Ana Claudia Pina

Nestor Tancredi

Maria Conchi

Ovin Concepcion

Alejandro Ania

Amaya
email: aamaya@fq.ed.uy

Conchi O Ania
email: conchi.ania@cnrs-orleans.fr

Alejandro Amaya

Stabilisation of sheep wool fibres under air atmosphere: Study of physicochemical changes

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

Introduction

Carbon fibres are lightweight materials with unique mechanical properties and high demand as consumer grade products for many different application fields including aerospace, textile, construction, adsorption, catalysis and energy storage [START_REF] Park | Carbon Fibers[END_REF][START_REF] Hiremath | Low cost textile-grade carbon-fiber epoxy composites for automotive and wind energy applications[END_REF][START_REF] Li | A mechanism study on preparation of rayon based carbon fibers with (NH4)2SO4/NH4Cl/organosilicon composite catalyst system[END_REF][START_REF] Al-Lami | Eco-efficiency assessment of manufacturing carbon fiber reinforced polymers (CFRP) in aerospace industry[END_REF][START_REF] Mechtcherine | Mineralimpregnated carbon fiber composites as novel reinforcement for concrete construction: Material and automation perspectives[END_REF], among others. Owing to the high cost and carbon footprint (energy-intensive) of the precursors commonly used to manufacture carbon fibres (e.g., polyacrylonitrile accounting for over 90% of current production, phenolic resins, polyimides) [START_REF] Zhang | A comparative study of two kinds of T800 carbon fibers produced by different spinning methods for the production of filament-wound pressure vessels[END_REF][START_REF] Khayyam | PAN precursor fabrication, applications and thermal stabilization process in carbon fiber production: Experimental and mathematical modelling[END_REF][START_REF] Sun | Precursor infiltration and pyrolysis cycle-dependent mechanical and microwave absorption performances of continuous carbon fibers-reinforced boron-containing phenolic resins for low-density carbon-carbon composites[END_REF], their niche market is mostly limited to high-performance structural materials. To reduce the high production costs, environmental impact and dependence on fossil sources in the manufacturing of carbon fibres, the search for alternative precursors from renewable sources has become a largely investigated topic with academic and industrial importance [START_REF] Ye | Separated 3D printing of continuous carbon fiber reinforced thermoplastic polyimide[END_REF]. Attempts towards more sustainable precursors for the synthesis of carbon fibres have mainly been directed to the use of biomass-derived precursors, polyolefins and natural fibres [START_REF] Milbrandt | Carbon Fiber from Biomass[END_REF].

Natural fibres are cheap, abundant and renewable; most importantly, they can be produced at low cost in many countries -including developing ones-and allow the reutilization of wastes generated in different production sectors (i.e., sustainability of wastes management within a circular economy policy). Yet the drawback is the high cost associated to their extraction from plants, along with the high-energy demand of the melt spinning process for the fabrication of the fibres [START_REF] Torres-Canas | Improved structure and highly conductive lignin-carbon fibers through graphene oxide liquid crystal[END_REF]. On the other hand, scarce studies report the use of natural fibres from animal origin (e.g., wool) as precursors of carbon fibres [START_REF] Hassan | Sustainable Production of Carbon Fiber: Effect of Cross-Linking in Wool Fiber on Carbon Yields and Morphologies of Derived Carbon Fiber[END_REF]. Despite the carbon content of wool is lower than that of polyacrylonitrile and other biomass-derived precursors (e.g., lignin, cellulose); due to their high protein content they are rich in heteroatoms, which could represent an advantage for particular purposes. Moreover, its relatively high sulphur prevents higher carbon loss during a thermal treatment. In addition, large amounts of wool fibre wastes are produced worldwide, and need to be disposed-off. Thus, wool fibres are an interesting natural sustainable alternative for the production of carbon fibres.

One of the main drawbacks of synthetic and cellulosic fibres is that during a thermal treatment, they fuse, decompose or degrade losing mechanical strength and morphology [START_REF] Hassan | Thermal, chemical and morphological properties of carbon fibres derived from chemically pre-treated wool fibres[END_REF], so the choice of the thermal processing pathway of natural fibres will control the features of the resulting material. A common procedure for the preparation of a material with fibre morphology and the desired mechanical strength is based on performing an oxidative thermal stabilisation of the fibre precursor at mild temperature. The reactions involved during the oxidative stabilisation step are mostly cyclization of nitrile groups (when using polyacrylonitrile as precursor) and dehydrogenation of saturated C-C bonds (for synthetic and cellulosic fibres). The original fibres are transformed into a cross-linked cyclized network of carbon-nitrogen rings. This new spatial disposition prevents the fusion of the fibres during further thermal treatments at high temperatures [START_REF] Torres-Canas | Improved structure and highly conductive lignin-carbon fibers through graphene oxide liquid crystal[END_REF]. The mechanisms involved during the oxidative stabilisation of wool fibres are not yet clarified, as most of the studies have focused on their use for the preparation of highly porous activated carbon fibres [START_REF] Pina | CO 2 capture and biomethane obtention using activated carbon filter of animal origin[END_REF], fabrics [START_REF] Hassan | Sustainable Production of Carbon Fiber: Effect of Cross-Linking in Wool Fiber on Carbon Yields and Morphologies of Derived Carbon Fiber[END_REF], and felts [START_REF] Pina | Supercapacitor Electrode Based on Activated Carbon Wool Felt[END_REF].

Considering this, the aim of this work is to investigate the oxidative stabilisation of wool fibres as precursors for the preparation of carbon fibres, by analysing in detail the chemical, morphological and structural transformations occurring during the oxidative treatment at various temperatures. The structure and morphology of the fibres were preserved for stabilization temperatures below 400 ºC, although the composition and type of functional groups changed. Sulphur and oxygen contents decreased with the stabilization temperature, whereas nitrogen content remained quite the same, indicating the decomposition of S-and O-moieties along with a rearrangement of N-groups. The thermal cleavage of di-sulphur bridges, crosslinking and loss of the secondary structure of the fibres are identified as the main reactions occurring during the oxidative stabilization. This information is critical to optimize the conditions that guarantee the preparation of materials with fibrous morphology and high carbon yield after the oxidative stabilisation (e.g., 60 wt.% carbon content upon stabilization at 300 ºC) and further carbonization at higher temperatures.

Experimental

Materials

Sheep Merino wool fibres (WF) provided by the Uruguayan wool secretariat (S.U.L.) were used as precursor. The as-received wool fibres (ca. 29 µm diameter) were already clean and spun.

Oxidative Stabilisation

The oxidative stabilisation of the as-received wool fibres was carried out under air atmosphere (100 mL min -1 ) at temperatures ranging from 200 to 300 °C (heating rate 15 °C min -1 ) and oxidation times between 30 to 120 minutes. The fibres were placed inside a tubular horizontal furnace (CARBOLITE) by rolling them up around a stainless steel hollow cylindrical mesh (ca. 4cm diameter, 23 cm length). After the stabilisation, the samples were cooled down to ambient temperature under N 2 atmosphere flow (ca. 100 mL min -1 ). The obtained stabilised samples were labelled as SF-T-t, where T and t stand for the temperature and time, respectively, of the oxidative treatment.

Characterisation

Thermogravimetric analysis of the pristine and stabilised wool fibres was carried out on 6-25 mg of the material in a thermobalance (SHIMADZU TGA-50) under air and inert atmospheres (flow 100 mLmin -1 , heating rate 10 °Cmin -1 ) from room temperature up to 900 °C, in a platinum crucible. To identify and quantify the gases evolved during the thermal decomposition of the materials, a thermobalance (NETZSCH STA 409 DC) coupled to a mass spectrometer was used; the thermogravimetric profiles were recorded under Ar atmosphere, and selected m/z signals were recorded and quantified (e.g., m/z 2, [START_REF] Pakkaner | Self-assembly behavior of the keratose proteins extracted from oxidized Ovis aries wool fibers[END_REF][START_REF] Terzyk | Further insights into the role of carbon surface functionalities in the mechanism of phenol adsorption[END_REF]44,46,64). Elemental analysis of samples dried at 105 °C was measured in a LECO CHNS-932 (C, H, N, S) and a LECO-VTF-900 (O) analyzers. The pH at the zero point charge was determined following a reverse mass titration method [START_REF] Noh | Estimation of the point of zero charge of simple oxides by mass titration[END_REF] by pH measurements (METTLER TOLEDO). Fourier-transform infrared (FTIR) spectroscopy studies were carried out with a Bruker Vertex 80v, using materials dispersed in -and pressed with-dry KBr (spectroscopic grade) in a 1:100 ratio (w/w). Transmission spectra were carried out between 4000-350 cm -1 (64 scans collected, resolution 4 cm -1 ). Spectral data were recorded under vacuum to avoid the interference of ambient moisture and CO 2 . Raman spectra were recorded in ambient conditions in a spectrometer (RENISHAW INVIA REFLEX) equipped with a 355 nm laser, and using an optical microscope with x50 long (LEICA DM2500) working distance objective (ca. 10 mm). Each spectrum was recorded with an integration time of 5 s in the 500-2400 cm -1 wavenumber range; the data presented represents the average of three measurements. Deconvolution of Raman spectra was carried out by a fitting procedure using a linear baseline and Gaussian/ Lorentzian functions. The morphology of selected samples was evaluated by scanning electron microscopy (JEOL JSM 5900L Scanning Electron Microscope); samples were observed as prepared, with metallic coating.

Results and discussion

Before selecting the conditions for the oxidative stabilisation, the thermal features of the pristine wool fibres (WF) were studied in a thermobalance under oxidizing and inert atmosphere (Figure 1). A first mass loss below 100 ºC observed in both atmospheres corresponds to the moisture of the wool fibres (ca. 1-3 wt.%). Under inert atmosphere, a sharp mass loss is observed between 200-500 °C, accounting for a ca. 60 wt. % of total mass loss. This feature is associated with the rupture of the hydrogen bonds of the peptide structure of the wool fibres, thereby undergoing condensation reactions rendering to a solid to melted phase change [START_REF] Hassan | Thermal, chemical and morphological properties of carbon fibres derived from chemically pre-treated wool fibres[END_REF]. The temperature range is in agreement with other studies reporting the thermal decomposition of wool [START_REF] Chen | Activated carbon powders from wool fibers[END_REF][START_REF] Pakkaner | Self-assembly behavior of the keratose proteins extracted from oxidized Ovis aries wool fibers[END_REF]. In contrast, the profile in air showed two distinct thermal events in the range 200-400ºC (Figure 1). Below 200ºC, the mass loss was negligible, pointing out the threshold oxidative temperature of the WF; a similar thermal fingerprint has been reported for several natural fibres [START_REF] Torres-Canas | Improved structure and highly conductive lignin-carbon fibers through graphene oxide liquid crystal[END_REF]. The mass loss in air between 225-375 °C accounted for ca. 40 wt.% of the overall mass loss. This has been associated with cyclization and saturation reactions [START_REF] Torres-Canas | Improved structure and highly conductive lignin-carbon fibers through graphene oxide liquid crystal[END_REF]. Above 480 °C, the mass loss rate increased (ca. 1.5 times faster compared to the first thermal event below 400 ºC) reaching a complete mass loss, characteristic of a combustion reaction. Based on the thermogravimetric profiles in air, a range of temperatures between 225-300 °C was selected to explore the oxidative stabilisation of the wool fibres under air atmosphere.

The thermal profiles in inert atmosphere of the wool fibres stabilised in air at various temperatures and residence times are shown in Figure 2. The samples are classified in two groups upon the final weight loss: G1 formed by samples SF-225-t, and SF-250-t; and G2 composed by samples SF-275-t and SF-300-t. As seen, the overall mass loss at 900 ºC decreased as the stabilisation temperature increased, following the trend: group G1 > group G2. The total mass loss of the fibres treated at 225 and 250 ºC (ca. 32 wt.%) was 1.6 times lower than for those stabilised at 275 and 300 ºC (ca. 53wt.%), regardless of the stabilisation time. The temperature corresponding to the main mass loss in the stabilised samples gradually moved towards higher temperatures, indicating that the stabilisation rendered indeed fibres more thermally stable than the pristine one. The mass loss occurring between 225-300 ºC is associated with the rupture of hydrogen-bond peptide helical structures, and the cleavage of disulphide bonds and the exothermic oxidation of the wool fibre, which is accompanied by solid-to-liquid phase changes (see calorimetric evidence in Fig. S1 in the Supplementary Information File) [START_REF] Cao | DSC Studies of the Melting Behavior of α-Form Crystallites in Wool Keratin[END_REF]. It should also be noticed that the obtained yields are higher than those reported upon chemical stabilisation of wool [START_REF] Hassan | Sustainable Production of Carbon Fiber: Effect of Cross-Linking in Wool Fiber on Carbon Yields and Morphologies of Derived Carbon Fiber[END_REF] or rayon fibres (ca. 20-30 wt.%) [START_REF] Huang | Fabrication and properties of carbon fibers[END_REF], indicating a more effective oxidative stabilisation. This might be due to the composition of the pristine wool fibres, richer in di-sulphur bonds located at the cysteine residues (85 wt.% keratin) [START_REF] Wojciechowska | Application of Fouriertransform infrared and Raman spectroscopy to study degradation of the wool fiber keratin[END_REF], compared to cellulosic and synthetic polymeric fibres.

Regarding the stabilisation time, this parameter did not seem to impact strongly on the yields, with similar values (within 5% difference) for the series treated at 30 and 60 min. Only for the highest temperature (i.e., 300 ºC) the reaction rate seemed to be independent of the time framework, with no significant differences in the yields for the samples stabilised at 30, 60 and 120 min.

The elemental composition of selected stabilised fibres is compiled in Table 1. The carbon content increased with oxidative stabilisation, at the expense of the decrease in hydrogen, oxygen and sulphur. Interestingly, the nitrogen content remained rather constant. In contrast, the sulphur content decreased with both the oxidation temperature and time, except for the samples treated at 300 °C that showed similar values for different times. The reduction in the sulphur content is linked to the oxidation of di-sulphur bonds of the cysteine residues in the wool that would decompose in the form of hydrogen sulphide and sulphur dioxide [START_REF] Savige | Oxidation of disulfides, with special reference to cystine[END_REF]. The decrease in oxygen and hydrogen contents with the stabilisation temperature is mainly associated with dehydrogenation and dehydration reactions. The infrared spectra of the fibres showed important differences upon the stabilisation. The characteristic signal at 2960 and 2920 cm -1 associated with CH groups was observed in all the samples. The band featured at 1111 cm -1 , present in all the samples is assigned to O-C stretching vibrations. The band at ~ 740 cm -1 was assigned to the presence of esters for SF-250 series onwards.

The broad band between 4000-3000 cm -1 can be interpreted as two contributions corresponding to N-H stretching vibration (~ 3300 cm -1 ) Amide A, and to O-H stretching (~ 3200 cm -1 ). In both cases, the frequency registered corresponds to H-bonded groups [START_REF] Pakkaner | Self-assembly behavior of the keratose proteins extracted from oxidized Ovis aries wool fibers[END_REF]. This band is observed for all the samples, regardless the temperature and time of the oxidative stabilisation. On the other hand, Amide B (Fermi resonance doublet ~ 3100 cm -1 ) peak, also related to N-H stretching vibrations, was no longer detected for sample SF-250-60 onwards. Amide I features (C=O stretching band ~ 1650 cm -1 ) were observed in the pristine fibres and up to the series SF-250 at the same frequency. Whereas amide II (out of phase combination of the NH in plane bend and the CN stretching vibration ~ 1500 cm -1 ) and amide III (in phase combination of the NH bend and the CN stretching vibration, ~ 1235 cm -1 ) were not detected for any of the samples stabilised at 300 ºC (SF-300 series). Amide IV (C=O-N bending mode ~ 620 cm -1 ) was detected for all samples [START_REF] Parker | Applications of infrared spectroscopy in biochemistry, biology and medicine[END_REF]. Thus, even though the signals associated with N-H and C-N (1200-1400 cm -1 ) bonds are present in all the samples, there are important differences in the nature of the groups. This is in agreement with the trend observed for the nitrogen content by elemental analysis (Table 1), and confirms that the N-groups are predominantly rearranged during the oxidation. On the other hand, the lack of Amide I and II groups for the samples stabilized at 300 ºC indicates the occurrence of conformational changes in the secondary structure of the fibres, most likely associated with the evolution of hydrogen bonds during dehydration reactions [START_REF] Parker | Applications of infrared spectroscopy in biochemistry, biology and medicine[END_REF].

A similar trend has been reported for the conversion of amino groups into amide bonds during crosslinking reactions [START_REF] Chandwani | Experimental study to improve anti-felting characteristics of merino wool fiber by atmosphere pressure air plasma[END_REF] and is consistent with the high content of amino acids in wool fibres. Such rearrangement of the N-moieties is supported by the appearance of a peak at 2221 cm -1 for sample SF-275 and onwards series, associated with carbodiimide bond (-N=C=N-). Carbodiimides are usually formed via isocyanates (-N=C=O) as intermediate sub product in the oxidation of amines.

In the case of proteins, two amino acid residues close to each other in the tertiary structure can form isocyanates, and these can undergo into carbodiimides generating new bonds between protein chains. The appearance of this group correlates with the gradual change observed in the 2500-1700 cm -1 range, indicating the presence of a new group. Sulphur containing groups are difficult to detect by FTIR analysis as they often appear as low intensity and nearby lying bands. To facilitate the identification of overlapped bands not resolved in the original spectra, we performed a derivative analysis of FTIR data [START_REF] Anthony | Second derivative F.t.-i.r. spectra of celluloses I and II and related monoand oligo-saccharides[END_REF]. As seen in Figure 4, the disulphide bonds stretching vibration band at ~522 cm -1 is detected for all samples, although it appears slightly shifted to higher frequencies for SF-300 series (~524 cm -1 ). As the wool is oxidized, structural changes could be expected in the cysteine residues. The bands at ca. 1020 and 1040 cm -1 can be assigned to cysteine-sulphonate moieties (Bunte´s salt) and cysteic acid, respectively; both were detected for WF with SF-300-60. On the other hand, cysteine monoxide and dioxide, featuring at 1076 and 1121 cm -1 respectively, were observed only for SF-300-60. These frequencies are in

Wavenumber (cm -1 )

Amide A Amide B Amide I Amide II Amide III Amide IV agreement with reported data for plasma treatment of wool fibres [START_REF] Chandwani | Experimental study to improve anti-felting characteristics of merino wool fiber by atmosphere pressure air plasma[END_REF], and thus confirm the oxidation of sulphur moieties upon the stabilisation in air. Additional characterisation of the changes in the wool fibres upon the oxidative stabilisation was carried out by temperature programmed desorption coupled to mass spectrometry, and the identification of the evolved gases in the samples. Figure 5 shows the profiles corresponding to the m/z 2, 18, 28, 44, 46, and 64 of the thermogravimetric profiles recorded in argon atmosphere. The evolution of signal m/z 64 (Figure 5 a) shows a broad emission band in the pristine wool fibres, spanning from 200-700 °C. The multi-feature profile suggests that all released sulphur is bonded to the carbon surface in various configurations/environments. According to literature, m/z 64 is assigned to sulphur-carbon complexes in thiol, sulphide or oxidized forms of carbon [START_REF] Anthony | Second derivative F.t.-i.r. spectra of celluloses I and II and related monoand oligo-saccharides[END_REF][START_REF] Puri | Carbon-sulphur surface complexes on charcoal[END_REF]. This is due to the high reactivity of thiol and sulphide configurations that can be easily oxidized into SO 2 evolving groups upon heating during the TPD assays.

To clarify the different contributions of the profiles, we performed a deconvolution of m/z 64 considering four thermal events (see insets in Figure 5) assigned to: thiol groups (ca. 180-250 ºC), disulphur (ca. 250-350 ºC), and oxidized sulphides (ca. 350-500 and 550-700 ºC) [START_REF] Gomis-Berenguer | Sulfur-mediated photochemical energy harvesting in nanoporous carbons[END_REF][START_REF] Terzyk | Further insights into the role of carbon surface functionalities in the mechanism of phenol adsorption[END_REF]. The first peak between 185-250 ºC, only observed for WF is associated with thiol groups belonging to cysteine residues (-SH). Above 200 ºC, thiols would be transformed into disulphur bonds (cystine type). The peak at 255 and 377 °C in WF can be associated with thiol groups present in methionine residues and as sulphur from cystine. The peaks at higher temperatures in WF would correspond to the presence of oxidized sulphur groups, such as cystine sulphonate (-SSO 3 -

) and cysteic acid (-SO 2 OH) as evidenced by FTIR (Figure 4). For the samples stabilised at 250 ºC (series SF-250-t), the contribution of disulphur bonds is still observed (250-350 ºC), with no evidence of thiol groups. The wide tail in the range 350-600 °C indicates oxidized forms of sulphur in different configurations; most likely esters, cystine monoxide, and cystine dioxide, in agreement with infrared data (Figures 3 and4). For the samples stabilised at 300 ºC (SF-300-t series), the contribution of unoxidized forms of sulphur can be discarded; for these samples, the main peak at ~ 400 °C suggests oxidized sulphur in various configurations (e.g. cystine monoxide to dioxide). Finally, a small peak centred at 610 °C is visible to all samples, although its contribution becomes higher with the temperature of stabilisation. A semi quantitative analysis inferred from the evolution of the areas of the peaks (Figure 6a) shows that the overall m/z 64 signal decreased for the stabilised samples. This is in agreement with the decrease in sulphur content detected by elemental analysis (Table 1).

The m/z 46 signal was assigned to nitrogen moieties decomposing as NO 2 ; the evolution for the stabilised samples also showed interesting features (Figure 5b). Deconvolution of the main peak was carried out considering three regions: 200-300 ºC, 300-350 ºC, and 350-550 °C. As in the case of m/z 64, pristine wool fibres showed the largest contribution (Figure 6a) in the entire interval. The normalized intensities of the m/z 46 signal did not vary much for the samples, although the profiles shifted towards higher temperatures with the stabilization temperature. This correlates with the data from elemental analysis (Table 1) where the nitrogen content remained rather unchanged. It suggests that N-containing groups mainly suffer structural/configuration modifications, rather than thermal decomposition upon the oxidative stabilisation. Taking into account that peptide bonds are very stable and that the amide groups are present for all samples (Figure 3), it may be inferred that the nitrogen in peptide bonds are not affected by the treatment. Therefore, the m/z signal likely comes from residues rather than the main chain. The second region in SF-250-t series overlaps with that of m/z 64 (ca. 310 °C), suggesting that it may be associated to the evolution of residues containing sulphur (cystine) linked to residues containing amines (e.g. arginine, proline, tryptophan). Furthermore, the shoulder at 390 °C clearly became a peak for SF-300-t series, implying a greater extent of oxidation. This peak temperature is in accordance with that corresponding to thermal degradation of carbodiimides, which presence was evidenced by FTIR and likely comes from oxidized primary amines (e.g. alanine, glycine, and isoleucine) [START_REF] Platnieks | Highly loaded cellulose/poly (butylene succinate) sustainable composites for woody-like advanced materials application[END_REF].

The m/z 44 signal is typically assigned to CO 2 evolving groups (Figure 5c); deconvolution also shows three zones: 180-290 ºC, 290-340 ºC, and 340-700 ºC. Unlike m/z 64 and 46, the profile of WF did not show the third contribution, indicating that the highest temperature peak is due to products of oxidation of the fibres. The first peak, only featured for WF, could correspond to residues with carboxylic groups (aspartic and glutamic acids). The second and the third zones would correspond to the emission of carboxylic groups, anhydrides, and lactones (serine residues), also supported by the contribution of m/z 28 signal. Figure 6b shows that the amount of CO 2 evolving groups remains rather constant for all the samples. The m/z 28 signal is assigned to the evolution of CO, since TPD-MS experiments were carried out under argon atmosphere (Figure 5d). Three main zones can be distinguished: 200-300 ºC, 300-500 ºC, and 500-800 ºC. The signal below 200 ºC has no clear origin, although it has been attributed to the decomposition of vicinal carboxyl groups [START_REF] Feng | Sulfur impregnation on activated carbon fibers through H 2 S oxidation for vapor phase mercury removal[END_REF]. Interestingly, the shape of the curves changed with the stabilisation treatments, with peaks featuring at higher temperature as the stabilisation temperature increased; also, the amount of CO evolved showed a maximum for the series SF-250-t (Figure 6b). For these series, the main contribution corresponds to the second peak (300-500 ºC), whereas for series SF-300-t, it corresponds to the 500-800 ºC zone. The latter can be related to carbonyl groups formed upon oxidation of threonine residues, quinones and ethers.

The release of water, m/z 18 signal (Figure 5e), was significant for all the samples, with several peaks in a wide range of temperatures between 100-700 °C, due to the different dehydration reactions involving adjacent surface groups (e.g., carboxylic, hydroxyl groups) [START_REF] Figueiredo | Modification of the surface chemistry of activated carbons[END_REF]. This is in agreement with the acidic character of the fibres (average pH values of ca. 4.8 ± 0.2), infrared data, and the detection of anhydrides in the m/z 28 and 44 profiles at similar temperatures). The contribution of m/z 18 evolved in the range 400-700 °C decreased for the SF-300-t series as the time of stabilisation increased. This can be connected to the lower presence of phenol groups, that would be oxidized to quinones [START_REF] Hotová | Oxygen surface groups analysis of carbonaceous samples pyrolysed at low temperature[END_REF]. The signal m/z 2 assigned to H 2 is linked to the cleavage of C-H and N-H bonds (Figure 5e). The signal was recorded above 500 °C in all cases, as expected for samples with high volatile matter content.

The amounts of evolved CO and CO 2 were quantified (Table 2); data showed relatively large quantities of both gases for all the stabilised fibres regardless of the stabilisation temperature. This indicates that the oxidative stabilisation creates O-containing groups with higher thermal stability, as also confirmed by the thermogravimetric analysis (Figure 1). The ratio CO/CO 2 increased up to 250 °C to then decrease for the treatments at 300 °C, remaining rather constant. Since the amount of CO 2 remains practically unchanged, this indicates that the stabilisation mainly favoured the formation of ethers and carbonyl/quinone groups. The structural changes induced upon stabilisation of the fibres were analysed by Raman spectroscopy (Figure 7). It should be mentioned that samples WF and SF 200-225 presented a high fluorescence component that prevented to record the Raman spectra. The spectra corresponding to the samples stabilised above 250 ºC are shown in Figure 7. The spectra of all the samples are quite similar, showing the typical fingerprint of disorganized carbon materials with the characteristic D and G bands (ca. 1360 and 1580 cm -1 , respectively) [START_REF] Fries | Raman spectroscopy and confocal Raman imaging in mineralogy and petrography[END_REF]. The broad profiles of the D and G bands and the absence of second order spectra indicate the amorphous character (short-range order) of the fibres. The broadness of G band also confirmed this observation, with full width at half-maximum values (ca. 89-97) comparable to those reported for stabilised pitch fibres [START_REF] Kim | Effect of carbonization temperature on crystalline structure and properties of isotropic pitch-based carbon fiber[END_REF]; the values for D band are higher due to a large contribution of defects. SEM (Figure 8) explored the morphology of the fibres upon stabilisation. For the pristine wool fibres, the two distinct zones of the precursor are observed: the inner marrow or cortex, and the external cuticle [START_REF] Wojciechowska | Application of Fouriertransform infrared and Raman spectroscopy to study degradation of the wool fiber keratin[END_REF]. For the stabilised fibres, the characteristic cellular arrangement of the cuticle was still recognizable, although the surface gradually became smoother as the stabilisation temperature increased. While the fibrous morphology was preserved between 225-300 ºC, the soft marrow gradually disappeared with the temperature. As it can be seen for sample SF-300-120, the stabilised fibres become hollow due to the loss of the cortex. The wall thickness of the hollow fibres was ca. 7.0 ± 0.5 µm. On the other hand, the diameter of the fibres was barely modified during the stabilisation, with an average diameter for the pristine wool fibres of ca. 26 µm, and of about 29 ± 2 µm for sample SF-300-120. At stabilisation temperatures above 400 ºC, the fibrous morphology was no longer preserved, as the wool fibres melted (Fig. S2). Most interestingly, further carbonization of the stabilized fibres at high temperature under inert atmosphere (ca. 800-900 ºC) allowed to obtain carbon fibres with a well-preserved fibrous morphology and a high carbon content. In contrast, direct carbonization of the wool fibres (non-stabilized) provokes the loss of the fibrous morphology.

Conclusions

The oxidative stabilization of wool fibres in air between 200-300 ºC allowed to obtain densified materials with a fibrous morphology and a carbon content of up to 60 wt.%. These stabilized fibres may be further carbonized at higher temperatures under inert atmosphere to increase carbon content and obtain carbon fibres (preserved fibrous morphology). Above 400 ºC, the morphology of the fibres is lost, and the material becomes fragile most likely due to the melting of the fibres. At lower temperatures, the morphology and structural features of the stabilized materials were rather similar, regardless of the stabilization conditions. A thorough characterization of the fibres stabilized up to 300 ºC for different times has revealed important changes in the composition due to the thermal evolution of the O-, N-and S-moieties present in the pristine fibres. Overall, the sulphur and oxygen contents decreased with the stabilization temperature, whereas the nitrogen content remained rather constant. This indicates different thermal degradation pathways for the surface groups of the wool fibres. The loss of sulphur upon the decomposition and oxidation of sulphur bridges in the pristine fibres, leading to different oxidized sulphur complexes, still binding polypeptide chains in most cases. Thiols and disulphur bridged species present in the wool fibres (e.g., cysteine, methionine residues) are gradually transformed into various oxidized sulphur complexes (e.g., sulphonate, cystine monoxide, and dioxide residues). After stabilisation at 250 ºC, thiol groups are no longer observed but disulphur bonds are still detected. In contrast, for the samples stabilised at 300 ºC sulphur is predominantly in oxidized complexes, with no evidence of thiols and a small contribution of disulphides. The amount of oxygen decreased, but remained relatively high in the stabilized fibres due to the formation of new groups (mainly ethers and quinone) during the cross-linking reactions occurring during the oxidative stabilization. Relatively large quantities of CO 2 and CO were released regardless of the stabilisation temperature, due to the decomposition of carboxylic, ethers, and carbonyl groups present or formed upon oxidation of various residues (e.g., threonine, serine). On the other hand, the rearrangement of N-groups would seem to be associated with the transformation of amides present in the pristine wool fibres into more stable carbodiimide groups. The release of water at various temperatures in the TPD-MS profiles confirmed the occurrence of dehydration reactions involving adjacent surface groups. This would be responsible for the loss of the secondary structure of the fibres. However, the polypeptide bond is not affected, and therefore, the primary structure remains unchanged. This allows to preserve the fibrous morphology of the stabilized materials.

Figure 1 .

 1 Figure 1. Thermogravimetric profiles (mass loss and derivative weight) under inert and air atmosphere of the pristine wool fibres.

Figure 2 .

 2 Figure 2. Thermogravimetric profiles under inert atmosphere of the wool fibres stabilised in air at various temperatures for (A) 60 and 120, and (B) 30 minutes.

Figure 3 .

 3 Figure 3. Infrared spectra of the pristine wool fibre and the series of samples stabilised at 60 and 120 minutes at various temperatures.

Figure 4 .

 4 Figure 4. Second derivative of the FTIR transmittance spectra of selected samples.

Figure 5 .

 5 Figure 5. TPD-MS profiles of selected m/z for the pristine and stabilised wool fibres. Insets illustrate differentiated emission zones for WF and SF-300-60.

Figure 6 .

 6 Figure 6. Total areas of the profiles corresponding to various m/z signals obtained from TPD-MS data.

Figure 7 .

 7 Figure 7. Raman spectra of selected stabilized samples; spectra have been shifted for clarity.

Figure 8 .

 8 Figure 8. SEM images of the pristine and stabilized wool fibres at various temperatures (magnification of the images: x270; x200 for the insets).

Table 1 .

 1 Chemical composition (wt.%, dry-ash free basis) of the pristine wool and the samples obtained after oxidative stabilisation at various temperatures and time.

	Sample	N	C	H	S	O
	WF	15.45	49.03	6.97	3.04	25.52
	SF-225-60	15.48	51.58	6.19	1.94	24.81
	SF-250-60	15.59	54.79	5.69	1.54	22.03
	SF-275-60	15.87	58.12	4.91	1.25	19.85
	SF-300-60	15.83	60.50	4.15	1.15	18.36
	SF-300-120 15.89	62.54	4.09	1.18	16.29

Table 2 .

 2 Quantification of the amounts of CO and CO 2 released obtained by integration of the m/z 28 and 44 peaks in the TPD profiles.

	Sample	CO (

µmol.mol -1 ) CO 2 (µmol.mol -1 ) CO/CO 2

	WF	1289	3602	0.36
	SF-250-30	2918	3571	0.82
	SF-250-60	3654	3532	1.03
	SF-300-30	2023	3452	0.59
	SF-300-60	1809	3444	0.53
	SF-300-120	2061	3726	0.55

Acknowledgements

This work was funded by ANII through a scholarship fund for national postgraduates (POS_NAC_2016_1_130887, PEDECIBA), and Region Centre Val de Loire (APR-IA, MOSAIK project). Ana Claudia Pina thanks CSIC for her scholarship to perform an internship in France.


Noelia Esteban

Conchi María L Ferrer

Maria Concepcion

Ovin Ania

José De

La Campa

Ángel E Lozano

Cristina Álvarez

Jesús A Miguel

Conchi O Ania

José G De La Campa

Porous Organic Polymers Containing Active Metal Centers for Suzuki-Miyaura Heterocoupling Reactions

Keywords: Porous organic polymers, bipyridine, Suzuki-Miyaura reaction, Pd catalyst, confined catalyst

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

INTRODUCTION

Suzuki-Miyaura cross-coupling reaction between haloderivatives and boronic acids catalyzed by Pd(0) compounds, which was awarded with the Nobel Laureate in 2010, 1,2 is one of the most reliable, efficient and practical methodologies for the formation of C-C bonds since its first reports in 1979. [2][3][4][5][6] In particular, the reaction is a versatile and powerful methodology, for example, in the construction of biaryl compounds and the substitution and modification of aromatic and heteroaromatic moieties. [6][7][8] In this context, the Suzuki-Miyaura reaction is used in many research fields because it permits the efficient discovery, development and synthesis of pharmaceutical and chemical compounds, of different kinds of engineering materials (liquid crystals, polymers, molecular wires, etc.), coordination chemistry materials, supramolecular chemistry, and diverse functional materials. 1,[8][9][10][11][12] Nowadays, one of the main concerns in scientific research, especially for pharmaceutical and chemical industries, is the search for greener, safer, environmentally friendly and efficient technologies. 13,14 Specifically, the use of recyclable heterogeneous catalysts for organic synthesis allows for the reduction of waste production and the optimization of the efficiency of the synthetic process, and therefore research in this area is of utmost importance.

Porous organic polymers, POPs, have shown great efficiency as new catalyst supports in heterogeneous catalysis, thanks to properties such as high microporosity and the possibility of incorporating high catalyst loads. 12,15 In particular, the inclusion of metal catalysts confined in microporous cavities has given rise to materials having excellent turnover number, TON, and turnover frequency, TOF, high regioselectivities, high recyclability and low metal contamination of the obtained products. It has been demonstrated that the confinement of catalyst produces a variation in the energetic and kinetic properties of the catalytic process, which improves its yield and selectivity.

The development of new POP materials with high thermal stability and high specific surface area is booming for advanced applications such as CO 2 capture, gas separation and storage, sensor preparation or heterogeneous catalysts. [16][17][18][19][20][21] These POP materials can be manufactured using a variety of different methodologies to achieve tailored properties. Olah et al. proposed the activation of electrophiles in Brønsted or Lewis super acidic media in order to obtain reactants that show a significantly higher electrophilic character. [22][23][24][25] Following Olah's methodology, Zolotukhin et al. obtained linear polymers by using a one-pot SEAr reaction of ketones having electron-withdrawing groups (for instance, isatin or trifluoroacetophenone) with diaromatic hydrocarbons (for instance, biphenyl or p-terphenyl). These linear polymers showed high molecular weight and exhibited good chemical and thermal stability (around 500 °C).

Furthermore, these polymers could be employed as gas separation membranes, which showed excellent values of permeability and selectivity. [26][27][28][29] Recently, in our research group, a new generation of porous polymer network were synthesized following Olah's methodology, by reaction of ketones having electron-withdrawing groups (i.e.; isatin and trifluoroacetophenone) with rigid trifunctional aromatic monomers (triptycene and 1,3,5-triphenylbenzene) employing triflic acid as a superacidic media. 30 These polymers showed high surface areas between 580 and 790 m 2 g -1 , high thermal stability and excellent CO 2 uptakes and were employed as fillers to prepare mixed matrix membranes displaying excellent gas separation performances. 21 Following this research line, we have now introduced a set of microporous polymer networks, POPs, having a rigid bipyridine moiety in its structure. These POPs were able to efficiently form confined Pd catalysts by simple immersion in a Pd(II) acetate solution. The catalysts thus obtained have been evaluated for the Suzuki-Miyaura reaction of a wide variety of haloaromatic derivatives and boronic acids with high yields and TOF and TON values.

EXPERIMENTAL PART

Materials

All reagents were purchased and used without any purification except 4,5-diazafluoren-9-one (DAFO), which was synthesized in our laboratory following the methodology described elsewhere. 31,32 The detailed synthesis of DAFO is described in the supporting Information section (SI).

Techniques

Fourier Transform Infrared (FTIR-ATR) spectra were recorded on a PerkinElmer Spectrum RX-I FT-IR spectrometer, equipped with an attenuated total reflection accessory (ATR) Pike Gladi ATR-210. Solid-state 13 C cross-polarization magic angle spinning NMR spectra (CP/MAS 13 C NMR) spectra were recorded on a Bruker Avance TM 400WB spectrometer, equipped with a 89 nm wide-bore and a 9.4 T superconducting magnet, operating at frequency of 100.6 MHz, using 1 ms contact pulses, a delay time of 3 s and a rotation rate of 11 kHz. Dynamic thermogravimetric analysis (TGA) were performed on a TA-Q500 instrument under a continuous nitrogen flow (60 mL min -1 ), operating with a Hi-Res method at a heating rate of 20 °C min -1 and sensitivity and resolution parameters of 1 and 4, respectively. Wide-Angle X-ray Scattering (WAXS) patterns were recorded in reflection mode at room temperature, using a Bruker D8 Advance diffractometer provided with a Goebel Mirror and a PSD Vantec detector. A stepscanning mode was employed for the detector, with a 2 step of 0.024°, at a rate of 0.5 s per step.

CuK  (wavelength  = 1.54 Å) radiation was used. Scanning Electron Microscopy (SEM) images were taken with a QUANTA 200 FEG ESEM on Au-metallized samples operating at an acceleration voltage of 1.5 kV in high vacuum and using the detection of secondary electrons method.

N 2 adsorption-desorption isotherms were measured at -196 °C in a volumetric analyzer (3Flex, Micromeritics), in the relative pressure range (P/P 0 ) from 10 -5 to 1 bar. Previously, samples were degassed under vacuum at 250 °C for 10 h to remove humidity, adsorbed gases and solvent from the samples. The adsorption isotherms were used to determine the specific surface area (S BET ) by applying the Brunauer-Emmet-Teller equation , the micropore volume (V micro ) using the Dubinin-Radushkevich (DR) equation, and the total pore volume (V total ) from the amount adsorbed at 0.99 relative pressure. Pore size distributions (PSD) were evaluated using the 2D-NLDFT-HS model [ref]. Each isotherm was recorded in duplicate on fresh sample aliquots, to guarantee the reproducibility (error was below 5 %).

The palladium content in the porous polymer catalysts was determined using a radial simultaneous ICP-OES Varian 725-Es device.

Synthesis of porous organic polymers

The methodology of formation of POPs was based on a reaction described recently. 30 Here, moreover, two reaction temperatures (room temperature and 60 ºC) and the use or not of chloroform, as a co-solvent, were tested. The 135TPB was combined with DAFO, isatin or a mixture of these monomers in molar DAFO/isatin ratios of 1/1 and 1/3.

As an example, the synthesis of 135TPB-DAFO-Isatin (4:3:3) polymer network at 60 ºC and using chloroform as co-solvent is described as follows:

An oven-dried three-necked Schlenk flask, 500 mL, equipped with a mechanical stirred and nitrogen inlet and outlet was charged with 1,3,5-triphenylbenzene, 135TPB, (2.24 g, 7.32 mmol), isatin (0.81 g, 5.49 mmol) and DAFO (1.00 g, 5.49 mmol). Anhydrous chloroform (15 mL) was added and monomers mixture was stirred at room temperature under a nitrogen blanket. The mixture was stirred and cooled at 0 °C, and then chilled TFSA (30 mL) was added dropwise with an addition funnel for 30-60 min. After the TFSA addition, the mixture was left to warm up to room temperature and maintained with mechanical stirring for 24 hours. Subsequently, the mixture was warmed to 60 °C and maintained for 96 h. Finally, the reaction mixture was poured onto cool distilled water, neutralized by adding a concentrated NaHCO 3 solution, filtered and consecutively washed with warm distilled water, methanol, acetone and chloroform. After filtering, the powder was dried at 180 °C and 60 mbar during 24 h. The material was obtained as a brown powder in quantitative yield (99%).

Synthesis of supported Pd(II) catalysts

As an example, the synthesis of 135TPB-DAFO-isatin (4:3:3) coordinated with palladium(II) acetate is described below:

A 25 mL oven-dried Schlenk flask, with nitrogen inlet and outlet was charged with 135TPB-DAFO-isatin (4:3:3) (0.75 g, 1.07 mmol of DAFO groups) and 10 mL dichloromethane. The mixture was dispersed with an Ultra-turrax disperser at 6000 rpm, then palladium acetate (0.24 g, 1.07 mmol) was added and, finally, the mixture was stirred in the dark for 72 h. The product was filtered and washed with dichloromethane and acetone. After drying it at 60 °C for 16 h under 60 mbar vacuum, the material was obtained as a brown powder in 99% yield.

General procedure for the Suzuki -Miyaura cross-coupling reaction

Aryl halide (0.50 mmol), arylboronic acid (0.75-0.60 mmol), inorganic base (1 mmol) and supported Pd (II) catalyst (0.5-1% mmol Pd) were added along with 5 mL of solvent (EtOH/water (2:3)) in a 25 mL two-necked round bottom flask and heated at 80 °C. The mixture was stirred by an ultrasonic probe under air. After the reaction was completed (between 0.5 and 2 h depending on the reactants), the mixture was extracted with dichloromethane, dried in a rotary evaporator and the conversion was evaluated by 1 H-RMN.

Catalyst recycling of Suzuki-Miyaura cross-coupling reactions

Recycling tests were made up as follows:

For the first cycle, aryl halide (0.50 mmol), boronic acid (0.60 mmol), Na 2 CO 3 (1 mmol), POP-supported Pd(II) catalyst (0.5% mmol) and 5 mL of solvent (EtOH/water (2:3)) were added to a flask, and the mixture was reacting with ultrasonic, mechanical or magnetic stirring for 1 h at 80 °C. Next, the mixture reaction along with 3 mL of EtOH was poured into a conical centrifuge tube and centrifuged at 4000 rpm for 5 min. The floating liquid was decanted and poured into a decantation funnel. The supported Pd(II) catalyst was centrifuged two more times with fresh EtOH and the floating liquid was also added to the decantation funnel. Then, the cross-coupling product was extracted in dichloromethane, the solvent was removed by rotatory evaporation and the final solid was characterized by 1 H-NMR to determine the conversion of the reaction.

For the following cycles, the same quantity of reactants with the recovered catalyst, which was taken from the centrifuge tube with the solvent, were added to the flask. The same procedure followed in the first cycle was consecutively carried out a certain number of times.

RESULTS AND DISCUSSION

Optimization of reaction conditions

The synthesis of POPs was achieved by reacting of a rigid trifunctional aromatic nucleophilic monomer having a D 3h symmetry (135TPB) with ketones having electron-withdrawing groups (DAFO, isatin or a mixture of these monomers in specific molar ratios) under superacid conditions employing TFSA as the acid promoter. The polymerization reaction occurs when the strong acid protonates the electron deficient ketone, and reacts with an aromatic nucleophile. In order to form a highly crosslinked network, a stoichiometric ratio of trifunctional monomer to bifunctional monomer (or mixed of bifunctional monomers in different molar ratios) was employed. With the target of controlling the amount of metallic catalyst in the POPs, a set of copolymers were attained by combining DAFO with isatin in molar DAFO/isatin ratios of 1/1 and 1/3. The structure of the materials and the acronyms used to refer to them are shown in Scheme 1. For comparative purposes, the homopolymer from 135TBP and isatin was also obtained using the reaction conditions previously reported. 30 The synthesis of these POPs was optimized on varying reactions conditions, such as reaction temperature (room temperature, RT, or 60 °C) and use or not of a co-solvent (chloroform). Table S1 summarize the reaction conditions tested in the synthesis of POPs along with the reaction yields. According to the findings, the reaction yield of all the POPs were high (> 90%), except for that of POP3 prepared at 60 ºC in TFSA. Particularly, the POPs were obtained in almost quantitative yields (99%) when a TFSA/CHCl 3 mixture was employed. However, in those with a higher content of DAFO, POP1 and POP2, it was necessary to hold the reaction temperature at 60 ºC for 96 h. All the POPs prepared were characterized by FTIR-ATR and CP/MAS 13 C-NMR (Figures S3-S10). Comparing the spectra, it was observed that the chemical structure of POP1 and POP2, having higher percentage of DAFO, were strongly depended on temperature and reaction medium. The thermal stability and surface area of these POPs were measured in order to pick out which of them would be employed as catalyst supports. The results of all the POPs obtained with high reaction yields are listed in Table 1. Moreover, the TGA thermograms of POPs, which were dried at 180 ºC for 24 h, are shown in Figures S11-S14. Two losses were observed: a first one associated to the adsorbed water below 200 ºC and a second one related to generalized degradation of material above 400 ºC. The DAFO-containing POPs that were prepared using a TFSA/CHCl 3 mixture showed an additional weight loss around 300 ºC, which could be presumably due to occluded solvent within the pores during the reaction.

According to the values summarized in Table 1, the copolymers prepared at 60 ºC and employing CHCl 3 as co-solvent presented excellent thermal stabilities, above 500 ºC, and the highest S BET , superior to 900 m 2 /g. Thus, these copolymers were chosen for making the supported Pd(II) catalysts. Hereafter, they will be referred to as SP-POP1, SP-POP2 and SP-POP3.

Characterization of catalyst supports

As highly crosslinked materials, all POPs were insoluble in organic solvent, in very low pK a acids and in high pK a bases, showing excellent chemical stability. ATR-FTIR and CP/MAS 13 C NMR spectra of the SP-POPs selected as catalyst supports were compared with the corresponding POP4 (135TPB-isatin) in Figures 1 and2, respectively. The absorption bands at 1712, 1467 and 1320 cm -1 were assigned to the 5-membered lactam rings coming from isatin.

The weak C=O stretching band around 1710 cm -1 in the spectra of SP-POP1 confirmed the reaction of DAFO. In the case of the copolymers, the band at 1570 cm -1 , which was attributed to The surface morphology of the three SP-POPs was explored by FE-SEM. Figure 4 compares the images of these catalyst supports with that of POP4 (135TPB-Isatin). The images of SP-POP1 and SP-POP2, having a higher content of DAFO, revealed very small size particles that form a homogeneous rough surface in which small agglomerates were visible. When the content of isatin was higher, such as in SP-POP3, the surface was rougher due to presence of a higher amount of agglomerates with irregular shapes. The porosity of SP-POPs was studied from the N 2 adsorption/desorption isotherms at -196 ºC, depicted in Figure 5. As indicated above, prior to measurements, the samples were dried at 250 ºC under vacuum for 10 h. The SP-POPs showed a high uptake at low relative pressures (P/P 0 < 0.01), which indicated the presence of micropores. 33 Furthermore, a significant hysteresis was observed in the desorption branch in the whole range of relative pressures. This behavior, indicative of the presence of constricted porosity, was more pronounced in sample SP-POP3, and it has already been reported in analogous materials derived from aromatic trifunctional monomers, such as triptycene and 135TPB, and ketones, such as isatin and 2,2,2trifluoroacetophenone. 30 The volume difference between the adsorption and desorption branches was higher in the case of SP-POP3, with an additional N 2 uptake at relative pressure P/P 0 = 0.22 of 24%. The SP-POP1 and SP-POP2, having the higher percentage of DAFO, showed a similar behaviour. The step increase in the amount of gas adsorbed at relative pressures above 0.98 is attributed to nitrogen condensation in the interstitial voids of the polymer particles.

Figure 5 (b andc) show the pore size distributions of the samples; as seen, all three SP-POPs exhibited a marked contribution of micropores, with a peak centred at 0.50 nm for SP-POP1 and SP-POP2 and at 0.61 for SP-POP3 and a tail of lower intensity spanning in the range 1-10 nm.

The main textural parameters and the skeletal density of SP-POPs are summarized in Table 2.

The porosity of all three SP-POPs was estimated to be close to 35%; the contribution of the microporosity to the total porosity was around 75% for samples SP-POP1 and SP-POP2, whereas it accounted for ca. 65% for sample SP-POP3. This corroborated the microporous character of these catalytic supports. a skeletal density (g cm -3 ) measurement by Helium picnometry; b Specific surface area (m 2 g -1 ) calculate by BET equation; c Total pore volume (cm 3 g -1 ) calculated at P/P 0 = 0.99; d Micropore volume (cm 3 g -1 ), calculated from DR equation; e Porosity (%) defined as ratio from V total to (V total + (1/density)); f microporosity fraction, defined as the ratio V micro /V total (%).

Characterization of Pd-supported catalysts

Palladium(II) coordination reaction of catalyst support was carried out by direct mixing the SP-POP with Pd(OAc) 2 , as already described in the Experimental section. Hereafter, these materials will be referred to as Pd@SP-POPx, where x is 1, 2 or 3 according to the catalyst support. The ATR-FTIR spectra (Figure 6) showed two peaks at 1700 and 1359 cm -1 that were assigned to the acetate groups of the Pd(OAc) 2 bipyridine complexes. 34 The asymmetric COO - stretching band at 1700 cm -1 was clearly visible in Pd@SP-POP1 because the catalyst support does not have carbonyl groups in its structure (cf. Figure 1). In the two other supported Pd(II) catalysts, this band overlapped with the C=O band derived from carbonyl group of lactam rings. The WAXS patterns of Pd-supported catalysts, depicted in Figure 7, showed very weak reflections around 12º and 33º, which could come from Pd(OAc) 2 . 35 Comparing the amorphous halos of Pd@SP-POPxs with those corresponding to the precursor porous polymers, significant changes in the patterns were observed, especially in the Pd@SP-POP1 and Pd@SP-POP2. In these cases, the patterns of supported Pd(II) catalysts showed broader maxima and a lower intensity ratio between them, as compared to those of precursor porous polymers. This finding seems to indicate that the formation of Pd(II) bipyridine complexes mainly led to a higher contribution of small intersegmental distances (high scattering angle side) to the global scattering and, then, a change in the packing of catalyst support could be possible. 3. The load amount of Pd embedded in all the SP-POPs was around 95%, related to the theoretical value calculated from their bipyridine content.

Table 3. Pd content of POP supported Pd(II) catalysts catalyst

Pd content (mg g -1 )

Experimental a Theoretical b Pd@SP-POP1 172 180

Pd@ SP-POP2 111 115 21 Pd@SP-POP3 64 67

a Determined by ICP-OES. b Calculated from its bipyridine content.

Catalytic activity

The cross-coupling reaction of 4-bromoanisol with phenylboronic acid was tested for all the Pd@SP-POP catalysts, employing classical Suzuki-Miyaura conditions. Yields were very high (quantitative), as seen in Table S2.

The Pd@SP-POP2 was picked out to evaluate its catalytic activity for Suzuki-Miyaura crosscoupling reaction of aryl halides with phenylboronic acid. This choice was made attending to its Pd load (around 10%), to the value of S BET and microporosity percentage of the precursor porous polymer relative to the two other supports studied (cf. Table 2). In addition, the cost of the material, the DAFO monomer is much more expensive than isatin, and SP-POP2 is economically more competitive than SP-POP1. SP-POP3 was not considered due to the relatively low value of Pd load.

In a first stage, the Suzuki-Miyaura coupling reaction of 4-bromotoluene with phenylboronic acid at 80 ºC, using aerobic conditions, was chosen as a model for optimizing reaction parameters such as the employed solvent and base. Different solvents and bases were tested, as seen in Table 4. Excellent yields of the cross-coupling product were obtained using DMF/H 2 O

(2/3) and EtOH/H 2 O (2/3) mixtures as solvent in presence of bases such as Cs 2 CO 3 and Na 2 CO 3 .

According to these findings and the essential goal of employing green reagents, the EtOH/H 2 O

(2/3) mixture and the Na 2 CO 3 were selected for this study. a Reaction conditions: 4-bromotoluene (1 mmol), phenylboronic acid (1.5 mmol), base (2 mmol), Pd@SP-POP2 (1% aryl derivative/catalyst) and solvent (10 mL). b Conversion values were obtained by 1 H-RMN.

In a second stage, a set of coupling reactions were made, reducing the reaction time and the amount of Pd@SP-POP2, employing other aryl bromides and chlorides with electron-donating groups (tert-butyl and OCH 3 groups), as shown in Table 5. In the case of the reaction of 4bromotoluene in EtOH/H 2 O (2/3), the reduction of reaction time to 1 hour (Entry 7) or lessening the amount of Pd-supported catalyst in half (Entry 8) led to lower conversions of reaction than that obtained previously (Entry 6). The reaction of 1-bromo-4-tert-butylbenzene with phenylboronic acid for 1 h resulted in 45% conversion (Entry 9), which was much lower than the conversions obtained for 1-bromotoluene (Entry 7) and 4-bromoanisol (Entry 11) under the same reaction conditions. This finding could be explained under the assumption of a lower diffusion transport of 1-bromo-4-tert-butylbenzene through the pores due to the low solubility of the aryl derivative in the solvent mixture. In order to solve this problem, the use of a solvent mixture able to dissolve more efficiently the reagents was considered. Thus, when an EtOH/H 2 O volume ratio of 3/2 was used, the conversion of coupling reaction was much higher (97%) (Entry 10). As to the 4-bromoanisol, the complete conversion achieved led us to attempt to change some other reaction parameter from Entry 11: 1) reducing the reaction time to 0.5 h (Entry 12) and 2) and lowering the amount of catalyst at 0.5 % mol (Entry 13). None of these changes significantly decreased the reaction yields and the conversions remained higher than 87%.

Aryl chloride derivative, 4-chorotoluene, was also tested to evaluate the differential catalytic activity of Pd@SP-POP2 in the Suzuki-Miyaura reaction between bromo and chloro substituents.

The reaction conversion of 4-chlorotoluene (Entry 14) was moderated (around 70%) as compared with its bromo analogue employing the same conditions (Entry 7). The use of a 3/2 EtOH/H 2 O mixture (Entry 15) did not improve the cross-coupling reaction conversion. The products of reaction were determined by GC-MS analysis and the formation of 4,4'dimethybiphenyl (homocoupling compound formed from 4-chlorotoluene) was observed. This unexpected fact may be interesting to employ this type of catalysts in homocoupling reactions. In view of the excellent conversions obtained using an excess of 1.5 mol of phenylboronic acid per 1 mol of aryl halide, the ratio of boronic acid/aryl bromide was reduced to 1.2, resulting also in high reactions conversions, as seen in Table 6.

The number of catalytic conversions (TON) and the frequency of conversions (TOF) for the reaction of 4-bromoanisol and phenylboronic acid were determined for a conversion value of 8%, using 0.023 % mol of Pd and a reaction time of 15 min. The obtained values were excellent with values of 1580 and 7900 h -1 for TON and TOF, respectively. 36 The effectiveness of the Pd-supported catalyst with bulky and difunctional aryl halides and phenylboronic acid and 4-methoxyphenylboronic acid was also tested. The reactions yielded excellent conversions, as seen in Table 7. In particular, Pd@SP-POP2 was able to produce a quantitative yield of the monomer 5'-butyl-m-terphenyl (Entry 20). The compound 4-4''dimetoxy-5'-t-butyl-m-terphenylene, which is precursor of 4-4''-dihydroxy-3,3´´-diamino-mterphenylene (Entry 22), which could be used as a monomer able to produce thermally rearranged materials, 37 was obtained with a yield of 81%. A reaction made in our lab, using 4methoxyphenilboronic acid, 5-t-butyl-1,3-dibromobenzene and tetrakis(triphenylphosphine)palladium(0) (4%), using a mixture of toluene/water at 100 o C, gave a yield around 75%. a Conversion values obtained by 1 H-RMN.

Scale-up of Suzuki-Miyaura reactions

After the optimization process, scale-up of two of Suzuki-Miyaura reactions tested were carried out by reacting 20-fold the quantity of reagents employed during the catalysis study.

The first scale-up synthesis of 4-methoxy-4'-methylbiphenyl (Entry 18) was made using 1% (0.10 mmol) of Pd@SP-POP2, 50 mmol of 4-bromotoluene, 20 mmol of Na 2 CO 3 , 60 mmol of acid 4-methoxyphenylboronic and a solvent mixture of 40 mL of EtOH and 60 mL of water. The 1 H-RMN Conversion of the reaction at 80 ºC after 2 h was >99% and the yield of the pure product was of 93%

The second scale-up synthesis of the precursor monomer, 4-4'-dimetoxi-5'-t-butyl-mterphenylene (Entry 22), was made by using 1% (0.058 mmol) of Pd@SP-POP2, 6 mmol of 1,3-dibromo-5-tert-butylbenzene, 24 mmol of Na 2 CO 3 , 14 mmol of acid 4-methoxyphenylboronic and a solvent mixture of 35 mL of EtOH and 24 mL of water. Because of the high load of compounds in the reaction batch, a combination of ultrasonic (25% of amplitude) and magnetic stirring was employed. 1 H-RMN Conversion of reaction at 80 ºC after 3 h was of 100% and the yield of the pure product was of 96% 3.5. Catalyst recycling in Suzuki-Miyaura coupling Recyclability of the heterogeneous catalyst was studied from the Suzuki-Miyaura reaction of 4-bromoanisole with phenylbonic acid (Entry 16). As described in Experimental Section, after each cycle, the catalyst was separated from reaction mixture by centrifugation and washed with

EtOH. The first catalytic recycling studies were cumbersome, because a loss of the heterogeneous catalyst after each cycle was observed, which could be due to the loss of very small size porous polymer particles (lower than 100 nm) during the catalyst recovery.

It was considered to use magnetic stirring instead of ultrasonic stirring because the latter as a dispersion method of the particles in suspension also produces the breaking down of polymer network aggregates and, consequently, the formation of smaller size particles, which could easily be lost during the separation and washing of catalyst. Interestingly, the use of magnetic stirring brought about a quantitative conversion for the first cycle (> 98%), similar to the value observed when ultrasonic stirring was employed. The conversion was also quantitative for the second cycle. From the third one, a lessening was observed, which was similar to that found for the fourth one.

Therefore, and because the recycling reactions employed a tiny amount of catalyst, the reaction was scale-up to employ 4% mol of Pd@SP-POP2 catalyst. The results showed that the Any funds used to support the research of the manuscript should be placed here (per journal style).

Notes

Any additional relevant notes should be placed here.

ACKNOWLEDGMENT (Word Style "TD_Acknowledgments"). Generally the last paragraph of the paper is the place to acknowledge people, organizations, and financing (you may state grant numbers and sponsors here). Follow the journal's guidelines on what to include in the Acknowledgments section.

Scheme 1 .

 1 Scheme 1. Chemical structures of POPs. The numbers next to the monomers indicate the moles

  the C=N stretching from DAFO, was not clearly visible. The CP/MAS 13 C NMR spectra confirmed the reaction of the ketones with the aromatic rings; all of them showed a signal at 60 ppm corresponding to the quaternary sp3 carbons. The more characteristic peaks derived from DAFO and isatin moieties are indicated in the spectra. The other signals, between 135-115 ppm, were assigned to aromatic carbons.

Figure 1 .

 1 Figure 1. ATR-FTIR spectra of POP-based supports. The POP4 spectrum has been included for

Figure 2 .

 2 Figure 2. CP/MAS 13C NMR spectra of POP-based supports. The POP4 spectrum has been

Figure 3 .

 3 Figure 3. WAXS patterns of POP-based supports. The POP4 pattern has been included for

Figure 4 .

 4 Figure 4. FE-SEM images of SP-POPs. The POP4 image is included for comparison.

Figure 5 .

 5 Figure 5. (a) N 2 adsorption (full symbols)/desorption (empty symbols) isotherms measured

Figure 6 .

 6 Figure 6. ATR-FTIR spectra of Pd-supported catalysts. The dashed lines indicate the position of the characteristic bands of briding COO -groups coming from Pd(OAc) 2 .

Figure 7 .

 7 Figure 7. WAXS patterns of Pd-supported catalysts. WAXS patterns of precursor porous

Table 1 .

 1 Reaction yield, thermal properties and surface area (S BET ) of POPs according to their

	reaction conditions.
	POP	Reaction conditions	Reaction yield (%)	Td (°C)	Char yield at 800 °C (%)	S BET (m 2 g -1 )
		120 h/RT
	POP1-RT			92	595	79	805
		TFSA
		24 h/RT, 96 h/60 ºC
	POP1-60			93	605	79	785
		TFSA
	POP1-60/CHCl 3	24 h/RT, 96 h/60 ºC
				99	545	76	920
	SP-POP1	TFSA/CHCl 3
		24 h/RT, 96 h/60 ºC
	POP2-60			93	545	79	550
		TFSA
	POP2-60/CHCl 3	24 h/RT, 96 h/60 ºC
				99	525	75	930
	SP-POP2	TFSA/CHCl 3
	POP3-RT/CHCl 3	120 h/RT, TFSA/CHCl 3		99	570	75	820
	POP3-60/CHCl 3	24 h/RT, 96 h/60 ºC
				99	565	77	935
	SP-POP3	TFSA/CHCl 3
	POP4-RT/CHCl 3	120 h/RT TFSA/CHCl 3	a	99	565	70	760
	a reaction conditions previously optimized 30

Table 2 .

 2 Density and porosity parameters from N 2 adsorption isotherms.

	SP-POP	density a	S BET b	V total	c	V micro	d	(%) Porosity e	Microporosity r
	SP-POP1 1.044	920	0.47		0.36		33	77
	SP-POP2 1.125	930	0.46		0.35		34	76
	SP-POP3 0.996	935	0.53		0.34		34	64

Table 4 -

 4 The optimization of reaction parameters for the Suzuki-Miyaura reaction of 4-

	bromotoluene with phenylboronic acid a

Table 5 -

 5 Suzuki-Miyaura reactions of aryl bromides and chlorides with phenylboronic acid

	catalyzed by Pd@SP-POP2 catalyst

a Conversion values obtained by 1 H-RMN. b The EtOH/H 2 O solvent ratio was 3:2.

Table 6 -

 6 Suzuki-Miyaura reactions of aryl bromides with arylboronic acid, varying the ratio

	between the reagents, catalysed by Pd@SP-POP2 catalyst
	Entry R	Y	B(OH) 2 /ArBr	Conversion a

a Conversion values obtained by 1 H-RMN. b EtOH/H 2 O solvent ratio was 3:2.

Table 7 -

 7 Suzuki-Miyaura reactions of bulky reagents catalysed by Pd@SP-POP2

	Entry Reaction

ABBREVIATIONS

CCR2, CC chemokine receptor 2; CCL2, CC chemokine ligand 2; CCR5, CC chemokine receptor 5; TLC, thin layer chromatography.

Funding Sources

heterogeneous catalyst retained its catalytic activity after five consecutive cycles; the reaction yield was of 95%, as seen in Figure 8.

CONCLUSIONS

New microporous porous organic polymers able to anchor palladium(II)-based catalysts were successfully prepared. Precursor porous organic polymers, POPs, used as catalyst supports were synthesized by a straightforward and high-yield reaction of 1,3,5-triphenylbenzene with ketones activated by electrowithdrawing groups, 4,5-diazafluoren-9-one (DAFO) and 2,3-dioxo-2,3dihydroindole (isatin), in super acidic media using a TFSA/CHCl 3 mixture, in which the strong acid acted as a reaction promoter. These POPs were prepared varying the content of DAFO and isatin in order to modulate the number of bipyridine ligands in the catalyst support. The POPs were characterized employing classical methods of macromolecular chemistry, showing astonishing high thermal stability, well above 475 o C, and high char yield. These materials were mainly microporous with BET superficial areas superior to 900 m 2 g -1 .

The POP-based Pd(II) catalysts were prepared by simple immersion of the POP in a Pd(II) acetate solution. These catalysts gave very high yields for Suzuki-Miyaura reaction, no matter the boronic acid or the aryl derivative employed during the synthesis. The cross-coupling compounds could be prepared in a green solvent (EtOH/H 2 O), using low loads of metal, and under aerobic conditions. Moreover, these heterogeneous catalysts could easily be recovered from reaction mixture by washing in EtOH and they could be reused for five consecutive cycles retaining their excellent catalytic activity.

Finally, the synthesis of cross-coupling compounds could easily be scaled-up to give more than 2 g amounts. In particular, it was feasible to make a bulky monomer precursor having a mterphenylene moiety, with better yields than those obtained by using a common Suzuki-Miyaura methodology.

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