domain: sdv spi spi.gproc

text:
Florian Chemarin

Marwen Moussa

Morad Chadni

Brigitte Pollet

Pascale Lieben

Florent Allais

Ioan-Cristian C Trelea

Violaine Athes-Dutour
email: violaine.athes-dutour@inra.fr

V Athès

New insights in reactive extraction mechanisms of organic acids: An experimental approach for 3-hydroxypropionic acid extraction with tri-n-octylamine

Keywords: Solvent extraction, organic acid, active solvent, tri-n-octylamine impurity, In Situ Product Recovery, 2 3 tertiary amine TOA: tri-n-octylamine Y: extraction yield Subscript

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

Introduction

It is now recognized that 3-hydroxypropionic acid (3-HP) is a valuable platform molecule that can be used as a building-block for the production of new polymers like polyesters and acrylic acid derivatives such as acrylates or acrylamides [START_REF] Werpy | Top Value Added Chemicals from Biomass, Volume 1-Results of Screening for Potential Candidates from Sugars and Synthesis Gas[END_REF]. This molecule is currently not widely used due to its exclusive and difficult synthesis via chemical pathways including transformation of petrochemical products such as propylene but is expected to exhibit a significant growth in the next years. Indeed, it has been referred as one of the Top 10 "chemical opportunities from biorefinery carbohydrates" [START_REF] Bozell | Technology development for the production of biobased products from biorefinery carbohydrates-the US Department of Energy's "Top 10" revisited[END_REF].

Lately, research has focused on the production of 3-HP using biotechnologies with natural [START_REF] Dishisha | Flux analysis of the Lactobacillus reuteri propanediol-utilization pathway for production of 3-hydroxypropionaldehyde, 3hydroxypropionic acid and 1,3-propanediol from glycerol[END_REF][START_REF] Burgé | Diversity of Lactobacillus reuteri Strains in Converting Glycerol into 3-Hydroxypropionic Acid[END_REF] and modified [START_REF] Kim | Enhanced production of 3-hydroxypropionic acid from glycerol by modulation of glycerol metabolism in recombinant Escherichia coli[END_REF][START_REF] Chu | Metabolic engineering of 3-hydroxypropionic acid biosynthesis in Escherichia coli[END_REF] micro-organisms able to convert glycerol or glucose into 3-HP [START_REF] Kumar | Recent advances in biological production of 3-hydroxypropionic acid[END_REF]. Other substrates like CO 2 [START_REF] Lan | Metabolic engineering of cyanobacteria for photosynthetic 3-hydroxypropionic acid production from CO2 using Synechococcus elongatus PCC 7942[END_REF][START_REF] Wang | Biosynthesis of platform chemical 3hydroxypropionic acid (3-HP) directly from CO2 in cyanobacterium Synechocystis sp. PCC 6803[END_REF] and C5 sugars [START_REF] Jung | Simultaneous conversion of glucose and xylose to 3-hydroxypropionic acid in engineered Escherichia coli by modulation of sugar transport and glycerol synthesis[END_REF][START_REF] Kildegaard | Production of 3-hydroxypropionic acid from glucose and xylose by metabolically engineered Saccharomyces cerevisiae[END_REF] are also currently studied. Biotechnology is considered as a promising route for the sustainable and efficient production of 3HP but one of the main drawbacks of this approach is the inhibitory effect the products can have on the micro-organisms, limiting productivity and final concentrations to quite low amounts, which is particularly true for the 3-HP production [START_REF] Kumar | Recent advances in biological production of 3-hydroxypropionic acid[END_REF]. In this context, in-situ product recovery (ISPR) seems to be a promising approach because the simultaneous removal of the product from the bioconversion medium could favor higher yields, productivity and concentrations. Liquid-liquid reactive extraction for organic acids' recovery is known to be particularly efficient and selective for dilute media as an ISPR technique in biotechnological processes and can prevent toxic metabolites and/or products from accumulating [START_REF] Jin | Extractive fermentation for enhanced propionic acid production from lactose by Propionibacterium acidipropionici[END_REF][START_REF] Wu | Extractive fermentation for butyric acid production from glucose by Clostridium tyrobutyricum[END_REF].

Current concentrations of 3-HP in bioconversion media for the best current biotechnological performances found in the literature do not exceed about 70 g/L [START_REF] Chu | Metabolic engineering of 3-hydroxypropionic acid biosynthesis in Escherichia coli[END_REF][START_REF] Zhao | Development of a two-step process for production of 3hydroxypropionic acid from glycerol using Klebsiella pneumoniae and Gluconobacter oxydans[END_REF]. For short chain carboxylic acids in dilute streams such as 3-HP, their hydrophilic character prevents their direct recovery in an organic solvent and a reagent is needed in the organic phase to trap the compound in a rather hydrophobic complex. In the case of hydrophilic carboxylic acids, these reagents can be hydrophobic basic molecules being able to interact with acidic groups [START_REF] Kertes | Extraction Chemistry of Fermentation Product Carboxylic Acids[END_REF]. Among such molecules, long chained aliphatic amines have been widely studied for a large set of carboxylic acids, such as hydroxycarboxylic acids like citric [START_REF] Bízek | Mathematical model of extraction of citric acid with amine[END_REF][START_REF] Bízek | Amine extraction of citric acid: effect of diluent[END_REF][START_REF] Keshav | Reactive Extraction of Citric Acid Using Tri-n-octylamine in Nontoxic Natural Diluents: Part 1-Equilibrium Studies from Aqueous Solutions[END_REF], glycolic [START_REF] Matsumoto | Synergistic extraction of organic acids with tri-n-octylamine and tri-n-butylphosphate[END_REF][START_REF] Datta | Reactive Extraction of Glycolic Acid Using Tri-n-Butyl Phosphate and Tri-n-Octylamine in Six Different Diluents: Experimental Data and Theoretical Predictions[END_REF], malic [START_REF] Uslu | Extraction of aqueous of malic acid by trioctylamine extractant in various diluents[END_REF][START_REF] Prochazka | Amine extraction of hydroxycarboxylic acids. 2. Comparison of equilibria for lactic, malic, and citric acids[END_REF] and tartaric [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Juang | Comparison of extraction equilibria of succinic and tartaric acids from aqueous solutions with tri-n-octylamine[END_REF][START_REF] Marinova | Separation of tartaric and lactic acids by means of solvent extraction[END_REF][START_REF] Marchitan | Modeling and optimization of tartaric acid reactive extraction from aqueous solutions: A comparison between response surface methodology and artificial neural network[END_REF] acids, C3 monocarboxylic acids like acrylic [START_REF] Tuyun | Reactive extraction of acrylic acid using trioctylamine (TOA) in versatile diluents[END_REF], propionic [START_REF] Wang | Equilibrium study on reactive extraction of propionic acid with N1923 in different diluents[END_REF][START_REF] Kumar | Experimental Data and Theoretical (Chemodel Using the Differential Evolution Approach and Linear Solvation Energy Relationship Model) Predictions on Reactive Extraction of Monocarboxylic Acids Using Tri-n-octylamine[END_REF][START_REF] Keshav | Reactive extraction of propionic acid using tri-n-octylamine[END_REF][START_REF] Aşçı | Extraction equilibria of propionic acid from aqueous solutions by Amberlite LA-2 in diluent solvents[END_REF] and pyruvic [START_REF] Senol | Influence of diluent on amine extraction of pyruvic acid using Alamine system[END_REF][START_REF] Marti | Equilibrium and Kinetic Studies on Reactive Extraction of Pyruvic Acid with Trioctylamine in 1-Octanol[END_REF][START_REF] Pal | Reactive Extraction of Pyruvic Acid Using Tri-n-octylamine Diluted in Decanol/Kerosene: Equilibrium and Effect of Temperature[END_REF] acids and even lactic acid, an α-hydroxylated C3 monocarboxylic acid (positional isomer of 3-HP) [START_REF] Prochazka | Amine extraction of hydroxycarboxylic acids. 2. Comparison of equilibria for lactic, malic, and citric acids[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Juang | Equilibrium studies on reactive extraction of lactic acid with an amine extractant[END_REF]. Amine basicity, steric hindrance and water solubility are relevant parameters and have an influence on extraction performances. For example, Kyuchoukov and Yankov [START_REF] Kyuchoukov | Lactic Acid Extraction by Means of Long Chain Tertiary Amines: A Comparative Theoretical and Experimental Study[END_REF] tested several amines for the extraction of lactic acid and their results show that, when amines are used in oleyl alcohol, extraction yields decrease when the steric hindrance of the amines increases. The contrary is observed when amines are used without oleyl alcohol as the active diluent. Kaur and Elst [START_REF] Kaur | Development of reactive extraction systems for itaconic acid: a step towards in situ product recovery for itaconic acid fermentation[END_REF] studied a wide range of amines and diluents for the extraction of itaconic acid. For all the diluents tested, they found that short chain amines (tributylamine, N,N-dimethyloctylamine) and primary amines (n-octylamine) had the lowest performances. On the contrary, long chain secondary and tertiary amines proved to be very efficient with very high extraction yields, probably due to their higher basicity and very low solubility in water. Secondary amines (di-n-octylamine) had better performances than tertiary amines (tri-n-octylamine) [START_REF] Kaur | Development of reactive extraction systems for itaconic acid: a step towards in situ product recovery for itaconic acid fermentation[END_REF], and this could be explained by the lesser extent of the steric hindrance around the reactive amine group, their higher basicity and higher density of amine groups.

Similar results were found in a previous study which also demonstrated that primary amines were not to be used in reactive extraction due to moderate extraction efficiency, high water solubility and high interfacial activity causing emulsion [START_REF] Reschke | Reactive Extraction of Penicillin II: Distribution Coefficients and Degrees of Extraction[END_REF].

The structure of the aliphatic amines is crucial for their role as extractants, but they cannot be used alone, due to their poor solvation ability and sometimes high viscosity [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretations of Data[END_REF]. Consequently, they are often used in mixture with organic solvents to increase their extraction performances. Diluents can be divided into two types: active and inert. Active diluents have chemically active functional groups such as hydroxyl or carbonyl groups that are able to interact with the amphiphilic acid-base complex through hydrogen bonds and dipole-dipole interactions [START_REF] Kertes | Extraction Chemistry of Fermentation Product Carboxylic Acids[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF]. Inert diluents such as hydrocarbons that cannot interact with diluted molecules show lower efficiency since they are not able to stabilize hydrophilic compounds in the organic phase. Solvent polarity decrease, e.g. when the length of the alkyl chain increases, tends to reduce the extraction efficiency and so does the steric hindrance around the active group. For example, secondary alcohols have lower solvation capacity than their primary isomers [START_REF] Kaur | Development of reactive extraction systems for itaconic acid: a step towards in situ product recovery for itaconic acid fermentation[END_REF][START_REF] Uslu | Reactive Extraction of Levulinic Acid by Amberlite LA-2 Extractant[END_REF].

Apart from the effects of amines and diluents' characteristics, the acid hydrophobicity [START_REF] Matsumoto | Synergistic extraction of organic acids with tri-n-octylamine and tri-n-butylphosphate[END_REF] and acidity [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretations of Data[END_REF] are key factors to evaluate the degree of extraction: the more hydrophobic and acidic the extracted acid, the higher the extraction yield [START_REF] Senol | Extraction equilibria of nicotinic acid using Alamine 336 and conventional solvents: effect of diluent[END_REF]. Differences in extraction yields even exist when comparing very similar acids like the positional isomers 3-hydroxypropionic acid and 2hydroxypropionic acid (lactic acid) [START_REF] Moussa | Reactive extraction of 3-hydroxypropionic acid from model aqueous solutions and real bioconversion media. Comparison with its isomer 2-hydroxypropionic (lactic) acid[END_REF]. Moreover, hydrophilic carboxylic acids being polar, they tend to bind together in apolar environments such as organic phases. It is known for example that they can form dimers, trimers, and so on [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretations of Data[END_REF]. This phenomenon is less likely to occur in more polar active diluents. In particular, protic diluents like alcohols or chloroform are known to prevent such phenomena because of their H-bond donor ability. The competition between the acid and the diluent to form H-bonds with free or complexed acid molecules tends to favor acid extraction through free and complexed acid monomers [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretations of Data[END_REF]. All the characteristics of the compounds found in these systems play an important role on the mechanisms of extraction.

Considering all these aspects, should be preferred for 3-HP extraction a water insoluble amine (high molecular weight) providing a high density of non-hindered reactive amine groups (low molecular weight). The optimum stays in the tertiary C8-C10 amines range [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF]. Therefore, we selected tri-noctylamine (TOA) to perform this study and investigated several diluents according to their physicochemical properties.

Since the purpose of an ISPR strategy is to keep the concentration of a product low to limit inhibition, in this study, 3-HP concentration will cover a rather dilute range of 0.0028 to 0.56 mol/L (0.25 to 50 g/L). The study focuses on phenomena occurring at low concentrations and related mechanisms such as the presence of impurities coming from the extractant phase.

Materials and methods

Theory

Physical extraction

Reaction 1 describes the acid partitioning between the aqueous and organic phases:

𝐴𝐻 𝑎𝑞 ⇌ 𝐴𝐻 𝑜𝑟𝑔 (𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 1)

Reactive extraction

The formation of acid-amine complexes is the major reaction involved in acid recovery from aqueous phase. In an aqueous phase, acids and bases can react together through reversible reactions to give the corresponding solvated free conjugated bases and acids. In an organic phase, the weak polarity and dielectric constant can prevent species from forming the corresponding solvated free ions and the acidic and basic molecules can stay bound to each other through acid-base interactions. These interactions can consist in H-bondings and/or ion pair formation according to the polarity of the solvent and the strength of the acid-base interactions. As a general scheme, several acid and amine molecules can form aggregates of different stoichiometries [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF]. The acid-amine complex formation can be described by the reaction 2:

𝑝 𝐴𝐻 𝑎𝑞 + 𝑞 𝑅 3 𝑁 𝑜𝑟𝑔 ⇌ �(𝐴𝐻) 𝑝 (𝑅 3 𝑁) 𝑞 � 𝑜𝑟𝑔 (𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 2)

Chemicals

Commercial products

Solutions of 3-hydroxypropionic acid were prepared by diluting a commercial product (28.1%wt solution in water) from TCI Europe, Zwijndrecht, Belgium. The organic phase consisted in tri-noctylamine (TOA, 98% purity) diluted either in n-hexanol (99% purity), n-octanol (99% purity), ndecanol (99% purity), n-dodecanol (98% purity), ethyl octanoate (98% purity), butyl hexanoate (98% purity), n-hexane (99% purity), n-decane (99% purity), n-dodecane (99% purity) from Sigma-aldrich, Saint-Louis, the USA, oleyl alcohol (82.4% purity, with impurities consisting mainly in other fatty alcohols like linoleyl and stearyl alcohols [START_REF] Swern | Purification of oleic acid, methyl oleate, and oleyl alcohol for use as chemical intermediates[END_REF][START_REF] Liebert | Final Report on the Safety Assessment of Stearyl Alcohol, oleyl Alcohol, and Octyl Dodecanol[END_REF]) from TCI Europe or refined sunflower oil (mainly triglycerides of oleic and linoleic acids, Lesieur, France).

In order to further investigate the influence of TOA purity on extraction efficiency, three different organic phases have been made with three different TOA purities. Two TOA solutions were purchased from the same commercial reference but had two lot numbers with different purity levels corresponding to the variations of production quality: 98.1%, solution 1, and 99.6%, solution 2, according to the certificates of analysis given by the supplier (Sigma-Aldrich). Solution 3 was obtained by purification of solution 2 as detailed below.

TOA purification

Purified TOA (solution 3) was produced as follows. The 99.6% TOA was washed using same volumes of H 2 SO 4 0.1 M. After centrifugation, 3 homogeneous phases were observed. Only the top phase was recovered and further washed with NaOH 0.3 M. Indeed, the phase in-between is suspected to be loaded in acid-amine complexes [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF] and to be sure that a maximum of impurities were removed from the TOA phase, only the top phase was recovered. These operations were repeated twice and the last TOA top phase was finally washed with deionized water. The pH of the final aqueous phase has been checked to be neutral, proving that no inorganic salts were left in purified TOA and this latter is called solution 3.

Experiments

Several experiments of reactive liquid-liquid extractions were performed for different initial concentrations of TOA and 3-HP. Unless specified, TOA of solution 2 (99.6% purity) was used. For each extraction, 10 mL of the aqueous phase and 10 mL of the organic phase were mixed in centrifuge tubes which were then manually shaken for 3 minutes. The stable emulsion formed was then left at 25 °C for one day before being shaken again for 3 minutes and centrifuged at 15557 g for 1 hour. Once the aqueous phase recovered, it was analyzed using High Performance Liquid Chromatography (HPLC) and the extraction yield was calculated as follows: 𝐴𝐻 ⇌ 𝐴 -+ 𝐻 + 𝐾 𝐴 𝐴𝐻 = 3.09 10 -5 [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] (𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 3)

𝑌 = [𝐴𝐻] 𝐻𝑃𝐿𝐶 𝑖𝑛𝑖 -[𝐴𝐻] 𝐻𝑃𝐿𝐶 𝑒𝑞 [𝐴𝐻] 𝐻𝑃𝐿𝐶 𝑖𝑛𝑖 (1
𝑝𝐻 𝑡ℎ = -log ⎝ ⎛ -𝐾 𝐴 𝐴𝐻 + �(𝐾 𝐴 𝐴𝐻 ) 2 + 4𝐾 𝐴 𝐴𝐻 [𝐴𝐻] 𝐻𝑃𝐿𝐶 2 ⎠ ⎞ (2)
All the extractions were made in duplicates. For some experiments, organic phases have been back

extracted with NaOH solutions (0.1-0.5 M) in order to confirm that mass balance was satisfied within 3% or less.

Experiments were centered on 0.011 mol/L (1 g/L) 3-HP and the optimal extractant concentration for this condition (0.46 mol/L = 20% v/v TOA) [START_REF] Moussa | Reactive extraction of 3-hydroxypropionic acid from model aqueous solutions and real bioconversion media. Comparison with its isomer 2-hydroxypropionic (lactic) acid[END_REF]. The initial acid concentration ranged from 0.0028 mol/L to 0.56 mol/L (0.25 to 50 g/L) for 0.46 mol/L (20% v/v) TOA in order to clearly observe limiting factors at low concentrations while having good quantification results in HPLC. Concerning the initial concentrations of amines, the whole range from 0 to 100% v/v (0 -2.3 mol/L) was studied with steps of 10%v/v (0.23 mol/L) for 0.011 mol/L (1 g/L) 3-HP.

Analytical methods

The 3-HP concentration in aqueous phases was determined using HPLC containing an Aminex HPX-87H H + exchange column (300x7.8 mm, Biorad, USA) and an H + Micro-Guard column (30mmx4.6mm, Biorad, USA). The mobile phase consisted in a 0.01N H 2 SO 4 solution circulating at a flowrate of 0.6 mL/min and the column temperature was set to 50 °C. An internal standard consisting in 10 g/L citric acid in water was added (50% v/v) to the samples and 20 µL of each sample were used for injection.

Signals were obtained using a UV detector with the specific wavelength of 210 nm for acids. The overall HPLC analysis uncertainty, defined as the coefficient of variation of the internal standard peak area, was evaluated to 1%.

Impurities identification and titration in the aqueous phase were performed using Ultra High

Performance Liquid Chromatography coupled with High Resolution Mass Spectroscopy (UHPLC-HRMS). Separations were performed using a Hypersil Gold C18 column (50X2.1 mM; 1.9 µm particle size; Thermo Fisher Scientific, USA) with an Ultimate 3000 pump and Thermo 3000 RS autosampler (Thermo Fisher Scientific, USA). The oven temperature was set to 25°C. The mobile phase was a mixture of LC/MS grade acetonitrile and water loaded with 0.1% formic acid (Optima, Thermo Fischer Scientific, USA) flowing at 250 µL/min. In a run, it first consisted in 2% acetonitrile for 4 minutes.

Then a gradient was applied to reach 98% acetonitrile in 6 minutes and the mixture was then stabilized at 98% acetonitrile for 5 minutes. The mobile phase was finally set back to initial conditions (2% acetonitrile) with a 2 minutes gradient.

Samples were diluted 100-fold in water (Optima LC/MS grade, Thermo Fischer Scientific, USA) and filtered before assay with syringe filters (nylon membrane, 0.2 μm porosity and 13 mm diameter, Merck Millipore, USA).

MS analysis was carried out with a Q-EXACTIVE Orbitrap mass spectrometer (Thermo Fisher Scientific, USA) in positive mode with a Heated ElectroSpray Ionization (HESI) probe using a high resolution full scan. The MS resolution was 70000 in a full MS mass range (m/z) from 50 to 700 AMU.

The ion transfer tube and vaporizer temperatures were set to 300°C. The electrospray voltage was set to 3 kV and the sheath, auxiliary and sweep gas (nitrogen) flow rates were set to 35, 10 and 0 arbitrary units, respectively.

IR analyses were performed using a Cary 630 Fourier Transform InfraRed spectrometer of Agilent Technologies (California, USA).

pH measurements were obtained from a SevenCompact pHmeter equipped with the Inlab Micro Pro-ISM combined pH electrode with temperature probe (Mettler Toledo, Switzerland).

Water concentration in the organic phase was determined using Karl-Fischer method with a 756 KF Coulometer (Metrohm, Switzerland). Methanol was the solvent for titration. Organic phase samples were analyzed in triplicate.

Statistical significance of differences between data was assessed using ANOVA tests at 0.05 level. First, a set of 11 solvents were selected for a screening regarding the reactive extraction of 0.011 mol/L (1 g/L) of 3-HP in aqueous phase using 0.46 mol/L of purified TOA (20% v/v) in the organic phase. Figure 1 provides the extraction yields for these solvents.

Results and discussion

Solvents screening

Three kinds of solvents were tested: alcohols, esters and alkanes with different lengths of alkyl chain and numbers of carbon atoms. They can be compared according to their extraction ability and physicochemical properties like polarity (dipole moment and relative permittivity) and H-bond ability. The immediate conclusion from the observation of Figure 1 is that alcohols have the highest extraction yields. Among alcohols, the shorter the chain, the higher the extraction yield: 91% for the C 6 hexanol, 53% for the C 18 oleyl alcohol. For alkanes and esters, the yield is independent of the chain length (ANOVA test with p=0.6 for both), which means that the chain length cannot be used as a good parameter to discriminate solvents. If all the n-alcohols have approximately the same dipole moment (i.e "molecular polarity") and H-bond ability due to the same primary hydroxyl groups (Table 1), they differ in their relative permittivity that is more related to the "bulk polarity" and hence to the molecular size and organization. In this case, the difference in solvent activity is given by their relative permittivity so that the smaller the n-alcohol the higher the yield. efficiently. n-hexane, n-decane and n-dodecane having the same values of relevant parameters (dipole moment, relative permittivity and H-bond ability), all provide the same efficiency as expected (5% yield). The same observations apply to the esters: both esters having the same relevant properties, they show equivalent performances (around 8% yield). This slightly, but significantly higher yield (ANOVA test with p<0.02), can be explained by the slightly higher polarity of the two esters compared to the three alkanes, which makes them a bit more active diluents. It is also interesting to compare esters and alcohols with similar polarities but different H-bond abilities like butylhexanoate and ethyloctanoate vs. oleyl alcohol and n-dodecanol. A much higher yield is obtained for both alcohols (7 times higher for oleyl alcohol than for the esters), which leads to the conclusion that the H-bond donor character of the solvent plays a key role, implying that alcohols stabilize the reactive system through H-bonding interactions. Ricker [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF] suggests that the acidic hydrogen of the carboxylic acid being involved in the acid-amine bond, carboxyl becomes a strictly acceptor group. This would explain such a difference between H-bond donors and acceptors as solvents of otherwise similar polarities. Similar results in the literature confirm the importance of the solvent polarity and specific H-bond donor character to favor the complex formation and increase the extraction yields [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Aşçı | Extraction equilibria of propionic acid from aqueous solutions by Amberlite LA-2 in diluent solvents[END_REF][START_REF] Kaur | Development of reactive extraction systems for itaconic acid: a step towards in situ product recovery for itaconic acid fermentation[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretations of Data[END_REF].

Based on these results, n-decanol and oleyl alcohol were chosen for further investigations. n-hexanol and octanol were ruled out due to their excessive solubility in water that is expected to be toxic for microorganisms under integrated extractive bioconversion (ISPR) [START_REF] Matsumoto | Toxicity of ionic liquids and organic solvents to lactic acid-producing bacteria[END_REF][START_REF] Vermuë | Toxicity of homologous series of organic solvents for the gram-positive bacteria Arthrobacter and Nocardia Sp. and the gramnegative bacteria Acinetobacter and Pseudomonas Sp[END_REF]. According to the literature review, some biocompatible vegetable oils, thanks to their low polarity and solubility in water, have been shown to be quite efficient for the removal of similar acids (propionic, pyruvic, lactic) from aqueous phases [START_REF] S� Liwiok | Spectroscopic and dielectric examination of the oleyl alcohol-oleic acid system[END_REF][START_REF] Hawranek | Infra-red dispersion of liquid trioctylamine[END_REF][START_REF] Yaws | Thermophysical Properties of Chemicals and Hydrocarbons, Chapter 19 Dipole moments -Organic compounds[END_REF]. But in our present results, sunflower oil was found to be inefficient for 3-HP extraction with TOA (less than 7% yield) whatever the proportions used because of its weak solvation properties.

Physical extraction

For physical extraction experiments, concentration of 3-HP were varied from 0.0028 to 0.56 mol/L (0.25 to 50 g/L) with no TOA in the organic phase. In n-decanol, a linear dependence (R²=0.96) of the equilibrium acid concentration in the organic phase with respect to that in the aqueous phase was found (results not shown). The slope, i.e. the partition coefficient (defined as the ratio of the molar concentrations at equilibrium), was evaluated to 0.020±0.001 by linear regression, meaning that physical extraction yield was around 2%. This is in line with Kertes and King's statement that the acid partition coefficient is independent on its concentration in the case of extraction using alcohols [START_REF] Kertes | Extraction Chemistry of Fermentation Product Carboxylic Acids[END_REF].

In oleyl alcohol, 3-HP partitioning was not detectable. Physical extraction yields were thus too low for practical purposes, confirming that the efficient liquid-liquid extraction of 3-HP needs to be driven by a chemical reaction. Figure 2 shows the extraction yield as a function of initial TOA concentration in the organic phase for 0.011 mol/L (1 g/L) of 3-HP initially in water, with n-decanol and oleyl alcohol as solvents. For both solvents, a bell-shaped profile can be observed. This profile illutrates the synergistic effect between TOA and these active diluents and thus the major role of the solvent. When used alone, neither TOA nor the solvents are able to extract 3-HP efficiently (less than 10% yield). However, a strong synergistic effect was observed when TOA was mixed with an active diluent. Indeed, TOA has poor solvation abilities to accumulate complexes whereas n-decanol and oleyl alcohol are not hydrophilic enough to extract such a hydrophilic compound in substantial proportions. When the amine and the solvent are used together, new hydrophobic properties of the complexed acid and new solvation abilities of the organic phase improve the extraction efficiency. Figure 3 provides IR spectra obtained from two different solutions: Figure 3A shows organic phases consisting in 20% v/v TOA in n-decanol and Figure 3B shows aqueous phases of the 3-HP commercial solution for comparison. When TOA is present in the organic phase (Figure 7A), characteristic infrared bands between 1300 and 1800 cm -1 prove the existence of a delocalized carboxylate group in the organic phase demonstrating strong interactions between the acidic proton of 3-HP and the amine leading to the formation of an ion pair. This observation is related to the formation of complexes involving 3-HP bonded to TOA. The fatty amine provides hydrophobic properties to the species formed, enabling a favorable distribution of the acid towards the organic phase. However, the anion, being such a polar part, needs a polar counterparts in the medium to be stabilized.

Effects of initial amine concentration

Alcohols, with polar and H-bond donor hydroxyl groups, are thus able to stabilize the hydrophilic anionic conjugate in the organic phase.

As a consequence, the extraction yield of 3-HP decreases at high amine concentrations, a behavior that aligns well with reported data for acetic [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF], lactic [START_REF] Kyuchoukov | Lactic Acid Extraction by Means of Long Chain Tertiary Amines: A Comparative Theoretical and Experimental Study[END_REF][START_REF] Choudhury | Study of lactic acid extraction with higher molecular weight aliphatic amines[END_REF], malonic [START_REF] Tamada | Extraction of Carboxylic Acids by Amines Extractant[END_REF], propionic [START_REF] Keshav | Reactive extraction of propionic acid using tri-n-octylamine[END_REF], succinic [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids by Amines Extractant[END_REF] and tartaric [START_REF] Marinova | Separation of tartaric and lactic acids by means of solvent extraction[END_REF] acids in active diluents. This behavior is not observed in the case of inert diluents in which the yield increases along with the amines concentration [START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF]. The greatest extent of extraction of 3-HP is reached around 0.46 mol/L (20% v/v) TOA in both active diluents at 0.011 mol/L

(1 g/L) of 3-HP initially with 68% and 51% yields for n-decanol and oleyl alcohol respectively. The maximum of the extraction yield corresponds to the optimal composition of the extracting phase. Moreover, water solubility appears to play a crucial role. As shown in Figure 4, the more the organic phase is loaded with TOA, the less water is soluble, water being practically insoluble in TOA.

Increasing the amount of TOA leads to a decrease of water content in the organic phase which could lead to a lesser stabilization of the complex and hence a lesser extraction efficiency. Tamada et al [START_REF] Tamada | Extraction of Carboxylic Acids by Amines Extractant[END_REF] reported that the amount of acid extracted and the stoichiometry of the complex in the organic phase depended on the water content in the organic phase. The amount of water soluble in the organic phase is supposed to stabilize the hydrophilic part of the complex in a rather apolar environment. Increasing the concentration of alcohol in the organic phase increases the water solubility and therefore the solvation effect of alcohols could be in fact partially due to the solvation of water. The balance between the respective contributions in complex stabilisation cannot be assessed based on the present results because the amounts of water and alcohol are correlated. Concerning the influence of the acid concentration, Figure 5 shows that the plot of the extraction yield against initial acid concentration is not monotonic. If we consider only the partitioning of the acid and the complex formation, such a profile is not expected. Indeed, the more diluted is the acid and the more in excess are the amines; this should lead to a higher extent of reaction 2 (acid-amine complex formation) and therefore the increase of extraction yield with the decrease of 3-HP concentration. This behavior was indeed observed for initial acid concentration levels between 0.11 and 0.56 mol/L (10 -50 g/L). But for the weakest concentration levels, a sharp decrease is observed and other phenomena are needed to explain such results. For example, in oleyl alcohol the extraction yield decreased from 51% to 40% and in n-decanol from 72% to 51% between 0.023 and 0.0041 mol/L (2 -0.4 g/L) of 3-HP initially. Deeper investigations on pH effects were undertaken in the case of n-decanol to understand these phenomena (section 3.5.). A further look at the water content of the organic phase after extraction proves that water is extracted in the organic phase jointly with the acid (Figure 6). Indeed, the more acid extracted, the more water in the organic phase. Indeed, water concentration reached higher amounts than its solubility in n-decanol with 20%v/v TOA (1.14 mol H 2 O/L, Figure 4) and in oleyl alcohol with 20%v/v TOA (0.50 mol H 2 O/L). Figure 6 shows that the amount of water in the organic phase varies linearly with the acid concentration. This tends to prove that, in the range studied, each 3-HP molecule carries the same average number of water molecules in the organic phase. This hydration number (given by the slopes of the regression lines in Figure 6) is around 2 for n-decanol and oleyl alcohol as solvents. This tendency of 3-HP to carry water molecules while being extracted in the organic phase could also partly explain why the extraction yield was very low with alkanes (section 3.1). Results are shown in Table 2 (solution 2). Considering the pH at the equilibrium in the aqueous phase ammonia and OA leading to impurities of around 2% DOA and 1% OA [START_REF]Encyclopedia of Chemical Technology, Alkanolamines to Antibiotics[END_REF]. This tends to confirm the presence of these impurities in the products in our study.

Effects of initial acid concentration

Influence of TOA impurities on extraction performances

pH at equilibrium

When acidic species are present in the aqueous phase, these TOA impurities (OA and DOA) undergo protonation and both deprotonated acid molecules and protonated amine molecules are released in the aqueous phase increasing the pH (Table 2, solutions 1 and 2). This phenomenon can be neglected at high acid concentration because impurities amount remains relatively low and the protonated form of TOA is practically insoluble in water [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF] so that relatively low amount of acid is neutralized in the aqueous phase. Consequently, the increase of the dissociated part of 3-HP is significant only for low concentrations and this phenomenon significantly decreases the extraction yield only for concentrations under 0.1 mol/L (10 g/L, Figure 5).

Potentiometric titration with colorimetry is often used to determine extraction yields in reactive

extraction with phenolphthalein as a pH indictor [START_REF] Keshav | Reactive Extraction of Citric Acid Using Tri-n-octylamine in Nontoxic Natural Diluents: Part 1-Equilibrium Studies from Aqueous Solutions[END_REF][START_REF] Datta | Reactive Extraction of Glycolic Acid Using Tri-n-Butyl Phosphate and Tri-n-Octylamine in Six Different Diluents: Experimental Data and Theoretical Predictions[END_REF][START_REF] Uslu | Extraction of aqueous of malic acid by trioctylamine extractant in various diluents[END_REF][START_REF] Tamada | Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling[END_REF][START_REF] Kumar | Experimental Data and Theoretical (Chemodel Using the Differential Evolution Approach and Linear Solvation Energy Relationship Model) Predictions on Reactive Extraction of Monocarboxylic Acids Using Tri-n-octylamine[END_REF][START_REF] Pal | Reactive Extraction of Pyruvic Acid Using Tri-n-octylamine Diluted in Decanol/Kerosene: Equilibrium and Effect of Temperature[END_REF]. However, the usual color turning point of phenolphthalein is at pH=8.2 [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] which means that acidic species at pH > 8.2 are not titrated. The pKa of n-octylammonium is 10.65 [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] which means that OA remains under its acidic form (n-octylammonium) above pH=8.2 and is not titrated. It implies that acidic species neutralized by n-octylamine are not taken into account in this titration method. From our results, it can be concluded that pH colorimetric titration is not appropriate to determine extraction yields when amines are used as extractants because it could lead to underestimated acid concentration in the aqueous phase and hence overestimating the yields, especially for low acid concentration.

Influence of TOA impurities on the extraction mechanisms

Three TOA solutions with different purity levels have been used in order to evaluate the impact of TOA impurities on the extraction process.

Analyses of aqueous phases after extraction proved that no significant TOA were released in the aqueous phases. However, OA and DOA were identified and quantified, with the former as the main amine in the aqueous phase (more than 90% of total amines concentration). Hence, total amount of amines (OA+DOA) in the aqueous phase is assimilated to OA to simplify further interpretation. The amount of OA varied between 0.031 and 9.1 mmol/L (Table 2), depending on the following operating conditions: TOA initial concentration in the organic phase, purity of TOA and 3-HP initial concentration in the aqueous phase. Amine transfer into the aqueous phase increases the pH. Accordingly, an amended formula should be used to calculate the pH by taking into account the amines. Knowing the total concentration of amines in the aqueous phase and their dissociation constants, the equilibrium pH of 3-HP solutions can then be calculated. If we consider that amines in the aqueous phase mainly consist in OA, the theoretical pH calculation becomes:

𝑝𝐻 𝑡ℎ,𝑂𝐴 = -log([𝐻 + ]) (3)
[H + ] being the physical solution of the following equation ( 4), based on 3-HP dissociation (reaction 3), n-octylammonium dissociation (reaction 5), water autoprotolysis (reaction 6) and the electroneutrality principle:

𝑂𝐴𝐻 + ⇌ 𝑂𝐴 + 𝐻 + 𝐾 𝐴 𝑂𝐴 = 2.24 10 -11 [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] (𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 5)

𝐻 2 𝑂 ⇌ 𝐻 + + 𝐻𝑂 - 𝐾 𝑤 = 10 -14 [START_REF] Lide | CRC Handbook of Chemistry and Physics[END_REF] (𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛 6)

𝐾 𝐴 𝐴𝐻 [𝐴𝐻] 𝐻𝑃𝐿𝐶 [𝐻 + ] + 𝐾 𝐴 𝐴𝐻 + 𝐾 𝑤 [𝐻 + ] - [𝐻 + ][𝑂𝐴] 𝐿𝐶𝑀𝑆 𝐾 𝐴 𝑂𝐴 + [𝐻 + ] -[𝐻 + ] = 0 (4)
As can be seen in table 2, values of pH th,OA are much closer to the pH measured experimentally.

Accordingly, the deviations observed between pH eq and pH th are directly related to the impurities in commercial TOA. Moreover, pH measurement can give, as a first approximation, reliable information about the amount of impurities (OA) transferred into the aqueous phase.

Since only non-dissociated 3-HP can be extracted by TOA in the organic phase, amines transfer into the aqueous phase and the consequent increase in pH eq reduces the amount of protonated 3-HP available for extraction. The final outcome of this phenomenon is the decrease of the extraction yield. Yield reduction caused by TOA impurities is illustrated in figures 5 and 7A for low acid concentrations.

In Figure 7A, the plot is given in logarithmic scale for initial 3-HP concentration in order to clearly observe the discrepancies for the small acid concentrations and the similarities for high acid concentrations. For a given TOA concentration in the organic phase (0.46 mol/L = 20% v/v), the three solutions provide equivalent yields at high acid concentration (>0.11 mol/L = 10 g/L). This was expected since the amount of amines released in the aqueous phase is small compared to the acid concentration, and its influence on the yield becomes negligible. However, for low acid concentrations, OA and DOA impurities present in TOA solutions have a strong impact. For example, at around 5mmol/L (0.5 g/L) 3-HP initially, the extraction yield is 23% for solution 1 (2.5 mmol/L OA), 58% for solution 2 (1.0 mmol/L OA) and 74% for solution 3 (0.0058 mmol/L OA). The yield decrease at low acid concentration with commercial TOA (solutions 1 and 2) is dramatic whereas with purified TOA (solution 3) the decrease is much smaller. Nevertheless, it still denotes the release of weak amounts of impurities, but with much lower importance than from unpurified TOA. In figure 7B, one can see a much smaller yield decrease for purified TOA (solution 3) than for solution 1 along with the increase of amine concentration, up to about 50% v/v TOA. This steeper decrease for unpurified TOA is due to impurities release, whereas for purified TOA only the solvating effect is noticeable.

In the case of purified TOA, results shown in Table 2 confirm that the purification of TOA was effective, dividing the amount of amines released in the aqueous phase by a factor 30 when compared to solution 2 and a factor 65 when compared to solution 1. This decrease makes the amount of amines practically negligible in the aqueous phase, as confirmed by pH measurements.

Besides the effects described above on the extraction chemistry, the diminution of the released amines by means of the purification step is particularly recommended if microorganisms are used in a biological process due to expected amine toxicity. For example, a similar lactate -long chains alkyl ammonium salt in an aqueous phase proved to be very toxic for several kinds of microorganisms [START_REF] Cybulski | Long-Alkyl-Chain Quaternary Ammonium Lactate Based Ionic Liquids[END_REF]. Purification of fatty tertiary amines can be reached for example with an adsorption process as described in a US patent [START_REF] Coenen | Purification of tertiary amines using an adsorbent[END_REF]. In continuous chemical processes, however, the presence of impurities would have less impact because the recycled organic phase would be washed by the continuous fresh aqueous flow and small amines would be readily depleted [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF].

Conclusion

This paper reports a comprehensive study of the reactive extraction of 3-HP, a valuable platform molecule. The experimental approach was designed in line with a potential application of reactive extraction for the in situ product recovery of this acid. It appeared from the solvent screening experiments, performed for the first time on 3-HP, that the type of solvent has a great influence on the extraction yield, with H-bond donor characteristic and polarity being the most important properties to increase the yield. This highlights strong interactions between the solvent and the complexes formed between acids and amines. Water solubility in the organic phase also seems to be

  ) with [𝐴𝐻] 𝐻𝑃𝐿𝐶 𝑖𝑛𝑖 being the total initial acid concentration in the aqueous phase according to HPLC analysis and [𝐴𝐻] 𝐻𝑃𝐿𝐶 𝑒𝑞 the same at equilibrium. pH of the resulting aqueous phases was measured and compared to the theoretical pH value, based on the 3-HP dissociation (reaction 3) and calculated as follows:

Figure 1 :

 1 Figure 1: Extraction yields for different solvents with 0.46 mol/L of TOA (20% v/v) and 0.011 mol/L of 3-HP (1 g/L). Error bars represent standard deviations (n=2).

Figure 2 :

 2 Figure 2: Extractions yields as a function of TOA concentration in decanol (open circles) and oleyl alcohol (filled circles), with 0.011 mol/L (1g/L) of 3-HP initially. Error bars represent standard deviations (n=2).

Figure 3 :

 3 Figure 3: IR spectra of organic phases after 3-HP extraction (53g/L in the organic phase, bold line) and after simple water contact (thin line) (A) and IR spectra of commercial aqueous phases of 3-HP before (bold line) and after (thin line) neutralization with NaOH (B)

Figure 4 :

 4 Figure 4: Water solubility in the organic phase as a function of TOA concentration (0 -2.3 mol/L) in decanol at 25°C. Error bars represent standard deviations (n=3) and are masked by the symbols.

Figure 5 :

 5 Figure 5: Extraction yields in decanol (open circles) and oleyl alcohol (filled circles) with 0.46 mol/L TOA (20% v/v) as a function of initial acid concentration. Error bars represent standard deviations (n=2).

Figure 6 :

 6 Figure 6: water co-extraction in decanol (open circles) and oleyl alcohol (filled circles) with 0.46 mol/L TOA (20% v/v) as a function of the acid concentration in the organic phase after extraction. Error bars represent standard deviations (n=3) and are masked by the symbols.

Figure 7 :

 7 Figure 7: Extraction yields for different TOA solutions (squares: solution 1 (98.1% purity); circles: solution 2 (99.6% purity); triangles: solution 3 (purified TOA)) in decanol as a function of initial acid concentration at 0.46 mol/L TOA (20% v/v) (A) and initial TOA concentration at 0.011 mol/L 3-HP (1 g/L) (B). Error bars represent standard deviations (n=2).

a

  major factor in complex stabilization. In active diluents like n-decanol and oleyl alcohol, a bellshaped profile of the extraction yield as a function of amine concentration is observed with very low extraction yields at high amine concentrations. This proves the major role of the solvent in the extraction process. In these solvents, the increase in initial acid concentration from 0.003 up to 0.1 mol/L (0.25 -10 g/L) led to an increase in extraction yield while a further increase from 0.1 to 0.56 mol/L (10 -50 g/L) led to the reduction of the extraction yield. This phenomenon has been shown to be due to amines transfer in the aqueous phase, with significant impact for low acid concentrations.Deviation of equilibrium pH from what should be expected with the remaining acid in the aqueous phase demonstrated the presence of impurities. We identified the release of n-octylamine and di-noctylamine coming from commercial TOA into the aqueous phase even with 99.6% purity. This transfer reduces extraction yields at low initial acid concentrations and increases equilibrium pH which is, accordingly, a reliable indicator of amines concentration in water. Given the extraction yield reduction and the suspected toxicity for microorganisms performing bioconversions, purification of TOA before use could be necessary. These results give insights in extraction mechanisms of organic acids, further clarifying the role of the organic solvent and water in the stabilization of the acid-amine complex and pointing out the role of the amine impurities in extraction yield reduction and biocompatibility of the extraction process. All this understanding is essential for further mechanistic and predictive modeling.

Table 1 :

 1 Physicochemical properties of the studied solvents[START_REF] S� Liwiok | Spectroscopic and dielectric examination of the oleyl alcohol-oleic acid system[END_REF][START_REF] Hawranek | Infra-red dispersion of liquid trioctylamine[END_REF][START_REF] Yaws | Thermophysical Properties of Chemicals and Hydrocarbons, Chapter 19 Dipole moments -Organic compounds[END_REF][START_REF] Marcus | Wiley series in solution chemistry: volume 4, The properties of solvents[END_REF]

a approximate values from pentylpentanoate b approximate values from octylacetate c estimated values from EPI Suite TM estimation program (Syracuse Research Corporation and the U.S. Environmental Protection Agency; copyright 2000).

Table 2 :

 2 amount of amines (OA+DOA) found in the aqueous phase after extraction and associated measured (𝑝𝐻 𝑒𝑞 ) and calculated (𝑝𝐻 𝑡ℎ and 𝑝𝐻 𝑡ℎ.𝑂𝐴 ) pH, as a function of initial for

	solutions 1 (98.1% of TOA purity), 2 (99.6% of TOA purity) and 3 (>99.6%: purified TOA)
			Solution 1 (TOA 98.1%)
	[𝐴𝐻] 𝑖𝑛𝑖	[𝑇𝑂𝐴] 𝑖𝑛𝑖	[𝑂𝐴] 𝐿𝐶𝑀𝑆	𝑝𝐻 𝑒𝑞	𝑝𝐻 𝑡ℎ	𝑝𝐻 𝑡ℎ,𝑂𝐴
	(mol/L)	(mol/L)	(mmol/L)
	0.0028	0.46	2.0	5.4	3.6	5.1
	0.011	0.46	2.6	4.6	3.4	4.5
	0.029	0.46	2.8	4.1	3.3	4.1
	0.058	0.46	2.7	3.8	3.1	3.8
	0.10	0.46	2.5	3.5	3.0	3.5
	0.21	0.46	2.9	3.2	2.9	3.3
	0.31	0.46	2.8	3.0	2.8	3.1
	0.50	0.46	2.8	2.8	2.6	2.8
	0.011	0.23	1.4	4.2	3.4	4.2
	0.011	0.69	3.6	4.9	3.4	4.6
	0.011	0.91	4.6	5.0	3.3	4.7
	0.011	1.4	6.4	5.2	3.3	4.9
	0.011	1.8	8.5	5.4	3.3	5.3
	0.011	2.1	8.7	5.4	3.3	5.3
			Solution 2 (TOA 99.6%)
	0.0041	0.46	1.0	4.8	3.6	4.5
	0.011	0.46	1.2	4.3	3.5	4.2
	0.023	0.46	1.2	4.1	3.4	3.9
	0.11	0.46	1.2	3.4	3.0	3.3
	0.23	0.46	1.2	3.2	2.8	3.0
	0.35	0.46	1.2	3.2	2.7	2.9
	0.55	0.46	1.3	2.9	2.6	2.7
	0.011	0.23	0.54	4.1	3.5	3.8
	0.011	0.69	1.5	4.5	3.5	4.3
	0.011	0.91	2.3	4.6	3.4	4.5
	0.011	1.4	3.4	4.5	3.4	4.5
	0.011	1.8	4.9	4.5	3.3	4.5
	0.011	2.1	5.4	4.5	3.3	4.5
		Solution 3 (purified TOA)
	0.0027	0.46	0.065	4.1	3.9	4.0
	0.011	0.46	0.039	3.7	3.6	3.6
	0.021	0.46	0.048	3.5	3.4	3.5
	0.053	0.46	0.040	3.2	3.2	3.2
	0.11	0.46	0.039	3.0	3.0	3.0
	0.21	0.46	0.029	2.8	2.9	2.9
	0.32	0.46	0.034	2.7	2.7	2.7
	0.53	0.46	0.031	2.5	2.6	2.6
	0.011	0.23	0.053	3.5	3.5	3.6
	0.011	0.69	0.062	3.6	3.6	3.6
	0.011	0.91	0.134	3.6	3.6	3.6
	0.011	1.4	0.191	3.6	3.5	3.6
	0.011	1.8	0.319	3.4	3.4	3.5
	0.011	2.1	0.318	3.4	3.3	3.4
	pH of aqueous solutions were measured for after extraction and compared to the calculated values.

Acknowledgements

The authors would like to thank AgroParisTech students of the 189 th promotion and Dr Claire Saulou-Bérion for having provided valuable experimental assistance, and the Région Champagne-Ardenne, the Conseil départemental de la Marne and Reims Métropole for financial support.

(𝑝𝐻 𝑒𝑞 ), it appears that for the lowest acid concentrations the experimental data are above the calculated values from acid concentrations at equilibrium alone (𝑝𝐻 𝑡ℎ , reaction 3, equation 2). This means that higher amounts of 3-HP molecules are deprotonated compared to what is expected with only 3-HP in water. When performing pH titrations of aqueous phases after acetic acid extraction by TOA in chloroform, Wardell and King [START_REF] Wardell | Solvent equilibriums for extraction of carboxylic acids from water[END_REF] also reported pH deviations. They attributed this phenomenon to the transfer of tri-n-octylamine in the aqueous phase in the presence of acid. The same was found for lactic acid extraction at low concentration with Alamine 336 [START_REF] Ju | Characteristics of lactic acid transport in supported liquid membranes[END_REF]. However, Ricker [START_REF] Ricker | Solvent Extraction of Wastewaters from Acetic-Acid Manufacture[END_REF] demonstrated that, even in 10% wt acetic acid in water, TOA solubility would not exceed 10 ppm (i.e. < 3 10 -5 mol/L) and suggested that pH deviation may have been due to impurities in commercial products like primary and secondary amines, lower-molecular-weight tertiary amines and starting materials for the amine synthesis.

When sodium hydroxide was added to our aqueous samples after extraction, a condensation of organic species occurred and the solution became cloudy as already reported by some authors [START_REF] Tamada | Extraction of Carboxylic Acids by Amines Extractant[END_REF][START_REF] Wardell | Solvent equilibriums for extraction of carboxylic acids from water[END_REF]. On the contrary, pure distilled water contacted with commercial TOA solution resulted in a neutral pH without any noticeable visual change when NaOH was added. This illustrates the fact that impurities are extracted in the aqueous phase under protonated form through reactive extraction with acidic species (see reaction 4) and that their partitioning as neutral forms can be neglected:

Identification of TOA impurities

Aqueous samples after extraction were analyzed with mass spectroscopy. Two main peaks appeared at m/z=130 and m/z=242 corresponding to both peaks of protonated n-octylamine (129 + 1 = 130 g/mol) and protonated di-n-octylamine (241 + 1 = 242 g/mol). Commercial samples of n-octylamine (OA) and di-n-octylamine (DOA) were also analyzed to confirm species identification. Several manufacturing processes exist for the production of fatty amines like nitrile hydrogenation or alcohols amination. For example, TOA has been produced by BASF using n-octanol amination with


Isabelle Liotard
email: isabelle.liotard@univ-paris13.fr

FabLab -a new space for commons-based peer production 1

Keywords: FabLab, innovation, collaborative economy, Do It Yourself

Over the last 20 years, companies' innovation practices have been revolutionised with the emergence of Web 2.0 and the possibilities offered by digital technology. Having paved the way with the concepts of open innovation (OI), Chesbrough (2003, 2006) and Von Hippel (2006) have conceptualised a trend that radically alters our classic design and research model. Challenging the traditional principle of innovation, which is internal and "closed", the concept of OI has shed light on brand new practices that aim to boost a company's innovative capacity through its relations and exchanges with the exterior. Even though this concept alone does not cover completely new tools (licence agreements or partnerships and networks are well-known mechanisms), it nonetheless remains that Open Innovation promotes new mechanisms that enable companies to open up to the outside world (in the widest sense of the term). Internetbased digital tools make it possible to create intermediation platforms and websites for companies whose aim is to seek out knowledge, skills and expertise beyond their own borders and beyond their well-identified circles of more or less direct partners (Liotard & Revest, 2017). The great strength of Web 2.0 is, then, to open the company up to the exterior, in the broadest sense because the "the exterior" now includes everyone (Internauts, students, employees, etc.), with the unprecedented characteristic of their having no previous connection with the company in question. Crowdsourcing now gives access to a great number of innovative proposals 2 , and contributes to bottom-up forms of innovation. However, these new practices are not the only ones to emerge, and other formats are now radically transforming innovation's traditional foundations. In particular, spaces known as FabLabs (FL) are currently springing up all over the world. This wave, instigated in 1998 by MIT professor Neil Gerhenfeld, has become widespread, and has led to the constitution of a network of FabLabs in both developed countries and the Global South 3 . Notably, these collaboration spaces, stemming from a desire to share knowledge and openings, call into question production (which becomes local), intellectual property (more open, based on open source files and pooling material), hierarchy (peer communities enable projects to be carried 1 This research takes place in EnCommun program financed by AFD 2 Flowers, Von Hippel, de Jong & Sinozic (2010), measuring rising innovations, esteem that there are probably 2 or 3 times more innovations originating from consumers than from industries themselves. 3

out and FabLabs are emerging as non-hierarchical, horizontal spaces), and lastly, the role of the individual in a certain number of initiatives. These digital manufacturing spaces make digitally-controlled machines and 3D printers available, representing not only a possibility for decentralised production and design for individuals but also offering open production spaces for both small and large companies, which also go there to seek training. The aim of this paper is to shed light on this new type of space and to define the different business models. It is based on a series of interviews we conducted between January and April 2017 with FabManagers from the Paris area and other regions in France.

1) FabLabs -where it all began: MIT's stroke of genius

In the beginning, the Fabrication Laboratory was devised at MIT by Professor Neil Gerhenfeld in 1998 and set up in 2001 with the help of his laboratory, the Center for Bits and Atoms. The aim of this initiative, (which at the start was a training module called How to make (almost) anything, teaching students how to use digitally-controlled equipment), was to pool in a single site equipment to craft and machine materials, and design electronic circuits and microprocessors. The idea, therefore, was based on learning how to use digitallycontrolled equipment to manufacture other machines. It was an immediate success, and to Neil Gerhenfeld's surprise, the course attracted not only engineers but also architecture students and artists (Capdevila, 2015;[START_REF] Bosqué | Fablabs, etc…Les nouveaux lieux de fabrication numérique[END_REF][START_REF] Menichinelli | Fab Lab, la révolution est en marche[END_REF]Mérindol et al, 2016). Its origins are fundamental. On the one hand, the FabLab network was to spread beyond the United States, in particular to developing countries (in India and Africa), thus allowing populations who were not necessarily pro-digital to appropriate this type of space. On the other hand, the founders of MIT's FabLab were to spearhead an architecture created from scratch to organise and manage these third places, going on to set up the FabFoundation and FabAcademy4 , the two pillars supporting the network. The first structure streamlines and manages projects, publishes international FabLab conference archives online and is responsible for coordinating the different spaces; the second manages MIT's training programme for the community so that everyone shares the same grammar and the same principles. In this context, MIT trains the trainers (known as gurus) who will be responsible for training other people in a cascade model. In view of its two structures, MIT plays a central role in the global network of FabLabs. It is the driving force behind the MIT Charter, requested by countless spaces. From the very beginning, the training mission was selected as the system's cornerstone. In addition, MIT is involved in the conferences that are held on the subject every two years. In this way, the community of practice generated by MIT appears to be driven by a dynamic of institutionalisation insofar as the MIT model spreads to and is adopted by the whole world [START_REF] Lhoste | Fablabs : l'institutionnalisation de tiers-lieux du « soft hacking[END_REF].

The MIT Charter Four basic conditions are required to become an FL under the MIT's umbrella: (1) allowing public access to the space: FabLabs must make technology and equipment accessible to everyone. (2) subscribing to the FabLab charter (see Box 1) (FabFoundation). (3) sharing tools (a minimum is required, including 3D printers) and procedures. (4) being part of the global community of FLs and not remaining isolated. (http://fabfoundation.org/). The MIT Charter (which can be found everywhere including on the FabFoundation site) covers a certain number of points. Having given a definition of the space, the Charter explains what should be found in the FabLab and the organisational and governance conditions of this space (what the responsibilities involve and who can use the space). In general, the stated objective of an MIT FabLab is not commercial, even though there are a few cases of projects developed in these spaces that have led to setting up companies. The charter stipulates that knowledge should be made available via open source and open design so that it can be used by anyone, and not bound by IP and patents. Commercial activity is possible, but there are conditions: "Commercial activities can be prototyped and incubated in a fab lab, but they must not conflict with other uses, they should grow beyond rather than within the lab, and they are expected to benefit the inventors, labs, and networks that contribute to their success". There is no formal authorisation from MIT accepting a space as being connected with this charter. As a rule, spaces are asked to register on the Fab Foundation site and self-evaluate their compliance with the four criteria required by the Charter (Botollier-Depois et al, 2014). In actual fact, a FabManager we interviewed stressed the importance of peer recognition and cooptation for FabLab candidates. In order to receive this "approval" or "knighting", the space's founder has to be recognised by other FabManagers. The same goes for the team involved in the candidate site. Peer validation ensures the coherence of the site with regard to the Charter's expectations, and constitutes acceptance. As such, the international conferences held every two years are very important because they enable FabLabs to make themselves known to others and forge relationships with people in the community.

2) The FabLab: a workshop in the middle of town

A FabLab is a workshop designed to be open, shared and collaborative. Its objective is to offer a physical space with digital tools for shared use so that individuals can design and invent. This space thus means that extremely diverse objects can be designed, prototyped, manufactured and tested. The target for this type of workshop is very wide because, potentially, individuals, researchers, students, designers, artists and companies can come to the FL5 . The workshop is offered so that individuals can progress rapidly from the concept to the prototype and then perfect a single product or series of products, sometimes with commercial perspectives [START_REF] Eychenne | innovation frugal, effectuation et fablabs : pour penser l'innovation différemment[END_REF][START_REF] Rumpala | Fab labs et makerspaces : entre innovation et émancipation ?[END_REF][START_REF] Bouvier-Patron | ) Étude exploratoire des Tiers-Lieux comme dispositif d'incubation libre et ouvert de projet, XXiiie conférence de l'AIMS Capdevila[END_REF]. Digital manufacturing (in a general or specialised space) can have a great many applications and numerous sectors are concerned [START_REF] Morel | Fab Labs l'usager-innovateur[END_REF] including for robots, 3D printers, accessories, artistic objects, games, prosthetic devices and implants, the building trade and the arms industry. At the moment, the term FabLab is widely used -and sometimes misused… It can be confused with other types of spaces with a similar philosophy but different users or business models (Vallat, 2016) (Box 1). Likewise, within the large family of FabLabs, they may be differences among the spaces, which do not all integrate the same realities. We shall return to this point a little later.

Box 1: FabLab, Makerspace, Hackerspace, Techshop, Living Lab: Do-It-Yourself Spaces

The term FabLab has now become a common noun designating any digital design space (Bosquet et al, 2014). Nevertheless, it can assume several forms targeting various publics and objectives and thus take on different terminology. People also talk of HackerSpace, TechShop, MakerSpace, or co-working space [START_REF] Eychenne | innovation frugal, effectuation et fablabs : pour penser l'innovation différemment[END_REF], Capdevila, 2015[START_REF] Bouvier-Patron | ) Étude exploratoire des Tiers-Lieux comme dispositif d'incubation libre et ouvert de projet, XXiiie conférence de l'AIMS Capdevila[END_REF]. For Mérindol et al ( 2016), Makerspaces and Hackerspaces are "open community spaces in which technology enthusiasts implement creative projects, discuss and learn in an often-digital universe designed to be open for appropriation or reappropriation by the user". It is a physical space for technological DIYers to exchange knowledge and skills and produce their own designs. TechShop is on a larger scale than a FabLab. It is a private space (and thus run by and for companies) where access is not open to everyone but granted via membership. Its equipment is more sophisticated. TechShop does not have a publication website as FabLabs do and each member is free to publish on his/her own website. Menlo Park TechShop in the United States is an example. In France, the Leroy Merlin TechShop opened in 2015. Living Lab is a real-size laboratory in which a group of multidisciplinary experts develop, deploy and test new technologies and strategies in response to global transformations. It involves more than just digital manufacturing, and encompasses spatial and social projects (Mérindol et al, 2016;Berthou & Picard, 2017). The aim of co-working spaces is to overcome independent workers' isolation and these spaces are often included in FabLabs' package of services (Vallat, 2016).

With the FabLab and Makers movement being relatively recent, it is too early to present the effects of this different form of innovation and production. As Menichelli et al (2014) point out, there are 4 possible levels of interpretation. (1) FabLabs can be seen as private spaces whose users are a handful of makers rebelling against the standard production format; (2) They can be considered as an innovation in technological education, allowing for "learning through practice"; (3) These spaces can be interpreted as a fun and contemporary crossroads between art, science and engineering; (4) Lastly, according to [START_REF] Anderson | Maker : the new industrial revolution[END_REF] and [START_REF] Rifkin | The Third industrial revolution[END_REF], FabLabs represent the "new industrial revolution" with, as their mainstay, the aim of empowering individuals and automating objects, abandoning mass production in favour of limited, local productions and offering individuals the freedom to participate in scientific research. From this perspective, this type of space helps reduce the head-on opposition between "knowing" and "doing", leading Monpère (2016) to evoke the notion of augmented craftsmanship: FabLabs, as micro-factories in the middle of town, allow designers and craftsmen and women to appropriate digital knowledge to rethink their practices, and to work jointly with specialists from other fields (engineers, coders, etc.).

The exponential growth of the number of FabLabs proves a keen interest in these spaces, whether from cities, companies or universities; the number of FabLabs in the world has greatly increased over recent years. Nevertheless, data are still somewhat irregular because some studies include all FabLabs in general while others count only those that fall within the scope of the MIT Charter. The FabFoundation site points out the existence of a network of 1,092 FabLabs in approximately 40 countries6 (this figure doubles every 18 months according to one of our contacts). The Makery site declared 551 of them in February 20177 and the report by Mérindol et al ( 2016) counted 364 of them in 2015. France ranks second for the number of FabLabs, behind the United States (Menichelli et al (2015), Makery site). This is confirmed by the figures supplied by the FabFoundation, according to which, in 2017, the United States had 145 sites, France 138 and Italy 131. In Europe, some of the first FabLabs to be set up launched a movement that became widespread, as illustrated by the initiatives developed in Barcelona, Toulouse and Grenoble. The first European FabLab appeared in Barcelona in 2007 under the auspices of Tomas Diez (architect) in collaboration with the IAAC School of Architecture [START_REF] Bosqué | Fablabs, etc…Les nouveaux lieux de fabrication numérique[END_REF]. The main idea involves implanting different FabLabs in several districts in the city to encourage entrepreneurship and innovation. Since then, the project has taken off in the city and now represents the notion of FabCity, in which digital manufacturing spaces serve the city and its needs in terms of energy, construction, etc., and work on ways for Barcelona to be self sufficient for a certain number of elements. One of the famous examples is the FabLab House, a self-sufficient building capable of producing its own energy and food. There are now 4 FabLabs in the city, each with a specific theme (energy production, building smart hives, etc.). Artilect and the Casemate were the first FabLabs in France ( 2009), set up in Toulouse and Grenoble. Artilect is a pre-incubation site whose aim is to offer the space economic viability (by leasing its machines and services). It currently has approximately 800 members including students, engineers, technicians, architects and biologists (Bosquet et al, 2014;Menichelli et al, 2015).

3) The project: digital manufacturing, exchange, learning commons and training

The "political" project of this type of system should be seen in conjuncture with the pooling of knowledge and skills between FabLab users, usually via open source. The goal is to encourage interactions between individuals; exchanges are numerous and enriched by the experience and knowledge of others. FLs therefore contribute to DIY insofar as they provide material assistance and an IT ecosystem to those who come with nothing but their ideas and intuition, and who proceed to realise them themselves (Box 2). Ultimately, with spaces such as these, the "inventor" re-appropriates his/her invention and control of it by personally supervising the transition from the idea to its production in a limited series [START_REF] Anderson | Maker : the new industrial revolution[END_REF]. He/she becomes a consumer-producer (Rifkin, 2014), the main actor in its creation and production, and thus experiences empowerment. In this way, FabLabs encourage creativity and innovation [START_REF] Suire | La performance des lieux de cocréation de connaissances : le cas des fablabs[END_REF]Mérindol et al., 2016;[START_REF] Lhoste | Fablabs : l'institutionnalisation de tiers-lieux du « soft hacking[END_REF]. Another prevailing aspect of FabLabs depends on their local dimension and integration in an area (Manzo & Ramella, 2015;[START_REF] Rumpala | Fab labs et makerspaces : entre innovation et émancipation ?[END_REF]. Very often, the objective is to deploy local resources to satisfy a local need for production and training. This is true to an even greater extent in FabLabs in the Global South: for example, sites in French-speaking parts of Africa set up major training programmes for young people and schoolchildren in the aim of introducing them to what could become a source of employment in the future, helping them move away from the aimlessness, poverty and delinquency of their neighbourhoods. Other African FabLabs focus on training for women so that they create employment and economic activity (these spaces assume a strong economic and social dimension). This aspect is emphasised in the MIT Charter according to which "labs enable invention by providing access to tools for digital fabrication" (Menichelli et al, 2015). Beyond this, in some African countries, FabLabs are a key factor to an approach that makes innovation possible in difficult or even hostile conditions, being based on resourcefulness (see Togo's FabLab 8 based on recycling to make 3D printers) (see [START_REF] Radjou | L'innovation frugale, comment faire mieux avec moins[END_REF], for an analysis of frugal innovation). In this way, these spaces -in addition simply to making digital tools available to facilitate design and innovation efforts -also represent another development path to which Global South countries have access (World Bank Report, 2014). FabLabs are therefore not only technical platforms but also assume a social and economic dimension, and are capable of shaking up traditional production formats. As Rumpala (2014) points out, with FabLabs, we are witnessing a kind of deconcentration of production: smaller units are being created, unlike 20 th century industrial workshops. Large investments are no longer mobilised thanks to the performance of digital equipment and the possibilities now offered by 3D printers. An "economy of contribution" is said to be emerging [START_REF] Bosqué | Fablabs, etc…Les nouveaux lieux de fabrication numérique[END_REF]. Production can now be carried out at a lower cost with the help of technologies (processing, storage, data transmission) that have reduced marginal costs and the constraints of communications between individuals (Altman, Nagle & Tuschman, 2013). To summarise, this third space is a space that enables people (i) to make (ii) to bring to fruition their ideas (iii) to pool knowledge, resources and skills (iv) to make the most of training and teaching tools and (v) to solve problems on a local scale (Le Roux & Morel, 2016). 8 The Woelab in Togo was created in August 2012. Its founder, Sénamé Koffi Agdodjinou, is still its manager. The objective is to offer a space devoted to design and manufacture that helps launch start-ups (11 have already been incubated). Sénamé is from the world of design, history of art and architecture. In 2010, he set up the Africaine d'architecture, the aim of which is to promote creations in this field. The current team is made up of 3 people who take it in turns to fill the different roles of executive manager, fabmanager and community manager. The project had no financial backing. In the beginning, the FabLab was equipped with a RepRap (self-replicating 3D printer), computers and small DIY tools. A few 3D printers, made using RepRrap, complete the equipment.

The space is open to anyone and totally free. It offers courses for children in 3D printing and coding.

Box 2: Commons-based projects A FabLab workshop is made up of a set of pooled digital machines available to the public in the aim of offering professional material to amateurs [START_REF] Eychenne | innovation frugal, effectuation et fablabs : pour penser l'innovation différemment[END_REF]. For example, the workshop will have CAD and CAM software, laser cutting machines (for parts in wood, chipboard, leather, cardboard, etc.), digital milling machines (for sculpting complex objects in thick materials), digital rippers (more powerful than milling machines), 3D printers (for making complex parts from plastic and bioplastic, moulds and models), vinyl cutting machines (for cutting paper, card, vinyl, textiles, copper film for circuit boards) and sometimes even more specific equipment (plasma cutter, electronics workshops, machines to prototype circuit boards, and 3D scanners; Diwo- [START_REF] Allain | Propriété intellectuelle et fablabs : quelle gestion de la propriété intellectuelle dans les fablabs et plateformes ouvertes de création numérique : propositions de pistes de réflexion, in La propriété intellectuelle et la transformation numérique de l'économie[END_REF]. Some spaces are better equipped than others. Some of these machines mean that MIT Charter certification can be obtained. ICI Montreuil is one of the largest spaces in the Paris region (1800 m²) and, for example, is equipped with a great many highly-specialised machines for many different trades. What's more, since FabLabs are part of a network, other types of commons-based projects can be implemented. Saclay's Digiscope mainly receives students working on research projects, and functions as a network with other spaces such as Stanford's FabLab, for example. Our interviews underscore the cooperation between these FabLabs, especially for educational projects and training. For example, EdFab offers training sessions in digital manufacturing jointly with the ICI Montreuil's FabManagers who are already well ahead in this field. Rennes' LabFab responded to a call for tender from the Paris Region and the Paris City Council jointly with ICI Montreuil to develop a five-month vocational training course with MOOC.

FabLabs' commons-based components and files: Arduino, RepRap, Raspberry Pi, Jerry Do-It-Together in Africa

With FabLabs being part of a network and supported by a community, founding projects play an important role in the network's structuring and development, as illustrated by the RepRap and Arduino projects. The principle is to share files within the network to form the basis of future innovations that are constantly being improved.

Arduino is an open source device. It is a miniscule circuit board combined with a microcontroller that enables amateurs and professionals to build devices that interact with their environment using sensors and actuators. The aim is, therefore, to give intelligence to objects such as thermostats, watering systems, irrigation, and artistic installations. Arduino plans can be used for free by anyone. The technology costs only 3 euros. From the very beginning, Massimo Banzi, whose project it was, wanted something simple to allow anyone simply to use the technology. Arduino enjoys the support of a strong community and documentation in all languages so that it can be transmitted. It is also used to build, for example, 3D printers, milling machines and laser cutters. Online courses are available so that people can learn how to use the equipment. The RepRap project makes files available that enable a 3D printer to print most of the parts for another 3D printer. It therefore involves the principle of replication (this also exists for other machines). De Jong and de Bruijn (2013) conducted a survey of the RepRap community and its 384 members. The study highlights the fact that members who are very connected to the community and have a lot of experience in building machines are those that innovate the most. The transmission of innovations in the community correlates strongly with the profile of the member instigating the innovation. Raspberry Pi is a minimalist computer. It is reduced to a single circuit the size of a credit card and can be connected to any screen, keyboard, or mouse (developed by the University of Cambridge). It has proved to be extremely useful for a great many applications in agriculture, home automation, heath and communications. Raspberry Pi users have created all kinds of things such as game consoles, alarm systems, etc. [START_REF] Radjou | L'innovation frugale, comment faire mieux avec moins[END_REF][START_REF] Bosqué | Fablabs, etc…Les nouveaux lieux de fabrication numérique[END_REF]. An inexpensive variant is the Jerry computer developed in Africa (our interviews; Fagbohoun, 2016): placed in a jerrycan (hence its name), the computer has a very simple design and is mainly intended for use in schools.

4) FabLabs' economic models

The interviews9 we conducted give us a better understanding of the FabLab environment, their diversity, and their different economic models, corroborating existing studies that state this reality. The report by Bottolier-Depois et al ( 2014) for the Ministry of the Economy, Industry and Digital Technology, based on a survey carried out in France and abroad, draws attention to a wide diversity in terms of legal frameworks10 , the type of target public, the type of workshop and the mode of funding. [START_REF] Eychenne | innovation frugal, effectuation et fablabs : pour penser l'innovation différemment[END_REF] categorises FabLabs into three groups. According to the author, FabLabs are (i) educational, (ii) "private business", or (iii) "general public pro/amateur".

(i) Educational FabLabs are linked to higher education establishments, e.g. the University of Cergy's FacLab (Nedjer-Guere & Gagnebien, 2015) or Rennes' LabFab. The target public is mainly students but these sites also receive all types of public (e.g. young or retired people) during an open and free slot in the week (Open Labs). These sites have an MIT Charter reference.

(ii) "Private-business" FLs are spaces supporting the interests of the companies that set them up, whether well-established firms, start-ups or even self-employed entrepreneurs. Several companies have now opened their FabLabs internally, e.g. Renault's Creative People Lab or EDF's I2R. These company FLs [START_REF] Lo | Fablab en entreprise : proposition d'ancrage théorique ; XIIIe conférence de l'AIMS Manzo[END_REF] are based on the need to cultivate employees' collective intelligence or collective innovation. In this type of space, only the company's employees (and not the general public in the widest sense) are invited to collaborate amongst themselves and foster innovative potential with their ideas. In the rest of our study, we shall not take into consideration this type of site, being limited to the perimeter of the company. The aim of these initiatives is to stimulate exchanges between different members of staff and encourage creative approaches that will undoubtedly give rise to projects. These company FabLabs also contribute to the development of knowledge management and knowledge transfer between employees (Nonaka & Takeuchi, 1995).

(iii) "General Public and Pro/Amateur" type FabLabs are sites backed by governments, development institutes or local authorities and private financers. They are neither situated within universities nor companies. They are open to the local area for various users and are set up in the middle of cities to be as close as possible to its needs. They offer services that may be either free or fee-paying, following a hybrid economic model. It is worth looking at this last category of third spaces more closely because, as we have seen in interviews, they are extremely diverse. FabLabs can be distinguished by target public, surface area, type of equipment on offer, type of membership, the possibility or not of free slots for everyone and the type of funding used to open the space. Some of them are not open to the general public and do not, therefore, offer free slots (ICI Montreuil, Usine IO, and Draft). Some FabLabs are created under the auspices of a competitiveness cluster but are also not open to the general public (EdFab). Funding sources may be different from one FabLab to another and may be hybrid (Draft). Lastly, some FabLabs play an incubation role (Usine IO).

5) Initial analyses

The diversity of FabLabs and their different characteristics help draw attention to the sites' various economic models. Without an economic model, they cannot survive unless they receive permanent public funding. The configurations we observed during our interviews (Table 1) point to two major possible models: (i) the free, educational, MIT Charter FabLab; (ii) the fee-paying FabLab, focused on start-ups, projects and training. Nevertheless, certain FabLabs combine the two formats by setting up some services that are free (e.g. free Open Lab days to publicise the site) and some that charge fees [START_REF] Eychenne | innovation frugal, effectuation et fablabs : pour penser l'innovation différemment[END_REF].

Hybrid funding to set up sites Our interviews show that spaces often combine their sources of income (public and private funding), thus corroborating the study conducted in 2014 by Bottollier-Depois et al. In the sample of respondents taken into account in this study, half had benefited from public subsidies and half had not. The majority of workshops had been funded by the founders' own funds (53%). 47% of the workshops had received donations (from their close networks). 22% had received funds via crowdfunding, 18% had received funds from partner companies and 16% from investment funds. The interviews we conducted drew attention to hybrid funding solutions among the various possible sources of funding. An interesting example is the Draft workshop in Paris. The two founders of the site (opened in 2014) obtained hybrid funding of 130,000 euros to set it up: own funds, classic bank loan, leasing for the machines, zero-interest loan and fundraising via KissKissBankBank (KKBB). ICI Montreuil, which opened in 2012, received very little public funding (5% from Montreuil's local council) at the start of the project. Its founder contributed private funds to the tune of 20% of the total amount and the rest was covered by Love Money from large companies, banks and artists (including SG, Bouygues, Renault and Sony). A grant was also obtained by winning a contest via France Active. For spaces that received public funding, the study conducted by Mérindol et al (2014) highlights the fact that the French Tech programme launched in 2013 to support France's digital ecosystem enabled Usine IO, for example, to benefit from public funding. This FabLab, set up by three founders in 2014, provides an example of yet another combination using funding from an angel investor and large companies. FabLabs are set up with help from public authorities that guarantee their funding, especially at the start of the project. Nonetheless, the sites must plan how they will then sustain themselves and find their own means of funding. The University of Cergy's FacLab set up a fee-paying University Diploma (3,000 euros per person) connected to a new profession (facilitator), the enrolments for which make up a large share of the site's funding. It also received financial support from the Orange Foundation and appeals for donations via its website. Rennes' LabFab, set up in 2012, received the backing of Rennes Métropole, which provided both financial and human resources. It also counts on developing MOOC and on partnerships for specific requirements (responding to calls for tender in collaboration with other FabLabs).

A differentiated public

The widespread view that FabLabs are open to anyone needs to be confronted with the reality of the facts. Although educational FabLabs have a genuine desire to be open to every category of the public, this is not the case for private structures. More specifically, the vocation of the former is to welcome a wide public (pupils, young people, students, retired people, job seekers, employees, etc.) so that they can discover digital manufacturing, try out machines, exchange knowledge, even tinker/hack, and to allow people changing careers to regain their self-confidence. This type of site therefore assumes an educational and social dimension. It creates a bridge between the world of digital manufacturing and the general public, offering training sessions given by members of the FabLab's team or by people using the site. In this way it forges bonds and fosters exchange. Users are thus users of the site but also occasionally its trainers. Giving one's time, running a workshop, contributing to a training course, and helping assemble a machine are a few examples of tasks that a FabLab contributor is likely to carry out in exchange for using the site. The aim of the space is to create connections by enabling people to meet, transmit their knowledge, and come together to work on projects (to design, make and learn collectively, as the FabLab announces). Open Lab slots are scheduled so that anyone can come and experiment with digital manufacturing. The vocation of the second category of FabLabs (fee-paying, with a specific public), on the other hand, is to run a workshop for projects to create start-ups and companies, and providing training opportunities. As soon as membership is payable rather than free (there are menus of fees depending on the type of services on offer)11 , these sites are not open to a wide public but to: (i) people wishing to develop their idea and prototype it; (ii) artisans and artists wishing to use the equipment and exchange with people from other professions to enrich their projects; (iii) freelance professionals (e.g. designers and architects); and (iv) employees of large companies coming to train in digital manufacturing and possibly produce limited series of objects. Usine IO hosts 300 projects a year: 70% are startups, 20% are collaborators from large corporations and 10% are independent users (designers, architects, students, etc.). Collaborators from large corporations come to develop their projects and prototype them. At ICI Montreuil, there are 63 skills (craftsmanship, design, etc.) and approximately 150 residents, spread over a space of 1800 m². EdFab, set up by the competitiveness cluster Cap Digital, opened its doors in March 2017. Located in La Plaine Saint Denis in MSH Paris Nord premises, EdFab's mission is to host companies (large and small, as well as start-ups) and other professionals (schools, etc.).

Fee-paying services central to the business model v. raised awareness of local needs Educational spaces such as the FacLab or LabFab do not ask for a membership fee from participants, or only a symbolic sum. Cergy's FacLab asks a €5 enrolment fee, for example. This approach squares with the desire to be an accessible space for as many people as possible, including schoolchildren. Supported by public funding, these sites nevertheless have also found a sustainable business model by means of associated fee-paying diplomas (see above) and projects set up with local actors such as city councils (introduction to digital manufacturing for schoolchildren, etc.). These sites can also act as the interface between external requests for services and FabLab participants. The space works as a go-between for a request (company, public stakeholder, etc.) and the skills of its contributors. The contributor is remunerated and the site takes a commission from the sum paid by the buyer. Private spaces offer equipment rental, the possibility of privatising the space and production on demand, a range of services (in response to requests from companies for training or production: Draft, ICI Montreuil), co-working space, etc. They offer training sessions and courses. Assistance may be given to advise companies for prototyping, or to provide support for incubator projects or respond to national or international calls for tender. All these services are fee-paying and can be found on the FabLab website's price list. These sites thus assume an economic dimension and are developed according to well-defined business models that help ensure their sustainability. In addition to standard training to teach people to use digitally-controlled equipment, other very "business-focused" training may be offered (how to draw up a business plan, setting up public relations, how to carry out a digital project, design thinking, etc.) as is the case at ICI Montreuil and EdFab. These training courses are offered either by FabLab staff and residents depending on their skills and knowledge (ICI Montreuil 12 ) or partly by external trainers (Draft, Usine IO). Some FabLabs even play the role of incubator and provide specialised support for people with projects, as illustrated by the case of Usine IO. The stated objective is to support the transition from the idea through to its industrialisation and accelerata project hardware 13 and offer contacts with manufacturers who may be interested in the project. This expertise raised 40 million euros in 2016. Other feepaying services supplement the business model, such as leasing co-working space, leasing rooms, providing contacts with the network of FabLab manufacturers, etc. Draft offers training in digital manufacturing for employees of large companies who wish to create small series for marketing or promotions, depending on the needs of their structure. EdFab has a dual objective: (i) to offer advice and support to people with projects from the training and education sector. EdFab supplies the site, equipment (two 3D printers and a laser cutting machine, command post for modelling software), training, and the opportunity to benefit from a panel of testers for prototypes; (ii) to act as the interface between companies requesting training for their employees and suitable service providers (from the centre's network of 1,400 members).

Partnerships and projects

Rennes' LabFab team is at the cutting edge for setting up MOOC for digital manufacturing. There are currently around 36,000 people enrolled throughout the world for these training courses, with a completion rate of 16%. The lab also works in partnership with ICI Montreuil and the Petit FabLab de Paris (following a call for projects from the Paris Region and the City of Paris) to devise professional training sessions (in partnership with Pôle Emploi). For 5

12 At ICI Montreuil, training is given by residents themselves depending on their skills and knowledge (wood, metal, leather, paper, plastic, etc.). ICI is responsible for linking those offering training (residents) and those requesting it: remuneration for the training is divided with 2/3 for the trainer and 1/3 for ICI. 13 To date, these projects developed in FabLabs have taken off: Timescope (virtual binoculars so that a site can be seen as it was in the past); Stanley Robotics prototyped its automated car garage system for Aéroport de Paris. months, partly classroom-based and partly distance learning (MOOC), people will be able to train in different fields (maker entrepreneur, embedded computer coding, modelling, electronics, how to use equipment, connected devices, etc.) with the possibility of specialising afterwards. ICI Montreuil also helps other partners, e.g. Cap Digital for training courses, or Usine IO.

Specific structures

FabLabs are of different sizes and thus require different organisational structures. Spaces such as the FacLab Draft are small spaces in which the founders, the FabManagers, play a major role in the system and have a small permanent team (reception, training, contact with external stakeholders etc.). Larger FabLabs (Usine IO or ICI Montreuil) have larger teams and have institutionalised certain practices. At Usine IO (16 permanent staff and 8 experts), the post of expert was created (they can be identified on site by their orange jackets) to provide support for people with projects so that they can benefit from the experience gained by the experts in the past when they themselves created start-ups or carried out projects. The structure of each site is based on a certain number of rules to be respected, whatever the size of the FabLab. There are "hard" rules and soft ones. Safety regulations are indispensable when using potentially dangerous equipment such as laser cutters, milling machines and 3D printers. When receiving a new person, the FabManager checks his/her competence to use the machines and may even ask for a level of certification (for very specific machines at ICI Montreuil e.g. when working with steel). Training sessions are offered at this stage. After this, organising safety regulations can be left to each FabLab workshop to define. For example, ICI Montreuil's wood and steel workshops are in charge of their own safety measures and manage this in accordance with the principle of trust. People who work there take responsibility for keeping a well-meaning but watchful eye on the correct running of the workshop. In addition, other types of regulations (hours, use of space, use of equipment, tidying up, etc.) complete the organisational structure.

Conclusion

Based on these initial findings, it can be observed that numerous FabLab models exist, and therefore a wide range of structures and economic models. Many sites are also halfway between being for-profit and not-for-profit organisations. These initial findings (which would be worth developing in greater depth with further interviews) are in line with the analysis by [START_REF] Menichinelli | Fab Lab, la révolution est en marche[END_REF] for which 4 main economic models coexist: (i) The facilitator model; (ii) The educational model; (iii) The incubator model; and (iv) The duplicated network model. In light of our preliminary observations, free, open, MIT-Charter FabLabs generally fall under the category of a model that is both facilitator and educational. Fee-paying FabLabs generally follow an incubator model but also have an educational and training objective. The borders between categories are porous. Research will continue by analysing the connection between FabLabs and learning commons. Although some sites appear to resemble learning commons (with equipment made available, courses organised to spread knowledge, work carried out jointly to install equipment or carry out a project, open access and network documentation), this is not necessarily the case for other FabLabs, which have a less open and more mercantile approach.

http://fabacademy.org/ and http://fabfoundation.org/

As we shall see later in the article, although the original FabLabs (MIT Charter) are destined for nonprofessionals, some of them target a more select public such as start-ups or even company employees (Company FabLab/Fablab d'entreprise, the Cap Digital project with avec EdFab, Draft and ICI Montreuil).

http://www.fabfoundation.org/index.php/fab-labs/index.html

http://www.makery.info/map-labs/

The interviews were conducted from January -April 2017 with 5 FabManagers of sites set up in the Paris Region and provinces. The data we gathered was supplemented by analyses of reports and studies.

In terms of legal frameworks, 46% of FabLabs are associations, 17% are set up in universities, 23% by private companies and 4% are run by local authorities.

http://www.icimontreuil.com/les-tarifs ; https://ateliers-draft.com/paris/forfaits-abonnements ; https://www.usine.io/abonnements/