Activated carbon functionalized with Mn(II) Schiff base complexes as efficient alkene oxidation catalysts: Solid support matters

Article abstract of DOI:10.1016/j.molcata.2014.05.038

A new synthetic methodology to covalently anchor Mn^II-Schiff- base catalysts onto activated carbon (ACox) has been applied resulting in heterogeneous Mn^II-L@ACox materials. These catalysts are effective and selective towards epoxides with H₂O₂, in the presence of CH₃COONH₄, as co-catalyst, providing TONs equivalent-to/or higher than the homologous Mn^II-L@SiO₂ catalysts for certain substrates. Moreover Mn^II-L@ACox catalysts are re-usable and kinetically faster than the corresponding Mn^II- L@SiO₂ catalysts resulting in considerably higher TOFs. Combining catalytic and EPR spectroscopic data we propose a catalytic reaction mechanism which elucidates the co-catalytic function of CH₃COONH₄ which is of key importance for the successful performance of the studied Mn ^II-catalysts.

Full text of DOI:10.1016/j.molcata.2014.05.038

Accepted Manuscript
Title: Activated Carbon Functionalized with Mn(II) Schiff
base Complexes as Efficient Alkene Oxidation Catalysts:
Solid Support Matters
Author: A. Mavrogiorgou M. Papastergiou Y. Deligiannakis
M. Louloudi
PII:
S1381-1169(14)00250-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.05.038
MOLCAA 9140
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
12-2-2014
20-5-2014
22-5-2014
Please cite this article as: A. Mavrogiorgou, M. Papastergiou, Y. Deligiannakis, M.
Louloudi, Activated Carbon Functionalized with Mn(II) Schiff base Complexes as
Efficient Alkene Oxidation Catalysts: Solid Support Matters, Journal of Molecular
Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.05.038
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Activated Carbon Functionalized with Mn(II) Schiff base
Complexes as Efficient Alkene Oxidation Catalysts: Solid
Support Matters
A. Mavrogiorgoua, M. Papastergioua,
Y. Deligiannakisb and M. Louloudia*
a Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
b University of Patras, Department of Environmental and Natural Resources
Management, Laboratory of Physical Chemistry, Pyllinis 9, 30100 Agrinio, Greece.
e-mail: mlouloud@uoi.gr
Page 1 of 33

Abstract
A new synthetic methodology to covalently anchor MnII-Shiff-base catalysts onto
activated carbon (ACox) has been applied, resulting in heterogeneous MnII-L@ACox
materials. These catalysts are effective and selective towards epoxides with H2O2, in
the presence of CH3COONH4, as co-catalyst, providing TONs equivalent-to/or higher
than the homologous MnII-L@SiO2 catalysts for certain substrates. Moreover MnII-
L@ACox catalysts are re-usable and kinetically faster than the corresponding MnII-
L@SiO2 catalysts resulting in considerably higher TOFs. Combining catalytic and
EPR spectroscopic data we propose a catalytic reaction mechanism which elucidates
the co-catalytic function of CH3COONH4 which is of key importance for the
successful performance of the studied MnII-catalysts.
Keywords: catalytic epoxidation; supported complexes; manganese complexes;
H2O2 activation; activated carbon, D-strain.
Page 2 of 33

1. Introduction
Among the desirable characteristics of a catalyst-support are stability,
inertness, reusability, high surface area, porosity and appropriate chemical structure.
Among a wide range of support-materials, so far, materials that appear to meet
optimally these characteristics optimally are Al2O3, SiO2 and carbon, mainly activated
carbon (AC) [1]. The potential of activated carbon as catalyst support is based on its
low-cost and surface chemical properties, which can be tailored appropriately. These
features have motivated researchers to immobilize, on AC surface, acetylacetonate-
[2-5], Schiff base- [6-8] and salen- [9-11] metal complexes for oxidation catalysis.
So far, heterogenisation of these complexes has been realized via their
adsorption or covalent binding on AC surface [2-11]. For covalent attachment of
metal complexes, to avoid leaching during catalysis, the oxygen functionalities of AC
surface allow application of a range of synthetic strategies: [i] direct attachment of
the catalyst either via binding of functional groups derived from the periphery of the
ligand [7], or via axial coordination of a metal centre onto phenolate groups of the
modified activated carbon [8]. The most applicable method involves the use of
bifunctional coupling agents which are able to bridge active groups of both the
modified/activated carbon surface and the metal complexes. In this context, diamines
[2-5,11] and cyanuric chloride [6,11] have been used as bifunctional binding agents.
[ii] in the case of silica based supports, heterogenisation of complexes often is done
via reaction of –OH surface silanol groups of SiO2 with bifunctional organosilanes
e.g. to form siloxane bonds. Recently, a similar procedure has been successfully
applied for the heterogenisation of a Rh-catalyst on AC with aminosilane as coupling
agent [12,13]. This heterogeneous catalyst has been tested for hydrogenation of
cyclohexene. Thus, so far aminopropyl-triethoxy silane (APTES) offers a tool to
functionalize AC in order to covalently attach an oxidation catalyst on carbon
matrices [11]. The silylation reaction of APTES with multi-walled carbon nanotubes
has been investigated recently confirming the binding of alkoxy-moieties to surfacial
–OH groups [14]. However, NH2-groups could also react with carbonyl surface
groups of the support resulting in a NH2-silicon side-polymerization which finally
reduces free NH2-groups desirable for a future catalyst attachment [14]. In the present
work, to avoid such drawbacks, we report a novel methodology to modify the AC
surface, synthesizing functional <ligand-organosilanes> ensuring their attachment
Page 3 of 33

only via their alkoxy-groups with surface –OH of AC. The, so obtained, carbon-based
heterogeneous catalysts have been evaluated as catalysts for alkene epoxidation.
In our previous works, we have exploited the surface properties of silica for
covalent immobilization of Mn(II)-Shiff base ligands for alkene epoxidation with
H2O2 (Scheme 1) [15, 16]. In general, the development of efficient heterogenised
manganese catalysts that incorporate H2O2 has limitations due to competitive H2O2
dismutation by both the Mn-center (catalase-type-activity) and the SiO2. The catalytic
efficiency of the heterogenised catalysts reported in [15, 16] was shown to be
switched-on by CH3COONH4 achieving remarkable effectiveness and selectivity
towards epoxides providing 150-900 TONs and 10 h-1-50 h-1 TOFs. Nevertheless,
these catalysts were not recyclable [16].
Insert Scheme 1
In the literature there are very few examples of Mn-based oxidation catalysts
immobilized on activated carbon; their anchoring has been achieved either via
adsorption [9,10,17], axial coordination to support [8, 18] or covalent attachment [11].
They have been evaluated for alkene oxidation using PhIO and NaOCl as oxidants,
nevertheless, with rather poor efficiency, i.e. 0.1-47 TONs and 0.1 h-1-11 h-1 TOFs. In
the present work, we show that covalently anchored Mn(II)-Shiff base catalysts onto
AC by the novel method described herein, achieve significant TONs and TOFs, and
are recyclable.
The main findings of the present work are: (i) the AC-anchored Mn-catalysts
are effective and selective towards epoxides, and, moreover (ii) they are re-usable and
kinetically faster than the corresponding SiO2-based catalysts which may be a
considerable advantage in using such catalysts. Electron Paramagnetic Resonance
[EPR] has been also used to study the coordination environment of the heterogeneous
Mn-catalysts and the interaction between the active catalyst centre and the activated
carbon support.
Page 4 of 33

2. Experimental
All substrates were purchased from Aldrich, in their highest commercial purity, stored
at 5 C and purified by passage through a column of basic alumina with methanol as
the eluting solvent, prior to use. Hydrogen peroxide was 30% aqueous solution.
Elemental analyses (C, H, N) were obtained using a Perkin Elmer Series II 2400
elemental analyzer. The manganese amount was determined by flame atomic
absorption spectroscopy on a Perkin-Elmer AAS-700 spectrometer. Infrared spectra
were recorded on a Spectrum GX Perkin-Elmer FT-IR System. Continuous-wave
(c.w.) Electron Paramagnetic Resonance (EPR) spectra were recorded with a Brucker
ER200D spectrometer at liquid N2 temperature, equipped with an Agilent 5310A
frequency counter. The spectrometer was running under a home-made software based
on LabView. Thermogravimetric analyses were carried out by Shimadzu DTG-60
analyser using a heat rate of 5 K min-1 and a flow rate of synthetic air carrier gas of 50
cm3 min-1. GC analysis was performed using an 8000 Fisons chromatograph with a
flame ionization detector and a Shimadzu GC-17A gas chromatograph coupled with a
GCMS-QP5000 mass spectrometer. The N2 adsorption–desorption isotherms were
measured at 77 K on a Sorptomatic 1990, thermo Finnigan porosimeter. Specific
surface areas were determined with the Brunauer–Emmett–Teller (BET) method
using adsorption data points in the relative pressure (P/Po) range of 0.05–0.30 and
assuming a closely packed BET monolayer. H2O2 was added by a digitally controlled
syringe pump type SP101IZ WPI over 1 h under stirring. Solution potential Eh was
measured by a Metrohm platinum redox electrode (type 6.0401.100).
Ligand Synthesis. Ligands L1 and L2 have been synthesized and characterized
by the methods detailed in our recent works [15,19].
Oxidation of activated carbon. 5 gr of activated carbon (AC) was refluxed with
90 ml 65% v/v HNO3 for 6h. The oxidized carbon (ACox) obtained was separated by
filtration, washed exhaustively with distilled water until pH~7 and dried under
reduced pressure at 40 C for 12 h. DRIFTS-IR (cm-1, selected peaks): 2955, 2869:
(C-H); 1708: (C=O); 1586: (C=C); 1251, 1214: (C-O). Thermal analysis: the
derivative thermogram of ACox shows a broad peak in the range of 130-420 °C
assigned to the decomposition of –COOH surface groups with a 10.2 % weight loss
Page 5 of 33

and an intense peak at 600 °C assigned to the combustion of carbon support. The
observed weight loss of ACox sample in the temperature range of 20-700 °C was
100%. The formed –COOH groups are ca. 2.2 mmol g-1 determined by
thermogravimetric and elemental analysis. The ACox had an average surface area of
ca. 687 m2 g-1, listed in Table 1.
Synthesis of Organosilane-ligands. (a) synthesis of organo-silanes L1-OS and
L2-OS. 1.2 mmol of L1 or L2 and 1.0 mmol of (3-glycidyloxypropyl)-
trimethoxysilane or (3-chloropropyl)-trimethoxysilane respectively were stirred for 48
h in 10 ml of MeOH under N2 at 60 C in order to generate L1-OS and L2-OS silane-
precursors. (b) formation of modified carbon materials. The L1@ACox and
L2@ACox materials were prepared through co-condensation of L1-OS or L2-OS
precursors with the –OH functional groups of oxidised carbon ACox. Accordingly,
0.5 g oxidised carbon ACox and 5 ml EtOH were added to the precursor L1-OS or
L2-OS solution, and the resulting slurry was stirred at 60 C for 24 h. The obtained
solids, L1@ACox and L2@ACox, were washed several times with MeOH and EtOH
and dried under vacuum at 60 C for 12 h.
L1@ACox: DRIFTS-IR (cm-1, selected peaks): 2943, 2866: (C-H); 1686:
(C=O); 1660: (C=N); 1587, 1495: (C=C); 1209: (C-O). Thermal analysis
provides a weight loss of 14.5 %, see Table 1, assigned to the decomposition of the
anchored organic ligand L1. This material showed an average surface area of ca. 45
m2 g-1, see Table 1.
L2@ACox: DRIFTS-IR (cm-1, selected peaks): 2962, 2873: (C-H); 1690:
(C=O); 1663: (C=N); 1587: (C=C); 1215: (C-O). Thermal analysis shows a
weight loss of 20.3 % corresponds to the decomposition of the anchored organic
ligand L2. This material had an average surface area of ca. 75 m2 g-1.
Anchoring of Mn-complexes. (a) organo-silanes metalation. The L1-OS and
L2-OS silane-precursors were formed according to the procedure described in the
previous paragraph. Then after cooling at room temeperature (260C), 1.2 mmol of
MnCl2.4H2O in 10 ml MeOH, were added to this solution. The obtained solution was
stirred overnight at room temperature leading to metalated precursors MnII-L1-OS
and MnII-L2-OS. (b) formation of carbon-based materials. The MnII-L1@ACox and
MnII-L2@ACox materials were prepared through co-condensation of MnII-L1-OS or
MnII-L2-OS precursors with the –OH functional groups of oxidised carbon ACox.
Page 6 of 33

For this, 0.5 g oxidised carbon ACox and 5 ml EtOH were added to the precursor
MnII-L1-OS or MnII-L2-OS solution, and the resulting slurry was stirred at 60 C for
24 h. The filtered solids, herein called MnII-L1@ACox and MnII-L2@ACox, were
washed several times with MeOH and EtOH and dried under vacuum at 60 C for 12
h.
MnII-L1@ACox: Metal loading: 0.30 mmol g-1. DRIFTS-IR (cm-1, selected peaks):
2941, 2871: (C-H); 1690: (C=O); 1665: (C=N);1587, 1493: (C=C);1214: (C-
O). Thermal analysis provides a weight loss of 18.5 %, see Table 1, assigned to the
decomposition of the anchored organic ligand L1. This material showed an average
surface area of ca. 2 m2 g-1, see Table 1.
MnII-L2@ACox: Metal loading: 0.40 mmol g-1. DRIFTS-IR (cm-1, selected
peaks): 2962, 2873: (C-H); 1693: (C=O); 1665: (C=N); 1587: (C=C);1215: (C-
O). Thermal analysis shows a weight loss of 20.4 % corresponds to the decomposition
of the anchored organic ligand L2. This material had an average surface area of ca.
21 m2 g-1. All parameters are listed in Table 1.
Catalytic Conditions. The catalytic experiments were performed in
acetone/MeOH (450 l /400 l) solvent mixture containing alkene (as substrate, 1
mmol), acetophenone or bromobenzene (as internal standard, 1 mmol), catalyst (1
μmol of Mn-complex) and CH3COONH4 additive (1 mmol). H2O2 (2 mmol) was
added by a digitally controlled syringe pump [SP101IZ WPI] over 30 min to the
catalytic solution. This corresponds to [catalyst : H2O2 : alkene : CH3COONH4] =
[1:2000:1000:1000 μmol]. 1 μmol of MnII-L1@ACox and MnII-L2@ACox weights
5 mg and 6 mg respectively based on the catalysts’ loading and their molecular
weights. The total volume of the catalytic reaction was 1 ml. Reactions were complete
within 5 h for MnII-L1@ACox and within 1.5 h for the MnII-L2@ACox catalyst.
The progress of the reaction was monitored by GC-MS, for 20μl samples periodically
taken from the reaction mixture. Quantitative analysis of the GC data was done by
comparing the integrals of the GC peaks vs. the internal standard integral, thus
providing the substrate conversion and product yield. When alcohol and ketone are
detected as allylic oxidation products, the sample was treated with PPh3 and measured
again by GC-MS according to Shul’pins’ procedure [20]. To establish the identity of
the epoxide product unequivocally, the retention time and spectral data were
compared to those of commercially available compounds. Blank experiments showed
Page 7 of 33

that without Mn-catalyst or CH3COONH4, oxidation reactions do not take place.
Control experiments were performed to substantiate that the observed catalytic results
are derived by the heterogeneous catalysts e.g. no leaching of the active component or
demetallation occurs during the catalytic processes. Thus, in typical catalytic
experiments, a) the solid catalysts were filtered after 1 h of reaction and b) the
catalytic reaction of the filtrate's solution was monitored by GC [16]. The data
showed that no evolution of the studied reactions was observed in the filtrate ensuring
that no leaching of the active supported Mn(II)-component occurs.
Recycling experiments. For recycling experiments, the heterogeneous catalysts were
tested with cyclohexene as substrate, for [catalyst:H2O2:cyclohexene:CH3COONH4] =
[1:2000:1000:1000 μmol], using the experimental conditions of the catalytic
experiments described above. After the end of each oxidation reaction, 5 h for
reactions catalysed by MnII-L1@ACox and 1.5 h for reactions by the MnII-
L2@ACox, the solid catalyst was separated from the reaction mixture by
centrifugation, washed three times with MeOH and dried under vacuum (40 oC, 12 h).
The mass of the recovered solid was estimated by mass-weighting. Importantly, we
underline that in reuse experiments, it has to be ensured that exactly the same mass of
solid is used in each cycle. In our experiments this was done for every material by
running 5 identical catalytic batches followed by washing/drying as detailed above.
Thus for the next use we could weigh exactly 5 or 6 mg of each material.
Subsequently the recovered solid catalyst was reused for a new oxidation reaction
under the same catalytic conditions.
Simulation of the EPR spectra: Numerical calculations of EPR spectra for 55Mn2+
(S=5/2, I=5/2) were performed assuming a Spin Hamiltonian has the form:
●
●
●
●
●
H =gβB S +S D S +I A S
[1]
where the first term is the Zeeman energy, the following two are the zfs (zero-field
splitting) interaction determined by D and E, the axial and rhombic zfs tensor
parameters respectively, the last term is the manganese hyperfine interaction. The six
main lines resolved in the EPR spectra centered at g=2.0, indicate that the zfs
(parametrized by D) of the MnII-complex is relatively small |D| « gβB0 (where the
resonant microwave quantum energy, hν, at X-band is ≈0.33 cm−1). The numerical
spectra were calculated using the EasySpin software version 4.5.5, running in a
Page 8 of 33

desktop computer [21]. After a systematic survey, we found that the spectral
lineshapes of our Mn-catalysts are determined by the value of the zero-field parameter
D in combination with D-strain. The notion of D-strain can be understood as follows:
small variations in metal–ligand coordination, bond-lengths/angles/strength may
result in a distribution of orbital energy levels, and in turn, a distribution in the zero-
field splittings, referred to as D-strain [22]. In our calculations D-strain was allowed
to span values between 20-200Gauss.
3. Results and discussion
Synthesis of the catalysts: The methodology applied here for anchoring of MnII-
L1 and MnII-L2 catalysts onto activated carbon uses silylation reaction of silane-
precursors, depicted in Scheme 2 (A and B). The key concept of the present method
is, first, a reaction of bifunctional, commercially available, alkoxy-silanes with our
ligang L1 or L2 producing a <L-silane> precursor which has the alkoxy-moiety
intact, and, second, silylation reaction of the so obtained <L-silane> with the surfacial
–OH groups of the activated carbon. This stepwise procedure avoids putative
undesirable side-reactions of bifunctional alkoxy-silanes with the activated carbon
surface groups.
Overall, the present synthetic procedure involves: (i) oxidation of the activated
carbon AC with concentrated HNO3; (ii) derivatization of the ligands L1 and L2 to
form <silane-L1>, <silane-L2> compounds; (iii) their metalation with MnII-ionic
salts; and (iv) anchoring of the metalated silane-precursors <silane-L-Mn> to the
oxidised activated carbon ACox.
Derivatization occurs via reaction of L1 and L2 with (3-glycidyloxypropyl)- or
(3-chloropropyl)-trimethoxysilane to generate L1-OS and L2-OS silane-precursors
respectively. Subsequently, their metalation results in MnII-L1-OS and MnII-L2-OS.
The propyl-trimethoxysilane moieties of MnII-L1-OS and MnII-L2-OS allows their
covalent attachment on the oxidize carbon ACox surface via hydrolysis and co-
condensation with the –OH functionalities of the ACox.
Insert Scheme 2
Alternatively, if the metalation step is detoured, the modified materials
L1@ACox and L2@ACox are obtained which bear the ligands L1 and L2 covalently
attached onto the ACox. However, we found that, that metalation of L1@ACox and
L2@ACox materials with MnII-compounds results in totally inactive materials in
Page 9 of 33

catalytic epoxidations. This is attributed to MnII-spoliation by the free –COOH groups
of the ACox blocking the desirable MnII-complexation with the ligands L1 and L2 in
order to form the active catalytic centre.
The DRIFTS-IR spectra of the MnII-L1@ACox and MnII-L2@ACox show
several bands corresponding to various structural units of the solid support as well as
from the ligands L1 and L2. More specifically, the bands at 1690 and 1693 cm-1 are
attributed to (C=O) stretching vibrations while the bands observed for both samples,
at ~1665 cm-1 are attributed to the (C=N) stretching of the organic ligands. These
vibrations are clearly shifted when compared with the non-metalated materials
L1@ACox and L2@ACox indicating coordination of the MnII with the imine-
nitrogen and keto-oxygen of the attached ligands L1 and L2 [15,16].
The derivative thermogram for MnII-L1@ACox shows a 28.7 % weight loss, in
the range 130-400 °C, which corresponds to a loss of anchored organic moieties and
surface oxidised functionalities. Taking into account that the parent ACox material
bears 10.2 % [w:w] –COOH surface groups which are decomposed in the same
temperature region, the additional weight loss of 18.5% is assigned to the
decomposition of the anchored organic ligand L1. The ligand-loading achived is ca.
0.30 mmol g-1, listed in Table 1. The thermogram exhibits also an intense peak at 460
°C assigned to the combustion of carbon support which is shifted to lower
temperature for the modified material MnII-L1@ACox. The total weight loss of
MnII-L1@ACox observed in the temperature range of 20-700 °C where all the
carbon has been combusted was 95.8%. The obtained solid residue of 4.2%
corresponds to SiO2 and MnO2 oxides, identified by IR spectroscopy with comparison
to reference SiO2 and MnO2 samples. Since the Mn-loading determined by FAA
spectroscopy found to be 0.30 mmol g-1 (2.6%), it allows us to calculate Si-loading of
0.27 mmol g-1.
The derivative thermogram for MnII-L2@ACox exhibits a 30.6% weight loss in
the range of 130-400 °C, which is assigned to a loss of anchored organic moieties and
surface oxidised functionalities. Since the parent ACox material bears 10.2 % [w:w] –
COOH surface groups, which are decomposed in the range of 130-400 °C, the
difference weight-loss of 20.4 % corresponds to the decomposition of the anchored
organic ligand L2 resulting in ligand-loading of 0.45 mmol g-1, see Table 1. The
thermogram shows also an intense peak at 450 °C that resembles that of the parent
Page 10 of 33

sample ACox, but with the maximum shifted to lower temperature for the modified
material MnII-L2@ACox; this peak is assigned to the combustion of the carbon
support. The observed weight loss of MnII-L2@ACox sample in the temperature
range 20-700 °C was 93.8 %. The solid residue, SiO2 and MnO2, corresponds to 6.2
%. Taking into account that Mn-loading determined by FAA spectroscopy found to be
0.40 mmol g-1 (3.5%), the Si-loading achived is ca. 0.45 mmol g-1. All these data are
summarised in Table 1.
Insert Table 1
Noticeably, after grafting, the resulting material shows a dramatic collapse of the
specific surface area, see BET values in Table 1. This shows that the MnII-L@ACox
hybrids should be conceptualised as compact blocks of compacted carbon layers with
practically zero porosity. Thus all the grafted Mn-L centers are localised on the
surface of the particles, since there are no pores available, thus making them readily
accessible to the catalytic substrate and the oxidant. As we argue in the following this,
characteristic might be correlated with the observed dramatic acceleration of the
catalytic performance of the MnII-L@ACox catalysts e.g. compared with the MnII-
L@SiO2 catalsysts .
Catalytic Epoxidation with H2O2: To evaluate the catalytic behaviour of the
prepared catalysts, oxidations of several alkenes have been carried out at room
temperature. As shown before for the parent homogeneous systems and the
corresponding silica-based heterogeneous systems [15-16,19], the present carbon-
based heterogeneous catalysts required also ammonium acetate as additive to generate
efficient catalytic system.
Catalytic Yield and TONs: According to Table 2, epoxidation of a wide range of
olefins proceeds with high conversion and selectivity for the epoxide product (the
mass balance is 98-100%) in most of the cases (Table 2). For example, oxidation of
cyclooctene provides a 100% selectivity for cis-cyclooctene epoxide with 61.0% and
52.0% yield and 100% m.b. catalysed by MnII-L1@ACox and MnII-L2@ACox
respectively (see Figure 1). Cyclohexene achieves moderate epoxide yields, ranging
from 45.6 to 56.0% catalysed by MnII-L1@ACox and MnII-L2@ACox respectively.
In the case of MnII-L1@ACox, allylic oxidation has also occurred in some extent,
Page 11 of 33

with the formation of 2-cyclohexene-1-ol and 2-cyclohexene-1-one. It is noticed that
the detected amounts of 2-cyclohexene-1-ol and 2-cyclohexene-1-one are identical to
that for PPh3-treated sample following the Shul’pins’ procedure [20]. This indicates
that if any hydroperoxide has been formed, as primary-product, this has been
completely transformed into the alcohol- and ketone-oxidation products [20, 23].
Hexene-1 is a rather hard oxidation substrate showing epoxide yields 7.6 and 8.5%
and selectivity 100% for the cis-epoxide.
Insert Table 2
Insert Figure 1
Styrene oxidation produces only epoxide (100% selectivity) with 42.4% and
47.6% yields. The catalysts showed analogous ability towards the oxidation of trans-
-methylstyrene (51.0 and 38.3% epoxide yield by MnII-L1@ACox and MnII-
L2@ACox respectively). The major products detected from oxidation of limonene,
were two epoxides (cis- and trans-) derived from epoxidation of the electron-rich
double bond in 1,2-position. Additionally, small amounts of products formed by
allylic oxidation of the limonene ring have been observed. These were identified as
the corresponding derivatives of cyclohexene-1-ol and cyclohexene-1-one. Epoxides
derived from the more accessible but less electron-rich double bond in 8,9-position
have not detected. The total yield of 1,2-epoxides was 76.7% and 64.9% catalyzed by
MnII-L1@ACox and MnII-L2@ACox respectively. The corresponding cyclohexene-
1-ol was detected from 12.9-14.5% and the corresponding cyclohexene-1-one 0.8% in
the case of MnII-L2@ACox. Thus, the epoxidation was clearly the main reaction path
resulting mainly in 1,2-epoxides (see Table 2, Figure 1). In the oxidation of cis-
stilbene catalysed by MnII-L1@ACox, the major product was cis-epoxide with
selectivity of 80% and yield 40.5%, while considerable amounts of trans-stilbene
epoxide (11.0%) has been also detected. Oxidation of cis-stilbene catalysed by MnII-
L1@ACox provides only cis-epoxide with 50.1 % yield and 100% selectivity.
When we compare the TONs achieved by the present carbon-based
heterogeneous catalysts MnII-L1@ACox and MnII-L2@ACox vs. the corresponding
silica-based heterogeneous catalysts MnII-L1@SiO2 and MnII-L2@SiO2 we notice
that MnII-L1@ACox achieves TON’s equivalent-to/or for certain substrates (e.g.
cyclohexene) outcompetes MnII-L1@SiO2 (Table 1 and Figure 2A). On the other
hand MnII-L2@ACox is equivalent-to/or outcompetes MnII-L2@SiO2 for certain
substrates (e.g. cis-stilbene, trans-β-methylestyrene) (Table 1 and Figure 2B).
Page 12 of 33

Insert Figure 2
Catalytic Kinetics, TOFs: The oxidation reactions catalysed by the carbon-based
catalysts were much faster than those catalysed by the corresponding silica-based
catalysts. Reactions catalysed by MnII-L1@ACox were practically accomplished
within 5 h and reactions catalysed by MnII-L2@ACox were accomplished within 1.5
h. For comparison, reactions catalysed by the corresponding silica-based MnII-
L1@SiO2 and MnII-L2@SiO2 catalysts were accomplished within 24 h and 18 h
respectively [15,16]. Taking into account these data, the carbon-based heterogeneous
catalysts show remarkably higher TOFs vs. the corresponding silica-based catalysts
(Figure 3A and B).
Insert Figure 3
Moreover, the present carbon-based heterogeneous catalysts show even higher TOFs
than the corresponding homogeneous systems: for homogeneous MnII-L1 catalyst,
TOFs ranged from 7.9 h-1-30 h-1 [15], while TOFs 15 h-1-182 h-1 (present data, see
Table 2) were achieved by MnII-L1@ACox; in a similar way, the homogeneous
MnII-L2 system showed TOFs from 54 h-1-257 h-1 [19], while MnII-L2@ACox
achieves TOFs 57 h-1-519 h-1 (Table 2). This clearly reveals that the activated carbon
support ACox has a beneficial influence on the anchored Mn-catalysts studied herein,
by decreasing dramatically the reaction time. We notice that -after grafting- the
resulting ACox shows a dramatic collapse of the specific surface area e.g. from 687
m2g-1 becomes 2 m2g-1 and 21 m2g-1 for MnII-L1@ACox and MnII-L2@ACox materials
respectively. Thus MnII-L@ACox hybrids consist of compacted carbon layers with
practically zero porocity (Scheme 3), which renders all the grafted Mn-L active
catalytic centers readily accessible to H2O2/CH3COONH4 and substrate. This
characteristic could be correlated with the observed dramatic acceleration of the
catalytic performance of the MnII-L@ACox catalysts.
Insert Scheme 3
Catalyst Recycling: The recyclability of the heterogeneous MnII-L1@ACox and
MnII-L2@ACox catalyst was examined by repeated recovering and reusing of the
solid
catalyst
for
the
oxidative
catalysis
of
cyclohexene.
A
[catalyst:H2O2:substrate]=[1:2000:1000] molar ratio was used in an acetone/MeOH
(450 l /400 l) solvent mixture. After each catalytic run, the catalyst was recovered
Page 13 of 33

by centrifugation, washed, dried and reused under the same experimental conditions -
as in Table 2. The catalytic data for MnII-L1@ACox and MnII-L2@ACox are
displayed in Figure 4.
Insert Figure 4
According to Figure 4, MnII-L1@ACox presented very good reusability i.e.,
<7% loss of the catalytic activity during the 2nd run and ~35% loss of the activity for
each of the following runs, that is, it worked with one third of its activity in a 4th
reuse. In total, MnII-L1@ACox provided more than 2000 TONs for cyclohexene
oxidation. MnII-L2@ACox presented analogous reusability and worked with 35% of
its activity in the 4th catalytic run. Thus MnII-L1@ACox and MnII-L2@ACox are
more stable than the corresponding silica-based MnII-L1@SiO2 and MnII-L2@SiO2
catalysts indicating that activated carbon support ACox protects the catalyst centers
against oxidative destruction.
Mechanistic Studies: [A] Redox: The time-course profiles of the MnII-
L1@ACox- and MnII-L2@ACox-catalysed oxidations of cyclohexene, in
conjunction with the observed redox potential of solution Eh (versus standard
hydrogen electrode SHE) are shown in Fig. 5. At the beginning of the reaction
catalysed by MnII-L1@ACox, Eh was +372 mV; after 1h, it dropped at +300 mV
with a 45.7% total oxidation yield and finally, after 5 h reaction time, it approached a
Eh=+279 mV with a 60.5% yield. In the case of MnII-L2@ACox-catalysed oxidation,
at t=0, Eh was +394 mV, then decreased to +273 mV (t=1 h), providing a product
yield of 49%, then Eh=+265 mV (t=1.5 h) with a 56% cyclohexene epoxide
formation. Similar time course profiles of the carbon-based catalysts have been
observed for all the substrates used herein.
Insert Figure 5
The present redox data show that during these catalytic reactions oxidative
equivalents are consumed in both systems. Interestingly, the kinetics of Eh evolution
correlate with the kinetics of the catalysis i.e. the MnII-L1@ACox system
accomplishes the catalytic reactions within 1.5 h while MnII-L2@ACox needs 5 h.
This shows that the same rate limiting steps are implicated in the redox events and in
catalysis.
[B] EPR spectroscopy: The EPR spectrum of ACox powder material bears radical
signals at g~2 whose linewidth (ΔΗ=8 Gauss and giso=2.0024) are characteristic of
Page 14 of 33

radicals (S=1/2) in amorphous carbon matrices with mixed sp2/sp3 hybridization [24-
26]. Spin counting using DPPH as spin standard [24-26] shows that the materials
contain 11020 µmoles of C-based radicals per gram of material. We underline the
acid-treated ACox powder was free of any EPR-detectable paramagnetic impurities
i.e. such as adventitious FeIII or MnII (data not shown).
Figure 6 shows 77K EPR spectra of MnII-L2@ACox material in 1:1 acetone:MeOH.
Spectrum (a) is the EPR for MnII-L2@ACox in the presence of 20µM H2O2
(Eh=360mV vs. SHE). The lineshape is typical for mononulcear Mn2+(S=5/2, I=5/2)
centres with well resolved allowed 55Mn(S=5/2, I=5/2) hyperfine splittings e.g. due to
the well understood mI dependence of the Mn55-splittings [27-29]. Noticeably the
MnII-L2@ACox material bears no-radical EPR signals. This shows that grafting of
the MnII-L2 complexes results in quenching of all surface radicals. This may be
correlated with the observed structural modification e.g. compacting of the carbon
matrix after grafting as evidenced by the collapse of the specific surface area, see
BET values in Table 1.
The EPR lineshape for the MnII-L2@ACox material is comparable to the EPR
spectrum analyzed previously by our group [19] for the homogeneous MnII-L2
complex as well as for the MnII-L2@SiO2 heterogeneous catalyst [16]. The A and D
values for MnII-L2@ACox derived by computer simulation are A=-94Gauss, D=161
Gauss material, are comparable with the those previously reported for the
homogeneous MnII-L2 complex [19] indicating that the grafting protocol that we
describe herein resulted in successful attachment of the Mn-L2 complex on the ACox
matrix.
Insert Figure 6
To better understand the structural significance of the spectral change observed
in Figure 6, we refer to the theoretical EPR spectra that are pertinent for our systems
studied herein. Figure 7A shows that for a D-strain=200Gauss, the EPR spectral
features are sensitive to the D-value in when DD-strain. Physically, this corresponds to
MnII complexes where the local ligand field shows strong heterogeneity.
Insert Figure 7
The theoretical spectra in Figure 7 reveal that for high D-strain systems, by increasing
the zero-field splitting parameter D, the spectrum undergoes two main changes [i] the
Page 15 of 33

outermost low-field and higher-field transitions are decreasing faster than the other
peaks. [ii] a broad bump [marked by the dashed lines and the vertical arrows] is
shifting downfield proportionally to the increasing D-value.
The effect of D on the relative intensities of the mI-transitions of 65Mn2+(S=5/2, I=5/2)
has been originally analyzed by Allen [29], who suggested a quite useful practical rule
based on the intensity ratio I5/2/I3/2 to get a reasonable estimate of D with no use of
computer simulations. Our present full-numerical calculations reveal that D-strain
may strongly affect the line intensities. On the other hand the observation of the low-
field bump may serve as a practical diagnostic tool for the variations in D-value is
systems with non-negligible D-strain. A full-detailed discussion of the spin physics of
this effect is out of the scope of the present paper, and will be analyzed in another
publication. Here, we focus on the use of the experimental EPR spectra, shown in
Figure 6, to understand the evolution of the Mn-coordination during the catalytic
cycle. Accordingly, after addition of the co-catalyst CH3COONH4, at t=1min, the
spectral changes i.e. decrease of the outermost sharp features at high- and low-field,
as well as the downshift of the low-field bump [see zoomed inset in figure 6], are
attributed to an increase of D to 185Gauss. Then at reaction times t=5min up to
30min, the Mn2+ spectral intensity decreases monotonically, indicating formation of
higher-oxidation Mn-states, as observed previously for the homogeneous Mn-L2
system under similar conditions [19]. After t=30min the Mn spectrum remains
unaltered. Importantly, at close inspection of the t=30min spectrum shows that this is
similar to the initial spectrum before addition of the co-catalyst.
These observations are interpreted as follows:[1] addition of H2O2 only, causes -
small- structural changes on the ligand field around the Mn2+ center, however no-
coordination or-redox advancement takes place. Thus, no catalytic evolution takes
place and the EPR spectrum remains unchanged (data not shown). [2] Addition of the
CH3COONH4 triggers a more important structural modification of the ligand field
around the Mn center e.g. increase in D, thus increase in ligand field strength. Now
the loss of MnII signals indicates rapid evolution towards Mn3+/Mn4+ states, becoming
EPR undetectable, and this results in decrease of the Mn2+ EPR intensity. [3]. Centres
that did not interact with H2O2/CH3COONH4 remain unchanged/inactive thus, at
30min their spectrum is the only part resolved in the EPR spectrum. The EPR data
show that these centers account for 20% of the total Mn2+ present.
Page 16 of 33

Thus CH3COONH4 plays a multiple role in the MnII/H2O2 system, as reported many
times previously [15-16, 19, 30-32]. This can be attributed to a dual acid-base role for
CH3COONH4 able to act as proton-donor and proton-acceptor [19]. An analogous
beneficial role of CH3COONH4 has been clearly demonstrated in Mn-Porphrin/H2O2
systems also [20, 33-34].
Herein, taking into account all the data we suggest a catalytic mechanism,
schematically depicted in Scheme 4, as follows: [i] CH3COO- abstracts proton from
H2O2 promoting its coordination to MnII and formation of MnII-OOH species (Scheme
+
4) and b) [ii] subsequently, NH4 , by acting as a proton-donor to MnII-OOH,
accelerates heterolytic O-O cleavage forming the active MnIV=O species which [iii]
catalyzes alkene epoxidation (Scheme 4).
Insert Scheme 4
4. Conclusion
A new synthetic methodology to covalently anchor our Mn(II)-Shiff base
catalysts onto activated carbon has been applied resulting in heterogeneous MnII-
L@ACox materials. This procedure involves functional <L-silane> precursors-which
have been pre-formed- and reaction of their alkoxy-groups with surface –OH from
activated carbon via the sol-gel chemistry. The, so obtained, carbon-based
heterogeneous catalysts [MnII-L@ACox] are effective and selective towards
epoxides with H2O2 in the presence of CH3COONH4 providing TONs equivalent-to/or
outcompetes MnII-L@SiO2 catalysts for certain substrates. Moreover, MnII-
L@ACox catalysts are kinetically faster than the corresponding MnII-L@SiO2
catalysts. BET data shows that after grafting the ACox matrix shows a dramatic
collapse of the specific surface with practically zero porosity. This feature could be
correlated with the observed dramatic acceleration of the catalytic performance of the
MnII-L@ACox catalysts which results in extremely higher TOFs compared vs. the
corresponding MnII-L@SiO2 catalysts probably by rendering all the grafted Mn-L
active catalytic centers readily accessible to H2O2, CH3COONH4 and substrate.
Moreover, MnII-L@ACox catalysts present very good reusability indicating that they
are more stable against oxidative destruction than the corresponding MnII-L@SiO2
catalysts which were no recyclable.
Overall, by combining catalytic and EPR spectroscopic data we suggest a
consistent catalytic mechanism which involves a) H-abstraction from H2O2 by
Page 17 of 33

+
CH3COO- and MnII-OOH formation and b) H-donation from NH4 to MnII-OOH
triggering MnIV=O generation which realizes alkene epoxidation.
Acknowledgment
This research has been co-financed by the European Union (European Regional
Development Fund – ERDF) and Greek national funds (ΕPΑΝ-ΙΙ), PEP–Sterea
Ellada–Epirus-Thessaly within Action SYNERGASIA-2011.
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Page 19 of 33

•MnII-L1@ACox
⁰MnII-L2@ACox
Figure 1. Distribution of oxidation products catalyzed by MnII-L1@ACox and MnII-
L2@ACox in the presence of H2O2. Reaction performed in CH3COCH3/CH3OH
(0.45:0.40 ml). See Table 2 for further details.
Page 20 of 33

A
Figure 2. Total turnover numbers for alkene epoxidations catalyzed by MnII-
L1@ACox / MnII-L1@SiO2 (A) and MnII-L2@ACox/ MnII-L2@SiO2 (B) in the
presence of H2O2. See Table 2 for further details.
Page 21 of 33

A
550
500
450
400
350
300
250
200
150
100
50
B
Mnˡˡ-L2@SiO₂
Mnˡˡ-L2@ACox
0
Figure 3 . TOF for alkene epoxidations catalyzed by MnII-L1@ACox / MnII-
L1@SiO2 (A) and MnII-L2@ACox/ MnII-L2@SiO2 (B) in the presence of H2O2. See
Table 2 for further details.
Page 22 of 33

Figure 4. Recyclability of MnII-L1@ACox and MnII-L2@ACox for cyclohexene
oxidation with H2O2.
Figure 5
Time dependence of cyclohexene epoxidation and solution redox potential for the
same reaction catalysed by MnII-L1@ACox [blue line, ■] and MnII-L2@ACox
[green line, ▲].
Page 23 of 33

MnL @ACox [H
O ]
2
2
2
MnL2@ACox [H2O2
CH3COONH4
]
MnL2@ACox [H2O2]
MnL2@ACox [H2O2]
x20
t=1 min
+CH3COONH4
t=5 min
t=30 min
2500
3000
3500
4000
4500
Magnetic Field (Gauss)
Figure 6. Experimental EPR spectra for MnII-L2@ACox in (a) Acetone / MeOH
1/1 (v/v), with addition H2O2, 1:2000 [Mn:H2O2] (b, c, d) after addition of
CH3COONH4, [Mn:H2O2: CH3COONH4=1:2000:1000]. at reaction time t=1min (a),
5min (b), and 30mi (c) Inset : zoom of spectra (a, b): increase of the D parameter is
manifested as a shift in the low-field bump marked by the arrows. Experimental
Conditions: Temperature 77 K, modulation amplitude 4 G, microwave power
12.5mW.
Page 24 of 33

D-strain=200G
E/D=0,01
A=-94G
D=150 Gauss
D=200
D=250
D=300
D=350
2250 2500 2750 3000 3250 3500 3750 4000 4250 4500
Magnetic Field (Gauss)
Figure 7. Theoretical EPR spectra for MnII(S=5/2, I=5/2) calculated by Easy spin
using the spin Hamiltonian (1) for Aiso=-94Gauss, E/D=0.01 and D-stain=200Gauss.
The spin package linewidth Iw was 1.3Gauss. By increasing the zero-field splitting
parameter D, the spectrum undergoes two main changes [i] the outermost low-field
and higher-field transitions are decreasing faster than the other peaks. [ii] a broad
bump [marked by the dashed lines and the vertical arrows] is shifting downfield
proportionally to the increasing D-value.
Page 25 of 33

SiO2
CH3
N
OH
N
N
H3C
H3C
O
O
O
SiO2
N
O
N
O
N
O
N
CH3
L1@SiO2
L2@SiO2
Scheme 1. Molecular structure of our Shiff base ligands covalently immobilised on
silica surface.
Page 26 of 33

H
N
N
N
OH
HO
L1
O
OMe
Si
MeO
O
OMe
O
O
O
O
Si
HO
N
N
N
OH
HO
L1-OS
MnII
HNO3
reflux, 6h
O
O
O
O
O
Si
C OH
HO
N
N
O
N
C
O
OH
Mn
O
C O
OH
MnII-L1-OS
O
C O
O
Si
N
HO
N
N
O
HO
Mn
C
O
O
O
C O
OH
MnII-L1@AC
Scheme 2A
Page 27 of 33

OH
N
O
N
N
N
H3C
CH3
O
CH3
H3C
O
O
O
O
Si
L2
OMe
Si
MeO
Cl
OMe
N
O
N
N
N
H3C
CH3
O
CH3
H3C
L2-OS
MnII
HNO3
reflux, 6h
O
O
O
O
Si
O
C OH
N
O
N
C
O
OH
N
Mn
N
H3C
CH3
O
C O
OH
H3C
CH3
MnII-L2-OS
H3C
N
O
C O
N
CH3
CH3
O
Si
HO
O
Mn
N
O
C
O
O
N
H3C
C O
OH
MnII-L2@AC
Scheme 2B
Scheme 2. Schematic representation of the synthesis of MnII-L1@ACox (A) and
MnII-L2@ACox (B) catalysts
Page 28 of 33

MnII
MnII
MnII
MnII
MnII
MnII
MnII
active catalytic center
Scheme 3: Morphological representation of MnII-L@ACox catalysts showing
compact blocks of carbon layers supports.
H2O2
MnII
CH3COO-
O
CH3COOH
MnII-OOH
MnIV
O
+
NH4
NH3
H2O
H
II
+
Mn -OO
H
MnII
active catalyst
ligand frame
Scheme 4: Suggested co-catalytic reaction mechanism of CH3COONH4 for the studied
MnII-catalysts.
Page 29 of 33