Anne-Laure Grevey
email: anne-laure.grevey@orange.fr

V Vignal

H Krawiec

P Ozga

K Peche-Quilichini

A Rivalan

F Mazière

A.-L Grevey

Anne-Laure Grevey

Microstructure and long-term corrosion of archaeological iron alloy artefacts

Keywords: carbon steel, SEM, X-ray diffraction, Raman spectroscopy, oxidation

published or not. The documents may come

Introduction

The smithing processes of ancient craftsmen in the western Mediterranean during the first half of the early Iron Age are not well defined [START_REF] Hortalà | Tecnología de las primeras manufacturas férricas en el nordeste de la Peninsula Ibérica -First iron technology in northern Iberian Peninsula[END_REF]. It is currently impossible to determine whether production was homogeneous across different geographical areas. Ascertaining whether the craftsmen selected materials with specific properties for a targeted application and whether they used heat treatments will provide some answers to this problematic.

To understand the smithing processes, it is essential to identify the microstructure of the alloy.

Metallographic etching is the best way to reveal the microstructure of Fe-C systems. This method has already been applied to a number of Portuguese items [START_REF] Renzi | Questioning research on early iron in the Mediterranean[END_REF]. However, metallographic etching cannot be used when items are highly oxidised.

With the aim of identifying the initial microstructure of highly oxidised items, microstructural observations were performed using optical microscopy (OM) [START_REF] Stepanov | Methodologies for the investigation of corroded iron objects: Examples from prehistoric sites in South-eastern Arabia and Western Iran[END_REF] and field-emission scanning electron microscopy (FE-SEM) [START_REF] Eliyahu-Behar | Reevaluating early iron-working skills in the Southern Levant through microstructure analysis[END_REF][START_REF] Kaufmann | Ferrous metallurgy from the Bir Massouda metallurgical precinct at Phoenician and Punic Carthage and the beginning of the North African Iron Age[END_REF][START_REF] Knox | Detection of iron carbide structure in the oxide remains of ancient steel[END_REF][START_REF] Monnier | A corrosion study of the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes techniques[END_REF]. Based on the OM analysis, four mechanisms of formation of ghost structures were proposed [START_REF] Stepanov | Methodologies for the investigation of corroded iron objects: Examples from prehistoric sites in South-eastern Arabia and Western Iran[END_REF]: [START_REF] Hortalà | Tecnología de las primeras manufacturas férricas en el nordeste de la Peninsula Ibérica -First iron technology in northern Iberian Peninsula[END_REF] intergranular corrosion (corrosion propagates along ferrite grain boundaries with the formation of an oxygen-rich component in grain boundaries and an oxygenpoor component in ferrite grains), [START_REF] Renzi | Questioning research on early iron in the Mediterranean[END_REF] inverse intergranular corrosion (grain boundaries are replaced by an oxygen-poor component, and ferrite grains by an oxygen-rich component), ( 3) pearlite (oxygenrich component) corroded in preference to ferrite (oxygen-poor component) and ( 4) ferrite (oxygenrich component) corroded in preference to pearlite (oxygen-poor component). FE-SEM observations at high spatial resolution revealed different ghost structures in sites with pearlite. In the first case, metallic cementite (Fe3C) was observed in the dense corrosion products [START_REF] Monnier | A corrosion study of the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes techniques[END_REF]. It has been proposed [START_REF] Rémazeilles | Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soil[END_REF] that cementite is nobler than ferrite (the pearlitic structure was composed of both) and that it acts as a cathode. Any cementite present in an initial microstructure remains intact, while any ferrite is oxidised. In the second case, the ghost structure consists of lamellar or globular pearlitic structures [START_REF] Eliyahu-Behar | Reevaluating early iron-working skills in the Southern Levant through microstructure analysis[END_REF][START_REF] Kaufmann | Ferrous metallurgy from the Bir Massouda metallurgical precinct at Phoenician and Punic Carthage and the beginning of the North African Iron Age[END_REF][START_REF] Knox | Detection of iron carbide structure in the oxide remains of ancient steel[END_REF], in which ferrite is oxidised and cementite is transformed to pseudomorph. There is thus no electrochemical driving force for the breakdown of Fe3C. While this compound can decompose to ferrite and graphite through graphitisation at high temperatures [START_REF] Schneider | Iron layer formation during cementite decomposition in carburising atmospheres[END_REF], these conditions would not have been present during burial in the soil. Graphitisation strongly depends on temperature, pressure and time [START_REF] Pierson | Synthetic carbon and graphite: carbonization and graphitization[END_REF][START_REF] Bharadwaj | Study of Graphitization in Carbon Steel Weldments for Remaining Life Assessment[END_REF]. Lower temperatures require considerably more time. For example, graphitisation in carbon steel is obtained within 20 hours at 650°C and within 1500 hours at 500°C [START_REF] Bharadwaj | Study of Graphitization in Carbon Steel Weldments for Remaining Life Assessment[END_REF]. The same conclusions were drawn in [START_REF] Reardon | Heat treatment of Steel[END_REF]: the lower the temperature, the longer the time. Therefore, we can assume that graphitisation can occur at extremely low kinetics at room temperature in archaeological iron alloy artefacts. There is no experimental evidence in the literature of this degradation process in archaeological items.

Research on the corrosion processes in iron archaeological items found in soil has only emerged fairly recently [START_REF] Monnier | A corrosion study of the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes techniques[END_REF][START_REF] Rémazeilles | Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soil[END_REF][START_REF] Dillmann | Characterization of protective rust on ancient Indian iron microprobe analyses[END_REF][START_REF] Dillmann | Advances in understanding atmospheric corrosion of iron. I. Rust characterisation of ancient ferrous artefacts exposed to indoor atmospheric corrosion[END_REF][START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF][START_REF] Hoerlé | Advances in understanding atmospheric corrosion of iron. II. Mechanistic modelling of wet-dry cycles[END_REF][START_REF] Chitty | Long-term corrosion resistance of metallic reinforcement in concretea study of corrosion mechanisms based on archaeological artefacts[END_REF][START_REF] Réguer | Buried iron archaeological artefacts: corrosion mechanisms related to the presence of Cl-containing phases[END_REF][START_REF] Demoulin | The evolution of the corrosion of iron in hydraulic binders analysed from 46-and 260-year-old buildings[END_REF][START_REF] Mercier-Bion | Electrical properties of iron corrosion layers formed in anoxic environments at the nanometer scale[END_REF][START_REF] Dillmann | Corrosion of Metallic Heritage Artefacts: Investigation, Conservation and Prediction of Long Term 48[END_REF][START_REF] Monnier | Localisation of oxygen reduction sites in the case of iron long term atmospheric corrosion[END_REF][START_REF] Saheb | Copper tracing to determine the micrometric electronic properties of a thick ferrous corrosion layer formed in an anoxic medium[END_REF][START_REF] Bellot-Gurlet | Raman studies of corrosion layers formed on archaeological irons in various media[END_REF]. It has been partly supported by the nuclear industry in relation to the storing of nuclear waste [START_REF] Rémazeilles | Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soil[END_REF][START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF][START_REF] Chitty | Long-term corrosion resistance of metallic reinforcement in concretea study of corrosion mechanisms based on archaeological artefacts[END_REF]. Studies have been conducted on archaeological items buried in different environments (atmospheric [START_REF] Monnier | A corrosion study of the ferrous medieval reinforcement of the Amiens cathedral. Phase characterisation and localisation by various microprobes techniques[END_REF][START_REF] Dillmann | Advances in understanding atmospheric corrosion of iron. I. Rust characterisation of ancient ferrous artefacts exposed to indoor atmospheric corrosion[END_REF][START_REF] Hoerlé | Advances in understanding atmospheric corrosion of iron. II. Mechanistic modelling of wet-dry cycles[END_REF][START_REF] Monnier | Localisation of oxygen reduction sites in the case of iron long term atmospheric corrosion[END_REF], anoxic [START_REF] Mercier-Bion | Electrical properties of iron corrosion layers formed in anoxic environments at the nanometer scale[END_REF], soil [START_REF] Rémazeilles | Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soil[END_REF][START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF] and chloride [START_REF] Réguer | Buried iron archaeological artefacts: corrosion mechanisms related to the presence of Cl-containing phases[END_REF]). The processes of how different oxides are formed from ferrite have been expounded in the literature [START_REF] Hoerlé | Advances in understanding atmospheric corrosion of iron. II. Mechanistic modelling of wet-dry cycles[END_REF][START_REF] Chitty | Long-term corrosion resistance of metallic reinforcement in concretea study of corrosion mechanisms based on archaeological artefacts[END_REF][START_REF] Réguer | Buried iron archaeological artefacts: corrosion mechanisms related to the presence of Cl-containing phases[END_REF][START_REF] Demoulin | The evolution of the corrosion of iron in hydraulic binders analysed from 46-and 260-year-old buildings[END_REF][START_REF] Mercier-Bion | Electrical properties of iron corrosion layers formed in anoxic environments at the nanometer scale[END_REF][START_REF] Dillmann | Corrosion of Metallic Heritage Artefacts: Investigation, Conservation and Prediction of Long Term 48[END_REF][START_REF] Kassim | Electron Microscope studies of iron corrosion products in water at room temperature[END_REF][START_REF] Kitick | Kinetics and mechanism of corrosion of mild steel in new types of ionic liquids[END_REF]. First, Fe(OH) + ions are formed [START_REF] Kitick | Kinetics and mechanism of corrosion of mild steel in new types of ionic liquids[END_REF]. This ion can transform through rapid oxidation into Fe(OH)2 + or through slow oxidation into Fea 2+ Feb 3+ Ox(OH)y. Fe(OH)2 + transforms into lepidocrocite -FeO(OH), while Fea 2+ Feb 3+ Ox(OH)y transforms through further slow oxidation into green rust (precipitate of Fe 2+ and Fe 3+ ) or through rapid oxidation into magnetite Fe3O4 [START_REF] Kitick | Kinetics and mechanism of corrosion of mild steel in new types of ionic liquids[END_REF]. The green rust stage is followed by the formation of lepidocrocite -FeO(OH), which then transforms into a final oxide. This final oxide can be goethite -FeO(OH), magnetite Fe3O4 or hematite -Fe2O3 depending on the pH and potential. In addition, siderite (FeCO3) can be found in carbonate environments [START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF]. While these various processes generally take place at the metal/dense product layer interface, they can also occur elsewhere (e.g. in cracks). In cases where chloride ions are present, akageneite -FeO(OH) and sometimes -Fe2(OH)3Cl can be formed [START_REF] Réguer | Buried iron archaeological artefacts: corrosion mechanisms related to the presence of Cl-containing phases[END_REF]. These are mainly found at the metal/dense product layer interface.

A key point in terms of the soil corrosion of archaeological items is the localisation of the reduction reactions. Different oxides (lepidocrocite, ferrihydrite and ferroxyhyte) can be reduced to magnetite, Fe(OH)2, and so on [START_REF] Monnier | Localisation of oxygen reduction sites in the case of iron long term atmospheric corrosion[END_REF]. This makes the oxygen reduction reaction unnecessary and promotes oxidation processes. Using the Cu 2+ /Cu (0) couple as a marker, one study [START_REF] Saheb | Copper tracing to determine the micrometric electronic properties of a thick ferrous corrosion layer formed in an anoxic medium[END_REF] has shown that an oxygen reduction reaction can occur at the surface of dense corrosion products composed of magnetite and ferrous carbonates (at the item surface as well as in any cracks). In this case, oxidation of the items may be limited by the diffusion of dissolved oxygen.

A model of the corrosion processes that occur in archaeological iron alloys found in soil has been proposed in the literature [START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF]. Using spatial-resolved techniques, relationships between the different oxides formed (mainly goethite -FeO(OH) and magnetite Fe3O4), and some chemical parameters (pH, presence of dissolved oxygen, presence of Fe 2+ or Fe 3+ ) were established. However, the authors only briefly discussed the role of cracks as a pathway for oxygen transport in the objects they examined.

This paper reports on a study of four objects excavated in southern France dating from the first half of the early Iron Age. They were selected because they still had a large metallic part and ghost structures. These objects had sustained soil corrosion in different environments (habitat and necropolis). The term "habitat" can be understood here as a residential dwelling. The term "necropolis" was preferred to that of "cemetery" because the latter has an overly restrictive Judeo-Christian connotation. Structural analyses were carried out at the macro-and microscales using OM, X-ray diffraction (XRD), micro-Raman spectroscopy and FE-SEM coupled with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The corrosion mechanisms identified are discussed below. Other parallel degradation mechanisms were also examined.

Method

Materials and sample preparation

Four objects were studied. Two of the objects (a shaft named object #1 and a slab named object #2) were excavated from a protohistoric habitat located in Corsica (Serra-di-Scopamène/Sorbollano) [START_REF] Peche-Quilichini | Les fouilles de Cuciurpula: la structure 1[END_REF]. The first was dated to between the 8 th and 7 th centuries BCE and came from an open space named "Rampe G". The second, which dated to the 7 th century BCE, came from a house named "Structure 1". These two objects were excavated in 2014 (object #1) and 2009 (object #2) in granite soil (pH around 6), and both were stored in minigrip in a standard reserve without special treatment. The other two objects were excavated from two protohistoric necropolises in southern France. One, object #3, was a knife dated to between the second and third quarter of the 7 th century BCE (grave 91, necropolis of La Rouquette, Puisserguier, Herault). Object #3 was excavated between 2003 and 2004. It was found in an ossuary vase and had therefore been in contact with burned human bones (all protected by a ceramic cover). The terrain was clay and slightly acidic. After stabilisation (same procedure as that used in [START_REF] Noth | Methods for Treating Marin Iron[END_REF]), it was packaged in a minigrip accompanied by Propasec desiccant bags (from Propagroup) and stored in a reserve with controlled atmosphere. The other, object #4, was a knife dated to between the 7 th and early 6 th centuries BCE (grave 991, necropolis of Le Causse, Labruguière, Tarn). Object #4 was excavated in 2010. It was located on a limestone bedrock (basic medium) of the burial grave and covered with silty clay. It was packaged in a minigrip and stored in a standard reserve without special treatment. These four objects were prepared for the XRD, micro-Raman, FE-SEM/EDS/EBSD and OM analyses. They were first cut using a diamond band saw, as shown in Fig. 1(a1), (b1), (c1) and (d1).

The cross-section surfaces (Fig. 1(a2-a3), (b2-b3) (c2-c3) and (d2-d3)) were then mechanically ground down to 4000 grit using silicon carbide (SiC) papers and polished with diamond pastes (down to 1 µm). The specimens were ultrasonically cleaned in acetone between each of these steps. For the EBSD measurements, vibratory polishing was carried out on the cross sections of objects #1, #2 and #4.

Surface observations

Optical images of the cross-section surfaces were recorded at the macroscale using a Nikon Eclipse LV150A upright metallurgical microscope. These optical images were assembled to produce two-dimensional reconstructions of the surfaces.

Surface observations at the microscale were performed using FE-SEM (JEOL JSM-7600F) coupled with EDS. The microscope operated at 15 kV and with a probe current of 3 nA. The working distance was set at 15 mm. Images were acquired in low angle backscatter electron image (LABE) mode. This detector is capable of producing qualitative compositional images with a very high degree of atomic number contrast.

The grain orientation was determined using FE-SEM coupled with the TSL EDAX OIM XM4 electron backscattered diffraction system. EBSD measurements were made on a surface area of 2.8×3.7 mm 2 with a step of 4 µm and an integration time of 23 ms per point.

X-ray diffraction measurements

XRD analysis was performed using CuKα (λ = 1.54 Å) as the radiation source. Measurements were carried out with a Bruker D8-A25-Advance diffractometer and a LynxEye detector. XRD diffractograms were fitted using the Topas software package and the Rietveld method (structural model) [START_REF] Ortiz | Fundamental parameters approach in the Rietveld method: a study of the stability of results versus the accuracy of the instrumental profile[END_REF]. The XRD spot size was set at 2 cm 2 . Hence, the surface of the four objects was completely irradiated over the full angular range (2 in the range of 15-100°).

Micro-Raman spectroscopy measurements

Micro-Raman spectra were taken in backscattering geometry using a Renishaw inVia Raman microscope (100× objective lens and numerical aperture NA = 0.85, spatial resolution 1 μm) with 633 nm HeNe excitation laser, 1 s exposure time, 1000 accumulations and laser power reduced by filters to 700 μW to avoid sample degradation due to heating effects. When the laser power is too high, lepidocrocite, siderite, magnetite, maghemite, ferrihydrite and wustite can form by oxidation [START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF]. A Peltier-cooled CCD matrix detector with a resolution of 1.3 cm -1 (diffraction gratings of 1800 grooves mm -1 ) was used for detection. The spectra were collected in fast mode at a fixed position of grating according to wavenumber ranges: i) 50-1300 cm -1 (oxide bands range); ii) 900-2000 cm -1 (the rest oxide bands, graphite D and G bands and amorphous carbon band range); iii) 2500-3250 cm -1 (graphite 2D band). The spectrum model profile was determined by fitting each fragment of spectrum following background correction with a pseudo-Voigt profile (linear combination of the Gaussian and Lorentzian curves) using Renishaw Wire ver. 3.4 software. Graphite 2D band is missing in spectra. 2D band is very sensitive to the stacking order of the graphene sp2 layers in graphite along the c-axis, which is what indicates the weakly oriented graphite present in analysed samples. Hence, the spectra in 2D range are not presented.

Results and discussion

Identification of corrosion products

Objects #1, #2 and #4 contained a large metallic part at each of their centres (13% of the total surface area of the cross section for object #2, and 27% for objects #1 and #4), the hatched regions in Fig. 1 (a2, b2, d2). For example, the metallic part of object #2 extended over a surface area of 1.8 × ~0.2 cm 2 . By contrast, object #3 contained only very small metallic particles (< 1% of the total surface area of the cross section): the largest particle was around 100 × 300 µm 2 ). In the objects' external zones, dense corrosion products were observed, corresponding to the grey regions in Fig. 1 (a3, b3, c3, d3). Fig. 2 shows that different phases (associated with different colours in optical and FE-SEM images in LABE mode) existed in these dense corrosion products.

A quantitative phase analysis was then performed by XRD. The XRD diffractograms obtained were complex (numerous peaks), making it difficult to identify all the phases (Fig. 3). This confirmed that all the oxides were mixed, leading to complex microstructures. Phase identification was also performed using micro-Raman spectroscopy. This technique offers some advantages when compared with XRD. It allows the measurement of both crystalline and amorphous compounds as well as the analysis of single sites. It is therefore easier to deconvolute the micro-Raman spectra (fewer compounds present). Fig. 4 and supplementary material 1 and 2 show the micro-Raman spectra in specific sites for objects #1, #2, #3 and #4. For objects #2 and #4, the micro-Raman measurements were performed in both the dark grey (zone A in Fig. 2 and supplementary material 1) and the light grey layers (zone B in Fig. 2 and supplementary material 1). For objects #1 and #3, the micro-Raman measurements were carried out in zones A, B, C and D, as shown in Fig. 2 and4, and supplementary material 2. These zones (A-D) were selected because they are characterised by different levels of grey on the FE-SEM images in LABE mode (see Section 2) and therefore by different levels of different chemical contrasts (composition, stoichiometry). The micro-Raman spectra were deconvoluted into individual bands using data from the literature: akageneite -FeO(OH) [START_REF] Bellot-Gurlet | Raman studies of corrosion layers formed on archaeological irons in various media[END_REF], ferrihydrite Fe5HO8*4H2O [START_REF] Kassim | Electron Microscope studies of iron corrosion products in water at room temperature[END_REF], goethite -FeO(OH) [START_REF] Oh | Characterization of iron oxides commonly formed as corrosion products on steel[END_REF][START_REF]RRUFF database: Goethite R[END_REF], hematite -Fe2O3 [START_REF] Bellot-Gurlet | Raman studies of corrosion layers formed on archaeological irons in various media[END_REF][START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF][START_REF] Oh | Characterization of iron oxides commonly formed as corrosion products on steel[END_REF], lepidocrocite -FeO(OH) [START_REF] Oh | Characterization of iron oxides commonly formed as corrosion products on steel[END_REF], maghemite -Fe2O3 [START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF][START_REF] Jacintho | Surface-enhanced Raman spectra of magnetic nanoparticles adsorbed on a silver electrode[END_REF], magnetite Fe3O4 [START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF][START_REF] De Faria | Raman microspectroscopy of some iron oxides and oxyhydroxides[END_REF][START_REF] Shebanova | Raman spectroscopic study of magnetite (FeFe2O4): a new assignment for the vibrational spectrum[END_REF] and siderite FeCO3 [START_REF] Hanesch | Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies[END_REF].

The quantitative phase analysis by XRD revealed that two main oxides were present, namely goethite (-FeO(OH)) and magnetite (Fe3O4)/maghemite (-Fe2O3), as reported in Table 1. It is not possible to distinguish Fe3O4 from -Fe2O3 using XRD. Phase identification by micro-Raman showed that maghemite was only present in object #1. In the other three objects, both compounds (Fe3O4 and -Fe2O3) were present. This quantitative phase analysis by XRD also revealed that a large quantity of akaganeite was present in object #2.

The main XRD peaks of lepidocrocite (at 2 = 27.04°) and akageneite (at 2 = 26.74°) were very close (2 of 0.3°). In addition, the second peak of lepidocrocite was located between the peaks associated with the two main compounds (goethite and magnetite). With the exception of object #2 (large amount of akageneite), it was therefore difficult to distinguish the two oxides (lepidocrocite and akageneite) and to give accurate quantities. This explains why there was no real correspondence between the XRD and micro-Raman results for these two compounds (Table 1). The micro-Raman measurements indicated that lepidocrocite was present in objects #1, #2 and #3 and that akageneite was present in objects #2 and #3.

Therefore, the XRD and micro-Raman measurements revealed that dense corrosion products formed during the soil corrosion of objects are systematically composed of goethite, magnetite (two main compounds) and lepidocrocite (small amount). Akaganeite (large amount in object #2), maghemite and ferrihydrite are also found in some objects. Hematite and siderite were only detected in object #1 (small amount) in this study. This finding confirms that goethite and magnetite are end products of the corrosion reaction and are thermodynamically stable [START_REF] Monnier | Localisation of oxygen reduction sites in the case of iron long term atmospheric corrosion[END_REF][START_REF] Saheb | Copper tracing to determine the micrometric electronic properties of a thick ferrous corrosion layer formed in an anoxic medium[END_REF].

Microstructure of corrosion products

The FE-SEM/EDS analysis of the dark grey layers visible in Fig. 2(d) yielded 62.1 at. % O (from the stoichiometric calculation) and 37.9 at. % Fe and of the light grey layers visible in Fig. 2(d) revealed 52.5 at. % O and 47.5 at. % Fe. The FE-SEM/EDS analysis therefore indicated that the dark grey and light grey layers corresponded to goethite (in theory: 67 at. % O and 33 at. % Fe) and magnetite (in theory: 57 at. % O and 43 at. % Fe), respectively. The micro-Raman and XRD analyses showed that other compounds could be present in these two layers. A stoichiometric analysis could not therefore be performed.

Low spatial resolution observations (at the macroscale) of the four objects #1-#4 revealed the existence of successive layers composed mainly of goethite and magnetite (Fig. 2). At the macroscale, the interfaces between the successive layers of magnetite and goethite seemed to be relatively regular. This marbling is discussed below. High spatial resolution observations indicated that the oxides (goethite and magnetite) were often mixed, with the mix composed of different percentages of each oxide (Fig. 2(b-d)). This indicates that corrosion mechanisms are complex and that they may depend on the object microstructure, the presence of cracks, and so on.

Corrosion mechanisms in the presence of soil water

According to the literature (Section 1), corrosion mechanisms result in the formation of -FeO(OH) and Fe3O4 at the metallic surface. This is consistent with the XRD and micro-Raman results reported in Table 1, which show that the main oxides present in the objects were -FeO(OH) and Fe3O4. Only a small quantity of -FeO(OH) was detected by XRD, suggesting that it had not yet transformed. At this stage, no crack initiates, and corrosion propagates uniformly along a front. This corrosion mechanism has already been described in the literature [START_REF] Neff | Corrosion of iron archaeological artefacts in soil: characterisation of the corrosion system[END_REF].

As corrosion had proceeded in the four objects, the formation of numerous dense oxides had resulted in the initiation and propagation of a number of cracks, as shown in Fig. 2 and Fig. 5(a).

These cracks had propagated in all directions (i.e. there were no preferential directions). As the cracks had emerged at the surface of these objects, they had promoted the propagation of soil water (with dissolved oxygen) inside them. At the same time as the cracks were propagating, an outward diffusion of Fe 2+ had occurred through dense oxides, from the metallic parts to the free surfaces (including crack walls). This Fe 2+ had then oxidised, and a new oxide had grown. The outward diffusion of Fe 2+ had been promoted through oxides containing a high density of defects (vacancies, dislocations, etc.). This was the case with the non-stoechiometric oxides, as in site 1 in Fig. 5(b). The FE-SEM/EDS analysis of site 1 yielded 44.5 at. % O and 55.5 at. % Fe. This corresponds to a non-stoichiometric compound. The oxide that had formed in the crack (site 2 in Fig. 5(b)) was Fe3O4 (FE-SEM/EDS analysis: 60.1 at. % O and 39.9 at. % Fe). As the oxide had grown, the crack diameter had decreased.

At the end of this process, the crack had become completely obstructed by the oxide (Fig. 5(c)) and soil water was no longer able to flow through it. The ghost structures on both sides of the crack fitted together perfectly (Fig. 5(d)). This confirmed that a new oxide had been formed in the crack due to the presence of soil water and that the ghost structure on both sides of the crack had not been destroyed by this process.

Taking into account the influence of a high density of cracks in the corrosion mechanisms, the dense corrosion products were composed of a mixture of the different oxides, as revealed by the high spatial resolution observations (Fig. 2(b-d)).

Intergranular oxidation

Intergranular oxidation was often observed in objects #1, #3 and #4 (Fig. 6). A high density of cementite was located at the grain boundaries of object #2. This particular case is investigated in Section 3.5. The supply of oxygen to the metallic parts in the objects had been controlled by inward diffusion through the dense oxides. Oxygen had penetrated the metallic grains (lattice diffusion) and the grain boundaries (grain boundary diffusion). As shown in Fig. 6, grain boundary diffusion occurs at a faster rate than lattice diffusion.

Grain boundary diffusion depends on the nature of the grain boundary. Grain boundaries of 3 ≤ Σ ≤ 29 are regarded as a coincidence site lattice (CSL) grain boundary (where Σ is the coincidence index), and those of Σ > 29 are regarded as a random grain boundary [START_REF] Kokawa | Grain-boundary structure and precipitation in sensitized austenitic stainless steel[END_REF]. The EBSD measurements carried out on object #1 showed that most of the grain boundaries were random grain boundaries (shown in black in Fig. 7(a)). However, coincidence site lattice (CSL) grain boundaries (shown in blue in Fig. 7(a)) were also found. These were uniformly distributed in the metal. The low energies associated with these grain boundaries do not favour fast diffusion. It was therefore expected that strong oxidation would be observed along random grains (shown by the white arrows in Fig. 6). Once a CSL grain boundary is reached, intergranular oxidation stops (see inserted image in Fig. 6(a) and (d)). Surface observations showed that oxygen had penetrated along grain boundaries to a maximum depth of 20 µm (object #4 (Fig. 6 (b)).

The presence of dislocations in the grains can increase lattice diffusion rates [START_REF] Atkinson | Grain boundary diffusionstructural effects and mechanisms[END_REF]. The EBSD measurements carried out on objects #1 and #4 showed that 50% of the grains had a grain orientation spread greater than 4° (Fig. 7(b) for object #1), indicating that the density of geometrically necessary dislocations was high in those grains. Oxygen penetration in grains with a high grain orientation spread (GOS) value had therefore been promoted.

Oxidation of sites containing pearlite and graphitisation of cementite

Cementite with a lamellar structure was observed in the dense oxides formed in objects #1-#4 (objects #2 and #4 in Fig. 8). This had initially been pearlite (the ferrite had been preferentially oxidised due to an inward diffusion of oxygen). It is a well-established fact [START_REF] Rémazeilles | Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soil[END_REF] that cementite is nobler than ferrite. When oxidation of the surrounding ferrite had been complete, the cementite had undergone substantial deformation, leading to the formation of microcracks (shown as circles in Fig. 8(a)).

The distribution of carbon, oxygen and iron in a site containing cementite embedded in the dense oxides is shown in Fig. 9(a-d) for a hypereutectoid object (object #2) and in Fig. 9(e-h) for a hypoeutectoid object (object #1). This was obtained by means of FE-SEM/EDS. Sites 1 correspond to cementite that had not yet been transformed (Fe and C were mainly found). In sites 2, only carbon was significantly detected. The micro-Raman analysis showed the formation of graphite and amorphous carbon (Fig. 10). Both these elements were mixed with oxides. Therefore, the FE-SEM/EDS and micro-Raman results revealed that graphitisation had occurred in objects #1-#4. The micro-Raman spectra were deconvoluted into individual bands using data from the literature, namely carbon graphite and amorphous carbon [START_REF] Ferrari | Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects[END_REF][START_REF] Cançado | Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies[END_REF]. The FE-SEM/EDS images also showed that graphitisation had started from the interface between the cementite and the dense oxides and had propagated into the cementite. This confirms that graphitisation, the formation of amorphous carbon and the oxidation of ferrite are concomitant processes.

Only Fe3C (and not graphite) was observed in the metallic parts of objects #1-#4. This means that the graphite observed in the corroded parts could not have resulted from graphitisation occurring during the elaboration process at high temperatures. This happens during burial in soil for a very long period. As already discussed in Section 1, we assumed that graphitisation can occur at extremely low kinetics at room temperature. This would require considerable time. This is compatible with the fact that these objects were buried for at least 2600 years (corresponding to nearly 23 millions hours).

Discussion of initial microstructure

In the absence of large metallic parts in archaeological iron alloy objects, the only way of determining their initial structure is to map the ghost structures (with graphite), non-oxidised cementite and former cracks. While the formation of numerous cracks, which are progressively filled with new oxides, can create areas in the dense corrosion products that exhibit no ghost structures, the structures are nevertheless always present on both sides of a crack, making the reconstruction of microstructure possible.

After etching in nital (where possible) and the mapping of ghost structures, non-oxidised cementite and former cracks in the four objects examined in this study showed that objects #1, #3 and #4 (Fig. 11 (a, c-d)) were composed of heterogeneous hypoeutectoid steel, while object #2 was made up of one part hypereutectoid steel and one part eutectoid, or close to eutectoid, steel (Fig. 11 (b)).

In objects #1 (Fig. 12 (a)), #3 (Fig. 12 (d-e)) and #4 (Fig. 8 (d)), the pearlite was always lamellar. This structure is appropriate for a tool, because lamellar pearlite is harder and less deformable than globular pearlite [START_REF] Allain | Microstructure-based behavior law for globular pearlitic steels[END_REF]. In object #2 (Fig. 12 (b-c)), lamellar pearlite was detected most of the time, but sometimes cementite in the process of spheroidisation was observed. Globular (or spheroid) cementite is obtained after long annealing [START_REF] Allain | Microstructure-based behavior law for globular pearlitic steels[END_REF][START_REF] Gordon | Metallographic study of iron artefacts from the eastern Transvaal, south Africa[END_REF] and could be appropriate for hypereutectoid steel because of the diminution of the strength induced by the spheroidisation.

In addition, cementite needles or cementite grain boundaries could be observed in object #2 (Fig. 12 (b)). Cementite needles have been observed in other studies [START_REF] Eliyahu-Behar | Reevaluating early iron-working skills in the Southern Levant through microstructure analysis[END_REF][START_REF] Stepanov | Early Iron age artefacts from southeastern Arabia: investigationg fabrication techniques using neutron tomography, optical microscopy, and SEM-EDS[END_REF] and can be formed after the precipitation of proeutectoid cementite (brittle) on austenitic grains boundaries during manufacturing processes [START_REF] Eliyahu-Behar | Reevaluating early iron-working skills in the Southern Levant through microstructure analysis[END_REF][START_REF] Spanos | The proeutectoid cementite transformation in steels[END_REF]. The numerous clusters of cementite needles observed at high spatial resolution, which are formed by the diffusion of carbon during the ageing process [START_REF] Munoz | Archaeologic analogues: microstructural changes by natural ageing in carbon steels[END_REF], did not correspond to our object. Finally, acicular overheat structures, which reduce ductility and toughness [START_REF] Colpaert | Metallography of steels. Interpretation of structure and the effects of Processing[END_REF], were observed in an area of object #3 (Fig. 11 (c) and 12 (e)).

The results of this study are in line with those for Catalonian objects [START_REF] Hortalà | Tecnología de las primeras manufacturas férricas en el nordeste de la Peninsula Ibérica -First iron technology in northern Iberian Peninsula[END_REF], which belong to the same socio-cultural area as objects #3 and #4. The use of steel with heterogeneous composition and sometimes with overheat structures is an indication that the properties of the material were fit for purpose. Furthermore, the first iron alloy knives from the early Iron Age, including objects #3 and #4, located in the area between the Rivers Rhône (France) and Ebro (Spain) were mainly discovered in necropolises as funeral offerings. In addition, the typological influence on these knives was possibly eastern [START_REF] Grevey | The origins of the first ferrous knives on the Iberian Peninsula and in Southern France: a typological viewpoint (9 th -7 th centuries BC)[END_REF]. Hence, the presence of brittle or deformable structures, their function as a funeral offering and the possible eastern influence could mean that the knives had a more symbolic than functional value. It is also possible to propose some interpretations regarding objects #1 and #2, which were found in a Corsican habitat. Object #1 (shaft easily workable) could have been some kind of semi-product, and object #2 (very brittle) may have been a manufacturing failure.

Conclusions

Four objects from the first half of the early Iron Age were investigated using OM, FE-SEM/EDS/EBSD, XRD and micro-Raman spectroscopy. The following conclusions can be drawn: 1) Dense corrosion products formed during the soil corrosion of objects are always composed of goethite, magnetite (the two main compounds) and lepidocrocite (small amount). In this study, other compounds were detected (including akageneite, maghemite, ferrihydrite, hematite and siderite) but not systematically.

2) Soil corrosion mechanisms depend on an object's microstructure. It is proposed that random grain boundaries provide a preferential pathway for oxygen penetration and oxidation. Soil corrosion can also be driven locally by galvanic coupling between ferrite and cementite (Fe3C, enriched in carbon).

3) At a certain stage of corrosion, cracks are formed in the dense corrosion products, providing new pathways for soil penetration. It was shown that a new oxide grows in the cracks due to the diffusion of iron ions through the dense product layer until obstruction occurs. The ghost microstructures on either side of the cracks are not affected by the formation of this new oxide. 4) For the time being, this study provides experimental evidence of the graphitisation process and the formation of amorphous carbon. In both these processes, either cementite decomposes in ferrite (which is oxidised, as mentioned above), graphite and amorphous carbon or there is direct oxidation of Fe to iron oxide/oxyhydroxide. The rates of both processes can change over time according to environmental conditions (e.g. supply of oxygen). Graphite and amorphous carbon make up the ghost structures.

5) The pearlitic structures of ancient iron alloys are observable until the destruction/decomposition of the items because of the stability of the graphite that makes up the structure during the last stage in the transformation of the cementite.

Fig. 1 .

 1 Fig. 1. Location of sampling sites where observations and analyses were performed. (1) = drawing of the object, (2) = schematic representation of the cross section (hatched regions : metallic iron), (3) = optical images : grey regions : dense corrosion products. (a) = object #1, (b) = object #2, (c) = object #3 and (d) = object #4. On the (1) drawings, the hatched drawing represents the metal part.

Fig. 2 .

 2 Fig. 2. Optical images of the dense corrosion products (oxidised parts) of (a) object #1, (b) object #4, (c) object #2, (d) object #3. Only the main oxide in the different zones is indicated here. A complete compilation of the oxides present are indicated in Fig. 4 (object #1), Fig. 5 (objects #2 and #4) and Fig. 6 (object #3). The red arrows show the cracks.

Fig. 3 .

 3 Fig. 3. XRD diffractograms of the four objects: (a) object #1, (b) object #2, (c) object #3 and (d) object #4.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Raman spectra results for object #1: (a) zone A, (b) zone B, (c) zone C and (d) zone D. The zones are indicated in Fig. 2. G = goethite, H = hematite, Mt = magnetite, L = lepidocrocite, Mh = maghemite, S = siderite.

Fig. 6 .

 6 Fig. 6. FE-SEM/EDS micrographs of intergranular oxidation observed in (a) object #1, (b-c) object #4 and (d) object #3. Random GB = random grain boundaries; CSL GB = lattice grain boundaries.

Fig. 7 .

 7 Fig. 7. (a) inverse pole figure and (b) GOS maps derived from the EBSD measurements on object #1. Grain boundaries are also represented: Random grain boundaries (shown in black) and CSL grain boundaries (shown in blue).

2 Fig. 9 .

 29 Fig. 9. (a-d) FE-SEM/EDS elemental mapping of a site containing pearlite embedded in the dense oxides in (a-d) object #2 and (e-h) object #1.

Fig. 10 .

 10 Fig. 10. (a-b) Raman spectra of the black boundaries (sites 2 in Fig.13). (c-d) Raman spectra of the white boundaries (sites 1 in Fig.13). G = goethite, H = hematite, Mh = maghemite, S = siderite, L = lepidocrocite, Gr = graphite, A = amorphous carbon (black line and points = experimental spectrum; dark blue, light blue, dark green, light green, red and purple lines = models spectrum).

Fig. 11 .

 11 Fig. 11. Illustration of mapping of ghost structures (graphitised and intact cementite in DPL) and structures revealed after nital etching (when possible) in (a) object #1, (b) object #2, (c) object #3 and (d) object #4. In blue: metallic part. (a) region with circles corresponds to lamellar pearlite in

Fig. 12 .

 12 Fig. 12. FE-SEM micrographs of example of microstructures of (a) object #1, (b-c) object #2, (d-e) object #3. (a) lamellar pearlite in hypoeutectoid steel, (b) hypereutectoid steel, (c) globular cementite, (d) graphitised lamellar pearlitic structures, (e) acicular overheat structures.

Supplementary data 1 .Supplementary data 2 .

 12 Raman spectra results for object #2: (a) zone A; (b) zone B. Raman spectra results for object #4: (c) zone A, (d) zone B. The zones are indicated in Fig. 2. G = goethite, H = hematite, Mt = magnetite, L = lepidocrocite, A = akaganeite. Raman spectra results for object #3. (a) zone A, (b) zone B, (c) zone C and (d) zone D. The zones are indicated in Fig. 2. G = goethite, H = hematite, Mt = magnetite, L = lepidocrocite, A = akaganeite, S = siderite, F = ferrihydrite.

(a3)

Acknowledgments

We thank Nicolas Geoffroy for assistance with XRD and Frederic Herbst for assistance with FE-SEM.

Funding Information

This study was conducted within the framework of LabEx ARCHIMEDE and was supported by the "Investissement d'Avenir" programme (ANR-11-LABX-0032-01, Agence Nationale de la Recherche, France).

List of abbreviations

Declarations

Data availability

All data generated or analysed during this study are included in this article.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Authors' contributions

A.-L. G and V. V. wrote this article and interpreted the results (major contributors in writing the manuscript).


E Bayart

S Deboeuf

F Corson

A Boudaoud

M Adda-Bedia

Measuring order in the isotropic packing of elastic rods

Keywords: numbers: 46.32.+x, 46.70.Hg, 61.43.-j

The packing of elastic bodies has emerged as a paradigm for the study of macroscopic disordered systems. However, progress is hampered by the lack of controlled experiments. Here we consider a model experiment for the isotropic two-dimensional confinement of a rod by a central force. We seek to measure how ordered is a folded configuration and we identify two key quantities. A geometrical characterization is given by the number of superposed layers in the configuration. Using temporal modulations of the confining force, we probe the mechanical properties of the configuration and we define and measure its effective compressibility. These two quantities may be used to build a statistical framework for packed elastic systems.

Packed elastic objects are ubiquitous in Nature and technology. For instance, DNA is packed in the cell nucleous or in viral capsids [START_REF] Arsuaga | Knotting probability of dna molecules confined in restricted volumes: Dna knotting in phage capsids[END_REF][START_REF] Katzav | A statistical approach to close packing of elastic rods and to dna packaging in viral capsids[END_REF], while growing tissues can be confined by their environment [START_REF] D'arcy-Thompson | On Growth And Form[END_REF][START_REF] Couturier | A global regulation inducing the shape of growing folded leaves[END_REF]. The optimization of folding is crucial in the design of self-deployable structures, such as tents or solar sails [START_REF] Miura | Proceedings of the 31st Congress of the International Astronautical Federation, volume IAF-80-A 31[END_REF], or in waste disposal. Like a granular pile, a confined plate can be either in a crystalline state, the stacked facets obtained by repeatedly folding a sheet into two, or in a disordered state, exemplified by a crumpled ball [START_REF] Tallinen | Deterministic folding in stiff elastic membranes[END_REF]. When a sheet is confined, the number of metastable configurations blows up [START_REF] Boué | Spiral patterns in the packing of flexible structures[END_REF]. Meanwhile, self-avoidance leads to jamming because it prevents the system from exploring the space of configurations. This raises the question of whether a confined sheet can be viewed as a glassy system, in the same class as a static granular medium [START_REF] Jaeger | Granular solids, liquids, and gases[END_REF]. Indeed, theoretical studies proposed thermodynamical approaches for packed rods [START_REF] Katzav | A statistical approach to close packing of elastic rods and to dna packaging in viral capsids[END_REF][START_REF] Boué | Folding of flexible rods confined in 2d space[END_REF][START_REF] Aristoff | Layering in crumpled sheets[END_REF]. It has been argued that a system which is confined isotropically experiences a configurationnal phase transition from a disordered to an ordered (nematic) state [START_REF] Katzav | A statistical approach to close packing of elastic rods and to dna packaging in viral capsids[END_REF][START_REF] Boué | Folding of flexible rods confined in 2d space[END_REF]. The geometric characterization of the nematic ordering has been developed in various numerical and experimental works [START_REF] Tallinen | Deterministic folding in stiff elastic membranes[END_REF][START_REF] Aristoff | Layering in crumpled sheets[END_REF][START_REF] Lin | Spontaneous emergence of ordered phases in crumpled sheets[END_REF]. On the experimental side, a difficulty in the study of crumpled balls [START_REF] Lin | Spontaneous emergence of ordered phases in crumpled sheets[END_REF][START_REF] Gomes | Fractal geometry in crumpled paper balls[END_REF][START_REF] Matan | Crumpling a thin sheet[END_REF][START_REF] Blair | Geometry of crumpled paper[END_REF][START_REF] Andresen | Ridge network in crumpled paper[END_REF][START_REF] Balankin | Scaling properties of randomly folded plastic sheets[END_REF] arises from the hand-generation of configurations. In this context, the confinement of a rod in a plane was an important and useful simplification [START_REF] Boué | Spiral patterns in the packing of flexible structures[END_REF][START_REF] Donato | Crumpled wires in two dimensions[END_REF][START_REF] Stoop | Morphological phases of crumpled wire[END_REF][START_REF] Deboeuf | Energy distributions and effective temperatures in the packing of elastic sheets[END_REF][START_REF] Adda-Bedia | Statistical distributions in the folding of elastic structures[END_REF]. However, drawing general conclusions from these experiments can be questioned because of issues such as friction between the periphery of the configuration and the container, the anisotropic injection of the rod in the container, plasticity or the impossibility of unfolding a configuration.

Here we reconsider the packing of a rod in a plane by proposing an original model experiment. In addition to a geometric description of the transition from a disordered to ordered configuration, we developed a mechanical characterization of the system allowing a global measure of the order without addressing the local geometrical properties of the folded configurations. To this purpose, we devised an experiment allowing us to reversibly confine a rod by a central force, deriving from an isotropic radial potential. As a consequence, there is no contact between the container and the periphery of the configuration, while the intensity of the forcing is controlled through the stiffness of the potential. We investigate the emergence of geometrical order through the stacking of layers. As this setup enables the temporal modulation of the confinement, we also probe the mechanical properties of configurations, and define an effective susceptibility of a configuration, which we associate with geometrical order. We thus obtain a coupled geometrical and mechanical characterization of the system. The principle of the experiment is as follows. A circular Hele-Shaw cell is filled with a liquid and entrained by a motor. A rod is inserted into the cell. The cell is slightly thicker than the rod, so that the rod cannot cross itself, constraining a two-dimensional folding. The liquid is denser than the rod. As a consequence, when the cell is rotated, the rod is submitted to a centripetal force and thus confined in a radial, parabolic potential of pressure, P (r) = P 0 + 1/2ρω 2 r 2 where r is a radial coordinate, ρ the fluid density and P 0 the pressure at the center of the cell. The influence of gravity is negligible in this setup. The cell was fixed in a vertical position on the axis of a 2kW motor, through a double ball bearing to avoid vibration transmission from the motor to the cell. The rotation velocity was controlled using an electronic frequency variator.We used a PDMS rod of circular section, h = 2± 0.2 mm in diameter and L = 3±0.01 m in length, density d = 1 and Young Modulus E = 1.0 ± 0.1 MPa, measured through the elongation of the rod under a force. The liquid was salt-saturated degassed water of density ρ l = 1.16 at ambient temperature. The cell was initially made of two disks of 50 cm in diameter: a stiff one in 10 mm-thick Duralumin (i), and a transparent one in 15 mm-thick Polycarbonate (ii), allowing us to observe the confined rod (Fig. 1a,b). Chamber 1 was filled with the liquid and the rod; the gap thickness was fixed at 2.5 mm by a ring of Plexiglas inserted between the two disks. The pressure gradient induced in the liquid by the rotation imposed a buckling of the disk ii, inducing a thinning of the gap. To have a permanent control of the gap thickness, a third disk in 15 mm-thick Polycarbonate (iii) is added to obtain a second chamber of thickness 1.5 cm; this chamber is filled with the same liquid and a communication between both chambers was created piercing two holes of 1 mm in diameter in disk ii, enabling the equilibration of pressure in the two chambers. Chamber 2 serves as a sacrificial chamber: when the cell is rotated, disk (iii) bends inwards (instead of disk (ii)) but the gap is thick enough so that it does not close at our maximal rotation velocity of 20 Hz. The chambers were filled through holes pierced in the edge of disks (i) and (iii) and the rod was gently inserted using a hole in the back of disk (i). The whole setup was placed in a dark room and lit with three stroboscopic lamps with diffusing screens. Movies were taken with a CCD camera. The duration of a flash was short enough (1 ms) to get sharp images of the rod, even at the higher velocities. Using a computer interface, the camera and stroboscopes were triggered with the same square periodic signal, while the variator was controlled using a DC voltage. To enhance contrast, we used a white rod on a dark back: an adhesive sheet of black plastic was laid on the Duralumin disk (i). Binary images of the rod were obtained by thresholding.

The initial configuration of the rod (Fig. 1a) is prepared using 8 magnetic beads of diameter 1.8 mm inserted into the first chamber and moved from outside with a magnet. The cell is then set in rotation; the time to reach the desired frequency is approximately 3 s. The control parameter is the frequency f of rotation (See EPAPS Document No. [number will be inserted by publisher] for a movie of a typical experiment.) For a given rotation frequency, a large number of folded configurations is accessible from the same initial configuration (Fig. ??). Indeed, the folding process is nondeterministic and the folded configuration is selected by the experimental noise at the very beginning of the experiment. We include in noise the influence of fluids flows in the cell and the friction between the rod and disks of the cell. A first flow is the transient flow to entertain fluid in a solid rotation. A second flow to consider is the ejection of fluid when to parts of the rod stack one on the other. We can convince that the timescales of these flows are much smaller than the timescales observed in our experiment necessary to the rod to reach a configuration of equilibrium, as explain below.

First, configurations can be characterized using the ra-dius of gyration,

R g = 1 L L 0 r 2 (s) ds ,
where s is the curvilinear coordinate of the rod and r(s) the distance to the cell axis. This quantity is directly calculated using the binary image of the confined rod. The radius of gyration decreases with time, rapidly when the rotation is started (Fig. 3a) and then more slowly, reaching a plateau value in a time lapse of the order of 10 3 s. This final value is not unique and differs according to realizations at a given frequency. As we only investigated equilibrium configurations, we had to wait 1800 s for each value of the frequency before taking measurements, to ensure that equilibrium is reached. When averaging over realizations, the mean radius of gyration R is found to be a decreasing function of confinement strength (Fig. 3b). It appears that, when the same experiment is repeated, a large diversity of sizes and geometries is observed. The aim of the present study is to quantify geometrical order in equilibrium states. To do so, we extract the skeleton of a folded configuration from binary images, for which void area into the pattern smaller than a arbitrary treshold are filled. This process did not influence results. Vertices are detected as self-contact points, ie points of the skeleton having three neighbors (Fig. 1c,d). We define branches as portions delimited by two vertices. A given branch may contain several layers of the rod: the thickness of the branch on the binary image directly yields the number of layers. The automatization of this analysis procedure allows to treat a large number of data. Inspired by observations (Fig. 4a), we first consider a geometrical definition of order using the number of superposed layers. Indeed, the configuration of absolute minimum of energy is a spiral [START_REF] Boué | Spiral patterns in the packing of flexible structures[END_REF], which can be qualified as very ordered and in which all layers are superposed and contained in a unique branch. Experimentally, this number of superposed layers is defined as the average number of layers per branch in a given configuration. We jointly measured the average number of layers per branch Nl/b and the radius of gyration of a configuration, as shown in Fig. 4b. The data roughly collapse on a single curve, independently of the confinement strength. Therefore, either the radius of gyration or the number of layers appear as better characterizations of the configuration. A disordered configuration, ie with small Nl/b , has a larger radius than a spiral ordered configuration. As in some previous experiments [START_REF] Lin | Spontaneous emergence of ordered phases in crumpled sheets[END_REF][START_REF] Deboeuf | Energy distributions and effective temperatures in the packing of elastic sheets[END_REF][START_REF] Adda-Bedia | Statistical distributions in the folding of elastic structures[END_REF], stacking appears as a distinctive feature of the confinement of rods, and our setup allows us to show that stacking decreases with the radius of the configuration. In the following we investigate the possibility of characterizing geometrical order without a detailed knowledge of the geometry of configuration. In other words, we seek an independent measure of order. We realized annealing experiments in which the confinement strength was repeatedly increased then decreased by varying the rotation velocity appropriately. At each step in rotation frequency, we waited 1800 s in order to reach equilibrium (Fig. 3a), so that an annealing experiment typically took 12 hours. After a few frequency steps, the radius of gyration followed approximately the same line (Fig. 5a). The first steps are irreversible, while the line is reversible. In other words, the system follows an irreversible branch in the (f, R g ) phase space before falling on a reversible branch. This behavior is reminiscent of the evolution of the density of a tapped granular pile according to the tapping acceleration, as reported in [START_REF] Nowak | Reversibility and irreversibility in the packing of vibrated granular material[END_REF]. In our case, each annealing experiment can be characterized with the intercept, R 0 , and with the slope, χ, of the reversible branch. Furthermore, we observed that no topological changes occurred along the reversible branch, as illustrated in Fig. 6: the relative positions of loops remain constant, and the configuration only seems to breath. As a consequence a given annealing experiment leads to a well-defined configuration, which can be characterized with R 0 and χ. R 0 corresponds to its effective radius, while χ defines an effective susceptibility of the configuration as it measures its its susceptibility to response to the variation of the confinement strength. The effective susceptibility was here defined as the response of the system to an external field which is the confinement potential and is not the same susceptibility usually defined in glassy system theory. When plotting these two quantities, the characteristic radius R 0 is found to be an increasing, roughly linear function of the susceptibility χ (Fig. 5b). The results are independent of the confinement (frequency) since, whatever annealing path, points seem to collapse on the same master curve. As we found above that an ordered configuration has a small radius of gyration (Fig. 4b), this second set of results indicates that an ordered configuration has a small susceptibility, and, conversely, a disordered configuration is highly compressible. A possible interpretation is that self-avoidance imposes a stringent constraint on ordered configurations for which more stacking means less freedom in exploring phase space; as a consequence an ordered configuration would be less compressible. In terms of mechanics, the solid-solid friction is more important in an ordered configuration than in a disordered one, while the area of part of rod in contact is higher. When the confinement pressure is decreased, friction opooses to the unfolding motion.

To summarize, we built an experiment allowing the two-dimensional confinement of a rod in a parabolic potential. This experiment allowed us to quantify order and disorder in a configuration, using a geometrical measure of stacking and a mechanical measure of effective susceptibility, this quantity being defined as the response of a system to an external field. We found that these two quantities are strongly correlated with the characteristic radius of the configuration. Although this effective susceptibility is strictly a susceptibility response to changes in rotation velocity, it can be readily generalized to other 3D systems such as crumpled balls. Indeed the definition and the measure of a nematic order in packed systems are issues still unsolved. In that context, a salient feature of susceptibility is that its measurement does not require a full knowledge of the geometry of the configuration. In other words, order could be inferred from the effective stiffness of the crumpled ball. Future work should address whether these quantities could be used in a thermodynamic approach to packing, or how these quantities could emerge in such an approach. Thus geometrical and mechanical properties appear as strongly entangled in the packing of sheets and rods.

We are grateful to J. Da Silva Quintas for his help in building the experimental apparatus.

FIG. 1 :

 1 FIG. 1: The experiment. (a) A rod of density 1 is confined in a circular Hele-Shaw cell, filled with saturated salted water of density 1.16. On the picture, the rod is in its initial configuration, before the cell has been set in rotation. (b) The cell is made of a superposition of three disks; the intermediate one (ii) is pierced with two holes to enable equilibration of pressure into the liquid. Rod is placed in the chamber 1 of controlled thickness, chamber 2 is a sacrificial room. The chambers are made watertight with flat and toric joints shown in dark. (c) Example of a equilibrium configuration obtained when a centripetal force is generated by the rotation of the cell around its axis. (d) Corresponding skeletonized image in which the number of layers per branch is defined.

FIG. 2 :

 2 FIG. 2: Panel of folded configurations resulting from experiments with same parameters. The rod is prepared in the initial configuration (Fig. ??a) then the rotation frequency is increased from 0 to 12 Hz in approximately 3 s. The large variety of geometry shows that the folding is a non-deterministic process.

FIG. 3 :

 3 FIG. 3: The radius of gyration. (a) The radius of gyration Rg as a function of time for three realizations in which the cell was launched from 0 to 14 Hz in 3 s. Radii reach a plateau value in about 10 3 s. The final radius differs according to realizations. (b) On average, the radius of gyration R decreases with the strength of confinement (quantified by the rotation velocity, f of the disk). Bars represent the width of radii distribution for a given rotation frequency.

FIG. 4 :

 4 FIG. 4: Stacking. (a) Order and disorder. Two extreme examples of equilibrium configurations obtained at two different frequencies, with a large and a small number of superposed layers, respectively. (b) Average number of layers per branch, N l/b , as a function of the radius of gyration Rg. Each point corresponds to one realization of the experiment. The colors of the symbols correspond to the imposed rotation velocity of the disk. An ordered configuration (large N l/b ) has a small radius of gyration.

FIG. 5 :

 5 FIG. 5: Susceptibility. (a) Evolution of the radius of gyration Rg during an annealing experiment. After a first irreversible brach, the radius follows a reversible branch (parallel to the dashed line). A configuration is characterized by the slope of the line, χ, and its intercept, R0, with the axis f = 0. (b) The characteristic radius R0 as a function of susceptibility χ. Each point corresponds to one realization of an annealing experiment. The error bars correspond to the uncertainties estimates of the linear fits as described in (a).

FIG. 6 :

 6 FIG. 6: Example of the evolution of a configuration during an annealing experiment. Rotation frequency was varied from 12 Hz (left) to 6 Hz (right), by steps of 2 Hz of 1800 s duration. Each colored area corresponds to a loop. The relative position of loops is invariant. The configuration only breathes under the perturbation.


María José Ginés-Molina

Nur Hidayahni Ahmad

Sandra Mérida-Morales

Cristina García-Sancho

Svetlana Mintova

Eng-Poh Ng
email: epng@usm.my

Pedro Maireles-Torres

Selective conversion of glucose to 5-hydroxymethylfurfural by using L-type zeolites with different morphologies

Keywords: biomass, glucose, 5-hydroxymethylfurfural, LTL zeolite, heterogeneous catalysis 1

The catalytic dehydration of glucose to 5-hydroxymethuylfurfural (HMF) requires the development of selective solid acid catalysts, bearing different types of acid-base active sites.

In the present work, the morphology of an L-type zeolite (LTL topology) has been modified in order to evaluate the influence of several protonated-form LTL zeolites with different morphologies on their stability and catalytic performance in glucose conversion to 5-HMF.

Physico-chemical characterisation of LTL-based catalysts has revealed that the three types of morphologies (needle, short rod and cylinder) are active, with complete glucose conversion and high HMF yield values. The addition of CaCl2 had a positive influence on the catalytic performance, improving the partition coefficient in the biphasic water:methyl isobutyl ketone system for efficiently extracting HMF formed from the aqueous phase. It was found that morphology influences the textural and acid properties of LTL zeolites, and hence their catalytic performance. The best catalytic results have been obtained with the NEEDLE-LTL, showing nanoparticles with a length of 4.03 nm and a width of 0.63 nm, which attains a HMF yield of 63.1%, at 175 ºC after 90 min of reaction, and a glucose conversion of 87.9%. The reusability study has revealed a progressive decrease in HMF yield after each catalytic cycle.

Different regeneration methods have been essayed (washing with solvents and calcination), without recovering the initial catalytic activity. The presence of organic molecules in the micropores has been demonstrated by TG-DTA analysis, which are difficult to remove even after a regeneration process at 550 ºC.

Introduction

In recent years, much attention is being paid to the development of environmentally and economically viable synthetic routes and technologies for producing chemicals and fuels from non-fossil carbon sources as alternative to fossil raw materials [1]. In this context, biomass is emerging as a very promising sustainable feedstock, being the only widely available and renewable carbon source [2,3]. Lignocellulosic biomass, mainly composed by lignin, cellulose and hemicellulose, with an estimated annual production about 2  10 11 metric tons, is the most abundant source of carbohydrates, but physico-chemical treatments are required for its use as a raw material [4]. Although lignocellulose is a sustainable resource for production of biofuels and chemicals, it is necessary that this does not interfere with the food chain. The hydrolysis of cellulose and hemicellulose leads to monomeric C5 and C6 sugars, which can be converted into important platform molecules, such as furfural and 5-hydroxymethylfurfural (HMF), respectively, which are the starting point for the synthesis of a large variety of biofuels and chemicals [5,6]. For instance, HMF can be transformed into 2,5-dimethylfuran [7] or levulinic acid [8,[START_REF] Rackemann | Biofuels, Bioprod[END_REF], among others, which are key intermediates for the synthesis of pharmaceuticals, polymers or biofuels.

Although the dehydration of fructose to HMF has largely been reported in the literature, glucose is preferred due to its abundance and low price [10]. There is not a general consensus about the mechanism of glucose dehydration to HMF, even though a generally accepted route based on: (i) isomerization of glucose to fructose, and (ii) dehydration of fructose to HMF [4].

The first step is considerably difficult and requires Lewis acid or basic sites, being the limiting factor for HMF production. The reaction may be performed either in water, organic solvents or ionic liquids, in particular polar aprotic solvents. Homogeneous catalysts such as sulphuric or hydrochloric acids can be effective for the hydrolysis of cellulose to glucose, and even for the dehydration of fructose to HMF. However, due to their corrosive properties which are hazardous for equipment, so they are gradually replaced by heterogeneous catalysts. Besides, heterogeneous catalysts allow their easy separation from solution, recovery and reuse [11][12][13].

Different solid acid catalysts have been tested for dehydration of glucose to HMF, such as γ-Al2O3 [14], zeolites [15], metal oxides like TiO2 or ZrO2 [12][13][14][15], mesoporous solids [16,17] inter alia.

Nevertheless, both solvent and catalyst must be considered as two key factors to attain high HMF yields from C6 carbohydrates. A common strategy is the use of biphasic systems for furfural production, because this approach allows higher HMF yields than those systems employing only water. Usually, the biphasic medium is formed by the addition of organic solvents (toluene, methyl isobutyl ketone, among other) to an aqueous solution, or the addition of miscible organic solvents like tetrahydrofuran (THF) [18], to a saturated salt solution, which allows to extract the HMF formed from the aqueous phase, preventing its further degradation and condensation [4].

In order to prevent these side reactions, Román-Leshkov et al. [19] employed inorganic salts in a biphasic system for dehydration of fructose to HMF and concluded that the saltingout effect leading to a higher partition coefficient, limiting the individual cationic or anionic contributions, so then it is feasible to correlate to the interaction of all ionic species. In this context, it has also been reported that divalent cations interact more strongly with saccharides than the monovalent ones [20]. Thus, Combs et al. [21] observed that alkaline earth metal cations can form bidentate complexes with glucose, which accelerated its transformation.

Later, our research group studied the beneficial effects of CaCl2 on glucose dehydration to HMF in the presence of Al2O3 as catalyst, in such a way that the addition of CaCl2 to the reaction medium notably improved the catalytic performance, even at very short reaction times, due to the interaction between Ca 2+ ions and glucose molecules, which favored the α-Dglucopyranose formation [14].

Concerning the use of zeolites for glucose dehydration, different acidic ZSM-5 zeolites (H-, Fe-and Cu-ZSM-5) were prepared and studied by modifying several experimental variables [15]. It was demonstrated the positive effect of the addition of inorganic salt (NaCl) to a biphasic (water/MIBK) system, since a glucose conversion of 80%, with a HMF yield of 42% was attained at 195 ºC, after 30 min, by using a H-ZSM-5 zeolite, which had the lowest Lewis/Brönsted ratio among the studied zeolites. Later, the H-ZSM-5 zeolite was compared with H-Y and H-Beta zeolites, in order to assess the influence of the textural properties on the catalytic performance in glucose dehydration [22]. Under similar experimental conditions, by using a H-Beta, the highest HMF yield (56%) was reached, thus demonstrating the benefit of mesoporosity in this catalytic process.

Recently, by using a bifunctional Cr/β zeolite, a high selectivity to HMF with a yield of 72%, was found at 150 ºC, after 90 min, by adding NaCl to a biphasic H2O/THF system [23].

After three consecutive catalytic cycles, the catalytic activity slightly decreased, but after a thermal treatment was almost recovered.

Morphological or textural characteristics play a highly important role when discussing catalytic activity in a chemical reaction [Y.J. Huang, G.R. Qi, L.S. Chen, Appl. Catal. A: Gen. 240 (2003) 263-271]. Nevertheless, the morphological roles of zeolite in glucose dehydration are still not completely understood and hence further investigation to reveal this effect in this reaction is utmost important. The aim of this work is a thorough study of glucose dehydration for HMF production using protonated L-type (H-LTL) zeolites with different morphologies, which have been characterized and their catalytic performance has been correlated with their textural and acid-base properties.

Results and Discussion

Catalyst Characterization

The choice of an LTL zeolitic framework to evaluate the influence of the morphology (short rod, cylinder and needle) on the catalytic performance in glucose dehydration to HMF arises from the dimensions of pore mouth and channels of this crystallographic structure (Figure 1). These allow the entrance of glucose molecules to reach active acid sites, and 5hydroxymethylfurfural can easily go out, leaving active sites available for new sugar molecules. The X-ray diffraction patterns of H-LTL zeolites exhibit many narrow diffraction peaks at 2 = XXX° (100), XXX° (200), XXX° (001), XXX° (210), XXX° (111), XXX° (220), XXX° (310), XXX° (301), etc. which can be assigned to the hexagonal LTL-type framework (PDF 98-007-4170) (Figure 2). In all cases, neither additional crystalline phase nor amorphization is detected and the crystallinity of the solids is preserved after ion-exchange and calcination treatments. Concerning the chemical composition of LTL zeolites, the Si/Al molar ratio values obtained from ICP-OES are close to 3, which are lower than that used in the synthesis gel (10) (Table 1). The comparison between the bulk and surface chemical composition data, deduced from ICP-OES and XPS, respectively, points an enrichment of Si on the surface of LTLzeolites. This could be explained by the higher Si/Al molar ratio (10) used in the synthesis of these zeolites. However, it must be taken into account the inherent error associated to the semiquantitative analysis by XPS. After the ion exchange process between K + and NH4 + ions, not all the alkaline cations are removed, which can be due to the existence of strong acid sites remaining dissociated and neutralized by K + cations, even in the presence of an excessive ammonium solution. Therefore, the K/Al molar ratio is lower than 1 for NEEDLE-LTL and ROD-LTL, indicating that, although acid sites are protonated, a fraction of sites is still occupied by K + ions. This fraction is even higher for the CYL-LTL, where the value is closed to 1 would point out that this morphology is the less favorable for the ion-exchange process. [24]. As regards the K 2p spectra, they exhibit the typical doublet with the K 2p3/2 at 293.3-293.7 eV, and a spectral separation of 2.8 eV, typical of K + ions [START_REF] Moulder | Handbook of X-Ray Photoelectron Spectroscopy[END_REF].

On the other hand, textural properties reveal that all zeolites maintain high surface area values, being the largest one for short rod morphology (Table 2). As expected, these zeolites are mainly microporous solids, with a percentage of microporous surface area higher than 95%. The microporous nature of these LTL-based zeolites can be easily confirmed by the shape of their adsorption-desorption isotherms of N2 at -196 ºC, which are Type I in the IUPAC classification, typical of microporous solids (Figure 4). The slight hysteresis loop could be associated to some mesoporosities generated during treatments used for synthesizing their protonated forms, probably associated to interparticular voids [START_REF] Ng | [END_REF]27], which is corroborated by the pore size distribution curves (Figure 4, bottom). The different morphologies are more clearly appreciated in the scanning electron micrographs (Figure 5), where micrometric particles are observed, whose dimensions and shapes fit in really well with short rods, needles and cylinders (Table 2). On the other hand, the chemical environment of aluminium has been analysed by 27 Al MAS-NMR spectroscopy (Figure 6). Extra The total acidity of catalysts was determined from ammonia temperature-programmed desorption (NH3-TPD), whereas the nature (Brönsted or Lewis) of acid sites was studied from pyridine adsorption coupled to FTIR spectroscopy. being strong acid sites predominant in all cases. This acidity order is the same found for Langmuir surface area values (Table 2). According to the 27 Al NMR data and NH3-TPD, it looks like the pyridine data of CYL and ROD are interchanged.

Catalytic study

These zeolites have been evaluated as acid catalysts in the dehydration of glucose to 5hydroxymethylfurfural, an important platform molecule for the synthesis of biofuels and high value-added chemicals. Although the mechanism involved in glucose dehydration to HMF seems to depend on type of catalyst, nature of solvent, among other parameters, it is broadly accepted that the rate-determining step is the isomerization of glucose to fructose [4]. For this reason, most of studies have used fructose as starting sugar. The isomerization process is catalyzed by Lewis acid or basic sites, whereas dehydration of fructose to HMF requires the participation of Brönsted acid sites. The production of HMF from glucose has two main drawbacks associated to the catalytic process, that is, the formation of by-products, such as soluble/insoluble polymers and humins and the rehydration of HMF for giving rise to levulinic and formic acids. These processes decrease the HMF yield and, in some cases, provoke the catalyst deactivation. In this sense, the use of organic co-solvents together with the aqueous phase containing carbohydrates has been usually reported as a suitable strategy for improving HMF yield. Among these organic solvents, methyl isobutyl ketone is one of most often used due to their benign physico-chemical properties [32]. In this work, a biphasic water-MIBK system was employed to study the dehydration of glucose to HMF.

Moreover, previous studies have demonstrated that the addition of inorganic salts to biphasic phases ameliorates the extraction of 5-HMF from biphasic systems [14,16,19]. In particular, calcium chloride exerts a positive effect on the catalytic performance, even at very short reaction times [14,33]. This was explained by the interaction between Ca 2+ ions and carbohydrates (glucose and xylose), modifying the corresponding anomeric equilibrium towards the anomer  more prone for the dehydration process, as was concluded from 1 H NMR spectroscopy data.

In this sense, the catalytic activity of the LTL-zeolites was studied with and without CaCl2 addition (Figure 9). The positive effect is clearly observed, since at 150 ºC, after 60 min of reaction and in the absence of inorganic salt, the conversion of glucose is lower than 20% and in this case the formation of HMF is barely detected (HMF yield < 1%). However, the addition of CaCl2 outstandingly improves the glucose conversion until values higher than 95% for ROD-LTL, with a HMF yield of 58.5%, whereas the rest of catalysts attain conversion and HMF yield values higher than 75% and 25%, respectively. Therefore, these data confirm previous results attained with other families of catalysts in the presence of inorganic salts [15,19]. ).

It is evident the improving effect of CaCl2 addition on the catalytic performance of this series of LTL-based catalysts. However, an additional catalytic study was accomplished in order to evaluate the contribution of CaCl2 to the overall catalytic activity. For this, CaCl2 (0.65 g gH2O -

1

) was put in contact with glucose in the biphasic system, and compared with the same system to which the NEEDLE-LTL catalyst was added. The data reveal that, in the absence of catalyst, after 3 h at 150 ºC, the glucose conversion was 47.1%, with a HMF yield of 16.8% (Figure 10). However, under similar experimental conditions, this catalyst exhibits better catalytic performance, attaining a conversion of 86.9% and a HMF yield of 44.0%. The catalytic activity in the presence of CaCl2 might be explained by the formation of -anomer in the presence of this salt, as it has been demonstrated in previous studies [14], which could easily be dehydrated due to the thermal contribution (non-catalyzed process) to the overall catalytic performance. The kinetics of the dehydration process with LTL-based catalysts were studied at 150 ºC (Figure 11), and similar conversion curves were obtained for the three catalysts, in spite of their different acid properties (Table 2) and morphologies. However, at intermediate reaction times, the HMF yield values are lower for CYL-LTL, which could be explained by considering its lower acidity, although its mesoporous character could compensate the lower concentration of acid sites in this catalyst. In this study, the highest HMF yield (50.3%) was attained after 300 min with the ROD-LTL catalyst. ).

By increasing the temperature until 175 ºC, the reaction time for attaining the highest HMF yield is shortened, being possible to obtain a HMF yield of 63.1% for a glucose conversion of 87.9% with the NEEDLE-LTL catalyst after only 90 min (Figure 12). The ).

Other experimental parameter evaluated has been the glucose:catalyst weight ratio, which has been varied between 1:1 and 10:1, by maintaining the amount of glucose and adding different amounts of NEEDLE-LTL. It can be observed that conversion of glucose raises with the catalyst loading until a weight ratio of 3:1, which can be explained by the increment of available acid sites for glucose dehydration (Figure 13). Nevertheless, a higher amount of catalyst (ratio of 1:1) does not improve the conversion, although a slightly higher HMF yield is attained. This could be explained by considering diffusional problems associated to the location of active sites in micropores. A key aspect in heterogeneous catalysis is the recovery of the solid catalyst to be used in successive catalytic runs. In order to carry out this study, a glucose:catalyst weight ratio of 1:1 was used, since a higher catalyst loading would minimize the loss of catalyst between cycles, produced by catalyst handling. The reaction was studied at 150 ºC, for 60 min (Figure 14).

After the first catalytic cycle, the solid catalyst was filtered and dried at 65 ºC. The catalytic data reveal a decrease in glucose conversion from 53.4 to 43.4%, although active sites involved in glucose dehydration to HMF seem to be almost deactivated, since HMF yield drastically diminishes from 15.5 to 3.7%. After the second run, a washing with water/acetone and filtration of the used solid was also carried out, recovering the glucose conversion, but the HMF yield is still lower. Finally, the used catalyst was calcined until 550 ºC for 2 h after a first catalytic cycle in order to remove the organic fraction. However, the initial catalytic performance continued without recovery, with values of glucose conversion and HMF yield of 100 and 4.52%, respectively.

In order to explain the reason of this behavior, the TG-DTA analyses of used and fresh NEEDLE-LTL zeolite were performed. The fresh zeolite shows a weight loss of 10% between room temperature and 200 ºC, associated to the removing of hydration water, without any remarkable weight variation until 900 ºC. However, the used catalyst exhibits a high weight loss in the same temperature range, but with two distinguishable steps, which could be associated to the elimination of hydration water and adsorbed MIBK, account for 20%. Then, a progressive weight loss is observed, but with an important sloop change between 700 and 900 ºC. Thus, a weight loss of 22% is observed between 200 and 900°C, and the high temperature required to remove organic molecules could be explained by the strong interaction with the catalyst surface inside the micropores. In this sense, a regeneration temperature of 550 ºC was not enough to remove organic species, which cover the active sites responsible of glucose dehydration to HMF. The covering of active sites by carbonaceous deposits, as main cause of the decrease in catalytic activity, has been already reported for zeolites [23]. Therefore, it can be concluded that the diffusion of glucose molecules to active sites present in LTL zeolites is favored by using needle and rod morphologies, but it is necessary to prepare hierarchical zeolites, where mesoporosity is a key feature for facilitating the access of reactants and the regeneration of active sites, and delaying the catalyst deactivation.

Experimental

Chemicals and materials

Potassium hydroxide pellets (85%) and ammonium nitrate were purchased from Merck, Darmstadt, Germany. Colloidal silica HS-40 was supplied by Sigma-Aldrich, Germany.

Aluminium sulfate hexadecahydrate (Al2(SO4)3 . 16H2O, 97%) was obtained from BDH Chemical Ltd, Poole, England. All chemicals were used without further purification. For the catalytic tests, the following chemicals have been utilized: glucose (Sigma-Aldrich, >99%), fructose (Sigma-Aldrich, >99%) and calcium chloride (VWR, 97%). Deionized water and methyl isobutyl ketone (MIBK, VWR, 98%) have been used as solvents. (needle and cylinder) but using hydrogels of different molar compositions as stated in Table 4. The protonated L-type zeolite crystalline solids with short-rod, cylindrical and needle shapes were denoted as ROD-LTL, CYL-LTL and NEEDLE-LTL, respectively.

Synthesis of L-type zeolites

Characterizations of LTL zeolites

XRD patterns were recorded using a Bruker-AXS D8 diffractometer operating at 40 kV and 10 mA (Cu Kα radiation, λ = 0.15418 nm). The surface morphology of samples was observed using a JEOL JSM-6701F FESEM microscope. The infrared (IR) spectra were acquired using a Perkin-Elmer System 2000 using the KBr method (Sample:KBr ratio = 1:50).

The chemical composition of zeolites was also determined with an Optima 8300 inductively coupled plasma-optical emission spectrometer (ICP-OES). Textural properties were determined by a Micrometrics ASAP 2010 nitrogen adsorption analyzer at -196 C. First, the sample (ca. 0.08 mg) was degassed under vacuum at 300 °C overnight. The surface area and pore size distribution of samples were estimated using the Langmuir and DFT models, respectively. The total pore volume of the solids was determined from the nitrogen adsorbed volume at P/Po = 0.990.

The FTIR spectra after pyridine adsorption were acquired using a Nicolet 2000 FTIR spectrometer. Initially, the zeolite powder (ca. 0.01 g) was ground and pressed into a selfsupporting wafer (area 2 cm 2 ) at 6.0 ton. The wafer was introduced into an IR vacuum cell and activated at 400 C for 5 h under vacuum (10 -3 mbar). The sample was cooled to 25 C before the background spectrum of zeolite was recorded. Pyridine vapor was adsorbed onto the sample for 3 min before the excess pyridine vapor was evacuated. The FTIR spectra were recorded at 25 C using 200 scans with a resolution of 6 cm -1 . The wafer was then heated at 150 C for 1 h to remove weakly bound pyridine before the second IR spectrum was recorded. The wafer was heated again to 300 C for 1 h before it was cooled to 25 C and scanned again with an IR spectrometer. The concentration of Lewis and Brönsted acid sites were calculated by using the molar integral extinction coefficients of εBrönsted = 3.03 cm µmol -1 and εLewis = 3.80 cm µmol -1

[reference should be included] X-ray photoelectron spectra were obtained with a Physical Electronics PHI 5700 spectrometer with non-monochromatic Mg Kα radiation (300 W, 15 kV, and 1253.6 eV) with a multi-channel detector. Spectra were recorded in the constant pass energy mode at 29.35 eV, using a 720 µm diameter analysis area. Charge referencing was measured against adventitious carbon (C 1s at 284.8 eV). A PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals.

Recorded spectra were always fitted using Gaussian-Lorentzian curves in order to determine accurately the binding energies of the different element core levels. 27 Al MAS NMR experiments were performed on a Bruker AV-400 (9.4 T) spectrometer (Rheinstetten, Germany), using a BL-4 probe with zirconia rotors. The spectra were obtained using a spinning speed of vR = 10 kHz, a pulse width of 1 μs corresponding to a π/12 rad. Pulse length, a relaxation delay of 1 s, and typically 1200 scans. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out to evaluate the total surface acidity of catalysts. After cleaning of catalysts (0.08 g) with helium up to 550 ºC and subsequent adsorption of ammonia at 100 ºC, the NH3-TPD was performed by raising the temperature from 100 to 550 ºC, under a helium flow of 40 mL min-1, with a heating rate of 10 ºC min-1 and maintained at 550 ºC for 15 min. The evolved ammonia was analyzed by using a TCD detector of a gas chromatograph (Shimadzu GC-14A).

Thermogravimetric analyses (TGA) were performed with a TGA/DSC 1 model (Mettler-Toledo), under air flow of 50 mL min -1 with a heating ramp of 10 ºC min -1 , from room temperature until 900 ºC. The carbon content of spent catalysts was measured with a LECO CHNS 932 analyser.

Catalytic reaction

The catalytic performance of LTL-type zeolites in glucose dehydration was evaluated in batch conditions, by using a glass pressure tube with thread bushing (Ace, 15 mL, pressure rated to 10 bars) under magnetic stirring. In a typical experiment, 0.15 g of glucose, 0.05 g of catalyst,

1.5 mL of deionized water and 3.5 mL of methyl isobutyl ketone (MIBK) were introduced into the reactor. Reactors were always purged with nitrogen, prior to the catalytic study, to minimize side reactions of HMF which decrease its yield. The mixture was heated with a thermostatically controlled oil bath. The reaction was quenched by submerging the reactor in a cool water bath, and the liquid phases were separated, filtered and the analysis of products was performed in

Figure 1 .

 1 Figure 1. (a) Structure of zeolite LTL view along [001] plane illustrating its hexagonal

Figure 2 .

 2 Figure 2. XRD patterns of (a) ROD-LTL, (b) NEEDLE-LTL and (c) CYL-LTL zeolites.

Figure 4 .

 4 Figure 4. N2 adsorption-desorption isotherms (top) and pore size distributions (bottom) of

Figure 5 .

 5 Figure 5. SEM images of zeolite LTL crystals synthesized of (a) ROD-LTL, (b) NEEDLE-

Figure 7

 7 displays the amount of ammonia desorbed in different temperature ranges, which have been assigned to weak (100-200 ºC), medium (200-300 ºC) and strong (300-550 ºC) acid sites. The total acidity follows the order:

Figure 7 .

 7 Figure 7. NH3 desorption of LTL-based zeolites, as a function of the strength: weak (100-200

Figure 8 .

 8 Figure 8. FTIR spectra of LTL zeolites after pyridine desorption at different temperatures (150 and 300 ºC)

Figure 9 .

 9 Figure 9. Effect of the addition of CaCl2 on the catalytic performance (Experimental

Figure 10 .

 10 Figure 10. Catalytic performance of CaCl2 and CaCl2/NEEDLE-LTL (Experimental

Figure 11 .

 11 Figure 11. Kinetics of glucose dehydration (Experimental conditions: 0.15 g glucose, 0.05 g

Figure 12 .

 12 Figure 12. Kinetics of glucose dehydration (Experimental conditions: 0.15 g glucose, 0.05 g catalyst, 1.5 mL water, 3.5 mL MIBK, Temperature= 175 ºC; CaCl2= 0.65 g gH2O -1).

Figure 13 .

 13 Figure 13. Influence of the glucose:catalyst (NEEDLE-LTL) weight ratio (Experimental

Figure 14 .

 14 Figure 14. Reusing study of NEEDLE-LTL after (1) drying at 65 ºC and (2) washing with

Figure 14 .

 14 Figure 14. TG-DTA analysis of fresh (black line) and used (red line) (glucose:catalyst weight

  Short-rod shape L-type zeolite was synthesized by dissolving KOH (3.018 g) and Al2(SO4)3.16H2O (1.442 g) with distilled water (18.208 g) in a polypropylene (PP) bottle. The mixture was magnetically stirred at room temperature to form a slightly cloudy solution. A clear silicate solution was prepared by mixing HS-40 (6.875 g) with distilled water (9.958 g) under stirring for 5 min. To avoid gelation, the silicate solution was added dropwise into the aluminate solution under vigorous stirring for 5 min. The solution with a final molar ratio of 10.0 K2O : 1 Al2O3 : 20 SiO2 : 800 H2O was further aged at room temperature for 18 h under stirring prior to crystallization at 180 °C for 3 days. The solid product was then filtered and purified with distilled water until pH 7 prior to freeze-drying. The zeolite powder in K-form (3.000 g) was then ion exchanged with ammonium nitrate (1.5 M, 100 mL) at 25 o C for 18 h before subjecting calcination at 480 o C for 4 h to produce protonated L-type zeolite (ROD-LTL). Similar procedures were used for preparing L-type zeolites with other morphologies

  both phases by high performance liquid chromatography (HPLC). A JASCO instrument equipped with quaternary gradient pump (PU-2089), multiwavelength detector (MD-2015), autosampler (AS-2055), column oven (co-2065) using a Phenomenex Luna C18 reversedphase column (250 mm × 4.6 mm,5 µm) and Rezex ROA-Organic Acid H + (8%) (300 mm × 7.8 mm, 5 µm), was employed. Both glucose and fructose were monitored using a refractive index detector for aqueous phase, while HMF production was monitored using a UV detector in both phases. The mobile phases consisted in pure methanol (flow rate 0.5 mL•min -1 ) for Luna C18 and deionized water (flow rate 0.35 mL•min -1 ) for Rezex ROA, being the columns at room temperature and 40 ºC, respectively. Author Contributions: Experiment, ; Data Curation, ……………; Writing-Original Draft Preparation, ………….;Writing-Review & Editing, ………….; Supervision, ………; Project Administration, …………...; Funding Acquisition, ……….

Table 1 .

 1 XPS and ICP-OES (* in parentheses) data of LTL zeolites, as determined from XPS analysis.

			Binding energy (eV)	Si/Al molar	K/Al molar
	Samples
		Si 2p Al 2p O 1s	K 2p3/2	ratio*	ratio*
	NEEDLE-LTL	102.8	74.3	532.1	293.3	4.23 (3.00)	0.45 (0.63)
	ROD-LTL	102.9	74.5	532.2	293.4	4.54 (2.90)	0.45 (0.55)
	CYL-LTL	103.2	74.6	532.4	293.7	4.43 (3.45)	1.28 (1.07)

X-ray photoelectron spectroscopy (XPS) has been used to get insights into the surface nature of LTL-based zeolites. In all cases, the binding energies of Si 2p (102.8-103.2 eV), Al 2p (74.3-74.6 eV) and O 1s (532.1-532.4 eV) are typical of these elements forming part of microporous aluminosilicates

Table 2 .

 2 Textural and acid properties of LTL zeolites

		SLangmuir	Smicro	Vp	Length	Width	Total acidity
		(m 2 g -1 )	(m 2 g -1 )	(cm 3 g -1 )	(m)	(m)	(mol m -2 ) *
	NEEDLE-LTL	248	235	0.100	4.03	0.63	6.73
	ROD-LTL	303	293	0.120	1.39	1.38	6.17
	CYL-LTL	219	199	0.090	2.18	0.97	4.88

*

Determined from NH3-TPD

  -framework octahedral Al species are associated to a resonance signal at a chemical shift near 0 ppm, whereas tetrahedral Al in crystallographic sites in the zeolite framework appears at about 60 ppm. This latter signal is observed in all cases, and the absence of broadening effect at lower chemical shift values could discard the existence of pentacoordinated or distorted tetrahedral aluminium, reported in other microporous aluminosilicates, which give rise to signals at 30 and 47 ppm, respectively[28- 30]. The small contribution at 0 ppm is associated to extra-framework Al species, but this is absent in the MAS-NMR spectrum of CYL-LTL, where all Al seems to be in tetrahedral

	coordination.
		CYL-LTL	T d	O h
		ROD-LTL
		NEEDLE-LTL
	150	100	50	0	-50
			27 Al (ppm)
	Figure 6.

27

Al Solid State Nuclear Magnetic Resonance spectra of LTL zeolites.

Table 3 .

 3 Surface acidity of zeolite LTL with different morphology at different temperature.

				Py-FTIR Acidity
				(µmol/g)
	Sample	Lewis (L)	Brönsted (B)	Total (L + B)	B/L
		150 °C 300 °C 150 °C 300 °C 150 °C 300 °C 150 °C 300 °C
	ROD-LTL	33.65	0.00	118.58	84.51	152.23	84.51	3.52	-
	CYL-LTL	126.08	32.08	128.07 152.85 254.15 184.93	1.02	4.76
	NEEDLE-LTL	120.55	31.76	161.39 154.50 281.95 186.26	1.34	4.86

Table 4 .

 4 Experimental conditions for the synthesis of L-type zeolites with different morphologies.

	Samples	Molar composition K2O Al2O3 SiO2	H2O	Aging (h)	Synthesis temperature ( o C)	Synthesis time (h)
	ROD-LTL	10.0	1	20	800	18	180	72
	NEEDLE-LTL 10.2	1	20	1100	18	180	72
	CYL-LTL	10.2	1	20	1030	18	180	72

Funding:

The authors are grateful to financial support from the Spanish Ministry of Economy and Competitiveness () and FEDER (European Union) funds. C.G.S. acknowledges the University of Málaga for a postdoctoral contract. Malaysia

Conflicts of Interest:

The authors declare no conflicts of interest.