This work reports the synthesis of Cr2O3-Al2O3 mixed oxide material via the sol-gel process. The characterization techniques used are X-ray diffraction, Nitrogen adsorption, scanning Electron Microscopy, UV-visible and FT-IR spectroscopy. The results show the presence of the boehmite phase and it cannot be possible to identify crystalline phase of Cr2O3. The surface BET of Cr2O3-Al2O3 is about 130.5 m2/g. The catalytic behavior of the mesoporous material is evaluated for the liquid- phase oxidation of cyclohexane with hydrogen peroxide as oxidant.  Cr2O3-Al2O3 was shown to be mild and efficient catalyst. The use of acetone as solvent presented an approximate 12.4% conversion with 89.66 % selectivity into cyclohexanol

CHOUKCHOU-BRAHAM, Abderrahim

TCHENAR, Yasmina Nesrine

English

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Chemistry and Materials Research, Vol.4 2013  Special Issue for International Congress on Materials & Structural Stability, Rabat, Morocco, 27-30 November 2013 42  Cr2O3-Al2O3 mixed oxide material: synthesis, structure and catalytic behavior for C-H activation. Yasmina Nesrine TCHENAR1 and Abderrahim CHOUKCHOU-BRAHAM1 1Tlemcen University, Department of chemistry, Tlemcen, Algeria Abstract. This work reports the synthesis of Cr2O3-Al2O3 mixed oxide material via the sol-gel process. The characterization techniques used are X-ray diffraction, Nitrogen adsorption, scanning Electron Microscopy, UV-visible and FT-IR spectroscopy. The results show the presence of the boehmite phase and it cannot be possible to identify crystalline phase of Cr2O3. The surface BET of Cr2O3-Al2O3 is about 130.5 m2/g. The catalytic behavior of the mesoporous material is evaluated for the liquid- phase oxidation of cyclohexane with hydrogen peroxide as oxidant.  Cr2O3-Al2O3 was shown to be mild and efficient catalyst. The use of acetone as solvent presented an approximate 12.4% conversion with 89.66 % selectivity into cyclohexanol.  1 Introduction Sol-gel processes have attracted the attention of many researchers in recent years [1-3]. In fact, the physical and chemical features of the materials obtained by these methods (e.g., particle sizes, surface areas and mechanical properties) can be changed according to the temperature, operating conditions, and to the used precursor. Nowadays, sol-gel methods are well established allowing the preparation of a board variety of supported metals, metal oxides, coating and composite materials with tailored properties. The obtained materials emerging from these processes can be used in selective heterogeneous catalysis, which represents an area of great economical interest. Here we wish to report about the sol-gel synthesis and characterization of Cr2O3-Al2O3 mixed oxide. It was used then as heterogeneous catalyst for the oxidation of cyclohexane with hydrogen peroxyde. Cyclohexane oxidation is currently of considerable commercial significance [4-6].  2 Experimental 2.1 Synthesis of sample The Cr2O3-Al2O3 material was prepared by the sol-gel method. An amount of Cr(NO3)3.9H2O was dissolved in water; the resulting solution was stirred and then a mixture of aluminum butoxide and butanol was added into solution while stirring. The obtained green gel was aged for 1 day, then dried at 120 °C and finally calcined at 300 °C for 6 h under an air flow.  2.2 Catalyst characterization The Cr2O3-Al2O3 material was characterized by X-ray powder diffraction (XRD) using a philips PW 3710 diffractometer with Cu Kα radiation (λ=1, 54060 Ǻ) in the range of 2ϴ= 20°-80°.  The specific surface area was determined from N2 adsorption isotherm at 77K using Nova 1000e (Quantachrome) instrument.  The scanning Electron Microscopy (SEM) was realized on TM- 1000 HITACHI instrument. The mixed oxide was also investigated by the UV-Vis and FT-IR spectroscopy using a Perkin Elmerr and an AVARAR 320 Thermo-Nicolet FT-IR spectrometers. 2.3 Catalytic cyclohexane oxidation reactions Cyclohexane oxidation reaction was carried out in a 250 ml three-necked flask. Typically, a mixture of 66.5 mmol cyclohexane, 66.5 mmol hydrogen peroxyde (H2O2), 25 ml solvent and 0.1g catalyst was magnetically stirred at 343 K for 6 h. The reaction mixture was analyzed by gas chromatography (GC), taking aliquots at different reaction times. A  Schimadzu GC-14 B gas chromatograph equipped with Apiezon L column (10% on Chromosorb P UNDMCS) and flame ionization detector (FID) was used. Consumption of H2O2 was determinate by iodometric titration.  3 Results an discussion 3.1 Characterization The X-ray pattern of Cr2O3-Al2O3 material (figure 1) exhibits diffraction lines of boehmite (JCPDS 21-1307) with well-defined peaks at 2θ= 14°, 28°, 38°, 46° and 64° which are assigned to (020), (120), (031), (131) and (231) planes, respectively [7, 8].  Chemistry and Materials Research, Vol.4 2013  Special Issue for International Congress on Materials & Structural Stability, Rabat, Morocco, 27-30 November 2013 43   Fig. 1. X-ray diffractogram of Cr2O3-Al2O3 Nitrogen adsorption/desorption shows that the simple displayed isotherm of type IV according to IUPAC classification. This result is characteristic of mesoporous solid. The BET surface area of Cr2O3-Al2O3 is about 130 m2/g. In figure 2, the SEM image of Cr2O3-Al2O3 material is shown. Geometrics grains coexisting with small grains lighter were observed.  Fig 2.  Scanning electron micrograph of Cr2O3-Al2O3 material. The diffuse reflectance UV-Vis spectrum of Cr2O3-Al2O3 mixed oxide (figure 3) exhibits a band around 267 nm characteristic of charge transfer transition from oxygen to Al3+ [9]. Two bands at 253 nm and 370 nm which are associated to charge transfer from oxygen to Cr (VI). The bands at 456 nm and 516 nm are attributed to Cr (III) in octahedral environment.  Fig 3.  DR-UV-Vis spectrum of Cr2O3-Al2O3 material. Figure 4 shows IR spectrum of Cr2O3-Al2O3 mixed oxide. The IR band at 3400 cm-1 is dues to OH stretching mode of hydroxyl groups, both from the inter-layer water molecules and from Al-OH groups formed by hydrolysis of residual alkoxyde [11]. The band appearing 2300 cm-1 around is due to the C-H stretching. The deformation band of H2O is situated at 1700 cm-1. The bands appearing between 1500- 1400 cm-1 are attributed to the carbonate or carboxylate surface compounds formed due to adsorption of atmospheric CO2 on the mixed oxide surface [12]. The IR band at 1050 cm-1 is assigned to Al-O-H bending vibration [12]. And band centered at 600 cm-1 can be due to Al-O stretching vibration [13]. We designated the absorption band at 480 cm-1 as Al-O bending vibration [13].  Fig 4.  FT-IR spectrum of Cr2O3-Al2O3 material.  3.2 Catalytic tests Cyclohexane oxidation was carried out in presence of a little amount of cyclohexanol as initiator and in different  solvents. It has been already shown that the cyclohexanol addition increased the catalyst activity [14].  Chemistry and Materials Research, Vol.4 2013  Special Issue for International Congress on Materials & Structural Stability, Rabat, Morocco, 27-30 November 2013 44  Cyclohexane was first used as reagent and solvent, but no conversion was noticed for the control reaction after the addition of hydrogen peroxide (H2O2), in the complete absence of both catalyst and initiator. The sole addition of catalyst did not produce any change, thus showing that the oxidation of cyclohexane remains a hard reaction at 70 °C. However, the addition of cyclohexanol leads to 1.85 % conversion into cyclohexanol (Cy-OH) and cyclohexanone (Cy=O) products (table 1). Table 1. Oxydation of cyclohexane by H2O2 Reaction conditions: 66.5 mmol cyclohexane, 66.5 mmol H2O2, 40 ml solvent, T= 343 K, t= 6h. When oxidation reactions with H2O2 were carried out with ethanol, acetone or acetic acid as solvent, a considerable increase in cyclohexane conversion was observed. It has been shown that the presence of a polar solvent in this kind of catalytic reaction makes the catalyst more active. Efficient removal of the oxidation products from active sites of the catalyst by the polar solvent is assumed to explain this weak activity [15]. In the acetone 12.36 % conversion was obtained with 89.66 % selectivity into cyclohexanol product. This conversion can be rationalized, taking into account that acetone and hydroperoxydes can react with each other to form dimethyldioxirane (DMDO: (CH3)2CO2) which is a powerful oxidizer for organic compounds [16]. 4 Conclusion The Cr2O3-Al2O3 mixed oxide can be prepared from aluminum butoxide and chromium nitrate by a simple sol-gel process. XRD analysis showed that this material presents a not well crystalline structure with the presence of only boehmite phase. BET analysis gives a surface area of about 130 m2/g. The presence of Cr (III) and Cr (VI) was assigned by UV-Visible. The mesoporous xerogel is a mild and efficient catalyst for the oxidation of cyclohexane into cyclohexanol and cyclohexanone products with H2O2 as oxidant and cyclohexanol as initiator. The presence of a polar solvent in this kind of reactions makes the catalyst more active. The use of acetone as solvent presented an approximate 12% conversion with 89.66% selectivity into cyclohexanol. References [1] K. M. Parida, G. K. Pradhan. Mater Chem & Phys  123, 427(2010). [2] A. K. Patra, A. Dutta, A. Bhaumik. ACS Appl. Mater & interfaces 4, 5022(2012). [3] J. Phalippou, L. Kocon. Techniques de l’ingénieur, “ Aérogels -Aspects fondamentaux”. (Editions T.I. Paris- France, 2013). [4] Y.N. Tchenar, D. Lahcene, R. Bachir, A. Choukchou-Braham, Research Journal of Pharmaceutical, Biological and Chemical Science. 2 (2011) 192. [5] Y.N. Tchenar, A. Choukchou-Braham, R. Bachir, Bull. Mater. Sci. 35, 673(2012). [6] M. Barraqué. Techniques de l’ingénieur, “cyclohexane CH”. (Editions T.I. Paris- France, 2013). [7] X. Yang, L. E. Erickson, K. L. Hohn, P. Jeevanandam, K. J. Klabunde; Ind. Eng. Chem. Res 45, 6169(2006). [8] S. El Korso, I. Rekkab, A. Choukchou-Braham, S. Bedrane, L. Pirault-Roy, C. Kappenstein. Bull. Mater. Sci 35, 1187 (2012). [9] A. C. Gluhoi, N. Bogdanchikova, B. E. Nieuwenhuys, J.Catal. 232, 96 (2005). [10] S. Shylesh, P.P. Samuel, A.P. Singh, Appl. Catal A: Gen. 318, 128 (2007). [11] C. P. Ricci, Z. N. Rocha, S. Nakagaki, K. A. D. F. Castro, A. E. M. Crotti, P. S. Calefi, E. J. Nassar, K. J. Ciuffi; Appl. Catal A: Gen. 389 147 (2010). [12] E.P. Reddy, R.S. Varma, J. Catal. 221, 93 (2004). [13] M. Mirzaee, M. M. Amini, M. Sadeghi, F. Y. Mousavi, M. Sharbatddaran, Ceramics- Silikáty 49 (1) (2005) 40. [14] A. Bellifa, D. Lahcene, Y. N. Tchenar, A. Choukchou-Braham, R. Bachir, S. Bedrane, C. Kappenstein, Appl. Catal A: Gen. 305, 1 (2006). [15] E. L. Pires, D. Arnold, U. Schuchardt, Amorphous silicates containing cerium: selective catalysts for the oxidation of cyclohexane, J. Mol. Catal. A : Chem. 169, 157 (2001).  [16] S. E. Kelly, in B. M. Trost (Ed). Comprehensive organic Synthesis. (Permongamon Press, Oxford 1993). Solvent Cy-OH (mmol) Cy=O (mmol) conversion (%) Cyclohexane 0.61 0.61 1.85 Ethanol 2.00 0.58 3.88 Acetone 7.37 0.85 12.36 Acetic acid 2.93 1.33 6.40 

Cr2O3-Al2O3 mixed oxide material: synthesis, structure and catalytic behavior for C-H activation

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Cr2O3-Al2O3 mixed oxide material: synthesis, structure and catalytic behavior for C-H activation

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