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Chiral manganese(III) Schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation
Carbon 43 (2005) 2096–2105 www.elsevier.com/locate/carbon
Chiral manganese(III) Schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation Ana Rosa Silva a, Vitaly Budarin b, James H. Clark b, Baltazar de Castro a, Cristina Freire a,* a
REQUIMTE/Departamento de Quı´mica, Faculdade de Cieˆncias, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal b Clean Technology Centre, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK Received 31 January 2005; accepted 16 March 2005 Available online 5 May 2005
Abstract Two chiral manganese(III) salen catalysts, bearing different chiral diamine bridges, were anchored by direct axial coordination of the metal centre onto the phenolate groups of a modified commercial activated carbon. The modification of the activated carbon was achieved by reaction between sodium hydroxide and surface phenol groups giving rise to phenolate groups (CoxONa), which were characterised by XPS, TG and TG-IR. Characterisation of immobilised manganese(III) salen catalysts onto CoxONa material by XPS, ICP-AES and TG-IR clearly point to reaction between carbon surface phenolate groups and the manganese(III) complexes through axial coordination of the metal centre to these groups. These materials were active and enantioselective in the epoxidation of styrene and a-methylstyrene in dichloromethane at 0 C using, respectively, m-CPBA/NMO and NaOCl. Only for a-methylstyrene comparable asymmetric inductions were found in the epoxide as the homogeneous phase reactions and catalyst reuse led to no significant loss of catalytic activity and enantioselectivity. 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Chemically modified carbons; Catalyst; Catalyst support; Catalytic properties
1. Introduction Manganese(III) Schiff base complexes with a N2O2 coordination sphere, generally known in the literature as manganese(III) salen complexes, have been reported as selective and efficient catalysts for the epoxidation of unfunctionalised alkenes in homogeneous phase [1,2], using a wide range of oxidants––iodosylbenzene, sodium hypochlorite, 3-chloroperoxybenzoic acid, hydrogen peroxide, etc [3]. Furthermore, when chiral
*
Corresponding author. Tel.: +351 22 6082890; fax: +351 22 6082959. E-mail address: [email protected] (C. Freire). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.03.024
Mn(III) salen complexes are used they have been shown to be highly enantioselective [3,4]. Currently, the heterogenisation of these homogeneous enantioselective catalysts on several supports is the object of intense research in order to make them recyclable as well as economical [5]. Moreover, the anchoring of Mn(III) salen complexes onto a support has been found to increase the catalyst stability [5], since the main deactivation process observed in homogeneous phase, formation of inactive dimeric l-oxo manganese(IV) species [1,3], is hindered by local site isolation of the complexes in the solid matrix [5]. Reports on the heterogenisation of chiral and non-chiral, symmetrical and unsymmetrical, Mn(III) salen catalysts have been centred on their covalent binding to organic
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
polymers and on their encapsulation, entrapment, adsorption and covalent attachment to porous inorganic supports, such as zeolites, MCM-41, Al-MCM-41 and clays [5]. Activated carbons are porous and inexpensive materials, extensively used in heterogeneous catalysis as supports [6], which possess several types of oxygen superficial groups, which may be selectively maximised by thermal and chemical processes [7], which can be used as building blocks for the covalent attachment of catalytic active species. With the aim of optimising procedures to prepare new carbon based heterogeneous catalysts, we have been researching several strategies to attach covalently salen transition metal complexes onto activated carbon [8–14]. We have found that the direct anchorage of hydroxyl functionalised Mn(III) salen complexes to the surface oxygen functional groups of an air oxidised activated carbon yielded heterogeneous catalysts which were selective and reusable in the epoxidation of styrene using PhIO as oxidant in acetonitrile at room temperature [12,13]. Recently, we have also found a simple and effective way of anchoring the Jacobsen complex (Fig. 1a) which yielded a heterogeneous catalyst in the asymmetric epoxidation of alkenes with similar %ee as that obtained using the homogeneous catalyst; no leaching of the catalyst from the surface was observed [14]. This new successful heterogeneous catalyst for the asymmetric epoxidation of alkenes, obtained using this simple anchoring procedure, prompted us to character-
(a)
(b)
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ise thoroughly the modifications made at the surface of the activated carbon by several techniques, as well as to check the effect of the size of manganese(III) salen complex in the anchoring process. Therefore, herein we report the anchoring of two chiral manganese(III) salen complexes (Fig. 1) onto activated carbon using the reported procedure (Fig. 2) and their catalytic activity in the asymmetric epoxidation of alkenes. The new materials, as well as their precursors, were characterised by ICP-AES, XPS, TG, TG-IR and nitrogen adsorption at 77 K.
2. Experimental section 2.1. Materials and reagents The starting carbon material was a NORIT ROX 0.8 activated carbon (rodlike pellets with 0.8 mm diameter and 5 mm length). This material has a pore volume of 0.695 cm3 g1 determined by porosimetry (corresponding to meso and macropores), a micropore volume of 0.359 cm3 g1 and mesopore area of 122 m2 g1 determined by N2 adsorption at 77 K (t-method) [8–13], an ash content of 2.6% (w/w), an iodine number of 1000 and mercury and helium densities of 0.666 and 2.11 g cm3, respectively. The activated carbon was purified by Soxhlet extraction with HCl 2 mol dm3 for 6 h, washed with deionised water until pH 6–7 and then dried in an oven, under vacuum, at 150 C for 13 h (C). The reagents and solvents used in the synthesis of the Schiff base complexes, in the modification of the activated carbon surface, in the anchoring of the metal complexes and in the catalytic experiments were used as-received. 3,5-Di-tert-butylsalicylaldehyde, (1R,2R)1,2-diaminocyclohexane, (1R,2R)-1,2-diphenylethylenediamine, styrene, a-methylstyrene, chlorobenzene, benzaldehyde and styrene epoxide, m-chloroperoxybenzoic acid (m-CPBA), 4-methylmorpholine N-oxide (NMO) and sodium hypochlorite solution were from Aldrich; manganese(II) chloride tetrahydrate from Sigma, and all solvents from Merck (pro analysi), except dichloromethane used in the catalytic experiments which was from Romil (HPLC grade). 2.2. Synthesis of the chiral Schiff bases and Mn(III) salen complexes
Fig. 1. Molecular structure and dimensions of the (a) CAT1 and (b) CAT2 homogeneous catalysts, calculated using the ‘‘MM2’’ method in Chem3D [16].
The ligands (1R,2R)-N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)cyclohexanediamine, (1R,2R)-H2(3,5-ditButsalhd), and (1R,2R)-N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)1,2-diphenylethylenediamine, (1R,2R)-H2(3,5-ditButsaldPh), were synthesised by refluxing an ethanolic solution of 3,5-di-tert-butylsalicylaldehyde with either (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-1,2-diphenylethylenediamine, as described elsewhere [2].
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105 CAT2
CAT1
OH Cox
NaOH 1 h reflux
ONa or CoxONa 8 h reflux, ethanol 8 h ethanol Soxhlet extraction; drying 120ºC
N
N Mn O
O O
CAT1@CoxONa CAT2@CoxONa
Fig. 2. Anchoring procedure for the chiral manganese(III) salen catalysts onto an modified air oxidised activated carbon.
The manganese(III) complexes, CAT1 (Jacobsen catalyst) and CAT2 (Fig. 1) were prepared by a procedure adapted from the literature [2], by refluxing ethanolic solutions of equimolar quantities of manganese(II) chloride tetrahydrate with the ligand for 2 h. The complexes were re-crystallised from acetonitrile; yield: 30–75%. CAT1, (1R,2R)-chloro-[N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)cyclohexanediaminate]manganese(III), MnC36H52N2O2Cl. ESI-MS, m/z = 599 (100%, [MnC36H52N2O2–Cl]+). FTIR, t/cm1: 2954 vs, 2908 m, 2868 m, 1612 vs, 1535 s, 1466 m, 1437 m, 1390 m, 1361 s, 1340 m, 1311 m, 1271 m, 1252 s, 1201 m, 1174 m, 1095 m, 1028 m, 837 m, 771 m, 748 m, 712 m, 642 m, 669 m, 542 m. CAT2, (1R,2R)-chloro-[N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)-1,2-diphenylethylenediaminate]manganese (III), MnC44H54N2O2Cl. ESI-MS, m/z = 697 (100%, [MnC44H54N2O2-Cl]+). FTIR, t/cm1: 2954 vs, 2904 m, 2868 m, 1614 vs, 1533 s, 1454 s, 1431 m, 1390 m, 1360 s, 1342 m, 1315 m, 1270 m, 1252 s, 1200 m, 1172 s, 1134 m, 1026 m, 1010 m, 970 w, 929 w, 914 m, 877 m, 852 m, 837 m, 779 m, 750 m, 698 s, 642 m, 577 m, 553 m. 2.3. Anchoring of chiral manganese(III) salen complexes onto the activated carbon An air oxidised activated carbon (3.01 g, Cox) [7,8,10,12–14] was refluxed with an aqueous solution of sodium hydroxide 0.0400 mol dm3 (100 cm3) for an hour; a decrease in the pH of the aqueous solution from 13 to 10 was observed [14]. This material (CoxONa) was washed with deionised water until constant pH (8) and then dried at 120 C in an oven, under vacuum [14].
Then 1.00 g of this material was refluxed for 8 h with an ethanolic solution of 0.200 mmol of CAT1 (Jacobsen catalyst) or CAT2 (Fig. 1). The anchoring process was monitored by UV–Vis spectroscopy and a decrease in intensity of the electronic bands of the manganese(III) salen complexes in the region 200–800 nm was observed. In order to remove physisorbed complex, the resulting materials were purified by Soxhlet extraction with ethanol for 16 h. Finally, the carbon nanocomposites were vacuum dried in an oven overnight at 120 C. 2.4. Physico-chemical measurements ESI mass spectrometry was performed at the ÔMass Spectrometry ServiceÕ, in the Department of Chemistry of the University of York (England). The FTIR-DRIFT spectra of the manganese(III) salen complexes diluted in KBr were obtained with a Bruker Equinox 55 spectrophotometer in the region 400–4000 cm1. The adsorption/desorption of the CAT1 (Jacobsen catalyst) or CAT2 (Fig. 1) remaining in solution was monitored by UV–Vis spectroscopy using a UNICAM UV300 spectrometer in the range 200–800 nm, using quartz cells with 1 cm optical path. Aliquots of 1.00 cm3 from the reaction mixture, diluted to 10.00 cm3, were used to record the spectra. Manganese ICP-AES analysis were carried out at ÔLaborato´rio de Ana´lisesÕ, IST, Lisbon (Portugal). X-ray photoelectron spectroscopy was performed at ÔCentro de Materiais da Universidade do PortoÕ (Portugal), in a VG Scientific ESCALAB 200A spectrometer using a non-monochromatized MgKa radiation (1253.6 eV). In order to correct for possible deviations
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
caused by electric charge of the samples, the graphitic C 1s peak at 284.6 eV was taken as internal standard [8–13]. Nitrogen adsorption measurements were carried out at 77 K using an ASAP 2010 volumetric adsorption analyser from Micromeritics. The BET specific surface areas (SBET) were evaluated using adsorption data in the relative pressure range from 0.06 to 0.15. Micropore volumes were calculated using t-plot (Vt-plot) (thickness ˚ ) and MP method (VDA). Mesopore areas 7.5–14 A ˚ were calculated using the BJH between 15 and 3000 A model [15]. TG-IR experiments were performed using a Netzsch STA 409 cell and TASC 414/3 controller attached to a Bruker Equinox 55 spectrophotometer and using about 30 mg of sample. The thermograms were recorded using a heating rate of 10 K min1 from room temperature till 1173 K and using a flow rate of the N2 carrier gas of 100 cm3 min1. Every 20 s an infrared spectrum in the region 400–4000 cm1 of the evolved fragments from the carbon materials was recorded. GC-FID chromatograms were obtained with a Varian CP-3380 gas chromatograph using helium as carrier gas and a fused silica Varian Chrompack capillary column CP-Sil 8 CB Low Bleed/MS (30 m · 0.25 mm id; 0.25 lm film thickness). The enantiomeric excesses (%ee) of the epoxides were determined using the same chromatograph but using a fused silica Varian Chrompack capillary column CP-Chiralsil-Dex CB (25 m · 0.25 mm d.i. · 0.25 lm film thickness). Conditions used: 60 C (3 min), 5 C min1, 170 C (2 min), 20 C min1, 200 C (10 min); injector temperature, 200 C; detector temperature, 300 C. The reaction parameters %C, TON, TOF and %ee were calculated using the following formula, where A stands for area of chromatographic peak: %C = {[A(alkene)/A(chlorobenzene)]t = 0 h [A(alkene)/A(chlorobenzene)]t = x h} · 100/[A(alkene)/A(chlorobenzene)]t = 0 h; TON = mmol of converted alkene/mmol Mn; TOF = TON/time of reaction; and %ee = [A(major enantiomer) A(minor enantiomer)] · 100/[A(major enantiomer) + A(minor enantiomer)]. 2.5. Catalysis experiments The activity of the catalysts in the epoxidation of alkenes was studied at 0 C (ice bath), under constant stirring conditions, and using 0.500 mmol of styrene or a-methylstyrene (substrate), 0.500 mmol of chlorobenzene (GC internal standard) and 0.100 g of heterogeneous catalyst in 5.00 cm3 of dichloromethane. Because styrene and a-methylstyrene show different reactivities with different oxidants, different experimental conditions were used: with a-methylstyrene 0.750 mmol of sodium hypochlorite were used as oxidant [16,17], whereas for styrene 2.50 mmol of m-chloroperoxybenzoic acid
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(m-CPBA) was used as oxidant with N-methylmorpholine (NMO) as co-oxidant [18]. During the experiments 0.05 cm3 aliquots were taken from solution, with a hypodermic syringe, filtered through 0.2 lm PTFE syringe filters, and directly analysed by non-chiral and chiral GC-FID. The catalysts were then washed sequentially by Soxhlet extraction with 150 cm3 of methanol and 150 cm3 of dichloromethane, for 2 h and dried under vacuum in a horizontal oven at 100 C, overnight. Then, they were reused using the same experimental procedures. To provide a framework for the results obtained using the heterogenised complexes, styrene and a-methylstyrene epoxidation were also carried out, under comparable experimental conditions to those described above, in homogeneous media using the same amount of the Mn(III) catalysts. The contribution of the support itself to the catalysis was found to be too small to be significant.
3. Results and discussion 3.1. Characterisation of the materials 3.1.1. Carbon modification Experimental evidence of activation of the surface phenol groups of the air oxidised activated carbon (Cox) with an aqueous solution of sodium hydroxide is given by the decrease in the pH of the aqueous solution at the end of the process. By XPS the presence of 1.15 mmol of sodium/g of material was also observed (Table 1). Furthermore, changes in the O 1s XPS spectra are observed upon reaction of the Cox with NaOH (Fig. 3). In the O 1s region, Cox shows an asymmetric band centred at 533.5 eV, due to C–OH and C–O–C groups, with a low energy shoulder at 531.6 eV due to C@O containing groups (ketone, lactone and carbonyl) [7,8,10,12–14]. When compared to the Cox O 1s region spectrum, carbon CoxONa shows a decrease in intensity at 533.5 eV and an increase in intensity at 531.6 eV, which can be explained through the formation of new functional groups at the surface of the activated carbon where the oxygen atom possesses more electronic density than the those that gave rise to them. Table 1 XPS elemental analysis of carbon based materials (at.%) Sample
C
O
N
Cl
Na
Mn
NORIT ROX 0.8 Cox CoxONa CAT1@CoxONa CAT2@CoxONa
92.08 89.28 86.78 85.27 85.40
6.1 9.94 10.33 11.57 11.41
0.38 0.35 0.68 0.78 0.98
0.14 0.17 0.74 0.71 0.60
0.90 – 1.47a 1.27b 1.29c
– – – 0.40 0.33
a b c
1.15 mmol of Na g1. 0.980 mmol of Na g1. 0.999 mmol of Na g1.
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105 Table 2 dTG and TG-IR data of carbon based materials
O1 s
Sample
dTG
542
540
538 536 534 532 binding energy (eV)
530
528
526
– 950 465
1056 950 –
– – –
– – –
CoxONa
1157 1050 (sh) 950 (sh) 420
– – 950 510
1156 1050 950 –
– – – –
– – – –
CAT1@CoxONa
1116 – 1000 (sh) 1030 930 – 810 500–800 635
1125 – 1000 (sh) – – – – – – 550–730
– – – – 550–620
CAT2@CoxONa
1097 990 (sh) 870
1110
– –
880 706 500–800 550
The dTG profile of carbon Cox, represented in Fig. 4, shows a peak at 1050 K and a peak at 950 K (Table 2). Upon modification of this material with sodium hydroxide changes are observed in the dTG profile (Fig. 4 and Table 2): a new peak at 1157 K and a decrease in intensity of the peak at 950 K, as well as new low temperature weight losses are observed. In order to gather more information about the modifications made on the car-
900 (sh) –
1110 K
dTG,%min-1
0.4
CAT2@COXONa COXONa
1097 K
0.2
0.0
0.0 200
400
600
800 T,K
1000
200
1200
400
600
800 T,K
1000 1200
CAT2
CAT1
0.4
1150 K COXONa COX 420 K
0.2
0.0 200
400
– –
521–671 540–650
bon materials surface chemistry, TG-IR experiments were performed. During the TG-IR experiments with the Cox and CoxONa materials only vibrational bands in the regions 2400–2250 and 2240–2030 cm1 were detected, which are due to carbon dioxide and carbon monoxide, respectively; the corresponding kinetic profiles are represented in Fig. 5.
0.2
dTG,%min-1
dTG,%min-1
900–850 3100–3020 cm1 cm1
1050 950 –
Fig. 3. XPS O 1s spectra of the materials Cox and CoxONa.
CAT1@COXONa COXONa
CO2 CO Cox
Cox CoxONa
544
TG-IR
600 800 T,K
1000
1200
Fig. 4. dTG of the Cox, CoxONa, CAT1@CoxONa and CAT2@CoxONa.
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
CO, mmol min-1 g-1
0.12
1160 K 1055 K
COX COXNa
955 K
0.10 0.08 0.06 0.04
dium hydroxide with carboxylic anhydrides occurred giving rise to carboxylate groups. Nitrogen adsorption measurements shows a decrease in the area of Cox upon reaction with sodium hydroxide (Table 4), and the porous distribution analysis by the MP method indicates that the sodium is homogeneously distributed throughout the micropores and mesopores.
0.02 0.00 -0.02 0.04
CO2, mmol min-1 g-1
2101
0.03 0.02 0.01 0.00 200
400
600
800
1000
1200
T, K Fig. 5. CO and CO2 TG-IR kinetic profiles for the Cox and CoxONa materials.
The CO and CO2 TG-IR kinetic profiles for the Cox material are very similar to the dTG profile (Fig. 4) and to the CO and CO2 TPD profiles already reported by us for this material [12]. Hence, in the CO TG-IR kinetic profile (Fig. 5) the broad shoulder at 950 K includes the decomposition of carboxylic anhydrides and phenol groups present at the surface of the material, whereas the sharp peak at 1050 K must be due to the decomposition of carbonyl/quinone groups [7]. The high temperature broad peak in the CO2 TG-IR kinetic profile (Fig. 5) corresponds to the decomposition of carboxylic anhydrides and lactone groups [7]. Both CO and CO2 TG-IR kinetic profiles of the CoxONa material (Fig. 5) show differences when compared to those of the Cox. In the CO TG-IR kinetic profile (Fig. 5) a decrease in intensity of the broad shoulder at 950 K is observed, which clearly indicate the disappearence of some of the phenol groups [7]. The new peak at 1156 K must correspond to the new phenolate groups. In the CO2 TG-IR kinetic profile (Fig. 5) a new peak at low temperatures is observed which may be due to carboxylic acids and carboxylate groups, formed from hydrolysis of carboxylic anhydrides promoted by the aqueous sodium hydroxide solution. XPS, TG and TG-IR data clearly indicate that reaction between sodium hydroxide with the phenol groups took place giving rise to phenolate groups, as shown in Fig. 2. The new phenolate groups formed at the surface of the carbon are more thermally stable than phenol groups, decomposing at about 1105–1160 K. TG and TG-IR experiments also suggest some hydrolysis of so-
3.1.2. Anchoring of the manganese(III) salen complexes Comparison of the XPS chemical analyses of the composite materials with its precursor (CoxONa) shows the presence of manganese besides a significant decrease of sodium content (Table 1). The manganese contents obtained by ICP-AES of the new materials were found to be 0.111 mmol g1 for the CAT1@CoxONa and 0.091 mmol g1 for the CAT2@ CoxONa (Table 3). Since CAT2 catalyst is bigger than the CAT1, a slightly lower amount of the first complex was anchored onto CoxONa material. It is noteworthy that a similar catalyst loading (0.107 mmol g1) was obtained in our previous report on the anchoring of the Jacobsen catalyst (CAT1) by the anchoring procedure reported herein, despite of the use of a higher concentration of the Jacobsen catalyst in the initial solution [14]; these results suggest that there is a limit for Jacobsen catalyst uptake onto the phenolate activated carbon surface (CoxONa). Moreover, a decrease of 0.17 mmol g1 in surface sodium content for the CAT1@CoxONa and a decrease of 0.15 mmol g1 for the CAT2@CoxONa were observed by XPS upon Mn(III) complex immobilisation onto CoxONa material (Table 1): these values are close to the manganese contents found by ICP-AES. These results are the first indication that manganese(III) salen complexes are anchored onto the surface of the activated carbon through their phenolate groups, by axial metal coordination, as represented in Fig. 2 [16]. The manganese contents obtained by XPS were found to be roughly three times higher than those obtained by ICP-AES: 0.308 mmol g1 for the CAT1@ CoxONa and 0.256 mmol g1 for the CAT2@CoxONa Table 3 Manganese contents (XPS and ICP-AES) of the heterogeneous catalysts carbon based materials before and after the catalysis experiments (mmol g1) Sample
XPSa
ICP-AES Before
After
CAT1@CoxONa CAT2@CoxONa
0.308 0.256
0.111 0.091
0.087b; 0.113c 0.073b; 0.076c
a
XPS/ICP
2.77 2.81
mmol Mn/weight of AC (from XPS data in Table 1) atom%Mn/ [atom% Mn · Ar(Mn) + atom% C · Ar(C) + atom% O · Ar(O) + atom% N · Ar(N) + atom% Cl · Ar(Cl) + atom% Na · Ar(Na)]. b After two successive catalytic cycles for styrene epoxidation with m-CPBA/NMO. c After two successive catalytic cycles for a-methylstyrene epoxidation with NaOCl.
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105
CO, mmol min-1 g-1
(Table 3). These results indicate that the manganese(III) salen complexes are mainly anchored at the external surface of the activated carbon matrix, which might be a consequence of the large dimensions of both molecules (Fig. 1). This phenomena was also observed in our previous report on the anchoring of the Jacobsen catalyst (CAT1), but a higher surface manganese content was observed (seven times higher) [14], probably due to the higher concentration of the initial Jacobsen catalyst solution. After manganese(III) salen complexes anchorage onto the CoxONa material changes in the dTG profiles are also detected (Fig. 4): new weight losses are observed between 500 and 800 K (Table 2), which corresponds to about 5% of sample weight loss, which is close to the weight of complex anchored, and thus might be correlated with the decomposition of the immobilised manganese(III) salen complexes. Moreover, a decrease in the intensity in the peak at 1156 K is detected, which might be attributed to the reaction of the manganese(III) salen complexes with the phenolate groups. During the TG-IR experiments strong vibrational bands in the regions 3020–2800 and 2240–2030 cm1 were detected, which correspond to C-H stretching vibrations from the tert-butyl groups and carbon monoxide, respectively. Other weak vibrations were also observed in the regions 3100–3020, 2400–2250 and 900– 850 cm1, which might correspond, respectively, to C–H stretching vibrations from the aldehyde fragment, carbon dioxide and C–H bending vibrations from phenyl rings. The CO and C–H stretching vibrations region kinetic profiles are represented in Fig. 6. Both materials show similar CO TG-IR kinetic profiles, but different from the CoxONa material (Fig. 5); 0.12 COXONa CAT2@COXNa CAT1@COXNa
0.10 0.08 0.06
the peak at 1156 K due to the phenolate groups disappeared, which confirms that the anchoring method shown in Fig. 2 took place. The C–H stretching vibrations must be due to fragments of CAT1 and CAT2 decomposition that include the tert-butyl groups, which present very strong absorptions in the infrared spectrum (vide experimental). For both materials these bands are observed at relatively low temperatures, which confirms that the low temperature weigh losses observed in the corresponding dTG profiles (Fig. 4) are due to the decomposition of the manganese(III) salen complexes anchored onto the CoxONa material. Sodium XPS analysis, Mn ICP-AES and TG-IR C-H fragment vibration region all give approximately the same amount of Mn(III) salen complexes anchored onto the CoxONa material. Considering that the complexes have been immobilised onto the surface of the activated carbon as a monolayer, taking into account the ICPAES values (Table 3) and the molecule geometrical parameters shown in Fig. 1, the areas occupied by both molecules can be estimated: 135 m2 g1 for CAT1 and 149 m2 g1 CAT2. These estimated areas agree well with the available surface area calculated using the BJH ˚ (Table 4 method for pores with diameter above 15 A and Fig. 1); it is noteworthy that these areas slightly change after complex immobilisation and hence, they are immobilised onto the surface of the activated carbon as a monolayer. We can then conclude that both catalysts are anchored onto the activated carbon surface by forming square pyramidal species in which the equatorial plane defined by MnN2O2 is almost parallel to the carbon surface (Fig. 2). The anchored Mn(III) salen complexes forming a monolayer on the pores with diam˚ of the activated carbon cause the blocketer above 15 A ˚, age of the micropores with diameter smaller than 15 A as can be seen by the decreases in the ABET area as well as in the micropore volumes (Table 4). 3.2. Catalytic experiments
0.04 0.02
The catalytic activity of the new materials (0.100 g) in the epoxidation of styrene, using 3-chloroperoxybenzoic
0.00 -0.02
A.U.C-H region
Table 4 Textural properties of carbon based materials Sample
ABET (m2 g1)
Ad>15 A˚a
Vmicropore (cm3 g1) Vt-plotb
200
400
600
800
1000
1200
T, K Fig. 6. CO and C–H region TG-IR kinetic profiles for the CoxONa, CAT1@CoxONa and CAT2@CoxONa materials.
NORIT ROX 0.8 Cox CoxONa CAT1@CoxONa CAT2@CoxONa a b c
676 922 797 744 759
128 157 132 167 168
0.323 0.435 0.377 0.355 0.371
VMPc
0.295 0.389 0.338 0.285 0.302 ˚ [15]. Determined by the BJH method, between 15 and 3000 A Determined by the t-method [7–11]. ˚ [15]. Determined by the MP method, between 2.5 and 5 A
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
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Table 5 Asymmetric epoxidation of alkenes at 0 C catalysed by the homogeneous and heterogenised quiral manganese(III) salen complexes Catalyst CAT1 CAT1@CoxONa
%Mn mola
Alkene/oxidant f
Styrene/m-CPBA-NMO a-Mestyrene/NaOClb Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb
CAT2 CAT2@CoxONa
Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb
2.3 2.1 2.1 2.4 2.1 2.1 1.8 1.8 1.8 1.8 1.8 1.8
Run
1st 2nd 1st 2nd
1st 2nd 1st 2nd
t (h)
%Cg
TONc
TOFd
%eee
1 24 4 4 24 24 1 24 4 4 24 24
99 83 59 50 17 16 79 69 48 60 22 24
45 47 29 21 11 8 47 46 26 35 12 13
45 2 7 5 0.5 0.3 47 2 7 9 0.5 0.5
45 60 31 9 57 56 60 30 5 1 28 40
a
Relative to styrene. Experimental conditions: 0.500 mmol of a-Mestyrene, 0.500 mmol chlorobenzene (internal standard), 0.100 g of heterogeneous catalyst and 0.750 mmol of NaOCl, in 5.00 cm3 of dichloromethane. c Total TON based on the styrene conversion. d TOF = TON/reaction time. e Determined by chiral GC. f Experimental conditions: 0.500 mmol of styrene, 0.500 mmol chlorobenzene (internal standard), 2.500 mmol NMO (co-oxidant), 0.100 g of heterogeneous catalyst and 1.000 mmol of m-CPBA, in 5.00 cm3 of dichloromethane. g Determined by GC. b
acid (m-CPBA) as oxygen source, and of a-methylstyrene, using NaOCl as oxidant, were assessed in dichloromethane at 0 C and the results are collected in Table 5. Both new materials acted as enantioselective heterogeneous catalysts in the epoxidation of styrene, using m-CPBA as oxidant and NMO as co-oxidant, and amethylstyrene, using NaOCl as oxidant. In the asymmetric epoxidation of styrene, using mCPBA as oxidant and NMO as co-oxidant, a drop in the %ee is observed for the CAT1@CoxONa catalyst, when compared to its corresponding homogeneous phase reaction. The CAT2@CoxONa catalyst also shows a drop in the %ee when compared to its corresponding homogeneous phase reaction, but more marked than for CAT1. This may be attributed to steric hindarences of the immobilised manganese(III) salen complexes during the course of the epoxidation reaction, as we found earlier that they are mainly anchored in the micropore–mesopore region. Since CAT2 is bigger than the CAT1 (Fig. 1), the higher %ee drop observed in the former case may be hence due to bigger steric hinderances of this complex inside the micropore–mesopore structure of the AC. It is noteworthy that the CAT1@CoxONa catalyst is less enantioselective than the one we reported previously [14], probably because the catalyst reported herein is more homogeneously distributed throughout the porous structure of the AC. In the case of asymmetric epoxidation of a-methylstyrene, using NaOCl as oxidant, comparable enantiomeric excesses as the reactions run with the homogeneous counterparts were obtained for both heterogeneous catalysts.
Since no drop in %ee was observed in the asymmetric epoxidation of a-methylstyrene using NaOCl as oxidant, when compared to the reactions run in homogeneous phase, (contrasting with the results obtained in the asymmetric epoxidation of styrene using m-CPBA/ NMO as oxidant system), we suggest that these results may be related to different susceptibilities to steric hindrance of the oxidising oxo-metallic intermediate species that are formed in the course of the epoxidation reaction on the surface of carbon. In fact, it has been described in the literature that the asymmetric epoxidation of unfunctionalised alkenes catalysed by manganese(III) salen complexes in homogeneous phase is very dependent on the experimental conditions used and, in particular, on the oxidant. Despite of the nature of the oxo-metallic intermediate species and the mechanism by which the oxygen transfer to the alkene takes place being still a matter of debate [19], a different mechanism may be involved for both oxidants. Moreover, the different nature of the oxidant leaving group, (which can interact with the carbon surface), and the fact that the mechanism may be susceptible to steric hindrance due to the localisation of the manganese(III) salen complexes in micropore–mesopore region, may be important factors to explain the different behaviour observed for the catalysts reported herein using two different oxidants. Nevertheless, under all experimental conditions the styrene conversions, turnover numbers and TOF are lower than the corresponding homogenous phase reactions, which is due to slow diffusion of the reactants into the AC porous structure, especially when NaOCl is used as oxidant because of the multiphase reaction system. This has already been observed for other manganese(III)
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salen catalysts heterogenised in other solid supports [16,17,20,21]. The catalysts were also reused using the same epoxidation procedures, after convenient purification process. Reuse of both heterogeneous catalysts in the epoxidation of styrene with m-CPBA/NMO resulted in a significant loss of enantioselectivity, and in the case of the CAT1@CoxONa, also of catalytic activity. In the epoxidation of a-methylstyrene with NaOCl no loss of enantioselectivity nor catalytic activity were observed. This may be due to the higher resistance of the manganese(III) salen catalysts under epoxidation conditions when NaOCl is used as oxidant, compared with mCPBA, for which decomposition of the catalyst has been claimed before for other manganese(III) salen catalysts heterogenised in other solid supports [20]. The CAT1@CoxONa acts as a better heterogeneous catalyst than CAT2@CoxONa in the asymmetric epoxidation of the alkenes, under the experimental conditions used. Remarkably, CAT2@CoxONa improves its catalytic activity on reuse under both experimental conditions. When NaOCl is used as an oxidant it gives also a higher enantioselectivity on reuse and exceeds the value obtained under homogeneous conditions. This may indicate a form of catalyst conditioning which is somehow improving its performance. The ICP-AES manganese analysis of both the heterogeneous catalysts confirm that after two successive catalytic cycles (Table 5) no significant loss of the manganese(III) complex took place during the experiments and that hence the axial coordination of the manganese(III) salen homogenous catalyst onto the surface phenolate groups of the activated carbon does not result in active phase leaching.
and therefore, both catalysts should show a square pyramidal geometry, with the equatorial plane (MnN2O2) almost parallel to the carbon surface. These materials were active and enantioselective in the epoxidation of styrene and a-methylstyrene in dichloromethane at 0 C using, respectively, m-CPBA/ NMO and NaOCl. With a-methylstyrene comparable asymmetric inductions were found in the epoxide as the homogeneous phase reactions, whereas with styrene a decrease in %ee was found. Nevertheless, the CAT1@ CoxONa catalyst acts as a better heterogeneous catalyst than CAT2@CoxONa in the asymmetric epoxidation of the alkenes. For epoxidation of a-methylstyrene using NaOCl, catalyst reuse led to no significant loss of catalytic activity and enantioselectivity, while for epoxidation of styrene with the other oxidant a significant loss of enantioselectivity was observed. For both heterogeneous catalysts no significant metal leaching was observed by ICP-AES after the two successive catalytic experiments, under all conditions used, showing that the anchoring method used is effective against active phase leaching.
Acknowledgement The authors thank NORIT N.V., Amersfoort, The Netherlands, for providing the activated carbon. This work was funded by Fundac¸a˜o para a Cieˆncia e Tecnologia (Portugal) and FEDER through project ref. POCTI/CTM/45449/2002. ARS thanks FCT and the European Social Fund for a Post-Doctoral fellowship. VB thanks INTAS (grant 00-291) and EPSRC for a ROPA grant.
4. Conclusions Two chiral manganese(III) salen catalysts, bearing the same aldehyde fragment and different chiral diamine bridges, were successfully anchored onto the surface of a commercial activated carbon using a simple procedure which consists in the axial coordination of the metal centre onto the AC phenolate groups. XPS, TG and TG-IR data clearly indicate that modification of the AC by reaction between sodium hydroxide with the surface phenol groups took place giving rise to phenolate groups, which were found to be more thermally stable than parent phenol groups. Characterisation of the manganese(III) salen heterogeneous catalysts by XPS, ICP-AES and TG-IR clearly indicate that complex anchoring proceeds through coordination of surface phenolate groups to the manganese(III) centre. Furthermore, nitrogen adsorption measurements suggested that the complexes are immobilised onto the surface of the activated carbon as a monolayer
References [1] Srinivasan K, Michaud P, Kochi JK. Epoxidation of olefins with cationic (salen)MnIII complexes––the modulation of catalytic activity by substituents. J Am Chem Soc 1986;108(9):2309– 20. [2] Silva AR, Freire C, de Castro B. Modulation of the catalytic activity of manganese(III) salen complexes in the epoxidation of styrene: Influence of the oxygen source. New J Chem 2004;28(2):253–60. [3] Katsuki T. Catalytic asymmetric oxidations using optically active (salen)manganese (III) complexes as catalysts. Coord Chem Rev 1995;140:189–214. [4] Jacobsen EN, Wu MH. In: Pfaltz A, Jacobsen EN, Yamamoto H, editors. Comprehensive asymmetric catalysis. Berlin: SpringerVerlag; 1999. p. 649–77. [5] Fan Q-H, Li Y-M, Chan ASC. Recoverable catalysts for asymmetric organic synthesis. Chem Rev 2002;102(10):3385– 466. [6] Rodrı´guez-Reinoso F. The role of carbon materials in heterogeneous catalysis. Carbon 1998;36(3):159–75.
A.R. Silva et al. / Carbon 43 (2005) 2096–2105 ´ rfa˜o JJM. Mod[7] Figueiredo JL, Pereira MFR, Freitas MMA, O ification of the surface chemistry of activated carbons. Carbon 1999;37(9):1379–89. [8] Silva AR, Freire C, de Castro B, Freitas MMA, Figueiredo JL. Anchoring of a nickel(II) Schiff base complex onto activated carbon mediated by cyanuric chloride. Micropor Mesopor Mater 2001;46(2–3):211–21. [9] Silva AR, Martins M, Freitas MMA, Valente A, Freire C, de Castro B, et al. Immobilisation of amine-functionalised nickel(II) Schiff base complexes onto activated carbon treated with thionyl chloride. Micropor Mesopor Mater 2002;55(3): 275–84. [10] Silva AR, Freire C, de Castro B, Freitas MMA, Figueiredo JL. Heterogenization of a functionalized copper(II) Schiff base complex by direct immobilization onto an oxidized activated carbon. Langmuir 2002;18(21):8017–24. [11] Silva AR, Martins M, Freitas MMA, Figueiredo JL, Freire C, de Castro B. Anchoring of copper(II) acetylacetonate onto an activated carbon functionalised with a triamine. Eur J Inorg Chem 2004(10):2027–35. [12] Silva AR, Vital J, Figueiredo JL, Freire C, de Castro B. Activated carbons with immobilised manganese(III) salen complexes as heterogeneous catalysts in the epoxidation of olefins: Influence of support and ligand functionalisation on selectivity and reusability. New J Chem 2003;27(10):1511–7. [13] Silva AR, Figueiredo JL, Freire C, de Castro B. Manganese(III) salen complexes anchored onto activated carbon as heterogeneous catalysts for the epoxidation of olefins. Micropor Mesopor Mater 2004;68(1–3):83–9. [14] Silva AR, Freire C, de Castro B. Jacobsen catalyst anchored onto an activated carbon as an enantioselective heterogeneous cata-
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lyst for the epoxidation of alkenes. Carbon 2004;42(14):3027– 30. Xie J, Wang X, Deng J. Modifying the pore structure of Pit-ACF with the chemical vapor deposition of methane and propylene. Micropor Mesopor Mater 2004;76(1–3):167–75. Xiang S, Zhang Y, Xin Q, Li C. Enantioselective epoxidation of olefins catalyzed by Mn (salen)/MCM-41 synthesized with a new anchoring method. Chem Commun 2002(22):2696–7. Zhang H, Xiang S, Li X, Zhong L, Han D, Wang H, et al. Confinement effect in enantioselective epoxidation of unfunctional olefins catalyzed by Mn(salen)/MCM-41 and Mn(salen)/SiO2. Poster presentation at 13th International Conference on Catalysis, Paris, France, 2004. Palucki M, Pospisil PJ, Zhang W, Jacobsen EN. Highly enantioselective, low-temperature epoxidation of styrene. J Am Chem Soc 1994;116(20):9333–4. Adam W, Roschmann KJ, Saha-Mo¨ller CR, Seebach D. cis-Stilbene and (1a,2b,3a)- (2-ethenyl-3-methoxycyclopropyl)benzene as mechanistic probes in the MnIII (salen)-catalyzed epoxidation: Influence of the oxygen source and the counterion on the diastereoselectivity of the competitive concerted and radicaltype oxygen transfer. J Am Chem Soc 2002(124):5068–73. Song CE, Roh EJ, Yu BM, Chi DY, Kim SC, Lee K-J. Heterogeneous asymmetric epoxidation of alkenes catalysed by a polymer-bound (pyrrolidine salen)manganese(III) complex. Chem Commun 2000(7):615–6. Bigi F, Moroni L, Maggi R, Sartori G. Heterogeneous enantioselective epoxidation of olefins catalysed by unsymmetrical (salen)Mn(III) complexes supported on amorphous or MCM-41 silica through a new triazine-based linker. Chem Commun 2002:716–7.
Chiral manganese(III) Schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation Ana Rosa Silva a, Vitaly Budarin b, James H. Clark b, Baltazar de Castro a, Cristina Freire a,* a
REQUIMTE/Departamento de Quı´mica, Faculdade de Cieˆncias, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal b Clean Technology Centre, Department of Chemistry, University of York, Heslington, York YO10 5DD, UK Received 31 January 2005; accepted 16 March 2005 Available online 5 May 2005
Abstract Two chiral manganese(III) salen catalysts, bearing different chiral diamine bridges, were anchored by direct axial coordination of the metal centre onto the phenolate groups of a modified commercial activated carbon. The modification of the activated carbon was achieved by reaction between sodium hydroxide and surface phenol groups giving rise to phenolate groups (CoxONa), which were characterised by XPS, TG and TG-IR. Characterisation of immobilised manganese(III) salen catalysts onto CoxONa material by XPS, ICP-AES and TG-IR clearly point to reaction between carbon surface phenolate groups and the manganese(III) complexes through axial coordination of the metal centre to these groups. These materials were active and enantioselective in the epoxidation of styrene and a-methylstyrene in dichloromethane at 0 C using, respectively, m-CPBA/NMO and NaOCl. Only for a-methylstyrene comparable asymmetric inductions were found in the epoxide as the homogeneous phase reactions and catalyst reuse led to no significant loss of catalytic activity and enantioselectivity. 2005 Elsevier Ltd. All rights reserved. Keywords: Activated carbon; Chemically modified carbons; Catalyst; Catalyst support; Catalytic properties
1. Introduction Manganese(III) Schiff base complexes with a N2O2 coordination sphere, generally known in the literature as manganese(III) salen complexes, have been reported as selective and efficient catalysts for the epoxidation of unfunctionalised alkenes in homogeneous phase [1,2], using a wide range of oxidants––iodosylbenzene, sodium hypochlorite, 3-chloroperoxybenzoic acid, hydrogen peroxide, etc [3]. Furthermore, when chiral
*
Corresponding author. Tel.: +351 22 6082890; fax: +351 22 6082959. E-mail address: [email protected] (C. Freire). 0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.03.024
Mn(III) salen complexes are used they have been shown to be highly enantioselective [3,4]. Currently, the heterogenisation of these homogeneous enantioselective catalysts on several supports is the object of intense research in order to make them recyclable as well as economical [5]. Moreover, the anchoring of Mn(III) salen complexes onto a support has been found to increase the catalyst stability [5], since the main deactivation process observed in homogeneous phase, formation of inactive dimeric l-oxo manganese(IV) species [1,3], is hindered by local site isolation of the complexes in the solid matrix [5]. Reports on the heterogenisation of chiral and non-chiral, symmetrical and unsymmetrical, Mn(III) salen catalysts have been centred on their covalent binding to organic
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
polymers and on their encapsulation, entrapment, adsorption and covalent attachment to porous inorganic supports, such as zeolites, MCM-41, Al-MCM-41 and clays [5]. Activated carbons are porous and inexpensive materials, extensively used in heterogeneous catalysis as supports [6], which possess several types of oxygen superficial groups, which may be selectively maximised by thermal and chemical processes [7], which can be used as building blocks for the covalent attachment of catalytic active species. With the aim of optimising procedures to prepare new carbon based heterogeneous catalysts, we have been researching several strategies to attach covalently salen transition metal complexes onto activated carbon [8–14]. We have found that the direct anchorage of hydroxyl functionalised Mn(III) salen complexes to the surface oxygen functional groups of an air oxidised activated carbon yielded heterogeneous catalysts which were selective and reusable in the epoxidation of styrene using PhIO as oxidant in acetonitrile at room temperature [12,13]. Recently, we have also found a simple and effective way of anchoring the Jacobsen complex (Fig. 1a) which yielded a heterogeneous catalyst in the asymmetric epoxidation of alkenes with similar %ee as that obtained using the homogeneous catalyst; no leaching of the catalyst from the surface was observed [14]. This new successful heterogeneous catalyst for the asymmetric epoxidation of alkenes, obtained using this simple anchoring procedure, prompted us to character-
(a)
(b)
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ise thoroughly the modifications made at the surface of the activated carbon by several techniques, as well as to check the effect of the size of manganese(III) salen complex in the anchoring process. Therefore, herein we report the anchoring of two chiral manganese(III) salen complexes (Fig. 1) onto activated carbon using the reported procedure (Fig. 2) and their catalytic activity in the asymmetric epoxidation of alkenes. The new materials, as well as their precursors, were characterised by ICP-AES, XPS, TG, TG-IR and nitrogen adsorption at 77 K.
2. Experimental section 2.1. Materials and reagents The starting carbon material was a NORIT ROX 0.8 activated carbon (rodlike pellets with 0.8 mm diameter and 5 mm length). This material has a pore volume of 0.695 cm3 g1 determined by porosimetry (corresponding to meso and macropores), a micropore volume of 0.359 cm3 g1 and mesopore area of 122 m2 g1 determined by N2 adsorption at 77 K (t-method) [8–13], an ash content of 2.6% (w/w), an iodine number of 1000 and mercury and helium densities of 0.666 and 2.11 g cm3, respectively. The activated carbon was purified by Soxhlet extraction with HCl 2 mol dm3 for 6 h, washed with deionised water until pH 6–7 and then dried in an oven, under vacuum, at 150 C for 13 h (C). The reagents and solvents used in the synthesis of the Schiff base complexes, in the modification of the activated carbon surface, in the anchoring of the metal complexes and in the catalytic experiments were used as-received. 3,5-Di-tert-butylsalicylaldehyde, (1R,2R)1,2-diaminocyclohexane, (1R,2R)-1,2-diphenylethylenediamine, styrene, a-methylstyrene, chlorobenzene, benzaldehyde and styrene epoxide, m-chloroperoxybenzoic acid (m-CPBA), 4-methylmorpholine N-oxide (NMO) and sodium hypochlorite solution were from Aldrich; manganese(II) chloride tetrahydrate from Sigma, and all solvents from Merck (pro analysi), except dichloromethane used in the catalytic experiments which was from Romil (HPLC grade). 2.2. Synthesis of the chiral Schiff bases and Mn(III) salen complexes
Fig. 1. Molecular structure and dimensions of the (a) CAT1 and (b) CAT2 homogeneous catalysts, calculated using the ‘‘MM2’’ method in Chem3D [16].
The ligands (1R,2R)-N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)cyclohexanediamine, (1R,2R)-H2(3,5-ditButsalhd), and (1R,2R)-N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)1,2-diphenylethylenediamine, (1R,2R)-H2(3,5-ditButsaldPh), were synthesised by refluxing an ethanolic solution of 3,5-di-tert-butylsalicylaldehyde with either (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-1,2-diphenylethylenediamine, as described elsewhere [2].
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105 CAT2
CAT1
OH Cox
NaOH 1 h reflux
ONa or CoxONa 8 h reflux, ethanol 8 h ethanol Soxhlet extraction; drying 120ºC
N
N Mn O
O O
CAT1@CoxONa CAT2@CoxONa
Fig. 2. Anchoring procedure for the chiral manganese(III) salen catalysts onto an modified air oxidised activated carbon.
The manganese(III) complexes, CAT1 (Jacobsen catalyst) and CAT2 (Fig. 1) were prepared by a procedure adapted from the literature [2], by refluxing ethanolic solutions of equimolar quantities of manganese(II) chloride tetrahydrate with the ligand for 2 h. The complexes were re-crystallised from acetonitrile; yield: 30–75%. CAT1, (1R,2R)-chloro-[N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)cyclohexanediaminate]manganese(III), MnC36H52N2O2Cl. ESI-MS, m/z = 599 (100%, [MnC36H52N2O2–Cl]+). FTIR, t/cm1: 2954 vs, 2908 m, 2868 m, 1612 vs, 1535 s, 1466 m, 1437 m, 1390 m, 1361 s, 1340 m, 1311 m, 1271 m, 1252 s, 1201 m, 1174 m, 1095 m, 1028 m, 837 m, 771 m, 748 m, 712 m, 642 m, 669 m, 542 m. CAT2, (1R,2R)-chloro-[N,N 0 -bis(3,5-di-tert-butylsalicylaldehyde)-1,2-diphenylethylenediaminate]manganese (III), MnC44H54N2O2Cl. ESI-MS, m/z = 697 (100%, [MnC44H54N2O2-Cl]+). FTIR, t/cm1: 2954 vs, 2904 m, 2868 m, 1614 vs, 1533 s, 1454 s, 1431 m, 1390 m, 1360 s, 1342 m, 1315 m, 1270 m, 1252 s, 1200 m, 1172 s, 1134 m, 1026 m, 1010 m, 970 w, 929 w, 914 m, 877 m, 852 m, 837 m, 779 m, 750 m, 698 s, 642 m, 577 m, 553 m. 2.3. Anchoring of chiral manganese(III) salen complexes onto the activated carbon An air oxidised activated carbon (3.01 g, Cox) [7,8,10,12–14] was refluxed with an aqueous solution of sodium hydroxide 0.0400 mol dm3 (100 cm3) for an hour; a decrease in the pH of the aqueous solution from 13 to 10 was observed [14]. This material (CoxONa) was washed with deionised water until constant pH (8) and then dried at 120 C in an oven, under vacuum [14].
Then 1.00 g of this material was refluxed for 8 h with an ethanolic solution of 0.200 mmol of CAT1 (Jacobsen catalyst) or CAT2 (Fig. 1). The anchoring process was monitored by UV–Vis spectroscopy and a decrease in intensity of the electronic bands of the manganese(III) salen complexes in the region 200–800 nm was observed. In order to remove physisorbed complex, the resulting materials were purified by Soxhlet extraction with ethanol for 16 h. Finally, the carbon nanocomposites were vacuum dried in an oven overnight at 120 C. 2.4. Physico-chemical measurements ESI mass spectrometry was performed at the ÔMass Spectrometry ServiceÕ, in the Department of Chemistry of the University of York (England). The FTIR-DRIFT spectra of the manganese(III) salen complexes diluted in KBr were obtained with a Bruker Equinox 55 spectrophotometer in the region 400–4000 cm1. The adsorption/desorption of the CAT1 (Jacobsen catalyst) or CAT2 (Fig. 1) remaining in solution was monitored by UV–Vis spectroscopy using a UNICAM UV300 spectrometer in the range 200–800 nm, using quartz cells with 1 cm optical path. Aliquots of 1.00 cm3 from the reaction mixture, diluted to 10.00 cm3, were used to record the spectra. Manganese ICP-AES analysis were carried out at ÔLaborato´rio de Ana´lisesÕ, IST, Lisbon (Portugal). X-ray photoelectron spectroscopy was performed at ÔCentro de Materiais da Universidade do PortoÕ (Portugal), in a VG Scientific ESCALAB 200A spectrometer using a non-monochromatized MgKa radiation (1253.6 eV). In order to correct for possible deviations
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
caused by electric charge of the samples, the graphitic C 1s peak at 284.6 eV was taken as internal standard [8–13]. Nitrogen adsorption measurements were carried out at 77 K using an ASAP 2010 volumetric adsorption analyser from Micromeritics. The BET specific surface areas (SBET) were evaluated using adsorption data in the relative pressure range from 0.06 to 0.15. Micropore volumes were calculated using t-plot (Vt-plot) (thickness ˚ ) and MP method (VDA). Mesopore areas 7.5–14 A ˚ were calculated using the BJH between 15 and 3000 A model [15]. TG-IR experiments were performed using a Netzsch STA 409 cell and TASC 414/3 controller attached to a Bruker Equinox 55 spectrophotometer and using about 30 mg of sample. The thermograms were recorded using a heating rate of 10 K min1 from room temperature till 1173 K and using a flow rate of the N2 carrier gas of 100 cm3 min1. Every 20 s an infrared spectrum in the region 400–4000 cm1 of the evolved fragments from the carbon materials was recorded. GC-FID chromatograms were obtained with a Varian CP-3380 gas chromatograph using helium as carrier gas and a fused silica Varian Chrompack capillary column CP-Sil 8 CB Low Bleed/MS (30 m · 0.25 mm id; 0.25 lm film thickness). The enantiomeric excesses (%ee) of the epoxides were determined using the same chromatograph but using a fused silica Varian Chrompack capillary column CP-Chiralsil-Dex CB (25 m · 0.25 mm d.i. · 0.25 lm film thickness). Conditions used: 60 C (3 min), 5 C min1, 170 C (2 min), 20 C min1, 200 C (10 min); injector temperature, 200 C; detector temperature, 300 C. The reaction parameters %C, TON, TOF and %ee were calculated using the following formula, where A stands for area of chromatographic peak: %C = {[A(alkene)/A(chlorobenzene)]t = 0 h [A(alkene)/A(chlorobenzene)]t = x h} · 100/[A(alkene)/A(chlorobenzene)]t = 0 h; TON = mmol of converted alkene/mmol Mn; TOF = TON/time of reaction; and %ee = [A(major enantiomer) A(minor enantiomer)] · 100/[A(major enantiomer) + A(minor enantiomer)]. 2.5. Catalysis experiments The activity of the catalysts in the epoxidation of alkenes was studied at 0 C (ice bath), under constant stirring conditions, and using 0.500 mmol of styrene or a-methylstyrene (substrate), 0.500 mmol of chlorobenzene (GC internal standard) and 0.100 g of heterogeneous catalyst in 5.00 cm3 of dichloromethane. Because styrene and a-methylstyrene show different reactivities with different oxidants, different experimental conditions were used: with a-methylstyrene 0.750 mmol of sodium hypochlorite were used as oxidant [16,17], whereas for styrene 2.50 mmol of m-chloroperoxybenzoic acid
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(m-CPBA) was used as oxidant with N-methylmorpholine (NMO) as co-oxidant [18]. During the experiments 0.05 cm3 aliquots were taken from solution, with a hypodermic syringe, filtered through 0.2 lm PTFE syringe filters, and directly analysed by non-chiral and chiral GC-FID. The catalysts were then washed sequentially by Soxhlet extraction with 150 cm3 of methanol and 150 cm3 of dichloromethane, for 2 h and dried under vacuum in a horizontal oven at 100 C, overnight. Then, they were reused using the same experimental procedures. To provide a framework for the results obtained using the heterogenised complexes, styrene and a-methylstyrene epoxidation were also carried out, under comparable experimental conditions to those described above, in homogeneous media using the same amount of the Mn(III) catalysts. The contribution of the support itself to the catalysis was found to be too small to be significant.
3. Results and discussion 3.1. Characterisation of the materials 3.1.1. Carbon modification Experimental evidence of activation of the surface phenol groups of the air oxidised activated carbon (Cox) with an aqueous solution of sodium hydroxide is given by the decrease in the pH of the aqueous solution at the end of the process. By XPS the presence of 1.15 mmol of sodium/g of material was also observed (Table 1). Furthermore, changes in the O 1s XPS spectra are observed upon reaction of the Cox with NaOH (Fig. 3). In the O 1s region, Cox shows an asymmetric band centred at 533.5 eV, due to C–OH and C–O–C groups, with a low energy shoulder at 531.6 eV due to C@O containing groups (ketone, lactone and carbonyl) [7,8,10,12–14]. When compared to the Cox O 1s region spectrum, carbon CoxONa shows a decrease in intensity at 533.5 eV and an increase in intensity at 531.6 eV, which can be explained through the formation of new functional groups at the surface of the activated carbon where the oxygen atom possesses more electronic density than the those that gave rise to them. Table 1 XPS elemental analysis of carbon based materials (at.%) Sample
C
O
N
Cl
Na
Mn
NORIT ROX 0.8 Cox CoxONa CAT1@CoxONa CAT2@CoxONa
92.08 89.28 86.78 85.27 85.40
6.1 9.94 10.33 11.57 11.41
0.38 0.35 0.68 0.78 0.98
0.14 0.17 0.74 0.71 0.60
0.90 – 1.47a 1.27b 1.29c
– – – 0.40 0.33
a b c
1.15 mmol of Na g1. 0.980 mmol of Na g1. 0.999 mmol of Na g1.
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105 Table 2 dTG and TG-IR data of carbon based materials
O1 s
Sample
dTG
542
540
538 536 534 532 binding energy (eV)
530
528
526
– 950 465
1056 950 –
– – –
– – –
CoxONa
1157 1050 (sh) 950 (sh) 420
– – 950 510
1156 1050 950 –
– – – –
– – – –
CAT1@CoxONa
1116 – 1000 (sh) 1030 930 – 810 500–800 635
1125 – 1000 (sh) – – – – – – 550–730
– – – – 550–620
CAT2@CoxONa
1097 990 (sh) 870
1110
– –
880 706 500–800 550
The dTG profile of carbon Cox, represented in Fig. 4, shows a peak at 1050 K and a peak at 950 K (Table 2). Upon modification of this material with sodium hydroxide changes are observed in the dTG profile (Fig. 4 and Table 2): a new peak at 1157 K and a decrease in intensity of the peak at 950 K, as well as new low temperature weight losses are observed. In order to gather more information about the modifications made on the car-
900 (sh) –
1110 K
dTG,%min-1
0.4
CAT2@COXONa COXONa
1097 K
0.2
0.0
0.0 200
400
600
800 T,K
1000
200
1200
400
600
800 T,K
1000 1200
CAT2
CAT1
0.4
1150 K COXONa COX 420 K
0.2
0.0 200
400
– –
521–671 540–650
bon materials surface chemistry, TG-IR experiments were performed. During the TG-IR experiments with the Cox and CoxONa materials only vibrational bands in the regions 2400–2250 and 2240–2030 cm1 were detected, which are due to carbon dioxide and carbon monoxide, respectively; the corresponding kinetic profiles are represented in Fig. 5.
0.2
dTG,%min-1
dTG,%min-1
900–850 3100–3020 cm1 cm1
1050 950 –
Fig. 3. XPS O 1s spectra of the materials Cox and CoxONa.
CAT1@COXONa COXONa
CO2 CO Cox
Cox CoxONa
544
TG-IR
600 800 T,K
1000
1200
Fig. 4. dTG of the Cox, CoxONa, CAT1@CoxONa and CAT2@CoxONa.
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
CO, mmol min-1 g-1
0.12
1160 K 1055 K
COX COXNa
955 K
0.10 0.08 0.06 0.04
dium hydroxide with carboxylic anhydrides occurred giving rise to carboxylate groups. Nitrogen adsorption measurements shows a decrease in the area of Cox upon reaction with sodium hydroxide (Table 4), and the porous distribution analysis by the MP method indicates that the sodium is homogeneously distributed throughout the micropores and mesopores.
0.02 0.00 -0.02 0.04
CO2, mmol min-1 g-1
2101
0.03 0.02 0.01 0.00 200
400
600
800
1000
1200
T, K Fig. 5. CO and CO2 TG-IR kinetic profiles for the Cox and CoxONa materials.
The CO and CO2 TG-IR kinetic profiles for the Cox material are very similar to the dTG profile (Fig. 4) and to the CO and CO2 TPD profiles already reported by us for this material [12]. Hence, in the CO TG-IR kinetic profile (Fig. 5) the broad shoulder at 950 K includes the decomposition of carboxylic anhydrides and phenol groups present at the surface of the material, whereas the sharp peak at 1050 K must be due to the decomposition of carbonyl/quinone groups [7]. The high temperature broad peak in the CO2 TG-IR kinetic profile (Fig. 5) corresponds to the decomposition of carboxylic anhydrides and lactone groups [7]. Both CO and CO2 TG-IR kinetic profiles of the CoxONa material (Fig. 5) show differences when compared to those of the Cox. In the CO TG-IR kinetic profile (Fig. 5) a decrease in intensity of the broad shoulder at 950 K is observed, which clearly indicate the disappearence of some of the phenol groups [7]. The new peak at 1156 K must correspond to the new phenolate groups. In the CO2 TG-IR kinetic profile (Fig. 5) a new peak at low temperatures is observed which may be due to carboxylic acids and carboxylate groups, formed from hydrolysis of carboxylic anhydrides promoted by the aqueous sodium hydroxide solution. XPS, TG and TG-IR data clearly indicate that reaction between sodium hydroxide with the phenol groups took place giving rise to phenolate groups, as shown in Fig. 2. The new phenolate groups formed at the surface of the carbon are more thermally stable than phenol groups, decomposing at about 1105–1160 K. TG and TG-IR experiments also suggest some hydrolysis of so-
3.1.2. Anchoring of the manganese(III) salen complexes Comparison of the XPS chemical analyses of the composite materials with its precursor (CoxONa) shows the presence of manganese besides a significant decrease of sodium content (Table 1). The manganese contents obtained by ICP-AES of the new materials were found to be 0.111 mmol g1 for the CAT1@CoxONa and 0.091 mmol g1 for the CAT2@ CoxONa (Table 3). Since CAT2 catalyst is bigger than the CAT1, a slightly lower amount of the first complex was anchored onto CoxONa material. It is noteworthy that a similar catalyst loading (0.107 mmol g1) was obtained in our previous report on the anchoring of the Jacobsen catalyst (CAT1) by the anchoring procedure reported herein, despite of the use of a higher concentration of the Jacobsen catalyst in the initial solution [14]; these results suggest that there is a limit for Jacobsen catalyst uptake onto the phenolate activated carbon surface (CoxONa). Moreover, a decrease of 0.17 mmol g1 in surface sodium content for the CAT1@CoxONa and a decrease of 0.15 mmol g1 for the CAT2@CoxONa were observed by XPS upon Mn(III) complex immobilisation onto CoxONa material (Table 1): these values are close to the manganese contents found by ICP-AES. These results are the first indication that manganese(III) salen complexes are anchored onto the surface of the activated carbon through their phenolate groups, by axial metal coordination, as represented in Fig. 2 [16]. The manganese contents obtained by XPS were found to be roughly three times higher than those obtained by ICP-AES: 0.308 mmol g1 for the CAT1@ CoxONa and 0.256 mmol g1 for the CAT2@CoxONa Table 3 Manganese contents (XPS and ICP-AES) of the heterogeneous catalysts carbon based materials before and after the catalysis experiments (mmol g1) Sample
XPSa
ICP-AES Before
After
CAT1@CoxONa CAT2@CoxONa
0.308 0.256
0.111 0.091
0.087b; 0.113c 0.073b; 0.076c
a
XPS/ICP
2.77 2.81
mmol Mn/weight of AC (from XPS data in Table 1) atom%Mn/ [atom% Mn · Ar(Mn) + atom% C · Ar(C) + atom% O · Ar(O) + atom% N · Ar(N) + atom% Cl · Ar(Cl) + atom% Na · Ar(Na)]. b After two successive catalytic cycles for styrene epoxidation with m-CPBA/NMO. c After two successive catalytic cycles for a-methylstyrene epoxidation with NaOCl.
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A.R. Silva et al. / Carbon 43 (2005) 2096–2105
CO, mmol min-1 g-1
(Table 3). These results indicate that the manganese(III) salen complexes are mainly anchored at the external surface of the activated carbon matrix, which might be a consequence of the large dimensions of both molecules (Fig. 1). This phenomena was also observed in our previous report on the anchoring of the Jacobsen catalyst (CAT1), but a higher surface manganese content was observed (seven times higher) [14], probably due to the higher concentration of the initial Jacobsen catalyst solution. After manganese(III) salen complexes anchorage onto the CoxONa material changes in the dTG profiles are also detected (Fig. 4): new weight losses are observed between 500 and 800 K (Table 2), which corresponds to about 5% of sample weight loss, which is close to the weight of complex anchored, and thus might be correlated with the decomposition of the immobilised manganese(III) salen complexes. Moreover, a decrease in the intensity in the peak at 1156 K is detected, which might be attributed to the reaction of the manganese(III) salen complexes with the phenolate groups. During the TG-IR experiments strong vibrational bands in the regions 3020–2800 and 2240–2030 cm1 were detected, which correspond to C-H stretching vibrations from the tert-butyl groups and carbon monoxide, respectively. Other weak vibrations were also observed in the regions 3100–3020, 2400–2250 and 900– 850 cm1, which might correspond, respectively, to C–H stretching vibrations from the aldehyde fragment, carbon dioxide and C–H bending vibrations from phenyl rings. The CO and C–H stretching vibrations region kinetic profiles are represented in Fig. 6. Both materials show similar CO TG-IR kinetic profiles, but different from the CoxONa material (Fig. 5); 0.12 COXONa CAT2@COXNa CAT1@COXNa
0.10 0.08 0.06
the peak at 1156 K due to the phenolate groups disappeared, which confirms that the anchoring method shown in Fig. 2 took place. The C–H stretching vibrations must be due to fragments of CAT1 and CAT2 decomposition that include the tert-butyl groups, which present very strong absorptions in the infrared spectrum (vide experimental). For both materials these bands are observed at relatively low temperatures, which confirms that the low temperature weigh losses observed in the corresponding dTG profiles (Fig. 4) are due to the decomposition of the manganese(III) salen complexes anchored onto the CoxONa material. Sodium XPS analysis, Mn ICP-AES and TG-IR C-H fragment vibration region all give approximately the same amount of Mn(III) salen complexes anchored onto the CoxONa material. Considering that the complexes have been immobilised onto the surface of the activated carbon as a monolayer, taking into account the ICPAES values (Table 3) and the molecule geometrical parameters shown in Fig. 1, the areas occupied by both molecules can be estimated: 135 m2 g1 for CAT1 and 149 m2 g1 CAT2. These estimated areas agree well with the available surface area calculated using the BJH ˚ (Table 4 method for pores with diameter above 15 A and Fig. 1); it is noteworthy that these areas slightly change after complex immobilisation and hence, they are immobilised onto the surface of the activated carbon as a monolayer. We can then conclude that both catalysts are anchored onto the activated carbon surface by forming square pyramidal species in which the equatorial plane defined by MnN2O2 is almost parallel to the carbon surface (Fig. 2). The anchored Mn(III) salen complexes forming a monolayer on the pores with diam˚ of the activated carbon cause the blocketer above 15 A ˚, age of the micropores with diameter smaller than 15 A as can be seen by the decreases in the ABET area as well as in the micropore volumes (Table 4). 3.2. Catalytic experiments
0.04 0.02
The catalytic activity of the new materials (0.100 g) in the epoxidation of styrene, using 3-chloroperoxybenzoic
0.00 -0.02
A.U.C-H region
Table 4 Textural properties of carbon based materials Sample
ABET (m2 g1)
Ad>15 A˚a
Vmicropore (cm3 g1) Vt-plotb
200
400
600
800
1000
1200
T, K Fig. 6. CO and C–H region TG-IR kinetic profiles for the CoxONa, CAT1@CoxONa and CAT2@CoxONa materials.
NORIT ROX 0.8 Cox CoxONa CAT1@CoxONa CAT2@CoxONa a b c
676 922 797 744 759
128 157 132 167 168
0.323 0.435 0.377 0.355 0.371
VMPc
0.295 0.389 0.338 0.285 0.302 ˚ [15]. Determined by the BJH method, between 15 and 3000 A Determined by the t-method [7–11]. ˚ [15]. Determined by the MP method, between 2.5 and 5 A
A.R. Silva et al. / Carbon 43 (2005) 2096–2105
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Table 5 Asymmetric epoxidation of alkenes at 0 C catalysed by the homogeneous and heterogenised quiral manganese(III) salen complexes Catalyst CAT1 CAT1@CoxONa
%Mn mola
Alkene/oxidant f
Styrene/m-CPBA-NMO a-Mestyrene/NaOClb Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb
CAT2 CAT2@CoxONa
Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb Styrene/m-CPBA-NMOf a-Mestyrene/NaOClb
2.3 2.1 2.1 2.4 2.1 2.1 1.8 1.8 1.8 1.8 1.8 1.8
Run
1st 2nd 1st 2nd
1st 2nd 1st 2nd
t (h)
%Cg
TONc
TOFd
%eee
1 24 4 4 24 24 1 24 4 4 24 24
99 83 59 50 17 16 79 69 48 60 22 24
45 47 29 21 11 8 47 46 26 35 12 13
45 2 7 5 0.5 0.3 47 2 7 9 0.5 0.5
45 60 31 9 57 56 60 30 5 1 28 40
a
Relative to styrene. Experimental conditions: 0.500 mmol of a-Mestyrene, 0.500 mmol chlorobenzene (internal standard), 0.100 g of heterogeneous catalyst and 0.750 mmol of NaOCl, in 5.00 cm3 of dichloromethane. c Total TON based on the styrene conversion. d TOF = TON/reaction time. e Determined by chiral GC. f Experimental conditions: 0.500 mmol of styrene, 0.500 mmol chlorobenzene (internal standard), 2.500 mmol NMO (co-oxidant), 0.100 g of heterogeneous catalyst and 1.000 mmol of m-CPBA, in 5.00 cm3 of dichloromethane. g Determined by GC. b
acid (m-CPBA) as oxygen source, and of a-methylstyrene, using NaOCl as oxidant, were assessed in dichloromethane at 0 C and the results are collected in Table 5. Both new materials acted as enantioselective heterogeneous catalysts in the epoxidation of styrene, using m-CPBA as oxidant and NMO as co-oxidant, and amethylstyrene, using NaOCl as oxidant. In the asymmetric epoxidation of styrene, using mCPBA as oxidant and NMO as co-oxidant, a drop in the %ee is observed for the CAT1@CoxONa catalyst, when compared to its corresponding homogeneous phase reaction. The CAT2@CoxONa catalyst also shows a drop in the %ee when compared to its corresponding homogeneous phase reaction, but more marked than for CAT1. This may be attributed to steric hindarences of the immobilised manganese(III) salen complexes during the course of the epoxidation reaction, as we found earlier that they are mainly anchored in the micropore–mesopore region. Since CAT2 is bigger than the CAT1 (Fig. 1), the higher %ee drop observed in the former case may be hence due to bigger steric hinderances of this complex inside the micropore–mesopore structure of the AC. It is noteworthy that the CAT1@CoxONa catalyst is less enantioselective than the one we reported previously [14], probably because the catalyst reported herein is more homogeneously distributed throughout the porous structure of the AC. In the case of asymmetric epoxidation of a-methylstyrene, using NaOCl as oxidant, comparable enantiomeric excesses as the reactions run with the homogeneous counterparts were obtained for both heterogeneous catalysts.
Since no drop in %ee was observed in the asymmetric epoxidation of a-methylstyrene using NaOCl as oxidant, when compared to the reactions run in homogeneous phase, (contrasting with the results obtained in the asymmetric epoxidation of styrene using m-CPBA/ NMO as oxidant system), we suggest that these results may be related to different susceptibilities to steric hindrance of the oxidising oxo-metallic intermediate species that are formed in the course of the epoxidation reaction on the surface of carbon. In fact, it has been described in the literature that the asymmetric epoxidation of unfunctionalised alkenes catalysed by manganese(III) salen complexes in homogeneous phase is very dependent on the experimental conditions used and, in particular, on the oxidant. Despite of the nature of the oxo-metallic intermediate species and the mechanism by which the oxygen transfer to the alkene takes place being still a matter of debate [19], a different mechanism may be involved for both oxidants. Moreover, the different nature of the oxidant leaving group, (which can interact with the carbon surface), and the fact that the mechanism may be susceptible to steric hindrance due to the localisation of the manganese(III) salen complexes in micropore–mesopore region, may be important factors to explain the different behaviour observed for the catalysts reported herein using two different oxidants. Nevertheless, under all experimental conditions the styrene conversions, turnover numbers and TOF are lower than the corresponding homogenous phase reactions, which is due to slow diffusion of the reactants into the AC porous structure, especially when NaOCl is used as oxidant because of the multiphase reaction system. This has already been observed for other manganese(III)
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salen catalysts heterogenised in other solid supports [16,17,20,21]. The catalysts were also reused using the same epoxidation procedures, after convenient purification process. Reuse of both heterogeneous catalysts in the epoxidation of styrene with m-CPBA/NMO resulted in a significant loss of enantioselectivity, and in the case of the CAT1@CoxONa, also of catalytic activity. In the epoxidation of a-methylstyrene with NaOCl no loss of enantioselectivity nor catalytic activity were observed. This may be due to the higher resistance of the manganese(III) salen catalysts under epoxidation conditions when NaOCl is used as oxidant, compared with mCPBA, for which decomposition of the catalyst has been claimed before for other manganese(III) salen catalysts heterogenised in other solid supports [20]. The CAT1@CoxONa acts as a better heterogeneous catalyst than CAT2@CoxONa in the asymmetric epoxidation of the alkenes, under the experimental conditions used. Remarkably, CAT2@CoxONa improves its catalytic activity on reuse under both experimental conditions. When NaOCl is used as an oxidant it gives also a higher enantioselectivity on reuse and exceeds the value obtained under homogeneous conditions. This may indicate a form of catalyst conditioning which is somehow improving its performance. The ICP-AES manganese analysis of both the heterogeneous catalysts confirm that after two successive catalytic cycles (Table 5) no significant loss of the manganese(III) complex took place during the experiments and that hence the axial coordination of the manganese(III) salen homogenous catalyst onto the surface phenolate groups of the activated carbon does not result in active phase leaching.
and therefore, both catalysts should show a square pyramidal geometry, with the equatorial plane (MnN2O2) almost parallel to the carbon surface. These materials were active and enantioselective in the epoxidation of styrene and a-methylstyrene in dichloromethane at 0 C using, respectively, m-CPBA/ NMO and NaOCl. With a-methylstyrene comparable asymmetric inductions were found in the epoxide as the homogeneous phase reactions, whereas with styrene a decrease in %ee was found. Nevertheless, the CAT1@ CoxONa catalyst acts as a better heterogeneous catalyst than CAT2@CoxONa in the asymmetric epoxidation of the alkenes. For epoxidation of a-methylstyrene using NaOCl, catalyst reuse led to no significant loss of catalytic activity and enantioselectivity, while for epoxidation of styrene with the other oxidant a significant loss of enantioselectivity was observed. For both heterogeneous catalysts no significant metal leaching was observed by ICP-AES after the two successive catalytic experiments, under all conditions used, showing that the anchoring method used is effective against active phase leaching.
Acknowledgement The authors thank NORIT N.V., Amersfoort, The Netherlands, for providing the activated carbon. This work was funded by Fundac¸a˜o para a Cieˆncia e Tecnologia (Portugal) and FEDER through project ref. POCTI/CTM/45449/2002. ARS thanks FCT and the European Social Fund for a Post-Doctoral fellowship. VB thanks INTAS (grant 00-291) and EPSRC for a ROPA grant.
4. Conclusions Two chiral manganese(III) salen catalysts, bearing the same aldehyde fragment and different chiral diamine bridges, were successfully anchored onto the surface of a commercial activated carbon using a simple procedure which consists in the axial coordination of the metal centre onto the AC phenolate groups. XPS, TG and TG-IR data clearly indicate that modification of the AC by reaction between sodium hydroxide with the surface phenol groups took place giving rise to phenolate groups, which were found to be more thermally stable than parent phenol groups. Characterisation of the manganese(III) salen heterogeneous catalysts by XPS, ICP-AES and TG-IR clearly indicate that complex anchoring proceeds through coordination of surface phenolate groups to the manganese(III) centre. Furthermore, nitrogen adsorption measurements suggested that the complexes are immobilised onto the surface of the activated carbon as a monolayer
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