Journal of Catalysis 255 (2008) 40–47 www.elsevier.com/locate/jcat Cobalt-substituted polyoxometalate pillared hydrotalcite: Synthesis and catalysis in liquid-phase oxidation of cyclohexanol with molecular oxygen Suman K. Jana a,b,∗ , Yoshihiro Kubota c , Takashi Tatsumi a,d,∗ a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-0583, Japan b SABIC Research and Technology Pvt. Ltd., Plot No. 5 & 6, Savli GIDC Estate, Manjusar, Gujarat 391775, India c Division of Materials Science and Chemical Engineering, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan d Japan Science and Technology Corporation, Saitama 332-0012, Japan Received 18 October 2007; revised 22 January 2008; accepted 22 January 2008 Available online 21 February 2008 Abstract Cobalt-substituted polyoxometalate (POM), α-[SiW 9 O 37 {Co(H 2 O)} 3 ] 10− , was heterogenized for its effective use as a solid catalyst in the liquid-phase reaction by inserting it in between the layers of basic Mg 3 Al-hydrotalcite. It was characterized by FT-IR, UV–vis, X-ray diffraction, N 2 adsorption–desorption and elemental analysis. A POM cluster was intercalated into the interlayer of layered double hydroxide (LDH), and the integrity of the cluster [SiW 9 Co 3 ] was maintained when the hydrotalcite precursor, Mg 3 Al-LDH-carbonate, was transformed into a mixed- oxide solid solution, followed by the formation of Mg 3 Al-LDH-hydroxide through hydrolytic reconstruction and Mg 3 Al-LDH-adipate via anion exchange by organoacid precursor before its use in the pillaring reaction under a stringent N 2 atmosphere. The intercalated cobalt-substituted POM cluster showed high catalytic activity and reusability in the liquid-phase selective oxidation of cyclohexanol to cyclohexanone using 1 atmosphere of molecular oxygen under mild reaction conditions.  2008 Elsevier Inc. All rights reserved. Keywords: Polyoxometalate; Cobalt; Hydrotalcite; Pillared; Cyclohexanol; Oxidation; Oxygen 1. Introduction Polyoxometalates (POMs) constitute a diverse class of inor- ganic oxo-metal clusters composed of early transition metals in their highest oxidation state [1]. They have tunable acidic and redox properties that can be controlled by changing the con- stituent elements. The use of POMs as oxidation catalysts has attracted attention because unlike organic porphyrins, which tend to oxidize and degrade after a few catalytic cycles, POMs resist oxidation due to the presence of W or Mo atoms in their * Corresponding authors. Address for correspondence of S.K. Jana is SABIC Research and Technology Pvt. Ltd., Plot No. 5 & 6, Savli GIDC Estate, Man- jusar, Gujarat 391775, India and of T. Tatsumi is Chemical Resources Labo- ratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-Ku, Yokohama 226-0583, Japan. Faxes: +91 2667 264245 (S.K. Jana), +81 45 924 5282 (T. Tat- sumi). E-mail addresses: suman@sabicind.com (S.K. Jana), ttatsumi@cat.res.titech.ac.jp (T. Tatsumi). highest oxidation state [1,2]. It is well known that heteropoly compounds can catalyze oxidation processes and that catalytic performance is greatly enhanced by the incorporation of transi- tion metals [3]. The transition metal-substituted POMs are more attractive and interesting catalytic materials, particularly when the metals occupy the lacunary or defect position of the POM structure. Such compounds can be viewed as transition metal complexes with an inorganic POM ligand that acts as a poten- tial multielectron acceptor. To date, lacunary POMs have been the most widely investigated oxidation catalyst because, besides having available open coordination sites on the transition met- als (expected to be beneficial for oxidative catalytic reactions), they have high thermal stability and can be readily synthesized. Although a number of catalytic applications of transition metal-substituted POM in the liquid-phase oxidation have been reported, those applications have been mostly in homogeneous catalytic reactions [4–10]. Thus, those catalytic routes were not environmentally benign. Because of the difficulty associated 0021-9517/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2008.01.022