Cyrille Galven

Valérie Albin

Simon Hubert

Virginie Lair

Armelle Ringuede

Marie-Pierre Crosnier-Lopez

Françoise Le Berre

Na 6 Li 4 MO 4 (CO 3 ) 4 (M = W, Mo) : an alternative electrolyte for high temperature electrochemical cell

Keywords: New oxycarbonate, MCFC, ionic conductivity, acentric structure, electrolyte

This paper deals with the synthesis and the characterization of two new acentric oxycarbonates Na6Li4MO4(CO3)4 (M = W, Mo). The structure solved by X-ray diffraction is described in the P-43m space group (a ≈ 7.15 Å). Electrical measurements performed on solid pellets and molten salt bath show that the conductivity is purely ionic. Both oxycarbonates present a stable electrical behavior with Ea close to 1.5 eV in the solid state and 0.21 eV above 600°C. s = 1,2 S.cm -1 (600°C) Ea = 0.21 ev

I. Introduction

In the current energy context, the reduction of greenhouse gases is a major concern. Among the ways considered to reduce environmental impact, hydrogen appears to be an unavoidable decarbonized vector. Its conversion will be the most interesting if it feeds a fuel cell. Among the fuel cell technologies developed, the molten carbonate fuel cell (MCFC) has already reached a certain maturity. The system has an electrical efficiency of around 50%, and the overall efficiency can be even higher, reaching 80%. These cells are used on an industrial scale, with projects in South Korea and the United States, for example. The industrial leader in MCFCs, Fuel Cell Energy, has now more than 50 installations worldwide following the start of commercialization of MCFC in 2003. The largest unit, 59 MW, is in Hwaseong, South Korea.

More recently, molten carbonate cells have attracted much attention for CO2 capture and separation. [START_REF] Mcphail | International Status of Molten Carbonate Fuel Cells Technology[END_REF][START_REF] Rashidi | Performance investigation of a combined MCFC system[END_REF][START_REF] Spinelli | Assessing the potential of molten carbonate fuel cell-based schemes for carbon capture in natural gas-fired combined cycle power plants[END_REF] Fuel cell is an electrochemical system that converts chemical energy into electricity, via an electrochemical reaction. It is composed of two electrodes, an anode and a cathode, where oxidation and reduction reactions occur respectively, and an electrolyte (ionic conductor) that separates them. In MCFC, the electrolyte is a molten mixture of alkali metal carbonatesusually a binary mixture at eutectic composition of lithium and potassium carbonates, or lithium and sodium carbonates. It is retained in a porous ceramic matrix, made of LiAlO2, electrically insulating. As the matrix is non-conductive, ionic conductivity is ensured by Li + , Na + or K + cations and carbonate anions through the molten carbonate phase. As references, it is worth mentioning the high level of molten carbonate eutectics conductivities,.e.g. 1.14 S.cm -1 and 1.82 S.cm -1 at 600°C respectively for the mixture Li2CO3-K2CO3 62:38 mol% and Li2CO3-Na2CO3 52:48 mol% [START_REF] Kojima | Electrical conductivity of Molten Li2CO3-X2CO3 (X: Na, K, Rb and Cs) and Na2CO3-Z2CO3 (Z: K, Rb and Cs)[END_REF] . Nevertheless, electrolyte management is one of the key points of the MCFC technology as it leads to degradation of performances. [START_REF] Morita | Degradation mechanism of molten carbonate fuel cell based on long-term performance: Long-term operation by using bench-scale cell and post-test analysis of the cell[END_REF][START_REF] Huijsmans | An analysis of endurance issues for MCFC[END_REF][START_REF] Patent | Mechanical strength improvement of aluminum foam-reinforced matrix for molten carbonate fuel cells[END_REF] For instance, the high corrosivity of carbonates limits the electrochemical devices lifespan because of Ni-based cathode dissolution or metallic bipolar plates corrosion. Moreover, electrolyte composition is modified during operation because of Li consumption due to in situ lithiation of NiO electrode (at short-term) and further evaporation, especially for potassium-based electrolytes. Then research on electrolytic membrane management is of great interest. Different options are considered such as the improvement of the matrix in terms of mechanical resistance but also ionic conductivity, the refuelling of the electrolyte with chosen additives, the modification of the alkali carbonates electrolyte itself. 8, 9, 10 Herein, we explore the modification of the electrolyte by adding, not alkaline or alkaline-earth elements, but transition metals, which are very common, less expensive. Molybdenum and tungsten oxides are frequently used in the industry as gas sensor, catalyst, electrochromics, lubricants… [START_REF] Arfaoui | New MoO3-CeO2-ZrO2 and WO3-CeO2-ZrO2 nanostructured mesoporous aerogel catalysts for the NH3-SCR of NO from diesel engine exhaust[END_REF][START_REF] Ramana | Growth, characterization and performance of bulk and nanoengineered molybdenum oxides for electrochemical energy storage and conversion[END_REF] Then, some of their physicochemical properties such as thermal and chemical stability, catalytic properties, corrosion resistance, etc., seem relevant to electrochemical application. Moreover, Lacorre et al. discovered fast oxygen ion-conducting electrolytes based on La2Mo2O9 (LAMOX). This electrolyte for Solid Oxide Fuel Cell (SOFC) presented interesting ionic conductivity (8.02 × 10 -2 S cm -1 at 800 °C). [START_REF] Georges | The LAMOX Family of Fast Oxide-Ion Conductors: Overview and Recent Results[END_REF][START_REF] Lacorre | Designing fast oxide-ion conductors based on La2Mo2O9[END_REF] More recently, LAMOX based electrolytes (La1.8Dy0.2Mo2-xWxO9) containing both Mo and W elements were synthesized. The electrical conductivity and the activation energy of these electrolytes under air atmosphere, at 800 °C were measured and the substitution of Mo by W led to an increase in conductivity. [START_REF] Srijith | Electrical conductivity studies on LAMOX based electrolyte materials for solid oxide fuel cells[END_REF] We present here two new ionic conductors oxycarbonates, Na6Li4MO4(CO3)4 (M = W, Mo).

After structural and physicochemical characterization, their ionic conductivity was measured by electrochemical impedance spectroscopy in the solid and molten forms under different gas atmospheres, at different temperatures in order to qualify their potential interest as electrolyte for high temperature electrochemical devices.

II. Experimental section

II.1. Synthesis

The two oxycarbonates Na6Li4MO4(CO3)4 (M= W, Mo) were prepared under powder form by solid state synthesis, starting from stoechiometric mixtures of dehydrated Na2CO3 (Acros Organics, 99.8 %), Li2CO3 (Alfa Aesar, > 99 %) and MoO3 (Alfa Aesar, 99.95 %) or WO3 (Alfa Aesar, 99.8 %). The resulting mixtures ( 1g) were pressed into pellets, placed in an alumina boat and heated at 495°C for 36 hrs with two intermediate grindings to obtain the desired well crystallized oxycarbonates.

The structure was solved from single crystal data for Na6Li4WO4(CO3)4. For this, single crystals were synthetized from melted mixture of stoichiometric composition heated 6 hrs at 550°C and slowly cooled (0.03°C.min -1 ).

The large quantity of sample required for conductivity measurements in the molten state ( 160g) led us to find another synthesis route to reduce the time of preparation. For this purpose, we used the melt casting way with the same precursors. After a mixing step of the starting compounds weighted in stoichiometric ratio, the mixtures (M = Mo and W) were introduced in a nickel crucible and heated to 575 °C for 45 minutes before being casted on a nickel foil. After grinding and in order to homogenize, the powder was then heated at 495°C for 24 hrs.

II.2. Analytical Measurements

II.2.1. Infrared (IR) Spectroscopy.

For IR spectroscopy, a Bomen Michelson MB120 FTIR spectrometer with a diamond-anvil cell as a microsampling device was used. The spectral resolution was 4 cm -1 in the 650-4000 cm -1 range.

II.2.2. Scanning Electron Microscopy (SEM)

The chemical analyses were performed on a JEOL JSM 6510LV microscope equipped with the EDS OXFORD system and using the AZtec software.

II.2.3. X-ray Diffraction.

The purity and the crystallinity of the powder samples were checked from Powder X-ray Diffraction (PXRD) patterns recorded in air with a PANalytical Empyrean diffractometer using Cu Kα radiation.

The refinements were performed with Fullprof software [START_REF] Rodriguez-Carvajal | Program FULLPROF Version[END_REF] using a pseudo-Voigt function to describe the peak shape. The background level was determined manually before being refined.

Thermal PXRD patterns were collected with the same diffractometer completed with a high temperature attachment XRK900. They were recorded under static air between 30 and 430°C every 50°C with the following experimental conditions: 10-120° 2θ, with a step of 0.013° and a counting time of 150 seconds per step.

The single crystal diffraction data were acquired on a Bruker APEX II Quazar (4-circle Kappa goniometer) single crystal diffractometer equipped with a micro-source (IµS microfocus source -Mo Kα) and a CCD 4 K detector. Included in the software package WINGX, the program SHELXL-2014/7 was used to solve and refine the crystal structure. 17 II.2.4. Second-Harmonic Generation (SHG) Measurement. A SHG test is commonly used to prove the crystal noncentrosymmetry. 18 If a significant second harmonic light is generated when the sample is irradiated by a short light pulse, then the SHG test is positive, and one can be sure that the crystal structure is non centrosymmetric. The test was performed on polycrystalline Na6Li4MO4(CO3)4 (M= W, Mo) samples using a microchip Q-switched Nd:YAG laser from Teem Photonics, which emits at the wavelength of 1064 nm short optical pulses of 0.6 ns (full width at half-maximum) and 10 μJ, at the repetition rate of 5 kHz. The pump laser beam was focused by a microscope objective ×6.3 onto the powder sample introduced in a Lindemann capillary tube with 0.7 mm internal diameter. This microscope objective was also used to collect the backscattered SHG light at 532 nm.

II.2.5. Thermal Analysis

Differential Scanning Calorimetry was carried out using a SETARAM SETSYS EVOLUTION in order to precisely determine the thermal behavior of Na6Li4MO4(CO3)4 (M= W, Mo). The sample (from 30 to 50 mg in a 90 µL alumina crucible) has been heated and cooled under different air/CO2 mixtures with a flow rate of 20 mL/min (CO2 100 vol%; CO2-air 70-30 vol%; and CO2-air 20-80 vol%) between 40°C and 800°C at a rate of 2°C min -1 .

II.3. Electrochemical measurements

Conductivity measurements were carried out by electrochemical impedance spectroscopy (EIS) using a Solartron Analytical Energylab potentiostat. Signal frequencies were scanned from 1MHz to 0.1 Hz, with 10 points per decade. The measurements were performed separately for both solid and molten phases, as a function of temperature, under atmospheric pressure. The temperature range for all the experiments was 250-470°C for the solid phase and 600-750°C for the molten phase referring to DSC analysis. All the electrochemical measurement were performed from the same powder batch.

II.3.1 Conductivity in the solid phase (250-470°C): Pellets of Na6Li4MO4(CO3)4 (M= W, Mo)

oxycarbonates were prepared from 0.5g of powder, first uniaxially pressed at 1000 bars (100 MPa) using a 10 mm die. The pellets were then isostatically pressed at 4500 bars (450 MPa) for 1mn and sintered at 495°C for 2h. Electrodes were deposited as thin gold films by RF sputtering technique on both parallel sides of the pellets. Then electrical behaviour was characterized through EIS using this 2-electrodes symmetric configuration in single atmosphere set-up. The chosen amplitude for EIS analysis is 100 mV, in respect to the system linearity. To ensure repeatability, measurements were repeated at least 3 times. A total flow rate of 65 mL.min -1 was used with 3 various gas compositions: CO2 100 vol%, CO2-air 70-30 vol%, and CO2-air 20-80 vol%.

II.3.2 Conductivity in the molten phase (600-750°C

): the electrochemical cell, an alumina cylindrical crucible placed in an alumina reactor, was described elsewhere. [START_REF] Lair | Theoretical predictions vs. experimental measurements of the electrical conductivity of molten Li2CO3-K2CO3 modified by additives[END_REF] The electrolyte consists of 75 g of the previous synthetized oxycarbonate, Na6Li4MO4(CO3)4 (M = W, Mo).

Oxycarbonate powders are dried before use and kept in a drying oven at 130°C. The system, at atmospheric pressure, is left at least 12 h, under atmosphere of interest before analysis to ensure its equilibrium. Two gold planar electrodes set in parallel, were used. Each plate has a surface of 1 cm 2 (1x1 cm 2 ) and is 0.5 mm thick (Au plate, 99.99%, AMTS, France). The distance between both electrodes is kept constant and equal to 2 cm. The gold plates were previously polished on SiC grinding paper (P1200, P2400, P4000) and then rinsed under ultrasonics in water-ethanol 50-50 vol% before each experiment. The signal amplitude used for EIS analysis is 5 mV, respecting the system linearity. A total flow rate of 50 mL.min -1 , was used with 3 various gas compositions: CO2 100 vol%, CO2-air 70-30 vol%, and CO2-air 20-80 vol%.

III. Results and discussion

In the aim to modify the chemical formulation of the classical electrolyte Li2CO3:Na2CO3 used in molten carbonate cells, we decided to add transition metals. We obtained for some compositions, PXRD patterns that corresponded to mixtures of known phases with in addition, a same set of intense lines. As these lines did not match to any pattern listed in the databases, we performed preliminary characterizations to determine the exact composition of this new phase and synthesize it as a single phase sample.

III.1. IR Spectroscopy

An IR spectrum (Figure 1) was recorded as described in the experimental part. This spectrum reveals three bands (1450, 1083 and 873 cm -1 ) characteristic of the presence of the CO3 2- carbonate group: the broad and intense band at 1450 cm -1 is attributed to the antisymmetric 3 vibration mode, while the two others, narrower bands at 1083 and 873 cm -1 are respectively attributed to the symmetric vibration mode 1 and the bending mode 2. 20

III.2. Scanning Electron Microscopy

The chemical analysis performed on the powder sample confirmed the simultaneous presence of sodium, tungsten (or molybdenum), oxygen and carbon with molar ratios Na/W ≈ 5.2 and Na/C ≈ 1.3. As Li can't be evidenced by EDS, we also performed a synthesis without lithium.

The resulting PXRD pattern does not show the set of intense Bragg reflections previously observed but corresponds to a mixture of known phases (Figure S1) implying thus the necessary presence of lithium in the target material.

III.3. Structure determination

III.3.1. Powder X-ray diffraction

After many attempts, we succeeded to obtain one pure sample with the following ratio: Na/Mo = 6 and Na/Li = 1.5. For this composition, all the diffraction reflections of the PXRD pattern could be indexed in a cubic cell with a parameter of 7.1508 Å (Figure 2).

III.3.2. Single crystal diffraction

The presence of light atoms as lithium and oxygen in the formulation led us to perform the structural determination from a single crystal (M = W). It was obtained as described in the experimental section and carefully selected for the X-ray diffraction data collection. The data analysis suggested the m-3m Laue class compatible with only two of the previous three acentric space groups: P432 (no.207) or P-43m (no.215). A starting model was quickly obtained in the P-43m space group with six atomic positions: 1b, 3c, 3d, 12i and two 4e positions. Given the relative scattering power of tungsten atom compared to that of the others, it was unambiguously located at 1b site while the other sites were assigned to Na (3c and 3d), C (4e) and O (4e and 12i) taking into account the usual anion-cation distances. Fourier difference syntheses allowed to complete the structure with one site (4e) in a tetrahedral environment which can then be associated to lithium cations according to Li-O distances. The resulting chemical formula is then Na6Li4WO4(CO3)4. With absorption correction, anisotropic displacement parameters and a weighting scheme, the final refinement converged to good reliability factors R = 0.0181 for the 348 data with I > 2s(I) and for all the data with a final difference synthesis featureless. The sample and crystal data for Na6Li4WO4(CO3)4 are given in Table 1 while Table 2 gathers information on the data collection and on the structure refinement.

Table 1: Sample and crystal data for Na6Li4WO4(CO3)4

Table 3 gives the atomic coordinates, the equivalent isotropic atomic displacement parameters and the bond valence sums while selected atomic distances and anisotropic atomic displacement parameters are given respectively in Tables 4 andS1. (mean distances are given in italic)

The refined crystal structure of Na6Li4WO4(CO3)4 is displayed in Figure 4. In this structure, the tungsten and the lithium atoms are both in a tetrahedral environment. The tungsten is surrounded by four O2 to form a perfect WO4 tetrahedron while the lithium is surrounded by one O2 and three O1. Each O2 corner is shared with one LiO4 tetrahedron, building thus WLi4O16 units isolated from each other and which can be noted WO24Li4O112 (Figure 4). The "free" O1 corners of the WLi4O16 units participate in the formation of CO3 2- carbonate group so that each CO3 2-group links three WLi4O16 units together (Figure 5). Such arrangement creates large cavities (coordination number 8) which accommodate the sodium ions: Na1 is located in a distorted cube while Na2 is in a trigonal dodecahedron (Figure 4).

From a distance point of view, the Table 4 shows that the mean distance for all cations are in good agreement with the Shannon table [START_REF] Shannon | Revised Effective Ionic Radii and Systematic Studies of Interatomie Distances in Halides and Chaleogenides[END_REF] (1.77, 2.53, 1.94 and 1.27 Å respectively for W, Na, Li and C taking into account their coordination number) leading consequently to bond valence sums close to the expected values (Table 3).

At this stage, it is interesting to ask whether this structure could have been described in the P432 space group (no.207) although the structure obtained in P-43m (no.215) seems quite relevant. A quick comparison of the positions of these two groups shows that the location of the tungsten and sodium atoms can be transposed easily directly from one group to the other:

1b for W and 3d, 3c for Na in both groups. For lithium and carbon, both are located on two 4e sites (x, x, x) in P-43m group with a x value for carbon (0.808) very close to the -x lithium value (0.209). These two positions could then correspond to one 8g position of the P432 group, meaning then that the four lithium atoms and the four carbon atoms would be statistically distributed over the eight equivalent positions of the 8g site while they are located on their own site in the P-43m space group. Concerning the two anionic positions, the 12i site (x, x, z with z  0.95) in the P-43m group could correspond to the 12i (0, y, y) site in the P432 group with z value set to zero while the 4e site for its part can be transposed to the 8g site with the same x value but a half occupancy rate. Then, since it's possible to transpose all the structure from the P-43m to the P432 space group, we performed a complete structure refinement in this new space group. We observed first that, with absorption correction, anisotropic displacement parameters and a weighting scheme, the final refinement converged to much higher reliability factors R = 0.0622 compared to 0.0181 in the P-43m space group. Secondly, the equivalent isotropic atomic displacement parameters for Li and C (8g site) factor is about 50 times higher than the two values observed for these cations in the P-43m space group. Moreover, a split of this 8g position is suggested by the SHELXL program. 17 This implies that the Li/C ordering highlighted in the P-43m space group can't be correctly transposed in P432 meaning that the P-43m space group should be chosen to describe the Na6Li4WO4(CO3)4 structure.

For Na6Li4Mo(CO3)4O4, the structure was successfully refined from PXRD data by directly applying the refined model for tungsten which is consistent with the similarity of the two PXRD patterns. The refining results for Na6Li4Mo(CO3)4O4 are given in Figure S3 and Table S2.

III.4. Thermal analysis

According to DSC analysis a very sharp endothermic peak has been observed for both oxycarbonates (Figures 6 andS4, respectively T2 and T4), characterizing the melting of the solid powder. The melting point of the molybdenum phase (Na6Li4MoO4(CO3)4) is 521°C whereas the melting point of the tungsten phase (Na6Li4WO4(CO3)4), is 528 °C. Those two melting points are quite close to the melting point of the classical eutectic carbonates, Li2CO2-Na2CO3 52:48 mol% which is 501°C. [START_REF] Jiang | Thermal stability of Na2CO3-Li2CO3 as a high temperature phase change material for thermal energy storage[END_REF] The PXRD patterns of the samples recorded before and after heating being identical (figure S5), we could conclude that the melting of our oxycarbonates is congruent, explaining then the route used to prepare the samples (see experimental section). It is worth noting that a very small and thin peak (T1, Figure 6) is observed in the molybdenum phase, at 503°C, and in the tungsten phase (512°C, T3, Figure S4), indicating that the powder may contain a small excess of Li-Na carbonates, from the synthesis. This value, higher than for eutectic composition may be explained by the presence of remaining lithium and sodium carbonate at a different composition, especially when many syntheses are carried out in order to obtain a huge amount of oxycarbonates. DSC analysis registered during the cooling step showed also two peaks corresponding to the recrystallisation of the two phases described before, with the same relative intensities as during heating. For both oxycarbonates, these two peaks are obviously slightly lower than the melting peaks.

Nevertheless, in order to highlight possible subtle structural changes, PXRD patterns were collected up to 430°C so as not to reach their melting temperature. For both oxycarbonates, no particular transformation was evidenced (Figure S6) except a classical left shift of the Bragg reflections, reflecting a progressive increase of the cell parameter with temperature. Using the P-43m space group, the PRXD patterns refinement was quite satisfactory in this temperature range and a linear evolution of the cell parameter was observed (Figure S7 and Table S3) leading to the expansion coefficient a = 2.1 10 -5 and 2.6 10 -5 °C-1 for Mo and W oxycarbonates respectively.

III.5. Electrochemical measurements

Conductivity measurements have been carried out using Electrochemical Impedance Spectroscopy (EIS) for both molten oxycarbonates (in the temperature range 600°C to 750°C) and pellets of oxycarbonates (in the range 250-470°C). As molten carbonate phases required CO2 to ensure the chemical equilibrium and avoid decarbonation of the system, different gaseous composition at atmospheric pressure were chosen. For each gas composition (20, 70 or 100 vol% of CO2), the impedance of the two oxycarbonates was measured at different temperatures, during one heating and cooling cycle. The material conductivity was calculated from the electrolyte resistance (intercept at high frequencies of the diagram and the real part Z') as illustrated in Figure S8 and Figure S9 for both, solid pellet and molten salt.

III.5.1. Oxycarbonates pellets (250°C ≤ T ≤ 470°C)

The conductivity variation of a Na6Li4MoO4(CO3)4 pellet varies linearly as a function of the reverse temperature whatever the CO2 content in the gas phase (Figure 7). Thus it follows a single Arrhenius law in all the temperature range explored for these solid pellets. Moreover, both heating and cooling cycle are overlayed without any hysteresis phenomenon and no influence of the CO2 content is observed (Figure 7). The activation energy calculated from the slope is 1.49 eV of this figure. This behavior is comparable for Na6Li4WO4(CO3)4

(Figure S10) and the activation energy is then a little bit slower, Ea = 1.45 eV. So, for both oxycarbonates, a stable electrical behavior is observed during a heating/cooling cycle whatever the CO2 content (from 20 to 100 vol%). The conductivity is therefore independent of the atmosphere, which is consistent for an electrolyte material, showing pure ionic conductivity in these two compounds. Its value of 10 -4 S.cm -1 at 480°C is nevertheless 10 times lower than YSZ (the state of art Solid Oxide Fuel Cell electrolyte) and 100 times lower than Gd-doped ceria, considered as a potential electrolyte in lower operating temperature range than with YSZ. [START_REF] Steele | Material science and engineering: The enabling technology for the commercialisation of fuel cell systems[END_REF] Furthermore, the activation energy determined for solid oxycarbonate compositions is highly higher compared to the ones associated to GDC or YSZ, 0.8 eV and 1 eV respectively. It is worth mentioning that the oxycarbonates were sintered at 495°C, much lower than the 1350°C required for GDC or YSZ, and the final density is de facto lower. Thus the conductivity of the oxycarbonate pellets is strongly affected by some porosity which cannot be avoided. It must be reminded that at higher temperature the two oxycarbonate phases are molten, thus cannot be densified.

III.5.2. Molten oxycarbonates (600°C ≤ T ≤ 750°C)

Above the melting point, (T>600°C), the conductivity variation of Na6Li4MO4(CO3)4 (M = W, Mo) follows also a single Arrhenius law in the whole temperature range whatever the CO2 gas content (Figures 8 andS11). It is important to mention that the same electrical behavior is observed during heating and cooling steps, as discussed for solid phases. That means that no decarbonation of the oxycarbonate phases occurs in the conditions used for the measurements, probably thanks to CO2 presence in the surrounding atmosphere. For both Na6Li4MoO4(CO3)4 and Na6Li4WO4(CO3)4, the ionic conductivity, 1.24 S.cm -1 and 1.06 S.cm -1 respectively for Moand W-based oxycarbonates are comparable to the usual molten carbonate eutectic mixtures:

slightly lower than (Li,Na)2CO3 conductivity, i.e. 1.82 S.cm -1 at 600°C but slightly higher than (Li,K)2CO3, i.e. 1.14 S.cm -1 at 600°C. The equivalent activation energy of 0.21 eV is typical of cations and anions transport mechanisms in these media. This value is consistent with classical molten carbonate eutectics, [START_REF] Kojima | Electrical conductivity of Molten Li2CO3-X2CO3 (X: Na, K, Rb and Cs) and Na2CO3-Z2CO3 (Z: K, Rb and Cs)[END_REF][START_REF] Lair | Theoretical predictions vs. experimental measurements of the electrical conductivity of molten Li2CO3-K2CO3 modified by additives[END_REF] meaning that these two new compounds could find potential applications such as electrolyte in high temperature fuel cell.

IV. Conclusion

The two new oxycarbonates, Na6Li4MO4(CO3)4 (M = W, Mo) presented in this paper have been characterized by complementary techniques: IR spectroscopy, SHG test, EDS and thermal analysis, X-ray diffraction. Their structure, solved from a single crystal of Na6Li4WO4(CO3)4, is described in the acentric P-43m space group with a = 7.1529(3)Å as cell parameter. It is composed of MLi4O16 units built from a MO4 tetrahedron sharing each of its corners with one LiO4 tetrahedron. Three MLi4O16 units are linked together by a CO3 2-group forming thus a 3D framework in which Na ions are inserted. From DSC and PXRD, we showed that the melting of the two oxycarbonates is congruent and around T ≈ 525°C. Electrical measurements were performed on solid pellets (T < 500°C) and molten salt bath (T > 600°C) in different atmospheres containing however CO2 to avoid decarbonation of the mixture. At low temperature (T < 500°C), both oxycarbonates present a stable electrical behavior during heating and cooling cycles, with an activation energy close to 1.5 eV. At high temperature (T > 600°C), when the oxycarbonates are melted, the activation energy is stable at 0.21 eV. It is worth noting that from conductivity value, the oxycarbonates containing W appears to be a little bit less performing than the one containing Mo.

Those results are quite promising in view of modifying classical electrolyte used in molten carbonate cells such as fuel cell or electrolysis cell. A lot of work is still required because ionic conductivity is not enough to fully qualify a composition as electrolyte. Nevertheless, as carbonates are quite corrosive towards NiO-based electrode and bipolar plates, oxycarbonates could have less impact. Another way would be to make a composite associating these new phases with solid oxide phase such as ceria-based ones in order to increase the conductivity even lowering the operating temperature. This assumption is already under work and the first results are highly promising.

Figure 1 :

 1 Figure 1: IR spectrum of Na6Li4WO4(CO3)4

Figure 2 :

 2 Figure 2 : Observed, calculated and difference PXRD patterns of Na6Li4MoO4(CO3)4 in the

Figure 3 :

 3 Figure 3: SHG signal for Na6Li4WO4(CO3)4 and Na6Li4MoO4(CO3)4

Figure 4 :

 4 Figure 4: View of the Na6Li4WO4(CO3)4 structure showing an isolated WLi4O16 unit, CO3 2-groups

  : 31.916(8) <C-O> = 1.286 Å Na2-O2 : 42.9370(9) <Li-O> = 1.90 Å <Na-O> = 2.609 Å

Figure 5 :

 5 Figure 5: Partial view of Na6Li4WO4(CO3)4 structure showing how a CO3 2-group connects three

Figure 6 :

 6 Figure 6: DSC curves of Na6Li4MoO4(CO3)4 oxycarbonate, (a) heating at 2°C.min -1 in 100% CO2

Figure 7 :

 7 Figure 7: Arrhenius diagram of Na6Li4MoO4(CO3)4 solid phase for three atmosphere

Figure 8 :

 8 Figure 8: Arrhenius diagram of the molten Na6Li4MoO4(CO3)4 phase as a function of the

Figures captionFigure 1 :

 1 Figures caption

Figure 2 :

 2 Figure 2: Observed, calculated and difference PXRD patterns of Na6Li4MoO4(CO3)4 in the Pm-

Figure 3 :

 3 Figure 3: SHG signal for Na6Li4WO4(CO3)4 and Na6Li4MoO4(CO3)4.

Figure 4 :Figure 5 :

 45 Figure 4: View of the Na6Li4WO4(CO3)4 structure showing an isolated WLi4O16 unit, CO3 2-

Figure 6 :

 6 Figure 6: DSC curves of Na6Li4MoO4(CO3)4 oxycarbonate, (a) heating at 2°C.min -1 in 100% CO2 and (b) cooling at 2°C.min -1 , in 100% CO2.

Figure 7 :

 7 Figure 7: Arrhenius diagram of Na6Li4MoO4(CO3)4 solid phase for three atmosphere

Figure 8 :

 8 Figure 8: Arrhenius diagram of the molten Na6Li4MoO4(CO3)4 phase as a function of the

Table 2 :

 2 Data collection and structure refinement for Na6Li4W(CO3)4O4

	Symmetry	Cubic
	Space group	P -4 3 m (no. 215)
	Cell parameter a /Å	7.1529(3)
	Volume /Å 3	365.97(5)
	Z	1
	Formula weight /gmol -1	633.59
	Densitycalc	2.966
	Temperature /K	296(2)
	Wavelength /Å	Mo K, 0.71073
	Crystal size /mm	0.132 0.210  0.389
	Absorption coefficient /mm -1 8.162

Table 3 :

 3 Atomic coordinates, equivalent isotropic atomic displacement parameters (Å

		Site s.o.f.	x	y	z	Ueq	 (exp)
	W	1b	1	0.5	0.5	0.5	0.0095(2)	6.3	(6)
	Na1 3d	1	0.5	0.0	0.0	0.0192(5)	1.0	(1)
	Na2 3c	1	0.0	0.5	0.5	0.0220(6)	1.0	(1)
	Li	4e	1	0.209(2)	0.209(2)	0.209(2)	0.012(4)	1.2	(1)
	C	4e	1	0.808(1)	0.808(1)	0.808(1)	0.013(2)	4.0	(4)
	O1	12i	1	0.2664(3) 0.2664(3) 0.9538(5) 0.0193(6)	2.1	(2)
	O2	4e	1	0.3587(7) 0.3587(7) 0.3587(7) 0.039(2)	2.0	(2)

[START_REF] Rashidi | Performance investigation of a combined MCFC system[END_REF]

) and bond valence sums () for Na6Li4W(CO3)4O4 (S.G.: P -4 3 m) (Ueq is defined as one third of the trace of the orthogonalized Uij tensor)

Table 4 :

 4 Selected interatomic distances (Å) for Na6Li4W(CO3)4O4

Acknowledgements

The authors thank Denis Mounier (IMMM, Le Mans) for SHG measurements. The X-ray diffractometer and the SEM used in this study belong to instrumental platforms of the "Institut des Molécules et Matériaux du Mans" (IMMM) : "Plateforme de Diffusion et Diffraction des Rayons-X" and "Plateforme de Microscopie Electronique" .

Associated Content


Léa Vilcocq

Nicolas Chaussard

Antonio Hernández Mañas

Olivier Boyron

Manel Taam

Frédérique Bertaud

Pascal Fongarland

Laurent Djakovitch

Potential of catalytic oxidation of Kraft black liquor for the production of biosourced compounds

Keywords: Klason phase: 0, 189 Klason phase: 0, 757 Acetic acid: 0, 5 Phenol: 0, 02 Succinic acid: 1, 2 AHA mix: 17, 66 Formic acid: 3, 91 Tartronic acid: 11, 35 Klason phase: 4, 61 Vanillin: 0, 73 Acetic acid: 1, 55 Phenol: 0, 12 Succinic acid: 3, 75 d)

Industrial Kraft black liquor from maritime pine was oxidised in aqueous, alkaline medium, under air, at 150°C, with or without CuO/TiO2 catalyst. The oxidation products were analysed by HPLC, elemental analysis, SEC, FTIR, NMR. The results showed the depolymerisation of lignin, the formation of phenolic compounds in low yields, with vanillin being the main phenolic compound, and the formation of aliphatic compounds in higher yields, with formic, succinic and tartronic acids being the main identified aliphatic compounds. The presence of the catalyst favoured the formation of phenolic and aliphatic compounds. Replacing Kraft black liquor by pure Kraft lignin as a starting material did not improve the performances of the catalytic oxidation, indicating that lignin purification may not be necessary under our conditions. Switching from a batch reactor to a fixed bed reactor, operating under similar conditions, did not increase the yields of oxidation products but did increase the productivity. This work demonstrates the potential of Kraft black liquor to produce a large panel of compounds, including phenolics, aliphatic acids and oxidised lignin, all of which valuable in chemical industry. For the first time, a catalytic process for the chemical valorisation of Kraft black liquor is presented.

Introduction

Kraft black liquor (KBL) is an industrial effluent produced during the Kraft process in pulp and paper mills. In the Kraft process, wood chips are delignified by cooking at 155-165°C in white liquor, an aqueous solution of soda and sodium sulphide, to produce pulp, a fibrous material rich in cellulose. The liquid residue is called black liquor (BL) and contains the reagents in excess (soda and sodium sulphide), and their by-products (sulphates, sulphites, thiosulphates) as well as degraded hemicellulose and lignin. 180 million tons of paper pulp are produced annually and each ton of pulp produces ca. 7 tons of weak (i.e. diluted) black liquor. [START_REF] Sixta | Handbook of Pulp[END_REF] This bio-waste is currently being burnt to regenerate inorganics in novel white liquor and to recover energy from combustion of organic compounds. The recovery boiler produces ca. 2 tons of biogenic CO2 per ton of pulp. [START_REF] Kuparinen | [END_REF] Moreover, it is often the bottleneck in the Kraft process. Therefore, bypassing some of the black liquor with an alternative process would be beneficial to pulp mills. In the long-term, consideration should be given to converting pulp mills into integrated forest biorefineries producing a wide range of commodities with low emissions of CO2. 3 During Kraft cooking, hemicelluloses are degraded with the peeling reaction to form a broad range of aliphatic acids: formic, acetic, glycolic, lactic and isosaccharinic acids were observed, among others. 4 Lignin is also degraded but retains a polymeric structure. However, transformations occur as some ether interunit bonds (β-O-4 bonds) are cleaved and new C-C bonds are formed. 5 Black liquors also contain high concentrations of inorganics derived from white liquor: sodium sulphate, carbonate or hydroxide are present. The pH is very high (superior to 13). Separation and valorisation of lignin from black liquor has been attempted in several pathways. Kraft lignin can be recovered by acidification and precipitation 6 (LignoBoost process) and commercialised as dispersant, binder, metal adsorbent, etc. 7,8 Kraft lignin can be valorised by different processes, such as gasification or pyrolysis, the latter producing bio-oil, biochar and biogas as products. 9,10 Under milder reaction conditions, hydrotreatment, solvolysis, [START_REF] Sebhat | Biomass Convers[END_REF] hydrolysis, or oxidation [START_REF] Almada | Biomass Convers[END_REF] can be applied to depolymerise lignin and produce of monomers in low yields (aromatics, phenolics, aliphatics), oligomers and gases. Among the technical lignins, Kraft lignin is the most recalcitrant and usually gives the lowest yields of products of interest, as Kraft cooking alters the structure of the lignin. One of the most interesting compounds produced from lignin is vanillin. This phenolic aldehyde can be obtained by depolymerisation and oxidation of guaiacyl units in lignin. 13 It has many applications in fragrances and flavours (F&F) industry and a high market value (20 $/kg in 2016 according to Fache et al. 14 , 10-15 $/kg according to Rinaldi et al. in 2016 15 ). The industrial production of vanillin relies for ca. 15 % on the oxidation of lignosulfonate with copper sulphate as a catalyst (Borregaard process). 14 Besides, the formation of aliphatic acids from lignin oxidation has been reported in a few articles: formic, acetic, succinic, oxalic, and glutaconic acids have been observed. 16 Many research teams have attempted to increase the productivity of lignin oxidation towards vanillin and phenolics. 15,[17][18][19][20][21][22][23][24][25] The oxidant (O2 or H2O2), the solvent (water or other), the pH, the operating conditions (reaction time, temperature, pressure, type of reactor) and the type of lignin, all have influence on the reaction performance. O2 has been identified as the cheapest and greenest oxidant, but also requires high temperatures and is not selective. Mild temperatures (100-190°C) and moderate partial O2 pressures (20-100 bar) are often applied. High pH is necessary for lignin solubilisation and increased reaction rates. In addition, lower pH values are associated with higher degradation rate of vanillin. Wet oxidation of lignin produces phenolic aldehydes (vanillin, syringaldehyde, p-hydroxybenzaldehyde), phenolic acids (vanillic, syringic, p-hydroxybenzoic acids), phenolic ketones (acetovanillone, acetosyringone, acetophenone), quinones, but also aliphatic monoacids (formic, acetic), diacids (oxalic, succinic, malonic, maleic), hydroxy-acids (lactic). Lignins with a low molecular weight and high β-O-4 content give higher yields in a shorter reaction time. Kraft lignins, with a low β-O-4 content, result in low yields of aromatic products. A catalyst can also be added to the reaction. 26 Metallic salts of Cu, Fe, Mn, Co, Zr have been tested. CuSO4 was identified as an interesting catalyst and has been used in industrial processes such as the Borregaard process. Organometallic complexes such as metallosalens and metalloporphyrins, which mimic the action of lignin peroxidases, are also promising. Solid catalysts have also been developed, based on Cu, Co, Fe, Zn, V, Mo, Re, Pd, Pt oxides or hydroxides, bulk or supported on various materials (Al2O3, C, TiO2, etc.). Copper-based solid catalysts have a high potential for lignin oxidation, as they are active, stable, inexpensive and non-noble materials. In particular, the production of phenolic aldehydes from Eucalyptus Kraft lignin with CuO catalyst, 27 and from softwood lignosulfonate with Cu(OH)2 catalyst 28 have been reported. Another category of interesting compounds in KBL is the mix of aliphatic acids. These ones can be isolated from KBL by various separation techniques (chromatography, membrane filtration, liquid-liquid extraction, distillation) but none of them has been industrialised so far. 4 Due to the complexity of the black liquor matrix, several separation operations are necessary to purify the aliphatic compounds. Besides, as an alternative to combustion in boiler, concentration and recovery of inorganics can be achieved by wet oxidation, membrane nanofiltration, or gasification. Direct conversion of black liquor is challenging because of the complexity of KBL. It is a concentrated, alkaline effluent, combining organic and inorganic components, as well as polymers and monomers with a broad variety of chemical functions. Black liquors can be transformed through different pathways to produce compounds of interest to the fuels and chemical industries: gasification of black liquor has been investigated as an alternative to combustion in recovery boiler. 29 Liquefaction of black liquor produces bio-oil as an alternative fuel. 30 The chemical valorisation pathway, in which biosourced compounds, either polymers or monomers, are transformed under mild reaction conditions (temperature inferior to 300°C, pressure inferior to 100 bar), has rarely been applied to crude black liquor. Few examples of catalytic reduction 31 or catalytic oxidation 32 have demonstrated the (partial) depolymerisation of lignin and sometimes the formation of phenolic compounds from lignin in black liquor, in yields inferior to 10 %. In a recent communication, our research teams demonstrated the feasibility of direct oxidation of black liquor to produce low yields of vanillin and phenolics; however with reasonable productivity. 33 In this work, the products of the catalytic oxidation of KBL have been studied in detail. The reaction was carried out with air, and a solid CuO/TiO2 catalyst. The formulation of the catalyst was based on elements from the literature and from our experience showing a good stability of TiO2 34 and CuO 35 in aqueous alkaline conditions. The role of different parts of the catalyst (support, metal) was elucidated. Our aim was to demonstrate the potential of KBL for the production of a wide range of biosourced compounds and materials by catalytic oxidation. To this end, reaction products were identified and characterized in detail: phenolic compounds, but also aliphatic compounds and residual lignin. Two types of reactors were compared: a batch stirred reactor with a catalytic slurry and a fixed bed continuous reactor. A study of the productivity of various compounds of interest was also attempted to provide some insights into the techno-economic prospects of KBL oxidation.

Experimental Materials

Kraft black liquor (KBL) employed in all the experiments was obtained from a Kraft pulp mill in France, which uses local maritime pine (Pinus pinaster) as biomass. Kraft Lignin (KL) was extracted from KBL in "Centre Technique du Papier" (CTP). KBL was acidified by bubbling CO2 until pH 9. The lignin suspension obtained was filtrated to collect the lignin. The lignin was purified by acid washing (diluted sulphuric acid in water at 2%) and finally air-dried. TiO2 Aerolyst 7711 support was kindly provided by Evonik. Cu(NO3)4, NaOH, HCl, HPLC acetonitrile grade, H2SO4, H3PO4, NaH2PO4 were purchased from Alfa Aesar, or Sigma Aldrich and used without further purification.

Catalyst preparation and characterization

The preparation and characterisation of CuO/TiO2 catalyst were described in our previous publication. 33 Briefly, TiO2 pellets were impregnated with an aqueous Cu(NO3)4 solution following the incipient wetness impregnation technique. The pellets were dried at 110°C overnight and then calcined at 550°C under air flow in a muffle furnace. After calcination, the pellets were crushed and sieved and the 90-200 µm fraction was retained for catalytic tests in batch reactor. For the continuous reactor, the pellets were used without crushing. The catalyst was characterised by XRD, ICP, and N2 physisorption for porosimetry (see ESI for details).

Catalytic oxidation test and post-treatment

The catalytic oxidation of KBL was carried out in a 300 mL stainless steel batch reactor (Vinci Technology). KBL was diluted in NaOH solution (10 g.L -1 ) up to a dry matter content equal to 5 g.L -1 . The reactor was filled with 150 mL of the diluted KBL solution and 0.75 g of catalyst (equivalent to 5% copper/lignin mass ratio). The reactor was sealed and tightened, purged three times with synthetic air and pressurised with air at 20 bar at room temperature. It was heated up to 150°C and time zero was set when this working temperature was reached (about 30 min). The reactor was stirred with a mechanical stirrer equipped with a Rushton turbine (1800 rpm). At the end of the reaction time (0 to 90 min), the reactor was cooled in an ice bath, depressurised and opened. The protocol for catalytic oxidation in a continuous reaction was described in a previous communication. 33 Briefly, A stainless steel tube was loaded with 20 g of CuO/TiO2 catalyst mixed with silicon carbide to form a bed 12.5 cm high in an isothermal zone between two layers of pure SiC at the inlet and outlet of the reactor. The reactor was sealed, placed in a tubular oven and connected to a feed of diluted KBL (same concentration as for batch reactions), fed with a pump, and air flow (15 NL.h -1 ), fed by a mass flow regulator. After the reactor, a backpressure membrane regulator maintained the reactor pressure at 80 bar. After the backpressure regulator, a sampling valve allowed liquid samples to be taken periodically. The liquid flow was controlled to keep the contact times (defined as the ratio of mass of catalyst to mass liquid flow) as short as possible. Whatever the reactor used, samples (50-100 mL) of the reaction medium were filtered under vacuum to remove solid residues. In the batch reactor, the catalyst was found in the solid residue; in the continuous reactor, filtration did not produce significant amount of solids. The filtrate was acidified with a 10% HCl solution until pH 1 to precipitate residual lignin. The precipitate was recovered by centrifugation at 4000 rpm for 20 min, washed with HCl solution and water and then dried under vacuum for 9 hours at 60°C. The recovered solid was called Klason phase (KP), "Klason" designating the acid precipitate in KBL and in product mixtures. The liquid separated in the first centrifugation was called liquid phase (LP).

Products analysis

Liquid phase -HPLC analysis. LP samples were filtered with syringe filters (0.22 µm) and analysed by HPLC on a Shimadzu apparatus equipped with a diode array UV detector (DAD) with two different methods.

In the first one, a Phenomenex Kinetex XB C18 column was used and the mobile phase was a gradient of acetonitrile and aqueous H2SO4 solution (10 mM) starting with 5 vol.% acetonitrile (0-6 min) and increasing to 30 vol.% acetonitrile (ramp from 6 to 16 min and step from 16 to 30 min). Phenolic compounds were identified and quantified by this method using commercial compounds as references. Identification was based on retention time and UV spectrum similarity. Quantification was based on external calibration with four levels of concentration in the range 0.02-0.1 g.L -1 , at 273 nm. In the second method, a Phenomenex Synergi RP-Hydro column was used and the mobile phase was a gradient of acetonitrile and aqueous solution buffered at pH 2.6 with a H3PO4/NaH2PO4 buffer, starting from 0 vol.% acetonitrile (0-15 min) and increasing to 22 vol.% acetonitrile (ramp from 15 to 48 min and step from 48 to 70 min). Aliphatic compounds were identified and quantified by this method using commercial compounds as references. Identification was based on retention time and UV spectrum similarity. Quantification was based on external calibration with four levels of concentration in the range 1-10 mM, at 210 nm. Several types of aliphatic acids can be found in crude KBL: simple carboxylic acids (e.g. formic, acetic), hydroxyacids (e.g. glycolic, lactic), iso-saccharinic acids. In our case, gluco-isosaccharinic acid (GISA) was detected but not quantified due to the lack of commercial standard. Klason phases. KBL, KL and KP samples were analysed by different techniques.

FT-IR analyses were carried out in an absorbance mode using a Nicolet IS5 equipped with a Thermo Scientific ID7-ATR accessory with diamond crystal. The sample was placed directly on the diamond and the IR signal was recorded with a resolution of 4 cm -1 , 32 scans and a spectral range of 4000 cm -1 to 525 cm -1 . The spectra obtained with this method were qualitatively analysed to characterise different structures in the lignin. HSQC NMR spectra were acquired on a Bruker AVANCE III 400MHz equipped with a BBFO probe (Z gradient). Samples were prepared by dissolving an aliquot (10-15 mg) in 600 mg of (CD3)2SO. Data were recorded over 48 scans at 50°C. A standard pulse sequence hsqcedetgpsp.3 was used. Size Exclusion Chromatography (SEC) analyses were performed on a Malvern Panalytical TDA Viscotek system equipped with three columns (Mixed C, 300 mm x 7 mm I.D. from Agilent Technologies). Klason phase samples were dissolved in THF and filtered at 0.45 µm before analysis. All samples were fully soluble in THF. 100 µL of sample at a concentration of 3 mg.mL - 1 was eluted in THF at a flow rate of 1 mL.min -1 at 35°C. Polystyrene standards from 500 g.mol -1 to 2 million g.mol -1 were used for calibration. Online detection was performed using a differential refractive index detector. The OmniSEC 5.12 software from Malvern Panalytical was used for the calculations. TGA analysis was performed on Mettler Toledo TGA/DSC STAR System apparatus, at 5 °C.min -1 , from 25 to 1000 °C, air as carrier gas. The moisture degree was determined by the weight loss at 100-115 °C and the ashes content by weight remaining at 1000 °C. Elementary analysis (CHNS) was performed on a Thermo Scientific FlashSmart apparatus. 2 mg of sample in aluminium crucibles were analysed for each KBL, KL and KP sample. Data were recorded using Eager Smart software.

Calculations

The lignin conversion was calculated considering that KP corresponds to unreacted lignin, according to the formula:

𝐶 (%) = ([𝐷𝑀] × % ) -[𝐾𝑃] [𝐷𝑀] × %
where [DM] is the dry matter concentration before starting the reaction (g.L -1 ), %lignin the percentage of pure lignin in the dry matter of KBL (i.e. 31 wt.%, see Table 1 for details) and [KP] the Klason Phase concentration at the end of the reaction (g.L -1 ). The yields of phenolic compounds were calculated on lignin mass basis, according to the formula:

𝑌 (𝑤𝑡. %) = [𝑖] -[𝑖] [𝐷𝑀] × %
where [i] is the mass concentration of phenolic compound i at the end of the reaction ([i]t) or before the reaction ([i]t=0) (g.L - 1 ). Phenolic compounds were assumed to be produced only from the degradation of lignin.

The yields of aliphatic compounds were calculated on carbon basis, according to the formula:

𝑌 (%𝐶) = [𝑗] -[𝑗] [𝐷𝑀] × %
where [j] is the carbon concentration of the aliphatic compound j at the end of the reaction ([j]t) or before the reaction ([j]t=0) (gC.L -1 ) and %C is the percentage of carbon in the dry matter of KBL (i.e. 31 %, see Table 1 for details). Yield calculations only take into account the formation of products during the reaction. The productivity of the oxidation products was calculated from the mass concentration of the products at the end of the reaction and the reactor features. For the batch reactor, productivity was calculated according to the formula:

𝑃 = [𝑖] × 𝑉 °𝑡 × 𝑉
where [i] is the mass concentration of a compound (kg.L -1 ), Vsol° is the volume of liquid solution in the reactor (L), t is the reaction time (h) and Vreactor is the volume of the reactor (m 3 ). Productivity unit is kg.m -3 .h -1 .

For the continuous reactor, the productivity was calculated following the formula:

𝑃 = [𝑖] × 𝑄 °𝑉
where [i] is the mass concentration of a compound (kg.L -1 ), Qsol° is the liquid flow of the feed solution in the reactor (L.h -1 ), and Vreactor is the volume of reactor (m 3 ). The unit of productivity is kg.m -3 .h -1 . Productivity calculations take into account the formation of products during Kraft cooking (i.e. before the catalytic oxidation reaction) and during catalytic oxidation. The repeatability of the catalytic test and analytical methods was evaluated on one catalytic test and showed good repeatability of the experimental methods (see ESI).

Results and discussion

Characterization of starting materials KBL and KL were characterised using a combination of several techniques (Table 1). KBL was characterised either as a solution or as a freeze-dried solid, depending on the technique used. KBL is an alkaline solution (pH = 13.7), concentrated (26 wt.% dry matter) and dense (density 1.1). KL has a low moisture content (4.5 wt.%). The ashes content varies from 39 % in KBL, corresponding to a large amount of inorganic material, to 2.6 % in KL, which contains only residual amounts of Na, K, S. The carbon content is 31 % of the dry matter of KBL. This corresponds to a highly oxygenated material. The lignin content in KBL was only 31 %, which implies that KBL contains ca. 30 % non-lignin organic compounds. On the other hand, KL is highly pure with a lignin content of 96 %. In KL, the percentage of carbon was 61 %, which corresponds to the carbon percentage in a guaiacyl unit (65 %), taking into account moisture and inorganic content in KL. The sulphur content is 1% in KBL and 2.2 % in KL, showing that sulphur tends to accumulate in lignin, either by physisorption of chemically bound thiol groups, sulphates and sulfones. Neither KBL nor KL contain nitrogen. To determine the carbohydrate content of the starting materials, a post-hydrolysis protocol was performed by adapting protocols from Theander & Westerlund 36 and NREL. [START_REF] Sluiter | Determination of Structural Carbohydrates and Lignin in Biomass[END_REF] Sugar concentrations before and after post-hydrolysis give the original content of free sugars and polysaccharides in starting materials (Table 2). The total carbohydrate content is below 1.5 wt.% for both KBL and KL, indicating that most of the carbohydrates from hemicellulose have been degraded during the Kraft cooking. FTIR analysis was performed on freeze-dried KBL and KL (Fig S1 in ESI). FTIR of KBL displays an atypical spectra, badly resolved (bands at 1580 cm -1 , 1492 cm -1 , 1411 cm -1 , 1313 cm -1 , 1260 cm - 1 , 1128 cm -1 , 1223 cm -1 and 1003 cm -1 ), probably due to interaction of lignin moieties with salts. [START_REF] Sebhat | [END_REF] However, Klason phase isolated from KBL and re-diluted in deionised water displays characteristic bands: 1423 cm -1 , 1509 cm -1 and 1591 cm -1 corresponding to the aromatic rings; 1265 cm -1 , 1194 cm -1 and 1027 cm

Catalytic oxidation of Kraft black liquor

Oxidation of KBL was carried out in batch mode with or without CuO/TiO2 catalyst. The reaction conditions were chosen on the basis of previous studies by our teams on pure lignin transformation. [START_REF] Almada | Biomass Convers[END_REF]40,[START_REF] Vilcocq | Waste & Biomass Valorization[END_REF] The temperature was set at 150°C, the pressure at 20 bar, stirring rate 1800 rpm and the copper/dry matter ratio at 5 wt.%. Effect of reaction time on lignin conversion and phenolics. Figure 1 depicts the evolution of lignin conversion as a function of reaction time. In the absence of copper catalyst, lignin conversion starts at 45 %, i.e. lignin depolymerisation has occurred during the heating ramp, and reaches 98 wt.% after 45 min. In the presence of CuO/TiO2 catalyst, lignin conversion starts at 54 wt.% and reaches 97 wt.% after 45 min. Both conversion profiles are similar and one can conclude that the copper catalyst plays only a minor role in lignin depolymerisation. Besides, the total yield of phenolics starts at 6.6 wt.% in the presence of catalyst and 5.0 wt.% in the absence of catalyst, then increases up to 8.5 wt.% in the presence of catalyst after 60 min or 7.2 wt.% in the absence of catalyst after 15 min. A plateau in phenolics production seems to be reached after 45 min in the presence of catalyst or after 15 min in the absence of catalyst. At longer reaction time (90 min), the phenolics yield decreases to 6.4 wt.% in the presence of catalyst.

The composition of the phenolics was detailed for both noncatalysed and catalysed reaction (Figure 2). The main phenolic compound was vanillin and it was produced with a yield of 2.1 wt.% at time zero without catalyst, then increased up to 3.6 wt.% in 15 min and was stable between 30 and 45 min. In the presence of CuO/TiO2 catalyst, the vanillin yield started at 3.4 wt.%, then increased up to 4.6 wt.% at 30 min before slowly decreasing for reaching 2.7 wt.% at 90 min. This decrease was attributed to the degradation of vanillin.

Minor phenolic compounds, such as vanillic acid, acetovanillone, 4-hydroxybenzoic acid, phenol and guaiacol were observed and quantified. 4-hydroxybenzaldehyde was only detected as traces in most of the samples. Other commonly observed compounds 26 were not detected: homovanillic acid, anisic acid, benzoic acid and catechol. The yield profiles were different for each phenolic compound. Vanillic acid started with a yield of 1.5 wt.% (without catalyst) or 1.6 wt.% (with catalyst) and decreased continuously over the reaction time, reaching the lowest level of HPLC quantification (0.1 g.L -1 ) after 30 min for the non-catalytic reaction and after 45 min for the catalytic reaction. For acetovanillone, the initial yield was 0.5 wt.% without catalyst, increasing to 1.1 wt.% after 30 min and then decreasing, whereas in the presence of catalyst, acetovanillone yield started at 0.5 wt.% and then remained stable for 60 min before decreasing. Less oxygenated compounds such as phenol and guaiacol are present as traces at the beginning of the reaction and are then produced up to 0.8 wt.% each (without catalyst, after 45 min) and up to 2.0 wt. and 1.7 wt.% in the presence of a catalyst in 60 min. The reaction pathways by which phenolics are produced and consumed during lignin oxidation have been already described in literature and particularly in our previous communication. 33 Vanillin, acetovanillone and vanillic acid all come from guaiacyl units in lignin whereas 4-hydroxybenzaldehyde could come from the degradation of other phenolic monomers or from hydroxyphenyl units. Phenol and guaiacol can be formed from the degradation of other phenolics or from a non-oxidative pathway of lignin cleavage. Each phenolic compound has its own degradation rate in oxidative conditions. Particularly, the fast oxidative degradation of vanillic acid when compared to vanillin have already been described under non-catalytic conditions. [START_REF] Casimiro | [END_REF] Therefore, it seems possible to tune the composition of the phenolic mix by balancing reaction conditions, and particularly reaction time.

An additional experiment was performed to determine the active species on the CuO/TiO2 catalyst. The TiO2 support was tested alone with the grain size and concentration corresponding to those used for CuO/TiO2 (Figure 3). The conversion value after 30 min was 89 wt.% after 30 min, equivalent to that obtained in the absence of catalyst (94 wt.%) or with CuO/TiO2 catalyst (92 wt.%). The phenolics yields are also shown on Figure 3. It can be noted that the values obtained in the presence of TiO2 are similar to those obtained without catalyst, i.e. 6.0 wt.% yield with TiO2 vs 7.1 wt.% yield without catalyst and 8.3 wt.% yield with CuO/TiO2. The detail of the phenolics composition is also similar for both the TiO2 catalyst and the run without catalyst (Figure 4). This confirms that copper oxide is the only active species of CuO/TiO2 involved in phenolic production during KBL oxidation. Composition of aliphatic compounds before and after KBL oxidation. Besides phenolics, a large variety of aliphatic compounds can be found in KBL before and after the reaction. These compounds are formed during the Kraft process by the oxidative degradation of polysaccharides in lignocellulose (i.e. cellulose and hemicellulose). The detailed composition of the monomers in KBL is given in ESI (Table S2): oxalic, tartronic, glycolic, formic, 3-hydroxypropionic, lactic, acetic, succinic, 2hydroxybutyric acids were detected, at concentrations consistent with other publications on black liquor. 4 During catalytic oxidation, the formation of aliphatic compounds can come from the degradation of phenolic compounds through ring opening reactions, or from the degradation of residual sugars and polysaccharides. Since the origin of the aliphatic compounds cannot be determined, their production was expressed as a carbon yield based on the global carbon content in KBL (see part 2.5. for details). Figure 5 depicts the production of aliphatic compounds during catalytic and non-catalytic oxidation of KBL. The production of aliphatic compounds starts during the heating ramp and yields simple acids (formic and acetic acids), but also in di-acids such as oxalic, malonic, malic and succinic acids. Hydroxy-acids (glycolic and lactic acids) were not produced at this stage (little degradation even occurred). This production is more important for catalytic oxidation and the distribution of acids is slightly different: more formic acid is produced at the beginning of catalytic oxidation and more tartronic acid in the absence of catalyst. During the course of the reaction, the global yield in aliphatic acids increases drastically during the first 45 min of reaction after which a slower evolution occurs. The main aliphatic products are succinic, tartronic, oxalic, formic and acetic acids. Their production increases continuously during 45 min, but is more important in the presence of a catalyst (48 C% vs 44 C% total yield). The production of acetic acid is more important in the absence of catalyst whereas the production of glycolic acid is slightly higher in the presence of catalyst. After a long reaction time (90 min), tartronic acid is degraded and more succinic acid is produced. Characterizations of Klason phases. At short reaction times (from 0 to 30 min), incomplete conversion was observed and Klason phases from catalytic and non-catalytic reactions were recovered and characterized. The data were compared with those of the initial KBL. Elemental analysis showed a low carbon content: from 55 % at 0 min to 60 % at 30 min without catalyst. The sulphur content varies from 1.1 % at 0 min to 0.8 % at 30 min without catalyst. This corresponds to a partial oxidation and partial desulphurisation of lignin.

SEC analysis (Table 3, Figure 6) shows two peaks corresponding to the presence of two types of compounds in all KP samples: oligomers with aan verage molar mass (Mw) between 900 and 1300 g.mol -1 . Monomers, dimers or trimers with an average molar mass inferior to the lowest calibration level (< 400 g.mol - 1 ). Masses of oligomers decrease with reaction time but are similar for reactions with or without catalyst. The ratio between the oligomer peak and the monomer/dimer/trimer peak also decreases with reaction time, indicating that the KP fractions contain only a minor part of oligomers after 30 min. The values of Mw for monomers are given as an indication only, as calibration with polystyrene is not suitable for lignin analysis: Constant et al compared several SEC methods for technical lignin analysis and showed that the results obtained were highly dependent on the type of column, solvent and sample preparation. 5 Furthermore, SEC is not suitable for the determination of small molar masses. It has already been reported that oligomers of lignin tend to deviate from PSS calibration curves during SEC analysis. 43 Therefore, in this paper, SEC results on molar masses were considered only semiquantitative and used for comparative purpose only. FTIR spectra of the Klason phases issued from non-catalytic runs (see Fig. S2 in ESI) still showed characteristic bands corresponding to lignin structure: aromatics at 1423 cm -1 , 1509 cm -1 and 1591 cm -1 ; guaiacyl moieties at 1265 cm -1 and 1027 cm - 1 ; unconjugated carbonyl groups at 1683-1693 cm -1 ; C-H stretching in CH2 and CH3 alkyl moieties 1452 cm -1 and ca. 1420 cm -1 . Surprisingly, for runs at 0 and 15 min spectra showed an increase of the relative intensities of the bands corresponding to C-O stretching (1027 cm -1 , 1044 cm -1 and 1076 cm -1 ). At the same time, the band at 1510 cm -1 , characteristic of aromatics, decreased slightly in relative intensity, attesting for a release of aromatic moieties from lignin during the reaction. The band at 1693 cm -1 increased, attesting an oxidation of the material. After 30 min, IR spectrum did not show anymore the increase of the bands associated to C-O stretching, and the band at 1510 cm -1 is almost absent. FTIR spectra obtained after a catalytic run of 30 min display almost the same spectra as the run carried out in the absence of catalyst. No significant differences were observed; spectra display all bands characteristic from the lignin structure, Data obtained from HSQC NMR and SEC analyses showed that lignin in KBL is strongly degraded during catalytic oxidation, whatever a catalyst was used or not, with a drastic decrease in oligomer content in accordance with the disappearance of inter-unit moieties. That contrasts in some extent with data obtained from FTIR showing that lignin structure was kept in the Klason phases; however, FTIR and NMR analyses both showed a decrease of the aromatic content that can be related to oxidation of the aromatics in residual lignin, corroborated by elemental analysis and the observation of conjugated carboxylic acids in HSQC NMR experiments. Such a hypothesis is supported by the increase of the band at 1693 cm -1 in FTIR corresponding to carbonyl groups and the relatively intense correlations corresponding mainly to guaiacyl units substituted by carbonyl moieties (G'2: H/C: 7.64/113.6 ppm, G'6: H/C: 7.33/120.3 ppm).

Catalytic oxidation of Kraft Lignin (KL) -comparative study

Lignin conversion and phenolics production. The catalytic oxidation of pure lignin was carried out under operating conditions similar to those used for KBL oxidation: 150°C, 20 bar air, 60 min, 5 g.L -1 dry matter, copper/dry matter ratio 5%. Lignin conversion and phenolics production are depicted on Figure 7. The level of lignin conversion was higher for KBL oxidation at 60 min (99 % vs 88 % for KL). Therefore, the results of KBL oxidation at comparable conversion levels were also included (results obtained at 30 min reaction time giving 92 % lignin conversion). The total yield of phenolics was similar, ca. 8.5 wt.%.

The distribution of phenolics was also similar for both starting materials after the same reaction time (Figure 7). Vanillin was produced with a slightly higher yield from KBL (4.0 wt.%) than from KL (3.7 wt.%). Other phenolic compounds were detected at the same level of production.

Our results show that the oxidation of KBL is as efficient as the oxidation of KL for the production of phenolics. Therefore, the lignin purification step may not be necessary for the valorisation of Kraft lignin. This finding may have strong implications for future industrial applications. Composition of aliphatic compounds. The composition of aliphatic compounds in the liquid phase after catalytic oxidation of KL or KBL was investigated (Figure 9). Large amounts of aliphatic acids were found in the liquid phase after KL oxidation. The distribution of aliphatic compounds was different from that of KBL oxidation products: large amounts of formic acid (18.5 C%) and acetic acid (5.2 C%) were observed from KL, whereas tartronic acid was not detected. The production of oxalic acid is also higher and hydroxy-acids (glycolic, lactic) were also produced in larger amounts. The global yield in aliphatics was 54 C%. The production of aliphatic acids from KL oxidation shows that a large part of aliphatic acids is actually formed from lignin, through ring opening reactions. Although lignin is often seen as a potential source of aromatic compounds, its valorisation into high value aliphatic compounds could also represent an interesting alternative.

Characterizations of Klason phases. Klason phases obtained after 60 min of oxidation of KL and 30 min oxidation of KBL were characterised.

Elemental analysis showed that KP from KL oxidation contains 56 % of carbon and 0.5 % of sulphur, indicating a partial oxidation and a partial desulphurisation of lignin during the reaction, as for KBL.

The SEC chromatogram of KP after KL oxidation shows two peaks corresponding to oligomers (Mw 1000 g.mol -1 ) and monomers/dimers/trimers (Table 4, Figure 10). The ratio between the two peaks is higher after KL oxidation than after KBL oxidation, even after 30 min, indicating that lignin degradation is more advanced during KBL oxidation. ppm), guaiacyl units substituted by carbonyl moieties (G'2: H/C: 7.65/113.1 ppm), and partly to guaiacyl unit (G5: H/C: 6.98/116.1 ppm) and p-hydroxyphenyl units (H2,6: H/C: 7.58/124.9 ppm) are observed.

Reactor study -Batch vs continuous reactor

In an attempt to intensify the process of catalytic oxidation of KBL, the reaction was transferred to a fixed bed reactor, with the same catalyst, temperature, and KBL concentration. The residence time and contact time were very different from one mode to another because of the intrinsic properties of each reactor: in the batch reactor, reaction times are long and the catalyst is diluted in the reactor whereas in the fixed bed reactor, reaction times can be shortened and the catalyst concentration in the reactor is high.

Lignin conversion and phenolics production. Lignin conversion during KBL oxidation was compared in both reactors following residence time (analogous to batch reaction time) and contact time (analogous to batch reaction time multiplied by the ratio of catalyst to feed concentration) (Figure 11). At equivalent residence time, lignin conversion was higher in the continuous reactor. At longer contact time, it was lower in the continuous reactor. One can conclude that contact time is irrelevant here and that lignin depolymerisation was more efficient in continuous reactor because of a better gas-liquid mixing, which would explain the higher performance at equivalent residence time. This hypothesis would imply that the CuO/TiO2 catalyst is not active for lignin depolymerisation, which was already demonstrated (see above). On the other hand, the total phenolics yield was much lower in the continuous reactor at equivalent residence time, and it was lower at longer contact time. The latter observation is in accordance with the previously observed degradation of phenolics at long contact times. Therefore, lignin conversion is mostly a homogeneous reaction that is little influenced by CuO/TiO2 whereas phenolics production and degradation are at least partly a catalytic reaction.

The detailed composition of phenolics yield in both batch and continuous modes is depicted on Figure 12. In the fixed bed reactor, the yield of vanillin corresponds to the lowest yield observed in the batch reactor at a long contact time; therefore, it seems that vanillin reaches a plateau around 2.5 wt.% after a certain contact time. Acetovanillone reaches a very low yield in continuous mode at long contact time. Surprisingly, vanillic acid is still present in continuous mode at long contact times. Phenol and are present with very low yields (inferior to 0.5 wt.%).

The comparison of batch results and continuous results shows that the production and degradation of phenolics is a complex phenomenon that cannot be fully explained by an effect of contact time, i.e. the CuO/TiO2 catalyst effect. The reactivity of phenolics is most probably influenced by homogeneous reactions in an alkaline, oxidative medium and by mass transfer effects, which differ from one reactor to another. Composition of aliphatic compounds. The production of aliphatic compounds was observed during the catalytic oxidation of KBL in both batch and continuous reactors (Figure 13). In the fixed bed reactor, the distribution of aliphatics changed at long contact time with a higher yield in tartronic acid and a lower yield in succinic acid. The yields of formic, acetic, glycolic, oxalic acids also decreased slowly. At long contact times, tartronic acid production decreased and oxalic acid production increased. Again, the formation and degradation of aliphatic acids seem to be complex and dependent not only on contact time (catalyst activity) but also on homogeneous reactions and/or reactor type.

Productivity for various compounds

Our work describes for the first time the formation of a broad range of products from the catalytic oxidation of black liquor: phenolic compounds, aliphatic compounds and lignin residues have been identified and quantified. This allows a first estimation of the productivity depending on the operating conditions (catalyst, time, starting material, reactor type) can be made (see Table S3 in ESI). The aim here is not to provide a precise techno-economic analysis but only to draw the reader's attention on the potential of KBL for the production of these compounds. The productivity of vanillin was 88 g.h -1 .m -3 from isolated Kraft lignin (Figure 14 a) but only 46 g.h -1 .m -3 from KBL under identical operating conditions (see ESI), due to the lower lignin content in KBL. However, reducing the reaction time is efficient to increase vanillin productivity up to 105 g.h -1 .m -3 at 30 min (Figure 14 b). The use of a continuous reactor did not lead to an increase in vanillin yield (see below). Nevertheless, it is very efficient to increase the productivity: 730 g.h -1 .m -3 can be obtained at low contact time (Figure 14 d). These trends can also be observed for phenol productivity, which reaches a maximum of 120 g.h -1 .m -3 in continuous reactor with KBL. The other phenolic compounds give low individual productivity. However, the mix of all phenolic compounds could also be valorised as a mixture. The global phenolics productivity reaches 220 g.h -1 .m -3 from lignin, 190 g.h -1 .m -3 from KBL in batch reactor, 1.27 kg.h -1 .m -3 in continuous reactor. A phenolics mixture can be isolated from other oxidation products, for example by liquid-liquid extraction. 18,44 Separation of one specific phenolic, e.g., vanillin, is more difficult and would require additional purification steps: liquid-liquid extraction, supercritical CO2 extraction, precipitation-crystallisation, membrane separation, solid adsorption have been studied for vanillin recovery after lignin oxidation. 45 A combination of several purification steps is usually required. Centrifugal Partition Chromatography has shown a great potential for separating different phenolic compouds. 46 These technologies could be adapted to recover vanillin from the KLB oxidation product mixture, with additional difficulties caused by the high concentration of sulphur and salts. The yields of aliphatic compounds are much higher than those of phenolics and consequently the productivities are higher. Formic acid, which could be easily recovered by distillation, gives a productivity between 1 kg.h -1 .m -3 in batch reactor (from lignin or from KBL) and reaches 4.2 kg.h -1 .m -3 in continuous reactor. Acetic acid productivity reaches 191 g.h -1 .m -3 from lignin and 240 g.h -1 .m -3 from KBL in batch reactor, and 1.5 kg.h - 1 .m -3 in continuous reactor. The volatility of acetic acid is also high enough to consider a distillation operation. Tartronic acid is not volatile and would require other techniques of purification (e.g., reactive distillation, electrodialysis, chromatography). Its productivity is null from lignin but reaches 11 kg.h -1 .m -3 from KBL in continuous reactor, making it a promising target molecule from KBL. Tartronic acid is a valuable compound for fine chemistry, used as a pharmaceutical, anticorrosion agent, etc. 47 Similarly, succinic acid would be demanding in terms of purification but its productivity reaches 593 g.h -1 .m -3 from lignin, and up to 3.75 kg.h -1 .m -3 from KBL in a continuous reactor. Succinic acid is a valuable compound that can be used for the synthesis of pharmaceuticals, adhesives, solvents, intermediates for polymer synthesis, and food additives. 48 An alternative could be the valorisation of a mixture of hydroxy-acids (AHA mix), which is less complicated to purify. 4 The productivity for this mix is the highest in a continuous reactor, reaching 17.86 kg.h -1 .m -3 from KBL.

The Klason phase could also be valorised as a material. Oxidised lignin can be applied, for example, to heavy metal removal 49 or as a plant fertilizer. 50 Its productivity reaches 293 g.h -1 .m -3 from lignin, or alternatively 757 g.h -1 .m -3 from KBL without catalyst, or 4.61 kg.h -1 .m -3 from KBL in a continuous reactor. It has the advantage of being easily removed from the product mixture by simple precipitation and filtration or centrifugation. Finally, the comparison of the productivities from lignin and from KBL at identical reaction times (data in ESI) evidences that lignin is a more productive starting material, given that KBL only contains ca. 30% of pure lignin (on a dry basis). The costs and energies associated with lignin purification prior to reaction should be taken into account to assess the advantage of using lignin as a starting material for the production of various commodities. The use of a solid CuO/TiO2 catalyst is advantageous for the productivity of phenolics but detrimental for the productivity of Klason phase. The most interesting finding is that the use of a continuous reactor allows the productivity of all products to be drastically increased. This demonstrates the interest of using intensified reactors in future forest biorefineries. The products obtained from KBL oxidation form a complex mixture, which would require several steps of purification. Some research efforts on separation technologies is necessary to achieve the production of valuable biosourced compounds from KBL.

Conclusions

The oxidation of Kraft black liquor was performed with or without a CuO/TiO2 catalyst, in an autoclave or in a fixed bed reactor. The oxidation of pure Kraft lignin was also investigated. The oxidation products were systematically analysed.

Our results showed that lignin depolymerisation was fast and was mainly due to non-catalytic reactivity. The resulting Klason phases contain degraded, oxidised and desulphurised lignin materials. Increasing the reaction time resulted in increased lignin depolymerisation. Using Kraft lignin instead of Kraft black liquor did not enhance lignin depolymerisation. Besides lignin, two families of products were analysed. Phenolic compounds were produced in low yields, with vanillin being the main phenolic compound. Vanillin and its derivatives tend to degrade at long reaction times. Using a CuO/TiO2 catalyst increased the yields of phenolics, with a maximum yield of 8 wt.% phenolics reached after 30 min at 150°C. Aliphatic compounds were produced in higher yields, with a maximum of 50 C% achieved at long reaction times with a copper catalyst. The composition of the aliphatic acids evolves with the reaction time, with tartronic, succinic and formic acids being the main acids analysed. Using Kraft lignin instead of Kraft black liquor did not lead to higher phenolic yields but slightly increased the yields of aliphatics, with different main acids (no tartronic and more oxalic acid). Switching from an autoclave to a fixed bed reactor resulted in different characteristic times: longer contact times and shorter residence times can be reached in continuous mode. This did not lead to an increase in the yields of phenolic and aliphatic compounds but to a different distribution in each family of products. These results illustrate that Kraft liquor oxidation is a complex network of reactions involving catalytic and noncatalytic steps. Finally, the calculation of productivity showed that a large panel of compounds can be produced from Kraft black liquor: small productions of phenolic mix or pure phenolic compounds (phenol, vanillin), together with larger productions of formic acid, acetic acid, mix of hydroxy-and di-acids and/or pure fluxes of succinic and tartronic acids, and oxidised lignin. Additional research effort is needed to purify these compounds, which are of interest in chemical, F&F, pharmaceutical industries. We hope that these results will provide an incentive for the design of new processes for the chemical valorisation of black liquor.

Figure 1 .

 1 Figure 1. Lignin conversion (symbols) and phenolic yields (bars) as a function of time during KBL oxidation for catalysed (a) and non-catalysed (b) reactions. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter mass ratio 5%.

Figure 2 .

 2 Figure 2. Yields of phenolic compounds as a function of time during KBL oxidation for catalysed (a) and non-catalysed (b) reactions. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter ratio 5%. Lines are only guides for the eyes. 4-HBA: 4-hydroxy-benzoic acid.

Figure 3 .

 3 Figure 3. Lignin conversion (symbols) and phenolic yields (bars) during KBL oxidation for non-catalysed and catalysed reactions. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter mass ratio 5%, 30 min.

Figure 4 .

 4 Figure 4. Yields of phenolic compounds during KBL oxidation for non-catalysed and catalysed reactions. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter mass ratio 5%, 30 min.

Figure 5 .

 5 Figure 5. Production of aliphatic compounds during catalytic (a) or non-catalytic (b) oxidation of KBL. Yields are based on carbon content in KBL. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter ratio 5%.

Figure 6 .

 6 Figure 6. SEC analysis of Klason phases during catalytic (a) or non-catalytic (b) oxidation of KBL. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter ratio 5%.

Figure 7 .Figure 8 .

 78 Figure 7. Lignin conversion (symbols) and phenolic yields (bars) during KBL and PL catalytic oxidation. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, 30-60 min, dry matter concentration 5 g.L -1 , copper/dry matter ratio 5 %.

Figure 9 .

 9 Figure 9. Aliphatic compounds production after PL and KBL oxidation. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, 30-60 min, dry matter concentration 5 g.L -1 , copper/dry matter mass ratio 5 %.

Figure 10 .

 10 Figure 10. SEC analysis of Klason phases during catalytic (a) or non-catalytic (b) oxidation of KBL. Operating conditions: 150°C, 20 bar air, stirring 1800 rpm, dry matter concentration 5 g.L -1 , copper/dry matter ratio 5%.

Figure 11 .Figure 12 .

 1112 Figure 11. Lignin conversion and phenolics production during KBL catalytic oxidation, represented as a function of residence time (a) or contact time (b). Operating conditions: 150°C, 20-80 bar air, dry matter concentration 5 g.L -1 , catalyst CuO/TiO2

Figure 13 .

 13 Figure 13. Aliphatic compounds production during KBL catalytic oxidation. Operating conditions: 150°C, 20-80 bar air, dry matter concentration 5 g.L -1 , catalyst CuO/TiO2.

Figure 14 .

 14 Figure 14. Sankey diagrams representing the productivity in kg.m -3 .h -1 of commodities from KL and KBL under different operating conditions: starting from KL, reaction time 60 min, catalyst CuO/TiO2 (a); starting from KBL, reaction time 30 min, catalyst CuO/TiO2 (b); starting from KBL, reaction time 15 min, no catalyst (c); starting from KBL, catalyst CuO/TiO2, continuous reactor (d). The grey part represents the non-identified materials and/or gaseous compounds, not analysed in this work.

Table 1 .

 1 Characterization of KBL and KL.

	KBL	KL

*analysis performed on freeze-dried KBL; [a] Lower Heating Value; [b]Acid Insoluble Lignin; [c] Acid Soluble Lignin

Table 2 .

 2 Carbohydrates content in raw KBL and KL, measured by post-hydrolysis.

	Content (wt.%, o.d. sample)	KBL	KL
	Glucose	0.18	0.04
	Xylose/Galactose/Mannose	0.04	0
	Arabinose	0.08	0.11
	Gluco-OS	0.1	0
	Xylo/Galacto/Manno-OS	0.55	0.31
	Arabino-OS	0.35	0.06
	Total carbohydrates	1.3 wt.%	0.5 wt.%

OS: oligosaccharides. Xylose, Galactose and Mannose were co-eluted.

Table 3 .

 3 SEC results on Klason phases after oxidation reactions.

		Reaction	Molar mass Mw	Ratio Peak
		time	(g.mol -1 ) a	1/Peak 2
			Peak 1	Peak 2
	With CuO/TiO2 catalyst	0 15 30	1298 886 1058	277 297 244	4.5 1.6 0.3
		0	1209	242	0.2
	Without catalyst	15	927	241	0.3
		30	1196	242	0.4

a Mass average molar masses (Mw) values were calculated with polystyrene calibration

  however with a relative lower intensity of the band at 1510 cm - 1 . HSQC NMR were recorded for the Klason phases (see Fig S8-S10, ESI) and compared, each other, to that of the Klason phase obtained at a reaction time of 0 min. For reaction carried out in the absence of the catalyst, HSQC spectra of the Klason phase obtained at initial reaction time display all characteristic correlations of lignin structure: In the oxygenated aliphatic

	regions, correlations are observed for methoxy groups (H/C:
	3.80/56.3 ppm); β-O-4 linkages (A) (H/C: 4.79/71.8 ppm),
	(AG) (H/C: 4.78/82.0 ppm), (A G/H) (H/C: 4.31/84.8 ppm)
	and (A) (H/C: 3.53/60.4 ppm), (A') (H/C: 4.12/67.4 ppm) and
	associated (A'') (H/C: 3.63/63.4 ppm); β-β linkages (B) (H/C:
	4.65/85.5 ppm), (B) (H/C: 3.08/54.7 ppm) and (B) (H/C: 3.72
	and 4.11/71.3 ppm); β-5 linkages gave correlations at (C)
	(H/C: 5.52/87.6 ppm) and (C) (H/C: 3.46/53.8 ppm).
	Aromatic region exhibits the characteristic correlations
	corresponding to guaiacyl units (G) (G2: H/C: 6.93/111.7 ppm,
	G5: H/C: 6.80/116.2 ppm, G6: H/C: 6.78/121.8 ppm); and
	those of p-hydroxyphenyl units (H2,6: H/C: 7.38/126.3 ppm).
	Correlations corresponding to guaiacyl units substituted by
	carbonyl moieties are also observed (G'2: H/C: 7.64/113.6
	ppm, G'6: H/C: 7.33/120.3 ppm).
	No correlations were observed for the inter-unit bonds, but
	some correlations corresponding to conjugated carboxylic acids
	were observed (H/C: 7.16-6.98/124.2 ppm and 6.85/127.4
	ppm).
	For the Klason phases issued from reaction conducted in the
	presence of the catalyst (see Fig S11, ESI), HSQC NMR spectra
	recorded after 30 min reaction showed almost the same
	situation, displaying only the correlation corresponding the
	methoxy groups (H/C: 3.87/56.8 ppm), and mostly to guaiacyl
	units substituted by carbonyl moieties (G'2: H/C: 7.55/113.1
	ppm. Little correlations corresponding to guaiacyl unit were
	observed (G5: H/C: 6.87/115.9 ppm). Like before, correlations
	attributed to conjugated carboxylic acids (H/C: 7.16-
	6.98/124.2 ppm and 6.85/127.4 ppm) were observed.

| 9 HSQC NMR spectra obtained after 15 min and 30 min reaction times without catalyst show highly degraded structures. Only correlations related to methoxy moieties (H/C: 3.85/56.8 ppm) and some of those corresponding to guaiacyl units (G2: H/C: 7.03/113.4 ppm, G5: H/C: 6.91/116.3 ppm, G'2: H/C: 7.49/112.2 ppm, G'6: H/C: 7.33/120.3 ppm) and phydroxyphenyl units (H2,6: H/C: 7.44/125.5 ppm) are observed.

Table 4 .

 4 SEC results on Klason phases after oxidation reactions on KBl and KL.

		Molar mass Mw (g.mol -1 ) a Ratio Peak 1/Peak 2
		Peak 1	Peak 2
	KBL, 30 min	1058	244	0.3
	KBL, 60 min	649	156	<0.1
	KL, 60 min	1074	252	0.7

a Mass average molar masses (Mw) values were calculated with polystyrene calibration

Acknowledgements

Laura Reyes is acknowledged for her help on KBL characterization. IRCELYON scientific and technical services are kindly acknowledged. Chantal Lorentz (IRCELYON) is warmly thanked for her help on NMR experiments. The industrial company Smurfit Kappa, and particularly Jérôme Moindrot, is thanked for providing black liquor. Evonik is thanked for providing TiO2 catalyst support. This work was funded by ANR (project Phenoliq ANR-16-CE06-0007), CNRS and UCBL. The authors thank AXELERA chemical and environmental cluster.

Notes and references

Conflicts of interest/data availability

There are no conflicts to declare. The datasets generated during and/or analysed during the current study are not publicly available but are available from the corresponding author on reasonable request.