [3] sensors, [4] biomedical imaging, [5] electron imaging, [3b, 6] gene delivery, [7] diagnosis, and therapy [8] due to their tunable properties and tunable particle sizes through appropriate choice of the core and shell structures in a material. Depending on the choice of the two components, the chemical and physical properties can be varied. [9] Bimetallic transition metal oxide core–shell nanoparticles have potential uses in catalysis and magnetic and optical applications that are quite distinct from those of their monometallic counterparts. [1a, 10] These core–shell nano- particles can be specifically designed for utilization as new classes of multifunctional materials. [5] Generally, the synthe- sis of porous Fe 2 O 3 by the soft-templating method is not convenient, and the resulting materials have low surface areas and poor crystallinity. Recently, we developed a highly efficient method for preparation of g-Al 2 O 3 nanoparticles by using a hydrothermal method and sodium salicylate as tem- plate. [11] Herein, use of these alumina nanoparticles as the core of the nanoparticle helps to form the core–shell struc- ture. These g-Al 2 O 3 nanoparticles were encapsulated by a thin shell of a-Fe 2 O 3 , and the resulting material showed high surface area due to self-aggregation of the tiny nanocrystals. These core–shell nanoparticles act as a Fenton catalyst in the presence of H 2 O 2 as oxidant and show high catalytic ac- tivity for conversion of cyclohexanone to adipic acid in a one-step reaction. Adipic acid is one of the most important dicarboxylic acids industrially and it has been widely used in the manu- facture of polymers for carpet fibers, furniture, tire rein- forcement, auto parts, clothing, and so forth. [12] Most indus- trial processes for the production of adipic acid involve nitric acid oxidation of cyclohexanol or cyclohexanol–cyclo- hexanone mixtures. [13] However, in this process, production of nitrous oxide (N 2 O) as an unavoidable chemical waste contributes significantly to global warming. Hence, a green synthetic route for the production of adipic acid is highly de- sirable. In 1998 adipic acid was produced by direct oxidation of cyclohexenes by using hydrogen peroxide as oxidant and sodium tungstate as catalyst. [12] To date most of the known reactions have involved tungsten-based catalysts [14] and hy- Abstract: Mesoporous nanoparticles composed of g-Al 2 O 3 cores and a- Fe 2 O 3 shells were synthesized in aque- ous medium. The surface charge of g- Al 2 O 3 helps to form the core–shell nanocrystals. The core–shell structure and formation mechanism have been investigated by wide-angle XRD, energy-dispersive X-ray spectroscopy, and elemental mapping by ultrahigh- resolution (UHR) TEM and X-ray photoelectron spectroscopy. The N 2 ad- sorption–desorption isotherm of this core–shell materials, which is of type IV, is characteristic of a mesopo- rous material having a BET surface area of 385 m 2 g 1 and an average pore size of about 3.2 nm. The SEM images revealed that the mesoporosity in this core–shell material is due to self-aggre- gation of tiny spherical nanocrystals with sizes of about 15–20 nm. Diffuse- reflectance UV/Vis spectra, elemental mapping by UHRTEM, and wide-angle XRD patterns indicate that the materi- als are composed of aluminum oxide cores and iron oxide shells. These Al 2 O 3 @Fe 2 O 3 core–shell nanoparticles act as a heterogeneous Fenton nanoca- talyst in the presence of hydrogen per- oxide, and show high catalytic efficien- cy for the one-pot conversion of cyclo- hexanone to adipic acid in water. The heterogeneous nature of the catalyst was confirmed by a hot filtration test and analysis of the reaction mixture by atomic absorption spectroscopy. The kinetics of the reaction was monitored by gas chromatography and 1 H NMR spectroscopy. The new core–shell cata- lyst remained in a separate solid phase, which could easily be removed from the reaction mixture by simple filtra- tion and the catalyst reused efficiently. Keywords: core–shell structures · heterogeneous catalysis · mesopo- rous materials · nanoparticles · oxi- dation [a] A. K. Patra, A. Dutta, Prof. Dr. A. Bhaumik Department of Materials Science Indian Association for the Cultivation of Science 2A and 2B Raja S. C. Mullick Road Jadavpur, Kolkata 700032 (India) E-mail: msab@iacs.res.in Homepage: http://www.iacs.res.in/matsc/msab/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301498.  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 12388 – 12395 12388