Sebastian Wahl

Sayed M El-Refaei

Ana Guilherme Buzanich

Patrick Amsalem

Kug-Seung Lee

Norbert Koch

Marie-Liesse Doublet

Nicola Pinna
email: nicola.pinna@hu-berlin.de

Norbert

Koch

O -A Stable and Highly Active Oxygen Evolution Catalyst Formed by Zinc Leaching and Tetrahedral Coordinated Cobalt in Wurtzite Structure

Keywords: oxygen evolution reaction, cobalt, zinc, Wurtzite, γ-Co(O)OH, Structure-Property Relationships

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Introduction

In 2015, the United Nations (UN) set 17 sustainable development goals (SDGs) to be reached by 2030. Goal 7, affordable and clean energy, wants to "Ensure access to affordable, reliable, sustainable and modern energy for all." One target is to "increase substantially the share of renewable energy in the global energy mix". [START_REF] Koh | Each potential was hold for 5 min, the current was averaged over this timeframe, and the Ohmic drop was determined using impedance spectroscopy[END_REF] One approach to reach this target, is the storage of excess solar and wind energy in chemical bonding, especially in the hydrogen-hydrogen bond. [2,3] The water splitting reaction, forming 2 H 2 and O 2 from 2 H 2 O, is based on a 4 electron transfer mechanism from oxygen to hydrogen. The reaction is hindered by the half-cell reaction at the anode, the OER, which takes place at a theoretical potential of 1.23 V at standard conditions. [4] Benchmarking papers as the ones by Jaramillo et al. show periodically the best oxygen evolution catalysts (OEC). [5][6][7] Among those, scarce and rare ruthenium and iridium oxides are still the best performing. However, during the last years, more and more catalysts based on earth abundant elements are registered in those benchmarks. By focusing on developing efficient water oxidation catalysts based on available metals like cobalt, nickel, manganese and iron, we come step by step closer to a large-scale realization of a circular energy economy based on hydrogen. [8] For cobalt-based systems, it is now proven that the active site involved in the OER is based on two or more redox-active cobalt atoms changing their oxidation states from +II to +IV and back.

Those are connected by µ-OH or µ-O bridges. [9,10] A structure widely known to provide those motifs is cobalt oxyhydroxide Co(O)OH. [11][12][13] Pfrommer et al. have shown, that a crystalline cobalt substituted zinc oxide can yield a stable OEC in alkaline electrolyte with enhanced activity, which is on the one hand based on the amorphization of the surface and the formation of Co(O)OH. [14] On the other hand, they agreed to the findings of others, that the presence of redoxinert Zn 2+ is "rendering structural support and/or providing cooperative effects" for the catalytically active cobalt. [15,16] Though, what was not yet discussed there, was the dissolution/leaching of zinc from the solid materials to the solution, which is expected at the elevated pH values in those studies. [17][18][START_REF] Jander | Lehrbuch der analytischen und praparativen anorganischen Chemie[END_REF] Correlating the leaching of Zn 2+ into the electrolyte with an enhanced OER activity was done first, to our knowledge, by Menezes et al. [13] They examined cobalt containing spinels towards their OER activities and showed that partly leaching of Zn 2+ from the tetrahedral sites leads to defective sites and thus increases the activity of the remaining cobalt in the octahedral sites of the spinel. Contrary to the hypothesis formed by the latter, Wang et al. studied the same system and stated, that Co 2+ in tetrahedral sites can be easily oxidized and form Co(O)OH, which then acts as the active phase, whilst Co 3+ in the octahedral sites is rather inactive towards the OER as it might be blocked by OH-groups. [START_REF] Wang | [END_REF] From this two opposing yet properly elaborated works, we got inspired to craft a material that combines the two identified driving forces. Thus, the material should provide a high amount of tetrahedral coordinated cobalt, preferably in oxidation state +II, and in addition hold a sufficient amount of Zn 2+ to be leached into the solution, leaving highly defective structures. In addition to the increase of the intrinsic activity, the material also should have a large number of accessible active sites. Inspired by Huang et al., who showed, that γ-Co(O)OH nanosheets outperform the bulk oxyhydroxide catalyst manifold and even overcome the IrO 2 catalyst, our material should exhibit also an exfoliated layered morphology . [21] Based on our expertise on the nonhydrolytic sol-gel synthesis of transition metal oxides, particularly cobalt and zinc based, in this work we focus on the synthesis of hexagonal Znstabilized Co II O in the Wurtzite structure. This combines all of the aforementioned properties. [22- 26] The synthesis involves a fast microwave-assisted solvothermal reaction of cobalt(III) acetylacetonate and zinc(II) acetate in benzyl alcohol. We analyzed the as-synthesized product thoroughly by high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (pXRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). We identified the changes the sample underwent by contact with the alkaline medium by HRTEM and pXRD, and validated the findings against the available literature. Then, we characterized the catalytic performance of the in situ formed catalyst towards the OER by electrochemical measurements and finally derived structure/activity correlation from ex situstudies.

By following this approach, we are able to provide a comprehensive study of one of the most active catalysts for the OER. The formed catalyst was able to maintain an overpotential η of less than 320 mV over a time frame of 98 h at a current density of 10 mA cm -2 in 1 M KOH.

Results and Discussion

Characterization of Zn 0.35 Co 0.65 O precatalyst

Zinc doped hexagonal cobalt oxide was synthesized by the microwave-assisted "benzyl alcohol route". This was achieved by using cobalt(III) acetylacetonate and zinc acetate as metal precursors and benzyl alcohol as oxygen source, solvent, reducing and surface stabilizing agent. [24,27,28] After 20 min at 210 °C, the sample was washed with ethanol, solid products were collected by centrifugation and the sample was dried in an oven at 70 °C overnight. A dark green powdered product, denoted as Zn 0.35 Co 0.65 O, was obtained.

The crystal structure was identified by pXRD. Only typical reflections of the hexagonal Wurtzite structure (ICSD PDF-number 01-074-0534) [29] were present in the pattern in Figure 1. Shifts in the positions of the reflections are not expected, since the ionic radii for Co 2+ and Zn 2+ are very similar (57 vs. 60 pm) and cobalt replaces the zinc in the structure. [30,31] HRTEM analysis shows agglomerates of sheets that form spheres of different sizes (Figure 2 a). This is in accordance to previous work of our group and proves the trend, that a higher amount of cobalt directs the formation of spherical agglomerates. [22] Power spectra analysis of the HRTEM (Figure 2 b) shows the crystallinity of the sample, which proves the hexagonal Wurtzite structure.

Energy dispersive X-ray spectroscopy (EDX) analysis yields a zinc to cobalt ratio of 0.35 to 0.65, confirming the stoichiometric implementation of both metals in the material. EDX mapping (see Figure SI-6) shows the uniform distribution of the metals in the assembly.

Insight in the oxidation state of the sample were gained by characterizing the chemical surface of the sample by XPS (Figure 3). The Co2p 3/2 peak is at 781 eV, with a shake-up satellite shifted by 5.8 eV. The Co2p 1/2 peak is shifted by 15.8 eV compared to Co2p 3/2 . Comparing this to reference compounds (see Figure SI-4 a) we can see clear difference to oxidation state +III, and from similar spectra in literature, we reason the oxidation state of cobalt is +II. [32,33] Further information on the oxidation state and the local atomic environment of the cobalt were provided by diffuse reflectance UV/Vis spectroscopy. The spectrum (Figure SI-3b) shows three typical absorption features at 579, 623 and 678 nm, which correspond to the transitions 4 A 2 (F) → 2 E(G), 4 A 2 (F) → 4 T 1 (P), and 4 A 2 (F) → 2 A 1 (G), respectively. Those are typical for Co 2+ in tetrahedral environment. [26,[34][35][36] To access the oxidation state and the local atomic environment of cobalt X-ray absorption spectroscopy (XAS) was used. In the X-ray absorption near-edge structure spectra (XANES, and commercially available CoO in rock salt structure. [37] The predominant cobalt oxidation state in the first reference is +III, and the edge position is 7720.6 eV (ZnCo 2 O 4 ). In the second reference, the predominant oxidation state is +II, and the edge position found is 7715.8 eV. The edge position of Zn 0.35 Co 0.65 O is at 7717.9 eV. This observation leads us to conclude, that the cobalt oxidation state in the material is less than +III, in comparison to the references. Since the pre-edge features in the XANES spectra are very prone to changes in crystal field symmetry, analysis of those also gives hints on the local structure of the absorbing atom. From the distinct pre-edge peak in the spectrum of Zn 0.35 Co 0.65 O we deduce tetrahedral coordination of the central cobalt atom. [38] Furthermore, we fitted the Wurtzite crystal system to the extended X-ray absorption fine structure (EXAFS) spectrum (R-range: 1.3 -5.0, Figure 4 b), where cationic positions were filled by 65 % Co 2+ and 35 % Zn 2+ , derived from EDX results. The first shell around the cobalt central atom at a reduced distance of 2.0 Å contains 4 oxygen atoms. The second shell consists in a distance of 3.2 Å of 6 atoms, which are cobalt and zinc in the ratio 2:1. Overlapping with the second shell, the third shell at a distance of 3.8 Å holds 9 oxygen atoms. The signal assigned to the forth shell at 4.5 to 4.6 Å is a summation of the signals of 2 zinc, 4 cobalt and 6 oxygen atoms, as well as a multi-leg scattering part within the second and third shell. With a R-factor of 0.0086 for this fit, we could further confirm the tetrahedral oxygen coordination of cobalt. [39,[START_REF] Calvin | XAFS for Everyone[END_REF] From the above mentioned analyses of the experimental data, we can safely conclude that in Zn 0. 35 Co 0.65 O cobalt is tetrahedrally coordinated and in oxidation state +II.

The homogeneity of the pre-catalyst material can be derived from the following considerations:

The pXRD (Figure 1) shows only one crystalline phase, and the HRTEM micrographs exhibited only one morphology (Figure 2a). The EDX mapping (Figure SI-6) showed a uniform distribution of Zn and Co over the whole sample, and the EXAFS fit (Figure 4b) shows that the second shell consists of Zn and Co in the ratio of 1:2. Thus, the material can be described as a one phase solid solution of Wurtzite (Co/Zn)O.

Characterization of the electrochemically active structure

When Zn 0.35 Co 0.65 O gets in contact with 1 M KOH (pH =13.8), it instantaneously changes color to a light brown. To figure out the origin of color change, we added some Zn 0.35 Co 0.65 O powder to 1 M KOH and stirred the dispersion at room temperature for 90 min. Then, we washed the sample with ethanol, collected the solid products by centrifugation, and dried it in an oven at 70 °C for 1 h. The pXRD-pattern in Figure 1 shows the typical reflections of hexagonal cobalt hydroxide Co(OH) 2 in Brucite-type layered structure (ICSD PDF-number 01-074-1057) [START_REF] Lotmar | [END_REF] and of hexagonal Wurtzite structure, leading to the conclusion, that the material is, at least at the surface, transformed to Co(OH) 2 . The formation of Co(OH) 2 is in agreement with the findings of Jang et al., who reported that hexagonal cobalt oxide transforms spontaneously to hexagonal cobalt hydroxide in contact with water. [42] Though, it is to mention that the incorporated Zn 2+ seems to stabilize the structure and prevents a fast conversion. TEM micrographs of the sample show platelets of around 50 nm in diameter with a thickness of around 4 nm (Figure 2c). The EDX element ratio of cobalt to zinc is 92.6 to 7.4 %, respectively. This indicates a loss of Zn from the structure, which is addressed in detail in the following section.

Electrocatalytic activity towards the OER was examined by rotating disk electrode (RDE) measurements in 1 M KOH at 1600 rpm. This can be attributed to the oxidation of Co 2+ to Co 3+ , which is a further proof that the asprepared material being in oxidation state +II. Since there is no equally sized reduction peak, we conclude an irreversible oxidation event. From the pXRD results we know that Co(OH) 2 is formed in situ. It has been described for other Co(OH) 2 systems, that this peak can be attributed to the irreversible oxidation of Co(OH) 2 to the cobalt oxyhydroxide Co(O)OH. [43][44][45] In the 4 th CV cycle, 2 oxidation/reduction pairs are visible, slightly above 1.0 V and around 1.4 V. They can be assigned to the reversible oxidation of Co 2+/3+ and Co 3+/4+ , respectively. From the higher current density of Zn 0.35 Co 0.65 O compared to the references, we conclude there are more accessible cobalt sites. To determine the electrochemically active surface area (ECSA), we used the method described by McCrory and measured the double-layer capacitance (see Figure SI-2). [7] We found that the ECSA was around 20-fold larger than for the reference catalysts. This is as well in very good agreement with the TEM observation of exfoliated layered structures.

Monitoring the electrocatalytic performance of Zn 0.35 Co 0.65 O compared to other cobalt containing catalysts in linear sweep voltammetry, it shows on the one hand a larger deviation between single measurements than the other catalysts. On the other hand, it has a significantly decreased average overpotential η of 322 mV compared to 365 mV for Co 3 O 4 , 369 mV for ZnCo 2 O 4 and CoO, respectively, and 382 mV for Co(O)OH at a current density of 10 mA cm -2 (Figure 5 d). The high activity is also reinforced by the low Tafel slope of 42.6 mV dec -1 (Table 1). Comparison of Tafel slopes shows that all reactions take place in the same kinetic regime around 40 mV dec -1 within a 10 mV dec -1 window. The examined materials have the same rate-determining step in the region where the Tafel slope was calculated. According to Shinagawa et al., the rate-determining step in this Tafel region could be either assigned to a high coverage of the low-coordinated surface metal centers by OH -species or to a high coverage of the surface with *OOH species. [46] Therefore, in the present article the Tafel slopes are provided for comprehensiveness of the electrochemical analyses, the in depth analysis of those is beyond the scope of this work.

To evaluate the long-term stability of the catalyst, we tracked the potential response in a chronopotentiometric measurement at a current density of 10 mA cm -2 on an Au-electrode.

During 98 h, the potential increased linearly from 1.534 to 1.546 V vs. RHE, which corresponds to an overpotential η of 306 to 318 mV. Linear fitting of the data resulted in a mean potential increase of 0.12 mVh -1 . By this, we were able to prove the long-term stability of our material (Figure 5 c).

Characterization of the material after electrolysis

After 1 h electrochemical reaction at a current density of 10 mA cm -2 , XPS measurements were performed directly on the screen printed electrode (Figure 3). The peaks of Co2p 3/2 and Co2p 1/2 are shifted to lower binding energies of 779.9 eV and 795.0 eV, respectively. Also, the first shake-up satellite is shifted by 10.0 eV towards lower energies (Figure 3a). From this, we deduce a change in oxidation state to +III. [32,33,47] The biggest change though is visible at the zinc edge.

Before the reaction the peaks for zinc were clearly observable, but after the reaction there is no more signal (Figure 3b). In alkaline media, ZnO first reacts to the insoluble Zn(OH) 2 . With an excess of hydroxide, the soluble [Zn(OH) 4 ] 2-complex is then formed. [START_REF] Jander | Lehrbuch der analytischen und praparativen anorganischen Chemie[END_REF] This indicates the leaching of Zn 2+ from the surface during the reaction.

In the pXRD pattern (Figure 1), three main reflections are visible. At 2 θ angles of 5.57, 16. 110) lattice planes of a slightly distorted γ-Co(O)OH. [48] From the Bragg reflection broadening, we derive a poor crystallinity of the oxyhydroxide. The asymmetric reflections of the (101) and ( 110) indicate a turbostratic disorder of the basal planes. [49] The d-value of 7.3 Å of the (003) lattice plane can be directly translated to the interlayer distance of the material.

The TEM micrograph of the sample after electrochemical characterization in Figure 2 (d) shows the main motifs are still layers, agglomerated to spheres. In contrast to the original sample, those layers are more exfoliated and the spheres are less separated from each other. Selected area electron diffraction of a large area (Figure 2 (e) shows weak Debye-Scherrer rings corresponding to interplanar distances of 2.42 and 1.44 Å, matching well the results of our pXRD analysis as well as the TEM analysis by Huang et al. [21] Here, it is to mention, that the sample is beamsensitive, which is in agreement with the reported recrystallization of Ni(O)OH under an electron beam (see SI for more information). [50] In contrast to the XPS measurements, where no zinc was detected on the materials surface, in EDX measurements 2.6 up to 6.5 atm.-% of zinc were detected (Table 2). Noteworthy, the amount of zinc decreased towards the edge of particle agglomerations (see Figure SI-8).

Analyzing the EDX mapping in figure 6, no such trend can be seen. Cobalt and oxygen are well dispersed over the whole structure. Due to the low abundance of the zinc in the sample, there are no areas detectable, where a position/zinc concentration correlation could be drawn. Only a uniform distribution of zinc in the examined structure can be seen. The high angle annular dark field (HAADF) micrograph in the same figure also shows the layered motifs as the bright field image. Although the shown agglomerate has a width of around 200 nm, the low phase contrast all over the agglomerate indicates a material of low density and high surface. A clear attribution of areas with higher Zn or Co content is not possible in the HAADF, as the electron density of the two elements is nearly the same.

From the XANES edge position at 7721.2 eV (Figure 4a), we deduce a predominant oxidation state of +III or slightly above in the sample after the electrochemical reaction. The distinct preedge feature indicates a non-symmetric geometry around the cobalt absorber. The EXAFS spectrum in Figure 4b clearly differs from the one before the reaction. The spectrum was fitted in the R-range from 1.0 to 3.5 Å by fixing the amplitude reduction factor S 0 2 to a value determined prior, as described by Ravel and Kelly. [START_REF] Ravel | AIP Conference Proceedings[END_REF] Thus, the degeneracies of the Zn 0. 35

Structure-activity correlations

Detection of Co(OH) 2 and Wurtzite crystal structures after 90 min in 1 M KOH is in contrast to the previously reported rapid total conversion of hexagonal CoO and points to a stabilizing influence of Zn. [42] It further proves that Co(OH) 2 is an important intermediate in the electrochemical formation of γ-Co(O)OH.

To explain the high activity of Zn 0.35 Co 0.65 O towards the OER, structure factors can be compared to reference-samples, thoroughly described in the literature. For all those samples, it is agreed that the OER activity arises from adjacent Co (III-IV) O(H) groups connected by µ 2 -O-bridges. [10,11,[START_REF] Favaro | [END_REF] The leaching of Zn 2+ in alkaline media leads to increased metal vacancies in the material, as proven by the EXAFS analysis of the sample after electrochemical reaction. The second coordination shell consists only of 4.6 metal atoms instead of 6.0. Comparable results were published by Menezes et al, who analyzed the leaching of Zn 2+ from the cubic spinel ZnCo 2 O 4 . [13] They conclude, that by leaching of Zn 2+ , more octahedral Co 3+ sites are accessible compared to activity. [START_REF] Wang | [END_REF] given by our observation, that Co(O)OH has the second highest ECSA, roughly five-fold higher than the other references, but the lowest OER activity. Thus, we deduct from our findings, that solely a higher accessibility of CoO 6 is not the reason for a high OER activity.

Instead, when starting from tetrahedral coordinated Co and transforming it to octahedral Co, Ovacancies and coordinately unsaturated CoO (6-x) -motifs are introduced in the structure. [START_REF] Wang | [END_REF] EXAFS analysis shows, that the oxygen coordination number is decreased to 5.6 instead of 6.0 in the first coordination shell. By this, as proven by density of states (DOS) studies by others on CoO (6-x)motifs on the surface of γ-Co(O)OH, the DOS near the Fermi level is increased. Thus, higher conductivity is achieved, which favors the electron transport from the active site. [21] Nevertheless, the extraction of Zn 2+ from the surface also contributes to the change of the electronic structure.

Co and O undergo an oxidation process and arrive to a higher valence. [53,54] Thus, the electrophilicity of surface-O can increase, which leads to a faster adsorption of OH -to form the *OOH species, which is the rate determining step, which is also in accordance to the calculated Tafel slope. In addition to this, it is nowadays accept that a lower overpotential is correlated to a higher adsorption stabilization energy of *OOH compared to *OH. [55][56][57][58] The remaining few percent of Zn 2+ in the structure play also an important role for the conductivity of the sample.

Ling et al. have recently shown, how 4-6 atm.-% of Zn in CoO can greatly enhance the ionic diffusion and electronic conductivity though the material. [59] Another contribution to higher conductivity might be also chemical bonding of the active material to the conducting support, obtained by the in situ formation of the active phase. [16,42] One more attribution to the increased OER activity, beside the above mentioned intrinsic factors, is the overall higher number of accessible cobalt sites. [60,61] The accessibility of those cobalt sites is further raised by the slightly enlarged inter-layer-distance of 7.3 Å in our γ-Co(O)OH. [21,43,60]

Conclusion

The successful synthesis of Zn 0.35 Co 0.65 O can connect this work to previous studies of our group on the field of cobalt-doped zinc oxides in Wurtzite structure by the non-hydrolytic benzyl alcohol route. [22] Herein, we report a fast, easy and reproducible one-pot synthesis for a zincstabilized cobalt oxide that, by its exfoliated morphology, acts as an excellent precursor catalyst for the OER.

In an alkaline solution with an applied anodic potential, the material undergoes structural transformation to γ-Co(O)OH: Edge-sharing octahedra are formed and act as the active site. A high defect concentration induced by the leaching of Zn 2+ and the structural transformation eases the accessibility of those active centers. Thus, an optimized energetic stabilization of the *OOH species, whose formation is the rate determining step, is achieved. Furthermore, the number of active sites is increased by the presence of exfoliated layers which provide a high surface area.

This leads to an exceptional activity for the OER, with an overpotential η of 306 mV at a current density of 10 mA cm -2 . During 98 h at the same current density, the overpotential just increased by 12 mV, proving the stability of the in situ formed catalyst. Further works should focus on different leaching agent and the optimal ratio of catalyst/leaching agent.

In summary, we could unify two contradicting hypotheses, [13,[START_REF] Wang | [END_REF] and we experimentally derived structure-activity correlations in a catalyst based solely on earth-abundant zinc and cobalt oxides.

Finally, by showing a high stability besides a high activity, our work possesses high potential to play a key-role in a sustainable hydrogen powered future.

Experimental

Materials

All reagents were used as received. Cobalt(III) acetylacetonate (98%) and cobalt(II) oxide (99.995%, metal basis; denoted as commercial CoO) were purchased from ABCR GmbH (Karlsruhe, Germany). 1 N potassium hydroxide solution was purchased from Carl Roth GmbH (Karlsruhe, Germany). Benzyl alcohol (99%), Nafion perfluorinated resin solution (5 wt-%) and zinc(II) acetate (99.99%) were purchased from Sigma Aldrich (Munich, Germany). Purified water with resistivity 18.2 MΩcm -1 was used for sample preparation.

Syntheses

Co 0.65 Zn 0.35 O was synthesized by adding cobalt(III) acetylacetonate (71 mg, 0.2 mmol, 2 eq.) and zinc acetate (18 mg , 0.1 mmol, 1 eq.) to benzyl alcohol (3 mL) in a 10 mL microwave reaction vial. The reaction mixture was stirred at room temperature for 2 h, until all precursors were dissolved. Subsequently, the mixture was heated in a microwave reactor (CEM Discover SP, Kamp-Lintfort, Germany) at 60 °C for 2 min followed by 210 °C for 20 min, and cooled down rapidly with compressed air. The solid product was collected by centrifugation, washed three times with ethanol and dried overnight at 70 °C. Averaged over 6 reaction, the yield was 18.8 mg of a dark green product (81.5 % yield on precursor's stoichiometry).

Syntheses of the reference catalysts can be found in the supporting information.

Characterization

Powder X-Ray diffraction patterns were obtained using a STOE STADI MP (STOE, Darmstadt, High resolution transmission electron microscopy (HRTEM) images were obtained using a FEI Talos F200S operated at 200 kV scanning/transmission electron microscope (S/TEM). Energy dispersive X-ray spectra (EDX) and maps were recorded with build in SuperX EDS detector.

EDX spectra were analyzed using FEI TEM Imaging & Analysis software version 4.17 SP1.

EDX mappings were analyzed using FEI Velox software version 2.6 with the Schreiber-Wims ionization cross-section model. The errors reported are taken from the reports generated by the software.

The XANES and EXAFS measurements of the samples before electrochemical reaction were performed at the BAMline (BESSY-II, Helmholtz Centre Berlin for Materials and Energy Berlin, Germany). The beam was monochromatized using a double-crystal monochromator (DCM). The size of the beam was 3 x 1 mm. The measurements were performed at Co-K edge (7709 eV) in transmission geometry, with two ionization chambers as detectors. The excitation energy was varied from 7614 eV to 8428 eV, with varying energy steps. For the pre-edge region, the energy was varied in 10 eV steps; for the region around the edge, energy was tuned in 0.5 eV steps and in the EXAFS region with a constant step in the k-space of 0.04 Å -1 . The associated uncertainty was experimentally determined by measuring the cobalt metal foil 10 times. A value of ±0.3 eV was obtained. For the measurement, the samples were mixed with boron nitride and fixed in plastic sample holders. Before collecting the sample spectra, a cobalt foil was used as a reference for the cobalt edge. The relative energies of the spectra were calibrated to the first infliction point of the cobalt metal absorption edge. XANES and EXAFS measurement of the sample after 1 h electrochemical reaction on a gold screen printed electrode (DropSense 250AT, Metrohm, Filderstadt, Germany) were measured at 8C nano-probe XAFS beamline (BL8C) of Pohang Light Source (PLS-II) in the 3.0 GeV storage ring, with a ring current of 400 mA. The X-ray beam was monochromated by a Si(111) double crystal where the beam intensity was reduced by 30% to eliminate the higher-order harmonics. The x-ray beam was then delivered to a secondary source aperture where the beam size was adjusted to be 0.5 mm (v) × 1 mm (h). XAFS spectra were collected in fluorescence mode. To determine the edge energy, the integral-method was used. [37] EXAFS data were processed by ATHENA and ARTEMIS from Demeter version 0.9.26. [39] This GUIs program belongs to the main package IFEFFIT (v. 1.2.12). [62] X-ray photoelectron spectroscopy (XPS) was performed in an ultrahigh vacuum chamber (base pressure 5.10-10 mbar) using a JEOL JPS-9030 set-up comprising a hemispherical photoelectron spectrometer and a monochromatic Al Kα (hν = 1486.6 eV) X-ray source. The XPS measurements were performed with an energy resolution of 0.7 eV as determined on a polycrystalline Ag 3d core level.

Electrochemical analyses

Electrochemical analyses were performed using a Bio-Logic VMP3 multichannel potentiostat/ galvanostat with a built-in EIS analyzer. The electrochemical behavior and activity of catalysts were measured in a three-electrode electrochemical cell using a 3 mm diameter glassy carbon rotating disc electrode (RDE, Autolab RRDE, Metrohm, Filderstadt, Germany) operated at 1600 rpm at 25 °C. A platinum wire was used as the counter electrode and a reversible hydrogen electrode (HydroFlex, gaskatel, Kassel, Germany) was used as a reference electrode. The electrodes were prepared by drop casting 3 µL of a catalyst ink onto glassy carbon. The ink was prepared by sonicating 1 mg catalyst powder with 490 µL ethanol, 490 µL purified water and 20 µL Nafion for 30 min.

The sample for the TEM and pXRD analyses was prepared by dropcasting an ethanolic dispersion of the catalyst onto a nickel foil, performing 1 h chronopotentiometry at a current ). [29,[START_REF] Lotmar | [END_REF]48] [33,63] The insets show the assigned surrounding atoms (b). Table 1. Comparison of electrochemical properties of different cobalt oxide based catalysts.

Tables

Catalyst ECSA

[cm²] R f a) η b)
[mV]

Tafel slope c)

[mVdec - Roughness: ECSA divided by geometric area (7.069 mm -2 ); b) at 10mA cm -2 in 1 M KOH; c) measured in 0.1 M KOH under quasi-stationary conditions

Figure 4 a

 4 Figure 4 a), the cobalt edge position in Zn 0.35 Co 0.65 O was compared to the references ZnCo 2 O 4

Figure 5 a

 5 Figure SI-3a). In the first cycle of the CV, an intense oxidation peak at around 1.15 V is visible.

7 and 28 .

 28 76 °, those reflections correspond to d-values of 7.3, 2.44 and 1.43 Å, respectively. Comparing this values to the ones reported by Bardé et al., we can assign the reflections to the (003), (101) and (

  Co 0.65 O after 1 h electrochemical reaction could be obtained. For a single-scatterer, the degeneracy equals the coordination number. The fit of the first three shells of Zn 0.35 Co 0.65 O after 1 h of electrochemical reaction was based on the hexagonal Co(O)OH structure without any dopant. The first shell around the absorbing Co-atom contains 5.6 O-atoms in a reduced distance of 1.91 Å. The second shell consists of 4.6 Co-atoms in a distance of 2.84 Å. The third shell holds 4.3 O-atoms in a distance of 3.44 Å. With an R-factor of 0.012, the fit indicates oxygen vacancies in the first shell and metal-ion vacancies in the second shell.

Co 3 O 4 ,

 34 and that tetrahedral voids favor the restructuring of the surface. To compare Zn 0.35 Co 0.65 O to their results, we synthesized ZnCo 2 O 4 . We also observed the dissolution of Zn 2+ from the spinel (see SI for detailed description), but in contrast to their work, we found the Zn 2+containing spinel oxide performed worse than the Co 3 O 4 reference. This finding is supported by the work of Wang et al, that compared CoAl 2 O 4 , Co 3 O 4 and ZnCo 2 O 4 towards their OER

  Germany) running at U = 40 kV and I = 40 mA with Mo Kα = 0.70930 Å) monochromatic radiation (using a Ge(111) monochromator) and a DECTRIS "MYTHEN 1K" detector. The patterns were recorded in 2 θ geometry in the range of 4 to 60 °.

Figure 1 .

 1 Figure 1. pXRD patterns of as prepared Zn 0.35 Co 0.65 O (red), product after 90 min stirring in 1 M KOH (orange) and product collected after 1 h Chronopotentiometry at 10 mA cm -2 . References

Figure 2 .

 2 Figure 2. HRTEM micrographs of Zn 0.35 Co 0.65 O: Sample before the electrochemical reaction (a

Figure 3 .Figure 4 .

 34 Figure 3. ex situ XPS on cobalt edge of Zn 0.35 Co 0.65 O sample before (red) and after (black)

Figure 5 .Figure 6 .

 56 Figure 5. Cyclic voltammograms in 1 M KOH at a scan rate of 20 mVs -1 showing initial and 4 th cycle of Zn 0.35 Co 0.65 O and Co(O)OH (a). Averaged LSVs in 1 M KOH at a scan rate of 5 mVs -1

  They concluded, that Co 3+ fully coordinated by 6 O(H) groups is mostly inaccessible for taking part in the OER. Our comparison to other cobalt oxides that are constructed of CoO 6 building blocks, namely CoO in rock-salt structure and Co(O)OH, shows similar results. Their activity is decreased compared to Co 3 O 4 and Zn 0.35 Co 0.65 O. Further proof of this theory is also

Table 2 .

 2 EDX Analysis of the Zn0.35Co0.65O before and after electrochemical reaction.

	Sample	Co atom%	Zn atom%
	before reaction	64.6 (±0.20) 35.3 (±0.17)
	after reaction, bulk 94.5 (±0.14) 5.4 (±0.04)
	after reaction, edge 97.4 (±0.34) 2.6 (±0.07)

density of 10 mA cm -2 , rinsing the foil with ethanol and scratching off the material. This was repeated several times.

Long-term stability tests were performed on screen printed electrodes (DropSense 250AT, Metrohm, Filderstadt, Germany) consisting of a gold working electrode with a diameter of 4 mm and a platinum counter electrode. A reversible hydrogen electrode (HydroFlex, gaskatel, Kassel, Germany) was used as the reference electrode. 2 µL of an ethanolic dispersion (5 mg/mL) of the catalyst were dropcasted on the gold electrode and dried in an air stream. Then, the electrode was placed in 150 mL of a 1 M KOH solution at room temperature, a current of 1.257mA was applied for 98 h (1 h for XPS measurement) and the potential was recorded every 0.5 s.

Electrocatalytically active surface area (ECSA)

The ECSA was determined by double-layer capacitance (C dl ) measurements in 1 M KOH.

Therefor, 5 cyclic voltammograms at each scan rate (10, 20, 30, 40, 50, 60, 80 and 100 mVs -1 )

were measured in the non-Faradaic region, which is a 100 mV potential window around the open circuit voltage of the system. For the last 4 cycles of each scan rate, the anodic and the cathodic currents at the open circuit voltage were averaged and plotted versus the scan rate. By applying linear fits to these and averaging the absolute slopes of the fits, C dl is calculated. The ECSA is C dl divided by the general specific capacitance C s , which is typically reported as 0.040 mFcm -2 in 1 M alkaline media. The origin of this value is discussed in depth in the supplementary information of the 2013 benchmarking paper of Charles C. L. McCrory. [7] To calculate the roughness R f , the ECSA is divided by the geometric area of the electrode.

Acknowledgement

Special thanks to Christoph Erdmann for measuring HRTEM and S/TEM.


Larissa V F Oliveira

Simona Bennici

Ludovic Josien

Lionel Limousy

Marcos A Bizeto

Fernanda Ferraz Camilo
email: ffcamilo@unifesp.br

Free-standing cellulose film containing manganese dioxide nanoparticles and its use in discoloration of Indigo Carmine dye

Keywords: cellulose film, manganese dioxide nanoparticles, Indigo Carmine dye, discoloration tests

Introduction

Manganese oxide nanomaterials, due to their unique structural and chemical properties, have been mainly investigated either as catalysts (D. [START_REF] Wang | Manganese dioxide as a catalyst for oxygen-independent atrazine dealkylation[END_REF] [START_REF] Shaabani | Porous chitosan-MnO 2 nanohybrid: a green and biodegradable heterogeneous catalyst for aerobic oxidation of alkylarenes and alcohols[END_REF] and as electrodes for lithium batteries [START_REF] He | Ultrathin MnO2 nanosheets supported on cellulose based carbon papers for high-power supercapacitors[END_REF] [START_REF] Lee | Carbon Nanotube/Manganese Oxide Ultrathin Film Electrodes for Electrochemical Capacitors[END_REF] Owing to their combined oxidant and adsorbent capabilities, MnO 2 nanoparticles their combined oxidant. [START_REF] Salam | Synthesis and characterization of novel manganese oxide nanocorals and their application for the removal of methylene blue from aqueous solution[END_REF] [START_REF] Han | Degradation of aqueous and soil-sorbed estradiol using a new class of stabilized manganese oxide nanoparticles[END_REF] [START_REF] Clarke | Oxidative breakdown of acid orange 7 by a manganese oxide containing mine waste: Insight into sorption, kinetics and reaction dynamics[END_REF].However, as this nanomaterial is normally obtained in the powdered form, its applicability is impaired due to the limited possibility of easyrecovering from the reaction medium that prevents its reuse or adequate disposal after the end of the treatment process. An alternative to overcome this problem is to support the nanoparticles into suitable solid matrices.(Y. [START_REF] Wang | In situ synthesis of MnO2 coated cellulose nanofibers hybrid for effective removal of methylene blue[END_REF] [START_REF] Maliyekkal | A novel cellulosemanganese oxide hybrid material by in situ soft chemical synthesis and its application for the removal of Pb ( II ) from water[END_REF] Cellulose, a renewable biopolymer, is an interesting material to use as a film to support the nanoparticles, since it has mechanical strength, durability, and water-resistance. [START_REF] Zhang | 1-allyl-3-methylimidazolium chloride room temperature ionic liquid: A new and powerful nonderivatizing solvent for cellulose[END_REF] [START_REF] Pang | Fabrication of cellulose film with enhanced mechanical properties in ionic liquid 1-allyl-3-methylimidaxolium chloride (AmimCl)[END_REF] Cellulose is the most abundant biorenewable material on earth and presents many advantages of its use including low cost, biodegradability and no toxicity. However, it has quite poor solubility and processability. It was just in 2002 that Rogers and coworkers first dissolved cellulose with an ionic liquid [START_REF] Swatloski | Dissolution of Cellose with Ionic Liquids[END_REF] and after it the possibility to produce cellulose films by the regeneration of the dissolved cellulose with an anti-solvent was opened. [START_REF] Isik | Ionic liquids and cellulose: Dissolution, chemical modification and preparation of new cellulosic materials[END_REF] Those films could be used as support for different materials like metal or oxide nanoparticles, solving the problem of nanoparticles transport and recycling in many different technological applications.

Indigo Carmine (IC), a colorant used in food, pharmaceutical industries and mainly in textile to blue jeans, becomes a serious pollutant in aquatic environments because it prevents the photosynthesis of aquatic plants and algae and beyond that it may be toxic to humans, causing itch, eye irritation, permanent damage to the cornea and respiratory tract, if inhaled. [START_REF] Othman | Study of photocatalytic oxidation of indigo carmine dye on Mn-supported TiO2[END_REF] [START_REF] Vautier | Photocatalytic Degradation of Dyes in Water: Case Study of Indigo and of Indigo Carmine[END_REF] Thus, many studies have been carried out to remove this dye of wastewater.

Manganese dioxide nanoparticles are very effective in promoting its degradation (Vidya Lekshmi, Yesodharan, & Yesodharan, 2017)(Sanchez-Botero, Herrera, & Hinestroza, 2017) [START_REF] Mylarappa | Electro chemical and photo catalytic studies of MnO2 nanoparticle from waste dry cell batteries[END_REF]. However, the limited capacity of recovering these nanoparticles, from the reaction medium, might be a problem.

Till now, only a few studies have examined the use of hybrid materials of cellulose and manganese dioxide to degrade the Indigo Carmine. In 2013, (Chacón-Patiño, [START_REF] Chacón-Patiño | Biocomposite of nanostructured MnO2 and fique fibers for efficient dye degradation[END_REF] the in situ synthesis of nanostructured MnO 2 onto natural fibbers was reported as the catalytic activity of this material to degraded the Indigo Carmine dye from water. The MnO 2 -nanocomposite was able to remove up to 98% of the colour present in the contaminated water samples in less than 5 minutes. This was one of the first report in which a biocomposite containing a transition metal oxide was able to degrade an organic pollutant without the need of addition of an oxygen source as oxidant such H 2 O 2 . In 2016, [START_REF] Blanco | Nanostructured MnO2 catalyst in E. crassipes (water hyacinth) for indigo carmine degradation[END_REF] the use of water hyacinth's dried matter as a support matrix for nano MnO 2 was reported as well its application in the removal of Indigo Carmine dye of wastewater. The degradation tests carried out in different pH indicated that the dye discoloration occurred only in pH 2.3. The effectiveness of the degradation reaction in acidic pH conditions is explained by the electrostatic interaction established between the negative charge of the IC structure with the positive charged MnO 2 oxide surface. Since these reported nanocomposites were obtained in the powdered or fibber forms, their applicability in wastewater treatment could be impaired due to the limited recovering capacity from the medium that prevents its reuse or adequate disposal.

In this sense, this paper describes a novel method to produce free-standing films of non-modified cellulose impregnated with manganese dioxide nanoparticles and their application in the discoloration of the indigo carmine dye upon exposure to ambient light. The use of a film instead of powdered material allows a straightforward isolation from treated water, avoiding the need of more complex separation methods, such as centrifugation or filtration. Reports about the production of such cellulose films containing MnO 2 are still scarce. In 2018, Danish and co-workers [START_REF] Danish | Assessment of cellulose acetate / manganese oxide thin film as adsorbent for selective extraction of flavone[END_REF] demonstrated the efficiency of cellulose acetate/manganese oxide thin films as adsorbents for selective extraction and detection of flavone in environmental waters.

The use of cellulose acetate has many disadvantages since it is a synthetic cellulose and has sensitivity to UV irradiation and is high swellable in water presence, which makes it use in water treatment not suitable. Recently, Pend and colleagues (Ruichao Peng, Honglei Zhang, Lin Gui, Zaikun Wu & Luo, 2019) fabricated a novel porous film by simply mixing mesoporous MnO 2 with porous cellulose prepared by LiOH/urea water system with freeze-thaw method. The composite film was composed of MnO 2 microspheres spread in the reticular structure of the cellulose. The composite film was used to degrade acid orange in wastewater. Compared with pure cellulose film, the composite film of MnO 2 @cellulose had a better catalytic degradation efficiency, since after 90 minutes the acidic orange dye degradation reached 76%.

Materials and Methods

Materials

Microcrystalline cellulose, 2-butanol (99%), potassium permanganate (99%), tetramethylammonium bromide (98%) and Indigo Carmine (97%) were obtained from Sigma-Aldrich and used as received without any further purifications. 1-chlorobutane (99%) and 1-methylimidazole (99%) were also obtained from Sigma-Aldrich but they were distilled before use.

Methods

Tetramethylammonium permanganate (TMAMnO 4 )

Tetramethylammonium permanganate was prepared by using a procedure already described in the literature. [START_REF] Benedetti | Electrostatic Layer-by-Layer Deposition and Electrochemical Characterization of Thin Films Composed of MnO 2 Nanoparticles in a Room-Temperature Ionic Liquid[END_REF]) Succinctly, 2.54 g of tetramethylammonium bromide (16.5 mmol) dissolved in 10.0 mL of deionized water was added dropwise in a solution of 2.37 g (15.0 mmol) of potassium permanganate dissolved in 25.0 mL of deionized water. The mixture was stirred vigorously for 1 hour at room temperature. The precipitate was isolated by filtration, recrystallized from water and dried. The final product was a purple powder with 77% yield.

Aqueous dispersion of manganese dioxide nanoparticles

The preparation of the aqueous dispersion of manganese dioxide nanoparticles has been already described in the literature. [START_REF] Benedetti | Electrostatic Layer-by-Layer Deposition and Electrochemical Characterization of Thin Films Composed of MnO 2 Nanoparticles in a Room-Temperature Ionic Liquid[END_REF] Briefly, 290 mg (1.50 mmol) of TMAMnO 4 was added in 30.0 mL of 2-butanol and 30.0 mL of deionized water. The mixture was gently stirred during 1 hour, at room temperature. Then, the mixture was poured into a separating funnel and the dark aqueous phase containing MnO 2 was separated and used. Water was removed from the dispersion in a rotary evaporator at 80°C and then the sample was dried in a dessicator containing silica gel. This sample was denoted as MnO 2 _powder.

1-butyl-3-methylimidazolium chloride (BMImCl)

This ionic liquid was prepared by the reaction between 1-chlorobutane and 1methylimidazole following a procedure already described in the literature.(da S. [START_REF] Roselaine | Production of self-supported conductive films based on cellulose, polyaniline and silver nanoparticles[END_REF]. The BMImCl was obtained as a white solid and exhibited melting point between 61-64 o C (80% yield). 1 H and 13 C NMR were used to confirm the structure.

Cellulose solubilization and cellulose film

5.00 wt% of microcrystalline cellulose (MCC) was dissolved in BMImCl at 60 °C under stirring during 24 h, giving a clear and viscous solution. Then, 5.00 g of this solution was spin-coated onto a glass Petri dish (15.00 cm diameter) at 500 rpm for 40 s. Then, water was added into the Petri dish to regenerate the cellulose as a film. The membrane was washed with water until the complete removal of the residual ionic liquid. The film was dried for several days in a desiccator with phosphorus pentoxide. This film was denoted as cellulose film.

Cellulose film containing manganese dioxide nanoparticles (CEL_MnO 2 NP)

5.00 g of cellulose solution in BMImCl (5.00 wt%) was placed in a Petri dish (15.0 cm diameter). Then, the same procedure described to produce the cellulose film was used, except for the drying step. After that, 30.0 mL of the aqueous dispersion of manganese dioxide nanoparticles were added to that Petri dish containing the wet cellulose film and they were kept in contact without stirring during 12 hours. Finally, the film was washed several times with water and dried during several days at room temperature, affording a dark brown cellulose film denoted as CEL_MnO 2 NP.

Indigo carmine discoloration tests

The tests were carried out by placing a piece of the film CEL_MnO 2 NP with 22 mg in 5 mL of the dye solution (20 ppm) upon ambient light ( irradiation of 4 tubular fluorescent light bulbs of the indoor laboratory illumination (16 W each located at approximately two meters above the reaction vial)) under constant stirring. The pH of the dye solution was adjusted to 2.0 with the addition of acetic acid. The discoloration of the solution was monitored by electronic absorption spectroscopy at UV-Vis, by accompanying the decreasing of the characteristic absorption band of Indigo Carmine at 610 nm. Additional tests were carried out in pH 3.0 and 4.0 using the same experimental conditions.

Characterizations

Transmission electron microscopy images were acquired in a JEM 2100 electron microscope from JEOL. The sample was prepared by depositing a drop of the MnO 2 dispersion onto a carbon-coated Cu microgrid.

Scanning electron microscopy images were acquired in a JSM-7401F microscope from JEOL equipped with an energy dispersive spectrometer (EDS). The samples were coated with a thin conductive layer of gold to ensure a good electrical path to ground, providing improved SEM image quality.

Scanning transmission electron microscopy (STEM) images were registered in a JSM-7900 JEOL microscope equipped with a Deben STEM detector, at an accelerating voltage of 30 kV. The sample was prepared by immersing vertically a piece of the cellulosic film containing the MnO 2 nanoparticles (CEL_MnO 2 NP sample) in a mixture of an epoxy resin and a hardener (EpoFix, Struers) for 24 hours.

Then, ultrathin slices were cut at with a cryogenic ultramicrotome (Leica, EM UC7).

These slices were left floating in water and then recovered onto TEM grids and dried at room temperature before observation.

Particle size distribution of the MnO 2 nanoparticle dispersion was assessed by using a Malvern Zetasizer Nano instrument.

Fourier transform infrared absorption spectra were recorded on an IR-Prestige-21 spectrometer from SHIMADZU, using ATR accessory (ATR-8200HA) in the range of 400-4000 cm -1 with 4.00 cm -1 of resolution.

Raman spectra were recorded with a RENISHAW InVia Raman spectrometer working with a multichannel detection. Laser excitation was provided by a 632.8 nm HeNe laser and the spectral resolution of 4.00 cm -1 . Electronic absorption spectra were registered in an Ocean Optics spectrophotometer model USB 4000 from OCEAN OPTICS. It was used quartz cuvettes with 1.00 cm of optical path.

Nuclear magnetic resonance spectra were acquired using a VARIAN INOVA 300 spectrometer, operating at 300 MHz for 1 H NMR and at 75 Hz for 13 C NMR.

Chloroform-D containing TMS has been used as the deuterated solvent and reference. X-rays diffraction patterns were registered on D8-Advance diffractometer from BRUKER, using Cu-K anode (0.1542 nm), operating at 40 keV and 40 mA.

Thermogravimetric analysis was carried out using DTG-60 thermogravimetric analyzer from SHIMADZU under airflow (100 cm 3 .min -1 ) at a heating rate of 20 ⁰C.min -1 and the temperature range of 25 ⁰C to 900 ⁰C.

X-ray photoelectron spectroscopy (XPS) data were obtained using a THERMO SCIENTIFIC K-Alpha spectrometer equipped with an aluminium monochromator Al Kα radiation (1486.6 eV).

Results and Discussion

An aqueous dispersion of manganese dioxide nanoparticles was prepared through the reduction of tetramethylammonium permanganate with 2-butanol (inset in Fig. 1A), which resulted in a dark brown dispersion whose electronic absorption spectrum did not show bands ascribed to the presence of MnO 4 -expected at 535 nm and 545 nm, indicating the complete conversion of the starting material to MnO 2 . [START_REF] Fujimoto | Sonolytical preparation of various types of metal nanoparticles in aqueous solution[END_REF] The absorption band at 320 nm was attributed to a ligand-to-metal charge-transfer (LMCT) of the MnO 2 nanoparticles (Fig. 1A). (Brock et al., 1999a) The measured zeta potential of -50.7 ± 2.25 mV assured the colloidal stability of the dispersion, since the nanoparticles were prepared in a pH higher than the isoelectric point of the MnO 2 (pH IEP around 4.5) [START_REF] Chibowski | Comparison of the influence of a kind of electrolyte and its ionic strength on the adsorption and electrokinetic properties of the interface: Polyacrylic acid/MnO2/electrolyte solution[END_REF]. At this pH, the MnO 2 nanoparticles have negative charges due to the ionization of terminal hydroxyl groups as schematically shown in The preparation of the cellulose self-standing film containing MnO 2 nanoparticles involved several steps as shown schematically in Figure 2. First, microcrystalline cellulose (MCC) was dissolved in the ionic liquid 1-butyl-3methylimidazolium chloride (BMImCl) at 60 o C during 24 h. Then, the produced dispersion (5 wt% of cellulose in ionic liquid) was placed in a Petri dish, where water was added in order to promote the cellulose regeneration in the form of a film. In the next step, the aqueous dispersion of manganese dioxide nanoparticles was added to the Petri dish containing the wet cellulose film and they were kept in contact during 12 hours, without stirring. The progress of nanoparticles impregnation was monitored by using electronic absortion spectroscopy in the visible range. Figure 2S (supplementary information) shows the temporal evolution of the dispersion spectra profile of samples, collected every one hour, along a period of 12 h. The intensity of the band at 315 nm, ascribed to the MnO 2 nanoparticles presence, decreased with the impregnation progress.

The plot of the absorbance band maxima at 315 nm as a function of impregnation time (Inset in Figure 2S) showed a straight step decrease in the first eight hours then remained practically unchanged after that. The direct regeneration of cellulose film using the MnO 2 dispersion was not possible due to the destabilization of the colloid caused by the ionic liquid presence. The impregnation in dried cellulose films was also not effective. After impregnation, the dark brown film was washed with water and dried. This film will be referred hereafter in the text as CEL_MnO 2 NP. shows peaks at 2θ=15 °, 22 ° and 34 ° ascribed, respectively, to the (110), ( 200) and

(004) planes, of the type-I cellulose [START_REF] Liu | Characterization of the regenerated cellulose films in ionic liquids and rheological properties of the solutions[END_REF][START_REF] Terinte | OVERVIEW ON NATIVE CELLULOSE AND MICROCRYSTALLINE CELLULOSE I STRUCTURE STUDIED BY X-RAY DIFFRACTION (WAXD): COMPARISON BETWEEN MEASUREMENT TECHNIQUES[END_REF], with around 91% of crystallinity. By regenerating the cellulose as a film (blue line -Fig 3), the polymer was converted to type II cellulose, that is amorphous. [START_REF] Ciolacu | Amorphous Cellulose -Structure and Characterization[END_REF] The presence of MnO 2 nanoparticles in the regenerated cellulose film was not detected by XRD (red line -Fig. 3), probably due to the small crystalline domains of the produced nanoparticles incorporated into the film and the high dispersion of them into the film. FTIR spectrum (Fig. 4 yellow rectangle) of cellulose film (black) shows a band at 901 cm -1 attributed to the skeletal mode vibrations at the 1,4--glycosidic linkages.

The C-O stretching vibrations are observed in the range between 1000 cm -1 and 1100 cm -1 . The symmetric and asymmetric C-H stretching in 2850 and 2935 cm -1 and O-H axial deformation seen as a large band above 3300 cm -1 are detected. [START_REF] Ciolacu | Amorphous Cellulose -Structure and Characterization[END_REF] , [START_REF] Liu | Characterization of the regenerated cellulose films in ionic liquids and rheological properties of the solutions[END_REF] All mentioned bands were coherent with the amorphous cellulose phase obtained through the dissolution of microcrystalline cellulose followed by regeneration with an anti-solvent. [START_REF] Ciolacu | Amorphous Cellulose -Structure and Characterization[END_REF] In the FTIR spectrum of cellulose film containing manganese dioxide nanoparticles (CEL_MnO 2 NPred line)

additionally to the bands of the biopolymer two bands at 1488 cm -1 and 950 cm -1 ascribed to the C-H and C-N vibrations of the tetramethylammonium cation (TMA + )

were also observed. These two bands were also identified in the FTIR spectrum of neat MnO 2 nanoparticles (Figure 3S) [START_REF] Bottqer | The infrared spectra of the crystalline tetramethylammonium halides[END_REF] , [START_REF] Harmon | Hydrogen bonding. IV. Correlation of infrared spectral properties with C-H⋯X hydrogen bonding and crystal habit in tetramethylammonium ion salts[END_REF]. Vibrational bands attributed to the MnO 2 were not seen in the FTIR spectrum, however, in the Raman spectrum (Fig. 4 blue rectangle) the bands at 564 cm -1 and 645 cm -1 attributed to Mn-O vibrations were detected. [START_REF] Xie | Effect of the crystal plane figure on the catalytic performance of MnO2 for the total oxidation of propane[END_REF] , (Tsubasa Uematsu, Yumi Miyamoto, Yoshiyuki Ogasawara, Kosuke Suzuki, 2016). These vibrations were also observed in the Raman spectrum of neat MnO 2 NP (Figure 4S). The Mn 2p XPS spectrum (Figure 6) shows two peaks at 642.1 eV and 653.8 eV attributed respectively to Mn 2p 3/2 and Mn 2p 1/2 , with spin-energy separation of 11.7 eV.

These peak values agree with those reported for MnO 2 indicating the predominant oxidation state of +4 for the Mn present in the film. [START_REF] He | Ultrathin MnO2 nanosheets supported on cellulose based carbon papers for high-power supercapacitors[END_REF] SEM image of the CEL_MnO 2 NP film (Fig. 7A inset) exhibited typical features of regenerated cellulose films with a smooth and uniform surface. These characteristics are probably due to the destruction of the cellulose crystalline structure, however, the fiber contours were observed in the regenerated films due to the maintaining of the strong hydrogen bonding of cellulose during film-forming [START_REF] Zhang | 1-allyl-3-methylimidazolium chloride room temperature ionic liquid: A new and powerful nonderivatizing solvent for cellulose[END_REF]) MnO 2 particles were not observed due to their small size with low electronic contrast.

However, the elemental analysis using EDS (Fig. 7A) reveals the presence of manganese at the surface due to signals at 5.8 and 6.4 keV besides the oxygen and carbon peaks, which are present in the grid and cellulose. SEM image of a fractured cross-section of the film shows a layered structure composed of hydrogen bond networks among cellulose chains (Fig. 7B inset). MnO 2 particles were not seen but the elemental analysis (Fig. 7B) confirmed their presence inside in the film and not only in the surface. STEM images of the CEL_MnO 2 NP film were obtained on the sample prepared with a specific procedure described in the experimental part. Fig. 7C shows that the MnO 2 particles are dispersed homogeneously in the cellulosic film all along the thickness. At higher enlargements (Fig 7D andE) it is possible to observe that these particles consist in the agglomeration of smaller MnO 2 nanoparticles, some of them being isolated. In Fig 7F,

the size of the small nanoparticles can be evaluated to be close to 10 nm. The particle size distribution of the MnO 2 suspension was performed by Dynamic Light Scattering in order to evaluate the size of the nanoparticles. Results show that the suspension presents a bimodal dispersion with particle average sizes corresponding to 8 and 79 nm, respectively. This datum confirms the STEM observations and the presence of both single and agglomerated MnO 2 nanoparticles in the cellulosic film. The color removal (% CR) was estimated by the percentage ratio between the absorbance at 610 nm before and after contact with the film, which equation was shown in the inset in Figure 8. % CR values higher than 90 % were considered as complete discoloration. After 25 minutes, complete color removal was observed. In neutral conditions, the tests were unsuccessful even after hours, due to the ineffective interaction of the manganese dioxide contained in the film and the dye molecules. As stated by [START_REF] Chacón-Patiño | Biocomposite of nanostructured MnO2 and fique fibers for efficient dye degradation[END_REF] at pH lower than the isoelectric point of the manganese dioxide (pH IEP = 4.5), the positive surface of the nanoparticles (Mn-OH 2 + groups) has favorable electrostatic interactions with the dye bearing a negative (anionic) charge. At neutral conditions, there is a repulsive electrostatic force, since both species are negatively charged (inset in Fig. 8). We also evaluated the influence of pH value on the discoloration time of indigo carmine at pH=3.0 and 4.0. The complete colour removal (%CR higher than 90 %) were achieved after 60 and 180 minutes, respectively,

showing the importance of the MnO 2 oxide surface charge on the discoloration behaviour of indigo carmine. Control tests were made under the same conditions, without the film or using a cellulose film, but no discoloration was observed, even after a long time (24 h). The mechanistic degradation of indigo carmine by the identification of the intermediary compounds produced during the degradation has already been studied in the literature by mass spectrometry [START_REF] Chacón-Patiño | Biocomposite of nanostructured MnO2 and fique fibers for efficient dye degradation[END_REF][START_REF] Zaied | Environmental Spontaneous oxidative degradation of indigo carmine by thin films of birnessite electrodeposited onto SnO 2[END_REF].

In addition, it was possible to reuse the film for at least 10 cycles observing only a decrease of 10% in the % CR from the first to last cycle (Fig. 9).

Conclusions

A free-standing non-modified cellulose film containing manganese dioxide nanoparticles was easily prepared. The film contains about 12 wt.% of manganese dioxide. This method can be used to immobilize other types of nanoparticles. The cellulose film containing manganese dioxide nanoparticles was capable of discoloring an Indigo Carmine solution efficiently under ambient light with no need of centrifugation and filtration steps to remove it from the dye solution. It could be reused at least 10 times with almost the same efficiency.

  Figure 1: (A) Electronic absorption spectrum of the MnO 2 dispersion. The inset shows a photograph of the vial containing the dispersion. (B) Simplified scheme of the surface charge of MnO 2 nanoparticles according to the pH of the aqueous dispersion. (C) TEM image of the MnO 2 nanoparticles. Inset shows the registered SAED pattern. (D) HRTEM image of MnO 2 nanoparticles. The highlighted areas represent crystalline regions of the nanoparticles.

Figure 2 :

 2 Figure 2: Scheme of the synthetic steps involved with the preparation of the CEL_MnO 2 NP film.

Figure 3

 3 Figure 3 displays a comparison between the registered XRD patterns of MCC,

Figure 3 :

 3 Figure 3: XRD patterns of MCC, cellulose film and CEL_MnO 2 NP film. Inset shows the intermolecular and intramolecular bonds in cellulose type I and cellulose type II. The hydrogen atoms were omitted.

Figure 4 :

 4 Figure 4: FTIR spectra (yellow rectangle) and Raman spectrum (blue rectangle) of cellulose and CEL_MnO 2 NP films. Inset shows the enlarged region between 900 and 1800 cm -1 .The amount of MnO 2 into the regenerated cellulose film was estimated by

Figure 5 :

 5 Figure 5: TG (solid lines) and DTG (dashed lines) curves of cellulose and CEL_MnO 2 NP films. The highlighted regions with coloured rectangles represent the main mass loss events.

  Figure 6: XPS spectrum of the CEL_MnO 2 NP film.

Figure 7 :

 7 Figure 7: SEM images and EDS analysis of the region circled in yellow of the top surface (A) and cross section (B) of the CEL_MnO 2 NP film (C-F) STEM images of the CEL_MnO 2 NP film at different magnification ((a) x14 000, (b) x50 000, (c) x100 000 and (d) x190 000). *peak at 2.1 KeV is from the thin layer of gold used to coat the sample. The indigo carmine discoloration test was carried out by placing 22 mg of a

Figure 8 :

 8 Figure 8: Absorbance decay of indigo carmine dye in water (20 ppm). Inset: Interactions between Indigo Carmine and MnO 2 in different pH and % CR equation.

Figure 9 :

 9 Figure 9: Absorbance decay of indigo carmine dye in water (20 ppm) after 25 min for different reuse cycles Inset: % CR values for 10 reuses.

Acknowledgments

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Brasil (CAPES) -Finance Code 001. The authors gratefully acknowledge the financial support from FAPESP (grant number 2018/20826-4), professor Vera Constantino (IQ-USP) for provided the use of the DLS/Zeta potential equipment and Brazilian Nanotechnology National Laboratory (LNNano) for the XPS experiments (project number XPS -24897).


Keywords: aerobic oxidation, amine, bioinspired catalyst, imine, metalloenzyme, NICE-2018, purpurogallin, sustainable chemistry

The goal of sustainable development has been accepted as a common policy in current society. In response to this challenge, the development of green processes which utilize environmentally benign oxidants, reduce chemical waste and handling costs, is highly desirable. Given the widespread importance of imines as pivotal synthetic intermediates and essential pharmacophores in numerous biologically active compounds, various catalytic methods allowing the aerobic oxidation of amines to imines have been developed. Recently, noticeable progress has arisen from the discovery of various quinone-based catalytic systems, inspired by copper amine oxidase enzymes (CuAOs), which are able to reproduce the selectivity of CuAOs for primary amines and even to expand the amine substrates scope. However, the need for synthesizing these catalysts prior use adversely affects the economics as well as the eco-friendly nature of the method. To surpass these drawbacks, the "second-order" biomimicry idea has been recently advanced to describe a system in which in situ modification of pre-catalyst components affords the active biomimetic catalyst. This minireview especially covers our recent contribution to the design of bioinspired quinone-based catalysts for the aerobic oxidation of amines to imines which has culminated in a dual bioinspired protocol as an example of "second-order" biomimicry.

Introduction

Copper amine oxidases (CuAOs) are a family of metalloenzymes which couples the selective oxidation of primary amines to aldehydes with the reduction of dioxygen to hydrogen peroxide through the synergistic action of a quinone-based organic cofactor, 2,4,5-trihydroxyphenylalanine quinone or topaquinone (TPQ, 1) and a copper ion [1]. Although 1 is derived from a conserved tyrosine in a self-processing, post-translational event implicating the dual requirement for copper ion and molecular oxygen, it is now admitted that the oxidation of primary amine is mediated by 1 without direct involvement of the copper ion (Scheme 1) [2]. This might play a role in orienting the TPQ ring correctly in the active site and in facilitating the reduction of dioxygen to hydrogen peroxide [3]. Support for the transamination mechanism of CuAOs catalysis shown in Scheme 1 arises from a range of crystallographic, spectroscopic and kinetic probes over two decades [2][3][4][5][6][7]. The crucial step of the process is the conversion of the initial iminoquinone "Schiff base substrate" 2 to an aminoquinol "Schiff-base product" 3, facilitated by abstraction of a proton from the α-carbon of the amine substrate. A conserved aspartate side chain has been found to lie in close proximity to 1, thereby implicating this residue as the catalytic base for proton abstraction. Product aldehyde is then released by hydrolysis, generating the reduced cofactor aminoquinol 4, which must be recycled to the initial oxidized cofactor 1 to complete the catalytic cycle. Quinone models that mimic the active site of copper enzymes have also yielded important insights into the transamination process by which these metalloenzymes operate using benzylamine as the model substrate [8][9][10][11][12][13]. In organic media, the intermediate 3 can undergo a direct addition of benzylamine affording the homocoupled imine product PhCH=NCH 2 Ph instead of the aldehyde formed in water. Due to the importance of imines as pivotal synthetic intermediates in a plethora of reactions and essential pharmacophores in various biologically active compounds, researchers have developed bioinspired quinone-based catalytic systems that use dioxygen, or, more preferably ambient air as the sole oxidant, with two objectives: first, to reproduce the activity and selectivity of CuAOs for unbranched primary amines; second, to expand the scope of amine substrates to alpha-branched primary amines, secondary amines and even tertiary amines including fused N-heterocycles [14][15][16]. This paper aims to provide a brief overview of this area which developed rapidly since 2012 by placing emphasis on our recent results.

Bioinspired aerobic catalytic systems which reproduce the activity of CuAOs

The organic chemists have found quinone-based small molecules able to reproduce the activity and selectivity of CuAOs for unbranched primary amines [17][18][19]. For example, Stahl and co-workers have shown that the tert-butyl analog of topaquinone 5 (1.5 mol%) acts as an efficient organocatalyst for the selective oxidation of primary benzylic amines to homocoupled imines, at room temperature under dioxygen atmosphere (Scheme 2) [20]. Oh and co-workers have described a cooperative catalytic system which utilizes 4-phenylo-naphthoquinone 6 (2-5 mol%) whose reactivity can be modulated through the nature of the cocatalyst. Using Cu(OAc) 2 as the cocatalyst (2-5 mol%), primary benzylic amines could be efficiently converted into homocoupled imines, at room temperature under dioxygen atmosphere [21]. The limitation of the reaction to primary benzylic amines allowed the preparation of cross-coupled imines by running the reaction in the presence of 2 equiv. of a second nonreactive primary amine. Upon increasing the catalyst loading to 5 mol% for 5 and 10 mol% for 6, cross-coupled products were formed with good to excellent selectivity. However, in the specific case of 6, the addition of TFA (20 mol%) as the cocatalyst was necessary to promote the crosscoupling reaction without any metal cocatalyst [21].

In this field, we have also described a bioinspired homogeneous cooperative catalytic system which is able to oxidize not only benzylic amines but also non-activated aliphatic primary amines at room temperature, under ambient air. The catalytic process combines two redox couples in a way reminiscent of CuAOs: the unstable ortho-iminoquinone organocatalyst 8, generated in situ from the corresponding ortho-aminoquinol 7 (2 mol%), is the substrate-selective catalyst, while the copper salt (0.2 mol%) serves as an electron transfer mediator by facilitating aerobic oxidation of 7 [22]. The oxidation process starts with atmospheric oxygen and continues in a cascade-like manner by passing the oxidation potential of dioxygen through the copper salt to the organic ortho-iminoquinone mediator 8, which finally oxidizes the amine substrate to the homo-coupled imine (Scheme 3).

The reaction mechanism is very close to the transamination process reported for natural CuAOs. Through the variation of the structure of the redox mediator 8, we showed that the catalytic process developed with electron-poor ortho-iminoquinone species bearing a 2-hydroxyl group. The presence of the active 2-hydroxyl group, which was engaged in an intramolecular hydrogen bond with the imine nitrogen to form a highly reactive Schiff base cyclic transition state (CTS) 9, constituted a prerequisite to the development of the catalytic process (Scheme 4). In the presence of a second amine B, the homo-coupled imine was further alkylated by the amine B through a transimination pathway leading to the cross-coupled imine product [23]. Its formation could be driven by continuous oxidation of the extruded amine A which was re-engaged in the transamination mechanism. As a consequence, just one equivalent of each amine is sufficient to achieve exclusive formation of the cross-coupled imine product through a dynamic self-sorting process [24].

In subsequent studies, we showed that the mild reaction conditions required for the generation of unstable cross-coupled alkylimines could be useful for acting as intermediates for the preparation of medicinally interesting heterocycles such as functionalized benzimidazole derivatives, in the presence of o-aminoanilines B (Scheme 5) [25][26][27]. By heating the coupling reaction mixture at 45 °C under air, in the presence of 4 mol% of 7 and by using CuBr 2 (0.4 mol%) as the copper catalyst, a large molecular diversity could be attained through the variation of both coupling partners leading to 1,2-disubstituted benzimidazoles derivatives in good yields.

MeOH 2 Cu (I) 2 Cu (II) 8 7 R 2 NH 2 NH 3 OMe Me 3 SiO B RT R 1 N R 2 R 1 NH 2 A R ′ N O R 1
Recently, Luo and co-workers have described the oxidative synthesis of heterocycles such as benzimidazoles, benzoxazoles and quinoxalines, which can be mediated by 4-methoxy-5-tert-butyl-o-quinone organocatalyst 10 (Scheme 6) without any metal cocatalysts [28]. At room temperature, the cross-coupled imines could also react in situ as a dienophile in Diels-Alder reactions with Danishefsky's diene to form the dihydro-4-pyridone derivatives (Scheme 5) [23]. This approach is interesting from the viewpoint of green chemistry because mild conditions are used, in particular ambient air as the oxidizing reagent, low reaction temperatures and low catalytic amounts of biocompatible copper salt and ortho-iminoquinone catalyst 8. In addition, the procedure is atom-economic, leaving ammonia as the sole by-product and tolerates large substrate scope including non-activated primary amines.

Bioinspired aerobic catalytic systems which surpass the activity of CuAOs

In the meantime, organic chemists have designed quinone-based catalytic systems capable of expanding the amine substrate scope. Alpha-branched primary amines are known to be poor substrates for the enzymes. The group of Luo [29] was the first to describe an effective ortho-quinone 10 as organocatalyst (10 mol%) which operates at room temperature, under one atmosphere of dioxygen without any metal cocatalyst (Scheme 6), showing the higher performance of ortho-quinone 10 vs. para-quinone 1 in the aerobic oxidation of alphabranched primary amines. Ortho-naphthoquinone 6 (10 mol%), described by Oh and co-workers for the oxidation of unbranched primary amines (Scheme 2), could also be used for the conversion of alpha-branched primary amines into imines [30]. However, whereas 10 promoted aerobic oxidation without any additives, the cooperative action of 6 and acetic acid (20 mol%), together with the dehydrating effect of 3 Å molecular sieves, was necessary for the interruption of the oxidation reaction at the ketimine stage.

More challenging, the possibility to expand the scope to the oxidation of secondary amines, which function as inhibitors of TPQ through the formation of irreversible covalent adduct [31], was extensively explored. This reaction generally required the development of cooperative catalytic systems. For example, the group of Kobayashi [32] has described a cooperative heterogeneous catalytic system that consists of immobilized platinum/iridium bimetallic nanocluster catalyst (0.5 mol%) and 4-tert-butyl-o-quinone 11 as the redox active catalyst (15-50 mol%) for the oxidation of secondary amines at 30 °C, under one atmosphere of dioxygen (Scheme 7). A slightly modified version of this protocol requiring the use of Rh nanoparticles (1 mol%) anchored on carbon nanotubes (RhCNT) as co-catalyst, together with 11 (10 mol%), was reported by Doris and coworkers. This catalytic system allowed the dehydrogenation of secondary amines including various nitrogen heterocycles under ambient air, at room temperature [33]. The group of Stahl has shown that the ruthenium/1,10-phenantroline-5,6-dione (phd) 12 complex catalyst (2.5 mol%), in combination with Cobalt (salophen) as a redox catalyst (5 mol%) allowed the oxidation of tetrahydroquinolines to quinolines under ambient conditions [34]. Furthermore, they have brought definitive evidence for a nonbiomimetic addition-elimination pathway involving a hemiaminal intermediate, which was characterized by NMR spectroscopy [35]. O-naphthoquinone 6 (10 mol%) was also able to dehydrogenate some fused N-heterocycles such as tetrahydroisoquinoline and 9,10-dihydroacridine, at room temperature with the assistance of Ag 2 CO 3 (20 mol%) as a cocatalyst [21]. Finally, o-quinone 10 (10 mol%) was capable to promote the oxidation of tetrahydroisoquinolines to dihydroisoquinolines at more elevated temperature (60 °C), but without any additives [29].

R NH 2 R 1 R N R R 1 R
Beyond the established reactivity of bioinspired catalytic systems with primary and secondary amines, the quinone-catalyzed aerobic oxidation of tertiary amines remained a challenge. Only, Stahl and co-workers had reported the oxidation of N-methylindoline in acetonitrile mediated by 12 as octahedral [Ru(phd) 3 ] 2+ complex (5 mol%) in combination with ZnI 2 (1 mol%) as the co-catalyst [35]. However, the yield was limited to 48 % whereas electron-deficient N-tosylindoline could not be oxidized under the same experimental conditions. Stahl and co-workers have just demonstrated that their [Ru(phd) 3 ] 2+ complex (2.5 mol%) is effective for the aerobic dehydrogenation of substituted indolines, provided the reaction is conducted in methanol, at 40 °C, in the presence of 5 mol% of Co(salophen) as an electron-transfer mediator (Scheme 8). The utility of the protocol has been proved in the preparation of pharmaceutical intermediates [36]. In the same vein, Luo and co-workers have reported the high efficiency of their o-quinone 10 (10 mol%) in tertiary amine dehydrogenation reactions [16,37]. Especially, cross-dehydrogenative-coupling reactions of N-aryl tetrahydroisoquinoline derivatives were performed in the presence of various nucleophiles (2 equiv.), at 60 °C, under aerobic neat conditions. Functionalization adjacent to the nitrogen atom could be obtained in high yields with nitroalkanes, dialkyl malonates, dialkylphosphonates, indoles and even ketones [37]. Finally, o-quinone 10 was found to be the only bioinspired organocatalyst able to oxidize the three classes of amines.

In summary, CuAOs have constituted a rich source of inspiration for the rational design of synthetic catalysts for the oxidation of amines to imines. Some catalysts are now able to reproduce the activity and selectivity for unbranched primary amines including non-activated native substrates for CuAOs, while others surpass the activity of CuAOs by expanding the substrate scope to alpha-branched primary amines, secondary amines and even tertiary amines. However, the procedures suffer still some drawbacks: first, the contamination of the final products by metal residues may constitute a problem in the synthesis of medicinally relevant compounds. Second, the majority of bioinspired catalysts has to be synthesized prior use affecting the economics as well as the eco-friendly nature of the methods. Therefore, the use of a commercially available inexpensive starting material working without any additives would be welcome to limit the production of waste. Unfortunately, commercially available quinone-based catalysts reminiscent of TPQ 1 are not sufficiently reactive to promote the aerobic oxidation of non-activated amines without any additives. New prospects are briefly presented in the last part of this review.

R 1 N R 2 O O 11 (Kobayashi) O O Ph 6 (Oh) R 1 N H R 2 N H N N N O O N N O O Co R R NH R 1 R N R 1 R OMe O O 10 (Luo)

A dual bioinspired protocol as an example of "second-order biomimicry"

Recently, the "second-order" biomimicry concept has been proposed to describe a system in which in situ modification or processing of precatalyst components affords the active biomimetic catalyst [38], as opposed to "first-order" biomimicry in which the active bioinspired catalyst is synthesized directly. Stahl and co-workers have illustrated this concept through the aerobic oxidation of alcohols to aldehydes. They have demonstrated that N,N′-di-tert-butylethylenediamine can serve as a precatalyst which undergoes an in situ oxidative self-processing step reminiscent of copper oxygenase enzymes affording the corresponding nitroxyl (Scheme 9). This can further act as the active cocatalyst that promotes, in cooperation with a copper salt, efficient aerobic alcohol oxidation in a way that mimics the activity of galactose oxidase. The protocol allows the in situ generation of highly reactive nitroxyl derivative which is not readily prepared or isolated due to its rapid decomposition. This new concept allows a more profound relationship between synthetic and naturally occurring catalytic systems. As TPQ cofactor 1 is formed via post-translational modifications of CuAOs active sites, recently, we have thought that this "second-order" biomimetic strategy could be applied to the development of bioinspired catalysts for the selective aerobic oxidation of primary amines to imines. At the starting point of our investigations, we have been drawn to the planar structure of natural purpurogallin (2,3,4,6-tetrahydroxy-5H-benzocyclohepten-5-one) 13 as a potential precursor of a highly reactive o-quinone-based organocatalyst 14 (Scheme 10). As for 1 and 8, purpurogallin 13 showed an active hydroxyl group at the 4-position which allowed the formation of a highly reactive H-bonded CTS 15 under single turnover. This facilitated the development of the transamination mechanism which engaged the active o-iminoquinone organocatalyst 17 without the need to regenerate 14. Interestingly, the catalytic process developed without the need of additional copper salt as a cocatalyst. However, the limited availability of natural purpurogallin 13 led us to concentrate on its well-established biosynthesis which involves the oxidative dimerization of pyrogallol 19 generated from gallic acid 18 [39,40]. Two molecules of ortho-quinone react through a [5 + 2] cycloaddition reaction followed by tautomerization to afford the benzotropolone scaffold 20 (Scheme 11). This intermediate leads to purpurogallin after nucleophilic attack by water, decarboxylation and oxidation steps.

N R 3 N N O O N N O O Co R 1 OMe O O 10 (Luo) R 2 N R 3 R 1 R 2 N Ar R N
Using pyrogallol 19 as the starting material, we have then developed a one-pot cascade reaction that mimics both purpurogallin biosynthesis (Scheme 11) and copper amine oxidase activity [START_REF] Largeron | Brought to you by | Bridgewater State University Authenticated | martine.largeron@parisdescartes[END_REF]. The reaction sequence uses inexpensive commercially available pyrogallol 19 as a precatalyst (8 mol%) and allows in situ formation of not easily accessible purpurogallin 13. This is further engaged in the CuAOs-like catalytic process for producing, under single turnover, the active biomimetic organocatalyst 17 which mediates the oxidative coupling of primary amines to imines under mild conditions (Scheme 12). Without any metal cocatalyst, this new protocol gives also access to 1,2-disubstituted benzimidazole derivatives, when o-aminoanilines are used as amine R′-NH 2 [START_REF] Largeron | Brought to you by | Bridgewater State University Authenticated | martine.largeron@parisdescartes[END_REF]. Finally, this dual bioinspired protocol, which utilizes inexpensive commercially available pyrogallol precatalyst for generating the active biomimetic organocatalyst in situ, constitutes an appropriate illustration of the potential of "second order biomimicry", in terms of atom economy while preserving the benefit of the eco-friendly nature of the bioinspired catalytic processes.

Active cocatalyst Precatalyst

Galactose oxidase mimicry

Catalytic alcohol oxidation

Conclusion

Due to the prevalence of imines as pivotal intermediates in the synthesis of fine chemicals and pharmaceuticals, tremendous research efforts have been made for developing CuAOs-like biomimetic catalytic systems for the aerobic oxidation of amines to imines, based on the utilization of environmentally benign oxidants. Through "first-order" biomimicry, synthetic catalysts have significantly increased the substrate scope of CuAOs allowing the oxidation of alpha-branched primary amines, secondary amines and even tertiary amines. On other side, "second-order biomimicry" has led to the utilization of pyrogallol as an inexpensive commercially available precursor of an active o-iminoquinone organocatalyst able to reproduce the activity and selectivity of CuAOs for the native non-activated primary amine substrates. Although numerous proteinaceous cofactors are post-translationally derived from encoded amino acids, surprisingly, "second-order" biomimicry is not typically considered in the design or development of bioinspired catalysts. A rational

Scheme 2 :

 2 Scheme 2: Aerobic oxidation of benzylic primary amines mediated by quinone catalysts 5 and 6.

R 1 = 4 mol% 5 :

 145 Scheme 5: Synthetic applications involving unstable cross-coupled alkylimines generated in situ from the Cu (II)/8 co-catalytic system.

Scheme 9 : 8 Scheme 10 :

 9810 Scheme 9: Second-order biomimicry for the catalytic oxidation of alcohols to aldehydes.

  Aerobic oxidation of primary amines mediated by topaquinone cofactor 1. The transamination process takes place in the active site of CuAOs.

	on-productive conformation		Productive conformation
					O		O					+ RCH 2 NH 3	O
																	OH
	O Cu (II)	O	-	O Rotation of 180°N -O H 2 O 2 Cu (II) 1 _	Asp	Cu (II)	O	-	+ _ NH 2 R	Asp
					O 2
						NH 4 +	Transamination process					H 2 O
					H 2 O
					RCHO
	HO						HO						O				H
	Cu (II)	O 4	-	+ NH 3 Asp _	H 2 O	Cu (II)	O 3	-	N H + _	R Asp	Cu (II)	O 2	-	N H +	H _	R Asp
	Scheme 1: Brought to you by | Bridgewater State University
								Authenticated | martine.largeron@parisdescartes.fr author's copy
													Download Date | 2/24/20 11:10 AM

  Established biosynthesis of Purpurogallin 13 from gallic acid. Example of "Second-order" biomimicry for the oxidative cross-coupling of primary amines to imines. Brought to you by | Bridgewater State University Authenticated | martine.largeron@parisdescartes.fr author's copy Download Date | 2/24/20 11:10 AM application of this concept should allow the discovery of efficient catalysts generated from simple, low-cost precursors introducing new opportunities in the future.

												O
												O
	HO	OH		-CO 2		HO	OH		O	HO	O	O
	HO			CO 2 H			HO				OH
		18						19
												[5 + 2] cycloaddition
												tautomerization
	HO	OH	O	OH	O		HO	OH	HO OH	HO	OH	HO O CO
	HO	13		-CO 2		HO	HO 2 C		H 2 O	HO	20
	Scheme 11: OH HO		HO	OH	O	OH			HN	OH O	OH
	2
	HO			HO							O
	(8 mol%) 19			(4 mol%) 13					17	Active organocatalyst
	Inexpensive commercially
	available precatalyst			R	NH 2	+	R′ NH 2	MeOH, air, RT	R	N	R′
											R′ = alkyl, cycloalkyl, 80-93 % yields
											R′ = aryl, 55 % yield
	Scheme 12:

Cu (I)Scheme

3: Cu (II)/8-mediated aerobic oxidation of primary amines to imines. Brought to you by | Bridgewater State University Authenticated | martine.largeron@parisdescartes.fr author's copy Download Date | 2/24/20 11:10 AM