Novel La3Fe(MoO4)6 phase: Magnetic properties and ethanol reactivity

Article abstract of DOI:10.1039/c5dt01444a

Single crystals of a new oxide, La₃Fe(MoO₄)₆, were grown from fluxes of oxide precursors, and a polycrystalline sample was also prepared by a standard solid state reaction. La₃Fe(MoO₄)₆ crystallizes in the orthorhombic space group Pbca with unit cell parameters a = 19.3164(11), b = 10.4143(5) and c = 22.0594(12) ?. This crystal structure exhibits a singular architectural type built on infinite chains of Fe(MoO₄)₄, each of them being surrounded by two isolated MoO₄ tetrahedra and three isolated La³⁺ cations. Fe³⁺ ions in La₃Fe(MoO₄)₆ are antiferromagnetically ordered below T_N = 6.6 K in chains and between chains, as refined from neutron diffraction data. Further the redox stability of this compound - pure powder - was checked using temperature-programmed X-ray diffraction under a controlled atmosphere; under air, we observed a reversible phase transition above 523 K. The same phenomenon was observed under a reductive atmosphere, followed by a destruction of the as-formed phase above 923 K owing to iron III to II reduction. Reactivity of ethanol was then evaluated to get insights into the redox properties of the material under working conditions. After 4 hours of reaction at 648 K, the ethanol conversion was 97% with a selectivity to acetaldehyde of ～60%, the other products being formaldehyde (～10%) and CO₂ (～30%), underlining a better acetaldehyde selectivity than that of the La-free conventional Fe₂(MoO₄)₃ catalytic formulation.

Full text of DOI:10.1039/c5dt01444a

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Novel La3Fe(MoO4)6 phase: magnetic properties
and ethanol reactivity†
Marie Colmont,*a Georgiana Bucataru,a Anita Borowiec,a Mickaël Capron,a
Franck Dumeignil,a,b Marielle Huvé,a Frédéric Capet,a Françoise Damay,c
Olivier Mentréa and Pascal Roussela
Cite this: Dalton Trans., 2015, 44,
14444
Single crystals of a new oxide, La3Fe(MoO4)6, were grown from fluxes of oxide precursors, and a polycrys-
talline sample was also prepared by a standard solid state reaction. La3Fe(MoO4)6 crystallizes in the ortho-
rhombic space group Pbca with unit cell parameters a = 19.3164(11), b = 10.4143(5) and c = 22.0594(12) Å.
This crystal structure exhibits a singular architectural type built on infinite chains of Fe(MoO4)4, each of
them being surrounded by two isolated MoO4 tetrahedra and three isolated La3+ cations. Fe3+ ions in
La3Fe(MoO4)6 are antiferromagnetically ordered below TN = 6.6 K in chains and between chains, as
refined from neutron diffraction data. Further the redox stability of this compound – pure powder – was
checked using temperature-programmed X-ray diffraction under a controlled atmosphere; under air, we
observed a reversible phase transition above 523 K. The same phenomenon was observed under a reduc-
tive atmosphere, followed by a destruction of the as-formed phase above 923 K owing to iron III to II
reduction. Reactivity of ethanol was then evaluated to get insights into the redox properties of the
material under working conditions. After 4 hours of reaction at 648 K, the ethanol conversion was 97%
with a selectivity to acetaldehyde of ∼60%, the other products being formaldehyde (∼10%) and CO2
(∼30%), underlining a better acetaldehyde selectivity than that of the La-free conventional Fe2(MoO4)3
catalytic formulation.
Received 16th April 2015,
Accepted 1st July 2015
DOI: 10.1039/c5dt01444a
www.rsc.org/dalton
of lone-pair (LP) cations or polyanions inducing an external s2
electron favorably arranged in NCS crystal structures.4,5
Introduction
The prospection for new inorganic materials is usually driven Rational substitution of a cation by another one less charged
by the ulterior motive of increasing the performance of par- that will thus induce vacancies in the structure is the classical
ticular applications. Scientists dream about the possibility to technique used to increase ionic conductivity in materials.6
design new materials with targeted properties. Nevertheless, if This modification can further potentially lead to changes in
developing a predictive strategy route is a smart approach, it is the catalytic reactivity of the as-formed compounds.
pretty often unsuccessful due to technical difficulties for syn-
Catalytic conversion of alcohols is nowadays of topical
thesis and also because the properties are often not as promis- interest, as some of them are readily obtained from bio
ing as thought. In this context, the nature of the atoms resources. Among them, ethanol is now massively produced
inherent to the material itself was tuned and their association for biofuel needs, but it is also being considered more and
with the structure studied enabling modification of properties. more as a platform molecule from which families of chemicals
For example, piezo-electric, pyroelectric, and ferroelectric1,2 can be derived.7 For instance, ethanol can be used as a starting
properties are strongly related to the achievement of non- material in the synthesis of different byproducts such as alde-
centrosymmetric (NCS)3 compounds favored by introduction hydes,8,9 acetals,10,11 ethers, esters or alcohols with longer car-
bonated chains through Guerbet chemistry.12,13 Such products
are of high importance in the context of biorefineries develop-
aUniversité Lille 1 Sciences et Technologies, UMR 8181 CNRS, Unité de Catalyse et
ment, as intermediate reactants in the downstream production
Chimie du Solide ‘UCCS’, F-59655 Villeneuve d’Ascq, France.
of alternative fuels or chemicals.14 Within this frame, FeMo-
E-mail: marie.colmont@ensc-lille.fr
bInstitut Universitaire de France, Maison des Universités, 103 Bd St-Michel, Paris,
based mixed oxides (namely Fe2(MoO4)3) are known for their
75005, France
specific redox behavior making them good catalysts for alcohol
cLaboratoire Léon Brillouin, UMR12, CEA Saclay, CEA-CNRS, 91191 Gif sur Yvette
oxidation.15,16
Cedex, France
In the present paper, we synthesized and characterized
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
the structure of a new lanthanum molybdenum iron oxide,
c5dt01444a
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La3Fe(MoO4)6. As a preliminary study, AFM coupling at very low action pattern (using JANA 2006 software18) led to a = 19.3212(3)
temperature was evidenced. Further, we examined the catalytic Å b = 10.4381(2) Å and c = 22.1252(3) Å which were in good
properties of this new material including La in a FeMo oxide- agreement with the lattice parameters refined from the single
based host structure. On a mechanically milled compound, a crystal data (see below).
conversion of 97% was observed at 650 K with a selectivity to
acetaldehyde of 62%. This clearly outperforms a conventional
Structural characterization
FeMo catalyst, which exhibits a much lower selectivity to acet-
aldehyde due to uncontrolled ethanol reaction from ca. 575 K.
Single crystal XRD study. Single crystals of this new phase
were collected using a Bruker Apex Duo diffractometer with a
Mo-IμS microfocus tube (λ = 0.71073 Å). The intensity data
were extracted from the collected frames using the program
SAINT-Plus 8.27b.19 The lattice parameters have been refined
from the complete data set. Absorption corrections have been
performed using multiscan methods using SADABS.20 The
crystal structure was solved by the charge flipping method
using SUPERFLIP21 and the data was refined with the
JANA200618 crystallographic suite. After absorption correction,
the 10 170 reflections (I > 3σ(I)) were merged in the Laue group
mmm leading to a merging factor Rint = 3.2% for 8270 reflec-
tions with I > 3σ(I). The refined orthorhombic lattice para-
meters of the title compound are: a = 19.3164(11) Å, b =
10.4143(5) Å, and c = 22.0594(12) Å. Analysis of the data set
revealed that the systematic absences were consistent with the
orthorhombic space group Pbca. All the relevant details of data
collection and pertinent data of the refinements are gathered
in Table S1.†
X-ray powder diffraction analysis of the powder sample has
been performed at room temperature in the angular range of
2θ 1–80° with the scan step width of 0.02° using a D8 Advance
Bruker AXS diffractometer in Bragg Brentano geometry
equipped with a 1D LynxEye detector. Full pattern matching
was performed using JANA 2006.18
High-temperature X-ray diffraction (HTXRD). The tempera-
ture stability of the title compound was checked using HTXRD
techniques on a Bruker D8 Advance diffractometer equipped
with a high-temperature Anton Paar XRK900 chamber and a
one dimensional X-ray detector (LynxEye) using Cu Kα radi-
ation under two different atmospheres, namely air and diluted
H2/N2 (3%) to simulate an oxidative and a reductive media,
respectively. Data were collected over the range 10–70° in 2θ,
with a 0.021° step and a time of 59 min per diagram from
room temperature to 1075 K. Diffractograms were obtained
every 50 K on heating and cooling.
Experimental section
Synthesis of La3Fe(MoO4)6
Single crystals. The flux growth technique was used to
obtain single crystals. Indeed, this method has been shown to
be particularly well suited in the exploratory crystal growth and
discovery of novel complex oxides.17 The temperature of syn-
thesis is imposed by the nature of the inorganic chemical used
as the solvent for crystallization, here MoO3.
The crystals were found in the polycrystalline residue from
the melt of 34 wt% of the (80% La(OH)3 + 10% Cr2O3 + 10%
Fe2O3) mixture in 66 (wt%) of MoO3 which was taken as a flux.
The protocol of heat treatment was deduced by carefully study-
ing ref. 17. The starting mixture was loaded into a gold tube,
sealed and then heated at 1175 K for 96 hours and finally
slowly cooled (40 °C h−1) for 20 h. The furnace was then
switched off and crystals were handily selected under a 60×
Nikon binocular.
Powder sample. The corresponding powder was obtained as
a pure polycrystalline phase from the stoichiometric mixture
of La(OH)3 (preferred to La2O3 which is highly hygroscopic)
and Fe2O3 and MoO3 heated at 1175 K in an alumina crucible,
after several intermediate grindings. The sample was promptly
removed from the furnace at elevated temperature to avoid any
residual presence of Fe2O3. La3Fe(MoO4)6 was obtained as a
single phase, as proved by the good pattern matching shown
in Fig. 1. The lattice parameters refined from this powder diffr-
Textural characterization
The specific surface areas (SSAs) of the calcined catalysts were
determined using the single-point BET (Brunauer, Emmett,
Teller) method, with N2 adsorption at liquid N2 temperature
and subsequent desorption at room temperature on a Micro-
meritics ASAP 2010 apparatus. The samples were outgassed at
473 K for 30 minutes prior to analysis.
Magnetic characterization
Magnetic susceptibility was measured using
a
MPMS
SQUID-VSM (Quantum Design) magnetometer. The tempera-
ture dependence variation of the magnetization was carried
out under a magnetic field of 0.1 T after cooling the sample in
Fig. 1 XRD profile and Rietveld refinement for La3Fe(MoO4)6 with
refined parameters a = 19.3212(3) Å b = 10.4381(2) Å and c = 22.1252(3) Å
at RT.
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a field of 0.1 T (FC, field cooling) or in the zero field (ZFC, zero
field cooling). Magnetization versus H was measured between
+3 and −3 T at 2 K.
Magnetic structure. Neutron diffraction experiments (ND)
were carried out from room temperature to low temperature
(1.5 K) on the G4.1 beam line of LLB (Saclay, France) with a
wavelength λ = 2.43 Å. The ND patterns were recorded in the
angular range 2θ = 9–80° with a 0.05° step, using approxi-
mately 8 g of powder introduced into a vanadium tubular
sample holder.
Catalytic test. Materials and apparatus: reaction of ethanol
oxidation was performed using a fixed bed reactor. Before the
experiment, 200 mg of the sample was mixed with 200 mg of
SiC in order to homogenize the temperature inside the cata-
lytic bed. The catalytic tests were carried out in a glass reactor
consisting of a 10 mm diameter tube connected online with a
GC-MS apparatus, which enabled analysis runs every 3 min.
The reactor was placed in an oven for which the temperature
was finely controlled using two thermocouples. The reaction
was performed in a range of temperatures between 600 and
675 °C under atmospheric pressure. The total gas flow rate was
maintained with a GHSV of 2256 h−1 during the reaction, and
the gas composition was kept at 13.2 vol% of ethanol, 6 vol%
of oxygen and 40.8 vol% of helium taken as a carrier gas. The
catalytic performances were evaluated by calculating the fol-
lowing parameters:
Fig. 2 (a) Projection of the structure of La3Fe(MoO4)6, along the b-axis.
It is built on isolated MoO4 tetrahedra (dark green), isolated La3+ cations
and Fe(MoO4)6 infinite chains (b) shown along the a axis.
ðn0 ꢀ nÞ ꢁ 100
Ethanol conversion ð%Þ : X ¼
;
n0
unit cell along the b-axis) and isolated MoO4 tetrahedra.
The resulting electroneutral formula is La3Fe(MoO4)6. In this
(La Fe Mo O) quaternary system, this compound is, beside
La(Fe0.667Mo0.333)O3,22 the second one referenced in the ICSD
database.
where n0 is the initial number of moles of ethanol and n is the
number of moles of unconverted ethanol.
nCx ꢁ nx
Selectivity to products ð%Þ : S ¼
;
nCEthðn0 ꢀ nÞ
The 1D [Fe(MoO4)6] units encountered in the title com-
where nCx is the number of carbons in the product x, nx is the
number of moles of the formed product x, and nCEth is the
number of carbon atoms in one ethanol molecule (i.e., 2).
pound can be compared to other iron molybdenum oxides
23
reported in the literature. Rb3FeMo4O15
exhibits 1D
Fe[Mo4O15] chains more or less identical but with one oxygen
atom in common with two MoO4 tetrahedra (Fig. 3a). FeO6
Crystal structure of La3Fe(MoO4)6
octahedra and MoO4 tetrahedra are also involved in 2D layers
24
A colourless needle like single crystal was isolated from the in KFe(MoO4)2 by sharing corners (Fig. 3b). In the other
inhomogeneous melt and mounted on a glass fibre. As afore- three examples: Fe2(MoO4)3 (Fig. 3d),25 Cs2Fe2(MoO4)3
mentioned, the observed extinction conditions undoubtedly (Fig. 3c)26 and Cs3FeMo4O15 (Fig. 3e),23 the connectivity
suggest the Pbca space group. The heavy atoms (3 La, 6 Mo between iron octahedra and molybdenum tetrahedra is rea-
and
1
Fe atoms) were easily localized using the lized by assembling oxygen corners together, building a final
Superflip21 method, whereas the remaining atoms (24 O) were 3D framework, more or less distorted by co-addition of
deduced from the inspection of successive difference Fourier counter cations.
maps. The structure refinement rapidly converged to the con-
The atomic positions, anisotropic displacement parameters
ventional reliability factor R1 = 0.0218.
and the principal distances are given in ESI S2 and S3.† The
The refined crystal structure obtained is shown in Fig. 2. It crystal-structure data for the La3Fe(MoO4)6 phase were de-
can be described as an association of one dimensional (1D) posited with the depository number CSD 429247.
chains of [Fe(MoO4)4], running along the b-axis, the chains
Structural complexity of the La3Fe(MoO4)6 structure has
being themselves built through the connection of FeO6 octa- been quantified using Shannon information theory.27 Theor-
hedra (dFe–O from 1.9124(20) to 2.0132(18) Å) sharing corners etically, structural complexity is strongly related to the atomic
with four MoO4 (dMo–O from 1.719 (2) to 1.8032(19)Å) tetra- arrangement of the unit cell using Shannon’ law. IG, total is
hedra. These chains are surrounded by cationic tunnels hosting equal to 1383.79 bits per unit cell (bit per u.c.), which classi-
La3+ (2 independent crystallographic sites, so 16 cations per fies it as a very complex structure. It is noteworthy that, only
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(Fig. 4c)) whereas no significant modification of a and c para-
meters are observed apart from the classical dilation phenom-
enon and (2) above 523 K some extra peaks appear (for
instance at 23.36, 23.89 and 29.66°). This phenomenon is
reversible on cooling and may be due to a partial decompo-
sition of the title phase leading to a modification of the b-para-
meter and with the appearance of a new phase. A more
detailed study is out of the scope of this preliminary
characterization. After this accident, the refinement of unit
cell parameters indicates a classical evolution with heating
and cooling (Fig. 4). This temperature evolution of both the
cell volume and of the different lattice constants allowed the
determination of the volume and of the linear thermal expan-
sion coefficients, α: αV = 28.6 × 10−6, αa = 11 × 10−6, αb = 16.1 ×
10−6, αc = 14.5 × 10−6 (K−1). The thermal expansion of the
material appears isotropic in all directions.
This study is also the proof that the high temperature form
of La3Fe(MoO4)6 has been stabilized and refined in this paper.
Under reductive atmosphere. The stability under a reductive
environment was checked by in situ X-ray diffraction of the
La3Fe(MoO4)6 powder sampled heated under a H2/N2 reductive
atmosphere (Fig. 5). Under this atmosphere, the material is
stable until 523 K, the temperature at which the same
phenomenon as under air is observed. Above 923 K, it starts to
reduce to La2(MoO4)3 and to an intermediate phase containing
iron. Another reduction is observed during the cooling
process, and leads to the formation of La7Mo(IV,V)7O30,
Mo(IV)O2, La2Mo2O9 and Fe(II)2Mo(IV)3O8.
Fig. 3 Several connections between FeO6 octahedra and MoO4 tetra-
hedra observed in the literature and compared to La3Fe(MoO4)6. (a)
Rb3FeMo4O15 building 1D units (corner sharing between FeO6 and
MoO4 tetrahedra are highlighted in yellow), (b) FeK(MoO4)2 with 2D
Fe(MoO4)2 cationic layers and finally (c) Cs2Fe2(MoO4)3, (d) Fe2(MoO4)3
and (e) Cs3Fe(Mo4O15) building a 3D network.
Magnetic properties
It is well known that iron molybdates may exhibit interesting
magnetic properties depending on the connectivity and
dimensionality between FeO6 and MoO4 subunits. For
instance, (Cs,Rb)Fe(MoO4)2 is an example of a triangular anti-
ferromagnet with an onset of 120° long range magnetic order
0.02% of inorganic structures are identified as very complex as below 4.5 K. It was announced as a possible multiferroic type
described in ref. 27. Here the complexity is due to the chemical II material with spontaneous polarization induced by the anti-
composition and the number of isolated groups gathered in symmetric magnetic exchanges.28,29 In La3Fe(MoO4)6, the Fe
the structure in terms of rods of FeO6 octahedra + 6 MoO4 connectivity is rather low and may suggest paramagnetism
sharing corners and isolated by La3+ cations and another set only. However, the present study shows a rather different sig-
of MoO4 tetrahedra. Such an arrangement is also closely nature, as detailed below.
related to the rather large unit-cell volume (V = 4437.62(41)Å3).
The temperature dependence of the magnetic susceptibility
of polycrystalline La3Fe(MoO4)6 is shown in Fig. 6. It evidenced
High temperature X-ray study and stability
a paramagnetic behaviour until a Neel like anomaly at TN
=
To simulate the different cycles and to check the stability of 6.6 K. Below this anomaly, AFM ordering occurs despite the
the solid prior to the catalytic experiment, an in situ X-ray diffr- poor connectivity of the Fe sub lattice (Fe–Fe = 9.65 Å along a,
action study versus temperature was performed under air and 5.25 Å along b, 11.03 Å along c). The Curie–Weiss law was
also under a reductive atmosphere. Accordingly, the sample refined between 300 K and 25 K giving μeff = 6.2μB/Fe3+ and
was heated from room temperature to 1073 K and then cooled θCW = −13.8 K. The μeff value is slightly higher than the theore-
to room temperature under two different atmospheres.
tical value (μeff = 5.92μB/Fe3+), probably related to the presence
Under air. Fig. 4a shows diffractograms measured for the of small impurities (the Morin transition of hematite is
polycrystalline sample of La3Fe(MoO4)6. All detected X-ray detected on χ−1 (T) at 260 K). The low negative value of θCW is
lines are consistent with the orthorhombic unit cell of the in good agreement with the expected AFM ordering below TN.
studied phase. With increasing temperature one can notice: (1) The magnetization at 2 K shown in the insert of Fig. 6 is
the displacement of peak positions related to a modification coherent with an AFM behaviour but shows an anomaly occur-
of the b-axis parameter occurring suddenly around 523 K ring at μ0H ∼ 2 Tesla, which could be due to minor Fe2O3
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Fig. 4 (a) High-temperature X-ray diffraction data for La3Fe(MoO4)6. A reversible transition is observed at 523 K during the heating and cooling
process as shown in the inserts, the blue diagrams correspond to the initial phase and the red ones to the HT phase, (b) cell volume and (c) b-axis
parameter evolution versus temperature for La3Fe(MoO4)6 giving the thermal expansion coefficient.
impurity. In this dilute Fe3+ system, the magnetic structure of
To solve the magnetic structure, only the Mx My and Mz
La3Fe(MoO4)6 is particularly interesting, especially the orien- components of a single site, and the appropriate represen-
tation and values of local moments of Fe3+ ions, in order to tation, are therefore needed. Only two representations lead to
conclude on the effects induced by very indirect exchanges, calculated magnetic intensities in relation with the experi-
Fe–(O)n–Fe.
A neutron diffraction study was performed at low tempera-
mental patterns Γ3 and Γ7.
Sx1 ꢀ Sx2 þ Sx3 ꢀ S4x ꢀ S5x þ S6x ꢀ S7x þ Sx8
ture (from RT to 1.5 K) on the G4.1 diffractometer, LLB labora-
tory, Orsay, France. The onset of the magnetic ordering was
confirmed through the appearance of magnetic satellites
between 4 K and 1.5 K on the neutron diffractograms. The
three main magnetic peaks appear at 14.86° (011), 16.44° (111)
and 20.78° (211) and can be indexed in the crystal unit cell
(propagation vector: k = (0,0,0)).
Γ3 : Sy ꢀ Sy ꢀ Sy þ Sy ꢀ Sy þ Sy þ Sy ꢀ Sy
1
2
3
4
5
6
7
8
Sz1 þ Sz2 þ Sz3 þ S4z ꢀ S5z ꢀ S6z ꢀ S7z ꢀ Sz8
Sx1 þ Sx2 þ Sx3 þ S4x ꢀ S5x ꢀ S6x ꢀ S7x ꢀ Sx8
Sy1 þ Sy2 ꢀ Sy3 ꢀ S4y ꢀ S5y ꢀ S6y þ S7y þ Sy8
Sz1 ꢀ Sz2 þ Sz3 ꢀ S4z ꢀ S5z þ S6z ꢀ S7z þ Sz8
Γ7 :
There is one single magnetic ion (Fe3+) in the Pbca unit cell, where Sid is the component along the d-axis of atom Fei. These
on the Wyckoff site 8c (x, y, z). A symmetry analysis performed two representations differ by spin reversal of half of the Fe
using the propagation vector k = (0,0,0), on the Pbca space ions, i.e. S1, S4, S5 and S7. Taking into account the very weak
group at the site 8c shows that the magnetic components of all magnetic satellites, the agreement factors are overestimated
8 sites are related by symmetry; the magnetic representation Γ and should be carefully considered. However the best refine-
can be decomposed into 8 irreducible representations of ment was obtained with the representation Γ7 (Rmagn = 24.9%)
dimension 1, contained 3 times each in Γ: Γ = 3Γ1 + 3Γ2 + 3Γ3 compared to Γ3 (Rmagn = 38.5%). In addition due to its very low
+ 3Γ4 + 3Γ5 + 3Γ6 + 3Γ7 + 3Γ8.
value the Sy component was considered in the second stage,
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Fig. 5 High-temperature X-ray diffraction data for La3Fe(MoO4)6 measured under a reductive atmosphere (N2/3%H2). The studied compound shows
a phase transition at 523 K (from blue to red part). Then, a transitional phase appears (in black) up to 1073 K after which iron and molybdenum are
reduced (green part).
component is not reversed which corresponds to an almost
total spin reversal. We note that along these two lateral direc-
tions magnetic coupling are very weak since no O2p overlap-
ping is expected taking into account long Fe–Fe distances
(>9 Å) and Fe–O–O–O–O–Fe paths. Here one should consider
long-distance local dipole–dipole magnetic interactions at the
origin of the 3D-ordering. The Γ3 model considered above
would be similar after shifting the spins of the chains by b/2.
These weak exchanges are responsible for the very low TN value
and the weak magnetic moments refined at T = 1.5 K show the
setting of antiferromagnetic ordering with partially ordered
moments. The particular orientation of the spins along the
pseudo-3 fold axis of Fe3+O6 tetrahedra is surprising. For such
isotropic magnetic ions (Fe3+, d5, L = 0, no LS coupling), ion
spin orientation is not expected to be influenced by the local
Fig. 6 ZFC/FC plots of M/H versus temperature at 103 Gauss. It shows
a paramagnetic behavior down to the Neel temperature: TN = 6.6 K.
axes (i.e., orbital overlapping), but should rather be held by
crystal axes.30,31 In La3Fe(MoO4)6, most of the magnetic inter-
actions are so weak, except along the chain AFM directions,
that local symmetry could predominate. Indeed, it was shown
very recently that the myth of an isotropic Cu2+ ion (S = 1/2)
has to be revised for a more magneto-anisotropic model due to
the spin–orbit coupling associated with their crystal-field split
d-states.32 Similar effects could be responsible for the anisotro-
pic Fe3+ ion local moments in the title compound.
Inset: magnetization M(H) at T = 2 K in mB per f.u.(G).
and its refinement in the Γ3 model is unstable and does not
converge. It follows that Γ7 appears most plausible leading to
Sx = 0.9(1), Sy = 0.0(1), Sz = 0.7(1) and M = 1.2(1)μB. The calcu-
lated pattern is shown in Fig. 7. The refined magnetic structure
is also shown in Fig. 7. Along the chains nearly parallel to the
b-axis two consecutive FeO6 (Fe1 and Fe5) are antiferromagneti-
cally coupled with Fe–O–O–Fe superexchanges (d(Fe–Fe) =
5.25 Å). Between two chains separated by a/2 (e.g. Fe1 and Fe4),
the spin components Sx are conserved while Sy and Sz are
inversed. Finally, along c/2 (e.g. Fe1 and Fe7) only the weak Sy
Catalytic properties
Two samples of La3Fe(MoO4)6 – described hereafter – were
tested for ethanol oxidation reaction at 600, 625 and 650 K. For
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Fig. 7 (a) NPD study at 4 K and 1.5 K. Main magnetic Bragg reflections are labelled, (b) refinement of the neutron diffraction pattern at 1.5 K using
the obtained magnetic structure (k = (0,0,0)), orientation of magnetic moments in (c) (c, b) and (e) (a, b) planes with (d) local 3-fold geometry of the
magnetic moment in polyhedra is also enhanced.
all the experiments, carbon balance was always in the range of
100 3%. The first one was a La3Fe(MoO4)6 sample used as-
prepared (at high temperature and hence with big crystal sizes,
Fig. 8a)), while the second one resulted from mechanical ball
milling of the same solid prior to the catalytic test (in order to
decrease the crystal size and hence to increase the specific
area and thus reactivity, Fig. 8b)). After the synthesis, the
material was attrited 4 hours (Netzch PE 075, 1000 rpm) in
ethanol using zirconia balls (2 mm diameter). This treatment
leads to an increase in the SSA, for which the evolution over
time is given in Table S4.† Note that an efficient attrition pro-
cedure needs at least 20 g of powder.
Fig. 8 represents the evolution of the ethanol conversion
versus the temperature for both samples (i.e., native and after
4 hours of attrition). As expected, the ethanol conversion
increased with temperature. However, while, e.g., at 650 K over
the attrited La3Fe(MoO4)6 powder a 97% ethanol conversion was Fig. 8 SEM micrographs of a La3Fe(MoO4)6 sample prepared by solid
state reaction (a) before and (b) after 4 hours attrition with precision of
specific surface area (SSA), (c) comparison of ethanol conversion for
La3Fe(MoO4)6 prepared with and without attrition and (d) selectivity to
different products for La3Fe(MoO4)6 prepared with attrition.
observed, the native non-premilled La3Fe(MoO4)6 sample was
less active with a conversion of only 34% under the same con-
ditions due to a lower SSA (0.4 m2 g−1 vs. 5.6 m2 g−1 for the
former – Table S4†). Ethanol oxidation on non-attrited La3Fe
(MoO4)6 exclusively leads to the production of a chemical
product obtained after partial oxidation, namely acetaldehyde.
Over the ball-milled sample other products were observed maldehyde, we propose the following interpretation: the CH3CH
(Fig. 8). The main product was still acetaldehyde, but CO2 and (OH)CH2COH aldol can be formed from ethanol and then retro-
aldolised to formaldehyde and acetone, as already reported.33
formaldehyde were also formed. To explain the apparition of for-
14450 | Dalton Trans., 2015, 44, 14444–14452
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tion Nationale de l’Enseignement Superieur et de la Recherche
are acknowledged for funding the X-ray diffractometers. Laur-
ence Burylo and Nora Djellal are thanked for their precious
technical help. This work was carried out under the framework
of the Multi-InMaDe project supported by the ANR (Grant ANR
2011-JS-08 003 01). Authors are grateful to Dr S. Colis for mag-
netic measurements.
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The second lanthanum iron molybdenum oxide referenced to
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Its composition (Fe + Mo for redox and La for basicity) and
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Acknowledgements
The Fonds Europeeen de Developpement Regional (FEDER),
CNRS, Region Nord Pas-de-Calais, and Ministere de l’Educa-
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