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Chitosan-Supported Palladium Catalyst. 1. Synthesis Procedure 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 Ales Cedex, France
Owing to its high sorption capacity for precious metals (especially palladium), the potential of the chitosan biopolymer for catalytic applications was investigated. The catalyst was prepared in two steps: sorption of the palladium, followed by chemical reduction. Two different supports derived from chitosan were tested: a glutaraldehyde cross-linked chitosan and a chitosan derivative that was prepared by preliminary reaction with a sulfate solution (sulfate crosslinking). Three different reduction treatments were tested: treatment with sodium borohydride, treatment with sodium formate, and reduction with hydrogen, which was generated in situ by reacting the material with a sulfuric acid solution in the presence of Zn. The catalytic activity was determined through a very simple reaction: the reduction of chromate in the presence of an electron donor (formate ions). Introduction Platinum group metals (PGMs) are widely used for catalysis.1,2 Considerable work has been carried out to develop their use in supported rather than homogeneous catalysis, using, for example, silica, alumina, activated carbon, or polymers, to avoid the possible loss of highvalue materials.3-6 Moreover, supporting catalytic metals on specially tailored supports can add new functionalities such as enantioselectivity.7-10 Recently, research into supported catalysis has been carried out on chitosan.11-17 Chitosan is derived from chitin, the most abundant biopolymer in nature after cellulose. Chitin is extracted from shrimp shells and transformed into chitosan by an alkaline deacetylation treatment.18 Chitosan is characterized as a heteropolymer consisting of D-glucosamine units and N-acetyl D-glucosamine. The main characteristic of the biopolymer is, therefore, its high amine function content. Its behavior in acidic solutions is cationic.19 The presence of amine functions leads to interesting chelating properties for metal cations,10-22 but the protonation of these amine functions (-NH3+) gives the polymer the behavior of a weakly basic anion exchanger.23-25 Many studies have also focused on the sorption of PGMs on chitosan.23,26,27 Sorption properties, i.e., sorption isotherms and/or kinetics, can be readily increased by the chemical grafting of new functional groups28,29 or by physical modifications in order to improve diffusion properties.21,30-33 Because chitosan is soluble in most mineral acids, it is necessary to increase its stability by means of a cross-linking treatment. Glutaraldehyde has been frequently used for stabilizing chitosan in acidic solutions.26,31 The strong ability of chitosan to sorb PGMs explains why many studies have focused on the use of chitosan for supporting catalysts, as pointed out above. Mabbett et al.34 have shown that the reduction of chromate ions to Cr(III) is much more efficient on socalled “biopalladium” than on a chemically produced mineral palladium. This biopalladium was produced by the bioreduction of palladium from aqueous solutions * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +33 (0)4 66 78 27 01.
at the surface of microorganisms. They observed that small crystallites of palladium precipitate at the surface of the bacteria and that the size of the crystallites is significantly smaller than those of the chemical palladium. They attributed the high efficiency of biopalladium for catalysis to the small size of these crystallites. The present study investigates the reduction of Cr(VI) by palladium supported on chitosan, using formate as the electron donor. The typical oxidoreduction equations are as follows:
Cr2O72- + 14H3O+ + 6e- T 2Cr3+ + 21H2O HCOO- + H2O T CO2 + H3O+ + 2eThe reduction properties were compared for different catalyst manufacturing processes in order to optimize reduction kinetics. The catalyst was synthesized by means of a two-step procedure. First, palladium was adsorbed on the chitosan-based sorbent, and then the loaded sorbent was submitted to a reduction process. Two different chitosan sorbents were tested: a glutaraldehyde cross-linked chitosan and a sulfated chitosan. Reduction was performed with several reducing agents: sodium borohydride, sodium formate, and hydrogen generated in situ (reaction of sulfuric acid with zinc). The results were compared to the kinetics obtained with palladium reduced from solutions using the same reducing procedures. Material and Methods Materials. Chitosan was supplied by Aber Technologies (Plouvien, France). It had already been characterized:26 the degree of deacetylation was 87%, and the molecular weight was 125 000 g mol-1. The chitosan was ground and sieved, and the 0-125 µm fraction was used for experimentation. NaBH4 and Na2SO4 were purchased from Fluka (Switzerland) as analytical-grade products. PdCl2 was purchased from Acros (USA). Other reagents (acids, zinc, and HCOONa) were supplied by Carlo Erba (Italy). Preparation of the Sorbents. Because chitosan is soluble in hydrochloric acid, it cannot be used as
10.1021/ie0201462 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/20/2002
Ind. Eng. Chem. Res., Vol. 41, No. 21, 2002 5159 Table 1. Catalyst Synthesis Procedure and Characteristicsa ref C1 C2: C3: C4: S1 S2: S3 S4:
C1/F C1/H C1/H S1/F S3/F
sorb. C C C C S S S S
red.
pH control
q
B B, F B, Hb B, Hc B B, F B B, F
no no no no no no yes yes
154 154 154 154 208 208 271 271
∆W (%) Pd sorption reduction content +14 +14 +14 +14 +49 +49 +39 +39
-18 -20 -20 -25 -29 -34 -8 -40
164 192 191 205 195 293 211 354
a
pH control: S catalyst/pH was controlled to 9 after adsorption of palladium, and the pH was then adjusted to 2. q: sorption capacity (mg of Pd g-1) calculated by the mass balance equation. ∆W (%) (sorption): variation of the mass of the solid during the sorption process. ∆W (%) (reduction): additional variation of the mass of the solid during the reduction process. Pd content: amount of palladium on the final solid, mg of Pd g-1 catalyst (assuming weight variation to be only due to the loss of organic material). B: NaBH4 reduction. F: formate reduction. H: Zn/H2SO4 reduction. b 200 mg of C1/100 mL of H2SO4 (1%, w/w) and 100 mg of Zn. c 200 mg of C1/100 mL of H2SO4 (1%, w/w) and 300 mg of Zn.
Figure 1. Chemical structure of chitosan and glutaraldehydecross-linked chitosan.
supplied: a cross-linking treatment is required. The chitosan was cross-linked with glutaraldehyde by contact with a glutaraldehyde solution (10%, w/w). The volume of the glutaraldehyde solution and the mass of the chitosan were set to reach a 1:1 molar ratio between the amine groups of the polymer and aldehyde functions of the cross-linking agent. Finally, the particles were abundantly rinsed to remove traces of unreacted glutaraldehyde and dried overnight at 60 °C. The catalysts resulting from this synthesis procedure will be referred to as C catalysts. Figure 1 shows the chemical structure of chitosan and glutaraldehyde-cross-linked chitosan. Alternatively, 1 g of chitosan was mixed with 1 L of a sodium sulfate solution (3 g L-1) for 48 h. The powder was then rinsed with demineralized water to remove excess sulfate ions on the polymer and finally dried overnight at 60 °C. The catalysts resulting from this preparation method will be referred to as S catalysts. Sulfate anions are sorbed to chitosan through electrostatic attraction (anion attraction by protonated amine groups in an acidic solution). Palladium Sorption. The sorbents (1 g) were mixed for 24 h with a 200 mg of Pd L-1 of palladium solution (2 L) 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 loaded sorbent particles were rinsed and dried overnight at 60 °C. The sorption capacity (amount of palladium adsorbed on the sorbent, q, mg of Pd g-1) was obtained by the mass balance equation. The actual amount of palladium present on the support could be calculated by taking into account the sorption of chloropalladate species.
Procedures for Palladium Reduction. Different agents were used for reducing the palladium: sodium borohydride, sodium formate, and in situ generated hydrogen (by reaction of sulfuric acid with zinc). For the reduction treatment with borohydride, the loaded sorbent was mixed with 500 mL of the reducing agent solution (1 g L-1) for 4 h; the pH was not adjusted. The preliminary treatment was shown to be necessary even in the case of reduction with formate. Indeed, when the loaded sorbent was reduced with sodium formate, the catalyst was less efficient than with the sodium borohydride treatment. The reduction with sodium formate was performed by mixing the loaded sorbent with 100 mL of sodium formate (8.5 g L-1; 0.125 M) at boiling point for 1 h. Because this drastic treatment can partially dissolve chitosan, it was necessary to measure the actual weight of the solid at the different stages of the synthesis process. The third treatment, which was considered as a complementary reducing treatment, consisted of bringing the loaded sorbent (200 mg) into contact with 100 mL of a sulfuric acid solution (1%, w/w) containing either 100 or 300 mg of zinc (provided as a fine powder). To optimize the process, several variables were changed to take into account, for example, the possibility to control the pH and the order of the treatment sequence. These differences are summarized in Table 1, together with the main characteristics of the catalysts. Palladium desorption was performed on glutaraldehyde-cross-linked chitosan saturated with palladium using hydrochloric acid at different concentrations. It appeared that even with a 5 M HCl solution palladium was not completely desorbed. With the 0.5 M HCl solution, desorption was less than 10%. In the pH range used for the present experiments (ca. above pH 2), no release of palladium was observed. Moreover, numerous studies have shown that the sorption of precious metals on biosorbents is followed by a partial reduction of the metals that induces an increase in the stability of loaded metals on these supports. The reduction procedure increased the stability of palladium on the chitosan support, and the catalyst can be considered as stable. Chromate Reduction. Chromate reduction was performed in a batch reactor with continuous agitation. The pH was monitored but not adjusted during the
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experiments. The pH was initially set at pH 2.7, using sulfuric acid for the pH control. When necessary, the pH was controlled with 1 M NaOH. The catalyst (10 mg, unless specified) was dropped into the chromate solution (100 mL) just before the beginning of the experiment, while the start-up was given by adding the electron donor (sodium formate at a concentration of 25 mM) to the reactor. Filtered samples were collected at fixed times after the addition of chromate and formate. The chromate concentration was measured by a direct colorimetric measurement at pH 3 using the wavelength 354 nm. The time for half-reduction (t1/2, min) of chromate was also determined and used to determine a coefficient of catalytic activity (CACcat, mmol of Cr(VI) min-1 g-1 of catalyst; CACPd, mmol of Cr(VI) min-1 g-1 of Pd). These coefficients were determined by the following formulas:
CACcat ) (C0/2)V/[t1/2m]
(1)
CACPd ) CACcat/pdc
(2)
where m is the amount of catalyst (g), V is the volume of the solution (L), and pdc is the palladium content of the catalyst (mg of Pd g-1; total mass of the catalyst). The initial catalytic activity coefficient was obtained by taking into account the extent of chromate reduction during the first initial minutes of contact (3, 5, or 10 min, depending on the experimental conditions). Results and Discussion Trial Tests. To verify whether the presence of the catalyst was necessary for the reduction of chromate, trial experiments were performed by mixing the Cchitosan and the S-chitosan with the formate and chromate solutions. The sulfated chitosan did not sorb chromate and the concentration did not change significantly throughout the contact time (even after 1 day of contact). With the glutaraldehyde-cross-linked chitosan, there was a low decrease in the concentration of chromate due to either a sorption mechanism or a reduction on the sorbent. This mechanism had previously been observed by Bosinco et al.36 and confirmed by Dambies et al.37 The presence of unreacted aldehyde moieties from the cross-linking agent on the modified biopolymer gives it reduction properties. However, this decrease did not exceed 5% after 75 min of contact time and was lower than 7% after 24 h of contact. The fraction of chromate sorption/reduction on the sorbent can be neglected under the selected experimental conditions. Blank experiments were also performed in the absence of catalyst. The presence of the electron donor was not sufficient to reduce chromate ions: the reduction reaction was not initiated, and the concentration remained constant overnight. Experiments were also performed with the catalyst in the absence of the electron donor, and the concentrations remained unchanged even after several hours. The simultaneous presence of palladium and the electron donor (sodium formate) was therefore required in order to significantly reduce the chromate. Catalyst Production. The variations in the weight of the solid throughout the synthesis procedure are given in Table 1 for different preparations and two sorbents. In most cases, the combination of adsorption and reduction operations resulted in a decrease in the weight of the solid, except in the case of sodium
Figure 2. Influence of the experimental procedure for the synthesis of the C catalyst on chromate reduction at pH 2.7.
borohydride reduction on Pd-loaded sulfated chitosan. As compared with sulfated chitosan, the glutaraldehyde cross-linking reduces the loss of sorbent during the reduction of the material with sodium formate. However, when the pH was controlled during the reduction process of the catalyst prepared from sulfated chitosan, the weight loss was significantly decreased. With sulfated chitosan, sorption capacities were significantly greater than with the cross-linked material: the increase varied from 30% to 70%, depending on the experimental conditions. Considering the Pd content of the final product, the sulfated chitosan appeared to be significantly better than cross-linked products by 2575%. In the case of sulfated chitosan, it was impossible to reduce the material by the in situ hydrogen production process because under such drastic conditions (i.e., very acidic pH and boiling temperature) chitosan material was partially soluble. Results show that the cross-linked material is significantly more stable than sulfated chitosan. The change in the weight of catalyst under formate reduction was more penalizing with sulfated chitosan than with cross-linked chitosan. However, it was then necessary to compare the activity of the materials to select the optimum catalyst, taking into account stability and weight loss during synthesis. Selection of Optimum Catalysts. The influence of the synthesis procedure on catalytic activity is presented in Figure 2 for C catalysts under the selected experimental conditions. Preliminary experiments were performed and showed that chromate reduction was significantly faster and better when the reaction was performed in acidic solutions. The influence of the pH was studied, and results (not shown) showed that the optimum pH was around 2.7. Results show that the reduction with sodium borohydride was not sufficient to prepare an efficient catalyst: long contact times were necessary to reach complete reduction of chromate (Figure 2). Complementary treatments were necessary to achieve fast kinetics. The catalysts may be ranked in the following order: C4 > C3 . C2 . C1. It is important to observe that the in situ generation of hydrogen produces more efficient catalysts than formate complementary treatment. Some of these catalysts were prepared by a combination of reduction steps (especially sodium borohydride and in situ generated hydrogen). It appeared that the pretreat-
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Figure 3. Influence of the experimental procedure for the synthesis of the S catalyst on chromate reduction at pH 2.7.
ment with sodium borohydride increased the efficiency of the catalyst. Though the exact reason for this improvement in the catalytic activity is not exactly known, several reasons may explain this positive effect: (a) sodium borohydride can be used for the hydrogenation of the imine function resulting from the reaction of aldehyde of glutaraldehyde with amine groups of chitosan; (b) the reaction with sodium borohydride enhanced palladium reduction. The difference in the reduction kinetics (and the catalytic activities) may be due to a different extent of palladium reduction on the support from Pd(IV) to Pd(II) and finally Pd(0). Increasing the amount of zinc used in the in situ generation of hydrogen resulted in higher catalytic activity. In the case of the S catalysts, after palladium reduction with borohydride, the catalytic activity was greater than that of C catalysts obtained under the same experimental conditions (Figure 3). This is especially evident in the case of the material that was reduced by a single treatment with borohydride; the differences are not so marked for formate-reduced products. However, kinetic curves almost overlapped for the S and C catalysts produced under the optimum conditions: S4 and C4. However, considering the higher palladium loading in the S4 catalyst compared to the C4 catalyst, it appeared that palladium use is more efficient in the C4 catalyst than in the S4 material. The control of the pH during the synthesis improved catalytic activity, especially in the case of the material that was produced with a single treatment with sodium borohydride. The complementary treatment with formate reduced the influence of the pH control on the catalytic activity. Moreover, the pH control during the synthesis reduced palladium loss. The reduction of palladium loaded on glutaraldehydecross-linked chitosan with hydrogen produced by reaction of sulfuric acid with a large amount of zinc was the optimum process for the synthesis of the catalyst: the catalyst was better than that obtained by formate reduction of loaded sulfated chitosan prepared under pH control (reprecipitation of the dissolved support). These experiments were performed using 10 mg of catalyst, containing variable amounts of palladium. Taking into account the actual palladium content of the different catalysts, results show that the catalytic activity was greater for C catalysts than for S catalysts.
Figure 4. Influence of the experimental procedure for the preparation of the chemically reduced palladium salt on chromate reduction at pH 2.7. Table 2. Influence of the Synthesis Procedure on Catalytic Activity Coefficients (Chromate Concentration, 30 mg of Cr(VI) L-1; Formate Concentration, 25 mM; Catalyst Dosage, 100 mg L-1)
catalyst
t1/2 (min)
C1 C2 C3 C4 S1 S2 S3 S4 M1a M2a
>240 47 31 13 163 21 101 19 >300 142
CACcat (µmol min-1 -1 g of catalyst)
CACPd (µmol min-1 -1 g of Pd)
CACPd0 (µmol min-1 g-1 of Pd)
