Jointly published by React.Kinet.Catal.Lett.Akadémiai Kiadó, Budapest Vol. 75, No. 2, 315-321and Kluwer Academic Publishers, Dordrecht (2002)

0133-1736/2002/US$ 12.00.© Akadémiai Kiadó, Budapest.

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RKCL3938

CLEAN SYNTHESIS OF ADIPIC ACID BY DIRECT OXIDATION OFCYCLOHEXENE IN THE ABSENCE OF PHASE TRANSFER AGENTS

Heng Jiang*, Hong Gong, Zhonghua Yang, Xiaotong Zhang andZhaolin Sun

Department of Material Science, Fushun Petroleum Institute, Fushun 113001, ChinaE-mail: hjiang78@hotmail.com

Received July 9, 2001In revised form November 6, 2001

Accepted November 26, 2001

Abstract

In the absence of phase-transfer agents, the ligand effects are studied for the cleansynthesis of adipic acid by direct oxidation of cyclohexene catalyzed byNa2WO4⋅2H2O with 30% hydrogen peroxide. In most cases, the isolated yield ofthe target product adipic acid is high if the ligand acidity is strong. Although theacidity of some phenolic ligands, L(+)ascorbic acid and 8-quinolinol is weak, theisolated yield of adipic acid is still high. It is demonstrated that the acid andcoordination effect of the ligand play the same important role in theNa2WO4⋅2H2O catalyzed oxidation of cyclohexene to adipic acid with 30%hydrogen peroxide. Kinetic investigations show that the hydrolysis ofcyclohexene oxide to 1,2-cyclohexandiol is the critical step and the acidity ofreaction system is important.

Keywords: Adipic acid, cyclohexene, catalytic oxidation, acidic ligand

INTRODUCTION

Adipic acid is an important intermediate utilized in the production of nylon-6,6. The usual industrial synthesis of this compound involves nitric acidoxidation [1]. However, N2O emission from nylon-6,6 production accounts for5 to 8% of the total amount released by man [2].

316 HENG JIANG et al.: ADIPIC ACID

For the catalytic oxidation of cyclohexene to adipic acid, earlier workersused 35% H2O2 and (N-n-C16H33pyridinum)3(PW12O40) or H2WO4 in tert-butylalcohol [3] or in other patented procedures either 40% H2O2 and [CH3(n-C8H17)3N]3PO4[W(O)(O2)2]4 in 1,2-dichloroethane [4] or 60% H2O2 and H2WO4

[5]. The byproducts were glutaric acid, peroxy acids, and 1,2-cyclohexanediol.With 35% H2O2 and a H2WO4 catalyst, only a trace amount of adipic acid wasobtained [6]. Oxidation with aqueous H2O2 as the oxidant is appreciated because water isthe sole expected side product [7]. In 1998, R. Noyori et al. described a veryefficient cleavage of olefins to carboxylic acids by Na2WO4⋅2H2O/[CH3(n-C8H17)3N]HSO4 with 30% hydrogen peroxide at 75-90°C in ca. 8 h [8]. As noorganic solvent and halide are involved, this economical method may be ofspecial industrial interest as an example of so-called „green chemistry”. Deng etal. employed peroxytungstate complexes as catalysts for the direct oxidation ofcyclohexene with 30% aqueous hydrogen peroxide [9,10]. Considering that thesynthesis of [CH3(n-C8H17)3N]HSO4 is complicated and tedious [11], we havereported the method of replacing [CH3(n-C8H17)3N]HSO4 with the simplesulfate and hydrochloride of a higher primary or tertiary amine [12]. Our furtherresearch showed that the acidity of ligand plays a very important role for theNa2WO4⋅2H2O catalyzed oxidation of cyclohexene with 30% aqueous hydrogenperoxide, and there is no need of phase-transfer agents.

EXPERIMENTAL

Materials and reagents

Analytical grade 30% aqueous hydrogen peroxide was purchased fromShanghai Yuanda Peroxide, Inc., and used as received. Chemically purecyclohexene was obtained from Shanghai Chemical Reagent No.1 Factory andwas distilled under Nitrogen before use. Analytical grade sodium tungstatedihydrate was purchased from Shenyang No.1 Chemical Reagents Factory. Theothers reagents are all analytical grade.

Catalytic oxidation

A 150 mL flask equipped with a magnetic stirring bar and a reflux condenserwas charged with 0.825 g (2.5 mmol) of Na2WO4⋅2H2O, 2.5 mmol of ligand,and 44.5 mL (440 mmol) of 30% aqueous H2O2. The mixture was vigorouslystirred at room temperature for 15 min and then 10.5 mL (100 mmol) ofcyclohexene was added. The biphasic mixture was then heated and refluxed for

HENG JIANG et al.: ADIPIC ACID 317

8 h with stirring at 1000 rpm. The homogeneous solution was allowed to standat 5ºC for 12 h, and the resulting white precipitate was separated by filtrationand washed with 10 mL of cold water. The product was dried at roomtemperature with a melting point of 151.0ºC to 152.0ºC. Adipic acid andbyproducts were determined on GC-MS (MAT90, Finnigan Company). In mostcases, the purity of adipic acid is more than 99.0% and there is no need torecrystallize it with water.

RESULTS AND DISCUSSION

As Hill pointed out [13], the ideal homogeneous oxidation catalyst should beselective, oxidatively and hydrolytically stable, oxidant green (O2 or H2O2) andsolvent green (H2O or CO2). Since the quaternary ammonium salt is toxic [14],our goal is to achieve real green oxidation of cyclohexene to adipic acidcatalyzed by Na2WO4⋅2H2O in the absence of quaternary ammonium salt. In the absence of quaternary ammonium salt as phase transfer agents, weinvestigated the effects of various ligands on the Na2WO4⋅2H2O catalyzedoxidation of cyclohexene to adipic acid with aqueous 30% hydrogen peroxide.The results are shown in Table 1. The pKa of ligands in Table 1 is the firstdissociation constant in aqueous solution at 25°C [15, 16]. It can be seen fromTable 1 that basic ligands (entries 40 - 44) and neutral ligands (entries 45 - 46)are not effective for the Na2WO4⋅2H2O catalyzed oxidation of cyclohexene. Inmost cases, the isolated yield of adipic acid (based on cyclohexene) is fairlyhigh when the pKa of the acidic ligand is in the range of 1 - 3. For example, theisolated yield of adipic acid increases in the sequence acetic acid (entry 10),formic acid (entry 9) and bromoacetic acid (entry 11). For aliphatic diacid(entries 12-15), the isolated yield of adipic acid decreases with increasingcarbon chain length because the acidity decreases in that order. Although thetarget product in the present catalytic system is adipic acid, it can also be usedas ligand. The same empirical rule can be observed from the aromaticcarboxylic acid ligands (entries 12-15). Strong acidic ligands such assulfosalicylic acid (entry 1) give excellent isolated yield of adipic acid. Theisolated yield of adipic acid is more than 75% when a relatively stronginorganic acid such as phosphoric acid (entry 34) and phosphorous acid (entry35) is employed as ligand. From these experimental facts, we believe that anacid effect of the ligand exists in the present catalytic system. Some exceptional circumstances appear in Table 1. Although the acidities ofphenols (entries 16-21), L(+)ascorbic (entry 3) acid and 8-quinolinol (entry 4)are very weak, the isolated yield of adipic acid is still very high. It can beinferred that ligand effects may exist for this reaction apart from the acidiceffect of ligands. Generally speaking, ligands can change the electronic and

318 HENG JIANG et al.: ADIPIC ACID

geometrical environment of the central metal atom. These changes affect thecoordination of reactant to the metal ion [17].

Phenols such as hydroquinone can be used as free radical inhibitors orantioxidants. They usually decrease the rate of free radical autoxidation. Fromthe experimental results for entries 16-21, we believe that the reactionmechanism may involve coordination catalysis.

Table 1

Ligand effect in Na2WO4-catalyzed oxidation of cyclohexene with 30% hydrogen peroxide forthe synthesis of adipic acid

No. Ligand [CAS RN] pKa Isolated yield ofadipic acid (%)

1 Sulfosalicylic acid [97-05-2] - 88.02 Salicylic acid [69-72-7] 2.98 81.53 L(+)ascorbic acid [50-81-7] 4.17 75.34 8-quinolinol [148-24-3] 5.10 75.95 ��picolinic acid [55-22-1] - 53.06 Nicotinic acid [59-67-6] - 53.37 Lactic acid [50-21-5] 3.88 8.08 Tartaric acid [87-69-4] 3.22 46.49 Formic acid [64-18-6] 3.77 57.910 Acetic acid [64-19-7] 4.75 13.911 Bromoacetic acid [79-08-3] 2.90 65.212 Oxalic acid [144-62-7] 1.23 80.413 Malonic acid [141-82-2] 2.85 72.914 Succinic acid [110-15-6] 4.21 18.415 Adipic acid [124-04-9] 4.43 23.716 Phenol [108-95-2] 9.95 77.417 Pyrocatechol [120-80-9] 9.45 77.718 Resorcinol [108-46-3] 9.44 74.119 Hydroquinone [123-31-9] 10.00 63.820 2-Aminophenol [95-55-6] - 62.121 2,4-Dinitrophenol [51-28-5] 4.09 71.822 Benzoic Acid [65-85-0] 4.21 54.523 Phthalic acid [88-99-3] 2.95 77.224 Anthranilic acid [118-92-3] 2.05 77.825 Acetyl acetone [123-54-6] 9.00 20.726 (S)-(+)-Arginine [74-79-3] - 16.827 L-Alanine [56-41-7] - 10.928 p-Toluenesulfonic acid [104-15-4] - 61.229 EDTA(2Na) [6381-92-6] - 51.930 Hydroxylamine hydrochloride [5470-11-1] - 63.831 Hydrazine dihydrochloride [5341-61-7] - 64.132 Hydrazine sulfate [10034-93-2] - 82.433 m-Phenylene diamine hydrochloride - 60.9

HENG JIANG et al.: ADIPIC ACID 319

Table 1 contd.

34 Phosphoric acid [14265-44-2] 2.12 76.835 Phosphorous acid [10294-56-1] 2.15 75.636 Metaphosphoric acid [37267-86-0] - 59.837 NaH2PO4

.2H2O [7558-80-7] - 12.438 Na2HPO4

.12H2O [7558-79-4] - ~039 Boric acid [11113-50-1] 9.24 10.540 Ethylidenediamine [107-15-3] - ~041 o-Phenylenediamine [95-54-5] - ~042 2,2'-Bipyridine [366-18-7] - ~043 1,10-Phenanthroline [66-71-7] - ~044 Quinoline [91-22-5] - ~045 n-C16H33(CH3)3N⋅Br [57-09-0] - ~046 (n-C4H9)4N⋅Br [1643-19-2] - ~0

According to the proposal of Noyori et al. [8], the reaction pathway involvesolefin epoxidation, vicinal diol oxidations, Baeyer-Villiger oxidation, andhydrolysis (Fig. 1). Usually, epoxide is found to be hydrolyzed favorably underacidic conditions [18]. Therefore, the role of the above acidic ligands is toaccelerate cyclohexene epoxidation and cyclohexene oxide hydrolysis. Theisolated yield of adipic acid increases significantly when the amounts of aceticacid and adipic acid as ligand are increased. These results demonstrate that thehydrolysis of cyclohexene oxide to 1,2-cyclohexandiol is the controlling step ofthe whole reaction and the acidic conditions in the reaction system is necessary.In the absence of acidic ligand, the isolated yield of adipic acid is about 30%when H2WO4 alone is employed as catalyst. However, the isolated adipic acidmust be recrystallized because H2WO4 is insoluble in water.

Fig. 1. Proposed reaction pathway for the synthesis of adipic acid by oxidation ofcyclohexene with 30% hydrogen peroxide (Noyori et al. [8])

CO O H

CO O H

H 2O 2 H 2OO

O H

O H

H 2O 2

O

O H

H 2O 2 O

O

O H

H 2O 2 O

O

O

H 2O

320 HENG JIANG et al.: ADIPIC ACID

In order to elucidate the reaction mechanism in Fig. 1, theNa2WO4⋅2H2O/C2H2O4⋅2H2O catalyzed oxidation of cyclohexene wasmonitored by GC. The kinetic curves are shown in Fig. 2. The cyclohexeneoxide produced in the initial stage is rapidly hydrolyzed to 1,2-cyclohexandiol,while 1,2-cyclohexandiol is converted to other intermediates. The yield ofadipic acid increases rapidly after 3 h.

Fig. 2. Kinetic curves for the oxidation of cyclohexene catalyzed byNa2WO4⋅2H2O/C2H2O4⋅2H2O with 30% hydrogen peroxide

Since unproductive decomposition of H2O2 is negligible under such W-catalyzed conditions, the oxidation requires only 4.4-fold amounts of H2O2 percyclohexene to obtain a satisfactory yield. Rapid stirring is necessary tofacilitate the biphasic reaction. The catalyst in the filtrate can be reused after itis concentrated on a rotary evaporator. For example, Na2WO4/H3PO4 can bereused at least four times (70% isolated yield). In the first run, the mainbyproducts in the filtrate are glutaric acid (2.0%) and 1,2-cyclohexandiol

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

Cyclohexene Cyclohexene oxide 1,2-Cyclohexandiol Adipic acid

Dis

trib

utio

n, m

ol%

Time, h

HENG JIANG et al.: ADIPIC ACID 321

(0.6%). After several times of reuse, the isolated adipic acid must berecrystallized because glutaric acid is cumulated in the reaction system.

REFERENCES

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