Ind. Eng. Chem. Res. 2010, 49, 4607–4613
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Hydroxylation of Phenol by Hydrogen Peroxide Catalyzed by Copper(II) and Iron(III) Complexes: The Structure of the Ligand and the Selectivity of ortho-Hydroxylation Edward A. Karakhanov,* Anton L. Maximov, Yulia S. Kardasheva, Vitaliy A. Skorkin, Sergey V. Kardashev, Ekaterina A. Ivanova,† Elena Lurie-Luke, Jeffrey A. Seeley, and Scott L. Cron Department of Chemistry, Moscow State UniVersity, Moscow, 119991, Russia
A number of complexes of copper(II) and iron(III) with different N,N and N,O ligands were tested as catalysts for the hydroxylation of phenol to dihydroxybenzene by hydrogen peroxide for the purpose of achieving a high catechol selectivity. Cu(II) complexes were demonstrated to give a high selectivity on catechol. The best selectivity was found for Cu(II) complex with 2,6-dihydroxypiridine. The best conditions for the selective formation of catechol were the reaction time of 15 min and a ratio of 2,6-dihydroxypiridine to copper greater than 3 (65 °C). The concentration of phenol and the reaction time had a dramatic influence on the catechol yield and selectivity for most selective catalysts. At high concentrations and reaction times, both the catechol yield and selectivity decreased, with tars being formed. Introduction Catechol is widely used in industry as an important intermediate in manufacturing pesticides and medicines and can also be used to produce perfumes (e.g., piperonal), dyes, photosensitive material, special inks, antioxidants and polymerization inhibitors, fungicides, light stabilizers, anticorrosive agents, and promoters. Catechol is used in the manufacture of the artificial flavors vanillin and ethyl vanillin. The world’s first fully synthetic “marine fragrance” Heliofresh, used in many popular perfumes as a trendy aroma, was successfully derived from catechol. Catechol is also used in the manufacture of the insecticides carbofuran and propoxur. The pharmaceuticals used in the treatment of Parkinson’s disease and hypertension, L-dopa and methyl L-dopa, are manufactured from catechol. The world production of catechol is more than 30 000 t.1 The current manufacturing process of catechol with hydroquinone involves a direct hydroxylation of phenol with peroxides. Only the Brichima process in Italy2 has used heavy metal compounds (e.g., small quantities of ferrocene and/or cobalt salts) as a catalyst (phenol reacts with 60% aqueous hydrogen peroxide at 40 °C). Catechol and hydroquinone are produced in the ratio of 1.5-4.1. The other methods include the reaction of phenol with 70% hydrogen peroxide or performic acid in the presence of phosphoric acid and catalytic amounts of perchloric acid at 90 °C (Rhone-Poulenc),3 hydroxylation of phenol with ketone peroxides (R-hydroxy hydroperoxides) formed in situ from a ketone and hydrogen peroxide in the presence of an acid catalyst (ratio phenol/hydrogen peroxide 20) (Ube, Rhone-Poulenc),4 and hydroxylation of phenol by hydrogen peroxide using TS-1 zeolite heterogeneous catalyst (Enichem).5 In all reported data for industrial production,6 the phenol concentration was high (more than 30%), phenol selectivity was less than 90% (for catalysis by transition metals not more than 85%), and hydrogen peroxide selectivity was 60-90% (lower for transition metal compound catalysts that deals with decomposition of hydrogen peroxide). Phenol * To whom correspondence should be addressed. Tel.: +74959395377. Fax: +7-495- 932-8846. E-mail:
[email protected]. † The Procter & Gamble Company.
conversions were not more than 25% on the basis of phenol. The amount of tars and byproduct was 10-20%. The lowest byproduct formation has been achieved using ketone peroxides (Table 1). The key problem of using transition metal catalytic systems deals with side reactions, especially when a high space time yield is achieved.8 Due to the complexity of the whole process, numerous reaction schemes have been found in the literature.9 In all reviewed literature, two types of byproduct, quinones and macromolecular tars, were formed (Figure 1).10-16 The overoxidation problem is solved, in industry, by using high phenol/hydrogen peroxide ratios along with high concentrations of phenol. Because the catechol and o-quinone formed are more easily oxidized than phenol, the reaction is carried out in a large excess of phenol. Thus, phenol/hydrogen peroxide molar ratios range from 2 to 20. The rate of the formation of catechol should be high enough to achieve high conversion of hydrogen peroxide in a short time to avoid overoxidation of catechol. A lower ratio leads to a higher conversion rate; however, if the conversion of phenol in such conditions is too high, then dihydroxybenzene will be further oxidized to produce acids and polymer tars and the selectivity of the reaction will be reduced.8 Therefore, under a high ratio of phenol/oxidant, phenol has an inhibition effect. The lower the conversion of phenol, the better the selectivity of the reaction to dihydroxybenzene. From the industrial point of view, it is preferable to have a high degree of conversion in order to keep the space time yield high. As mentioned above, a conversion should be Table 1. Comparison of the Phenol Hydroxylation Processes6-8 Rhone-Poulenc HC(O)OOH+ Ube industries process and HClO4, Brichima Enichem Rhone-Poulenc catalyst H3PO4 Fe(II)/Co(II) TS-1 ketone + acid phenol/H2O2 ratio % phenol conversion % phenol selectivity % H2O2 based yield % tar
20
3-10
4-10
20
5
10
25
4-4.5
90
80
88
90-95
70
50-75
70
80-90
10
20
12
5-10
10.1021/ie902040m 2010 American Chemical Society Published on Web 04/23/2010
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Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010
Figure 1. Phenol oxidation pathways.
preferably 5-30% through-put desirable catechol out. On the basis of this, we considered that a low ratio of phenol/hydrogen peroxide (greater or equal to 2) is preferable. The experiments’ design described in this article was based on this assumption. The most common oxidizing agents used in phenol hydroxylation are hydrogen peroxide, H202, and alkyl hydroperoxides.9e,17 Organic peroxides are more potent oxidizing agents than hydrogen peroxide; however, they are not environmentally friendly; can be explosive, corrosive, and toxic; and present extreme fire hazards. Many organic peroxides give off flammable vapors when decomposing. Further, because they are such strong oxidizing agents, combustible materials contaminated with residual organic peroxides can catch fire very easily and burn very intensely.18 The drawback of hydrogen peroxide is its low selectivity. Hydroxylation of aromatic compounds by H202 either through electrophilic (H2O2/acid/ketone)19 or free radical processes (Fenton’s 4 or Udenfriend’s s reagent)20 is usually not selective because of the further oxidation of the phenol ring due to the introduction of the activating OH group. This leads to formation of a mixture of acids and polymeric compounds.9e The reaction yields hydroquinione and catechol. The main challenge of catechol production by hydroxylation is a low selectivity toward catechol. The industrial catalysts give selectivity on catechol from 50 to 70%. On the basis of the above, the main drawbacks of these methods are the formation of high quantities of hydroquinone and o-quinone as byproducts and low yields on H2O2.21 In spite of a wide number of metal complexes that were tested as hydroxylation catalysts, only very few investigations have looked into the impact of ligand structure on selectivity for phenol’s oxidation. This publication deals with the hydroxylation of phenol to catechol, catalyzed by copper and iron complexes with some commercially available or easily synthesized enamine ligands (N,N and N,O ligands), and the study of ligands structure influence on ortho-selectivity in hydroxylation.
2. Experimental Section 2.1. Ligands 1-12. Ligands 1-12 were of commercially available reagent-grade quality. The purification of solvents was done according to literature procedures.22 2.2. Ligands 1s-9s. The ligands 1s-9s were synthesized according to the published procedures.23 About 10 mmol of the corresponding diamine dissolved in 8 mL of dry methanol was added into solution of 4 mmol of corresponding aldehyde dissolved in dry methanol. The mixture was allowed to stir at room temperature for 2 h. For compounds 1s-4s and 9s, the crude yellow precipitate was filtered and washed with 2-3 mL of cold methanol. The product was recrystallized from dichloromethane. The yields were from 89 to 96%. For compounds 5s-8s, the solvent was removed by evaporation. The remaining yellow oil was washed by diethyl ether repeatedly, and the precipitate was recrystalized from chloroform/hexane. The yields were from 80 to 85% 2.3. NMR and ESI Measurements. The 1H NMR measurements were recorded on a Bruker DXP 300 AVANCE spectrometer. Element analyses were performed on a Carlo Erba Element Analyzer (type 1106). ESI-MS spectra were studied using an Agilent 1100 SL Ion trap mass-spectrometer. 2.4. Hydroxylation of Phenol. A total of 4 mL of solution of catalyst (Cu(SO4)2 · 5H2O or Fe(NO3)3 · 9H2O and ligand) was added to 1 mL of solution of a corresponding quantity of phenol in water. To the reaction mixture kept under the corresponding temperature was added 1 mL of hydrogen peroxide solution. The concentration of hydrogen peroxide changed in such way as to achieve the desired molar ratio of phenol to H2O2. After the desired time, the reaction mixture was homogenized by acetone and analyzed by HPLC with UV and MS detectors (Agilent 1100 SL Ion trap). The quantities of phenol, catechol, and hydroquinone were determined using anisol as an internal standard.
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Figure 3. Synthesis of enamyne type ligands. Figure 2. Commercially available ligands 1-12.
3. Results and Discussion 3.1. Ligand Selection. A number of different types of ligands have been examined to identify a ligand that would provide the highest selectivity. In this article, we focus on metal complexes with N,O ligands and O,O ligands with aromatic fragments (Figure 2), taking inspiration from enzymes, recognized as active and selective oxidation catalysts, wherein the copper or iron coordinates to a number of N (e.g., from histidine) and O (e.g., carboxyls and hydroxyl groups) atoms. A number of ligands (1-7) were used without any modification (Figure 2). These ligands are the simplest models of such catalysts and provide the possibility to elucidate the impact of catalyst selectivity as a function of coordination environment. In a number of works, it has been demonstrated that complexes of iron and copper containing N,N (e.g., dipyridil, N,N-bis[2(2-pyridyl)-amin) or N,O ligands (salen type and bis(2-hydroxybenzyl)ethylenediamine type) have been active in hydroxylation of aromatic compounds by oxygen or hydrogen peroxide. These ligands form the (µ-ν2:ν2-peroxo)dicopper(II) complexes or high valence iron complexes that prove to be active toward selective hydroxylation of phenol.24 It has been shown that copper ions can increase the selectivity of hydroxylation,25 and in most cases, substituted phenols have been used as the substrates in such studies. The orientation of donor atoms in ligands 1-11 promotes the formation of chelates and prevents the oxidation of phenol catalized by noncoordinated ions. The aromatic fragment of ligands can enhance the oxidation and the reduction of the metal ion in the catalytic cycle. The ligands (5-12) that contain a hydroxyl or an amino group linked with an aromatic fragment can induce oxidation by scavenging free radicals and forming semiquinone-like particles.26 The charge transfer from the ligand to the metal ion could increase the catalytic activity.27 The synthetic Schiff-base ligands containing N,N- and N,Ochelate fragments are a family of attractive oxidation catalysts for a variety of organic substrates due to their cheap and easy synthesis as well as chemical and thermal stability.23 These ligands widely vary in their structures, flexibility, electronic nature, and the presence of additional donor atoms besides imino nitrogen. Further tuning of their coordination characteristics can be achieved by varying the nature of the aromatic fragment
Table 2. Hydroxylation of Phenol in Homogeneous System Catalyzed by Cu(II) and Fe(III) Complexesa copper complexes
iron complexes
selectivity conversion phenol selectivity conversion phenol on catechol to dihydroxybenzene on catechol to dihydroxybenzene [%] based on H2O2 [%] ligand [%] based on H2O2 [%] 1 2 3 4 5 6 7 8
94 91 90 87 95 89 91
3 4 2 4
