Characterization of clay intercalated cobalt-salen catalysts for the oxidation of p-cresol
Graphical abstract
Intercalation of cobalt-salen into montmorillonite clay was confirmed by various characterization techniques. The most convincing evidence for intercalation was provided by TGA, XANES and EXAFS. Formation of an additional Co–O bond (bond length of 0.199 nm) as observed from EXAFS studies confirmed the host–guest relationship between cobalt-salen and the montmorillonite clay. This catalyst showed an excellent oxidation activity for p-cresol.
Introduction
Due to their ability to bind molecular oxygen reversibly, cobalt metal complexes catalyze a variety of oxidation reactions in solution that lead to dioxygenations [1], [2] monooxygenations, [3], [4] and dehydrogenations [5], [6], [7] that mimic biological oxidations. However, the major drawbacks of such oxidations are: (i) poor selectivity to the desired product, since a variety of oxygenated products are formed due to the highly reactive nature of free radical intermediates; (ii) fast deactivation of homogeneous catalysts due to formation of μ-oxo dimers; [8] (iii) carry-over of trace metal impurities into the product stream during catalyst separation protocol. These drawbacks can be overcome by using a heterogeneous catalyst that, upon separation, can be recycled easily [9], [10]. Several heterogeneous catalysts containing cobalt and/or some other metals such as copper or manganese supported on molecular sieves, carbon or resins have been reported [11], [12], [13], [14], [15]. Zeolite catalysts such as CoAPO-5 and CoAPO-11 were found to dissolve in the reaction media, while in other cases leaching of cobalt was found to be >50% under reaction conditions [12]. In almost all these reports, large excess of catalyst (up to 8 mol%) was used. Heterogenization of metal complexes has been carried out using supports such as zeolite, clays, silicious materials and activated carbon [16], [17], [18], [19], [20], [21], [22], [23], [24]. Among these supports, clays offer better advantage due to the possibility of substrate diffusion in only two-dimensional space instead of three-dimensional volume. This increases the encounter frequencies between reactants, leading to enhancement in reaction rates at very mild conditions as well as to reduction of the extent of undesired reactions [25]. In our previous work, we have reported that the cobalt-salen intercalated into montmorillonite catalyst gave a TON as high as 150 with a selectivity of 90% to the oxyfunctionalized products for air oxidation of p-cresol under ambient pressure conditions [26]. In this paper, we report the detailed characterizations so as to confirm the intercalation of cobalt-salen complex into the montmorillonite clay. The most convincing evidence for intercalation has been provided by TGA, XANES and EXAFS. A distinct shift in decomposition temperature for cobalt-salen intercalated into montmorillonite, as well as the formation of an additional Co–O bond (bond length of 0.199 nm) as observed from EXAFS studies, confirmed the host–guest relationship between cobalt-salen and montmorillonite clay. Among the various Co-Schiff bases, Co-salen intercalated into the montmorillonite clay showed the highest oxidation activity. Effects of NaOH concentration and various solvents on the conversion and selectivity pattern have also been studied.
Section snippets
Materials
p-Cresol was supplied by Loba Chemie, while sodium hydroxide was obtained from Merck. Analytical grade as well as HPLC grade methanol was obtained from M/s Runa Chemicals, India. Commercially available montmorillonite was obtained from Sigma–Aldrich, Banglore, India. Cobalt acetate, salicyladehyde, ethylene di-amine, o-phenylenediamine, and acetophenone were purchased from S.d. fine chemicals, India
Schiff base ligand preparation
Schiff bases were prepared by the reported condensation method [27]. The stoichiometric amount of
Catalyst characterization
The DRUV–vis spectra of Co(salen)-mont showed two peaks at 380 and 288 nm due to d–d transition and to ligand charge transfer, respectively. These peaks showed a distinct blue shift from the 482 and 337 nm values of the parent Co-salen (Fig. 1). The shifting of d–d transition band to the higher energy region clearly means that the in-plane ligand field around the metal ion is becoming stronger, indicating the intercalation of the complex into the montmorillonite clay. Another indication of
Conclusion
Among various solid catalysts screened, clay intercalated cobalt-salen showed an excellent activity for the liquid phase air oxidation of p-cresol under ambient pressure conditions. Hence, this catalyst was thoroughly characterized and the intercalation of the complex into the clay matrix was confirmed by the following results:The shifting of d–d transition band of the parent Co-salen to the higher energy region indicated that the in-plane ligand field around the metal ion was becoming stronger
Acknowledgments
The authors wish to acknowledge the DST-JSPS program under which this work was carried out. One of the authors VSK wishes to thank CSIR for the award of a senior research fellowship to him. X-ray absorption measurements were done by the approval of the PAC committee (proposal No: 2008G018).
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