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Chitosan-Supported Palladium Catalyst. 3. Influence of Experimental Parameters on Nitrophenol Degradation Thierry Vincent and Eric Guibal* Ecole des Mines d’Ale` s, Laboratoire Ge´ nie de l’Environnement Industriel, 6 avenue de Clavie` res, F-30319 Ale` s Cedex, France Received March 3, 2003. In Final Form: June 27, 2003 Glutaraldehyde cross-linked chitosan was loaded with palladium and then reduced by means of an in situ hydrogen generation procedure (Zn in sulfuric acid solution) to prepare a chitosan-supported palladium catalyst. This catalyst was successfully used to degrade nitrophenol in dilute solutions in the presence of sodium formate as the hydrogen donor. The optimum initial pH was below pH 4. The pH strongly increased during the reaction. The influence of the initial concentration of nitrophenol and sodium formate was studied in order to determine the minimum molar ratio between these compounds to achieve the complete conversion of the nitrogenous product. The pseudo-first-order equation was shown to fit degradation kinetics in most cases; however, in some cases it was necessary to use a variable-order equation in order to model the kinetics. Decreasing catalyst particle size increased degradation rate; the kinetic parameter varied linearly with the reciprocal of the diameter, indicating that film diffusion may partially contribute to the kinetic control of the reaction. The kinetic parameter linearly increased with catalyst dosage, while increasing the palladium loading in the catalyst slightly increased degradation kinetics but the catalytic activity did not increase proportionally. Catalytic activity appears to be restricted to external catalyst layers.
Introduction Water pollution by phenol and phenolic compounds is of great public concern. Nitrophenols are some of the most refractory pollutants that can occur in industrial wastewaters. More particularly, nitrophenols and their derivatives result from the production processes of pesticides, herbicides, insecticides, and synthetic dyes and also occur in the pharmaceutical and petrochemical industries. The United States Environmental Protection Agency (USEPA) lists nitrophenols among the top 114 organic pollutants. Several processes have been developed to recover these pollutants from dilute solution and prevent their discharge into the environment. Among the several currently known physical, chemical, and biological methods for wastewater treatment, adsorption still continues to be the most widely used process. Most of the work has been carried out on activated carbon and carbon black samples.1 However, the cost of these adsorbents has focused the research toward cheap and easily obtainable unconventional adsorbents, such as clinoptilolite2 or charred sawdust.3 The main disadvantage of these sorption processes is related to the difficulty in regenerating and recycling the sorbent after saturation, and the technique finally consists of a one-way transfer of the pollutant from a disperse medium to a concentrated phase. Complete decomposition of the pollutant would be more environmentally friendly. Several alternative processes have been studied to reach a partial or complete degradation of nitrophenol compounds such as biological degradation4 and chemical5,6 or photochemical oxidation.7,8 * To whom correspondence should be addressed. Phone: + 33 (0)4 66 78 27 34. Fax: + 33 (0)4 66 78 27 01. E-mail: Eric.Guibal@ ema.fr. (1) Chern, J.-M.; Chien, Y.-W. Water Res. 2002, 36, 647. (2) Sismanoglu, T.; Pura, S. Colloids Surf., A 2001, 180, 1. (3) Dutta, S.; Basu, J. K.; Ghar, R. N. Sep. Purif. Technol. 2001, 21, 227. (4) Arcangeli, J. P.; Arvin, E. Water Sci. Technol. 1995, 31, 117. (5) Adams, C. D.; Cozzens, R. A.; Kim, B. J. Water Res. 1997, 31, 2655. (6) Goi, A.; Trapido, M. Chemosphere 2002, 46, 913.
Many studies have also focused on the use of catalytic processes for the degradation of nitrophenol.9-11 Catalysis may be performed in homogeneous systems; however, in the case of expensive catalysts such as those involving precious and strategic metals,12,13 it is important to recover the metals at the end of the process. For this reason, supported catalysis has been extensively investigated during the past few decades. Though activated carbon and other minerals (alumina, silica gel) are the most used supports, polymeric supports have recently attracted considerable attention both for their versatile conditioning and also for the forms of selectivity they can bring to the reaction.14 These polymers, due to their stereospecificity, can present selective catalytic properties.15 Natural polymers have recently been the subject of many studies for their application in catalysis. For example, chitosan has recently been used as a support for catalysis.16-21 Indeed, this amino-polysaccharide is characterized by its high affinity for metal ions, which can be adsorbed either by chelation mechanisms (metal cations in near-neutral (7) Chen, D.; Ray, A. K. Water Res. 1998, 32, 3223. (8) Andreozzi, R.; Caprio, V.; Insola, A.; Longo, G.; Tufano, V. J. Chem. Technol. Biotechnol. 2000, 75, 131. (9) Pintar, A.; Levec, J. Chem. Eng. Sci. 1994, 49, 4391. (10) Choudhary, V. R.; Sane, M. G.; Tambe, S. S. Ind. Eng. Chem. Res. 1998, 37, 3879. (11) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247. (12) Macquarrie, D. J.; Gotov, B.; Toma, S. Platinum Met. Rev. 2001, 45, 102. (13) Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M. J. Mol. Catal. A: Chem. 2001, 173, 3. (14) Altava, B.; Burguete, M. I.; Garcı´a-Verdugo, E.; Luis, S. V.; Vicent, M. J.; Mayoral, J. A. React. Funct. Polym. 2001, 48, 125. (15) Felix, G. J. Chromatogr., A 2001, 906, 171. (16) Chiessi, E.; Pispisa, B. J. Mol. Catal. 1994, 87, 177. (17) Jin, J.-J.; Chen, G.-C.; Huang, M.-Y.; Jiang, Y.-Y. React. Polym. 1994, 23, 95. (18) Han, H.-S.; Jiang, S.-N.; Huang, M.-Y.; Jiang, Y.-Y. Polym. Adv. Technol. 1996, 7, 704. (19) Zeng, X.; Zhang, Y.; Shen, Z. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2177. (20) Yin, M.-Y.; Yuan, G.-L.; Wu, Y.-Q.; Huang, M.-Y.; Jiang, Y.-Y. J. Mol. Catal. A: Chem. 1999, 147, 93. (21) Quignard, F.; Choplin, A.; Domard, A. Langmuir 2000, 16, 9106.
10.1021/la034364r CCC: $25.00 © 2003 American Chemical Society Published on Web 08/16/2003
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solutions) or by ion-exchange mechanisms (metal anions in acidic solutions).22-25 High sorption capacities have been achieved for precious metals and platinum group metals: sorption levels as high as 3 mmol Me g-1 are commonly reached with gold, platinum, and palladium, under mild experimental conditions.26,27 The reduction of loaded metals by a suitable procedure (using sodium borohydride and/or in situ generated hydrogen) enables loaded metals to be present on the support at the zerovalent oxidation state, which is appropriate for many hydrogenation procedures.28 This kind of catalyst has been investigated for chromate reduction using sodium formate as the electron donor: chromate reduction may result from a hydrogen transfer mechanism and/or by production of hydrogen from formic acid oxidation.29 This support has also been carried out for chlorophenol dehalogenation under similar experimental conditions, using sodium formate as the hydrogen donor.30 The present work focuses on the testing of a chitosansupported palladium catalyst for the degradation of nitrophenols using sodium formate as the hydrogen donor. The reaction produced several subproducts including aminophenol. The study first investigated the influence of pH on the reaction kinetics; the influence of other experimental parameters such as catalyst dosage, nitrophenol concentration, formate concentration, catalyst particle size, and catalyst loading was then studied at the optimum pH (ca. pH 3). Degradation kinetics was modeled using several models such as a pseudo-first-order kinetic equation, a variable-order equation, and also in some particular cases a “reversible” reaction equation. The variation of the kinetic coefficients with experimental parameters was considered. The conversion yield was found to correlate approximately to the sodium formate/ nitrophenol molar concentration ratio. Finally, a first attempt to test the selectivity of the system was tried by comparing the degradation kinetics for several different nitrophenols under selected conditions. Material and Methods Materials. Chitosan was supplied by Aber Technologies (Plouvien, France). It had previously been characterized:17 the degree of deacetylation was 87% measured by Fourier transform infrared spectroscopy using the method described by Baxter et al. on chitosan film,31 and the molecular weight was MWn ) 125.000 g mol-1 (MWw ) 191.000 g mol-1), using size-exclusion chromatography (TSK-gel G4000 PWXL and TSK-gel G6000 PWXL columns) coupled with a light scattering photometer (Dawn F. Wyatt Technology). Chitosan was ground and sieved in order to separate it in samples of different particle sizes: 0 < G1 < 125 µm < G2 < 250 µm < G3 < 500 µm < G4 < 710 µm. Another sample (G0) was prepared by grinding the G1 fraction and sieving the final powder to select the 0-63 µm size fraction. (22) Bassi, R.; Prasher, S. O.; Simpson, B. K. Sep. Sci. Technol. 2000, 35, 547. (23) Inoue, K. In Environmental Marine Biotechnology; Fingerman, M., Nagabhushanam, R., Thompson, M. F., Eds.; Recent Advances in Marine Biotechnology, Vol. 2; Oxford & IBH: New Delhi, 1998; pp 6397. (24) Guibal, E.; Jansson-Charrier, M.; Saucedo, I.; Le Cloirec, P. Langmuir 1995, 11, 591. (25) Guzman, J.; Saucedo, I.; Revilla, J.; Navarro, R.; Guibal, E. Langmuir 2002, 18, 1567. (26) Guibal, E.; Vincent, T.; Larkin, A.; Tobin, J. M. Ind. Eng. Chem. Res. 1999, 38, 4011. (27) Ruiz, M.; Sastre, A.; Guibal, E. React. Funct. Polym. 2000, 45, 155. (28) Drelinkiewicz, A.; Hasik, M. J. Mol. Catal. A: Chem. 2001, 177, 149. (29) Vincent, T.; Guibal, E. Ind. Eng. Chem. Res. 2002, 41, 5158. (30) Vincent, T.; Spinelli, S.; Guibal, E. Ind. Eng. Chem. Res., in press. (31) Baxter, A.; Dillon, M.; Taylor, K. D. A.; Roberts, G. A. F. Int. J. Biol. Macromol. 1992, 14, 122.
Vincent and Guibal NaBH4 and nitrophenols (NP: 2-NP, 3-NP, 4-NP) were purchased from Fluka (Switzerland) as analytical grade products. PdCl2 was purchased from Acros (USA). Other reagents (acids, zinc, HCOONa) were supplied by Carlo Erba (Italy), as analytical grade products. Sorbent Preparation. Since chitosan is soluble in hydrochloric acid, it cannot be used as supplied, and a cross-linking treatment is required. Chitosan was cross-linked with glutaraldehyde by contact of chitosan with glutaraldehyde solution (10%, w/w). The volume of glutaraldehyde solution and the mass of chitosan were set to reach a 1:1 molar ratio between the amine groups of the polymer and the aldehyde functions of the crosslinking agent. The sorbent has not been characterized after crosslinking treatment, and more specifically the number of free amine groups has not been determined. However, previous studies have shown that the excess of glutaraldehyde (compared to chitosan amine groups) did not significantly affect palladium sorption properties.32 The availability of amine groups is a key parameter in the case of chelation mechanisms, but this parameter is less important in the case of ion-exchange mechanisms. Finally, the particles were abundantly rinsed to remove traces of unreacted glutaraldehyde and dried at 100 °C. Palladium Sorption. The sorbent (1 g) was mixed for 24 h with a palladium solution (2 L) at a concentration of 200 mg Pd L-1 at pH 2. The solutions were then filtered, and the residual concentration of palladium in the solution was measured using inductively coupled plasma atomic emission spectrometry (ICP-AES; Jobin-Yvon JY 2000, Longjumeau, France). The sorption capacity (amount of palladium adsorbed on the sorbent, q, mg Pd g-1) was obtained by the mass balance equation. Procedures for Palladium Reduction. The reduction treatment consisted of bringing the loaded sorbent (200 mg) into contact with 100 mL of sulfuric acid solution (1% w/w) and 300 mg of zinc (provided as a fine powder). By taking into account the change in the weight of the solid during the sorption and reduction steps, it was possible to calculate the amount of palladium contained in the final product. The palladium content was close to 105 mg Pd g-1. A digestion/mineralization procedure (contact of the catalyst with a hydrochloric acid and hydrogen peroxide mixture) was used to disrupt the polymer and dissolve the metal. ICP analysis confirmed the actual palladium content: 102 mg Pd g-1 catalyst. Other samples were prepared with higher concentrations of palladium in the loading bath and for different particle sizes. The same procedure was used for the determination of palladium content, which was 153, 144, 132, and 124 mg Pd g-1 for G1, G2, G3, and G4, respectively. Though a partial degradation of the polymer cannot be rejected during acidic treatments (especially during the procedure of Pd reduction), several parameters including the use of sulfuric acid (soluble samples being almost insoluble in sulfuric acid solutions), the glutaraldehyde cross-linking, and the stabilization due to palladium sorption contribute to limit polymer dissolving or degradation and to increase its stability. The difference in the palladium content in the final product (after reduction procedure) and in the loaded sorbent (after sorption procedure) was negligible, indicating a negligible loss of material. Characterization of the Catalysts. Transmission electron microscopy (TEM) was used to measure the size of Pd nodules or crystals. Catalyst particles were included in a liquid resin; after cross-linking of the resin, thin slices of resins (60 µm) were cut using a microtome. TEM observations showed that the size of metal crystals was close to 4-5 nm. This is a critical parameter for catalytic activity: the smaller the size of catalyst nodules, the greater the activity.33 The particles were highly dispersed in the material though a slight gradient was observed between the center and the periphery of the particles. Some aggregates were also observed: the agglomeration of small nodules led to the formation of large palladium particles (around 30 nm). (32) Ruiz, M.; Sastre, A. M.; Guibal, E. React. Funct. Polym. 2000, 45, 155. (33) Mabbett, A. N.; Yong, P.; Baxter-Plant, V. S.; Mikheenko, I. P.; Farr, J. P. G.; Macaskie, L. E. In Biohydrometallurgy: Fundamentals, Technology and Sustainable Development; Ciminelli, V. S. T., Garcia, O., Jr., Eds.; Wiley: Amsterdam, 2001; Vol. 11B, pp 335-342.
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X-ray photoelectron spectroscopy (XPS) analysis was also performed in order to determine the oxidation state of palladium on the catalyst. It was shown that only 50-60% of palladium was reduced from Pd(II) to Pd(0). Procedure for Nitrophenol Degradation. Nitrophenol was degraded, unless otherwise specified, by contact of 50 mL of a nitrophenol solution, containing the organic compound at a concentration of 50 mg L-1 (0.36 mM) and sodium formate at a concentration of 25 mM, with 10 mg of catalyst. Unless specified, nitrophenol was used as 3-nitrophenol, 3-NP. The pH was initially controlled at pH 3 (unless specified) using a molar sulfuric acid solution. Samples were collected at selected contact times and filtered. The filtrates were analyzed using a UV spectrophotometer (Varian 2050) and by measuring the absorbance of the solutions at 332 nm, after the sample (1 mL volume) had been acidified with 20 µL of sulfuric acid solution (5% w/w). Test experiments were performed to check the actual influence of the catalyst and hydrogen donor and to verify for reactions at high relative temperature that this parameter was not sufficient to accelerate the reaction. Determination of Kinetic Parameters. The half-time reaction (t1/2) was systematically determined to compare the time required to halve the initial concentration of nitrophenol. Additionally, three other parameters were determined: (a) the catalytic activity (mmol nitrophenol min-1 g-1 Pd), (b) the initial kinetic coefficient (k0, min-1), and (c) the overall kinetic coefficient (k, min-1). The catalytic activity coefficient, which may also be called intrinsic or specific rate, was obtained by determining the amount of nitrophenol transformed in a given contact time (2 min) divided by the time and the actual mass of Pd contained in the catalyst. The kinetic equations were obtained from the LangmuirHinshelwood kinetic model (L-H). Considering that the hydrogen donor (formate ions) was in excess compared to nitrophenol, it might be assumed that the reduction rates were independent of the formate concentration. The first approximation was to fit the data to a first-order rate law:
dC(t) ) -k0C(t) dt
or
ln
[ ]
C(t) ) -k0t C0
(1)
where C0 and C(t) (mg L-1) are the initial substrate concentration and the concentration at time t, respectively. The parameter k0 (min-1) is the kinetic parameter for the first-order kinetic equation. This equation was used to fit experimental data during the initial stage (within the first 5-6 min of contact) for the initial kinetic parameter, and the slope of the curve was used to calculate the kinetic parameter, k0. The overall kinetic parameter was also calculated by the same method; however, the experimental data were fitted to the firstorder equation over the concentration (time) range corresponding to a 10-fold decrease in the initial concentration. In this case, the kinetic parameter was determined by nonlinear regression analysis using the Levenberg-Marquardt algorithm included in the Mathematica software application. However, in some cases, for example, lack of sodium formate in the course of the reaction, the apparent first-order equation failed to fit experimental data and it was necessary to use the L-H equation. Two cases have been identified corresponding to (a) a variable-order equation and (b) a reversible equation, as defined by Schu¨th and Reinhard.34 When the data cannot be fitted using the simple first-order model, in the case of supported catalysis, Schu¨th and Reinhard use the following equation for modeling the kinetics:34
-k1C(t) dC(t) ) dt 1 + k2C(t)
(2)
where k1 (min-1) and k2 (L mg-1) are the kinetic parameters. This type of rate law is often observed for heterogeneous reactions in batch reactors with a constant volume and accounts for reactions that shift in order from 0 to 1 as the substrate is (34) Schu¨th, C.; Reinhard, M. Appl. Catal., B 1998, 18, 215.
utilized. Integration of the differential equation gives the following solution:
t)
[[ ]
C0 1 ln + k2(C0 - C(t)) k1 C(t)
]
(3)
Kinetic parameters were determined using a nonlinear regression analysis tool. The first term (left side of the right-hand side of the equation) represents a pseudo-first-order kinetic equation, while the second term represents a zero-order kinetic equation. When the equilibrium concentration, Ceq, did not tend to 0, Schu¨th and Reinhard modeled catalytic kinetics using the following equation:34
dC(t) -k1(C(t) - Ceq) ) dt 1 + k2C(t)
(4)
In this case, integration of the differential equation gives the following equation:
t)
[(
[
C0 - Ceq 1 (1 + k2Ceq) ln k1 C(t) - Ceq
])
+ k2(C0 - C(t))
]
(5)
Kinetic parameters were also determined using a nonlinear regression analysis tool. Fitted parameters are summarized in the tables given in the Supporting Information and were used for plotting the modeled curves on the figures.
Results and Discussion Products of the Reaction and Approach of the Reaction Mechanism. Experiments have been performed with standard conditions consisting of 50 mL of solution at pH 3 containing 50 mg L-1 of substrate (nitrophenol; aminophenol, AP; or phenol) and a 25 mM concentration of sodium formate. The solution was mixed with 10 mg of catalyst for 24 h, and samples were regularly collected by filtration and analyzed by UV spectrophotometry to follow the evolution of the reaction. Samples were also collected for identification of intermediary products by solid-phase microextraction (SPME)-mass spectrometry. It appears that the process can be separated in (at least) two steps: (a) the reductive hydrogenation of nitrophenol into aminophenol and (b) the oxidation of aminophenol into subproducts that depend on experimental conditions (for example, muconic acid and oxidized derivatives). The transformation of 3-NP leads to the same products of degradation as 3-AP, while 2-NP and 4-NP were altered into the same products of degradation as 2-AP. Therefore, the position of the substituents on the phenolic ring strongly controls the products of the reaction. The second step usually occurs after 45-60 min of contact (after sodium formate consummation and disappearance of reductive conditions); the complete degradation of AP usually requires several hours of contact under selected experimental conditions (at least 24 h). Since, under similar experimental conditions, phenol is not substantially degraded, it seems that the degradation of aminophenol does not proceed by a degradation of aminophenol into phenol followed by its subsequent degradation. The degradation of phenol may only occur during the substitution steps on amino groups of aminophenol; therefore, the orientation of the reaction is controlled by the position of the substituents on the phenolic ring. The homogeneous hydrogenation of aromatic nitro compounds by palladiumsynthetic polymer complexes has been investigated using hydrogen gas, and several compounds have been identified as reaction products: aminophenol, hydroxyaniline, and so forth.35,36 On a Pd/carbon catalyst, in the presence of
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hydrogen gas, Choudhary et al. only observed aminophenol formation during the hydrogenation of nitrophenol.10 The same hydrogenation product was obtained using a Pt/C catalyst by reaction of hydrogen gas on nitrophenol solution.37 In the case of nitrotoluene hydrogenation using palladium supported on alumina material and hydrogen gas as the hydrogen donor, several complex mechanisms have been identified with intermediary products.38 Due to the low kinetics of aminophenol degradation and taking into account the time scale of nitrophenol degradation, the system has been simplified considering in this preliminary study that nitrophenol is mainly transformed into aminophenol and that the subsequent oxidation reactions can be neglected. Further studies will focus on the investigation of the reaction mechanism, on the full identification of product reactions, and on the effect of experimental conditions (concentration of nitrophenol and sodium formate) on the orientation of the reaction. However, these topics are not within the scope of the present study that aims at validating the use of chitosan as a support for catalytic reactions. In the case of chlorophenol degradation, the analysis of the products of degradation using gas chromatographymass spectrometry (GC-MS) facilities and the purge & trap procedure showed that the main products were cyclohexanol and/or cyclohexanone compounds. This indicates that the reaction did not occur by a simple hydrogenation of nitrophenol to aminophenol but rather that the reaction continued until phenol production occurred followed by dearomatization of phenol to produce cyclohexanol or cyclohexanone products. This is consistent with results cited by Baumgarten et al. and Morales et al. for the dechlorination of chlorophenols using palladium and platinum catalysts, respectively.39,40 However, this dearomatization reaction only occurred as a secondary reaction of the hydrogenation procedure, as in the present case. In the case of the photocatalytic degradation of nitrophenol on TiO2, Theurich et al. and San et al. obtained a wider diversity of subproducts (including substituted catechol and substituted quinine among others), the type of products changing with the pH of the reaction.41,42 Urbano and Marinas stated that the hydrogen transfer mechanism is not fully understood.43 Since in terms of electronegativity hydrogen occupies a central position in the periodic table, in the reactions involving its transfer, hydrogen may appear as a proton, atom, or hydride depending on reagents and conditions. Actually, in many reduction reactions with hydrogen donors, it may not be easy to decide just how hydrogen is transferred. Most often hydrogen gas is used as the hydrogen donor, and the use of formate for the reduction of phenolic compounds is frequently reported in the literature. In the case of the dehalogenation of halobenzenes, Aramendia et al. pointed out that the efficiency of the hydrogenation process is (35) Xu, S.; Xi, X.; Shi, J.; Cao, S. J. Mol. Catal. A: Chem. 2000, 160, 287. (36) Xi, X.; Liu, Y.; Shi, J.; Cao, S. J. Mol. Catal. A: Chem. 2003, 192, 1. (37) Vaidya, M. J.; Kulkarni, S. M.; Chaudhari, R. V. Org. Proc. Res. Dev. 2003, 7, 202. (38) Rajashekharam, M. V.; Nikalje, D. D.; Jaganathan, R.; Chaudhari, R. V. Ind. Eng. Chem. Res. 1997, 36, 592. (39) Baumgarten, E.; Fiebes, A.; Stumpe, A. React. Funct. Polym. 1997, 33, 71. (40) Morales, J.; Hutcheson, R.; Cheng, F. J. Hazard. Mater. 2002, B90, 97. (41) Theurich, J.; Linder, M.; Bahnemann, D. W. Langmuir 1996, 12, 6368. (42) San, N.; Hatipoglu, A.; Koc¸ tu¨rk, G.; C¸ inar, Z. J. Photochem. Photobiol., A 2002, 146, 189. (43) Urbano, F. J.; Marinas, J. M. J. Mol. Catal. A: Chem. 2001, 173, 329.
Vincent and Guibal
Figure 1. Influence of pH on 3-NP degradation kinetics (3-NP concentration, 50 mg L-1; formate concentration, 25 mM; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1; CD, 200 mg L-1).
controlled by the relative adsorption coefficient between sodium formate and the halobenzene at the catalyst surface.44 A strong sorption of the substrate drastically reduces the sorption of the hydrogen donor (sodium formate) and then reduces the reaction rate. Therefore, we can suggest the following simplified reaction scheme: (a) adsorption of both sodium formate and the phenolic substrate at the surface of the catalyst; (b) decomposition of formate into hydrogen and carbon dioxide; (c) reaction of hydrogen with the substrate at the surface of the catalyst; (d) release of degradation products (or subsequent degradation). The number of subproducts resulting from nitrophenol degradation makes the interpretation of kinetic data very complex; for this reason, we focused on the kinetics of nitrophenol disappearance rather than the appearance of subproducts. In a first attempt to identify the limiting steps of the process, we only considered the transformation of nitrophenol into aminophenol as the test reaction and we used simplified kinetic models described by eqs 1-5 for their simulation. Influence of pH. The pH is an important variable in the evaluation of catalytic reactions since it affects surface charge, dissociation of the substrate, and also dissociation of the hydrogen donor, when it acts as a chemical reagent (hydrogen transfer mechanism). Indeed, depending on the surface properties of the catalyst, its overall charge may control the sorption of the substrate or that of the hydrogen donor. It may also influence the desorption kinetics of the products of the catalytic reaction. Chitosan is characterized by its pKa, which depends on its degree of deacetylation. For the chitosan sample used in this study, the intrinsic pK was close to 6.5. Below pH 5-5.5, more than 90% of amine groups are protonated and thus available for sorption of anionic species. The pKa of the formic acid/ formate couple being close to 3.8, formate anions are adsorbed in the pH range 3-5.5. The pKa of nitrophenol is close to 7.1. Nitrophenolate is only predominant in neutral or alkaline solutions, but under these pH conditions, chitosan is almost completely deprotonated. Gallezot et al. have shown that formic acid can be totally oxidized (into CO2) by Pt/C catalyst and that this reaction is accompanied by a strong increase in the pH, shifted under selected experimental conditions (atmospheric air pressure, 50 °C), from pH 3 to pH 5.5.45 In the present study, the pH of the solution was strongly increased up to pH 7-8 with an initial pH above (or equal to) pH 3. Figure 1 shows that degradation kinetics overlapped when the initial pH was pH 3 or pH 4. When the initial (44) Aramendia, M. A.; Borau, V.; Garcia, I. M.; Jime´nez, C.; Marinas, A.; Marinas, J. M.; Urbano, F. J. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3, 465. (45) Gallezot, P.; Laurain, N.; Isnard, P. Appl. Catal., B 1996, 9, 11.
Chitosan-Supported Palladium Catalyst
pH was increased to pH 5, a significant decrease in the initial degradation rate was observed followed by a progressive decrease and after 1 h of contact the degradation efficiency tended to 60%. Acidic conditions were not sufficient to allow the sorption of the substrate and the reagent. At pH 2.2, the degradation rate was also decreased (even compared to degradation kinetics at pH 5) and nitrophenol concentration decreases but did not reach a plateau within the selected contact time. Several reasons may explain this low efficiency: the weak decomposition of formate and/or the low sorption of reagent and substrate on the catalyst due to the competition of competitor ions (brought by sulfuric acid). Andreozzi et al. also observed a significant decrease in the degradation rate of 4-nitrophenol using a photocatalytic oxidation system (on TiO2) at increasing pH.8 However, in this case, complete degradation of the substrate was also obtained at a pH as high as pH 8.5, though much more slowly. In the reduction of nitrate, Pru¨sse et al. observed that increasing the pH resulted in a decrease of the selectivity: at low pH nitrate ions were reduced to nitrogen, while at increased pH nitrate was transformed into ammonium and nitrite.46 They compared nitrate reduction using both hydrogen gas and formate as the hydrogen donor and observed that the latter was more efficient and more selective, possibly due to the buffering effect of formate (which neutralizes inhibiting OH- generated during nitrate reduction). The exact mechanism of the action of the formate was not completely elucidated, and two possible mechanisms were proposed: (a) reaction of hydrogen produced during formate decomposition or (b) transfer hydrogenation. The kinetic data confirmed that an initial pH between pH 3 and pH 4 limited the half-reaction time; the kinetic coefficients were comparable, while the activity coefficients were slightly higher at pH 4 (Table 1 in the Supporting Information). However, pH 3 was preferred for other experiments since the pH varied less than at pH 4 during nitrophenol degradation. Influence of Agitation. An important parameter in the design of the catalytic system and the determination of the limiting step is the velocity of agitation. Nitrophenol degradation kinetics was not significantly decreased by increasing the agitation speed from 100 to 750 rpm (100200-300-500-750 rpm, not shown). The curves perfectly overlapped below a 10-min contact time and slightly diverged at a greater contact time: the half-time of reaction varied by less than 2 min around a mean value of 14 min, while the time required for a 10-fold decrease of the relative concentration was decreased from 35 to 25 min by increasing the velocity of agitation from 100 to 750 rpm. Therefore, the velocity of agitation has only a limited effect on reaction kinetics. This is a first indication of the limited effect of external mass transfer resistance on the control of the reaction kinetics. Indeed, with respect to the diffusion mechanism in sorption processes, agitation usually controls the thickness of the film around the sorbent particles and thus the resistance to external diffusion.47 It is interesting to observe that high agitation speeds are not required to limit the effect of external diffusion as it occurs with some other systems involving hydrogen gas as the hydrogen donor.10,38,48,49 This conclusion needs to be confirmed by studying the influence of (46) Pru¨sse, U.; Ha¨hnlein, M.; Daum, J.; Vorlop, K.-D. Catal. Today 2000, 55, 79. (47) Helfferich, F. Ion Exchange, 2nd ed.; Dover: Mineola, NY, 1995. (48) Rajashekharam, M. V.; Chaudhari, R. V. Ind. Eng. Chem. Res. 1999, 38, 906. (49) Khilnani, V. L.; Chandalia, S. B. Org. Proc. Res. Dev. 2001, 5, 263.
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Figure 2. Influence of catalyst dosage on 3-NP degradation kinetics (pH 3; 3-NP concentration, 50 mg L-1; formate concentration, 25 mM; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1).
other parameters such as particle size, solute concentration, and catalyst dosage in order to verify whether the process was rather controlled by intraparticle diffusion. Influence of Catalyst Dosage. As expected, increasing the amount of catalyst speeded the degradation of nitrophenol (Figure 2). Similar observations have been made by Gallezot et al. and Pintar et al.9,45 Lea and Adesina observed an optimum catalyst dosage in the case of the photocatalytic degradation of nitrophenol and attributed the decrease in kinetic rate at high loading to the decrease in the light intensity transmitted to substrate molecules.50 The time for half-reaction varied with the reciprocal of the catalyst dosage, while the kinetic parameters (including both the initial and the overall kinetic coefficients) varied linearly with catalyst dosage (Table 2 in the Supporting Information). Kinetic parameters vary as a function of catalyst dosage (CD) according to the following equations:
k1 ) 5.36 × 10-4 CD
R2 ) 0.999 (6)
k0 ) 3.53 × 10-4 CD + 0.019
R2 ) 0.994 (7)
Using Pd/C catalyst and hydrogen gas as the hydrogen donor (and methanol as the solvent), Choudhary et al. also observed a linear variation of the initial rate of nitrophenol degradation with increasing catalyst loading (amount of Pd per unit of volume of the solution).10 Vaidya et al. also observed a linear variation of the initial kinetic rate with catalyst loading (catalyst amount) using Pt/C catalyst, using hydrogen gas as the hydrogen donor.37 In the case of dinitrotoluene hydrogenation using Ni catalyst (supported on zeolite), the initial rate of transformation also varied linearly with catalyst loading,48 while in the case of Pd supported on alumina, the variation of the initial rate was not linear (indicating in this case that the reaction is of second order in selected experimental conditions). In the case of dinitrobenzene hydrogenation using Pd/C catalyst, the initial degradation rate also linearly varied with the catalyst amount.49 Since the process was not controlled by external diffusion (limited effect of particle size), the researchers conclude that the overall reaction rate is controlled by the reaction at the surface of the catalyst. The present results are consistent with the results obtained by Felis et al. on parachlorophenol dehalogenation on Ru/C catalysts.51 The reciprocal of the overall kinetic rate also varied linearly with the reciprocal of (50) Lea, J.; Adesina, A. A. J. Chem. Technol. Biotechnol. 2001, 76, 803. (51) Felis, V.; De Bellefon, C.; Fouilloux, P.; Schweich, D. Appl. Catal., B 1999, 20, 91.
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Figure 4. Influence of the concentration ratio F/NP on the degradation yield for different formate concentrations (pH 3; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1; CD, 200 mg L-1).
Figure 3. Influence of 3-NP concentration and formate concentration on 3-NP degradation kinetics (pH 3; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1; CD, 200 mg L-1).
catalyst dosage. This was not verified with the initial degradation rate. Zhang and Chuang also observed that the ratio Concentration/Removal rate (homogeneous to the kinetic rate, min-1) varied with the reciprocal of the catalyst concentration in the case of black liquor oxidation on palladium-based catalysts. They used the slope of the linearized curve for the calculation of the combined resistance to external diffusion, internal diffusion, and reaction rate, while the intercept served to determine the adsorption resistance.52 Whatever the catalyst dosage, the pseudo-first-order equation fitted kinetic curves well. Increasing the dosage of catalyst significantly reduced the catalytic activity coefficient but not proportionally to catalyst dosage variation. Low sorbent dosage allowed the optimum use of palladium loaded on the catalyst but significantly increased the time required to achieve the complete degradation of nitrophenol. Influence of the Molar Ratio of Formate/Nitrophenol on Degradation Yield. The excess of the hydrogen donor (sodium formate) versus nitrophenol concentration was varied in order to determine the minimum excess required to achieve the complete degradation of 3-NP, under selected experimental conditions. Figure 3 shows data obtained with different formate concentrations, varying the concentration of 3-NP, at room temperature (22 °C ( 1 °C). At low formate concentration, increasing 3-NP concentration resulted in a decrease of the degradation rate together with the equilibrium concentration of the substrate (Table 3 in the Supporting Information). Under these experimental conditions, the excess of formate was not sufficient to completely degrade 3-NP. Increasing the formate concentration resulted in complete degradation of 3-NP, though, as expected, increasing the concentration of 3-NP slightly decreased degradation efficiency. Figure 4 summarizes the evolution of the conversion rate with the molar ratio F/3-NP for different formate concentrations at T ) 22 °C. Increasing the excess of formate increased the degradation efficiency. However, the effect of this excess strongly depended on the initial (52) Zhang, Q.; Chuang, K. T. Appl. Catal., B 1998, 17, 321.
Figure 5. Influence of formate concentration on 3-NP degradation kinetics at T ) 22 °C (pH 3; 3-NP concentration, 50 mg L-1; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1; CD, 200 mg L-1).
concentration of formate. The positive effect of the formate excess was very sensitive at low F/3-NP, between 4 and 9; the increase of the degradation was less significant above 9. It seems that a 10-fold excess of formate (compared to 3-NP concentration) was sufficient to reach the maximum of nitrophenol elimination. Similar conclusions can be reached considering the initial degradation rates (k0 in Tables 2 and 3 in the Supporting Information). Standard conditions for further experiments were established using a 3-NP concentration of 50 mg L-1 and a sodium formate concentration of 25 mM, though less favorable experimental conditions were used, where appropriate, to measure the impact of other experimental parameters. Influence of Sodium Formate Concentration. Though this parameter was partially investigated in the other sections, a more exhaustive study was performed under selected experimental conditions (CD ) 200 mg L-1, pH 3, 3-NP concentration ) 50 mg L-1) at room temperature (Figure 5; and Table 4 in the Supporting Information). As stated in the section on the influence of the F/3-NP ratio, decreasing formate concentration for a fixed substrate concentration resulted in a decrease in the degradation efficiency and the kinetic rates. Decreasing the formate concentration resulted in a decrease of the kinetic parameters k0, k1, and CAC (catalytic activity coefficient), while the half-reaction time was increased. The decrease of the CAC was especially significant below 12.5 mM, while above this level the CAC was only slightly decreased. The catalytic activity was shown to vary as a power law as a function of the concentration of sodium formate (eq 8). Similar trends were observed for the degradation of aromatic nitro compounds.10,38,48
CAC ) 0.44 F0.452
R2 ) 0.937
(8)
Influence of Nitrophenol Concentration. Changing the concentration of 3-nitrophenol did not affect the
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Helfferich observed that in the case of ion-exchange processes controlled by film diffusion resistance the sorption rate varied linearly with substrate concentration.47 This may be an indication of a marginal contribution of film diffusion resistance to the control of 3-NP degradation rate on this catalyst. The reciprocal of CAC was plotted versus the reciprocal of nitrophenol concentration, and the relationship was found to be linear. A similar relationship was observed by Bahnemann et al. and Meng et al. for the photocatalytic degradation of chlorophenol and chlorobenzoate, respectively.53,54 They used the slope and intercept of these plots to obtain the reaction rate constant and the apparent adsorption constant. Frequently, the initial rate of degradation varies as a power law of the concentration of the substrate, and at high concentration the kinetic rate tends to level off.10,37,38 Vaidya et al. observed that the hydrogenation of nitrophenol on Pt/C catalyst using hydrogen gas shows a zeroorder dependence on hydrogen partial pressure at high
nitrophenol concentration while the initial rate strongly decreased at low substrate concentration (below 0.4 mol L-1).37 In the present work, the linear variation can be attributed to the concentration range selected for experimental work. A higher initial concentration would be necessary to reach the plateau. Influence of Catalyst Loading. Palladium content within the catalyst was varied by increasing the concentration of the solution used for loading the glutaraldehyde cross-linked chitosan during the sorption step. Experiments were performed using the standard catalyst dosage (ca. 200 mg L-1). At increasing palladium loading, the degradation rate hardly increased (not shown) (Table 6 in the Supporting Information). The time for half-reaction was slightly decreased and kinetic rates were only increased by 20-40% despite a doubling in palladium content. The CAC, which takes into account palladium content, was significantly decreased. This confirms that high palladium loading is not required to optimize the process. This result may be explained by a reaction limited to the surface of the catalyst. This is consistent with results obtained at varying catalyst dosages. A compromise must be obtained between the optimum use of palladium loaded on the carrier (using the CAC) and the time required to achieve the degradation of nitrophenol. Influence of Catalyst Particle Size. A reaction limited to the external surface of the catalyst has been postulated to explain the weak effect of increasing palladium loading on the particle. Changing the size of catalyst particles may bring complementary information on kinetic controlling steps. Figure 7 (and Table 7 in the Supporting Information) summarizes the data obtained by varying the diameter of catalyst particles and using the same total amount of catalyst. Increasing the particle size strongly increased the time for half-reaction, from 3 to 25 min for an increase in mean particle size from 32 to 605 µm. Both the CAC and the degradation rates decreased with increased catalyst particle size. For the largest particles, the two-parameter equation appears to fit the experimental points better rather than the pseudo-first-order equation. This decrease in the reaction rate is common in supported catalysis. In the case of the degradation of aromatic nitro compounds, Choudhary et al. also observed a decrease of the degradation rate when increasing the size of catalyst particles.10 They used the effectiveness factor (η) to establish the optimum particle size for minimum contribution of intraparticle diffusion resistance: a particle size less than 50 µm appeared to be optimum, the effectiveness factor being close to 1. Satterfield and Sherwood have pointed out the role of diffusion in catalysis.55 Lommerts et al. also observed a marked limitation in catalytic rates with increasing particle size in the synthesis of methanol over a copper/ zinc alumina catalyst and related these limitations to internal mass transport limitations.56 In the present study, both k0 and k1 varied linearly with the reciprocal of the mean diameter of the particles (Figure 7). This information is very important since it enables the contribution of diffusion-controlling steps to be evaluated. Helfferich has stated that in ion-exchange processes, when the ionexchange rate varies with the reciprocal of the particle size, rather than the square of the reciprocal, the main controlling transfer resistance is due to film diffusion
(53) Bahnemann, D. W.; Theurich, J.; Lindner, M. Langmuir 1996, 12, 6368. (54) Meng, Y.; Huang, X.; Wu, Y.-Q.; Wang, X.; Quian, Y. Environ. Pollut. 2002, 117, 307.
(55) Satterfield, C. N.; Sherwood, T. K. The Role of Diffusion in Catalysts; Addison-Wesley: Reading, MA, 1963. (56) Lommerts, B. J.; Graaf, G. H.; Beenackers, A. A. C. M. Chem. Eng. Sci. 2000, 55, 5589.
Figure 6. Influence of the concentration of 3-NP on its degradation kinetics and linearization of kinetic parameters versus 3-NP concentration (pH 3; formate concentration, 25 mM; temperature, 22 °C; particle size, G1; CL, 105 mg Pd g-1; CD, 200 mg L-1).
degradation efficiency under the selected experimental conditions: the excess of formate was sufficient to allow the complete elimination of the substrate (Figure 6; and Table 5 in the Supporting Information). As nitrophenol concentration increased, the pseudo-first-order kinetic equation became progressively less accurate at modeling experimental data, and the two-parameter equation was more suitable. Considering the ratios k1/k2 calculated from Table 5 (Supporting Information) allowed the equivalent pseudo-first-order kinetic parameter to be obtained, assuming high substrate concentration. This calculated equivalent parameter could only be used for comparing the coefficients at the earliest contact time. The values obtained for concentrations ranging from 100 to 200 mg L-1 varied between 11.1 and 13.2 min-1. These values were close to those obtained at initial nitrophenol concentrations of 25 and 50 mg L-1. The mean value for the five concentrations was 11.88 ( 1.07 min-1. Increasing nitrophenol concentration significantly increased both the time for half-reaction and the CAC, while it decreased the initial rate of degradation. Indeed, the initial rate of degradation (k0) was found to linearly decrease with the concentration. The kinetic parameters vary according to the following equations:
1/CAC ) 27.2 [NP] + 0.160
R2 ) 0.994
k0 ) 0.106 - 3.40 × 10-4 [NP]
R2 ) 0.971 (10)
(9)
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Figure 7. Influence of catalyst particle size on 3-NP degradation kinetics (left panel) and kinetic parameters (right panel) (pH 3; 3-NP concentration, 50 mg L-1; formate concentration, 25 mM; temperature, 22 °C; CL, 105 mg Pd g-1; CD, 200 mg L-1).
(rather than intraparticle diffusion).47 So it seems that film diffusion resistance played an important role in the control of the degradation kinetics. Alternatively, another cause could be that the catalytic mechanism is limited to the external layer of the catalyst; in this case, the degradation rate will be proportional to the ratio of the external surface area to the volume of the solution, which is homogeneous to the reciprocal of the diameter of the particle. This would be consistent with the results obtained in the preceding section that focused on the influence of metal loading in the particle. Moreover, we observed that increasing the catalyst dosage (catalyst concentration in the solution) resulted in linearly increasing degradation rates. This is also consistent with a reaction limited to the surface of the catalyst.49 This limitation of the catalytic mechanism at the surface of the solid may be explained by (a) very poor intraparticle diffusion properties, (b) very high degradation efficiency at the surface of the catalyst limiting the contribution of internal sites, and/or (c) heterogeneous reduction of palladium during the preparation procedure for large size sorbent particles. Previous studies of palladium and platinum sorption on chitosan derivatives have shown by SEM-EDAX (scanning electron microscopy coupled with energy dispersive analysis through X-ray) spectroscopy that the metals were homogeneously distributed throughout the mass of the catalyst. However, the oxidation state of palladium in the catalyst was not checked (by XPS analysis) throughout the whole mass.26,57 A sample of the G4 fraction was ground, and a G0b sample, with a particle size in the 0-95 µm range, was prepared. This sample was tested for nitrophenol degradation under comparable conditions, and the results (Figure 7) show that grinding the catalyst significantly improved degradation kinetics, though it was not possible to match the degradation velocity observed with the G0 sample. The kinetic curve tended to the curve obtained with the G1 fraction. This indicates that palladium is almost homogeneously reduced throughout the whole mass of the material and that the decrease in degradation rates is mainly related to surface reactions (due to restrictions in intraparticle diffusion and/ or very high degradation efficiency at the surface of the catalyst). Since only the external layers of the surface are involved in the catalytic reaction, the catalytic activity is underestimated by taking into account the whole mass of sorbent. This could explain the relatively lower catalytic activity of these materials compared to more conventional materials.37 A better use of the catalytic metal should be obtained by using the polymer as a thin coating layer at the surface of inert material or using thin membranes such as those developed in our lab (catalytic hollow fibers, work in preparation). (57) Guibal, E.; Van Offenberg Sweeney, N.; Vincent, T.; Tobin, J. M. React. Funct. Polym. 2002, 50, 149.
Influence of the Type of Nitrophenol. Degradation reactions have been performed using different nitrophenols (2-NP, 3-NP, and 4-NP) under comparable experimental conditions (not shown). Though some differences may be observed in their degradation rates (halflife, kinetic parameters, and CAC), the differences are not very marked (Table 8 in the Supporting Information). Based on the slight differences observed, it appears that the degradation rates vary according to the following order: 4-NP > 3-NP > 2-NP. However, the differences are not marked enough to establish selectivity in their degradation patterns. Complementary experiments (not shown) have been performed on 4-nitroaniline showing a faster degradation of this compound compared to nitrophenol. So the structure of the substituents on the benzene ring significantly influences their degradation rates. Moreover, preliminary results in the determination of degradation products have shown that the type of nitrophenol (position of substituents) may also control the degradation products. Conclusion The strong affinity of chitosan (cross-linked with glutaraldehyde) for palladium chloro-species and the strength of the chitosan-palladium interaction, which prevents metal leaching during catalytic reactions, has been the motive for testing chitosan loaded with palladium for catalytic reactions. The abundant resource of this renewable material and the stereospecific properties that its structure can bring may explain the recent interest in using this material for catalytic processes. Another important reason for interest in this material is its versatility: it can be conditioned in forms as different as beads, fibers, membranes, and hollow fibers or as a coating material on mineral membranes that may be helpful in the design of new catalytic systems. The ability of palladium supported on chitosan to be used for the degradation of nitrophenol in the presence of sodium formate (as the hydrogen donor) has been proved. Though the degradation performance appears lower by 1 order of magnitude to that obtained with other systems (such as Pd/C catalyst using hydrogen and methanol as the solvent),10 a direct comparison of kinetic parameters is difficult since the experimental conditions are completely different (nitrophenol concentration, solvent, hydrogen donor, etc.). Nitrophenol is first degraded in aminophenol before being transformed in different products depending on experimental conditions (oxidative or reductive experimental conditions, type of nitrophenol). The reaction proceeds by hydrogen transfer: both the substrate and the hydrogen donor adsorb on the surface of catalyst nodules, and the degradation of formate into carbon dioxide and hydrogen induces the reduction of nitrophenol.
Chitosan-Supported Palladium Catalyst
The reaction appears to be more efficient in acidic solutions: an initial pH below pH 4-5 appears to be optimum. The pH strongly increases during catalytic degradation, possibly due to degradation of formate into CO2. A large excess (about 10 times) of sodium formate compared to nitrophenol is required to reach the maximum degradation for a given substrate concentration. The weak effect of agitation speed on degradation kinetics indicates that external film diffusion is not a strongly limiting parameter. The study of the effect of catalyst particle size, catalyst dosage, and catalyst loading on degradation kinetics leads to the conclusion that internal catalytic sites play a limited role in the conversion process and that the reaction should be limited to the external surface of the catalyst. The optimum use of the catalyst will be achieved using microparticles or depositing the supporting polymer on an inert material with a large superficial area before metal uptake and reduction. Current experiments are performed using palladiumloaded chitosan hollow fibers instead of particles in order to reduce the influence of diffusion limitations and to allow
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the catalytic system to be used in a continuous mode more easily than in fixed-bed systems (influence of diffusion and hydrodynamic limitations). Moreover, the hollow system allows optimum control of the chemical system (pH, concentration, etc.) by controlling both the reactive compartment (sodium formate, hydrogen) and the solution compartment (the effluent to be treated flowing in the lumen of the fiber). Acknowledgment. We thank A.N.VA.R. (Agence Nationale de Valorisation de la Recherche, French Ministry of Industry) for financial support in the development of chitosan-based catalysts. Supporting Information Available: Tables showing the influence of pH, catalyst dosage, concentration, palladium loading, particle size of catalyst, and type of nitrophenol on kinetic parameters for 3-NP degradation. This material is available free of charge via the Internet at http://pubs.acs.org. LA034364R
