Ind. Eng. Chem. Res. 1998, 37, 2647-2653
2647
Liquid-Phase Cooxidation of Cyclohexane and Cyclohexanone over Supported Cerium Oxide Catalysts Cheng-Shen Yao and Hung-Shan Weng* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republic of China
CeO2/γ-Al2O3 is found to be the best catalyst for the liquid-phase cooxidation of cyclohexane and cyclohexanone among several supported cerium oxide catalysts. By using this catalyst, the conversions of cyclohexane and cyclohexanone exceed those by other supported catalysts under the same reaction conditions. Furthermore, the yield of the desired products surpasses those in our previous studies. In addition, this catalyst is resistant to acidity and is more stable in an acidic medium than other heterogeneous catalysts in our previous works. The induction period, as a characteristic of the free-radical reaction, is not observed if γ-Al2O3 is used as a catalyst. The -caprolactone is not formed via a free-radical mechanism but, instead, the Baeyer-Villiger mechanism. Experimental results also indicate that water profoundly influences the elusion of metals from solid catalysts; however, no cerium eludes from the CeO2/γ-Al2O3 catalyst under the present reaction conditions. The formation of dibasic acids and -caprolactone is also discussed. Introduction The commercial output of adipic acid is produced by a two-step process employing air and then nitric acid as oxidizing agents (Castellan et al., 1991). Loder (1940) initially proposed the one-step process using air instead of nitric acid; Asahi Chemical Industry and Gulf Oil Co. later improved it (Tanaka, 1974a,b; Schulz and Onopchenko, 1979). In both processes, some byproducts, including cyclohexanol, cyclohexanone, -caprolactone, succinic acid, glutaric acid, and water, are formed though in smaller quantities. Carbon oxides (CO and CO2) are gaseous byproducts. The one-step process is advantageous: (1) there is no risk of corrosion and (2) no nitrogen oxide must be recovered, thereby reducing the investment costs. However, this process yields somewhat conflicting results and has not found industrial applications. Homogeneous catalysts are used in either of the above processes. On the other hand, developing a commercially viable heterogeneous catalyst to alleviate some of the drawbacks in the homogeneous system has received extensive attention (Caloyannis and Graydon, 1971; Sadana and Katzer, 1974; Srivastava and Srivastava, 1975; Hronec, 1986; Shen and Weng, 1988a,b; Lin and Weng, 1993; Jun and Lee, 1993; Schuchardt et al., 1995; Spinace et al., 1995). According to those investigations, the most widely used catalysts in the liquid-phase oxidation of hydrocarbons are transition-metal oxides, among which lanthanides and actinides have the highest activity for catalytic oxidation of hydrocarbons due to their formation of nonstoichiometric metal oxides (Peter, 1981). Cerium oxide, which is by far the most abundant and inexpensive rare-earth metal oxide, has been employed as a catalyst for wet oxidation of acetic acid, ammonia, and poly(ethylene glycol) (Imamura and Doi, 1985; Imamura et al., 1986). In a study on the liquid-phase oxidation of cyclohexanone (Shen and Weng, 1990), we found that cerium * Corresponding author. E-mail: ncku.edu.tw. Fax: 886-6-2344496.
[email protected].
oxide catalyst had a similar activity but was more stable in an acidic medium than the immobilized cobalt catalyst (Shen and Weng, 1988a). In another study, introducing cerium-exchanged zeolite (CeX) as a catalyst in the liquid-phase oxidation of cyclohexanone led to a higher activity than that with cerium oxide as a catalyst (Yao and Weng, 1992). However, the cerium ion slightly dissolved in the acetic acid and, thus, the catalyst activity was lowered to some extent. Recently, in our preparatory experiments on liquidphase oxidation of cyclohexanone, applying the same kinds of unsupported cerium oxides as catalysts yields a similar activity and dibasic acid selectivity as those in our previous works (Shen and Weng, 1988b; Juang, 1990; Lin and Weng, 1993) despite changing the calcination temperature and cooling mode in the catalyst preparation. Such observations resemble those of applying the supported cerium oxide catalysts. In this work, six different kinds of supports are used to prepare supported cerium oxide catalysts to catalyze the liquid-phase cooxidation of cyclohexane and cyclohexanone. This work attempts to find an appropriate supported catalyst with the highest activity, selectivity, and stability. The routes for forming -caprolactone and dibasic acids and the effect of water content on the conversion and product distribution are discussed. Furthermore, the performances of these supported catalysts are compared with those of other types of catalysts. Experimental Section Chemicals. γ-Alumina, Kieselgel 60 (Merck, for synthesis), titanium dioxide, vanadium oxide(V) (Nippon Shinyaku, extra pure), Celite-545 (Wako, extra pure), and binder-free Linde sodium 13X zeolite (NaX, Lot No. 69856) were used as the catalyst supports. Preparation and Characterization of Catalysts. Supports were heated in a vacuum oven at 110 °C for 24 h and, then, were impregnated with transition-metal nitrates (Merck, GR) by an incipient wetness method.
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2648 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 1. Schematic diagram of the experimental apparatus. SV: shut-off valve. D: dryer. TV: three way shut-off valve. RD: rupture disk. S: stirrer. M: variable-speed motor. PI: pressure indicator. MV: magnetic valve. TC: thermal couple. R: autoclave with magnetic stirrer.
Next, the catalyst precursors were left to stand overnight and subsequently calcined at 550 °C in an air stream for 6 h. The prepared catalysts were then cooled to room temperature in a desiccator and made available for use. The method of preparation of the ceriumexchanged Linde 13X catalyst has been fully described (Yao and Weng, 1992). The binding energies of cerium on the catalysts were measured with a VG Escalab 210 photoelectron spectrometer, which was equipped with a magnesium anode operated at 15 kV and 20 mA. The catalysts were pelletized beforehand with a pellet die at a hydraulic pressure of 400 kg/cm2. All the binding energies were referred to as 284.6 eV of C(1s) in the supported catalyst. Then, X-ray diffraction (XRD) measurement was performed on a Rigaku D/MAX III.V diffractometer with a copper target operated at 30 mA and 40 kV with a 3 × 5 cm glass trayer and a scanning rate of 2°/min. The surface area and pore size distribution of catalysts were then determined with a Micromeritics ASAP 2400 surface analyzer. Finally, metal ions in the solution were determined by using an ICP (Jobin-Yvon THR1000). Equipment and Reaction Procedure. The experimental setup (Figure 1) consists of a 600-mL stainless steel autoclave (Parr 4563M) with a four-blade magneticdriven stirrer and proportional-integral-differential controller. Experiments were performed under a batchwise constant-temperature and -pressure mode. The reaction feed, 125 mL in volume, was charged into the reactor with the catalyst. It consisted of cyclohexane, cyclohexanone (as reactants), chlorobenzene (as internal standard), and acetic acid (as solvent, in some experimental runs, 5-10 mL of acetic anhydride was added as a water adsorbent) which were all purchased from Fluka (puriss. p.a. ACS; >99%). The stirrer speed was set to 300 rpm. After purging the air in the reactor with a nitrogen stream, the heater was turned on to heat the reaction mixture to a preset temperature. Next, the oxygen was admitted until the desired pressure was obtained, thereby initiating the reaction. During the reaction, the oxygen pressure was maintained constant by regulating the oxygen input via a needle valve to ensure that the fluctuation of the system pressure did not exceed (0.1 kg/cm2. The liquid samples were taken periodically and centrifuged for subsequent analysis. The reaction samples were then injected into a gas chromatograph (Shimadzu GC-8A) which was equipped with a flame ionization detector and a fused silica WCOT capillary column (Alltech; 0.53 mm × 30 m) containing SE-30 (thickness: 1.2 µm) as a stationary phase and nitrogen as a carrier with a progamming rate
of 5 °C/min from 80 to 120 °C. Then, dibasic acids were esterified with a BF3-methanol complex (20%; Merck, for synthesis) in an oil bath at 110 °C for 2 min before the sample was submitted to the same gas chromatograph with a 3 mm × 5 m stainless steel column (10% SE-30 on Chromosorb W, 80-100 mesh) with a programming rate of 5 °C/min from 130 to 160 °C (Metcalfe and Schmitz, 1961; Stovick, 1981). The gas chromatographs were recorded with a Chem-Lab data station interface. The esterification derivatives were then identified by a GC/MS (VG 70-250). Finally, the water content was measured on a Karl Fischer titrator (Orion AF8). Hydranal composite 5K as a Karl Fischer reagent and methanol (max. H2O < 0.005%) as a titration medium were purchased from Riedel-de Haen (Germany). Results and Discussion In our preparatory experiments, nitrate solutions of group VIII are impregnated on γ-Al2O3 for preparing the supported metal oxide catalysts. According to the activity test and inductively coupled plasma measurements of metal ions in the residue solution, the supported cerium oxide catalyst has the highest activity for the cooxidation reaction of cyclohexane and cyclohexanone and is not dissolved in the acetic acid under the reaction conditions. Consequently, the following sections focus primarily on the supported cerium oxide catalysts. As the support may actually contribute to catalytic activity and/or may influence the activity of active species, i.e., cerium oxide in this study, due to the interaction, six different kinds of supports are used to prepare supported cerium oxide catalysts; their performances are compared as well (Stiles, 1987). In addition, an attempt is made to clarify the effects of support on the catalytic properties of the supported cerium oxide catalysts. In doing so, the bare supports are also employed as catalysts for the same reaction. Physical and Surface Properties. Table 1 indicates that the BET surface areas of bare supports were reduced when they were impregnated with CeO2 or exchanged with Ce(III). In our previous studies, it has been shown that the BET surface area of the cerium oxide catalyst plays an important role in the activity and product distribution in the liquid-phase oxidation of cyclohexanone (Shen and Weng, 1990). The larger the BET surface area on the cerium oxide catalyst, the higher the conversion of the cyclohexanone. However, in the supported cerium oxide catalysts, the effects of the BET surface area on the catalytic activity and product distribution are still quite vague. The most prominent examples are CeO2/Kieselgel (SBET ) 426 g/m2) and CeO2/Celite (SBET ) 1.37 g/m2). Though the BET surface area of the former largely exceeds that of the latter, the activities (induction period) and dibasic acid yields of CeO2/Kieselgel are far less than those of CeO2/Celite. From XRD measurement, cerium oxides (fluorite) may be regarded as a tiny crystal deposited on the support’s surface. Haneda and Mizushima (1993) observed the same structural character by fluorescence measurement. X-ray photoelectron spectroscopies of Ce(3d) spectra of cerium oxide on supports are complicated. However, the cerium binding energies are only slightly different from each other. As shown in Table 1, all five kinds of supported cerium oxide catalysts, except Ce/ZX, have a Ce(3d5/2) binding energy featured
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2649 Table 1. Some Physical Properties of the CeO2-Supported Catalysts catalyst CeO2/γ-Al2O3 CeO2/Celite CeO2/TiO2 CeO2/Kieselgel 60 CeO2/V2O5 Ce/ZX (3.1 mequiv/g) CeO2d
BET, m2/g (125.6)a
124.3 1.31 (1.37) 34.6 (42.5) 463.4 (426.2) 2.5 (3.1) 542.3 (543.8) 23.80
pore size, nm (by BJH adsorption) (6.62)a
6.30 51.59 (73.04) 9.35 (12.32) 4.87 (3.52) 12.8 (16.92) 5.21 (11.82) 14.65
binding energy,b eV
XRD structure
882.9 882.6 882.0 882.3 882.7 883.3 882.5
fluorite/spinel fluorite/chistobalite fluorite/anatase amorphous c fluorite
a Values in parentheses are those of the support. b The Ce(3d ) was taken for comparison of the binding energy of cerium ions over 5/2 the supports. c From the XRD spectrum, the structure of Ce/ZX is basically the same as those of Na/ZX except that the numbers of the diffracted lines of Ce/ZX are less and smaller than those of Na/ZX. Thus, in the structure of Ce/ZX, there are much more “disordered” unit celsl than in Na/ZX. d CeO2 was prepared according to the procedure described by Shen and Weng (1990).
at around 882.5 eV which is the one measured for CeO2. It may be concluded that these catalysts are all in the same Ce(IV) state. To sum up, the surface characters of the supports have decisive effects on the conversion and products distribution of the liquid-phase cooxidation of cyclohexane, yet the BET surface area and CeO2 crystallinity affect the reaction, but the correlation is still not clear enough. Induction Period. After the reaction started, the reaction pressure remained constant for a period of time and then decreased rapidly. This constant-pressure period is referred to herein as the “induction period”. This phenomenon is a characteristic of the free-radical chain reaction (Emanuel et al., 1984). Semenov (1941, cited by Berezin and Denisov, 1966) formulated the theory of branching and degenerate-branching chain reactions to quantitatively account for the characteristics of the kinetics while oxidizing the organic substances with molecular oxygen. According to this degenerate-branching chain theory, the induction period is the time period for which a steady-state hydroperoxide (cyclohexyl hydroperoxide and ketocyclohexyl hydroperoxide) concentration is attained. However, this is not always true since, occasionally, no induction period is observed (Caloyannis and Graydon, 1971; Mukherjee and Graydon, 1967; Sadana and Katzer, 1974; Varma and Graydon, 1973). Herein, we did not observe an induction period if γ-Al2O3 and V2O5 were used as catalysts. According to some investigations, the induction period appeared only at higher catalyst loadings (Varma and Graydon, 1973; Neuberg et al., 1972; Neuberg et al., 1974). In a study on the aqueous-phase catalytic oxidation of phenol over copper oxide, Sadana and Katzer (1974) observed no induction period; however, the induction period would appear when some kind of support was added. The induction period for the liquid-phase autoxidation of cyclohexane lasts more than 6 h under the same reaction conditions. In this study, introducing cyclohexanone as a cooxidant can also shorten the induction period to 3.5 h. As Table 3 indicates, the use of a catalyst can further reduce the induction period, regardless of whether an induction period is observed. For the case of Kieselgel 60, the increase in the induction period might be attributed to the termination of free radicals during the initiation. Concentration vs Time Curves. Figure 2 displays typical concentration vs time curves of the liquid-phase cooxidation of cyclohexane and cyclohexanone over the CeO2/γ-Al2O3 catalyst. The cyclohexanone concentration gradually decreased once the reaction began and, then, leveled off when the products (dibasic acids and caprolactone) arrived at maximun values. On the other
hand, the cyclohexane concentration remained constant for a while (induction period) and then diminished. Notably, the reaction rate of cyclohexane in acetic acid was significantly lower than that of cyclohexanone. Under the same reaction conditions, the conversion of cyclohexanone in 6 h might exceed 70% when no cyclohexane was added. The addition of cyclohexane, although decelerating the reaction rate of cyclohexanone, shortens the induction period of the cyclohexane oxidation. The products in the liquid-phase cooxidation of cyclohexane and cyclohexanone are adipic acid, caprolactone, succinic acid, glutaric acid, and other undetectable minor byproducts. -Caprolactone is a raw material used to synthesize biotin (one of the constituents of vitamin B complex). Glutaric acid is a feedstock of plasticizers. In addition, succinic acid has many applications such as in seasonings, fine chemicals, dyes, and neutralizers. All of these compounds are also regarded as the desired products in the following discussion. Carbon oxides (including CO and CO2) are gaseous byproducts. Performances of Supported Cerium Catalysts. Table 2 compares various kinds of supported cerium catalysts in terms of the activities and products distribution on liquid-phase cooxidation of cyclohexane and cyclohexanone. This table also reveals the performances of some bare supports as catalysts. Compared with the bare supports, most of the supported CeO2 catalysts yield a much lower fraction of the undesired products and a higher yield of adipic acid in addition to a shorter induction period. V2O5 and CeO2/V2O5 are the most active among the bare supports and the supported cerium oxide catalysts, yielding the highest conversion rates of cyclohexane and cyclohexanone. However, they dissolve into the reaction mixture and collapse during the reaction. Although CeO2/Celite yields a high fraction of the desired products, its activity is inadequately high. Adding CeO2 to the TiO2 catalyst significantly increases the fractional yield of adipic acid. Nevertheless, the fractional yield of the desired products is not obviously improved. Using CeO2/Kieselgel yields higher conversion rates of cyclohexane and cyclohexanone than using the bare Kieselgel despite the fact that the fractional yields of dibasic acids still remain quite low. Interestingly, Kieselgel produces a lot of -caprolactone but no dibasic acids. Among these supported cerium catalysts, CeO2/γAl2O3 and CeX have the highest conversion of cyclohexane and cyclohexanone. Besides, the fractional yield of adipic acid is nearly the same as those using the immobilized Co-resin and CoAPO-5 molecular sieve. However, a small amount of cerium ion may elude from
2650 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 2. Typical concentration-time curves of the liquid-phase cooxidation of cyclohexane and cyclohexanone over supported cerium catalyst at 110 °C and 15 atm. Catalyst: CeO2/γ-Al2O3 (5%). Loading: 2.4 g/L. Table 2. Comparison of Activities and Products Distribution on Liquid-Phase Cooxidation of Cyclohexane and Cyclohexanone over Various Catalystsa catalyst
induction period, h
CeO2/γ-Al2O3 (5%) γ-Al2O3 CeO2/Celite (5%) Celite CeO2/Kieselgel (5%) Kieselgel 60 CeO2/TiO2 (5%) TiO2 CeX (3.1 mequiv/g) Linde 13X CeO2/V2O5 (5%)e V2O5e no catalyst
1 0 0.5 3 3.5 5.5 1 1 2 2.5 0 0 3.5
conversion,b % ANE ONE 36 29 27 33 23 0 33 26 36 17 63 79 21
40 56 30 39 29 16 47 30 55 8 75 76 30
CY
desired product,c % CA S G
A
undesired productd
5 4 5 7 13 11 5 5 6 22 5 7 8
25 18 27 24 36 82 32 25 20 29 4 7 4
42 39 46 33 28 0 23 26 49 11 52 47 32
8 21 7 23 9 7 19 24 8 15 11 9 45
14 7 14 9 10 0 15 13 11 20 17 19 11
6 11 1 4 4 0 6 4 6 3 11 11 0
a Reaction conditions: [ANE] ) 1.78 M, [ONE] ) 0.74 M, T ) 110 °C, P 2 O2 ) 15 kg/cm , reaction time ) 6 h (induction period excluded), vol. of reaction feed ) 125 mL, wt. of catalyst ) 2.4 g/L. b ANE ) cyclohexane, ONE ) cyclohexanone. c CY ) cyclohexyl acetate, CA ) -caprolactone, S ) succunic acid, G ) glutaric acid, A ) adipic acid. d Undesired product ) 100% - (CY + CA + S + G + A). e It was dissolved into the reaction mixture and collapsed during the reaction.
CeX in the course of reaction, especially when the water content is large. Comparison between CeO2/γ-Al2O3 and Other Heterogeneous Catalysts. Table 3 summarizes the performances of some heterogeneous catalysts which have been investigated in our laboratory. Among these catalysts, the CoAPO-5 catalyst exhibits a unique catalytic property. This catalyst can oxidize cyclohexane without adding cyclohexanone as a coreactant. Despite this advantage, the fractional yield of the desired product is low. This drawback should be overcome before the catalyst is to be employed for a commercial process. The CeO2/γ-Al2O3 catalyst yields a higher cyclohexane conversion and a much higher fraction of -caprolactone than the unsupported CeO2 catalyst, although the conversion of cyclohexanone is markedly lower. As a
higher conversion of cyclohexane is more important than that of cyclohexanone, we consider that yielding a higher cyclohexane conversion is an important characteristic of the CeO2/γ-Al2O3 catalyst. Notably, no cerium eludes from both catalysts. Although Co-resin and CeX yield higher conversions of cyclohexane and cyclohexanone than CeO2/γ-Al2O3, Co-resin produces a larger amount of undesired products and CeX gives a smaller quantity of adipic acid than CeO2/γ-Al2O3. Furthermore, active metals may elude from these two catalysts. In conclusion, the γ-Al2O3-supported cerium oxide catalyst has the following merits: (1) The catalyst’s stability exceeds that of other catalysts. No cerium ion is emitted from the catalyst structure. (2) The fractional yield of the desired products exceeds those employing other heterogeneous catalysts. (3) This catalyst yields
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2651 Table 3. Comparison of Activities and Products Distribution on Liquid-Phase Cooxidation of Cyclohexane and Cyclohexanone over Solid Catalysts catalyst catalyst loading, g/L [ANE], M [ONE], M rxn temp, °C PO2, atm rxn time, h induction period, h conv. of ANE/ONE, % dibasic acids yield, % adipic acid yield, % CA yield, % undesired products, % ion dissolution, % investigator
CeO2/γ-Al2O3 CeO2/Celite CeX CeX (7%) Co-resin Co-resin CoAPO-5 CoAPO-5 (2.9 mequiv/g) (3.1 mequiv/g) CeO2 CeO2 (3%) 16.7 0 1.16 98 15 5 0.2 0/60 81 55 0 19 8.6a b
23.1 0.75 0.35 98 15 5 0.25 53/51 79 66 0 21
