Langmuir 2006, 22, 7091-7095
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Exploitation of Electrostatic Field Force for Immobilization and Catalytic Reduction of o-Nitrobenzoic Acid to Anthranilic Acid on Resin-bound Silver Nanocomposites Subhra Jana, Surojit Pande, Sudipa Panigrahi, Snigdhamayee Praharaj, Soumen Basu, Anjali Pal, and Tarasankar Pal* Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India ReceiVed March 7, 2006. In Final Form: May 30, 2006 A new solid-phase catalyst has been designed and reported here for the catalytic reduction of o-nitrobenzoic acid to anthranilic acid. Electrostatic field force helps immobilization, in turn deposition of silver nanoparticles onto solid resin surfaces and reduction of o-nitrobenzoic acid through effective catalysis. While characterization of catalyst particles has been performed by different physical methods (XRD, XPS, SEM, TEM, and EDX) in a worthwhile fashion, selective reduction of o-nitrobenzoic acid has also been achieved conveniently (∼95%). Different thermodynamic parameters for the reduction reaction have been presented from varied experimental conditions. Novelty of this work lies with the catalytic efficiency of nanometer size silver particles immobilized solid-phase matrix for one step synthesis of anthranilic acid over bulk silver.
1. Introduction Fundamental knowledge and understanding of the physical and chemical properties of nanoscale metal particles is of considerable scientific as well as technological relevance. The reactivity of the planer surface has been well investigated, but there are many new unexplored aspects involved in the reactivity of nanoparticle surfaces.1 The properties of nanoparticles are of general importance in heterogeneous catalysis, surface science, and electrochemistry. A large surface-to-volume ratio and the enhanced surface atomic catalytic activity make metallic nanoparticles attractive catalysts compared to bulk materials.2-4 Such materials also receive attention because of their intrinsic sizedependent properties, e.g., their geometric structure, catalytic activity as well as magnetic property etc., and applications.5-8 One distinct property of nanoparticles is the large number of less coordinated atoms present at the surface of these particles.9 Metal nanoparticles on a variety of solid supports have been synthesized and found to be good catalysts for many reactions, e.g., silver and gold nanoparticles supported on polymer have been found to catalyze the reduction of aromatic nitro compounds.10 Similarly zinc and platinum nanoparticles supported on a mesoporous zeolite matrix exhibited high aromatizing activity in the conversion of lower alkanes.11 Paulas and his group12 described oxygen reduction on carbon-supported Pt-Ni alloy catalysts in aqueous acidic electrolyte at low temperature. Nanoscale cobalt particles * To whom correspondence should be addressed. E-mail: tpal@ chem.iitkgp.ernet.in. (1) Meier, J.; Friedrich, K. A.; Stimming, U. Faraday Discuss. 2002, 121, 365. (2) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Ri, H. C.J. Am. Chem. Soc. 2002, 124, 12094. (5) Jovin, T. M. Nat. Biotechnol. 2003, 21, 32. (6) Huynh, W. U.; Dittmer, J. J. Science 2002, 295, 2425. (7) Tessler, N.; Medvedev, V.; Kazes, M.; Kan, S. H.; Banin, U. Science 2002, 295, 1506. (8) Klimov, V. I.; et al. Science 2000, 290, 314. (9) van Hardeveld, R.; Hartog, F. Surf. Sci. 1969, 15, 189. (10) Praharaj, S.; Nath, S.; Ghosh, S. K.; Kundu, S.; Pal, T. Langmuir 2004, 20, 9889. (11) Vosmerikov, A. V.; Ermakov, A. E.; Vosmerikova, L. N.; Fedushchak, T. A.; Ivanov, G. V. Kinet. Catal. 2004, 45, 215.s. (12) Paulas, U. a.; Wokaun, G. C.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2002, 106, 1869.
dispersed in charcoal were used as catalyst for the PausonKhand reaction and so on. The important advantage of solid phase over solution phase synthesis13,14 is the simplified purification procedure and easy handling of multiple reaction vessels. As a result, such a solid phase synthetic technique has become an extremely powerful tool to the scientists in combinatorial chemistry research15-17 for accelerated drug discovery.18-20 Despite catalysis, such composite materials may find a wide range of applications in the areas of microelectronic devices, nonlinear optics, electrochemical sensor, bioanalysis21-23 chemical sensors,24 and capsules for controlled release of therapeutic agents.25 Anthranilic acid is a commercially important intermediate for synthesis of different dyes, perfumes and medicinal substances such as anticancer agents26 etc. Up to now several groups have devoted their efforts to synthesize it. In 1957, Lesiak et al. had synthesized anthranilic acid from o-nitroethylbenzene.27 Jan Bakke et al. prepared it from o-nitrotoluene.28 With the help of the Ullman reaction, Cook also synthesized it. Srinivasan, in 1957, described the synthesis of anthranilic acid from Shikimic acid-5-phosphate and L-glutamine.29 (13) Pradhan, N.; Pal, A.; Pal, T. Langmuir 2001, 17, 1800. (14) Worden, J. G.; Dai, Q.; Shaffer, A. W.; Huo, Q. Chem. Mater. 2004, 16, 3746. (15) Dong, A. G.; Wang, Y. J.; Tang, Y.; Ren, N.; Yang, W. L.; Gao, Z. Chem. Commun. 2002, 350. (16) Mayer, A. B. R.; Grebner, W.; Wannemache, R. J. Phy. Chem. B 2000, 104, 7278. (17) Worden, J. G.; Shaffer, A. W.; Huo, Q. Chem. Commun. 2004, 518. (18) Czatnik, A. W.; DeWitt, S.H. Eds.A Practical Guide to Combinatorial Chemistry; American Chemical Society: Washington, DC, 1997. (19) Jung, G., Ed.; Combinatorial Peptide and Nonpeptide Library; VCH: New York, 1996. (20) Chaiken, I. M., Janda, K. D., Eds.; Molecular DiVersity and Combinatorial Chemistry; American Chemical Society: Washington, DC. (21) Lu, C.; Wu, N.; Jiao, X.; Luo, C.; Cao, W. Chem. Commun. 2003, 1056. (22) He, L.; Musick, M. D.; Nicewarner, S. R.; Salinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 9071. (23) Shipway, A. N.; Lahav, M.; Blonder, R.; Willner, I. Chem. Mater. 1999, 11, 13. (24) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 892. (25) Caruso, F. Chem. Eur. J. 2000, 6, 413. (26) European Patent Office, Patent number: RU2187496, 2002. (27) Lesiak, T.; Schittek, W. Przemysl Chemiczny 1957, 13, 456. (28) Bakke, J.; Heikman, H.; Nystroem, G. Acta Chem. Scand. 1972, 26, 355. (29) Srinivasan, P. R. J. Am. Chem. Soc. 1959, 81, 1772.
10.1021/la0606300 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/30/2006
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Jana et al.
In this article, we have explored for the first time a facile approach for the catalytic reduction of o-nitrobenzoic acid (oNBA) to anthranilic acid (o-aminobenzoic acid) over a resinbound silver nanocomposite as catalyst in the presence of sodium borohydride. Such composite material has been synthesized through immobilization of specific silver precursor ions on the surface of anion-exchange resin beads followed by wet chemical reduction. Electrostatic field force out of the charged resin particles has been considered for both catalyst preparation and product synthesis. The as-synthesized catalyst particles were characterized by SEM, TEM, EDX, XPS, and XRD analysis. Thus, the present work is related to a simple and straightforward technique for the synthesis of resin-bound silver nanocomposites and its catalytic application to produce anthranilic acid. Again, at the end of the reaction, the catalyst particles remain active but get separated from the reaction mixture. To the best of our knowledge, there exists no other report of the exploitation of solid-phase nanocatalysts for the synthesis of anthranilic acid. 2. Experimental Section 2.1. Reagents and Instruments. All of the reagents used in this work were AR grade. Double distilled water was used throughout the experiment. Anion-exchange resin, SERALITE-SRA-400, was purchased from Sisco Research Laboratory, India. Silver nitrate (AgNO3) and HCl were obtained from Merck. o-NBA (from Aldrich) was used as received. A sodium borohydride (NaBH4, Sigma) solution was prepared freshly in ice-cold distilled water each time just before use. All UV-visible absorption spectra were recorded using a SPECTRASCAN UV 2600 digital spectrophotometer (Chemito, India) taking the solution in a 1-cm quartz cuvette. XRD was done in a PW1710 diffractometer, Philips, Holland, instrument. The XRD data were analyzed using JCPDS software. The XPS study was performed with ESCALAB-MK-II, U.K. SEM analysis was performed with a JEOL, JSM 5800 instrument, and an EDX machine (Oxford, Link, ISIS 300) was attached to the instrument to obtain its particle morphology. Transmission electron micrographs were obtained with an H-9000 NAR instrument, Hitachi, using an accelerating voltage of 300 kV. GC-MS analysis was carried out using a GCMS-QP5050A (Shimadzu Corporation, Koyoto, Japan) quadrupole mass spectrometer. The NMR spectrum was recorded using a Bruker 200 MHz instrument, and the solvent used was d6acetone. 2.2. Preparation of the Resin-Bound Silver Nanocomposite. The synthetic protocol of the resin-bound silver nanocomposite is as follows. The silver precursor, [AgCl2]-, complex was prepared by dissolving 0.3 g of solid AgCl in concentrated HCl solution in an ultrasonic bath. Next, the soluble silver precursor ions were allowed to exchange with Cl- ions of the neat chloride form of anion-exchange resin beads (R+Cl-) and kept overnight. The resin beads, on which silver precursor ions were immobilized, were washed several times with water to drain out the liberated HCl solution and un-exchanged [AgCl2]- and reduced with a freshly prepared ice-cold aqueous solution of sodium borohydride. The reduction of the attached silver precursor ions leads to silver nuclei and nanoparticles deposition onto the polystyrene bead. The as-prepared shining reddish black silver coated beads, [R(Ag)0]+Cl-, were washed thoroughly with distilled water, dried at room temperature (25 °C) under vacuum, and employed as the solid-phase catalysts for the reduction of o-NBA (Scheme 1). The solid matrixes remain stable for months. The mechanism of the above synthetic procedure is represented below. AgNO3 + HCl f AgClV + HNO3 AgCl + HCl f H+[AgCl2][H]
R+Cl- + H+[AgCl2]- f R+[AgCl2]- 98 [R(Ag)0]+Cl-
Scheme 1. Schematic Representation for the Synthesis and Catalytic Application of Resin-Bound Silver Nanocomposites
2.3. Catalytic Reaction. In a standard quartz cuvette of 1 cm path length, 2.5 mg of solid catalyst was taken along with aqueous solution of o-NBA (0.1 mM), and the volume of the solution was made up to 2 mL with water. Next, 0.2 mL of 0.1 M aqueous NaBH4 solution was added to the reaction mixture, and time dependent absorption spectra were recorded using a UV-visible spectrophotometer at 27 ( 2 °C.
3. Results and Discussion Anion-exchange resin is a polymer containing quaternary ammonium groups or amines as an integral part of the polymer lattice with an equivalent number of anions such as chloride, hydroxyl, or sulfate ions.30 SERALITE-SRA-400, a cross-linked polystyrene containing quaternary ammonium groups as an integral part, was supplied in the chloride form with an ionexchange capacity 3.5 mmol g-1. Silver precursor ions, i.e., large AgCl2- complex ion as it bears a negative charge, are effectively exchanged with the anion-exchange resin and immobilized on the resin beads by an electrostatic attraction even at a very low concentration,30 but they are not immobilized onto a cationexchange resin. 3.1. Characterization of the Catalyst Particles. 3.1.1. SEM Analysis. Features of elemental composition and morphology of resin-bound silver nanocomposites were investigated by scanning electron microscopy (SEM) assisted energy-dispersive X-ray analysis (EDX). Figure 1a shows the SEM image of a silver nanoshell coated resin bead. A calculation has been done to evaluate the shell thickness.31 An average thickness of ∼30 ( 5 nm of silver layer has been obtained on each polystyrene bead after their complete coverage. EDX spectra shown in Figure 1b also attribute it to the presence of metallic silver (3.31% silver loading) in the as-synthesized resin particles. 3.1.2. Transmission Electron Microscopy Studies. Transmission electron microscopy (TEM) was employed to visualize the size and shape of nanoparticles. Figure 2 represents the TEM image of silver nanoparticles on the surface of the resin beads. The silver coated catalyst particles were cryogrinded to fine powder. Then a drop of alcoholic suspension of the powder was placed on a 200 mesh carbon coated copper grid and dried under vacuum to evaporate the solvent. Next the photograph of the silver particles revealed spherical silver nanoparticles of ∼25 nm diameter in the resin matrix. 3.1.3 X-ray Photoelectron Spectroscopy. The direct evidence for metallic silver deposition on the surface of resin particles was investigated by X-ray photoelectron spectroscopy (Figure 3). The catalyst particle exhibits two specific peaks with binding (30) Vogel, I. QuantitatiVe Inorganic Analysis, 3rd ed.; The English Language Book Society and Longman: London, 1969; p 704. (31) Nath, S.; Ghosh, S. K.; Kundu, S.; Praharaj, S.; Panigrahi, S.; Basu, S.; Pal, T. Mater. Lett. 2005, 59, 3986.
From o-Nitrobenzoic Acid to Anthranilic Acid
Langmuir, Vol. 22, No. 16, 2006 7093
Figure 3. XPS spectra of [R(Ag)0]+Cl- particles.
Figure 1. (a) SEM image of a single silver nanoshell coated resin bead. (b) EDX spectra of resin-bound silver nanoparticles.
Figure 4. XRD pattern of (a) free resin beads and (b) resin-bound silver nanocomposites.
Figure 2. Transmission electron micrograph of silver nanoparticles on the surface of resin beads.
energies of 369 and 375 eV due to Ag3d5/2 and Ag3d3/2 electrons of Ag0 respectively. These peaks suggest the chemical deposition of Ag0 on the surface of polystyrene beads. 3.1.4. X-ray Diffraction Study. Figure 4, panels a and b, shows the X-ray diffraction (XRD) pattern of neat and silver nanoshell coated polystyrene beads, respectively. The five diffraction peaks above 30° are indexed to the (111), (200), (220), (311), and
(222) planes respectively, indicating the formation of metallic silver nanoparticles on the surface of the polystyrene beads due to the complete reduction of immobilized silver precursor ions with NaBH4 solution.31 The crystal size of the silver shell deposited over a resin bead was calculated as ∼28 nm from XRD line width, FWHM using Schrrer equation. Again, the XRD pattern of silver nanoshell coated resin beads is similar to that of original metallic silver, confirming that the resin beads are covered by a silver nanoshell.13 3.2. Product Characterization. 3.2.1. UV-Visible Spectroscopy. Anthranilic acid has been synthesized by catalytic reduction of o-NBA over resin-bound silver nanocomposites, and the progress of the catalytic reaction was studied spectrophotometrically. Figure 5 shows typical UV-visible absorption spectra for the successive reduction of o-NBA by the catalyst particles. An aqueous solution of o-NBA shows a distinct spectral profile with an absorption maximum at ∼267 nm as shown in trace a (Figure 5). Upon the addition of a freshly prepared icecold aqueous solution of NaBH4 to this solution, there was no shift of the λmax value at all; only a slight decrease in the absorbance value was noted, and the peak position remained unaltered for a couple of days in the absence of any catalyst particle. After the addition and proper mixing of 2.5 mg of the resin-bound silver nanocomposite in the reaction mixture, it had been observed
7094 Langmuir, Vol. 22, No. 16, 2006
Figure 5. UV-visible spectra for (a) o-NBA and (b-f) successive reduction of o-NBA catalyzed by silver nanoshell coated cationic polystyrene beads at an interval of 2 min Conditions: [o-NBA] ) 0.1 mM; [NaBH4] ) 0.1 M; [R(Ag)0]+Cl- ) 2.5 mg.
Figure 6. Absorption spectra of (a) authentic vs (b) product anthranilic acid.
that the peak at ∼267 nm gradually red-shifted and finally moved to ∼307 nm as shown in trace f (Figure 5). Thus, the progress of the reaction is visualized from the red shifting of the peak from ∼267 to ∼307 nm. This new peak at ∼307 nm has been attributed to anthranilic acid and was verified by superimposing the spectra of the authentic sample under identical reaction conditions as shown in Figure 6. However, the reaction does not proceed at all in the experimental time scale while the neat resin (without any silver loading) was used under identical reaction conditions in the presence of NaBH4. It may be spelt out that when both of the groups in the benzene ring are either electron withdrawing or electron donating the effect of the group causing larger shift is always considered for the shift of the λmax value. On the other hand, if one group is electron donating and the other electron withdrawing, the calculated λmax would usually be much lower than the observed λmax.32 The above facts are asserted in the case of para substituents, but similar results were observed for the present case. Here the effect of the -NO2 group would be much more pronounced than that of the -CO2H group, as it would cause a larger shift of the λmax value, and the calculated value closely relates to that of the observed one. Again, the observed λmax value for anthranilic acid is higher than the calculated value due to the presence of an auxochrome, -NH2, which shifts the ethylenic and benzenoid bands to longer wavelengths because of the n-π conjugation. When the concentrations of the solutions are kept the same, it has been (32) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectromertic Indentification of Organic Compounds, 5th ed.; John Wiley & Sons: New York, 1991; pp 306-310.
Jana et al.
Figure 7. Change in wavelength (∆λ) vs time plot for the reduction of o-NBA in the presence of solid catalyst particles. Conditions: [o-NBA] ) 0.1 mM; [NaBH4] ) 0.1 M; [R(Ag)0]+Cl- ) 2.5 mg.
observed that o-NBA has a higher absorbance than the corresponding amino compound, i.e., anthranilic acid, because of the reduction of the chromophoric -NO2 group. 3.2.2. GC-MS Analysis. Gas chromatography (GC) and mass spectrometry (MS) make an effective combination for chemical analysis. The formation of anthranilic acid was confirmed by GC-MS analysis of the product. The GC-MS analysis of the ether extract of the product from aqueous reaction mixture showed that anthranilic acid (retention time )12.7 min) is the sole product. Real time monitoring could not be done with the aqueous reaction mixture, and at the end of the reaction (after 9 min), we failed to identify any trace of o-NBA. We ascertained the percent yield (95%) spectrophotometrically using a calibration graph of varied concentrations of o-aminobenzoic acid. The mass spectrum showed its molecular ion peak in the highest mass region at m/z 137(M+•). The other prominent peaks are formed at m/z 119 (M-18) and 92 (M-45) due to the loss of H2O and -CO2H group, respectively. 3.2.3. NMR Study. To authenticate the formation of anthranilic acid as the product 1H NMR study was done at the end of the reaction. The 1H NMR spectrum of the product isolated from the aqueous reaction mixture showed that anthranilic acid is the sole product. Real time 1H NMR analysis could not be performed as the reaction was carried out in water. In the spectrum, the aromatic proton para to the amino group and meta to the carboxyl group appeared in the most upfield region at δ 6.4-6.8 as a multiplet. Within this multiplet is embedded the peak due to the aromatic proton ortho to the amino group and meta to the carboxyl group and the amino proton. In the most downfield region at δ 7.86 appeared, as a double doublet, the aromatic proton ortho to the carboxyl group and meta to the amino group. The aromatic proton meta to the amino group and para to the carboxyl group appeared as multiplet in the range of δ 7.1-7.5. The 1H NMR spectrum obtained for our product is comparable to that of the authentic anthranilic acid. 3.3. Kinetics of the Reaction. The rate of the nanocompositecatalyzed reaction has been found to be pseudo-first-order with respect to the substrate concentration. Here, change in wavelength (∆λ) versus time is plotted (taking Figure 5 into consideration) for the reduction of o-NBA by the catalyst particles, shown in Figure 7. The activation energy of this reduction process is calculated to be 32 kJ mol-1 when the reaction was carried out at four different temperatures (10, 25, 40, and 55 °C). In case of heterogeneous/microheterogeneous catalysis, the reaction rate generally increases linearly with the amount of the
From o-Nitrobenzoic Acid to Anthranilic Acid
Figure 8. UV-visible absorption spectra for successive reduction of o-NBA by [R(Ag)0]+Cl-. Conditions: [o-NBA] ) 0.1 mM; [NaBH4] ) 0.1 M; [R(Ag)0]+Cl- ) 2.0 mg.
catalyst.33 To study the reduction reaction, the amount of catalyst was varied keeping other parameters constant. It was observed that with the increased amount of catalyst (resin-bound silver nanocomposite) the rate also increases. When the reaction was carried out taking 2.0 mg of catalyst particles, the following type of curves (Figure 8) were observed. Here some featureless curves, initially during the progress of the reaction, are obtained and such types of curves become prevalent even with a smaller amount (
