Heteronuclear Macrocyclic Iron−Copper Complex Catalyst Covalently

Ind. Eng. Chem. Res. 2007, 46, 4787-4798

4787

Heteronuclear Macrocyclic Iron-Copper Complex Catalyst Covalently Bonded to Modified Alumina Catalyst for Oxidation of Cyclohexane M. Jhansi L. Kishore, and Anil Kumar* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur-208016, India

In this paper, we report the synthesis of FeCuL1 and FeCuL2 complexes of 2,6-diformyl-4-methylphenol and 1,2-phenylenediamine (L1) and 2,6-diformyl-4-methylphenol and 1,3-diaminopropane (L2). The FeCuL1 complex has been covalently bonded to alumina and used as a catalyst for the oxidation of cyclohexane. The complex catalyst thus-prepared was found to be thermally stable and was completely characterized by CHN, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDAX), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and NMR spectroscopy. The oxidation of cyclohexane has been carried out using molecular oxygen without any solvent, coreactant, or cocatalyst in the temperature range (398-483 K) with cyclohexanone as the only product. The GC-MS of the product formed showed the total conversion was as much as 7.7%. In order to explain the experimental data, a possible reaction mechanism has been proposed and rate constants (assuming single phase) at different temperatures were determined using an optimal curve fitting by applying genetic algorithm. The rate constants thus-determined were found to be functions of temperature only. From experiments carried out at different temperatures, we found that, for every rate constant, an Arrhenius-type relation could be established. Introduction Cyclohexanone, prepared by the oxidation of cyclohexane, is an important raw material for the polyamide industry for the production of nylon 66 and caprolactum for the production of nylon 6.1,2 The functionalization of unactivated C-H bonds of cyclohexane requires high pressure and temperature, and a number of catalysts have been developed. In this reaction, various oxidizing agents having active oxygen have been used, such as peroxides (like hydrogen peroxide,3,4 iodosobenzene,5 t-butyl peroxide,6 and ozone7), solvents (such as heptanol, 2-methylpropanal, and acetaldehyde), and cocatalysts8 (such as acetic acid, chloroacetic acid, and trifluoroacetic acid), and are being employed to obtain high conversions. For the commercial processes, cobalt (as acetate or palmitate) is the usual catalyst with molecular oxygen as the oxidant, and the product formed is an approximately equimolar mixture of cyclohexanol and cyclohexanone. Other catalyst systems using oxygen as the oxidant, such as iron and ruthenium catalyst,8 nanostructured iron and cobalt catalyst,9 immobilized cobalt catalyst,10 polymersupported cobaltous palmitate,11 and cobalt salen complex supported on silica12 have been reported in the literature. Very few data on kinetic rate constants have been reported in the literature. Several reactions have been proposed to explain various product formations in the oxidation of cyclohexane, but in all these, for the determination of rate constants, only principal reactions (forming cyclohexanone and cyclohexanol) have been considered. There is only one study reported on the noncatalytic oxidation of cyclohexane,13,14 in which 154 elementary reactions were considered for the formation of principal products and the rate constants for these reactions were calculated from Statistical Mechanical computations. The reported mechanisms for catalytic oxidation of cyclohexane consist of a chain reaction involving hydroperoxy free-radical initiation, propagation, and termination steps. The literature reports three simplified kinetic models * To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Kinetic models of cyclohexane oxidation.

involving irreversible reaction steps, and two of these models have been utilized in data fitting.15-17 In the first model (shown in Figure 1a), cyclohexane forms a hydroperoxide intermediate which is then converted into cyclohexanone, cyclohexanol, and an unidentified product (D). In the second mechanism (shown in Figure 1b), the formation of intermediate is not considered,

10.1021/ie0612055 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

4788

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 2. Steps of preparation of catalyst.

but further oxidation of cyclohexanol is terminated by the reaction with boric acid, forming boric esters. Kharkova et al.18 suggested an exhaustive model for noncatalytic oxidation based on experimental data reported in the literature and estimated the rate constants and the concentration of the intermediate free radicals RO2•, RO•, R•, and OH•. Poherecki et al.19 suggested a different catalytic model for cyclohexane oxidation, as is shown in Figure 1c. Unlike earlier models, a lumped kinetic model having 10 irreversible rate constants was proposed, and by assuming quasi-steady-state approximation, concentrations of

intermediate species were eliminated from the mathematical model. Using these reaction schemes, the determined rate constants were found to be dependent on species and catalyst concentration. Moden et al.20 reported the rate constants based on the redox properties of the active metal sites where cyclohexyl hydroperoxide is an intermediate in cyclohexanol and cyclohexanone formation using O2 as the oxidant and using MnAPO-5 catalyst. Loncarevic et al.21 studied the isothermal and nonisothermal oxidation of cyclohexane using polymer-supported (copolymer

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4789

Figure 3. FTIR of the final complex. Table 1. CHN and SEM-EDAX Analysis of the Complex theoretical C H N Fe Cu C H N Fe Cu

Figure 4. Structure of FeCuL2 complex.

of poly-4-vinylpyridine with divinylbenzene) catalysts with different contents of metal ions in the temperature range 110170 °C. The rate constants obtained were found to depend upon the concentrations of reacting species and the content of metal ions. Their dependence on the concentrations of species was explained to arise from complex interactions between the reaction medium and the heterogeneous catalyst. Suresh and co-workers22-25 report cyclohexane oxidation and claimed23 that the reaction medium consists of two phases (gas and liquid). They reported that the reaction was autocatalytic and zero order in oxygen, and a kinetic model was developed based on simplification of the accepted free radical scheme. They studied the interaction of kinetics and mass transfer and showed that, in an autocatalytic system, the dissolved oxygen level rises to saturation and falls as the rate of reaction increases. A study of the literature indicates the use of multimetallic catalysts to improve the catalytic efficiency and specificity, and this study is an effort in this direction. We further observed that, using multimetallic catalysts, the heat of mixing for different salts determines the state of the metal on the support26 (as ideal solution, solid solutions, ordered solution, mono- or

experimental

SEM-EDAX

FeCuL1(NO3)3(CH3COO)2H2O 43.64 43.66 5.7 3.119 10.48 8.158 4.95 28.66 FeCuL2(NO3)3(CH3COO)2H2O 39.8 39.914 4.03 3.995 11.62 11.931 3.04 23.86

biphasic or surface alloys) and, in this way, affects the performance of the catalyst. The use of multimetallic complexes for catalysis is a step toward developing a system where ∆Hm has no role. Multimetallic complexes have been known in the early development of modern chemistry, and the literature has mostly focused on their preparation and properties. These complexes are known to provide new reactivity patterns, because the interactions between the metals in these complexes help in promoting reactions, as they will have greater oxidizing power. Shul’pin studied the oxidations of alkanes near room temperature using hydrogen peroxide catalyzed by monometallic transition metal complexes of vanadium, gold, and manganese,27,28 which give their corresponding alcohols and ketones. In this paper, we report the synthesis of a macrocyclic FeCu complex that is covalently bonded to modified alumina. As a result of this, the thermogravimetric (TG) analysis shows that the thermal stability of the bound complex improves from 285 °C (for the complex) to 625 °C (of the final catalyst). We have studied the oxidation of cyclohexane with this catalyst at different temperatures (125-210 °C) and pressures (7-35 atm) and found that the major product formed for our catalyst was cyclohexanone with small amounts of cyclohexanol in the ratio 16:1. We have then proposed a new reaction mechanism and determined the rate constants by optimization of the concentrations of all the components of the reaction mixture. Our study shows that the rate constants can be expressed in the usual

4790

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 5. FTIR of the final catalyst.

Figure 6.

1H

NMR spectrum of FeCuL2 complex.

Arrhenius form and are independent of the concentrations of the species in the reaction mass. 2. Experimental Section 2.1. Preparation of Catalyst. 2.1.1. Step 1: Preparation of FeCu Complex. The 2,6-diformyl-4-methylphenol needed for cyclic complexing agent is prepared following the procedure given in ref 29. The NMR spectrum (in Figure 6) of the dialdehyde we prepared shows singlets at 11.42 (phenolic), 10.2

(aldehydic), 7.74 (aromatic), and 2.36 ppm (methyl), and it is consistent with that of the assigned structure and matches with that given in ref 29. The reactions forming the cyclic complex with the zirconium are shown in step 1 of Figure 2, and the procedure of its preparation is given below. 2.1.1.1. Formation of FeL′ in Reaction 1 of Step 1. To 50 mL of N,N-dimethylformamide at 313 K, 2,6-diformyl-4methylphenol (0.95 g, 0.012 mol) is added. 1,2-phenylenediamine (0.65 g, 0.006 mol) is added to this solution. To this solution, ferric nitrate (2.42, 0.006 mol) is added and the

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4791

Figure 7. Molecular structure of the complex (prepared using ferric nitrate). Table 2. Crystal Data and Structure Refinement Parameters empirical formula formula weight temperature wavelength crystal system, space group unit-cell dimensions volume Z, calculated density absorption coefficient F(000) crystal size theta range for data collection reflections collected/unique completeness to theta ) 28.30 absorption correction refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole

C24H22CuFeN6O9 657.87 293(2) K 0.710 69 Å orthorhombic, P-1 a ) 8.565(5) Å, R ) 90.000(5)° b ) 18.262(5) Å, β ) 90.000(5)° c ) 32.333(5) Å, γ ) 90.000(5)° 5057(3) Å3 8, 1.728 Mg/m3 1.482 mm-1 2680 0.1 × 0.1 × 0.05 mm 2.32 to 28.30° 32164/6232 [R(int) ) 0.0688] 99.0% empirical (SADABS) full-matrix least-squares on F2 6232/0/355 1.053 R1 ) 0.0645, wR2 ) 0.1518 R1 ) 0.0794, wR2 ) 0.1597 1.737 and -1.235 e‚A-3

precipitate of FeL′ complex formed is filtered, washed with diethyl ether, and dried. The Fourier transform infrared (FTIR) spectrum shows the presence of functional groups CdN at 1690 cm-1, CdO at 1614 cm-1, and C6H5 O at 1223 cm-1. 2.1.1.2. Formation of FeCuL′ in Reaction 2 of Step 1. The cupric acetate (1.448, 0.008 mol) is dissolved in methanol (20 mL) at ambient temperature, and FeL′ (2.74 g, 0.008 mol) is added. The solution is stirred for 0.5 h, and the crystals of the complex appear. These are collected by filtration, washed with diethyl ether, and dried. The FTIR spectrum of this complex shows CdO at 1618 cm-1, CdN at 1508 cm-1, and C6H5 O at 1224 cm-1. 2.1.1.3. Formation of FeCuL1 in Reaction 3 of Step 1. The FeCuL′ (2 g) is dissolved in 30 mL of methanol, and 1,2phenylenediamine (0.336 g) is added. The crystals that appear are collected by filtration, washed with diethyl ether, and dried. The FTIR spectrum shows only CdN at 1526 cm-1, and the CdO at 1607 cm-1 peak appears as it forms CdN with 1,2phenylenediamine (shown in Figure 3). The complex was characterized by the CHN and the energy-dispersive X-ray (EDX) analysis for elemental composition and TG analysis for thermal stability and is discussed in the next section. We have also prepared a similar complex using diaminopropane as the cycling agent represented as FeCuL2 and shown in Figure 4. This complex has been used for characterization purposes; however, catalytic studies were not carried out using

Figure 8. SEM photograph of FeCuL2 complex crystals. Table 3. Selected Bond Lengths (Å) and Angles (deg) Cu(1)-N(1) Cu(1)-O(1) Cu(1)-O(2) Cu(1)-N(2) Fe(2)-N(3) Fe(2)-O(1) Fe(2)-O(2) Fe(2)-N(4) N(1)-Cu(1)-O(1) N(1)-Cu(1)-O(2) O(1)-Cu(1)-O(2) N(1)-Cu(1)-N(2) O(1)-Cu(1)-N(2) O(2)-Cu(1)-N(2) N(3)-Fe(2)-O(1) N(3)-Fe(2)-O(2) O(1)-Fe(2)-O(2) N(3)-Fe(2)-N(4) O(1)-Fe(2)-N(4) O(2)-Fe(2)-N(4)

1.956 (4) 1.960 (3) 1.961 (3) 1.980 (4) 1.949 (4) 1.967 (3) 1.970 (3) 1.981 (4) 92.11 (14) 169.78 (14) 77.67 (13) 97.73 (15) 169.30 (14) 92.47 (14) 91.74 (14) 168.41 (14) 77.30 (12) 99.38 (15) 166.90 (14) 91.96 (14)

this because this complex cannot be covalently bonded to the alumina support. 2.1.2. Step 2: Preparation of Modified Alumina. The alumina after drying at 773 K has been shown30,31 to have hydroxyl groups on its surface. Phenylisocyanate is prepared according to ref 32 by reacting benzoyl chloride with sodium azide at 273 K in the presence of benzene (eq 1, step 2 of Figure 2). The liquid and solid phases obtained are separated, and the liquid formed, i.e., phenyl isocyanate (confirmed by matching its FTIR for the presence of NdCdO group) is reacted with the dried alumina for 4 h at ambient conditions (eq 2, step 2 of Figure 2). The FTIR spectrum shows -NH group at 3337 cm-1, CdO at 1650 cm-1, and -OH at 3466 cm-1. The carbamated alumina (3 g) is reacted with 50 mL of 1,2-dichloroethane in the presence of ZnCl2 (5 mg) at 353 K for 2 h (eq 3, step 2 of Figure 2). The product is washed and dried, and its FTIR spectrum shows carbamate group -CONH- at 2341 cm-1 and -Cl at 694 cm-1. 2.1.3. Step 3: Covalent Bonding of the Complex to Alumina. The complex prepared (FeCuL1) in step 1 is dissolved in methanol and reacted with the modified alumina at 333 K for 4-6 h in the presence of a Lewis acid catalyst ZnCl2 (eq 4, step 3 of Figure 2).33,34 The alumina catalyst thus-obtained is washed and dried. The FTIR of the final catalyst showing the

4792

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007

Figure 9. SEM photographs of the catalyst.

bonding of the complex at the Cl group is shown in Figure 5, where the peak for -Cl is disappeared.35 In order to confirm that the complex is indeed chemically bonded with the modified alumina, we have also carried out the similar bonding of the complex with a small-molecularweight compound like t-butanol. In the first step, phenyl isocyante is reacted with t-butanol. Its FTIR shows the phenyl group -CH- at 3035 cm-1 and aliphatic -CH2- at 2940 cm-1. In the next step, it is reacted with dichloroethane, and its FTIR shows the presence of -Cl at 780 cm-1. The final step consists of binding the complex with the carbamated t-butanol compound. The FTIR of the final product shows the reduction of the peak corresponding to the Cl group, in this way suggesting that the FeCu complex is attached to the carbamate modified t-butanol. 2.2. Reaction Studies. The oxidation of cyclohexane has been carried out in a batch reactor made of stainless steel of 500 mL volume. The reactor is provided with an inlet for introducing gas and collecting the sample, a pressure gauge to monitor the pressure in the reactor, a thermocouple to measure the temperature inside the reactor, and a furnace to heat the reactor to the required temperature. The temperature is maintained using an

on-off temperature controller. Cyclohexane (100 mL) is fed into the reactor along with 1 g of catalyst. The reactor is then closed and O2 is filled in the reactor at a pressure of 100 psi with a cyclohexane-to-oxygen ratio of 1:10. The reaction is carried out in the temperature range of 398-483 K using FeCuL1/Al2O3 catalysts with a reaction time of 8 h. 3. Characterization of the Catalyst The complex prepared is characterized by FTIR and NMR spectroscopy, and its composition has been confirmed by CHN analysis and scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDAX) analysis. The TG analysis of complex and catalyst were taken to test the thermal stability. The single-crystal X-ray analysis of the complex has been carried out, and its electronic structure has been derived. The products obtained after reaction were analyzed by gas chromatography (GC) using a fused silica capillary column, 0.25 mm × 50 m film thickness 0.25 micron with flame ionization detector, and the gas chromatography-mass spectroscopy (GCMS) was carried out using a Shimadzu QP-2000 instrument.

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 4793

Figure 10. TPD profile of the catalyst.

(JEOL-400)) of the FeCuL2 complex in d6-DMSO (dimethyl sulfoxide) is taken, which is shown in Figure 6. The NMR shows singlets at 9.149 (phenolic), 3.378, and 2.072 ppm (aliphatic). The higher intensity at 2.072 ppm shows that the methyl peaks are overlapped with the aliphatic protons of diaminopropane. The peaks are observed only in the 0-10 ppm range, suggesting that the magnetic spin due to the unpaired electrons of iron and copper get cancelled.

Figure 11. FTIR of the catalyst before and after saturation with ammonia. The arrows above show the active sites on the catalyst surface.

4. Results and Discussion 4.1. Characterization of the Complex. We have already given the IR of the complex (shown in Figure 3) at different stages of its preparation, confirming the formation of complex in Section 2.1. It shows the CdN at 1512 cm-1, which is seen in the structure of the FeCu complex of step 1 of Figure 2. The complex exists as FeCu(C2H3OO)2(NO3)3L1, where L1 is the ligand ((CH3C6H2OCHNC6H4)2), and (C2H3OO)2(NO3)3 is present from the metal salts ferric nitrate and copper acetate used during the complexation. The theoretical values of C, H, and N are calculated from the above structure and compared with the experimental values given in Table 1. In view of the above, the SEM-EDAX analysis of the complex has been carried out by coating it with gold under vacuum to make the sample conducting for electrons. The EDAX analysis gave C and N values and is also reported in Table 1. The difference between the theoretical and experimental values is