Synthesis of heterogeneous copper complex catalyst for oxidation of cyclohexane using molecular oxygen.
Cyclohexanone and cyclohexanol are important intermediates in the manufacture of caprolactam (serving as a monomer for nylon 6 polymer formations) and adipic acid (serving as a monomer for nylon 66 polymer formation; Berezin et al., 1966). The oxidation of cyclohexane is carried out industrially at a temperature of 150-180[degrees]C and pressure of 1.0-1.6 MPa in presence of Co salts (naphthenate, stereate, oleate) as catalyst. The cyclohexane conversion is kept low (about 3-4% per pass) as the cyclohexanol and cyclohexanone formed are more susceptible for further oxidation to C[O.sub.2] (Berezin et al., 1966; Emanuel et al., 1967). In literature, catalytic oxidation studies have also been conducted using oxidants such as hydrogen peroxide, t-butyl hydrogen peroxide (TBHP) other than molecular oxygen (Arends et al., 1997; Carvalho et al., 1997; Schuchardt et al., 2001). Catalyst systems studied other than cobalts salts are metal oxides, metal cations incorporated in inorganic matrices such as silica, alumina, zirconia, active carbon, zeolites (Lin and Weng, 1994) aluminophosphates (Sakthivel and Selvam, 2002), and CoAPO-5 catalyst. The use of carboxylic acids (except formic acid) as the solvent is necessary and the use of propionic acid gives the highest reaction rate (Steeman et al., 1961). Heterogeneous catalyst of cyclohexane exhibit leaching of active metal ions, extreme reaction conditions (2 MPa pressure and 177[degrees]C temperature), and low activity (Suresh et al., 1988a). An induction time, generally observed in the case of air oxidation of cyclohexane, is reduced by adding promoters or co-reactants such as acetaldehyde, cyclohexanone, cyclohexanol, and azobis (isobutyronitrile; AIBN; Wen et al., 1997).
The mechanisms (see Figure 1) suggested in the literature assume that cyclohexyl hydroperoxide (CHHP) is the intermediate formed in the presence of transition metal salts (Suresh et al., 1988b; Tolman et al., 1989; Wen et al., 1997) and is a multistage, free radical chain reaction, comprising of initiation, chain propagation, and chain termination step. Tolman et al. (1989), Spielman (1964), and Alagy et al. (1974) developed a reaction scheme consisting of 154 reactions which is impractical to analyze as it requires the determination of as many number of rate constants simultaneously with high accuracy. Hence lumped kinetic models which require lesser rate constants (Gange et al., 1981; Pohorecki et al., 1992, 2001; Ponec, 2001; Anisia and Kumar, 2007) are useful in analyzing reaction data.
[FIGURE 1 OMITTED]
In our present work, a macrocyclic binuclear monometallic copper complex has been synthesized and is ionically bonded to the zirconium pillared montmorillonite clay through ion exchange. The oxidation of cyclohexane using heterogeneous complex catalyst without solvent and cocatalyst has been studied in the temperature range 145-200[degrees]C. In this article, it is shown that the Cu-Cu Homonuclear macrocyclic complex catalyst serves as an effective catalyst for the oxidation of cyclohexane and gives faster reaction with product specificity different from Fe-Cu complex catalyst reported in our earlier work (Shul'pin, 2002).
Synthesis of CuCuL1 2([CH.sub.3]COO) [L1=[([CH.sub.3][C.sub.6][H.sub.2][(CH).sub.2] [O.sub.2][N.sub.2][C.sub.6][H.sub.4]).sub.2]] Macrocyclic Complex
The 2,6-diformyl-4-methylphenol needed for the macrocyclic complex was prepared following the procedure given in literature (Serwicka and Bahamwoski, 2004). The NMR Spectrum of the dialdehyde that we prepared shows singlets at 11.42 (phenolic), 10.2 (aldehydic), 7.74 (aromatic), and 2.36 ppm (methyl) and is consistent with the assigned structure and matches with that given in literature (Serwicka and Bahamwoski, 2004). In order to prepare the macrocyclic ligand, the 2,6-diformyl-4-methylphenol is reacted with 1,2-phenylenediamine in two stages as follows and this gives two identical N202 sites on the formed complex.
[FIGURE 2 OMITTED]
To a 50 ml, of NN-dimethylformamide at 40[degrees]C, 2,6-diformyl-4-methylphenol (1.95 g, 0.012 mol) and 1,2-phenylenediamine (0.65 g, 0.006 mol) were added. To this solution cupric acetate (2.4 g, 0.012 mol) was added and the solution was stirred till the cupric acetate dissolved completely. The solution was kept for 1 h and then diethyl ether was added after which a precipitate appeared. The precipitate was collected by filtration, dried and its FTIR spectrum in Figure 2a shows the presence of functional groups C=N at 1533 [cm.sup.-1] and C=0 at 1668 [cm.sup.1].
The CuCuL1' (1.8 g, 0.0034 mol) obtained from the previous step was dissolved in 30 ml, of methanol and to this solution, 1,2-phenylenediamine (0.37 g, 0.0034 mol) was added. The solution was kept for 1 h and to this, diethyl ether was added. The precipitate that appeared was collected by filtration and dried. The FTIR spectrum shown in Figure 2b gave C=N at 1512 [cm.sup.-1] while a weak C=O peak appearing at 1664 [cm.sup.-1].
[FIGURE 3 OMITTED]
Preparation of the Heterogeneous Catalyst
The acid (using HCl) treated montmorillonite was procured from Ashapura Minechem Ltd. (Mumbai, India) and was first pillared using zirconium ions and then was intercalated with the complex as shown in Figure 3. The clay (20 g) was subjected to swelling by adding water (1 L) to the clay, and stirring it for 5 h and the mixture was finally centrifuged and dried. In the next step, the clay was treated with NaCl solution (1 M) and was aged for 24 h. The 20 g clay was separated, dried, and then refluxed with freshly prepared zirconium oxychloride (0.1 M) solution for 24 h at 100[degrees]C to obtain zirconium pillared montmorillonite. It was separated and dried after loading zirconium salt and its final weight was 28 g (an increase of 8 g). The final step is the intercalation of the complex in the clay layers and is shown in this figure. The clay (20 g) from this step was taken and refluxed with the Cu-Cu complex (1 g) dissolved in acetonitrile (250 mL) for 24 h at 80[degrees]C. The final catalyst thus obtained was separated, washed with acetone, dried and an increase in weight of 0.9 g was observed.
The oxidation reactions were performed in a high-pressure stainless steel reactor with a capacity of 300 ml, equipped with gas delivery system and sampling lines. The reactor was initially charged with 100 ml, cyclohexane, 1 g of catalyst, and 0.35 MPa oxygen, then heated to the required temperature for the desired reaction time using oxygen as the oxidant. An on/off controller was used for controlling the temperature with a chrome alloy thermocouple for temperature sensing. The products obtained after reaction were analyzed by gas chromatography (GC) using a fused silica capillary column (0.25 mm x 50 m long film having thickness 0.25 [micro]m) with flame ionization detector and the gas chromatography-mass spectroscopy (GC-MS) was carried out using Shimadzu QP-2000 instrument.
Test for the Absence of Cyclohexyl Hydroperoxide in the Reaction Mass
The presence of CHHP in the product mixture can be demonstrated by adding excess triphenylphospine to the product formed due to which the former is converted to alcohol and can be detected as a peak corresponding to the alcohol. The GC analysis of the reduced sample confirmed the presence of CHHP molecule (Gange et al., 1981; Shul'pin, 2002). In the presence of our catalyst, on carrying similar experiments by adding triphenylphospine, the peak intensity of the cyclohexanol in the GC analysis of the original product and the reduced sample remains unchanged. Thus, it was concluded that in our case there is no CHHP in the product mixture.
CHARACTERIZATION OF THE CATALYST
FTIR Analysis of the Complex
The FTIR analysis was carried out on a Bruker Vector 22 instrument in the 4000-400 [cm.sup.-1] wave number range. The samples were ground with KBr and pressed to 1 mm thick film. Examination of the FTIR spectra was useful in showing the formation of the complex and its various intermediates based on the frequencies of the C=N (1533 [cm.sup.-1]) and C=O bond (1668 [cm.sup.-1]). The C=O bond weakens in the final step of the complex synthesis when CuCuL1' is reacted with 1, 2-phenylenediamine.
CHN Analysis of the Macrocyclic Complex
The CHN analysis was carried out in an elemental analyzer (CE 440 Leimann Labs, Inc., Exeter Analytical Inc., North Chelmsford, MA). Helium was used as the carrier gas and 3-5 mg of the sample was required for this analysis. The percent of Carbon, Hydrogen, and Nitrogen present in the complexes were experimentally determined (Carbon 57.6%, Hydrogen 3.9%, and Nitrogen 6.9%) and these values were compared with calculated theoretically. Since the complex was prepared using cupric acetate the complex was assumed to have (C[H.sub.3]COO-) group and its molecular formula was taken as CuCuL1 [(C[H.sub.3]COO).sub.2] [L1=[(C[H.sub.3][C.sub.6][H.sub.2][(CH).sub.2][O.sub.2][N.sub.2] [C.sub.6][H.sub.4]).sub.2]] and the theoretical values were calculated as Carbon 57%, Hydrogen 3.91%, and Nitrogen 7.83% which was within 5% of the experimental values.
Thermogravimetric Analysis (TGA)
The TGA analysis was done using Perkin-Elemer instrument in [N.sub.2] atmosphere. The CuCuL1 complex was heated from 40 to 900[degrees]C at the rate of 10[degrees]C/min and it was found that the complex was stable upto 250[degrees]C (Figure 4). Similarly the TGA of the final catalysts was done by heating the sample from 50 to 800[degrees]C at the rate of 10[degrees]C/min and it was found that it was stable till 600[degrees]C (Figure 5). One can explain the enhanced stability from the binding of the complex in the clay as follows. As shown in Figure 6, the montmorillonite clay comprised of negatively charged layers composed of two tetrahedral Si-O sheets. These formed sandwiched octahedral Al sheet with oxygen atoms at the apex shared by the octahedra with the tetrahedra. The excess negative charge was due to the partial substitution of the [Al.sup.3+] by [Mg.sup.2+] and in order to achieve electroneutrality, the layer charge is compensated by the presence of cations such as [Na.sup.+] and [Ca.sup.2+] in the interlayers (Bedioui,1995; Serwicka and Baharnwoski, 2004). These are held together by weak dipolar and van der Waal forces and the distance between them is known as basal spacing or c-spacing (Bedioui, 1995). In Figure 6, the complex has been shown to be within the clay and the energy of interaction between them is equivalent to the energy needed to break an ionic bond. There has been a considerable interest in the literature to predict the state of adsorbate in porous material and determine the phase equilibria of these (Gelb et al., 1999). In order to model these, it was assumed that the liquid was in a cage formed by the porous substances. The state of the adsorbate depended upon the pore geometry, the level of interaction between the liquid and pores and the chemical and geometrical heterogeneity. The observed chemical and thermal stability of the complex ionically bonded in the final catalyst may well be attributed to the cage (Figure 6) effect produced by the clay layers.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Energy Dispersive X-Ray Analysis (EDAX)
The EDAX analysis was carried out using FET QUANTA 200 Scanning Electron Microscope (SEM) and for this, the samples were first coated with gold under vacuum. It was found that the final catalysts contained 4.66 wt% copper.
Small Angle X-Ray Diffraction Analysis
Small angle X-ray diffraction measurements were done on ARL X'TRA X-ray diffractometer (Thermo Electron Corporation, Waltham, MA) equipped with Cu K[alpha] ([lambda] = 0.154 nm) radiation. The voltage and the current applied to the X-ray tube were 45 kV and 20 mA, respectively. The sampling width was set at 0.05[degrees] and the scanning speed was 3[degrees]/min (2[theta]=2[degrees]-30[degrees]). The X-ray diffraction patterns of the original montmorillonte clay and the final catalyst (CuCuL1 complex supported on montmorillonite clay) are given in Figure 7. The d-spacing was calculated from the 2[theta] value of the peak corresponding to plane (001) and it was found that the d-spacing of the original montmorillonite was 16.35 [Angstrom](2[theta]=5.4[degrees]) while the d-spacing of the final catalyst was 30.84 [Angstrom](2[theta]=2.86[degrees]). From this, it could be concluded that after the complex was loaded on montmorillonite there was an increase in its d-spacing and hence the complex was successfully intercalated between the layers of the clay.
RESULTS AND DISCUSSION
It was first demonstrated that there was no oxidation reaction in presence of copper nitrate salt. For this, cyclohexane was charged with the copper salt in the batch reactor, pressurized with oxygen and then reacted. The reactor was found to give no conversion of cyclohexane. The oxidation of cyclohexane with molecular oxygen in presence of Cu complex catalyst was conducted in the temperature range 145-190[degrees]C to get a high conversions. The above temperature range was chosen because below 145[degrees]C, the conversion was very low while above 190[degrees]C, though the conversion was high, a large amount of undesired products (D) were formed. From the GC and GC-MS analysis it was found that cyclohexanone was formed as the major product while the by-products were cyclohexanol and cyclohexene along with the undesired product (D). Based on the feed and the product concentration the % overall cyclohexane conversion = [N.sub.RC]/[N.sub.RF] x 100, % cyclohexanone selectivity = [N.sub.PC]/[N.sub.RC] x 100, and % cyclohexanone yield = [N.sub.PC]/[N.sub.RF] x 100 were calculated. In these, [N.sub.RC] was the number of moles of cyclohexane consumed, [N.sub.RF], the number of moles cyclohexane fed and [N.sub.PC], the number of moles of cyclohexanone formed. Similarly the selectivity and the yield for cyclohexanol and cyclohexene were calculated. The reaction was conducted for 8 h and a drastic change in conversion and yield were observed during the first 2.5 h of reaction. The overall conversion increased from 9% to 23.6% when the temperature was increased from 145 to 190[degrees]C (480 min reaction time). At 145[degrees]C, only cyclohexanone was formed as the product and after 480 min, the conversion was 9% and the selectivity towards the formation of cyclohexanone was 90.5%. When the temperature was increased to 160[degrees]C, cyclohexanol and cyclohexene were formed in small amounts along with cyclohexanone as the major product. With increase in temperature, the selectivity towards the formation of cyclohexanone and cyclohexanol decreased. Cyclohexene was formed only in small amounts and its selectivity was below 10% in the temperature range studied. The yield of cyclohexanone and cyclohexanol increases with increase in temperature so the decrease in the selectivity can be attributed to the increase in rate of formation of the undesired product (D).
[FIGURE 8 OMITTED]
In our earlier work (Anisia and Kumar, 2007) oxidation of cyclohexane was carried out using Fe-Cu complex catalyst. This gave very low rate of reaction but highly specific to the formation of cyclohexanone. In the present Cu-Cu complex catalyst the reaction was atleast twice as fast but cyclohexene and cyclohexanol were formed in addition to the main product. In this case the conversions of cyclohexane as well as the yield of the products were found to approach a steady state value with increase in time at all reaction temperatures. To confirm that the metal complex is not leaching at the reaction conditions studied, we carried out the following experiments.
I. The oxidation reactions were carried out using the spent catalyst and the conversion was found to be the same as in the case of the fresh catalyst. The reaction mass was checked for any copper salt and was shown to have no metal.
II. From the product, the catalyst was filtered and the product mixture was once again subjected to the same temperature and pressure. The overall conversion was measured before and after the catalyst was filtered and found to be unchanged indicating that there is no leaching of the active species.
Reaction mechanism that have been found in literature are now discussed. Moden et al. (2006) have investigated the kinetics and mechanism of cyclohexane oxidation in presence of MnAPO-5 catalyst. They have proposed that CHHP is an intermediate in cyclohexanol cyclohexanone formation. Their combined rates of formation were found to be first order in ROOH concentration and proportional to the redox active Mn sites. Nunes et al. (2005) have studied the mechanism and kinetics of cyclohexane oxidation (with iodosylbenzene) catalyzed by supramolecular manganese (III) porphyrins. They proposed a mechanism in which the cyclohexyl radical and OH groups combine rapidly to form cyclohexanol, which is further oxidized to cyclohexanone. The free radical mechanism of cyclohexane oxidation occurs through the formation of CHHP intermediate, which decomposes to cyclohexanol and cyclohexanone (present in almost equimolar amounts).
Since CHHP was shown not to be formed in the reaction mass, a reaction mechanism (Figure 8) has been proposed with intermediate in the adsorbed state. Oxygen is adsorbed on the ligand site (step 1) of the catalyst and is assumed to reach equilibrium with oxygen to form an activated species in the reaction mass. The reaction mechanism of this figure is based on the product distribution obtained from the experiments that were conducted and some of the pathways leading to the formation of the products were taken to be reversible in nature as the concentration of the products approached almost steady state values. The cyclohexane molecule in presence of the activated catalyst first forms a cyclohexyl radical anion intermediate, A (step 2). Intermediate A reacts with oxygen molecule forming a peroxy radical anion intermediate, B with the catalyst (step 3). This intermediate B forms cyclohexanone as shown in step 4 of Figure 8. This also can react with another molecule of cyclohexane forming cyclohexanone and cyclohexanol (see step 5). The intermediate A reacts with oxygen forming cyclohexanone in step G and cyclohexanol in step 7 and cyclohexane in step 8. Unidentified side products (D) are also formed from intermediate A (step 9) and cyclohexanone (step 10).
[FIGURE 9 OMITTED]
Following the reaction mechanism, we can write mole balance equations for each component of the reaction as given in Table 1. Using these equations, we carried out simulation employing Runge-Kutta 4 method (as needed for the Genetic Algorithm (GA) in this specific code for optimal curve fitting) with [DELTA]t= 0.01 min for numerically stable solution and calculated the concentrations of each component for 8 h of reaction time. The results were optimized with the experimental values by using GA code and for this the objective function (OF) (given below) was written as the sum of squares of the difference of simulated and experimental values of cyclohexane, cyclohexanone cyclohexanol, and cyclohexene.
OF = [([[CH].sub.sim] - [[CH].sub.exp]).sup.2] + [([[CHone].sub.sim] - [[CHone].sub.exp]).sup.2] + [([[CHol].sub.sim] - [[CHol].sub.exp]).sup.2] + [([[CHene].sub.sim] - [[CHene].sub.exp]).sup.2] (1)
In this study, the fitness function is taken as 1/(1 + OF) and the value of a string is known as the string's fitness which is evaluated. The crossover and mutation probability were varied and finally taken at 0.9 and 0.05, respectively. The random population was created using a random number generator with a random seed equal to 0.887. The optimization was done for different temperatures and the results of fitting the data of 180[degrees]C is shown in Figure 9. The simulated concentrations overlap the experimental data and the best fit activation energy and Arrhenius constants were determined and reported in Table 2 In Figures 10 and 11, the concentrations of the intermediate species obtained from simulation have been reported and it is observed that these concentration fall due to fall in amount of oxygen (Figure 12) present in the reaction mass. This suggests that a higher oxygen pressure would favour the forward reaction giving high yield of the products.
[FIGURE 10 OMITTED]
[FIGURE 11 OMITTED]
[FIGURE 12 OMITTED]
In the present work, a macrocyclic binuclear monometallic Copper complex has been synthesized and has been ionically bonded with zirconium pillared montmorillonite. From the small angle X-ray diffraction patterns it can be concluded that the complex is intercalated in the layers of montmorillonite as there is an increase in the d-spacing (from 16.35 to 30.84 [Angstrom]) after loading of the complex in the clay. The heterogeneous catalyst, thus prepared, was stable at high temperatures as confirmed by the TG analysis. This catalyst has been tested for its catalytic activity with the oxidation of cyclohexane, in which cyclohexanone was obtained as a major product and the by-products were cyclohexanol and small amounts of cyclohexene. It was also confirmed that the metal complex was not leaching under the conditions in which the reaction was conducted. The overall conversion increases from 9% to 23.6% when the temperature was increased from 145 to 190[degrees]C (480 min reaction time). A reaction mechanism has been proposed based on the product distribution. The optimal rate constants were determined using GA and the concentrations obtained from simulation were matched with the experimental data.
Manuscript received March 5, 2007; revised manuscript received March 10, 2008; accepted for publication July 8, 2008
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K.S. Anisia and A. Kumar * Department of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India
* Author to whom correspondence may be addressed. E-mail address: email@example.com
Table 1. Rate equation for each component No Rate equation 1 d[[C.sub.6][H.sub.12]]/dt = -K[k.sub.1][[C.sub.6][H.sub.12]] + [k.sub.2][[C.sub.6][H.sup.x-.sub.11] - [k.sub.5][[C.sub.6][H.sub. 12]][[C.sub.6][H.sub.11] O[O.sup.x-]] 2 d[[C.sub.6][H.sup.x-.sub.11]/dt = [k.sub.1][[C.sub.6][H.sub.12] - [k.sub.2][[C.sub.6][H.sup.x-.sub.11] - [k.sub.3][[C.sub.6][H.sup. x-.sub.11][[O.sub.2]] - [k.sub.6][[C.sub.6][H.sup.x-.sub.11] [[O.sub.2]] + [k.sub.7][[C.sub.6][H.sub.10]O] - [k.sub.8][[C.sub. 6][H.sub.x-.sub.11][[[O.sub.]].sup.0.5] + [k.sub.9][[C.sub.6][H. sub.12]O] 3 d[[C.sub.6][H.sub.11]O[O.sup.x-]]/dt = [k.sub.3][[C.sub.6][H.sup. x-.sub.11][[O.sub.2]] - [k.sub.4][[C.sub.6][H.sub.11]O[O.sup.x-] - [k.sub.5][[C.sub.6][H.sub.12]][[C.sub.6][H.sub.11]O[O.sup.x-] 4 d[[C.sub.6][H.sub.10]O]/dt = [k.sub.4][[C.sub.6][H.sub.11]O[O. sup.x-] + [k.sub.6][[C.sub.6][H.sup.x-.sub.11][[O.sub.2]] - [k. sub.7][[C.sub.6][H.sub.10]O] - [k.sub.12][[C.sub.6][H.sub.10]O] 5 d[[C.sub.6][H.sub.12]O]/dt = [k.sub.5][[C.sub.6][H.sub.12]][[C. sub.6][H.sub.11]O[O.sup.x-]] + [k.sub.8][[C.sub.6][H.sup.x-. sub.11][[[O.sub.2]].sup.0.5] - [k.sub.9][[C.sub.6][H.sub.12]O] 6 d[[O.sub.2]]/dt = -[k.sub.3][[C.sub.6][H.sup.x-.sub.11]] [[O.sub.2].sup.0.5] 7 d[[O.sub.2]]/dt = -[k.sub.3][[C.sub.6][H.sup.x-.sub.11][[O.sub. 2]] - [k.sub.6][[C.sub.6][H.sup.x-.sub.11]][[O.sub.2]] + [k.sub. 7][[C.sub.6][H.sub.10]O] - [k.sub.8][[C.sub.6][H.sup.x-.sub.11] [[O.sub.2].sup.0.5] + [k.sub.9][[C.sub.6][H.sub.12]O] - [k.sub. 10][[C.sub.6][H.sup.x-.sub.11]][[[O.sub.2]].sup.0.5] 8 d[D]/dt = [k.sub.11][[C.sub.6][H.sup.x-.sub.11]] + [k.sub.12] [[C.sub.6][H.sub.10]O] Table 2. Arrhenius dependence on rate constants as determined from the experimental data Rate constant E/R In A [Kk.sub.l] 5.80E+03 9.00E+00 [k.sub.2] 6.60E+03 1.60E+01 [k.sub.3] 1.20E+04 2.70E+01 [k.sub.4] 1.00E+04 2.40E+01 k 1.40E+04 3.10E+01 [k.sub.7] 9.30E+03 9.20E+00 [k.sub.8] 1.50E+04 3.10E+01 [k.sub.9] 6.40E+03 4.70E+00 [k.sub.10] 1.50E+04 2.90E+01 [k.sub.11] 2.10E+04 4.00E+01 [k.sub.12] 6.20E+03 -1.10E+00
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|Author:||Anisia, K.S.; Kumar, A.|
|Publication:||Canadian Journal of Chemical Engineering|
|Date:||Dec 1, 2008|
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