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Nitration of Phenol over a ZSM-5 Zeolite M. Arshadi,† M. Ghiaci,*,† and A. Gil‡ Department of Chemistry, Isfahan UniVersity of Technology, Isfahan, 8415683111, Iran, and Department of Applied Chemistry, Los Acebos Building, Public UniVersity of NaVarra, Campus of Arrosadia, E-31006, Pamplona, Spain
In this paper, liquid phase nitration of phenol over a solid acid catalyst, ZSM-5 zeolite, was investigated by nitric acid (60%) under various solvent and temperature conditions. The performance of the nitration in ortho and para positions of phenol is suggested to derive from the amount of catalyst and the solvent effects. The loading level of the substrate inside or outside the pores of the zeolite makes the relatively high selectivity to the ortho or para product. However, the experimental results are accounted for by the important role of solvent and the amount of ZSM-5 zeolite in ortho/para selectivity. 1. Introduction Zeolite-catalyzed shape-selective catalysis is a promising way for the synthesis of symmetrically substituted isomers for industrial chemicals and advanced materials. Types of zeolite, adjustment of pore radii and acidity, and/or deactivation of external acid sites are key factors for the enhancement of shape selectivity. Meanwhile, it is well-known that the shape selectivity of ZSM-5 is due to the properties of medium pore size. A Mobil research group has proposed that the high para selectivity of modified H/ZSM-5 zeolites is due to the product selectiVity; i.e., the intracrystalline diffusivity of the para isomer is much higher than that of ortho and meta isomers.1-4 However, Yashima et al. proposed that the primary product in the alkylation of alkylbenzene is the para isomer due to restricted transition state selectiVity inside ZSM-5 pores, and that the other isomers are produced through the subsequent isomerization of already produced para isomer.5-7 Several investigators have reported nitrating agents such as concentrated nitric acid, a two-phase system (ether-phenol/ water-NaNO3-NaNO2-H+),8 an ionic complex of N2O4 with 18-crown-6,9 nitration-oxidation with metallic HSO4- and NaNO3,10 nitrocyclohexadienone as the nitrating agent,11 acid anhydrides/triflates,12,13 metal nitrates,14-17 nitrogen oxides,18,19 other sources of NO2+ (nitronium salts,20,21 N-nitropyridinium salts,22 nitrogen oxide,23,24 and peroxynitrite25), and organic nitrating agents (acetyl nitrate, benzoyl nitrate, and trimethylsilyl nitrate).26-30 Recently, various nitrating salts such as Bi(NO3)3 · 5H2O,31 VO(NO3)3,14 Fe(NO3)3 · 9H2O,32 (Me4N)NO3,33 Mg(NO3)2 · 6H2O,34 and NaNO335 have also been reported. Regioselective nitration using solid acid catalysts36-42 is also used. Due to the attachment of the acid function to a solid surface, corrosion would be of less interest. Solid acids play efficiently the role of sulfuric acid in the phenol nitration reaction, assisting in the formation of nitronium species. Therefore, some researchers have investigated solid acids such as MCM-4143 and other zeolite frameworks,44 sulfonated ionexchanged resins (polystyrenesulfonic acid),45 clay supported metal nitrates,46 Fe3+ on K-10 montmorillonite,47 modified silica,48 silica-alumina and supported acids,49 and sulfated titania.50 The nitration of phenol selectivity to p-nitrophenol in high yields has been obtained by Tomasz et al.51 over metal * To whom correspondence should be addressed. Tel.: +98311 391 3254. Fax: +98311 391 3250. E-mail:
[email protected]. † Isfahan University of Technology. ‡ Public University of Navarra.
oxide solid acid catalysts. They have shown that the selectivity to p-nitrophenol changes with the catalysts and reaction conditions. Also, in some cases, the reaction has been performed in expensive media such as ionic liquids52 and microemulsions.53 The nitration of aromatic compounds is one of the most widely studied reactions, which has found extensive applications in the synthesis of a variety of fine chemicals. Nitroaromatic compounds are important chemicals that have applications as solvents, dyes, pharmaceuticals, agrochemicals, explosives, and plastics in industry. In particular, both o- and p-nitrophenols are intermediates in the synthesis of azo dyes and a number of pesticides, mainly insecticides (o-nitrophenol: carbofuran, phosalon; p-nitrophenol: parathion, parathion-methyl, fluorodifen) and, to a lesser extent, herbicides (p-nitrophenol: nitrofen, bifenox). The corresponding aminophenol gained by reduction is used as a photographic developer (o-aminophenol) and as an intermediate in the synthesis of the tuberculostatic 4-aminosalicylic acid and the analgesic 4-acetaminophenol (paracetamol).54 Nitration of phenols using the method of nitric acid in sulfuric acid has generally given complex mixtures containing o- and p-nitrophenols, dinitrated phenols, and products of the phenolic oxidation. Further, it is noteworthy that the typical yield of direct nitration never exceeds 60% because of the above-mentioned side reactions in most cases,55 which make these existing processes uneconomical. Environmental considerations nowadays would not allow such a means of disposal, and large amounts of unwanted byproduct. Additionally, regioselectivity is also one of the important issues and the typical ortho/para ratio is in the range 1.4-1.5.55 Obviously, there is a need for more regioselective control of electrophilic aromatic substitution. A survey of the literature shows the lack of 100% ortho nitration under the mild reaction conditions; however, there are a few reports of almost regioselective ortho nitration of phenols.14,29-50 Several solid acids tested so far mainly give p-nitrophenol as the selective product. However, shape-selective properties of medium pore size zeolites like ZSM-5 are well-known.56,57 This work describes our efforts involving the use of solid support ZSM-5 with a Si/Al molar ratio of 60 as catalyst in the direct nitration of phenol with nitric acid in the solution phase using various commonly used solvents. We have shown that, by changing the reaction conditions, selectivities to o-nitrophenol and p-nitrophenol are substituted and are forcefully dependent on the amount of the catalyst, solvent, and reaction conditions. Furthermore, in this work we have shown that with the proper
10.1021/ie901862q 2010 American Chemical Society Published on Web 05/03/2010
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
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Scheme 1
choice of medium one can achieve very high regioselectivity through the nitration reaction of phenol over ZSM-5. 2. Experimental Section 2.1. Materials. All reagents were purchased from Merck or Fluka and were used without further purification, except that solvents were treated according to standard methods. ZSM-5 zeolite with a SiO2/Al2O3 molar ratio of 60 was synthesized using tetrapropylammonium bromide (TPABr) as a template. A specific surface area of 321 m2/g was measured for this material. The structure and surface characteristics of the prepared zeolite sample were determined using X-ray diffraction (XRD) spectroscopy and scanning electron microscopy. The XRD powder pattern of as-synthesized sample is the same as those published previously58 and shows very good crystallinity. The crystals are well-defined and spherically shaped with a crystal size of around 300 nm. Elemental analysis for the SiO2/Al2O3 ratio in the prepared sample was performed by inductively coupled plasma emission spectroscopy. 2.2. Characterization Techniques. The acidity of the zeolite was evaluated from the Fourier transform IR (FTIR) spectra of adsorbed pyridine and 2,6-dimethylpyridine, measured using a JASCO FT/IR (680 plus) spectrometer. Approximately 40-50 mg of material, which had been previously calcined in air at 773 K for 4 h, was pressed (for 3 min at 15 metric tons/cm2 pressure under approximately 10-2 Torr vacuum) into a selfsupporting wafer of 0.9 mm diameter. The pretreated wafer was conducted with 26 mm Hg of pyridine for 3 h at 423 K and evacuated for 3 h at 423 and 573 K. After each treatment, an IR spectrum was recorded at room temperature. The number of Brønsted acid sites was determined by means of the integrated molar extinction coefficient values of 2.22 and 1.67 cm/µmol for the infrared absorption bands of pyridinium at 1450 and 1545 cm-1, respectively.59 The FTIR spectra of phenol in various solvents were recorded at 1 and 2 cm-1 resolution using a 0.016 mm KBr liquid cell for the solutions. 2.3. Reaction Condition. The nitration of phenol (Scheme 1) was carried out in the liquid phase in a 50 cm3 two-necked, round-bottomed flask immersed in a thermostated bath and equipped with a reflux condenser and a magnetic stirrer. A typical reaction run was as follows: a known amount of catalyst, preactivated in air for 2 h at 673 K, was suspended in a solution of 5 cm3 of freshly distilled 1,2-dichloroethane (solvent), 2 mmol of nitric acid (60%), and 2 mmol of phenol. The reaction mixture was heated to several reaction temperatures and stirred for 2 h. When the reaction was complete, the reaction mixture was filtered. The reaction products were analyzed using an Agilent 6890 gas chromatograph (GC) system equipped with a flamer ionization detector (FID). An HP-50 capillary column connected to the FID was used for the analysis. The reaction was performed in various solvents such as n-hexane, cyclohexane, CCl4, diethyl ether, dichloroethane (DCE), CH2Cl2, acetone, CH3CN-ethyl acetate (1:1), CH3CN, methanol, ethanol, acetic acid, and water.
3. Results and Discussion The effects of various operating parameters, such as the amount of the catalyst, the temperature, and the nature of the solvent, on the performance of the ZSM-5 zeolite catalyst were investigated. o-Nitrophenol (2) and p-nitrophenol (3) (Scheme 1) were the major reaction products obtained, together with small amounts of p-benzoquinone (4), 2,4-dinitrophenol (5), and 2,6dinitrophenol (6). The GC analysis did not show any mnitrophenol in effluents. Vione et al.60 observed m-nitrophenol via aqueous phase hydroxylation of nitrobenzene, but the absence of m-nitrophenol in our work implies that this potential pathway is negligible as a byproduct source of nitrophenols. A wide variety of solvents have been applied for this reaction including n-hexane, cyclohexane, 2,2,4-trimethylpentane (TMP), CCl4, diethyl ether, dichloroethane (DCE), CH2Cl2, acetone, CH3CN-EtOAc (1:1), CH3CN, methanol, ethanol, acetic acid, and water, and the results are summarized in Table 1. There were no reactions also at reflux temperature and even at a longer reaction time when ethanol, methanol, and water were used as solvents. The lowest conversion (2.7%) was obtained when 0.5 g of the catalyst was used and acetone was the solvent of choice. In solvents such as CCl4 and TMP, when the weight of the catalyst was 0.1 g, the lowest (32%) and highest (90.5%) conversions of phenol were obtained, respectively. Possibly, a nonpolar solvent such as CCl4 provides an added advantage that the byproduct, water, can be retained in the cages of the zeolite but blocks the active acid sites and decreases the conversion of phenol. Preferential formation of o-nitrophenol is observed in all solvents over 0.1 g of catalyst with the exception of cyclohexane. When the reaction was carried out in CCl4 or CH3CN-EtOAc, very high selectivity (99.9%) of the nitration reaction at ortho position was observed. The formation of similar amounts of both nitrophenol isomers (o- and p-nitrophenols) is found in cyclohexane. A higher selectivity toward nitration at ortho position indicates that the nitration reaction in some solvents, such as CCl4, CH3CN-EtOAc, and CHCl3, proceeds to a perceptible degree on the external surface of the ZSM-5 zeolite or occurs in the bulk of solvent. Therefore, it should be noted that, in these systems, the steric factors and dipole moment of the solvent probably play an important role especially in the aggregation of the reactants, conversion of phenol, and selectivity to o- and p-nitrophenols. The ZSM-5 structure consists of a three-dimensional channel system with relatively large channel intersections and pore openings limited by 10-membered rings. The dimension of the channels (5.3 Å × 5.6 Å and 5.1 Å × 5.5 Å) should be ideal for the para-selective nitration of phenol to p-nitrophenol (kinetic diameter of o-nitrophenol, 8.1 Å; of p-nitrophenol, 6.7 Å61). The zeolite should have been highly selective to p-nitrophenol in the case that transition-state selectivity was responsible for the selectivity enhancement.5-7 Thus, to demonstrate this idea, and in order to increase the ratio of para/ortho isomers, the
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Table 1. Nitration of Phenol with Nitric Acid on ZSM-5 Zeolitea
solvent
catalyst wt (g)
T (K)
conversionb (%) phenol
cyclohexane cyclohexane cyclohexane CCl4 CCl4 CCl4 n-hexane n-hexane n-hexane TMPc TMPc,d CHCl3 CHCl3 CHCl3 acetone acetone diethyl ether acetic acid acetic acid acetic acid dichloroethane dichloroethanef dichloroethane CH3CN-ethyl acetate CH3CN-ethyl acetate CH3CN-ethyl acetate CH3CNf CH3CN ethanol MeOH water water
0.1 0.5 0.5 0.1 0.5 0.5 0.1 0.5 0.5 0.1 0.5 0.1 0.5 0.5 0.5 0.5 0.5 0.1 0.5 0.5 0.1 0.5 0.5 0.1 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
298 298 reflux 298 298 reflux 298 298 reflux 298 298 298 298 reflux 298 reflux 298 298 298 reflux 298 298 reflux 298 298 reflux 298 reflux 298 298 298 reflux
86.0 88.7 92.0 32.2 69.2 97.4 76.3 77.3 83.3 90.5 100 70.5 78.6 95.5 2.70 3.60 79.4 76.7 87.0 91.5 75.4 85.9 90.6 79.1 91.5 98.9 84.8 96.1 NRe NR NR NR
selectivity (%) ONP
PNP
42.2 99.1 92.6 100 54.5 46.7 62.0 80.1 83.6 75.2 12.8 91.88 79.2 30.9 99.9 78.0 7.54 66.5 52.3 48.1 80.3 61.6 50.2 99.9 78.3 10.8 43.4 49.3
57.8 0.90 2.71 38.9 47.2 30.0 11.4 7.36 24.1 83.2 0.67 11.6 64.4 tracee 22.0 87.1 29.0 46.6 51.2 10.8 38.2 45.7 trace 21.6 88.2 53.5 45.6
BNQ
DNP
4.17
0.52
4.40 4.02 8.00 8.41 9.04 0.66 1.30 7.45 7.11 3.60
2.11 1.90
0.21 4.49 0.06
5.15
8.81 0.70 0.50 0.68 3 3.40
2.70 1.91 1.10
1.04 0.65 3.60 0.23 1.70
ortho/para ratio 0.73 110 34.1 999< 1.40 0.98 2.07 7.02 11.3 3.12 0.15 137 6.82 0.47 999 3.54 0.08 2.29 1.12 0.93 7.43 1.61 1.09 999 3.62 0.12 0.81 1.08
a Reaction conditions: phenol/HNO3, 1:1; reaction time, 2 h. ONP, o-nitrophenol; PNP, p-nitrophenol; BNQ, benzoquinone; DNP, dinitrophenol. The products were extracted with ether and analyzed by GC. Mass balances were excellent (>90-95%). c TMP, 2,2,4-trimethylpentane. d When the reaction is conducted with 2,2,4-trimethylpentane as the solvent, p-nitrophenol remains within the zeolite. e NR, no reaction. Trace )
