Performance of NiO and Ni-Nb-O active phases during the ethane ammoxidation into acetonitrile

Article abstract of DOI:10.1039/c3cy00415e

There is a need to develop catalysts for the direct ammoxidation of ethane to acetonitrile. This work reports a series of bulk Ni-Nb-O catalysts with increasing niobium content, analysing the structure and the effect on acidity and redox properties and how these relate to Ni-Nb interaction. Niobium doping affects NiO lattice leading to progressively smaller unit cell sizes. The Nb/Ni atomic ratio determines the formation of two Nb-Ni-O mixed phases identified by HRTEM, and which Raman bands are identified near 850 cm^-1 and 800 cm^-1 for a Nb-poor phase and near 800 cm^-1 for a Nb-rich Ni-Nb-O one. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00415e

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ARTICLE TYPE
Performance of NiO and Ni-Nb-O active phases during the ethane
ammoxidation into acetonitrile
a
b
c
a
Elizabeth Rojas, Juan J. Delgado , M. Olga Guerrero-Pérez and Miguel A. Bañares
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
There is a need to develop catalysts for the direct ammoxidation of ethane to acetonitrile. This work
reports a series of bulk NiꢀNbꢀO catalysts with increasing niobium content, analysing the structure and
the effect on acidity and redox properties and how these relate to NiꢀNb interaction. Niobium doping
affect NiO lattice leading to progressively smaller unit cell sizes. The Nb/Ni atomic ratio determines the
formation of two NbꢀNiꢀO mixed phases identified by HRTEM, and which Raman bands are identified
1
0
ꢀ
1
ꢀ1
ꢀ1
near 850 cm and 800 cm for a Nbꢀpoor phase and near 800 cm for a Nbꢀrich NiꢀNbꢀO one.
Introduction
Unlike propane ammoxidation, ethane ammoxidation is not that
extensively studied, in fact, there are just few studies dealing with
the ethane ammoxidation reaction (15ꢀ19). Since SbꢀVꢀO
catalytic system has demonstrated to be active and selective for
the propane ammoxidation route (1), we also investigated the
possibility of using those catalysts for the ethane ammoxidation
process, but they proved to be not efficient. The combination of
theoretical and experimental data demonstrated that adsorbed C2
molecules are displaced by much stronger adsorption of ammonia
molecule on both, antimony and vanadium sites at the surface of
the vanadium antimonite (20ꢀ22). Those data confirm that
another catalytic system must be developed for the ethane
ammoxidation process. Aliev and Sokolovskii studied the ethane
ammoxidation to acetonitrile over CrꢀNbꢀMoꢀO catalyst at 350–
Nitriles, like acrylonitrile and acetonitrile, are useful chemical
intermediates (1). Acetonitrile is used as solvent in many
industrial processes, such as high performance liquid
chromatography or for the butadiene extraction in hydrocarbon
streams. It is also used in several organic and inorganic syntheses,
such as the synthesis of flavones and flavonol pigments.
Acetonitrile can also be used to make acrylonitrile by reaction
with methane (2) or methanol (3). Currently, acetonitrile is
obtained as a byꢀproduct during the propylene ammoxidation
process to obtain acrylonitrile, delivering 2ꢀ4 l of acetonitrile per
5
5
6
6
7
7
8
0
5
0
5
0
5
0
1
2
2
3
3
4
4
5
0
5
0
5
0
5
1
00 l of acrylonitrile produced. Acetonitrile can also be produced
by other methods, such as the dehydration of acetamide (4), but
these do not have commercial relevance at this moment in time.
5
00 ◦C, achieving 10% selectivity to acetonitrile at 18% ethane
conversion (15). Later, Catani and Centi investigated ethane
ammoxidation over alumina supported Nb–Sb oxides; reporting a
selectivity close to 50% for conversions of 40% at 530 ºC (23).
More recently, better performances have been described with
cobalt ion exchanged zeolites. Yꢀzeolite and dealuminated USY
showed a yield to acetonitrile close to 15% at 24% ethane
conversion at 450ºC (24). Li and Armor reported significantly
improved yields to acetonitrile, near 26%, at 47% ethane
conversion at 475 ºC over a catalyst based on Co exchanged into
Beta zeolite (18).
Currently, there is no method for the direct commercial synthesis
for acetonitrile, which creates a need for such a direct process,
being ethane a most attractive raw material.
Alkane feedstocks are currently produced by steam cracking of
various petroleum fractions, delivering an easy supply of ethane,
which, in addition, is the second major component in natural gas
(5). Alkane ammoxidation reactions, in which the hydrocarbon is
oxidized in the presence of ammonia, are increasingly attracting
interest as a route to obtain nitriles from alkane feedstocks
(1,6,7). In recent years, propane ammoxidation has attracted
Lemonidou and coꢀworkers reported that niobiumꢀpromoted NiO
catalysts are able to activate the ethane molecule during the
oxidative dehydrogenation reaction (25ꢀ26). Our group reported
some preliminary results showing that NiꢀNbꢀO bulk oxide
catalysts are also promising during the direct ammoxidation of
ethane to acetonitrile (19). Since that study indicated that NiO
catalyst was not able to incorporate the nitrile group into ethane
molecule in absence of niobium in the catalyst formulation, we
much attention and efficient catalysts have been developed (9ꢀ
13). Such technology is now available at a commercial scale
until very recently Asahi Kasei Chemical opened a 200.000
tonne/year propane ammoxidation plant in Thailand (14).
Propaneꢀbased technology would replace the propyleneꢀbased
technology. But the reaction conditions to activate alkanes are
more energy demanding than those of olefins (8), which has a
negative effect on selectivity.
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investigated supported NiO/Nb O
ammoxidation (27ꢀ28). Two different synthesis methods for those
supported catalysts were studied, a conventional impregnation
catalysts for ethane
broadening of a peak at halfꢀheight, that is, broadening due to the
crystallite dimensions (29).
2
5
(27), and a novel dry mixing method (28). In both cases, these 60 Ammonia chemisorption experiments were performed using an
5
studies show that NiO oxide phase is required along with the
presence of a mixed NiꢀNbꢀO phase to obtain efficient ethane
ammoxidation. Thus, the objectives of this paper are to
investigate the performance of bulk NiꢀNbꢀO catalysts with
ASAPꢀ2000 apparatus. The catalysts (0.2 g) were pretreated at
250ºC for 0.5 h and then cooled to 100ºC under He flow to
remove adsorbed species. The pretreated samples were exposed
to 5% NH /He, with subsequent flushing with helium at 100ºC
3
respect to that of supported NiO/Nb O catalyst series to uncover 65 for 1 h to remove the physisorbed ammonia. The acidity values
2
5
10
the nature and characterize the two active phases which
coexistence appear necessary for ethane ammoxidation, NiO and
NiꢀNbꢀO mixed phase.
were determined from the difference between ammonia
adsorption and desorption isotherms.
Xꢀray photoelectron spectra were recorded on a VG Escalab
Experimental procedures
70 200R electron spectrometer using a Mg K
XꢀRay exciting source
α
(1254.6 eV, 10kV, 100W) of a twin anode in the constant
Preparation of Samples
analysed energy mode, with a pass energy of 25 eV for general
spectra and 75 eV for acquisition of spectral regions. Spectra
were recorded for Ni 2p, Nb 3d, C 1s and O 1s regions. The
1
2
2
3
5
0
5
0
Bulk NiꢀNbꢀO catalysts were prepared by evaporation of
solutions of the corresponding precursors. Ammonium niobium
soluble complex (CBMMꢀNiobium Products) was added to an 75 accuracy of the BE was ±0.1 eV. The samples in the form of selfꢀ
aqueous solution of nickel acetate tetrahydrate (>99%, Aldrich)
in the appropriate proportion depending the desired Ni/Nb atomic
ration. This solution was kept under stirring at 80ºC during 1h to
ensure complete dissolution and good mixing of the starting
supporting wafers of 10 mm diameter were outgassed overnight
in the preparation chamber of the spectrometer and subsequently
transferred to the analysis chamber. The pressure of the main
ꢀ
5
chamber during spectra acquisition was maintained at ca. 1 x 10
compounds. The resulting solution was dried in a rotatory 80 Torr. The residual pressure in the turboꢀpumped analysis chamber
ꢀ
9
evaporator at 80ºC under a reduced pressure of 10ꢀ40 mmHg. The
resulting solid was dried at 120ºC for 24 h and then calcined at
450ºC during 5 h, at a heating rate of 5 ºC/min in a flow of
synthetic air and then cooled to ambient temperature inside the
was kept below 7 x l0 Torr during data collection. Each spectral
region was signalꢀaveraged for a given number of scans to obtain
good signalꢀtoꢀnoise ratios. Although surface charging was
observed on all the samples, accurate binding energies (BE) were
oven. Bulk NiO was prepared by calcination in synthetic air of 85 determined by charge referencing with the C 1s line at 284.6 eV.
nickel acetate at 450°C for 16 h and bulk Nb O used as reference
Data processing was performed with the XPS peak program, the
spectra were decomposed with the least square fitting routine
provided with the software with Gaussian/Lorentzian (90/10)
product function and after subtracting a Shirley type background.
90 Atomic surface ratios were obtained using the area under the
curve of Ni 2p, Nb 3d, C 1s and O 1s coreꢀlevel photoemission
spectra with the atomic sensitivity factors reported by Wagner et
al. (30).
2
5
was prepared by calcinations in synthetic air of hydrated niobium
pentoxide at 400 ºC by 12 h. The nomenclature of catalysts was
as follows: Ni Nb where y and x indicate the atomic content of
y
x;
Ni and Nb, respectively.
Characterization
Nitrogen adsorption isotherms (ꢀ196 ºC) were recorded by an
automatic Micromeritics ASAPꢀ2000 apparatus. Prior to the
adsorption experiments, samples (ca. 0.2 g) were outgassed under
35
40
45
50
55
95
Raman spectra were run with a singleꢀmonochromator Renishaw
System 1000 equipped with a cooled CCD detector (ꢀ73ºC) and
an Edge filter, which removes the elastic scattering. The samples
−3
vacuum of 10 Pa at 140ºC during 2 h to remove adsorbed
species. BET areas were computed from the adsorption isotherms
2
(
0.05 < P/Po < 0.27), taking a value of 0.164 nm for the crossꢀ
+
were excited with the line from an Ar laser (488 nm) operating
section of the adsorbed N molecule. ICP elemental chemical
ꢀ1
2
at 10 mW; spectral resolution was ca. 4 cm ; spectra acquisition
analysis of the niobiumꢀpromoted NiO catalysts was made on a
PerkinꢀElmerꢀ3300 DVꢀDisgreggation using Nb and Ni elemental
standards (Alfa Aesar). Around 0.1 g of sample were dissolved
with nitric, chloride, and fluoride acids in a microware milestone
MLS 1200 MEGA with a maximum power of 650W.
1
1
1
00 consisted of 10 accumulations of 30 s. The signal was collected
by using a microscope Raman spectrometer (Renishaw Microꢀ
ꢀ
1
Raman System 1000) in the 100–1500 cm Raman shift range.
The spectra were obtained under dehydrated conditions (50
ml/min synthetic air, ca. 200ºC) in a hot stage (Linkam TSꢀ1500).
05
Xꢀray diffraction patterns were recorded on
Krystalloflex Dꢀ500 diffractometer using CuKα radiation (λ=
.15418 nm) and a graphite monochromator. Working conditions
a Siemens
TPR experiments were carried out on a TPD/TPR 2900
(Micromeritics) instrument. In a typical experiment ca. 20 mg of
0
ovenꢀdried samples (110 ºC for 12 h) was taken in a Uꢀshaped
quartz tube (i.d. 6 mm). Prior to TPR studies, the catalyst sample
were 40kV, 30 mA and scanning rate of 2°/min for Bragg's
angles (2θ) from 10º to 90º. The crystallite size of the synthesized
10 was pretreated in an inert gas (He, 50 ml/min) at 300 º C for 2 h.
After preꢀtreatment, the sample was cooled to ambient
temperature and the carrier gas consisting of 5% hydrogen
balance argon (30 ml/ min), was allowed to pass over the sample
and the temperature was increased from ambient temperature to
nickel catalysts, D , was calculated from the major diffraction
c
peaks of the base of (200) using the Scherrer formula:
Dc=(Kλ)/(Bcosθ); where K is a constant (ca. 0.9) assuming
particles spherical; λ is the Xꢀray wavelength used in XRD (λ=
1
.5418 Å); θ is the Bragg angle; B is the pure diffraction
2
|
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DOI: 10.1039/C3CY00415E
1
000 ºC at a heating rate of 10 ºC /min. Hydrogen concentration
yi ni
in the effluent stream was monitored with a thermal conductivity
detector.
Selectivity of product P
:
i
Si =
∑i yi n
i
5
0
5
0
5
0
5
0
5
HighꢀResolution Transmission Electron Microscopy (HRTEM)
and HighꢀAngle Annular Dark FieldꢀScanning Transmission
Electron Microscopy (HAADFꢀSTEM) images were recorded on
a JEOLꢀ2010F instrument. The spatial resolution at Scherzer
defocus conditions in HRTEM mode is 0.19 nm, while the
HAADFꢀSTEM studies were performed using an electron probe
of 0.7 nm of diameter and a diffraction camera length of 10 cm. It
should be pointed that the chemical composition of the sample
was studied in STEM mode using an Energyꢀdispersive Xꢀray
spectrometer (Oxford Instrument, Inca Energyꢀ200).
X * Si
Yield of product Pi; Yi =
1
00
where y and y are the mole fractions of products P and C H ,
respectively; n and n are the number of carbon atoms in each
i E
i
E
i
2
6
5
5
6
0
5
0
1
1
2
2
3
3
4
4
molecule of product P and C H , respectively, and all the terms
i
2
6
were evaluated for the exit stream. The major products of ethane
ammoxidation reaction over niobiumꢀpromoted NiO catalysts are
C H N, C H , CO . The turnover frequency (TOF) data were
calculated by considering the amount of acetonitrile produced per
Ni atom (determined in the XPS analyses). Since this is closer to
the concentration of Ni on the surface of the catalysts and, thus,
accessible to reactants. We consider it more accurate to determine
the TOF value based on XPS than on ICP results, which indicate
the bulk Ni content.
2
3
2
4
2.
Catalytic measurements
Activity tests were performed with a tubular quartz microreactor
(OD =9 mm, ID =6.8 mm, length= 290 mm) coupled to an online
gas chromatograph (Varian CPꢀ3800) equipped with both flame
ionization and thermal conductivity detectors. The GC columns
consisted of a molecular sieve 5A and a Porapak Q. To prevent
participation of homogeneous reactivity, the reactor was designed
to minimize gasꢀphase activation of propane, minimizing void
volume before and after the catalytic bed. The catalytic tests were
made using 0.2 g of powder sample with particle dimensions in
the 0.250ꢀ0.125 mm range. The catalytic bed (ca. 1 cm) was
located between two quartz wool beds and pretreated in air flow
at 200 °C for 1 h. The axial temperature profile was monitored
by a thermocouple sliding inside a quartz tube inserted into the
catalytic bed. Tests were made using the following reaction feed
Results
Figure 1 shows the XRD patterns of fresh and used catalysts. All
diffractograms present peaks at 37.31°, 43.47°, 62.98°, 75.61°,
and 79.59º, which correspond to the (111), (200), (220), (311),
(222) crystalline planes of NiO with cubic rock salt structure and
a lattice constant of a = 4.1694 Å (Table 1)(JCPDS 4ꢀ835) (26).
This NiO pattern is detected for all the fresh samples, being
6
7
7
8
8
5
0
5
0
5
sharper as the niobium content increases. Ni0.7Nb0.3, Ni0.6Nb0.4
,
Ni0.4Nb0.6 and Ni0.35Nb0.65 samples exhibit additional diffraction
peaks after ammoxidation reaction, uncovering some structural
arrangement induced by catalytic reaction. The intensity of this
reactionꢀinduced phase diffraction phase increases with Nb
content (Figure 1). Millet’s group also detected those peaks in
composition (% volume): 25 % O , 9.8% ethane, 8.6 % NH in
2
3
balance helium. The total flow rate was 20 ml/min, corresponding
ꢀ
1
to a gas hourlyꢀspace velocity (GHSV) of ca. 3000 h . The
quantity of catalyst and total flow were determined in order to
avoid internal and external diffusion limitations (31). The
accuracy of the analytical determinations was checked for each
test by verification that the carbon balance (based on the ethane
converted) was within the cumulative mean error of the
determinations (±10%). Yields and selectivities of the reaction
products were determined on the basis of the moles of ethane
feed and products, considering the number of carbon atoms in
each molecule.
NiOꢀNb O5 catalysts with Nb contents above 15 wt.% and
identified it as NiNb O (32). It should be noted that these signals
2
2
6
are not present in the catalysts with the highest niobium content
Ni0.2Nb0.8 used). In fresh and used samples with Nb contents
higher than 15ꢀ20 %, it appears a broad band near 27º that have
been attributed to an amorphous Nbꢀrich phase by Lemonidou et
al (26,33). The intensity of such band increases with Nb content.
None of the diffraction pattern of crystalline Nb O oxide is not
detected in any sample; this is indicative that niobium oxide
would remain as an amorphous phase or that it would be present
in very small crystalline domains, not large enough to generate a
diffraction pattern (ca. < 4 nm).
(
2
5
Ethane conversion, selectivities, and yields are defined as
follows:
∑i yi n
i
Conversion of C H : X =
2
6
y n +
∑i yi ni
E
E
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ARTICLE TYPE
*
N iO (J C P D S : 4-8 3 5)
* N iO (J C P D S : 4 -8 3 5 )
*
•
N i-N b -O m ix e d p h a s e
*
•
N i-N b -O m ix e d p h a s e
*
*
N i0 .2 N b 0.8 u s e d
*
*
*
*
N i0 .2 N b 0.8 fre s h
•
•
•
*
*
N i0 .7 N b 0 .3 u s e d
•
N i0 .7 N b 0 .3 fre s h
N i0 .8 N b 0 .2 u s e d
•
•
•
•
N i0 .3 5 N b 0 .6 5 u s e d
N i0 .3 5 N b 0 .6 5 fre s h
N i0 .8 N b 0 .2 fre sh
•
N i0 .4 N b 0 .6 u s e d
N i0 .8 5 N b 0 .15 u s e d
N i0.4 N b 0 .6 fre s h
N i0 .8 5 N b 0 .1 5 fre s h
N i0 .9 N b 0 .1 u s e d
N i0 .6 N b 0 .4 u s e d
N i0 .6 N b 0 .4 fre s h
N i0 .9 N b 0 .1 fre s h
N iO
2
0
40
60
80
20
40
60
80
2
θ
/ º
2 θ / º
Fig.1. XRD patterns of fresh and used catalysts
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b
a
c
c
a
b
a
c
Ni0.7Nb0.3 used
Nb O fresh
2
5
Ni0.7Nb0.3 fresh
Ni0.8Nb0.2 used
Ni0.2Nb0.8 used
Ni0.2Nb0.8 fresh
Ni0.8Nb0.2 fresh
Ni0.35Nb0.65 used
Ni0.85Nb0.15 used
Ni0.35Nb0.65 fresh
Ni0.85Nb0.15 fresh
Ni0.4Nb0.6 used
Ni0.4Nb0.6 fresh
Ni0.9Nb0.1 used
Ni0.9Nb0.1 fresh
Ni0.6Nb0.4 used
Ni0.6Nb0.4 fresh
NiO fresh
1
400
1200
1000
800
600
400
200
1400
1200
1000
800
600
400
200
-
1
-1
Raman shift/ cm
Raman shift/ cm
2 5
Fig.3. Raman spectra of fresh (left) and used (right) catalysts. a (NiO), b (Nb O ), c (Ni-Nb-O)
(
by
size, D
(nm)
c
NH
m )
3
/
5
0
5
The NiO lattice constant values and NiO average crystal sizes are
listed in Table 1. Pure NiO crystallizes in the cubic rock salt
structure, with a lattice parameter of a = 4.1694 Ǻ. The niobium
incorporation progressively contracts the lattice parameter; NiO
and Ni0.9Nb0.1 exhibit the largest NiO crystal cell size. The BET
2
DRX)**
4.1694
4.1692
4.1602
4.1666
4.1664
4.1634
4.1572
4.1514
4.1444
NiO
0
20
48
35
17
17
20
19
17
11
15
1.9
2.0
2.3
2.8
3.8
3.1
3.5
3.5
3.3
0.4
Ni0.9Nb0.1
Ni0.85Nb0.15
Ni0.8Nb0.2
Ni0.7Nb0.3
Ni0.6Nb0.4
Ni0.4Nb0.6
Ni0.35Nb0.65
Ni0.2Nb0.8
0.111
0.176
0.250
0.428
0.666
1.500
1.857
4
57
65
65
66
97
94
84
46
68
2
1
1
area obtained for NiO oxide is quite low (20 m /g) but increases
upon niobium incorporation, reaching a maximum for Ni0.6Nb0.4
sample. On the other hand, NiO crystallite size in the mixed
oxides is smaller than in the pure oxide (Table 1). This may
reflect the incorporation of Nb cations into the NiO lattice
2+
5+
structure. This is possible since the ionic radii of Ni and Nb
cations are quite similar (0.69 Å and 0.64 Å, respectively). The
acidity of the samples calculated for ammonia adsorption are
shown in Table 1, as expected, acidity increases with progressive
Nb incorporation.
2 5
Nb O
*
, SBET; surface Area, ** Determined considering the (200) peak higher
intensity
2
0
25
Table 1. Physicochemical characteristics and composition of pure
NiO and Ni-Nb-O niobium-promoted NiO catalysts.
Figure 2 shows the TPR profiles of reference materials and of
fresh catalysts. The TPR profile of nickel oxide exhibits a
reduction peak near 335 ºC with a shoulder near 266 ºC. This
reduction peak has been attributed to the Ni ꢀNi reduction
Nb/Ni
atomic
ratio
NiO
lattice
constant
NiO
average acidity
crystal (µmol
Total
S
BET
Catalyst
2
2+
0
(
m /g)*
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step, whereas the shoulder at lower temperature has been related
20
Fig.2 Temperature- programmed reduction profiles catalysts and
NiO and Nb reference materials.
3
+
2+
2 5
O
to the reduction of Ni to Ni , which may be present in nonꢀ
stoichiometric NiO (34ꢀ36). The shoulder around 266 ºC is not
visible in the samples after niobium incorporation. The fact that
Figure 3 shows the Raman spectra of fresh and used catalysts.
ꢀ1
The Raman bands near 504 and 1050 cm are assigned to NiO
3
+
5
0
5
XPS data (Table 3) that indicated that Ni species population
phase and, as expected, their intensity decreases with niobium
3+
decreases with niobium content, suggests that the Ni sites
present at the surface of NiO would reaction with niobium into
ꢀ1
2
3
3
4
5
0
5
0
content. The Raman bands near 910, 664 and 248 cm , belong to
Nb O oxide, and are visible in fresh and used Ni0.35Nb0.65 and
2
5
2+
Ni phase. A new reduction peak at 370 ºC becomes increasingly
apparent with niobium addition. This new reduction peak must to
be related to the reduction of Nbꢀcontaining NiO domains,
probably to the mixed NbꢀNiꢀO phases detected by XRD (Figure
Ni0.2Nb0.8 samples. Nb O was not detected by XRD, indicating
2
5
that their crystal sizes are not big enough to generate a diffraction
pattern. Two additional bands are detected near 797 and 854 cm ,
which are more intense in used niobiumꢀcontaining samples and
grow stronger as niobium content increases. These bands have to
1
ꢀ1
1
). Such phases are already apparent to Raman spectroscopy at
very low niobium loading (Figure 3). The additional weak
asymmetric reduction peak near 855 ºC, in catalysts with high Nb
be due to NbꢀOꢀNi mixed phases, in line with XRD data (Figure
ꢀ1
1
). Only one Raman band near 800 cm is detected in this region
1
contents (Ni0.35Nb
reduction of the Nb cations in Nb O (35), these must be very
and Ni Nb0.8), has been assigned to the
0.65 0.2
at higher Nb contents (Ni Nb ). These Raman bands in the
0
.2
0.8
5+
ꢀ1
2
5
790ꢀ850 cm range have been assigned to vibrations of bridging
small domains since they do not generate Xꢀray diffraction
pattern.
NbꢀOꢀNi bonds (37). Thus, in the NiꢀNbꢀO vibrations range (790ꢀ
ꢀ1
8
50 cm ), only two Raman bands are detected for samples with
low niobium contents; whereas only one is apparent at higher
niobium contents. This trend may indicate that two different Nbꢀ
NiꢀO phases form.
Nb O5
2
Table 2 and Figure 4 show the XPS and the bulk (ICP)
atomic compositions. Niobium surface concentrations are higher
than the corresponding bulk values, indicating that the surface is
Ni0.2Nb0.8
45 niobiumꢀenriched. Table 3 shows the binding energy (BE) values.
Ni 2p3/2 in NiO possesses two bands at 853.8 and 855.7 eV,
which correspond to Ni and Ni , respectively, along with two
satellites at 862.2 and 867 eV [38]. The peak area values of these
two peaks are listed in Table 3, illustrating an evolution of the
2+
3+
Ni0.35Nb0.65
2
+
3+
5
0
Ni and Ni nearꢀsurface populations. These data indicate a
3+
Ni0.6Nb0.4
progressive depletion of Ni species by niobium, probably
associated with the formation of NbꢀNiꢀO structures; such phases
must essentially be constituted by Ni and Nb ions (such as
NiNb O , Ni Nb O , Ni Nb O , etc.). The BE values obtained for
2+
5+
Ni0.9Nb0.1
2
6
4
2
9
3
2
8
5+
55
niobium species are characteristic of Nb (Figure 4) [37].
NiO
2
00
400
600
800
Temperature (ºC)
Table 2. Bulk atomic (by ICP) and surface atomic compositions (by XPS) for fresh and used catalysts.
Atomic composition (by ICP)
%)
Surface composition (by XPS)
(
( %)
Nb
ꢀ
2.7
5.4
6.0
6.6
10.4
7.9
17.3
16.8
15.5
20.8
25.2
25.9
23.1
30.3
Catalyst
Ni
50
57.7
Nb
ꢀ
1.8
O
50
40.5
Ni
O
NiO
43.3
42.6
39.0
38.2
37.2
33.7
33.2
26.4
20.2
29.1
20.0
21.7
20.6
22.2
19.0
56.7
54.8
55.5
55.8
56.2
56.0
58.9
56.3
63.0
55.4
59.2
53.1
53.5
54.6
50.7
Ni0.9Nb0.1 fresh
Ni0.9Nb0.1 used
Ni0.85Nb0.15 fresh
Ni0.85Nb0.15 used
Ni0.8Nb0.2 fresh
Ni0.8Nb0.2 used
Ni0.7Nb0.3 fresh
Ni0.7Nb0.3 used
Ni0.6Nb0.4 fresh
Ni0.6Nb0.4 used
Ni0.4Nb0.6 fresh
Ni0.4Nb0.6 used
Ni0.35Nb0.65 fresh
Ni0.35Nb0.65 used
69.4
58.7
40.3
29.3
30.5
22.3
3.0
3.7
27.6
37.6
54.8
64.3
55.0
63.5
4.9
6.4
14.5
14.2
6
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Ni0.2Nb0.8 fresh
Ni0.2Nb0.8 used
15.8
21.3
62.9
8.9
12.0
39.2
34.9
51.9
53.1
8
54.4(63.8%)
2+
530.8 (30.9%)
*
O.S; oxidation state
HRTEM images of Ni0.9Nb0.1 corroborate the presence of large
nanoparticles, which digital diffraction pattern (DDP) of the
selected area corresponds to that of NiO (Figure 5A). It should be
noted that these particles show a slightly smaller size than those
present in the pure nickel oxide. NiO particles were found in all
the samples and their particle size decreases with increasing
niobium loading; this is consistent with the surface area and XRD
trends (Table 1). On the other hand, HRTEM (Figure 5B) shows
that some areas of the fresh samples, including those with low
niobium content, are covered by an amorphous phase, being
increasingly evident at higher niobium loadings. Figure 5C shows
a typical HAADFꢀSTEM image of sample Ni0.9Nb0.1, where
larger (20ꢀ50 nm) and smaller particles (4ꢀ6 nm) coexist.
According to XRD patterns (Figure 1), no Nb O particles big
1
2
2
3
3
4
4
5
0
5
0
5
0
5
2
5
enough to give a diffraction pattern are detected by HRTEM. All
samples exhibit a similar trend but the amount and size of the
larger particles decreases with increasing niobium content.
Chemical analyses of selected areas (0.5 nm) were performed by
taking advantage of the possibility to control the beam position
operating in STEM mode. Thus, Figure 5F includes XꢀEDS
spectrum of the areas marked in Figure 5C. According to these
results, there is an enrichment of niobium in the smaller particles,
which is consistent with the results previously reported by
Heracleous et al. [40]. Interestingly, the samples used in the
catalytic run show an increase in the crystallinity of the niobium
amorphous layer. Figure 5D and 5E clearly show this evolution.
Although commonly the Nbꢀrich samples still shows an illꢀ
defined structure (Figure 5E), some wellꢀdefined crystals can be
found. In both cases, the elemental analyses confirm niobium
enrichment in those areas. The DDP in Figure 5 is consistent with
that of NiNb O phase (JCPDS 31ꢀ906, ICDS 37212) along with
2
6
Fig. 4. XPS fresh samples (pure NiO and Ni0.9Nb0.1 , Ni0.6Nb0.4 and
Ni0.4Nb0.6 catalysts) for A) Nb 3d and B) Ni 2p3/2 lines.
the [2 5 1] zone axis. The poor crystallinity of the sample in
Figure 5E, makes it difficult to precisely identify a phase; the
observed spacing (2.8ꢀ2.9 Ǻ) cannot be assigned to NiO but it
may be related to the presence of [0 2 0] reflections of the
5
0
NiNb O phase (JCPDS 31ꢀ906).
Table 3. XPS binding energy data and contribution from the total
area of the peaks.
2
6
1
50
5+
*
Nb (3d 5/2)
Catalysts
Ni 2p3/2
O.S.
O 1s
8
8
8
8
55.7 (56.8%)* 3+
529.1 (64.2%)
530.3 (35.8%)
529.3 (54.5%)
530.9 (45.5%)
NiO
ꢀ
53.8 (43.2%)
56.1 (45.4%)
54.1 (54.6%)
2+
3+
2+
55
206.8
Ni0.9Nb0.1 fresh
Table 4. Ethane conversion , selectivity to the different products,
TOF and productivity of catalysts. 425ºC. *TOF: acetonitrile produced
per surface Ni atom as calculated by XPS
Ni0.9Nb0.1 used 855.4(44.8%)
53.4(55.1%)
Ni0.6Nb0.4 fresh 856.1(25.0%)
53.8(75.0%)
Ni0.6Nb0.4 used 856.3(30.6%)
54.5(69.3%)
Ni0.4Nb0.6 fresh 856.2(36.2%)
3+
2+
3+
2+
3+
2+
3+
207.1
206.5
206.4
206.6
529.4 (100%)
8
529.2 (66.4%)
530.5 (33.5%)
529.4 (100%)
60
8
8
529.3 (69,1%)
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DOI: 10.1039/C3CY00415E
Selectivity (%)
Ethane
conversion
Nb/Ni
atomic ratio
Catalyst
TOF
(s )*
Productivity
((gACE/gcat*h)
-1
(%)
CO
2
Ethylene
57.4
ACN
10.7
35.3
16.2
15.7
13.0
12.2
5.5
Methane
0.3
NiO
0
41.9
55.1
79.8
75.9
73.2
69.3
62.7
49.4
27.2
1.0
31.6
50.2
45.2
36.7
30.9
25.0
21.7
5.9
0.06
0.16
0.13
0.10
0.13
0.12
0.07
0.02
0.05
--
0.26
0.68
0.48
0.37
0.34
0.33
0.14
0.04
0.04
0.02
Ni0.9Nb0.1
Ni0.85Nb0.15
Ni0.8Nb0.2
Ni0.7Nb0.3
Ni0.6Nb0.4
Ni0.4Nb0.6
Ni0.35Nb0.65
Ni0.2Nb0.8
0
0
.111
.176
14.5
0.1
38.3
0.3
0
.25
47.5
0.1
0
0
.428
.666
56.0
0.1
62.7
0.1
1
.5
72.8
0.0
1
.857
4
91.5
2.6
0.1
4.3
91.7
4.0
0.0
2 5
Nb O
ꢀꢀ
6.8
64.0
28.3
0.9
both NiO and mixed NiꢀNbꢀO mixed phase are required in order
to deliver efficient ethane ammoxidation.
Fig. 5. HRTEM images of fresh samples Ni0.9Nb0.1 (A) and Ni0.2Nb0.8
B), as well as a low magnification HAADF image of Ni0.9Nb0.1 (C).
(
A
5
HRTEM micrographs of samples Ni0.9Nb0.1(D) and Ni0.2Nb0.8 (E)
after the catalytic run and (F) X-EDS spectrum of the marked areas.
NiO
Ni0.9Nb0.1
80
60
40
20
0
Ni0.85Nb0.15
Ni0.8Nb0.2
Ni0.7Nb0.3
Ni0.6Nb0.4
Ni0.4Nb0.6
Ni0.35Nb0.65
Ni0.2Nb0.8
Nb O5
2
350
375
400
425
450
Temperature (ºC)
B
20
15
Figure 6A shows ethane conversion values vs. reaction
temperature in the all catalysts. Ni0.6Nb0.4, Ni0.7Nb0.3, Ni0.8Nb0.2
,
10
15
20
25
Ni0.85Nb0.15 and Ni0.9Nb0.1 samples are more active than pure NiO
for the studied temperature range; conversely, the samples with
higher niobium content (Ni0.35Nb0.65 and Ni0.2Nb0.8) present lower
activity. Figure 6B shows the yield to acetonitrile vs. reaction
temperature. The performance of Ni0.9Nb0.1 is quite different than
that of the rest of the catalysts, presenting the highest yield to
acetonitrile. Table 4 shows the selectivities to main products
along with the TOF values, calculated per Ni site assuming that
all Ni sites visible to XPS are exposed. These values also show
that Ni0.9Nb0.1 catalyst exhibits the highest TOF value. Thus, the
incorporation of small niobium quantities into the catalyst
formulation enhances both activity and performance to
acetonitrile, while it would also increase the activity per nickel
site; affording yield values close to 19.1 %, which is a promising
value. The promoting effect of niobium appears limited to low
loadings. These results are in line with those reported previously
10
5
0
360
380
400
420
440
Temperature (ºC)
3
0
Fig. 6. A) Ethane conversion vs. temperature, B) Yield to acetonitrile
vs. reaction temperature. Reaction conditions: total flow, 20 ml/min;
feed composition (% volume), C
3 8 2 3
H /O /NH (9.8/25/8.6), 200 mg of
catalyst.
35
The stability vs. timeꢀonꢀstream of the best performing Ni0.9Nb0.1
and Ni0.6Nb0.4 catalysts has been also investigated (Figure 7). The
data show that ethane conversion values for Ni0.9Nb0.1 catalyst
remain essentially constant during more than two days of
with supported NiO/Nb O catalysts (27ꢀ28), that showed that
2
5
8
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DOI: 10.1039/C3CY00415E
operation at 450ºC. In the case of Ni0.6Nb0.4 catalysts, some
activation occurs during the first hours of reaction, this should
selectivity to acetonitrile since the NiO is not selective for
acetonitrile formation. This is the NbꢀNiꢀO mixed phase detected
reflect the extensive rearrangement of NbꢀNiꢀO species into Nbꢀ 45 in NiO/Nb O supported catalysts. These data show that
2 5
NiꢀO mixed oxide during reaction; in line with the Raman spectra
and XRD characterization data of the used samples.
extensive addition of niobium generates smaller niobiumꢀrich
particles that are not active for ethane ammoxidation.
5
0
Fig.7. Ethane conversion and yield to acetonitrile as a function of
time-on-stream for Ni0.9Nb0.1 and Ni0.6Nb0.1 catalysts. (Reaction
conditions: total flow, 20 ml/min; feed composition (% volume),
The amount of NiꢀNbꢀO particles with small size (Raman
ꢀ
1
band near 800 cm ) is higher at higher niobium contents; Nb O
2
5
5
5
6
0
5
0
oxide is also detected by Raman spectroscopy (cf. Figure 3) and
TPR analyses (cf. Figure 2). The crystal size of these Nb O
1
C
2
H
6
/O
2
/NH
3
/He (9.8/25/8.6/56.6), 200 mg of catalyst, T=450ºC).
2
5
particles is small, in line with the lack of Xꢀray diffraction pattern
cf. Figure 1), and HRTEM images, which show the presence of
an amorphous Nb O layer, which crystallinity increases upon
(
8
0
0
0
0
0
2
5
use in reaction. Thus, small NbꢀNiꢀO particles are detected at
high niobium contents (Ni0.35Nb0.65 and Ni0.2Nb0.8); the BET area
values decrease, as well as the NiO average crystal size, and the
6
4
2
Ni0.9Nb0.1 (Ethane conversion)
Ni0.9Nb0.1 (Yield to acetonitrile)
formation of segregated Nb O5 oxide is detected. Ammonia
2
Ni0.6Nb (Ethane conversion)
0.4
chemisorption data show that the samples with high niobium
content present higher acidity (cf. Table 1), as expected.
Ni0.6Nb0.4 (Yield to acetonitrile)
The changes in the catalysts morphology and composition
upon niobium incorporation correlate with changes in the
catalytic performance. The catalyticꢀpromoting effect of niobium
is limited to low loading and is lost when well defined niobiumꢀ
0
10
20
30
40
50
TOS (h)
[
34,35]
6
5
containing phases form
. At low loadings, mixed NiꢀNb
oxide phases are beginning to form; niobium modulates redox
properties and increases acidity (cf. Table 1), rendering a
catalytic system that is more efficient for the ammoxidation
reaction. Ethane ammoxidation need sites to activate both, ethane
Discussion
Raman spectroscopy evidences two NbꢀNiꢀO structures,
ꢀ
1
characterized by modes near 850 and 800 cm ; both bands are 70 and ammonia, and such sites must be nearby for an efficient
detected at low niobium contents, while the former red shifts and
is the only present at higher niobium contents (Figure 3), indeed,
formation of the nitrile molecule.
1
2
2
3
5
0
5
0
ꢀ
1
at high niobium loadings, only the band near 800 cm is apparent
after reaction for all samples. The relative intensities of these
bands evolve with niobium content. Supported catalysts, in which
NbꢀNiꢀO forms at the interphase between the active phase and the
support (28), exhibit only the phase lowꢀNb contents phase. In
addition to Raman spectroscopy, HRTEM trends along with the
The acidity is related with the ability of the catalysts to adsorb
ammonia during reaction. However; high acidity would lead to a
very strong adsorption of ammonia, limiting its reactivity. This
would explain why the samples with high Nb contents are not
selective to acetonitrile formation (cf. Table 4), although they are
highly selective to ethylene. The progressive increase of acidity
upon addition of niobium would account for a better activation of
ammonia, necessary for nitrile formation. However, it may also
limit the presence of nearby adsorbed hydrocarbon molecules,
necessary to combine with ammonia into the nitrile.
7
5
2
+
3+
evolution of the XPS Ni /Ni ratio (Table 3) indicate also that
different NbꢀNiꢀO structures form, depending on the niobium
content. Also all characterization data uncover a rearrangement of
NbꢀNiꢀO structures upon use in reaction (cf. Figure 1 and Figure
80
ꢀ
1
3
). Since only the band near 800 cm is visible at high Nb
ꢀ
1
contents, the 800 cm Raman band should correspond to the Nbꢀ
rich NbꢀNiꢀO mixed phase. According to the HRTEM results,
two distinct NbꢀNiꢀO phases exist in the catalyst series; these
exhibit different sizes. One is in the 20ꢀ50 nm range and the other
2+
NbꢀNiꢀO phases induce the formation of Ni species removing
3+
nonꢀstoichiometric Ni ions that appear to be located at the
surface of NiO particles (cf. XPS analyses). Selective removal of
3+
2+
in the 4ꢀ6 nm range. The one with smaller size range is richer in 85 Ni ions by stabilization into Ni upon niobium doping would
niobium; this is consistent with the results reported by Heracleous
et al. . Thus, the Raman band near 800 cm must correspond
to the mixed phase with smallest particle size.
progressively decrease redox properties in NiO system thus
modulating its oxidation capacity. An incipient phase (i.e., a
phase that is beginning to develop) between nickel and niobium
oxides would thus modulate redox properties. In such a sense, the
relevance of highlyꢀdefective NbꢀNiꢀO phases would be critical
to facilitate the appropriate redox cycle of nickel ions. Such a
phase would present properties distinct from those of pure NiO or
Nb O , but still not those of a fully developed mixed NiꢀNbꢀO
[
33]
ꢀ1
3
5
The niobiumꢀrich NbꢀNiꢀO phase is not active for ethane
ammoxidation (cf. Figure 6), while those catalysts with low
niobium loading are efficient for this reaction. Ni0.9Nb0.1 delivers
the best performances for ethane transformation into acetonitrile,
outperforming the rest of the series (cf. Figure 6 and Table 4),
although the selectivity to COx is high. Thus, niobium
incorporation into the NiO lattice delivers larger particles (20ꢀ50
nm) (detected by HRTEM) and enhances both the activity and the
9
0
2
5
phase. Both oxidation states would be involved in the reaction.
Such a trend would be consistent with the promotion of oxygen
4
0
95
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Page 10 of 10
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DOI: 10.1039/C3CY00415E
1
0
G. Centi, R.K. Grasselli, F. Trifiro, Catal. Today 1992, 13, 661.
exchange reported by Lemonidou et al. in this system using
[
33]
11 R. K. Grasselli, Catal. Today 1999, 49, 141.
12 M. O. GuerreroꢀPérez, J.N. AlꢀSaeedi, V.V. Guliants, M.A. Bañares,
isotopic labelling
.
60
Appl. Catal. A 2004, 260, 93.
Niobium doping increases acidity while modulates redox
properties. This would lead to an efficient activation of ammonia
minimizing the risk to oxidize it to nitrogen. Thus, there is a need
1
1
3
4
M. O. GuerreroꢀPérez, J. L. RivasꢀCortés, J.A. DelgadoꢀOyagüe,
J.L.G. Fierro, M.A. Bañares, Catal. Today. 2008, 139, 202.
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for a balance between redox and acidic sites, which appears to 65 15 S. M. Aliev, V. D. Sokolovskii, React. Kinet. Catal. Lett. 1978, 9, 91
exist at low niobium content.
16 Y. Li, J. N. Armor, Chem. Commun. 1997, 2013
1
1
1
7
8
9
Y. Li, J.N. Armor, J. Catal. 1998, 173, 511.
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2009, 10, 1555
Conclusions
7
0
Niobiumꢀcontaining NiO catalysts are an efficient formulation for
the direct ammoxidation of ethane to acetonitrile. Two different
mixed NbꢀNiꢀO phases have been identified, with different Nb/Ni
atomic ratios. A Nbꢀpoor NbꢀNiꢀO phase, characterized by two
2
0
E. Rojas, M. Calatayud, M.O. GuerreroꢀPérez, M.A. Bañares, Catal.
Today 2010, 158, 178.
10
15
20
25
21 E. Rojas, M. Calatayud, M.O. GuerreroꢀPérez, M.A. Bañares, Catal.
Today, 2012, 187, 212.
75 22 E. Rojas, M. Calatayud, M.A. Bañares, M.O. GuerreroꢀPérez, J.
ꢀ
1
ꢀ1
Raman bands near 850 cm and 800 cm , which corresponds to a
mixed phase with particle size in the 20ꢀ50 nm range; and a Nbꢀ
rich NbꢀNiꢀO phase with, presents smaller particle size and
Phys. Chem. C, 2012, 116, 9132
2
2
3
4
R. Catani, G. Centi, J. Chem.Soc. Chem. Commun. 1991, 1081
B. Wichterlová, Z. Sobalik, Y. Li, J.N. Armor, Stud. Surf. Sci. and
Catal. 2000, 130, 869
ꢀ
1
characterized by a single broad Raman band near 800 cm . The
catalytic data show that the Nbꢀpoor phase is efficient for ethane
ammoxidation, while the Nbꢀrich one is not efficient for nitrile
formation, but delivers very good performance to ethylene. The
incorporation of a small amount of niobium into the NiO lattice
induces important changes into the oxide structure that enhances
the activity and the selectivity for ethane ammoxidation to
acetonitrile. This appears related to an increase in acidity and
modulation of redox properties due to an interaction between Ni
and Nb through the formation of mixed NiꢀNbꢀO phases.
80
25 E. Heracleous, A.A. Lemonidou, J. Catal. 2010, 270, 67
2
2
6
7
E. Heracleous, A.A. Lemonidou, J. Catal. 2006, 237, 162
E. Rojas, M.O. GuerreroꢀPérez, M.A. Bañares, Catal. Lett. 2013, 143,
3
1.
2
2
3
8
9
0
F. RubioꢀMarcos, E. Rojas, R. LópezꢀMedina, M.O. GuerreroꢀPérez,
M.A. Bañares, J.F. Fernandez, ChemCatChem 2011, 3, 1637.
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C.D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H.
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90 31 M.O. GuerreroꢀPérez, M.A. Peña, J. L. G. Fierro, M. A. Bañares, Ind.
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3
3
3
2
3
4
B. Savova, S. Loridant, D. Filkova, J.M.M. Millet, Appl. Catal. A
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(
Acknowledgements
9
5
The Spanish Ministry of Science and Innovation (CTQ2011ꢀ
2
5517ꢀE and MAT2008ꢀ00889) is acknowledged for the financial
support. E.R. thanks CONACYT (Mexico) and ICyTDF
Mexico) for her predoctoral fellowship. The authors thank R.
30
35 B. Solsona, P. Concepción, S. Hernández, B. Demicol, J.M. López
(
Nieto, Catal. Today 2012, 180, 51
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