Molecular approach to prepare mixed MoW alumina supported hydrotreatment catalysts using H4SiMo: NW12- nO40 heteropolyacids

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Nikulshin and C. Lamonier, Catal. Sci. Technol., 2018, DOI: 10.1039/C8CY00672E.

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Molecular approach to prepare mixed MoW alumina supported

hydrotreatment catalysts using H₄SiMo_nW_12-nO₄₀heteropolyacids

M. S. Nikulshina,^a,b, P. Blanchard^a, A. Mozhaev^b, C. Lancelot^a, A. Griboval-Constant^a, M. Fournier^a,

E. Payen^a, O. Mentré^a, V. Briois^c, P.A. Nikulshin^b* and C. Lamonier^a*

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

H₄[SiMo_nW_12-nO₄₀

] heteropolyacids (HPA) are interesting precursors for the preparation of alumina supported

hydrotreating catalysts to introduce simultaneously both metals while maintaining a Mo-W nanoscale proximity. Two

heteropolyacids with n = 1 and 3 have been synthesized and used for the first time for preparing hydrotreatment catalysts.

The crystal structure refinement has been performed and evidences the formation of β-H₄[SiMo₃W₉O₄₀] with three

ordered Mo sites forming a face. The purity of the samples in aqueous solution has been shown from Raman and

polarography characterizations. These heteropolyacids were then impregnated on alumina to prepare supported MoW

based catalysts. As references, catalysts with same Mo/W ratios have been prepared using monometallic H₄SiMo₁₂O₄₀and

H₄SiW₁₂O₄₀HPA (mixture of these 2 HPA in the impregnating solution). EXAFS characterization after drying performed

4-

simultaneously at the Mo K and W L_IIIedges indicate mixed heteropolyanion SiMo_nW_12-nO₄₀preservation at the alumina

surface even if a partial decomposition to Keggin lacunary species could not be excluded and evidences after sulfidation

the mixed “MoW-S₂” slabs formation. Better catalytic hydrogenation properties for dibenzothiophene

hydrodesulfurization and naphthalene hydrogenation have been obtained when using β-H₄[SiMo₃W₉O₄₀] which is

explained

by

the

formation

of

the

mixed

“MoW-S₂”

active

phase.

heteropolyanions can be synthesized directly depending on the

medium pH. It is also possible to prepare substituted derivatives in

which one or more metal atoms (Mo, W, etc.) of the dodeca-

condensed structure are replaced by different atoms. Substitution

can take place with an element whose oxidation number is much

lower than that of the metal atom it is replacing (e.g. cobalt-II,

nickel-II, iron-III…)^2,3, or with an element that can have similar

properties to that of the substituted atom, as with vanadium-V in

the case of the acid H₄PMo₁₁VO₄₀⁸. Sanchez et al.⁹have thereby

isolated potassium and tetrabutyl ammonium salts of mixed

Introduction

The Keggin-type heteropolyanion or polyoxometallate (POM)

XM₁₂O₄₀^n-(with M = Mo, W and X a non-metallic atom) consists in a

regular XO₄tetrahedron surrounded by 12 MO₆octahedra, which

are connected by shared edges to form trimetallic M₃O₁₃groups

joined together by their vertices¹. Depending on the synthesis

conditions, various isomers (as α,β,γ), obtained by a π/3 rotation of

a M₃O₁₃group around C₃axis, can be isolated². At solid state, these

ions and their counter-cations form a crystalline network, with a

symmetry depending on the hydration of the solid and the nature

of the counter-cation². The "lacunary" Keggin structure corresponds

to a Keggin structure in which one or more sites previously

occupied by molybdenum or tungsten atoms are vacated. This new

family corresponds to an M/X ratio of 11^3-5or 9 ^6,7, obtained by

extracting one or three metal atoms from a single M₃O₁₃group or

from different M₃O₁₃groups. These monovacant or polyvacant

4-

SiMo₁W₁₁O₄₀and SiMo₃W₉O₄₀^4-heteropolyanion. Reduction study

on mono substituted Keggin SiMo^VI₁W^VI

synthesized with

11

microtube shape has also been recently reported by Shen and al.¹⁰

with the formation of stable reduced SiMo^V₁W^VI₁₁microtube.

In the field of hydrotreatment catalysis, Keggin and Keggin-

derivated heteropolymolybdate and heteropolytungstate structures

have been successfully used to replace conventional precursors as

ammonium heptamolybdate (NH₄)₆Mo₇O₂₄(AHM) and ammonium

metatungstate (NH₄)₆H₂W₁₂O₄₀(AMT) to prepare oxidic precursors

of metal sulfide catalysts^11-15. From a catalytic point of view, the use

of mixed metals Keggin-type POM may enhance activity due to the

introduction of a second metal in the POM cluster. Mixed HPA

(3+x)-

precursors PMo_(12-x)V_xO₄₀

generally used in the field of mild

oxidation catalysis have been prepared for their use in

hydrodemetallation catalysis¹⁶or diesel hydrotreatment¹⁷

. The

synthesis of cobalt salt of substituted PCoMo₁₁O₄₀H^6-has been

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developed leading to HDS catalysts with improved catalytic room temperature (RT), the potassium salt was precipitated by

performances compared to those of conventional catalysts solid KCl addition (about 50 g) and separated by filtration.

prepared from AHM and cobalt nitrate¹⁸. For hydrocracking catalytic The synthesis of the potassium salt α-K₄[SiMo₁W₁₁O₄₀] was

2+

application, NiW ASA (Amorphous Silica Alumina) supported sulfide then performed at RT following reference⁹.

A MoO₂

catalysts were also prepared from heteropolytungstate Ni salts as molybdenum solution was prepared by addition of 6mL of

the substituted Ni₃PW₁₁NiO₄₀H or the lacunary Ni₄SiW₁₁O₃₉salt¹²

nitric acid (13M) in 10 mL of a Na₂MoO₄solution (1M). Then 10

.

For the CoMo system as well as for the NiW one, the vicinity of both g of α-K₈[SiW₁₁O₃₉] were added by small fractions. The

elements in the HPA salt at the molecular level was able to potassium salt of SiMo₁W₁₁O₄₀^4-precipitated as a yellow solid.

generate a great amount of Co-MoS₂or Ni-WS₂active phase after In order to allow better yield, it was filtered after 3 hours and

sulfidation leading to improved catalytic performances^11,19

.

purified by recrystallization in distilled water.

Another way to develop active hydrotreating catalysts was carried We observed that the mixed corresponding free acid was

out in 2001 with the use of bulk mixed tri-metallic (Ni)MoW sulfide. sufficiently stable to be crystallized from aqueous solution by using

This new type of catalyst was at least three times more efficient the “etherate” method². A concentrated aqueous solution of the

than any previous hydrotreating catalyst, conventionally used in mixed MoW potassium salt was strongly acidified with HCl 12M and

industry²⁰

expensive MoW mixed supported catalysts prepared by heavy etherate phase was decomposed by addition of water. The

impregnating on support classical precursors (together or remaining solution is let one night (around 12 hours) in a fume

. Further researches recently proposed to use less shaken with excess of diethyl oxide at RT. After separation, the

a

sequentially) such as ammonium heptamoybdate and ammonium hood allowing vapor ether extraction. After removal of ether the

metatungstate^21-23or ammonium thiomolybdate (NH₄)₂MoS₄and aqueous solution was evaporated at 4 °C until α-H₄[SiMo₁W₁₁O₄₀

]

24

ammonium thiotungstate (NH₄)₂WS₄

. Mechanical mixture of heteropolyacid crystallization.

supported NiMo-NiW catalysts has also been proposed²⁵. Whatever

the preparation procedure except mechanical mixture, Ni promoted β-H₄[SiMo₃W₉O₄₀

mixed MoW catalysts revealed most of the time better catalytic

]

performances than that of their monometallic counterpart.

We propose here a new approach to prepare MoW catalysts based salt β-Na₉[SiW₉O₃₄H] was prepared according to reference²⁷.

on the use of mixed MoW heteropolyacids H₄SiMo_nW_12-nO₄₀12 g of sodium metasilicate (Na₂SiO₃, 5H₂O) were dissolved in

In the first step the lacunary Keggin polyoxotungstate sodium

,

allowing to start from a single molecular entity to introduce 250 mL of cold water (12°C) and 150 g of sodium tungstate

together Mo and W. We synthesized two mixed heteropolyacids (Na₂WO₄, 2H₂O) were added. Hydrochloric acid (6M, 95 mL)

H₄[SiMo₁W₁₁O₄₀

] and H₄[SiMo₃W₉O₄₀] that were extensively was slowly added into the vigorously stirred solution. A silica

characterized at the solid state combining X-Ray Diffraction and XAS residue was filtered off if necessary. The solution was left to

(X-Ray Absorption Spectroscopy) at the Mo K and W L_IIIedges. stand at RT at least one hour while stirring and let in the fridge

Purity of these mixed HPA together with their stability in aqueous at 4°C during two days until precipitation.

solution was checked by Raman spectroscopy and polorography. The synthesis of the mixed potassium salt β-K₈[SiMo₃W₉O₃₉]

Characterizations by Raman and XAS were also performed after was performed following ref⁹. A MoO₂²⁺molybdenum solution

deposition on alumina support in order to check the preservation of was prepared by addition of 60 mL of hydrochloric acid (12M)

the mixed heteropolyacids. Catalytic results of the mixed supported in 40 mL of a Na₂MoO₄solution (1M). Then 27 g of β-

system were compared to that obtained using monometallic Na₉[SiW₉O₃₄H] were added by small fractions of 3 g. After 30

H₄SiMo₁₂O₄₀and H₄SiW₁₂O₄₀HPA and their mixtures with Mo/W min, the resulting pale yellow solution was precipitated by 5 g

ratio = 1/11 and 3/9. Catalysts EXAFS characterization at the Mo K of solid KCl to form K₄[SiMo₃W₉O₄₀] that was filtered off.

and W L_IIIedges after sulfidation have been used to explain the The H₄[SiMo₃W₉O₄₀] heteropolyacid is also prepared by

different hydrodesulfurization and hydrogenation properties.

dissolving the mixed potassium salt in water and extraction by

ether and HCl. Crystals of β-H₄[SiMo₃W₉O₄₀] were formed after

evaporation at 4°C.

Experimental

Synthesis of mixed H₄[SiMo_nW_12-nO₄₀

(n=1,3)

] heteropolyacids (HPA)

Preparation of the supported oxidic precursors

Impregnating solutions were prepared by dissolving mixed

heteropolyacids in water solution. For comparison purposes,

monometallic impregnating solutions were prepared using the

Keggin heteropolyacids H₄SiMo₁₂O₄₀and H₄SiW₁₂O₄₀prepared

according to references.^3,28Bimetallic reference catalysts were

also prepared using impregnating solutions obtained by mixing

H₄SiMo₁₂O₄₀and H₄SiMo₁₂O₄₀in water solution with a Mo/W

ratio of 1/11 and 3/9 corresponding to the mixed MoW

α-H₄[SiMo₁W₁₁O₄₀

]

In the first step, the lacunary Keggin polyoxotungstate

potassium salt α-K₈[SiW₁₁O₃₉] was prepared following

reference²⁶. 182 g of sodium tungstate (Na₂WO₄,2H₂O) and 11

g of silicate (Na₂SiO₃,5H₂O) were dissolved in 300 mL of hot

water (60°C) in which was added 195 mL of hydrochloric acid

(4M). After boiling this solution during one hour and cooling at

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heteropolyacids and were denoted Ref Mo₁W₁₁and Ref at 3.9 atoms /nm², leading to different theoretical Mo/W

Mo₃W_9,respectively. loadings as reported in Table 1. All prepared impregnation

The oxidic precursors were prepared by incipient wetness solutions had a close pH value around 0.25 due to the same

impregnation of alumina support [γ-Al₂O₃(Norton), specific concentration of protons. After impregnation and 2 h of

area: 240 m²/g, pore volume: 0.9 mL/g] with the maturation in a wet atmosphere in order to let the species

aforementioned impregnating solutions.

diffuse into the alumina extrudates, the solids were dried

In any case for comparison purpose, the HPA concentration overnight at 100°C without further calcination.

was kept at 0.145 mol_HPA.L^-1to set the (Mo+W) surface density

Table 1. Composition of precursors and prepared catalysts.

MoO₃

(wt %)

18.0

-

WO₃

(wt %)

-

Precursor(s)

Catalyst nomenclature

H₄SiMo₁₂O₄₀

H₄SiW₁₂O₄₀

SiMo₁₂/Al

SiW₁₂/Al

26.2

24.2

20.1

24.2

20.1

H₄SiMo₁W₁₁O₄₀

SiMo₁W₁₁/Al

SiMo₃W₉/Al

Ref Mo₁W₁₁

Ref Mo₃W₉

1.4

H₄SiMo₃W₉O₄₀

4.2

H₄SiMo₁₂O₄₀+H₄SiW₁₂O₄₀

1.4

4.2

Characterization techniques

Raman Spectroscopy

and refined. Multiscan absorption correction was applied using

SADABS²⁹. The structure was solved using Superflip³⁰and

refined using Jana 2006³¹. Specificities for each collection and

refinement are given below in the crystal structure dedicated

The Raman spectra of the samples were recorded at RT using sections. Further details of the crystal structures may be

an Infinity XY Horiba Jobin-Yvon Raman microprobe equipped obtained from the Fachinformationszentrum Karlsruhe, 76344

with a photodiode array detector. The exciting laser source Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666;

was the 532.16 nm line of a Nd-YAG laser. The wavenumber e-mail:

crysdata@fiz-karlsruhe.de,

http://www.fiz-

accuracy was 4 cm^-1.

karlsruhe.de/request_for_deposited_data.html) on quoting

the depository numbers CSD–431628 and CSD–431629 for the

SiMo₁W₁₁O₄₀H₄and SiMo₃W₉O₄₀H₄hydrates, respectively. The

quantification of the water molecules is discussed in the

TGA and DSC analysis

Thermogravimetric analysis was carried out with a STA 449 F3 dedicated section.

Jupiter. The measurements were conducted in air with heating

from RT to 600°C and a heating rate of 10°C/min. Samples of X-Ray Absorption Spectroscopy

about 30 mg were introduced into a small corundum crucible,

an empty crucible was used as the standard for thermal Mo K and W L_IIIedges EXAFS (Extended X-ray Absorption Fine

analysis.

Structure) measurements were carried out at the ROCK beamline of

the SOLEIL synchrotron source³²using Si(111) monochromator. The

measurements were done at RT on powdery sample pressed into

pellets when characterizing bulk HPA and dried catalysts at the

Single crystal X-Ray Diffraction

Single crystals suitable for XRD experiments have been isolated oxide state. Sulfide catalysts were characterized after in-situ

from the crystallization products and sticked on a glass fiber sulfidation using the catalytic cell available at SOLEIL ³³. Powdered

using vacuum grease. Crystals of β-H₄[SiMo₃W₉O₄₀].9H₂O are dried oxide catalysts were loaded in the cell and the in-situ

air stable and can be handled without precaution. On the sulfidation was performed with a flow of 10% H₂S in H₂at

opposite, crystals of the α-H₄[SiMo₁W₁₁O₄₀].30H₂O are very atmospheric pressure from RT to 400°C with a plateau of two hours

unstable under exposure in air and rapidly decompose. In this according to the activation procedure applied in catalytic tests.

case, crystals were placed in grease, rapidly mounted and the For the whole measurements, XAS spectra were obtained in

data were collected under nitrogen flow at 100 K without any transmission mode with ionization chambers as X-Ray detectors.

decomposition.

Both crystal structures have been refined using single crystal nitrogen (50:50) for measurements at the Mo K edge and (73:27) at

XRD data collected on DUO-Bruker SMART apex the W L_IIIedge. Mo and W edges EXAFS regions of the spectra were

diffractometer using the Mo Kα radiation by least square extracted and analyzed using Athena and Artemis software

minimization on F_hkl

Intensity data are corrected for Lorenz- packages³⁴. For Mo K edge, k³χ(k) EXAFS signals were Fourier

Polarization effects and absorption, then the structure was solved transformed in R-space pseudo-radial distribution functions using a

The ionization chambers were filled with a mixture of argon and

a

.

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Kaiser−Bessel window between k_min= 4.8 Å⁻¹and k_max= 10.5 Å⁻¹for the decomposition of the Mo 3d and W4f spectra of the sulfided

and dk window sill parameter corresponding to the sloped region of solids has been described elsewhere ^12,36. The Mo3d spectrum is

Fourier transform is equal to 2. For W L_IIIedge, FT of the k³χ(k) decomposed in three doublets regarding Mo (three Mo species)

EXAFS data were carried out from 3.5 to 16.8 Å⁻¹with a dk and two S2s components associated to two sulfur species S^2-and

parameter equal to 2. The inverse Fourier Transformation of the (S₂)^2-as usually reported for Mo(W) and Co(Ni)Mo hydrotreatment

EXAFS at the Mo K edge and W L_IIIedge were fitted using catalysts. The three well known Mo doublets whose Mo3d_5/2

appropriate single and multiple scattering EXAFS paths generated components are placed at about 229.0, 230.5 and 233.0 eV

from the cif files derived from crystallographic structures. During according to three oxidation states of molybdenum, Mo^IV, Mo^Vand

fitting of mixed bulk samples using Artemis, the distances between Mo^VIrespectively. The first contribution is attributed to MoS₂phase,

the concerned atoms and the coordination numbers were the second one is attributed to Mo^Vsurrounded by oxygen and

constrained by the related parameters extracted from the XRD sulfur atoms (intermediate sulfidation phase) and the contribution

single crystal data to check the transferability of amplitude and at higher binding energy is assigned to Mo^VIoxide species. The W4f

phase functions which will be further applied for catalysts EXAFS spectra could also be decomposed into three doublets whose

analysis. In this frame, the amplitude reduction factor S₀², which W4f_7/2components are attributed to three different W species: W^IV

takes into account multielectronic effects, together with the enot sulfide (WS₂) with BE at about 32.2 eV), W^Voxysulfide with BE at

parameter which allows to match the k-scale of theoretical phase about 33.5 eV and W^VIoxide with BE at about 36 eV. The

and amplitude functions with the one of the experimental EXAFS decomposition of Mo3d and W4f XPS spectra allowed obtaining the

signals related to the user choice of the E₀position on the edge, relative atomic % of each corresponding Mo (W) species noted

2

were first determined on the bulk HPA for the Mo and W edges. S₀

and enot values were after kept fixed for catalysts fitting. The

number of fitted points was below the number of independent

%Mo_i(%W_i) according to the following formula:

%Mo_i= [A_i(Mo3d_5/2+ Mo3d_3/2) /∑(A_i(Mo3d_5/2+ Mo3d_3/2))] × 100

points according to the Nyquist criterion³⁵. Typical accuracies for where A_i(Mo3d_5/2+ Mo3d_3/2) is the area of one Mo doublet,

the determination of the coordination numbers and distances are

10 to 20 % and 1 to 2 %, respectively.

and %W_i= [A_i(W4f_7/2+ W4f_5/2) /∑(A_i(W4f_7/2+ W4f_5/2))] × 100

where A_i(W4f_7/2+ W4f_5/2) is the area of one W doublet.

Polarography

Catalytic activity

The polarographic measurements were performed on a Tacussel The catalytic properties were evaluated using

a fixed-bed

(Pol 150) electrochemical analyzer monitored by a Tracemaster 5 microreactor featuring a high-pressure flow system. 0.6 g of sieved

programmer. A conventional single-compartment cell with standard catalyst (0.25 – 0.50 mm) was diluted with 1 cm³of low-surface-

three-electrodes configuration was used. Pt was used as an auxiliary area sieved carborundum (0.2 – 0.4 mm) and placed in the centre of

electrode, while Ag/AgCl was used as a reference electrode. The the reactor (the reactor had an internal diameter of 8 mm). Before

working electrode was a glassy carbon disk (using a rate of stirring testing, the catalysts were sulfided in a flow of 10% H₂S in H₂at

was equal to 840 rpm) allowing the heteropolymolybdates study 400°C for 2 h. A mixture of DBT (1500 ppm S), naphthalene (3 wt.

with reduction waves observed in the range -0.1 and 0.5 V. %), hexadecane (as an internal standard, 1 wt. %) and toluene (as a

Tungsten compounds being less reducible than molybdenum solvent) was used as a model feedstock for evaluation of HDS and

compounds, the study of H₄SiW₁₂O₄₀that requires a mercury hydrogenation (Hyd) performances. Catalysts were tested under

dropping electrode creating a higher overvoltage for reaching lower the following conditions: 320°C, 3.0 MPa hydrogen, 10 h^-1liquid

potential (between -1.1 and -0.2 V) could not be performed. hourly space velocity (LHSV) and a 500 NL/L volume ratio of

Deoxygenating of the solutions was carried out using argon as inert hydrogen to feed. The liquid product compositions of the samples

gas. The concentration in molybdenum atoms was equal 10⁻³M in collected every 0.5 h were determined using a Crystall-5000 Gas

aqueous electrolytic solutions HCl (1 M)/ethanol (equivolume).

Chromatograph (GC) equipped with a 30 m OV-101 column. The

reaction products were identified by matching retention times with

those of commercially available standards and by GC/MS analysis

using a Finnigan Trace DSQ. The HDS reaction was allowed to

XPS analysis

The composition and chemical state of the elements were studied proceed for 12 h to evaluate the deactivation of the catalyst.

using X-ray Photoelectron Spectroscopy (XPS) on a KRATOS Axis However, all catalysts exhibited stable performance, achieving a

Ultra spectrometer equipped with a monochromatic Al Kα (1486.6 steady state after 5 – 6 h. All catalysts were tested two times to

eV) X-ray radiation (powered at 100 watts). The analysis chamber exclude the possible error and average values of reactants

was operated under ultrahigh vacuum with a pressure close 10^-10conversion and selectivity ratios were calculated. The rate

Torr. The binding energy (BE) values were referred to the Al_2pat constants of the pseudo-first-order reactions of the DBT HDS and

74.6 eV. Simulation of the experimental photopeaks was carried out naphthalene Hyd were determined using the following equations:

using a mixed Gaussian/Lorentzian peak fit procedure according to

F_Naph

w

F_DBT

w

,

(1)

k_Hyd= −

ln(1− x_Naph)

k_HDS= −

ln(1− x_DBT)

the software supplied by Casa XPS. The methodology implemented

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where K_HDSand K_Hydare the pseudo-first-order reaction constants selectivity was calculated according to the reaction network for DBT

for the DBT HDS and naphthalene Hyd (mol g^-1h^-1) respectively, X_DBTHDS (Scheme 1):

and x_Naphare the conversions (%) of DBT, and naphthalene

respectively, F _DBTand F_Naphare the reactant flows in moles (mol h^-1)

k_HYDC_CHB+ C_BCH

S_{HYD / DDS}

=

and w is the weight of the catalyst (g).

k_DDS

C_BP

,

(2)

The HDS products from DBT include biphenyl (BP) via the direct

desulfurization (DDS) pathway, as well as cyclohexylbenzene (CHB)

and bicyclohexyl (BCH) from the HYD pathway. The HYD/DDS

where C_CHB, C_BCH, and C_BPare the concentrations (wt. %) of CHB,

BCH, and BP in the reaction products, respectively.

BP

Slowly

DDS

S

BCH

CHB

HYD

DBT

S

THDBT

Scheme 1. Reaction network of the HDS of DBT.

maps. They are often associated to large thermal parameters due to

local disorder, e.g. Mo/W, protonation or not. To validate the

presence of water molecules in between the polyanions, the

occupancies of oxygen atoms (Ow) was refined and rejected if

significantly lower than 100%. It means that the hydration rate

given above stem from crystal refinements, but additional water

molecules could exist in some partially occupied available voids.

Collection and refinement parameters are given in Table 2. Intra

HPA cation-anion bond distances are reported in SI (Table 1 SI).

Results and discussion

HPA Crystal Structures:

For both compounds, the space group was determined by means of

the systematic extinctions and several models have been tested in

case of ambiguities. The charge flipping method located most of the

atoms with their chemical nature on the basis of their coordination

and convergence of the refinement. Complementary anions,

including water molecules have been located on Fourier-difference

Table 2. Crystal data, data collection and structure refinement parameters for single crystal of α-H₄[SiMo₁W₁₁O₄₀] and β-

H₄[SiMo₃W₉O₄₀] poly-hydrates.

Formula

α-H₄[SiMo₁W₁₁O₄₀]∙30H₂O

β-H₄[SiMo₃W₉O₄₀].9H₂O

Diffractometer: Bruker ApexII, Duo, λ =0.7107 Å (Mo Kα)

Crystal System

Tetragonal

P-4n2

12.735(1)

-

Monoclinic

P2₁/m

Space group

a (Å)

10.1951(2)

14.4697(2)

14.8556(2)

100.3961(8)

2155.52(6)

2

b (Å)

c (Å)

18.079(1)

-

β(°)

V (Å³)

2932.2(4)

2

Z

ρ_calc(g cm^–3

μ (mm^–1

)

3.8133

21.85

4.28

)

24.89

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N collected Reflection

N ind Reflections (all)

R_int(%)

126997

3013

29958

5729

4.14

5.01

1.96-26.42

15≥h≥-15

22≥h≥-22

Least squares on F

0.0229

1.98-28.62

13≥h≥-13

19≥k≥-19

20≥l≥-19

θ

min-max

hkl range

Refinement method:

R₁[I > 3σ(I)]^a

0.0284

0.0410

0.0439

0.0448

unit

a

wR₂

0.0239

R₁(all data)

0.0275

wR₂

0.0276

Weighting scheme

goodness-of-fit

unit

0.43

1.92

N. refined parameters

largest diff. peak and hole [e Å^–3

175

176

]

1.37, –1.12

2.64, –2.31

2

^aR₁=∑||F_o|−|F_c||/∑|F_o|; wR₂={∑[w(F_o²−F_c)₂]/∑[w(F_o²)²]}^1/2

α-H₄[SiMo₁W₁₁O₄₀]. 30 H₂O:

After data collection at 100K, the crystal structure was satisfactorily creates local off centering in the water sub-array.

refined in the tetragonal P-4n2 symmetry. The lattice parameters

and final R values are a= 12.735(1) Å, c=18.079(1) Å, Z= 2, R_obsβ-H₄[SiMo₃W₉O₄₀]. 9 H₂O:

=2.29%, wR_all=2.75%. The refinement of the statistic disorder over The crystal structure adopts the monoclinic P2₁/m symmetry with

the three 8(i) general positions occupied by metal ions matches well lattice parameters and final

R

values a=10.1951(2) Å,

the experimental predicted formula and was fixed to ideal Mo:W b=14.4697(2) Å, c=14.8556(2) Å, β=100.3961(8)°, R_obs=2.84%, wR_obs

=1/12 : 11/12 in the last refinement cycles. Some external oxygen =4.10%. In the solid, the polyanions are separated by ~9.6 Å leading

(O1, O5, O6) and water molecules have very large thermal to a much smaller hydration ratio than in the previous case. In this

parameters which mainly picture local disordering due to the compound one rare characteristic concerns the full ordering

statistic Mo/W disorder and distribution of bond lengths. Si-O detected between Mo and W sites. The β-SiMo₃W₉O₄₀is obtained

distances are 1.624 Å while the three independent W/MoO₆from a β-SiW₉O₃₄H lacunary structure further completed by 3 Mo

octahedra are typical with one short (~1.66 Å) four intermediate atoms. The β-SiW₁₂O₄₀structure differs from the Keggin α one by

(~1.80-2.01 Å) and one long (~ 2.40 Å) bonds. The number of rotating one W₃O₁₃group by 60° under a ternary axis and in this

crystallization water molecules is important leading to separation case each WO₆of this rotated group is sharing oxygen corners with

between heteropolyanions around ~12.8 Å (measured from Si to Si), WO₆octahedra belonging to the same W₁₃O₁₃group. As shown in

as shown Fig 1 (a). The morphology of the polyanion is typical of the the mixed HPA structure presenting three adjacent Mo octahedra,

α-form with four W₃O₁₃groups surrounding symmetrically (T_dthe initial β-[SiW₉O₃₄H] structure was obtained by removing three

symmetry) the SiO₄tetrahedron. In each W₃O₁₃group, WO₆adjacent W atoms from three different W₃O₁₃groups (that differ

octahedra are linked by edge-sharing while the octahedra belonging from the rotated W₃O₁₃) in agreement with the β-[SiW₉O₃₄H]^9-

3-

to two different W₁₃O₁₃groups are sharing oxygen corners. From structure reported by Robert and Tézé³⁷and the α-PMo₉O₃₄H₆

the SiW₁₂O₄₀structure any WO₆can be removed and be replaced structure determined by Strandberg³⁸, showing in both cases three

4-

8-

]

by one MoO₆octahedron to form the lacunary α[SiW₁₁O₃₉

adjacent metallic sites. Based on these literature data, Sanchez et

structure. The announced 30 H₂O hydration amount was deduced al.⁹assumed that in the β SiMo₃W₉O₄₀the three Mo atoms

from the water molecules present in a sufficient amount for refined belonged to three different groups without performing any

occupancies greater than 30% and with viewable Debye Waller structural determination.

parameters and should approach rather well the true composition.

4-

It is clear that the local protonation of the external oxygen anions

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Figure 1. Crystals and detailed heteropolyanion structures and formation of α-H₄[SiMo₁W₁₁O₄₀] (a,b,c) and β-H₄[SiMo₃W₉O₄₀] (d,e,f,g).

Bulk HPA analysis

1.91, 2.38 Å) resulting from a distorted oxygen octahedral

coordination whereas the second shell contains the contribution of

W, Si and Mo backscattering neighbors at 3.40, 3.58 and 3.81 Å

respectively. The second neighbors at 3.40 Å are W atoms from the

same M₃O₁₃group while Mo neighbors belonging to another M₃O₁₃

appeared at higher distance (3.81 Å). Around W (Table 4), the first

coordination shell of bulk H₄[SiW₉Mo₃O₄₀] presents similar oxygen

contributions than that observed around Mo due to similar size of

six-fold coordinated Mo⁶⁺(0.59 Å) and W⁶⁺(0.60 Å) cations³⁹. The

second coordination shell around W contains five contributions, the

second neighbors around W are shared between W (N=1.33) and

Mo (N=0.66) at 3.36 and 3.40 Å from the same M₃O₁₃group while

W neighbors belonging to another M₃O₁₃group are shared between

W from the 60° rotated W₃O₁₃group and the three others groups

leading to two different distances equal to 3.74 Å and 3.68 Å

respectively.

EXAFS analysis of bulk HPA

H₄[SiMo₃W₉O₄₀]

β-H₄[SiW₉Mo₃O₄₀] EXAFS oscillations both at the Mo K and at the W

L_IIIedges have been successfully refined (see supporting

information) taking into account the radial distribution functions of

neighbors deduced from the crystallographic data determined from

XRD single crystal analysis. In Tables 3 and 4 are reported the main

parameters of the least square fitting process at the Mo K and W L_III

edges, the number of neighbors N, the distance R (Mo-X for Table 3

and W-X for Table 4), the Debye-Waller factor σ² and the R factor

which measures the relative misfit of the simulation with the

experimental data. In agreement with N and R crystallographic data

also reported in Table 3 the first shell around the Mo atom contains

three contributions due to three Mo-O different distances (1.68,

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Table 3. Structural parameters determined by least square fitting of the inverse Fourier Transform (ꢀR = 1-3.9 Å) of the Mo K edge EXAFS

signals for bulk H₄[SiW₉Mo₃O₄₀] and H₄[SiW₁₁Mo₁O₄₀] HPAs and their corresponding dried supported catalysts with S₀²=1.0 and enot = 8.70

eV for a E₀value used for EXAFS extraction equal to 20012.6 eV. Values given in bracket correspond to the values range determined from

single crystal XRD data. Italic parameters were kept fixed during the minimization process.

Backscatterer

N

R (Å)

σ² x 10³(Å²)

R-factor

0.0076

Bulk H₄[SiMo₃W₉O₄₀

1.68 (1.67)

1.91 (1.87-2.00)

2.38 (2.37-2.39)

3.40 (3.35-3.37)

3.58 (3.55-3.57)

3.81 (3.72-3.78)

SiMo₃W₉/Al

1.75

]

O

1.0 (1)

4.0 (4)

1.0 (1)

2.0 (2)

1.0 (1)

2.0 (2)

1.7

10.1

0.5

6.3

2.0

4.3

O

W

Si

Mo

O

3.0

2.0

1.0

2.0

1.0

2.0

7.0

1.0

2.02

O

2.37

1.5

0.0027

W

Si

3.42

10.6

5.9

3.60

Mo

3.82

13.3

Bulk H₄[SiMo₁W₁₁O₄₀

1.79 (1.66-1.85)

2.00 (1.95-2.40)

3.47 (3.51-3.53)

3.49 (3.52-3.53)

SiMo₁W₁₁/Al

1.74

]

O

3.0 (3)

4.0 (4)

1.0 (1)

17.2

2.7

5.1

3.5

0.0259

0.0670

W

Si

O

3.3

2.7

4.0

1.0

4.0

3.9

5.5

1.96

W

Si

3.40

3.32

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Table 4. Structural parameters-determined by least square fitting of the inverse Fourier Transform (

ꢀ

R = 1-4 Å) of the W L_IIIedge EXAFS

signals for bulk H₄[SiW₉Mo₃O₄₀] HPA and its corresponding dried supported catalyst with S₀²=0.76 and enot = 8.6 eV for a E₀value used for

EXAFS extraction equal to 10206.70 eV. Values given in bracket correspond to the values range determined from single crystal XRD data.

Italic parameters were kept fixed during the minimization process.

Backscatterer

N

R (Å)

σ² x 10³(Å²)

R-factor

Bulk H₄[SiMo₃W₉O₄₀

1.69 (1.69-1.73)

1.90 (1.88-1.94)

2.29 (2.30-2.36)

3.36 (3.30-3.36)

3.40 (3.35-3.37)

3.51 (3.50-3.55)

3.68 (3.64-3.66)

3.74 (3.71-3.74)

SiMo₃W₉/Al

1.72

]

O

1.0 (1)

4.0 (4)

1.1

5.0

2.5

6.3

7.8

0.2

4.4

2.1

O

1.0 (1)

W

Mo

Si

1.33 (1.33)

0.66 (0.66)

1.0 (1)

0.1290

W

1.33 (1.33)

0.66 (0.66)

O

1.0

4.0

0.7

5.5

4.7

6.6

5.2

5.3

4.1

3.2

1.89

O

1.0

2.32

W

Mo

Si

1.33

0.66

1.0

3.36

0.1111

3.48

3.51

W

1.33

0.66

3.65

3.77

H₄[SiMo₁W₁₁O₄₀]

IR, Raman and TGA-DSC analysis (SI)

α-H₄[SiMo₁W₁₁O₄₀] EXAFS oscillations at the Mo K edge have also IR and Raman spectra of H₄[SiMo₁W₁₁O₄₀], H₄[SiMo₃W₉O₄₀],

been successfully refined (see supporting information (SI)) using the H₄[SiMo₁₂O₄₀] and H₄[SiW₁₂O₄₀] are reported in SI. The IR spectra as

radial distribution function extracted from crystallographic data well as the Raman ones of mixed HPA are very close and look like

deduced from XRD single analysis. In Table 3 the disordered that of H₄SiW₁₂O₄₀indicating that in the mixed HPA Mo- and W-

octahedral coordination of Mo is described with two contributions oxygen vibrational modes cannot be observed separately. TGA-DSC

of three oxygen neighbors at 1.79 and 2.00 Å. At the W L_IIIedge, due analysis (see SI) shows that the thermal behavior of the mixed HPA

to the α symmetric structure, in addition to the Si contribution, only and silicotungstic acid is very similar and that the substitution of W

a unique W contribution at 3.47 Å is present in the second by Mo slightly decreases the thermal stability of the mixed HPA

coordination shell. It was not possible to refine EXAFS oscillations at compared to that of silicotungstic acid.

the W L_IIIedge for distances above the first coordination shell by

keeping R and N crystallographic values. In this symmetric α HPA impregnating solutions analysis

structure, the part of multiple scattering is so important that a high

contribution of paths involving at least 3 atoms as W-O-W has to be Synthesis of mixed MoW HPA was devoted to the preparation of

taken into account making the parameters to fit higher than the aqueous impregnating solutions for the preparation of MoW

number of independent points in the fit.

hydrotreatment catalysts possessing a MoW nanoscale proximity. It

was then key point to check the preservation of mixed

heteropolyacids after their dissolution in water.

These EXAFS results confirmed the formation of the mixed HPA.

a

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Raman spectroscopy

mixed solids with no formation of separate monometallic

heteropolyanion and ii) their preservation after dissolution in

Raman spectra of impregnating solutions (Fig. 2) are in good water. As for Raman characterization at the solid state, the

agreement with those of the corresponding bulk compounds wavenumber of M=O_tvibration (main peak of the Raman spectrum)

reported in SI. The similarity between the Raman spectra of the corresponds to a weighted average of the Mo=O_tand W=O_t

solid and of the solution of mixed HPA indicates i) the purity of the wavenumbers.

H₄SiMo₃W₉O₄₀

H₄SiMo₁W₁₁O₄₀

H₄SiW₁₂O₄₀

H₄SiMo₁₂O₄₀

1100

100

200

300

400

500

600

700

800

900

1000

Raman shift(cm^-1)

Figure 2. Raman spectra of HPA based impregnating solution.

Polarography

with half-wave potentials recorded at 0.280, 0.190 and –0.020 V

respectively. The global shape of the polarogram and the half-wave

The electrochemistry of heteropolycompounds in solution has been potentials values together with their differences are in agreement

studied for a long time by polarography.^2,3,40It consists in observing with literature data^1-3,28and confirm the presence of α-H₄SiMo₁₂O₄₀

the reduction waves (current (I)-potential (E) curves) corresponding isomer. Polarography is one of the techniques allowing us to

to successive electron exchanges. In this work, due to the use of the differentiate between α and β isomers (the first half-wave potential

glassy carbon disk as working electrode, the polarogram of is expected around 0.380 V for the β-H₄SiMo₁₂O₄₀isomer). For the

H₄SiW₁₂O₄₀solution where reduction waves appear between -0.3 two mixed HPA solutions, polarograms are clearly different from

28

and -1.0 V cannot be observed. Fig. 3 shows the polarograms of that of α-H₄SiMo₁₂O₄₀and only one “2-electron” reduction wave

the three HPA solutions obtained by dissolving bulk H₄SiMo₁₂O₄₀

,

(corresponding to the O-II) reduction is visible without any

H₄SiMo₁W₁₁O₄₀and H₄SiMo₃W₉O₄₀in a buffer medium. In the contribution of H₄SiMo₁₂O₄₀anion. It means that all the waves are

Keggin- like structure the electrons are delocalised over the entire significantly shifted toward lower potential due to the presence of

polyanion, and Mo-O-Mo bridges allow the paired electrons to W in mixed HPA and the two other waves are shifted before -0.2 V

move according to the bipolaron model⁹.The degrees of reduction and thus do not appear on the Figure 3. Taking into account that

of the heteropolymolybdates are referred to as 0, II, IV, VI… in H₄SiMo₃W₉O₄₀mixed HPA is a β isomer the presence of W greatly

which the roman number indicates that the anion is reduced by 0, influences the Mo redox potential allowing to distinguish

2, 4, 6 electrons or more. In acidic medium, three “2-electron” unambiguously between both mixed HPA. Moreover, the two well-

reduction waves (O-II, II-IV, IV-VI) can be seen in the polarogram of defined polarograms of mixed samples indicate the presence of one

H₄SiMo₁₂O₄₀under our experimental conditions (AgCl/Ag reference type of HPA in solution in agreement with the existence of pure

electrode and HCl 1 mol.L^-1/ethanol 50-50 vol electrolyte solution) MoW mixed HPA.

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-100

-80

-60

-40

-20

0

H₄SiMo₁₂O₄₀

IV ↔ VI

H₄SiMo₃W₉O₄₀

II ↔ IV

H₄SiMo₁W₁₁O₄₀

0 ↔ II

0.5

0.4

0.3

0.2

0.1

0

-0.1

-0.2

E(V)

Figure 3. Polarograms of Mo containing HPA in 0.5 M ethanol/HCl mixture.

Characterization of supported oxidic precursors

Raman spectroscopy

demonstrating that tungsten based HPA are more stable than the

molybdenum ones upon pH increase^3,43

Indeed, tungsten

.

containing catalysts show different behaviour since the spectra of

the oxidic precursors are very similar to those of their bulk

To understand the behaviour of these HPA after deposition on the precursor. For the three W based catalysts, after maturation and

support, Raman spectra of catalysts after maturation and drying drying only a shift (≈20 cm ^-1) of the main line (M=O_tvibration)

with that of the corresponding bulk HPA are reported on Fig. 4. towards lower wavenumbers is observed (from 1000 to 982 cm^-1for

Different behaviours depending on the catalyst can be observed. SiW₁₂/Al; from 998 to 979 cm^-1for SiMo₁W₁₁/Al and from 997 to

For SiMo₁₂/Al the spectrum of the solid after maturation does not 977 cm^-1for SiMo₃W₉/Al). Such shifts have already been observed

4-

correspond to that of the bulk SiMo₁₂O₄₀anion and new Raman after drying but in a lesser extent in the case of ASA supported

lines are clearly observed at 948, 564, 350 and 215 cm^-1. These lines heteropolytungstate and were attributed to interaction with the

are unambiguously assigned to the formation of the well-known support¹². Moreover, analysis of the low Raman shift region (below

AlMo₆O₂₄H₆^3-Anderson heteropolyanion species that is formed 260 cm^-1) shows similar lines in shape and position for the bulk and

through decomposition of SiMo₁₂Keggin structure on the support the corresponding maturated and dried W based HPA catalysts. The

due to the “buffer effect” of alumina and Al³⁺extraction from the line around 220-230 cm^-1is attributed to the stretching mode of the

support^41,42. This species is maintained after drying and well- W(Mo)-O vibration (where

dispersed on the alumina surface according to the broad Raman characteristic of Keggin structure²⁸. The global Raman analysis of

lines of the spectrum¹¹

During impregnation the important catalysts after maturation and drying suggests the preservation of

O links W(Mo) and Si) and is

.

parameter is the pH of the solution in the pore of alumina which the mixed MoW heteropolyanion species on alumina support even

evolves due to the PZC of alumina (around 7). The respective if the broadness of the Raman peaks could prevent a clear

stability of tungsten and molybdenum based heteropolyacids in discrimination between SiW₁₁lacunary structure and SiW₁₁Mo

aqueous solutions at various pH have been studied in the literature Keggin structure.

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SiMo₁₂/Al

SiW₁₂/Al

982

949

215

152

218

181

906

347

Dried

517

523

562

564

Dried

948

980

215

178

350

220

Mat.

988

961

1000

252

167

222

629

973

924

161

226

906

Bulk

540

Bulk

100 200 300 400 500 600 700 800 900 1000

Raman shift (cm^-1)

100 200 300 400 500 600 700 800 900 1000

Raman shift (cm^-1)

SiMo₁W₁₁/Al

SiMo₃W₉/Al

980

982

150 224

900

221

890

577

Dried

Mat.

Bulk

Dried

979

977

217

232

152

161

905

885

180

217

Mat.

567

998

997

972

923

226

581

173

581

884

Bulk

100 200 300 400 500 600 700 800 900 1000

Raman shift (cm^-1)

100 200 300 400 500 600 700 800 900 1000

Raman shift (cm^-1)

Figure 4. Raman characterisation of catalysts after maturation (Mat.) and drying (Dried) and their corresponding bulk HPA (Bulk).

phase. This is a strong evidence of the preservation of this HPA on

EXAFS analysis

alumina after drying. As aforementioned, for H₄SiMo₁W₁₁due to

important multiple scattering contributions for this symmetric

In order to confirm the preservation of the mixed HPA on alumina structure it was not possible to fit the EXAFS data at the W L_IIIedge.

upon impregnation and drying, EXAFS experiments of dried catalyst Nevertheless, from Fig. 5 (a) and (b) it is clear that the bulk starting

have been performed at the W L_IIIand Mo K edges. For easier HPA matches very well with its catalyst, indicating unambiguously it

comparison purposes Fig. 5 reports EXAFS oscillations as well as is maintained on alumina surface.

Fourier Transform moduli at both edges for the dried catalysts and Regarding the Mo side (Fig. 5 (c) and (d)) if we compare dried

their corresponding bulk HPA. At the W L_IIIedge, Fig. 5 (a) and (b) catalysts with the bulk starting materials, similarities are observed

show clearly that EXAFS signals and the corresponding Fourier in both EXAFS oscillations and FT even if the fitting results reported

transforms moduli of the dried samples SiMo₁W₁₁/Al and in Table 3 show some variations in the distances between Mo and

SiMo₃W₉/Al are both in phase with that of their corresponding its neighbours compared to that observed in the bulk materials.

mixed HPA. A successful fitting of SiMo₃W₉/Al EXAFS data with These results obtained from the Mo side could be explained by a

crystallographic data of its bulk starting material has been then partial decomposition of SiMo₃W₉HPA with the removal of one Mo

obtained and reported in Table 4 attesting that the local order atom

around W is similar in the supported catalysts to the one of the bulk

from

the

Keggin

structure.

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Figure 5. EXAFS oscillations at (a) W L_IIIedge, (c) Mo K edge and the corresponding Fourier Transform modulus at (b) W L_IIIedge (d) Mo K

edge for dried mixed MoW catalyst in red and its bulk mixed HPA in blue.

Characterization of supported sulfide catalysts

EXAFS analysis

contribution is located around 1.9 Å (phase-shift not corrected) on

the FTs of the EXAFS signals and corresponds to S neighbors with a

characteristic Mo-S or W-S distance equal to 2.41 Å, identical to

that reported for bulk sulfided MoS₂or WS₂samples^44,22. The

second coordination shell, attributed to a metallic neighbor (Mo or

W), is characterized by a distance around 3.17 Å, again identical to

that of the bulk sulfide material^42,44. Nevertheless, for this second

contribution a significant change in magnitude is observed at both

edges especially for the mixed catalyst compared to the

monometallic ones. Slabs size of the MoS₂or WS₂phases could

All prepared Mo(W)/Al catalysts were in-situ sulfided during

synchrotron measurement sessions, with the same operating

conditions than those used for activation of the solids prior to

catalytic evaluation. The Fourier transform moduli data recorded

for the samples at the end of the sulfidation are presented in Figure

6 and the corresponding fit EXAFS parameters are gathered in Table

5 and 7. At the Mo and W edges of all samples, the first

influence the magnitude of the second coordination shell,

decrease of the slabs size could be then reflected by the decrease

N_Mo-Moor N_W-Wcoordination number. Results of TEM

a

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characterization after liquid phase sulfidation showed that coordinated which clearly demonstrates the quantitative formation

SiMo₁₂/Al, SiW₁₂/Al and SiMo3W9/Al catalysts presented similar of mixed Mo-W sulfide slabs. The formation of such mixed slabs

average slabs sizes with values of 4.7, 4.9 and 4.8 Å respectively⁴⁵

.

may be favored by the nanoscale proximity of both metals in the

It could be expected that gas sulfidation could also lead to similar heteropolyanionic structure. Recent literature discussed about

average slabs length for these three catalysts. Nevertheless, besides mixed MoW disulfide phase formation when using different

the effect of dispersion (slab size) on the coordination number, the preparation and/or sulfidation procedures. Starting from separate

local ordering of the MoS₂phase may also influence the Mo and

W precursors to prepare bulk Ni promoted and

coordination number and the introduction of some disorder unpromoted catalysts and using as sulfidation procedure 10 vol.%

(through the Debye-Waller factor) in the position of Mo atoms in a H₂S into H₂under at 18 bar, Hein et al.^47,48demonstrated by XAS the

MoS2 slab can explain low Mo–Mo coordination numbers⁴⁶

.

formation of mixed slabs from EXAFS refinements at the Mo K and

Moreover, the larger decrease observed for mixed HPA based W L_IIIedges. Haandel et al.²²also deduced through EXAFS analysis

catalysts could be due to the presence of W neighbors for Mo and mixed slabs formation for Ni-Mo-W catalysts prepared from

vice-versa, that is due to the destructive interference of the conventional precursors and activated under gas sulfidation. This

individual scattering paths including either the absorbing atom and mixed phase has been recently clearly visualized for the first time

Mo or the absorbing atom and W which are out of phase^44,22

.

thanks to HAADF images revealing through Z contrast W and Mo

atoms in the same sulfide slabs⁴⁵. This study was performed on

SiMo₃W₉/Al and its corresponding reference catalysts activated by

liquid phase sulfidation and showed mixed MoWS₂phase in both

cases with the presence of monometallic slabs for the reference

catalyst. In our case using gas sulfidation we can deduce

predominant mixed sulfide phase formation using mixed HPA while

monometallic WS₂and MoS₂phases are the main ones using

separate monometallic HPA.

It is thus necessary to fit the data to obtain reliable quantitative

local coordination of Mo and W atoms (Table 5 and 6). For the

Mo_xW_12-xsolids, a Mo-W contribution had to be considered for an

optimum fitting of the EXAFS spectra, with 2.9 and 1.5 W neighbors

at the Mo edge and 0.7 and 1.5 Mo neighbors at the W edge, for

SiMo₁W₁₁/Al and SiMo₃W₉/Al respectively. Contrariwise,

introducing a Mo-W contribution does not improve the fitting in

mixed MoW reference catalysts. Sulfide catalysts prepared from

mixed heteropolyanions present Mo and

W together closely

Mo K edge

W L_IIIedge

SiW₁₂/Al

SiMo₁₂/Al

Ref Mo₃W₉

SiMo₃W₉/Al

k (Å^ꢀ1)

SiMo₁₂/Al

Ref Mo₃W₉

SiMo₃W₉/Al

SiW₁₂/Al

Ref Mo₃W₉

SiMo₃W₉/Al

R (Å)

Figure 6. EXAFS at the Mo K edge and W L_IIIedge and the corresponding Fourier transforms of sulfided Mo(W)/Al catalysts.

14 | J. Name., 2012, 00, 1ꢀ3

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Table 5: Parameters obtained from the fit of the Mo K edge EXAFS spectra of the sulfided Mo(W)/Al catalysts at the Mo K edge (Δk=3.5-15

Å^-1, R-range 1.5-3.35 Å, S²₀= 1.05 and enot = 1.6 ± 2 eV for a E₀value used for EXAFS extraction equal to 20004.3 eV).

Backscatterer

N

R (Å)

SiMo₁₂/Al

σ² x 10³(Å²)

R-factor

0.0007

Mo-S

5.1 ± 0.2

3.3 ± 0.3

2.41 ± 0.01

3.17 ± 0.01

4.3 ± 0.3

5.3 ± 0.4

Mo-Mo

SiMo₃W₉/Al

Mo-S

5.3 ±1.1

1.3 ±0.8

1.5 ±1.0

2.41 ±0.01

3.16 ±0.02

3.15 ±0.01

4.0 ±1.3

3.7 ±2.1

Mo-Mo

Mo-W

0.0111

0.0081

Ref Mo₃W₉

Mo-S

5.1 ±0.6

3.5 ±1.7

2.41 ±0.01

3.17 ±0.01

3.7 ±0.6

7.8 ±2.8

Mo-Mo

Table 6: Parameters obtained from the fit of the W L_IIIedge EXAFS spectra of the sulfided Mo(W)/Al catalysts at the W L_IIIedge (Δk=4.3-16.3

Å^-1, R-range 1.4-3.4 Å, S²₀= 0.87 and enot = 8.90 ± 2.0 eV for a E₀value used for EXAFS extraction equal to 10218.8 eV).

Backscatterer

N

R (Å)

SiW₁₂/Al

σ²x 10³(Å²)

R-factor

0.0091

W-S

4.3 ±0.4

3.0 ±0.6

2.41 ±0.01

3.2±0.6

4.3 ±0.8

W-W

3.16 ±0.007

SiMo₃W₉/Al

W-S

4.8 ±0.5

1.7±0.9

1.5 ±1.1

2.41 ±0.01

3.17 ±0.01

3.15 ±0.01

3.4 ±0.7

3.7 ±2.1

W-W

W-Mo

0.0111

0.0106

Ref Mo₃W₉

W-S

4.7 ±0.5

2.6±0.9

2.41±0.01

3.16 ±0.01

3.4 ±0.7

4.7 ±1.4

W-W

XPS

varying from 4.2 to 18 MoO₃wt % has no influence on the

sulfidation rate. The Ref Mo₃W₉catalyst possesses the highest MoS₂

Figure 7 shows the decomposition of the Mo3d and the W4f spectra sulfidation rate reaching 91 %. A low sulfidation degree of tungsten

for sulfided SiMo₁₂/Al, SiW₁₂/Al, SiMo₃W₉/Al and Ref Mo₃W₉(49 %) is observed for SiW₁₂/Al catalyst that could be attributed to a

catalysts. This decomposition allows to obtain the relative amounts strong W-O bond compared to the Mo-O one that could delay the

of the different Mo and W species that are gathered in Table 7. The formation of WS₂phase compared to that of MoS₂phase at a given

sulfidation rate corresponding to the MoS₂atomic % is very high sulfidation temperature^50,51

. The presence of Mo in catalyst

(above 80 %) in the three Mo containing catalysts, compared with formulation clearly leads to increase W sulfidation that reaches 61

sulfidation rate reported in the literature for conventional HDS and 67 % for SiMo₃W₉/Al and Ref Mo₃W₉respectively.

catalysts (around 60 %)⁴⁹. It shows that the molybdenum content

J. Name., 2013, 00, 1ꢀ3 | 15

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160

22

Mo3d_5/2(MoS₂)

Mo3d_3/2(MoS₂)

150

140

130

120

110

100

90

20

18

16

14

12

10

8

Mo3d_3/2(MoS₂)

Mo3d_5/2(Mo⁶⁺

Mo3d_3/2(Mo⁶⁺

Mo3d_5/2

(MoS_xO_y)

)

Mo3d_5/2

Mo3d_5/2(Mo⁶⁺

Mo3d_3/2(Mo⁶⁺

)

(MoS_xO_y)

)

S 2s

(sulfide)

S 2s

Mo3d_3/2

(MoS_xO_y)

(sulfide)

)

Mo3d_3/2

(MoS_xO_y)

S 2s

(oxysulfide)

S 2s

(oxysulfide)

80

6

240

236

232

228

224

220

240

236

232

228

224

220

Binding energy (eV)

SiMo₁₂/Al

SiMo₃W₉/Al

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

Mo3d_5/2(MoS₂)

W4f_7/2(WS₂)

W4f_5/2(WS₂)

Mo3d_3/2(MoS₂)

Mo3d_5/2

(MoS_xO_y)

W4f_7/2(W⁶⁺

)

Mo3d_5/2(Mo⁶⁺

)

S 2s

(sulfide)

W4f_5/2(W⁶⁺

)

W4f_5/2

(WS_xO_y)

Mo3d_3/2

(MoS_xO_y)

S 2s

(oxysulfide)

W4f_7/2

(WS_xO_y)

80

70

44

40

36

32

28

240

236

232

228

224

220

Binding energy (eV)

Ref Mo₃W₉

SiW₁₂/Al

14

12

10

8

W4f_5/2(WS₂)

W4f_7/2(WS₂)

W4f_5/2(WS₂)

12

10

8

W4f_7/2(W⁶⁺

)

W4f_5/2

(WS_xO_y)

W4f_7/2(W⁶⁺

)

W4f_5/2

(WS_xO_y)

W4f_5/2(W⁶⁺

)

W4f_5/2(W⁶⁺

)

6

W4f_7/2

(WS_xO_y)

W4f_7/2

(WS_xO_y)

4

2

44

40

36

32

28

44

40

36

32

28

Binding energy (eV)

SiMo₃W₉/Al

Ref Mo₃W₉

Figure 7. XPS Mo 3d and W 4f spectra recorded for Mo(W)/Al catalysts; in blue: Mo(W)⁶⁺oxide contributions; in pink: Mo(W)S_xO_ycontributions; in green: Mo(W)S₂

contributions (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

J. Name., 2013, 00, 1ꢀ3 | 16

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Table 7. Relative at.% of Mo and W species in sulfided Mo(W)/Al catalysts.

Catalysts

MoS₂

82

-

MoO_xS _y

Mo^VI

WS₂

-

WO_xS_y

-

W^VI

-

12

-

6

-

SiMo₁₂/Al

SiW₁₂/Al

49

61

67

14

37

26

21

81

91

14

7

5

2

13

SiMo₃W₉/Al

Ref Mo₃W₉

15

Table 8. Catalytic properties of prepared Mo(W)/Al catalysts in DBT HDS and naphthalene Hyd.

Rate constant

DBT HDS

(%)

Selectivity ratio

S_HYD

Naphthalene Hyd

(%)

Rate constant

Catalysts

k_HDS×10^-5

(mol h^-1g^-1)

45.1

/

k_Hyd×10^-5(mol h^-1g^-1)

DDS

SiMo₁₂/Al

51.9

22.2

49.2

52.9

20.6

32.1

1.7±0.2

2.6±0.2

3.2±0.4

2.9±0.3

1.8±0.2

40.5±0.5

23.8±0.3

42.6±0.4

47.5±0.5

24.4±0.2

27.3±0.3

157.3

SiW₁₂/Al

15.5

82.4

SiMo₁W₁₁/Al

SiMo₃W₉/Al

Ref Mo₁W₁₁

Ref Mo₃W₉

41.8 (17.9*)

46.4 (22.9*)

14.2 (17.9*)

23.9 (22.9*)

168.6 (88.6*)

195.2 (101.1*)

85.0 (88.6*)

96.7 (101.1*)

*The additive quantities, which were calculated using the values for monometallic SiMo₁₂/Al₂and SiW₁₂/Al

Catalytic results HDS performance of pure Mo catalyst (51.9 %) with four times less

Hydrotreatment (HDT) catalytic properties in dibenzothiophene molybdenum.

(DBT) hydrodesulfurization (HDS) and naphthalene hydrogenation Concerning Hyd ability, conversion of 42.6 % is obtained for

(Hyd) are reported in Table 8.

SiMo₁W₁₁/Al and of 47.5 % for SiMo₃W₉/Al, this later value being

The conversion of the reactants over all catalysts largely vary from significantly higher than that measured for pure Mo catalyst (40.5

20.6 to 52.9 % for dibenzothiophene HDS and from 23.8 to 47.5 % %). SiMo₃W₉/Al catalyst thus demonstrates the highest activity in

for naphthalene Hyd. Compared to SiW₁₂/Al, the presence of a both studied reactions. Comparison of reaction rates between

small amount of molybdenum in Ref Mo₁W₁₁has no significant SiMo₃W₉/Al and Ref Mo₃W₉shows for the mixed HPA catalyst. a

effect on both HDS and Hyd properties. However, replacement of relative increase evaluated at 65 % in DBT HDS and 74 % in

three tungsten atoms by molybdenum in Ref Mo₃W₉leads to an naphthalene hydrogenation. This observation supports the

increase in the catalytic activity in HDS of DBT from 22.2 to 32.1 and formation of a mixed Mo-W active phase developing different

from 23.8 to 27.3 in Hyd of naphthalene. Taking into account the catalytic properties. The mixed HPA based catalyst also presents the

Mo and

W atomic ratio together with their corresponding highest selectivity ratio S_HYD/DDS(Table 8), with a value of 3.2

sulfidation rates previously calculated by XPS, the HDS rate constant showing that the presence of the mixed sulfide phase leads to

of Ref Mo₃W₉catalyst (23.9 10⁵mol h^-1g^-1) is almost a linear improved hydrogenation properties. Such properties are required

combination between that of SiMo₁₂/Al and SiW₁₂/Al (22.9 10⁵mol for the treatment of heavy fuels^52-54

.

h^-1g^-1for the calculated value). Naphthalene hydrogenation rate Mixed MoW catalysts have been studied by Thomazeau et al.²⁴who

constant of Ref Mo₃W₉(96.7 10⁵mol h^-1g^-1) appears to be even prepared MoW catalysts by incipient wetness co-impregnation of

lower than the calculated value (101.1 10⁵mol h^-1g^-1). When using alumina with solutions of separate precursors, ammonium

a mixed heteropolyanionic precursor, the addition of Mo in the heptamolybdate (NH₄)₆Mo₇O₂₄and ammonium metatungstate

SiMo_xW_12-x/Al catalysts has a large beneficial effect in both HDS and (NH₄)₆H₂W₁₂O₄₀

. They reported a continuous variation of the

Hyd. HDS conversions are 49.2 % for SiMo₁W₁₁/Al and 52.9 % for thiophene HDS activity for the alumina Mo_1-xW_xS₂series, in

SiMo₃W₉/Al, reaching then the

agreement with the behaviour of separate MoS₂and WS₂phase as

we noticed for Ref Mo₃W₉catalyst. Other authors reported in the

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case of separate Mo and W precursors that the thiophene HDS The work was also supported the Ministry of Education and

performance of Ni promoted mixed MoW with low Mo content Science of the RF under project No. 14.586.21.0054 (project

(Mo/W≤1) were comparable to that of the catalysts containing only unique identifier RFMEFI58617X0054). M. Nikulshina thanks

44

W, as we observed for our Ref Mo₁W₁₁

.

French Embassy in Russia for the Vernadsky fellowship and

Our catalytic results allow us to show that the origin of the catalytic Haldor Topsøe Company for the grant to perform PhD thesis.

improvement observed for Mo_xW_12-xis not only due to the

simultaneous presence of Mo and W as in Ref catalysts but also

Supporting Information Available: Intra HPA cation-anion bond

arises from close interaction of both metals in the mixed HPA

distances. Bulk H₄[SiMo₁W₁₁O₄₀

oscillations at the Mo and at the

experimental data. X-Ray crystallographic files (CIF). Bulk

]

and H₄[SiMo₃W₉O₄₀

] EXAFS

precursor leading to the formation of mixed Mo_xW_1-xS₂active

phase, as supported by XAS analysis.

K

W

L_IIIedges: FIT and

H₄[SiMo₁W₁₁O₄₀], H₄[SiMo₃W₉O₄₀], H₄[SiMo₁₂O₄₀] and H₄[SiW₁₂O₄₀

IR and Raman spectra and TGA-DSC analysis.

]

Conclusions

Notes and references

In this study, mixed MoW heropolyacids α-H₄[SiMo₁W₁₁O₄₀]

and β-H₄[SiMo₃W₉O₄₀] have been synthesized from their

corresponding mixed potassium salts starting from mono and

tri-vacant heteropolytungstates. Crystallographic structures

have been obtained and the corresponding data were used to

fit with success EXAFS oscillations at the Mo K edge showing

typical Mo-W distances for both mixed heteropolyacids. The

purity of the samples was checked by characterizations at the

liquid state using Raman spectroscopy and polarography.

These mixed HPA were used for preparing hydrotreatment

catalysts. After deposition on alumina, mixed HPA seems to be

preserved even if a partial decomposition to lacunary Keggin

species may occurs and their use ensure the nanoscale

proximity between Mo and W. This close vicinity between Mo

and W explains the formation after sulfidation of mixed MoW

sulfide slabs for mixed HPA based catalysts as demonstrated

by EXAFS fitting at the Mo K and W L_IIIedges. Catalytic

performances in dibenzothiophene HDS and naphthalene

hydrogenation have been evaluated. Catalytic results confirm

the beneficial use of mixed HPA and particularly β-

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Conflicts of interest

There are no conflicts to declare.

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Higher catalytic conversions and different selectivity ratio are explained by the mixed (MoW)S₂active phase

formation when using mixed MoW heteropolyacid as starting material

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Molecular approach to prepare mixed MoW alumina supported hydrotreatment catalysts using H4SiMo: NW12- nO40 heteropolyacids

DOI: 10.1039/c8cy00672e

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