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Colloids and Surfaces A: Physicochem. Eng. Aspects 329 (2008) 190–197

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

ost-synthesis organo-sylanation of mesostructured FSM-16 for chiral Mn(III)alen catalyst anchoring

ankaj Dasa,1, Ana Rosa Silvaa,2, Ana P. Carvalhob, João Piresb,∗, Cristina Freirea,∗,3

REQUIMTE, Departamento de Química, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, PortugalDepartamento de Química e Bioquímica and CQB, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal

r t i c l e i n f o

rticle history:eceived 16 April 2008eceived in revised form 3 July 2008ccepted 7 July 2008vailable online 15 July 2008

a b s t r a c t

The mesoporous FSM-16 material was chemically modified with two spacers, namely 3-aminopropyltriethoxysilane (APTES) or 4-triethoxysilylaniline (TESA) to anchor the Jacobsen catalystderivative (CAT 1) through coordinative bond of the amine group of the spacer to the metal centre. Theresulting heterogeneous catalysts were tested in the asymmetric epoxidation at low temperature, ofthree alkenes – styrene, �-methyl-styrene and 6-cyano-2,2-dimethylchromene – using m-CPBA/NMO

eywords:esoporous-solid

SM-16rgano-sylanationn(III) salen catalyst

as oxygen source. The nitrogen elemental analysis showed that a much higher amount of APTES thanTESA was anchored onto the FSM-16. The FSM-16 material with grafted APTES has the double amountof CAT 1 anchored than the FSM-16 material with grafted TESA. The pore size distribution analysis ofthe nitrogen adsorption isotherms shows that TESA, APTES and CAT 1 catalyst are anchored inside thepores of the FSM-16 material. Under the same experimental condition in the asymmetric epoxidationof styrene, �-methyl-styrene and 6-cyano-2,2-dimethylchromene, the APTES based materials showed

versi

mtss

bbstsiaci

better activity in both, con

. Introduction

In 1990s, an approach to prepare porous solids from layered sil-cates by the interlayer cross-linking of the silicate layers by theeaction with alkyl trimethyl ammonium ions was proposed [1–3].he layered silicate used was kanemite and the resulting mate-ial, the FSM-16 solid, presented a regular structure with channelype mesopores [1–3]. FSM-16 is structurally similar to the MCM-1 solid [4,5], presenting a uniform mesoporous channel structure,ut FSM-16 has higher structural stability than MCM-41 [3]. For cer-ain applications, such as catalysts support, and although the typef porosity is eventually the more important issue, the structuraltability can also be a relevant property.

A relatively high number of studies can be found in the litera-

ure concerning the heterogenization of manganese complexes inilica-like supports, as reviewed elsewhere [6,7]. Nevertheless, theumber of works that deal with chiral complexes, as is the case ofhe present work, is much smaller. Additionally, the catalytic perfor-

∗ Corresponding author. Tel.: +351 217500898; fax: +351 217500088.E-mail addresses: jpsilva@fc.ul.pt (J. Pires), acfreire@fc.up.pt (C. Freire).

1 Actual address: Department of Chemistry, Dibrugarh University, Dibrugarh86004, Assam, India.2 Actual address: Unilever R&D, Port Sunlight, Bebington, United Kingdom.3 Tel.: +351 220402590; fax: +352 220402 659.

i[

Jdptdowoc

927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.07.008

on and enantiomeric excess, than TESA based materials.© 2008 Elsevier B.V. All rights reserved.

ance of immobilized chiral complexes is highly sensitive to both,he nature of the support and the organic functionalization of theurface, which justifies the need for the extensive study of variousupports and organic functionalization methodologies.

FSM-16 material was previously used in a very limited num-er of studies concerning the immobilization of complexes [8–10]ut not aiming enantioselective reactions. In the present work wetudy, to our knowledge for the first time, the possibilities of usinghe FSM-16 material for organo surface modification and then asupport for the Jacobsen complex, which is a chiral catalyst. Ass well known, enantioselectivity is induced by voluminous lig-nds of the Jacobsen complex which is an excellent homogeneousatalyst for the asymmetric epoxidation of alkenes. This reactions a powerful strategy for the synthesis of chiral intermediatesn the pharmaceutical as well as in the agrochemical industries11–13].

This work follows our study on the immobilization of theacobsen onto various types of mesoporous materials, possessingifferent textural properties and herein we endeavor to explore theroperties of the FSM-16 material after post-synthesis modifica-ion with organosiloxanes for the anchoring of the Jacobsen catalyst

erivative: (R,R)-[Mn(3,5-dtButsalhd)]ClO4 (CAT 1). The methodol-gy pursed was to modify the surface of the FSM-16 by reactionith two spacers, namely 3-aminopropyltriethoxysilane (APTES)

r 4-triethoxysilylaniline (TESA), followed by the anchoring of thehiral Mn(III) salen complex by coordination of the metal centre to

P. Das et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 329 (2008) 190–197 191

of CA

ti�ia

2

2

Tu

bdd((ep(Me

2

2

i[u(rsawi(

2F

8swc[C

(dAwud1

2

aaufaf

rwcpaerow

da

(Jauuw

MErlc

Scheme 1. Strategy used in the immobilization

he nitrogen groups of spacers, as depicted in Scheme 1. The result-ng catalysts were then tested in the epoxidation reaction of styrene,-methyl-styrene, whose epoxides are widely used in fine chem-

stry, and also of 6-cyano-2,2-dimethylchromene which epoxide isn important biologically active compound [14].

. Experimental

.1. Parent materials, reagents and solvents

The parent FSM-16 was kindly given by Dr. Shinji Inagaki, fromoyota Central R&D Labs. The material, kept in a desiccator, wassed as received, and will be labeled in this text as F1.

The manganese(III) complex (R,R)-(−)-N,N′-Bis(3,5-di-tert-utylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chlori-e abbreviated below as (R,R)-[Mn(3,5-dtButsalhd)Cl] (genericallyenoted the Jacobsen catalyst), 3-aminopropyltriethoxysilaneAPTES), 4-triethoxysilylaniline (TESA), styrene, �-methylstyrene�-Me-styrene), 6-cyano-2,2-dimethylchromene (6-CN-chrom-ne), chlorobenzene, benzaldehyde, styrene epoxide, m-chloro-eroxybenzoic acid (m-CPBA) and 4-methylmorpholine N-oxideNMO) were purchased from Aldrich. All solvents used were from

erck (pro analysi), except dichloromethane used in the catalyticxperiments which was from Romil (HPLC grade).

.2. Preparation of materials

.2.1. Anchoring of TESA or APTES onto FSM-16FSM-16 was functionalized with TESA or APTES by follow-

ng a method of post-synthesis as described in the literature15]. Briefly, to 1.5 g of FSM-16 (F1), dried overnight at 120 ◦Cnder vacuum, 50 cm3 of dry toluene and 3.8 mmol of TESA0.8 cm3) or APTES (0.9 cm3) were added and the suspensions wereefluxed, under nitrogen atmosphere, for 24 h. After cooling, theolids were filtered and Soxhlet extracted with toluene for 6 hnd then dichloromethane for further 6 h. Finally, the materialsere dried at 120 ◦C, under vacuum, overnight. The functional-

zed materials obtained were designated as F2 (for TESA) and F4for APTES).

.2.2. Immobilization of CAT 1 onto the organo-functionalizedSM-16 materials

The (R,R)-[Mn(3,5-dtButsalhd)Cl] (Jacobsen catalyst, 0.05 g,0 mmol) was dissolved in 50 cm3 of acetonitrile, and to this

olution 0.02 g (95 mmol) of AgClO4 was added. The solutionas stirred for 4 h at room temperature. After filtration of silver

hloride and solvent removal a reddish brown complex (R,R)-Mn(3,5-dtButsalhd)]ClO4 was obtained, which was denoted asAT 1.

wotsu

T 1 into FSM-16 using TESA and APTES spacers.

A solution of complex (R,R)-[Mn(3,5-dtButsalhd)]ClO4, CAT 150 mg or 72 �mol for F2 and F4, respectively) in 100 cm3 ofichloromethane was refluxed for 12 h with 0.75 g of F2 or F4.fter cooling, the solid materials were filtered, Soxhlet extractedith dichloromethane for 24 h and then dried at 120 ◦C, under vac-um, overnight to get the immobilized CAT 1. The materials wereesignated as F3 (FSM-16/TESA/CAT 1) and F5 (FSM-16/APTES/CAT).

.3. Characterization methods

Nitrogen adsorption isotherms at −196 ◦C were measured inn automatic apparatus (Asap 2010; Micromeritics). Before thedsorption experiments the samples were outgassed under vac-um during 2.5 h at 150 ◦C. Microporous volumes were estimatedrom the t-method [16] and mesoporous volumes from the amountsdsorbed at high relative pressures (p/p0 ∼ 0.97), and specific sur-ace areas were obtained by the BET method [16].

Nitrogen contents were obtained by elemental analysis at ‘Labo-atório de Análises’, IST, Lisboa (Portugal). The bulk Mn contentas determined by atomic absorption spectroscopy in a Pye Uni-

am SP9 spectrometer. Typically one sample of 20 mg of solid,reviously dried at 100 ◦C, was mixed with 2 cm3 of aqua regiand 3 cm3 of HF for 2 h at 120 ◦C, in a stainless steel autoclavequipped with a polyethylene-covered beaker (ILC B240). Aftereaching room temperature the solution was mixed with about 2 gf boric acid and finally adjusted to a known volume with deionizedater.

Powder X-ray diffractograms were obtained in a Philips PX 1820iffractometer using Cu K� radiation, with a step size of 0.005 (2�◦)nd a time per step of 2.5 s.

FTIR spectra of the materials were obtained as KBr pelletsMerck, spectroscopic grade) in the range 400–4000 cm−1, with aasco FT/IR-460 Plus spectrophotometer; all spectra were collectedt room temperature, after drying the pellets in a oven overnight,sing a resolution of 4 cm−1 and 32 scans. For the spectra of the liq-id samples TESA and APTES, a Perkin Elmer KBr cell with 0.2 mmas used.

X-ray photoelectron spectroscopy was performed at “Centro deateriais da Universidade do Porto” (Portugal), in a VG Scientific

SCALAB 200A spectrometer using a non-monochromatized Mg K�adiation (1253.6 eV). All the materials were compressed into pel-ets prior to the XPS studies. In order to correct possible deviationsaused by electric charge of the samples, the C1s line at 285.0 eV

as taken as internal standard. The elemental contents of the vari-us samples were calculated from the areas of the relevant bands inhe high-resolution XPS spectra, which were also curve-fitted usingymmetric Gaussian curves, after fitting to a Shirley background,sing XPSpeak version 4.1.

1 ysicochem. Eng. Aspects 329 (2008) 190–197

2

mauc(ocmuFoiacdlc

emwCucclfi(d(l%z[e[×

Fp

Table 1Nitrogen and manganese contents determined from elemental analysis and XPS andtextural properties of the studied samples

Material N (mmol g−1) Mn (�mol g−1) Textural propertiesb

EA XPSa AAS XPSa ABET (m2/g) Vmeso (cm3/g)

F1 0.13 1323 0.95F2 0.4 0.36 – 876 0.58F3 <0.2 0.43 4.3 (3.5) 5.2 1048 (681) 0.71 (0.40)F4 1.9 1.57 739 0.41F5 1.4 1.25 9.6 (6.1) 21 689 (391) 0.35 (0.18)

a N or Mn amount per weight of sample (before catalytic tests) calculated from XPSdata in Table 3: mmol N or Mn/weight of sample = at% N or Mn/[at% C × Ar(C) + at%N × Ar(N) + at% O × Ar(O) + at% Mg × Ar(Mg) + at% Si × Ar(Si) + at% Cl × Ar(Cl) + at%M

o

3

3

itcihotnatrthe case of the use of TESA, the porous volume of the samplewith complex (F3) is higher than the sample without com-plex (F2) is not straightforward; this is most probably relatedwith the removal of some TESA species, which were less firmly

92 P. Das et al. / Colloids and Surfaces A: Ph

.4. Catalytic experiments

The catalytic activity of the FSM-16 based materials in the asym-etric epoxidation of styrene, �-methylstyrene (�-Me-styrene)

nd 6-cyano-2,2-dimethylchromene (6-CN-chromene) was eval-ated at −5 ◦C at atmospheric pressure in batch regimen, underonstant stirring, using a mixture of m-chloroperoxybenzoic acidm-CPBA) as oxidant and N-methylmorpholine (NMO) as co-xidant. In the experiments 0.2 mmol of alkene, 0.2 mmol ofhlorobenzene (internal standard), 90 mg of catalyst, 0.4 mmol of-CPBA and 1.0 mmol of NMO in 5.00 cm3 of dichloromethane weresed. After 4 h of reaction the product analysis was made by GC-ID analysis using the internal standard method. After each cyclef utilization the catalyst was washed several times under reflux-ng condition with dichloromethane to remove occluded reactantsnd products; the purification of the catalysts after each catalyticycle by Soxhlet extraction was avoided due to the material losturing this type of procedure. After drying the recovered cata-

yst at 100 ◦C in an oven, overnight, it was reused under identicalondition.

The analysis of the products obtained after the catalyticxperiments was done by GC-FID (using the internal standardethod) on a Varian CP-3380 gas chromatograph equippedith a fused silica Varian Chrompack capillary column CP-Sil 8B Low Bleed/MS (30 m × 0.25 mm id; 0.25 �m film thickness),sing helium as carrier gas. The enantiomeric excesses in per-entage (ee%) of the epoxides were determined using the samehromatograph but using a fused silica Varian Chrompack capil-ary column CP-Chiralsil-Dex CB (25 m × 0.25 mm d.i. × 0.25 �mlm thickness). Conditions used: 60 ◦C (3 min), 5 ◦C min−1, 170 ◦C2 min), 20 ◦C min−1, 200 ◦C (10 min); injector temperature, 200 ◦C;etector temperature, 300 ◦C. The reaction parameters such as C%conversion, in percentage) and ee% were calculated using the fol-owing formula, where A stands for area of chromatographic peak:C = {[A(alkene)/A(chlorobenzene)]t = 0 h − [A(alkene)/A(chloroben-ene)]t = x h}× 100/[A(alkene)/A(chlorobenzene)]t = 0 h and %ee =A(major enantiomer) − A(minor enantiomer)] ×100/[A(major

nantiomer) + A(minor enantiomer)]. %Sepoxide = A(alkene) × 100/A(alkene) +

∑A(other reaction products)], TON = %C × %Sepoxide

mmol (alkene)t = 0 h/mmol Mn, TOF = TON/reaction time.

ig. 1. Nitrogen adsorption–desorption isotherms at −196 ◦C for the various sam-les.

Ff

n × Ar(Mn) + at% Na × Ar(Na)].b From the nitrogen adsorption isotherms at −196 ◦C. In parenthesis values

btained after catalysis with �-methylstyrene.

. Results and discussion

.1. Textural characterization

In Fig. 1, the low temperature nitrogen adsorption–desorptionsotherms in the various samples are given, and the respectiveextural parameters obtained from these curves, such as the spe-ific surface area (ABET) and the porous volume, are collectedn Table 1. As can be seen the curve for the parent FSM-16as the general shape described in the literature for this typef mesoporous ordered material, namely, the steep increase inhe adsorbed amounts for relative pressures approaching 0.4 isoticed [2,3,17]. After reaction with TESA or APTES and the link-ge to the complex, the adsorption capacities are reduced, buthe general isotherm profile, characteristic of a mesoporous mate-ial, is kept. The explanation for the experimental fact that, in

ig. 2. Pore size distributions obtained from the nitrogen adsorption data by NLDFTor the various samples.

P. Das et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 329 (2008) 190–197 193

Table 2XPS data (atom %) of the studied samples

Materials C N O Si Cl Mn

F1 4.89 0.25 61.75 33.11 – –F2 7.72 0.72 57.34 34.22 – –FFF

bcs

o(o[nmcopowta

Tpopeothmu[w

e

Table 3Curve fitting data of the XPS spectra in the relevant regions of the studied samples

Sample Binding energy (eV)a

Si2p O1s C1s N1s

F1 103.8 (2.5) 533.1 (2.5)284.9 (2.9)

400.0 (3.3)287.8 (3.0)

F2 103.8 (2.4) 533.0 (2.4)284.9 (2.3)

400.0 (2.5)286.6 (3.0)

F3 103.7 (2.3) 533.0 (2.3)284.9 (2.2)

400.0 (3.1)286.7 (3.0)

F4 103.1 (2.5) 532.5 (2.4)284.8 (2.6) 399.5 (2.6)286.6 (2.5) 401.7 (2.3)

F

ttsirt

wpl

3

3

mAtplex anchoring there is a decrease in the bulk N quantity, which may

TA

C

C

F

F

3 11.87 0.83 55.77 31.28 0.24 0.014 13.78 3.00 52.30 30.62 0.31 –5 15.40 2.37 51.96 29.90 0.33 0.04

onded to the walls, during the experimental operations thatonducted to the bond of the complex (see chemical analy-is).

The diffraction peaks (not shown) of the parent material wherebserved at 2� = 2.3, 4.0, 4.5 and 6.0 and are due to the (1 0 0), (1 1 0),2 0 0) and (2 1 0) indices of the hexagonal cell [18]. For the hexag-nal cell [1] where the a0 parameter is given by a0 = (2/

√3) d1 0 0

1,5] which, in the present case, gives a0 = 4.46 nm, a value that isear those reported in literature for similar materials [1–3,17]. Foraterials such as FSM-16, due to their regular porosity, the widths

an be estimated from the DRX data, by subtracting the thicknessf the pore walls [17] that gives, for the sample in this work, aore width of 3.7 nm. Other authors proposed a different method-logy, also based in geometric considerations, to estimate the poreidths of regular hexagonal mesophases. This was done by using

he porosity ε that is related with the mesoporous volume (Vmeso)nd the density (�) of the sample by ε = (Vmeso�)/(Vmeso� + 1) [13].

he pore width is then given by a0

√(2

√3)/(�)ε [5] which, in the

resent study, and by making the assumption [5] that the valuef � is 2.2 g/cm3, gives the value of 3.3 nm. In the context of theresent work, the characterization of the pore dimensions is a rel-vant question since the treatments of the parent solid with APTESr TESA and the subsequent bond of the metal complex may changehe pore widths and, eventually more important, the sample thatas initially uniform pore widths may present, after the variousodifications, a broad distribution of pore sizes. In this way, we

sed in present work the Non-Linear Density Functional Theory

19,20] to obtain the pore size distributions for the various samples,hich are given in Fig. 2.

As can be seen from Fig. 2, the parent FSM-16 presents, asxpected, a narrow pore size distribution with a maximum near

btib

able 4symmetric epoxidation of alkenes using CAT 1 catalyst anchored on FSM-16 by TESA and

atalyst Alkene %Mnb Cycle

AT 1 Styrene – –�-Me-styrene – –6-CN-chromene – –

3Styrene 0.2

1st2nd

�-Me-styrene 0.21st2nd

6-CN-chromene 0.2 1st

5Styrene 0.4

1st2nd

�-Me-styrene 0.41st2nd

6-CN-chromene 0.51st2nd

a Reactions were carried out with 90 mg of heterogeneous catalyst, in dichloromethaneb Relative to alkene.c Relative to alkene; determined by GC-FID against internal standard (chlorobenzene),d Epoxide selectivity determined by GC-FID, see in Section 2.4 for details.e Total TON based on the alkene conversion, see in Section 2.4 for details.f TOF = TON/reaction time.g Determined by chiral GC-FID, see in Section 2.4 for details.

5 103.1 (2.5) 532.7 (2.3)284.9 (2.4) 399.2 (2.8)286.3 (3.0) 401.4 (3.0)

a Values between brackets refer to the FWHM of the bands.

he 4 nm, which is only slightly above the values obtained fromhe geometric considerations discussed above. In the case of theamples modified with TESA (F2 and F3) the pore size distributions still narrow, although a shift to lowest values is noticed. Theseesults indicate that the reaction with TESA, and the anchorage ofhe complex, occurred inside the pores.

Concerning the pore size distributions of the samples modifiedith APTES (F4 and F5) and in a certain way similarly to what hap-ened with the samples with TESA, the distributions are shifted to

ower values, but they are also broad.

.2. Chemical analysis and spectroscopic methods

.2.1. Chemical analysisThe nitrogen elemental analysis and manganese contents deter-

ined by AA are collected in Table 1. A much higher amount ofPTES was anchored onto the FSM-16 (F4) than TESA (F2), being

he anchoring efficiencies of 76 and 16%, respectively. Upon com-

e attributed to some leaching of the spacers under the experimen-al conditions of complex immobilization procedure, as describedn literature [11–14]. The FSM-16 material with APTES is capa-le of anchoring roughly the double amount of CAT 1 (F5) than

APTES linker and using m-CPBA/NMO as oxidanta

%Cc Sepoxide (%)d TONe TOF (h−1)f ee%g

59 87 135 135 3856 78 110 110 3433 94 71 35 67

22 64 72 18 149 31 11 3 2

32 23 36 9 2030 18 26 6 314 93 61 15 42

20 69 33 8 318 51 8 2 5

24 20 13 3 3324 18 9 2 4

9 100 18 4 5514 80 23 6 11

at −5 ◦C; alkene:m-CPBA:NMO = 1:2:5; reaction time = 4 h.

see in Section 2.4 for details.

1 ysicoc

tcta

3

rtvF(oasabo

tilisatt

rmw

94 P. Das et al. / Colloids and Surfaces A: Ph

he FSM-16 material with TESA (F3). As can be checked in theatalytic experiments section, catalytic results are in line withhe amounts of Mn anchored onto the modified FSM-16 materi-ls.

.2.2. XPSThe chemical composition obtained by XPS for the FSM-16 mate-

ials are collected in Table 2 and the results of the deconvolution ofhe high-resolution spectra are collected in Table 3; the most rele-ant spectra and their curve fittings are shown for F1, F4 and F5 inig. 3. The high-resolution XPS spectra of parent FSM-16 materialF1) show bands in the Si2p region at about 103.8 eV, in the rangef the values discussed by other authors for these materials [21]

nd a symmetrical band at 533.1 eV in the O1s region, which corre-pond to silicon and oxygen from the SiO4 framework. Low intensend broad bands at the C1s region and N1s are also observed foroth materials that are most probably due to the presence of somerganic impurities resulting from the synthesis of the materials.

dmNsp

Fig. 3. Si2p and N1s XPS high-resolution spectra for F1, F

hem. Eng. Aspects 329 (2008) 190–197

Upon surface functionalization with TESA (F2) and APTES (F4)here is an increase in surface carbon content besides of the increasen nitrogen content due to the anchoring of both spacers. For F4,ower amount of nitrogen was obtained by XPS than by EA (Table 4),ndicating that the APTES is mostly located within the internal poretructure of the material, whereas for F2 approximately the samemount of nitrogen was obtained by XPS and EA, indicating thathe TESA molecule is more homogeneously distributed throughouthe sample.

Upon complex anchoring onto the functionalized FSM-16 mate-ials (F3 and F5) there is an increase of surface carbon andanganese contents, but for F3 the nitrogen content increaseshereas for F5 decreases. The latter result corroborates the AE

ata, justified before as due to partial spacer leaching, but the for-er result is in some way unexpected when compared with thecontent from AE. However, the result for F3 may suggest that

ome of the TESA molecules leached from inner pores during com-lex anchoring may be trapped (probably due to their rigidity) at

4 and F5 samples and corresponding curve fittings.

P. Das et al. / Colloids and Surfaces A: Physicoc

Fc

tt

tcFcw

3

3stasmbiTtb8l

S2trX

oi11t

otrsoa

ebsbbdsm1oi

3

1sulc

aimcpeit

Coig

itoMmb

ig. 4. FTIR spectra of the studies materials, and of free TESA, APTES and CAT 1omplex: (a) 4000–2000 cm−1 and (b) 2000–400 cm−1 regions.

he material outer pores, leading to an increase in surface N con-ent.

The manganese values obtained by XPS are much higher thanhose obtained by bulk analysis, AA (Table 1), suggesting that theomplex is anchored into the outer pores for both materials, F3 and5: for F3 it is probably a consequence of the TESA high surfaceoncentration and for F5 due to the APTES higher concentrationithin the material.

.2.3. FTIRThe spectrum of FSM-16 exhibits a broad band at around

460 cm−1, which is characteristic of hydrogen-bonded O–Htretching, and moderately intense band around 1637 cm−1 dueo O–H bending associated with the water molecules physicallybsorbed at the FSM-16 surface (Fig. 4a). In addition, FSM-16 alsohows a very intense and large band at 1084 cm−1, due to asym-etric stretching of the Si–O–Si linkage of the SiO4 tetrahedra. This

and is very intense and is common to various types of silicas and,n the context of the present work, it is not much informative.

herefore, this band is saturated in Fig. 4b to increase the rela-ive absorption other less strong, but more informative bands. Theand at 970 cm−1 is due to the Si–OH stretching, and the band at02 cm−1 is due to the O–Si–O stretching. The very intense and

arge band at 458 cm−1 is due to the bending mode of the bulk

d

hpl

hem. Eng. Aspects 329 (2008) 190–197 195

i–O–Si (Fig. 4b) [22]; the presence of very low intense bands at940–1880 cm−1, which may be assigned to C–H stretching vibra-ions, may be due to the presence of some organic impuritiesesulting from the synthesis of the materials, as also detected inPS C1s spectrum (see above).

FTIR spectrum of FSM-16 after TESA grafting (F2) shows besidesf the bands between 2880 and 2940 cm−1 due to the C–H stretch-ng of –CH moieties of the grafted TESA, a shoulder at about600 cm−1, due to N–H scissoring (bending), and a new band at510 cm−1, which are typical of the TESA molecule (Fig. 4), provinghat it was grafted onto the surface of the material.

In the F3 material, that is, upon CAT 1 immobilization onto F2,nly the new band at 635 cm−1 can be unambiguously assigned tohe CAT 1 due to the strong absorbing bands of the supports in theegion where the strongest bands should appear. Furthermore, thehoulder at 1600 cm−1 that appeared after TESA functionalizationf FSM-16 now disappears probably due a band shift caused by thexially anchoring of the CAT 1.

After APTES grafting the spectra (F4) (Fig. 4) also shows sev-ral changes, e.g., the broad band around 3450 cm−1 sharpens, newands appear between 2880 and 2940 cm−1 attributable to the C–Htretching of –CH2 moieties and of the grafted APTES, and newands at 1556 cm−1 due to N–H scissoring (bending) and severalands between 1400 and 1500 cm−1 (1471, 1448, 1412 and 1390)ue to C–H bending showing that molecule was grafted onto theurface of the material. Upon CAT 1 immobilization onto the F4aterial a new band is observed at 628 cm−1, which is typical of CAT

. Furthermore, in this spectrum (F5 material) a shift to 1546 cm−1

f the N–H scissoring (bending) of the APTES is observed, suggest-ng that the complex was anchored axially to this group.

.3. Catalytic activity

The catalytic activity data of CAT 1 immobilized in the FSM-6 based supports (F3 and F5), for the asymmetric epoxidation oftyrene, �-Me-styrene and 6-CN-chromene carried out at −5 ◦C bysing m-CPBA/NMO as oxidant is summarized in Table 3. The evo-

ution of the selectivity and the enantiomeric excess for the severalatalysts is given in Fig. 5 for each epoxidation reaction.

One relevant feature is that, for all the alkenes tested, CAT 1nchored onto the APTES based material (F5) gives better epox-de enantioselectivity (ee%) than anchored onto the TESA based

aterial (F3); this is probably due to the higher flexibility of APTESompared with TESA (rigid spacer), that will ease the metal com-lex to acquire the necessary conformation to induce the productnantioselectivity in the subsequent cycle. For all the alkenes thencrease in the catalyst performance from F3 to F5, correlates withhe amounts of Mn anchored.

The maximum ee% (55%) has been obtained for the alkene 6-N-chromene with F5 catalyst being lower than the value of 67%btained by us for the homogeneous reaction (cf. the CAT 1 samplen Table 3 and Fig. 5). The alkene ee% order in each material is,enerally: 6-CN-chromene > �-Me-styrene > styrene.

For both catalysts, and in general, the order of alkene conversions: �-Me-styrene > styrene > 6-CN-chromene and the order of selec-ivity is the reverse: 6-CN-chromene > styrene > �-Me-styrene. Therder for the TON and TOF is: styrene > 6-CN-chromene > �-e-styrene. Therefore, CAT 1 anchored onto the FSM-16 basedaterials is more efficient and active in the epoxidation of styrene,

ut this alkene yields the lowest enantioselectivities, as already

iscussed above.

The most active catalyst for all the alkenes is F3 yielding theighest TON and TOF of all the catalyst tested (Table 3), but itresents the lowest ee% values of the series (Fig. 5). Neverthe-

ess, this proves that the heterogeneous epoxidation of alkenes by

196 P. Das et al. / Colloids and Surfaces A: Physicoc

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ig. 5. Selectivity in epoxide (open rectangles) and enantiomeric excess (closedectangles) for the studied systems. For comparison purposes, the results in homo-eneous phase (CAT 1) are given also.

anganese(III) salen complexes works efficiently with very lowanganese molar percentages in the reaction mixture. It should

lso be noticed that F3 catalyst has the higher surface area andesoporous volume of the four catalysts studied (Table 1). Compar-

son with literature data, besides the fact that there is now previousata concerning the use of FSM-16 as support, is not always easy.

n fact, the confinement effects of various supports as well as thearious methodologies of immobilization that can be employedan be substantially different. Nevertheless it can be stated thathe TOF values in the present work are normally lower than thoseecently found in MCM-41 type materials [23]. The exceptions arehe TOF values for 6-CN-chromene which compare favorable withhe literature values [14].

With the reutilization of all the heterogeneous catalysts in aurther catalytic cycle there is a decrease of epoxide enantioselec-ivity and selectivity and, generally, of alkene conversion, indicatingxidative degradation of the anchored catalyst under the experi-ental conditions, as has been reported in the literature [24].

hem. Eng. Aspects 329 (2008) 190–197

The characterization of the catalysts after the two cycles of epox-dation of the alkenes was made only for some catalysts due to theow amounts of sample available. The obtained values for the spe-ific surface area and mesoporous volumes, as well as the final Mnontents, are given in Table 1, as an example, for the samples afteratalysis of �-methylstyrene. The sequence in the values of the sur-ace area and mesoporous volume is maintained in the samplesefore and after catalysis (F3 > F5), although a decrease in thesearameters is observed. In the case of the surface area this reduc-ion is about 40%. Concerning the Mn amounts in the samples afteratalysis, it is interesting to note that more than 75% of the Mn con-ent is still present in the sample with TESA, while in the sampleith APTES the reduction was more pronounced, but the retained

mount is still of 64% of the initial value.

. Conclusions

The Jacobsen derivative catalyst was anchored into FSM-16hrough metal axial coordination to the nitrogen atom of two spac-rs, APTES and TESA, previously grafted into the parent material.higher amount of APTES was anchored onto the FSM-16 than

ESA. Upon anchoring of CAT 1 a decrease of nitrogen content inhe FSM-16 modified materials was observed, probably due to par-ial leaching of the grafted TESA and APTES spacers during the

odification procedures. The FSM-16 material with grafted APTESresented a double amount of anchored CAT 1 when compared tohe FSM-16 material with grafted TESA.

The heterogeneous catalysts were active in the asymmet-ic epoxidation of styrene, �-methyl-styrene and 6-cyano-2,2-imethylchromene. The anchored CAT 1 onto the APTES basedaterial showed better conversion and enantioselectivity (ee%)

han anchored onto the TESA based material. For both catalystshe order of alkene ee% is, generally: 6-CN-chromene > �-Me-tyrene > styrene, being the alkene conversion% in the reverse order.or the studied systems, the conversion and the enantioselectiv-ty showed to be in line with total Mn content, but not with theotal surface area, or the uniformity of the pore size distribution,hus pointing out the lack of relation with eventual confinementffects.

cknowledgments

This work was funded by FCT Fundacão para a Ciênciaa Tecnologia (FCT) and FEDER, through the project ref.

OCI/CTM/56192/2004 and PPCDT/CTM/56192/2004. PD thanksCT for a Post-Doctoral fellowship. Thanks are due to Dr. Shinjinagaki, from Toyota Central R&D Labs. for the FSM-16 sample.

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