Construction of bifunctional co/h-zsm-5 catalysts for the hydrodeoxygenation of stearic acid to diesel-range alkanes

Article abstract of DOI:10.1002/cssc.201800670

Bifunctional Co/H-ZSM-5 zeolites were prepared by a surface organometallic chemistry grafting route, namely, by the stoichiometric reaction between cobaltocene and the Br?nsted acid sites in zeolites. The catalyst was applied to a model reaction of the catalytic hydrodeoxygenation of stearic acid (SA). The cobalt species existed in the form of isolated Co^{2 +} ions at the exchange positions after grafting, transformed to CoO species on the surface of the zeolite, stabilized inside the zeolite channels upon calcination in air, and finally reduced by hydrogen to homogeneous clusters of metallic cobalt species approximately 1.5 nm in size. During this process, the Br?nsted acid sites of the H-ZSM-5 zeolites were preserved with a slight-ly reduced acid strength. The as-prepared bifunctional catalyst exhibited an approximately 16 times higher activity for the hydrodeoxygenation of SA (2.11 g_SA g_cat¹ h¹) than the reference catalyst (0.13 g_SA g_cat¹ h¹) prepared by solid-state ion exchange and a high C₁₈ /C₁₇ ratio of approximately 24. The remarkable hydrodeoxygenation performance of the bifunctional Co/H-ZSM-5 was owed to the effective synergy between the uniformed metallic cobalt clusters and the Br?nsted acid sites in H-ZSM-5. The simplified reaction network and kinetics of the SA hydrodeoxygenation catalyzed by the as-prepared bifunctional Co/H-ZSM-5 zeolites were also investigated.

Full text of DOI:10.1002/cssc.201800670

Accepted Article
Title: Construction of Bifunctional Co/H-ZSM-5 Catalysts for the
Hydrodeoxygenation of Stearic Acid to Diesel-range Alkanes
Authors: Guangjun Wu, Nan Zhang, Weili Dai, Naijia Guan, and
Landong Li
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To be cited as: ChemSusChem 10.1002/cssc.201800670
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10.1002/cssc.201800670
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Construction of Bifunctional Co/H-ZSM-5 Catalysts for the
Hydrodeoxygenation of Stearic Acid to Diesel-range Alkanes
Guangjun Wu,[a] Nan Zhang,[a] Weili Dai,[a] Naijia Guan, [a,b] and Landong Li*[a,b]
Abstract: Bifunctional Co/H-ZSM-5 zeolites were prepared by
surface organometallic chemistry grafting route, namely by the
stoichiometric reaction between cobaltocene and the Brønsted acid
sites in zeolites, and applied to the model reaction of stearic acid
catalytic hydrodeoxygenation. Cobalt species existed in the form of
isolated Co2+ ions at exchange positions after grafting, transformed to
CoO species on the surface of zeolite and stabilized inside zeolite
channels upon calcination in air, and finally reduced to metallic cobalt
species of homogeneous clusters of ca. 1.5 nm by hydrogen. During
this process, the Brønsted acid sites of H-ZSM-5 zeolites could be
preserved with acid strength slightly reduced. The as-prepared
bifunctional catalyst exhibited a ~16 times higher activity in stearic
acid hydrodeoxygenation (2.11 gSAgcat-1h-1) than the reference catalyst
(0.13 gSAgcat-1h-1) prepared by solid-state ion exchange, and a high
C18/C17 ratio of ~24 was achieved as well. The remarkable
hydrodeoxygenation performance of bifunctional Co/H-ZSM-5 could
be explained from the effective synergy between the uniformed
Decarboxylation, decarbonylation and hydrodeoxygenation
(HDO) are then developed for the upgrading of triglycerides and
fatty acid to diesel-range alkanes, namely the second-generation
biofuels.[1,2b,5] Various types of catalysts have been investigated
and well summarized recently.[1,2b,6] Among these catalysts, non-
noble metal catalysts, such as Ni,[6c,7] Co,[7f,7g,7h] W,[8] Mo,[7g] Fe,[9]
are particularly attractive, due to their sufficient activity, good
stability and low price. From the view of atomic economy, HDO
process, with less C atom losses, is more desirable for the
upgrading of triglycerides and fatty acid in comparison to
decarboxylation and decarbonylation processes.[6d] In HDO
process, the removal of the hydroxyl group should be the final
step, which was proposed to be the rate determining step.[10] As
the hydroxyl group can be easily removed on an acid site via
dehydration, acidic supports are favored for HDO process. In this
context, bifunctional or multifunctional catalysts on acidic
supports are preferred, which have been widely investigated for
metallic cobalt clusters and the Brønsted acid sites in H-ZSM-5 zeolite. the upgrading of triglycerides and fatty acids in HDO
The simplified reaction network and kinetics of stearic acid
hydrodeoxygenation catalyzed by the as-prepared bifunctional Co/H-
ZSM-5 zeolites were also investigated.
process.[6d,7,10a] Besides, the properties of support materials could
influence the size and distribution of metal particles, which could
significantly modulate the catalytic performance of bifunctional or
multifunctional catalysts.[7a,7c] Zeolites, the well-known solid acids
with high specific surface area and widely industrial applications,
are most promising supports for HDO catalysts, which have been
extensively investigated not only for the HDO but also for other
biomass upgrading processes.[6d,11]
Introduction
Triglycerides, composed of one mole of glycerol and three moles
of fatty acids, are considered as high-energy density liquid
molecules of biomass that can be used for the production of liquid
fuels.[1] Transesterification reaction is developed as the first-
generation technology for triglycerides upgrading to fuels (bio-
diesel). Typically, transesterification process be realized with acid
or base catalysts, which has been commercialized around the
world.[1,2] Despite of the advantages of biodegradability, reduced
exhaust emissions, high flash point, and excellent lubricity,[3] the
as-produced biodiesel shows some disadvantages of higher
oxygen contents, low oxidative stability, and weak cold flow
properties, which severely limit its utilization.[4]
For bifunctional or multifunctional catalysts containing redox
metal centers and acidic zeolite supports, it is most important to
promote the synergy between different sites.[11] The preparation
of bifunctional or multifunctional catalysts could significantly
influence the synergy between different active sites and,
accordingly, show great impacts on their HDO performance.
Several approaches, e.g. liquid/solid-state ion-exchange,
impregnation, and deposition-precipitation, are commonly
employed in the preparation of zeolite supported catalysts.
Recently,
a
simple and reproducible route of surface
organometallic chemistry (SOMC) reaction has been developed
for the preparation of iron-containing zeolites.[12] Through SOMC
reaction, the iron species are grafted on zeolite surface through
the reaction between ferrocene and Brønsted acid sites in zeolites
and the specific iron sites could be adjusted at the atomic and
molecular level to derive well-defined catalysts for target
reactions.[13]
[a]
Dr. G. Wu, N. Zhang, Dr. W. Dai, Prof. N. Guan, Prof. L. Li
School of Materials Science and Engineering & National Institute for
Advanced Materials
Nankai University
Tianjin 300071, PR China
E-mail: lild@nankai.edu.cn
Prof. N. Guan, Prof. L. Li
Key Laboratory of Advanced Energy Materials Chemistry of the
Ministry of Education
On the basis of above background and inspired by current
achievements, we herein report the construction of bifunctional
Co/H-ZSM-5 catalysts via SOMC reaction and their applications
in stearic acid HDO. The properties of cobalt species and H-ZSM-
5 zeolite support are well characterized by multiple techniques
and the synergy between them are clearly illustrated by catalytic
[b]
Collaborative Innovation Center of Chemical Science and
Engineering
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10.1002/cssc.201800670
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data. Such bifunctional metal/zeolite catalyst system might be
extended to other complicated reactions of biomass upgrading.
via grafting, significant decreases in the intensities of Brønsted
acidic hydroxyl groups and hydroxyl groups bound to Lewis acid
centers were observed, indicating the interaction between
cobaltocene and these hydroxyl groups. Meanwhile, new bands
due to cyclopentadienyl groups appeared: stretching vibration of
=C-H at 3125 cm-1 and the vibrations of olefinic C=C at 1410 and
1505 cm-1. These results clearly demonstrated the successful
grafting of cobaltocene to H-ZSM-5 host, which is also reflected
by the color change from white to light purple. With the increasing
temperature from 373 to 673 K, distinct decreases in the
intensities of IR bands due to cyclopentadienyl groups were
observed, indicating the gradual removal of these groups upon
thermal treatments.
Results and Discussion
Preparation of Co/zeolite via SOMC grafting
The physicochemical properties of Co/zeolite samples prepared
by SOMC grafting and solid-state ion exchange were summarized
in Table 1. It was seen that cobalt species could be introduced to
various zeolite hosts via SOMC grafting and the Co/Al ratio of ~1
indicates the stoichiometric reaction between cobaltocene and
the Brønsted acid sites in zeolites. No significant loss in cobalt
species could be observed during the subsequent calcination and
reduction processes, in great contrast to the case of Fe/zeolite.[13b]
Moreover, the surface areas and pore volumes, as measured by
low temperature N2 adsorption/desorption, were well preserved
during the SOMC grafting, calcination and reduction processes
(Table 1), which should be good for the catalytic applications.
Co/H-ZSM-5-IR
Co/H-ZSM-5-GR
Co/H-ZSM-5-IC
Table 1. Physicochemical properties of Co/zeolite samples under study
Co
loading
(%)
Pore
volume
(cm3/g)
SBET
(m2/g)
Sample
Si/Al[a]
Co/Al[a]
H-ZSM-5
23.7
23.9
24.1
24.0
23.8
23.9
23.8
23.7
23.5
24.8
25.3
/
/
435
422
437
439
403
371
389
366
392
526
516
0.38
0.37
0.37
0.37
0.36
0.36
0.35
0.34
0.41
0.21
0.52
Co/H-ZSM-5-G
Co/H-ZSM-5-GC
Co/H-ZSM-5-GR
Co/H-ZSM-5-I
Co/H-ZSM-5-IC
Co/H-ZSM-5-IR
Co/H-EU-1-GR
Co/H-mordenite-GR
Co/H-beta-GR
Co/H-USY-GR
3.93
3.82
3.80
4.41
4.19
4.25
3.73
4.31
4.45
4.08
0.95
0.92
0.92
1.05
0.99
1.00
0.93
1.00
1.09
1.04
Co/H-ZSM-5-GC
H-ZSM-5
10
20
30
40
50
60
70
2 Theta [deg.]
Figure 1. XRD patterns of zeolite samples employed in this study.
[a] molar ratio.
H
H
i-O
The XRD patterns of Co/H-ZSM-5 samples were shown in
Figure 1. Clear diffraction lines of H-ZSM-5 were observed for all
samples,[14] and the diffraction lines of cobalt species (cobalt
oxides and cobalt metal) could not be distinguished due to their
low intensities and/or the overlap by the diffraction lines of H-
ZSM-5 zeolite. It could also be observed that the intensities of
diffraction lines of H-ZSM-5 zeolite decreased upon the
introduction of cobalt species and subsequent calcination.
Nevertheless, the framework of ZSM-5 zeolite was well preserved,
consistent with the results from N2 adsorption/desorption
analyses.
FTIR spectroscopy was employed for an insight of the reaction
between cobaltocene and the Brønsted acid sites of H-ZSM-5,
and the results were shown in Figure 2. In the hydroxyl region,
three bands, i.e. isolated silanol groups at 3740 cm-1, hydroxyl
groups bound to Lewis acid centers at 3700 cm-1 and Brønsted
acidic hydroxyl groups at 3605 cm-1,[15] were observed for parent
H-ZSM-5 zeolite. Upon the introduction of cobaltocene to zeolite
S
O
-
H
0
Al
O
Al
-
H
4
7
3
H
o-
C
-
C
=
0
0
7
i-O
S
5
3
6
5
2
1
3
6
3
5
0
6
3
673 K
573 K
473 K
373 K
298 K
Co/H-ZSM-5-G
H-ZSM-5
1380
3800
3600
3400
3200
3000
2800 1800
1600
1400
Wavenumbers / cm-1
Figure 2. FTIR spectra of Co/H-ZSM-5-G at increasing temperatures.
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The temperature-programmed reaction of Co/H-ZSM-5-G was
performed and the results were shown in Figure 3. In flowing He,
the decomposition of cobaltocene on H-ZSM-5 started at 423 K
and the two mass fragments corresponding to C5H6 (m/z=66) and
C5H5• (m/z=65) were detected as the products. No cobalt-
containing fragments, e.g. Cp2Co+ (m/z=188) and CpCo•
(m/z=124), could be observed. These results suggested that
cobaltocene was stabilized on H-ZSM-5 below 423 K while it
decomposed at > 423 K via the loss of one cyclopentadienyl group,
in good agreement with in situ FTIR results (Figure 2). In flowing
O2, CO2 (m/z=44) was detected as the exclusive product from
cobaltocene decomposition and a two-stage CO2 emission was
observed at 553 and 703 K, respectively. It appeared that
cobaltocene grafted on H-ZSM-5 was more stable in flowing O2
than that in flowing He. According to the temperature-
programmed reaction results, the organic species in Co/H-ZSM-
5-G could be completely removed through calcination in flowing
O2 at 773 K and Co/H-ZSM-5-GC sample could be successfully
prepared.
satellite features at ~802 and ~784 eV indicated the presence of
Co2+ in these samples.[16] According to the nonlinear least-
squares fitting results, Co2+ ions were the dominating species on
H-ZSM-5 after calcination, similar to previous report on ion-
exchanged Co-ZSM-5.[17]
Co/H-ZSM-5-G
Co/H-ZSM-5-GC
Co/H-ZSM-5-GR
Co/H-ZSM-5-IR
m/z=65
m/z=66
m/z=44
O2
He
40
35
30
25
20
15
10
5
40
35
30
25
20
15
10
5
350 400 450 500 550 600 650 700 750
300 350 400 450 500 550 600 650 700 750
Co/H-ZSM-5-GR
Co/H-ZSM-5-IR
Temperature / K
Temperature / K
Figure 3. Temperature-programmed reaction profiles of Co/H-ZSM-5-G in
flowing He or O2.
Characterization of Co/H-ZSM-5
The TEM images of selected Co/H-ZSM-5 samples were shown
in Figure 4. For Co/H-ZSM-5-G, clear lattice fringes of H-ZSM-5
zeolite were observed and no cobalt species could be
distinguished although the presence of cobalt was confirmed by
energy dispersive spectroscopy analysis (ca. 4.2 %, not shown
here). Upon the removal of organic species through calcination at
773 K (Co/H-ZSM-5-GC), the aggregation of cobalt species did
occur and tiny cobalt oxide clusters (vide infra) were observed to
disperse on the surface of H-ZSM-5 and/or inside the channels.
Subsequent reduction treatment did not result in significant
changes in the size of cobalt species and homogeneous clusters
(should be metallic cobalt, vide infra) with average size of 1.5 nm
were obtained for Co/H-ZSM-5-GR. In contrast, large cobalt
clusters with size of 7-20 nm were observed on H-ZSM-5 support
for Co/H-ZSM-5-IR. The TEM observations clearly demonstrated
the advantage of the grafting route to homogeneous tiny cobalt
clusters on H-ZSM-5 support.
0
0
0
5
10
15
20
0
5
10
15
20
Cluster Size / nm
Cluster Size / nm
Figure 4. TEM images of Co/H-ZSM-5-G, Co/H-ZSM-5-GC, Co/H-ZSM-5-GR
and Co/H-ZSM-5-IR; The cobalt cluster size distribution in Co/H-ZSM-5-GR and
Co/H-ZSM-5-IR.
Co 2p3/2
Co 2p3/2
Co/H-ZSM-5-IC
Co/H-ZSM-5-GC
Co 2p3/2
satellite
Co 2p3/2
satellite
Co 2p1/2
Co 2p1/2
Co 2p1/2
satellite
Co 2p1/2
satellite
The surface states of cobalt species in Co/H-ZSM-5-GC and
Co/H-ZSM-5-IC were investigated by means of XPS analysis. As
shown in Figure 5, similar binding energy values at 795.1 and
779.9 eV corresponding to Co 2p1/2 and 2p3/2 of Co3+ and/or Co2+
were observed for Co/H-ZSM-5-GC and Co/H-ZSM-5-IC. The
810
805
800
795
790
785
780
775
770
810
805
800
795
790
785
780
775
770
Binding energy / eV
Binding energy / eV
Figure 5. Co 2p XPS of Co/H-ZSM-5-GC and Co/H-ZSM-5-IC.
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The H2-TPR profiles of Co/H-ZSM-5-GC and Co/H-ZSM-5-IC
are shown in Figure 6. Typically, three hydrogen consumption
peaks at ~500, ~650 and ~900 K were clearly distinguished for
both samples. According to previous H2-TPR results on Co-ZSM-
5 samples,[18] these hydrogen consumption peaks should be due
to the reduction of CoOx species on the surface of zeolite, CoOx
species stabilized inside zeolite channels and isolated Co2+ ions
at exchange positions, respectively. Since Co2+ ions were the
dominating species on H-ZSM-5 (Figure 5), the total H/Co ratio of
~2 in the temperature range of 373-1273 K indicated the complete
reduction of Co2+ to metallic Co. A significant higher proportion of
CoOx species stabilized inside zeolite channels was obtained in
Co/H-ZSM-5-GC than that in Co/H-ZSM-5-IC, which should be
originated from the different preparation methods. These CoOx
species were efficiently stabilized by the zeolite channels and
their aggregation upon reduction treatment could be greatly
suppressed, as confirmed by TEM observations (Figure 4). It
could be seen from our H2-TPR results that CoOx species on the
surface of zeolite or stabilized inside zeolite channels species
could be fully reduced to metallic cobalt at 773 K in flowing H2/Ar
(the typical reduction treatment conditions) while Co2+ ions at
exchange positions could not. That is, a large proportion of
metallic cobalt and a small proportion of Co2+ ions existed in Co/H-
ZSM-5-GR and Co/H-ZSM-5-IR catalysts for the HDO reaction.
via SMOC grafting or solid-state ion exchange followed by
calcination, the ammonia desorption peak corresponding to
strong Brønsted acid sites in H-ZSM-5 disappeared while a small
peak centred at 673 K appeared (Figure 7, left-hand chart), which
should be due to the desorption of ammonia interacting with Lewis
acidic CoOx species (all the Brønsted acid sites occupied by
CoOx species, Figure 2). Through reduction treatment, the
intensities of ammonia desorption peaks at 673 K increased
distinctly (Figure 7, right-hand chart). It was revealed from TPR
analysis (Figure 6) that a majority of cobalt species in Co/H-ZSM-
5-GC and Co/H-ZSM-5-IC could be reduced to metallic cobalt in
flowing H2/Ar at 773 K, accompanied by the recovery of Brønsted
acid sites in H-ZSM-5. Since the interaction between ammonia
and metallic cobalt was rather weak, the ammonia desorption
peak at 673 K in Co/H-ZSM-5-GR and Co/H-ZSM-5-IR should be
mainly originated from the ammonia interacting with Brønsted
acid sites in H-ZSM-5 zeolite. Compared with parent H-ZSM-5,
the strength of Brønsted acid sites in Co/H-ZSM-5-GR and Co/H-
ZSM-5-IR decreased significantly, i.e. ammonia desorption peak
shifted from 743 to 673 K, which should be explained from the
influence of adjacent metallic cobalt species.[20]
Co/H-ZSM-5-GC
Co/H-ZSM-5-IC
Co/H-ZSM-5-GR
Co/H-ZSM-5-IR
H/Co=0.4
Co/H-ZSM-5-GC
H/Co=1.1
H/Co=0.5
H-ZSM-5
H-ZSM-5
H/Co=0.7
400 500 600 700 800 900 1000
400 500 600 700 800 900 1000
Temperature / K
Temperature / K
Co/H-ZSM-5-IC
H/Co=0.8
Figure 7. NH3-TPD profiles of H-ZSM-5, Co/H-ZSM-5-IC, Co/H-ZSM-5-GC,
Co/H-ZSM-5-IR and Co/H-ZSM-5-GR.
H/Co=0.4
400
600
800
1000
1200
On the basis of the above-mentioned characterization results,
it could be concluded that cobalt species existed in the form of
CoO species on the surface of zeolite and stabilized inside zeolite
channels and isolated Co2+ ions at exchange positions in Co/H-
ZSM-5-GC. Through reduction treatment, a majority of these
cobalt species could be reduced to metallic cobalt species, which
appeared as homogeneous clusters of ca. 1.5 nm in Co/H-ZSM-
5-GR. The Brønsted acid sites in H-ZSM-5 could be preserved in
Co/H-ZSM-5-GR while the acid strength is reduced to some
extent. The Co/H-ZSM-5-GR was established as a bifunctional
material consisting both metallic cobalt centers and Brønsted acid
sites, which should be a promising catalyst for the HDO reaction.
Temperature / K
Figure 6. H2-TPR profiles of Co/H-ZSM-5-GC and Co/H-ZSM-5-IC.
The acidic properties of Co/H-ZSM-5 zeolites were evaluated
by means of NH3-TPD and the results were shown in Figure 7.
Two ammonia desorption peaks at 493 and 743 K were observed
for parent H-ZSM-5, corresponding to the weak and strong acid
sites, respectively. It was generally acknowledged that the high
temperature ammonia desorption peak was originated from the
desorption of ammonia strongly interacting with Brønsted acid
sites in H-ZSM-5 zeolite.[19] With the introduction of cobalt species
Stearic acid HDO catalyzed by Co/H-ZSM-5
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The catalytic HDO of stearic acid over Co/H-ZSM-5-GR and
Co/H-ZSM-5-IR was investigated, and the results were shown in
Figure 8. Co/H-ZSM-5-IR exhibited very low activity in the reaction
with stearic acid conversion rate of 0.13 gSAgcat-1h-1 at 533 K. 1-
investigated for the HDO of stearic acid, and the results were
shown in Figure 9. Co/H-EU-1 and Co/H-mordenite exhibited
remarkable activity, similar to the case of Co/H-ZSM-5, while
Co/H-USY and Co/H-Beta exhibited distinct lower activity.
Obviously, the nature of zeolite supports could significantly
influence the catalytic activity of cobalt catalysts, which was
probably due to the different strength and distribution of acid sites
in different zeolites. In a whole, Co/H-ZSM-5-GR could be
optimized among all Co/zeolites investigated and it would be
employed as a model catalyst for further kinetic study.
Octadecanol
(C18H37OH),
n-Octadecane
(n-C18H38),
i-
Octadecane (i-C18H38), Octadecene (C18H36) and n-Heptadecane
(n-C17H34) were detected as the products. 1-Octadecanol was
observed as the dominating product in the early stage of reaction.
With the progress of reaction, the percentage of 1-Octadecanol
decreased while the percentage of n-Octadecane increased,
accompanied by the appearance of Octadecene and i-
Octadecane. The ratio of C18/C17 in the product is calculated to be
~6 after reaction for 240 min.
Co/H-ZSM-5-GR exhibited a ~16 times higher activity (2.11
gSAgcat-1h-1) than Co/H-ZSM-5-IR under identical reaction
conditions (0.13 gSAgcat-1h-1). Similar to the case of Co/H-ZSM-5-
IR, 1-Octadecanol was observed as the dominating product in the
early stage of reaction, which was further converted to alkanes
with the progress of reaction. The ratio of C18/C17 in the product
was calculated to be ~24 after reaction for 240 min, in great
contrast to the value of ~7 obtained with Co/H-ZSM-5-IR. The
high stearic acid conversion rate as well as the high C18/C17 ratio
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Co/H-EU-1-GR
Co/H-USY-GR
C18H37OH
n-C18H38
i-C18H38
C18H36
Stearic acid
C17H36
C18H37OH
n-C18H38
i-C18H38
C18H36
C17H36
Stearic acid
0
20 40 60 80 100 120 140 160 180 200 220 240
0
20 40 60 80 100 120 140 160 180 200 220 240
Reaction Time / min
Reaction Time / min
100
80
60
40
20
0
100
100
80
60
40
20
0
100
C18H37OH
n-C18H38
i-C18H38
C18H36
in the product made Co/H-ZSM-5-GR
nonprecious metal catalyst for the conversion of stearic acid to
diesel-range alkanes.
a very promising
Co/H-beta-GR
Co/H-mordenite-GR
80
60
40
20
0
80
60
40
20
0
C17H36
Stearic acid
C18H37OH
n-C18H38
C18H36
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Stearic acid
Co/H-ZSM-5-GR
Co/H-ZSM-5-IR
C18H37OH
n-C18H38
i-C18H38
C18H36
C17H36
Stearic acid
C16H34
C17H36
C18H37OH
n-C18H38
i-C18H38
C18H36
0
20 40 60 80 100 120 140 160 180 200 220 240
0
20 40 60 80 100 120 140 160 180 200 220 240
Reaction Time / min
Reaction Time / min
Stearic acid
C17H36
Figure 9. HDO of stearic acid over Co/zeolite-GR. Reaction conditions: 1.0 g
stearic acid, 100 mL n-Heptane, 0.2 g catalyst, 3 MPa H2, temperature = 533 K.
0
20 40 60 80 100 120 140 160 180 200 220 240
0
20 40 60 80 100 120 140 160 180 200 220 240
Reaction Time / min
Reaction Time / min
Reaction network and kinetics
Figure 8. HDO of stearic acid over Co/H-ZSM-5-GR and Co/H-ZSM-5-IR.
Reaction conditions: 1.0 g stearic acid, 100 mL n-Heptane, 0.2 g catalyst, 3 MPa
H2, temperature = 533 K.
The reaction network of stearic acid HDO was simplified in
Scheme 1. Stearic acid could undergo the hydrogenation of
carbon-oxygen double bond to generate 1-Octadecanol, which
could be further dehydrated to Octadecene. The hydrogenation of
carbon-carbon double bond in Octadecene led to the formation of
n-Octadecane. Alternatively, 1-Octadecanol could undergo
dehydrogenation to n-Octadecanal, which could be further
converted to n-Heptadecane via decarbonylation. For the
conversion of stearic acid to n-Heptadecane, stearic acid could
directly undergo decarboxylation or undergo decarbonylation to
Heptadecene followed by carbon-carbon double bond
hydrogenation. The intermediate product Octadecene could be
isomerized to i-Octadecene, followed by hydrogenation to i-
Octadecane. The n-Octadecane could also be directly isomerized
to i-Octadecane.
The HDO of stearic acid was known to occur over bifunctional
catalysts containing both acid sites and metal functional centers.
In this study, both Co/H-ZSM-5-GR and Co/H-ZSM-5-IR
appeared to be bifunctional catalysts containing Brønsted acid
sites and cobalt centers (see characterization results). However,
the cobalt particle size in Co/H-ZSM-5-GR (1.5 nm) was much
smaller than that in Co/H-ZSM-5-IR (7-20 nm), which hinted to
more accessible cobalt centers and a better synergy between the
cobalt centers and the Brønsted acid sites in Co/H-ZSM-5-GR.
These features should be responsible for the much higher activity
observed for Co/H-ZSM-5-GR in contrast to Co/H-ZSM-5-IR.
Through SOMC grafting route, cobalt catalysts on other zeolite
For the stearic acid HDO catalyzed by Co/H-ZSM-5-GR, 1-
Octadecanol was observed as the dominating product in the early
stage of reaction and, therefore, the hydrogenation of carbon-
oxygen double bond in stearic acid to 1-Octadecanol was
proposed as the dominating step. Afterwards, the formation and
supports,
i.e.
Co/H-EU-1
(SiO2/Al2O3=24),
Co/H-USY
(SiO2/Al2O3=25), Co/H-beta (SiO2/Al2O3=25) and Co/H-mordenite
(SiO2/Al2O3=25), could be prepared. These catalysts were also
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10.1002/cssc.201800670
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consumption of Octadecene was observed (Figure 9), indicating
the dehydration of 1-Octadecanol to Octadecene and the
subsequent hydrogenation to n-Otadecane. The relatively low
amount of Octadecene observed and its quick conversion
revealed that the 1-Octadecanol dehydration and Octadecene
hydrogenation were very fast steps. Mass spectroscopy analysis
reveals that carbon monoxide was the exclusive carbon
containing gas product and, therefore, the direct decarboxylation
of stearic acid can be completely excluded. Besides, no
Heptadecene was observed in the liquid phase, excluding the
possibility of stearic acid decarbonylation-hydrogenation to n-
Heptadecane. Thereupon, the only possible pathway of n-
Heptadecane formation was the dehydration of 1-Octadecanol
followed by decarbonylation, which was in good agreement with
the observation of carbon monoxide as gas product. However, no
intermediate product n-Octadecanal could be detected in the
liquid phase. A possible explanation was that n-Octadecanal
existed in the form of adsorbed species on the catalyst surface,
which underwent very quick conversion to other products.[21]
Heptadecane (C17H36), were investigated according to the
following equations:
푑퐶푆퐴
= −푘1퐶푆퐴
푑푡
푑(퐶C
+ 퐶C
)
18H36
18H38
= 푘2퐶
C18H37OH
푑푡
푑퐶
C17H36
= 푘3퐶
C18H37OH
푑푡
푑퐶
C18H37OH
= 푘1퐶푆퐴 − (푘2 + 푘3)퐶
C18H37OH
푑푡
Table 2. Hydrodeoxygenation of stearic acid over Co/H-ZSM-5-GR at different
temperatures
Product selectivity (%)
Temperature Time
Stearic acid
(K)
(min) Conversion (%)
C18H37OH C18H38 C18H36 C17H36
513
523
533
543
100
80
37.8
39.2
40.4
45.9
85.9
80.9
43.3
27.6
0.2
3.9
13.4
14.2
17.4
18.3
0.3
0.9
4.6
5.1
C17H36
CO2
60
34.7
48.9
40
hydro-
genation
hydrogenation
dehydration
C16H33CH=CH2
C18H37OH
C17H35COOH
C18H38
k2
k1
H2
H2
H2O
H2O
Table 3. Rate constants and apparent activation energy values of key
pathways in the hydrodeoxygenation of stearic acid over Co/H-ZSM-5-GR
H2
C17H35CHO
CO
hydrogenation
H2
Rate constants (s-1)
Temperature
C15H31CH=CH2
H2
C15H31C(CH3)=CH2
i-C18H38
(K)
k1
k2
k3
CO
C17H36
1.01×10-4
1.34×10-4
1.88×10-4
3.54×10-4
94
2.65×10-5
4.67×10-5
3.35×10-4
1.02×10-3
298
6.38×10-7
2.35×10-6
2.97×10-5
7.72×10-5
392
C17H36
513
523
k3
Scheme 1. The reaction network of stearic acid HDO.
533
543
Generally, the hydrogenation, dehydrogenation and
decarbonylation steps take place on transition metal centers while
the dehydration and isomerization steps on Brønsted acid sites.[22]
For Co/H-ZSM-5-GR, the presence of Brønsted acid sites and
metallic cobalt centers as well as their effective synergy made it a
robust bifunctional catalyst for stearic acid HDO to diesel-range
alkanes.
According to the reaction network in Scheme 1, an empirical
kinetic model for stearic acid HDO over Co/H-ZSM-5-GR could be
established. It should be noted that the internal diffusion of
molecules inside zeolite channels could not be assessed, and, in
this context, the kinetic data reported here will include the effects
of internal diffusion. Besides, considering that the reaction was
performed in much excess hydrogen, the effects of gas
component on the kinetic model could be ignored.[23]
Ea (kJ/mol)
HDO of stearic acid over Co/H-ZSM-5-GR at 513-543 K was
performed and the activity data were summarized in Table 2. With
increasing reaction temperature, stearic acid conversion and
alkane selectivity increased while 1-Octadecanol selectivity
decreased. On the basis of these data, the rate constant values
at different temperatures, i.e. k1, k2 and k3, could be calculated,
as shown in Table 3. Arrhenius plots in Figure 10 revealed that
the apparent activation energy value for stearic acid conversion
to 1-Octadecanol was 94 kJ/mol, much lower than the conversion
of 1-Octadecanol to Octadecane (298 kJ/mol) and Heptadecane
(392 kJ/mol). That is, the first hydrogenation of stearic acid to 1-
Octadecanol was much easier than the further conversion of 1-
Octadecanol to alkanes, in good agreement with experimental
observations (Figure 8). Meanwhile, the conversion of 1-
Octadecanol to Heptadecane was distinctly difficult than the
conversion of 1-Octadecanol to Octadecane, corresponding to
the lower selectivity to Heptadecane than Octadecane.
Three reaction pathways were considered (Scheme 1, framed
by red, green and blue dot-line, respectively), and the conversion
rates of major products, i.e. stearic acid (SA), 1-Octadecanol
(C18H37OH), Octadecane (C18H38), Octadecene (C18H36) and
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10.1002/cssc.201800670
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-6
elevated temperature and then reacts with the Brønsted acid sites in
zeolites. In a typical experiment, a zeolite sample of 2.5 g was placed in a
quartz reaction chamber connected to a vacuum line and treated under
vacuum of 5*10-2 Pa at 573 K for 4 h. After cooling to 323 K, 0.5 g
cobaltocene was added into the reaction chamber, which was heated to
and kept at 423 K under vacuum for 24 h to insure the complete surface
reaction between cobaltocene and the Brønsted acid sites in zeolites. The
resulting solid (labelled as Co/zeolite-G) was subjected to calcination in
flowing air at 773 K for 6 h and labelled as Co/zeolite-GC. The Co/zeolite-
GC sample was further subjected to reduction in flowing 10%H2/Ar at 773
K for 6 h and then labelled as Co/zeolite-GR.
-8
-10
-12
For reference, Co/H-ZSM-5 sample was also prepared by solid-state ion
exchange. In
a typical experiment, 2.5 g H-ZSM-5 zeolite was
k1
mechanically mixed with 0.5 g cobalt acetate in the glove box and the
mixture was placed in a sealed reactor. The reactor was heated to and
kept at 573 K for 2 h. The resulting solid was thoroughly washed with
deionized water, dried at 353 K overnight (labelled as Co/H-ZSM-5-I),
calcined in flowing air at 773 K for 6 h, and labelled as Co/H-ZSM-5-IC.
The Co/zeolite-IC sample was further subjected to reduction in flowing
10%H2/Ar at 773 K for 6 h and then labelled as Co/zeolite-IR.
k2
k3
-14
1.82
1.84
1.86
1.88
1.90
1.92
1.94
1.96
1/T *1000
Figure 10. Arrhenius plots of different reaction pathways in stearic
acid conversion catalyzed by Co/H-SZM-5-GR.
Characterization techniques
The exact cobalt loadings in Co/zeolite samples were analyzed on an IRIS
Advantage inductively coupled plasma atomic emission spectrometer
(ICP-AES). The specific surface areas and pore volumes of samples were
determined through N2 adsorption/desorption isotherms at 77 K collected
on a Quantachrome iQ-MP gas adsorption analyzer. The X-ray diffraction
(XRD) patterns of Co/zeolite samples were recorded on a Bruker D8
ADVANCE powder diffractometer using Cu-K radiation ( = 0.1542 nm)
at a scanning rate of 4o/min in the region of 2θ = 5-50o. Transmission
electron microscopy (TEM) images of samples were taken on a FEI Tecnai
G2 F20 electron microscope at an acceleration voltage of 200 kV. A few
drops of alcohol suspension containing the sample were placed on a
carbon-coated copper grid, followed by evaporation at ambient
temperature. Fourier transform infrared (FTIR) spectra of Co/H-ZSM-5-G
in flowing He were measured on a Bruker Tensor 27 spectrometer with
128 scans at a resolution of 2 cm−1. A self-supporting pellet made of
sample was placed in the reaction chamber and heated to designed
temperatures for recording the spectrum. The temperature-programmed
reaction of Co/H-ZSM-5-G in flowing He or O2 was performed on quartz
tube reactor and the products were on-line analyzed by a Pfeiffer Omnistar
GSD 320 mass spectrometer. X-ray photoelectron spectra (XPS) of
Conclusions
Co/H-ZSM-5 zeolites have been successfully prepared through
SOMC grafting, followed by calcination in air and reduction in
hydrogen. TEM and H2-TPR results reveal that cobalt species
exist in the form of homogeneous metallic clusters of ca. 1.5 nm
stabilized by zeolite, while the presence of Brønsted acid sites
with slightly reduced acid strength compared to parent H-ZSM-5
is confirmed by NH3-TPD analyses. That is, the as-prepared
Co/H-ZSM-5-GR zeolites appear to the bifunctional catalysts
containing metallic cobalt clusters and Brønsted acid sites, and
their effective synergy can be expected.
Co/H-ZSM-5-GR is very active in the HDO of stearic acid to
diesel-range alkanes with a reaction rate of 2.11 gSAgcat-1h-1, being
~16 times higher than that of Co/H-ZSM-5-IR prepared through
solid-state ion exchange (reaction rate of 0.13 gSAgcat-1h-1).
Meanwhile, a remarkably high C18/C17 ratio of ~24 in diesel-range
alkane products is achieved, indicating the neglectable carbon
losses during stearic acid conversion. Kinetic analysis results
reveal that hydrogenation of stearic acid to 1-Octadecanol (with
apparent activation energy of 94 kJ/mol) is much easier than the
further conversion of 1-Octadecanol to alkanes, while the lower
apparent activation energy value of 1-Octadecanol conversion to
Octadecane (298 kJ/mol) than that to Heptadecane (392 kJ/mol)
can well explain the high ratio of C18/C17 in the diesel-range alkane
products.
Co/zeolite samples were recorded on
a Kratos Axis Ultra DLD
spectrometer with a monochromated Al-Kα X-ray source (hν= 1486.6 eV),
hybrid (magnetic/electrostatic) optics and a multi-channel plate and delay
line detector. All spectra were recorded by using an aperture slot of
300×700 microns. Accurate binding energies (±0.1 eV) were determined
with respect to the position of the adventitious C 1s peak at 284.8 eV. The
temperature-programmed desorption of ammonia (NH3-TPD) experiments
were performed on a Quantachrome ChemBET 3000 chemisorption
analyzer. A sample of ca. 0.1 g was pretreated in flowing He at 673 K for
1h, cooled to 373 K in He and saturated with 5% NH3/He. After that, the
sample was purged with He for 30 min to eliminate the physisorbed
ammonia. NH3-TPD was then carried out in flowing He in the temperature
range of 373-1073 K at a heating rate of 10 K/min. The temperature-
programmed reduction by hydrogen (H2-TPR) of samples were performed
on a chemisorption analyzer (Quantachrome ChemBET 3000) with
10%H2/Ar at a heating rate of 10 K/min from 373 to 1273 K. Prior to
reduction, the sample (100 mg) was calcined in dry air at 673 K for 1 h.
Experimental Section
Catalyst Preparation
Alumino-silicate zeolites in their H-form with similar SiO2/Al2O3 ratio of 25,
i.e. H-ZSM-5, H-EU-1, H-USY and H-modenite, were provided by Sinopec
and employed as supports for cobalt species. Co/zeolite samples were
prepared by organometallic grafting method.[12] During the grafting process,
the organometallic cobalt precursor cobaltocene (Cp2Co) sublimates at
Catalytic reaction and product analysis
The HDO of stearic acid (Adamas, 99%) was performed in a high-pressure
stainless autoclave (Xinyuan Chemical Machinery, Series CJK, 300 mL)
at a stirring rate of 750 rpm (adequate to exclude the external diffusion
limitations). In a typical experiment, 0.2 g catalyst, 1.0 g stearic acid and
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10.1002/cssc.201800670
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100 mL solvent n-Heptane were well mixed in the autoclave and purged
with pure N2 at room temperature. The autoclave was rapidly heated to
desired temperature and H2 was introduced at 3.0 MPa to initiate the
reaction.
After reaction, the liquid organic products were analyzed by gas
chromatography (Shimadzu GC-2010) and gas chromatography-mass
spectrometry (Shimadzu GCMS-QP2010 SE), both with a RXI-5MS
column (30 m, 0.25 mm i.d., stationary phase thickness 0.25 μm).
Eicosane was used as an internal standard for quantification. The following
temperature program was employed: Isothermal heating at 323 K for 5 min,
heating to 573 K with a rate of 10 K/min, and isothermal heating at 573 K
for 10 min. The gas products were qualitatively analyzed with a mass
spectrometer (Pfeiffer Omnistar GSD 320).
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FULL PAPER
Bifunctional Catalyst: Bifunctional
Co/H-ZSM-5 zeolites was prepared
by surface organometallic chemistry
grafting route. The as-prepared
bifunctional catalyst exhibited a ~16
times higher activity (2.11 gSAgcat-1h-
1) in stearic acid hydrodeoxygenation
to alkanes than the reference
Guangjun Wu, Nan Zhang, Weili Dai,
Naijia Guan, and Landong Li*
Page No. – Page No.
Construction of Bifunctional Co/H-
ZSM-5 Catalysts for the
Hydrodeoxygenation of Stearic Acid
to Diesel-range Alkanes
catalyst prepared by solid-state ion
exchange and a remarkable
selectivity to C18 alkanes with a
C18/C17 ratio up to ~24.
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