Antoine Maignan

Ramzy Daou

Emmanuel Guilmeau

David Berthebaud

Tristan Barbier

Oleg Lebedev

Sylvie Hebert
email: sylvie.hebert@ensicaen.fr

Sylvie Hébert

Thermoelectric properties, metal-insulator transition, and magnetism: Revisiting the Ni1-xCuxS2 system

come

1-Introduction

NiS2 crystallizes in the pyrite structure like other di-chalcogenides such as CoS2 and FeS2 [1]. The description of its electronic structure has evolved with time. NiS2 was first described as a prototypical example of a Mott-Hubbard insulator [2][3][4][5][6][7]. More recently, theory and experimental (photoemission) studies [8,9] pointed towards a charge-transfer insulator with a band gap between the S anion 3p band at the highest valence states and Ni 3d states at the bottom of the conduction band. Taking into account this peculiarity, the ground state is predicted to be insulating [8], in contrast to Density Functional Theory (DFT) calculations which indicate a metallic state [10]. The insulating state is actually very close to a metallic one with a large Fermi surface and heavy electrons, as revealed recently by transport data as a function of pressure where a metallic regime is recovered below 50K above 3 GPa [11].

In such a charge-transfer insulator scenario, the role of the (S2) 2-dimers has to be considered as their bands lie across the band gap between occupied S-p and unoccupied Ni-d states in a broad region of d-p hybridization [8]. All these studies also demonstrated that both electronic and magnetic properties of NiS2 were strongly intertwined [8,9,12,13].

The magnetic properties change a lot across the pyrite series: CoS2 is an itinerant ferromagnet with almost 1µB per Co 2+ (ref. [14] and references therein) for a d 7 band filling, whereas NiS2 is an insulating antiferromagnet with almost 1µB per Ni 2+ (d 8 ) (ref. [13,15,16]). Moreover, CuS2, due to the d 9 electronic configuration of divalent Cu, exhibits a metallic character and becomes a superconductor below Tc = 1.5 K [17].

Although most of the studies in line with the Mott-Hubbard scenario interpreted the effect of Co (or Cu) for Ni substitution in NiS2 as a way to remove (or add) electrons in the half-filled eg broad band [3][4][5], most recent ones pointed towards a possible charge disproportionation as the mechanism responsible for the insulating state [15]. The NiS2 magnetic behaviour is complex, with two different commensurate antiferromagnetic (AF) states, M1, below TN1 = 39.2 K, and M2, below TN2 = 29.75 K [15]. The existence of a weak ferromagnetic component, which sets in below TN2 and is thus associated to M2, can be observed by magnetic susceptibility measurements.

Its origin has long been debated since the pyrite cubic Pa3 " symmetry does not allow such a component. The coexistence of 4 different AF domains below TN1 could explain it, if one of these magnetic structures is slightly changed [15] (for instance by a stoichiometry deviation). Interestingly, as the Se for S substitution suppresses concomitantly the M2 phase and the insulating state, the presence of the weak ferromagnetic component related to M2 is taken as a signature of the insulating state [15]. Apart from this charge ordering scenario, the low T transport properties have to be discussed with caution, as reports on NiS2 [18] and FeS2 [19,20] crystals of pyrite show that a surface layer with conducting states is responsible for apparent transitions in the T dependence of the electrical resistivity below ~ 150K. These surface states can lead to lower resistivity values at 5K, compared to the charge ordered state.

Taking into account these different developments and the reports on an insulator to metal transition induced by only few at.% Cu substitutions for Ni in NiS2 crystals [3] or ceramics [6], we have revisited the electronic, thermal and magnetic properties of the Ni1-xCuxS2 system with x varying up to 0.07.

Our results are consistent with the recent ones for NiS2-xSex [15]: with x increasing in Ni1-xCuxS2, the signature of the AF M2 state on the magnetic susceptibility curves is rapidly suppressed and from x = 0.05, a metal-like ground state is obtained. The aim of this work is also to investigate a possible reduction of thermal conductivity in these pyrites by the substitution of Ni by Cu. If an efficient reduction mechanism can be found in this family of materials, pyrites would be very interesting for thermoelectric applications as a very large power factor has already been measured in CoS2 [14].

In the following, we report on an experimental study of the effect of Cu substitution in NiS2 ceramic samples which shows a charge localization below about TN2 at small x (<0.05), replaced by a metallic state starting below about 100 K as soon as x ≥ 0.05 in the Ni1-xCuxS2 series, and the electrical resistivity (r) at 300 K which decreases by 2 orders of magnitude for only 2 at.% Cu substitution. As already reported in ref. [3], the Cu substitution impacts the thermopower (S, Seebeck coefficient), but it is also found that this substitution is efficient at reducing the total thermal conductivity (к), a bulk physical parameter, even though the electrical resistivity (r) decreases, indicating that the lattice contribution to к is strongly reduced. These results are discussed in light of the surface effects reported for crystals of pyrites, which might bias the interpretation of the transport properties, and also of Cu-induced melting of the charge ordered AF phase.

2-Experimental

The Ni1-xCuxS2 compounds, for 0 ≤ x ≤ 0.07 and using Δx step of 0.01, were prepared by direct reaction of the pure elements Ni, Cu and S. In order to avoid air exposure, the following operations were all performed in an Ar glove-box. After weighting the latter (1 g in total) according to the (1-x) : x : 2 ratio, the powders were mixed, cold pressed in the form of bars and set in silica tubes that were sealed under primary vacuum. These closed vessels were heated at 973 K for 12 h. For the three compositions (x = 0.00, x = 0.01 and x = 0.07), 10 g were also reacted to provide the powder quantities needed for Spark Plasma Sintering (SPS). For these densification steps, after grinding, the powders were sintered at 1023 K for 30 minutes under 64 MPa using graphite dies of 10 mm diameter. The geometrical densities were found to be greater than 90 % of the crystallographic ones. The purity of the samples was checked by room temperature powder X-ray diffraction using an X'Pert Pro Panalytical diffractometer using Cu Ka radiation in a 2θ range of 10 -120°.

Characteristic diffraction peaks of NiS2 without additional ones were obtained (Figure 1), corresponding to a refined (LeBail refinements) unit cell parameter a = 5.688(1) Å from x = 0.00 to a = 5.684(1) Å for x = 0.07. Consistently with the similar ionic radii of Cu and Ni cations, no deviation of the unit cell parameter superior to 0.005 Å was obtained showing a rather unchanged volume and thus a constant bandwidth. The copper content was checked by EDX coupled to electron diffraction in a 300kV-Tecnai transmission electron microscope. Though the Cu contents were in the limit of the accuracy of the technique, these observations confirmed the homogeneous distribution of Cu in the matrix and the lack of Cu containing impurities. In the following, the compositions correspond to the nominal ones. The NiS2 end member is known to be very sensitive to the sulfur non stoichiometry "NiS2-δ": its unit cell a parameter at room temperature is rather constant, a=5.688 Å, as δ varies from 0.1 to 0.03 in the sulfur substoichiometric region, but a decreases abruptly for -0.1≤ δ ≤0 sulfur excess (corresponding to the formation of Ni vacancies) to a ≈ 5.677 Å [6]. Our measured a = 5.688(1) Å value, consistent with the value a = 5.689 Å reported for the nominal NiS2 sample used for neutron diffraction in ref. [15], strongly suggests that our nominal NiS2 composition leads to a sulfur substoichiometric sample as expected from ref. [6]. As x increases, the almost constant value of the a parameter is in good agreement with [6].

The magnetic measurements were performed using either 5T-SQUID magnetometer or ACMS option of a physical properties measurements system (PPMS equipped of a 9 T magnet), both set-ups from Quantum Design. The T-dependent magnetic moment of all the samples was collected within an external applied field of 10 -2 T, and 1T for x = 0.06. For T ≤ 300 K, a fourprobe technique in the PPMS was used to measure the electrical resistivity (ρ) as a function of temperature (T). For T > 300 K, ρ and the Seebeck coefficient (S) were simultaneously recorded within a partial He pressure using an Ulvac Riko ZEM3 system. The low T (T ≤ 300 K) Seebeck coefficient values were measured using a home-made probe (steady state method) inserted in the PPMS. The thermal transport option (TTO) of the PPMS was used to measure the thermal conductivity (к) on the SPS densified bar-shaped samples. For the latter, the surfaces of the obtained pellets were removed to avoid carbon contamination. This allowed also to discard possible sulfur deficient surface layers which could play a role in the transport of pyrites as discussed in the results section. It must be emphasized that the comparison of the resistivity values for the samples before and after SPS densification showed similar values. This allowed to confirm that the role of conducting surface on the apparent transport measurements is much less critical in bulk polycrystalline samples as compared to crystals. Electrical contacts were manually deposited using a silver epoxy on bars of typical 10x2x2mm size. This similar geometry ensured that all ρ changes as a function of x (Cu content) correspond to equivalent change in the electrical resistivity. In the first report of surface effects in NiS2 crystals [18], the evidence for it was that the resistance was not changing but the resistivity was in crystals of different thicknesses for T below about 100 K. Thus, the present observed ρ changes for samples of similar geometry at 300K would imply that the conducting layers at the surface change with the Cu content. This means that if the transport is dominated by the surface, the Cu substitution is also efficient in systematically and reproducibly changing the surface. As seen in the following, as the Cu content also impacts the thermopower which is less sensitive than ρ (or the Hall effect, not measured) to the surface, and taking into account that the surface removal does not change the resistivity values, we are confident that for Cu at least the relative change between compounds in the transport properties can be discussed.

3-Results

3.1-NiS2: an antiferromagnetic insulator

The ρ value at 300 K is close to 1.1 Ω . cm for the SPS densified sample (top panel of Fig. 2). This value is very close to the resistivity reported for NiS2 ceramics [6]. More precisely our sample corresponds to the nominal "NiS1.945" sample of that study, since as T decreases, ρ values are found to increase to reach values close to 5 Ω . cm at 5 K in both samples. It must be emphasized that these rather flat curves differ greatly from those reported for crystals showing several orders of magnitude increase as T decreases even if the ρ value at 300K is similar [4]. Also, for our ceramic, the ρ(T) curve shows a change of slope below about 150 K.

The T-dependent magnetic susceptibility (χ) curve (middle panel of Fig. 2), shows the appearance of a weak ferromagnetic component associated with the M2 AF phase below TN2 = 29.75 K [15]. The presence of the second transition at TN1 can be observed on the inverse of susceptibility (χ -1 (T), same figure), as previously observed in [6]. Furthermore, the lack of Curie-Weiss behavior beyond 40 K up to 300 K is consistent with a previous study that showed that this linear regime in the χ -1 (T) could be reached only above 350 K [13]. This was shown to come from surviving AF order. At 10 K, consistently with the AF ground state, the weak ferromagnetic component associated to M2 is small (Fig. 3), as attested by the value M(5T) = 0.025µB / f.u. from the M(H) curve. Just above TN2 (T = 30 K in Fig. 3), i.e. in the M1 AF phase, this component is suppressed confirming its relation to the M2 AF phase. However, at T = 30 K, the ferromagnetic-like component of M2 is restored by application of a magnetic field as seen in the inset of Fig. 3, where a ferromagnetic component is induced at about 6 T. This indicates some spin flop of the M1 phase. Nevertheless, this weak ferromagnetic component evaluated from the χ(T) curves is found to differ from study to study. In order to compare our sample to those of others studied by neutron diffraction, one has to quantify the magnetization from the M(H) curve below TN2 as demonstrated by Thio et al [16]. At 10K, the present NiS2 sample shows a M0 magnetization (M= M0+ χH) of 0.016 µB/f.u. which is similar to the value for the (111) component in the crystal of ref. [16]. Together with the TN2 signature from the χ(T) curve determined at 29.5K, these values demonstrate that our sample is representative of NiS2.

As suggested in ref. [15] to explain the thermo-structural study results, Ni could occupy two sites of different charge and magnetic moment in the M2 phase, indicating a charge disproportionation (or charge ordering). In that respect, the slight tendency to localization at low T might be related to the M2 phase. However, as changes in the resistivity at low T were ascribed to surface effects in pyrites [18][19][20], this correspondence is to be taken with caution.

In contrast, the S(T) curve (Fig. 2, bottom) shows S ~ 0 μV . K -1 values for T < 100K with a T region where S<0 (inset of bottom panel in Fig. 2). In that low T region, the groundstate of stoichiometric NiS2 (Ni 2+ : t2g 6 eg 2 ) being characteristic of an eg half-filled band system with well splitted eg orbitals, any small deviation from stoichiometry should give either p-or n-type contribution to S. As shown in ref. [3][4][5], the Co (or Cu) for Ni substitution allows the eg broad band to be less (or more) than half-filled, the transport being thus dominated by electrons (holes). As shown below, our NiS2 sample being slightly less than half-filled, electrons are expected to dominate S at low T.

-Ni1-xCuxS2: inducing the semimetal state by hindering the M2 AF setting

The effect on ρ of the Cu for Ni substitution in NiS2 is spectacular as ρ300K decreases by about two orders of magnitude for only 7 at.% Cu substitution (Fig. 4a). A similar magnitude was reported for single crystals of same compositions in Ref. [3]. Concomitantly, the localized state observed below ≈ 30 K up to x = 0.04 is replaced by a metal-like T dependence for x ≥ 0.05, reaching r5K ≈ 4 mΩ . cm for Ni0.93Cu0.07S2. Thus, the Cu substitution induces a change of the "apparent" ground state from insulator (x = 0.00) to metal (x ≥ 0.05). The resistivity values are consistent with the previous reports [6]. But surface effects might dominate the transport at low T [18][19][20], and, for all the samples corresponding to x > 0.00, the ρ(T) curves exhibit a broad maximum separating two different conducting regimes, with a maximum at ~110 K for x = 0.07, these properties would thus deserve more measurements to be interpreted definitively.

The change of conducting regime below TN2 induced by Cu is apparently connected to the weak ferromagnetic-like component of the M2 AF phase: its signature on the χ(T) curves is rapidly suppressed as x increases as illustrated in Fig. 4b (field cooling process). (χ5K -χ30K) decreases very rapidly from 8.10 -4 emu . g -1 for NiS2 (main panel) to only 3.10 -6 emu . g -1 for Ni0.95Cu0.05S2 (inset). This is confirmed also by the M(H)5K curves (x = 0.03, x = 0.05 and x = 0.06 in Fig. 5) which become linear without a hysteretic component for x = 0.05 and x = 0.06 showing that the weak ferromagnetic component related to the M2 phase is suppressed.

The χ(T) curves (inset of Fig. 4b) also reveal a second magnetic transition near 120 K which could correspond to the systematic maximum on the ρ(T) curves in this T region. This result is consistent with that of Krill et al. [6] obtained also for ceramic samples but prepared by a very different technique. The presence of coupled magnetic and transport transitions in both series of samples makes hardly believable that surface effects dominate the transport in the same manner, as it would imply similar surface states. Thus, cooling the samples from 300 K, it is likely that a small ferromagnetic component sets in first in this high T region, and, for the x < 0.05 samples, is then followed by a second weak ferromagnetic component below TN2 related to the M2 phase. The measurements in a larger magnetic field of 1T instead of 10 -2 T tend to suppress the 120 K anomalies which is in favor of this ferromagnetic feature. From these χ(T) curves, nothing can be said about the M1 AF phase.

The phase diagram presented in Figure 6 summarizes the main findings.

As the Cu concentration increases, the "apparent" electrical resistivity and the low T magnetic susceptibility simultaneously decrease. From x = 0.02 to 0.05, the signature of the magnetic transition at TN2 is progressively suppressed. For larger x, x > 0.05, the samples present a metallic behavior at low T which must have some connection to the disappearance of the AF canting (AFM -M2 phase). The second magnetic transition at T ~ 120K seems more delicate to interpret though it was also observed in other samples of Cu substituted NiS2 [6]. This high T transition might be related to the one observed in [13] in NiS2, where the increase of AFM short range correlations were detected by inelastic neutron scattering below 150K.

3.3-Thermopower.

NiS2 is known to have a S(T) curve with a bell shape as the one shown in figure 7 for the NiS2 SPS ceramic. Below ≈ 100 K, S values are nearly 0 (see also the S(T) curve in the bottom panel of Fig. 2) and, beyond that temperature, they start to increase reaching positive values as large as S = +200µV . Κ -1 at 300K and then to decrease as T further increases to reach negative values above 500 K, saturating at S = -25 µV . K -1 . A similar behavior for NiS2 was reported in Ref. [4]. According to these authors, at very high T, when the T-independent entropy term contribution dominates, the negative S values for NiS2 can be assigned to a collapse of the eg bands where electrons dominate due to a possible sulfur non-stoichiometry. Similarly, the maximum S value for NiS2 on the S(T) curve depends on the exact Ni/S stoichiometry [4].

For a stoichiometric sample, the high-T extrapolated S∞ should be exactly S∞ = 0 as the Ni eg band is half-filled. To explain the value of S∞= -25 μV . K -1 at 700 K (Fig. 7), one can make a comparison with the data reported in ref. [4] for Ni1-xCoxS2. As Co 2+ (d 7 ) is substituted for Ni 2+ (d 8 ), the Ni eg band becomes less than half-filled and thus S is dominated by electrons from which it results a n-type character. In that respect, the S∞<0 values for our NiS2 indicates a doping similar to that induced by Co 2+ as if the Ni eg band is less than half-filled. Indeed, this S value is similar to that of Ni0.98Co0.02S2 [4]. On the opposite, for the Ni1-xCuxS2 series [3], d 9 Cu 2+ giving electrons to the halffilled eg band, the transport becomes dominated by holes. This explains the sign change of S at high-T experimentally observed in Fig. 7 for the x = 0.01 and x = 0.07 Ni1-xCuxS2 samples.

As T decreases, the S increase in NiS2 was interpreted [3][4][5] as the splitting increase of the eg band where S becomes dominated by holes in the lower eg band. For the present series, as Cu is substituted for Ni, the bell tends to fall out rapidly and the temperature at which the maximum occurs is pushed higher (Fig. 7). These results are consistent with the previous study [3] showing that the Cu substitution affects S more rapidly than the Co substitution [4] or than selenium for sulfur substitution [5]. For only 1 at.% Cu, the S maximum is shifted down to + 90µV . K -1 at 450 K (instead of a S maximum at 300 K for the pristine compound) and the negative T-independent S values at higher T are no longer observed even at 650 K.

The product SmaxTmax is classically used to determine the value of the gap even if corrections should be used when considering narrow gap systems like here [21]. As Cu content increases, this term decreases, in good agreement with a reduction of the gap. The gap is found to be close to 120 meV for x = 0, similar to previous results [22] and decreases to 78 meV for x = 0.01, and 11 meV for x = 0.07.

Interestingly, for the x = 0.07 composition, the T dependence of S at even lower T gives additional information. Below the linear T regime, S shows a shallow upturn in the region below 100 K with S<0 which could be related to the T region of the r maximum observed for several compositions (Fig. 4a).

However, this S<0 regime is unexpected as the minority holes are expected to dominate the Seebeck. For this high Cu concentration (x=0.07), it might be that the Cu orbitals start to participate to the transport, as CuS2 is a metal [17], the description based on the more or less half-filled eg orbital of Ni becoming erroneous.

From the thermopower data, no obvious relation with the magnetic transition can be deduced. The almost zero values of S observed for NiS2 suggest a very metallic behavior, and in that respect, the proximity to a metallic state with heavy carriers revealed by pressure [11] could explain these very small thermopower values. Also the perfect combination of two different bands of carriers in a narrow gap semiconductor can explain the S(T) curve. The S(T) model used in ref. [3][4][5] to describe Ni1-xCoxS2, Ni1-xCuxS2 and NiS2-xSex was based on an eg band splitting by the electronic correlations.

However, this model failed to reproduce the S(T) properties below 100K. A new model based on the charge transfer insulator description with two AF transitions and possibly a Ni charge ordering would be worth developing.

-Thermal conductivity (κ)

As expected from the simple cubic pyrite structure, where the M 2+ metallic cation lies in octahedral coordination of S anions, the latter forming S2 2-di-anion sticks, the thermal conductivities of these materials are rather large at 300K as illustrated by FeS2: values of κ ≈ 23 W . K -1. m -1 and κ ≈ 45 W . K -1. m -1 were reported for a mineral compound [START_REF] Clauser | Thermal Conductivity of Rocks and Minerals[END_REF] and single crystals [START_REF] Popov | [END_REF], respectively. Also, we measured κ value of ~ 11 W . K -1. m -1 in a polycrystalline dense CoS2 ceramic [14].

As shown in Fig. 8 for the SPS densified ceramics of NiS2, the value of κ = 6.7 W . K -1. m -1 at 300K is lower than those of FeS2 and CoS2. This might reflect the existence of defects in NiS2 related to its non-stoichiometry invoked to explain the S<0 values for T>500K (Fig. 7). Remarkably, the Cu for Ni substitution is very efficient at reducing κ (Fig. 8). From κ = 6.7 W . K -1. m -1 for NiS2, the values at 300 K decrease rapidly with x to reach κ = 2.5 W . K -1. m -1 for Ni0.93Cu0.07S2. Note that κ decreases even though ρ decreases by 2 orders of magnitude which is consistent with phonon-dominated κ. This is confirmed by the calculations of the electronic (κel) parts of the total thermal conductivity (κ) using the Wiedemann-Franz law and a Lorenz number L0 = 2.45*10 8 W . Ω . K -2 . At 300K, this leads to κel /κ ratios of about 1/100 and 1/10 for NiS2 and Ni0.93Cu0.07S2, respectively.

Considering the small mass difference between Ni and Cu, and the small level (7%) of substitution, the origin of this κ reduction by a factor of 2.5 at 300K cannot be simply ascribed to the alloying effect. This is supported by recent κ data reported for the Co1-xFexS2 pyrite series (x≤0.3) showing that the lattice part (κlatt) of the thermal conductivity is unchanged even for 30% substitution [25]. In that respect, the lack of κ data for CuS2 makes difficult the extraction of the important parameters governing the thermal conductivity in the pyrite series. As FeS2 is a diamagnetic semiconductor but exhibits a large RT κ, much larger than the value reported for CoS2 which is an itinerant ferromagnet, it seems that there exists no direct relation between ρ and κ, the latter being driven principally by phonons.

As T decreases below TN2, the thermal conductivity strongly increases showing that the AFM alignment favors the phonon propagation in NiS2. This leads to a peak in k at ~ 12K, which can not be related to a classical peak at ~ qD / 5, as qD = 445K in NiS2 [26]. As x increases, this upturn disappears. The same effect was previously observed in the pyrites Co1-xFexS2 as x increases [25] and is commonly observed in other materials such as filled skutterudites [START_REF] Nolas | Thermal conductivity, Theory, Properties and Applications[END_REF]. The impact of Cu substitution is thus very efficient to reduce the thermal conductivity also in the magnetically ordered state of these pyrites.

-Discussions and concluding remarks

For thermoelectric properties, the bell shape S(T) curve of NiS2 is interesting as S exhibits very large values at the top of the dome though counterbalanced by a too low electrical conductivity. The latter can be drastically increased by Cu 2+ for Ni 2+ doping as shown by the present results with room temperature ρ values decreasing from few Ω . cm to few mΩ . cm. Concomitantly, the S maximum values rapidly decrease as x increases in Ni1-xCuxS2, the T at the maximum shifting from 300 K for NiS2 to 450 K for Ni0.99Cu0.01S2. As the high positive S value of the pristine compound is related to near perfect half filling of the eg orbitals of 3d 8 Ni 2+ , the substitution by Cu 2+ creates a conduction dominated by holes. In the high T, this is confirmed by S > 0 values contrasting with the S < 0 and T-independent values observed for the Ni1-xCoxS2 samples, i.e. when the eg band is less than half-filled.

The S(T) curves for T ≤ 300 Κ confirm that, for x > 0.00, a metalliclike regime is at play over a broad T range with S ~ T, despite that the ρ(T) curves exhibit a (dρ/dT)<0 slope from T > ~120 K. Nevertheless, since the T dependence is very small, as for Ni0.93Cu0.07S2 ρ decreasing only from 4 mΩ . cm at 300 K to 3mΩ . cm at 650 K, these materials can be classified as semimetals.

The third physical quantity to be considered for thermoelectricity is the thermal conductivity. Our measurements performed on selected dense ceramics are consistent with phonons dominated κ with rather large values.

However, the NiS2 κ value is lower than those of other FeS2 and CoS2 pyrites [START_REF] Clauser | Thermal Conductivity of Rocks and Minerals[END_REF][START_REF] Popov | [END_REF]14], κ decreasing as the number of d electrons increases. Moreover, the Cu substitution for Ni is very efficient to reduce κ, 7 at.% Cu for Ni inducing a reduction by a factor of 2.5 at 300K. This effect, much greater than that of Fe-substituted CoS2 [25], supports the peculiar role of Cu in sulfides as compared to Fe, Co or Ni. The Cu efficiency in reducing κ might be related to the fact that CuS2 does not form in normal conditions but requires high pressure [START_REF] Nolas | Thermal conductivity, Theory, Properties and Applications[END_REF]. In that respect, as Cu in binary sulfides, such as Cu2S and CuS, adopts coordination other than octahedral, its substitution at NiS6 octahedral sites might be energetically unfavorable. This would imply that the atomic structure near Cu might not remain cubic pyrite, creating local structural distortions responsible for this strong impact on the lattice part of κ.

While the combination of these physical quantities (ρ, S, κ) leads to very low thermoelectric figures of merit, the low T measurements still give interesting insights in the complex physics of these materials. The insulating AF M2 ground state of NiS2 is rapidly replaced by a metallic AF state M1 when x ≥ 0.05 in Ni1-xCuxS2. Thus, this substitution induces an insulator to metal transition. Concomitantly, the signature of the AF transition at TN2 ≈ 30K is progressively suppressed as x increases which demonstrates that the insulating low T state is related to this AF phase. The isothermal M(H)T=5K curves also support this trend. In addition to these coupled electrical and magnetic transitions, there exists for NiS2 a clear change in the thermal conductivity below TN2 suggesting a spin/charge/phonon coupling for this T region.

Alternatively, the existence of a Ni charge ordering would be also consistent with the present observations. Additional measurements at low T are now necessary to confirm the hypothesis of Ni disproportionation. The reciprocal magnetic susceptibility is also given in the middle panel (○).

An enlargement of S at low T is given as an inset in the bottom panel.

Figure captions : Fig. 1 :

 :1 Figure captions:

Fig. 2 :

 2 Fig. 2: NiS2: T dependence (below 300K) of ρ (top; inset : the ln (r) data as a function of 1/T), χ (field-cooling mode, 10 -2 T, middle, (□)) and S (bottom).

Fig. 3 :

 3 Fig. 3: NiS2: isothermal H dependence of the magnetization M collected up to 5T. Inset: M(H)T=30K curve up to 9T.

Fig. 4 :

 4 Fig. 4: Ni1-xCuxS2: (a) ρ(T) and (b) χ(T) registered in field-cooling mode (10 - 2 T). x values are labelled in the graphs.

Fig. 5 :

 5 Fig. 5: Ni1-xCuxS2 : M(H)5K. x values are labelled in the graphs.

Fig. 6 :

 6 Fig.6 : Phase diagram of Ni1-xCuxS2 presenting as a function of x the values of r at 5K and of c (left-scale), and the values of the magnetic transitions TN2 and Tonset (right scale) extracted from the c(T) curves in Fig.4b.

Fig. 7 :

 7 Fig. 7: S(T) curves of the SPS densified Ni1-xCuxS2 samples (x=0.00, x=0.01 and x=0.07) as a function of T. x values are labelled in the graph.

Fig. 8 :

 8 Fig. 8: κ(T) curves of the SPS densified Ni1-xCuxS2 samples (x=0.00, x=0.01 and x=0.07). x values are labelled in the graphs.

  Figure 2

  Figure 4a

  Figure 6

Acknowledgments

This work is supported by the French Agence Nationale de le Recherche (ANR) through the Labex program EMC3 (ANR-10-LABX-09-01) and the COVATIS project.


Enrico Catizzone
email: enrico.catizzone@unical.it

Stijn Van Daele

Micaela Bianco

Alessandro Di Michele

Alfredo Aloise

Massimo Migliori

Valentin Valtchev
email: valentin.valtchev@ensicaen.fr

Girolamo Giordano

Alessandro Di

Catalytic application of ferrierite nanocrystals in vapour-phase dehydration of methanol to dimethyl ether

Keywords: Ferrierite, Crystal morphology, Brønsted-Lewis acidity, Methanol conversion, DME, Coke deposition

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

INTRODUCTION

Zeolites are crystalline microporous aluminosilicates whose catalytic properties are largely explored for decades. The ever-growing success of zeolites as industrial catalysts is mainly resulting from their unique molecular shape-selectivity, owing to a well-defined regular microporous structure, combined with tunable active sites.

Generally, both Brønsted and Lewis type acid sites are simultaneously present in a zeolite and their concentration, distribution, strength and location affect the overall activity, product selectivity and deactivation of the catalyst [START_REF] Corma | State of the art and future challenges of zeolites as catalysts[END_REF]. Unfortunately, the exceptional shape selectivity of zeolites is coupled with severe intracrystalline mass transfer limitations due to comparable size of the diffusing molecules and the zeolite channels. The interaction between diffusing molecules and channel walls, known as configurational diffusion, hinders the reaction rate and reduces the catalyst efficiency.

In order to overcome this problem, one of the strategies to be adopted in order to reduce mass transfer limitation is to synthesise zeolites with a secondary pore system of larger channels [START_REF] Pérez-Ramírez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF]. In this class of materials, known as hierarchical or mesoporous zeolites, intrinsic micropores are connected with a mesoporous channel net, promoting the diffusion to/from the active sites. During the last decade, several methods have been developed for hierarchical zeolites preparation, using bottom-up or top-down approaches [START_REF] Pérez-Ramírez | Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design[END_REF][START_REF] Verboekend | Design of hierarchical zeolite catalysts by desilication[END_REF][START_REF] Qin | Chemical equilibrium controlled etching of MFI-type zeolite and its influence on zeolite structure, acidity, and catalytic activity[END_REF]. Depending on the adopted procedure, pore architecture, position and topology of mesopores of zeolites can be varied and thus their catalytic performances [START_REF] Milina | Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts[END_REF]6].

The reduction of the crystal size to nanometric scale is another option largely explored in order to reduce the diffusion path length and thus to increase catalyst effectiveness [START_REF] Corma | Delaminated zeolite precursors as selective acidic catalysts[END_REF][START_REF] Valtchev | Porous nanosized particles: preparation, properties, and application[END_REF]. Yang et al. [START_REF] Yang | Nanosizeenhanced lifetime of SAPO-34 catalysts in methanol-to-olefin reactions[END_REF] showed that the crystal size strongly affects both product distribution and catalyst deactivation, during methanol (MeOH) conversion to olefins over SAPO-34 catalysts (CHA-type structure). Namely, the utilization of nano-sized SAPO-34 remarkably enhanced the olefins yield and catalyst life time and the positive effect of nano-zeolites in methanol-to-olefin conversion was also reported by other authors [START_REF] Nishiyama | Size control of SAPO-34 crystals and their catalyst lifetime in the methanol-to-olefin reaction[END_REF][START_REF] Dai | Mechanism of the deactivation of SAPO-34 materials with different crystal sizes applied as MTO catalysts[END_REF][START_REF] Chen | The effect of crystal size of SAPO-34 on the selectivity and deactivation of the MTO reaction[END_REF][START_REF] Hirota | Light olefins synthesis from methanol and dimethyl ether over SAPO-34 nanocrystals[END_REF][START_REF] Firoozi | The effect of micro and nano particle size of H-ZSM-5 on the selectivity of MTP reaction[END_REF]. The superiority of nano-sized zeolite in catalysis has also been demonstrated for other zeolite-catalysed reactions such as n-hexane cracking [START_REF] Konno | Kinetics of n-hexane cracking over ZSM-5 zeolites-effect of crystal size on effectiveness factor and catalyst lifetime[END_REF], hydrocracking of heavy vacuum gas oil [START_REF] Landau | Hydrocracking of heavy vacuum gas oil with Pt/H-beta-Al 2 O 3 catalyst: effect of zeolite crystal size in the nanoscale range[END_REF][START_REF] Camblor | Mild hydrocracking of vacuum gasoil over NiMo-Beta zeolite catalysts: the role of the location of the NiMo phases and the crystallite size of the zeolite[END_REF], Fischer-Tropsch process [START_REF] Martínez | The influence of ZSM-5 zeolite composition and crystal size on the in situ conversion of Fischer-Tropsch products over hybrid catalysts[END_REF], alkane and xylene isomerization [START_REF] Lee | Synthesis and catalytic behavior of ferrierite nanoneedles[END_REF][START_REF] Ratnasamy | Influence of crystal size of HZSM-5 on activity and shape selectivity in xylene isomerization[END_REF], biomass pyrolysis [START_REF] Zheng | Effect of crystal size of ZSM-5 on the aromatic yield and selectivity from catalytic fast pyrolysis of biomass[END_REF], ethanol to propylene [START_REF] Meng | The effect of crystal sizes of HZSM-5 zeolites in ethanol conversion to propylene[END_REF] and others [START_REF] Herrmann | Effect of the crystal size on the activity of ZSM-5 crystals catalysts in various reactions[END_REF].

Nano-sized zeolite crystals can be obtained via a careful optimization of the synthesis gel composition and crystallization conditions, as summarized by Mintova et al. [START_REF] Mintova | Advance in nanosized zeolites[END_REF][START_REF] Grand | Mechanism of zeolites crystal growth: new findings and open questions[END_REF] and Di Renzo [START_REF] Renzo | Zeolites as tailor-made catalysts: control of the crystal size[END_REF]. One key factor is the ability to induce simultaneous nucleation events, producing a zeolite with narrow particle size distribution. Such conditions were achieved for several industrial relevant zeolites by fine control of the reaction parameters (e.g. gel composition, alkalinity, dilution, temperature, gel aging, etc.) [START_REF] Choi | Stable singleunit-cell nanosheets of zeolite MFI as active and long-lived catalysts[END_REF][START_REF] Mintova | Advance in nanosized zeolites[END_REF][START_REF] Grand | Mechanism of zeolites crystal growth: new findings and open questions[END_REF][START_REF] Renzo | Zeolites as tailor-made catalysts: control of the crystal size[END_REF][START_REF] Valtchev | Tribochemical activation of seeds for rapid crystallization of zeolite Y[END_REF][START_REF] Chen | In situ and post-synthesis control of physicochemical properties of FER-type crystals[END_REF][START_REF] Kamimura | Seed-assisted, OSDA-free synthesis of MTW-type zeolite and "Green MTW" from sodium aluminosilicates gel systems[END_REF][START_REF] Li | Aging effects on the nucleation and crystallization kinetics of colloidal TPA-silicalite-1[END_REF][START_REF] Zhang | Synthesis of zeolite NaA at room temperature: the effect of synthesis parameters on crystal size and its size distribution[END_REF].

Recently, the crystallization of zeolites in the presence of a surfactant was reported as an unconventional method to prepare nano-sized MFI and MOR crystals [START_REF] Iwakai | Preparation of nano-crystalline MFI zeolite via hydrothermal synthesis in water/surfactant/organic solvent using fumed silica as Si source[END_REF][START_REF] Tago | Preparation for size-controlled MOR zeolite nanocrystal using water/surfactant/organic solvent[END_REF][START_REF] Chauhan | Synthesis of zeolite ZSM-5: effect of emulsifiers[END_REF].

Different surfactants, anionic, cationic or non-ionic molecule, can be employed. SLS was proposed as an emulsifier to obtain highly crystalline nano-sized FER crystals in the range 200-400 nm when piperidine is used as a structure directing agent. CTABr or Span 60 will lead to the formation of an amorphous phase and 1-3 µm large ferrierite crystals, respectively [START_REF] Chauhan | Synthesis of zeolite ferrierite-Role of emulsifiers[END_REF].

Recent works showed that the FER-type zeolite is promising acid catalyst to produce dimethyl ether (DME) via both methanol dehydration [START_REF] Catizzone | Dimethyl ether synthesis via methanol dehydration: effect of zeolite structure[END_REF][START_REF] Catizzone | From 1-D to 3-D zeolite structures: performances assessment in catalysis of vapour-phase methanol dehydration to DME[END_REF][START_REF] Catizzone | Ferrierite vs. γ-Al 2 O 3 : the superiority of zeolites in terms of water-resistance in vapour-phase dehydration of methanol to dimethyl ether[END_REF] and one-pot CO 2 hydrogenation [START_REF] Bonura | Catalytic features of CuZnZr-zeolite hybrid systems for the direct CO 2 -to-DME hydrogenation reaction[END_REF][START_REF] Frusteri | Direct CO2-to-DME hydrogenation reaction: new evidences of a superior behaviour of FER-based hybrid systems to obtain high DME yield[END_REF][START_REF] Bonura | Acidity control of zeolite functionality on activity and stability of hybrid catalysts during DME production via CO 2 hydrogenation[END_REF]. DME has a bright future ahead as a reliable alternative fuel for diesel engines due to its high cetane number and soot-free emissions [START_REF] Catizzone | CO 2 recycling to dimethyl ether: state-of-the-art and perspectives[END_REF]. Moreover, DME can also be an intermediate in the olefins and fine chemicals production, and it is considered as a reliable energetic vector to introduce renewable energy in the chemical industry [START_REF] Centi | Opportunities and prospects in the chemical recycling of carbon dioxide to fuels[END_REF][START_REF] Azizi | Dimethyl ether: a review of technologies and production challenges[END_REF], since it can be produced from biomass or CO 2 .

Compared with other zeolite catalysts (e.g. beta, MFI, MOR, MTW, EU-1), the ferrierite shows a higher DME selectivity and resistance to deactivation by coke deposition [START_REF] Catizzone | From 1-D to 3-D zeolite structures: performances assessment in catalysis of vapour-phase methanol dehydration to DME[END_REF][START_REF] Bonura | Acidity control of zeolite functionality on activity and stability of hybrid catalysts during DME production via CO 2 hydrogenation[END_REF]. The effect of acidity of the FER-type zeolite on methanol dehydration to DME reaction was also investigated revealing that a higher Lewis acid concentration will increase the catalyst effectiveness [START_REF] Catizzone | The effect of FER zeolite acid sites in methanol-to-dimethyl ether catalytic dehydration[END_REF]. The beneficial use of nanosized zeolites in methanol dehydration to dimethyl ether reaction was recently investigated by Rownaghi et al. [START_REF] Rownaghi | Selective dehydration of methanol to dimethyl ether on ZSM-5 nanocrystals[END_REF] using ZSM-5 zeolites. The authors showed that by reducing the crystal size from 1.2 µm to 0.12 µm, both the DME selectivity and the MeOH conversion improved significantly.

According to that, the aim of this study is to investigate the behaviour of nanosized FER-zeolites in DME synthesis. The crystals size was varied from nano-to micronsized dimension and the impact of crystals size on the number and distribution of active sites was studied in vapour phase methanol dehydration.

EXPERIMENTAL

Samples preparation

All FER-type zeolite samples were prepared by using pyrrolidine (Py, Aldrich) as structure directing agent (SDA) adopting the synthesis procedure described elsewhere [START_REF] Chen | In situ and post-synthesis control of physicochemical properties of FER-type crystals[END_REF] and using a gel with the following molar composition: All the syntheses were carried out in 60 ml PTFE-lined stainless steel autoclaves using a tumbling oven (tumbling speed = 20 rpm). The crystallization of micron-sized FERtype zeolite (named M-FER) was carried out at 180 °C for 120 h. With the aim to reduce the crystal size, sodium lauryl sulfate (SLS, Aldrich) was used as a surfactant in the sampled labelled as NP-FER. A SLS/Al 2 O 3 molar ratio equals to 0.3 was used and crystallization was carried out at 180 °C for 75 h.

The effect of seeds was also evaluated by adding 3 wt. % seeds (H-form NP-FER) with respect to silica in the above reported synthesis gel. Crystallization of this system was performed at 160 °C for 60 h and before synthesis, the gel was aged at 80 °C for 48 h under stirring condition (stirring rate: 500 rpm). The solid synthesized following this from this procedure is reported as NC-FER.

As standard procedure, after synthesis, the solid was recovered by suction filtration and washed several times with distilled water. Organic SDA was removed by calcination in air at 550 °C for 8 h. After this treatment, the H-form of the synthesized zeolites was prepared by an ion-exchange with NH 4 Cl solution [START_REF] Catizzone | Dimethyl ether synthesis via methanol dehydration: effect of zeolite structure[END_REF], followed by calcination in air (550 °C for 8 h).

Physicochemical characterization

Powered X-ray diffraction patterns of synthesised samples were obtained with APD 2000 Pro GNR instrument (Cu Kα radiation, λ=1.5406, 40 kV, 30 mA, 5-50° 2θ range, scanning step 0.2°•s -1 ). The crystal size and shape were studied using a scanning electron microscope (FEI model Inspect). Nitrogen adsorption/desorption isotherms were carry out on H-form samples at 77 K with a Micromeritics ASAP 2020. The specific surface area was calculated using the BET equation, whilst the micropore volume and external surface were estimated by the t-plot method. Thermogravimetric analysis (TG/DTA) was carried out in air on a DTG-60 Shimadzu instrument in the range 25 °C -850 °C (heating rate: 5 °C•min -1 ). The chemical composition of the samples was determined via atomic absorption spectroscopy (GBC 932). 27 Al-NMR spectra of H-form samples were acquired on a Bruker Advance 500 MHz spectrometer [START_REF] Aloise | Catalytic behaviour in propane aroamtization using Ga-MFI catalyst[END_REF].

The in-situ FT-IR technique was used to study the surface acidity of the samples. FTIR measurements were carried out on a Nicolet Magna 550-FTIR spectrometer at 4 cm -1 optical resolution with a DTGS detector. Samples to be analysed were pressed as selfsupported wafers (diameter 13 mm, 18 mg). Before the spectrum acquisition at room temperature, each sample was pre-treated in the IR cell connected to the vacuum line at 393 K (1 K/min) for 4 h and at 723 K (1 K/min) for 5 h under 10 -6 torr. Deuterated acetonitrile (CD 3 CN) was used as a probe to estimate total acidity and Brønsted/Lewis distribution as this base molecule is able to diffuse in both 8-and 10-membered rings of FER structure. 2,6-dimethyl pyridine and tert-butyl nitrile were used as probe molecules to estimate the number of Brønsted and Lewis acid sites located on the external crystal surface. Probe molecules were adsorbed at room temperature (295 K) with a saturation pressure of 0.5 torr.

Catalytic tests

Vapor-phase methanol dehydration reaction was carried out in a lab-scale apparatus, using a quartz tubular reactor [START_REF] Migliori | Kinetic analysis of methanol to dimethyl ether reaction over H-MFI catalyst[END_REF]. A mixture of methanol/nitrogen (methanol content 0.06 mol/mol) was fed to the reactor containing 70 mg of catalyst (pellets form 300-500 µm) with a total flow rate of 60 Nml•min -1 . Catalytic tests were carried out in the 120-280 °C range under atmospheric pressure. Stability tests were performed at 280 °C for a duration up to 60 h. Reactor out-stream composition was analyzed by an on-line gaschromatograph equipped with flame ionization detector [START_REF] Migliori | Kinetic analysis of methanol to dimethyl ether reaction over H-MFI catalyst[END_REF].

Coke analysis

The amount of carbonaceous solid (named coke) deposited during time-on-stream tests at 280 °C was determined via thermogravimetric analysis of exhausted samples [START_REF] Catizzone | Dimethyl ether synthesis via methanol dehydration: effect of zeolite structure[END_REF].

The composition of coke was measured via GC-MS analysis following a dissolving/extracting procedure [START_REF] Catizzone | From 1-D to 3-D zeolite structures: performances assessment in catalysis of vapour-phase methanol dehydration to DME[END_REF]. The morphology and distribution of coke on zeolite surface was also investigated by Field Emission Gun Electron Scanning Microscopy LEO 1525 ZIESS, after metallization with Cr and elemental composition was determined using a Bruker Quantax EDS.

RESULTS AND DISCUSSION

Physicochemical properties of investigated samples

XRD analysis showed that all samples are highly crystalline FER-type materials, neither other phases nor amorphous material was identified (Figure 1). SEM images of M-FER, NP-FER and NC-FER are shown in Figure 2(A)-(C), respectively. M-FER shows stacked plate-like crystals with a well-developed {1 0 0} face with following dimensions: length 8-12 µm, length/width ratio around 2 µm and thickness 0.1 µm (Figure 2A). The use of SLS surfactant resulted in the formation of non-stacked platelike crystals with size ranging between 300 and 500 nm (Figure 2B). The addition of seeds allowed to decrease the crystal size to about 100 nm (Figure 2C). Chemical analysis data reported in Table 1 show that the samples exhibit a similar bulk aluminium content (AC), but a higher aluminium content on the surface (AS) was found, with major evidence for smaller crystals. The analysis of the local structure by 27 Al MAS NMR is presented in Figure S.1 (Supplementary Information), revealing that both, tetrahedral and octahedral aluminium species, are present in the investigated samples, with a chemical shift of about 54 ppm and 0 ppm, respectively. The relative concentration of octahedral aluminium increases as the crystal size decreases from ca.

14% of the largest crystals to ca. 21% for the smallest ones (Table 1). The higher AS in the nanocrystals is obviously related to the presence of extra-framework aluminium on their outer surface.

The N 2 adsorption/desorption isotherms at 77 K presented in Figure 3 reveal some differences in the porosity and the texture of the investigated samples. M-FER exhibits a type-I isotherm, where a second uptake is observed at 0.7-0.8 relative pressure with hysteresis. in the later is a consequence of stacking of platelets of about 100 nm thickness [START_REF] Bonilla | Desilication of ferrierite zeolite for porosity generation and improved effectiveness in polyethylene pyrolisis[END_REF][START_REF] Rownaghi | Uniform mesoporous ZSM-5 single crystals catalyst with high resistance ton coke formation for methanol deoxygenation[END_REF]. NP-FER exhibits a less evident hysteresis loop probably due to limited agglomeration between the particles [START_REF] Leofanti | Surface area and pore texture of catalysts[END_REF]. On the contrary, NC-FER exhibit a type-H4 hysteresis loop at lower relative pressure (starting from about 0.4) typical of agglomerates of particles forming mesopores with uniform size and shape [START_REF] Rownaghi | Uniform mesoporous ZSM-5 single crystals catalyst with high resistance ton coke formation for methanol deoxygenation[END_REF][START_REF] Leofanti | Surface area and pore texture of catalysts[END_REF][START_REF] Rownaghi | Methanol to gasoline-range hydrocarbons: influence of nanocrystal size and mesoporosity on catalytic performances and product distribution of ZSM-5[END_REF][START_REF] Rownaghi | Yield of gasoline-range hydrocarbons as a function of uniform ZSM-5 crystal size[END_REF]. As reported in Table 1 both BET surface area and micropore volume are typical of highly crystalline FER-type materials [START_REF] Chen | In situ and post-synthesis control of physicochemical properties of FER-type crystals[END_REF]. The differences in micropore volume could be related to the presence of extra-framework aluminium species inside the pores, since samples with a higher concentration of extra-framework aluminium species exhibit lower micropore volume. The analysis of the local structure of aluminium by 27 Al MAS NMR is presented in Figure S.1 (Supplementary Information), revealing that both, tetrahedral and octahedral aluminium species, are present in the investigated samples, with a chemical shift of about 54 ppm and 0 ppm, respectively. The relative concentration of octahedral aluminium increases as the crystal size decreases from ca.

14% of the largest crystals to ca. 21% for the smallest ones (Table 1). Figure 4(A) shows the FT-IR spectra of investigated FER-type samples prior to the adsorption of the probe molecule. The sharp band at 3746 cm -1 is associated with terminal silanol groups located on the external surface. The shoulder at 3720-3730 cm -1 originates from silanols located inside the pores. The band at 3600 cm -1 is characteristic of bridging Si(OH)Al Brønsted sites. The broad shoulder around 3650 cm -1 is typically related to highly distorted or extra-framework aluminium species that can act as Lewis acid sites [START_REF] Trombetta | An FT-IR study of the internal and external surfaces of HZSM5 zeolite[END_REF][START_REF] Khitev | Synthesis and catalytic properties of hierarchical micro/mesoporous materials based on FER zeolite[END_REF][START_REF] Datka | Acid properties of NaKHferrierites of various exchange degrees studied by IR spectroscopy[END_REF][START_REF] Cañozares | Isomerization of n-butene over ferrierite zeolite modified by silicon tetrachloride treatment[END_REF][START_REF] Peixoto | Influence of different processes of dealumination on acid properties of an H-ferrierite zeolite[END_REF]. Spectra reported in Figure 4 reveal that all the samples show similar bands in the 3720-3746 cm -1 region, suggesting similar concentration and location of silanol groups. On the contrary, FT-IR spectra differ in the 3600-3680 cm -1 region: the band height at 3600 cm -1 follows the order M-FER>NP-FER>NC-FER suggesting that M-FER possess a higher concentration of bridging Si(OH)Al Brønsted sites than the other samples. NC-FER contains a more pronounced band at 3630-3680 cm -1 region showing a higher concentration of extra-framework aluminium species for nanosized sample, in agreement with the 27 Al MAS NMR data.

In order to better discriminate between Lewis and Brønsted acid sites, in the 3600-3680 cm -1 region the bands entirely disappear after the adsorption of CD 3 CN, indicating that the acid sites of investigated samples are fully saturated (Figure 4(B). A small number of terminal silanols are still present after CD 3 CN adsorption, confirming the low-acidic character of these groups, whilst internal silanols (bands at around 3725 cm-1) totally disappear.

Spectra analysis suggests that d 3 -acetonitrle exhibits at least five stretching modes of ν(C≡N) in the region 2220-2360 cm-1 as reported in Figure S.2. IR band at about 230 cm -1 is ascribed to coordinatively bonded CD 3 CN on electron acceptor aluminium sites that act as Lewis centres whereas the band at 2296-2298 cm -1 derives from coordination of probe molecule with bridging OH groups that act as Brønsted acid sites. The presence of a ν(C≡N) stretching band at 2275-2280 cm -1 is associated to the interaction of CD 3 CN with zeolite defects (i.e., silanols) whereas the presence of physisorbed and liquid phase CD 3 CN is characterized by the bands at 2265 cm -1 and 2250 cm -1 , respectively [START_REF] Wichterlová | Determination and properties of acid sites in H-ferrierite: a comparison of ferrierite and MFI structures[END_REF][START_REF] Pazé | Acetonitrile as probe molecule for an integrated 1 H NMR and FTIR study of zeolitic Brønsted acidity: interaction with zeolites H-ferrierite and H-beta[END_REF]. The number of bridging Brønsted sites was calculated from the area of the band observed at 2296 cm 1 by adopting an extinction coefficient of 2.05 cm/µmol, whilst the number of Lewis acid sites was calculated from the area of the band centered at 2322 cm 1 by adopting an extinction factor of 3.6 cm/µmol [START_REF] Wichterlová | Determination and properties of acid sites in H-ferrierite: a comparison of ferrierite and MFI structures[END_REF].

In order to probe the external surface of ferrierite crystals two bulky molecules, 2,6dimethyl pyridine (lutidine) and tert-butylnitrile (pivalonitrile), were employed. The difference spectra after saturation with lutidine are presented in Figure 5(A). Besides the bands characteristic of lutidine, a negative signal due to the consumption of silanols is observed at 3747 cm -1 . This result shows that the majority of silanols are situated at the external surface of ferrierite, which is in agreement with the data reported by van Donk et al. [START_REF] Van Donk | Probing the accessible sites for n-butene skeletal isomerization over aged and selective H-ferrierite with d 3 -acetonitrile[END_REF]. The relative magnitude of the signal at 3600 cm -1 , representing the consumption of Brønsted acid sites, points out that only a small percentage of the total Brønsted acid sites is consumed by lutidine. By analysing the spectra in the 1700-1500 cm -1 range both bands specific for lutidine:Lewis at 1580-1603 cm -1 and lutidine:Brønsted at 1630-1650 cm -1 appear. As reported elsewhere [START_REF] Jacobs | Active sites in zeolites[END_REF][START_REF] Morterra | Revisiting the use of 2,6dimethylpyridine adsorption as probe molecule for the acidic properties of metal oxides[END_REF] lutidine is strongly adsorbed on Brønsted even after outgassing at high temperature whilst readily desorbs from the Lewis sites because of limited interaction by steric hindrance. For this reason, lutidine is considered a suitable probe molecule to estimate Brønsted sites [START_REF] Morterra | Revisiting the use of 2,6dimethylpyridine adsorption as probe molecule for the acidic properties of metal oxides[END_REF][START_REF] Thibault-Starzyk | Quantification of enhanced acid site accessibility in hierarchical zeolitesthe accessibility index[END_REF]. The Brønsted acid sites on the external surface were estimated by the band at 1630-1650 cm -1 using the molar extinction coefficient reported by Onfroy et al. [START_REF] Onfroy | Quantitative IR characterization of the acidity of various oxide catalysts[END_REF]. In order to quantify external Lewis acidity probing with tert-butylnitrile was employed. On the other hand, due to overlapping signals, quantification of the Brønsted acidity by tertbutylnitrile molecule is hindered. Based on the literature data both, the molar extinction coefficient and the identification of different bands, was performed [START_REF] Sadowska | Accessibility of acid sites in hierarchical zeolites: quantitative IR studies of pivalonitrile adsorption[END_REF][START_REF] Montanari | Use of nitrile sas probe molecules for the accessibility of the active sites and the detection of complex interactions in zeolites through IR spectroscopy[END_REF]. As reported in Figure 5(B), the coordination between the probe molecule on a Lewis site is found at 2305 cm -1 . Although a high percentage of Lewis acidity is observed in all three samples, no surface enrichment is detected.

The quantification of acid sites resulted from FT-IR analysis is summarized in Table 2.

NP-FER sample shows the highest Lewis acid sites concentration followed by NC-FER and M-FER. On the contrary, the highest bridging Brønsted acid sites concentration is measured for M-FER followed by NP-FER and NC-FER. Note that the investigated samples possess a similar aluminium content. Therefore, the set of data points out that the smaller crystals contain less Brønsted sites than larger ones. We relate this result with the synthesis conditions used to obtain nanocrystals that might cause a lower aluminium incorporation [START_REF] Cai | Direct dimethyl ether synthesis from syngas on copper-zeolite hybrid catalysts with a wide range of zeolite particle size[END_REF]. Although NC-FER exhibits the highest aluminium content on the external surface, the lower amount of external acid sites was found on this sample, indicating that not all the external aluminium species possess acidity properties [START_REF] Cai | Direct dimethyl ether synthesis from syngas on copper-zeolite hybrid catalysts with a wide range of zeolite particle size[END_REF].

For all the samples, less than 2% of total acid sites are located on the external surface of zeolite crystals suggesting that the majority of acid sites, either Brønsted-or Lewistype, are mainly located in the pore volume. The latter result permits to assume that the effect of the external surface acidity is negligible in comparison with the internal acidity on catalytic behaviour of investigated zeolites.

The effect of crystal size on catalytic activity

The methanol conversion is reported in Figure 6 in the reaction temperature range 120°C -280°C. The following activity order NP-FER>NC-FER>M-FER can be observed up to 220 °C whilst all catalysts exhibit a similar conversion, close to the equilibrium thermodynamic value, at high temperatures. Since the observed differences in catalytic activity can be related to both acidity and crystal size, a turnover frequency (TOF) analysis was employed to shed more light in the catalytic reaction. Both intrinsic reaction rate and an accurate counting of active sites are necessary in order to estimate a TOF value. It is well-known that only an apparent reaction rate can be calculated when zeolites are used as catalyst due to severe intracrystalline mass transfer limitation; furthermore, both Brønsted and Lewis acid sites catalyse dehydration of methanol to DME, even if Lewis acid sites show a higher activity [START_REF] Catizzone | The effect of FER zeolite acid sites in methanol-to-dimethyl ether catalytic dehydration[END_REF][START_REF] Boudart | Turnover rates in heterogeneous catalysis[END_REF]. Therefore, in this work, an apparent turnover frequency (TOF app ) value is calculated by considering the observable methanol conversion rate and both Brønsted and Lewis acid sites number calculated from FT-IR analysis. Silanols sites were not included since they are not active towards such reaction under the investigated conditions. TOF app is calculated by the following equation: number, respectively. The apparent TOF analysis was performed with conversion data at 180°C [START_REF] Catizzone | From 1-D to 3-D zeolite structures: performances assessment in catalysis of vapour-phase methanol dehydration to DME[END_REF] and the estimated values are reported in Table 3. The obtain values showa positive effect of reducing crystal size on the efficiency of the catalyst in terms of apparent turnover rate since TOF app increases from 45 h -1 to 65 h -1 by decreasing the crystal size from micro-to nano-metric scale. A recent work reports that Lewis acid sites exhibit higher activity than Brønsted acid sites during methanol to DME reaction leading to higher TOF value for FER-type zeolites with higher Lewis acid sites concentration [START_REF] Catizzone | The effect of FER zeolite acid sites in methanol-to-dimethyl ether catalytic dehydration[END_REF]. The results obtained in this work report that, although NC-FER possesses a LAS concentration much lower than NP-FER, it exhibits a higher apparent TOF indicating that the control of mass transfer limitation in zeolites plays a crucial role in Methanol-to-DME reaction.

For reaction temperatures up to 220 °C, DME was detected in the reactor out-stream, in agreement with previous studies [START_REF] Catizzone | Dimethyl ether synthesis via methanol dehydration: effect of zeolite structure[END_REF][START_REF] Catizzone | From 1-D to 3-D zeolite structures: performances assessment in catalysis of vapour-phase methanol dehydration to DME[END_REF][START_REF] Catizzone | Ferrierite vs. γ-Al 2 O 3 : the superiority of zeolites in terms of water-resistance in vapour-phase dehydration of methanol to dimethyl ether[END_REF]. At 240 °C, traces of hydrocarbons, mainly methane, ethylene and propylene are formed, causing a DME selectivity drop; which is more critical for larger crystals while no by-products were detected when NC-FER was tested (see Table 3).

By increasing the reaction temperature to 280 °C, DME selectivity decreases to 95% and 90% for NP-FER and M-FER, respectively.The DME selectivityis about 99% for NC-FER. For both, NP-FER and M-FER, by-products were quantified from GC analysis whilst similar analysis was not possible for NC-FER because of too low signals. Methane, ethene and propene were mainly formed as by-products. In particular, the distribution of by-products loop is reported in Table 3. The results clearly show that the ethene fraction is similar in M-FER and NP-FER samples whilst the yield of methane was higher on the NP-FER sample, probably related to its higher Lewis acid sites concentration [START_REF] Catizzone | The effect of FER zeolite acid sites in methanol-to-dimethyl ether catalytic dehydration[END_REF].

It is known that the formation of hydrocarbons from methanol, involves a complex mechanism (such as hydrocarbon pool in the case of olefins) where methanol might be dehydrated to DME and then both DME and methanol are converted to hydrocarbons [START_REF] Bjørgen | Conversion of methanol to hydrocarbons over zeolite H-ZSM-5: on the origin of the olefinic species[END_REF][START_REF] Tian | Methanol to olefins (MTO): from fundamentals to commercialization[END_REF][START_REF] Haw | The mechanism of methanol to hydrocarbon catalysis[END_REF][START_REF] Stöcker | Methanol-to-hydrocarbons: catalytic materials and their behavior[END_REF][START_REF] Möller | The use of a jet loop reactor to study the effect of crystal size and the cofeeding of olefins and water on the conversion of methanol over ZSM-5[END_REF]. The obtained selectivity results suggest that in case of nano-sized crystals it is possible to reduce the residence time inside the crystal and, as a consequence, formed DME is able to effectively diffuse outside the crystal reducing the occurrence of consecutive reactions [START_REF] Chen | The effect of crystal size of SAPO-34 on the selectivity and deactivation of the MTO reaction[END_REF][START_REF] Möller | The use of a jet loop reactor to study the effect of crystal size and the cofeeding of olefins and water on the conversion of methanol over ZSM-5[END_REF]. On the contrary, on large crystals (e.g. M-FER), more side reactions can occur. Time-On-Stream tests carried out at 280 °C showed that all catalysts keep a nearly constant methanol conversion grade for 60 h (Figure S.3, Supporting Information). Smaller crystal (NP-FER and NC-FER) showed stable DME selectivity up to 60 h. The selectivity of micron-sized crystals (M-FER) increased from 0.90 to 0.98 after 60 h.

Coke analysis

The amount of coke deposited on the investigated catalysts during methanol dehydration at 280 °C monitored via Thermogravimetric/Differential Thermal Analysis (TG/DTA) is presented in Figure 7. It clearly appears how the coke deposition is favourably affected by the increase in crystal size: the measured amount of deposited coke is 75 mg coke /g cat , 69 mg coke /g cat and 46 mg coke /g cat for M-FER, NP-FER and NC-FER, respectively, after 60 h of Time-on-Stream.

In order to obtain quantitative information on the coke formation rate, the experimental data was fitted to a first order model [START_REF] Catizzone | Dimethyl ether synthesis via methanol dehydration: effect of zeolite structure[END_REF]:

            t e C C 1 (1)
where C is the specific coke amount (mg coke /g cat ), C ∞ the plateau value and τ the characteristic time (h). From this equation, the initial coke deposition rate r 0 (mg coke /(g cat •h)) can be estimated as:

     C dt dC r t 0 0 (2)
The experimental data were fitted via a nonlinear regression by using a commercial software (Curve Expert Professional 2.0) and the model parameters values are summarized in Table 4. The calculated kinetic parameters suggest that the crystal size strongly affects the kinetics of coke formation, in terms of either carbon amount or deposition rate. In fact, by decreasing the crystal size both C ∞ and r 0 are reduced as a consequence of the utilization of nanosized zeolite in methanol-to-DME dehydration.

In order to establish the composition of the deposited carbon, a coke extraction procedure was carried out following the method developed by Guisnet and Magnoux [START_REF] Guisnet | Organic chemistry of coke formation[END_REF]. No insoluble carbon particles were observed, suggesting that all the carbonaceous deposits can be referred to "soluble coke" which can be analyzed via GC-MS technique.

Figure 8 reports the GC-MS spectra for the extracted carbonaceous deposits for the samples after 5 h, 15 h and 60 h of reaction. This analysis revealed that no polyaromatics were detected and the coke-forming compounds are poly-substituted benzenes with a grade of substitution depending on reaction time and crystal size. Over the micron-sized ferrierite crystal (M-FER), the coke is manly composed from dimethyl benzene (Di-MB) and tetra-methyl benzene (Tetra-MB) with traces of hexamethyl benzene (Hexa-MB). The same identity of coke molecules are detected for NC-FER, whereas NP-FER only forms Tetra-MB.

Di-and Tetra-MB can be accommodated in ferrierite cages, whilst bulkier molecules, such as Hexa-MB, are most probably formed on the external surface [START_REF] Olsbye | Conversion of methanol to hydrocarbons: how zeolite cavity and pore size controls product selectivity[END_REF][START_REF] Liu | Computational screening of zeolite catalysts for MTO reaction[END_REF]. The formation of Hexa-MB on the NC-FER sample might be related to the crystal morphology of this sample. The clustering of nano-crystals may induce inter-crystalline mass transfer limitations, higher retention time, and thus increase the probability of a transformation of precursors molecules into Hexa-MB. Hexa-MB is also detected in M-FER, where stacking along the pinacoidal plane is largely observed. In contrast the small single crystals of the NP-FER sample ensure a fast release of coke precursors from the crystal surface. Consequently no Hexa-MB is observed. As the reaction time increases, Tetra-MB becomes the main observed carbonaceous species for NP-FER, while larger M-FER and NP-FER crystals contain tetra-MB and di-MB molecules.

Figure 9 presents the DTA profiles for the series of catalysts after 5 h, 15 h and 60 h reaction at 240 °C. The exothermic peaks are related to the combustion of carbonaceous species, revealing that this effect occurs at different temperatures and depends on both, the reaction time and crystal size. This is in agreement with the earlier discussed GC-MS analysis as, when Hexa-MB is present, which is the case after 5 h of reaction for NC-FER and M-FER samples, a combustion peak is present at a relatively low temperature (i.e. 285°C and 274 °C, respectively). This peak can then be associated with the combustion of Hexa-MB located on the external surface of the crystals. In fact, when Hexa-MB is absent, the combustion phenomena take place at a higher temperature and it can be attributed to the thermal degradation of Di-or Tetra-MB molecules trapped within the micropores. On the other hand, if Tetra-MB is the only present specie, as in the case of NP-FER after 15 h reaction, the exothermic peaks appear at ca. 344 °C, 435°C and 590 °C suggesting different locations and force of interaction between the molecule and FER framework.

On the basis of DTA profiles, the carbon deposit can be classified as low temperature coke (LTC) and high temperature coke (HTC) associated to carbon deposit burning below or above 500 °C, respectively. According to this classification, it is possible to calculate the amount of LTC and HTC from thermogravimetric analysis and assess the effect of reaction time on HTC/LTC. Figure 10 shows that HTC/LTC ratio increases as a function of reaction time in the following order M-FER>NP-FER>NC-FER. The data unambiguously show that the decrease of crystal size leads to reduction of HTC.

Consequently, the deposited carbon can be eliminated by using milder regeneration conditions. In order to gain a better understanding of the location of the carbonaceous species, SEM-EDX analysis was performed (Figure 11). The EDX mapping clearly reveals that the external coke is homogenously distributed on NC-FER crystals while large carbonaceous agglomerates form on zeolites with larger crystals. This data also confirm that the zeolite crystal size/morphology play a fundamental role in coke deposition in terms of amount, composition and distribution.

CONCLUSIONS

The vapour-phase dehydration of methanol to dimethyl ether on FER-type zeolite crystals was studied. The set of experimental data show that the decrease of crystal size from 5-10 µm to 100 nm leads to an increase of methanol apparent turnover frequency due to reduced intracrystalline mass transfer limitation and improved acid sites accessibility. At 240 °C DME selectivity increases from 0.96 to 0.98 by reducing the crystal size from 5-10 µm to 300-500 nm whereas no by-products were detected for smaller nano crystals., By-products distribution analysis indicated that methane is preferably formed foron catalysts with higher Lewis sites concentration. The thorough assessment of the carbon deposition process revealed that: (i) The coke consists of polymethyl benzene (PMB) molecules whose composition depends on both the zeolite crystal morphology and the reaction time. The formation of Hexa-MB is observed during the first 5 h for samples with intracrystalline voids, while only Di-MB and Tetra-MB are observed for longer reaction time. The relative Di-MB/Tetra-MB ratio decreases with the reduction of crystal size. Furthermore, (ii) the coke deposition can be reduced and slowed down in rate by using zeolites with smaller crystals and (iii) the regeneration of nano-sized zeolite catalysts can be performed at a lower temperature. As a general conclusion, this work showed that the nano-sized FER-type crystals are a much more efficient catalyst in methanol dehydration to DME with respect to their micron-sized counterparts. ()

ACKNOWLEDGMENTS TABLES CAPTIONS

M-FER 73.9 ± 0.9 2.9 ± 0.

  flow rate (e.g. expressed in µmol/h), MeOH x the methanol conversion grade (dimensionless) and B and L are Brønsted and Lewis acid sites

FIGURES CAPTIONSFigure 1 -

 1 FIGURES CAPTIONS

Figure 2 -

 2 Figure 2 -SEM images of studied samples.

Figure 3 -

 3 Figure 3 -Nitrogen adsorption/desorption isotherms of studied samples.

Figure 4 -

 4 Figure 4 -FT-IR spectra of studied samples after evacuation at 723 K for 4h (A) and

Figure 5 -

 5 Figure 5 -Difference FT-IR spectra of studied samples after saturation with 2,6-

Figure 6 -

 6 Figure 6-Methanol conversion as a function of reaction temperature.

Figure 7 -

 7 Figure 7-Coke content in zeolite catalysts deposited at 240°C as a function of TOS. .

Figure 8 -

 8 Figure 8 -GC-MS spectra of carbonaceous deposit extracted from soluble coke of

Figure 9 -

 9 Figure 9 -DTA profiles ofstudied samples after 5 h, 15 h and 60 h reaction at 240 °C.

Figure 10 -

 10 Figure 10 -HTC/LTC ratio as a function of reaction time.

Figure 11 -

 11 Figure 11 -EDX mapping images of studied samples after 60 h reaction at 240 °C.

Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6

 123456 Figure 1

Figure 7 Figure 8

 78 Figure 7

Figure 9 Figure 10 Figure 11

 91011 Figure 9

Table 1 -

 1 Physicochemical characteristics of the series of studied zeolite samples.

Table 2 -

 2 Acidic properties of the series of studied samples.

Table 3 -

 3 Apparent turnover frequency at 180 °C, DME selectivity at 240°C and 280 °C, and by-products distribution at 280 °C

Table 4 -

 4 Coke formation: kinetic parameters for 1 st order model.

Table 1 Sample

 1

			Al/(Si+Al)
		Average crystal size		(mol%)		Al EF c	S BET d	V mic	e
					AS/AC ratio
		(µm)	In crystal	On surface (AS) b		(%)	(m 2 /g)	(m 2 /g)
			(AC) a
	M-FER	5-10	9.6	11.4	1.2	14	332	0.134
	NP-FER	0.3-0.5	10.4	13.0	1.3	17	314	0.125
	NC-FER	0.1	9.4	13.3	1.4	21	304	0.122

a Determined by AA b Determined by EDX c Percentage of aluminium in octahedral coordination estimated via 27 Al-NMR d BET surface area e Micropore volume calculated by t-plot method a Determined by FT-IR after adsorption of d 3 -acetonitrile; b Determined by FT-IR after adsorption of 2,6-dimethyl pyridine; c Determined by FT-IR after adsorption of tert-butylnitrile.

Table 3 SAMPLE TOF app at 180 °C (h -1 )

 3

	DME Selectivity	By-products loop distribution at 280 °C
	(%)		(%)
	240 °C 280 °C	Methane	Ethene	Propene

The authors gratefully acknowledge Dr. Pietro Argurio (University of Calabria) for chemical analysis via atomic absorption spectroscopy and Dr. Paola Lanzafame and Dr.

Georgia Papanikolaou (University of Messina) for chemical analysis via EDX.


Shichen Yan

Xinyu Guan

Hui Li

Daohao Li

Ming Xue

Yushan Yan

Valentin Valtchev

Shilun Qiu

Qianrong Fang

Three-Dimensional Salphen-Based Covalent Organic Frameworks as Catalytic Antioxidants

The development of three-dimensional (3D) functionalized covalent organic frameworks (COFs) is of critical importance for expanding their potential applications. However, the introduction of functional groups in 3D COFs remains largely unexplored. Herein we report the first example of 3D Salphenbased COFs (3D-Salphen-COFs) and their metal-containing counterparts (3D-M-Salphen-COFs), the later being further used as catalytic antioxidants. These Salphen-based COFs exhibit high crystallinity and specific surface area in addition to excel

lent chemical stability. Furthermore, the Cu(II)-Salphen COF displayed high activity in the removal of superoxide radicals. This study not only presents a new pathway to construct 3D functionalized COFs, but also promotes their applications in biology and medicine.

Covalent organic frameworks (COFs), an emerging class of crystalline porous materials, are composed of pure organic skeletons connected by strong organic bonds. [1][2][3][4][5] . Since the pioneering work of Yaghi in 2005, various COFs with different backbones and functional groups have been synthesized and employed as platforms for applications such as gas adsorption and separation, [6][7][8] optoelectronics, 9,10 heterogeneous catalysis, [11][12][13] and a number of others. [14][15][16][17][18][19][20] Yet most of the reported COFs are two-dimensional (2D) frameworks with eclipsed stacking structures. 21,22 Threedimensional (3D) COFs have recently attracted much attention due to their unique porous structures and outstanding performances. [23][24][25][26][27][28] We have reported a series of 3D COFs by the meticulous design of building units, including 3D ionic COFs for selective ion exchange, 29 3D ionic liquid containing COFs with high gas separation ability, 30 3D carboxy-functionalized COFs for selective extraction of lanthanide ions, 31 and so on. [32][33][34] It must be noted, however, that 3D COFs are still limited so far, and thus the development of 3D COFs, especially 3D functionalized COFs, is important for improving their structural diversity and widening their potential applications.

Herein, we addressed the above issues by designing 3D Salphen-based COFs (3D-Salphen-COFs) and their corresponding metal-containing isostructures (3D-M-Salphen-COFs). The obtained COFs showed high crystallinity, open channels, good chemical stability, and large specific surface area. Furthermore, various metal ions (Cu(II), Mn(II) and Eu(III)) were successfully incorporated into the Salphen-based COFs, as the Cu(II)-Salphen COF exhibited the best performance in eliminating deleterious oxygen-derived free radicals. To the best of our knowledge, this study is the first case of 3D Salphen-based COFs and the application of COF materials as catalytic antioxidants. Our strategy for constructing 3D-Salphen-COFs embraces the form of both COF frameworks and Salphen units synchronously. As shown in Scheme 1a, 2-hydroxybenzaldehyde (HBA) and 4,5dichlorophenylene-1,2-diamine (DCPDA) afford the model Salphen unit, which can be further assembled into Mn(II)-Salphen unit via the complexation (see the Supporting Information, SI, Section S1). On the basis of this reaction, we synthesized a tetrahedral salicylaldehyde-based monomer, tetrakis(3-formyl-4hydroxylphenyl)methane (TFHPM). By the condensation of TFHPM and amino-based linkers, 4,5-difluorophenylene-1,2diamine (DFPDA) or DCPDA, two extended 3D Salphen-based COFs, denoted as JUC-508 and JUC-509 respectively, can be obtained (Scheme 1b). Furthermore, JUC-509 can be gathered into 3D-M-Salphen-COFs by the introduction of Mn(II), Cu(II) or Eu(III) ions, denoted as JUC-509-Y (Y = Mn, Cu or Eu, Scheme 1c). Based on this design, TFHPM can be defined as a 4connected node, and DFPDA or DCPDA can act as a 2-connected node, which results in the 3D network with the noninterpenetrated dia topology due to the steric effect of Salphen unit (Scheme 1d). 35 Typically, 3D-Salphen-COFs were synthesized by suspending TFHPM with DFPDA or DCPDA in the mixed solvent of dioxane and mesitylene in the presence of acetic acid followed by heating at 120 °C for 3 days. Complementary characterizations have been employed for detailed structural definition of new materials. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of 3D-Salphen-COFs showed flowerlike microstructure composed of prismatic several micrometers long crystals (Figures S1-S4). Peaks around 1616 cm -1 for JUC-508 and 1618 cm -1 for JUC-509 in Fourier transform infrared (FTIR) spectra indicated the formation of imine linkages, and the disappearance of peaks at 1658 cm -1 for both 3D-Salphen-COFs confirmed the complete transformation of aldehyde groups, which is similar to that from the model Salphen unit (Figures S5 andS6). The solid state 13 C cross-polarization magic-angle-spinning (CP/MAS) NMR spectra further confirmed the presence of carbon from the C=N bond at 163 ppm for JUC-508 and 161 ppm for JUC-509 (Figures S8 andS9). According to the thermogravimetric analysis (TGA), both 3D-Salphen-COFs are stable up to about 350 °C under nitrogen (Figures S10 andS11). Notably, their crystalline structures can be preserved in the aqueous solutions at pH = 1~14 (Figures S15 andS16). The crystal structures of 3D-Salphen-COFs were resolved by the powder X-ray diffraction (PXRD) measurements in conjunction with structural simulations (Figure 1). After a geometrical energy minimization by using the Materials Studio software package 36 213) and (204) Bragg peaks of space group I-42d, respectively. The refinement results matched well with the observed patterns with good agreement factors (Rωp = 6.88% and Rp = 3.59% for JUC-508; Rωp = 8.72% and Rp = 5.46% for JUC-509). We also considered the alternative structures based on the 2fold interpenetrated dia net from the same space group of I-42d for both 3D-Salen-COFs; however, their simulated patterns did not match to the experimental ones (Figures S17-20). On the basis of the above results, these Salphen COFs were proposed to have the expected architectures with non-interpenetrated dia topology, which show microporous cavities with a diameter of about 1.2 nm (Figure 2 and Figure S21). Remarkably, almost all of 3D COFs with dia topology show multifold interpenetrated structures, and thus the successful preparation of 3D-Salphen-COFs with noninterpenetrated nets will benefit the development of 3D COFs with large pores. The porosity and surface areas were measured by N 2 adsorption and desorption analysis (Figure 3). Both materials show a sharp uptake at a low pressure of P/P 0 < 0.05, which is characteristic of the microporous material. The inclination of isotherms in the 0.8-1.0 P/P 0 range and slight desorption hysteresis show the presence of textural mesopores, which is the consequence of the agglomeration of COF crystals. 34 The Brunauer-Emmett-Teller (BET) equation was applied over the 0.05 < P/P 0 < 0.30 range of the isotherms, revealing BET surface areas of 1513 m 2 g -1 for JUC-508 and 1443 m 2 g -1 for JUC-509 (Figures S25 andS26). Pore size distributions of 3D-Salphen-COFs calculated by nonlocal density functional theory (NLDFT) showed pore with sizes of 1.3 nm for JUC-508 and 1.2 nm for JUC-509, respectively (Figure 3, inset), which is in good agreement with that of the proposed model (1.2 nm).

Encouraged by their high surface area and abundant Salphen units, a series of 3D-M-Salphen-COFs were subsequently acquired by immersing JUC-509 in a methanol solution of relative metal salts, giving JUC-509-Y (Y = Mn, Cu or Eu). The metal complexation of Salphen COFs could be easily visualized by their color change (Figure S28). Meanwhile, scanning electron microscope-energy dispersive X-ray spectra (SEM-EDS) analysis exhibited the uniform distribution of metal ions in the crystals (Figure S29). The ICP analysis of the resulting M-Salphen COFs showed that most of the Salphen pockets have been occupied by the metal ions (see Section S1 in SI). FTIR spectroscopy indicated the strong interaction between Salphen-COF and metal ions based on the shift of the IR bands for the imine groups from 1618 cm -1 (JUC-509) to 1608 cm -1 (JUC-509-Cu), 1606 cm -1 (JUC-509-Mn) or 1606 cm -1 (JUC-509-Eu), which is similar with the results from the model Salphen unit to Mn(II)-Salphen unit (from 1614 cm -1 to 1603 cm -1 , Figure S7). X-ray photoelectron spectroscopy (XPS) suggested Cu and Mn were in the +2 oxidtion state and Eu was in the +3 oxidation state (Figures S30-32). In addition, the retained PXRD patterns of 3D-M-Salphen-COFs pointed out that the crystalline structure was preserved after the metal incorporation (Figure 2 and Figures S22-24). The porosities were evaluated by N 2 adsorption and desorption analysis, which showed values slightly lower than that of the pristine material (Figure S27). We further explored the application of M-Salphen COFs in the removal of superoxide radical anion (O 2

•-). It is well known that the overproduction of O 2

•-has been shown to contribute to tissue damage and injury. 37 Salens or Salphens with redox-active metal centers are considered as major catalytic antioxidants that catalyze the dismutation reaction of O 2

•-similar to superoxide dismutases (SODs). 38 However, the employments of traditional Salphens are strongly limited because of stability and recyclability issues. Herein, 3D-M-Salphen-COFs were chosen as effective oxygen radical removal agents due to their high surface areas, chemical stability and reusability.

Typically, O 2 •-was generated under sunlight in the presence of methionine and riboflavin, and further reacted with nitrotetrazolium blue chloride (NBT) to form a deep blue complex, which exhibited a characteristic absorption at about 560 nm in the UV-Vis spectroscopy. 39 As shown in Figure 4, 3D-M-Salphen-COFs had different activity in the elimination of superoxide radicals accompanied by the change of catalyst concentration. Among them, JUC-509-Eu had no obvious antioxidant activity, and JUC-509-Cu displayed the best performance (almost 100% clearance rate in only 0.875 mg/ml), which can be attributed to the high catalytic effect of Cu(II) ion. 40 Furthermore, their cyclic voltammetric (CV) measurements demonstrated that JUC-509-Cu showed the best redox process in 3D-M-Salphen-COFs (Figures S33-35). As a comparison, the metal-free pristine material, JUC-509, was tested under the same condition, and showed no evidence for the removal of O 2

•-(Figure 4d and4e), which further demonstrated the critical role of redox-active metal centers in antioxidant activity. ICP analysis of the filtrate after the reaction revealed almost no leaching of metal ions (see Section S1 in SI). PXRDs showed that the recovered solid remained crystalline and structurally intact (Figures S22-24). Most importantly, these M-Salphen COF catalysts can be readily recycled and reused at least three times without lost of activity (Figure 4f).

In conclusion, we have for the first time designed and synthesized 3D-Salphen-COFs and 3D-M-Salphen-COFs. These Salphen-based COFs showed high crystallinity, BET surface area, and chemical stability. Furthermore, the Cu(II)-Salphen COF displayed high performance in the removal of superoxide radicals and could be reused without significant activity loss. 3D Salphenbased COFs successfully prepared in this work may not only open a new pathway to create 3D functionalized COFs, but also expand the applications of COF materials in biology and medicine.

ASSOCIATED CONTENT Supporting Information

Synthetic procedures, SEM, TEM, FTIR, solid state 13 C NMR, TGA, and BET plots. This material is available free of charge via the internet at http://pubs.acs.org.

Scheme 1 .

 1 Scheme 1. Strategy for preparing 3D-Salphen-COFs a

Figure 1 .

 1 Figure 1. PXRD patterns of JUC-508 (a) and JUC-509 (b).

Figure 2 .

 2 Figure 2. Structural representations of JUC-509 (a and b) and metal-containing JUC-509 (c and d).

  based on non-interpenetrated dia net for 3D-Salphen-COFs, the unit cell parameters were obtained (a = b = 19.153 Å, c = 41.988 Å and α = β = γ = 90° for JUC-508; a = b = 18.983 Å, c = 42.202 Å and α = β = γ = 90° for JUC-509). Furthermore, full profile pattern matching (Pawley) refinements were carried out on the experimental PXRD patterns. Peaks at 5.41, 7.19, 8.02, 10.31, 10.84 and 11.65° for JUC-508 correspond to the (101), (103), (112), (200), (202) and (211) Bragg peaks of space group I-42d (No. 122), respectively; peaks at 5.10, 7.80, 8.37, 9.31, 1021, 12.17 and 12.53° for JUC-509 correspond to the (101), (112), (004), (200), (202), (

Figure 3 .

 3 Figure 3. N 2 adsorption-desorption isotherms of JUC-508 (a) and JUC-509 (b). Inset: pore-size distribution calculated by fitting on the NLDFT model to the adsorption data.

Figure 4 .

 4 Figure 4. Adsorption spectra of JUC-509-Cu (a), JUC-509-Mn (b), JUC-509-Eu (c) and JUC-509 (d) in antioxidant secestration experiments. (e) The clearance rate based on different concentration. (f) Recyclability study of JUC-509-Cu, JUC-509-Mn, JUC-509-Eu and JUC-509.

ACKNOWLEDGMENT

This work was supported by National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076, and 21571078), 111 project (B07016 and B17020), Guangdong and Zhuhai Science and Technology Department Project (2012D0501990028), and the program for JLU Science and Technology Innovative Research Team. Q.F and V.V. acknowledge the support from Thousand Talents program (China).

AUTHOR INFORMATION

Corresponding Author *qrfang@jlu.edu.cn

Notes

The authors declare no competing financial interests.