[Botavina et al., 7, Catal. Sci. Technol., 2017, pagg 1690–1700]
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Insights into Cr/SiO
2catalysts during dehydrogenation of propane: an operando XAS
investigation
M. Botavina,
aC. Barzan,
aA. Piovano,
aL. Braglia,
aG. Agostini,
b,†G. Martra,
aand E. Groppo
a*In situ and operando XAS spectroscopy have been applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO2-DHS catalyst during propane dehydrogenation in non-oxidative and different oxidative conditions. The spectroscopic data have been compared to
the catalytic performances. Although the majority of the specialized literature ascribes the propane dehydrogenation activity mainly to CrIII
species and only a few works report the presence of small amounts of CrII
species, our data unequivocally demonstrate the co-presence of both CrIII
and CrII
species during the propane dehydrogenation reaction. The relative amount of the two chromium phases has been estimated by analyzing both the XANES and EXAFS spectra with a two-phases fitting approach. It was found that the CrIII
-phase prevails when oxygen is present in the reaction mixture and when the reaction is performed at low flow or in static conditions. The amount of the CrIII
-phase correlates well with the propane conversion. In contrast, the selectivity to propene correlates with the amount of the CrII
-phase, at least when propane dehydrogenation is performed in oxidative conditions. Although other phenomena surely contribute to the overall catalytic performances of Cr/SiO2 catalysts, our results indicate that the relative proportion of the CrII and CrIII
-phases play a role in influencing the selectivity of the propane oxidative dehydrogenation reaction.
1. Introduction
Propene and ethene are among the most important building blocks in the chemical industry, being extensively used for the production of both large volume commodities and specialty chemicals. At present, the industrial production of light olefins is almost entirely based on non-oxidative processing of oil by-products, such as steam cracking and fluid-catalytic-cracking.1-3
These processes are high-energy demanding and scarcely selective toward the production of a single olefin. For these reasons, alternative on-purpose technologies are gaining an increasing interest. Among them, the catalytic dehydrogenation of propane is the most direct and selective way to produce propene. Small amounts of propane are contained in the shale-gas, which nowadays provides an economical alternative to crude oil. Although some of the reaction features (e.g. the strong endothermic character of the reaction) still limit the full development of commercial processes, the current market conditions and the high price difference between propane and propene incentivize the research in this field.1-3
In this scenario, the catalytic oxidative dehydrogenation (ODH) of propane is a promising alternative process, which does not present the thermodynamic limitations shown by non-oxidative dehydrogenation, operates at much lower temperatures, and minimizes coke formation, with a consequent increase of the catalyst stability.1,4-10 In this process,
an oxidant is added in the reagents mixture with the function to combust the hydrogen formed during the reaction, thus ensuring a full conversion of the paraffin. The main challenge is to avoid the occurrence of consecutive and parallel combustion reactions in order to maintain a high olefin selectivity. In this respect, the use of carbon dioxide as a mild oxidant has attracted particular attention. Indeed, CO2 is
thermodynamically stable and kinetically inert, is cheap and abundant, and has a high heat capacity that can be useful to minimize the hot spot phenomena usually leading to undesired side reactions.8,11,12 It has been suggested that CO
2 may
improve the yield of propene by two independent pathways: 1) participating as an oxidizing agent in the red-ox cycle involving
the catalyst (following a pathway in agreement with the Mars-Van Krevelen mechanism), and 2) removing the hydrogen produced in the non-oxidative dehydrogenation of propane by the reverse water–gas shift reaction.11,13-18
Although several catalyst’s formulations have been proposed for ODH of propane,1,4-10,19-24
supported chromium oxide-based materials have been pointed among the most promising catalysts.25-38
It is worth noticing that CrOx-based
catalysts are industrially employed also in the direct -dehydrogenation of propane.1,3
Several factors influence the catalytic performances, such as the chromium loading, the type of the support, and the preparation method. In particular, a high chromium dispersion seems fundamental to achieve a high catalytic activity. For this reason, high surface area materials are preferentially used as supports, among which alumina, silica and mesoporous silica-based materials (such as mesoporous MCM-4130,31
and SBA-1515
). Understanding the nature of the active chromium sites in chromium-based catalysts under ODH (or direct dehydrogenation) conditions is fundamental to optimize the catalyst properties and to limit the catalyst deactivation, but it is extremely challenging because it requires the application of operando characterization techniques. Up to now, diffuse reflectance UV-Vis and Raman spectroscopies have been mainly applied,28,39-43
in a few cases directly into the dehydrogenation reactor.44-49
On freshly prepared catalysts a number of surface species with different oxidation state (+6, +5, +3, +2) and speciation (chromates, polychromates, crystalline and amorphous chromia) have been spectroscopically identified, although at high chromium dispersion the +6 oxidation state prevails.15,30,31,37,39-45,47,50-58
It is widely accepted that in reaction conditions the reagent mixtures reduce the chromium oxide species with the sequential formation of adsorbed acetone, formates and acetates and the gradual deposition of hydrocarbons (either aliphatic and unsaturated/aromatics).58
However, the oxidation state, the coordination and the speciation of the reduced chromium species are still debated. The dehydrogenation activity has been mostly assigned to dispersed and coordinatively unsaturated CrIII
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also small amorphous CrIII clusters and CrII species have beendetected and suggested as active sites.17,18,27,31,32,57-66 Several
cycles between various red-ox chromium species have been proposed, including CrVI/CrIII,27,31,60 CrV/CrIII,61 and CrIII/CrII.17,32,59
Determining the individual contributions of these species to the catalytic performance has proven to be extremely challenging.
Recently we have reported on the synthesis, systematic physical-chemical characterization and catalytic testing of a series of Cr/SiO2 materials prepared by direct hydrothermal
synthesis (DHS), having a chromium loading in the 0.25- 2.00 wt% range, targeting ODH conditions as closest as possible to industrial applications.34,36 XANES data of reacted catalysts
indicated that when CO2 is used as oxidant in ODH of propane,
the average oxidation state of the chromium sites can be as low as +2, whereas it is close to +3 in the presence of O2 as
oxidant. In this work, we have extended our spectroscopic investigation in order to gain more insights in the oxidation state and local structure of the chromium species during ODH of propane as a function of the reagents composition and of the reaction conditions. Our attention was focused on the catalyst with the highest chromium loading (2 wt%), hereafter labelled as 2.0Cr/SiO2-DHS, that exhibited the best catalytic
performances. We selected X-ray Absorption Spectroscopy (XAS) as the election tool to monitor the changes underwent by the chromium species during the reaction. Indeed, XAS technique is element selective and allows to monitor the actual state of the chromium sites even in the presence of coke. At first, we performed a detailed XANES and EXAFS investigation on the 2.0Cr/SiO2-DHS catalyst recovered after ODH of propane
in the presence of different oxidants. Successively, the same catalyst was monitored by operando XANES spectroscopy. To the best of our knowledge, in very few works63 XAS
spectroscopy has been applied in operando conditions to monitor the behaviour of the chromium species during ODH (or dehydrogenation) of propane. Most of the times, XAS data refer to catalysts recovered after the reaction and often it is not clear if they have been exposed to air or not, thus increasing the confusion on the red-ox state of the chromium species. The spectroscopic data are then compared to the catalytic performances, allowing us to postulate a correlation among the oxidation state of the chromium sites during the ODH of propane and the selectivity in propene.
2. Experimental
2.1 Catalysts preparation and treatments
The catalyst was prepared by direct hydrothermal synthesis (DHS), following the protocol usually employed to obtain mesoporous MCM-41 structures, according to the procedure described elsewhere.35 Briefly, cetyltrimethylammonium
bromide (Sigma-Aldrich), pure fumed silica (Aerosil 300, Degussa) and chromium nitrate (Sigma-Aldrich) were used as structure directing agent, silica and chromium source, respectively. Tetramethylammonium hydroxide (Sigma-Aldrich) was used to maintain a pH of ca. 11.0. The relative amount of Cr(NO3)3 was adjusted in order to attain a chromium loading of
2.0 wt%. The resulting sample will be referred as 2.0Cr/SiO2-DHS.
The synthesis gel was stirred at 35 °C (water bath) for 2 h, transferred into a teflon lined autoclave and kept at 100 °C for 48 h. The obtained powder/liquid mixture was filtered at room temperature and carefully washed with distilled water (three doses of 50 ml). The sample was then dried at 100 °C and placed into a gas-flow furnace, where the temperature was raised to 600 °C (heating rate: 2 °C/min) under flowing N2 (1
atm, 2 L/min). The flow was then switched to O2 (1 atm, 2
L/min) for calcination for 8 h. The final cooling to room temperature occurred under O2 flow. A previous investigation
conducted on a series of analogous materials having chromium loading in the range 0.25 – 2.0 wt% indicated that the 2.0Cr/SiO2-DHS catalyst does not retain the ordered mesoporous
structure characteristic of MCM-41 materials, although it still shows a very high specific surface area (670 m2/g, total pore
volume of 0.41 cm3/g).35
Two series of experiments were performed with different set-ups. In the first series (Series 1-), the catalyst (in the powder form) was activated inside a quartz tube in O2
atmosphere (equilibrium pressure PO2 = 100 mbar) at 600 °C for
1 hour (two times for 30 minutes, oxidation step in Table 1). Successively, the reaction tube was degassed and the catalyst was treated at the same temperature in the presence of different reagent mixtures (propane ODH step in Table 1). In all the cases the propane ODH reactions was run for 30 minutes; then the reaction cell was degassed at 600 °C, and
Table 1. Summary of the experimental conditions adopted during experiments of Series 1- (static conditions) on the 2.0Cr/SiO2-DHS.
Entry
Treatment
Reaction Temp
(°C)
Gas Mixture
(% mol)
Equilibrium
Pressure (mbar)
Time
(n° x minutes)
1-ox
Oxidation
600
Pure O
2100
2 x 30
1-a
Propane
DH and ODH
600
15% C
3H
8, 85% N
2100
1 x 30
1-b
15% C
3H
8, 30% CO
2, 55% N
21-c
15% C
3H
8, 3% O
2, 82% N
21-d
15% C
3H
8, 30% CO
2, 3% O
2, 52% N
21-red
Reduction
350
Pure CO
100
2 x 30
the catalyst cooled down to room temperature in dynamic vacuum. The same catalyst was also reduced in the presence of CO at 350 °C, in order to have a reference for CrII
sites
(reduction step in Table 1). In that case, a typical protocol for the conversion of CrVI
into CrII
was followed, on the basis of a long standing experience in the field of ethylene
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polymerization on the parent Phillips catalyst.67 After activationin O2 at 600 °C, the sample was cooled down to 350 °C, treated
in CO (PCO = 100 mbar) and outgassed at the same
temperature. The treatment in CO was repeated twice (30 minutes each step), followed by degassing at 350 °C, in order to remove chemisorbed CO. The sample was finally cooled to room temperature in dynamic vacuum. Table 1 summarizes the samples measured during experiments of Series 1- and the corresponding nomenclature.
In a second series of experiments (Series 2-) the catalyst was measured directly in reaction conditions, i.e. in the presence of reagents and products at 600 °C. The catalyst was measured in the form of powder inside a quartz capillary 1.5 mm in diameter and 10 μm wall thickness, placed in between two small regions of quartz wool. The capillary was connected to a gas-dosing system with mass-flow controllers to flow different gas mixtures, and inserted inside an oven equipped with a temperature controller. The reaction products were qualitatively detected with a mass spectrometer placed at the end of the capillary. The experimental details (gas mixture, total flow, reaction temperature) and samples nomenclature are summarized in Table 2. A typical operando experiment was performed according to the following sequence: a) oxidative pre-treatment in dried air (21% O2 in He, 40 ml/min),
increasing the temperature up to 600 °C (10°C/min) and leaving the sample at 600 °C for 30 min; b) changing the flow from O2 to the reactant mixture (different compositions, either
40 ml/min or 8 ml/min) for 1 hour (first run); c) successive regeneration was achieved by switching back to dried air for 30 min; d) a second ODH run was performed by changing the flow again to reagents mixture. At the end of every step, the catalyst was flushed with He flow for 5 min.
2.2 Characterization techniques
Cr K-edge XAS spectra were collected at the BM23 beamline at the European Synchrotron Radiation Facility (ESRF, Grenoble, F). The white beam was monochromatized using a Si(111) double crystal; harmonic rejection was performed by using silicon mirrors (4 mrad). The intensity of the incident beam was monitored by an ionization chamber. The spectrum of the reference Cr2O3 (Sigma-Aldrich) was measured in transmission
mode, whereas the XAS spectra of the 2.0Cr/SiO2-DHS samples
were collected in fluorescence mode by silicon drift detector. For experiments of Series 1-, the catalyst powder was recovered at the end of each reaction without exposure to air and introduced inside an argon-filled glove-box, where it was pressed in the form of a self-standing pellet. The pellet was placed inside a small cryostat, closed under argon atmosphere. Finally, the cryostat was transferred to BM23 beamline. Measurements were performed at liquid He temperature and in dynamic vacuum (pressure lower than 1.0 10-6 Torr) in order
to avoid contamination of the samples. This experimental set-up allowed us to collect both XANES and EXAFS spectra of high quality. The XANES part of the spectra was acquired with an energy step of 0.4 eV and an integration time of 2 s/point. The EXAFS part of the spectra was collected up to 15 Å-1 with a
variable sampling step in energy, resulting in Δk = 0.05 Å-1, and
an integration time that linearly increases as a function of k from 5 to 20 s/point to account for the low signal-to-noise ratio at high k values. For each sample, six equivalent EXAFS spectra were acquired and averaged before the data analysis. The extraction of the χ(k) functions was performed using Athena program. Once extracted, the k3-weighted χ(k) functions were
Fourier transformed in a variable Δk range (Δk = 1.0 – 13.0 Å-1
range for samples after catalysis, and Δk = 1.0 – 11.0 Å-1 for
oxidized and reduced samples). The fits were performed in R-space in a variable ΔR range, using the Arthemis program. Phase and amplitudes were calculated by FEFF6.0 code, using as input the structure of Cr2O3. In experiments of Series 2-,
quick XANES spectra were acquired with an energy step of 0.4 eV and an integration time of 2s/point, up to k = 5 Å-1 in order
to allow an easy and reliable normalization. Each XANES spectrum required an acquisition time of about 12 minutes. The XANES spectra were normalized using the Athena program. The adopted experimental set-up (low Cr loading, fluorescence acquisition, capillary setup, quite small scattering volume) did not allow to collect EXAFS spectra with a satisfactory signal-to-noise ratio.
Table 2. Summary of the experimental conditions adopted during experiments of Series 2- (flow conditions) on the 2.0Cr/SiO2-DHS.
Entry
Treatment
Reaction
Temp
(°C)
Gas Mixture
(% mol)
Equilibrium
Pressure (atm)
Flow
(ml/min)
a2-ox
Oxidation
600
21% O2, 79% He
1
40
2-a / 2-a’
Propane
DH and ODH
600
15% C3H8, 85% He
1
40 / 8
2-b / 2-b’
15% C3H8, 30% CO2, 55% He
2-c / 2-c’
15% C3H8, 3% O2, 82% He
2-d / 2-d’
15% C3H8, 30% CO2, 3% O2,
52% He
2-red
Reduction
350
2.5% CO, 97.5% He
1
10
aIn order to allow a complete mixture of the gases, a flow of 100 ml/min was set in the line, and successively split in two parts through a needle-valve just before the
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3. Results and Discussion
3.1 A summary of the catalytic performances
The catalytic performances of the 2.0Cr/SiO2-DHS catalyst in the
ODH of propane in the presence of different oxidants and in the direct dehydrogenation reaction have been deeply discussed in our previous paper. Briefly, we found that the 2.0Cr/SiO2-DHS catalyst performs very well in the ODH of
propane, being the initial propane conversion values of ca. 70, 75 and 80% in the presence of CO2, O2 or CO2+O2, respectively,
to be compared to ca. 60% in the absence of oxidants (direct dehydrogenation). A certain catalyst deactivation occurs at long reaction time that, however, is not irreversible. Indeed, independently on the dehydrogenation conditions, the behaviour of the catalyst in the first run coincides with that observed at subsequent 5-8 runs after regeneration. The highest selectivity in propene was observed in the presence of CO2 as an oxidant: an initial propene yield of ca. 47% was
observed, which remains as high as ca. 40% after 1000 min of reaction. At the same time, the ethene yield remains quite low along the all duration of the test, suggesting that propane ODH with CO2 as an oxidant on Cr/SiO2 catalysts can be of actual
interest for the production of C2 and C3 olefins mixture highly
rich in propene. The selectivity in propene was much lower in the absence of oxidants and drastically decreases also when O2
is present in the reaction mixture. In this latter case, ethylene becomes the main product, likely resulting from an enhanced cracking of propane in the presence of O2.
Our finding that the selectivity in propene reaches the highest value when the reaction is performed in the presence of CO2 is in good agreement with the specialized literature. The
role of CO2 in the ODH of propane has been a matter of debate.
It has been demonstrated that it plays a double role.11,13-18 From
one side, it sequesters the hydrogen produced in the direct dehydrogenation of propane through the reverse water–gas shift reaction, hence shifting the reaction equilibrium towards higher paraffin conversion. On the other side, it participates as an oxidizing agent in the red-ox cycle involving the catalyst. On these basis, it is expected that the red-ox behaviour of the chromium species might be different in the presence of different oxidants. To clarify the possible correlation between the type of oxidant, the oxidation state and the coordination of the chromium species on one side, and the selectivity in ODH of propane on the other side, we undertook a detailed XAS investigation, as follows.
3.2 Spectroscopic properties of the catalysts recovered at the end of the propane direct and oxidative dehydrogenation reaction
3.2.1 The two extremes: oxidized and CO-reduced 2.0Cr/SiO
2-DHS catalysts. Before discussing the properties of the catalyst in
reaction conditions, it is worth to comment the main properties of 1-ox and 1-red samples, which are at the two extremes in terms of oxidation state and are useful for the successive analysis. Figure 1 shows the normalized Cr K-edge XANES spectra (part a) of samples 1-ox (black) and 1-red (grey), and their derivative signals (part b). Both spectra are very similar to those reported in the literature for CrVI/SiO
2 and
CrII/SiO
2 samples having lower chromium loading and
employed for ethylene polymerization (Phillips catalyst). 30,51,55,67-75 In particular, the spectrum of 1-ox shows an intense pre-edge
peak centered at 5994 eV, characteristic of CrVI species in a T d
-like symmetry; the edge value, evaluated at the maximum of the derivative curve, is at 6007 eV. In the spectrum of 1-red, the edge is downward shifted to 6002 eV, testifying that the treatment in CO caused a reduction of the CrVI sites. In
addition, an intense and well resolved pre-edge
Figure 1 – Parts a) and b): Normalized XANES spectra and corresponding derivative signals of samples 1-ox and 1-red. The inset in part a) shows a magnification of the pre-edge region. Part b) has been limited vertically to appreciate the signal of 1-red.
Figure 2 – Fourier Transforms no phase corrected in both modulus (part a) and imaginary parts (part b) of the k3
-weighted χ(k) functions (FT in the Δk = 1.0 – 11.0 Å-1
range) for samples 1-ox and 1-red. Experimental data are shown as dotted curves, the best fits (ΔR = 1.0 – 3.0 Å range) are reported with full lines. The insets show the small clusters used to fit the EXAFS data of 1-ox and 1-red, with indication of the ligand distances as obtained from the fits.
peak is observed at 5994.2 eV, previously assigned to a Cr1s →
Cr4p transition, and considered the fingerprint of isolated CrII
sites.51,68,69,72 Finally, two weak pre-edge features are present at
5988 and 5990 eV: the same peaks were previously reported for CrII/SiO
2 Phillips catalyst and assigned to Cr1s → (Cr3d + O2p)
dipole-forbidden transitions.72 In the following, we will
consider the XANES spectrum of 1-red as representative of highly uncoordinated CrII sites. Previous experience67 and the
specialized literature76 demonstrate that the adopted
procedure stoichiometrically converts all the mono-chromates to CrII sites and CO
2, which is easily removed at the reduction
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The Fourier Transforms of the k3-weighted χ(k) functions ofsamples 1-ox and 1-red are shown in Figure 2 in both modulus (part a) and imaginary parts (part b). The details of the EXAFS data analysis are given in the Supporting Information, the best fits are reported in Figure 2 (full curves), and the results of the fit are summarized in Table 3. The spectrum of 1-ox (black dotted curve) is dominated by an intense signal centered around 1.1 Å (not corrected in phase). Only very weak signals are observed at longer distance, providing an evidence that the chromium sites are essentially isolated. According to the fit, the local structure of the chromium sites in 1-ox is well represented by structure ox in Figure 2a: in average, the chromium sites are in the form of mono-chromates, with two oxygen ligands at 1.60 Å (double-bonded) and two oxygen ligands at 1.78 Å (single-bonded). It is interesting to observe that the Debye-Waller factors are quite small for both ligands (indicative of a quite homogeneous situation), but smaller for the Cr=O contribution, as expected because the double-bond is more rigid than the single-one.
The spectrum of 1-red (dotted grey curves) is characterized by a much weaker first shell signal at longer distance (1.3 Å). In average, each chromium site is surrounded by two oxygen ligands at a distance of 1.90 ± 0.02 Å, and by two silicon ligands at 2.71 ± 0.03 Å. In addition, one oxygen ligand at 2.13 ± 0.04 Å is necessary to fit the data, which likely belong to surface siloxane bridges weakly bonded to the chromium sites. Similar local environments have been previously found for other CrII/SiO
2 samples,51,72,75 although the average number of the
second-shell oxygen ligands and their distance changes from author to author. This structural variability might be a
consequence of the different type of silica support, of the synthesis method, and/or of the activation procedure. It is worth noticing that most of the literature agree in the description of the local structure of the chromium sites when they are in the +6 oxidation state, while slightly different structures are reported for the same Cr/SiO2 samples when
chromium is in the +2 oxidation state. This indicates that, once reduced, the chromium sites are much more flexible than CrVI
and can accommodate at the silica surface, trying to minimize their energy. These small structural differences in CrII sites
reflect the structural flexibility of supported metal sites, which is often the key to explain their catalytic performances.77
The XANES and EXAFS spectra of 1-ox and 1-red indicate that in the fully oxidized and fully reduced catalysts the chromium sites are grafted in the monomeric form. This was further confirmed by DR UV-Vis measurements (as reported in our previous work).36 In this respect, there are no differences
with respect to what previously reported in the literature for other Cr/SiO2 samples, irrespective of the type of silica and of
the synthesis procedure.30,51,67,68,71,74,78,79
3.2.2 The 2.0Cr/SiO2-DHS catalyst after propane direct and
oxidative dehydrogenation reaction: effect of the reagents
mixture. During experiments of Series 1-, the 2.0Cr/SiO2-DHS
catalyst was recovered at the end of a series of propane direct and oxidative dehydrogenation reactions performed in static conditions in the presence of different reagent mixtures, as summarized in Table 1. XAS spectra were acquired without exposing the samples to air; hence they should reflect the status of the catalyst in action. The normalized XANES spectra
Table 3. Optimized parameters in the EXAFS analysis (performed in the ΔR = 1.0 – 3.0 Å range, FT in 1.0–11.0 A-1 k-range) for samples 1-ox and 1-red. Variables without error have
been fixed.
Sample
ligand
ΔE (eV)
S
02N
R (Å)
σ
2(Å
2)
R-factor
1-ox
O (Cr=O)
3 ± 5
0.8 ± 0.2
2
1.60 ± 0.03
0.004 ± 0.003
0.08
O (Cr-O)
2
1.78 ± 0.04
0.006 ± 0.005
1-red
O short
0 ± 3
0.8
2
1.90 ± 0.02
0.010 ± 0.002
0.09
O long
1
2.13 ± 0.04
0.006 ± 0.004
Si
2
2.71 ± 0.03
0.008 ± 0.003
Figure 3 – Normalized XANES spectra (part a) and corresponding derivative signals (part b) of the 2.0Cr/SiO2-DHS catalyst recovered after direct dehydrogenation of propane (1-a)
and after ODH of propane in presence of different oxidants (1-b, 1-c, 1-d). Also the spectrum of Cr2O3 reference is shown for comparison. Inset in part a) shows a
magnification of the pre-edge region.
and their derivative signals for the 2.0Cr/SiO2-DHS catalyst
treated in different reaction conditions are shown in Figure 3, compared to that of the Cr2O3 reference compound. The XANES
spectrum of the catalyst recovered after direct propane dehydrogenation (sample 1-a), has the edge (evaluated at the maximum of the derivative curve, Figure 3b) at 6002 eV and is characterized by an intense and broad pre-edge peak centered around 5996 eV, and by two weak pre-edge peaks at 5988 and 5990 eV. All these features are indicative of the presence of isolated CrII species, as testified by the close similarity of the
spectrum with that of 1-red (Figure 1). Nevertheless, the feature at 5996 eV is less intense than for 1-red and in the white-line region a weak peak around 6006 eV is indicative of the co-presence of a small fraction of Cr2O3-like species.71 The
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XANES spectrum of the catalyst after ODH of propane in thepresence of CO2 (sample 1-b) is practically undistinguishable
from the previous one.
In contrast, when the ODH reaction is performed in the presence of O2 (sample 1-c) the XANES spectrum shows a
double edge at 5999 and 6003 eV (see derivative signal, Figure 3b), very similar to that of Cr2O3. In the pre-edge region two
weak pre-edge peaks are observed at 5990 and 5993 eV, as in the spectrum of Cr2O3. Finally, instead of the intense pre-edge
peak around 5996 eV characteristic of CrII species, only a
shoulder is present. Hence, also in this case CrII and CrIII sites
co-exist in the sample, although CrIII seem to prevail. The
spectrum of the catalyst recovered after ODH reaction in the presence of O2+CO2 (sample 1-d) is even more similar to that
of Cr2O3. In particular, in the white-line region two peaks are
observed at 6008 and 6010 eV, which are characteristic of the multiple-scattering contributions of Cr2O3-like particles. As for
sample 1-c, a shoulder is observed instead of the peak at 5996 eV characteristic of CrII species.
In all the cases, the XANES spectra suggest that two chromium phases are co-present in the 2.0Cr/SiO2-DHS catalyst
during propane ODH reaction: i) a Cr2O3-like phase, where the Table 4 – Quantitative estimation of the relative amount of CrIII-phase and CrII-phase
present in the 2.0Cr/SiO2-DHS catalyst reacted in different ODH reaction conditions, as
determined by fitting the XANES spectra shown in Figure 3 with a linear combination of the two reference phases. The fits were performed on the normalized XANES spectra, in the ΔE = -20 – 15 eV region, without constraints.
phase
Samples
1-a
1-b
1-c
1-d
Cr
III-phase
0.31 ±
0.01
0.35 ±
0.01
0.69 ±
0.02
0.74 ±
0.02
Cr
II-phase
0.73 ±
0.01
0.69 ±
0.02
0.30 ±
0.03
0.28 ±
0.02
chromium sites have a +3 oxidation state and display a pseudo-octahedral coordination; and ii) a CrII-containing phase, where
the chromium sites are highly uncoordinated as for isolated CrII
sites obtained after CO-reduction (1-red sample). In the following, we will call these two different chromium phases as
CrII-phase and CrIII-phase. The amount of each phase present in
the catalysts was evaluated by fitting each spectrum with a linear combination of the spectra of Cr2O3 and 1-red samples.
The results of the fits, summarized in Table 4, demonstrate that during the propane ODH reaction the starting CrVI sites are
reduced to a mixture of CrIII and CrII, whose relative proportion
depends on the ODH reaction conditions: without oxidants (sample 1-a) or in the presence of CO2 (sample 1-b), about 30%
of the chromium sites are reduced to CrIII, whereas the fraction
of CrIII increases to about 70% when the reaction is performed
in the presence of O2 or O2+CO2 (samples 1-c and 1-d).
Additional structural information have been obtained by analyzing the Fourier Transforms of the k3-weighted χ(k)
functions collected with the XANES spectra discussed above (Figure 4). The spectra of catalysts 1-a, 1-b, and 1c are very similar among each other and are dominated by a first shell signal centered around 1.5 Å (not corrected in phase), as observed for Cr2O3 although 1/3 less intense (note that the
spectrum of Cr2O3 is divided by 3). In addition, a broad signal
around 2.6 Å is observed, which is similar to the second shell signal characteristic of Cr2O3, but less intense and shifted to
Figure 4 – Fourier Transforms in both modulus (part a) and imaginary parts (part b) of the k3-weighted χ(k) functions (FT in the Δk = 1.0 – 13.0 Å-1 range) of samples 1-a, 1-b, 1-c and 1-d. Experimental data are shown as dotted curves, the best fits (ΔR = 0.0 – 4.0
Å range) are reported with full lines. Also the spectrum of Cr2O3 (multiplied by a factor
of 0.65) is shown for comparison. The vertical dotted line in part b) highlights the second shell signal around 2.6 Å characteristic of Cr2O3.
Table 5. Optimized parameters in the EXAFS analysis (performed in the ΔR = 0.0 –4.0 Å range, FT in 1.0–13.0 A-1 k-range) for samples 1-a, 1-b, 1-c and 1-d. Variables without error
have been fixed.
Sample
Ligand
X (Cr
III%)
ΔE (eV)
N
R (Å)
σ
2(Å
2)
R-factor
1-a
O
II0.35 ± 0.06
0 ± 1
2
1.98 ± 0.06
0.0034
0.041
O
III6
2.014 ± 0.009
0.010
Cr
1.9 ± 0.8
3.04 ± 0.02
0.012 ± 0.005
1-b
O
II0.39 ± 0.09
0 ± 2
2
1.97 ± 0.08
0.0034
0.044
O
III6
2.021 ± 0.008
0.010
Cr
1.3 ± 0.6
3.05 ± 0.02
0.009 ± 0.004
1-c
O
II0.63 ± 0.07
-1 ± 2
2
1.97 ± 0.09
0.0034
0.072
O
III6
2.00 ± 0.02
0.010
Cr
1.3 ± 0.9
3.02 ± 0.02
0.007 ± 0.005
1-d
O
II0.65 ± 0.1
-2 ± 4
2
1.93 ± 0.3
0.0034
0.10
O
III6
2.00 ± 0.02
0.010
Cr
1.4 ± 0.9
2.97 ± 0.03
0.009 ± 0.007
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Journal Name
longer distance (well evident in the Imm(FT), Figure 4b). Thespectrum of sample 1-d is more similar to that of Cr2O3. In this
case, the second shell signal is almost at the same distance as for Cr2O3 and also a third shell signal is observed around 3.2 Å,
which is absent in the other cases.
On the basis of the XANES results discussed above, the EXAFS data have been analyzed considering the co-presence of a CrII-phase and of a CrIII-phase, having the following
properties: i) the CrII-phase contributes only in the first shell
signal with two oxygen ligands (OII in Table 5), being the
corresponding signal quite weak; ii) the CrIII-phase contributes
to the first shell signal with six oxygen ligands (OIII in Table 5),
and also to the second shell one, with a number of chromium ligands (Cr in Table 5) that depends on the dimension of the Cr2O3-like particles. Details on the EXAFS data analysis are given
in the Supporting Information. Briefly, the following variables were fitted: i) a single ΔE0 common to all the paths; ii) the
fraction x of the CrIII-phase, being defined as (1-x) the fraction
of CrII-containing phase; iii) the second shell N
Cr coordination
number; iv) the Cr-OII, Cr-OIII and Cr-Cr distances and v) the
Debye-Waller factor for the Cr-Cr contribution, σ2
Cr. The results
of the analysis are summarized in Table 5 and the best fits are shown in Figure 4 as full curves.
The fraction x of the CrIII-phase present in each catalyst as
a function of the reaction conditions is in very good agreement with the values determined by the XANES analysis (Table 4). Hence, the co-presence of two chromium phases is fully confirmed. Their relative amount was estimated by means of two independent analysis, the former mainly based on the evaluation of the chromium oxidation state and the latter on the structure around the absorbing chromium sites. It is also interesting to observe that the second shell Cr-Cr contribution is characterized by a NCr coordination number much smaller
than for Cr2O3, also when the relative fraction of this phase is
considered. This suggests that in the CrIII-phase the CrO 6
octahedra are, in average, organized in dimers or trimers. In conclusion, the XAS data collected on the 2.0Cr/SiO2-DHS
catalyst recovered after direct dehydrogenation of propane and after ODH in the presence of different oxidants suggest that during the catalysis two chromium phases coexist, whose relative proportion depends on the reaction environment: i) a CrII-phase, where the chromium sites are in the +2 oxidation
state and are surrounded by two oxygen ligands; and ii) a CrIII
-phase, where the chromium sites belong to small CrO6
octahedra partially aggregated, although not yet organized as for Cr2O3.
3.3 The catalyst observed during direct and oxidative propane dehydrogenation reaction
In order to demonstrate that experiments of Series 1- were representative of the situation in reaction conditions, a second series of experiments was performed by collecting quick-XANES spectra on the same catalyst in reaction conditions. First of all, the XANES spectra of samples 2-ox and 2-red are practically indistinguishable from those of samples ox and
1-red (not reported). Different propane dehydrogenation
reaction conditions were employed, keeping constant the reaction temperature (600 °C) and varying the reagents
Figure 5 – Normalized XANES spectra collected during ODH of propane on the 2.0Cr/SiO2-DHS catalyst in presence of different reagent mixtures and at a different total
flow (40 ml/min and 8 ml/min, parts a) and b), respectively). The insets show a magnification of the pre-edge region.
Table 6 – Quantitative estimation of the relative amount of CrIII- and CrII-phases present
in the 2.0Cr/SiO2-DHS catalyst reacted in different ODH reaction conditions, as
determined by fitting the XANES spectra shown in Figure 5 with a linear combination of the two reference phases. The fits were performed on the normalized XANES spectra, in the ΔE = -20 – 15 eV region, without constraints.
phase
Samples
2-a
2-b
2-c
2-d
Cr
III-phase
0.24 ±
0.02
0.33 ±
0.02
0.44 ±
0.02
0.48 ±
0.02
Cr
II-phase
0.80 ±
0.02
0.71 ±
0.02
0.59 ±
0.02
0.53 ±
0.02
phase
Samples
2-a’
2-b’
2-c’
2-d’
Cr
III-phase
0.47 ±
0.02
0.61 ±
0.02
0.67 ±
0.02
0.72 ±
0.02
Cr
II-phase
0.56 ±
0.02
0.41 ±
0.02
0.37 ±
0.02
0.31 ±
0.02
composition and the total reagents flow (Table 2). It was found that at 600 °C the reaction started immediately. As a consequence, in all the cases the first XANES spectrum (collected in about 12 minutes) was already the “final” one, and no intermediate species were observed, in agreement with previous literature.63 The XANES spectra remain constant even
after several hours of reaction, suggesting that the slow deactivation observed during the catalytic tests has to be ascribed mainly to coke deposition and not to a change in the properties of the chromium sites. In the adopted experimental conditions the amount of coke accounted for about 2-3% by weight, as reported in our previous work.36 After regeneration
of the catalyst in oxygen flow at 600 °C the XANES spectra were the same as for the fresh catalyst. More important, in very general terms, all the XANES spectra collected during the reaction do show the co-presence of a CrII-phase and a CrIII
-phase, in agreement with the results of experiments of series
1-. This finding definitely demonstrates that in reaction
conditions (i.e. in the presence of reagents at 600 °C) a mixture of CrII and CrIII species are present, which may contribute to the
Journal Name
ARTICLE
Some differences in the relative amount of the CrII-phaseand CrIII-phase were observed in the XANES spectra as a
function of both the reaction mixture and the total flow, as shown in Figure 5. As done for experiments of Series 1-, the relative amount of CrII and CrIII-phases was evaluated by fitting
the experimental XANES spectra with a linear combination of the two reference spectra, Cr2O3 (reference for CrIII species),
and 1-red (reference for the CrII-phase). The results are
summarized in Table 6. At a constant reactants flow, the relative amount of the CrIII-phase increases in the order: C
3H8 <
C3H8 + CO2 < C3H8 + O2 < C3H8 + CO2 + O2. These results indicate
that the relative amount of CrII and CrIII-phases present in the
catalyst during the reaction depends on the balance between the reducing ability of propane and the oxidizing power of the oxidants present in the reactant mixtures. Propane has a strong reducing ability at 600 °C and in the absence of oxidants the chromium species would be in the +2 oxidation state. Nevertheless, a small amount of the CrIII-phase is also
observed, and explained by considering that during the direct dehydrogenation of propane a small amount of CO2 is
produced, which enters in the feed composition. In the presence of the oxidants, the amount of the CrII-phase
decreases in favor of the CrIII-phase, following the oxidizing
ability of the oxidants.
The reported data indicate that by keeping constant the feed composition, the relative amount of the CrIII-phase is
greater at low flow. This might suggest that the red-ox ability of the reactants is influenced by the contact time: low flows (long contact times) favor the action of the oxidant (CO2 or O2, or
both), whereas high flows (short contact times) favor the action of reductants (C3H8). It must be noticed that this
conclusion is strictly valid only if the reaction is not limited by diffusion processes from the bulk gas phase to the catalyst surface. Previous kinetics experiments in un-stationary field demonstrated that the oxidants have a lower affinity towards the catalyst surface than the hydrocarbons.80 These findings
support the conclusion that the oxidants need more time to act with respect to propane.
4. Conclusions
XANES and EXAFS spectroscopies have been applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO2-DHS catalyst during
propane dehydrogenation in non-oxidative and different oxidative conditions. All the XANES and EXAFS spectra have been analyzed by considering the co-presence of two chromium phases in all the catalysts: 1) a CrIII-phase, similar to
the Cr2O3 reference, where the Cr sites are in the +3 oxidation
state and surrounded by six oxygen ligands in the first coordination shell and by a limited number of chromium ligands in the second coordination shell; and 2) a CrII-phase,
where the chromium sites are in the +2 oxidation state, isolated and highly uncoordinated (only two oxygen ligands in the first coordination shell). The majority of the specialized literature ascribes the propane dehydrogenation activity mainly to CrIII species, almost isolated or weakly aggregated,
although other works report the presence of small amounts of CrII species. However, to the best of our knowledge this is the
first work reporting a two-phases XAS analysis for a Cr/SiO2
catalyst employed in the propane dehydrogenation reactions. The detection of CrIIIO
x oligomers in reaction conditions indicates that, at 600 °C and in the presence of the reagents, the chromium sites are partially mobile on the silica surface. Indeed, the activated catalyst before the reaction (sample 1-ox) shows exclusively monomeric chromate species. The high mobility of dispersed chromium sites at the silica surface at high temperature and in oxidizing conditions is well known to people working in the field of the Phillips catalyst activation, and has some important implications for its commercial usage. Max Mc Daniel76,81 has elegantly shown that chromium is
mobile during the activation of the Phillips catalyst, migrating within each silica particle, and even from one particle to another. The exact mechanism behind the chromium mobility is still under debate, although water, or perhaps surface silanol groups, seem involved in the migration. Interestingly, in our experimental conditions (where a mixture of oxidants and reductants are simultaneously present, as well as water) the chromium mobility leads to the aggregation of only a fraction
Figure 6 – Part a): Initial propane conversion (full circles) and propene selectivity (empty circles) for the 2.0Cr/SiO2-DHS catalyst in different reaction conditions (T = 600 °C, space
velocity = 200 h−1
, 5 cm3
of catalyst). Parts b)-d): Relative fraction of the CrIII
-phase (full circles) and CrII
-phase (empty circles), and sum of the two fractions (full squares) in different reaction conditions, as determined by fitting the XANES spectra with a linear combination of the two reference phases. In part b), also the amount of the CrIII-phase as determined
ARTICLE
Journal Name
of the chromium sites (the CrIII), while the sites with loweroxidation state (CrII) remain isolated.
Figure 6 summarizes the main results, and compares the catalytic performances of the 2.0Cr/SiO2-DHS catalyst (Figure 6a)
with the relative amount of the CrII and CrIII-phases, as a
function of the reaction conditions (static of flow conditions) and of the reactants mixture. We have found that the CrIII
-phase prevails when oxygen is present in the reaction mixture and when the reaction is performed at low flow (Figure 6c) or, as a case limit, in static conditions (Figure 6b). The amount of the CrIII-phase correlates well with the propane conversion
(Figure 6a, full circles). In contrast, the selectivity to propene (Figure 6a, empty circles) seems to correlate with the relative amount of the CrII-phase, at least when propane
dehydrogenation is performed in oxidative conditions. Although other phenomena surely contribute to the overall catalytic performances of Cr/SiO2 catalysts, our results indicate
that the relative proportion of the CrII and CrIII-phases play a
role in influencing the selectivity of the propane ODH reaction.
Acknowledgements
The authors are grateful to Olivier Mathon and Sakura Pascarelli (BM23 at ESRF) for their friendly assistance during the XAS experiments. University of Torino, Compagnia di San Paolo (projects ORTO11RRT5 and Progetto di Ateneo/CSP 2014, Torino_call2014_L1_73) and RFBF-ASP (grant no. 07-03-951581) are acknowledged for the funding.
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