Activated vs. pyrolytic carbon as support matrix for chemical functionalization: Efficient heterogeneous non-heme Mn(II) catalysts for alkene oxidation with H2O2

Article abstract of DOI:10.1016/j.molcata.2016.08.033

Two types of heterogeneous catalytic materials, Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox, have been synthesized and compared by covalent grafting of a catalytically active [Mn^II-L₃imid] complex on the surface of an oxidized activated carbon (Cox) and an oxidized pyrolytic carbon from recycled-tire char (PCox). Both hybrids are non-porous bearing graphitic layers intermixed with disordered sp²/sp³ carbon units. Raman spectra show that (I_D/I_G)_{activatedcarbon} > (I_D/I_G)_{pyrolyticcarbon} revealing that oxidized activated carbon(Cox) is less graphitized than oxidized pyrolytic carbon (PCox). The Mn^II-L₃imid@Cox and Mn^II-L₃imid@PCox catalysts were evaluated for alkene oxidation with H₂O₂ in the presence of CH₃COONH₄. Both showed high selectivity towards epoxides and comparing the achieved yields and TONs, they appear equivalent. However, Mn^II-L₃imid@PCox catalyst is kinetically faster than the Mn^II-L₃imid@Cox (accomplishing the catalytic runs in 1.5 h vs. 5 h). Thus, despite the similarity in TONs Mn^II-L₃imid@PCox achieved extremely higher TOFs vs. Mn^II-L₃imid@Cox. Intriguingly, in terms of recyclability, Mn^II-L₃imid@Cox could be reused for a 2th run showing a ～20% loss of its catalytic activity, while Mn^II-L₃imid@PCox practically no recyclable. This phenomenon is discussed in a mechanistic context; interlinking oxidative destruction of the Mn-complex with high TOFs for Mn^II-L₃imid@PCox, while the low-TOFs of Mn^II-L₃imid@Cox are preventive for the oxidative destruction of the Mn-complex.

Full text of DOI:10.1016/j.molcata.2016.08.033

Accepted Manuscript
Title: Activated vs. Pyrolytic Carbon as Support Matrix for
Chemical Functionalization: Efficient Heterogeneous
non-Heme Mn(II) Catalysts for Alkene Oxidation with H2O2
Author: A. Simaioforidou M. Papastergiou A. Margellou D.
Petrakis M. Louloudi
PII:
S1381-1169(16)30368-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.033
MOLCAA 10020
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
27-5-2016
29-8-2016
30-8-2016
Please cite this article as: A.Simaioforidou, M.Papastergiou, A.Margellou,
D.Petrakis, M.Louloudi, Activated vs.Pyrolytic Carbon as Support Matrix for
Chemical Functionalization: Efficient Heterogeneous non-Heme Mn(II) Catalysts
for Alkene Oxidation with H2O2, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.08.033
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Activated vs. Pyrolytic Carbon as Support Matrix for Chemical
Functionalization: Efficient Heterogeneous non-Heme Mn(II)
Catalysts for Alkene Oxidation with H2O2
A. Simaioforidou, M. Papastergiou, A. Margellou, D. Petrakis, M. Louloudi*
Dedicated to Professor Georgiy B. Shul’pin on the occasion of his 70th birthday.
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
e-mail: mlouloud@uoi.gr (M. Louloudi)

GRAPHICAL ABSTRACT
:
C
Pyrolysis
PC
Cat.1
Cat.2
Highlights




► Mn-catalysts were covalently grafted onto activated carbon and pyrolytic carbon
from recycled-tire char.
► These catalysts were effective and selective towards epoxides with H2O2 and
CH3COONH4.
► The catalyst grafted onto pyrolytic carbon was kinetically faster, however was
practically no recyclable.
► This phenomenon is discussed in a mechanistic context.
Abstract:
Two types of heterogeneous catalytic materials, MnII-L3imid@Cox and MnII-
L3imid@PCox, have been synthesized and compared by covalent grafting of a catalytically
active [MnII-L3imid] complex on the surface of an oxidized activated carbon (Cox) and an
oxidized pyrolytic carbon from recycled-tire char (PCox). Both hybrids are non-porous
bearing graphitic layers intermixed with disordered sp2/sp3 carbon units. Raman spectra
show that (ID/IG)activated carbon> (ID/IG)pyrolytic
revealing that oxidized activated
carbon
carbon(Cox) is less graphitized than oxidized pyrolytic carbon (PCox). The MnII-
L3imid@Cox and MnII-L3imid@PCox catalysts were evaluated for alkene oxidation with
H2O2 in the presence of CH3COONH4. Both showed high selectivity towards epoxides and
comparing the achieved yields and TONs, they appear equivalent. However, MnII-
L3imid@PCox catalyst is kinetically faster than the MnII-L3imid@Cox (accomplishing the
catalytic runs in 1.5h vs. 5h). Thus, despite the similarity in TONs MnII-L3imid@PCox
achieved extremely higher TOFs vs. MnII-L3imid@Cox. Intriguingly, in terms of
recyclability, MnII-L3imid@Cox could be reused for a 2th run showing a ~20 % loss of its
catalytic activity, while MnII-L3imid@PCox practically no recyclable. This phenomenon is
discussed in a mechanistic context; interlinking oxidative destruction of the Mn-complex with
high TOFs for MnII-L3imid@PCox, while the low-TOFs of MnII-L3imid@Cox are
preventive for the oxidative destruction of the Mn-complex.
Keywords: Mn(II) catalysts; covalent immobilization; H2O2; catalytic epoxidation; activated,
pyrolytic carbon

1. Introduction
In nature, manganese can frequently be found in the catalytic redox center of several
enzymes [1-2]. Mn-complexes bearing non-heme ligands i.e. such as salen-derived ligands
have been promise as versatile catalysts in olefin epoxidation [3-5]. Manganese complexes of
1,4,7-triazacyclononane (tacn) or 1,4,7-trimethyl-1,4,7triazacyclononane (tmtacn) ligands,
which were originally synthesized and studied as models for the oxygen evolving centre of
photosystem II and for manganese catalase [6,7], were also employed as bleaching- [8] and
epoxidation [9-12] -catalysts using H2O2 as oxidant. There is still great research interest for
efficient non-heme Mn complexes in catalytic epoxidations of olefins [13-20].
Besides efficiency of Mn-catalysts, stability and reusability constitute equally important
parameters for catalytic reactions. Thus, recovery and reuse of the used catalysts is a
challenging task that has to be considered. In this context, catalyst heterogenization on a solid
support provides advantages such as easy handling, easy product separation, catalyst recovery
and reuse, and lower waste’s production [21-23].
A variety of carbon materials have been used as catalyst-supports in heterogeneous
catalysis such as graphite, carbon black, activated carbon (C) and pyrolytic carbon (PC) [24-
26]. Carbons appear to be a very flexible support since they have high surface area and high
porosity [27]. It has been shown that the surface area and porosity of carbons may be
important factors for the preparation of the corresponding catalysts by influencing their
features properties [24, 28]. Within this context, the role of surface chemistry of carbons is
also determinant [28]. When oxidation takes place, the surface of carbon becomes rich in a
variety of oxygen-based functional groups i.e. carboxyl, carbonyl, lactonic and phenolic, at a
broad range of concentrations which depend on the method of activation, the used reagents
and the temperature of the process [29-33]. These functionalities can affect the synthesis of
supported catalysts onto the carbons, by controlling the acid-base and hydrophilic character of
the carbon surface [34-37]. So far, heterogenization of metal complexes has been realized via
their adsorption or covalent grafting on carbon surfaces [38-47].
For efficient heterogeneous catalysts via grafting of metal complexes onto carbon, the
choice of the appropriate spacer group consists of another key parameter. This spacer should
be bifunctional i.e. to react with both the surface groups of the carbon and the metal complex,
to bridge them. Moreover spacer length, flexibility and degree of surface coverage [22, 48-50]
will ultimately affect the catalytic performance.
In the literature, there are very few examples of Mn oxidation catalysts immobilized on
carbon. More specifically, their anchoring has been achieved either via adsorption [45-46,
51], axial coordination to support [44, 52] or covalent attachment [44]. It is obvious that the
covalent bond formation between the metal complex and the support ensures minimum
catalyst leaching during catalytic process [53].

Recently [25] we have reported an efficient protocol to covalently immobilized metal
complexes onto the carbon surface, by generating a [metal complex-silane] precursor which is
attached to the surface OH groups of the carbon [25]. The so-obtained Mn(II) heterogeneous
catalysts have been proven to be effective and selective towards epoxides with H2O2 in the
presence of CH3COONH4 [25]. Moreover they presented very good reusability indicating that
they are stable against oxidative destruction [25]. The same protocol has been applied very
recently by our group [26] to graft Mn(II)-catalyst onto the surface of PC for oxidative
degradation of methyl orange [26].
It is obvious that -due to the rapid development of automobile industry- the amount of
used tires is growing continuously. Thus, to face economic, managerial, environmental issues
of used tires pyrolysis is considered to be the most environmentally acceptable and promising
treatment method with low pollution emission. Pyrolytic char is one of the main products
obtained by the pyrolysis process and its’ applications are still being sought [54-56]. Thus, a
post-production use of carbons and chars from waste tire is an attractive technology [57-63].
These materials have physical and textural characteristics similar to those of certain
commercial carbon blacks [52, 64]. Hence pyrolytic carbon char (PC) has great potential
applicability, however so far applications in catalysis are still being scarce [26, 64-66].
Here, a non-heme Mn(II) complex was covalently immobilized onto commercial
activated carbon (C) and pyrolytic carbon char (PC). The Mn-complex bears a bio-inspired
ligand with three imidazole moieties (L3imid). The so-derived MnII-L3imid@Cox and MnII-
L3imid@PCox, hybrid materials have been evaluated for alkene epoxidation catalysis. The
main findings of the present work are (i) the activated carbon bears more structural defects
than the pyrolytic one which is smoother, (ii) both MnII-L3imid@Cox and MnII-
L3imid@PCox, catalysts are selective towards epoxide formation and, based on the achieved
yields and TONs, none shows a clear superiority, and (iii) MnII-L3imid@PCox, is kinetically
faster than MnII-L3imid@Cox, (iv) higher TOFs achieved by MnII-L3imid@PCox are
correlated with faster oxidative deactivation of the material.
2. Experimental
Activated commercial carbon(C) was purchased from Fluka (05120). The char (pyrolytic
carbon, PC) used in this work was obtained via pyrolysis of used rubber tires. 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. Analytical grade acetone, methanol and ammonium acetate were from Aldrich and
Merck respectively. Hydrogen peroxide was 30% aqueous solution. (3-glycidyloxypropyl)-
trimethoxysilane was purchased from Fluka.

Infrared spectra were recorded on GX Perkin-Elmer FT-IR System. Continuous-wave
(c.w.) X-ray diffraction (XRD) patterns were performed with a D8 Advance Bruker X-ray
diffractometer using Cu Ka radiation (k = 0.154 nm) as the X-ray source, scanning from 10°
to 60° at a rate of 0.03° /2 sec for the activated carbon and from 10° to 80° at a rate of 0.02° /2
sec for the pyrolytic carbon. Thermogravimetric analysis were carried out with a Shimadzu,
DTG-60 instrument, at a heating rate of 5 °C min-1 and a flow rate of synthetic air gas of 50
cm3min-1. Raman spectra were recorded on a Renishaw 1000 micro-Raman spectrometer at
532 nm wavelength incident laser light. The N2 adsorption-desorption isotherms were
measured at 77K on an Autosorb-1, Quantachrome porosimeter. Specific surfaces areas were
determined with the Brunauer-Emmett-Teller (BET) method using adsorption data points.
Scanning electron microscopy (SEM) was observed in a Jeol JSM 5600 system operating at
20kV.
GC analysis was performed using a Shimadzu GC-17A gas chromatograph coupled with
a GCMS-QP5000 mass spectrometer. Gas chromatography was performed using a SPB-5
column (30 m, 0.25 mm i.d, 0.25 μm). The temperature program for cyclohexene,
cyclooctene, styrene, limonene ((4R)-1-methyl-4-prop-1-en-2-ylcyclohexene), cyclopentene,
1-methyl-1-cyclohexene and alpha-pinene (4,6,6-trimethylbicyclo[3.1.1]hept-3-ene) was: 80
°C for the first 5 min, followed by a 10 °C min-1 temperature gradient to 150 °C for 5 min.
The temperature program for trans-β-methylstyrene ((1E)-1-propen-1-ylbenzene) and cis-
stilbene ((Z)-stilbene) was: 80 °C for the first 5 min, followed by a 10 °C min-1 temperature
gradient to 220 °C for 1 min. For hex-1-ene: 50 °C for the first 5 min followed by a 10 °C
min-1temperature gradient to 150 °C for 5 min. Injector and interface temperatures were set at
230 °C and 290 °C, respectively. Chromatography grade helium was used as the carrier gas,
linear velocity= 36.8 cm s-1 at column pressure 64.5 kPa.
2.1 Ligand Synthesis
The synthesis and characterization of the L3imid ligand has been described in full detail in
our previous work [18].
2.2. Oxidation of activated carbon C and pyrolytic carbon
Activation of commercial activated carbon (C) and pyrolytic carbon (PC) has been
achieved by oxidation with nitric acid (HNO3) leading to the formation of surfacial carboxylic
groups (–COOH) [25]. 5g of C was refluxed with 90ml 65% v/v HNO3 for 6h. The oxidized
carbon (Cox) obtained was separated by filtration, washed exhaustively with distilled water
until pH~7 and dried under reduced pressure at 40 °C for 12 h. The same procedure was
employed for the oxidation of PC. Here after the obtained oxidized carbons are abbreviated as
Cox (oxidized commercial activated carbon) and PCox (oxidized pyrolytic carbon). DRIFTS-

IR: (cm-1, selected peaks): C: 3430: ν(O-H); 2922, 2853: v(C-H); 1582, 1460 ν(C=C); 1167
ν(C-O). Cox: 3419: v(O-H); 2959, 2919, 2850: v(C-H); 1715: ν(C=O); 1580, 1472 ν(C=C);
1176 ν(C-O) (Fig.1.A). PC: 3438: ν(C-OH); 2920, 2853: ν(C-H); 1632: ν(C=C); 1385: ν(C-
OH); 1105: ν(C-C) (Fig.1). PCox: 3438: v(O-H); 2917, 2848: v(C-H); 1704: ν(C=O); 1536,
1461 ν(C=C); 1382 ν(-CH2-); 1106 ν(C-O)(Fig 1.B).
2.3. Synthesis of precursor silane-compounds and metalation of carbon materials
1.2 mmol of L3imid and 1 mmol of (3-glycidyloxypropyl)-trimethoxysilane were stirred
for 24 h in 40 ml of MeOH at 65 °C in order to generate L3imid-P silane precursor. Into this
solution, 1mmol of MnCl2.4H2O dissolved in 5 ml MeOH was added and the reaction mixture
was stirred overnight at room temperature to form the metalled precursor MnII-L3imid-P.
Then, 300 mg of Cox or PCox were added and the obtained slurry solutions were refluxed at
65 °C for 24 h. The resulting materials MnII-L3imid@Cox or MnII-L3imid@PCox were
washed several times with MeOH, EtOH and finally dried under vacuum at 40 °C for 24 h.
MnII-L3imid@Cox: 3419: v(O-H); 3218: ν(N-H); 2922, 2861: v(C-H); 1679: ν(C=N);
1546,1458: v(C=C); 1175ν(C-O) (Fig.1.A). MnII-L3imid@PCox: 3438: v(O-H); 3228: ν(N-
H); 2917, 2848: v(C-H); 1680: ν(C=N); 1536,1455ν(C=C); 1106 ν(C-O) (Fig.1.B).
2.4 General procedure for catalytic epoxidation of olefins and terpenes
The catalytic experiments were carried out at room temperature (25±0.5 °C) with a molar
ratio [catalyst: H2O2: alkene: CH3COONH4] = [1:2000:1000:1000μmol]. The standard
protocol for catalytic epoxidation reactions is the following: in a solution of acetone/methanol
(v/v=450μl/400μl), alkene substrate (1mmol), acetophenone or bromobenzene as internal
standard (1mmol), catalyst (1 μmol) and CH3COONH4 as additive (1mmol) were added. The
last reagent H2O2 (2mmol) was first diluted in methanol (1:1) and then was added by a
digitally controlled syringe pump [SP1011Z WPI] over 30 min. The mass of the catalysts
used was based on the Mn loading of MnII-L3imid@Cox and MnII-L3imid@PCox. The total
volume of catalytic reactions was approximately 1ml. The progress of the reaction was
monitored by GC-MS, by removing small amounts of the reaction mixture. To establish the
identity of the epoxide products unequivocally, the retention time and spectral data were
compared to those of commercial samples. Blank experiments showed that without Mn-
catalyst or CH3COONH4, epoxidation reactions do not take place. Reactions were completed
within 5 h for MnII-L3imid@Coxand within 1.5 h for the MnII-L3imid@PCox catalyst.
When alcohol and ketone were detected as allylic oxidation products, the sample was treated
with PPh3 and measured again by GC-MS according to Shul’pin’s procedure [67]. In
oxidation catalysis with H2O2, this procedure is essential given that there is always the

possibility of alkyl hydroperoxides formation. These peroxides can decompose in the case of
direct injection to the GC instrument with the formation of the corresponding alcohol and
ketone with rapture of C-H bonds. As a result, the quantification of alcohol and ketone cannot
reflect their real amounts. To be sure that we didn't have these transformations in our catalytic
systems, we estimated by GC the concentrations of ketone and alcohol before and after the
treatment of the reaction samples with triphenylphosphine.
2.5 General procedure for recycling experiments
The first run of the catalytic reactions were performed under the catalytic conditions
described above by using cyclooctene as substrate. After the end of each catalytic oxidation
which corresponded at 5 h for the reactions catalyzed by MnII-L3imid@Coxand 1.5 h for
those catalyzed by MnII-L3imid@PCox, the solid catalysts were filtered off, washed three
times with MeOH and dried under vacuum. The recovered solid catalysts were reused in a
next catalytic reaction under the same catalytic conditions. In recycling experiments, it was
ensured that the same mass of the solid catalysts was used in each run which was estimated by
mass-weighting. This was realized for both catalysts by running 5 identical catalytic batches
followed by washing/drying as detailed above.
3. Results and discussion
3.1. Synthesis and characterization
Carbon materials with covalently grafted MnII -Schiff base complexes were prepared by
the followed synthetic process: i) Oxidation of the activated carbon C and pyrolytic carbon
PC with HNO3; ii) generation of the precursor compound L3imid-P which was formed by
reaction of the ligand L3imid with the commercial available (3-glycidyloxypropyl)-
trimethoxysilane; iii) its metalation with MnII ions salts; and (iv) anchoring of the metalated
silane-precursor MnII-L3imid-P onto the oxidized activated Cox and pyrolytic PCox carbon
which occurs via hydrolysis and co-condensation reactions with the –OH functional groups of
the support materials. Scheme 1 displays the synthesis process of the MnII-L3imid@Cox and
MnII-L3imid@PCox.
To prepare active catalytic materials, the immobilization of the preformed Mn-complex is
the appropriate way i.e. instead of ligand immobilization and subsequent metalation. This
protocol avoids Mn-coordination by free –COOH groups of the oxidized supports (Cox and
PCox), that would result in unreactive Mn-carboxylate species [25, 26].
FT-IR: In Figure 1 the IR spectra of the pristine materials, oxidized materials and synthesized
catalysts are showed. In more details, when the IR spectra of the oxidised materials Cox and
PCox are compared to the parent carbon supports C and PC respectively, new bands are

observed attributable to the formed–COOH surface groups. More specifically, in the spectra
of Cox and PCox the bands observed at 1715, and 1176 cm-1 for Cox and at 1704, and 1106
cm-1 for PCox are assigned to ν(C=O) and ν(C-O) stretching vibrations, respectively. The IR
spectral data indicate the successful formation of –COOH. Accordingly, the IR spectra of
MnII-L3imid@Cox and MnII-L3imid@PCox (Fig.1) show several bands corresponding to
various structural units of the solid support as well as from the ligand L3imid. The v(C=N)
and v(N-H) stretching vibrations respectively are shifted when compared with the
corresponding spectrum of the organic ligand L3imid [18]. This indicates coordination of
manganese with the imine-nitrogen and the imidazole groups of the immobilized ligand
L3imid (Fig. 1).
TGA: The derivative thermogram of activated carbon Cox show a broad peak in the range
of 80-450°C assigned to the decomposition of –COOH surface groups with a 10.0% weight
loss and an intense peak at 600°C assigned to the combustion of carbon support. The formed
–COOH groups are ca. 2.2 mmol g-1(Table 1). For pyrolytic carbon PCox the peak which
observed in the range of 80-450°C was assigned to the decomposition of –COOH surface
groups as well with a 10.8% weight loss;thus, theresulted –COOH groups are ca. 2.4mmol g-
1(Table 1). The thermograms of pristine Cox and PCox materials show a 10.0% and 10.8%
weight-loss in the range of 70-400 °C attributable to the decomposition of surface COOH
groups, while the MnII-L3imid@Cox and MnII-L3imid@PCox show a 18.8 % and 25.0%,
weight loss respectively in the same temperature range; this additional weight loss of 8.8%
and 14.2% is assigned to the decomposition of the anchored organic ligand L3imid moieties.
The total combustion of MnII-L3imid@Cox corresponds to 93.9% total weight loss and the
obtained solid residue of 6.1% is assigned to inorganic salts (3.6%), SiO2 (1.1%) and
MnO2(1.4%). The loading achieved is ca. 0.16 mmolg-1 determined by thermogravimetric and
elemental analysis. Accordingly, the combustion of MnII-L3imid@PCox provides a total
weight loss of 87.5% and the obtained solid residue of 12.5 % corresponds to inorganic salts
(9.2%), SiO2(1.6%) and MnO2(1.7%). The loading achieved is ca. 0.19 mmolg-1 determined
by thermogravimetric and elemental analysis.
Surface Area-Porosity: The Nitrogen physisorption isotherms for C and Cox are shown in
Fig.2(A) where volume of nitrogen adsorbed and desorbed are plotted against the relative
pressure P/PO. According to IUPAC classification the isotherms of C and Cox samples can be
classified as a mixture of I and II type [68-69]. Such behaviour is characteristic of penetration
in micropores followed by a monolayer multilayer adsorption and interparticle capillary
condensation [70].The specific surface area calculated using BET method was found to be
equal to 857 m2/g and 758 m2/g for C and Cox respectively. Accordingly, the pore volume by

nitrogen adsorption was measured 0.56 cm3/g and 0.49 cm3/g. The pore size distribution of C
and Cox, see inset in Fig. 2(A),was calculated with BJH method. This distribution curve
indicates one sharp peak at pore radius ~19Å, which shows the microporous character of the
two samples. The isotherm of MnII-L3imid@Cox which was prepared after grafting of the
entire MnII-L3imid-complex onto Cox indicates a non-porous material, see Table 1. The
specific surface area shows a dramatic collapse, e.g., from 758 m2g-1 in activated carbon Cox
to 53 m2g-1 in MnII-L3imid@Cox (Table 1).
The isotherms of PC, PCox and MnII-L3imid@PCox (Fig. 2(B)) are type II, indicating
nonporous materials; the specific surface area is 22, 35 and 42 m2g-1, respectively. The
observed slight increase in the surface area could be attributed to the change of the size of the
bulk grains caused during the treatment with HNO3 and the grafting procedure.
XRD: Figure 3(A) shows the XRD patterns obtained for the activated carbon-based
materials, i.e. C, Cox, and MnII-L3imid@Cox. All diffractograms display two intense
doublet feature centered at 2θ ~ 25° and 43°, characteristic of a disordered carbonaceous
structure [71]. The peaks observed exactly at 26.4°, 43.3° and 44.0° are assigned to the (002),
(100) and (101) reflections, respectively, of the graphitic part of the materials [71], i.e. the
(002) one is due to the stacking structure of aromatic layers while the (100) formed by sp2
hybridized carbons[71]. The 24.4° reflection witnesses the packing distance of saturated
structures i.e. such as aliphatic side chains attached at the edge of coal crystallites [72].The
observed XRD patterns suggest that all the studied materials have structures intermediate
between graphite and amorphous carbon i.e. called turbostratic structure or random layer
lattice structure [72].
Figure 3(B) shows the XRD patterns obtained for the materials PC, PCox, and MnII-
L3imid@PCox.Two non-splitted diffraction peaks at 25.0° and 43.0° were observed on the
all the pyrolytic-based carbon materials, corresponding to the (002) and (100/101) planes of
the hexagonal graphite[73]. The width of the peaks indicates disordered sp2/sp3carbon
structures between the graphitic layers for all samples. However, in case of pristine PC the
pattern exhibits additional peaks attributable to inorganic components, i.e. ZnS and ZnO
originating from the tires manufacturing. For comparison, a literature XRD pattern for ZnO is
superposed in Fig. 3(B) [74]. Upon HNO3 treatment of PC, these additional bands disappear
from the diffractogram of the oxidized char PCox, which now appears to be identical to MnII-
L3imid@PCox. The broadening and slight decrease in the intensity of the peak at 2θ~44° of
the layered graphitic sp2-domains of MnII-L3imid@PCox as well as of MnII-L3imid@Cox
vs. the corresponding peak of PCox or Cox confirms the successful modification of the Cox
and the PCox matrices by the [MnII-L3imid] catalytic component. This is further supported

by the XRD patterns of both MnII-L3imid@PCox and MnII-L3imid@Cox which do not
contain sharp peaks from non-immobilized crystallites of the [MnII-L3imid] complex.
Raman: Raman spectra of the C, Cox and MnII-L3imid@Cox are shown in Figure 4(A).
All spectra exhibit two intense peaks, the D band assigned to carbon atoms with sp3
hybridization due to defects or distortion of the crystal lattice and the G band attributed to an
active vibration E2g of sp2 hybridized carbon atoms. For the pristine C, the G band shows at
1601 cm-1 while the D band at 1353 cm−1. Τhe intensity of ID/IG ratio is equal to 0.93. After
the oxidation treatment, the intensity of the D band increases in the spectrum of Cox; this is
attributed to the increase of the sp3carbons, resulting in disorder of the Cox. Τhe intensity of
ID/IG ratio increases now to 0.98. In the spectrum of the MnII-L3imid@Cox,the intensity of G
band remains stable while the intensity of the D band increases. The ID/IG ratio increases from
0.98 for Cox to 1.05 for MnII-L3imid@Cox, suggesting an increase at the concentration of
the structural defects, probable due to the formation of new bonds between the carbon matrix
and the active catalytic component [MnII-L3imid].
The Raman spectra of PC, PCox and MnII-L3imid@PCox are shown in Figure 4(B).
The intensity ratio of PC is ID/IG=0.69.The G band shows at 1585 cm-1 while the D band at
1348 cm−1. Upon oxidation treatment (PCox spectrum) and grafting procedure (MnII-
L3imid@PCox spectrum) the intensity of both peaks (G and D band) at 1589 cm-1, 1380 cm-1
and 1589 cm-1, 1354 cm-1 are decreased resulting in intensity ratio ID/IG=0.77 and ID/IG=0.83
respectively. Overall, these findings suggest well-accomplished oxidation and
functionalization procedures of the pristine PC material resulting in increase of sp3-hybridised
carbon defect sites. In summary, based on these data, the ID/IG ratios follow the order
(ID/IG)C< (ID/IG)Cox< (ID/IG)Mn-L3imid@Coxand(ID/IG)PC<(ID/IG)PCox< (ID/IG)Mn-L3imid@PCox
clearly indicating that any chemical treatment of the pristine materials decreases their
graphitic sp2 character.
Comparing the ID/IG ratio of activated vs. pyrolytic carbon for any chemical process,
we observed that always (ID/IG)activated carbon> (ID/IG)pyrolytic carbon
,
i.e., (ID/IG)C> (ID/IG)PC, (ID/IG)Cox> (ID/IG)PCox and (ID/IG)Mn-L3imid@Cox>(ID/IG)Mn-L3imid@PCox
.
This indicates that commercial activated carbon bears more sp3 carbon atoms i.e. as
confirmed also by XRD. Thus, the low content of such aliphatic structures into the pyrolytic
matrixes suggests that –probably– they did not survive during pyrolysis.
SEM: Representative SEM images of the pristine matrices, acid activated samples and
catalysts are shown in Fig. 5. Significant differences between the surface topography of C and
PC carbon are observed in SEM (Fig. 5a, d); activated carbon C has a more multilayered
structure than the pyrolytic carbon PC which seems to be smoother probably due to the

transformation of its morphology during pyrolysis. Moreover, it is observed that both pristine
matrices have irregular shape.
SEM images of HNO3-treated samples are shown in Fig.5b, e; it is evident that this treatment
does not significantly affect the surface of Cox, since no obvious morphological and textural
changes are observed vs. C; however, PCox particles are smoother than the pristine PC
particles. Fig. 5c, f displays SEM images of the catalytic materials MnII-L3imid@Cox and
MnII-L3imid@PCox. Covalent anchoring of the active catalytic component [MnII-L3imid]
onto the Cox and PCox caused a significant cracking of the particles, i.e. Fig. 5 C reveals the
presence of smaller particles. The morphology of the surface is slightly different, and it is
clear that after the anchoring procedure the surface of the particles became smoother.
3.2. Catalytic evaluation
Products’ detection and quantification:The catalytic behavior of the MnII-L3imid@Cox was
investigated in olefin epoxidation by H2O2 and then compared to that of MnII-L3imid@PCox
under identical conditions (Table 2). The present carbon-based heterogeneous catalysts, to
become active for alkene epoxidation with H2O2 require ammonium acetate as additive in
analogous way to other non-heme Mn-catalysts [18,25,26,50]. This is attributed to a dual
acid-base behavior of CH3COONH4 by acting as proton-donor and proton-acceptor
[18,67,75]. More precisely, according to the catalytic mechanism which was proposed by us
recently [i] CH3COO-abstracts proton from H2O2 promoting its coordination to MnII and the
+
formation of MnII-OOH species and [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 [25]. The catalytic oxidations were carried out at room
temperature in acetone:MeOH (v/v=450μl/400μl) solution and the used molar ratio of
catalyst:H2O2:CH3COONH4: substrate was equal to 1:2000:1000:1000 (μmoles). According
to Table 2, epoxidation of 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). To
ensure that the observed catalytic results were derived by the heterogeneous catalysts and no
leaching of the active catalytic component occurs during the catalytic processes, control
experiments were carried out. Specifically, a possible leaching of the Mn(II)-complexes from
the carbon support was tested by the 'filtration method' according to M.H. Valkenberg, W.F.
Holderich [76]. In typical catalytic experiments after 30 min the solid catalysts were filtered;
the catalytic reaction of the filtrate's solution was monitored by GC [53]. The data showed
that no evolution of the studied reactions was observed in the filtrate. This ensures that no
leaching of the active Mn(II)-catalyst occurs substantiating that the obtained catalytic results
derive exclusively from the heterogeneous catalysts.

Based on our catalytic data provided in Table 2, in the epoxidation of cyclooctene by MnII-
L3imid@Cox and cis-cyclooctene epoxide (9-
MnII-L3imid@PCox,
oxabicyclo[6.1.0]nonane) was the main product with a 100% selectivity and 60.7 and 75.6%
yield respectively (Fig.6). In the catalytic oxidation of cyclohexene, cyclohexene oxide (7-
oxabicyclo[4.1.0]heptane) was clearly the major product (yields 82.7 and 51.5%); however,
small amounts of allylic oxidation products such as has cyclohex-2-en-1-ol (0.6, 1.9%) and
cyclohex-2-en-1-one (3.5, 2.9%) have been also detected. It is clarified that the quantified
amounts of cyclohex-2-en-1-ol and cyclohex-2-en-1-one are identical to those obtained after a
PPh3-treatment according the Shul'pins’ procedure [67] which ensures that putative
hydroperoxides formed during catalysis as primary products have been completely
transformed into the corresponding alcohol/ketone [67-75,77]. Hex-1-ene, which is a rather
hard oxidation substrate, showed epoxide yield from 11.3-16.3%.
The epoxidation of trans-β-methylstyrene ((1E)-1-Propen-1-ylbenzene), cis-stilbene ((Z)-
stilbene), styrene and cyclopentene produces only the corresponding epoxides with a 100%
selectivity, demonstrating that the olefin epoxidation by MnII-L3imid@Cox and MnII-
L3imid@PCox is highly stereospecific resulting in yields of [53.3%, 46.1%], [51.7%, 33.3
%], [58.1%, 48%] and [22.5%, 62.6%] respectively.
Two epoxides (cis- and trans-) were the main products derived from epoxidation of the
electron-rich double bond in 1,2-position of limonene ((4R)-1-methyl-4-prop-1-en-2-
ylcyclohexene). The total yield of 1, 2-epoxides was 67.5% and 81.4% catalysed by MnII-
L3imid@Cox and MnII-L3imid@PCox, respectively. Small amounts of the alcohols p-
mentha-1,8-dien-2-ol (1-methyl-4-prop-1-en-2-ylcyclohex-2-en-1-ol) and p-mentha-2,8-dien-
1-ol ((3R,6S)-3-methyl-6-prop-1-en-2-ylcyclohexen-1-ol) formed by allylic oxidation of the
limonene ring have also been observed and have been identified (11.6-15.5%). The formation
of epoxides derived from the more accessible but less electron-rich double bond in 8,9-
position has not been observed. Thus, the epoxidation was clearly the main reaction path
resulting mainly in 1,2-epoxides (see Table 2, Figure 6).
In the oxidation of alpha-pinene (4,6,6-trimethylbicyclo[3.1.1]hept-3-ene) catalysed by
MnII-L3imid@Cox, the main product was the epoxide with 40% yield; when the MnII-
L3imid@PCox used as catalyst, the epoxide yield found to be 57.4% and small amounts of 2-
pinen-4-ol(4.1%) and 3-pinanone (3.1%) have been also detected. MnII-L3imid@Cox
exhibited higher catalytic activity towards the oxidation of 1-methyl-1-cyclohexene resulting
in yield 77.4% (100% selectivity) compared to 54.1% of the reaction catalyzed by MnII-
L3imid@PCox which provided also small amounts of ketone (0.5%) and allyl alcohol
(3.3%).

Evaluating the catalytic performance of the catalysts MnII-L3imid@Cox and MnII-
L3imid@PCox based on the achieved TONs, we do not observe a clear superiority of any of
them (Table 2, Fig. 7).
Kinetic Data: The time-depended cyclooctene oxidation catalyzed by the heterogeneous
catalysts, in conjunction with the observed redox potential of solution Eh (vs. standard
hydrogen electrode SHE) are shown in Fig.8. At the beginning of the reaction catalyzed by
MnII-L3imid@Cox, Eh was + 337 mV; after 1 h, it dropped at + 225 mV with a 27 % epoxide
yield and finally, after a reaction time of 5 h, it approached a value of Eh= +230 mV with 60.7
% yield. In the case of MnII-L3imid@PCox–catalyzed epoxidation, at reaction time t=0,
Ehwas + 338 mV, at t=1 h it decreased at +210 mV providing a product yield of 65% and at t=
1.5 h, Eh approached +205 mV with a 75.6% cyclooctene epoxide formation. Analogous
kinetic profiles were observed for all the studied substrates. Thus overall, oxidations catalysed
by MnII-L3imid@Cox were practically accomplished within 5 h and reactions catalysed by
MnII-L3imid@PCoxwere accomplished within 1.5 h. The resulting TOFs, listed in Table 2,
visualised in Fig. 9, clearly demonstrate the superiority of MnII-L3imid@PCox vs. the MnII-
L3imid@Cox. Notice that the TOFs achieved by MnII-L3imid@PCox ranged from 109-646
h-1 while the TOFs by MnII-L3imid@Cox was by far inferior, ranging from 22-174 h-1.
Catalytic Recycling: The heterogeneous catalysts MnII-L3imid@Cox and MnII-
L3imid@PCox were tested for their recyclability by repeated recovering and reusing of the
solid catalysts in new catalytic reactions. As pilot reaction, the catalytic oxidation of
cyclooctene was chosen and a [catalyst: H2O2: cyclooctene: additive: CH3COONH4] =
[1:2000:1000:1000] molar ratio was used in an acetone/MeOH (450 μl /400 μl) solvent
mixture. After each catalytic run, the catalyst was recovered by centrifugation, washed, dried
and reused under the same catalytic conditions. The recycling catalytic data for both MnII-
L3imid@Cox and MnII-L3imid@PCox are displayed in Fig. 10.
According to Fig.10 MnII-L3imid@Cox presented a good reusability especially during a
second run showing a ~20 % loss of the catalytic activity during the 2th run and > 50 % loss of
the activity for each of the following re-uses. MnII-L3imid@PCox is able to work only for a
single run presenting a total loss of its catalytic activity (>90%) during the 2nd re-use. In total,
MnII-L3imid@Cox provided 1300 TONs for cyclooctene oxidation instead of 831 TONs by
MnII-L3imid@PCox. This behaviour is assigned to oxidative destruction of the active
catalytic component [MnII-L3imid] during catalysis [78]; the majority of the MnII-centrers are
still associated to the carbonaceous matrices after the 1st run for MnII-L3imid@PCox and the
2nd for MnII-L3imid@Cox, however, they do not show the specific coordination environment
of [MnII-L3imid] in analogous way, as we have shown vey recently [78] using EPR

spectroscopy. Thus, the much higher TOFs and efficiency of MnII-L3imid@PCox vs. MnII-
L3imid@Cox is counteracted by its higher auto-oxidation rate.
4. Conclusion
Covalent immobilisation of a catalytically active [MnII-L3imid] complexon the surface of
commercial available activated and pyrolytic carbon has been achieved. The ligand L3imid
bears a Schiff base backbone and three imidazole residues. The preformed [MnII-L3imid]
complex was transformed into a silane-precursor MnII-L3imid-P which was grafted onto
oxidized activated Cox or pyrolytic PCox carbon via sol-gel type chemistry, resulting in the
MnII-L3imid@Cox and MnII-L3imid@PCox functionalized materials.
Both hybrids are non-porous materials with disordered sp2/sp3graphitic carbon structures. Cox
bears more sp3 carbon atoms vs. PCox indicating that, for the pyrolytic matrixes, these
aliphatic structures did not survive during pyrolysis. Raman shows (ID/IG)activated carbon
>
(ID/IG)pyrolytic carbon, indicating higher graphitic character of pyrolytic carbon than the
activated carbon. Accordingly, further chemical treatment of Cox and PCox materials
decreased their graphitic character, retaining however, the general order (ID/IG)activated carbon
>
(ID/IG)pyrolyticcarbon. Noticeably, under chemical treatment, the particles became smoother and
significant cracking occurred.
Both MnII-L3imid@Cox and MnII-L3imid@PCox showed high selectivity towards
epoxides. However, MnII-L3imid@PCox catalyst is kinetically faster (1.5h vs. 5h) than the
MnII-L3imid@Cox resulting in extremely higher TOFs vs. MnII-L3imid@Cox. In terms of
recyclability, MnII-L3imid@Cox can reused for a 2th run showing a ~20 % loss of its catalytic
activity, while MnII-L3imid@PCox was practically no recyclable. Thus, the much higher
rapidity and efficiency of MnII-L3imid@PCox vs. the MnII-L3imid@Cox is counter paid by
higher oxidative destruction.
Acknowledgment
This work is financially supported by the “SYNERGASIA Program” 11SYN_5_682
(O.P. Competitiveness & Entrepreneurship; EPAN II), ROP Macedonia, Thrace, ROP Crete
and Aegean Islands, ROP Thessaly, Mainland Greece, Epirus, ROP Attica.

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C
Cox
(A)
MnII-L3imid@Cox
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
PC
PCox
MnII-L3imid@PCox
(B)
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Figure 1.IR spectra of the (A) activated carbon (C), the oxidized activated carbon (Cox), the catalyst
MnII-L3imid@Cox and (B) of the pyrolytic carbon (PC), the oxidized pyrolytic carbon (PCox) and the
catalyst MnII-L3imid@PCox.

C
Cox
(A)
400
350
300
250
200
150
100
50
MnII-L3imid@Cox
0,016
0,012
0,008
0,004
0,000
Cox
C
0
20
40
60
80
100
Radius (Å)
0
0,0
0,2
0,4
0,6
0,8
1,0
P/P0
(B)
300
250
200
150
100
50
PC
PCox
MnIIL3imid@PCox
0
0,0
0,2
0,4
0,6
0,8
1,0
P/P0
Figure 2. N2 adsorption-desorption isotherms for (A) C, Cox, MnII-L3imid@Cox, (inset) Pore size
distribution curve for C and Cox and (B) PCo, PCox and MnII-L3imid@PCox.

C
Cox
(A)
MnII-L3imid@Cox
10
20
30
40
50
60
2-theta (degree)
ZnO (79-0205)
(B)
PC
PCox
II
Mn -L imid@PCox
3
10
20
30
40
50
60
70
80
2-theta (degree)
Figure 3.X-ray diffraction (XRD) characterization for (A) activated carbon (C), oxidized activated
carbon (Cox), MnII-L3imid@Cox; (B) for pyrolytic carbon (PC), oxidized pyrolytic carbon (PCox)
and MnII-L3imid@PCox.

C
Cox
G band
D band
(A)
MnII-L3imid@Cox
I
/I = 1.05
D G
I /I = 0.98
D G
I
/I = 0.93
D G
3000
2500
2000
1500
1000
500
Raman Shift (cm-1)
G band
D band
PC
PCox
MnII-L3imid@PCox
(B)
I
/I = 0.83
D G
I
/I = 0.77
D G
I
/I = 0.69
D G
3000
2500
2000
1500
1000
500
Raman Shift (cm-1)
Figure 4. Raman spectra of (A) activated carbon based materials: C, Cox, MnII-L3imid@Cox and (B)
pyrolytic carbon based materials: PC, PCox and MnII-L3imid@PCox.

Figure 5. SEM micrographs of (a) C, (b) Cox, (c) MnII-L3imid@Cox,(d) PC, (e) PCox and (f) MnII-
L3imid@PCox.

100
90
80
70
60
50
40
30
20
10
0
II
Epoxide
Alcohol
Ketone
Mn -L imid@Cox
3
II
Mn -L imid@PCox
3
e
c
e
e
e
n
ne
e
x
e
n
n
e
n
n
e
e
e
b
il
t
n
e
e
x
n
e
-
t
e
-
e
x
n
e
ze
e
e
e
c
n
r
3
-
t
p
h
n
t
o
e
y
e
h
o
l
-
1
-
e
s
-
t
oh
n
e
o
l
l
b
)
S
c
y
c
e
l]
p
Z
lo
x
y
(
y
y
h
]
c
o
l
c
y
C
c
l
C
n
y
lc
He
e
y
.1
.1
-
1
2
y
h
t
C
-
3
[
p
n-
e
e
o
lo
-
r
c
M
-
1
p
-
y
1
)
ic
lb
E
(
[
prop-
y
-
h
4
4
t
-
l
e
hy
im
t
r
t
-
me
,6
,6
-
1
-
)
R
4
(
Figure 6. Distribution of oxidation products catalyzed by the MnII-L3imid@Coxand MnII-
L3imid@PCoxin the presence of H2O2. Reactions performed in CH3COCH3 / CH3OH (0.45 / 0.40 ml).
See Table 2 for further details.

1000
800
600
400
200
0
II
Mn -L imid@Cox
3
II
Mn -L imid@PCox
3
ne
ne
ne
ne
yre
ne
e
-
ne
nte
e
e
t
e
-
xene
xene
xene
e
bene
l
3
e
e
i
t
-
x-1
St
s
-
ooc
benze
oh
cl
oh
oh
l
]
l
)
Z
op
cl
He
hept
cl
(
]
Cy
nyl
cyc
cyc
Cy
1
.
1
e
Cy
-
yl
-
1
-
hyl
t
.
3
e
n-2
o[
e
-
l
M
-
1
cyc
p-1
E)-prop
bi
[(
pro
-
hyl
met
4
-
hyl
4
ri
t
-
6
,
met
-
6
,
1
-
R)
4
(
Figure 7. Total turnover numbers for alkene epoxidations catalyzed by MnII-L3imid@Cox and MnII-
L3imid@PCox in the presence of H2O2. Reactions performed in CH3COCH3 / CH3OH (0.45 / 0.40 ml).
See Table 2 for further details.

80
70
60
50
40
30
20
10
0
340
320
300
280
260
240
220
200
180
= MnII-L3imid@Cox
= MnII-L3imid@PCox
-5
0
5
10
15
20
25
Time (h)
Figure 8.Time dependence of cyclooctene epoxidation and solution redox potential for the same
reaction catalyzed by MnII-L3imid@Cox (■) and MnII-L3imid@PCox (●).

1000
900
800
700
600
500
400
300
200
100
0
MnII-L3imid@Cox
MnII-L3imid@PCox
ne
ne
ne
ne
yre
ne
e
-
ne
nte
e
e
t
e
-
xene
xene
xene
e
bene
l
3
e
e
i
t
-
x-1
St
s
-
ooc
benze
oh
cl
oh
oh
l
]
l
)
Z
op
cl
He
hept
cl
(
]
Cy
nyl
cyc
cyc
Cy
1
.
1
e
Cy
-
yl
-
1
-
hyl
t
.
3
e
n-2
o[
e
-
l
M
-
1
cyc
p-1
E)-prop
bi
[(
pro
-
hyl
met
4
-
hyl
4
ri
t
-
6
,
met
-
6
,
1
-
R)
4
(
Figure 9.TOFs for alkene epoxidation catalyzed by MnII-L3imid@Cox and MnII-L3imid@PCox in
the presence of H2O2. Reactions performed in CH3COCH3 / CH3OH (0.45 / 0.40 ml). See Table 2 for
further details.

II
Mn -L imid@Cox
3
80
70
60
50
40
30
20
10
0
800
700
600
500
400
300
200
100
0
II
Mn -L imid@PCox
3
1stuse
2nduse
3rduse
4thuse
1st use 2nd use 3rd use 4th use
Figure 10.Recyclability of MnII-L3imid@Coxand MnII-L3imid@PCox for cyclooctene oxidation
with H2O2. Catalytic conditions as those described in Table 2.

Scheme 1: Schematic representation of the synthesis of MnII-L3imid@Cox and MnII-L3imid@PCox
catalysts.