Jenny Jouin
email: jenny.jouin@unilim.fr

Helene Celerier

Lila Ouamara

Nicolas Tessier-Doyen

Sylvie Rossignol

Study of the formation of acid-based geopolymer networks and their resistance to water by time / temperature treatments

Keywords: Phosphoric acid, Metakaolin, Mechanisms of formation, XRD, FTIR, High Temperature, Water resistance, Amorphous network

teaching and research institutions in France or abroad, or from public or private research centers.

Study of the formation of acid-based geopolymer networks and their resistance to water by time/temperature treatments

Introduction

Geopolymers, in their alkaline-based form, are now known since many years and are the subject of numerous studies due to their relative low cost and good mechanical performances [START_REF] Davidovits | Geopolymer: chemistry and applications[END_REF]. They usually result from the dissolution of an aluminosilicate source in an alkaline silicate solution [START_REF] Gharzouni | Effect of the reactivity of alkaline solution and metakaolin on geopolymer formation[END_REF], leading to the formation of a tridimensional network combining SiO4 and AlO4 tetrahedra. However, recently, acid-based geopolymers are gaining an increasing interest from the scientific community. These materials are related to the phosphate cements, which have been studied for a longer time [ 3 ]. Although this subject is quite recent, several studies, especially using phosphoric acid, have indeed been carried out considering various aluminosilicate sources and different Si/Al, Al/P or Si/P ratios [START_REF] Tchakouté | Mechanical and microstructural properties of metakaolin-based geopolymer cements from sodium waterglass and phosphoric acid solution as hardeners: a comparative study[END_REF][START_REF] Celerier | Composition and properties of phosphoric acid-based geopolymers[END_REF][START_REF] Douiri | Structural, thermal and dielectric properties of phosphoric acid-based geopolymers with different amounts of H3PO4[END_REF]. These materials exhibit remarkable mechanical properties [START_REF] Tchakouté | Mechanical and microstructural properties of metakaolin-based geopolymer cements from sodium waterglass and phosphoric acid solution as hardeners: a comparative study[END_REF] and a high temperature resistance up to 1450°C [START_REF] Le-Ping | Preparation of phosphoric acid-based porous geopolymers[END_REF].

Moreover, a previous study [START_REF] Celerier | Influence of various metakaolin raw materials on the water and fire resistance of geopolymers prepared in phosphoric acid[END_REF] has stated that their properties (thermal and water resistance) are governed by their composition and synthesis temperature [START_REF] Bewa | Water resistance and thermal behavior of metakaolin-phosphate-based geopolymer cements[END_REF][START_REF] Cheng-Yong | Thermal resistance variations of fly ash geopolymers: foaming responses[END_REF] and can be closely related to the existence of different amorphous oxide networks in the material.

The synthesis of these acid-based geopolymers starts with the dissolution of an aluminosilicate source in an acidic medium, the most common one being phosphoric acid [START_REF] Louati | Acid based geopolymerization kinetics: Effect of clay particle size[END_REF].

A recent study showed that the subsequent release of Al 3+ and its reaction with H3PO4 then leads to the formation of Al-O-P-and Si-based networks [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF]. However, the structural organization of the networks forming during the geopolymerization is not clearly defined yet.

The initial reactive mixture containing aluminum, silicon and phosphorus, any phase forming in the different phase diagrams belonging to the Al2O3-SiO2-P2O5 system might indeed appear and eventually crystallize. In these binaries, some phases are however more prone to form and they will thus be described in the following.

Firstly, when considering the P2O5-Al2O3 binary, the addition of Al2O3 modifies the structure of the original phosphate glass, breaking the P-O-P bonds in favor of the more stable P-O-Al bonds. For low aluminum concentrations, it leads to the formation of Al(PO3)3, whereas AlPO4 is favored for higher aluminum concentrations [START_REF] Metwalli | Cation effects on anion distributions in aluminophosphate glasses[END_REF]. In the geopolymer mixtures, it indeed seems that AlPO4 occurs when the temperature increases, as observed in previous papers [START_REF] Tchakouté | Mechanical and microstructural properties of metakaolin-based geopolymer cements from sodium waterglass and phosphoric acid solution as hardeners: a comparative study[END_REF][START_REF] Louati | Acid based geopolymerization kinetics: Effect of clay particle size[END_REF]. Various polymorphs have however been reported for different systems, such as berlinite-, cristobalite-or tridymite-type structures [START_REF] Graetsch | High-temperature phase transitions and intermediate incommensurate modulation of the tridymite form of AlPO4[END_REF]. In the case of the SiO2-Al2O3 system, an increase of temperature up to 1000°C results in the crystallization of mullite (3Al2O3-2SiO2), which has been observed by some authors after a high temperature treatment of acid-based geopolymers [START_REF] Le-Ping | The phase evolution of phosphoric acid-based geopolymers at elevated temperatures[END_REF][START_REF] Gualtieri | Preparation of phosphoric acidbased geopolymer foams using limestone as pore forming agent -Thermal properties by in situ XRPD and Rietveld refinements[END_REF]. The SiO2-P2O5 binary is not encountered in a lot of cases, but the additional formation of Si-O-P networks in geopolymers was reported for some compositions [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF][START_REF] Perera | Relative strengths of phosphoric acid-reacted and alkali-reacted metakaolin materials[END_REF] and led to the crystallization of SiO2 and phosphosilicates (SiP2O5 [START_REF] Dupree | Magic angle spinning NMR of alkali phospho-alumino-silicate glasses[END_REF]) in the case of high Si/P ratios [START_REF] Le-Ping | The phase evolution of phosphoric acid-based geopolymers at elevated temperatures[END_REF][START_REF] Khabbouchi | Interaction of metakaolin-phosphoric acid and their structural evolution at high temperature[END_REF]. Finally, numerous studies [START_REF] Aramendía | DRIFT, and FT-Raman characterization of SiO2-AlPO4-B2O3 ternary catalytic systems[END_REF][START_REF] Cody | Silicate-phosphate interactions in silicate glasses and melts: I. A multinuclear ( 27 Al, 29 Si, 31 P) MAS NMR and ab initio chemical shielding ( 31 P) study of phosphorous speciation in silicate glasses[END_REF][START_REF] Digiovanni | Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join[END_REF][START_REF] Kosinski | Raman and NMR spectroscopy of SiO2 glasses co-doped with Al2O3 and P2O5[END_REF] have been devoted to glasses belonging to the SiO2-Al2O3-P2O5 system. Their conclusions confirm what is observed in the different binaries, namely i) the strong affinity between Al 3+ and P 5+ , which promotes the formation of Al-O-P bonds, ii) the existence of free amorphous silica, and, iii) when the stoichiometry allows it, the formation of the unfavorable Si-O-P bonds.

The present work aims to better understand the network organization during the geopolymerization of a mixture of metakaolin and phosphoric acid, with an enhanced view at the early stages of the reaction. The reactive mixture was calcined at different times and temperatures, and then characterized by Fourier Transform InfraRed (FTIR) and X-Ray Diffraction (XRD). The goal is to be able to build acceptable structural models for the geopolymerization process that occurs in a fully disordered state at room temperature. The identified networks were compared with the water resistance of the calcined samples.

Materials and methods

Samples preparation

The samples were synthesized from an ACS grade solution of 85 wt% of phosphoric acid produced by VWR (Radnor, USA) and the metakaolin denoted M3, produced by Imerys (Limoges, France) which was studied in previous works by Gharzouni et al. [START_REF] Gharzouni | Control of polycondensation reaction generated from different metakaolins and alkaline solutions[END_REF][START_REF] Gharzouni | thesis: Contrôle de l'attaque des sources aluminosilicates par la compréhension des solutions alcalines[END_REF]. Before the synthesis, kaolinite, with a Si/Al molar ratio equal to 1, was calcined at 750°C for 4 hours to obtain M3 metakaolin. Then the acid-based geopolymer was synthesized at room temperature by mixing it with phosphoric acid, keeping an Al/P ratio of 1. The complete synthesis procedure is described in a previous study [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF]. This particular composition, with a very long consolidation time (21 days), was chosen in order to ensure a sufficient time between the measurements and the possibility to observe as many steps as possible occurring during the overall reaction. Immediately after the mixing, samples composed of the reactive mixture were taken off and thermally treated at different temperatures (200, 300, 500, 800 and 1000°C) in a furnace with a 5°C.min -1 slope and a 1 h dwelling time. This experimental procedure was repeated at different stages of the reaction (0, 2, 5, 10, 15, 20 and 25 days). Figure 1 represents a graphical overview of the selected samples, highlighting which samples were used for the two studies presented in this paper. The first one is the characterization of the early stages of geopolymerization whereas the second concerns the time and temperature dependency for the crystallization of phases. The heat-treated samples were systematically hand-ground in an agate mortar for the structural characterizations, and kept in their bulk form for the estimation of their water resistance.

Samples characterization

The XRD patterns were recorded with a Bruker D8 Advance diffractometer using CuKα radiation. The acquisitions were carried out with a 2θ-angle varying from 5 to 60° with a step size of 0.02° and an equivalent time per step of 50 s. Their analysis was then performed using the powder diffraction file (PDF) database of the International Center for Diffraction Data to determine the various crystalline phases occurring within the samples. The list of the crystalline phases identified in this work, along with their notation in this paper, is given in Table 1. The decomposition of the diagrams was conducted using the Peakoc software [START_REF] Masson | PEAKOC profile fitting software v1.0[END_REF] in order to extract the contributions from both crystalline and amorphous phases in the consolidating binder. The refinement was done between 5 and 60° using a Voigt function that takes into account the CuKα1 and CuKα2 wavelengths of the XRD source. In addition, the continuous background was fitted with a 2 nd order polynomial. with a resolution of 4 cm -1 . The data were analyzed on the OMNIC software provided by Nicolet Instrument.

A selection of heat-treated samples was immersed in deionized water with a liquid/solid volume ratio equal to 15. The pH value of the water, originally at 6.5, was measured after 1 day with a pHmeter. Then the external visual aspect of the samples was observed after 7 days of static immersion in water: if coarse cracks or disaggregation could be observed on a specimen, it was classified into non-resistant group, whereas if no defect was visible, the specimen was classified into the water-resistant category.

Results

Description of the samples before heating

The full study of the consolidation of the sample at room temperature was previously published [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF]. However, for an easier comprehension of the present results, a brief summary of the sample's characteristics at room temperature will be given here. During the geopolymerization, the evolution from the reactive mixture, e.g. metakaolin and phosphoric acid, to the consolidated sample, can be followed by XRD and FTIR. It can be done, as shown for example in Figure 2, by detecting specific contributions and studying their respective intensity in the diagram. In this case, the first contribution is present in the metakaolin and progressively disappears while the second one is increasing its intensity. The same was observed with FTIR. Associated with a Nuclear Magnetic Resonance (NMR) spectroscopy study ( 31 P, 29 Si, 27 Al), the main networks of the geopolymer were identified. After the dissolution of the metakaolin and the bonds formations, it is indeed composed of Al-O-P networks, with minor contributions of Si-O, Si-OH and Si-O-P. After heating, the Al-O-P part of the sample crystallizes, while the silicate part remains unchanged.

Study 1: early stages of the geopolymerization (0 and 2 days)

The first temperature dependent study was focused on the very early stages of geopolymerization, i.e. immediately after the mixing of the reagents (0 day) and at the beginning of the reaction (2 days). Each sample was calcined at 200, 300, 500, 800 and 1000°C, crushed into powder in an agate mortar and pestle, and then characterized by FTIR and XRD.

Figure 3 and Figure 4 present the obtained results. For the ease of reading, only the 600 to 1400 cm -1 part of the FTIR spectra are presented here as it corresponds to the characteristic wavenumbers of the metal-oxygen bonds. The corresponding full spectra are available in supplementary material (S1 and S2).

First, the FTIR spectra of the original mixture, calcined at different temperatures, are shown in Figure 3a. For the lowest temperature treatment, i.e. at 200°C, five contributions are observed at 1199, 1092, 984, 802 and 669 cm -1 and attributed to Si-O-Si [START_REF] Ting | Phosphate content affects structure and bioactivity of sol-gel silicate bioactive glasses[END_REF], P-O [START_REF] Burrell | Aluminium phosphate adjuvants prepared by precipitation at constant pH. Part I: composition and structure[END_REF], P-O from H2PO4 2-[29], Si-O-Si [START_REF] Hk Ting | Phosphate content affects structure and bioactivity of sol-gel silicate bioactive glasses[END_REF] and O-Si-O [START_REF] Koloušek | Preparation, structure and hydrothermal stability of alternative (sodium silicate-free) geopolymers[END_REF], respectively. All of these bands correspond to the reagents, testifying that the reaction has hardly begun at this stage. With the increase of the temperature (300°C and 500°C), and the subsequent increase of the energetic input, new bands appear at 1286, 974 and 926 cm -1 and are attributed to Al(PO3)3 [START_REF] Chukanov | Infrared spectroscopy of minerals and related compounds[END_REF] compounds, and Al-OH-Al bonds [START_REF] Frost | A vibrational spectroscopic study of the phosphate mineral vantasselite Al4(PO4)3(OH)3•9H2O[END_REF] for the last one. When the temperature reaches 800°C, the Al(PO3)3 signature disappears and is replaced by contributions from two different AlPO4 polymorphs [START_REF] Rokita | The AIPO4 polymorphs structure in the light of Raman and IR spectroscopy studies[END_REF] (1232 cm -1 and 1113 cm -1 corresponding respectively to the orthorhombic and the triclinic forms). Finally, at 1000°C, the formation of the two AlPO4 polymorphs is complete (1121 cm - 1 for the triclinic and 1232, 728, 712 cm -1 for the orthorhombic [START_REF] Rokita | The AIPO4 polymorphs structure in the light of Raman and IR spectroscopy studies[END_REF]). In addition, P=O (1321 cm -1 [START_REF] Rokita | Spectroscopic studies of the amorphous SiO2-AlPO4 materials[END_REF]) and Si-O-P (1188 cm -1 [START_REF] Kamal | Structural and thermal properties of monolithic silica-phosphate (SiO2-P2O5) gel glasses prepared by sol-gel technique[END_REF]) bonds are evidenced at 1000°C. These compounds were already identified by a joint FTIR and NMR spectroscopy in a previous study [START_REF] Celerier | Relation between working properties and structural properties from 27 Al, 29 Si and 31 P NMR and XRD of acid-based geopolymers from 25 to 1000[END_REF]. 1 for the peaks attribution.

These results are in good agreement with the XRD study as shown in Figure 3b. First, all the samples display weak contributions of quartz and anatase, originating from the starting metakaolin. Apart from these crystallized minerals, the sample is amorphous at 200°C and only a very broad contribution is visible at approximately 25°. Peaks from orthorhombic AlPO4 start to emerge at 300°C and are clearly visible at 500°C. In addition, triclinic AlPO4 (O1) is formed at 800°C, while the orthorhombic polymorph is overlapped due to this scarce presence. Finally, at 1000°C, orthorhombic Al12P12O48 (O2) and triclinic AlPO4 (T) are well crystallized, while the amorphous part of the sample tends to disappear. The Al(PO3)3 (H) polymorph, although detected by FTIR, is not seen here. The long-range organization of this phase is probably not sufficient to generate peaks of sufficient intensity.

After a calcination at 200°C, the FTIR spectrum of the 2-days old sample (Figure 4a) is characterized by bands at 1097 cm -1 (P-O), 802 cm -1 (Si-O-Si) and 669 cm -1 (O-Si-O), which were previously identified in the 0-day sample. Additional bands, however, appear at 1232 cm - indicating that it was probably also present at lower temperatures. Moreover, bands from triclinic AlPO4 (1121 cm -1 ) as well as Si-O-P bonds (1188 cm -1 ) are identified. Finally, at 1000°C, P=O (1321 cm -1 ) and a few berlinite (hexagonal AlPO4) (650 and 625 cm -1 ) bonds are formed in addition to the previous ones [START_REF] Frost | A vibrational spectroscopic study of the phosphate mineral vantasselite Al4(PO4)3(OH)3•9H2O[END_REF].

The identification of crystalline phases by XRD (Figure 4b) is coherent with FTIR.

Peaks from the orthorhombic polymorph of AlPO4 (O1) can be seen in the 200°C to 800°C temperature range. From 800°C, the triclinic polymorph of AlPO4 (T) is formed and at 1000°C only the peaks from the latter and the orthorhombic Al12P12O48 (O2) are identified. It can be noted that the phase corresponding to Al-OH-Al bonds , Al(PO3)3 (H) and the berlinite (B) observed on FTIR spectra could not be detected, probably due to their low volume fraction in the sample or highly disordered nature. 1 for the peaks attribution.

To summarize, in the first steps of the geopolymerization (0 and 2 days), the predominant identified compounds are Al(PO3)3 and orthorhombic AlPO4 (O1) at low temperature (200 to 500°C). With the increase of the temperature, Al(PO3)3 and AlPO4 (O1) disappear, and triclinic AlPO4 (T) is formed from 800°C and Al12P12O48 (O2) at 1000°C. Finally, large amounts of disordered material remain in the sample, with the exception of the ones calcined at 1000°C.

Study 2: formation of partially organized networks

In order to follow the crystallization of the different phases in the sample until its complete consolidation, estimated at 21 days [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF], additional samples of the reactive mixture 1000°C for consolidation times of 0, 5, 10, 15, 20 and 25 days. See Table 1 for the peaks attribution.

The XRD measurements were complemented by a systematic FTIR spectroscopy analysis, whose results are presented in Figure 6. As stated previously, only the 600 to 1400 cm -1 part of the FTIR spectra are presented here, the full spectra being available in supplementary material (S3, S4 and S5). Firstly, at 200°C, the evolution of the presence of the orthorhombic polymorph of AlPO4 is confirmed by the intensity of its band situated at 1232 cm -1 as shown in Figure 6a. The band at 926 cm -1 , corresponding to Al-OH-Al bonds is in agreement with the presence of AlP3O8(OH)2. However, contrarily to what was observed by XRD, this compound is identified as soon as 2 days after the preparation of the reactive mixture.

Moreover, the occurrence of the berlinite-type AlPO4 from 5 days on, whose contribution strengthens with time, is confirmed by the increasingly intense bands at 749, 706, 698 and 650 cm -1 [START_REF] Chukanov | Infrared spectroscopy of minerals and related compounds[END_REF]. Secondly, at 500°C (Figure 6b), there is a clear contribution of Al(PO3)3, in the FTIR spectra, with bands at 1112, 749, 732, 706, 698 and 650 cm -1 [START_REF] Chukanov | Infrared spectroscopy of minerals and related compounds[END_REF] visible from 5 days on. The presence of berlinite-type AlPO4 is also confirmed by its well-defined contributions in the same time range. Moreover, AlP3O8(OH)2, represented by Al-OH-Al bonds (926 cm -1 ) is also visible on all the spectra, although it was difficult to observe in XRD due to its low proportion in the sample. No clear signature from the orthorhombic phases, namely Al16P16O64 (O3) and orthorhombic AlPO4 (O1) were evidenced in the FTIR spectra as they are probably overlapped by other contributions due to the similarities of the considered structures. Finally, at 1000°C, the strong presence of triclinic AlPO4 (1121 cm -1 ) and orthorhombic Al12P12O48 (1232, 728, 712 cm -1 ), as well as weak contribution of berlinite-type AlPO4 (650, 625 cm -1 ) were confirmed, the triclinic AlPO4 being the most visible. Moreover, P=O and Si-O-P bonds are observed similarly to our previous study [START_REF] Celerier | Relation between working properties and structural properties from 27 Al, 29 Si and 31 P NMR and XRD of acid-based geopolymers from 25 to 1000[END_REF].

Discussion

In this study, the crystalline phases identified are mainly composed of aluminum and phosphorus. In order to verify this assumption, as the reagents also contain silicon, the decomposition of the XRD contributions of the silica-based network (S) and of the geopolymerbased network (G) was performed. The method used is described in a previous work [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF] and an example of the results is presented in Figure 7. The identified crystalline structures, most of which belong to the zeolites family, have already been refined by other authors, and they are listed in Table 2, including some structural data and cif file references. From a structural point of view, they can be divided into three main categories. The first one, being berlinite-type [START_REF] Brill | On the chemical bond type in AlPO4[END_REF], triclinic tridymite-type [START_REF] Graetsch | Two Forms of aluminium phosphate tridymite from X-ray powder data[END_REF] and orthorhombic AlPO4 [START_REF] Achary | Preparation, phase transition and thermal expansion studies on low-cristobalite type Al1-xGaxPO4 (x = 0[END_REF] show some up-down-up-down structures made of an alternation of corner sharing AlO4 and PO4 tetrahedra [START_REF] Bennet | Structural features of aluminophosphate materials with Al/P=1[END_REF]. They are described as strict 6-ring structures, and are analogous of the polymorphs of silica. The level of distortion of these structures defines the symmetry of the phase. The second group, with Al12P12O48 [START_REF] Richardson Jr | AlPO4-25: Framework topology, topotactic transformation from AlPO4-21, and high-low displacive transition[END_REF] and Al16P16O64 [START_REF] Keller | Synthesis, structures of AlPO4-C and AlPO4-D and their topotactic transformation[END_REF] zeolitetype structures, is made of 8-rings structures of corner sharing AlO4 and PO4 tetrahedra, linked together by smaller 4-rings or 4+6-rings entities. Chains alternating aluminum and phosphorus cations characterize them both. When previously studied, they were synthesized by the thermal treatment and dehydration of hydrated AlPO4 prepared by hydrothermal synthesis [START_REF] Yvoire | Etude des phosphates d'aluminium et de fer trivalent. I. L'orthophosphate neutre d'aluminum[END_REF],

similarly to AlP3O8(OH)2. Finally, Al(PO3)3 [START_REF] Fratzky | Crystal structure of aluminium cyclononaphosphate Al3P9O28[END_REF] and AlP3O8(OH)2 [START_REF] Lyutsko | Investigation of crystal structure of aluminium dihydridophosphate AlH2P3O10-II[END_REF], consist in structures made of corner sharing AlO6 octahedra and PO4 tetrahedra. Concerning the phosphate part of the structures, Al(PO3)3 is characterized by P4O8 groups while the other one exhibits some open P3O10 phosphates structural units. Moreover, in both cases the AlO6 are isolated from each other and only connect to PO4. Figure 8 summarizes all the phases identified in the samples, with the exception of Figure 8e, which recalls which samples have been synthesized. For each of them, the intensity of the black color of the dots represents the intensity of their contribution to the diagram, and the continuous grey area highlights the samples for which this phase is predominant. First, considering the 6-rings structures (Figure 8a, b and c), phases of different symmetry were identified in every diagram. At short times and low temperature, the slightly distorted O1 phase briefly dominates, but is replaced, still for low temperatures, by a better organized B phase starting at 10 days. This reorganization can be the reason of the slight modification of the geopolymerization rate that was observed at 11 days in a previous study performed on the same sample [START_REF] Mathivet | Acid-based geopolymers: understanding of the structural evolutions during consolidation and after thermal treatments[END_REF]. Such a transformation was also observed by Kominami et al. [START_REF] Kominami | Crystallization and transformation of aluminum orthophosphates in organic solvent containing a small amount of water[END_REF] during a high temperature hydrolysis in an organic solvent. The formation of the metatstable phase evolving into berlinite was then attributed to a weak dissolution in the solvent used. At 1000°C, the age of the reactive mixture is not significant, as such a high amount of energy always favors the crystallization of T phase. This evolution is well documented, in the phase diagram of AlPO4, as the phase transition berlinite to tridymite is reported at 815°C in aluminophosphates [START_REF] Rey | Ultrarotabsorption von AlPO4 und SiO2 in Abhängigkeit von Fehlordnung und Temperatur[END_REF],

which is coherent with our observations.

The second group, with the zeolite-type 8-ring structures, is represented in Figure 8d and f. Depending on the calcination temperature, two different structures are observed. At 1000°C (Figure 8d), Al12P12O48, with its 8+6+4 rings network, is identified in all the diffraction diagrams, regardless of the reaction time. This phase, also known as AlPO4-25, was previously studied by Richardson et al. [START_REF] Richardson Jr | AlPO4-25: Framework topology, topotactic transformation from AlPO4-21, and high-low displacive transition[END_REF] and is the result of the temperature induced (500°C) dehydration and structural reorganization of AlPO4-21. At 500°C another structure, Al16P16O64

(or AlPO4-D) with its pure 8+4-rings network is more visible with the increasing reaction time (Figure 8d). Its formation and structure was studied by Keller et al. [START_REF] Keller | Synthesis, structures of AlPO4-C and AlPO4-D and their topotactic transformation[END_REF], from the calcination at 250°C of AlPO4-C, which originated from the dehydration of AlPO4-H3, or from the direct dehydration of AlPO4-H6. AlPO4-25 and AlPO4-D share relatively similar structures, and differ mostly by their formation temperature. We can thus suppose that in the sample studied in this work, they were crystallized from the same amorphous network. Moreover, whatever the structure, no signature of the crystalline hydrated phase was evidenced, but FTIR bands corresponding to hydrated phases can be seen in Figure 6, and a part of these contributions could correspond to amorphous hydrated phases with a tetrahedral chaining close to the ones observed in AlPO4-21 or AlPO4-H6 for example. The amorphous part of the diagrams is indeed clearly visible, as shown in Figure 8h. Finally, the remaining phases are shown in Figure 8g and i. Al(PO3)3, with its Al 3+ in 6coordination is well represented at 500°C, whatever the age of the reactive mixture, as seen in Figure 8g. This reveals that after its dissolution in the acid, a part of the aluminum tends to keep its original octahedral neighborhood. This is also true for hydrated AlP3O8(OH)2 (Figure 8i), observed as a minority phase in samples calcined at 200°C and 500°C.

It is interesting to point out that, whatever the phases that crystallize, all these open structures are made of small rings, with only 4, 6 or 8 entities connected, when aluminophosphate zeolites can present very large openings [START_REF] Baerlocher | Atlas of Zeolite Framework Types[END_REF]. This is also true at low calcination temperatures when the system only received a small quantity of thermal energy. We can thus suppose that the original amorphous network is probably made of relatively compact aluminophosphate rings. Moreover, it seems that a small part of the aluminum stays in its octahedral configuration, but the majority of the reagent belongs to two types of tetrahedralbased networks that coexist in the sample, as shown by the simultaneous crystallization of silica isostructural AlPO4 polymorphs (6 rings), and zeolite-type structures based on 8 rings structures.

To verify whether these treatments would modify the properties of the geopolymer, the water resistance of the calcined samples was tested, and the pH value of their immersion water was measured after 1 day. The results are presented in Figure 9, the original pH of deionized water being 6.5. Note that if not calcined, after its complete consolidation (21 days) this sample is thermal resistant but not water resistant [START_REF] Celerier | Influence of various metakaolin raw materials on the water and fire resistance of geopolymers prepared in phosphoric acid[END_REF]. All the calcined samples, with the exception of the less than 10 days-old mixtures calcined at 200°C, are water resistant. The water resistant samples systematically contain crystallized phases, such as cristobalite-or berlinite-type AlPO4. , while the non-resistant samples are amorphous. In previous studies [START_REF] Celerier | Influence of various metakaolin raw materials on the water and fire resistance of geopolymers prepared in phosphoric acid[END_REF][START_REF] Celerier | Relation between working properties and structural properties from 27 Al, 29 Si and 31 P NMR and XRD of acid-based geopolymers from 25 to 1000[END_REF] the water resistance of different geopolymers prepared in phosphoric acid was characterized, showing that this property was not systematic. Some samples indeed were water resistant but not fire resistant or, as the present example, fire resistant but not water resistant. In this work, some characteristics of water resistant samples were pointed out. The water resistance was composition and consolidation temperature dependant. Moreover, an NMR study showed that a higher degree of ordering in the networks would favor water resistance [START_REF] Celerier | Relation between working properties and structural properties from 27 Al, 29 Si and 31 P NMR and XRD of acid-based geopolymers from 25 to 1000[END_REF]. This last point is confirmed in our study, as the ordering of the structure leads to its crystallization.

Moreover, the pH values measured in immersion water show that the time of reaction does not significantly influence the quantity of acid released in the environment, and the calcination temperature is a more critical parameter. The samples calcined at 200°C indeed liberate a larger amount of acidic groups while increasing the calcination temperature leads to a more limited release. This indicates that these species are not strongly bonded to the geopolymeric network, as its formation does not modify the phenomenon. The intensity of the amorphous contribution, centered at approximately 24°, can also be related to this pH value. As can be seen in Figure 5, this contribution is well visible at 200°C, intermediate at 500°C and it disappears at 1000°C. The transition from amorphous to crystalline phases in the material thus seems to be the main reason for the limited release of acidic species in the water.

Conclusion

During their lifetime, acid-based geopolymers can involve various and evolving configurations of phases within their structure, closely depending both on the time and on the temperature of an eventual thermal treatment. This work presents the identification of the networks that form during the polymerization of an acid-based geopolymer by heat-treating the reactive mixture at different ages. By studying the phase transformations induced by time and temperature, it was aimed to understand the steps occurring during the geopolymerization process, and was focused more precisely on the type of networks that are formed at the early stages of the reaction. The FTIR and XRD structural characterizations showed that seven different aluminophosphate-based crystalline phases could appear, associated or not with an amorphous phase. 3-dimensional networks with relatively small rings characterize these compounds; they are probably a good representation of the opening of the structures in the amorphous network. Moreover, three different networks, one based on aluminum in its octahedral configuration, especially visible at the beginning of the geopolymerization, and two based on the tetrahedral configuration of aluminum, were identified in the reactive mixture and later on in the geopolymer. The crystalline phases were identified by both XRD and FTIR, while Si-O-P and P=O networks were only observed via FTIR due to their high level of disorder. The water resistance of the calcined samples was tested, and is greatly improved by the thermal treatment even at low temperature. The release of unbounded acidic species in the water is also strongly related to the prior calcination temperature of the samples.

Bibliography

Figure 1 :

 1 Figure 1: various consolidation times and temperatures used for the thermal treatment of the

Figure 2 :

 2 Figure 2 : Diffraction diagrams of the metakaolin and the consolidated geopolymer.

Figure 3 :

 3 Figure 3: a) FTIR and b) XRD measurements of 0 day-aged reactive mixture thermally

1

  and 926 cm -1 and correspond to the orthorhombic form of AlPO4 and Al-OH-Al bonds, respectively. At 300 and 500°C, the predominant formation of Al(PO3)3 is visible with clear contributions at 1286 and 974 cm -1 . Their presence may overlap the possible position of orthorhombic AlPO4, making its presence unclear, but the contributions of the other bonds are still visible. With the increase of the temperature to 800°C, Al(PO3)3 contributions and Al-OH-Al bonds disappear and orthorhombic AlPO4 is visible again at 1232, 728 and 712 cm -1 ,

Figure 4 :

 4 Figure 4: a) FTIR and b) XRD measurements of 2 days-aged reactive mixture, thermally

  were considered for different reaction times (0, 2, 5, 10, 15, 20 and 25 days) and calcined at 200, 500 and 1000°C. The XRD diagrams of the calcined materials are presented in Figure5.At 200°C (Figure5a), whatever the duration, the sample always contains a part of amorphous phase, visible by the broad contribution centered at approximately 25°. No crystalline phase appears at 0 day, with the exception of quartz and anatase originating from the M3 metakaolin, which are visible in all the diffractograms. Then, the orthorhombic polymorph of AlPO4 (O1) appears from 2 days. The intensity of its contributions increases up to 5 days, then becomes less preponderant, and slowly decreases from 10 to 25 days. This decrease is concomitant with both the emergence of AlP3O8(OH)2 (Hy) and the increasing presence of berlinite (B), the hexagonal polymorph of AlPO4 that starts to crystallize in the 5-days sample. When the sample is fully solidified (25 days), berlinite is the predominant phase with minor contributions from orthorhombic AlPO4 and AlP3O8(OH)2.At 500°C (Figure5b), as seen previously the orthorhombic polymorph of AlPO4 (O1) crystallizes in the 0-day sample. The intensities of the related peaks then increase until 5 days and then remain stable afterwards. Contrarily to what happens at 200°C, Al(PO3)3 (H) appears with a weak intensity after 2 days. At this temperature, it is visible but remains with a very low intensity for longer consolidation times. AlP3O8(OH)2 (Hy) is identified from 5 days until the complete geopolymerization reaction. Moreover, at this temperature and starting at 5 days, a new orthorhombic aluminophosphate, with the composition Al16P16O64 (O3), is formed. The intensities of its contributions increase with the consolidation time and stay stable from 15 days on. Finally, after a 500°C thermal treatment, the fully consolidated sample (25 days) contains a mixture of the orthorhombic AlPO4, Al16P16O64 and berlinite-type AlPO4 as majority phases, with the contribution of AlP3O(OH)2 as a minority phase and a very weak signature of Al(PO3)3. At 1000°C (Figure5c), the amount of amorphous phase in the sample is barely noticeable. Starting at 0 days, all the samples contain two crystalline aluminophosphate compounds, namely triclinic AlPO4 (T) and the orthorhombic Al12P12O48 (O2). Their intensities remain stable within the ageing of the reactive mixture. Starting at 5 days, the berlinite-type polymorph of AlPO4 appears but keeps a relatively low intensity. Finally, when fully consolidated, the predominant phase is the triclinic polymorph AlPO4, with lower contributions from Al12P12O48 and berlinite-type AlPO4.

Figure 5 :

 5 Figure 5: XRD diagrams of the reactive mixture thermally treated at (a) 200, (b) 500, and (c)

Figure 6 :

 6 Figure 6: FTIR results of reactive mixture thermally treated at a) 200, b) 500, and c) 1000°C

  It shows the results obtained for the reactive mixture (0 day), calcined at 200°C (amorphous) and 800°C (amorphous and crystalline). If we define IS as the intensity of the Si-based network contribution and IG the intensity of the geopolymer-based network contribution, the evolution of the IG/IS ratio gives an information about the type of network that crystallizes while the other tends to remain amorphous. This ratio changes from 6.6 to 2.6, indicating that the amorphous Si-based network is not modified at the first stages of the reaction. For the fully crystallized samples, since Al-Si-O and Al-P-O compounds are isostructural, the original aluminophosphate compounds were considered.

Figure 7 :

 7 Figure 7: Decomposition of the different contributions in the XRD diagrams measured after

Figure 8 :

 8 Figure 8: Predominance of each phases (a) AlPO4 orthorhombic, (b) AlPO4 triclinic, (c)

Figure 9 :

 9 Figure 9: characterization of water resistance through pH values of the water in which

Table 1 :

 1 List of the crystalline phases discussed in this paper.

	Name	Chemical Formula	Cristal system	JCPDF	Symbol
	Berlinite	AlPO4	Hexagonal	01-089-4201	B
	Phosphotridymite	AlPO4	Triclinic	00-050-0054	T
	Phophocristobalite	AlPO4	Orthorhombic 00-011-0500	O1
	Aluminum phosphate	Al12P12O48	Orthorhombic 00-041-0566	O2
	Aluminum phosphate	Al16P16O64	Orthorhombic 00-041-0562	O3
	Aluminum phosphate	Al(PO3)3	Hexagonal	00-015-0364	H
	Aluminum phosphate hydroxyde	AlPO3O8(OH)2	Monoclinic	01-089-4201	Hy
	Quartz	SiO2	Trigonal	00-046-1045	Q
	Anatase	TiO2	Tetragonal	00-021-1272	A
	Fourier-transform infrared spectroscopy spectra of the samples, mixed in KBr, were
	recorded in transmittance mode on a ThermoFisher Scientific Nicolet 380 infrared
	spectrometer. The IR spectra were recorded after 64 scans over a range of 400 to 4000 cm -1

Table 2 :

 2 List of the identified crystalline phases, including some of their structural data.

	Symbol	Formula	ICSD	Alternate	Al	Rings	Reference
			reference	name	coordination	structure
	B	AlPO4	44880	Berlinite	[4]	6	[38]
	T	AlPO4	280307	Tridymite	[4]	6	[39]
	O1	AlPO4	97546	-	[4]	6	[40]
	O2	Al12P12O48	108832	AlPO4-25	[4]	8 + 4	[42]
	O3	Al16P16O64	69358	AlPO4-D	[4]	8 + 6 + 4	[43]
	H	Al(PO3)3	409479	-	[6]	none	[45]
	Hy	AlPO3O8(OH)2	39432	-	[6]	none	[46]


Racha Abed

Ali Abdine

Lucas Pagès

Marc Taillefer
email: marc.taillefer@enscm.fr

Florian Monnier
email: florian.monnier@enscm.fr

Hydroarylation of N-Allenyl Derivatives Catalyzed by Copper

Keywords: allenes, copper, hydroarylation, allylic compounds, arenes, homogeneous catalysis

A additive-free copper catalytic system was used to perform the addition of (hetero)aryl nucleophiles to N-allenyl derivatives. This intermolecular C-C bond formation occurs with both complete regio-and stereoselectivity to afford the linear (E) allylic product with moderate to excellent yields. This atom economical method is the first example of hydroarylation of allenes catalyzed by copper.

Allylic or vinylic molecules are highly valuable intermediates [START_REF] Ma | [END_REF] which are easy-to-prepare through transition metal-catalyzed hydrofunctionalization of monosubstituted allenes. [2][3][4] This atom economical strategy usually occurs without any prefunctionalization and with very high chemo-, regioand stereospecificity. Hydrofunctionalization of allenes mainly allowed the selective formation of C-N, C-O and C-C bond. [2,3,[5][6][7][8] The latter being performed through the addition of pronucleophiles such as malonates, [9][10][11][12] nitriles, [13][14][15][16] alkynes, [17][18][19] alkenes [20- 22] and aryl derivatives. The reaction of aromatic compounds with allenes, known as hydroarylation reaction, consists in adding an aryl moiety to a C-C double bond, and constitutes a powerful tool to form a Csp 3 -Csp 2 bond in regio-and stereoselective manner.

The first example of hydroarylation of allenes with ArH catalyzed by a transition metal was described by the group of Panunzi with a platinum(II) catalyst. [23] Since then, a lot of different transition metals (Sc, [24,25] Pd, [26] Rh, [27][28][29][30] Ir, [31] Co, [32] Ru, [33] Ni, [34] Mn [35,36] and Fe [37] ) have also been used to perform hydroarylation of allenes with aryl nucleophiles (Scheme 1), especially from cationic gold(I) catalysts that are well known as π-acid complexes. [38][39][40][41][42][43] Most of these hydroarylation methods are limited to electron-rich aryl nucleophiles and afford the linear product i.e. the anti-Markovnikov product (Scheme 1, equation 1). Other groups reported hydroarylation reactions with various nucleophilic partners such as aryl boronic acids ArB(OH)2, [44] aryl boronic esters ArB(OR)2 [45] or aryl halides ArX [46] allowing an ipso-arylation. These methods allowed the addition of a wide scope of aryl partners selectively on the central carbon of the allene, affording then either the endo-or the exo-olefinic product (Scheme 1, equation 2). These two regio-divergent pathways are providing interesting products that could be further functionalized through either the C-C double bond or the (hetero)aryl group.

Scheme 1. Metal-catalyzed hydroarylation of allenes: known methods and pathway described herein with Cu catalysis

Herein, we report an unprecedented copper-catalyzed hydroarylation of allenes illustrated by the addition of aryl and heteroaryl derivatives on N-allenyl compounds. Based on our previous works on coppercatalyzed hydrofunctionalization of allenamides, [47][48][49][50][51][52][53][54][55] we began to study the model reaction between Nallenyl-2-pyrrolidinone 1a with 1,3,5trimethoxybenzene 2a under various experimental conditions (Table 1). a Reaction conditions: 1a (0.5 mmol), 2a (1.2 equiv, 0.6 mmol) and catalyst (0.025 to 0.1 mmol) were placed in a screw tube under argon in 1 mL of solvent for 18h at 25 to 100°C. b NMR yields using 4-iodoanisole as internal standard. c The mixture was reacted for 10 hours. d Reaction performed with 1 mmol, 2 equiv of 2a.

We initially tested the reaction at 25 °C with 20 mol% of CuI as a catalyst in various solvents: 1,4-dioxane, THF, DMF and acetonitrile (entries 1-4, table 1). The latter was then considered as our reference solvent as it gave the desired product 3aa in 23% yield. A control experiment was conducted to prove that no reaction is occurring without any copper salts (entry 5, table 1). Different copper sources were then investigated, and Cu(CH3CN)4PF6 allowed the best rate of formation of 3aa (entries 6-9, table 1). We observed that the higher the temperature, the better the yield up to 100°C (entries 9-11, table 1). We then reduced the catalyst loading, and observed at this temperature no major difference between 10 or 20 mol% of Cu(CH3CN)4PF6 while the yield significantly decreased with only 5 mol% of copper (entries 12-13, table 1). Tests performed at 100 °C with 10 mol% of Cu(CH3CN)4PF6 showed that the reaction is not complete after 10 hours (entry 14, table 1). Finally, adding 2 equiv instead of 1.2 equiv of aryl reagent 2a allowed us to further increase the yield of 3aa (entry 15, table 1). The reaction was found to be totally stereo-(only the (E) product was obtained) and regioselective as the addition occurred only on the terminal carbon of the allene. With these optimized conditions (entry 12 and 14, table 1) in hand, we first explored the scope of the method for the hydroarylation of allenamide 1a with various readily available (hetero)aryl compounds 2a-i (Scheme 2). Noteworthy that obtained compounds are enamide-type molecules which constitute a versatile building block for medicinal chemistry and for total synthesis of natural products. [56] a Reaction conditions: 1a (0.5 mmol), 2a-i (1.2 equiv, 0.6 mmol, unless otherwise notified), Cu(CH3CN)4PF6 (10 mol%, 0.05 mmol), CH3CN (0.5M, 1mL), argon, 100°C, 18h. Isolated yields. b Reaction performed with 2 equiv, 1 mmol of aryl partner 2.

Scheme 2. Cu(CH3CN)4PF6-catalyzed hydroarylation of various (hetero)aryl compounds 2a-i with allene 1a. a

The reaction of both 1,3,5-trimethoxybenzene 2a and 1,2,4-trimethoxybenzene 2b with 1a gave the formation of 3aa and 3ab, respectively in quantitative (>99%) and in 90% isolated yields. Whereas 1,3dimethoxybenzene 2c afforded 3ac with a lower yield of 67%, 1,4-dimethoxybenzene and anisole did not yield the desired product. Different types of N-(pyrrole, indole) or S-containing (thiophenes) heteroaromatic compounds were also successfully engaged in the reaction with 1a, affording products 3ad to 3ag in medium to excellent yields. Other heteroaromatic substrates did not provided any product under these conditions, such as substituted pyridines, toluene and 1,3,5-trimethylbenzene. The meta-substituted N,N-dimethylanilines 2h and 2i afforded the corresponding products 3ah and 3ai in moderate yields. It is interesting to notice that 3ah could be potentially involved in further cross-coupling reaction through its aryl bromide moiety. Noteworthy that analysis of 1 H NMR spectra clearly showed the exclusive formation of one product with the (E) configuration of the double bond (see supporting information). Other N-allenyl derivatives bearing either a carbamate 1b or a sulfonamide moiety 1c were then successfully engaged in this copper-catalyzed hydroarylation, affording the corresponding products in good to high yields (Scheme 3). These substrates 1b and 1c afforded the corresponding linear (E) products with electronrich arene (3ba, 3ca), thiophene derivatives (3be), Nphenyl pyrrole (3bf), indole derivatives (3bg, 3cg) and aniline derivatives (3bh, 3ch) in moderate to good yields with a total control of regio-and stereoselectivity.

Based on our previous studies [49,51] and mechanisms described in the literature, [38,43,57] we suggest as the first step of the process the coordination of the copper to the N-allenyl derivative through the assisting group and the central carbon of the allene. We assume that the electron rich aryl is then reacting as a nucleophile on the terminal position of the allene. This electrophilic aromatic substitution (SEAr) leads to the formation of a plausible Wheland intermediate, which undergoes into protodecupration to afford selectively the (E)-product. The need of electron donating groups in adequate positions on the (hetero)aryl substrate is consistent with this mechanism proposal (Scheme 4). In summary, we reported an unprecedented coppercatalyzed hydroarylation of allenes illustrated by the addition of (hetero)aryl nucleophiles on N-allenyl derivatives which afford selectively (E)-allylic compounds with medium to excellent yields in a total atom economical fashion. Various readily available aromatic compounds were successfully engaged in the reaction with allene substrates, showing the good tolerance of the methodology. Nevertheless, we noticed that electron-rich substrates are required for the reaction to proceed, which is consistent with the literature and SEAr-type mechanism. Further investigations on the potential of copper-catalyzed systems for hydrofunctionalization of allenes will be reported in due course.

Experimental Section

Detailed experimental procedures and characterization data for all new compounds are provided (PDF).

General procedure for hydroarylation of N-allenyl derivatives: An oven-dried Schlenk flask of appropriate size equipped with a magnetic stirring bar is placed under vacuum then back-filled with argon. This procedure is repeated three times. Under a stream of argon, Tetrakis(acetonitrile)copper(I) hexafluorophosphate Cu(CH3CN)4PF6 (10 mol%) is added, followed by the (hetero)aryl nucleophile (1.2 equiv or 2 equiv as mentionned in main text), acetonitrile (0.5M, 1 mL) and the allene (0.5 mmol, 1 equiv). The Schlenk flask is then sealed under a positive pressure of argon, stirred and heated at 100°C for 18h. After allowing the reaction to cool down to room temperature, either 1,3,5-trimethoxybenzene (0.33 equiv, 0.165 mmol, 27.8 mg) or 4-iodoanisole (0.33 equiv, 0.165 mmol, 38.6 mg) is added as internal standard. The reaction mixture is then diluted in ca. 3 mL of dichloromethane and extracted with water. The organic layer is dried over anhydrous magnesium sulfate MgSO4 and the solvent is removed under vacuum. The residue is then purified by triethylamine NEt3 treated silica gel column chromatography.

Scheme 3 .

 3 Scheme 3. Extension to other N-allenyl derivatives and cross reactivity. a

Scheme 4 .

 4 Scheme 4. Plausible mechanism for the copper-catalyzed hydroarylation of N-allenyl derivatives.

Table 1 .

 1 Hydroarylation of 1a with 2a: Selected data for reaction parametric study. a

		solvent	T	[Cu]	x	yield
		(0.5M)	(°C)			(%) [b]
	1	1,4-dioxane	25	CuI	20	0
	2	THF	25	CuI	20	10

Acknowledgements

Financial support provided by École Nationale Supérieure de Chimie de Montpellier (ENSCM, PhD fellowship for RA and LP), Institut Universitaire de France (IUF, FM) and Centre National de la Recherche Scientifique (CNRS) are warmly acknowledged with thanks.

FULL PAPER Hydroarylation of N-Allenyl Derivatives Catalyzed by Copper

The regio-and stereoselective addition of (hetero)aryl nucleophiles to N-allenyl derivatives is described under copper catalysis. represents the first example of hydroarylation of allenes catalyzed by copper.

KEYTOPIC: Allene hydroarylation

Racha Abed Ali Abdine, Lucas Pagès, Marc Taillefer,* and Florian Monnier*