Synthesis of heterogeneous copper complex catalyst for oxidation of cyclohexane using molecular oxygen

Canadian Journal of Chemical Engineering, Dec, 2008 by K.S. Anisia, A. Kumar

INTRODUCTION

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).

EXPERIMENTAL STUDIES

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]

CuCuL1'

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].

CuCuL1

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].