Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
Synthesis, characterization and catalytic properties of a chromium silicate xerogel Rosenira Serpa da Cruz a'+, Martin de M. Dauch a, Ulf Schuchardt a and Rajiv Kumar b alnstituto de Quimica, Universidade Estadual de Campinas- CP 6154- 13083-970-CampinasSP, Brazil bCatalysis Division, National Chemical Laboratory- Pune 411008- India A mixed oxide xerogel, Cr203-SiO2, was prepared by the sol-gel method, characterized and investigated in cyclohexane oxidation. According to diffuse reflectance UV-vis, IR, DRX, TPR and EPR measurements, Cr 3+ ions are located in octahedral sites in the silicate framework, with no evidence for a separate Cr203 phase. Leaching experiments suggested that the catalytic activity in cyclohexane oxidation is due to the leaching of chromium.
1. INTRODUCTION In order to meet the increasing ecological and economic regulations for the development of chemical processes, more attention is being paid to the use of heterogeneous catalysts. Controlled synthesis of the catalytic material and appropriate tuning of the catalytic site is necessary to achieve the high selectivity required in chemical production processes. Sol-gel chemistry has emerged as the most important and versatile method for the preparation of inorganic and mixed organic-inorganic materials [ 1]. The sol-gel approach to material synthesis is based on hydrolysable molecular precursors. Hydrolysis and polycondensation reactions lead to the formation of oxo polymers or metal oxides. These fundamental chemical processes are influenced by several parameters which, once they are understood for a particular chemical system, allow the control of the homogeneity (or controlled heterogeneity) and nano- and micro-structure of the derived material. Chromium is widely used as catalyst in oxidation reactions. Up to now traditional oxidation methods employing stoichiometric or even excess quantities of inorganic oxidants such as potassium dichromate have been predominantly applied both in industry and in small scale. Although chromium substituted molecular sieves have been used to catalyze the oxidation of different substrates, leaching of chromium to the liquid phase during oxidation reactions has been observed [3, 4]. Another much less studied approach to redox metal containing heterogeneous catalysts is the use of amorphous metallosilicate aerogels and xerogels prepared by sol-gel methods. In this work we present our results on the synthesis and
+Present address: Departamento de Ci~ncias Exatas e Tecnol6gicas, Universidade Estadual de Santa Cruz, Rodovia Ilh6us-Itabuna-Km 16, 45650-000, Ilh6us-Ba, Brazil
characterization of an Cr203-SiO2 xerogel. This material.was investigated in the oxidation of cyclohexanol and cyclohexane with t-butyl hydroperoxide (TBHP) as the oxidant.
2. EXPERIMENTAL 2.1. Synthesis Tetraethyl orthosilicate, 98% (TEOS), Aldrich; chromium (III) acetylacetonate, Alfa Inorganics; ethanol, 99,8% and hydrochloric acid, 37%, Merck were used as received. The catalysts were prepared by a single step acid-catalyzed sol-gel method  which permits structural control of the mixed oxides during the preparation. The amount of (TEOS + metal source) was typically 50 mmol. If not mentioned explicitly, the standard procedure was the following: TEOS is placed in a 100 ml PP beaker equipped with a magnetic stirring bar and mixed with the corresponding amount of the metal precursor compound dissolved in ethanol. Subsequently, the water and acid is introduced by adding aqueous HCI (8 mol L "l) dropwise to the stirred solution at room temperature. The solution was stirred for 5 minutes and then placed in the hood to allow for a slow evaporation of the volatiles. Gelation usually occurred after 4-5 days. After gelation was completed, the sample was heated in an oven in the following way: room temperature up to 223 K (24 h at 223 K); up to 423 K (12 h) and up to 523 K (12 h), always with a 15 K min 1 heating rate. Finally the samples were cooled down to room temperature, crushed and sieved ( 100 mesh). 2.2. Characterization The chemical composition of the samples was determined by X-ray fluorescence spectroscopy (Spectrace TX-5000), using mechanical mixtures of SiO2 and chromium (III) acetylacetonate for the calibration curves. The carbon and hydrogen contents of the final samples were determined with a Perkin Elmer 2400 C/H/N analyser. FT-IR measurements were performed on a Perkin Elmer 1600 instrument. X-ray diffraction patterns were measured using a Shimadzu XD-3A diffratometer with CuK~ radiation in the range of 2e = 10-50 ~ with a scanning rate of 2~ min -l. The UV-vis spectra in the DRS mode were obtained on a Varian Cary 5 spectrometer in the range of 200 to 700 nm, on powdered samples. The spectrum of SiO2 was taken for background subtraction. The surface areas, the micropore volume and the pore size distributions were calculated from the N2 adsorption-desorption isotherms at liquidN2 temperature in a Omnisorb 100 CX (Coulter Corporation, USA) instrument. Prior to the adsorption measurements, the samples were activated at 473 K for 8 h in high vacuum. The ESR spectra were measure at room temperature using a Bruker ESR spectrometer(200D). Temperature-programmed reductions of the samples were carried out in a homebuilt instrument using 3 vol% H2-N2 mixture at a heating rate of 10 Kmin l.
2.3. Catalytic experiments The catalytic oxidation reactions were carried out in a three-necked flask equipped with a condensor and magnetic stirring. The temperature of the reaction was maintained using an oil bath. In a typical experiment, the solid catalyst was added to the substrate in the specified solvent followed by the addition of TBHP in cyclohexane.
1039 Atter reaction, the mixture was quenched to room temperature and the catalyst was separated by filtration. The products were analyzed by GC. Cyclohexane, cyclohexanone and cyclohexylhydroperoxide (CHHP) were quantified using calibration curves. As CHHP is partially decomposed upon injection, its concentration was estimated by double analysis, before and after reducing CHHP to cyclohexanol with PhaP. The oxidation products were identified using an HP 5980 gas chromatograph coupled to an HP 5970B mass spectrometer. To verify if acids were formed, the reaction mixture was esterified before analysis. Two leaching experiments were performed to determine if the activity observed for the catalysts in the liquid phase oxidation of cyclohexane and cyclohexanol with TBHP was due to heterogeneous or homogeneous catalysis. In the first experiment, the solid catalyst was initially stirred with a solution of TBHP, filtered at reaction temperature after 4 h reaction time and the substrate was added to the filtrate. This solution was allowed to react at the same temperature for another 20 h. The second experiment consisted in the filtration of the catalyst from the reaction mixture at 80~ after 4 h reaction time, then a fresh portion of TBHP was added to the filtrate which was allowed to react for another 20h. 3. RESULTS AND DISCUSSION The physicochemical properties of the samples are given in Table 1. Table 1. Physicochemical properties of the samples. Material
metal (mol %)
BET surface area ( m 2 g-l) 364 450
240 200ft. I,U)
e Io.o~ 0 0
. 111 18 20
Relative P r e s s u r e (PIPo)
Fig 1. N2 adsorption isotherm of Cr203-8iO2 and the micropore distribution.
average poresize diameter (nm) 1.1 1.6
The samples showed a narrow monomodal pore-size distribution and a type I isotherm in the N2-adsortion. Figure 1 shows the isotherm and pore size distribution of Cr203-SiO2. The organic residue resposible for the carbon and hydrogen contents could be originated for realkoxylation of the hydroxyl groups in the surface and/or by incorporation of the alkoxide ligands as well as the solvent in the silica matrix . All the samples were X-ray amorphous, showing only broad reflections.
1040 Since the metallosilicates contain- 98-99% SiO2, no large differences between their FT-IR spectra and that of SiO2 were expected. Only the bands common to all silicate structures at-~1080, 790 and 450 cm "1, ascribed to the strectching and bending vibrations of Si-O bond were observed. Of special interest is the band at ~ 930-960 cm "l, which has been attributed to one of two vibrations: (SiO3)Si-OH units in the xerogel and associated to Si-OH vibrations due to the hydroxylated surface ; or the Si-O stretching in the polarized Si-O-M bond. In our case, this band at 945 cm "1 is also present in Cr-free xerogels, indicating that at least for the most part it is due to the hydroxylated surface in the xerogel. Then, this band cannot be an indication of isomorphic Cr-substituion in mixed oxide xerogels. The IR spectrum also shows that the acetylacetonate ligand is clearly lost. The DR-UV-vis spectra of the Cr203-SiO2, Cr203 and a mechanical mixture of Cr203 + SiO2 are given in Figure 2.
The spectra of Cr203+SiO2 and C[203 show two bands at 600 and 466 nm, assigned to 4TE---~4T5 and 4T2----~4T4[F]
i \ i \ i \
// /'/ /" bA
30 20 ..... . ' , " ,
transitions of chromium(Ill) in an octahedral coordination, respectively . The spectrum of Cr203-SiO2 indicates the presence of chromium in two different coordination spheres. The absence of peaks of reduction by H2 in the temperature-programmed reduction thermogram indicates the absence of a separate Cr203 phase. Additionally, evidence for this hypothesis was obtained from UV-vis diffuse reflectance spectroscopy and XRD. The EPR spectrum of Cr203-SiO2 shows a broad peak at g= 1.975, which is attributed to small clusters of Cr 3+ invisible to XRD  or associated with Cr3+in the framework, irrespective of coordination (octahedral or tetrahedral).
Fig. 2. Diffuse reflectance spectra of (a) mechanical mixture of Cr203 + SiO2, (b) Cr203-SiO2 and
(C) Cr203. The results of cyclohexane oxidation with TBHP in cyclohexane catalyzed by Cr203SiO2 are presented in Table 2.
1041 Table 2 Cyclohexane oxidation over Cr203-SiO2 a Product distribution (mol%) Temperature (~
TON b Cyclohexanone
70 95 85 3 80 120 75 2 100 185 69 0 aReaction contitions: cyclohexane, 95 mmol, TBHP in cyclohexane 9.5 2,3 10-2 mmol Cr, 8h. b Moles of oxygenated products per mol of chromium. c The major products are bicyclohexyl and adipic acid.
10 2 6 17 0 31 mmol, Cr203-SiO2'
Data of Table 2 show that the total yield of oxidation products increases as the temperature is increased. The ratio cyclohexanone/cyclohexanol increases when the temperature increases from 70 to 100~ implying that cyclohexanol is continually oxidized at higher temperatures. In order to confirm this hypothesis, a parallel experiment using cyclohexanol as the substrate was performed at 80~ The oxidation of cyclohexanol, giving 46% conversion and cyclohexanone in 100% selectivity was indeed observed. The selectivity of CHHP decreases and becomes 0 at 100~ suggesting that CHHP is an unstable intermediate.
reaction time (h)
Fig. 3. Effect of catalyst separation on the cyclohexane oxidation at 80~ Reaction conditions as in Table 2.
In order to determine if the catalysis was indeed heterogeneous, a few leaching experiments were performed following the procedure described by Lempers and Sheldon  and discussed in detail in a recent review . The activity of the liquid phase of the reaction after removing the solid catalyst was examined and showed variable catalytic activities, sometimes higher (o) sometimes lower(A) than the ones in the presence of the solid (m), e.g., cyclohexane was continuously converted to cyclohexanone (Figure 3), strongly suggesting that chromium species leached from Cr203Si02 are responsible for the catalysis.
In an additional experiment, the catalyst was stirred with the substrate at the reaction, temperature, filtered off and TBHP added to the filtrate. Since no catalysis was observed in the filtrate even after 24h, one cannot say that leaching of the chromium was promoted by substrate/solvent. As, however, leaching is observed in the presence of the oxidant, be do not agree with Neumann et al.  and Maier et al.  who reported that mixed oxides prepared by sol-gel method are true heterogeneous catalysts in oxidation reactions. The used catalysts were dried at 120~ for 2 h, and then used for recycling experiments. The conversion of cyclohexane in three consecutive reactions was very closed to that of a fresh catalyst. This result can give erroneous conclusions about leaching because minimal amounts of leached Cr can account for the observed catalysis as observed by Sheldon et al. [ 14] during liquid phase oxidations using CrAPO-5, CrAPO-11 and CrS-1 as catalysts. 4. CONCLUSIONS The study of the oxidation of cyclohexane in the presence of a Cr203-SiO2 xerogel as catalyst has shown that the catalytic activity is due to the chromium leached into the reaction medium. In spite of the very small amount of dissolved chromium, a high conversion catalysis was observed. ACKNOWLEDGMENTS Financial support by FAPESP and CNPq and supply of raw material by NITROCARBONO S.A. are gratefully acknowledged. We also thank Dr. D. Srinivas, NCL-Pune, India for the EPR spectra and Dr. R. Buffon for helpful discussions and revision of the manuscript.
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