From Hydrocolloids to High Specific Surface Area Porous Supports for

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From Hydrocolloids to High Specific Surface Area Porous Supports for Catalysis Romain Valentin,† Karine Molvinger†, Christophe Viton‡, Alain Domard‡, and Franc¸ oise Quignard*,† Laboratoire des Mate´ riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS-UM1 Institut Gerhart FR 1878, 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France, and Laboratoire des Mate´ riaux Polyme` res et des Biomate´ riaux, UMR-CNRS 5627, Universite´ Claude Bernard Lyon 1, Baˆ t ISTIL, 15 Bd Andre´ Latarjet, 69622 Villeurbanne Cedex, France Received April 13, 2005; Revised Manuscript Received June 5, 2005

Polysaccharide hydrogels are effective supports for heterogeneous catalysts. Their use in solvents different from water has been hampered by their instability upon drying. While the freeze-drying process or airdrying of hydrocolloid gels led to compact solids with a low surface area, drying the gel in CO2 beyond the critical point provided mesoporous materials with a high specific surface area. Their effectiveness as a support for catalysis was exemplified in the reaction of substitution of an allyl carbonate with morpholine catalyzed by the hydrosoluble Pd(TPPTS)3 complex. The influence of water on the catalytic activity and the properties of the support was evidenced. Introduction The synthesis of heterogeneous catalysts is a field of continuing interest for fine chemistry. Although some of the organometallic complexes exhibit remarkable catalytic properties in terms of activity and selectivity, their heterogenization is currently an economical as well as a toxicological and environmental challenge. Polysaccharides, which have been known for many years to be supports for enzymatic catalysts, now receive more attention as supports for metal catalysts. They fulfill most of the requisite properties: insolubility in the majority of the organic solvents and presence of numerous and different surface functionalities such as hydroxy, carboxy, or amino groups. Most of the results reported in the literature concern the use of chitosan, produced by the N-deacetylation of chitin, an important natural biopolymer, as a support for metal particles. These materials were used for the reduction of chromate,1 phenol,2,3 and nitroaromatic compounds.4,5 Functionalization of chitosan provided catalysts for cyclopropanation of olefins,6 oxidation of alkylbenzene,7 and Suzuki and Heck reactions.8 The amphiphilic nature of polysaccharides has been used to prepare different supported aqueous phase catalysts for the palladium catalyzed allylic substitution.9,10 Although the specific surface area is of major importance for a support of the catalyst, very few attempts to increase this parameter are reported in the literature. Clark et al.11 described the use of expanded corn starch for liquid-phase organic reactions. These authors formed an expanded starch gel network by gelatinization in hot water, then by aging the sample several weeks at low temperature; BET surface areas over 100 m2 g-1 were thus achieved. Recently, in the laboratory, we * Corresponding author. E-mail: [email protected]. † UMR 5618 ENSCM-CNRS-UM1 Institut Gerhart FR 1878. ‡ Universite ´ Claude Bernard Lyon 1.

reported that chitosan microspheres dried in supercritical CO2 conditions affording an easy access to the functional groups of this natural polymer were used as a catalyst for the synthesis of a monoglyceride by fatty acid addition on glycidol.12 Their specific surface areas determined from the nitrogen sorption-desorption isotherms at 77 K were close to 110 m2 g-1. A recent review summerized the main advances in this field published over the last 15 years.13 Polysaccharides are hydrocolloids able to give rise to strong physical hydrogels in water, thanks to the formation of a three-dimensional network of polymer chains. The present study describes a detailed investigation of the behavior of the corresponding hydrocolloid aerogel microspheres as a catalyst support. The stability of the obtained catalyst was investigated in terms of textural stability as well as in terms of catalytic performance. The work, centered on the study of alginate aerogels, was extended to other natural hydrocolloids to obtain some ideas on the influence of the chemical structure of the support. The catalytic properties of the polysaccharide-palladium solids were determined for the reaction of coupling of the allyl-methyl carbonate with morpholine. Experimental Procedures Materials. The characteristics of the three different sodium alginates used in this work are reported in Table 1. The guluronic residue14 content was determined by 13C NMR. The weight-average molecular weights Mw of the samples were determined by size exclusion chromatography (SEC) coupled online with a multi-angle laser light scattering (MALLS) detector. SEC was performed by means of an IsoChrom LC pump (Spectra Physics) connected to Protein Pack glass 200 SW and TSK gel 6000 PW columns. A

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Table 1. Characteristics of Alginate Samples sample

provenance

guluronic %

Mw (g mol-1)

η (dL/g)

G-20 G-45 G-76

ProtanalFMC Biopolymer Cecalgum S500 SANOFI Satialgine SG 800 Systems Bio-Industries

20.0 ( 0.4 45.0 ( 0.9 76.0 ( 1.5

200 000 ( 15 000 400 000 ( 30 000 400 000 ( 30 000

484 847 795

Waters R 410 (Waters-Millipore) differential refractometer and a multi-angle laser-light scattering detector, operating at 632.8 nm (Wyatt Dawn DSP), were connected online. The dynamic viscosity was determined with solutions containing 0.6-0.1% w/v of any alginate at 25 °C in 0.1 M NaCl, with a HAAKE RS100 rheometer. Intrinsic viscosity [η] values were estimated accordingly to Launey et al.15 by analyzing the concentration dependence of the reduced viscosity. [η] was then estimated by extrapolating the plot of the reduced viscosity versus C at zero concentration. Pd(OAc)2 (95% purity), acetonitrile (anhydrous,99%), methyl-allyl carbonate, and morpholine (98%) were used as received from Aldrich, and TPPTS (P(m-C6H4SO3Na)3‚ 3H2O) was from Strem. Preparation of Beads. Alginate Beads. Sodium alginate was dissolved in distilled water at a concentration of 2% (w/w). The polymer solution was added dropwise at room temperature to the stirred CaCl2 (Aldrich) solution (0.24 M) using a syringe with a 0.8 mm diameter needle. The microspheres were cured in the gelation solution for 15 h. Carrageenan Beads. A 2.5% (w/w) κ-carrageenan stock solution was prepared by dispersing κ-carrageenan (Eucheuma cottonii SIGMA 90%) in ultrapure water at 80 °C for 30 min. The principle of bead formation was the thermoand ionotropic gelation of κ-carrageenan hot droplets falling into a cold saline (KCl) solution. The 2.5% stock solution of κ-carrageenan thermostated at 80 °C was dropped into a 0.6 M KCl solution at 5 °C under stirring using a syringe with a 0.8 mm diameter needle. The gel beads were aged for 12 h in this solution at 5 °C without stirring and hence washed with cold water.16 Chitosan Beads. An aqueous solution of chitosan (Mahtani Chitosan PVT, LTD) characterized by its degree of acetylation (DA) of 5%, as the molar ratio of remaining acetyl groups measured by NMR spectroscopy and a weightaverage molecular weight of Mw ) 200 000 g/mol measured by light scattering, was obtained by dissolving 1 g of chitosan in 100 mL of a solution of acetic acid that was 0.055 mol L-1. This solution was dripped into a NaOH solution (4 N) through a 0.8 mm syringe needle. The chitosan beads were stored in the alkaline solution for 2 h and then filtered and washed with water. The microspheres were dehydrated by immersion in a series of successive ethanol-water baths of increasing alcohol concentration (10, 30, 50, 70, 90, and 100%) for 15 min each.17 Finally, the microspheres were dried under supercritical CO2 conditions (74 bar, 31.5 °C) in a Polaron 3100 apparatus. Synthesis of the Catalyst. All experiments were performed under strict exclusion of oxygen using the standard Schlenk tube techniques. In a typical experiment, Pd(OAc)2 (3.2 mg, 0.014 mmol) and TPPTS (48.1 mg, 0.084 mmol) were dissolved under

magnetic stirring in deaerated water (2.36 mL). This solution was maintained at 40 °C for 30 min. A total of 300 mg of alginate microspheres was outgassed under vacuum and stored under argon. Then, they were impregnated with anhydrous ethanol for 1 h and poured into the catalytic solution. After 30 min at 40 °C, the beads were dehydrated by immersion into two succesive baths of pure ethanol and dried under supercritical CO2 conditions. The Pd content was determined by ICP. Characterization. Scanning electron micrographs (SEM) on the dried microspheres were obtained with a Hitachi S 4500 apparatus after platinum metallization. Nitrogen adsorption/desorption isotherms were recorded with a Micromeritics ASAP 2010 apparatus at 77 K, after outgassing the sample at 353 K under vacuum until a stable 3 × 10-5 Torr pressure was obtained without pumping. Catalytic Tests. All experiments were performed under strict exclusion of oxygen using the standard Schlenk tube techniques. In a typical experiment, a solution of methylallyl carbonate (9.9 µL, 8.6 × 10-5 mol) and morpholine (7.6 µL, 8.6 × 10-5 mol) was prepared in acetonitrile (2.9 mL) in a Schlenk tube. The solid was introduced into a round-bottomed flask fitted with a condenser and a septum. When necessary, the amount of water needed to reach the desired ratio H2O/catalyst was added with a syringe, and the system was left for equilibration for 30 min at room temperature. The solution of reactants was then introduced on the solid, and the temperature was raised to 50 °C by immersion in a preheated oil bath. This step corresponded to t ) 0 of the catalytic test. The course of the reaction was monitored by periodic samplings and quantitative analysis by GC (BP20 column). After the reaction, the solution was filtered off at the temperature of the reaction and used for a homogeneous catalytic test. The solid was washed with acetonitrile and reused for the successive runs. Results and Discussion Textural Properties of Alginate Beads. Alginates are abundant polysaccharides produced by brown algae mainly constituted of (1 f 4) linked β-D-mannuronic (M) and R-Lguluronic (G) residues (Figure 1), according to three kinds of sequences (M)m, (G)n, and (M,G)x. Alginates differ by their M/G ratio. They are used extensively for the entrapment of biologically active materials.18 The use of alginates as immobilizing agents, in most applications, lies in their ability to form heat-stable strong gels with divalent cations, especially Ca2+. Gelling of alginates occurs when the divalent cations take part in the interchain binding between G blocks leading to a three-dimensional network. The properties of alginate gels are then influenced by the ratio and sequencing of uronic monomers,19 the concentration

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Figure 1. Structure of alginate repeating units (M, mannuronic, and G, guluronic acids). Table 2. Average Bead Size and Mean Square Error (mm) for Three Different Alginates at Different Stages of Preparation

hydrogel alcogel aerogel xerogel

G-20

G-45

G-76

3.03 ( 0.11 2.76 ( 0.13 2.81 ( 0.24 0.87 ( 0.08

2.72 ( 0.17 2.53 ( 0.13 2.50 ( 0.14 0.83 ( 0.03

2.76 ( 0.13 2.63 ( 0.19 2.35 ( 0.11 0.90 ( 0.04

of cations in the maturation bath, and the time of maturation.20,21 Three different alginates were used for this study, with different guluronic fractions: G-20 (20% guluronic), G-45 (45% guluronic), and G-76 (76% guluronic). The gels were formed by interchain complexation of Ca2+ cations. The concentration of the cation in the gelling bath and the time of maturation were chosen according to literature data18 to form a stiff gel. Thus, whatever the alginate, the beads were aged for complete maturation for 15 h in a 0.24 M Ca2+ aqueous bath. Two different procedures were followed to dry the microspheres. One consisted of the evaporation of the solvent under vacuum at room temperature. This procedure led to a xerogel. The second consisted in the extraction of the solvent and evaporation above the critical point. This procedure led to an aerogel. Supercritical CO2 was chosen because of its low critical point of 31.5 °C, at 70 bar. This technique is commonly processed with inorganic solids to achieve very high specific surface areas.22 This procedure releases the porous texture quite intact by avoiding the pore collapse phenomenon. For all alginate samples, the average sphere diameter at the different stages of preparation was measured:23 the data are reported in Table 2. For all alginate samples, the exchange of ethanol by water brought about a size shrinkage lower than 10%. The supercritical CO2 drying allowed us to form the aerogel from the alcogel and induced almost no size change of the spheres of G-20 and G-45. In the case of G-76, the size change was at the limit of the mean square error. The influence of the drying procedure on the morphology of the microspheres is illustrated in Figure 2. Gel beads obtained from alginate G-20 are represented at different stages of preparation: hydrogel beads (Figure 2a), alcogel beads obtained after exchange of water by ethanol (Figure 2b), aerogel beads obtained after supercritical CO2 drying (Figure 2c), and xerogel beads obtained from the alcogel by ethanol evaporation at room temperature (Figure 2d). At the

Figure 2. G-20 alginate beads at different steps of their preparation. (a) Hydrogel, (b) alcogel, (c) aerogel from supercriticalal CO2-dried alcogel, and (d) xerogel from room temperature-dried alcogel. Grid square side 1 mm.

macroscopic scale, it is clear that the spherical character of the beads is maintained in both procedures of drying, but an important shrinkage occurred for the xerogel (Figure 2d), while the aerogel maintained the size of the beads. The difference between the two procedures of drying is obvious when we observe the inner part of the microspheres. Scanning electron micrographs of cross-sections of the aerogel and xerogel spheres from alginate G-20 are reported in Figure 3. The difference between the two procedures of drying is obvious when we observe the inner part of the microspheres. Scanning electron micrographs of cross-sections of the aerogel and xerogel spheres from alginate G-20 are reported in Figure 3. The xerogel spheres are hollow spheres (Figure 3a), while in the aerogel spheres (Figure 3b), the inner part of the sphere still reflects the presence of the threedimensional network of polysaccharide fibrils (Figure 3c). Information about several textural properties24,25 was obtained by nitrogen adsorption-desorption isotherms. The adsorption-desorption isotherms of N2 at 77 K on the three alginate aerogels are presented in Figure 4. All isotherms of Figure 4 are of type IV at the borderline with type II in the IUPAC classification, typical of mesoporous solids with strong adsorbent-adsorbate interaction, thus indicating the presence of large mesopores with a size distribution that continues into the macropore domain. Nevertheless, the porosity is more in the range of macroporos-

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Figure 3. SEM picture of a cross-section of a G-20 bead: xerogel (a), aerogel (b), and inner part of the aerogel (c) at different magnifications.

Figure 4. N2 adsorption-desorption at 77 K isotherms of G-20 (open triangle), G-45 (black triangle), and G-74 (open square) aerogels.

ity as clearly observed in the SEM picture than in the range of mesoporosity. The adsorption at low relative pressure allowed us to evaluate the specific surface area of the samples by the BET method,26 assuming a monolayer of N2 molecules to cover 0.162 nm2. Specific surface areas up to 500 m2 g-1 were obtained in a reproducible way. The textural properties of the three alginates are reported in Table 1. When the gels were lyophilized, or dried by evaporation of the solvent, the specific surface area never exceeded 1 m2 g-1. The texture was stable upon storage without particular conditions. The difference measured in the specific surface area after 10 months storage was lower than 10%. Catalyst Synthesis and Characterization. The catalyst synthesis involved the preparation of an aqueous solution of Pd (OAc)2 and 5 equiv of TPPTS. The reduction of Pd(II) to Pd(0) occurred readily, as demonstrated by 31P NMR spectroscopy, and led to the formation of Pd(TPPTS)3. Part of the phosphine was simultaneously oxidized (OTPPTS) (eq 1). This reaction was close to completion before the solution was poured onto the solid.27 It can be described as follows: Pd(OAc)2 + 4TPPTS f Pd(TPPTS)3 + OTPPTS + 2AcOH (1) Catalysts were then prepared by impregnation of various alginate beads with the catalytic solution, in relation with their different methods of preparation: (i) impregnation of the hydrogel beads, (ii) impregnation of the alcogel beads, and (iii) impregnation of the aerogel beads. All the solids were then dehydrated and dried under CO2 supercritical conditions. As the palladium complex is sensitive to oxygen, all the steps had to be done under an inert atmosphere and in a deaerated solvent. This requisite could not be respected during the CO2 supercritical drying.

Procedure i presented some drawbacks among which was the loss of an important amount of Pd (TPPTS)3. This problem could only be solved when the catalytic solution impregnated the alcogel beads (procedure ii). Surprisingly, the catalytic tests revealed that procedure iii led to inactive catalysts. At the same time, an important modification of the morphology of the alginate beads was observed. We thus investigated the behavior of the aerogels toward rehydration in terms of textural modifications. Scanning electron microscopy observations were associated with the analysis of N2 sorption isotherms after CO2 supercritical drying. These techniques allowed us to determine the best procedure to preserve the mesoporous character of the dried beads. After rehydration, the microspheres were dehydrated again according to the previously described procedure. The samples were labeled (G-xx W E) for the sequence water-ethanol. The consequences of a direct rehydration on the textural properties of the aerogel microspheres are reported in Figure 5 (a-c). Although the spherical morphology of the beads was preserved, it is clear in the SEM pictures that the internal texture had been dramatically modified on rehydrating. The spheres became hollow spheres; thus, all the fibrils collapsed to form a shell. The N2 sorption isotherms were still typical of mesoporous materials, but the mesoporosity reflected the texture of the shell. To that extent, the solid G-74 exhibited the highest mesoporosity. To maintain the texture of the aerogel, a rehydration had to be done following the reverse procedure of the dehydration. Aerogel beads were first impregnated by an ethanol solution followed by exchange with water. Both morphologies and internal textures were thus preserved as evidenced in Figure 5d for G-76 alginate. The N2 sorption isotherms before and after this treatment were identical. The crosssection of the microsphere still reflected the three-dimensional network of the alginate fibrils. This result suggested that procedure ii could be adapted to aerogel beads. This would present an interesting alternative (procedure iv). Because of the sensitivity of palladium (0) toward oxygen, procedure ii forced us to prepare the initial alginate solution in deaerated water. On the contrary, this was not necessary in procedure iv (impregnation of the aerogel beads (1) by ethanol-(2) by the aqueous catalytic solution). Some preliminary experiments were performed to select the procedure of synthesis according to the best catalytic properties in terms of activity but also of stability.



Figure 5. SEM micrographs of G-20WE (a), G-45WE (b), G-74WE (c), G-74EWE (d), and N2 adsorption-desorption isotherms at 77 K of G-20 (a), G-45 (b), G-74 (c and d) (black triangles) G-20 WE (a), G-45WE (b), G-74WE (c), and G-74 EWE (open triangles). Table 3. Catalytic Properties in the Reaction of Substitution of Methyl-Allyl Carbonate [1] with Morpholine [2]: Percent Yield at tr ) 5 min, [1]/[2]/Pd ) 12.5:12.5:1, T ) 50 °C, and [1] ) 30 mmol L-1

run 1 run 2 run 3

cata 1

cata 2

98 ( 2 93 ( 2 78 ( 2

98 ( 2 98 ( 2 96 ( 2

The catalytic properties of alginate based catalysts were tested in the reaction of substitution of methyl-allyl carbonate with morpholine (eq 2), in acetonitrile, and in conditions determined as optimal for the polysaccharide supported Pd(TPPTS)3 previously reported systems.10

After each catalytic test, the solution was filtered off at the temperature of the reaction. The solutions showed no activity at all when they were subjected to fresh reactants, thus confirming that the palladium complex did not leach into the organic solution. The solid was washed with acetonitrile and reused for three successive runs. The results for the solid prepared by impregnation of alcogel beads (cata 1) and for the solid prepared by impregnation of aerogel beads (1), by ethanol (2), and by the catalyst solution (cata 2) are reported in Table 3. For the first run, the catalytic performances of both catalyts were similar, but it seemed that the stability of the catalysts was better when prepared by impregnation of dried beads. All the following experiments were conducted on catalysts prepared by impregnation of aerogel beads. Because of the

high activity of the catalysts, the substrate/palladium ratio was increased to 100. The catalyst synthesis consisted of the impregnation of polysaccharide aerogel beads by ethanol before exchange with the aqueous catalytic solution (procedure iv). The beads remained in the catalytic solution for 30 min at 40 °C, the best conditions to ensure a good diffusion of the catalytic complex. Afterward, the beads were dehydrated in ethanol. Different catalysts were prepared with the three alginates G-20, G-45, and G-76. The amount of palladium involved in the synthesis was calculated to reach 1% palladium loading in the catalyst. Although the catalyst Pd(TPPTS)3 was insoluble in ethanol, part of the catalytic complex remained in the liquid phases in all steps of the synthesis. The yield of palladium incorporation was always comprised between 30 and 50%. The corresponding aerogels were prepared, and their textural characterization was performed. The corresponding nitrogen adsorption-desorption isotherms are given in Figure 6. The same scale was chosen for all isotherms. For clarity, the y-axis was limited to 800. The complete isotherms for G-45 and G-74 are given in Figure 4. The impregnation with the catalytic solution only slightly modified the textural properties of the materials except for sample B, where an important decrease in the specific surface area was noticed. The small differences observed in Figure 6 between both isotherms, for G-20 and G-76 alginate, may arise from the contribution of the amount of Pd (TPPTS)3 to the weight of the sample. The BET surface areas were thus calculated only taking into account the polysaccharide weight. The values obtained were close to those of the initial alginate specific surface area values. These results suggested that fibrils of

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Figure 6. N2 adsorption-desorption isotherms at 77 K of alginate aerogel (black symbols) and Pd/alginate aerogel (open symbols) (a) G-20, (b) G-45, and (c) G-76. Table 4. Textural Properties of the Catalysts

% Pd wt SBET (m2 g-1) SBET (m2 g-1)a a

G-20

Pd/G-20

337 ( 5

0.37 ( 0.02 275 ( 5 314 ( 5

G-45

Pd/G-45

438 ( 5

0.48 ( 0.02 312 ( 5 371 ( 5

G-74

Pd/G-74

453 ( 5

0.36 ( 0.02 396 ( 5 450 ( 5

Specific surface area with respect to the mass of alginate only.

Figure 7. Catalytic activities of Pd/G-20 (diamond), Pd/G-45 (square), and Pd/G-74 (triangle) in the reaction of substitution of methyl-allyl carbonate [1] with morpholine [2] in CH3CN-[1]/[2]/Pd ) 100:100:1,T ) 50 °C, and [1] ) 30 mmol L-1.

alginate were coated with the catalytic species. The textural data of the different solids are reported in Table 4. Catalytic Properties. The catalytic properties of the three solids (Pd/G-20, Pd/G-45, and Pd/G-74) were compared. The results are reported in Figure 7. The three solids were very similar in terms of catalytic activity. The reaction was completed in 30 min; a yield of 100% was reached. The curves obtained for solids G-20 and G-76 are perfectly superimposable. The solid Pd/G-45 seemed to be less active. Considering the hydrophilic properties of both the support and the catalyst, the effect of water addition was studied as for supported aqueous phase catalysis.28 When the catalysts were prepared by impregnation of the lyophilized polysaccharide materials, there was no activity in absence of water, suggesting that the catalytic sites were not accessible. Because of the swelling of the support with water, the activity increased with the increased sorption of the amount of water.10 The best activity was generally obtained with a ratio H2O/catalyst of 1 (g g-1). This behavior has to be related to the role of water as a plastifier of polysaccharides and then to its important contribution to the molecular mobility

through the increase of the free volume, therefore to an increase of the accessiblity in the amorphous domains. In Figure 8, it is clear that water had no effect on the activity of Pd/G-20 and Pd/G-76 catalysts and only a limited effect on Pd/G-45 solid. It is interesting to notice that Pd/ G-45 was the solid for which impregnation led to the most important modification of the textural properties (Figure 6b). It is also the solid with the highest palladium loading and the solid exhibiting the lowest activity. Thus, several hypotheses could be suggested. In the hypothesis of a coating of the alginate fibrils with the palladium complex, the dispersion of the catalytic sites may be a factor of influence on the catalytic properties. Because of the hydrosoluble character of the complex, water could contribute to the dispersion of palladium by the solubilization of Pd (TPPTS)3. A calculation of the steric hindrance of the complex was done.29 The surface occupied by the molecule was estimated to 260 Å2. The fraction of surface occupied by the complex was thus 22% for Pd/G-20, 15% for Pd/G-74, and 30% for Pd/G-45. The fraction of occupied surface was below the monolayer and that the dispersion could not be a drawback is this case. The modification of the textural properties of Pd/G-45 reflected a small shrinkage of the polymer structure during the drying process. In this case, some palladium sites could be nonaccessible, due to their entrapment into some shrunken fibrils. Water swelled the polymeric support, allowing the accessibility to these sites. When compared to the nonporous polysaccharide where no activity at all was noticed in absence of water, these systems exhibited a high activity. Influence of the Chemical Structure of the Support. The same procedure of catalyst synthesis was applied to other polysaccharides such as κ-carrageenan and chitosan. Carrageenans constitute a group of natural polysaccharides extracted from red marine algae (Rhodophyceae). Their structure is constituted of a linear chain of alternating (1f 3)-linked R-galactose-4-sulfate and (1f 4)-linked 3,6-βanhydrogalactose.30 Chitosan, obtained by deacetylation of



Figure 8. Effect of water on the activity of Pd/G-20 (diamond), Pd/G-45 (square), and Pd/G-74 (triangle): without water (open symbols) and mH2O/mcata ) 1 (black symbols) in the reaction of substitution of methyl-allyl carbonate [1] with morpholine [2] in CH3CN. [1]/[2]/Pd ) 100: 100:1, T ) 50 °C, and [1] ) 30 mmol L-1. Table 5. Influence of the Support in the Catalytic Properties of Polysaccharide Supported Palladium Complex

a

catalyst

TONa

Pd/G-20 Pd/G-45 Pd/G-74 Pd/carrageenan Pd/chitosan

540 ( 10 480 ( 10 546 ( 10 192 ( 5 41 ( 2

TON: mol product mol Pd-1 h-1

chitin (from squid pens or shrimp shells), is a linear copolymer of linked β- (1 f 4) glucosamine and Nacetylglucosamine, with a proportion of the second residue below 70%. These supports thus differ by the nature of the chemical functionality present in the polysaccharide network: carboxylates for alginates, sulfates for carrageenans, and primary amines for chitosan. The results expressed as the number of molecules of product produced per molecule of Pd per hour (turnover number) are reported in Table 5. This turnover was calculated from the yield reached in 10 min of reaction time (linear part of the curves yield as a function of time). The differences reflected the importance of the chemical properties of the support. Activities were correlated to electrostatic properties of the support. Carrageenans only bear one sulfate group per disaccharide unit, while two carboxylic groups are present on the disaccharide units of alginates. In chitosan aerogels, 70% of the amino groups of the polysaccharide are accessible,12 and the sensitivity of the allylic substitution reaction to the pH of the medium could explain the very low activity obtained when the Pd complex was supported on this basic polysaccharide. Nevertheless, the hypothesis of a direct interaction of the palladium complex with the amino groups of chitosan, which would inhibit the activity in the reaction, could not be excluded. Influence of the Solvent. The previous experiments had been done in a nitrile solvent, known to be perfectly suitable for palladium complex catalyzed reactions.31,32 Nevertheless, it is of interest to try to convert all the chemical processes into more environmental friendly procedures. In this objective, the catalytic tests were conducted in ethanol. The activities described by the curves of Figure 9 are similar in both solvents. A second run in ethanol led to the same yield for a 70 min reaction time. The stability of the catalytic species is the major advantage involved in the use of a nitrile

Figure 9. Comparative catalytic activity of Pd/G-20 in the reaction of substitution of methyl-allyl carbonate [1] with morpholine [2] in CH3CN (open triangles) and EtOH (black triangles) [1]/[2]/Pd ) 100:100: 1, T ) 50 °C, and [1] ) 30 mmol L-1.

solvent. In this work, it seems that supporting the catalyst had an efficient stabilizing effect. Asymmetric Induction. In addition to their availability and biodegradability, polysaccharides could be considered as cheap chiral pools for chemical synthesis, as far as the later property could be exploited (i.e., in enantioselective catalytic reactions). Several attempts have been reported in the literature, especially with chitosan. Guo-Li Yuan et al.33 claimed very high enantioselectivities (up to 100% ee) in the asymmetric hydrogenation of ketones with a chitosanpalladium complex supported onto silica. Huang et al.34 immobilized osmium tetroxide on a chitosan-silica composite to catalyze hydroxylation of olefins, and Xue et al.35 used a cobalt catalyst immobilized on chitosan-silica composite support for the asymmetric hydration of 1-octene to prepare (S)-(+)-2-octanol. The metal-catalyzed allylic substitution reaction differs from the aforementioned reactions. The general catalytic cycle of the reaction offers at least five opportunities to induce enantioselectivity.36 The bond formation or cleavage occurs outside the coordination sphere of the metal, and the reaction is sensitive to a chiral cavity more than to a chiral site.36-38 Eq 2 was chosen as a test for asymmetric induction by the support. No enantiomeric excess was obtained whatever the catalyst described previously. The catalytic reaction was not sensitive to the primary chirality of the polysaccharide. A secondary chirality due to the formation of a helical conformation is only present in the crystalline parts of the materials, a part not accessible to reactants.

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Conclusion Polysaccharide gels are easily formed in aqueous solutions. They present an extremely open framework of fibrils. Alcohol exchange and CO2 supercritical drying have proven to be efficient methods to preserve the volume and microscopic texture of the gel. Porosity, diffusion, and accessibility are critical parameters for the preparation of polysaccharide supported catalysts: aerogels provided macroporous solids with specific surface areas in the range of 300-500 m2 g-1, easily obtained in a reproducible manner. These solids were used to prepare some heterogeneous catalysts by a particular process of impregnation of the aerogel from an aqueous solution of Pd(TPPTS)3. The solids were proven to be efficient and recyclable catalysts of the reaction of allylic substitution. Acknowledgment. Thanks to Re´gion Languedoc-Roussillon for financial support. References and Notes (1) Vincent, T.; Guibal, E. Ind. Eng. Chem. Res. 2002, 41, 5158. (2) An, Y.; Yuan, D.; Huang, M. Y.; Jiang, Y. Y. Macromol. Symp. 1994, 80, 257. (3) Tang, T. M.; Huang, M. Y.; Jiang, Y. Y. Macromol. Rapid Commun. 1994, 15, 527. (4) Han, H. S.; Jiang, S. N.; Huang, M.; Jiang, Y. Y. Polym. AdV. Technol. 1996, 7, 704. (5) Vincent, T.; Guibal, E. Langmuir 2003, 19, 8475. (6) Wang, H.; Sun, W.; Xia, C. J. Mol. Catal. A 2003, 206, 1995. (7) Chang, Y.; Wang, Y. P.; Su, Z. X. J. Appl. Polym. Sci. 2002, 83, 2188. (8) Hardy, J. J. E.; Hubert, S.; Macquarrie, D. J.; Wilson, A. J. Green Chem. 2004, 6, 53. (9) Quignard, F.; Choplin, A.; Domard, A. Langmuir 2000, 16, 9106. (10) Buisson, P.; Quignard, F. Aust. J. Chem. 2002, 55, 73.

Valentin et al. (11) Doi, S.; Clark, J. H.; Macquarrie, D. J.; Milkowsi, K. Chem. Commun. 2002, 2632. (12) Valentin, R.; Molvinger, K.; Quignard, F.; Brunel, D. New J. Chem. 2003, 27, 1690. (13) Guibal, E. Prog. Polym. Sci. 2005, 30, 71. (14) Grasdalen, H. Carbohydr. Res. 1983, 118, 255. (15) Launey, B.; Doublier, J. L.; Cuvelier, G. Functional Properties of Food Macromolecules; Mitchell, J. R., Ledward, D. A., Eds.; Elsevier Applied Science: London, 1986; p L. (16) Sipahigil, O.; Dortunc, B. Int. J. Pharm. 2001, 228, 119. (17) Martinsen, A.; Storrø, I.; Skjåk-Bræk, G. Biotech. Bioeng. 1992, 39, 186. (18) Dhoot, N. O.; Wheatley, M. A. J. Pharm. Sci. 2003, 92, 679 and references therein. (19) Smidsrød, O. Faraday Discuss. Chem. Soc. 1974, 57, 263. (20) Velings, N.; Mestdagh, M. M. Polym. Gels Networks 1995, 3, 311. (21) Ouverx, C.; Velings, N.; Mestdagh, M. M.; Axelos, M. A. V. Polym. Gels Networks 1998, 6, 393. (22) Pierre, A. C.; Pajonk, G. M. Chem. ReV. 2002, 102, 4243. (23) Valentin, R.; Molvinger, K.; Quignard, F.; Di Renzo, F. Macromol. Symp. 2005, 222, 93. (24) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity; Academic Press: London, 1982. (25) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by powders and porous solids; Academic Press: San Diego, 1999. (26) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (27) Dos Santos, S.; Tong, Y. Y.; Quignard, F.; Choplin, A.; Sinou, D.; Dutasta, J. P. Organometallics 1998, 17, 78. (28) Arhancet, J. P.; Davis, M. E.; Merola, J. S.; Hanson, B. E. Nature 1989, 339, 454. (29) The Pd(TPPTS)3 structures were obtained by Molecular Mechanics (MM) optimization (Cerius2, [1] universal force field 1.02). Molecular modeling software for materials research: Accelerys; Biosym Technologies: San Diego, 1993. (30) Barbotin, J. N.; Nava Saucedo, J. E. In Polysaccharides; Dumitriu, S., Ed.; Marcel Dekker: New York, 1998; p 749. (31) Trost, B. M. L. Asymmetric Allylic Alkylation Reactions; WileyVCH: New York, 2000; p 593. (32) Sigismondi, S.; Sinou, D. J. Mol. Catal. A 1997, 116, 289. (33) Yuan, G. L.; Yin, M. Y.; Jiang, T. T.; Huang, M. Y.; Jiang, Y. Y. J. Mol. Catal. A 2000, 159, 45. (34) Huang, K.; Liu, H.-W.; Dou, X.; Huang M.-Y.; Jiang, Y.-Y. Polym. AdV. Technol. 2003, 14, 364. (35) Xue, L.; Zhou, D.-J.; Tang, L.; Ji, X.-F.; Huang, M.-Y.; Jiang, Y.Y. React. Funct. Polym. 2004, 58, 117. (36) Trost, B. M. Acc. Chem. Res. 1996, 29, 355. (37) Trost, B. M.; Van Vanken, D. L. Chem. ReV. 1996, 96, 395. (38) Trost, B. M.; Crawley, M. L. Chem. ReV. 2003, 103, 2921.

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From Hydrocolloids to High Specific Surface Area Porous Supports for

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