Chitosan supported phthalocyanine complexes: Bifunctional catalysts with basic and oxidation active sites

Applied Catalysis A: General 309 (2006) 162–168 www.elsevier.com/locate/apcata

Chitosan supported phthalocyanine complexes: Bifunctional catalysts with basic and oxidation active sites Alexander B. Sorokin b,*, Franc¸oise Quignard a,**, Romain Valentin a, Ste´phane Mangematin b a

Laboratoire des Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS-UM1 Institut Gerhardt FR 1878, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b Institut de Recherches sur la Catalyse, UPR 5401, CNRS, 2, av. A. Einstein, 69626 Villeurbanne Cedex, France Received 10 January 2006; received in revised form 30 March 2006; accepted 30 March 2006 Available online 19 June 2006

Abstract The immobilisation of water-soluble metallophthalocyanine complexes on chitosan aerogel microspheres affords new bifunctional catalysts which have been used for the aerobic oxidation of b-isophorone. Chitosan is both the support of the metal complex and the organic base necessary for the reaction. These new materials have been characterized by nitrogen adsorption/desorption, scanning electron microscopy, diffuse reflectance UV–vis and 15N NMR spectroscopies. The influence of the reaction parameters on the efficiency of heterogeneous oxidation of b-isophorone, an important substrate for the preparation of flavour and fragrance fine chemicals has been studied. Combination of basic and oxidation sites in one solid material provided a more efficient heterogeneous clean oxidation of b-isophorone by O2 without any waste. # 2006 Elsevier B.V. All rights reserved. Keywords: Heterogeneous oxidation; Supported catalyst; Phthalocyanine; Aerobic oxidation; Chitosan; Isophorone

1. Introduction Chitosan is a polysaccharide derived from chitin, a linear chain of acetylglucosamine groups, generally extracted from crab shell or squid pen. Chitin is the largest biomass polysaccharide component along with starch and cellulose. Chitosan is obtained by removing most of the acetyl groups from chitin. This process gives rise to amine groups, and chitosan can be considered as a natural solid base (Scheme 1). The use of chitosan in catalysis has received increasing interest for the past 5 years. Most of the applications deal with the affinity of this polysaccharide to metal ions. These materials were used for the reduction of chromate [1], of phenol [2,3] and nitroaromatic compounds [4]. Functionalisation of chitosan provided catalysts for cyclopropanation of olefins [5], oxidation of alkylbenzene [6], Suzuki and Heck reactions [7]. The

* Corresponding author. Tel.: +33 4 67 16 34 60; fax: +33 4 72 44 53 99. ** Corresponding author. E-mail addresses: [email protected] (A.B. Sorokin), [email protected] (F. Quignard). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.03.060

hydrophilic nature of this polysaccharide has been used to prepare supported aqueous phase catalysts for the palladium catalyzed allylic substitution [8]. A recent review summarized the main advances in this field published over the last years [9]. Chitosan hydrogel is easily shaped in microspheres by dropping an acidic solution of chitosan into an alkaline solution. We demonstrated recently that the corresponding aerogels obtained after a CO2 supercritical drying afforded a porous material with a surface area close to 150 m2 g 1. The amino groups were accessible and were demonstrated to catalyze the glycidol opening with lauric acid [10]. The high surface area of this material coupled with the presence of amino groups makes it an interesting material for the preparation of supported bifunctional catalysts. Phthalocyanine complexes are readily available at the large scale and we have recently showed that iron phthalocyanine complexes covalently anchored onto silica were efficient catalysts for the selective oxidation of aromatic compounds [11,12] and alkynes [13]. Allylic oxidation of highly substituted cyclic olefins, which is competitive to epoxidation, is also an important target affording valuable synthetic products. While efficient homogeneous methods of the oxidation of b-isophorone

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163

Scheme 1. Chemical structure of chitosan.

(b-IP) to ketoisophorone (KIP), an important intermediate for the preparation of flavours and fragrances (Scheme 2) have been proposed [14–16], the heterogeneous approach provided only low KIP yields [15,16]. For example, Murphy and Baiker reported 100% conversion and 67% yield of KIP in homogeneous oxidation of b-IP in the presence of Cu(salen), whereas only 52% conversion and less than 6% KIP yield were obtained with the same complex supported onto SiO2. Evidently, there is a need for a clean aerobic heterogeneous oxidation method for this demanding oxidation. In fact, this complex cyclic olefin contains several oxidazable sites and several types of oxidation along with isomerisation and structural rearrangements can occur, e.g. epoxidation, allylic oxidation. To obtain KIP one needs to avoid other possible reaction by appropriate choice of catalytic system. Recently we have prepared iron phthalocyanine–TiO2 hybrid catalyst which was efficient in heterogeneous oxidation of b-IP providing 57% yield of KIP [17]. However, it should be noted that all these oxidations were performed in the presence of an external base, triethylamine. The immobilisation of metallophthalocyanines onto chitosan enables one to get a material having two kinds of sites. This bifunctional catalyst could allow avoiding the addition of external base; the amino groups of chitosan are expected to play this role. We report herein the immobilisation of tetrasulfophthalocyanine metal complexes (MPcS) on chitosan aerogel microspheres for the selective heterogeneous oxidation of b-isophorone (b-IP). For comparison, analogous solids were prepared with alginate, a polysaccharide with no amino groups. 2. Experimental Chitosan was purchased from France Chitin and was purified as described in literature [18]. It is characterized by its degree of acetylation (DA), defined as the molar ratio of remaining acetyl groups measured by NMR spectroscopy and, a weight-average molecular weight of Mw = 200,000 g/mol,

determined by size exclusion chromatography (SEC) coupled on line with a multi-angle laser light scattering (MALLS) detector. Chitosan gel microspheres were formed from an aqueous solution of chitosan by dissolving 2 g of chitosan in 100 mL of a 0.055 M acetic acid solution. Total dissolution was obtained by stirring overnight at room temperature. Gelation of the millimetric microspheres was obtained by dropping the chitosan solution into a 4 M NaOH solution through a 0.8 mm gauge syringe needle. The chitosan microspheres were left in the alkaline solution for 2 h, filtered and washed with distilled 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. Finally, the microspheres were dried under supercritical CO2 conditions (74 bar, 31.5 8C) in a Polaron 3100 apparatus. This process led to chitosan aerogel microspheres. Micrometric chitosan microspheres were produced by processing in the encapsulator INOTECH an acetic acid solution of 2 wt.% chitosan with an adapted flow rate and frequency. The polymer droplets were hardened in a bath of 500 mL NaOH (4 M) for 2 h before filtration, washing and drying. The diameter of the aerogel spheres was close to 200  20 mm. N-methylated chitosan microspheres were prepared by adding 1.9 g (63 mmol, 10.5-fold excess to amine groups) of paraformaldehyde to 0.95 g of chitosan dry aerogel microspheres (5.9 mmol of amine groups) in methanol then refluxed for 2 h. After cooling, 1.43 g of NaBH4 were added slowly and stirred for 2 h [19]. The beads were thus washed and dried under supercritical conditions. Alginate microspheres were prepared by dissolving sodium alginate (sigma low molecular weight) in distilled water at a concentration of 2% (w/w). This polymer solution was added dropwise at room temperature to the stirred CaCl2 solution (0.24 M) using a syringe with a 0.8 mm diameter needle. The microspheres were cured in the gelation solution during 15 h. Then, the microspheres were dried under supercritical CO2 conditions as previously described. 2.1. Tetrasulfophthalocyanine complexes

Scheme 2. Oxidation of b-isophorone.

Iron and cobalt tetrasulfophthalocyanine complexes were prepared and purified according to modified method of Weber and Busch [20,21].

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Chitosan-based metal phthalocyanine (M = Fe, Co) catalysts were prepared by incipient wetness impregnation of chitosan aerogel beads with an aqueous solution of sulphonated phthalocyanines of Fe or Co. In a typical experiment 0.50 g of chitosan aerogel beads was impregnated with 4.05 g of an aqueous solution of Co-phtalocyanine (20.8 mg; 4.5 mmol L 1; 36.14 mmol g 1 chitosan). After 30 min, the beads were washed in ethanol (5  10 ml) and dried under supercritical CO2 conditions (Co loading 30.5 mmol g 1). With this amount of Co-phtalocyanine, the yield of incorporation was quantitative, with higher loading the yield of incorporation does not exceed 50%. The different samples were labelled MPcS@polysaccharides, with M = Fe or Co and polysaccharide: chitosan or alginate. The metal phthalocyanine contents of the prepared materials were determined by metal analyses using an inductively coupled plasma-mass spectrometry method. 2.2. Characterization Diffuse reflectance UV–vis spectra were recorded on a Perkin-Elmer Lambda 14 spectrophotometer. NMR spectra of solids were acquired on a Bruker DSX 400 spectrophotometer. 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. The reaction products were identified and quantified by GC–MS (Hewlett-Packard 5973/6890 system; electron impact ionization at 70 eV, He carrier gas, 30 m  0.25 mm mm cross-linked 5% PH ME siloxane (0.25 mm coating) capillary column, HP-5MS) and GC (Agilent 4890D system, N2 carrier gas, 15 m  0.25 mm cross-linked 5% PHME siloxane (0.25 mm coating) capillary column, HP-5MS) methods.

2.3. Typical procedure for catalytic oxidation Catalyst (mass corresponding to 1 mmol of MPcS) was added to a solution of b-IP (0.1 mmol) in acetonitrile (1 ml). The reaction was performed at 80 8C under oxygen atmosphere (2 bar) for 24 h. The products were identified by GC–MS and quantified by GC using authentic samples and biphenyl internal standard. Recycling experiments were performed at 0.4 mmol scale. 3. Results and discussion 3.1. Synthesis and characterization of the solids The squid pen chitosan used here was characterized by its degree of acetylation (DA), which is the percent of remaining acetyl groups. The absence of the signal at ca. 110 ppm in 15N NMR spectrum due to the amide group and the absence of peak at 173 ppm for C O group in 13C NMR spectrum indicated complete deacetylation (DA < 1.5%) and thus a maximum amino groups number on the support. Chitosan-based metal phthalocyanine catalysts were prepared by incipient wetness impregnation of chitosan aerogel beads with an aqueous solution of sulphonated phthalocyanines of Fe or Co. This procedure was derived from a recently published synthesis of another hydrosoluble complex, Pd(TPPTS)3, supported on polysaccharide [22]. According to these results, only the aerogels prepared by supercritical drying were efficient in catalysis. We demonstrated that the CO2 supercritical drying allowed the best accessibility to the metal complex [22] and to the amino groups of chitosan [10]. The solids containing phthalocyanine complexes were characterized by diffuse reflectance UV–vis spectra. The spectra of the MPcS@polysaccharides are shown in Fig. 1. The controlled covalent linkage of FePc(SO2Cl)4 onto aminomodified silica has been described in the previous publication [23]. Depending on the experimental conditions it was possible to obtain FePcS either in a monomer or in a dimer form. UV–vis spectroscopy is a suitable method to identify these species since

Fig. 1. Diffuse reflectance UV–vis spectra of (a) FePcS@chitosan; (b) FePcS@alginate; (c) CoPcS@chitosan (0.48% Co); (d) CoPcS@chitosan (0.18% Co).

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it is well established that the FePcS dimeric form shows absorption maximum near 630–635 nm whereas the monomer form exhibits a peak in the region of 680 nm. The solids FePcS@polysaccharides showed a sharp Q band at 626–632 nm corresponding to the spectrum of the dimeric species. The diffuse reflectance UV–vis spectra of the CoPcS@chitosan showed a large Q band indicating the presence of the monomeric species along with a dimeric species (shoulder at 600–620 nm) [24–26]. Interestingly, while FePcS was supported onto polysaccharides in a dimeric form, the monomeric and dimeric species content was comparable in the case of CoPcS indicating a lower tendency of CoPcS to form aggregated species. The amount of the CoPcS dimeric species

Fig. 2.

15

165

increased when increasing the loading of phthalocyanine complex. In an attempt to increase the basicity of the support, methylation of the chitosan was achieved. We applied the procedure published by Sondengam et al. [19] for the methylation of primary or secondary amines to chitosan aerogel microspheres. While 15N NMR of chitosan exhibited a single signal at 0.7 ppm characteristic of the NH2 group, the methylation process afforded a new and unique signal at 34 ppm corresponding to NMe2 functional groups (Fig. 2). The methylation was quantitative with a selectivity of 100% toward the formation of the dimethylchitosan product.

N NMR of (a) chitosan; (b) dimethyl-chitosan.

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A.B. Sorokin et al. / Applied Catalysis A: General 309 (2006) 162–168 Table 1 Textural properties of the solids Catalysts

Complex content (mmol g 1)

SBET (m2 g 1)

Chitosana Chitosan FePcS@G-76 FePcS@chitosan FePcS@chitosan CoPcS@chitosan CoPcS@chitosan CoPcS@chitosan a N-dimethyl chitosan CoPcS@N-dimethyl chitosan

0 0 15 19 82.3 30.5 81.4 103.5 0 80.0

184  5 149  5 362  5 223  5 99  5 149  5 142  5 179  5 151  5 nd

a

Fig. 3. N2 adsorption–desorption isotherms of CoPcS@chitosan 0.48% Co (black triangles) and chitosan (empty triangles).

Information about textural properties was obtained from nitrogen adsorption–desorption isotherms at 77 K [27,28]. The isotherms were typical of solids with strong adsorbent– adsorbate interaction. Two characteristic examples of the studied catalysts are represented in Fig. 3. Surface areas were evaluated by the BET method by assuming a monolayer N2 molecule to cover 0.162 nm2. The textural properties of the solids are reported in Table 1. After impregnation with metallophthalocyanines, the textures of the aerogels materials were slightly modified as observed in the scanning electronic micrographs (Fig. 4), suggesting a coating of the chitosan fibrils by the complex, but the textural properties were maintained. The surface areas reported in Table 1 were very close before and after impregnation. 3.2. Catalytic properties The catalysts were tested in the heterogeneous oxidation of bisophorone resulting in the formation of ketoisophorone (KIP), allylic alcohol 1 and a-isophorone, product of isomerisation

Micrometric size beads.

(Scheme 2). The results in terms of activity and selectivity are listed in Table 2. A control experiment without the catalyst and base exhibited a very minor oxidation. In the presence of a strong base like triethylamine, the only reaction observed was the isomerisation of b-IP to a-IP. The basicity of chitosan alone was not strong enough to efficiently catalyze this isomerisation reaction. The experiments performed with FePcS@alginate and FePcS@chitosan showed 48% conversions of b-isophorone in both cases. However, while FePcS@alginate catalyzed the reaction of isomerisation of b-IP to a-IP (39% yield) along with only 3% KIP yield, FePcS@chitosan provided more selective oxidation of b-IP to KIP. This finding indicated that combination of the FePcS catalyst with base sites of chitosan gave a material with improved catalytic properties. Although the conversion was still low, the KIP yield (up to 17%) was superior to that previously published for heterogeneous oxidation mediated by Mn(salen) and Cu(salen) complexes supported onto silica (6% KIP yield) [16]. Hoping to increase the b-IP conversion and selectivity of bIP oxidation to KIP, we decided to prepare and to test a CoPcS@chitosan material. In fact, cobalt phthalocyanines have been reported to be efficient catalysts for aerobic oxidation of sulfur compounds [29]. Indeed, CoPcS@chitosan performed much better than FePcS@chitosan giving 62% conversion and

Fig. 4. SEM pictures of a diametral cut of microspheres: (A) before impregnation and (B) after impregnation with the catalytic complex sample CoPcS@chitosan 0.48% Co.

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Table 2 Heterogeneous oxidation of b-isophorone catalyzed by supported metallophthalocyanines Catalyst (1 mol%), complex content (mmol g 1)

Conversion (%)

Yield (selectivity) (%) KIP

a-IP

1

<1 0 6 (20) 3 (6) 17 (35) 6.7 (39.0)

<1 35 5 39 1 No data

2 <1 7 No data

CoPcS-chitosan (81.4 mmol g 1), different reaction temperature 60 8C 43 80 8C 62 100 8C 90

27 (63) 30 (48) 31 (34)

5 2 1

6 6 7

CoPcS-chitosan, different complex loading CoPcS-chitosan, 30.5 mmol g 1 CoPcS-chitosan, 81.4 mmol g 1 CoPcS-chitosan, 103.5 mmol g 1

56 62 51

23 (41) 30 (48) 25 (49)

5 2 2

5 6 6

CoPcS-chitosan (81.4 mmol/g), different catalyst/substrate ratio 1% 2% 4%

62 73 85

30 (48) 33 (45) 39 (46)

2 2 1

6 9 8.5

Recycling with 2 mol% of CoPcS-chitosan (81.4) Run 1 Run 2 Run 3

73 61 50

33 (45) 28 (46) 18 (36)

2 2 –

9 8 6

CoPcS-methyl-chitosan 2 mol%

49

22 (45)

1

12

No catalyst, no amine No catalyst, NEt3 Chitosan FePcS-alginate FePcS-chitosan Mn(salen)–SiO2 [16]

<5 37 30 48 48 17.2

48% KIP selectivity. Otherwise, we attempted to modify amine sites of chitosan by methylation to increase the basicity. We have successfully transformed NH2 groups to N(CH3)2 groups by the treatment with paraformaldehyde as evidenced by 15N NMR technique. However, this modification did not improve the catalytic properties of the catalyst. The effect of temperature was studied in the range of 60– 100 8C, the field of thermal stability of chitosan, using CoPcS@chitosan as a catalyst. As expected, increasing temperature increased the conversion but decreased the selectivity in KIP. The optimal reaction temperature was found to be 80 8C for improved selectivity. Three solids were prepared with a different loading of complex: 30.5, 81.4 and 103.5 mmol g 1. All the CoPcS@chitosan materials obtained exhibited a high activity in the range of 51–62% of conversion and selectivity for the KIP formation in the range of 41–49%. Some effect of the complex loading could be evidenced. The catalytic performance of the solid seemed to be optimum for a loading of 81.4 mmol g 1 then it decreased with a higher amount of complex. A maximal content of complex necessary for complete covering of support surface can be estimated from the dimensions of complex and specific surface of chitosan support. The size of the square phthalocyanine ligand ˚  20 A ˚ . Consequently, the surface can be estimated as 20 A ˚ 2. According occupied by one molecule of CoPcS is about 400 A to this estimation, a monolayer of complex will be obtained with 81 mmol g 1. Above this value stacking of phthalocyanine molecules may occur and some catalytic sites may be not

accessible to the substrate. This could explain the decrease of the catalytic performances of the highly loaded solid. It has to be pointed out that the complex loading not only had an influence on the accessibility to the active sites, depending on the dispersion of the catalytic species, but also that it changed the ratio between substrate and amine groups. When the CoPcS content increased from 81.4 to 103.5 mmol g 1, the CoPcS/amine ratio only increased from 1:69 to 1:53. The effect of phthalocyanine clustering certainly overweight the increasing of complex loading. We studied the influence of the catalyst/substrate ratio on bIP oxidation. Increasing the catalyst content from 1 to 2 and 4 mol% resulted in increasing the b-IP conversion to 73 and 85%, respectively. The KIP yields also increased to 33 and 39%, but the KIP selectivity was essentially the same (45– 48%). The recyclability of the heterogeneous catalyst has been also studied using 2 mol% of CoPcS@chitosan catalyst (81.4 mmol g 1). After completing the first run the catalyst was isolated by filtration and washed with MeCN. On reuse, an approximate 10–20% decrease of the catalytic activity was observed between runs, but the KIP selectivity remained fairly constant for successive runs. The poor selectivity of b-IP heterogeneous oxidation has been explained by isolation of the supported oxidation sites and external base in solution leading to different reaction pathways [16]. Complex cyclic olefins can undergo different side reactions (epoxidation, isomerisation, structural rearrangements) leading

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to very poor selectivity to the desired a,b-unsaturated ketone. Immobilisation of the oxidation catalyst onto a support bearing basic sites enhances the accessibility to both sites and thereby diminishes side reactions. Although further work is still required to optimise the distribution of the basic and oxidation sites onto the support to improve activity and selectivity of this allylic oxidation, the yield of KIP already obtained with this novel heterogeneous catalyst (up to 39%) is higher than 6% KIP yields published previously for this demanding heterogeneous oxidation [16]. 4. Conclusion The immobilisation of metallophthalocyanines onto chitosan enables one to get a material having two kinds of sites. This material is a promising heterogeneous catalyst for the oxidation of b-isophorone to ketoisophorone. This novel method corresponds to criteria of green chemistry and sustainable development: (i) using of dioxygen as an oxidant; (ii) using of support from inexpensive renewables; (iii) combination of basic and oxidation sites in one solid material to avoid an addition of external base or other additives resulting in no wastes and (iv) easy separation of catalyst from reaction mixture and possibility of recycling. Acknowledgments The authors gratefully acknowledge the support of D. Cot for electron microscopy and Re´gion Languedoc-Roussillon for financial support. References [1] T. Vincent, E. Guibal, Ind. Eng. Chem. Res. 41 (2002) 5158. [2] L.-M. Tang, M.-Y. Huang, Y.-Y. Jiang, Macromol. Rapid Commun. 15 (1994) 527.

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