Ahmed Ould Saleck

C Mercier

C Follet

Olivier Mentré

Abderrazzak Assani

M Saadi

L El Ammari

Synthesis, crystal structure and magnetic behavior of a new calcium magnesium and iron orthophosphate Ca2MgFe2(PO4)4

Introduction

The orthophosphate compounds have received a great attention owing to its structural richness. All structures of those compounds are based on the connection of PO4 tetrahedral groups with MOn polyhedra (M generally represents transition and /or alkaline earth metals).

According to its structural type and the nature of the metal cations, many physical and chemical properties can be observed for these class of materials such as: electrochemical [START_REF] Trad | Phosphates de fer de structures originales comme matériaux d'électrode positive pour batterie au lithium ou au sodium[END_REF][START_REF] Kim | LiFePO4 with an alluaudite crystal structure for lithium ion batteries[END_REF][START_REF] Kim | Alluaudite LiMnPO4 : a new Mn-based positive electrode for Li rechargeable batteries[END_REF][START_REF] Essehli | Alluaudite Na2Co2Fe(PO4)3 as an electroactive material for sodium ion batteries[END_REF][START_REF] Masquelier | Polyanionic (Phosphates , Silicates , Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries[END_REF], magnetic [START_REF] Riou-Cavellec | Magnetic iron phosphates with an open framework[END_REF][START_REF] Hadouchi | Synthesis, Crystal Structure and Properties of a New Phosphate, Na2Co2Cr(PO4)3[END_REF][START_REF] Harbaoui | Synthesis and Structural, Electrical, and Magnetic Properties of New Iron-Aluminum Alluaudite Phases β-Na2Ni2M(PO4)3 (M = Fe and Al)[END_REF] and catalytic properties [START_REF] Miller | Oxidative dehydrogenation of ethane over iron phosphate catalysts[END_REF][START_REF] Moffat | Phosphates as catalysts[END_REF][START_REF] Kacimi | Structural features of AgCaCdMg2(PO4)3 and AgCd2Mg2(PO4)3, two new compounds with the alluaudite-type structure, and their catalytic activity in butan-2-ol conversion[END_REF]. Moreover, It is well known that the orthophosphate compounds doped by the rare-earth elements exhibit a very interesting luminescence properties [START_REF] Yang | phosphate phosphor Ba3Y(PO4)3 : Sm 3+[END_REF][START_REF] Pelova | Luminescence of alkali-earth phosphates activated with manganese and cerium[END_REF][START_REF] Bedyal | Effects of cationic substitution on the luminescence behavior of Dy 3+ doped orthophosphate phosphor[END_REF][START_REF] Donker | On the Luminescence of Some Tin-Activated Alkaline-Earth Orthophosphates[END_REF].

In this context, our focus of interest is axed on the tetra-orthophosphate based materials. On the basis of our bibliographic analysis, those phosphates can be represented with the general formula, M3M'3(PO4)4, A2(M/M')5(PO4)4 or A3M"3(PO4)4 (A, M/M' and M" represent mono-, bi-and trivalent cations respectively). Accordingly, it has been found that the members of the M3M'3(PO4)4 phosphates group, M3Cu3(PO4)4 (M = Ca, Sr and Pb) and Ca3Mg3(PO4)4, crystallize in the monoclinic system with different space groups [START_REF] Effenberger | Sr3Cu3(PO4)4, Pb3Cu3(PO4)4, BaCu2(PO4)2• H2O, and Ba2Cu(PO4)2• H2O : Crystal Structures and Topological Relationships[END_REF][START_REF] Drillon | 1D ferrimagnetism in copper(II) trimeric chains: Specific heat and magnetic behavior of A3Cu3(PO4)4 with A = Ca, Sr[END_REF][START_REF] Anderson | The Crystal Structure of Ca3Cu3(PO4)4[END_REF][START_REF] Wu | Synthesis and color-tunable luminescence properties of Eu 2+ and Mn 2+ -activated Ca3Mg3(PO4)4 phosphor for solid state lighting[END_REF]. However, the tetraorthophosphate A2(M/M')5(PO4)4, namely Na2Cd5(PO4)4 [START_REF] Ben Hamed | Synthesis , crystal structure and spectroscopic characterization of a new cadmium phosphate , Na2Cd5(PO4)4[END_REF], Na2Ca5(PO4)4 [START_REF] Celotti | A misunderstood member of the nagelschmidtite family unveiled : structure of Ca5Na2(PO4)4 from X-ray powder diffraction data[END_REF], Na2Mg5(PO4)4 [START_REF] Yamakawa | Un monophosphate de magnésium et de sodium[END_REF], Na2Zn5(PO4)4 [START_REF] Ji | A new structure type of phosphate : Crystal structure of Na2Zn5(PO4)4[END_REF], Li2Cu5(PO4)4 [START_REF] Cui | Synthesis , crystal structure and optical properties of Li2Cu5(PO4)4[END_REF] and Cs2Zn5(PO4)4 [START_REF] Guo | Cs2Zn5(PO4)4 and Cs3La(PO4)2: Two Cs-Containing Phosphates with Three-Dimensional Frameworks[END_REF], known till now, represent a structurally rich group since those phosphates adopt different crystal structure in triclinic, monoclinic, quadratic or hexagonal systems. Moreover, a great deal of interest has been accorded to the third tetra-orthophosphate group, K3Cr3(PO4)4 [START_REF] Kouass | Tripotassium Trichrome (III) Tetraphosphate K3Cr3(PO4)4: Synthèse, Étude Structurale, Caractérisation et Conductivité Ionique, Phosphorus. Sulfur[END_REF], Na3Fe3(PO4)4 [START_REF] Trad | A Layered Iron(III) Phosphate Phase, Na3Fe3(PO4)4 : Synthesis, Structure, and Electrochemical Properties as Positive Electrode in Sodium Batteries[END_REF] and Na3V3(PO4)4 [START_REF] Liu | Novel 3.9 V Layered Na3V3(PO4)4 Cathode Material for Sodium Ion Batteries[END_REF], which crystallize in the orthorhombic and monoclinic system respectively, owing to its interesting properties in the electrochemistry applications as positive electrodes for sodium batteries [START_REF] Trad | A Layered Iron(III) Phosphate Phase, Na3Fe3(PO4)4 : Synthesis, Structure, and Electrochemical Properties as Positive Electrode in Sodium Batteries[END_REF][START_REF] Liu | Novel 3.9 V Layered Na3V3(PO4)4 Cathode Material for Sodium Ion Batteries[END_REF]. Besides, it is worth to mention the rarely encountered tetra-phosphates: (i) the bi-and trivalent cations based tetraphosphates Pb3Fe2(PO4)4 [START_REF] Sqalli | Synthesis, crystal structure and magnetic properties of Fe2Pb3-xBax(PO4)4 (0≤x<3)[END_REF] and Ba3Bi2(PO4)4 [START_REF] Masse | Structure of tribarium dibismuth tetrakis(phosphate)[END_REF] both adopt a monoclinic symmetry and (ii) the only tetra-orthophosphate NaCaFe3(PO4)4, grouping mono-, di-and trivalent cations which crystallizes in the orthorhombic system [START_REF] Hidouri | Crystal structure, magnetic properties and Mossbauer spectroscopy of[END_REF].

In the present work, we aim to develop the synthesis, the crystal structure determination from single crystal x-ray diffraction data and magnetic properties of a new calcium, magnesium and iron phosphate, namely Ca2MgFe2(PO4)4.

Experimental

Synthesis

Both single crystal and powder of the title phosphate Ca2MgFe2(PO4)4 were synthesized from stoichiometric mixtures of CaCO3 (Fisher Chemical 98%), MgO (Fisher Chemical 95%), Fe2O3 (Fisher Chemical 99.99%) and NH4H2PO4 by the solid-state reaction. Two stoichiometric mixtures were progressively heated up in a platinum crucible at 300°C, 400°C, 500°C and 600°C. The first product was melted at 980°C and maintained at this temperature during 10 min before being slowly cooled to 300°C at a rate of 5°C/h. The final product consisted of transparent single crystals of the title compound. While, the second mixture has undergone heat treatments up to 700 °C where it is kept for 6 days to obtain the Ca2MgFe2(PO4)4 pure powder.

Experimental Techniques

The Ca2MgFe2(PO4)4 structure was determined at room temperature by single crystal X-ray diffraction using a four-circles diffratometer Bruker X8 equipped with an Apex II CCD detector and graphite monochromator for Mo Kα radiation, λ = 0.71073 Å. The software APEX2 was used for data collection and SAINT for cell refinement and data reduction.

Absorption corrections were made using the SADABS program [START_REF] Sheldrick | SADABS version 2004/1, A Program for Empirical Absorption Correction[END_REF] from equivalent reflections on the basis of multiscans. The crystal structure was solved using direct method and refined by SHELXT 2013 [START_REF] Sheldrick | SHELXT -Integrated space-group and crystal-structure determination[END_REF] and SHELXL 2013 [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] programs incorporated in the WinGX [START_REF] Farrugia | WinGX and ORTEP for Windows: an update[END_REF] program. For structural drawing, DIAMOND [START_REF] Putz | DIAMOND-Crystal and molecular structure visualization. Crystal Impact-GbR[END_REF] was used. The X-ray diffraction diagram (XRD) of Ca2MgFe2(PO4)4 powder was collected using Panalytical X'pert Pro diffractometer, with Bragg-Brentano geometry (-2) and a X-ray tube with a copper anticathode (K1 = 1.54056 Å ; K2 = 1.54441 Å). The diffraction pattern was recorded in 2 = 10 ° -100 ° angular range with a measurement step of 0.02 ° and a counting time of 30 s pro step. The Ca2MgFe2(PO4)4 powder XRD diagram was refined using pattern matching method with Jana 2006 program [START_REF] Petrícek | Crystallographic computing system JANA2006: General features[END_REF]. The Infrared spectrum of Ca2MgFe2(PO4)4 was recorded between 400 and 4000 cm -1 region using the JASCO FT/IR-4600 spectrometer. The spectrum measurement is made on pellet prepared from a mixture of 1% of Ca2MgFe2(PO4)4 in KBr. The susceptibility versus temperature (H = 0.05 and 0.1 T) and magnetization versus field (at various temperatures) data have been collected on a polycrystalline sample between 1.8 K and 400 K until H = 9T, using a PPMS Dynacool (Quantum Design) system. Zero field cooled and Field Cooled routines have been employed for (T).

Results and discussion

Crystal structure determination

The crystal structure of Ca2MgFe2(PO4)4 was determined from single crystal XRD data in the orthorhombic system with the Pbca space group and the unit cell parameters: a = 9.3357(7) Å ; b = 8.7340(6) Å and c = 29.422(2) Å. The Fe1, Fe2, Ca1, Ca2, Mg1 , P1, P2, P3 and P4 atoms are localized using the direct method by means of SHLEXS97 program [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF]. After the refinement of those cationic positions, sixteen O 2-anions have been localized in a Fourier difference map and refined by the least square method. All these ions are in fully occupied general positions (8c) of the Pbca space group. However, the structure presents a disorder in two cationic sites. Indeed, the Fe2 site is shared by 75% of iron and 25% of magnesium atoms, so called Fe2/Mg1 site. The Mg1 site is occupied by 75% of Mg and 25 % of Fe, so named Mg2/Fe3 site. The validity of this model is corroborated by the final reliability factors obtained: R = 1.9 % and wR = 4.8 % with S = 1.08, in accordance with the crystallographic sites occupation and the charge neutrality. Crystal data and structure refinement parameters for Ca2MgFe2(PO4)4 are summarized in Table 1. Atomic coordinates, occupancies and equivalent isotropic atomic displacement parameters, anisotropic displacement parameters for the studied phosphate are detailed in Table 2 andTable 3.

The Fe1 atom is surrounded by six oxygen atoms to form an octahedral environment, Fe1O6, with the Fe1-O distances ranging from 1.973(1) Å to 2.079(1) Å as shown in Table 4. The Fe2/Mg1 site presents a rectangular-based pyramidal geometry (Fe2/Mg1)O5. Significant Fe2/Mg1-O distances are summarized in Table 4. Five oxygen atoms surround the magnesium and iron atoms localized on the Mg2/Fe3 site giving arise to triangular based bipyramidal geometry (Mg2/Fe3)O5. The Mg2/Fe3-O distances are extending from 1.991(1) Å to 2.050 (1) Å (Table 4) and the O1 -Mg2/Fe3-O13 angle is equal to 169.39(6) °.

Two of each polyhedron P2O4, P3O4, (Fe1)O6 and (Mg2/Fe3)O5 consecutively share vertices so as to construct the first kind of ring (Fig. 1a). Each ring is surrounded by eight other identical rings to form the first type of sheet in the (a, b) plane (Fig. 1b). A second kind of ring is made from the alternated connection of four P4O4 and four rectangular-based pyramid (Fe2/Mg1)O5 by means of corners (Fig. 1c). This latter ring is interconnected to eight other like rings to build a second type of sheet parallel to the plane (a, b) (Fig. 1d).

Two consecutive first kind layers are connected to each other with the P1O4 tetrahedra so as to build double layers (Fig. 2a). Those double layers are connected by the second type of sheet (Fig. 2b) leading to a three-dimensional network delimiting cavities occupied by Ca 2+ cations (Fig. 3).

Based on our bibliographic analysis and to our knowledge, the present compound, Ca2MgFe2(PO4)4, shows an original structure which is similar to that of the only orthophosphate cited in the literature NaCaFe3(PO4)4 [START_REF] Hidouri | Crystal structure, magnetic properties and Mossbauer spectroscopy of[END_REF]. However, the most important difference between the two structures lies in the disorder of the two cationic sites in the structure of the studied phosphate.

Powder X-ray diffraction analysis

The powder of the title phosphate was obtained after heat treatment at 700 °C during 6 days.

Its corresponding XRD patterns profile was refined using "Le bail Method" [START_REF] Bail | Whole powder pattern decomposition methods and applications: A retrospection[END_REF] of Jana 2006 program [START_REF] Petrícek | Crystallographic computing system JANA2006: General features[END_REF]. The success of the refinement is attested by the good accordance between the observed (Yobs) and the calculated (Ycalc) patterns as shown in Fig. 4. The final reliability factors obtained for this powder model are Rp = 1.59, Rwp = 2.69 and S =1.81.

Infrared spectroscopy (IR)

Fig. 5 shows the infrared spectrum of Ca2MgFe2(PO4)4. The observed absorption bands at 1148 cm -1 , 1084 cm -1 and 1043 cm -1 can be assigned to ν3 of P-O antisymmetric stretching mode. Besides, the band situated at 1222 cm -1 can be likely assigned to the ν3 mode of the shorter P-O. The band at 980 cm -1 is associated to ν1 of P-O symmetric stretching mode. The absorption bands at 646 cm -1 , 603 cm -1 , 555 cm -1 and 552 cm -1 can be assigned to the ν4 symmetric modes. The peak around 473 is attributed to ν2 mode. The presence of only isolated tetrahedra (PO4 3-) is confirmed by the absence of absorption bands between 700 cm -1 and 900 cm -1 which are the characteristic of the P-O-P angular deformation absorptions [START_REF] Cui | Synthesis , crystal structure and optical properties of Li2Cu5(PO4)4[END_REF].

Magnetic properties

The magnetic properties of the title compounds are complicated by both the non-trivial iron connectivity in this dense phase, mostly mediated by PO4 bridges and by the mixed Fe/Mg occupancy within the Fe 3+ (S = 5/2, L = 0) magnetic sublattice. For simplification, we assume here that the main bulk properties are driven by the fully occupied Fe1 and 75% occupied Fe2 sites. Fe3 being 25% occupied, we considered its influence as a local decoration of the main magnetic framework, subject to its influence, but enable to rule out the exchanges in a cooperative manner due to its high degree of dilution in the crystal. The ZFC and FC (T) plots collected at 0.H = 0.05T are given Fig. 6a. The 7K-400K region obeys a Curie weiss (CW) law:  = c/(T -CW) and was fitted by eff = (8C) = 5.83 B/Fe 3+ and CW = -38.5 K.

The effective moment is in perfect agreement with the spin only value for Fe 3+ (5.91 B/Fe 3+ ) which refutes any significant orbital contribution as expected for L = 0 single ions. The negative CW temperature validate predominant validate antiferromagnetic exchanges, as expected by Goodenough-kanamori rules from any kind of Fe 3+ -O-Fe 3+ super-echanges (SE) significantely different from 90° and gor Fe 3+ -O-O-Fe 3+ super-super exchanges (SSE). As a matter of facts SSE paths predominate in Ca2MgFe2(PO4)4 while only the Fe1(100%)-O-Fe3(25%) SE is achieved. One counts 8 and 7 SSE-neighbors around Fe1 and Fe2 respectively, leading to the average J/Kb Fe-Fe coupling of -1.75K using the mean field approximation θCW = Ʃi zi S(S+1) Ji, Fe-Fe/3Kb with z the average number of neighbors (=7.5) and S = 5/2. This weak mean value does not take into consideration the distribution of exchange values in the structure and one should empirically hierachize them as discussed later, for a comprehensive interpretation. Their interaction is responsible for the complex low-T behavior resumed in Fig. 6b which shows: i) a magnetic ordering responsible for the the sharp (T) anomaly at TN = 13.3 K. ii) a second smeared transition at Tt = 5.7 K with appearing at lower temperature of a net magnetic moment, as evidenced by the ZFC/FC divergence. This effect is marked in M(H) plots in the first 0→9T and second 9T→0T branches below TN.

-Here, during the field induced spin alignement (1 st branch), a weak metamagnetic/spin flop transition is observed around Hmeta ≈ 2.7T at 2K as shown from the peak on dM/dH, see Fig. 6b, reminiscent of the closing of a low-field blocked spin configuration. This transition is prohiminent below Tt.

-Decreasing the field (2 nd branch), this contribution is retained above a critical field HC. We found HC ≈ 0.1T at TN-10K, HC ≈ 0.056T at 5K, and HC ≈ 0.012T at 2K assorted with a remanent moment at zero field at the later temperature. This behavior is reminiscent of spin frustration that plausibly occurs as described below.

With the aim to give a plausible scenario, we should examine carefully the SSE's in competition. We checked that the strongest expected ones (i.e. according to the empirical criteria that they display the smallest dihedral/torsion Fe-O-O-Fe angles [START_REF] Whangbo | Spin exchange interactions and magnetic structures of extended magnetic solids with localized spins : theoretical descriptions on formal , quantitative and qualitative levels[END_REF] to enhance the magnetic delocalization across p orbitals) are effective in the (a,c) layers dealing : strong J1(Fe1O-O-Fe1), J2(Fe1-O-O-Fe2) and weakest J3(Fe2-O-OFe2) and J4(Fe2-O-O-Fe2), see Table 5. All together they define non-frustrated pseudo-hexagonal AFM layers, shown Fig. 7 where white and black Fe-sites denote up and down colinear spins with an arbitrary direction. We recall that that the neglectable orbital contribution for Fe 3+ (L = 0) suggests anisotropic magnetic single ions, oriented by spin-lattice coupling (i.e. along specific crystallographic axes) rather than by local d-overlapping [START_REF] Melot | Magnetic Structure and Properties of the Li-Ion Battery Materials FeSO4F and LiFeSO4F[END_REF], such that a collinear spin-structure is presumed. Along the b-axis, the couplings between the layers also occur by SSE exchanges (Fe1-O-O-Fe1 and/or Fe2-O-O-Fe2), that we found in smaller extent in the crystal structure, according to the dense distribution of Fe ions within the (a,c) leaves. Fe3 (25% occ.) weakly perturbate the layers contribution below TN but their contribution turns to significant on further cooling down to Tt or increasing the field until Hmeta. In fact, the 25% of Fe3 are "locally" coupled to Fe1 by strong AFM super-exchanges J5 with Fe3-O-Fe1 = 104.3°, but also coupled to Fe2 by significant SSE J6 exchanges with Fe3-O-O-Fe2 = 6.2°. In Fine, it forms a statistic distribution (25%) of frustrated J2-J5-J6 triangles, shown in red Fig. 6. It is probable that the Fe3 spin orientation is dominated by the SE J5 below TN but should rapidly flop to a frustrated equilibrium position when all JS come in competition below Tt. Such a situation validates an easy field-induced re-alignement, at the metamagnetic transition.

Conclusion

In this paper, we have reported a new calcium magnesium and iron orthophosphate,

Table 5

Geometry of the paths within the main in-plane magnetic exchanges and their estimated strengths

Ca2MgFe2(PO4) 4 ,Fig. 1 :Fig. 2 :Fig. 3 :Fig. 4 : 4 4000Fig. 5 :Fig. 6 :Fe1Fig. 7 :

 412344567 Fig. 1: a) First kind of ring with eight polyhedra. b) First type of sheet. c) Second kind of ring with eight polyhedra; d) Second type of sheet

Fe

Table 1

 1 Crystal data and structure refinement for Ca2MgFe2(PO4)4

	Crystal data

Table 2 :

 2 Atomic coordinates, occupancies and equivalent isotropic displacement parameters (Å 2 ) for

	Ca2MgFe2(PO4)4
	Atom	Occupation	x	y	z	Uéq
	Fe1	1	0.64595 (3)	0.60846 (3)	0.196251 (8)	0.00527 (6)
	Fe2/Mg1	0.75/0.25	0.69856 (3)	0.54472 (3)	0.043350 (9)	0.00511 (6)
	Mg2/Fe3	0.75/0.25	0.32487 (5)	0.77518 (6)	0.12837 (1)	0.0105 (1)
	P1	1	0.35795 (5)	0.81205 (5)	0.23506 (1)	0.00462 (8)
	P2	1	0.45056 (5)	0.44136 (5)	0.11940 (1)	0.00579 (9)
	P3	1	0.88616 (5)	0.41175 (5)	0.13281 (1)	0.00555 (8)
	P4	1	0.40656 (5)	0.73416 (5)	0.01686 (1)	0.00601 (9)
	Ca1	1	1.03281 (4)	0.66505 (4)	0.19113 (1)	0.00787 (7)
	Ca2	1	0.39729 (4)	1.08148 (5)	0.06107 (1)	0.01321 (8)
	O1	1	0.2661 (2)	0.7733 (2)	0.19375 (4)	0.0094 (2)
	O2	1	0.3659 (1)	0.9861 (1)	0.24327 (4)	0.0082 (2)
	O3	1	0.3019 (1)	0.7360 (1)	0.27836 (4)	0.0078 (2)
	O4	1	0.5108 (1)	0.7488 (2)	0.22902 (4)	0.0095 (2)
	O5	1	0.5201 (2)	0.4530 (2)	0.16660 (4)	0.0101 (3)
	O6	1	0.5531 (2)	0.4634 (2)	0.08035 (5)	0.0163 (3)
	O7	1	0.3915 (2)	0.2750 (1)	0.11648 (4)	0.0096 (2)
	O8	1	0.3227 (2)	0.5504 (2)	0.11595 (4)	0.0131 (3)
	O9	1	1.0475 (1)	0.4555 (2)	0.13681 (4)	0.0114 (3)
	O10	1	0.8150 (1)	0.5065 (2)	0.16978 (4)	0.0106 (3)
	O11	1	0.8736 (2)	0.2393 (2)	0.13965 (4)	0.0176 (3)
	O12	1	0.8425 (2)	0.4595 (2)	0.08498 (4)	0.0154 (3)
	O13	1	0.3592 (2)	0.8103 (2)	0.06101 (4)	0.0120 (3)
	O14	1	0.5663 (2)	0.6958 (2)	0.01539 (5)	0.0127 (3)
	O15	1	0.3826 (2)	0.8546 (2)	-0.02110 (4)	0.0127 (3)
	O16	1	0.3081 (1)	0.5947 (1)	0.00867 (4)	0.0095 (2)

Table 3

 3 Anisotropic displacement parameters (Å 2 ) for Ca2MgFe2(PO4)4

	Atom	U 11	U 22	U 33	U 12	U 13	U 23
	Fe1	0.0057 (1) 0.0050 (1) 0.0051 (1) -0.00013 (9)	-0.00009 (8)	-0.00064 (8)
	Fe2/Mg1	0.0050 (1) 0.0058 (1) 0.0046 (1) -0.0001 (1)	-0.0001 (1)	0.00068 (9)
	Mg2/Fe3	0.0135 (2) 0.0113 (2) 0.0067 (2) -0.0051 (2)	0.0016 (2)	-0.0025 (2)
	P1	0.0053 (2) 0.0040 (2) 0.0045 (2) 0.0001 (2)	0.0003 (1)	0.0000 (1)
	P2	0.0059 (2) 0.0059 (2) 0.0055 (2) -0.0014 (2)	0.0001 (2)	0.0006 (1)
	P3	0.0069 (2) 0.0048 (2) 0.0050 (2) 0.0010 (2)	0.0004 (2)	0.0005 (1)
	P4	0.0068 (2) 0.0049 (2) 0.0063 (2) 0.0006 (2)	0.0005 (2)	0.0002 (1)
	Ca1	0.0083 (2) 0.0084 (2) 0.0069 (1) -0.0002 (1)	0.0005 (1)	-0.0003 (1)
	Ca2	0.0144 (2) 0.0131 (2) 0.0121 (2) -0.0029 (2)	0.0006 (14)	-0.0038 (1)
	O1	0.0102 (6) 0.0125 (6) 0.0055 (5) -0.0025 (5)	-0.0016 (4)	-0.0008 (4)
	O2	0.0133 (6) 0.0041 (5) 0.0072 (5) -0.0002 (5)	0.0010 (5)	-0.0001 (4)
	O3	0.0079 (6) 0.0085 (6) 0.0070 (5) -0.0013 (5)	0.0007 (4)	0.0029 (4)
	O4	0.0075 (6) 0.0104 (6) 0.0107 (5) 0.0028 (5)	0.0022 (5)	-0.0003 (5)
	O5	0.0118 (6) 0.0101 (6) 0.0082 (5) -0.0036 (5)	-0.0036 (5)	0.0002 (4)
	O6	0.0153 (7) 0.0215 (8) 0.0121 (6) -0.0055 (6)	0.0075 (5)	-0.0002 (5)
	O7	0.0130 (7) 0.0073 (6) 0.0087 (5) -0.0030 (5)	-0.0001 (5)	-0.0005 (4)
	O8	0.0113 (7) 0.0137 (6) 0.0145 (6) 0.0045 (5)	-0.0019 (5)	-0.0013 (5)
	O9	0.0069 (6) 0.0144 (7) 0.0128 (6) 0.0012 (5)	0.0011 (5)	-0.0045 (5)
	O10	0.0102 (6) 0.0101 (6) 0.0115 (6) 0.0017 (5)	0.0036 (5)	-0.0035 (5)
	O11	0.0379 (9) 0.0058 (6) 0.0091 (6) 0.0003 (6)	0.0045 (6)	0.0007 (5)
	O12	0.0184 (7) 0.0192 (7) 0.0086 (6) 0.0057 (6)	-0.0025 (5)	0.0032 (5)
	O13	0.0162 (7) 0.0110 (6) 0.0087 (6) -0.0001 (5)	0.0029 (5)	-0.0023 (5)
	O14	0.0072 (6) 0.0107 (6) 0.0200 (6) 0.0007 (5)	0.0004 (5)	-0.0008 (5)
	O15	0.0122 (7) 0.0115 (6) 0.0143 (6) 0.0027 (5)	0.0030 (5)	0.0065 (5)
	O16	0.0095 (6) 0.0081 (6) 0.0110 (6) -0.0011 (5)	0.0000 (5)	-0.0007 (5)

Table 4

The bond lengths (Å) for crystal Ca2MgFe2(PO4)4


Kai C Szeto

Nicolas Merle

Julien Trébosc

Mostafa Taoufik

Régis M Gauvin

Frédérique Pourpoint

Laurent Delevoye

Caveat on the Actual Robustness of Heteronuclear NMR Methods for Probing the Surface of γ-Alumina and Related Catalysts

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Introduction

Transitional aluminas, such as -, -or -Al2O3, hold a prominent position as catalysis-related materials. [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] They are currently used in several major industrial processes as catalysts or as catalyst supports. [START_REF] Busca | Surface, and Catalytic Properties of Aluminas[END_REF] In this context where interfacial phenomena are instrumental, it is essential to understand their surface chemistry. As illustrated recently in a critical review by Busca,[START_REF] Busca | The Surface of Transitional Aluminas: A Critical Review[END_REF] this topic has fueled debate for decades, owing to the complexity of these materials and the importance of experimental factors on their textural and spectroscopic features, to mention only a few. The reactivity of transitional aluminas owes much to the presence of both hydroxyl groups and Lewis acidic sites. The population of these two families (in terms of nature and density) strongly depends on the transitional phase (crystallinity) and on the thermal treatment the material was subjected to. A complex situation results from restructuration upon high temperature outgassing, a procedure which is typical for alumina activation prior to use in catalysis or catalyst preparation. This may occur via hydroxyl co-condensation or condensation with neighboring sites, forming new surface species. [START_REF] Knözinger | Catalytic Aluminas: Surface Models and Characterization of Surface Sites[END_REF][START_REF] Wagner | Observation of Distinct Surface AlIV Sites and Phosphonate Binding Modes in γ-Alumina and Concrete by High-Field 27 Al and 31 P MAS NMR[END_REF] Experimental investigations on these systems have been performed with several distinct spectroscopic techniques. Considering the hydroxyl network, infrared spectroscopy has been routinely used to study these materials and their transformation, resulting in extensive literature on the topic. [START_REF] Morterra | A Case Study: Surface Chemistry and Surface Structure of Catalytic Aluminas, as Studied by Vibrational Spectroscopy of Adsorbed Species[END_REF] These elements, combined with theoretical / empirical considerations, have resulted in several contrasting assignments regarding the terminal or doubly/triply bridging character of the aluminum hydroxyl groups, and the coordination number of the Al centers involved. Among the other spectroscopic methods used in this context, solid-state NMR is a highly efficient technique that can afford structural information at the molecular level.

The alumina surface sites mainly concerned by this technique are the AlOH, which can be amenable to both [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] H and [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al NMR, both nuclei being highly NMR-sensitive. Since De Canio's report on the 1 H NMR spectrum of γ-Al2O3, the chemical shift of the hydroxyl signal is known to be a good parameter to assess the terminal vs. bridging character. [START_REF] De Canio | Solid-State 1 H MAS NMR Characterization of γ-Alumina and Modified γ-Aluminas[END_REF] Furthermore, [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR is also suitable when investigating the local structure around the aluminum centers. Indeed, for this half-integer quadrupolar nucleus (S=5/2) with a 100% natural abundance, the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al chemical shift is dependent on the coordination number, with tetra-(AlT), penta-(AlP), and hexa-coordinated (AlH) centers resonating around 60, 40 and 0 ppm, respectively. Additionally, the determination of anisotropic parameters such as the quadrupolar coupling constant (CQ), which results from the interaction of the electric quadrupolar moment of the nucleus with the electric field gradient, provides information on the local geometry in contrast to spin 1/2 nuclei. The reliable assessment of the CQ magnitude is thus essential. The [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR spectra are often broadened by the presence of the second-order quadrupolar coupling which affects the lineshape, unfortunately leading to strong overlapping of signals when dealing with multiple sites. The MQMAS experiment was thus proposed to separate the contributions of isotropic and anisotropic interactions, in order to retrieve both the chemical shift and quadrupolar parameters for each individual site. However, for the characterization of the γ-Al2O3 surface, MQMAS yields poor results as surface aluminum sites are lost in the dominant signal arising from these bulk aluminum sites (mostly tetra-and hexa-coordinated). When considering the issues associated with the study of catalysts where one has to probe the very surface of the material, an approach is needed that selectively provides the spectral data for the top layer(s) of Al centers. To achieve this, methods relying on 1 H-magnetization transfer are particularly well suited thanks to the presence of hydroxyl groups at the surface of alumina. The most classical method derived from spin 1/2 is Cross Polarization (CP)-MAS, which is used to characterize alumina-type materials. [START_REF] Morris | 27 Al Cross Polarization of Aluminas. The NMR Spectroscopy of Surface Aluminum Atoms[END_REF][START_REF] Coster | Acid Sites and Surface Aluminum in Aluminas and Zeolites: A High-Resolution NMR Study[END_REF][START_REF] Barrow | Surface Selective 1 H and 27 Al MAS NMR Observations of Strontium Oxide Doped γ-Alumina[END_REF] As described below in the case of γ-Al2O3, CP-MAS is subject to severe limitations when applied to a quadrupolar nucleus such as [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al. [START_REF] Ashbrook | Spin-Locking of Half-Integer Spin Nuclei[END_REF] An alternative method used to study alumina and derived materials is D-HMQC, which does not rely on spin-locking. [START_REF] Trébosc | Through-space R 3 -HETCOR experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR[END_REF] It can be implemented with a dipolar recoupling scheme in order to highlight those sites which are in the vicinity of protons. [START_REF] Grekov | Solid-State NMR of Quadrupolar Nuclei for Investigations into Supported Organometallic Catalysts: Scope and Frontiers[END_REF] Additionally, other recent pulse sequences, such as R-INEPT and PRESTO, are gaining interest as an alternative to transfer magnetization between 1 H and [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al, but their application is still minor in the literature. [START_REF] Giovine | Magnetization Transfer from Protons to Quadrupolar Nuclei in Solid-State NMR using PRESTO or Dipolar-Mediated Refocused INEPT Methods[END_REF][START_REF] Perras | PRESTO Polarization Transfer to Quadrupolar Nuclei: Implications for Dynamic Nuclear Polarization[END_REF][START_REF] Venkatesh | Proton Detection of MAS Solid-State NMR Spectra of Half-Integer Quadrupolar Nuclei[END_REF] Regarding the characterization of complex materials, the impact of 1D data remains limited, owing to strong overlap of NMR signals from quadrupolar nuclei. All these methods can thus be adapted into a two-dimensional (2D) heteronuclear correlation (HETCOR) experiment, which is essential to obtain more detailed information. However, such heteronuclear correlations must be both efficient and robust. The latter point is critical when dealing with quadrupolar nuclei, for which lineshape analysis is a key element in the characterization process. When dealing with catalytically relevant surfaces, local constraints due to specific site structure may induce the occurrence of anisotropic parameters of high magnitude. It is therefore essential to be able to rely on methods that provide undistorted lineshapes for nuclei experiencing large quadrupolar coupling. Generally speaking, assignments and structural interpretations must be taken with extreme caution.

The first section of this contribution includes a critical assessment of the two most frequently used (on alumina-derived materials) sets of strategies, namely CP-MAS and D-HMQC, to reach through-space 2D HETCOR spectra when a half-integer quadrupolar nucleus is involved. In the following, special focus is placed on the robustness on both experiments rather than on sensitivity, which has been widely documented elsewhere. [START_REF] Venkatesh | Proton Detection of MAS Solid-State NMR Spectra of Half-Integer Quadrupolar Nuclei[END_REF][START_REF] Tricot | Structural Characterisation of Phosphate Materials: New Insights into the Spatial Proximities between Phosphorus and Quadrupolar Nuclei using the D-HMQC MAS NMR Technique[END_REF] In particular, we show that for alumina-derived materials, even though 2D 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS spectra can be obtained within a reasonable time, the robustness of such experiments must be questioned when an overview of the materials surface is required. The efficiency of D-HMQC is also addressed and a new surface-selective [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al high-resolution pulse sequence is proposed to demonstrate the wide range of quadrupolar coupling constants experienced by the various [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al hydroxyl sites. In the second part, the robustness of the D-HMQC correlation sequence is demonstrated with two examples of -alumina-modified catalysts for applications in olefin polymerization and hydrogenation. We show that D-HMQC outperforms CP-MAS when dealing with systems featuring high CQ values.

Experimental Methods

The -alumina (from Rhône-Poulenc) was first calcined at 500 °C under a N2/O2 flow overnight, then partially dehydroxylated at 500 °C under high vacuum (10 -5 Torr) for 12 hours. After this treatment, it was stored and handled in an argon-filled glove box. The specific surface area measured by BET (N2) was 200 m 2 .g -1 . The XRD spectrum (Figure S1) clearly confirms the structural assignment as -Al2O3, in comparison with the diffraction pattern, and matches that of the reference 00-029-0063 (-Al2O3) from ICDD (International Center for Diffraction Data). The infrared spectrum is also in line with reported data (Figure S2). [START_REF] Busca | Surface, and Catalytic Properties of Aluminas[END_REF] Catalysts Al2O3-H and Al2O3-Zr were synthesized as described in references 19 and 20 respectively.

For solid-state NMR analysis, the highly air-sensitive samples were packed into 3.2-mm zirconia rotors inside an Ar-filled glovebox, and sealed with tightly fitting Kel-F caps. 1 H MAS NMR, 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS NMR and 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC MAS NMR spectra were acquired on a Bruker Avance III 800 spectrometer ( 1 H, 800.13 MHz; [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al:, 208.50 MHz). For all experiments, the spinning frequency was 20 kHz. For 1 H MAS NMR spectra, 16 scans were collected using a π/2 pulse excitation of 3.0 μs. For all 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS NMR, contact time was set at 1 ms. Several Hartmann-Hahn conditions were scanned and are therefore specified in the corresponding figure captions. D-HMQC experiments were set up with an [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al spin echo selective to the central transition, with pulses of 8 and 16 μs, and a 1 H π/2 pulse of 3 μs on either side of the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al π pulse. The SR4 1 2 dipolar recoupling scheme [START_REF] Brinkmann | Proton-Selective 17 O-1 H Distance Measurements in Fast Magic-Angle-Spinning Solid-State NMR Spectroscopy for the Determination of Hydrogen Bond Lengths[END_REF] was applied for 500 μs. The MQ-D-HMQC pulse sequence starts with two hard pulses of 3.0 and 1.25 μs at a radio-frequency (RF) field strength of 90 kHz on the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al channel, for triple-quantum excitation and reconversion respectively, followed by the D-HMQC scheme (with experimental parameters as stated above).

Results and Discussion

Critical assessment of CP vs. HMQC MAS NMR towards partially dehydrated γ-Al2O3 surface characterization

This section is devoted to the use of heteronuclear correlation methods in the case of Al2O3-500, i.e. γ-Al2O3 which has been subjected to dehydroxylation at 500 °C under vacuum for 12 hours. This is considered as a benchmark system, since such a treatment is classical in alumina-related catalytic studies. [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF][START_REF] Busca | The Surface of Transitional Aluminas: A Critical Review[END_REF][START_REF] Knözinger | Catalytic Aluminas: Surface Models and Characterization of Surface Sites[END_REF] Its surface features different types of hydroxyl environments related to aluminium sites with various coordinations. For example, the surface aluminium sites can present CQ values spreading over a large range from 4-6 MHz to a higher value of 12 MHz. [START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF] Of course, this does not rule out the possibility that other types of Al centers featuring larger quadrupolar coupling constants are present but not observed due to the broadness of their [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al NMR signal under our experimental conditions (magnetic field of 18.8

T, spinning speed of 20 kHz). [START_REF] Bryant | Structural Characterization of MAO and Related Aluminum Complexes. 1. Solid-State 27 Al NMR with Comparison to EFG Tensors from ab Initio Molecular Orbital Calculations[END_REF] As mentioned above, we compare two of the main strategies (CP and D-HMQC) commonly applied to highlight 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al proximities. In particular, we critically assess their ability to provide bidimensional HETCOR spectra, which yield sound structural information.

The CP pulse sequence (Fig 1a) consists of a magnetization transfer through a spin-lock pulse, generally between the high gyromagnetic ratio nucleus (typically 1 H, with γH) and the low gyromagnetic one (γX), the gain being proportional to the ratio γH/γX. First, it provides signal enhancement and also allows correlations to be determined, as magnetization transfer operates via the dipolar coupling between the 1 H and X nuclei. However, it is well documented that the CP experiment suffers from several drawbacks, mainly owing to the complex spin dynamics of the quadrupolar nucleus X during spin-locking (vide infra). [START_REF] Ashbrook | Spin-Locking of Half-Integer Spin Nuclei[END_REF][START_REF] Vega | CP-MAS of Quadrupolar S= 3/2 Nuclei[END_REF] The 1 H T1 relaxation delay is also a major factor when considering CP experiments. Indeed, the recycling delay between two transients must be about 3 to 5 times higher than the actual 1 H T1 value. This is a key parameter when considering quantitative measurements. Based on our experience with such materials, we have frequently observed long to extremely long relaxation times for hydroxyl protons, with recycling delays reaching 1200 sec. [START_REF] Szeto | Selective grafting of Ga(i-Bu)3 on mesoporous H-ZSM-5 by surface organometallic chemistry for catalytic propane aromatization[END_REF][START_REF] Gallo | Ligand exchange-mediated activation and stabilization of a Re-based olefin metathesis catalyst by chlorinated alumina[END_REF][START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] This factor is very dependent on technical conditions, namely the possibility of adventitious introduction of air (i.e. of O2 and H2O) in the rotor, for instance when spinning under an air flow with a loose rotor. In our studies, materials and catalysts were handled under rigorously inert conditions: the samples were handled strictly in the exclusion of air until being introduced into the stator, having packed the rotor in an argon-filled glovebox. However, we occasionally observed a T1 decrease upon extended residence time in the instrument when spinning under air, though with no detectable 1 H NMR spectrum difference. For instance, this is demonstrated in the case of Al2O3-500, where [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] H T1 evolves from 130 to 45 sec after 6 days of spinning in the magnet (Figure S3). This observation is due to the introduction of O2 in the rotor, with a paramagnetic effect on T1. [START_REF] Mirhej | Proton Spin Relaxation by Paramagnetic Molecular Oxygen[END_REF][START_REF] Froix | Effect of Molecular Oxygen on the N.M.R. Relaxation Time of Some Aromatic Polymers[END_REF][START_REF] Klinowski | The Origin of 29 Si Spin-Lattice Relaxation in Zeolites: a Means of Rapide Acquisition of N.M.R. Spectra and of Probing Internal Sites in Microporous Catalysts[END_REF] To our knowledge, there is no mention of such long recycling delays in the 1 H NMR studies on alumina-related materials reported in the literature. This may be due to insufficient attention to this parameter or most probably to short exposure to air upon sample preparation.

In most studies dedicated to the investigation of alumina surface, only 1D spectra have been recorded, which is of limited interest for structural analysis. Some of them even reported an incredibly high number of transients, i.e. several tens of thousands. [START_REF] Wischert | Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View[END_REF] Surprisingly, only a single article reported a 2D 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS spectrum of alumina. [START_REF] Paluch | Application of 1 H and 27 Al Magic Angle Spinning Solid State NMR at 60 kHz for Studies of Au and Au-Ni Catalysts Supported on Boehmite/Alumina[END_REF] However, a quick look at the 1 H MAS NMR of the sample indicates the presence of large quantities of adsorbed water (signal at 4.5 ppm), giving rise to a fully hydrated surface with expected short relaxation times, while being detrimental to resolution in the 1 H dimension. With a material featuring short 1 H T1, (dehydrated -alumina denoted Al2O3-500 * , with 1 H NMR spectrum devoid of apparent differences with the pristine material, Figure S3), we were also able to record an 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al correlation spectrum using the1 H→ [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS pulse sequence (Figure 1, top), with a good signal-tonoise ratio and within a reasonable experimental time (15 hours). This spectrum features several correlations between various types of protons (with chemical shifts ranging from 0.5 to 4.5 ppm, thus corresponding to µ 1 -HOAl, µ 2 -HOAl2 and µ 3 -HOAl3) and aluminum centers (with tetra-, penta-and hexacoordinated configurations). First, correlations emerge between specific pairs of proton and aluminum thanks to the resolution in the 1 H dimension. This provides a further layer of understanding compared to the 1D CP-MAS NMR spectrum (Figure 1b). The topology of the hydroxyl network in terms of the nature of the hydroxyls (terminal, doubly and triply bridging) and coordination number of aluminum (4, 5 or 6) can be obtained from this data. This was the case in a previous study by some of us on Al2O3-500, combining of Al2O3-500. f) CP-MAS pulse sequence, g) 2D [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al→ 1 H CP-MAS correlation spectrum (Note that this spectrum was flipped so that [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al axis matches that of spectrum c) to facilitate side-by-side visual comparison) and h) 1D When dealing with materials as complex as aluminas and studying their surface, the CP approach quickly demonstrates its limitations. Indeed, and surprisingly enough since this is well known to groups proficient in using solid-state NMR for material analysis, optimization of the Hartmann-Hahn conditions when a quadrupolar nucleus is involved is not straightforward, so CP-MAS is definitely not a robust experiment. For instance, we recorded the 2D [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al → 1 H CP MAS NMR spectra of Al2O3-500 using specifically chosen sets of experimental conditions, i.e. low and high power Hartmann-Hahn conditions and different [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al carrier frequencies (Figure S4).

The previous CP-MAS HETCOR spectra can also be compared to that obtained using the D-HMQC pulse sequence. It has been demonstrated on several occasions that the HMQC sequence is more robust than CP, especially at high magnetic field and high spinning frequency. [START_REF] Tricot | Structural Characterisation of Phosphate Materials: New Insights into the Spatial Proximities between Phosphorus and Quadrupolar Nuclei using the D-HMQC MAS NMR Technique[END_REF] In particular, the HMQC sequence avoids the spin locking period of CP, which is a severe hurdle for quadrupolar nuclei. The D-HMQC spectrum of Al2O3-500, Figure 2 (top), is very informative as explained in a previous article where it was combined with 1 H-1 H DQSQ to propose a topology of the alumina surface. [START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF] In short, each region of the 1 H spectrum, which corresponds to terminal (-0.2 -0.5 ppm), doubly (0.8 -2.5 ppm) and triply (2.5-6 ppm) bridging hydroxyls, gives rise to specific correlation patterns with [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al nuclei. The latter feature a diversity of sites which differ more by their quadrupolar coupling constant than by the chemical shift for a given coordination number (AlT, AlP or AlH). Figure S5 gives an overview of the main families of aluminum environment, associated with their NMR parameters.

We comment here only on the shape of the terminal hydroxyl group's signal, which shows a secondorder quadrupolar pattern with a quadrupolar coupling constant of 11.5 MHz, as deduced from the bestfit simulation (Figure 2c). The sites give rise to the most salient feature of the 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC spectrum and are associated with the most reactive hydroxyl species of the  1 -AlT-OH type, which feature a (O-H) stretching frequency of 3774 cm -1 . [START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF], They show higher reactivity compared to other OH groups, [START_REF] Busca | The Surface of Transitional Aluminas: A Critical Review[END_REF][START_REF] Joubert | Molecular Understanding of Alumina Supported Single-Site Catalysts by a Combination of Experiment and Theory[END_REF][START_REF] Vimont | Infrared Spectroscopic Study on the Surface Properties of -Gallium Oxide as Compared to Those of γ-Alumina[END_REF] and some authors have proposed that they are located in the vicinity of Lewis acidic sites, which accounts for their specific reactivity. [START_REF] Liu | DRFT-IR Studies of the Surface of -Alumina[END_REF] From the NMR point of view, the 1 H and [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al chemical shifts are not efficient descriptors for those reactive sites. In the present case, the latter show a large quadrupolar coupling constant, indicating severe local constraints, maybe due to a distorted tetra-coordinated AlO4 arrangement. of Al2O3-500 with (a, f) corresponding pulse sequences, b) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al projection from 1 H range [-0.5, 5 ppm], c) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al projection of terminal hydroxyl groups, from [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] H range [-0.5, 0.5 ppm] with best-fit simulation of AlT-OH sites (CS=69 ppm, CQ= 11.5 MHz, Q=0.26), d) 2D 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC spectrum, e) 1 H DEPTH MAS spectrum, g) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MQ-D-HMQC projection, h) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MQ-D-HMQC projection of tetracoordinated aluminum sites with best-fit simulation of the AlT-OH sites (CS=69 ppm, CQ= 11.5 MHz, Q=0.26), i) 2D

1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MQ-D-HMQC spectrum, j) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al isotropic dimension. Spectra were obtained in d) 18 hours (number of transients=512 for 64 t1 increments, recycling delay D1=2 s) and i) 38 hours (number of transients=4944 for 28 t1 increments, recycling delay D1=1 s).

In Figure 3, we compare [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al slices extracted from heteronuclear correlation spectra of Al2O3-500 with the 1 H chemical shift of -0.2 ppm, which accounts for terminal Al-OH (correlated with tetracoordinated and hexacoordinated aluminum at 60 and 10 ppm, respectively: Figure 3a is extracted from the 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC spectrum (Figure 2d), and Figure 3b from the 2D [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al → 1 H CP MAS NMR spectrum (Figure 1c). The former (Figure 3a) has been modeled using a quadrupolar lineshape with a CQ value above 11

MHz. In the case of the CP extracted slice (Figure 3b), the lineshape cannot be fitted using a pure secondorder quadrupolar lineshape. The outcome was notably dependent on the experimental conditions, and particularly on the very influential position of the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al carrier frequency. As cross-peaks were detected erratically (Figure S2), the lineshape appears as of either Gaussian or Czjzeck type (Figure 3b), or closer to a second-order quadrupolar coupled signal (Figure 3c), whether carrier frequency is set off or on resonance to the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Both CP and D-HMQC experiments under MAS conditions give rise to the correlation of 1 H and [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al, with a low resolution in the quadrupolar nucleus dimension, i.e. retaining a broadening from the secondorder quadrupolar coupling. A further development of the MQMAS experiment, as previously attempted for CP, [START_REF] Fernandez | 27 Al{ 1 H} Cross Polarization Triple-Quantum Magic Angle Spinning NMR[END_REF] can be proposed here for the HMQC sequence. The MQMAS experiment provides a highresolution spectrum of half-integer quadrupolar nuclei by removing second-order quadrupolar broadening in the indirect dimension of a 2D spectrum. In Figure 2e, we present a new pulse sequence where the D-HMQC is preceded by a MQMAS filter. This results in a high-resolution [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al spectrum which provides information on the very first layer(s) of the alumina surface. The most striking element is the observation of a strongly quadrupolar coupled signal with no chemical shift distribution (red box in Figure 2h), which we assign to the µ 1 -HOAlT moiety. Best-fit simulations (using the dmfit program) are fully in line with the previously determined NMR parameters (chemical shift of about 69 ppm, CQ value of about 11.5

MHz, purple line in Figure 2g). This is direct evidence of the presence of strongly quadrupolar coupled µ 1 -HOAlT sites on the alumina surface.

Probing the HMQC MAS NMR approach on γ-Al2O3-supported catalytic systems

In the previous section, we compared experimental data for an archetypical catalytic support using two strategies to reach 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al correlation spectra, namely CP-MAS and D-HMQC. We demonstrated that the CP-MAS approach is inappropriate to study alumina surface, having explored a large number of hydroxyl sites with different NMR characteristics. The main issue with CP-MAS is not so much related to the overall sensitivity of the sequence, since 2D correlation spectra can be obtained within acceptable experimental time, but rather to the reliability of the CP transfer. This leads to signal distortion or, worse, to signal loss. Any further reasoning based on such an approach is thus jeopardized.

The applicability of the HMQC when studying catalytically relevant materials was further exemplified in the case of alumina-supported aluminum hydride Al2O3-H, which is used as an olefin polymerization and hydrogenation catalyst. [START_REF] Mazoyer | Heteronuclear NMR Correlations to Probe the Local Structure of Catalytically Active Surface Aluminum Hydride Species on γ-Alumina[END_REF] This catalytic material has been prepared in two steps from γ-Al2O3, first by grafting of AliBu3, then by hydrogenolysis at high temperature (400 °C). This procedure, followed by mass balance analysis, elemental analysis and IR spectroscopy, affords an alumina-derived material devoid of surface hydroxyls, with Al-H sites as the sole type of surface protons. Such sites give rise to remarkable NMR features: indeed, the 1 H MAS NMR spectrum comprises an asymmetrical multiplet at 3.3 ppm which stems from the combination of 1) a large scalar coupling with a spin 5/2 nucleus ( 1 J( 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al) = 376 Hz, assessed from J-resolved experiment), and 2) a large quadrupolar coupling constant for the Al center (CQ of about 15 MHz), giving rise to a second-order quadrupolar-dipolar cross-term (Figure 4a). [START_REF] Harris | Quadrupolar Effects Transferred to Spin-½ Magic-Angle Spinning Spectra of Solids[END_REF] Interestingly, this large CQ value is unambiguous experimental proof of a quadrupolar coupling constant of a surface Al center. We previously studied Al2O3-H using the J-HMQC heteronuclear correlation experiment, and demonstrated that these hydride sites are mostly on tetra-and pentacoordinated Al centers. The J-HMQC sequence is selective, as it provides information through covalent bonds. In addition, we now use the complementary D-HMQC sequence to investigate the material through dipolar interaction and to probe the second coordination sphere of the aluminum hydrides. The 2D 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC spectrum is presented in Figure 4f. It features an intense correlation between the hydride signal at 3.3 ppm (as evidenced from the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al-decoupled 1 H spectrum, Figure 4b)

and the three types of Al centers (AlT, AlP and AlH). This spectrum slightly differs from the J-HMQC correlation map (Figure S5). Indeed, as we rely on dipolar interactions, not only can we observe the Al-H fragments, but also the neighboring Al centers located in the second coordination sphere. This is clearly evidenced by comparing the Dand J-filtered [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al NMR signals (Figure 4d and e, respectively). It appears that the D-HMQC also reveals the contribution of the aluminum centers with lower CQ values, which are mostly in tetra-and hexacoordinated configuration. This is evidenced by the sharper contributions at about 66 and 6 ppm (Figure 4e). Interestingly, the maxima of these two signals differ not only from that of the Al-H, but also from that of the AlT and AlH from the Al2O3-H material's [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR spectrum (Figure 4c). This agrees with the fact that their chemical environment differs from that of the bulk, owing to their proximity with the surface. Furthermore, AlP are also observed, although to a lesser extent compared to the other two Al configurations. A tentative best-fit simulation is in line with this lineshape resulting from the combination of strongly quadrupolar coupled AlT and AlP (as first coordination sphere), along with bulk-like, distributed AlT and AlH, with a significantly minor contribution from AlP (see Figure S8).

Additionally, 1D 1 H→ [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP-MAS spectra were also obtained for different Hartmann-Hahn conditions and carrier frequencies (Figure S9). The signal intensity is erratic and spectral lineshapes are occasionally distorted. Even if specific experimental conditions lead to a spectrum comparable to that obtained with D-HMQC, the lack of robustness for the CP-MAS experiment observed here is another argument against a broad applicability. 1 H MAS spectrum with [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al RA-MP decoupling [START_REF] Delevoye | Resolution enhancement using a new multiple-pulsedecoupling sequence for quadrupolar nuclei[END_REF] , c) [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR, d) { 1 H} J-HMQC-filtered [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR, e) { 1 H} D-HMQC-filtered [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR and f) 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al 2D D-HMQC MAS NMR spectra.

Spectrum in f) was acquired in 21 hours (number of transients=1536 for 48 t1 increments, recycling delay D1=1 s).

A further illustration of the versatility of the 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC method for the detailed investigation of alumina-based catalysts can be found in the field of heterogeneous olefin polymerization catalysts. Group 4 metallocenes on various supports are used industrially in major (co)polymerization processes, and the structural studies on these catalysts are still a hot topic, especially when it comes to precise understanding of the nature of the interaction between the grafted organometallic fragment and the inorganic -Al2O3 support. [START_REF] Severn | Bound but Not Gagged" Immobilizing Single-Site α-Olefin Polymerization Catalysts[END_REF] Cp2ZrMe2 (Cp: cyclopentadienyl) supported on γ-Al2O3-500 (Al2O3-Zr) was previously studied using IR, mass balance analysis and EXAFS as well as 1 H MAS and [START_REF] Trébosc | Through-space R 3 -HETCOR experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR[END_REF] C CP MAS NMR. [START_REF] Jezequel | Supported Metallocene Catalysts by Surface Organometallic Chemistry. Synthesis, Characterization, and Reactivity in Ethylene Polymerization of Oxide-Supported Mono-and Biscyclopentadienyl Zirconium Alkyl Complexes: Establishment of Structure/Reactivity Relationships[END_REF] Even if its catalytic performances are modest compared to its best industrial counterparts, the characterization of such an archetypical system is highly relevant when investigating supported catalysts. However, so far, understanding of the interaction between these organometallic species and an inorganic support remains limited. To fill this gap, there is clearly a need for a robust heteronuclear correlation method that could provide insight into the nature of the Al centers surrounding the metallocene species and the residual hydroxyl network. In order to demonstrate the D-HMQC potential in alumina-based catalysts characterization, we focus here on the catalyst prepared from Cp2Zr(CD3)2. Compared to Cp2Zr(CH3)2, this simplifies the 1 H spectral data by avoiding overlap of the Zr-Me signals with that of the residual Al-OH. [START_REF] Jezequel | Supported Metallocene Catalysts by Surface Organometallic Chemistry. Synthesis, Characterization, and Reactivity in Ethylene Polymerization of Oxide-Supported Mono-and Biscyclopentadienyl Zirconium Alkyl Complexes: Establishment of Structure/Reactivity Relationships[END_REF] The 1 H NMR MAS spectrum of Al2O3-Zr, (Figure 5a), is rather uninformative. It features an intense peak at 6.2 ppm, accounting for the cyclopentadienyl protons, and a broad, featureless signal from 0.5 to 5 ppm. On the contrary, the 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC MAS NMR spectrum is very informative. It features cross-peaks for both Cp and hydroxyl protons, with distinct patterns. In the case of Cp protons, the corresponding [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al trace (Figure 5c) provides a direct view of the neigboring aluminum centers: these are mostly tetra-and hexa-coordinated (in a rough 1:1 proportion). The pentacoordinated Al centers account for a minor yet significant share of the signal. The lineshape of the AlT and AlP denotes a strong quadrupolar coupling for these nuclei. This differs from the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al MAS NMR signal of Al2O3-Zr (Figure S10), thus showing that the D-HMQC sequence filters the signal of the surface aluminum from that of the bulk. Along the same line, the weak, broad 1 H hydroxyl signal gives rise to correlations mostly with hexacoordinated aluminum centers, and to a lesser extent with tetra-and pentacoordinated Al (Figure 5d).

The position of the signal in the 1 H dimension is in line with assignments as doubly and triply bridging hydroxyls (around 2 and 2.5-5 ppm, respectively). The distinctive terminal AlTOH signal is no longer observed, indicating a full reaction of these sites. Therefore, we may conclude on the higher reactivity of the terminal hydroxyl (on AlT centers) and to a much lesser extent, doubly bridging groups (with a preference for those connected to AlT and AlP). This illustrates how this heteronuclear correlation can be harnessed to benefit from its robustness by revealing 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al proximities, with information on the Al site coordination. This provides further key information on the grafting sites. Clearly, in this example, a 1D

1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al correlation spectrum (CP-MAS or D-HMQC filtered) could not provide such specific information by itself, while it is made possible with the 2D 1 H- [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC. In addition, the precise reactivity pattern deduced from this spectroscopic evidence could not be deduced either from the 1 H MAS NMR only or from the infrared spectrum (Figure S11).

Conclusion

Owing to their importance in major catalytic processes, transitional aluminas are attracting considerable

attention, yet the nature of their surfaces is still a matter of debate. In this context, heteronuclear correlation methods in NMR are instrumental, as they afford molecular level information on both 1 H and [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al centers. Extensive discussion revolves around the anisotropic [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al NMR parameters, such as the quadrupolar coupling constants, which are considered as a major descriptor of the local configuration of the [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al nucleus. Our objective was to critically address the efficiency of two widely used methods (CP and D-HMQC), with a strong emphasis on their robustness, especially in terms of quadrupolar lineshape reproducibility.

We first show that the classical and most commonly used method, i.e. cross-polarization, does not meet the reproducibility criteria. Even if bidimensional spectra can be obtained, the outcome is severely dependent on the experimental conditions. It is clear that a quick analysis of the above-described 2D CP-MAS correlation spectra, with a non-critical eye, would probably lead to some wrong conclusions about the local structure at the surface of alumina, or about the (in)visibility of the surface aluminum sites. This is clearly not acceptable for the widespread use of 1 H→ 27 Al CP-MAS (or [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al→ 1 H CP-MAS). In addition, we also confirm that lineshape distortion is a major drawback, an obvious fact for NMR spectroscopists.

In spite of this, many papers in the field of materials characterization and catalysis rely extensively on this approach, even in very recent years.

On the other hand, the D-HMQC method, which does not rely on spin-locking of the quadrupolar nucleus, faithfully reproduces the structural complexity of an archetypical alumina surface. This was further exemplified in selected study examples of the surface of alumina-based catalysts by [START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al NMR.

D-HMQC as a heteronuclear method provides unambiguous, robust information regardless of the CQ magnitude: on a spectrum, one can readily observe sites featuring CQ values ranging from 5-6 MHz (bulklike signal) up to 15 MHz (Al-H signal).D-HMQC approach has already provided unprecedented information on (modified) alumina surfaces.

From the wider perspective of partially dehydroxylated alumina surfaces and NMR studies of derived catalytic materials, the main issues do not concern the excitation of quadrupolar nuclei featuring (potentially) large CQ values, but rather the frequency of the spectrometer, as high fields are mandatory when attempting to study such complex materials. Further work will include methodological and technical improvements for the implementation of INEPT and PRESTO in such a context. This should lead to further improvements in sensitivity compared to the HMQC experiment, maybe by considering the use of indirect proton detection. [START_REF] Venkatesh | Proton Detection of MAS Solid-State NMR Spectra of Half-Integer Quadrupolar Nuclei[END_REF][START_REF] Carnahan | Probing O-H Bonding through Proton Detected 1 H-17 O Double Resonance Solid-State NMR Spectroscopy[END_REF] Progress can also be expected from current developments in Dynamic Nuclear Polarization (DNP), provided that 1) the gain achieved at high field is comparable to that obtained at moderate field, [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF] and 2) samples are not altered upon preparation with solvents and radicals, bearing in mind that -alumina surfaces are extremely sensitive to adsorption phenomena. From the catalytic materials point of view, there is a definite need to implement efficient and reliable methods that can take our understanding to new levels. The current hurdles include sensitivity or the ability to reproduce strongly quadrupolar-coupled sites, i.e. those that are considered as the most relevant to catalytic reactions.

Figure 1 .

 1 Figure 1. Top: 1 H→ 27 Al spectra of Al2O3-500 * . a) CP-MAS pulse sequence, b) 1D 27 Al CP-MAS

Figure 2 :

 2 Figure 2: 1 H-27 Al D-HMQC MAS NMR (top) and 1 H-27 Al MQ-D-HMQC MAS NMR spectra (bottom)

  Al tetracoordinated region. The tentative fit of the slice in Figure 3c (dashed line) with two Gaussian lineshapes is mathematically correct but structurally irrelevant. The solid line represents a poor simulation of this experimental lineshape, though being close to the actual NMR parameters, as deduced from the D-HMQC experiment.Based on CP HETCOR, all aluminum sites correlating with surface hydroxyls would show low quadrupolar couplings, below 6-7 MHz, which are values far below those obtained from D-HMQC or those reported from DFT calculations.[START_REF] Wischert | Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View[END_REF] This lineshape distortion is thus a major drawback of the CP approach, as shown in earlier studies.[START_REF] Tricot | Structural Characterisation of Phosphate Materials: New Insights into the Spatial Proximities between Phosphorus and Quadrupolar Nuclei using the D-HMQC MAS NMR Technique[END_REF]

Figure 3 .

 3 Figure 3. Slices extracted from heteronuclear correlation spectra of Al2O3-500 at 1 H chemical shift of -

Figure 4 .

 4 Figure 4. Synthesis and selected NMR data on Al2O3-H: a) 1 H MAS NMR, b) { 27 Al} J-HMQC-filtered

Figure 5 .

 5 Figure 5. Synthesis and selected NMR data on Al2O3-Zr: a) 1 H MAS NMR, b) 1 H-27 Al 2D D-HMQC

  [START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] H CP-MAS spectrum. (e) and (i)[START_REF] Pearson | Aluminum Oxide (Alumina), Activated[END_REF] H DEPTH MAS spectrum. Both 2D CP-MAS spectra were obtained in 15 hours (number of transients=512, recycling delay D1=2 s). The pulse sequence starts with a 1 H π/2 pulse of 2.65 μs ( 1 H→[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al) and a central-transition (CT) selective[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al π/2 pulse of 8.5 μs ( 27 Al→ 1 H), followed by Hartmann-Hahn conditions set to c)  𝐴𝑙[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] ,𝐶𝑇 = 44 kHz,  𝐻

		1	= 24 kHz
	and g)  𝐴𝑙 27 ,𝐶𝑇 = 32 kHz,  𝐻 1	= 12 kHz, with the 27 Al carrier frequency at 40 ppm.

H-[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al D-HMQC (see below) and 1 H-1 H DQ-SQ MAS NMR along with DFT calculations.

A critical analysis of the[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al lineshapes extracted from the 2D correlation spectra is provided below when discussing the D-HMQC spectrum.As discussed above, (very) long 1 H relaxation delays of several seconds up to 130 s (or more as observed in specific cases) are encountered when samples have not been exposed to a trace amount of air. When considering 1 H→[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP as a magnetization transfer process, this is prejudicial to the recording of CP spectra of a sufficient S/N ratio within a reasonable amount of time. An alternative approach to overcome the issue associated with large 1 H T1 is to transfer magnetization from[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al to 1 H, so that the recycling delay is set to[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al T1, which is generally shorter than that of 1 H. This should lead to the same heteronuclear correlation information as the 1 H→[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al CP experiment though at the expense of signal intensity, as the gain is now given by the ratio γAl/γH. Figure1(bottom) shows[START_REF] Zhang | Enhanced Metathesis Activity and Stability of Methyltrioxorhenium on Amorphous Alumina: Role of the Local Grafting Environment[END_REF] Al→ 1 H CP-MAS correlation spectrum for Al2O3-500, which features a 1 H T1 value of 130 s. The correlation map is overall similar to that of the above-described 1 H→ 27 Al CP-MAS spectrum. Most interestingly, recording the 1 H→ 27 Al CP-MAS spectrum of this material would take a few months if one had to recycle every 3 to 5 times T1.

ASSOCIATED CONTENT

Supporting Information. Additional experimental details and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * laurent.delevoye@univ-lille.fr, regis.gauvin@univ-lille.fr

Author Contributions

The manuscript was written thanks to contributions from all authors.

ACKNOWLEDGMENT

Chevreul Institute (FR 2638), CNRS, Ministère de l'Enseignement Supérieur et de la Recherche, Région Nord -Pas de Calais and FEDER are acknowledged for supporting and partially funding this work.

Financial support from the TGIR RMN THC Fr3050 is gratefully acknowledged.