Molecular Mn-catalysts grafted on graphitic carbon nitride (gCN): The behavior of gCN as support matrix in oxidation reactions

Accepted Manuscript Molecular Mn-catalysts grafted on graphitic carbon nitride (gCN): the behavior of gCN as support matrix in oxidation reactions Anastasia Simaioforidou, Yiannis Georgiou, Athanasios Bourlinos, Maria Louloudi PII: DOI: Reference:

S0277-5387(18)30366-8 https://doi.org/10.1016/j.poly.2018.06.048 POLY 13257

To appear in:

Polyhedron

Received Date: Accepted Date:

9 March 2018 26 June 2018

Please cite this article as: A. Simaioforidou, Y. Georgiou, A. Bourlinos, M. Louloudi, Molecular Mn-catalysts grafted on graphitic carbon nitride (gCN): the behavior of gCN as support matrix in oxidation reactions, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.06.048

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Molecular Mn-catalysts grafted on graphitic carbon nitride (gCN): the behavior of gCN as support matrix in oxidation reactions

Anastasia Simaioforidou1, Yiannis Georgiou1, Athanasios Bourlinos2, Maria Louloudi1*

Dedicated to Professor Spyridon Perlepes on the occasion of his 65th birthday.

1) Department of Chemistry, University of Ioannina, GR-45110 Ioannina, Greece 2) Department of Physics, University of Ioannina, GR-45110 Ioannina, Greece

e-mail: [email protected] (M. Louloudi)

Abstract We report a synthetic methodology for covalent grafting of metal complexes with Schiff base ligands, on gCN as support. Based on this, the La@gCN-MnII and Lt@gCN-MnII materials have been prepared. Their physicochemical characterization suggested the successful immobilization of the entire [MnII-Schiff base] complexes on gCN without significant disturbance of ordered structure of the pristine gCN. The La@gCN-MnII and Lt@gCN-MnII were evaluated as catalysts for olefins’ epoxidations with H2O2. We found that they are efficient, selective towards epoxide formation and interestingly, the Lt@gCN-MnII was kinetically faster than the La@gCN-MnII. That is, the grafting process and the support environment offered by gCN maintain the catalytic performance of grafted molecular catalysts. Moreover, a detailed recycling study of the present catalytic materials has been done to investigate their robustness focusing on the textural features of the recovered and recycled catalysts. Our data showed an important matrix exfoliation occurs during oxidation catalysis indicating a limited stability of gCN as support material in oxidative media.

Keywords: graphitic carbon nitride; Mn(II) catalysts; grafting; alkene epoxidation; exfoliation

1. Introduction Manganese Schiff base complexes present often good catalytic behavior in epoxidation reactions with high activity and selectivity [1]. However, they are easily deactivated, in liquid homogeneous catalytic reactions, due to the formation of μ-oxo dimmers and other polymeric species [2, 3]. Moreover, catalyst separation from the reaction products and medium is usually energy- and cost-intensive. To manage these problems/limitations, immobilization of the catalysts onto a solid support constitutes an applicable approach [1, 4]. Thus, the obtained supported heterogeneous catalysts are easily handled and convenient for catalyst recovering and recycling procedures resulting in improved processing steps and environmentally friendly manufacturing [5, 6]. Evaluating traditional inorganic materials as supports, certain restrictions are noted mostly due to the nature of their active sites resulting in limitations to the designed applications. On the other hand organic polymeric supports have the advantage of flexibility for optimization but traditionally suffer from substantial swelling and stability problems [7]. Within this context the heterogenization of Mn complexes by their anchoring onto the surface of carbon-based materials as activated carbon, pyrolytic carbon and CMK matrices, have already shown their potential in catalytic epoxidations [8-11]. Continuing our exploration in carbon based materials as supports, we consider that the use of the so-called gCN which is the most stable allotrope of carbon nitride is a challenge. Carbon nitride seems to be a very attractive support, this polymeric semiconductor is mainly composed of carbon and nitrogen, and its organic chemistry provides ways to modify its reactivity without having to change the overall composition too much. Thus, the fabrication of graphitic carbon nitride materials (for simplicity: g-C3N4 or gCN) with different textural structures and surface morphologies is expected to affect the catalytic performances [12]. The framework topology identified in gCN is in fact presumably a defect-rich, Nbridged ‘poly(tri-s-triazine)’. As the s-triazine ring (C3N3) is aromatic, it is proven that a conjugated, two-dimensional polymer of s-triazine form a p-conjugated planar layers like that of graphite [13-16]. Since it bears a high concentration of active sites it is suitable for the immobilization of metal complexes by using for example an appropriate bifunctional linker [17]. To this front, our work here presents the covalent grafting of Mn-Schiff base complexes onto gCN aiming to investigate the potential of the obtained heterogeneous catalysts in alkene epoxidations with the use of eco-friendly H2O2 as oxidant. For readership completeness, it is note that carbon nitride itself or after modifications was used as catalyst in oxidation of alkanes [16, 18, 19], olefins [20, 21] or alcohols [22]. Among olefins which have been subjected to oxidation in the presence of carbon nitride catalysts were toluene, ethylbenzene and some substituted benzylic aromatics. The activity of bulk gCN was very low, while doping of boron atoms in the matrix of gCN promotes the

catalytic activity, for instance, from a 2% toluene conversion to a 6.3% with the doped material [19]. Interestingly, Min et al. [20] reported the UF-MCN material, which was synthesized by nanocasting with urea–formaldehyde by polymerization over the disk shaped mesoporous silica (INC-2) as a hard template and which showed catalytic activity toward oxidation of cyclic olefins using H2O2. The conversion of cyclic olefins was ranged from 65– 80% within 10 h at 75 ◦C with selectivity 40–90% towards epoxides depended on substrate size. Ding et al. [21] showed that transition metals including Fe, Co, Ni, Mn, and Cu can be successfully included into a gCN matrix by a simple soft-chemical method without destroying the graphic structure of the host; among the studied doped materials, Co and Fe modified carbon nitrides were proven to be active for the epoxidation of styrene with O2. In 2018 Bian et al. [23] immobilized a molybdenum-Schiff base complex onto GO surface and combine them with the gCN polymer resulting in Mo-GO/gCN composite which was tested in a photocatalytic epoxidation showing good photo-catalytic activity. Thus, it is obvious that catalytic studies using gCN as support are rather scarce in the literature. Within this, chemical surface modification of gCN by covalent grafting of catalytically active metal complexes and evaluation of the catalytic behavior of the hybrid structure seems to be interesting. Herein, we report the synthesis and characterization of gCN supported [MnII-Schiff base] complexes via the covalent grafting methodology. The catalysts La@gCN-MnII and Lt@gCN-MnII were evaluated as catalysts for olefins’ epoxidations. The catalytically active component [MnII-Schiff base], which is designed and well-studied by our group, functions in epoxidations via the oxygen-transfer by a MnIV=O transient intermediate. The present study aims to investigate any positive effect of gCN support in catalytic epoxidations with H2O2 and in general to evaluate its behavior in oxidative media. The catalysts La@gCN-MnII and Lt@gCN-MnII were found to be selective towards epoxide formation, while Lt@gCN-MnII is kinetically faster than the La@gCN-MnII. Significantly, we demonstrate that important matrix exfoliation occur during oxidation catalysis resulting in limited stability of gCN as support material.

2. Experimental 2.1. Starting Materials The chemicals used for the synthesis and functionalization of carbon nitride were purchased from Sigma Aldrich and Merck. All substrates were purchased from Aldrich, in their highest commercial purity, stored at 5 °C and passed through a column containing active alumina to remove peroxide impurities. Analytical grade acetone, methanol and ammonium

acetate were from Aldrich and Merck respectively. Hydrogen peroxide was 30% aqueous solution (Aldrich). 2.2. Ligand Synthesis Ligands La and Lt have been synthesized and characterized by methods detailed in ours works [24, 25]. 2.3 Carbon Nitride Synthesis The most common precursors used for chemical synthesis of gCN are reactive nitrogenrich and oxygen-free compounds containing pre-formed C−N core structures, such as triazine and heptazine derivatives, most of them being unstable, difficult to obtain and/or highly explosive. The synthesis of single-phase sp3-hybridized carbon nitrides is a challenging task due to their low thermodynamic stability [26, 27]. Not surprisingly, it seems that the defect materials are much more valuable than the ideal one, in particular for catalysis, which requires surface defects. Thus, the synthesis of gCN with defects is an interesting topic, when the material is going to be used in catalysis. Based on above, the method used for the synthesis of carbon nitride (codenamed gCN) was according to Daikopoulos et al. [28]. FT-IR (cm-1, selected peaks): 3384: ν(N-H); 3206: ν(N-H); 1622: ν(C=N); 1401: ν(C-C); 1283: ν(C-N); 616: ν(N-H). Average surface area, ca. 10 m2 g-1. 2.4. Synthesis of modified carbon nitrides 1st Process: 500 mg of gCN and 0.5 mmol of dichloro(3-chloropropyl)methylsilane were refluxed for 1 day in 10 ml of toluene under N2 at 80 °C in order to generate gCN-Cl. To the solution, 0.5 mmol of La, which were dissolved in 20 ml propylcarbonate and 3 ml MeOH, were added and they left to reflux for 2 days at 60 °C. The obtained material La@gCN, was washed several times with MeOH and finally dried at 40 °C for 1day. La@gCN: FT-IR (cm-1, selected peaks): 1785: ν(C=O); 1619: ν(C=N); 800: ν(Si-CH3); 776: ν(Si-C). Average surface area, ca. 15.6 m2 g-1. To a suspension of 200 mg of La@gCN in MeOH, 0.35 mmol of MnCl2.4H2O dissolved in 15 ml MeOH were added and they were left to stir overnight at room temperature leading to La@gCN-MnII. La@gCN-MnII: Metal loading: 0.20 mmol g-1. FT-IR (cm-1, selected peaks): 1743: ν(C=O); 1625: ν(C=N). Average surface area, ca. 9 m2 g-1. 2nd Process: 500 mg of gCN and 0.5 mmol of epichlorohydrin were refluxed for 1 day in 10 ml of toluene under N2 at 80 °C in order to generate gCN-epoxy. To the solution, 0.5 mmol of Lt, which were dissolved in 5 ml toluene and 5 ml MeOH, were added and they left to reflux for 1 day at 60 °C. The obtained modified material Lt@gCN was washed several times with MeOH and subsequently dried at 40 °C for 1 day. Lt@gCN: FT-IR (cm-1, selected peaks): 3208: ν(N-H); 1623: ν(C=N). Average surface area, ca. 12 m2 g-1. To a suspension

200 mg of Lt@gCN in MeOH, 0.44 mmol of MnCl2.4H2O dissolved in 15 ml MeOH were added and they were left to stir overnight at room temperature leading to Lt@gCN-MnII. Lt@gCN-MnII: Metal loading: 0.37 mmol g-1. FT-IR (cm-1, selected peaks): 3229: ν(N-H); 1630: ν(C=N). Average surface area, ca. 13 m2 g-1. 2.5 Physical Measurements All samples were characterized by powder X-ray diffraction (XRD) on a D8 Advance Bruker X-ray diffractometer at room temperature, using monochromatized Cu- Ka radiation (k = 0.154 nm) as the X-ray source. Data were collected over the 2θ range of 5–80°, 0.02 steps and 2 sec counting time per step. Inter-planar distances were calculated using the Bragg equation. SEM images were obtained on a Jeol JSM 5600 system operating at 20kV. The samples were placed in a sample holder and metalized with gold. The N2 adsorption–desorption isotherms were measured at 77 K on an Autosorb-1, Quantachrome porosimeter. The program consisting of both an adsorption and desorption branch and typically ran at −196 °C after samples were degassed at 80 °C for 12 h under vacuum (10–4 mbar) before the measurements. Specific surface area SBET was determined with the Brunauer–Emmett–Teller (BET) method. TGA traces were obtained under synthetic air with a Shimadzu, DTG-60 instrument, at a heating rate of 5 °C min-1 and a flow rate of 50 cm3 min-1. Raman spectra were recorded using a Raman microscope system (Renishaw, System 1000) consisting of an optical microscope (Leica) coupled to a Raman spectrometer. Infrared spectra were recorded in KBr pellets with a FT–IR spectrometer of Nicolet iS5 model. IR spectra were recorded in the range of 4000–400 cm−1 using a resolution of 4 cm−1 and 32 scans. GC analysis was performed using and 8000 Fisons chromatograph with a flame ionization detector and a Shimadzu GC-17A gas chromatograph coupled with a GCMS-QP5000 mass spectrometer using a SPB-5 column (30 m, 0.25 mm i.d, 0.25 μm). The epoxidation reactions were followed with temperature program for cyclohexene, cyclooctene, styrene, limonene, cyclopentene, 1-methyl-1-cyclohexene and alpha-pinene was: 80 °C (5 min), 10 °C min-1, 150 °C (5 min). For trans-β-methylstyrene was: 80 °C (5 min), 10 °C min-1, 220 °C (1 min). For hex-1-ene: 50 °C (5 min), 10 °C min-1, 150 °C (5 min); Injector temperature: 230 °C interface temperature: 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. Solution potential Eh was measured by a Metrohm platinum redox electrode (type 6.0401.100). 2.6. General procedure for catalytic oxidation of olefins In a typical procedure, catalytic experiments were carried out in a 5 mL vial containing 2 mmol of H2O2, that were first diluted in methanol (1:1) and were added digitally using

controlled syringe pump [SP1011Z WPI] over 30 min in a mixture of acetone/MeOH (450 μl /400 μl). Acetophenone or bromobenzene (as internal standard, 1 mmol), the olefin (as substrate, 1 mmol), CH3COONH4 (as additive 2 or 2.5 mmol) and finally the catalyst (1 μmol of Mn-complex) were added. The oxidation reaction was under magnetic stirring at room temperature. The solid catalyst was then filtered, and the organic phase was analyzed by gas chromatography coupled with a mass spectrometer. All the reactions were performed at least two times and the oxidation products were compared with authentic samples. Moreover, when allylic epoxidation products were detected a PPh3-treatment according to the Shul'pins’ procedure [29] was employed to ensure that putative hydroperoxides formed during catalysis as primary products have been completely transformed into the corresponding alcohol/ ketone [29-31]. 2.6.1 Catalysts reuse and stability The reusability of heterogeneous catalytic systems was checked in the multiple consecutive oxidation of cyclooctene as described above. At the end of each reaction, the catalyst was separated and washed with methanol and then, dried at 60 °C for 24 h before reuse. However, after each catalytic run, the mass of recovered catalyst was much lower that the started catalyst due to gCN exfoliation. To ensure that we dispose the required mass of recycled catalyst for the next catalytic reuse, 50 identical catalytic batches of La@gCN-MnII and Lt@gCN-MnII respectively were run, followed by centrifugation/washing/drying as described above. Such recovered solid catalysts were reused for new catalytic reactions under the same catalytic conditions. 2.6.2. Kinetic Studies The time dependence of the cyclooctene epoxidation catalyzed by the supported catalysts, in conjunction with the observed redox potential of solution Eh (standard electrode Metrohm), were studied. After the end of each Eh measurement the catalytic reaction was monitored by GC-MS in order to analyse the progress of the product yield and the solution was left to stir. The same procedure was employed till the Eh was stable and the product yield was reached a maximum. Thereafter, the solution was left to stir and tested again after 24 h to crosscheck the final recorded values for product yield and Eh. 2.7 EPR study Three mg of solid catalytic material inserted in 3 mm ID quartz tubes (Willmad Glass) were used for EPR measurements. To estimate Mn-leaching during catalysis, the catalytic mixture was centrifuged, and 120 μl of the liquid supernatant were measured frozen at 77 K in liquid N2. Quantitative determination of the MnII-concentration was done by comparison with a standard solution of MnCl2 in MeOH, using the double integral of the EPR signal.

3. Results and discussion 3.1 Synthesis of the catalysts Modification of gCN was achieved by reaction of its terminal amino-groups with the bifunctional linkers dichloro(3-chloropropyl)methylsilane or epichlorohydrin resulting in formation of gCN-Cl and gCN-epoxy solids respectively (Scheme 1). Subsequently, reaction of these materials with the ligands La and Lt led to their covalent grafting and the obtained post-modified carbon nitride matrices were coded as La@gCN and Lt@gCN respectively. Interaction of immobilized ligands with solution of MnCl2 provided the metallated La@gCNMnII and Lt@gCN-MnII catalytic materials characterized in the following.

II

II

Scheme 1: Schematic representation of the synthesis of La@gCN-Mn and Lt@gCN-Mn .

3.2. Powder X-ray diffraction (XRD) gCN La@gCN

Intensity

La@gCN-MnII

gCN Lt@gCN Lt@gCN-Mn

10

20

30

40

50

60

70

II

80

ο

2θ( )

Fig. 1. XRD patters (up) gCN, La@gCN, La@gCN-MnII and (down) gCN, Lt@gCN and Lt@gCN-MnII

The typical XRD pattern of the gCN and the hybrids materials are shown in Fig. 1 and we easily found that there are two prominent peaks at ca. 27.1° and 12.0°, respectively. In Fig. 1, the most intense peak at 27.1° corresponds to an interlayer stacking distance of d002 = 0.338 nm and can be indexed as the (002) stacking peak of the conjugated aromatic ring. Further discussing the stacking distance (d002), it was found that it is slightly shorter than that of crystalline graphite (0.353 nm) [32, 33]. That is, the packing density of g-C3N4 perpendicular to the layer direction is higher, which may be caused by a stronger π–π stacking or van der Waals forces between carbon and nitrogen atoms in the adjacent layers. This could be the reason why the thermal and chemical stability of carbon nitride is relatively higher than of other polymers. The other peak observed at 12.6°, indexed as (100), and may be attributed to an in-plane structural packing motif, and accordingly, the distance was be calculated to be 7.0 Å, very close to tris-s-triazine unit (7.1 Å); this further supports the aspect that carbon nitride is constituted by heptazine rather than triazine ring [12, 34-36]. This peak is still observed for La@gCN, La@gCN-MnII, Lt@gCN and Lt@gCN-MnII (Fig. 1) indicating that the ordered structure of pristine material was retained after the grafting of the [MnII-Schiff base] catalysts on their surface. However, the observed loss in intensity could be due to the disturbance of the packing and potential undulations of the single layers after the grafting of the catalytic active centers [37].

3.3. Electron microscopy

Fig. 2. SEM images: a) gCN, b) La@gCN, c) La@gCN-MnII, d) Lt@gCN and e) Lt@gCN-MnII

SEM images given in Fig. 2 demonstrate the morphologies of as-prepared gCN, modified carbon nitrides matrices and catalytic materials. Fig. 2a shows that pristine carbon nitride has a layered structure with large grains. In Fig. 2 b-e, it is evident that during each modification process the size of the material changes, however, its morphology remains layered. For example, the obtained materials La@gCN and Lt@gCN (Fig. 2b, d) which bear covalently the organic ligands, contain smaller particles than the initial gCN; moreover, the metallation caused additionally a significant breakage as shown in Fig. 2c, e. Given that the as-prepared gCN is a partly imperfect polymer-like structure, it could be broken up from its defects by protonation/grafting processes, while the characteristic interplanar stacking structure remains showing that delamination is far from being complete. However, this happens in an important degree, as we show hereafter, under oxidative conditions.

3.4 FT-IR study The FT-IR spectra of the developed catalysts show characteristic bands, which are attributed to various structural components of the present materials, i.e. to the support scaffold as well as to the grafted Schiff base ligands La and Lt. When the FT-IR spectra of La@gCN and Lt@gCN were compared to those of the parent material gCN, new bands can be observed which are attributed to the anchored ligands. More specifically, in the spectra of grafted ligand La@gCN, Fig 3(a), the band observed at 1785 cm-1 is attributed to (C=O) stretching vibrations of the Schiff base ligand La; the band at 1617 cm-1 is assigned to the ν(C=N) vibrations with contributions from the ligand La as well from the gCN matrix. The bands at 800 and 776 cm-1 are attributed to (Si-CH3) and (Si-C) vibrations from the linker [38, 39].

Relative Transmittance

gCN La@gCN II La@gCN-Mn

gCN Lt@gCN Lt@gCN-MnII 4000 3500 3000

2000 1500 1000 500 -1

Wavenumbers (cm ) Fig. 3. FTIR spectra of: (up) gCN, La@gCN, La@gCN-MnII; (down) gCN, Lt@gCN and Lt@gCN-MnII

Finally, in the spectrum of La@gCN-MnII the ν(C=N) vibration is being clearly shifted to 1626 cm-1, while the band at 1785 cm-1 is absent when compared with the nonmetalated material La@gCN probably due to its shift after metalation; this behavior indicates coordination of the MnII with the imine-nitrogen and keto-oxygen of the attached ligand La [8, 40, 41]. In the FT-IR spectra of Lt@gCN the bands at 3208 and 1627 cm-1 are attributed to the (N-H) and (C=N) stretching vibrations from the ligand Lt and the gCN matrix. However, in the FT-IR spectrum of metalated material Lt@gCN-MnII these bands are clearly shifted at 3197 and 1623 cm-1 when compared with the corresponding non-metalated material Lt@gCN, indicating coordination of MnII to nitrogen of (N-H) and (C=N) sites of the ligand Lt preferably due to its chelation capacity.

3.5 Thermal analysis Thermal gravimetric analysis (TGA) of gCN is shown in Fig. 4. The initial weight-loss close to 100 °C is ascribed to physically adsorbed water. The slow sublimation of carbon nitride occurs at 286 °C and it completely decomposes at 600 °C. The low decomposition temperature of gCN indicates a rather low matrix condensation degree since a complete decomposition for the majority of carbon nitrides occurs at 750 °C without material residue [42].

(a)

100

o

529 C o

512 C

(b)

La@gCN-MnII

80 o

o 540 C 570 C

gCN

o

o

313 C 302 C

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603 C

La-@gCN

gCN

o

o

513 C

o

exo

317 C

40 20

TG%

DTA

La@gCN-MnII La@gCN

endo

o

529 C

(c)

0 100

(d)

Lt@gCN Lt@gCN-MnII

80 gCN

60 Lt@gCN-Mn

II o

314 C

gCN

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o

529 C o

o

603 C

311 C

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o

515 C o

295 C

0

Lt@gCN

100

200

300

400

500 o

Temperature ( C)

600

700

100

200

300

400

500

600

700

o

Temperature ( C)

Fig. 4. Differential thermal analysis (DTA) curves (a), (c) and corresponding thermogravimetric analysis (TGA) curves (b), (d) of gCN, La@gCN, La@gCN-MnII, Lt@gCN and Lt@gCN-MnII.

This thermal stability is, however, one of the highest found for an organic material and for instance higher than those of typical high-temperature polymers and aromatic polyamides and polyimides. Note that the thermal stability of carbon nitride depends to preparation methods which are probably related to different degrees of polymerization [14, 43-46] . The thermograms of La@gCN-MnII and Lt@gCN-MnII, in the same temperature range, show additionally a 23.0% and 29.0% weight loss which is assigned to the decomposition of the anchored organic ligands moieties. The total combustion La@gCN-MnII corresponds to 98.3% total weight loss and the obtained solid residue of 1.7% is assigned to MnO2. Ligand and metal loading achieved is ca. 0.20 mmolg-1 determined by both thermogravimetric and elemental analysis. Accordingly, the combustion of Lt@gCN-MnII provides a total weight loss of 96.5% and the obtained solid residue of 3.5 % corresponds to MnO2. Ligand and metal loading achieved is ca. 0.37 mmolg-1 determined by both thermogravimetric and elemental analysis. 3.6 Raman The Raman spectra of gCN, La@gCN, La@gCN-MnII, Lt@gCN and Lt@gCN-MnII are shown in Fig. 5. Upon a first assignment, the Raman doublet at 1531 (G band) and 1346 cm-1 (D band) is indicative of sp2- and sp3-hybridized carbons respectively [47]. Additionally, the pristine gCN exhibits a broad band at 2837 cm-1, which is typical for nitrogenous carbons with high N content [47], and a weak doublet at 2219 cm-1 assigned to sp bonded-CN groups [48]. In this case it is indicated that nitrogen is incorporated into the carbon matrix through

sp2 and sp3 C-N bonds, which provide also analogous G and D Raman bands. Therefore, in the Raman spectrum of gCN, the G and D bands are derived by both C-C and C-N bonding [49]. Grafting and metalation procedures affect by decreasing the intensity of both G and D bands at 1531 and 1346 cm-1 resulting in G band prevail. In analogous way, the band at 2837 cm-1 which is attributed to N content is decreased too while the weak doublet at 2219 cm-1 is no more detectable. Overall, these findings suggest a well-accomplished and consistent modification of the pristine gCN matrix which results in decrease of graphite component of the matrix in conjunction with increase of the disorder [48, 50]. G-band 1531

gCN La@gCN La@gCN-Mn

D-band 1346

II

2837

Intensity (a.u.)

2219

gCN Lt@gCN Lt@gCN-Mn

II

4000 3500 3000 2500 2000 1500 1000 500

Raman Shift (cm-1) Fig. 5. Raman spectra of: (up) gCN, La@gCN, La@gCN-MnII; (down) gCN, Lt@gCN and Lt@gCN-MnII

3.7 Catalytic activity The synthesized and characterized catalytic materials La@gCN-MnII and Lt@gCN-MnII were applied for the epoxidation of olefins with H2O2 in the presence of CH3COONH4 as additive at room temperature (25 °C) with a molar ratio of [catalyst/ H2O2/substrate/ additive] equal to [1:2000:1000:2000] and [1:2000:1000:2500] μmol. The yields and selectivity of catalytic epoxidations are shown as graphs in Fig. 6 (a) and they are given also in Supplementary Information in Table S1. A set of control experiments have been also run to be crosschecked the catalytic activity of La@gCN-MnII and Lt@gCN-MnII, i.e., either without CH3COONH4, or H2O2, or without the active Mn-site by using the as-prepared gCN. In the absent of CH3COONH4, H2O2 or the active Mn-site [MnII-La] and [MnII-Lt] epoxidation reactions didn’t occur. Focused especially on the role of CH3COONH4, our group

has clarified that it acts as proton-donor and proton-acceptor promoting heterolytic O-O cleavage of MnII-OOH transient intermediate [24, 25, 29].

Fig. 6. a) Distribution of oxidation products b) Total turnover numbers of the epoxidation of alkenes catalyzed by the La@gCN-MnII and Lt@gCN-MnII in the presence of H2O2. Reaction conditions: cat:H2O2:substrate:additive =1:2000:1000:2000 and 1:2000:1000:2500 μmol respectively, at room temperature in 1ml acetone/ methanol (v/v 0.45:0.4) as solvent.

Comments on the data of Table S1 could are that La@gCN-MnII and Lt@gCN-MnII showed a high catalytic activity in the epoxidation of cyclooctene, cyclohexene, limonene, 1methyl-1-cyclohexene and cyclopentene. Cyclooctene epoxide was the sole product with 53 and 55.5% yield and 100% selectivity, respectively. In the epoxidation of cyclohexene the major product was the corresponding epoxide with 85.2% yield obtained by the Lt@gCNMnII. Allylic epoxidation products such 2-cyclohexen-1-one (5.6-7.6%) and 2-cyclohexen-1ol (11.1-10.4%) were traced too. It is clarified that the quantified amounts of 2-cyclo1hexene-ol and 2-cyclo1-hexen-1-one are identical to those obtained after a PPh3-treatment according to the Shul'pins’ procedure [29] which ensures that putative hydroperoxides formed during catalysis as primary products have been completely transformed into the corresponding alcohol/ ketone [29-31]. The epoxidation of linear hex-1-ene promoted with just 7.8-10.3% yield. Pinene oxide was the main product (37.1% yield), in catalytic epoxidation of a-pinene, while small amounts of 2-pinen-4-ol (4.2-4.8%) and 3-pinanone (4.7-5.3%) have been also detected. In the case of 1-methyl-1-cyclohexe, the corresponding epoxide was produced very effectively (52.2 – 58.5%) with co-formation of additional products, i.e., 1-methylcyclohex-2-en-1-ol (8.6-9.2%) and 2-methylcyclohex-2-en-1-one (2.12.3%). Oxidation of trans-β-methylstyrene produced 39.4 and 35.5% trans-β-methylstyrene oxide with 100% selectivity catalyzed by La@gCN-MnII and Lt@gCN-MnII respectively. Styrene afforded low conversion (28.4-37.4%) mostly due to the presence of more electron withdrawing groups connected to the double bond. As a result of the electrophilic attack on the 1,2-double bond of the limonene molecule, 1,2-epoxylimonene is formed with 57.9-65.8% yield as a mixture of -cis and -trans epoxides. Comparing the catalytic performance of both

La@gCN-MnII and Lt@gCN-MnII catalysts based on the achieved TONs, we do not observe a clear superiority of any of them (Table S1, Fig. 6 (b)). 3.8 Kinetic Studies-TOFs

Fig. 7. A) Evolution of the cyclooctene epoxidation vs. time and solution redox potential for the same reaction, B) Turn over frequency of the epoxidation of alkenes catalyzed by La@gCN-MnII and Lt@gCN-MnII in the presence of H2O2. Reaction conditions: cat:H2O2:substrate:additive =1:2000:1000:2000 and 1:2000:1000:2500 μmol respectively, at room temperature in 1ml acetone/ methanol (v/v 0.45:0.4) as solvent.

The time dependence of the cyclooctene oxidation catalyzed by the La@gCN-MnII and Lt@gCN-MnII catalysts in conjunction with the observed redox potential of solution Eh (vs. standard hydrogen electrode SHE) are shown in Fig.7 (a). At the beginning of the reaction catalyzed by La@gCN-MnII, Eh was + 325 mV; after 2.5 h, it dropped at + 175 mV with a 51 % epoxide yield. In the case of Lt@gCN-MnII –catalyzed epoxidation, at reaction time t=0, Eh was + 399 mV, at t=1 h it decreased at +198 mV providing a product yield of 55.5%. Thus overall, oxidations catalysed by La@gCN-MnII were practically accomplished within 2.5 h and reactions catalysed by Lt@gCN-MnII were accomplished within 1 h. The resulting TOFs, listed in Table 1, visualised in Fig. 7(b), clearly demonstrate the superiority of Lt@gCN-MnII vs.the La@gCN-MnII. Accordingly, notice that the TOFs achieved by Lt@gCN-MnII ranged from 78-582 h-1while the TOFs by La@gCN-MnII range from 41-325 h-1.

3.9 Recycling studies The stability and reusability of the two catalysts was also studied. Catalysts’ recycling experiments were run with cyclooctene as substrate under the same conditions as the ordinary catalytic reactions. After the end of each catalytic reaction, the solid catalysts were recovered from the reaction mixture by direct filtration or centrifugation, washed, dried and reused in the next run under the same catalytic conditions. However, we have to notice that after each catalytic run, the mass of recovered catalyst was much lower that the started catalyst probably due to gCN exfoliation. Thus, to ensure that we dispose the required mass of recycled catalyst for the next catalytic reuse, 50 identical catalytic batches of La@gCN-MnII and Lt@gCNMnII respectively were run, followed by centrifugation/washing/drying as described above. Nevertheless, such recovered catalysts maintained their activity and selectivity upon each reuse. After each run, manganese leaching into the filtrate was examined by EPR and the structure of the recovered solids was analyzed by XRD. As we show hereafter, partial oxidative destruction of catalytically active Mn-complexes occur in conjunction with significant matrix exfoliation during oxidation catalysis. Taking into account that free MnII species shows very limited catalytic performance [30], we confirm that the observed catalytic performance is derived from the solid catalysts La@gCN-MnII and Lt@gCN-MnII which bear intact the [MnII-La] and [MnII-Lt] active components. 100 90 80

La@gCN-MnII Lt@gCN-MnII

Yield(%)

70 60 50 40 30 20 10 0

e e e e e se sh us d us h us h us h us Fre 1st u nd 4t 5t 6t 3r 2

Fig. 8. The recovery and reuse of La@gCN-MnII and Lt@gCN-MnII. The reaction conditions are as follows: 1:2000:1000:2000 and 1:2000:1000:2500 μmol, Cat: H2O2: cyclooctene: additive and reactions were run at room temperature for 2 and 4 h. Acetone: methanol (0.45:0.4) as solvent.

On the other hand, the reduced stability of gCN as support material, leads us to wonder if the reusability performance being shown has some real meaning since the heterogeneouscharacter of the catalytic materials is rather lost with the achieved mass recovery which was found to be <50%.

3.10 EPR Spectroscopy In Fig. 9 A, B (solid black lines) we present 77 K EPR spectra of the ‘as prepared’ unused catalysts La@gCN-MnII and Lt@gCN-MnII. The gCN-based materials are characterized by a broad Gaussian derivative centered at g ∼2 and show 6-line EPR spectra with resolved MnII (S = 5/2, I = 5/2) hyperfine splitting including, however, a moderate surface clustering. The resolved semi forbidden 55Mn-hyperfine features i.e., the weak sharp lines between the main sextet lines, are typical for the monomeric MnII–L complex, showing that these signals originate mainly from the active catalytic component [MnII-Schiff base] grafted on a solid matrix [10, 51]. The EPR spectra of recovered La@gCN-MnII and Lt@gCN-MnII materials (solid colored lines) after several catalytic uses demonstrate also a MnII-spectrum originated from the same active [MnII-Schiff base] moiety.

Fig. 9. EPR spectra for (A) Lt@gCN-MnII and (B) La@gCN-MnII catalysts before use (solid black lines) and after catalytic uses (solid colored lines); dotted lines referred to corresponding EPR spectra of liquid supernatant of reaction mixture. (C) μmol of MnII-ions per gram of used catalyst detected into liquid supernatant after each catalytic use. EPR conditions: temperature 77 K, modulation amplitude 10 Gptp, microwave power 20 mW.

In Fig. 9 C we present the μmol of MnII per gram of used solid catalyst, which were leached in the liquid supernatant during each catalytic use; that is, the detected by EPR μmol of MnII were normalised per gram of solid catalyst to facilitate direct comparison with the initial MnII loading. Based on these data it is demonstrated that, during the first use, 40 μmol of MnII were liberated from one gram of pristine La@gCN-MnII which had metal loading 0.20 mmol per gram. Similarly, 58 μmol of MnII were leached from one gram of pristine Lt@gCN-MnII with Mn loading 0.37 mmolg-1. Manganese leaching was also detected during each catalysts’ re-use. From the lineshape of the EPR spectra we conclude that the leached MnII-species corresponds to free MnII solvated ion (Fig. 9 A, B (dotted –lines)) probably due to the partial oxidative destruction of the active catalytic site [MnII-Schiff base]. That is, the observed catalytic performance is derived from the solid catalysts La@gCN-MnII and Lt@gCN-MnII which retain intact the [MnII-La] and [MnII-Lt] active components, since, as we have published, the free MnII ion shows very low catalytic activity towards epoxidations [30]. 3.11 Interaction of Radicals of the Organic matrix with MnII-La and MnII-Lt The organic matrix bears stable radicals [28, 52], which can be served as probes to determine the geometrical distance of the MnII-La and MnII-Lt complexes from the bulk gCN matrix. The magnetic interactions between the MnII-complexes and the radicals of the matrix are responsible for the strong line-broadening of the radicals of gCN [28, 52], see Fig. 10. This line broadening was found to be different after each reuse of La@gCN-MnII and Lt@gCNMnII. As analysed previously in detail [52, 53] from this line broadening we can estimate an average distance between the radicals and the MnII-ligand. This is based on the relation ΔΗdipolar=D/R3 where ΔΗdipolar is the EPR line broadening of the radical due to the dipolar interaction with the MnII spin. R represents [Mn-radical] distance and D is a constant D=βe2μοgNPgradical/4πħ. From this relationship we estimate that the R is much larger than 10 Å for the pristine unused catalytic materials La@gCN-MnII and Lt@gCN-MnII. This [Mn-radical] distance was found to be decreased from 10Å to 4Å after the 1st use of the solid catalysts and it was progressively further decreased less than 4Å after more reuses of catalysts. These findings indicate that the MnII-La and MnII-Lt centres are more tightly associated with the matrix after each reuse of the solid catalyst; this is in accordance with the observed matrix exfoliation after each reuse of the catalysts resulting in shorter distance between the survived manganese centres and matrix sheets.

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Fig. 10. EPR spectra of the radicals in Lt@gCN-MnII and La@gCN-MnII materials recorded at 77K. Inset graphs: Progressive microwave saturation profiles for the radicals detected by EPR. Experimental conditions: microwave frequency 9.51 GHz, modulation amplitude 4.00 Gpp.

5. Conclusion Herein, we report a synthetic methodology for covalent grafting of organic functionalities, and therefore metal complexes, on gCN. Based on this, the La@gCN-MnII and Lt@gCN-MnII materials have been prepared. On molecular level, we have demonstrated that is feasible and the successful immobilization of the entire [MnII-Schiff base] complexes on gCN, which serves as support, has been evidenced by various physicochemical methods. As concerned the textural features of the gCN support, the ordered structure of pristine material was retained after the grafting of the [MnII-Schiff base] components, however, disturbance of the packing and potential undulations of the single layers after the grafting of the catalytic active centers were suggested by XRD. Moreover, the obtained functionalized

materials La@gCN-MnII and Lt@gCN-MnII contain smaller particles due to breakage of the pristine gCN. As gCN is a partly imperfect polymer-like structure, it could be broken up from its defects during grafting process. The La@gCN-MnII and Lt@gCN-MnII were evaluated as catalysts for olefins’ epoxidations with H2O2. We found that they are efficient, selective towards epoxide formation and kinetically fast as we have previously observed for catalysts grafted onto carbonaceous-based supports [8-11]. Interestingly, the Lt@gCN-MnII was faster than the La@gCN-MnII. Among the aims of the present work was to evaluate the behavior of gCN as support in catalytic epoxidations in the presence of H2O2. For this, an extensive study has been accomplished which contained catalysts’ recycling experiments, characterization of solid catalysts pre- and after-use and EPR investigation of liquid catalytic supernatant. Based on these, we showed that delamination of gCN happens in an important degree. EPR data suggested that partial oxidative destruction of catalytically active Mn-complexes occur also during catalysis resulting in 15-20% manganese release as free MnII solvated ion. However, our data confirm that the observed catalytic performance is derived from the solid catalysts La@gCN-MnII and Lt@gCN-MnII which bear intact the [MnII-La] and [MnII-Lt] active components. Overall, the present study showed that chemical modification of gCN by covalent grafting of functionalities can be achieved without significant disturbance of ordered structure of the pristine gCN and maintaining the functionality of the grafted moieties. However, an important matrix exfoliation occurs during oxidation catalysis indicating a limited stability of gCN as support material in oxidative media.

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A novel synthetic methodology for covalent grafting of [MnII-Schiff base] complexes on gCN, which serves as support, is reported. The so-obtained oxidation catalysts were found to be selective towards epoxide formation and kinetically fast. The recycling experiments revealed that an important matrix exfoliation occurs during oxidation catalysis.