Manganese complexes with triazenido ligands encapsulated in NaY zeolite as heterogeneous catalysts

Article abstract of DOI:10.1016/j.ica.2012.09.027

Two different methods were described for the preparation of manganese complexes with triazenido derivative ligands encapsulated in NaY zeolite. The catalytic behaviour of the heterogeneous catalysts was evaluated for styrene and cyclohexanol oxidation rea

Full text of DOI:10.1016/j.ica.2012.09.027

Inorganica Chimica Acta 394 (2013) 591–597
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Manganese complexes with triazenido ligands encapsulated in NaY zeolite
as heterogeneous catalysts
⇑
´
Iwona Kuzniarska-Biernacka, António M. Fonseca, Isabel C. Neves
Centro de Química, Departamento de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 April 2012
Received in revised form 26 June 2012
Accepted 11 September 2012
Available online 4 October 2012
Two different methods were described for the preparation of manganese complexes with triazenido
derivative ligands encapsulated in NaY zeolite. The catalytic behaviour of the heterogeneous catalysts
was evaluated for styrene and cyclohexanol oxidation reactions. Encapsulation was achieved by two
methods: (i) ion-exchange from an ethanol solution containing both bis(p-tolyl)triazenido ligand and
manganese ions, with the ligand: Mn 3:1 molar ratio (Method A) and (ii) the flexible ligand method
(Method B). The heterogeneous catalysts were characterized by FTIR spectroscopy, X-ray photoelectron
spectroscopy (XPS), powder X-ray diffraction (XRD) and chemical analyses. The results show that both
preparation methods lead to effective encapsulation of the Mn{bis(p-tolyl)triazenido} (1:1) complex in
the NaY zeolite framework. The heterogeneous catalysts show activity in oxidation of styrene and cyclo-
hexanol with tert-butyl hydroperoxide (tBuOOH) as an oxygen source. The catalysts are chemo-selective
in cyclohexanol oxidation, whereas in styrene oxidation reaction leads to styrene oxide and
benzaldehyde.
Keywords:
Bis(p-tolyl)triazenido ligand
Manganese complex
Encapsulation
NaY
Heterogeneous catalysts
Oxidation reactions
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
tions with the inserted chemical compounds [3–5]. Zeolite cages
increase the protection of complex molecules from decomposition
Selective catalytic oxidation of alcohols to carbonyls is one of
the most important chemical transformations in industrial chemis-
try. Carbonyl compounds such as ketones and aldehydes are the
precursors for many drugs, vitamins, and fragrances and they are
also important intermediates for many complex syntheses [1,2].
The catalytic epoxidation of terminal and electron-deficient olefins
also constitutes a challenging task in organic chemistry. Transition
metal complexes are successfully used to catalyse these reactions
in homogeneous reaction media. The separation and recycling of
homogeneous catalysts is problematic, often making the entire cat-
alytic process economically unviable for industrial processes. The
heterogenization of homogeneous catalysts has therefore become
an important strategy to obtain supported catalysts that retain
the homogeneous catalytic sites with the advantages of easy sepa-
ration and recycling [3]. As a consequence, zeolites have been pre-
ferred hosts for the encapsulation/immobilization of transition
metal complexes [3,4].
and irreversible dimerization by providing steric constraints
around the molecule [5–7]. Y zeolite is frequently chosen as a host
to encapsulate metal complexes because of its large supercages
(11.8 Å) with smaller pore openings (7.4 Å) suitable for the accom-
modation of metal complexes and three-dimensional pore network
desirable for easy reactant accessibility and product diffusion [3–
7]. The host supplies additional stability of the incorporated metal
complex, thereby extending the physical and chemical resistance
of the catalyst [8].
The development of novel ligands capable of forming stable
complexes that can be used as effective catalysts in organic
synthesis is an important field in organometallic chemistry [9].
Nitrogen-donor ligands have received much attention, including
the b-diketiminate and the amidinate ligand systems [10–12].
Minor attention has been given to triazenides ligands [13,14].
The triazenides are able to form strong metal–nitrogen bonds
and successfully stabilise the complexes formed. These complexes
were successfully synthesised [15–20]. However, few of them have
been developed as homogeneous catalysts [21]. To the best of our
knowledge, there are no studies with these complexes as heteroge-
neous catalysts in oxidation reactions.
Zeolites are hydrated aluminosilicate crystalline solids with
very regular microporous structures in which active and chemi-
cally interesting compounds can be included. Apart from the space
constraints imposed by the zeolite, their exchangeable properties
due to the negative charge of the framework and the distribution
of the positive charges of the cations can lead to specific interac-
The usual method for the introduction of metal complexes into
the zeolite structure is the flexible ligand method. In this approach,
the ligand is added to the metal ion exchanged in zeolite [22–29].
In situ complex preparation within the zeolite also has been used.
In this method, the ligand and the metal ion are added simulta-
⇑
Corresponding author.
E-mail address: ineves@quimica.uminho.pt (I.C. Neves).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.09.027

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592
neously to the liquid phase inside the zeolite with subsequent
coordination [1,30–34]. It may be expected that different encapsu-
lation methods would result in different catalytic properties of the
resulting heterogeneous catalysts [30,35,36]. Initial studies used
the in situ method and showed that the Y-encapsulated transition
metal complexes presented a good catalytic activity in the oxida-
tion of alcohols and alkenes [31–34]. In this research two different
methods were used for in situ encapsulation of manganese com-
plexes with the triazenido ligand in NaY zeolite framework. Cata-
lytic studies were performed in the liquid phase for the oxidation
of cyclohexanol and styrene, using tert-butyl hydroperoxide
(tBuOOH) as an oxygen source.
The extracted sample was further washed with a solution of NaCl
in deionised water to ion-exchange undesired free metal ions.
The resulting catalyst, designated as Mn(dtta)-YA, was dried in an
oven at 60 °C overnight under reduced pressure.
2.3.2. Method B
NaY Zeolite was first ion-exchanged with an aqueous solution
of manganese(II) sulphate monohydrate (0.01 mol/L, liquid/so-
lid = 130 mL/g) at pH 5.3 with stirring for 12 h at room tempera-
ture. The solid obtained was washed with deionised water and
dried at 80 °C overnight under reduced pressure. Mn-Y solid
(1.5 g) was suspended in a solution of 1.35 g (6 mmol) of ligand
in 50 mL of ethanol and then six drops of triethylamine were
added. The mixture was stirred for 24 h at room temperature.
The resulting solid was filtered off and washed with deionised
water and ethanol, then dried at 60 °C under reduced pressure
overnight. The solid was Soxhlet extracted with ethanol and again
with dichloromethane to remove the unreacted ligand, till the elu-
ate became colourless. The extracted sample was further washed
with a solution of NaCl in deionised water to ion-exchange unde-
sired free metal ions. The catalyst, Mn(dtta)-YB, was dried in an
oven at 60 °C overnight under reduced pressure.
2. Experimental
2.1. Materials and reagents
NaY zeolite (Si/Al = 2.83) in powdered form was obtained from
Zeolyst International. The powder it was calcined at 500 °C for 8 h
under a dry air stream prior to use. All chemicals and solvents used
were reagent grade and purchased from Aldrich: manganese(II)
chloride tetrahydrate (MnCl2Á4H2O), manganese(II) sulfate mono-
hydrate (MnSO4ÁH2O), triethylamine, acetonitrile, dichlorometh-
ane, ethanol, tert-butyl hydroperoxide solution – 5.0–6.0 M in
decane (tBuOOH), chlorobenzene, cyclohexanol and styrene. Spec-
troscopic grade potassium bromide (KBr) used for the FTIR pellets
preparation was supplied by Merck.
2.4. Characterisation methods
NMR spectra were obtained with a Varian Unity Plus Spectrom-
eter at an operating frequency of 300 MHz for 1H NMR and
75.4 MHz for 13C NMR or with a Bruker Avance III 400 at an oper-
ating frequency of 400 MHz for 1H NMR and 100.6 MHz for 13C
NMR using the solvent peak as internal reference at 25 °C. X-ray
photoelectron spectroscopy (XPS) was performed at the Centre
for Scientific Support Technology and Research, University of Vigo,
Spain, using a VG Scientific spectrometer ESCALAB 250 iXL with
2.2. Synthesis of 1,3-bis(tolyl)triazene and free complex
1,3-Bis(tolyl)triazene (dtta) was synthesized following the
literature procedures [37]. Triazene was obtained by coupling of
p-toluidine, in toluene using isoamyl nitrite and isolated by
crystallization from ethyl acetate/hexane (4:1) at 0 °C with a yield
of 78%. As yellow crystals; m.p. 162 °C. FTIR (KBr,
(N@N)]; 3198 [
(N–H)]. 1H NMR (300 MHz, CDCl3): d = 2.30 (s,
m
/cmÀ1): 1612
non-monochromatised Al K
a radiation (1486.6 eV). Charge refer-
[m
m
encing was done by setting the lower binding energy C 1s photo-
peak at 285.0 eV. The atomic concentrations were determined
from the XPS peak areas using the Shirley background subtraction
technique and Scofield sensitivity factors. The XPS high-resolution
spectra were fitted using a Gaussian–Lorentzian line shape, Shirley
background [38] and damped non-linear least-squares procedure.
The line-width of the Gaussian peak (full-width at half maximum,
FWHM) was optimised for all components on each individual high-
resolution spectrum. Chemical analysis of C, H and N were carried
out on a Leco CHNS-932 analyzer. High-resolution mass spectra
(HRMS) were obtained with a GV AutoSpec spectrometer using
an m-nitrobenzyl alcohol (NBA) matrix. Mn loading on zeolitic
samples was evaluated according to the SMEWW 3120 method,
using Inductive Coupled Plasma (ICP) performed in the Instituto
Superior Técnico in Portugal. Powder X-ray diffraction patterns
(XRD) were recorded using a Philips Analytical X-ray model
PW1710 BASED diffractometer system. The solids samples were
exposed to the Cu Ka radiation at room temperature in a 2h range
between 5° and 65°. Fourier Transform Infrared (FTIR) spectra of
the materials were obtained as KBr pellets, mixed in 1:150 ratio
(material:KBr), in the range of 500–4000 cmÀ1, with a BOMEM
MB 10 spectrophotometer. All spectra were collected at room tem-
perature, with a resolution of 4 cmÀ1 and 32 scans. The electronic
UV–Vis absorption spectra of the free complex was collected in the
range 600–200 nm in a Shimadzu UV/2501PC spectrophotometer
using quartz cells at room temperature. The GC-FID chromato-
grams were obtained with a SRI 8610C chromatograph equipped
with CP-Sil 8CB capillary column. Nitrogen was used as the carrier
gas. The identification of reaction products was confirmed by GC–
MS (Varian 4000 Performance).
6H, 2CH3), 7.0 and 7.3 (AX pattern, 8H, JAX = 9 Hz, C6H4CH3), 7.9
(s, 1H, NH). 13C NMR (100.6 MHz, CDCl3): d = 142.8 (CAr–N), 138.7
(CAr–H), 132.8 (CAr–H), 129.5 (CAr–H), 128.1 (CAr–CH3), 123.2
(CAr–H), 21.2 (Ar–CH3). Anal. Calc. for C14H15N3: C, 74.64; H, 6.71;
N, 18.65. Found: C, 74.51; H, 6.64; N, 18.81%.
Free complex was synthesized by addition of a solution of dtta
(76 mg, 0.34 mmol) in 25 mL of ethanol to a solution of manga-
nese(II) chloride tetrahydrate (20 mg, 0.10 mmol) in the same sol-
vent. The reaction mixture was stirred and then triethylamine
(0.5 mL, 3.5 mmol) was added. A 4 mL of hexane was added to
the resulting brown solution and brown-green crystals started to
precipitate after 10 h. The solid was filtered off, washed with
diethyl ether and hexane, and then dried under vacuum to obtain
a yield of 74% (0.04 g). Anal. Calc. for MnClC28H28N6: C, 62.40; H,
5.24; N, 15.59. Found: C, 62.27; H, 5.18; N, 16.02%. FTIR (m/
cmÀ1): 1525 [
m(N@N)]. UV/Vis (kmax/nm): 290 and 360. HRMS
(NBA): m/z = 538.14228 [M]+; Calc. for MnClC28H28N6 538.14389.
2.3. Methods for the preparation of the heterogeneous catalysts
2.3.1. Method A
A solution of dtta ligand (77 mg, 0.34 mmol) and manganese(II)
chloride tetrahydrate, (20 mg 0.10 mmol) in 25 mL of ethanol was
added to NaY zeolite suspension (1.0 g in 25 mL of ethanol). A vol-
ume of 0.5 mL (3.58 mmol) triethylamine was added to the mix-
ture with stirring for 12 h at room temperature. The solid
fraction was filtered off. Uncomplexed ligand and the complex
molecules adsorbed on the external surface were removed through
Soxhlet extraction with ethanol until the eluate became colourless.

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593
2.5. Catalytic experiments
ido ligand is coordinated in chelating form. Elemental analysis and
spectroscopic results are in agreement with the assumed structure,
where presented two chelates dtta ligands and one chloride atom.
NaY zeolite was used as a host in different encapsulation meth-
ods in order to obtain stable heterogeneous catalysts for oxidation
reactions (Scheme 2).
In method A, the manganese ion and dtta ligand were added
simultaneously to zeolite suspension in ethanolic medium. In
method B, the in situ encapsulation was achieved by the ion ex-
change of the manganese ion in aqueous medium followed by
the insertion of the dtta ligand in the framework of the zeolite
upon its complexation.
2.5.1. Oxidation of styrene
Epoxidation of styrene was studied under an argon atmosphere
with constant stirring at room temperature. Briefly, 0.10 g
(0.1 mmol) of styrene, 0.10 g (0.1 mmol) of chlorobenzene (inter-
nal standard) and 0.10 g of heterogeneous catalyst were mixed in
5.0 mL of acetonitrile; tBuOOH (0.3 mL of 5.5 M in decane solution,
oxygen source) was progressively added to the reaction at a rate of
0.05 mL minÀ1. During the experiments, 0.1 mL aliquots were ta-
ken from solution with a hypodermic syringe, filtered through
0.2 lm syringe filters and directly analysed by GC. After the reac-
tion cycle, the catalysts were washed, dried and characterized.
The heterogeneous catalysts were characterized by infrared
spectroscopy (FTIR), powder X-ray diffraction (XRD), chemical
analyses and X-ray photoelectron spectroscopy (XPS).
2.5.2. Oxidation of cyclohexanol
The FTIR spectra obtained for NaY and the catalysts are illus-
trated in Fig. 1. The spectrum of NaY zeolite is characterized by a
very intense broad band at ca. 3450 cmÀ1 with a poorly resolved
shoulder at ca. 3600 cmÀ1 which can be attributed to the hydroxyl
groups in the supercages and in the sodalite cages respectively
[40,41]. In the low energy region the spectrum showed a band at
1640 cmÀ1 characteristic of d(H2O) mode of absorbed water [42].
The band at ca. 1020 cmÀ1 is usually attributed to the asymmetric
stretching of Al–O–Si chain of zeolite. The symmetric stretching
and bending frequency bands of Al–O–Si framework of zeolite ap-
pear at ca. 727 and 513 cmÀ1, respectively [43].
The FTIR spectra of Mn(dtta)-YA and Mn(dtta)-YB are dominated
by the strong bands attributed to the zeolite structure. No shift or
broadening of the zeolite vibration bands is observed upon encap-
sulation of the complex. This provides further evidence that the
zeolite structure remains unchanged after the encapsulation
methods.
The reaction was carried out in 4.0 mL of acetonitrile at 40 °C
( 5 °C) under an argon atmosphere with constant stirring, and
the composition of the reaction medium was cyclohexanol
(0.6 mL, 5.8 mmol), chlorobenzene as internal standard (0.6 mL,
5.9 mmol) and 0.10 g of heterogeneous catalyst. The oxidant,
tBuOOH (2.0 mL of 5.5 M in decane solution), was progressively
added to the reaction medium at a rate of 0.1 mL minÀ1 using a
KD Scientific Syringe Pump: KDS 200P. The reaction products were
analysed and identified as mentioned above.
At the end of each run, the heterogeneous catalysts were
sequentially extracted/centrifuged three times with 10.0 mL of
ethanol and two times with 10.0 mL of acetonitrile, and then dried
in an oven at 100 °C overnight before a new catalytic cycle was ini-
tiated and characterized by FTIR.
3. Results and discussion
In addition to these strong bands originating from the zeolite,
the FTIR spectra of the catalysts show the bands in the 1600–
1200 cmÀ1 region where NaY does not absorb and they are attrib-
uted to the presence of the complex. These bands have frequencies
3.1. Synthesis and characterization
The free complex synthesized in this study by reaction of man-
ganese chloride with bis(p-tolyl)triazene (dtta) in ethanol is de-
scribed in Scheme 1.
Manganese chloride tetrahydrate reacts with 1,3-bis(p-
tolyl)triazene in the presence of triethylamine, in refluxing ethanol,
to obtain the brown air-stable complex [MnCl(1,3-bis(tolyl)tria-
zenido)2], [MnCl(dtta)2]. The infrared spectrum shows the band
of absorptions expected for coordinated triazenido ligand. In par-
that are close to those of the free complex: 1473 and 1400 cmÀ1
.
Also, in the high energy region, Mn(dtta)-YA and Mn(dtta)-YB cata-
lysts show bands unequivocally assigned to the encapsulated com-
plex at 2690, 2740, 2800 and 2995 cmÀ1. The presence of these
bands at frequencies different from those of the free complex, sug-
gests that the manganese complex has a different coordination
sphere. This may be a consequence of the physical constraints im-
posed by the host and due to the host–guest interactions with NaY
framework as observed for copper(II) complexes entrapped in NaY
[1,23] and NaBEA zeolite [44].
Preservation of NaY zeolite structure of the catalysts was mon-
itored by X-ray powder diffraction. XRD patterns of NaY, and
Mn(dtta)-YA and Mn(dtta)-YB catalysts were recorded at 2h values
between 5° and 65° (Fig. 2).
ticular the
3198 cmÀ1 is absent in the complex spectrum and the vibration
band attributed to (N@N) is shifted from 1612 (free ligand) to
1525 cmÀ1 (complex). This shift observed has been attributed to
m(N–H) vibration band of the free triazene at
m
p
-backdonation due to p-bonding between the d orbitals of the
metal and p⁄ orbital of triazenido group, which leads to a variation
in N@N bond order [39]. These observations show that the triazen-
All catalysts exhibit the typical and similar pattern of highly
crystalline Y zeolite. XRD patterns of Mn(dtta)-YA and Mn(dtta)-
YB present over 80% of crystallinity. A clear change occurred in
the relative peak intensities of 331, 311 and 220 upon introducing
the Mn{bis(p-tolyl)triazenido} complex. In the case of NaY, the or-
der of peak intensity was found: I331 > I220 > I311, while in both het-
erogeneous catalysts, the order of peak intensity became
I331 > I311 > I220. This can be attributed to a redistribution of intra
zeolite charge balancing cations [45]. These observations indicate
that under encapsulation conditions, the synthesis of the complex
by different methods has a minor impact on the crystallinity of the
zeolite host [46].
H3C
CH3
CH3
Cl
NH
N
N
N
EtOH/rf.
Et3N
N
MnCl2.4H2O
+
N
Mn
N
N
N
Chemical analyses of the catalyst samples were carried out and
summarized in Table 1.
H3C
CH3
CH3
The Mn/N ratio was found to be 1.63 and 2.04, for Mn(dtta)-YA
and Mn(dtta)-YB, respectively. Both ratios are much higher than
Scheme 1. Synthesis of free complex [MnCl(1,3-bis(tolyl)triazenido)2].

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Mn2+
Mn2+
Mn2+
dtta ligand
Scheme 2. Schematic representation of the methods used to encapsulate Mn(dtta) complex in NaY.
(A)
(B)
1
1
2
3
4
2
3
4
4000 3500 3000 2500 2000 1500 1000 500
wavenumber/cm-1
1600 1400 1200 1000 800 600
wavenumber/cm-1
Fig. 1. FTIR spectra of free complex (1), catalysts with encapsulated complex, Mn(dtta)-YA (2) and Mn(dtta)-YB (3), in the ranges: (A) 4000–500 cmÀ1 and (B) 1850–500 cmÀ1
;
in both spectra, correspond to the FTIR spectrum of NaY (4).
stoichiometry. However, for both catalysts, the presence of non-
coordinated manganese species was also observed.
X-ray photoelectron spectroscopy experiments were performed
in Mn(dtta)-YB catalyst. Low resolution XPS spectrum shows the
presence of oxygen, silicon, aluminium and sodium from NaY zeo-
lite. However, the high resolution XPS spectrum shows bands in Si
2p region at 102.88 eV, in Al 2p region at 74.72 eV and in O 1s re-
gion at 532.56 eV corresponding respectively to silicon (typical for
Si atoms with different chemical environments, such as SiO4 and
terminal Si–OH groups), aluminium (from the tetrahedral AlO4
groups), oxygen from zeolite framework and a band centred at
1071.0 eV in the Na 1s region corresponding to sodium located in
the zeolite matrix [47]. Apart from these features, a single peak
is observed in N 1s region at 400.5 eV. This peak position indicates
that nitrogen atoms (from the ligand) are coordinated to the metal
centre involving sp2 N donors [48].
(c)
(b)
(a)
5
15
25
35
2 theta
45
55
65
Fig. 2. Powder XRD patterns of: (a) NaY, (b) Mn(dtta)-YA and (c) Mn(dtta)-YB.
In Mn 2p region the high resolution XPS spectrum exhibits dif-
ferent peaks: two main peaks centred at 642.40 and 646.20 eV in
the Mn 2p3/2, and 654.13 and 658.07 eV in the Mn 2p1/2. These val-
ues are related to two different Mn species presence in the zeolite
in agreement with chemical analysis. Moreover, elemental sulfur
was also detected (BE = 169.5 eV), suggesting that the SO4 ions
are adsorbed on the surface of the zeolite as the values were close
to the detection limit for elemental analysis, i.e. less than 0.01%.
the theoretical Mn/N ratio, determined for the free complex syn-
thesized (Mn:dtta:Cl is 1:2:1). On the other hand, based on theo-
retical values of C (48.03%), H (4.30%), N (12.00%) and Mn
(15.69%) for [MnCl2(dtta)] molecule (Mn:dtta:Cl is 1:1:2), the the-
oretical Mn/N ratio is 1.31, suggesting that the confined Mn com-
2À
plex in zeolite forms
a Mn/ligand complex with a 1:1

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595
Table 1
Chemical composition of free manganese complex and catalysts obtained.
Sample
C
H
N
Mna
Mn/N
[MnCl(dtta)2]
Mn-Y
Mn(dtta)-YA
Mn(dtta)-YB
62.27(62.34)b
5.18(5.24)b
ND
ND
ND
16.02(15.59)b
ND
0.22
0.54
NDc(10.19)b
1.20 (218.42)
0.36 (65.53)
1.10 (200.22)
–(0.65)b
–
1.63
2.04
NDc
0.87
2.23
a
b
c
Obtained from ICP, values in parenthesis total manganese loading in
Theoretical values.
ND, not determined.
l
mol/g (loading = %ICP/(MMn[ mol] Â 100%)).
l
The combined results from all the techniques used show that
Table 3
Oxidation of cyclohexanol catalysed by heterogenized Mn{(1,3-bis(p-tolyl)triazeni-
do)} complex in the presence of tBuOOH.
the manganese complex encapsulated in zeolite supercages by dif-
ferent methods has a different sphere of coordination from the free
complex. These results indicate that manganese is coordinated by
one bidentate dtta ligand and the other coordination sites are occu-
pied by water molecule and zeolite framework oxygen [5,22–
26,31–33].
Substrate
Entry
Catalyst
Run
t (h)a
%Cb,c
Cyclohexanone
%Sb,d
%
gb,e
7
8
9
10
11
NaY
Mn-Y
Mn(dtta)-YA
1
1
1
2
1
48
48
2
2
6
3
6
44
4
100
100
100
100
–
3
6
44
4
OH
Mn(dtta)-YB
1
–
3.2. Catalytic activity
a
b
c
Reaction time at which the substrate conversion start become constant.
Determined by GC against internal standard.
Cyclohexanol conversion (%C) calculated as %C = {[A(cyclohexanol)/A(chloro-
The catalytic activity of the heterogeneous catalysts was evalu-
ated by oxidation of styrene and cyclohexanol using acetonitrile as
solvent in the presence of tBuOOH (oxygen source). The blank
experiments were also carried out under the same conditions.
The catalytic behaviour of the homogeneous phase was not per-
formed (using free complex as catalysts). As a result, the metal
coordination mode in the free complex is different from that of
the encapsulated catalyst and this makes it difficult to compare di-
rectly the catalytic activities in homogeneous and heterogeneous
phases.
Table 2 summarizes the catalytic results for the oxidation of
styrene (under optimized conditions). NaY and Mn-Y catalysts (en-
tries 1 and 2) are almost inactive and the benzaldehyde is the ma-
jor oxidation product. The high benzaldehyde selectivity observed
suggests that the acidic nature of zeolite matrix plays a role and
leads to epoxide ring opening.
It is clear that the catalytic activities improved in the presence
of the encapsulated Mn complex (entries 3 and 5) prepared by
two different encapsulation methods. As expected, when the coor-
dination mode changes from oxygen (Mn-Y) to the nitrogen atoms
from dtta ligand (Mn(dtta)-YA/B) the catalytic activities of the
encapsulated complex are enhanced. This may be due to electronic
changes induced in the metal centre upon dtta complexation. The
enhanced activity of encapsulated catalysts may also be due to the
synergy between the catalytic behaviour of the metal complexes
and the zeolite host.
benzene)]t
h  [A(cyclohexanol)/A(chlorobenzene)]t
h} Â 100/[A(cyclohexa-
=
0
= x
nol)/A(chlorobenzene)]t
.
0 h
=
d
Product selectivity (%S) calculated as %S = A(product) Â 100/[A(product) +
P
A(other reaction products)].
e
Product yield (%g) calculated as %g = %C Â %S/100; where A stands for chro-
matographic peak area.
3) and Mn(dtta)-YB (entry 5), respectively. Both catalysts lead to
similar styrene conversion. The conversion decreases slightly in
the second reaction cycle for both catalysts (entries 4 and 6).
Mn(dtta)-YA, (entry 4), shows the highest styrene oxide selectivity
in the first catalytic cycle. Nevertheless, a substantial decrease sty-
rene oxide yield is observed for Mn(dtta)-YA. This may be due to
some catalyst deactivation and/or manganese complex leaching
into reaction media during the first run. However, for Mn(dtta)-
YB (entry 6), both catalytic cycles present the same oxidation prod-
uct. The decrease in the substrate conversion, usually observed
upon immobilization of molecular catalysts into porous supports,
may be a consequence of various factors including diffusion
restrictions of the substrate through the porous structure of the
support and the electronic changes induced in the metal centre
through the chemical modifications of the ligands/metal centre
as a consequence of the immobilization procedures used [49,50].
However, the catalysts show different activity in cyclohexanol
oxidation. The catalytic results obtained for cyclohexanol oxida-
tion, under optimized conditions, are summarized in Table 3.
The increase in styrene conversion resulted from the presence
of the Mn complex by a factor of 9.8 and 8.0 for Mn(dtta)-YA (entry
Table 2
Oxidation of styrene catalysed by heterogenized Mn{(1,3-bis(p-tolyl)triazenido)} complex in the presence of tBuOOH.
Substrate
Entry
Catalyst
Run
t (h)a
%Cb,c
Styrene oxide
Benzaldehyde
%Sb,d
%
gb,e
%Sb,d
%
gb,e
1
2
3
4
5
6
NaY
Mn-Y
Mn(dtta)-YA
1
1
1
2
1
2
48
48
48
48
48
48
4
9
39
36
32
29
0
0
45
14
38
38
0
0
18
5
12
11
100
100
55
86
62
4
9
21
31
20
18
Mn(dtta)-YB
62
a
b
c
Reaction time at which the substrate conversion start become constant.
Determined by GC against internal standard.
Styrene conversion (%C) calculated as %C = {[A(styrene)/A(chlorobenzene)]t
h  [A(styrene)/A(chlorobenzene)]t
h} Â 100/[A(styrene)/A(chlorobenzene)]t
.
0 h
=
0
=
x
=
P
d
e
Product selectivity (%S) calculated as %S = A(product) Â 100/[A(product) + A(other reaction products)].
Product yield (%
g
) calculated as %
g
= %C Â %S/100; where A stands for chromatographic peak area.

´
596
I. Kuzniarska-Biernacka et al. / Inorganica Chimica Acta 394 (2013) 591–597
(B)
1
(A)
1
2
3
2
3
4
4
5
6
5
6
4000 3500 3000 2500 2000 1500 1000 500
wavenumber /cm-1
1600 1400 1200 1000 800 600
wavenumber /cm-1
Fig. 3. FTIR spectra of the catalysts (1) Mn(dtta)-YA – as prepared, (2) entry 4, (3) entry 9, (4) Mn(dtta)-YB – as prepared, (5) entry 5 and (6) entry 11; for abbreviation see
Tables 2 and 3 in the ranges (A) 4000–500 cmÀ1 and (B) 1850–500 cmÀ1
.
NaY catalyst presents low catalytic activity (entry 7). Mn-Y cat-
alyst, NaY loaded with manganese(II) sulphate (entry 8), leads to
low cyclohexanol conversion. Mn(dtta)-YA catalyst exhibit cata-
lytic activity in the 1st cycle (entry 9) but during their re-use a
drastic reduction (40% lower than initial) of cyclohexanol conver-
sion and cyclohexanone yield was observed at the end of the 2nd
cycle (entry 10). Mn(dtta)-YB catalyst is almost inactive and leads
to negligible substrate conversion (entry 11). This experiment
was duplicated and the results were found to be reproducible with
a maximum error of ca. 1% relatively to the substrate conversion
average. As a consequence of the obtained %C values, Mn(dtta)-
YA catalyst was re-used only once, whereas Mn(dtta)-YB was not
recycled. The low conversion of cyclohexanol, when Mn(dtta)-YB
was used as catalyst, could be related with the sulphate species ad-
sorbed in external surface. These species restrict the access of the
substrate and the tBuOOH to active Mn(III)–dtta complex. This re-
sult is supported by XPS of Mn(dtta)-YB where the presence of sul-
phate species is clearly showed. The sulphate species can interact
with hydroxyl group from cyclohexanol. This fact is reported in lit-
erature where similar behaviour was observed for alcohols [51].
In order to get more insights into the recycled catalysts, FTIR
spectra were obtained and depicted in Fig. 3. After catalytic oxida-
tion of cyclohexanol in the presence of Mn(dtta)-YA some band
broadening is observed in the frequency range where the vibration
bands for the complex occur (Fig. 3 B).
entrapment of manganese complexes without changes in the zeo-
lite framework. Analysis of the data shows that the Mn ion is coor-
dinated by the nitrogen atoms of the bis(p-tolyl)triazenido ligand
in the host structure with a stoichiometry of 1:1. The other sites
in the metal centre are supplied by the zeolite and water molecule
inside NaY. Both heterogeneous catalysts showed similar styrene
conversion and moderate styrene oxide selectivity. Mn(dtta)-YB
can be reused almost without a decrease in its catalytic activity.
In contrast, the Mn(dtta)-YB catalyst was not active in oxidation
of cyclohexanol.
Acknowledgments
I.K.-B. thanks FCT for her contract within the program Ciência
2007. Dr. C.S. Rodriguez (C.A.C.T.I., Vigo University, Spain) is grate-
fully acknowledged for performing and interpreting the XPS anal-
yses. We thank Dr. A.S. Azevedo (Departamento de Ciências da
Terra, Minho University, Portugal) for collecting the powder dif-
fraction data. This work was supported by the Centro de Química
(University of Minho, Portugal), through the Project No. F-COMP-
01-0124-FEDER-022716 (refª FCT PEst-C/QUI/UI0686/2011), FED-
ER-COMPETE, Fundação para a Ciência e Tecnologia (FCT-Portugal).
The NMR spectrometer Bruker Avance III 400 is part of the National
NMR Network and was purchased within the framework of the Na-
tional Program for Scientific Re-equipment, contract REDE/1517/
RMN/2005 with funds from POCI 2010 (FEDER) and FCT.
The results indicate that some complex leaches into the reac-
tion medium and/or some degradation of the complex under cata-
lytic reaction conditions occur. This might explain the substantial
reduction in cyclohexanone yield in the second reaction cycle (en-
try 10). For both heterogeneous catalysts studied, FTIR spectra in
the range typical for the zeolite matrix do not show significant
changes after the catalytic reaction. This suggests that no struc-
tural changes took place to the host during the catalytic reaction.
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