Investigation of the Isolated Cr(VI) Species in Cr/MCM-41 Catalysts and its Effect on Catalytic Activity for Dehydrogenation of Propane

Article abstract of DOI:10.1002/cctc.201801598

Cr-modified MCM-41 with different loadings were prepared and tested in the propane dehydrogenation (PDH) reaction. As for Cr/MCM-41 system, the general knowledge was that high Cr loadings resulted in the reduction of silica surface hydroxyl groups, thus leading to the formation of inactive Cr(III) species, as clusters and/or crystalline Cr₂O₃. However, in the present work, isolated Cr(VI) states were identified in the Cr/MCM-41 catalysts, instead of Cr(III) species, using UV-vis and H₂-TPR. These isolated Cr(VI) species are difficult to reduce, causing the decreased activity for propane conversion and low selectivity towards C₃H₆ in the PDH process. In contrast, a good dispersion of the Cr species was obtained using a chromium column adsorption process, and the absence of the isolated state of Cr(VI) species on this sample contributed to high activity toward PDH process. The investigation of the isolated Cr(VI) species is very important for the Cr-based PDH catalyst system.

Full text of DOI:10.1002/cctc.201801598

Accepted Article
Title: Investigation of the Isolated Cr(VI) Species in Cr/MCM-41
Catalysts and its Effect on Catalytic Activity for Dehydrogenation
of Propane
Authors: Dedong He, Yaliu Zhang, Shuang Yang, Yi Mei, and
Yongming Luo
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Investigation of the Isolated Cr(VI) Species in Cr/MCM-41
Catalysts and its Effect on Catalytic Activity for Dehydrogenation
of Propane
Dedong He,[a,b] Yaliu Zhang,[a] Shuang Yang,[b] Yi Mei,[a] and Yongming Luo,*[b]
Abstract: The modified MCM-41 by Cr with different loadings were catalysts are mainly tested in process. On the one hand, various
[8]
prepared and tested for propane dehydrogenation (PDH) reaction. Cr catalysts with supports of Al2O3,[6] ZrO2,[7] and SiO2 have
As for Cr/MCM-41 system, the general knowledge was that high Cr
been studied. It is accepted that the type of the support can
loadings resulted in the reduction of silica surface hydroxyl groups, influence the PDH catalytic activity, and mesoporous silica-
thus leading to the formation of inactive Cr(III) species, as clusters based material, MCM-41, with high surface area and uniform
and/or crystalline Cr2O3. However, the difference showed in the pore size is preferentially used as support for a preparation of
present work was the observation of the isolated Cr(VI) states
instead of Cr(III) species in the Cr/MCM-41 catalysts, as evidenced
by the UV-vis and H2-TPR results. These isolated Cr(VI) species
different kinds of catalysts.[8] In fact, Cr supported on MCM-41
catalyst has been tested and proved to be a highly active
catalyst for oxidative dehydrogenation of propane,[9] and its
were difficult to be reduced, and causing the decreased activity application for dehydrogenation of isobutane and ethane has
toward propane conversion and low selectivity of C3H6 for PDH
process. On the contrary, a good dispersion of the Cr species was
also been reported.[10] However, to our best knowledge, few
works are available in the literatures concerning PDH reaction
obtained from chromium column adsorption process, and the over Cr/MCM-41 catalyst. On the other hand, understanding the
absence of the isolated state of Cr(VI) species on this sample exact nature of active Cr species under PDH condition is
contributed to high activity toward PDH process. The ivestigation of critically important and seem to be the fundamental for
the isolated Cr(VI) species deserved significant importance for Cr- controlling the catalyst properties.[8] The present literature results
based PDH catalyst system.
indicate that the redox cycle between active Cr(VI) and Cr(III)
species has been proposed to be responsible for the activity of
dehydrogenation reaction. Moreover, the presence of oxidized
surface Cr species with higher-valence state (e.g. Cr(VI)) seems
to be necessary for the production of active Cr sites, and could
be generally regarded as active centers.[7a, 9a, 10, 11]
Introduction
Recently, growing demand for propene has spurred an
increased interest in the dehydrogenation reaction of propane.[1]
Since the product of propene is an important feedstock in the
manufacture of many chemicals, such as propylene oxide,
polypropylene and acrylonitrile.[2] In particular, the recent
conversion of propane from shale gas has anticipated to
decrease the cost of production and made it to be more
commercial and attractive, due to the vast amounts and growing
It is considered that active Cr(VI) species predominate at low Cr
loadings, while high loadings result in the formation of inactive
12]
Cr(III) species, as clusters and/or crystalline Cr2O3,[9b,
on
which not all Cr sites are accessible to the reactants and
producing negative effect on the catalytic activity.[9b] In fact, as
for these reported literatures, the general knowledge is that low
Cr loadings contribute to good dispersion of Cr species, and as
a result, mononuclear Cr(VI) as well as some Cr(III) species with
coordinative vacancies are obtained. But high Cr loadings are
expected to cause the formation less active polymeric Cr(III)
species. The formation of inactive Cr(III) species is attributed to
the consumption of silica surface hydroxyl groups, which can
provide anchoring sites for the stabilization of Cr(VI) species.[13]
Consequently, some of the Cr(VI) species are converted to
inactive Cr(III) species, accompanied by the depletion of surface
hydroxyl groups at higher Cr loadings.[14] In our present work, to
verify the effect of Cr loadings on the nature of the Cr species,
Cr/MCM-41 catalysts with different weight of Cr loadings (5, 10,
and 15wt%) were prepared. Surprisingly, it is found that at high
Cr loading, not all the Cr(VI) species are expectantly converted
to inactive Cr(III) species. Instead, some of the inactive Cr(VI)
species regarded as isolated Cr(VI) species are observed on
15%Cr/MCM-41 catalyst. Furthermore, these isolated Cr(VI)
species, which exhibits difficulty to be reduced, cuase the
decreased activity for PDH process. However, as far as we
know, few reports are focued on investigating the presence of
isolated Cr(VI) species and their effects on the catalytic activity
of PDH reaction. Thus, our current work is mainly to study the
isolated Cr(VI) species in the Cr/MCM-41 catalysts and
analyzing its effect on catalytic activity for dehydrogenation of
production
of
shale
gas
worldwide.[3]
In
general,
dehydrogenation reaction can be classified into oxidative and
non-oxidative processes. Though the former shows the
advantage of reducing coke formation, it is still associated with
the problem that the oxidation degree is hard to be controlled
during actual industrial application, thus leading to safety issues
and low propene yield inevitably.[4] Consequently, non-oxidative
propane dehydrogenation (PDH) process has been gained much
attention and realized in commercial applications.[5]
As far as the PDH catalyst system is concerned, Cr-based
[a]
[b]
Dr. D. He, Dr. Y. Zhang, Dr. Y. Mei
Faculty of Chemical Engineering
Kunming University of Science and Technology
Kunming, 650500 (P. R. China)
E-mail: environcatalysis222@yahoo.com
Dr. S. Yang, Dr. Y. Luo
Faculty of Environmental Science and Engineering
Kunming University of Science and Technology
Kunming, 650500 (P. R. China).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201801598
ChemCatChem
FULL PAPER
propane, which deserves significant importance for Cr-based
PDH catalyst system.
experiments are carried out using mechanical mixtures of Cr2O3
with MCM-41 support, and almost no catalytic activity is seen in
the PDH reaction, as shown in Figure S1 in Electronic
Supplementary Information (ESI). Therefore, a detailed analysis
towards the effect of bulk Cr2O3 phase on the activity of PDH
process will not be discussed further. The UV-vis spectra of the
Cr/MCM-41 samples are compared in Figure 2B. From Figure
2B, UV bands at about 270 and 360 nm express O-Cr(VI)
charge transfer transitions of chromates species.[17] Specifically,
the intense peak centered at about 360 nm, which is
characteristic for isolated Cr(VI) oxide,[11c] is more pronounced in
the 15Cr/MCM-41 catalyst, indicating more isolated Cr(VI)
species are presented in the case of the samples with a high Cr
content. Additionally, two bands at about 460 and 600 nm
assigned to octahedral Cr(III) in CrOx clusters or Cr2O3 are
mainly found for all samples.[9a] Actually, the band located at
around 600 nm could be assigned to the presence of crystalline
Cr2O3.[17] Thus, the obtained UV-vis results is in accordance with
the XRD data.
Results and Discussion
Figure 1 shows the catalytic activity for PDH over Cr/MCM-41
catalysts. As a result, all the samples exhibit a similar trend as a
function of time in terms of propane conversion and propene
selectivity. The decrease in the activity within initial 40 min of
running time could be attributed to the coversion of part of the
active Cr(VI) sites and the formation of coke during the PDH
reaction,[4b,15] as revealed in the followings. Sample 10Cr/MCM-
41 obtains the best catalytic performance with a propane
conversion of 40% and a propene selectivity of 80% after 1 h of
reaction, whereas sample 15Cr/MCM-41 shows the poorest
catalytic performance with regard to propane conversion and
propene selectivity. To explain the above results, further
characterization studies are carried out and given.
Figure 2. (A) XRD patterns, (B) UV-vis spectra, and (C) H2-TPR profiles of
Cr/MCM-41 catalysts.
The H2-TPR profiles of the synthesized Cr/MCM-41 samples are
displayed in Figure 2C. Over the 5Cr/MCM-41 and 10Cr/MCM-
41 catalysts, one broad reduction peak at about 420 oC
considered as a result of the reduction of Cr(VI) species
interacted with the support can be seen.[17] Furthermore, an
additional reduction band centered at about 280 oC appears
clearly from the 15Cr/MCM-41 sample, which is given account
for the reduction of Cr(VI) species dispersed on crystalline α-
Cr2O3.[12, 17] This result is in line with the XRD and UV-vis data
that suggest the presence of crystalline Cr2O3 for the tested
sample. In particular, one high temperature reduction peak
Figure 1. Catalytic activity of Cr/MCM-41 catalysts for (A) propane conversion,
and (B) propene selectivity.
The XRD patterns of the Cr/MCM-41 catalysts with different Cr
contents are given in Figure 2A. It is observed that the diffraction
lines of Cr2O3 are clearly found in the XRD patterns of all the
tested samples,[16] and the peak intensity increases with
increasing the Cr content from 5% to 15%, suggesting the good
crystallinity of Cr2O3 at high Cr loadings. Despite this, catalytic
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located at about 540 oC can be detected in the case of the
15Cr/MCM-41 sample, supposedly caused by the reduction of
the isolated Cr(VI) species, which is reported to be difficult to be
reduced.[11c] Thus, the H2-TPR data combined with the UV-vis
results confirm part of Cr(VI) species exists as the isolated state
in the sample of 15Cr/MCM-41. Besides, mechanical mixtures of
CrO3 with MCM-41 solid (with Cr content of 1, 5 and 15%) are
designed to shed some further light on the nature of active Cr
species in the Cr/MCM-41 system, since no interaction between
active Cr(VI) species and MCM-41 support is obtained from this
mechanical mixing process, and consequently, the isolated state
of Cr(VI) species are likely to appear. From the characterization
results of UV-vis and H2-TPR as demonstrated in Figure S2 in
ESI, it is shown that the isolated Cr(VI) species, as marked in
Figure S2A, lead to the reduction peak divide into two parts
(Figure S2B), thus further confirming our current conclusion.
Figure 3 illustrates the presence of both Cr(VI) and Cr(III) states
in the synthesized samples, and peak fitting of the Cr2p BE
region of the 10Cr/MCM-41 and 15Cr/MCM-41 samples shows
that a higher relative percent of Cr(VI) are presented on the
15Cr/MCM-41 catalyst. Actually, an actual Cr(VI) content over
the 10Cr/MCM-41 and 15Cr/MCM-41 catalysts are calculated,
and it seems that H2 comsumption (regarded as the peak area
from H2 reduction process) is positively related to the actual
Cr(VI) content. However, the H2 comsumption over the
15Cr/MCM-41 sample is lower than it deserves (as shown in
Figure 3C), possibly resulted from the existence of isolated state
of Cr(VI) species, and causing an inhibition of reduction process.
Figure 3. (A) and (B) Cr 2p XP spectra of Cr/MCM-41 catalysts; (C) H2
consumption and actual Cr(VI) content over Cr/MCM-41 catalysts.
Figure 4. (A) and (B) FT-IR spectra of Cr/MCM-41 catalysts.
Figure 4 is the FT-IR results for the Cr/MCM-41 samples. From
Figure 4A, a typical band at about 640 cm-1, which is absent in
the MCM-41 support, is observed over the 10Cr/MCM-41 and
15Cr/MCM-41 catalysts, indicating the formation of Cr2O3 phase.
Figure S3 in ESI shows the Si 2p, O 1s and Cr 2p XPS curves of
the Cr/MCM-41 catalysts. From Figure S3B, the intense O 1s XP
spectra of the 15Cr/MCM-41 sample located at low binding
energy (BE) arises from the contribution of lattice oxygen,[18]
which symbolizes the possible generation of crystalline Cr2O3
form,[10a] as illustrated by the XRD, UV-vis and H2-TPR results.
Moreover, as seen in Figure S3A and Figure S3B, the shift in the
Si 2p and O 1s BE of the 15Cr/MCM-41 sample is likely caused
by the fact the local environment in CrOx domain is different,[19]
as revealed that some unexpected isolated state of Cr(VI)
species are presented. Besides, the increase in the intensity of
the Cr 2p XP spectra in Figure S3C confirms the increase in the
Cr loadings. In addition, an examination of Cr2p XP spectra in
[16]
Moreover, a weak IR band around 905 cm-1, attributed to a
Cr(VI)-O vibration,[20] is seen in the tested Cr/MCM-41 samples,
which is in accordance with the UV-vis and XPS results. Besides,
a FT-IR band around 960 cm-1 is correlated with the superficial
hydroxyl groups from Si-O bond vibration.[21] The shift of this
band to the low wavelength location, when Cr content increases
to 15wt%, is related to the replacement of OH groups.[20]
Furthermore, it is noted that the high-frequency region from
3200 to 3700 cm-1 can be assigned to the stretching vibration of
SiO-H bond.[22] The decrease in the intensity of this band in the
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case of the 15Cr/MCM-41 sample further reflects the
consumption OH groups during Cr impregnation process. As
mentioned above, high Cr loadings may cause the depletion of
surface hydroxyl groups. This effect, as evident from the FT-IR
analysis, decreases the stabilization of the Cr(VI) species on the
support.[13] Consequently, some of the Cr(VI) species are
converted to inactive Cr(III) species, accompanied by the
depletion of surface hydroxyl groups. As a result, inactive Cr(III)
species as well as some unexpected isolated Cr(VI) states are
found. The diagram for the formation of isolated state of Cr(VI)
species on Cr/MCM-41 is exhibited in Figure 5, on which not all
the Cr(VI) species are converted to inactive Cr(III) species, while
some of the inactive Cr(VI) species regarded as isolated Cr(VI)
species are observed on 15Cr/MCM-41 catalyst. The isolated Cr
can be located on the support, interface, steps and edges, et.al.
On the other hand, the high amount of OH groups allows us to
expect a better stability of the catalyst by suppressing the coke
deposition during the reaction.[2b,10b] Thus, a decrease in the
activity for PDH process over the 15Cr/MCM-41 catalyst with low
intensity of OH groups is obtained.
attributed to the good dispersion of Cr and the absence of
isolated state of Cr(VI) species.
Figure 6. STEM/Mapping images of the 15Cr/MCM-41 catalysts.
Figure 5. The diagram for the formation of isolated state of Cr(VI) species on
Cr/MCM-41.
The STEM/Mapping images as well as SEM/ampping analysis
are respectively shown in Figure 6, Figure S4 and Figure S5 in
ESI, which indicates the difference in the dispersion of Cr
between 10Cr/MCM-41 and 15Cr/MCM-41 samples. From
Figure 6, it is clearly found that the introduced Cr species are
agglomerated on the MCM-41 support. Thus, it could be
concluded that the poor dispersion of Cr species over the
15Cr/MCM-41 catalyst gives rise to the formation of isolated
Cr(VI) states. In fact, it’s worth mentioning that another method
for the synthesis of Cr/MCM-41 catalyst, called as Chromium
Column Adsorption process and reported in our previous
work,[13c] shows a better dispersion of Cr species over the
Cr/MCM-41 sample, as displayed in Figure S6 in ESI (according
to the XRF data, the Cr content from this method is close to
15%wt, restuls are not shown). Surprisingly, no characterization
of the isolated state of Cr(VI) species are seen from the H2-TPR
profile (as shown in Figure S7B in ESI), and meanwhile, its
activity towards PDH reaction is higher than that over the
15Cr/MCM-41 sample, as exhibited in Figure S7A in ESI). The
o
observation of the reduction band at about 280 C evidences the
Figure 7. (A) CH4, (B) C2H6 and (C) C2H4 selectivity of Cr/MCM-41 catalysts.
formation of crystalline α-Cr2O3 over the sample from the
Chromium Column Adsorption process, which indicates a high
content of Cr contains on this catalyst. But to our surprise, the
high activity towards PDH process over this sample is mainly
Figure 7 exhibits the selectivity of the produced CH4, C2H6 and
C2H4 over these Cr/MCM-41 catalysts. From Figure 7, it is
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clearly seen that higher amounts of CH4 and C2H4 are shown on
the 15Cr/MCM-41 sample, possibly resulted from the side
Cr species,[12a] present simultaneously at the 15Cr/MCM-41
sample, which indicates that high Cr loading leads to polymeric
Cr species. As a matter of fact, from the above analysis, these
polymeric Cr not only contains the inactive Cr(III) species, but
also includes the formed isolated Cr(VI) species, which shows
low catalytic activity for PDH. Meanwhile, Raman spectra toward
the spent 15Cr/MCM-41 catalysts are shown in Figure 9B. It is
seen that Raman signals at about 1380 and 1610 cm-1,
representing for the accumulation of coke deposit over the spent
catalyst,[23] are found in the spent 15Cr/MCM-41 sample. From
Figure 9B, it is noted that the existence of coke deposit has
affected the expression of Cr signals. Hence, further analysis for
the spent 15Cr/MCM-41 sample is not given. Additionally, C3H8-
TPD tests of the 10Cr/MCM-41 and 15Cr/MCM-41 samples are
carried out. From Figure 9C, it is observed that 10Cr/MCM-41
sample shows a more intense C3H8 desorption peak, which
suggests the better catalytic activity of this sample. Moreover,
the intensity of the C3H6 (desorption product) over the
10Cr/MCM-41 sample is higher than that of the 15Cr/MCM-41
sample, which can be applied to explain the poor selectivity of
C3H6 over the 10Cr/MCM-41.
reaction of C3H8(g) ⇌ CH4(g) + C2H4(g) (∆rHΘ
= 81.7 kJ/mol),
298K
instead of the occurrence of the main reaction of C3H8(g) ⇌
C3H6(g) + H2(g) (∆rHΘ
= 123.8 kJ/mol). From this point, it is
298K
deduced that the isolated state of Cr(VI) species function as the
inactive one during PDH process. Besides, from Figure S1, it is
known that catalyst of mechanical mixing of Cr2O3 with MCM-41
shows no catalytic activity towards PDH, but an improved
activity is observed for its oxidized phase (by oxidizing the
o
sample with air at 600 C, as shown in Figure 8). However, from
Figure 8B and Figure 8C, the existence of the isolated Cr(VI)
state results in the lower selectivity of C3H6 during the reaction.
Thus, these results claerly show that the generated isolated
Cr(VI) species lead to the decreased activity toward propane
conversion and low selectivity of C3H6 for PDH process, due to
the low catalytic activity of the generated isolated Cr(VI) species.
Figure 8. (A) C3H8 conversion, (B) selectivity of the produced hydrocarbons of
over Cr2O3-MCM-41 catalyst (mechanical mixing), and (C) UV-vis of the
Cr2O3-MCM-41 catalyst (mechanical mixing) and oxidized Cr2O3-MCM-41
catalyst (mechanical mixing).
Figure 9. Raman spectra of (A) Cr/MCM-41 catalysts; (B) spent 15Cr/MCM-41
catalysts; (C) and (D) C3H8-TPD and C3H6-TPD profiles of Cr/MCM-41
catalysts.
To further clarify the type of the formed Cr species, Figure 9A
displays the Raman spectra obtained for the 10Cr/MCM-41 and
15Cr/MCM-41 samples. As seen in Figure 9A, 10Cr/MCM-41
and 15Cr/MCM-41 show an intense band at about 982 cm-1,
which is attributed to the symmetric stretching of surface
monochromate species.[12a] Furthermore, the bands located at
about 400, 545 and 600 cm-1, respectively characteristic of the
bending mode of CrO2 units, the symmetric stretching mode of
Cr2O3 crystals as well as the symmetric stretching of polymeric
Conclusions
To summarize, Cr/MCM-41 catalysts with different Cr loadings
were prepared and tested for the PDH reaction. It was shown
that some inactive Cr(VI) states regarded as the isolated Cr(VI)
species were found over the high Cr content Cr/MCM-41
catalyst, accompanied by the reduction of silica surface hydroxyl
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groups. Instead, the previous general knowledge is that high Cr
loadings can cause the generation of inactive Cr(III) species, as
clusters and/or crystalline Cr2O3. Therefore, understanding and
investigating the characters of the formed isolated Cr(VI)
species in Cr/MCM-41 catalysts can be used to realize its
negative effect on catalytic activity for PDH process, which
deserved significant importance for Cr-based PDH catalyst
system.
The National Natural Science Foundation of China (U1402233,
21667016 and 21267011) is gratefully acknowledged for
financial support to this research work.
Keywords: Cr/MCM-41. Propane Dehydrogenation. Isolated
Cr(VI) Species. Cr Dispersion. Chromium Column Adsorption.
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Experimental Section
The mesoporous MCM-41 was prepared in our laboratory as previously
reported.[13c] The Cr modified MCM-41 catalysts with different weight of
Cr loadings (5, 10, and 15wt%) were synthesized by the wet
impregnation method, due to its superiority of low cost and operational
easy. Appropriate amounts of (NH4)2CrO4 in aqueous solution were
mixed with 2 g of MCM-41 samples and vigorously stirred for 1 h. After
impregnation, the resulting mixtures were dried at 90 oC for 6 h and
subsequently calcined at 550 oC for 6 h. All of the prepared catalysts
were pressed, crushed and sieved before the activity test.
[3]
[4]
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determine the crystal structure of the obtained catalyst samples. X-ray
photoelectron spectroscopy (XPS) were recorded on a PHI 5000 Versa
Probe II spectrometer to study the surface atomic states of chromium
species, and carbon calibration was done by referring C1 s peak at 284.6
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Nicolet 560 IR spectrometer, using KBr plate in the range of 500-4000
cm-1. H2-temperature programmed reduction (H2-TPR) were performed
on a apparatus equipped with a thermal conductivity (TCD) detector. 50
mg of catalyst was pretreated in pure Ar flow at 400 oC for 1 h, and
reduced from 100 to 700 oC (at a ramping rate of 10 oC/min) with H2
(10%)/Ar flow. Scanning electron microscopy (SEM) were observed
using an S-4800 SEM (Hitachi S-4800, Japan), and elemental mapping
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elemental mapping were also conducted.
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The PDH reaction was operated in a fixed-bed micro-reactor under
atmospheric pressure, and the reaction temperature was fixed at 630 oC.
Prior to reaction, 0.4 g of the catalyst was preheated at 630 oC in a
stream of N2 flow for 0.5 h. Afterwards, the feed gas consisted of
C3H8(14%)/N2 was introduced into the reactor, with the total flow rate of
29 mL/min. Products of PDH were analyzed by an online gas
chromatograph (GC) equipped with TCD, and FID (flame ionization
detector). The conversion of propane and selectivity of propene were
defined based on follows:
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C
[C3H 8]in C
[C3H 8]out 100%
C3H8 Conversion =
C
[C3H 8]in
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C3H6 Selectivity =
C
[C3H 6]out
100%
C
[C3H 6]out  (2/3)C[C 2 H 6]out  (2/3)C[C 2 H 4]out  (1/3)C[CH 4]out
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whereC[C3H8]in is the initial concentration of C3H8 before the reaction, and
C[CxHy]out is the output concentration of hydrocarbons during the reaction.
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10.1002/cctc.201801598
ChemCatChem
FULL PAPER
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The observation of the isolated Cr(VI)
states instead of Cr(III) species in the
Cr/MCM-41 catalysts, caused the
decreased activity toward propane
conversion and low selectivity of C3H6
for PDH process.
D. He, Y. Zhang, S. Yang, Y. Mei, Y.
Luo*
Page No. – Page No.
Investigation of the Isolated Cr(VI)
Species in Cr/MCM-41 Catalysts and
its Effect on Catalytic Activity for
Dehydrogenation of Propane
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