Niobium-containing lindqvist isopolyanions [NbxW6-xO19](2+x)- used as precursors for hydrodesulfurization catalysts with isomerization properties

Article abstract of DOI:10.1002/ejic.201403160

Lindqvist isopolyanions [Nb_xW_6-xO₁₉]^(2+x)- (x = 0-4 and 6) were prepared and their spectroscopic and thermal properties were determined by Raman and IR spectroscopy as well as TGA/DSC. The structure of the [NbW₅O₁₉]^3- anion obtained as single crystal was determined. Ni-promoted alumina-supported hydrodesulfurization (HDS) catalysts were prepared from the best soluble NbW polyoxometalates. In the calcined catalysts, better dispersion of the metallic species is observed when using NbW isopolyanions instead of the conventional ammonium metatungstate. The presence of niobium was expected to introduce acidity leading to isomerization property in classical NiW HDS catalysts. In HDS reaction conditions (under hydrogen pressure and sulfided environment) the cyclohexane isomerization into methylcyclopentane activity of niobium-based catalysts was found up to 5 times superior to that of conventional NiW catalyst, showing the beneficial effect of niobium for this reaction. [Nb_xW_6-xO₁₉]^(2+x)- Lindqvist type isopolyanions were synthesized and characterized to prepared NbWNi-based alumina-supported hydrodesulfurization catalysts. Nb-containing catalysts showed improved CC6 isomerization properties under HDS conditions (H₂ pressure and H₂S presence) compared to those of a reference NiW catalyst related to the acid properties induced by niobium.

Full text of DOI:10.1002/ejic.201403160

FULL PAPER
DOI:10.1002/ejic.201403160
Niobium–Containing Lindqvist Isopolyanions
(
2+x)–
[
Nb W O ]
Used as Precursors for
x
6–x 19
Hydrodesulfurization Catalysts with Isomerization
Properties
[
a,b]
[a]
Karolina Bouadjadja-Rohan,
Christine Lancelot,
[a]
[b]
Michel Fournier, Audrey Bonduelle-Skrzypczak,
Antoine Hugon, Olivier Mentré, and Carole Lamonier*
[b]
[a]
[a]
Keywords: Heterogeneous catalysis / Niobium / Tungsten / Isomerization / Lindqvist isopolyanion
Lindqvist isopolyanions [Nb
were prepared and their spectroscopic and thermal proper-
ties were determined by Raman and IR spectroscopy as well
x
W
6–x
O
19](2+x)– (x = 0–4 and 6)
the conventional ammonium metatungstate. The presence of
niobium was expected to introduce acidity leading to isomer-
ization property in classical NiW HDS catalysts. In HDS reac-
tion conditions (under hydrogen pressure and sulfided envi-
ronment) the cyclohexane isomerization into methylcyclo-
pentane activity of niobium-based catalysts was found up to
5 times superior to that of conventional NiW catalyst, show-
ing the beneficial effect of niobium for this reaction.
3
–
as TGA/DSC. The structure of the [NbW
5
O
19
]
anion ob-
tained as single crystal was determined. Ni-promoted alu-
mina-supported hydrodesulfurization (HDS) catalysts were
prepared from the best soluble NbW polyoxometalates. In
the calcined catalysts, better dispersion of the metallic spe-
cies is observed when using NbW isopolyanions instead of
Introduction
anions are represented by few families of compounds,
among the major groups a few can be listed: Lindqvist
Polyoxometalates (POMs) or polyanions are molecular
metal oxides having different structures and proposing a
combination of interesting chemical and physical proper-
M MЈ O ](2+x)–, Keggin [XM O ] , Anderson
n–
[
[
x
6–x 19
n–
12 40
n–
XM O H ] and Dawson [X M O ] , where M(MЈ) is
6
24
6
2
18 62
a metallic cation and X a non-metallic one. Niobium can
form, among others, Lindqvist-type structure due to similar
values of ionic radius of niobium(V) (0.64 Å) and tung-
sten(VI) (0.60 Å) in six-coordination according to ionic ra-
[
1–8]
ties. These materials can be largely applied in catalysis
in
the field of oxidation, isomerization or hydrodesulfurization
reactions. Polyoxometalates constitute a family of mineral
condensed ions in the form of isopolyanions and hetero-
polyanions. Isopolyanions M O (M is most frequently a
metallic atom such as W and Mo) are formed during con-
densation of MO4 oxoanions. This process takes place
during acidification of starting solution with the elimi-
nation of water molecules and with creation of μ-oxo bonds
between metal ions. In the presence of other XO4
[9]
dii values reported by Shannon. Those elements can be
t–
m
y
combined in the frames of one structure forming
(2+x)–
[10]
[
Nb W O ]
isopoly compounds.
Figure 1 repre-
x
6–x 19
n–
sents Lindqvist-type isopolyanion structure with six
metallic atoms (depicted as large grey spheres) and 19 oxy-
gen atoms (small spheres) which are divided into three sub-
groups: six terminal oxygen atoms bonded to only one met-
p–
oxoanions (where X is different from M, being frequently
a non-metallic atom such as B or Si), metallic atoms are
associated around the second oxoanion and then, after
polycondensation, an heteropolyanion is formed. Poly-
[
a] Unité de Catalyse et Chimie du Solide, UCCS, UMR CNRS
8
5
181, Université Lille Nord de France,
9655 Villeneuve d’Ascq, France
E-mail: carole.lamonier@univ-lille1.fr
uccs.univ-lille1.fr
http://uccs.univ-lille1.fr/
[
b] IFP Energies Nouvelles,
Rond-Point de l’Echangeur de Solaize, BP3, 69360 Solaize,
France
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403160.
Figure 1. Lindqvist-type isopolyanion structure.[19]
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allic atom (noted O ), 12 atoms forming bridges (noted O ) in which tungsten anions are progressively replaced by ni-
t
b
[
31]
and one central oxygen atom being surrounded by six atoms obium.
of metal (noted Oc).[
11–14]
The Figure 2 gathers the Raman spectra of the complete
(2+x)–
Hydrodesulfurization (HDS) of petroleum feedstocks for Lindqvist family [Nb W O ]
, x = 0, 1, 2, 3, 4, 6
x
6–x 19
sulfur removal is a catalytic process generally performed and allows to observe similarities in spectra shape from one
with Ni(Co)W(Mo) alumina-supported catalysts, in which compound to another as proposed by Rocchiccioli-Delt-
[
31]
–1
the active phase consists in well dispersed WS (MoS ) cheff et al. Raman high-frequency peaks (850–1000 cm )
2
2
nanocrystallites decorated with Ni(Co) atoms. These active are attributed to the metal-oxygen terminal vibrations for
phases are obtained through the sulfidation of an oxidic all oxygenated anions of elements of V A and VI A
precursor prepared by incipient wetness impregnation of groups.[32] In the Raman spectra of [Nb W O ]
(2+x)–
x
6–x 19
alumina with an aqueous solution of ammonium meta- HPAs, the evolution of the Nb=O and W=O band wave-
t
t
tungstate (ammonium heptamolybdate) and nickel nitrate numbers can be explained by the replacement of W par Nb
cobalt nitrate). Instead of these classical W(Mo) and that leads formally to symmetry lowering as for instance for
(
3–
4–
Ni(Co) precursors, heteropolyanions (HPA) compounds [NbW O ] with C4v symmetry and [Nb W O ] with
5
19
2
4
19
containing Co and Mo or Ni and W atoms in the same C2v symmetry. In this case, redistribution of potential en-
entity were used as precursors of HDS catalysts; the corre- ergy of the anion leads to the partial mixture of W=O and
t
sponding enhancement of catalytic performance when com- Nb=Ot vibrators. In consequence, force constants of
5–
6–
pared with catalysts prepared from conventional precursors Nb=O vibrators e.g. in [Nb W O ] or [Nb W O ] are
t
3
3
19
4
2
19
was attributed to better promotion due to the proximity of expected to be displayed at higher wavenumber that for
the active phase metals in the HPA entity.[
15–18]
[Nb O ] and correlatively force constants of W=O vi-
8–
6
1
9
t
In HDS of heavy fuels, acidity was identified as a factor brators are expected to be displayed at lower frequencies.
accelerating hydrodesulfurization through for example Evolution of W=O frequencies towards lower frequencies
t
[
19–23]
isomerization of refractory sulfur-containing species.
by increasing Nb content is clearly observed in Figure 2.
2
–
Niobium, due to its acidic character, might be particularly For the [W O ] isopolyanion TMA salt, υ W = O ter-
6
19
s
t
interesting in combination with tungsten for HDS catalysis. minal vibration is displayed at 1001 cm– and shifts towards
1
[
24–26]
Niobium has been used as a dopant of HDS catalysts
lower wavenumber when decreasing tungsten loading,
or as a support modifying agent.[ Due to the possibility reaching 929 cm for K [Nb W O ]. (Figure 2). Peak cor-
27]
–1
6
4
2
19
to form niobium sulfide, it was also proposed as an inde- responding to the asymmetric tungsten–oxygen vibration
pendent hydrotreatment catalyst.[
28–30]
υ W = O also shifts toward lower wavenumber when
as
t
–1
The aim of this work was to prepare niobium–tungsten- decreasing tungsten loading, with values of 977 cm for
based Lindqvist isopolyanions as precursors for synthesiz- [W O ] and 937 cm for [Nb W O ] isopolyanions.
ing hydrodesulfurization catalysts with acid properties: the Comparison of [W O ] with [Nb O ] Raman spectra
2–
–1
6
19
3
3
19
8–
2–
6
19
6
19
initial proximity of Nb and W could lead to more efficient shows M=O vibrations frequencies higher for tungsten
t
bifunctonnal catalysts thanks to an optimized distribution than for niobium which is in accordance with more ionic
(
2+x)–
of both acid and HDS sites. A series of [Nb W6–xO ]
character of Nb=O . For K [Nb O ] isopolyanion υ Nb =
x
19
t 8 6 19 s
–1
anions was synthesized and characterized by Raman and O characteristic peak is observed at 883 cm . This band is
t
Infrared spectroscopy. Thermogravimetric analysis was per- noted also for other Nb-containing isopolyanions, at
formed to determine the general formulas of the prepared 880 cm–1 for [Nb W O ] , around 893 cm
6–
–1
for
compounds. The most soluble salts were selected to prepare [Nb W O ] and at 914 cm for [NbW O ] . This evol-
4
2
19
–1
5–
3–
3
3
19
5
19
alumina-supported NbW-Ni-promoted catalysts to assess
their catalytic performance in isomerization of cyclohexane
to methylcyclopentane.
Results and Discussion
Characterization of Isopolyanions
Vibrational Spectroscopy
There are only very few references dealing with the topic
of NbW Lindqvist isopolyanions characterization by mean
[
31]
of vibrational spectroscopy. Rocchiccioli-Deltcheff et al.
observed significant analogies between recorded Raman
spectra for various NbW Lindqvist-type isopoly com-
pounds and observed constant evolution towards the Ra-
man spectrum of the last compound of the family
19](2+x)– isopolyanions
O
Figure 2. Raman spectra of [Nb
x
W
19
6–x
6–
8
–
[
Nb O ] . Thus, they proposed the hypothesis that
8–
5–
6
19
salts: (a) [Nb O ]
(b) [Nb W O ]
(c) [Nb W O ]
(d)
6
19
4
2
3
3
19
(2+x)–
4–
3–
2–
[
Nb W O ]
anions possess hexatungstate structure [Nb
2
W
4
O
19
]
(e) [NbW
5
O
19
]
(f) [W
6
O
19] .
x
6–x 19
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ution is more difficult to point out for less intense Nb=Ot Single-Crystal and Powder X-ray Diffraction for
3–
vibrations for which a tightening of υ NbO and υ NbO
[NbW O ] salt
s
t
as
t
5 19
peaks can be observed. Indeed, regardless of the tungsten
Among four NbW synthesized isopolyanions,
content in mixed isopolyanion NbW, υ W = O terminal
3–
s
t
[
NbW O ] salt was obtained as single crystal. Its crystal-
5
19
vibration is the dominating one in agreement with stronger
polarisability of W=O in comparison with Nb=O .
lographic structure is formed of one independent Lindqvist
HPA, two independent TMA molecules and one indepen-
dent potassium countercations arranged within a cubic ge-
ometry [Space group 225 – International Tables for Crystal-
t
t
Peaks observed for all the compounds from about
–
1
–1
8
00 cm to 400 cm correspond to υMOb vibrations.
–1
Peaks displayed at low frequencies from 235 cm for
¯
lography – Fm3m, R1(F) for all reflexions: 2.14%]. In the
2
–
–1
8–
[
W O ] till 281 cm for [Nb O ] correspond to sym-
6 19 6 19
anions, Nb and W are disordered over a 24(e) position and
–1
metric stretching υ MO . They are observed at 267 cm for
s
c
their relative occupancies are refined to Nb/W
=
5
–
–1
4–
[
Nb W O ] and at 256 cm for [Nb W O ] and corre-
3 3 19 2 4 19
2
2(1)%:78(1)%. In the last cycles of refinement the ratio was
spond to υ WO vibration. Furthermore, for TMA-contain-
s
c
fixed to the expected 16.6:83.3% without significant
changes in the convergence process. If one considers the
Lindqvist unit as a super-octahedron, each of its six exter-
–
1
ing salts, a peak at 946 cm is attributed to an asymmetric
C–N stretching vibration[
33,34]
and it can be observed on
spectra c, d and e on Figure 2. For the TMA salt of
+
nal species is bounded to a K cations [K–O = 2.8557(7)],
5
–
[
Nb W O ] this peak is incorporated into larger peak
+
3
3
19
such that K is at the center of a KO octahedron (Fig-
6
containing also υ WO vibration.
as
t
ure 4). Also eight TMA molecules surround each HPA with
their C3 faces parallel to the faces of the super-octahedron.
Infrared spectra (Figure 3) also display characteristic
bands of metal–oxygen bonds[
32,35]
also confirming success-
ful formation of [Nb W O ](
2+x)–
isopoly compounds.
x
6–x 19
We observe characteristic bands of Infrared spectra (Fig-
–
1
2–
ure 3) at 982 cm for [W O ] isopolyanion. This band
6
19
shifts toward lower wavenumber with decreasing tungsten
–
1
3–
content till 958 cm for [NbW O ] . Not all the W=O
5
19
t
vibrations are easily observable due to weak coupling be-
tween W=O vibrators that are quite distant one from an-
t
[
32]
other
Nb=Ot vibration band is observed at about
–
1
8–
856 cm for [Nb O ] and moves toward higher wave-
6 19
number with decreasing niobium content.
Figure 4. Interactions between the isopolyanionic units and the
counter-cations K+ and N(CH
)N .
+
3
The Figure 5 shows a projection of the crystal structure
along [101] with evidences of infinite columns of HPA. A
general formula can be proposed – TMA K[NbW O ],
2
5
19
which confirms the formation of the expected HPA
3–
NbW O
and shows also that the salt is a mixture of
5
19
Figure 3. Infrared spectra of [Nb
x
W
19
6–x
6–
O
19](2+x)– isopolyanions
8–
5–
salts: (a) [Nb
6
O
19
]
(b) [Nb
4
W
2
O
]
2–(c) [Nb
3
W
3
O
19
]
(d)
4–
3–
[Nb
2
W
4
O
19
]
(e) [NbW
5
O
19
]
6
(f) [W O19] .
As previously observed for Raman spectra, a decrease of
W=O bands wavenumber and an increase of Nb=O fre-
t
t
quencies with increasing tungsten loading (decreasing ni-
obium content) can also be highlighted for IR spectra. A
band corresponding to the presence of the cation is ob-
–
1
served around 795 cm for compounds containing signifi-
–
1
cant amounts of TMA. Bands at 700–810 cm and at 530–
–
1
5
4
80 cm correspond to υMOb vibrations and at 400–
Figure 5. Arrangement of isopolyanions in line along the [101] di-
rection.
–
1
40 cm to υMO vibrations.
c
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TMA and potassium cations, the potassium coming from
the K [Nb O ] niobium precursor.
8
6
19
Furthermore, an XRD diagram was calculated from
crystal structure parameters and was compared with an ex-
perimental XRD diagram obtained for powder material
(Figure 6). It is clearly noticed that both diagrams are iden-
tical (with one additional peak for the bulk compound at
about 17.5 °C corresponding to traces of physisorbed water
or impurities) which leads to the conclusion that even for
bulk compound the structure is identical to the one defined
for single crystal. This observation confirms that the syn-
thesis is quantitative and the whole powder obtained corre-
sponds to TMA K[NbW O ], a mixed TMA and potas-
2
5
19
sium salt. The presence of potassium and/or sodium, com-
ing from niobium K [Nb O ] and tungsten Na WO ·2H O
8
6
19
2
4
2
Figure 7. TGA (solid line) and DSC (dashed line) for ammonium
precursors has already been reported by Dabbabi and metatungstate.
[
35]
[36]
Boyer and Kaezer-França et al. during the formation
of Lindqvist isopolyanion cesium or TMA salts.
Figure 6. Comparison of theoretical and experimental X-ray
Figure 8. TGA (solid lines) and DSC (dashed lines) curves for
pattern for TMA
2
K[NbW
5
O
19].
K [Nb O ] (a and aЈ) and K [Nb W O ] (b and bЈ).
8
6
19
6
4
2
19
Thermogravimetric Analysis and Differential Scanning
Calorimetry
Thermal stability of synthesized bulk compounds was
evaluated by thermogravimetric analysis (TGA). TG curves
8
–
are reported for potassium salts of [Nb O ]
and
of
6
19
6
–
[
Nb W O ] ,
TMA-potassium
mixed
salts
4
2
19
(2+x)–
[
Nb W O ]
(x = 3,2,1) and (NH ) H W O (used
x
6–x 19
4 6 2 12 40
for the preparation of reference catalysts) on Figure 7, Fig-
ure 8, and Figure 9. Respective DSC analyses were also car-
ried out, results obtained for (NH ) H W O ,
4
6
2
12 40
3
–
5–
[
NbW O ] and [Nb W O ] salts are presented.
5 19 3 3 19
For (NH ) H W O , its weight loss is followed under
4
6
2
12 40
nitrogen (Figure 7), its decomposition can be described by
the following equation:
Figure 9. TGA (continuous lines) curves of of TMA salts
(
4 6 2 12 2 3 3 2
NH ) H W O40·nH O Ǟ 12 WO + 6 NH + (4 + n) H O
(
2+x)–
of [Nb
x
W
6–x
O
19
]
isopolyanions: (a) TMA
(c) TMA [Nb 19]. DSC of
5
O19] (dashed line bЈ).
4 2 4 19
[Nb W O ]
(
b) TMA
2
K[NbW
5
O
19
]
5
3 3
W O
The TGA curve shows three weight losses corresponding
TMA
2
K[NbW
to loss of water molecules and/or ammonia, characterized
by endothermic peaks in DSC (the third one being certainly
incorporated into the exothermic peak). Assuming the ini- ammonia molecules and 2 water molecules. After removal
tial compound formula (NH ) H W O ·4H O, first loss of 6 ammonia and 6 water molecules, at 340 °C a plateau
4
6
2
12 40
2
of 2.3% corresponds to the release of 4 water molecules is reached and corresponds to the formation of hydrated
whereas the second weight loss (4.6%) is attributed to 6 oxide WO ·2H O. Two remaining water molecules are elim-
3
2
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inated after 380 °C as indicated by the last weight loss ob- tinuous weight losses could be due to the removal of 2
served in TG curve. Simultaneously, crystallization of tung- TMA molecules indicating the presence of potassium in
sten trioxide WO is represented by the exothermic peak in this salt for which the proposed formulae is
3
the DSC curve. The same range of temperature for WO
TMA K [Nb W O ]·5H O.
Calculations based on weight losses obtained from TGA
results allowed us to estimate the release of water and TMA
3
2 3 3 3 19 2
[
37]
crystallization was also reported by Marashi et al.
paratungstate thermal decomposition.
for
From Figure 8 displaying the thermogravimetric profile molecules. This estimate lets proposing general formulas
8
–
of [Nb O ] potassium salt, up to 220 °C we observed the and hydration degree of the NbW salts which are presented
6
19
removal of crystalline water from the structure of the hy- in Table 1.
drated K [Nb O ]·nH O. After 220 °C a plateau is reached.
8
6
19
2
Assuming the formation at high temperature of the sepa-
Table 1. General formulae based on TGA calculations for NbW
compounds.
rated oxides Nb O and K O, this total water loss of 12.3%,
2
5
2
confirmed by two endothermic peaks at 80 and 210 °C, oc-
curs in two continuous steps corresponding to the release
General formula
8
–
6–
[Nb O ]
K [Nb O ]·9H O
6
4
3
2
19
8
6
19
2
of 9 water molecules. The TGA curve of [Nb W O ] po-
4
2
19
6
–
[Nb
[Nb
[Nb
W
W
W
2
3
4
O
O
O
19
19
19
]
]
]
K
TMA
6
[Nb
4
W
2
O
2
19]·7H O
tassium salt shows a continuous weight loss until 350 °C.
This weight loss is very important until 100 °C and is corre-
5–
4–
–
2
K
3
[Nb
3
W
3
O
W
19]·5H
19]·3H
19]·1H
2
O
TMA0.5 [K3.5Nb
2
4
O
2
O
3
lated with an endothermic peak observed in DSC curve. [NbW
From the total weight loss of 9.7%, assuming the formation
of Nb O , WO and K O, the number of water molecules
5
O
19
]
TMA
2
K[NbW
5
O
2
O
2
5
3
2
It is observed that with increasing niobium content, the
hydration degree increases, from 1 for [NbW O ] till 9
for [Nb O ] isopolyanion. Niobium-based solutions are
known to be easily hydrolyzed and this behavior seems in
relation to niobium affinity for water molecules.
is evaluated to 7 indicating the initial salt formulation
3
–
K Nb W O ·7H O. It is noted that no exothermic peak
5
19
6
4
2
19
2
8–
corresponding to WO3 crystallization is shown by DSC
curve.
6
19
The TGA curves of TMA-potassium mixed salts are re-
ported in Figure 9. In the case of TMA K[NbW O ] which
2
5
19
Solubility
formulae has been determined by XRD measurement, a
slight first weight loss correlated to the endothermic peak
of the DSC curve is attributed to water loss. Assuming the
formation of WO , Nb O and K O this weight loss (1.7%)
Order of magnitude of solubility values in water at room
temperature (r.t.) and at 70 °C for isopolyanions is given in
Table 2. The solubility was measured by addition of known
water volume (ca. 10 mL in the case of Nb W O salt) to
known compound quantity (typically 0.200 g) until com-
plete dissolution of the last compound grain under agita-
tion.
3
2
5
2
is attributed to the loss of 1 molecule of H O. Between
2
3
3
19
400 °C and 420 °C, an important weight loss is observed
and correlated with an exothermic peak in the DSC curve.
The exothermicity is explained by the combustion of TMA
that formally corresponds to the loss of (TMA O) accord-
2
ing to the following equation:
Table 2. Solubility values for [Nb
6
W
6–x
O
19](2+x)– HPA salts.
2 5 3 2 5 2 2
TMA K[NbW O19] Ǟ 5 WO + 0.5 Nb O + 0.5 K O + TMA O
–
1
–1
Solubility, W [molL ] Nb [molL ]
The corresponding weight loss (10.6%) is in agreement
with the theoretical weight loss (10.8%) confirming clearly
the complete formula TMA K[NbW O ]·1H O.
19]3–
–3
–4
–4
[
[
[
[
NbW
5
O
room temp.
70 °C
2.0ϫ10
4.0ϫ10
4.0ϫ10
8.0ϫ10
–
3
2
5
19
2
19]4–
19]5–
5.0ϫ10–3
2.5ϫ10–3
Nb
Nb
Nb
2
3
4
W
W
W
4
O
r.t.
70 °C
4
–
For [Nb W O ] salt, the weight loss occurs in several
2
4
19
0.02
0.01
steps. The first step is related to water removal, after that a
continuous weight loss is observed in two steps: a first one
between 380 and 440 °C and a second one with a slower
rate between 440 and 560 °C. This total continuous weight
loss (6.1%) is too small for explaining the removal of four
molecules of TMA and formally corresponds to the weight
3
2
O
r.t.
0.05
0.1
0.05
0.1
7
0 °C
r.t.
0 °C
O
19]6–
0.07
0.15
0.14
0.3
7
loss of 2 TMA O. Taking into account the presence of po-
One can easily notice that those compounds have a very
2
3–
tassium in this compound as shown for [NbW O ] salt, low solubility, even when increasing the temperature.
5
19
6–
5–
this weight loss is then attributed to the release of 0.25 [Nb W O ] potassium salt and [Nb W O ] mixed
4
2
19
3
3
19
TMA O. In this case, the first weight loss (4%) is due to TMA-K salt are the most soluble ones. Thus, they were
2
the loss of 3 water molecules and proposed formula would chosen for preparation of the isopolyanions-based catalysts.
be TMA0.5K3.5[Nb W O ]·3H O.
The solubility of isopolyanions was also verified in other
2
4
19
2
5–
The TGA curve corresponding to the [Nb W O ] salt solvents such as ethanol, DMSO, DMF, acetonitrile but
3
3
19
shows also three weight losses. In the same way as for water appeared to be the best solvent even if the solubility
4
–
[
Nb W O ] salt, the first weight loss (6.8%) is attributed remains low. Infrared spectra of NbW water solutions
2 4 19
to the release of 5 molecules of water, whereas the two con- showed species preservation after dissolution.
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Characterization of Oxide Catalysts
isopolyanion-based catalyst the dispersion is homogeneous
throughout the interior of the extrudate whatever the ele-
ment, while for the reference catalyst, heterogeneous disper-
sion of Ni and W is observed throughout the extrudate. For
the isopolyanion-based catalyst niobium and tungsten are
introduced in one molecular entity and it can be considered
that the heteropolyanionic precursor use is beneficial for
homogeneous elements dispersion.
Due to low solubility of synthesized isopoly compounds,
only the two most soluble ones were selected for catalysts
5
–
6–
preparation: [Nb W O ] TMA-K salt and [Nb W O ]
3
3
19
4
2
19
potassium salt. Despite their solubility remaining low, WO3
loading on prepared catalysts was set on 8 wt.-%. The pres-
ence of a promoter as nickel is required to form NiWS
active HDS phase during sulfidation procedure. In our case
nickel was introduced using nickel nitrate solution during a
second impregnation step as described in the experimental
section to obtain a Ni/W ratio of 0.3 for the catalysts. All
the catalysts were characterized in oxidic form by Raman
spectroscopy in dried or calcined form.
Raman Spectroscopy
The catalysts in oxidic form were characterized by Ra-
man spectroscopy. Figure 10 shows the Raman spectra of
Nb4W2Ni and Nb3W3Ni catalysts in the dried and cal-
cined state. For all spectra, observed peaks are large. First,
for dried samples (Figure 10, b and d), few bands are ob-
Figure 11. EMPA results for Nb3WNi C and reference catalysts.
–
1
–1
served, at about 946 (970) cm and 878 (885) cm and also
–
1
at 271 (269) cm . The latter well-defined peak correspond-
Cyclohexane Isomerization
ing to υ MO vibration suggests that the isopolyanion is
s
c
The isomerization activity of the niobium-containing
HPA-based catalysts was evaluated in cyclohexane (CC6)
isomerization to give methylcyclopentane on sulfide cata-
lysts under hydrodesulfurization conditions (hydrogen pres-
sure and sulfur presence). The performance of these solids
was compared to that of a reference NiW catalyst without
niobium (same W and Ni/W ratio). Figure 12 shows that
Nb-containing catalysts exhibit higher isomerization ac-
tivity than the Nb-free catalyst, in particular the
Nb3W3NiC catalyst that appears 5 times more active than
the reference. A slight increase in CC6 conversion is ob-
served in the calcined catalysts when comparing with the
performance of the dried solids. Nb4W2Ni is less active
than Nb3W3Ni despite its higher quantity of niobium, it
can be related to the presence of a larger quantity of potas-
sium issued from the use of the initial potassium salt
K Nb W for preparing Nb4W2Ni catalyst.
preserved after impregnation, maturation and drying. For
calcined catalysts, only bands at high wavenumber are ob-
served indicating that isopolyanionic structure is not pre-
served after calcination at 450 °C. These bands correspond
both to polytungstate and polyniobate phases.
6
4
2
Figure 10. Raman spectra for (a) Nb4W2Ni C (b) Nb4W2Ni D
(c) Nb3W3Ni C (d) Nb3W3Ni D, where C: calcined catalysts, D:
dried catalysts.
Electron Probe Micro Analysis
Figure 11 shows EPMA results allowing the observation
of active elements (W, Ni and Nb when present) dispersion
in the support extrudate for Nb3W3Ni calcined catalyst
and for a reference catalyst prepared from conventional pre-
Figure 12. Isomerization conversion for Nb3W3 and Nb4W2 cata-
cursors (same W and Ni/W ratio). Each image represents a lysts (D:dried and C: calcined) compared with NiW catalyst per-
trilobal alumina extrudate in cross section (around 1.5 mm) formance. Reaction conditions: 350 °C hydrogen pressure
60 bars)- feed composition: 4 wt.-% of sulfur, 20.0 wt.-% of tolu-
-
(
of calcined catalysts. The elements concentration is repre-
sented by color coding – blue representing the lowest con-
centration, green and yellow for increasing concentration,
ene and 74.1 wt.-% of cyclohexane.
XPS results (not presented here) have shown that in the
reaching red color for the highest element loading. For the studied catalysts niobium entities were unsulfided after the
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applied sulfidation procedure (350 °C during 2 h) and pres- hydrogenosulfite was added and the pH was adjusted to 8 with
6
m hydrochloric acid. The solution was placed at 0 °C overnight.
ent only an oxidic environment. Brönsted acid sites are re-
ported to be involved in cyclohexane isomerization to meth-
Needle-shape crystals of final product were dried under vacuum.
[
38]
ylcyclopentane. We can assume that the polyoxoniobiate [Nb W O ] TMA-K Mixed Salt: Tetramethylammonium bromide
3
3
19
phase supported on alumina observed by Raman spec- [(CH
3
)
4
N]Br (≈ 227 mol) was added to the filtrate after separation
[Nb 19] needles from synthesis described above until
complete precipitation of [Nb
of K
6
4 2
W O
troscopy possesses hydroxy groups developing Brönsted
acidity requested for CC6 isomerization.
5
–
3
W
3
O
19
]
salt.
All other [Nb
19](2+x)– salts were synthesized by modifying
x
6–x
W O
above described syntheses parameters such as Nb/W ratio, pH and
temperature. The general method of preparation consists in acidifi-
cation of the solution of sodium tungstate and potassium hexa-
niobate with set Nb/W ratios, syntheses being performed under
heating. In the group of studied compounds, the higher the pH of
Conclusions
This study focused on the base of Nb-containing Lind-
qvist isopolyanions [Nb W O ]
(2+x)–
. By varying synthe-
x
6–x 19
sis parameters such as Nb/W ratio, pH or temperature, iso- preparation, the lower content of tungsten is reached. Thus,
3
–
4–
polyanions with x = 0–4 and 6 were prepared. The Raman [NbW
5
O
19
]
is prepared at pH 2.0 and [Nb
2
W
4
O
19
]
at pH 5–6.
4
–
5–
6–
and IR results clearly showed formation of desired com- Stability zones of [Nb
2
W
4
O
19] , [Nb
3
W
3
O
19
]
4 2 19
and [Nb W O ]
3
–
niobotungstates are overlaid, making their preparation and separa-
tion fragile. The separation of these compounds is based primarily
on differences in solubility.
pounds. [NbW O ] salt was obtained as a single crystal,
5
19
X-ray Diffraction and crystallographic study were per-
formed in order to specify the structure of this compound.
The XRD theoretical pattern was determined and com- Sodium tungstate dihydrate and niobium oxide were purchased
from Sigma and Aldrich and all other reactants used during syn-
thesis from Fisher Scientific.
pared with an experimental one. Those two pattern being
identical, one can conclude that the determined structure
corresponds not only to single crystal but is also applicable Catalysts Preparation: The niobium–tungsten catalysts, promoted
to the bulk material. TGA-based calculations let proposing with nickel, are obtained by sulfidation of oxidic precursors ob-
general formulas; all compounds contain alkaline element tained by incipient wetness impregnation of alumina with impreg-
nating solutions based on heteropoly compounds or conventional
precursors as described in the following paragraph.
and the hydration degree increases with increasing niobium
content. The solubility of synthesized compounds was
evaluated in several solvents and water appeared to be the Preparation of the Impregnating Solutions: Impregnating solutions
best one, even though the solubility remained low. This pa- were prepared according to the following procedure:
rameter was a decisive one during selection of materials for
catalysts preparation. The two most soluble compounds –
a) Conventional impregnation solution was prepared by dissolution
of ammonium metatungstate (AMT).
6
–
5–
[
Nb W O ] potassium salt and [Nb W O ] TMA-K
4 2 19 3 3 19
b) Isopolyanions-based impregnation solutions were obtained by
isopolyanion salt dissolution in water.
salt – were selected to prepare catalysts promoted with
nickel (Ni/W = 0.3), that were tested in cyclohexane isomer-
ization after sulfidation. Catalyst prepared from
c) Solution of nickel precursor was obtained by nickel nitrate dis-
solution in water, the Ni/W ratio being set on 0.3.
5
–
[
Nb W O ] TMA-K salt was 5 times more active than a
3 3 19
free niobium reference catalyst showing that the presence Preparation of the Oxidic Precursors: The oxidic precursors were
prepared by incipient wetness impregnation of alumina extrudates
of niobium in hydrodesulfurization catalysts generates
2
–1
–1
(
specific area: 265 m g , total pore volume: 0.75 mLg ) with pre-
Brönsted acidic sites involved in isomerization reaction,
that is recognized for helping HDS of refractory sulfur-con-
taining molecules.
vious impregnating solutions. After impregnation and 2 h of matu-
ration in a humid atmosphere in order to allow the species dif-
fusion, the solids were dried for 8 h at 90 °C, nickel was introduced
by post-impregnation of the nickel nitrate solution and then the
solids were calcined at 450 °C under air. Isopolyanion-based cata-
lysts were prepared in four impregnations due to low precursors
solubility. Every impregnation was performed after a drying step.
Table 3 resumes prepared catalysts, used precursors and nomencla-
ture. Dried catalysts are noted by a symbol D and calcined ones by
symbol C. Following the same procedure Calcined NiW reference
catalyst is prepared from ammonium metatungstate (AMT) solu-
tion and nickel nitrate solution impregnations after intermediate
Experimental Section
19](2+x)– Lindqvist-type isopo-
Isopolyanions Synthesis: [Nb
lyanions were prepared following a protocol of Dabbabi and
x 6–x
W O
[
35]
Boyer after modifications. In all syntheses, potassium hexaniob-
ate K [Nb 19], prepared by alkaline fusion of potassium hydrox-
8
6
O
ide (0.98 mol) and niobium oxide (0.05 mol), was used as starting
compound. An example of synthesis of potassium and tetrameth-
ylammonium (TMA) salts are given below:
K
1
6
[Nb
50 mL of distilled water with addition of 6 mL of hydrogen perox-
ide. The solution was heated at 60 °C and sodium tungstate dihy-
drate (Na WO ·2H O) dissolved in 50 mL of distilled water was
4 2 8 6
W O [Nb O19] was dissolved in
19]: 4ϫ10–3 mol of K
Table 3. Prepared catalysts, used precursors and nomenclature.
Catalysts
Precursors
State
Nb3W3NiD
Nb3W3NiC
Nb4W2NiD
Nb4W2NiC
NiW
Nb
Nb
Nb
Nb
3
3
4
4
W
W
W
W
3
3
2
2
O
O
O
O
19 salt, Ni(NO
19 salt, Ni(NO
19 salt, Ni(NO
19 salt, Ni(NO
3
3
3
3
)
)
)
)
2
2
2
2
dried
calcined
dried
calcined
calcined
2
4
2
then added. The solution was kept at 60 °C under vigorous stirring
until complete dissolution of reactants. Then, the solution was
acidified with 8 mL of 6 m hydrochloric acid, kept boiling for
3 2
AMT, Ni(NO )
10 min and then cooled down to 60 °C. 25 mL of 2 m sodium
Eur. J. Inorg. Chem. 2015, 2067–2075
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FULL PAPER
2 5
Table 4. Crystal data, data collection and structure refinement parameters for single crystal of TMA K[NbW O19].
Crystal symmetry
Space group
cubic
Fm3m (no. 225)
a = 13.8203(1)
2639.69(3)
¯
Unit cell [Å]
3
Volume [Å ]
Formula
Z
C
4
8 24 2 1 1 5 19
H N K Nb W O
M
F(000)
w
, calculated density
1503.5, 3.78205
2574
Data collection
Equipment
Radiation Mo-K
Scan mode
Bruker X8
0.71069
ω/φ-scan
2.55–32.47
α
[Å]
Recorded angular range θ [°]
Recording reciprocal space
–20 Յ h Յ 20; –18 Յ k Յ 20; –20 Յ l Յ 20
16393
291
271
Number of measured reflections
Number of independent reflections
Number of independent reflections [IϾ3σ(I)]
–1
μ [mm ] (λ = Mo-K
α
)
22.358
Absorption correction
R merging factor [%]
Semiempirical (Sadabs)
3.60
Refinement parameters
Software, refinement type
Number of refined parameters
Jana 2000, least squares on F
18
R
1
(F) all, [IϾ3σ(I)] = Σ||F
o
| – |F
c
||/Σ|F
o
| [%]
2.14,2.05
2.50, 2.50
2
2 1/2
o c o
wR (F) all, [IϾ3σ(I)] = [Σw(F – F ) /Σw(F ) ] [%]
Weight
2
1/σ
Isotropic secondary extinction
Max./min. Δρ [e/Å ]
Gaussian, 0.083 (8)
1.47/–1.50
3
drying. WO
all catalysts.
3
loading was set on 8 wt.-% and Ni/W ratio on 0.3 for
diffractogram was calculated from the single-crystal refined model
using Powder-cell.[
42]
Precursors and Catalysts Characterization: All compounds were
characterized using several techniques. Raman and Infrared spec-
troscopy confirmed the structure of obtained compounds. Resis-
tance to heating and decomposition under high temperature was
determined by thermogravimetric analysis. Finally, for
Thermogravimetric Analysis: The TGA/differential scanning calo-
rimetry experiments were carried out on a Universal V2.6D
thermoanalyzer from TA Instruments. The analyses were carried
out in air flow, with a heating rate of 10 °C/min and the tempera-
ture was raised till 700 °C.
5
NbW O
19]3– anion obtained as single crystal, the structure was
[
Cyclohexane (CC6) Isomerization in the Presence of Sulfur: CC6
Isomerization activity can be evaluated in the presence of sulfur
with dried ou calcined catalysts sulfided in situ by dimethyl disul-
fide (DMDS) added to the charge. In the catalytic test temperature
determined by XRD and crystallographic study.
Raman Spectroscopy: The Raman spectra of the samples main-
tained at room temperature were recorded using a Raman micro-
probe equipped with a photodiode array detector. The exciting la-
ser source was the 532 nm line of a Nd-YAG laser. The wave-
range DMDS is totally decomposed into CH
is composed of 5.9 wt.-% of DMDS (giving 4 wt.-% of sulfur),
0.0 wt.-% of toluene and 74.1 wt.-% of cyclohexane. The test is
4 2
and H S. The feed
–1
number accuracy was 4 cm .
2
Infrared Spectroscopy: Spectra were recorded using a Nicolet 510
performed in gas phase at a pressure of 60 bars in fixed bed reactor.
The procedure is composed of sulfidation followed by catalytic test.
The first step takes place from room temperature to 350 °C with
Fourier Transform IR spectrometer. The samples were analyzed in
the 400–4000 cm–1 spectral range using the KBr pellet technique
–
1
–1
with 1 wt.-% of the sample in KBr. In this work we present spectra
in the range 400–1100 cm–1 where characteristic tungsten and ni-
obium bands are present.
temperature increase of 2 °Cmin , GHSV = 4 h and H
2
/HC ratio
–1. The catalytic test is carried out at 350 °C with GHSV
= 450NLL
–
1
= 1 h and the same H2/HC ratio as during the sulfidation step.
The isomerization product methylcyclopentane and the cyclohex-
ane reactant are analyzed by gas chromatography. After sulfid-
ation, analysis of effluent through on-line gas chromatography ev-
ery 2 h after the first measurement was performed. A Scheme (S1)
showing all catalytic events is included in supporting information.
Single-Crystal and Powder X-ray Diffraction (XRD): A single crys-
tal suitable for X-ray diffraction was isolated from the bulk and
mounted on a glass fiber. The data collection was performed using
a Bruker X8 diffractometer equipped with an Apex 4k CCD detec-
tor. The absorption corrections were performed using the semi-
empirical method implanted in SADABS.[ Crystal structure was
39]
[40]
solved using the direct method with SHELXS and refined using
Acknowledgments
[
41]
JANA2000.
Data for the data collection and structure refine-
ment are listed in the Table 4.
The authors would like to acknowledge Olivier Delpoux from IFP
Energies Nouvelles for his kind and helpful technical assistance
and expertise in Raman spectroscopy of catalysts and Nathalie Lett
from IFP Energies Nouvelles for catalytic tests.
The homogeneity of the grown single crystals was confirmed by
powder XRD after crushing a significant amount of crystals using
a D8-Bruker diffractometer with a Cu-K
α
radiation. The theoretical
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