Claudia Adam
email: adam@jamstec.go.jp

Valerie Vidal
email: valerie.vidal@ens-lyon.fr

Javier Escartín
email: escartin@ipgp.jussieu.fr

80-Myr history of buoyancy and volcanic fluxes along the trails of the Walvis and St. Helena hotspots (South Atlantic)

Keywords: hotspot, mantle plume, buoyancy fluxes, temporal evolution PACS: 91.45.Fj, 91.45.Jg, 91.50.Ga, 93.30.Mj

Walvis and St. Helena are the only long-lived hotspot chains in the South Atlantic.

Therefore, their characterization is important to constrain the processes associated with mantle plume formation, their temporal evolution, and the interaction with plate and mantle dynamics in the region. We study the temporal evolution of plume buoyancy and magma production rate along both hotspot chains, which are constrained from the swell and volume of volcanic materials emplaced along the chain.

The regional depth anomaly is calculated by correcting the 2' bathymetry grid of Smith & Sandwell (1997, Science, 277) for thermal subsidence and sediment loading.

We separate the topography associated with volcanism and the swell surrounding the hotspot chains using the MiFil filtering method (Adam et al., 2005, Geochem. Geophys. Geosyst., 6). We then estimate the temporal variations associated with

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Hotspot chains are the result of the interaction of stationary zones of mantle upwelling (plumes) with an overlying drifting plate [START_REF] Wilson | A possible origin of the Hawaiian Islands[END_REF]. Two main surface manifestations are generally associated with such a plume-lithosphere interaction: (1) a prominent volcanic chain which displays a linear age progression, as plate motion drags the seamounts away from their zone of emplacement above the plume; and (2) a large positive depth anomaly (typically 100's of kilometers in extent and ∼100's of meters in height) called a swell [START_REF] Crough | Hotspot swells[END_REF]. The hotspot swells may be produced by several phenomena. Dynamic topography, associated with vertical forces due to plume uplift, may generate a swell (a few hundred meters in height) in the youngest part of the hotspot chain, where the lithosphere drifts over the plume head. The swell at older parts of the hotspot chain may have two origins: 1) A thinner than 'normal' lithosphere, due to reheating at the time the lithosphere was overriding the hotspot plume. This effect vanishes through time, as the lithosphere cools and subsides while it is driven away from the plume. 2) Stocking of plume material within or beneath the lithosphere or crust (underplating), which has a permanent effect. Since no underplating has been identified along St. Helena and Walvis chains, and since the dynamic topography is spatially restricted along the youngest part of these chains, we attribute the main cause of the swell to the long-term thermal effect of the plume.

The swell is a direct consequence of the buoyant plume upwelling, and therefore, it is commonly used as the parameter to quantify the hotspot strength [START_REF] Sleep | Hotspots and mantle plumes: some phenomenology[END_REF][START_REF] Courtillot | Three distinct types of hotspots in the Earth's mantle[END_REF][START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF]. However, this parameter is inadequate to study the history of a hotspot chain and the underlying plume dynamics, as the swell amplitude decreases with time due to lithospheric cooling. The volcanic material accumulated along the hotspot chain, which is not affected by this cooling, is a direct record of the volcanic output to the seafloor. However, while being a good indicator of hotspot activity, the volume of extrusives may somewhat underestimate the total melt production, as other processes may operate such as melt entrapment in deep lithospheric levels, or lateral melt migration (i.e., lithospheric cracks) and emplacement away from the volcanic chain. The combined analyses of both swell and volcanic volumes accumulated along the chain is therefore most adequate and necessary to constrain long-term plume activity [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF].

In this study, we use the available bathymetric [START_REF] Smith | Global sea floor topography from satellite altimetry and ship depth soundings[END_REF], sediment thickness and seafloor age data [START_REF] Müller | Digital isochrons of the world's ocean floor[END_REF] to correct for thermal subsidence and sediment loading, and obtain a seafloor depth anomaly around the Walvis and St. Helena hotspot chains. We then separate the swell and volcanic topography components from the residual bathymetry [START_REF] Adam | MiFil: A method to characterize seafloor swells with application to the south central Pacific[END_REF]. We then estimate the temporal evolution in production rate of volcanic material Q v (including both the volcanic edifices and compensating root due to plate flexure) and in the swell flux Q s . We compare these results with those from Hawai'i [5], the most typical example of a hotspot chain.

2 Tectonic setting

Description of the Walvis and St. Helena chains

Walvis and St. Helena are the two main hotspot chains found in the South Atlantic. They are formed by wide and continuous zones of diffuse volcanism (oceanic islands, seamounts and small ridges), that locally reach a width of more than 500 km. It is generally considered that these volcanic chains have been created by mantle plumes interacting with the overlying African plate, and that the locus of present-day activity is situated in the proximity of the Islands of Tristan da Cunha for the Walvis Ridge. For St. Helena, the youngest age of volcanism (2.6 Ma) is reported at Josephine seamount [START_REF] Courtillot | Three distinct types of hotspots in the Earth's mantle[END_REF][START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF] (see Figure 1). This is consistent with the reported general age progression of volcanism away from the St. Helena and Gough islands and towards the African continent [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF].

The Walvis Ridge extends WSW some 2800 km from the southwest African continental margin towards the Mid-Atlantic Ridge near Tristan da Cunha and Gough islands (Figure 1). Based on morphological and structural characteristics, the chain displays different tectonic settings at the time of volcanic emplacement and loading. The northeastern part (east of 3 • E) has a ridge-like, elongated morphology rising 2-3 km above the surrounding seafloor. Largescale block faulting in this area is prominent, as inferred from the gravity, bathymetry and seismic data [START_REF] Goslin | Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure[END_REF][START_REF] Detrick | An analysis of Isostasy in the World's Oceans, 3. Aseismic Ridges[END_REF]. Further southwest, the ridge splits into two branches, one trending N-S and a second one trending NE-SW, both composed of individual seamounts and guyots. The N-S trending branch terminates near 34 • S, whereas the oblique branch continues WSW towards the present-day hotspot location on the eastern flank of the Mid-Atlantic Ridge.

The initial manifestation of the Walvis hotspot occurred at the opening of the South Atlantic 130 Ma ago, when it contributed to the emplacement of the Paraná (Southeast Brazil) and Etendeka (southwest Africa) flood basalts, 127.5 to 139 Ma old [START_REF] Gibson | Timescales and mechanisms of plumelithosphere interactions: 40 Ar/ 39 Ar geochronology and geochemistry of alkaline igneous rocks from the Paraná-Etendeka large igneous province[END_REF]. The hotspot was located initially on the American plate, crossed the axis 80-70 Ma ago, and progressively separated from it to its present-day location on 7 Ma-old African plate.

Evidence for this transition from on-axis to intraplate volcanism is found in many geophysical and geochemical observables. The geochemical analyses evidence a westward migration of the spreading axis, away from the plume [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] Schilling | Hotspot-migrating ridge interaction in the South Atlantic[END_REF]. The structural differences between the northeast (oldest) and southwest (youngest) sections of the chain are also reflected in their compensation mechanisms. While the older part is compensated by over-thickening of the oceanic crust [START_REF] Goslin | Geophysical study of the easternmost Walvis Ridge, South Atlantic: Deep structure[END_REF][START_REF] Detrick | An analysis of Isostasy in the World's Oceans, 3. Aseismic Ridges[END_REF], the western part is instead regionally compensated by the bending of a lithosphere with an effective elastic thickness of 5-8 km [START_REF] Detrick | An analysis of Isostasy in the World's Oceans, 3. Aseismic Ridges[END_REF]. These different compensation mechanisms reflect the transition with time from a near-axis emplacement with a thin lithosphere and over-thickened crust to an intraplate setting with a thicker lithosphere. This transition is also observed in the mode of volcanic emplacement, that produces a province of intermittent, isolated volcanism rather than a structurally continuous ridge.

The St. Helena chain is less well-defined, and is characterized by a broad and ill-defined band of scattered seamounts and volcanic ridges, with a shorter history than that of Walvis, but with a similar progressive separation of the hotspot from the axis. The oldest dated sample along the St. Helena chain is 81 Ma [START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF], dredged on a seamount situated at the northeastern extremity of the chain (Figure 1). The Cameroon line, northeast of this volcano, seems to form a bathymetric extension of the St. Helena chain into the Gulf of Guinea and linking with the African continent. However, the Cameroon lineament does not show any age progression [START_REF] Fitton | The Cameroon Line, West Africa: a comparison between oceanic and continental alkaline volcanism[END_REF], and therefore cannot be directly linked to a hotspot origin. The origin of the St. Helena chain on the African plate is associated with the emplacement of the Brazilian seamounts on the American plate [START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF]. Geochemical [START_REF] Schilling | Hotspot-migrating ridge interaction in the South Atlantic[END_REF][START_REF] Schilling | Upper mantle heterogeneities and dynamics[END_REF] and seismological studies [START_REF] Zhang | Ridges, hotspots and their interactions as observed in seismic velocity maps[END_REF] indicate a migration of the St. Helena plume toward the spreading axis. Circe seamount (Figure 1) has been proposed as the site of an active hotspot by Schilling et al. [START_REF] Schilling | Hotspot-migrating ridge interaction in the South Atlantic[END_REF]. It is dated at 6.6 Ma, and may be part of a hotspot trail whose present-day location should be some 130 km SW of Circe seamount [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF].

Age evolution

The distribution of the available volcanic ages (Figure 1) shows a large scatter, with many volcanic edifices located in the same area displaying differences in age of more than 10 Ma. For example, at 34 • S, we observe a 44.6 Ma volcanic age between sites dated at 64.4 Ma (south) and 61.6 Ma (north). This age scattering can be explained by diffuse volcanism, due either to a broad plume (several hundreds of km in diameter) or the presence of a cluster of plumes [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF]. Independent geophysical data support the latter hypothesis, since numerous small-wavelength elongated features appear in filtered geoid and topography maps [START_REF] Fleitout | Small-wavelength geoid and topography anomalies in the South Atlantic Ocean: A clue to new hot-spot tracks and lithospheric deformation[END_REF], which could indicate the presence of multiple narrow plumes. The likely presence of additional hotspot chains in nearby areas, such as the Circe seamount and proposed associated trail , is consistent with the probable presence of several hotspots [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF][START_REF] Schilling | Hotspot-migrating ridge interaction in the South Atlantic[END_REF]. Some features uncorrelated with the Walvis and St. Helena plumes have also been reported, as Vema (18 Ma) and 7 East (91 Ma) seamounts south of the Walvis chain. Haxel & Dziak [START_REF] Haxel | Evidence of explosive seafloor volcanic activity from the Walvis Ridge, South Atlantic Ocean[END_REF] have detected volcano-acoustic signals around a seamount situated on the northern edge of the Walvis track at 5.22 • W 32.96 • S (see Figure 1). These signals are interpreted as volcanogenic explosions suggesting present-day magmatic activity. The authors support the hypothesis that this phenomenon is not directly related to the Walvis mantle plume, but rather to extensional fracture zones along inactive segments of the chain.

The Walvis and St. Helena chains may have recorded the decrease in velocity of the African plate, that occurred between 19 and 30 Ma [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF]. According to O'Connor et al. [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF], the migration rate along the St. Helena chain changed from 30 mm/yr to 20 mm/yr at 19 Ma. This change occurred at 30 Ma along the Walvis chain, with a reduction from 31 mm/yr to 22 mm/yr. However, the plate velocity diminution has been computed considering that the chain follows a continuous NW direction [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF], without taking into account hotspot tracks deduced from the rotation poles. Moreover, the slowdown is estimated solely on the two younger ages available for each chain, and is therefore poorly constrained.

Data analysis

We first compute the depth anomaly throughout the study area. Using the global age grid of Müller et al. [START_REF] Müller | Digital isochrons of the world's ocean floor[END_REF] and the GDH1 thermal subsidence model of Stein & Stein [START_REF] Stein | A model for the global variation in oceanic depth and heat flow with lithospheric age[END_REF], we compute the thermal subsidence, which we substract from the bathymetry [START_REF] Smith | Global sea floor topography from satellite altimetry and ship depth soundings[END_REF] (Figure 1). The resulting depth anomaly is then corrected for sediment loading (see [START_REF] Adam | MiFil: A method to characterize seafloor swells with application to the south central Pacific[END_REF] for computational details), to obtain the residual bathymetry. In order to test different cooling models, we have also performed the computation with the Parsons & Sclater [START_REF] Parsons | An analysis of the variation of ocean floor bathymetry and heat flow with age[END_REF] thermal subsidence model. The results (e.g., flux vs. time estimates) are shifted relative to each other but preserve the variations and their amplitudes [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF]. Interpretation of the buoyancy and volcanic flux variations in time are therefore independent of the thermal model used, even if their absolute value remains unconstrained.

In order to separate the swell (H S ) and volcanic edifices (H V ) components from the resulting depth anomaly grid, we adapted the MiFil method developped by Adam et al. [START_REF] Adam | MiFil: A method to characterize seafloor swells with application to the south central Pacific[END_REF]. This low-pass filtering method effectively removes the topography associated with volcanic edifices from the depth anomaly, and gives the topography corresponding to the swell component H S , as shown for the St. Helena and the Walvis chains in Figures 2a and2b, respectively. The volcanic component H V is thus the difference between the residual bathymetry and the swell component H S .

The H S and H V data can then be used to compute both the swell volume and the volume of erupted material (volcanic edifices). The swell volume corresponds to the volume between the reference depth anomaly (zero) and the swell H S . To estimate the volume of erupted material, we consider the vol-canic topography, and calculate the associated compensating root. We assume that the volcanic loading is supported by an elastic lithosphere [START_REF] Watts | Gravity anomalies and flexure of the lithosphere: A three-dimensional study of the Great Meteor Seamount, northeast Atlantic[END_REF] with an elastic thickness of 7 km, as deduced from the age of the crust at the time of loading. Note that the choice of this compensation (regional), rather than a local (Airy) compensation, does not have a significant influence on the final results, as the computed volcanic volumes are unchanged [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF].

We then compute the variation of the buoyancy (Q S ), inferred from swell volume variations [START_REF] Sleep | Hotspots and mantle plumes: some phenomenology[END_REF], and volcanic (Q V ) fluxes through time for each of the two volcanic chains in a fashion similar to that of Vidal & Bonneville [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF].

We translate a rectangular box of fixed size (1000 km long across-track and 20 km wide along-track) along the main axis of the chains to compute the variation of the swell and volcanic material along this trend. We chose to approximate the main axis by the straight traces shown in Figure 2, since the hotspot tracks calculated from rotation poles [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] Duncan | Hotspots, mantle plumes, flood basalts and true polar wander[END_REF][START_REF] Müller | Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks[END_REF] do not match either the swell or the volcanic chain trends. The length of the computing box, in the direction transverse to the trend, is taken large enough (1000 km) to include all the topographic features created by the hotspot and/or all its volcanic root. Therefore, volume calculations are scarcely affected by using other paths (e.g. theoretical hotspot track) for our computation instead of the straight line presented here [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF].

We also introduce the buoyancy flux (B) temporal evolution along the main axis trends. The buoyancy flux is commonly used to quantify the rate of swell formation [START_REF] Sleep | Hotspots and mantle plumes: some phenomenology[END_REF]:

B = (ρ m -ρ w )AV p (1)
where ρ m and ρ w are mantle and seawater densities respectively, A is the area of the swell in a vertical plane transverse to the hotspot track, and V p the plate velocity in the hotspot frame. B can therefore be written as B = (ρ m -ρ w )Q s .

Results

We identify swells associated with both the St. Helena and Walvis chains, and an additional one associated with the Circe seamount (Figure 2), north of St. Helena's trail.

St. Helena swell -The St. Helena swell is composed of two circular highs, one centered at longitude 6.5 • W, the other at longitude 9 • W (Figure 2a). This observation suggests that two plumes may be at the origin of the St. Helena chain. It is also interesting to note that the dated volcanoes are located slightly south of these maxima, indicating a probable influence of weakness zones of the lithosphere. The strong increase in swell amplitude, northeast of the chain (>50 Ma), will not be discussed further here, since its origin is not related to a plume, but more probably to continental processes (see section 2.1).

Circe swell -The swell has a maximum ∼400 m amplitude and a 300-400 km width (Figure 2a). The swell maximum is located 150 km SW of the 6.6 Ma Circe seamount, which is part of an independent hotspot trail, as suggested by O'Connor et al. [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF].

Walvis swell -The Walvis chain shows several maxima along its track. The high associated with the presumed present-day hotspot location has an amplitude of ∼1400 m and a diameter >500 km, with a maximum located NE of Gough Island and ESE of Tristan da Cunha (Figure 2b). The dated volcanoes are here again located on the edge of the swell and not on its maximum. The middle section of the track, between 28 • S and 37 • S, shows two adjacent swells, with a somewhat smaller amplitude of ∼1200 m, and a width of ∼400 km. The easternmost swell is elongated along a SW-NE direction, whereas the western one has a trend close to the NS direction. Note that both trends are parallel to the volcanic chain.

Important swell amplitudes are found in the NE part of both St. Helena and Walvis chains (Figure 2). Considering the ages of the volcanism associated with this part of the chains, these older swells (>50 Ma for St. Helena and >65 Ma for Walvis) have a too important amplitude to be considered as the result of the plume expression in an intraplate setting (see section 2.1). Indeed, after the plume reheating, the lithosphere cools and subsides, and the subsequent mean swell amplitude should decrease. Figure 2 shows that the swell in the older part of both chains does not follow this tendency, thus indicating that other physical processes are at stake. Therefore, the parameters we used to characterize the Walvis and St. Helena hotspot swells are not adequate to characterize the NE depth anomaly, which implies different phenomena, maybe of continental origin, occurring at different spatial scales [START_REF] Van Der Linden | Walvis Ridge, a piece of Africa?[END_REF]. Hence, these parts of the swells (>50 Ma for St. Helena and >65 Ma for Walvis) will not be further studied.

The hotspot tracks, deduced from rotation poles (displayed in Figure 2b), match only the overall alignment and orientation of the swells along the central and northeasternmost portions of the chain, but are not consistent with the location of the swell at the southwesternmost end of the chain (Figure 2b).

Indeed, all the models choose Tristan da Cunha as the present-day location of the plume, whereas the swell morphology clearly indicates that Gough is the most probable origin. Note that the shift between the plume head em-placement and the swell maximum is a classical feature of a hotspot chain.

Indeed, the plume requires some time (typically, a few million years) to influence the thermal structure of the lithosphere, and hence generates a swell whose maximum is located a few hundred kilometers from the present-day plume head location, in the direction of plate motion (NE, here). On the other hand, hotspot volcanism is associated with the ascent of melt over the plume head, through the lithosphere and to the seafloor, and hence occurring 'upstream' from the swell maximum. Gough is therefore the best candidate for the present-day Walvis plume location. Since the tracks deduced from rotation poles do not accurately describe the plume trajectory, we choose to describe it by the linear trends displayed in Figures 2a and2b. We assume a linear age progression of volcanism due to the drift of the African plate along them.

Figure 3 shows the swell amplitude along the trend, as a function of distance from the zone of active volcanism. The reported ages of volcanism [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF] show an overall age progression, but the large scatter in the data suggests that the chain formation is complex, with active volcanism extending over a large area. Distance along the proposed axes of both chains can be used as a proxy for age if we assume a constant velocity of the African plate of 29 mm yr -1 [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF]. O'Connor et al. [START_REF] O'connor | First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma[END_REF] also report a substantial deceleration of the African plate speed at 20-30 Ma, but this event is poorly constrained, and is not systematically observed in other plate motion models [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] Duncan | Hotspots, mantle plumes, flood basalts and true polar wander[END_REF][START_REF] Müller | Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks[END_REF]. We thus chose to extrapolate the volcanic ages along the trend (bottom x-axis in Figures 3 and4), with a constant migration rate of 29 mm yr -1 . We also represent in Figure 4 the buoyancy flux B, commonly used to quantify the rate of swell formation (see section 3).

The swell along the volcanic chains (Figure 3) does not display the typical decay in amplitude with distance (age) from the present-day position of the hotspot. We observe instead a maximum in the swell amplitude at ∼800 km (∼30 Ma) and ∼500 km (∼15-20 Ma) along St. Helena and Walvis track, respectively. Note that the width of the swell profile along the St. Helena track is due to the presence of two swell maxima, slightly shifted off-axis (see above section, and Figure 2a). Another large bump is observed along the Walvis chain, between 35 and 60 Ma. In the next section, we analyse the temporal variations of the deduced buoyancy fluxes.

Discussion

Temporal evolution of plume fluxes

Swell flux Q s -

The calculated buoyancy fluxes as a function of volcanic age are shown in Ma for Walvis) are not interpreted (see swell amplitude discussion, section 4).

The Hawai'i chain, which has a shorter life span (< 45 Ma) than that of the St. Helena (∼ 60 Ma) and Walvis chains (∼ 80 Ma), is probably the best characterized hotspot trail [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF], and can be used as a reference in our study.

Note that we ignore here the Emperor part, where no swell is reported, and whose volcanic flux is much smaller [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF]. Hawai'i shows two important peaks at 3 and 15 Ma. It is important to note that the 3 Ma peak may not be a "real" one since plumes require some time (typically a few million years) to influence the thermal structure of the lithosphere. The swell may then not be totally formed yet at the youngest extremity. Therefore, the age difference between the two peaks in the Hawaiian swell (Figure 4, bottom) could still increase in the next few million years.

Volcanic flux -

The St. Helena chain displays a peak at 10 Ma, preceded by another peak at 30 Ma, in the magma production rate Q v . Based on both the lack of correlation between variations in Q v and Q s (Figure 4), and on the presence of two marked swells (Figure 2a), we propose that the St. Helena trail is formed by two hotspots, instead of a single one. Recent evidence of the existence of two plumes beneath the younger part of St. Helena chain, given by tomography [START_REF] Montelli | A catalogue of deep mantle plumes: New results from finite-frequency tomography[END_REF], favor this hypothesis.

The Walvis chain also shows a peak at 10 Ma in the magma production rate Q v , correlated with the main peak in the buoyancy flux profile (Figure 4). This main peak is preceded by two secondary peaks in Q v , at 39 and 59 Ma, also correlated with two smaller peaks in Q s at 38 and 54 Ma (Figure 4 and above discussion on Q s ). This correlation between Q v and Q s strongly suggests a common origin for both fluxes, with a single plume that peaked in activity at 10 Ma with pulsations 10-20 Ma apart. These pulsations are superimposed on an apparent long-term waning of the plume. We see indeed a progressive overall decrease in Q v since a maximum at about 60 Ma. This decrease in the volcanic activity is also apparent in the progressive diminution of the size and density of seamounts at the seafloor (Figure 1). In comparison, the production of magma increases drastically during the last 30 Ma for Hawai'i (Figure 4). A

peak is observed at 15 Ma along this chain with a pronounced increase to the present time, suggesting that the plume is waxing and may have not reached yet its maximal activity.

Both St. Helena and Walvis show short-term variations in magma production rate Q v , with a periodicity of about 5 m.y., which has also been reported for Hawai'i [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF].

Evolution of fluxes -

The most significant result of this study is that the Walvis plume does not appear to be steady state, but rather shows pulsations at 10-20 m.y.. This periodicity cannot be determined for St. Helena, as it is likely the result of the activity of two plumes and their temporal variations. A possible explanation for these pulsations is the tilt of the plume conduit due to large-scale mantle advection [START_REF] Steinberger | Plumes in a convecting mantle: Models and observations for individual hotspots[END_REF][START_REF] Whitehead | Instabilities of fluid conduits in a flowing Earth -Are plates lubricated by the asthenosphere ?[END_REF], as they may develop oscillations if their dip is larger than 60 • [START_REF] Whitehead | Instabilities of fluid conduits in a flowing Earth -Are plates lubricated by the asthenosphere ?[END_REF]. This mechanism has been also invoked to explain the scatter in volcanic ages along other hotspot chains such as Louisville [START_REF] Koppers | Implications of a nonlinear 40 Ar/ 39 Ar age progression along the Louisville seamount trail for models of fixed and moving hot spots[END_REF]. In this study, we suggest that the observed periodicity of 10-20 m.y. may be the first estimate of the frequency of these oscillations, deduced from observed data.

Walvis also shows a marked decrease in volcanic flux Q v through time which may reflect a progressive and steady waning of the plume and its associated activity during the last 60 Ma or more; its extinction could occur in 30 Ma if this decline continues at a similar rate. The Walvis plume conduit is not imaged by tomography models [START_REF] Montelli | Finitefrequency tomography reveals a variety of plumes in the mantle[END_REF][START_REF] O'neill | On the uncertainties in hot spot reconstructions and the significance of moving hot spot reference frames[END_REF], which may be also an indicator of its weakening and possible extinction. Alternatively, there is a progressive increase of the age of the lithosphere over which the volcanism is emplaced due to the migration of the ridge away from the hotspot. The resulting thickening of the lithosphere with time could play a role in this reduction of Q v , but this interpretation is not supported by the lack of such a pattern along the St. Helena chain.

The shorter-term variations with a periodicity of ∼5 m.y. observed in the magma production rate Q v for St. Helena and Walvis (Figure 4) have a too small periodicity to be associated with a plume tilting phenomenon, which can sustain tilts up to 60 • , depending on the surrounding mantle viscosity [START_REF] Steinberger | Plumes in a convecting mantle: Models and observations for individual hotspots[END_REF]. No evidence of the decorrelation of both the 10-20 and 5 m.y. pulsations can be affirmed. One possible explanation, however, could be the presence of solitary waves within the plume conduit [START_REF] Whitehead | Magma waves and diapiric dynamics[END_REF]. These perturbations have been invoked

to explain both isotopic heterogeneities [START_REF] West | Isotopic constraints on the origin of Hawaiian lavas from the Maui Volcanic Complex, Hawaii[END_REF] and a similar 5-7 m.y. periodicity in volcanism [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF] along the Hawai'i chain (Figure 4).

Total volumes and fluxes

We have computed the cumulative volcanic and swell volumes for the St. Helena and Walvis chains, and reported the estimation of swell volume for Circe in Table 1; owing to the lack of data, we cannot provide an estimate of its volcanic flux. In Table 1 are also reported the values found for Hawai'i (from [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF]). We have also calculated the mean fluxes along each chain, which we prefer over the present-day values normally reported in the literature, as they integrate the activity and strength of hotspots over their lifespan.

Hawai'i has the greatest mean volcanic and swell fluxes, thus indicating that it is the most vigorous of all the identified plumes. Walvis and St. Helena have similar volcanic fluxes but Walvis' swell flux is three times greater than that of the St. Helena system. As St. Helena results from the combination of two plumes, we can deduce that these are two small ones. The low swell flux associated with Circe suggests that it is a weak and/or small plume.

Conclusion

In this paper we have presented the first characterization of the swell and volcanic fluxes and their temporal evolution along the Walvis and St. Helena hotspot trails. Both hotspot trails are associated with very broad swells, with the volcanic activity slightly offset relative to the swell maxima. For Walvis, the swell maximum shows a marked offset with respect to the proposed present-day hotspot location inferred from rotation pole reconstruction (Tristan da Cunha), suggesting that the trajectory of this plume is not accurately described by these models. We propose that Gough is the most probable present-day hotspot location. We also find a modest swell associated with the Circe seamount, north of the St. Helena trail, supporting the proposed presence of a hotspot whose location is still undetermined, and an associated volcanic chain.

The Walvis hotspot appears to be the result of a single, waning plume, that shows variations in the volcanic and swell flux at a 10-20 m.y. time-scale, and additional, shorter-term variations of about 5 m.y. in the volcanic flux. In contrast, the St. Helena chain appears to be the result of two hotspots, which precludes the interpretation of calculated volcanic and swell fluxes in terms of temporal variability of a single plume. This chain shows also a volcanic periodicity of 5 m.y., similar to that observed along the Hawai'i chain. The 10-20 m.y. periodicity may be due to the tilting of the mantle plume conduit under mantle advection, whereas the 5 m.y. variabilities could be explained by instabilities in the conduit itself.

The estimation of the mean swell and volcanic fluxes makes it possible to compare the relative strength of the South Atlantic plumes. The strongest is Walvis, followed by the two St. Helena plumes, and probably Circe. The Hawai'i plume (Pacific), which is the reference hotspot in our study, remains the most vigorous among all these plumes. (see Figure 1), as a function of distance from the present-day hotspot location (upper x-axis). Available ages of volcanism (gray arrows and numbers in Ma [START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF]) and calculated ages assuming a constant velocity (29 mm yr -1 ) for the African plate (lower x-axis) are also shown along the horizontal axis. Table 1. Total volumes and volume fluxes of volcanic material and swell for Circe, St. Helena and Walvis (this study), and Hawai'i (from [START_REF] Vidal | Variations of the Hawaiian hot spot activity revealed by variations in the magma production rate[END_REF]). Note that the total fluxes here are calculated as a mean flux along the chain.

FIGURE & TABLE CAPTIONS

                                               

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Figure 4 .

 4 Figure 4. For St. Helena, we observe a single wide bump centered at ∼30 Ma, whose width can be explained by the presence of two adjacent swells (see previous section). We observe several maxima in the swell flux along the Walvis chain: a marked maximum at ∼10 Ma and two additional ones of smaller amplitude at 38 and 54 Ma. These secondary maxima are subdued in Figure 4, due to the scaling, but they are well-visible on the Walvis swell map (Figure 2b). The older parts of the buoyancy flux (>50 Ma for St. Helena and >65

Fig. 1 .

 1 Fig.1. Bathymetry of St. Helena and Walvis chains (2' bathymetry grid from[START_REF] Smith | Global sea floor topography from satellite altimetry and ship depth soundings[END_REF]), and published volcanic ages (in million years, from[START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] O'connor | South Atlantic hot spot plume systems: 1. Distribution of volcanism in time and space[END_REF]). Black lines indicate the traces used to study the temporal evolution of hotspot fluxes (see text for discussion). The white and black stars indicate the present-day location of the Walvis plume inferred by Courtillot et al.[START_REF] Courtillot | Three distinct types of hotspots in the Earth's mantle[END_REF] and Steinberger[START_REF] Steinberger | Plumes in a convecting mantle: Models and observations for individual hotspots[END_REF], respectively. The white dot on Walvis northern edge indicates the location of presumed present-day underwater volcanism as inferred from hydrophone events[START_REF] Haxel | Evidence of explosive seafloor volcanic activity from the Walvis Ridge, South Atlantic Ocean[END_REF].

Fig. 2 .

 2 Fig. 2. Calculated swell associated with the St. Helena and Walvis chains. a) Numbers correspond to age of volcanism in million years [9,10]. b) For readability, we only report the most recent volcanic ages (< 1 Ma) along the Walvis chain. Hotspot tracks deduced from rotation poles[START_REF] O'connor | Evolution of the Walvis Ridge-Rio Grande Rise Hot Spot System: Implications for African and South America Plate Motions Over Plumes[END_REF][START_REF] Duncan | Hotspots, mantle plumes, flood basalts and true polar wander[END_REF][START_REF] Müller | Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks[END_REF] are also reported for comparison (see text). In both maps, the 2500 m isobath (thin black line) marks the position of the ridge and volcanic islands, and the bold black line indicates the traces used to study the temporal evolution of buoyancy and volcanic fluxes along both chains. The filter parameters to obtain the swell map are r=45 km and R=350 km for the minimization and filtering (see[START_REF] Adam | MiFil: A method to characterize seafloor swells with application to the south central Pacific[END_REF] for details).

Fig. 3 .

 3 Fig. 3. Swell amplitude along the St. Helena (top) and Walvis (bottom) trends

Fig. 4 .

 4 Fig. 4. Calculated buoyancy (gray) and volcanic (black) fluxes for the St. Helena (top) and Walvis (middle) chains, as a function of the interpolated age of active volcanism (see text). Gray arrows point the secondary maxima at 38 and 54 Ma in the Walvis buoyancy flux (see discussion). For comparison, the buoyancy and volcanic flux for Hawai'i are given in the bottom graph (from [5]). Note the change in vertical scale, for readability.

Figure 1

 1 Figure 1 Click here to download Figure: Adam_Fig1.pdf

Figure 2

 2 Figure 2 Click here to download Figure: Adam_Fig2.pdf

Acknowledgments This work is partially supported by a Grant-in-Aid for Scientific Research (16253002) from the Japan Society for the Promotion of Science (C. Adam), and benefited from a CNRS/ATI grant to J. Escartín. Discussions with M. Cannat, J. Lin, G. Ito and Y. Fukao during the early stages of the project contributed to its development and completion. We are grateful to J. O'Connor for kindly answering questions about the African plate motion and age reliability, and F. Vivanco for his help in data transfer. Comments by the editor G. D. Price and reviewer W. Sager greatly helped us to clarify and improve this manuscript.


Franck Tessier

Erwan Ray

François Cheviré

Laurent Lemaître

Fabien Bonnier

Delphine Bazer-Bachi

Vincent Lecocq

Transesterification of vegetable oils by AlPOxNy heterogeneous catalysts

Keywords: Transesterification, nitridation, aluminophosphates, heterogeneous catalysis, vegetable oils

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

Introduction

Metallic phosphates of the M III PO 4 type with M = B, Al, Fe, Ga, Mn form an interesting class of materials [START_REF] Moffat | Topics in Phosphorus Chemistry[END_REF][START_REF] Moffat | [END_REF]. Both cations -phosphorus and the metal-are in tetrahedral sites and tetrahedrons are linked to build 3D-networks analogous to those existing in various cristallized polymorphs of silica. Bondings in these MPO 4 phosphatesare mainly covalent, with a stronger covalent behavior of phosphate ionscompared to that of silicate ones [START_REF] Moffat | [END_REF].This family of phosphates has excellent properties, for example in catalysis, as bifunctionnal acid and basic catalysts. They present a significant reactivity for several reactions such as alcohol dehydration, alcenes isomerisation, cracking of aromatic molecules as well as the Knoevenagel condensation [START_REF] Kearby | Proc. 2 nd . Int. Catal. Cong[END_REF][START_REF] Mattex | [END_REF][START_REF] Kearby | [END_REF][6][7][8][9].

The introduction of nitrogen N 3-within an oxide catalyst modifies the acid-base properties of the surface and allows to perform catalytic reactions leading to the synthesis of pure chemical substances (aldehydes or sulfones) [10][11][12][13][14][15][16][17][18][19][20]. Steins claimedthat the nitrogen/oxygen substitution within a high specific surface area oxide should involve an increase of the Lewis basicity in relation to the lower electronegativity of nitrogen [21]. By tuning the nitrogen content of these materials, the acid-base character can be adjusted to give rise to a large panel of catalytic applications.

"AlPON"-type materials, developed in the early 90's are representative of the most studied nitridophosphates for catalysis [22]. AlPO 4 precursors are often prepared using the hydrogel route to form X-rays amorphous and high specific surface areaspowders (350-400 m 2 .g -1 ).SEM analysis shows an homogeneous texture with aggregates made of particles with size around tens of nanometers. The oxide precursor is then modified by the progressive nitrogen insertion.

"AlPON" results from the reaction of AlPO 4 under flowing ammonia between 650°Cand800°Cand remains X-ray amorphous. A large domain of AlPO x N y compositions (1.5 < x < 4; 0 < y < 1.65) exists according to the chosen temperature and nitridation step.Using ammonia temperature programmed desorption (TPD), Conanec has showna decrease in the acidity of "AlPON" samples with increasing nitrogen content [19,22]. Moreover, under specific experimental conditions, CO 2 TPD enabled to evidence a stronger basic behavior for "AlPON", as observed in the Knoevenagel reaction test [19]. This reaction between a methylenic compound and an aldehyde or a ketone is largely studied as a standard basicity test [23,24]. AlPONs have shown during the condensation reaction of malonitrile or ethylcyanoacetate and benzaldehyde a higher activity compared to that of MgO, a well-known basic catalyst reference. AlPONs present both basic sites originating from amino groups at the surface (-NH-, -NH 2 ) and also acid sites linked to the metal. Consideringthe interest of this aluminophosphate family, we have studied the behavior of such heterogeneous catalysts -both AlPO 4 and "AlPON" -in the transesterification reaction of vegetable oils. The final products are fatty acid esters (FAME) entering the composition of biodiesel and glycerine, a by-product(generally 10 to 15 wt.%) that can be further recycled and valued in industryif highly pure. In this work are presented the catalytic performances along with the characterizations of the powders including NMR and infrared spectroscopy.

Experimental section

Precursors syntheses.

Hydrogel route. This synthesis method was described by Kearby [START_REF] Kearby | Proc. 2 nd Int. Congr. Catal[END_REF]and used in our laboratory by Conanec and Peltier to prepare AlPO 4 and GaPO 4 [22,[START_REF] Peltier | [END_REF]. Considering the case of AlPO 4 (Figure 1): phosphoric acid H 3 PO 4 (1 M) is added in a stoichiometric quantityto an aluminum chloride AlCl 3 , 6H 2 O solution. The mixture is then cooled in an ice bath and propylene oxide is poured dropwise in a large excess (3 to 30 times)under vigorous stirring.The very acidic pH of the solution (pH1) increases progressively.Starting from pH  2.5 a thick and translucent gel forms after few hours restat ambient conditions. The gel is then washed with isopropanol in order to dissolve chlorinated alcohols resulting from the reaction. After several washing steps, the remaining product is dried in an oven during 15 hours at 120°C. After grinding and calcination between 550 and 650°C the ensuing powder is X-rays amorphous.

Cooling at 0°C

Progressive addition of propylene oxide

Formation of the gel

Isopropanol washing

Calcination at650°C

Amorphous citrate route. The citrate route is also a method of interest to prepare high specific surface area phosphate precursors [27]. Aqueous solutions of aluminum chloride and ammonium dihydrogen phosphate corresponding to anAl/P ratio = 1 were mixed under stirring. Then, citric acid (C 6 H 8 O 7, Merck, >99%) dissolved in a minimum amount of water was added to each solution, the addition being followed by a 30 min stirring step at 120°C. Since the complexation of cations by citric acid is improved at pH ≥ 7, the acidic solutions were neutralized using an ammonia solution (25%, Merck) [28]. The solutions were then mixed together and stirred at 150°C for 20 min to promote chelate formation. The liquid was progressively heated up to 250°C to eliminate solvents and start the organics combustion, leading after 5 h to an expanded black solid residue. The solid was finally ground and calcined at 550°C in air in an alumina crucible for elimination of residual carbon. A heating step at 1000°C during few hours enables to confirm the presence of crystalline AlPO 4 with the ratio Al/P=1. Thermal ammonolysis. Nitridation reactions were carried out in alumina boats placed inside an electric tubular furnace through which ammonia gas flowed with a flow rate of 20 L.h -1 . The temperature was raised to 800 °C range at a heating rate of 10 °C.min -1 . After 15 to 50 h reaction time, the furnace was switched off and the nitrided powders were allowed to cool to room temperature under nitrogen atmosphere [29].

X-ray diffraction.XRD powder patterns were recorded using a Philips PW3710 diffractometer operating with Cu K  radiation ( = 1.5418 Å). X'PERT softwares -Data Collector and Graphics, and Identify -were used, respectively, for pattern recording, analysis and phase matching.

Elemental analysis. Nitrogen and oxygen contents were determined with a LECO ® TC-600 analyzer using the inert gas fusion method. Nitrogen was measured as N 2 by thermal conductivity and oxygen as CO 2 by infrared detection. The apparatus was calibrated using Leco ® standard oxides, Si 2 N 2 O and -TaN as a nitrogen standard [30]. Nitrogen contents from 6 to 15 wt.% were measured depending on the nitridation temperature. Corresponding chemical formulas were calculated as AlPO x N y .

Energy-dispersive X-ray analysis (EDX) has been used for identifying the elemental Al:P contents of the products. The EDX analysis system works as an integrated feature of a scanning electron microscope (JEOL JSM 6400). Specific surface area.A flowsorb II 2300 Micromeritics apparatus was used to determine the specific surface area of the powders by the single point method. Before measurement, the samples were outgassed under He/N 2 flow between 100 and 200 °C for 30 min. Analyses performed with the ASAP 2010 (Micromeritics) give access to the porosimetry, adsorption isotherm and also to the specific surface area by the multipoint method. 27 Al NMR was set to 12 KHz with a /16 pulse and a 0.5 sec delay between two pulses. An aluminum nitrate solution was used as a reference for the chemical shifts. The separation between the contributions of the sites occupied by aluminum is often complex due to the quadrupolar feature of this atom.

TGA analysis.The

DRIFT analysis.DRIFT spectra were collected with a FT spectrometer (Nicolet TM 380) using a deuterated triglycine sulphate (DTGS) detector working at 4 cm -1 resolution. The sample is placed inside a controlled environment chamber (Spectra-Tech 0030-103). Undiluted samples are heated from room temperature to 500°C at 10°C.min -1 under nitrogen flow. Data are presented in absorbance mode rather than on Kubelka-Munk units because of the undiluted feature of the samples.

Transesterification reaction. The transesterification reaction between triglycerides and alcohols can be catalyzed by both acids and bases to prepare methyl esters of vegetable oils and a byproduct glycerol [31,32].Experiments were performed in a 100 mL stainless steel batch reactor.

The reaction medium was stirred with a magnetic stirring bar and heated with a heating magnetic stirrer. Reactants (commercial alcohols and edible-grade rapeseed oil) and catalyst (0.5 g) were first introduced in the reactor. The molar alcohol/oil ratio was set at 27.5, as the nature of the alcohol could change for the study. The total mass of liquid was set at 50 g. The reaction medium was then stirred at 200 rpm and heated at 453 K. The time t = 0 was assigned when the temperature reached 453 K, which explains nonzero conversion at t = 0.

The initial reaction medium and the reaction effluents form a biphasic mixture at ambient temperature (however, it is monophasic at 453 K), the polar phase containing mainly glycerol and unreacted alcohol, and the apolar phase containing mainly FAME (or other esters when alcohols other than methanol are used) and glycerides. Sample of 2 mL were collected manually from the reaction medium, at different reaction times, for analysis. The samples were washed with brine in order to remove glycerine and the alcohol. Eight drops of the apolar phase (containing glycerides and FAME) were then diluted in 3 mL of analytical-grade THF, and the sample was analyzed by gel permeation chromatography on a Waters HPLC apparatus equipped with three Waters Styragel columns (THF) with a molar mass range of 0-1000 g•mol -1 . These columns were placed in a thermostated oven at 313 K. Detection was made using a Waters 2414 refractive index detector. The results gave the relative composition of the apolar phase in triglycerides, diglycerides, monoglycerides, and FAME. At the end of the reaction, the reaction medium was collected and filtered on a 0.1 μm Teflon filter, and extra alcohol was evaporated.

The apolar phase (containing ester and nonconverted glycerides) and the polar phase (containing glycerine and ethers) were separated and analyzed by ICP-OES, to analyze eventual catalyst leaching products. The apparatus used was a Thermo Jarrell Ash Iris Advantage inductively coupled plasma optical emission spectrometer. The apolar-phase samples were diluted in xylene and directly injected, whereas the polar-phase samples were diluted in ethanol and injected via a cold (-10 °C) nebulization chamber.

Zinc aluminate (ZnAl 2 O 4 ) was chosen as a reference catalyst, as the use of this mixed oxide has been described in a heterogeneous industrial transesterification process [33].

Results and discussion

Preparation of nitridoaluminophosphates

AlPO 4 was prepared either via the hydrogel or the amorphous citrate routes. The first methodusually contributed to the synthesis of AlPO 4 , GaPO 4 or AlGaPO 4 [22,[START_REF] Peltier | High Temp. Mater. Processes[END_REF][START_REF] Peltier | [END_REF]. Both synthetic approaches are efficient to prepare reactive, X-rays amorphous and high specific surface area powders. After thermal reaction under ammonia, the corresponding surface area of the oxynitride is smaller, but in a minor extent,compared to that of the precursor. Conanec showed that the evolution of the specific surface does not depend on the nitrogen enrichment, nor the reaction step under NH 3 but is rather driven by the heating kinetics of the nitridation furnace [START_REF] Conanec | [END_REF]. Indeed, when considering a fastheating rate, the presence of adsorbed water at the surface of the phosphate precursor will not be eliminated at low temperature and at high temperature it will follow a sintering of the powder along with a decrease in the specific surface area. Our samples were all subjected to a heating rate of 10°C/min. The nitrogen content is essentially dependent on the nitridation step varying from 10 to 50 h. The temperature of the reaction was raised to 800 °C. Above this maximum, a release of phosphorus is observed and lead to defective compositions with ratios Al/P > 1. EDX analyses confirmeda lower phosphorus content for sample nitrided over 800°C. Indeed, during the calcination under air at 1100 °C of oxynitride samples prepared between 800 and 1000 °C from AlPO 4 , mixtures of AlPO 4 + Al 2 O 3 were identified by X-rays analysis, proving the release of phosphorus throughout the nitridation treatment. The main features of these white samples prepared and tested for the transesterification reaction are gathered in Table 1. nitridophosphatecompositions with increasing nitrogen contents. The sample prepared with the highest nitrogen content results from a "citrate route" precursor.The purpose of this workwas to study the impact of nitrogen contents and specific surface areas on the structure and the surface of the catalysts, as well as on the transesterification catalytic activity.

NMR characterizations

Due to the amorphous feature of the samples, NMR 31 P and 27 Al analyses were undertaken to characterize any modification of the aluminum and phosphorus environments with nitridationin samples listed in Table 1.

Figure 2 : NMR 27 Al spectra of AlPO 4 and corresponding oxynitrides.

NMR 27 Al spectrum of AlPO 4 displays a major peak at 40 ppm and a second one less intensive at -10 ppm, as well as a shoulder at 10 ppm (Figure 2). According to different authors, the main peak is attributed to aluminum in tetrahedral environment [36][37][38][39][40]. Taking into account the amorphous and hygroscopic behaviors of the powders, peaks at -10 and 10 ppm are linked to aluminum in octahedral site and 5-fold coordinated sites respectively. Indeed, structural disorder [36,40] and/orthe presence of OH groups [37,38] may involve the formation of supplementary peaks. Figure 3 more than 15wt. % nitrogen [22] and assigned to a nitrided aluminum tetrahedral environment.An attempt to deconvoluate the signal of this sample gives the following quantitative distribution: 10-15% (52 ppm), 25-30% (30 ppm) and 60% (-10 ppm). But it is not obvious to do a simple determination with such a quadrupolar 27 Al nuclei and the presence of asymmetric peaks. This result needstherefore to be taken with precaution. glasseswhere the components at -20, -10 and 0 ppm were assigned respectively to PO 4 , PO 3 N and PO 2 N 2 tetrahedra [41].

Aluminum and phosphorus atoms are principally in tetrahedral sites in AlPO 4 and "AlPON". The progressive nitrogen insertion in AlPO 4 is followed by a displacementof the resonance signals of 27 Al and 31 P towards higher chemical shifts.

Infrared characterizations

The adsorption of probe molecules followed by infrared spectroscopyis largely used to determine the nature, the number and the force of acid and basic sites at the surface of a catalyst [42]. The evidence of the acidity is quite easy using a basic probe molecule while the determination of the basicity from acid probes is more difficult. Indeed all acid molecules may contain nucleophilic (basic) atoms interacting also with acid sites at the surface. The IR spectrum of AlPO 4 in the -OH region (3000-4000 cm -1 ) has two characteristic bands at 3796 cm -1 and 3679 cm -1 (Figure 4). The first one is attributed to the vibrational mode of -OH groups linked to P and the second one is due to vibration modes of -OH linked to tetrahedral Al [22,[43][44]. Acetonitrile (CH 3 CN) is an amphoteric molecule suitable for our study to detect both acid or basic sites in the 2350-2250 cm -1 and 2250-2050 cm -1 infrared windows respectively [45]. The adsorption of acetonitrile revealed the presence of three kinds of acidic sites (2333 cm -1 , 2304 cm -1 and 2274 cm -1 ) (Figure 5). However, it is not possible to distinguish with this probe Brønsted from Lewis acid sites.Carbon monoxyde (CO) is a basic probe and allow to differentiate between such acidic sites.The adsorption of CO at -196°C evidencesdifferent sites of AlPO 4 . Figure 6shows the bands which result from the interaction of CO with Lewis acidic sites (2215 cm -1 )and at least two types of Brønsted sites at 2169 cm -1 and 2140 cm -1 , which were not distinguished by 31 P NMR. The introduction of nitrogen has a clear impact on the IR spectrum with the appearance of a large band centered around 3370 cm -1 with several shoulders between 3400 and 3600 cm -1 . Figure 7 gives an illustration for the sample containing 8.7 N wt.%. This large band is attributed to the vibrational stretching modes of NHx species [46]. According to the observations of Climent et al., one of the shoulders located at 3480 cm -1 corresponds to NH 2 groups [47]. As for the precursor case, two fine bands at 3796 and 3676 cm -1 indicate the presence of hydroxyl groups linked respectively to aluminum and phosphorus atoms in tetrahedral sites. With increasing nitrogen content, we note a decrease in the relative intensity of  PO-H band (around 3680 cm -1 ) compared with  AlO-H band. Ammonia interacts preferentially with P-OH sites at the surface of the aluminophosphates to form P-NHx species. With acetonitrile adsorption we observe for the nitrided sample (6.7 N wt.%) the same bands (2335, 2305 and 2272 cm -1 ) related to acid sites as displayed by the oxide precursor (Fig. 5). By comparison with AlPO 4 , the intensity of the bands at 2272 and 2305 cm -1 are less intensive, proving the influence of the nitridation on some of these acid sites.With increasing nitrogen content, a supplementary band (acid site) is detected at 2364 cm -1 (2257 cm -1 is a physisorption band) (Fig. 8). The adsorption spectrum of acetonitrile on the sample with 15.3 N wt.% is quite different from the others, with the presence of only two acidic sites at 2329 cm -1 and 2301 cm -1 .

Figure 8: Adsorption of acetonitrile on AlPON -8.7 N wt.% sample.

It has to be noticed that no basic sites were evidenced neither by acetonitrile adsorption or CO 2 adsorption (not shown). This probe molecules could not be efficient to reveal the basic sites of AlPON, revealed otherwise by the presence of -NHx groups visible in the IR spectrum.

The adsorption of CO on nitrided samples confirms the presence of two types of acidic sites. A Lewis site at 2203 cm -1 and three Brønsted sites at 2167, 2152 and 2140 cm -1 . The nitridation also modifies the profile of the Brønsted acidity region around 2150 cm -1 in intensity (maybe linked to the modification of extinction coefficient or to a lower number of acid sites, as it can be thought regarding the decrease of the P-OH band intensity) and with the appearance of a band at 2152 cm -1 (Fig. 9).

Figure 9: Adsorption of CO on AlPON-6.7 N wt.% sample.

With higher nitrogen content (8.7 N wt.%) the spectra display two Lewis sites 2218 and 2196 cm -1 and three Brønsted sites (2168, 2154, 2140 cm -1 ). At higher nitrogen content (15,3 N wt.%) the CO adsorption spectrumindicates a very weak proportion of Lewis acidic sites (2193 cm -1 ) compared with Brønsted sites at 2169, 2149 and 2138 cm -1 (Fig. 10). If the presence of acid centers in the aluminophosphates was evidenced for both oxide and oxynitride catalysts, basic sites are more difficult to characterize. Infrared spectroscopy with acetonitrile adsorption never reveals, under our experimental conditions, basic sites and moreover the basicity was not even shown through our CO 2 TPD experiments. Despite of this impossibility to clearly detect basic sites, we have tested these (nitrido)aluminophosphates in heterogeneous catalysis for the transesterification reaction in order to study the influence of the nitrogen content on the catalytic activities.

Catalytic activity for the transesterification reaction.

A large part of the production of biodiesel is performed usingsoluble basic homogeneous catalysts,i.e. sodium hydroxide or sodium methoxide. In this case, the alcohol is deprotonated by the base and transformed into an alcoholate. However, several purification steps are necessary to reach a high level of purity for the targeted esters (biodiesel) and glycerine in agreement with the required standards. Indeed, glycerine is polluted with alkali salts or alcoholates so that the purification process has a major cost of the same order of that necessary to prepare esters.

Unlike homogeneous catalysts, heterogeneous catalysts present the appreciable advantage to produce esters and glycerine without any trace of catalyst which make them easier to purify.

Basic heteregoneous catalyst based on zinc aluminate is the core of the IFPEN/Axens Esterfip-H® process, which is the only industrial heteregenous catalyst for the transesterification of vegetable oils. This basic heterogeneous catalyst allows high activities as well as high selectivities for a high glycerine purity. Acid catalysts tend to be more active, but the formation of ethers is possible due to the presence of Brönsted acid sites at their surface, thus degrading the overall selectivity of the reaction.

In order to take advantage of the two types of heterogeneous catalysts -acid and basic -in the transesterification reaction, nitridophosphates AlPO x N y are catalysts of choice for whichthe acido-basicity can be modifiedby tuning the nitrogen content in these solid solutions. The catalytic activity was measured for all samples according to the procedure described in the experimental section. Compared to the performance of the referenceZnAl 2 O 4 (Table 3), the AlPO 4 catalyst exhibits a much higher conversion yield of triglycerides into FAME. However, after reaction,the resulting glycerine presents a high ether (methoxyglycerol and dimethoxyglycerol)content close to 16 wt. %. It is necessary to limit drastically the formation of such phases to keep a high level of purity. In order to combine both an important production of FAME and high purity glycerine, nitrided-AlPO 4 catalysts were tested under the same experimental conditions. Despite of a lower conversion rate observed on Figure 12(AlPO x N y 6.7 and 8.7wt. % N), the ether content measured in the glycerine phase is considerably lower (< 0,1 %), suggesting a high purity. Thus, the insertion of nitrogen was efficient to modify the acidobasicity of the surface of the catalyst and to orientate the reaction towards pure final products.

The use of AlPO x N y oxynitrides allows to attain significant catalytic results, 70% 65% conversion rates for the samples containing 6.7 and 8.7 wt.% nitrogen respectively. For this transesterification reaction both acid and basic sites are active to catalyse the reaction, but each of them manifests a different selectivity. Basic catalysts are the most selective whereas acid ones lead often to the formation of glycerol ethers affecting the formation of glycerine. Moreover, Lewis acid sites are more active for transesterification while Brönsted one are less selective. The ideal case is to favor the creation of Lewis acid sites and basic sites.Unfortunately, the comprehension of the formation of Lewis and Brönsted acid sites is really difficult; nitrogen seems however to impact some acid sites and contributes to the formation of basic sites.

The increase of the nitrogen content may be correlated to the reaction time and the ensuing decrease of the specific surface area, which one influences the catalytic activity through the number of active sites. In Table 4 are reported the kinetics constants related to the transformation of triglycerides by considering the hypothesis: the methanol concentration is kept constant and iso-mass compared to triglycerides. These constants are calculated per unit of surface to avoid any effect of BET surface variation.

Table4: Velocity constant of the reaction (triglycerides conversion) per unit of surface. Nitridophosphates AlPO x N y with 6.7 and 8.7 wt.% nitrogen are characterized by the highest velocity constants in comparison with their oxide precursor although their catalytic activitiesare lower (see Figure 12). The catalyst with 15.3 wt.% nitrogen is less efficient than ZnAl 2 O 4 . This observation indicates finally that in the iso-mass conditions the triglycerides conversion is more important when using AlPO x N y catalysts, although a too much important nitrogen substitution may involve a decrease in the catalytic activity.This assumption does not take account of the powder morphology that controls also the catalysis reaction.

These phosphate catalysts arestable in the reactor. No leaching was reported (dissolution of the catalyst in the formed phases). Indeed, in the ester phase, the aluminum content is lower than 1 ppm and phosphorus lower than 5 ppm. In the glycerine, aluminum and phosphorus are measured between 10 and 50 ppm. This rate is even lower than 10 ppm for phosphorus originating from AlPO 4 catalyst. Thus, both esters and glycerine we aim to prepare are of a high purity, except when using AlPO 4 where noxious ethers are formed.

As the acido-basic sites are extremely sensitive to air and water, a catalytic test was performed first by activating AlPO x N y catalysts (with 6.7 wt.% N) under vacuum for 2 hours, and then they were loaded in the reactor under inert atmosphere (Figure 13). The activated catalyst appears less active than that stored in air without any precaution.This difference may be explained by the disappearance of some acido-basic sites (hydroxyles, carbonates…) due to the pretreatment. These sites of medium or weak strength may be active in transesterification. This hypothesis was not proven by infrared study because the activation is needed to substract the water signal.

Influence of the reacting alcohol

The impact of the choice of the alcohol to react with the vegetable oils in presence of the catalyst AlPON (6.7 wt.% N) is shown on figure 14. Quantities of alcohol and vegetable oils have been adapted in order to keep a molar ratio alcohol/oil = 27.5 and a total mass of 50 g. The pKa of each alcohol/alcoholate couple is given in brackets. Thus, as AlPON is an acid-base catalyst, the transesterification reaction may be driven by a basic or acid-base mechanism. The difference between ethanol and n-propanol is quite small regarding the activities, an acid mechanism may characterize them. Indeed, despite of an important pKa difference, the esters yield are similar. It seems that the reaction does not require the deprotonation of the alcohol on a basic site. The steric hindrance within the alcohols affect also the catalytic activity as branched alcohols lead only to poor conversion rates (less than 20% esters within 6 hours with tert-butanol).

Table5: Triglycerides conversion kinetics constantsper unityof surface. Converted per unit of surface, GaPO 4 offers the highest conversion kinetics constant (Table 5). Group III based phosphates (with Al and Ga) are active for the transesterification reaction. Tantalum-based phosphate does not have any interest for this reaction. Titanium was also tested with TiPON samples, but unfortunately give rise to an important leaching where the catalysts is dissolved at the end of the reaction.

Conclusion

Nitridoaluminophosphates have been prepared with different nitrogen contents (from 6 to 15 wt.%) and their spectroscopic characterizations were performed using infrared and NMR.

Infrared analyses indicate the presence of Lewis and Brønsted sites at the surface of all catalysts.

Although the basicity of AlPON was evidenced by CO 2 TPD (Conanec et al.), no basic sites was properly detected on the nitrided phases under our experimental conditions. 31 P and 27 Al NMR studies show that phosphorus and aluminum atoms are mainly in tetrahedral sites of the "AlPON" structure. The progressive nitridation of AlPO 4 results in the shift of the resonance peaks towards higher chemical shifts, a typical feature encountered with nitrogen insertion.

AlPO 4 used as an acid catalyst provided the best catalytic activity compared to nitrided phosphates. However the triglycerides conversion velocity constants (per unit of surface) prove that some of the AlPON catalysts (containing 6.

Figure 1 :

 1 Figure 1 : Synthesis of AlPO 4 .

  represents NMR 31 P spectrum of AlPO 4 where a single peak centered at -25 ppm corresponds to tetrahedral phosphorus P(OAl) 4 . Peaks at 20 ppm and, in a less extent, at -70 ppm are related to the rotation bands of the rotor. Regarding the nitrided samples, we observe on Figure 2,with increasing nitrogen content, a progressive decrease in the intensity of the peak at 40 ppm, while no change characterizes the component at -10 ppm. The composition of the tetrahedral environment evolves continuously while the octahedral sites remain unchanged (-10 ppm). Nitrogen affects significantly only the tetrahedral environment of aluminum. For sample AlPO 2.20 N 1.20 ,the tetrahedral component (40 ppm) splits clearly into two contributions one close to 30 ppm and the other at 52 ppm. This second peak was also observed by Conanec et al. for nitrided aluminophosphates containing

Figure 3 :Figure 3

 33 Figure 3 : NMR 31 P spectra of AlPO 4 and corresponding oxynitrides

Figure 4 :

 4 Figure 4: IR spectrum of AlPO 4 .

Figure 5 :

 5 Figure 5: adsorption of acetonitrile on AlPO 4 sample.

Figure 6 :

 6 Figure 6: Adsorption of CO on AlPO 4 surface.

Figure 7 :

 7 Figure 7: IR spectrum of AlPON (8.7 N wt.%).

Figure 10 :

 10 Figure 10: Adsorption of CO on AlPON-15,3 N wt.% sample.

Figure 11 :

 11 Figure 11: Transesterification reaction scheme.

  Figure 12displays diverse catalytic behaviors for the triglycerides conversion depending on the tested catalyst amongZnAl 2 O 4 (spinel-type), the oxide AlPO 4 and three oxynitrides AlPO x N y with different anionic contents. ZnAl 2 O 4 is a basic reference catalyst known for its high stability during the transesterification. At first sight, the most active catalyst is AlPO 4 with a conversion rate of 77% after only 6 hours of reaction. With increasing nitrogen content, we observe a decrease in the conversion rate of the triglycerides into methyl esters. The most nitrided sample with N= 15.6% presents only a yield of 16% after 6 hours reaction.

Figure 12 :

 12 Figure 12: Triglycerides conversion rateswith different catalysts vs. time.

Figure 13 :

 13 Figure 13: "AlPON" (%N wt = 6.7) activation under vacuum.

Figure 14 :

 14 Figure 14: Activity of the catalyst "AlPON" (%N wt = 6.7)vs. a panel of alcohols.

  7 and 8.7 wt. % N) are more efficient for the transesterification reaction. The insertion of nitrogen within an acid catalyst has an impact on acid sites and more than likely on the formation of basic sites. Moreover acid catalysts like AlPO 4 lead to the formation of ethers that impair the quality of glycerine, but AlPONs bring a solution to overcome this pollution without affecting conversion rates. The choice of the alcohol and the cation plays a major role on the values of the conversion rates. AlPONs accommodate very well with methanol. AlPO x N y nitridophosphates, with tunable acido-basicity, producedsignificant better catalytic results in comparison with a ZnAl 2 O 4 catalyst reference.

Table 1 :

 1 Characterization of the samples testedfor the transesterification reaction.

	Sample	Synthesis route	Temperature and step nitridation	Specific (m 2 .g -1 ) surface	% O (

wt.%) % N (wt.%)

	We	have	selected	one	aluminophosphate	(hydrogel	route)	and	three
		AlPO 4	hydrogel		-	342		50.9	-
	AlPO 3.16 N 0.56	hydrogel		800°C -10h	244		40.2	6.7
	AlPO 2.93 N 0.71	hydrogel		800°C -15h	232		39.1	8.7
	AlPO 2.20 N 1.20	citrate		800°C -50h	165		37.5	15.3

Table 2 :

 2 Triglycerides conversion in presence of AlPO 4 .

			Sampling (h)
	0 b	2	4	6

Table 3 :

 3 Triglycerides conversion in presence ofZnAl 2 O 4 .

		Sampling (h)
	0 b	4	6

  CatalystSpecific surface (m 2 .g -1 ) k (s -1 . m -2 ) x 10 4

	AlPO 4	340	17
	GaPO 4	130	31
	TaPO 5	280	4
	ZnAl 2 O 4	-	11

Influence of the cation

Considering the substantial results measured for the catalytic activities of aluminophosphates (AlPO 4 et "AlPON") for the transesterification reaction, more phosphates based on different cations have been prepared and evaluated in the same conditions, for example GaPO 4 and TaPO 5 whose results are reported on figure 15. The main characteristics for this test are given in Table 5.