Sara Gharib-Abou

Aymen Amine Ghaida

Guillerme Assadi

Abdelkrim Costa

Dominique Bouzaza

Wolbert

Gharib-Abou Ghaida

Aymen Amine

Costa Guillerme

Bouzaza Abdelkrim

Wolbert Dominique

Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: butyraldehyde oxidation, by-products identification and ozone valorization

Keywords: Pilot scale, Sequential coupling processes, DBD plasma, photocatalysis, TiO 2 /MnO 2 Catalysis

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Volatile organic compounds (VOCs) constitute one of the most important family of chemicals involved in atmospheric pollution, causing damage to environment and human health [START_REF] Karuppiah | Abatement of mixture of volatile organic compounds ( VOCs ) in a catalytic non-thermal plasma reactor[END_REF][START_REF] Quoc An | Application of atmospheric non thermal plasma-catalysis hybrid system for air pollution control: Toluene removal[END_REF][START_REF] Subrahmanyam | Catalytic non-thermal plasma reactor for total oxidation of volatile organic compounds[END_REF], and need, consequently, to be eliminated. Nevertheless, gas effluents containing low VOCs concentrations are not efficiently treated by conventional industrial processes, for which high power is usually required (i.e. thermal oxidation or catalytic oxidation) [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF]. Thus, alternative solutions were investigated over the years, among which nonthermal plasma (NTP) is proved to be an effective technology for the treatment of such effluents with low concentrations particularly [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][START_REF] Guo | Toluene decomposition using a wireplate dielectric barrier discharge reactor with manganese oxide catalyst in situ[END_REF]. Dielectric barrier discharge (DBD) is commonly used to create NTP, by applying electrical energy between the two electrodes of the reactor, where at least one of which is covered with dielectric material [START_REF] Tang | Comparative Study between Single-and Double-Dielectric Barrier Discharge Reactor for Nitric Oxide Removal[END_REF], in order to create high energy electrons (1-10 eV) [START_REF] Guo | Toluene decomposition using a wireplate dielectric barrier discharge reactor with manganese oxide catalyst in situ[END_REF], under moderate conditions (i.e. room temperature and atmospheric pressure) [START_REF] Karuppiah | Abatement of mixture of volatile organic compounds ( VOCs ) in a catalytic non-thermal plasma reactor[END_REF][START_REF] Kim | nonthermal plasma and catalyst : A review[END_REF].

These electrons are more likely to collide with gas molecules other than the pollutants since their concentrations are low. This results in the production of reactive species such as ions, free radicals and excited species able to react with the pollutants and oxidize them into less harmful compounds [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF][START_REF] Delagrange | Combination of a non-thermal plasma and a catalyst for toluene removal from air : Manganese based oxide catalysts[END_REF][START_REF] Pollution | DC Corona Electric Discharges for Air Pollution Control. Part 1[END_REF]. However, debate is still widely open concerning the merits and demerits of this technology. On one hand, nonthermal plasma permits the oxidation of VOCs at relatively low energy cost [START_REF] Quoc An | Application of atmospheric non thermal plasma-catalysis hybrid system for air pollution control: Toluene removal[END_REF][START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF], but, on the other hand, it has many disadvantages such as low energy efficiencies, poor selectivity to CO 2 even when high conversion rate is reached, and undesirable byproducts formation (e.g. NO x , ozone, etc.) [START_REF] Guo | Toluene decomposition using a wireplate dielectric barrier discharge reactor with manganese oxide catalyst in situ[END_REF][START_REF] Pollution | DC Corona Electric Discharges for Air Pollution Control. Part 1[END_REF][START_REF] Roland | Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 2. Ozone decomposition and deactivation of ??-Al2O3[END_REF]. Among promising techniques to overcome these limitations, combining plasma NTP with suitable heterogeneous catalysts has been proved to improve the efficiency of VOCs abatement [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF], as a synergy effect between NTP and catalytic action is greatly expected [START_REF] Karuppiah | Abatement of mixture of volatile organic compounds ( VOCs ) in a catalytic non-thermal plasma reactor[END_REF]. Two configurations of plasma-catalysis are widely used so far: single-stage (also called plasma-driven catalysis PDC or in-plasma catalysis IPC), where catalysts are placed directly in the discharge zone, and two-stage (plasma-enhanced catalysis PEC or post-plasma catalysis PPC), where catalysts are placed downstream the reactor [START_REF] Jarrige | Plasma-enhanced catalysis of propane and isopropyl alcohol at ambient temperature on a MnO2-based catalyst[END_REF]11].

The aim of the present work is to study the decomposition of butyraldehyde at pilot scale using surface dielectric barrier discharge (SDBD) and different (photo) catalysts made of titanium dioxide (TiO 2 ), manganese dioxide (MnO 2 ), and a mixture of both, in order to determine their catalytic efficiency when coupled with plasma NTP. Moreover, the novelty of this study is the investigation of different configurations in order to optimize and valorize the residual ozone. Finally, butyraldehyde reaction by-products are identified and a reactional pathway is suggested.

Material and methods

Pilot-scale reactor

The reactor consists of a glass chamber (length L = 1000mm, width l = 135mm and height H = 135 mm), inside of which two glass plates acting as the dielectric media hold, on the inner side, the catalytic media and high voltage grids (stainless steel, rectangular shaped and 2 mm thickness) and on the outer side, a copper plate forming the outer electrode (1 mm thick and 800mm in length). The electrodes are connected to a generator and an amplifier. A coil of capacitors with a total capacity of 2.5 nF is placed between the copper electrode and the connection to ground in order to collect the charges created in the reactor. These two plates are placed one opposite the other at an adjustable distance (Figure 1).

To generate the plasma, high voltage is applied to the reactor. The applied voltage is generated by a generator (BFi OPTILAS) as a sinusoidal waveform up to 10V and then The reactor can be used also as a photocatalytic reactor and as a plasma DBD-photocatalytic reactor. Eight lamps (Philips PL-S 9W / 10 / 4P) continuously emitting between 300 and 460 nm with a maximum at 365 nm are placed equidistant from each other in the inter-plate space.

The photocatalytic medium is interposed between the stainless steel grid electrode and the dielectric barrier in the plasma active area. It should be noted that the plasma does not activate the photocatalyst in the reactor. The UV lamps arranged in the reactor, permit the activation of the photocatalyst. Two configurations (continuous and sequential combined system) were tested (Figure 1).

Catalysts

The supported material has been provided by Ahlstrom Research and Services [START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF]. It is further named Glass Fiber Tissue (GFT) containing colloidal silica, a variable percentage of titanium dioxide and manganese dioxide nanoparticles and inorganic fibers. In fact, titanium dioxide or manganese dioxides have been deposited on inorganic fibers by impregnation using an industrial-size press (Figure 2). A dry mixture of 50 wt% colloidal silica and different wt% of titanium dioxide nanoparticles (PC500 Millennium) and manganese dioxide is suspended in pure water. In order to ensure the deposition of 13 g/m 2 of dry TiO 2 and/or MnO 2 on fiber support, the suspension is composed of 40% of dry powder and 60% of pure water. PC500 TiO2 nanoparticles are 5-10 nm in diameter and are of pure anatase form. The specific area of TiO 2 nanoparticles is 300 m 2 /g. The specific area of MnO 2 nanoparticles is 377 m 2 /g. The coating process consists in impregnating fibers using industrial size press. The press is employed to impregnate fibers with the suspension; then, they are dried (Figure 2). Material preparation has been performed by Ahlstrom Research and Services.

Figure 2. Diagram of the impregnation technique for GFT synthesis "size press"

Three catalysts media were produced: GFT with 100 % TiO 2 , GFT with 100% MnO 2 and GFT with 25% MnO 2 + 75% TiO 2 .

Polluted air flow generation

Dry air flow is obtained by an air compressing system. Butyraldehyde, in liquid state, is directly injected in the flow through a syringe/pump system and the pollutant feed is done continuously through a septum. A heating tape is wrapped around the pipe at the injection zone to ensure good evaporation of the pollutant.

Homogenization of the air/pollutant mixture is ensured by a static mixer placed between the syringe/pump system and the reactor. The experiment is carried out at room temperature and atmospheric pressure. The temperature and relative humidity are measured by a TESTO sensor.

The main air flow can be generated by the internal network of compressed air when dry air is needed (5% relative humidity) or by using ambient air when working at a higher flow rate.

The compressed air network enables moisture to be controlled by varying the flow in a packed air-water countercurrent column. Thus, it is possible to obtain a range of relative humidity (RH) from 5 to 90%. The entering air flow is measured in real time by a mass flow meter (Bronkhorst In-Flow) calibrated normal cubic meter per hour on the range 0-20 Nm 3 .h -1 .

Pollutant and by-products analysis

The pollutant analysis can only start once the air flow in the reactor reaches equilibrium. In other words, a certain time is required after the pollutant is injected in the air flow crossing the reactor, so that its concentration stabilizes. Once the outlet concentration is stabilized and the catalyst is loaded, plasma DBD is generated and oxidation begins. At this point, samples for pollutant analysis can be taken. Two openings with septum permit taking gas samples at the entrance and exit of the reactor. Analysis of butyraldehyde is performed using a gas chromatography coupled to a Fisons flame ionization detector (GC-FID). A column Chrompact FFAP-CB (25m in length and 0.32mm outer diameter) corresponding to the volatile fatty acids is used. Nitrogen is the carrier gas and constitutes the mobile phase. All injections are done manually with a syringe of 1 ml and were repeated at least three times.

The byproducts generated during the DBD plasma oxidation of butyraldehyde are identified and evaluated by Gas Chromatograph-Mass spectrometer (GC-MS) (Perkin Elmer Clarus 500) equipped with an infrared (IR) detector. The temperature conditions in the oven, the injection chamber and the detector are, respectively, 100, 120 and 200°C. Due to their low concentrations, byproducts are concentrated in a Carbotrap (25ml) then removed by thermal desorption unit coupled with GC-MS [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF].

CO 2 , CO and O 3 analysis

CO 2 outlet concentrations are measured by a Fourier Transform Infrared (FTIR) spectrophotometer brand Environment SA (Cosma Beryl® reference 100, Cosma® Igny, France). The measurement accuracy is about 5% [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF]. CO outlet concentrations are measured by a NO/CO_ZRE gas analyzer. CO 2 and CO selectivity's are calculated according to the following equations: A standard iodometric titration method is used to estimate the concentration of the ozone formed during oxidation reactions by DBD plasma. Thus, at the reactor exit, a constant air flow of 285 L.h -1 is bubbled in a potassium iodide (KI) solution at 10 -2 M. I -is oxidized into I 2 and thus gives a yellow solution. After, a standard iodometric titration method is used to estimate the downstream ozone formation [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF].

Results and discussion

To follow the degradation of butyraldehyde, two parameters were selected: Conversion rate (CR), which was calculated as follow:

(3) Elimination Capacity (EC) which was estimated according to the equation:

(4)
Where C in and C out are the inlet and the outlet BUTY concentrations (mg.m -3 ), respectively. m TiO2 is the amount of catalyst deposited on glass fiber tissue.

Depending on the need, one or the other will be used for expressing the results obtained during experiments.

Butyraldehyde oxidation: GFT with 100 % of TiO 2

We started our investigations by studying BUTY elimination by photocatalysis (UV/TiO 2 ) and plasma DBD separately, then by coupling both technologies.

Figure (3) shows variation of the elimination capacity (EC) of butyraldehyde by

photocatalysis versus air flow rate and BUTY inlet concentration. On figure (4) the influence of relative humidity (RH) is represented. It can be noted that butyraldehyde EC increases when air flow rate and inlet pollutant concentration are increased. Knowing that photocatalysis reaction is often assimilated to a pseudo-first order reaction, this means that a greater flow of pollutant (due to the increased air flow rate or inlet concentration) naturally induces an increase in the degradation kinetics [START_REF] Assadi | Removal of trimethylamine and isovaleric acid from gas streams in a continuous flow surface discharge plasma reactor[END_REF][START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: Influence of the mass transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF]. Of course, for a constant amount of catalyst, the percentage of degradation and mineralization will decrease when these operating parameters increase. The EC also varies with relative humidity and is optimal when approaching 25-30% for all pollutant inlet concentrations tested. However, it is more visible for high inlet concentrations (120 and 160 mg/Nm 3 ). When RH exceeds a certain threshold, butyraldehyde EC decreases remarkably.

In fact, RH has two opposite effects in photocatalysis alone. Initially, the presence of water molecules in the air improves the EC of BUTY due to the dissociation of H 2 O, forming new reactive species (H • and HO • ). Secondly, when RH rises too much, a competition effect between the pollutant and water molecules for the active sites on the photocatalyst surface predominates, diminishing consequently EC [START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF][START_REF] Assadi | Modeling and simulation of VOCs removal by nonthermal plasma discharge with photocatalysis in a continuous reactor: Synergetic effect and mass transfer[END_REF]. BUTY abatement is significantly better when coupling processes than with Photocatalysis or DBD alone, whatever the injected energy, air flow rate or inlet pollutant concentration are.

With DBD alone, we note that BUTY conversion rate increases with the increase of injected energy into the reactor (figure 5). This is due to an increase in the amount of radicals and atomic oxygen produced. Indeed, the increase of the applied voltage in the reactor increases the degree of ionization, and consequently the amount of reactive species produced. The probability that the pollutant is attacked by radicals or electrons is higher [START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF]. Clearly, BUTY removal is from 3 to 10% higher when coupling processes than using photocatalysis or plasma separately. From the graphic above (Figure 5), the CR increases and then drops. In fact, when RH increases, the competitive effect towards the active sites of water and the consumption of actives species of plasma becomes predominant and thus the CR decreases. We note an optimum at around 30% of RH. In our case the values of CR are 29.97%, 34.34% and 70.76% with photocatalysis, DBD and coupling respectively. In addition, it must be pointed that coupling photocatalysis with DBD helps maintaining a relatively high CR of butyraldehyde. For example, for RH = 80%, coupling insures a CR equal to ~63%, while CR with photocatalysis and DBD does not exceed 23.05 and 27.47% respectively. This synergistic effect is observed under different experimental conditions. These observations are similar to those obtained by several groups of researchers [START_REF] Assadi | Photocatalytic oxidation of trimethylamine and isovaleraldehyde in an annular reactor: Influence of the mass transfer and the relative humidity[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Guo | Effect of manganese oxide catalyst on the dielectric barrier discharge decomposition of toluene[END_REF][START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF].

Intermediates identification

We wanted to follow closely the degradation of butyraldehyde, so we decided to identify the degradation by-products by gas chromatography coupled to mass spectrometry (GC-MS).

Three samples were taken for butyraldehyde after each treatment process. Analyses of outlet unpolluted air flow composition after its treatment with each of the three processes have also been made in order to suppress any doubt while identifying butyraldehyde intermediates. The results are shown in figure 6.

Figure 6. By-products identified by GC-MS following butyraldehyde oxidation by the combined system

Compounds present in the outlet air were detected by GC-MS. Regarding butyraldehyde as pure substance and its oxidation by combined system; the detected degradation by-products were acetone (1), ethyl acetate (2), acetic acid (3), propionic acid (4) and butyric acid [START_REF] Guo | Toluene decomposition using a wireplate dielectric barrier discharge reactor with manganese oxide catalyst in situ[END_REF].

Butyraldehyde is also detected at the exit of the reactor at 32.6 ppm. The same intermediates as in combined system were detected when plasma was used alone.

These results are comparable to those obtained by [START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF] [START_REF] Ye | Photocatalytic oxidation of aldehydes: Byproduct identification and reaction pathway[END_REF], where they were able to detect, after treatment by photocatalysis, the presence of acetic, propionic, and butyric acids, but in liquid phase. They detected other compounds in gas phase: propionaldehyde and acetaldehyde.

Since different oxidation mechanisms occur when plasma treatment is on, we suggest the following reactional mechanism for butyraldehyde degradation (Figure 7), where ethyl acetate and butyric acid derive directly from butyraldehyde and could dissociate into acetic acid, acetone and propionic acid (all detected with GC-MS), respectively, according to the represented reaction pathway.

CO and CO 2 selectivity, and ozone formation

Figure 8 shows that photocatalysis is the most selective to CO 2 among the three processes tested. We also note that introducing the photocatalyst (TiO 2 ) in the plasma can improve the selectivity to CO 2 . The production of CO is generally low and its concentration rarely exceed 10%. As for ozone, huge amounts are produced with plasma alone and with coupled processes.

However, we also notice that for the same injected energy (12.8 J.L -1 ), coupling ensures a higher BUTY conversion rate while decreasing generated O 3 quantity. This behavior could be explained by the fact that O 3 is decomposed in the presence of external UV light to the planar reactor [START_REF] Assadi | Modeling and simulation of VOCs removal by nonthermal plasma discharge with photocatalysis in a continuous reactor: Synergetic effect and mass transfer[END_REF].

Effect of adding MnO 2 catalysis: continuous and sequential configurations

Ozone is known to be an inevitable by-product of plasma. Interactions between highly energetic electrons and molecular oxygen produce atomic oxygen. This latter interacts then with molecular oxygen in the bulk gas to form ozone as follows [START_REF] Atkinson | Evaluated kinetic and photochemical data for atmospheric chemistry: Volume IV – gas phase reactions of organic halogen species[END_REF]:

e -+ O 2 → e -+ • O + • O (a) • O + O 2 + M → O 3 + M (b)
where M is a third body for discharging excess energy of the reaction, which can be N 2 , O 2 or a surface.

Although coupling DBD/photocatalysis processes reduces ozone generation like we discussed previously, the concentrations emitted remain high (about some dozens of ppm) and dangerously threatening human health. In fact, this oxidizing gas, even at relatively low concentrations, is capable of damaging bronchiolar and alveolar cells and interacting with some receptors and some protein molecules, lipids or certain enzymes [START_REF] Pascal | Effets à court terme de la pollution atmosphérique sur la mortalité[END_REF]. Short-term inhalation of ozone at concentrations that occur in urban environment causes acute conduit artery vasoconstriction [START_REF] Brook | Inhalation of fine particulate air pollution and ozone causes acute arterial vasoconstriction in healthy adults[END_REF]. Due to its dangerous effects, it would be more benefic to minimize its generation to avoid forming intermediate products that may be more toxic than the original pollutant.

A second aim of the present study is, consequently, to minimize as much as possible and valorize ozone production by implying it in the decomposition of pollutants. Thus, a series of experiments was carried out under plasma, by testing different catalysts compositions and their positions in the reactor.

Continuous configurations (plasma and catalysis in-situ)

In our tentative for reducing ozone emissions by the DBD process, a series of BUTY oxidation experiments was carried out by testing different catalyst media. Three media was tested, TiO 2 as a photocatalyst for VOCs removal [12-15, 17, 24-26], MnO 2 for ozone degradation [START_REF] Zhu | Volatile organic compounds decomposition using nonthermal plasma coupled with a combination of catalysts[END_REF] and the combination of TiO 2 /MnO 2 to enhance catalytic activity [START_REF] Van Durme | Efficient toluene abatement in indoor air by a plasma catalytic hybrid system[END_REF].

Figure 9 shows the influence of each catalyst mentioned above in butyraldehyde removal and ozone formation. We note that MnO 2 alone is very reactive when activated by DBD alone. Compared to TiO 2 alone or DBD alone, it improves butyraldehyde abatement by a factor of 1.9, but surprisingly, it increases also ozone production by a factor of 2.1. Probably this is due to the fact that the presence of MnO 2 in the discharge zone may affect the discharge characteristics: an increase in the production of active species [START_REF] Van Durme | Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: A review[END_REF] could have helped reducing the amount of consumed ozone for butyraldehyde oxidation. Indeed, Ozone is created from atomic oxygen in a reaction to three bodies according to reaction (b). When introducing the fibers into the plasma zone, it expands the solid surface in contact with the plasma. This increases the probability that oxygen enhances with third body for producing of ozone. Presumably oxygen atoms are adsorbed on the surface and react with O 2 . Accordingly, the ozone concentration increases [START_REF] Jin | Manganese Oxide-Surface Modified Titanium(IV) Dioxide as Environmental Catalyst[END_REF][START_REF] Thevenet | Plasmacatalyst coupling for volatile organic compound removal and indoor air treatment: a review[END_REF][START_REF] Allegraud | Décharge à Barrière Diélectrique de Surface : physique et procédé[END_REF].

When hybridized with TiO 2 , it reduces the generation of ozone by a factor of 7.2, but on the other hand, it decreases by a factor of 1.7 butyraldehyde removal compared to MnO 2 alone. In a first conclusion, we can note that hybrid catalyst seems to have the highest ozone removal rate. However, since optimization of the reactor performance is our main objective, it would be idealistic if we can achieve a highest removal rate with the lowest ozone generation. The combined catalyst (75% TiO 2 + 25% MnO 2 ) permits to have the lowest ozone outlet concentration when placed in the discharge zone, but it decreases pollutant removal rate by almost the half. Thus, further investigation was conducted in order to study the reasons behind this decrease in BUTY conversion rate. Experiments with UV/TiO 2 , UV/MnO 2 and UV/TiO 2 /MnO 2 were performed (Figure 10). CR of butyraldehyde was compared to that obtained with DBD/TiO 2 -and/or-MnO 2 (Figure 9). Based on the obtained results, we note that the presence of MnO 2 seems to completely inhibit the photocatalytic activity of TiO 2 . A clear explanation for this finding has not been established yet. However, some researchers working on the photocatalytic activity of TiO 2 combined with MnO 2 in liquid phase have found similar results [START_REF] Van Durme | Efficient toluene abatement in indoor air by a plasma catalytic hybrid system[END_REF][START_REF] Li | Influence of MnO2 on the photocatalytic activity of P-25 TiO2 in the degradation of methyl orange[END_REF]. They admit that the photocatalytic activity of TiO 2 under UV irradiation decreases when its surface is modified by MnO 2 particles. These particles affect O 2 reduction. This leads to a decrease of UV radiations and therefore their absorption by TiO 2 [START_REF] Jin | Manganese Oxide-Surface Modified Titanium(IV) Dioxide as Environmental Catalyst[END_REF]. Heterojunctions formation between MnO 2 and TiO 2 particles seems responsible for altering the chemical status of Ti 4+ and O 2-sites in the crystalline phase of TiO 2 [START_REF] Li | Influence of MnO2 on the photocatalytic activity of P-25 TiO2 in the degradation of methyl orange[END_REF]. This poisoning effect of TiO 2 by MnO 2 might be present in the gas phase and could explain our results.

Sequential combinations

Since MnO 2 placed in the discharge zone was not efficient for reducing ozone generation by DBD process, a second series of experiments was carried out, where MnO 2 is placed downstream of the discharge zone. Reactor was in PPC configuration. Samples were taken from the middle zone (Part I) and the exit of the reactor (Part II), i.e. before and after contact with catalyst surface. Results obtained showed a huge reduction in ozone emissions along with improvement in butyraldehyde CR (Figure 11). This observation suggests that ozone produced in the discharge zone is dissociated on MnO 2 surface, generating more reactive species contributing to BUTY oxidation [START_REF] Thevenet | Plasmacatalyst coupling for volatile organic compound removal and indoor air treatment: a review[END_REF].

Relative humidity, when increased, enhances ozone reduction significantly, but not BUTY oxidation, CR decreases slightly (Figure 11). However, this increase in ozone reduction is not clearly seen when inlet air flow rate is increased from 2 to 4 Nm 3 .h -1 (results not shown). This could be due to the short residence time of butyraldehyde in the reactor. Thus, working with a packed-bed of MnO 2 beads, instead of GFT, which could provide an increased contact of BUTY with the catalyst surface, might be a better alternative.

Conclusions

As a conclusion, we can note firstly that plasma/photocatalysis combination with UV light provides better performance concerning Buty degradation. Secondly, UV/DBD/TiO 2 configuration results in a slight ozone reduction, an increase in CO 2 selectivity and a synergistic effect was observed.

Influence of MnO 2 and MnO 2 /TiO 2 media was also studied. When using DBD/MnO 2 combination with no external UV light, a better butyraldehyde oxidation in IPC configuration was obtained but huge amounts of ozone was produced. Under PPC configuration, lower ozone amounts but also lower butyraldehyde decomposition were obtained.

It seems that PPC configuration using MnO 2 catalyst without external UV light could be the best compromise between good butyraldehyde decomposition and low ozone production.

Further optimization of the process is still required in order to obtain the same promising results for higher air flow rates.

Acknowledgments:

This work is supported by the French National Research Agency (ANR). Alexander

Vorontsov (Boreskov Institute of Catalysis, Russia) is gratefully acknowledged for highlighting discussions.

  amplified by an amplifier (TREK 30/40) to achieve 30 kV. The DBD plasma is obtained by subjecting the electrodes to a sinusoidal high voltage ranging from 0 to 30 kV at a frequency of 50 to 200Hz. The applied voltage (U app ) and the voltage across the capacitors (U m ) are measured by two probes Optilas connected to a digital oscilloscope (Lecroy wave Surfer 24 Xs 200 MHz).

Figure 1 .

 1 Figure 1. Sectional drawing and Schema of NTP coupled with catalysis in planar reactor.

  ] 0 and [ COV] s are the inlet and outlet concentrations of BUTY respectively (ppmv), n (C|COV) is the number of the stoichiometric coefficient of the removal reaction. In our case n is equal to 4.

Figure 3 .

 3 Figure 3. Variation of EC with inlet concentration at different flow rates (SE = 17 J.L -1 , T = 20°C, RH = 5%, I = 20 W.m -2 )

Figure 4 .Figure 5

 45 Figure 4. Variation of EC with RH at different inlet concentrations (SE = 17 J.L -1 , Q air = 2 m 3 .h -1 , I = 20W.m -2 )

Figure 5

 5 Figure 5 also compares BUTY behavior when oxidized by photocatalysis and plasma DBD separately, to its oxidation behavior due to coupling photocatalysis/plasma DBD versus RH variation for a given injected energy.

Figure 5 .

 5 Figure 5. Variation of conversion rate with relative humidity vs. different processes using GFT with 100% TiO 2 (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 17 J.L -1 , RH = 5-7%, T = 18-20°C)

Figure 7 .

 7 Figure 7. Suggested reaction scheme of BUTY oxidation by combination of GFT with 100% TiO 2 and DBD

Figure 8 .

 8 Figure 8. CO 2 and CO selectivity's, and removal rate of oxidized butyraldehyde by photocatalysis, plasma DBD and combined system (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 12.8 J.L -1 , RH = 5-7%, T = 18-20°C).

Figure 9 .

 9 Figure 9. Effect of catalyst composition on BUTY conversion rate and the amount of formed ozone by plasma-catalytic monobloc system (IPC) (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , E inj = 12.96 J.L -1 , RH = 5-7 %, T = 18-20°C)

Figure 10 .

 10 Figure 10. Comparison of the photocatalytic performance of different media for BUTY oxidation (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , RH = 5-7 %, T = 18-20°C)

Figure 5 .

 5 Figure 5. Effect of RH on BUTY conversion rate and ozone formation in PPC for the same BUTY inlet concentration ( Qair = 2 Nm3.h-1, [C4H8O] = 50 mg.Nm-3, Einj = 12.96 J.L-1,T = 18-20°C) On the left:, RH = 5-7 %, On the right: RH = 55 %.

Figures:Figure 6 .

 6 Figures:

Figure 7 .Figure 8 .

 78 Figure 7. Diagram of the impregnation technique for GFT synthesis "size press"

Figure 9 .

 9 Figure 9. Variation of EC with RH at different inlet concentrations (SE = 17 J.L -1 , Q air = 2 m 3 .h -1 , I = 20W.m -2 ).

Figure 5 .

 5 Figure 5. Variation of conversion rate with relative humidity in function of different processes using GFT with 100% TiO 2 (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 17 J.L -1 , RH = 5-7%, T = 18-20°C).

Figure 6 .

 6 Figure 6. By-products identified by GC-MS following butyraldehyde oxidation by the combined system.

Figure 7 .

 7 Figure 7. Suggested reaction scheme of BUTY oxidation by combination of GFT with 100% TiO 2 and DBD.

Figure 8 .

 8 Figure 8. CO 2 and CO selectivity's, and removal rate of oxidized butyraldehyde by photocatalysis, plasma DBD and combined system (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 12.8 J.L -1 , RH = 5-7%, T = 18-20°C).

Figure 9 .

 9 Figure 9. Effect of catalyst composition on BUTY conversion rate and the amount of formed ozone by plasma-catalytic monobloc system (IPC) (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , E inj = 12.96 J.L -1 , RH = 5-7 %, T = 18-20°C).

Figure 10 .Figure 11 .

 1011 Figure 10. Comparison of the photocatalytic performance of different media for BUTY oxidation (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , RH = 5-7 %, T = 18-20°C)

Figure 10 .

 10 Figure 10. Sectional drawing and Schema of NTP coupled with catalysis in planar reactor.

Figure 12 .

 12 Figure 12. Variation of EC with inlet concentration at different flow rates (SE = 17 J.L -1 , T = 20°C, RH = 5%, I = 20 W.m -2 ).

Figure 13 .

 13 Figure 13. Variation of EC with RH at different inlet concentrations (SE = 17 J.L -1 , Q air = 2 m 3 .h -1 , I = 20W.m -2 ).

Figure 5 .

 5 Figure 5. Variation of conversion rate with relative humidity in function of different processes using GFT with 100% TiO 2 (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 17 J.L -1 , RH = 5-7%, T = 18-20°C).

Figure 6 .

 6 Figure 6. By-products identified by GC-MS following butyraldehyde oxidation by the combined system.

Figure 7 .

 7 Figure 7. Suggested reaction scheme of BUTY oxidation by combination of GFT with 100% TiO 2 and DBD.

Figure 8 .

 8 Figure 8. CO 2 and CO selectivity's, and removal rate of oxidized butyraldehyde by photocatalysis, plasma DBD and combined system (Q air = 2 Nm 3 .h -1 , [C 4 H 8 O] = 80 mg.Nm -3 , E inj = 17 J.L -1 , RH = 5-7%, T = 18-20°C).

Figure 9 .

 9 Figure 9. Effect of catalyst composition on BUTY conversion rate and the amount of formed ozone by plasmacatalytic monobloc system (IPC) (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , E inj = 12.96 J.L -1 , RH = 5-7 %, T = 18-20°C).

Figure 10 . 4 Nm

 104 Figure 10. Comparison of the photocatalytic performance of different media for BUTY oxidation (Q air = 4 Nm -3 .h -1 , [C 4 H 8 O] = 50 mg.Nm -3 , RH = 5-7 %, T = 18-20°C)


Fethi Abbassi

Sebastien Mistou

Olivier Pantale

Ali Zghal

FE simulation and full-field strain measurements to evaluate the necking phenomena

Keywords: Metal forming, local necking, diffuse necking, tensile test, FEM, DIC, ductile damage;

During airbag production, many parts are obtained by plastic deformation such as the lid of the gas generator. Since the airbag gas generator is a significant vehicle safety part, several pressure tests have been developed in order to prove the reliability of this product. A ductile fracture problem initialized by a localization of the deformation is observed along the blending radius. In order to understand the mechanical processes involved in the forming operation, a numerical and experimental study was developed. A tensile test specimen extracted from the sheet of study, undergoes elastic deformation that is followed by a transition to plastic deformation. Although during this stage of the test the deformation is stable, the strains begin to localize within a relatively broad zone known as diffuse necking. The stable deformation, with continuously rising load, is followed by the instability whereby a local neck or shear band is produced due to strong localization of deformation. The localization is explainable by the inhomogeneity in macroscopic material properties in the specimen. The necking behavior is a vital precursor to the final failure. A Digital Image Correlation (DIC) method is used in 2D and 3D to evaluate the necking and localization of deformation, which shows clearly the two stage of necking phenomena. Moreover, a numerical modeling was developed of tensile test under ABAQUS commercial software. In this model we use Gurson Tvergaard Needleman (GTN) ductile model of damage to predict the necking phenomenon. Furthermore the results from FE simulations are compared with experimental results from uniaxial tensile tests. Finally we develop the numerical model of the lid of the gas generator obtained by forming process.

Introduction

Airbags, like safety belts contribute greatly reducing the death rates on the roads [START_REF] Fresnel | Numerical prediction of the structural failure of airbag inflators in the destructive testing phase[END_REF]. Some pieces of airbag are obtained by metal forming. The occurrence of defects in sheet metal forming is an important problem for mass production (i.e. automotive industry). In this context, many methods have been developed in order to identify the mechanical behavior of thin ductile sheets. The standard tensile test is one of the most important engineering procedures used to characterize the mechanical behavior of materials. This simple test provides the intrinsic stressstrain relationship during a uniform loading history up to a certain point. In the deep drawing process, the metal used for manufacturing is selected according to the formability deduced from the forming limit diagram (FLD). Generally, this FLD is computed from a plastic theory and necking conditions. This latest phenomenon is considered as a vital mechanism for fracture prediction in forming processes. When a sheet is increasingly thinned, two different plastic instabilities appear: diffuse and local necking. In 1885, Considère proposed the first criterion of diffuse necking suitable for uniaxial tension which is related to the maximum force. Concerning the industrial point of view, during the processes of airbag production, many parts are obtained by plastic deformation, as for example the lid of the gas generator illustrated in figure 1. Since the airbag gas generator is a significant safety part in automotive industry, many pressure tests have been developed in order to prove the safety of this product. These tests involve a dynamic internal pressure loading that can be finished by a fracture in bending radius zone. In several cases, a ductile fracture problem initialized by a localization of the deformation is observed along the sensitive area. In order to understand the mechanical behavior involved in the forming process, an experimental study has been conducted to identify the material properties of the S360 steel.

Experimental investigation

Digital Image Correlation and Stereo-correlation

In order to obtain a measurement out of plane, a 3D image correlation technique (stereo correlation) is used. On the surface of specimens, a random pattern is developed using white and black paints, two calibrated cameras are used to record two images of the undeformed surface and two images of the deformed surface. The SIC analysis by Aramis software based on the calculus of the matrix which relate the coordinates system of the two cameras. Fig. 2a show the configuration of the two camera points P1 and P2 used to calculate object point P (x,y,z) in the tensile sample. Example of the results of stereo image correlation (SIC) is presented in Fig. 2-b.

Digital Image Correlation (DIC) is one of the most important non-contacting methods to measure full-field displacement. Full-field displacement and strain measurements are widely used in laboratories conditions and for experimental applications in industries. These techniques are summarized in acquire images of an object, store image and analysis to extract the local displacement. Principally, 2D Digital image correlation can be used to measure displacement and strain fields of planar surfaces. The setup of this technic inquires only one camera, where the displacement is obtained by comparing the random pattern of grey levels in two images during forming.

During the tensile test, two cameras are operating separately, the first one is fixed in front of tensile sample while the second is positioned to measure the displacements filed in the profile view as shown in Fig. 3-a.

Tensile test

In the first section of this paper, we presente an experimental work to characterize a deep drawing steel S360 used in airbag part production. The determination of the displacement and deformation field is carried out by images correlation. Fig. 3 describes the geometry of specimen and the experimental procedure and illustrates the equipment used during these tests. Control and data acquisition is performed by computer interfaces and commercial software. Two different configurations are used as presented in previous sections. The direct result acquired during tensile test is the variation of force according to the displacement. Then this relation is usually changed to one between the true stress and the true strain and this later is used as the hardening law. In this work, to identify the S360 deep drawing steel, in order to determine the stress-strain curve, two virtual sections are selected in the central zone of the surface of the initial specimen and the relative displacement of these sections is tracked during the loading process to compute the strains for all the digital images captured by the cameras. The hardening material parameters have been identified, with respect to the Swift equation:

n K ) ( 0 H H V (1)
where K is the strength coefficient, n is the strain-KDUGHQLQJ H[SRQHQW DQG İ0 is a pre-strain. The experimental YDOXH RI İ0 is very small and could be calculated from the values of K and n, (K=741.3, n=0,174, İ0= 0,017). Fig. 4, show the necking formation that develops starting from a certain level of deformation. Two phases of necking are visible in the successive digital images of the sample surface during the tensile test on the DIC results computed by the Aramis software. The necking evolution starts by a symmetric diminution of the width of the specimen. This one is called the diffuse necking. If the last phenomenon is preceded, the severe thickness reduction that mainly takes place at the center of the sample causes a sudden initiation of a crack that immediately propagates in an unstable form along only one shear band. This later is called the local necking. In figure 4-b, we observe that the deformation is quickly heterogeneous in the tensile direction. For a given value of the load, there are several values of the strain in the sample surface.

Finite elements simulation

A Finite Element Method applied to predict some phenomena such as necking. In this model we consider a material elasto-plastic behavior with ductile damage type modified Gurson. The Gurson-Tvergaard-Needleman (GTN) approach model the different stages of damage evaluation in material. The yield function describing the plastic constitutive model is represented as flows:

0 1 2 3 cosh 2 , , 2 3 2 1 2 2 1 • © § ) ) (f q ( ı ı q q f ı ı f ı ı ij * y m * y m (2)
Table 1. GTN model parameters used in numerical modeling All required parameters for this numerical modeling presented in table 1. Fig. 5 shows the results of numerical model that a good correlation was observed with the experimental results.

f 0 q 1 q 2 q 3 İ N S N f N f F f c 0.001 1.

Conclusions

The aim of this work is to characterize S360 deep drawing steel used in automotive industry, study of the necking phenomena and the material formability. A Digital Image Correlation (DIC) technique in 2D and 3D are used in the experimental investigation. This advanced measurement system is used for reliable identification of the mechanical behavior of sheet metal by tensile tests. Moreover, a large study of diffuse and local necking and the evolution of this phenomenon are carried out. The material behavior law assumed to follow the Swift hardening law and a damage ductile model type Gurson used to predict the experimental test results using FE method. DIC and SIC contribute greatly to understand some mechanical material behavior present during mechanical loading especially in the localization of deformations. The numerical modeling of mechanical test shows a good correlation between numerical and experimental results. A virtual manufacturing process of airbag part was developed under Abaqus software that shows the critical damage in the bending radius where it observed the fracture during the destructive test.

Fig. 1 .

 1 Fig. 1. Example of S360 steel application: the lid of gas generator of airbag

Fig. 2 .

 2 Fig.2. (a) Stereo correlation principle, (b) Example of stereo correlation results.

Fig. 3 (

 3 Fig.3 (a) tensile test with cameras in position 1, (b) stereo correlation position, (c) tensile test specimen geometry

Fig. 4

 4 Fig. 4 Tensile test results: (a) DIC results 2D and 3D, (b) 3D curve of deformation in the spesemen surface

Fig. 5 Fig. 6 :

 56 Fig.5 Necking in tensile test obtained by FEM with GTN damage. After determination of 360S steel behavior and validation by tensile test a numerical model of the manufacturing of the lid of the gas generator was developed .Fig. 6-a show the tools used in the forming process and Fig 6-b show the numerical results. The critical deformation and damage localized in the bending radius where we observed the crack during destructive test.


Lucas Le Goanvic

Jean-Luc Couturier

Jean-Luc Dubois

Jean-François Carpentier

Lucas Le Goanvic

Ruthenium-Catalyzed Hydroformylation of the Functional Unsaturated Fatty Nitrile 10-Undecenitrile

Keywords: Hydroformylation, Isomerization, Ruthenium, Biphephos, 10-Undecenenitrile

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

If rhodium is irrefutably the most efficient metal to promote olefin hydroformylation, one of the most widely applied homogeneously-catalyzed processes in industry [1], its very high and volatile price has urged investigation on other metals [2]. In 1977, the relative activities of the unmodified metal carbonyl complexes in hydroformylation were suggested as follows: Rh ≫ Co > Ir, Ru > Os > Pt > Pd ≫ Fe > Ni [START_REF] Pospech | [END_REF]. However, recent reports have shown that those old assumptions should be re-examined; for instance, the activity ratio of rhodium-to-iridium is in fact much closer to 1 than the 10,000:1 ratio initially predicted [4,5]. Ruthenium may also offer an interesting compromise between price and activity, as it is currently ca. 15 and 12 times cheaper than rhodium and iridium, respectively [START_REF]For examples of recent developments in Ir-catalyzed hydroformylation[END_REF], and its activity in an oxo process is generally announced as one of the best (with iridium) among all alternative metals.

The first investigations on Ru-catalyzed hydroformylation began as early as in 1965 with Wilkinson's brief report on hydroformylation of 1-pentene using the mononuclear zerovalent complex Ru(CO) 3 (PPh 3 ) 2 as catalyst precursor (100-120 °C, 100 bar, CO/H 2 = 1:1) [7]. The authors subsequently presented more detailed results for 1-hexene hydroformylation with the same catalyst system and other related mononuclear Ru-phosphine complexes; at a quite high catalyst loading ([olefin]/[Ru] = 100), the turnover frequency (TOF) reached 0.075 min -1 , with a claimed 100% chemoselectivity for the aldehydes in most cases, although the linear-to-branched ratio was low (l/b = 2.0-2.9) [8]. The Ru(II) dihydrido dicarbonyl complex Ru(H) 2 (CO) 2 (PPh 3 ) 2 was proposed as the principal active species. Ru 3 (CO) 12 proved to be a modest precursor (24% conv.) under the studied conditions but increased conversion was obtained upon addition of 1 equiv. (vs. Ru) of PPh 3 (88% conv.) or, even better, P(OPh) 3 (95% conv.); however, those systems were all less active than the mononuclear complexes. Meanwhile, Schulz and Bellstedt also reported hydroformylation of propylene with Ru 3 (CO) 12 to afford 94% of conversion, but the final mixture contained less than 25% of aldehydes [9].

Examples of 1-hexene hydroformylation conducted in an ethanol-water (80:20) mixture using water-soluble complexes of the type K[Ru(EDTA-H)Cl] were reported in 1988 [10]. At a high catalyst loading ([olefin]/[Ru] = 140; 130 °C, 50 bar CO/H 2 1:1), these led to full conversion of the olefin (overall TOF = 0.2 min -1 ) and exclusive formation of linear heptanal.

Surprisingly, ruthenium-catalyzed hydroformylation with the ligands most often used nowadays in combination with rhodium, i.e. diphosphines and diphosphites, was not reported until recently. It is only in 2012 that Nozaki and coworkers reported on combinations of {RuCp(acac)} 2 with Xantphos or Bisbi diphosphines, or the A4N3 diphosphite (Figure 1) [11]. The latter diphosphite ligand allowed reaching increased chemo-(up to 66% aldehydes) and regioselectivities (l/b up to 79) in the hydroformylation of 1-decene (100 °C, 20 bar CO/H 2 1:1); the side-products were essentially isomerized (internal) olefins (19%) and a slight amount of the hydrogenation product (1.5%). The catalyst loading was, however, quite high ([olefin]/[diphosphite]/[Ru] = 40:2:1) and overall TOFs were about 0.025 min -1 . Domino hydroformylation-hydrogenation reactions, to end up with the corresponding alcohols instead of the aldehydes, were also developed. Besides examples relying on rhodium complexes to achieve hydroformylation and ruthenium complexes for the hydrogenation reaction [11,12], Beller and coworkers developed the first such domino reaction with the same ruthenium catalyst. Using Ru 3 (CO) 12 or Ru(methylallyl) 2 (COD) as precursor, combined with 1 equiv. (vs. Ru) of a 2-phosphino-substituted imidazole ligand, at a [olefin]/[Ru] ratio of 167, 130 °C and 60 bar CO/H 2 (1:5), they achieved full conversion of 1-octene (overall TOF = 0.13 min -1 ) to provide 87% of alcohol (l/b = 10), along with 9% of octane and less than 1% of the intermediate aldehyde [13,14].

In previous studies, we reported the use of Rh-Biphephos [START_REF] Wu | For a related Ru 3 CO 12 -monophosphite catalyst system enabling the reverse water-gasshift (RWGS) domino hydroformylation-reduction of 1-olefins using a CO 2 /H 2 (1:1) mixture instead of CO/H 2[END_REF] and Ir-Biphephos [5c] catalyst systems for the tandem isomerization-hydroformylation [16,17] of the unsaturated fatty nitrile 10-undecenitrile (1) (Scheme 1), as a route toward biosourced polyamide-12.

Those systems performed at very high substrate-to-catalyst ratio (20,000-100,000) and yielded the desired linear aldehyde (2) with high chemo-and regioselectivities up to 93% and 99%, respectively. However, significant amounts of undesired isomerization products (1-intx) along with minute amounts of the hydrogenation product (4) were formed, which eventually plague both conversions and selectivities for the desired linear aldehydes (Scheme 1). Preliminary experiments showed that potentially interesting results could be also reached with ruthenium catalysts, although the activities were apparently much lower [5c]. Herein we report full details on the isomerization-hydroformylation of 10-undecenitrile with such ruthenium-based systems. A variety of precursors and ligands, as well as regular reaction parameters (solvent, temperature, syngas pressure, substrate concentration), have been screened.

Experimental Section

General features

All reactions involving Ru-phosphine catalysts were performed under an inert atmosphere (argon) using standard Schlenk techniques. Solvents (toluene, THF, etc.) were purified over alumina columns using a MBraun system. RuCl 2 (PPh 3 ) 3 , RuCl 2 (p-cymene) and Ru 3 (CO) 12 were generously provided by Umicore Co and stored in the glove box. RuCl 2 (DMSO) 4 was synthesized according to the literature procedure. [18] Biphephos and A4N3 diphosphite ligands were purchased from Strem Chemicals and MCAT, respectively, and used as received (stored in the glove box). 10-Undecenenitrile (typically 94% pure, contains 6% of 9undecenitrile (1-int-0) and other internal isomers (1-int-x), as determined by NMR) was supplied by Arkema; it was first eluted through a short alumina column and vacuum-distilled (Kügelrohr distillation) at 125 °C under 0.03 mm Hg prior to use. 1 H and 13 C NMR spectra were recorded on Bruker AC-300 and AM-400 spectrometers. 1 H and 13 C chemical shifts were determined using residual signals of the deuterated solvents and were calibrated vs. SiMe 4 .

General Procedure for Hydroformylation Reaction

In a typical experiment, the ruthenium precursor RuCl 2 (PPh 3 ) 3 , as a 1.0 g.L -1 toluene solution (0.72 mL, 0.75 µmol) was added on Biphephos (11.8 mg, 15.0 µmol) in a Schlenk flask. 10-undecenitrile (2.479 g, 15.0 mmol) in the desired solvent (toluene or acetonitrile, 15 mL) was added onto the resulting mixture. The solution was transferred under argon into a 90 mL stainless-steel autoclave under argon, equipped with a magnetic stir bar cross. The reactor was sealed, charged with CO/H 2 at the desired pressure at room temperature, stirred (800 rpm) and then heated with silicon oil set at the desired temperature. During the reaction, aliquots were sampled at regular time intervals to monitor the conversion and selectivities by NMR. After the appropriate reaction time, the reactor was cooled to room temperature and vented to atmospheric pressure. The solution was analyzed by NMR (after evaporation of solvent). The conversion of 1 into 1-int-x and 2-5, as reported in Tables 12345, was calculated taking into account the quantity of internal isomers (1-int-x) initially present in the substrate:

Conv(1) = ([2] t + [3] t + [4] t + [5] t + [1-int-x] t -[1-int-x] 0 ) / [1] 0 .
The reported TOF values are overall values calculated from the conversion at total reaction time: TOF = conv × 20,0000 / time.

The NMR characteristics for 10-undecenenitrile (1), its internal isomers (1-int-x), the hydroformylation products (2 and 3) and the hydrogenation product (4) have been reported previously [5c,15]. Typical 1 H NMR signals for the linear alcohol ( 5) were observed at δ = 3.62 (t, J = 6 Hz, 3H, HOCH 2 CH 2 -) ppm.

Results and Discussion

First hydroformylation experiments were performed at low catalyst loading ( [1] 0 /[Ru] = 20,000; initial 1/1-int-x ratio = 94:6) using different chloro Ru(II) (RuCl 2 (PPh 3 ), RuCl 2 (pcymene), RuCl 2 (DMSO) 4 ) and Cl-free Ru(0) (Ru 3 (CO) 12 ) precursors in combination with Biphephos, diphosphite A4N3 or triphenylphosphite. For the sake of comparison, the experimental conditions used were those optimized in the hydroformylation of 1 using Rh(acac)(CO) 2 -Biphephos [START_REF] Wu | For a related Ru 3 CO 12 -monophosphite catalyst system enabling the reverse water-gasshift (RWGS) domino hydroformylation-reduction of 1-olefins using a CO 2 /H 2 (1:1) mixture instead of CO/H 2[END_REF].

Preliminary Notes

It is here important to point out that the chemo-/regioselectivities remained constant over time, and no obvious change in the kinetic regime was noted, indicating the stability of all these catalytic systems over the reaction course. This indicates, in particular, that these Ru catalyst systems were not affected by the HCl released from the chloro precursors, which is in contrast with the highly sensitive Rh-based systems. Also, all the experiments reported in the forthcoming tables were at least duplicated, showing a good reproducibility. One necessary condition is required to achieve such reproducibility, that is the use of freshly prepared (i.e., no older than one week) stock solutions of the Ru precursor, stored in the dark. Because of the low catalyst loading, such stock solutions were needed to introduce accurately small amounts of the Ru precursor. These stock solutions are perfectly clear (RuCl 2 (PPh 3 ) 3 : orange, RuCl 2 (DMSO) 4 : yellow); yet, we observed that over days-weeks, upon exposure to light, some of these solutions can turn blackish, suggesting the formation of metal colloids/nanoparticles; in those cases, we observed that the chemoselectivity for aldehydes can dramatically drop from 75% down to 60% (mainly in favor of hydrogenation and also isomerization products), while the regioselectivity slightly dropped from 99.0:1.0 down to 98.5:1.5. We also noted that old, still clear solutions to the naked eye can lead to decreased selectivities.

Catalyst Precursor

Significant differences in terms of activity were noted among the four precursors investigated.

Representative results are summarized in Table 1. Both RuCl 2 (PPh 3 ) 3 and RuCl 2 (DMSO) 4 led to the higher activities with, respectively, important and full conversion of the substrate under the chosen conditions (TOF HF = 5 and 6 min -1 , entries 1 and 4, respectively). On the other hand, the chemoselectivity for aldehydes was very similar for all systems. This selectivity is quite comparable to that obtained with the Rh-Biphephos catalytic system [START_REF] Wu | For a related Ru 3 CO 12 -monophosphite catalyst system enabling the reverse water-gasshift (RWGS) domino hydroformylation-reduction of 1-olefins using a CO 2 /H 2 (1:1) mixture instead of CO/H 2[END_REF], so as the regioselectivity (2/3; l/b up to ca. 120); only the system based on RuCl 2 (p-cymene) was somewhat less regioselective. The close similarity of the regioselectivity achieved with Clcontaining and Cl-free (Ru 3 (CO) 12 ) precursors is noteworthy (vide supra). The 1-int-0/1-int-x (x > 0) ratio indicates the isomerization ability of the system: the lower this ratio, the more important the isomerization of the double bound inside the carbon chain. In fact, those Ru systems proved moderately isomerizing, since most of them led essentially to 9-undecenitrile with quite minor amounts of more internal isomers. A consequence of this low isomerizing ability is that, even upon long exposure (75 h) after full conversion of 1, the amount of internal isomers 1-int-x did not decrease significantly. The final amount of hydrogenated product (4) is similar in Ru systems as in the Rh ones (ca. 2-5%). Noteworthy, in contrast to Rh-based catalysts but in line with Ru systems [10][11][12][13][14], small amounts (<0.5 mol%) of alcohol products (5)19 can be observed. Further experiments were conducted using the two most efficient precursors, i.e., RuCl 2 (PPh 3 ) 3 and RuCl 2 (DMSO) 4 .

Solvent

The dependence of the performance of the RuCl 2 (PPh 3 ) 3 -and RuCl 2 (DMSO) 4 -Biphephos systems on the solvent nature was evaluated with toluene, acetonitrile, DMF, diglyme and 1,2-dichloroethane (DCE). The results are summarized in Table 2. The trends observed with RuCl 2 (PPh 3 ) 3 are identical to those with RuCl 2 (DMSO) 4 . Hence, acetonitrile, DMF and toluene featured similar results in terms of conversion of 1 and chemoselectivity for aldehydes. Nevertheless, if the regioselectivity was as good as the one obtained with toluene for acetonitrile, DMF induced a slightly more important amount of branched aldehyde. On the other hand, both 1,2-dichloroethane and diglyme led to low substrate conversion and also a slightly lower regioselectivity. There is no apparent correlation between the solvent polarity and the catalytic performance. Hence, this screening allowed us to highlight two couples of efficient precursors -RuCl 2 (PPh 3 ) 3 and RuCl 2 (DMSO) 4and solvents-toluene and acetonitrile.

Ligands

The A4N3 ligand (Figure 1) used by Nozaki et al. [11] and triphenylphosphite, a simple monophosphite ligand, were evaluated in comparison with Biphephos, in combination with RuCl 2 (PPh 3 ) 3 (similar results were obtained upon using RuCl 2 (DMSO) 4 ) under the same reactions conditions than the standard experiments presented above; the amounts of ligands were adjusted to match the same phosphorus-to-metal ratio. Unsurprisingly, the P(OPh) 3based system exhibited a lower activity and chemoselectivity and a much poorer regioselectivity (l/b = 4.5) as compared to the two other systems based on diphosphite ligands.

The A4N3 ligand exhibited a slightly lower activity (TOF = 3-5 min -1 ) and also a slightly decreased regioselectivity than the equivalent Ru-Biphephos system. The same trend was also observed in the case of Rh-based catalysts in previous studies carried in our lab [START_REF]minor products in this process, only the linear alcohol derived from the major linear aldehyde is considered[END_REF]. On the other hand, very poor performances were observed with Zhang's tetraphosphine [START_REF] Le Goanvic | [END_REF].22

[Ligand]/[Ru] ratio

In our standard conditions, a ligand-to-Ru ratio of 20 is used. In the case of Rh-Biphephos catalysis, this condition proved to be necessary to prevent catalyst decay and decreased activities and selectivities, assumed to arise from the formation of rhodium aggregates [START_REF] Wu | For a related Ru 3 CO 12 -monophosphite catalyst system enabling the reverse water-gasshift (RWGS) domino hydroformylation-reduction of 1-olefins using a CO 2 /H 2 (1:1) mixture instead of CO/H 2[END_REF].

With ruthenium, the amount of ligand introduced may not need to be necessary as high as in the case of rhodium to prevent such phenomenon. Thus, experiments at [Biphephos]/[Ru] ratios in the range 0-20 were carried out; the results are summarized in Table 4. Experiments conducted without Biphephos ligand proved to be inefficient to perform hydroformylation of undecenitrile in 88 h; at the end of these experiments, a black suspension was recovered, suggesting the formation of ruthenium aggregates. Formation of these aggregates was prevented by stabilization of the metal center with the diphosphite ligand. In these experiments, no impact was observed regarding the selectivities, i.e., the l/b ratio and %HF did not change significantly when the [Biphephos]/[Ru] was modified, indicating that the same active species is at work in all cases. However, a noticeable increase in the catalytic activity was noted when the [Biphephos]/[Ru] ratio increased. Although this may appear counterintuitive at first sight, this observation can be rationalized by taking into account that, at such a high substrate-to-catalyst ratio (20,000), excess ligand induces larger amounts of active species, possibly by counter-balancing competitive coordination of the substrate to the metal precursor.

Temperature, Pressures

Considering the moderate activity of the Ru-based systems, experiments performed at higher temperatures with the RuCl 2 (PPh 3 ) 3 /Biphephos system were first envisioned. The compositions profiles and selectivities obtained at 140 °C, instead of 120 °C, are presented in Figure 2 and Table 5. As expected, an increase of the global reaction rate was observed at 140 °C. However, the isomerization process was much more favored at this temperature, as compared to 120 °C; the amount of internal olefins increased from 22% to 40% and the selectivity in hydroformylation products concomitantly dropped from 76% down to 58%. The significantly larger amount of internals olefins that migrated at least twice (1-int-x; x = 0/1 + = 88:12) underscores the importance of the isomerization process. The relative pseudo zerothorder rates [ 23 ] of hydroformylation vs. isomerization, as determined from the initial rates in Figure 2 and expressed as k HF /k Iso , decreased from 4.6 at 120 °C down to 1.4 at 140 °C; this corresponds to a difference in activation energies E a,Iso -E a,HF of ca. 8 kJ.mol -1 . It is noteworthy that even at the latter high temperature, conversion of the internal isomers 1-int-x to aldehydes did not proceed at a noticeable rate (see Figure 2); this evidences the impossibility, at least with the present catalyst systems, to achieve a tandem isomerizationhydroformylation process [17]. Yet, the increase in temperature from 120 to 140 °C only had a minimal impact on the l/b ratio and, more surprisingly, on the chemoselectivity (in particular, larger amounts of alcohols may have been anticipated since this is a consecutive, more energy-demanding process).

Modification of the total and relative pressures is likely to influence the formation of active species and/or lead to different catalytic species, and eventually affect activities and selectivities [24]. Representative results of such variations in the RuCl 2 (PPh 3 ) 3 /Biphephos system are presented in Table 6. When the total syngas pressure was varied in the range 10-40 bars, at a constant 1:1 CO/H 2 ratio, the reactions proceeded with just slightly decreased activity (from 5.0-6.1 min -1 down to 3.5 min -1 ) and quite similar selectivities (entries 25, 26 and 29). A much more significant decrease of the catalytic activity was observed at 90 bars, where the TOF dropped down to 0.6 min -1 ; meanwhile, the amount of branched aldehyde was doubled while the chemoselectivity remained constant (entry 30). The same trends were observed upon changing the CO/H 2 ratio to 3:1 at a total pressure of 20 bars (entry 28).

Obviously, excess CO is detrimental. On the other hand, when the CO/H 2 ratio was set at 1:3, the activity somehow decreased but most noticeably, the chemoselectivity for aldehydes dramatically decreased; in particular, larger amounts of alcohols 5 were formed, a nonunexpected result (entry 27).

Recycling

Attempts to recycle the catalyst and eventually improve on the catalytic productivity were conducted. We used the same procedure as the one positively evaluated for the analogous Rh-Biphephos system [15a]: the vacuum distillation of the crude reaction mixture can be readily achieved, allowing complete elimination of toluene solvent, and recovery of analytically pure aldehydes (along with residual internal olefins) and of a solid residue that contains the catalyst/ligand. To prevent deteriorated performance, the solid residue was concentrated and recovered under an inert atmosphere. Representative results obtained using this procedure are gathered in Table 7; the runs were conducted over long time period (124-190 h) to achieve high conversions. The results evidence that effective recycling can be achieved, maintaining a good chemo-and regioselectivity in favor of the linear aldehyde over at least three runs; the overall TON thus reached 55,000. The addition of a novel charge of fresh Biphephos in the recycling runs does not appear essential (compare run 2 and run 3). Yet, a 31 P NMR monitoring of the reaction mixture was also performed, indicating that the Biphephos ligand slowly degrades under the reaction conditions (see the Supporting Information, Figure S1); similar observations were made with the Rh-Biphephos catalyst system [15a].

Conclusions

Combination of Ru(II) metallic precursors with Biphephos affords a selective catalytic system for the hydroformylation of 10-undecenitrile. The selectivity data: 75% of hydroformylation and l/b ratio up to 99:1, compare favorably with the very good performance of the rhodiumand iridium-based catalysts we previously reported [5c,15]. Formation of the hydrogenated olefin and of the alcohols resulting from the reduction of the desired aldehydes can be prevented by an adequate choice of the temperature and CO/H 2 pressures. These good chemoand regio-selectivities are balanced with the lower activity (non-optimized hydroformylation TOF HF of 2-7 min -1 ), which are decreased by ca. 1-2 orders of magnitude (and not 5 orders as initially anticipated) in comparison with equivalent Rh-Biphephos systems (TOF HF = ca. 80 min -1 ), although the lower prices of ruthenium (ca. 15 times cheaper than Rh and Ir) must also be taken into account. This reduced activity did not hamper to achieve very high productivities (effective TurnOver Numbers for aldehydes, TON, up to 15,000 for batch experiments and up to 55,000 upon recycling). A limitation of these ruthenium systems is their incapacity, under the conditions suitable for selective hydroformylation, to promote isomerization of internal olefins, and hence to achieve a tandem isomerizationhydroformylation process as the Rh-Biphephos system is amenable to; we assume that this reflects the lower isomerizing ability of putative Ru-hydride species generated in these systems as compared to the corresponding Rh-hydride species in the Rh-based systems. Also, these ruthenium-based systems seem to be more versatile than the rhodium ones, as important isomerization or loss of regioselectivity can occur more easily if freshness of the catalyst precursors is not perfectly controlled.

Supporting Information Available

31

P{ 1 H} NMR monitoring of reaction media. [a] Reaction conditions: [1] 0 /[1-int-x] 0 (94:6) = 5.0 mmol, [1] 0 /[Ru] = 20,000, [Biphephos]/[Ru] = 20, toluene (5 mL), CO/H 2 = 1:1, P tot = 20 bar, T = 120 °C. [b] Distribution (mol-%) of remaining 1, internal alkenes 1-int-x (residual or formed during the reaction; x=0/1 + refers to the positioning of the internal C=C bond, x = 0 being 9undecenitrile and x = 1+ referring to 8-, 7-, …undecenitriles; please refer to Scheme 1), aldehydes 2 and 3, hydrogenated product 4, and alcohols 5 resulting from aldehydes reduction, as determined by 1 H NMR analyses. [c] Regioselectivity as determined by the linear-to-branched aldehyde ratio. [d] Conversion of 1 into 1-int-x and 2-5, calculated taking into account the quantity of 1-int-x initially present in the substrate:

Conv = ([2] t + [3] t + [4] t + [5] t + [1-int-x] t -[1-int-x] 0 ) / [1] 0 . [e]
Chemoselectivity as determined by the percentage of hydroformylation among all other competitive processes. [f] Overall TOF determined from the conversion of 1 over the whole reaction time. [a] See Table 1. [a] Metallic precursor: RuCl 2 (PPh 3 ) 3 + 20 equiv. of ligand, except P(OPh) 3 , 40 equiv.; otherwise, see Table 1.

[h] [%] b [%] b x=0/1 + [%] b (l/b) c [%] b [%] b [%] d [%] e [
[h] [%] b [%] b x=0/1 + [%] b (l/b) c [%] b [%] b [%] d [%] e [
Table 4: Influence of the ligand-to-metal ratio in the hydroformylation of 10-undecenitrile by the RuCl 2 (PPh 3 ) 3 /Biphephos system. a

[a] See Table 1. [a] See Table 1 for experimental conditions except for T. [g] TOF value calculated at its maximal slope.

Figure caption

  Figure caption

Figure 1 .Figure 2 :Scheme 1 .

 121 Figure 1. Diphosphine and diphosphite ligands used for ruthenium-catalyzed

Table caption Table 1 .

 caption1 Variation of precursor in Ru-catalyzed hydroformylation of 10-undecenitrile. a

	Entry	Ru precursor Time	1	1-int-x	1-int-x 2+3	2/3	4	5	Conv. 1	HF TOF f
			[h]	[%] b [%] b x=0/1 + [%] b (l/b) c	[%] b [%] b	[%] d [%] e [min -1 ]
	1	RuCl 2 (PPh 3 ) 3	48	10	22	98:2	63 99.1:0.9	4	0.3	89	76	6
	2	RuCl 2 (p-cymene) 48	55	15	99:1	27 98.5:1.5	3	0.4	42	69	3
	3	[Ru(COD)Cl 2 ] n 48	62	13	nd	23 99.1:0.9	2	traces	34	72	2
	4	RuCl 2 (DMSO) 4 48	0	24	92:8	71 98.9:1.1	5	0.5	100	75	7
	5	Ru 3 (CO) 12	64	45	16	99:1	36 99.2:0.8	2	0	51	75	3

Table 2 :

 2 Solvent comparison in the Ru-catalyzed hydroformylation of 10-undecenitrile. a

	Entry	Ru precursor	solvent	time	1	1-int-x 1-int-x 2+3	2/3	4	5	Conv. 1	HF TOF f

  min-1 ]

	6	RuCl 2 (PPh 3 ) 3	toluene	48	10	22	98:2	63 99.1:0.9	4	0.3	89	76	6
	7	RuCl 2 (PPh 3 ) 3	CH 3 CN	48	10	22	97:3	64 99.2:0.8	4	0.3	89	77	6
	8	RuCl 2 (PPh 3 ) 3	DMF	50	22	19	97:3	56 98.7:1.3	3	0	76	80	5
	9	RuCl 2 (PPh 3 ) 3	diglyme	44	41	20	99:1	37 98.8:1.2	2	0	55	72	4
	10	RuCl 2 (PPh 3 ) 3	DCE	48	77	14	96:4	4	98.0:2.0	4	traces	16	36	1
	11	RuCl 2 (DMSO) 4	toluene	48	0	24	92:8	71 98.9:1.1	5	0.5	100	75	7
	12	RuCl 2 (DMSO) 4	CH 3 CN	48	1	25	93:7	70 99.1:0.9	4	0	99	76	7
	13	RuCl 2 (DMSO) 4	DMF	47	5	31	94:6	60 98.6:1.4	4	0	95	68	7
	14	RuCl 2 (DMSO) 4 diglyme	48	70	11	nd	16 97.5:2.5	3	nd	25	68	2

Table 3 :

 3 Ligand comparison in the Ru-catalyzed hydroformylation of 10-undecenitrile. a

	Entry	Ligand	Time	1 1-int-x 1-int-x 2+3	2/3	4	5	Conv. 1	HF	TOF f

Table 5 :

 5 Influence of temperature in the hydroformylation of 10-undecenitrile by the RuCl 2 (PPh 3 ) 3 /Biphephos system. a [%] b [%] b [%] d [%] e [min -1 ]

	Entry [L]/[Ru] Time	1	1-int-x 1-int-x 2+3	2/3	4	5	Conv. 1	HF	TOF f
			[h]	[%] b	[%] b x=0/1 + [%] b	(l/b) c	[%] b [%] b [%] d [%] e [min -1 ]
	18	0	88	92	7	99:1	0	-	0.4	0	1	0	-
	19	2.4	90	59	12	97:3	27 99.1:0.9	2	0	37	79	1.4
	20	6.0	53	49	17	99:1	32 98.8:1.2	2	0.3	47	71	2.9
	21	10.5	73	40	22	97:3	35 99.1:0.9	3	0	57	66	2.6
	22	20	48	10	22	97:3	64 99.1:0.9	4	0.3	88	76	6.1

b [%] b x=0/1 + [%] b (l/b) c

Table 6 :

 6 Influence of total and relative pressures in the hydroformylation of 10-undecenitrile by the RuCl 2 (PPh 3 ) 3 /Biphephos system. a[a] See Table1except for the P value.

Table 7 :

 7 Recycling of the RuCl 2 (PPh 3 ) 3 -Biphephos system over 3 runs in the hydroformylation of 10-undecenenitrile. a See Table1.[g] 20 equiv of Biphephos (vs. Ru) were added to the solid residue before starting the 2 nd run.

	Run	Biphephos	time	1	1-int-x 2+3	2/3	4	5	Conv. 1	HF	TOF f
			[h]	[%] b	[%] b	[%] b	(l/b) c	[%] b [%] b [%] d	[%] e [min -1 ]
	1	-	190	14	31	45	98.8:1.2	5	5	84	57	1.6
	2	+20 equiv g	150	6	28	62	99.3:0.7	3	1	94	71	2.2
	3	-	124	3	27	64	98.9:1.1	5	1	99	72	2.7
	[a]

Acknowledgements

This work was supported by the French National Research Agency, ANR project MEMCHEM (14-CE06-0022-02) and Arkema Co. We thank Umicore Co. (Dr. Angelino Doppiu) for a generous gift of RuCl 2 (PPh 3 ) 3 , RuCl 2 (p-cymene) and Ru 3 (CO) 12 .