Highly selective production of linear 1-heptadecene from stearic acid over a partially reduced MoO: X catalyst

Article abstract of DOI:10.1039/d0cc01307b

Here, a MoO_x-T based catalyst was developed by a simple reduction of MoO₃ precursors at different temperatures. Interestingly, a partially reduced MoO_x-600 catalyst obtained by reducing the MoO₃ precursor at 600 °C shows the co-existence of a mixture of different valence states (0, +4, ～+6) of Mo, and as a result, provides a superior catalytic activity.

Full text of DOI:10.1039/d0cc01307b

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Highly Selective Production of Linear 1-Heptadecene from Stearic
Acid over a Partially Reduced MoOx Catalyst
Zihao Zhang,a,b,e Yafei Li,a Francis Okejiri,d Miaomiao Liu,d Hao Chen,a Jixing Liu,d Kequan Chen,c
Received 00th January 20xx,
Accepted 00th January 20xx
Xiuyang Lu,a Pingkai Ouyanga,c and Jie Fu*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Here, MoOx-T was developed by a simple reduction of MoO3 precursors at selectivity to LAO production over heterogeneous catalysts, as the
different temperatures. Interestingly, partially reduced MoOx-600 obtained most realistic catalytic system from an industrial production
by reducing MoO3 precursor at 600 °C, shows the co-existence of a mixture standpoint, is quite low and is very difficult to improve.5, 14, 15
of different valence states (0, +4, ~+6) of Mo, and as a result, provides a
The side reactions, such as isomerization and decarboxylation to
saturated alkanes, is perhaps the most important factor limiting high
selectivity to LAO production in heterogeneous systems. For
example, Lopez-Ruiz and coworkers studied decarbonylation of
heptanoic acid using Pt/C and Pd/C catalysts, and achieved a
relatively high α-selectivity of 57% over the gas-phase operations but
< 15% α-selectivity in the liquid-phase operations (Scheme 1).16, 17
Although > 90% selectivity to LAO production has been reported over
a Lewis-acid gamma-Al2O3 or WOx–Al2O3 catalyst, the reactants,
however, are only limited to unsaturated fatty acids.18 To overcome
these challenges, a very recent study from Jensen and co-workers
employed a commercial Pd/C catalyst (Scheme 1), through optimized
phosphine ligand - DPEphos (molecular structure shown in Fig. S1,
ESI†) mediated strategy, and achieved high selectivity to LAO
production from saturated fatty acids conversion.12 Considering the
high cost of noble metal – Pd catalyst, the development of rather
cost-effective, non-noble metal-based, heterogeneous catalyst for
highly selective production of LAOs from fatty acid conversion, is
highly desirable from economic standpoint.
superior catalytic activity.
Linear α-olefins (LAOs) with a terminal C=C double bond are
important building blocks for the manufacture of a variety of
different value-added chemicals, such as polyolefins, detergents,
lubricants, plasticizers, and synthetic oils.1-4 At present,
a
predominant preparation strategy for the manufacture of LAOs
involves the oligomerization of ethylene derived mostly from
petroleum resources. This production strategy, however, has three
main limitations: 1) the ethylene feedstock is mainly derived from
fossil resources; 2) control growth of the carbon chain length to
obtain products with a single carbon chain length is a difficult task to
accomplish, and 3) the as-obtained LAOs are certainly of even-chain
length.5-7 Biomass-derived fatty acids abound in low-cost waste
cooking oils and animal fats which can be converted to valuable
hydrocarbons, including LAOs by mere deoxygenation.8-10 This
deoxygenation of fatty acids has been identified as a complementary
strategy to oligomerization of ethylene and can be used to obtain
odd-chain LAOs from even-chain fatty acids.1 This unique
complimentary strategy is not limited by the above-mentioned
challenges.11-13 Different catalytic systems such as heterogeneous,
homogeneous, and biocatalysis have been investigated for the
selective transformation of fatty acids to LAOs. However, the
a. Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College
of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027,
China. Email: jiefu@zju.edu.cn
b. Institute of Zhejiang University - Quzhou, 78 Jiuhua Boulevard North, Quzhou
324000, China
c. State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing
211816, China
d. Department of Chemistry, The University of Tennessee, Knoxville, TN 37996, USA.
e. Institute for Integrated Catalysis, Pacific Northwest National Laboratory, 902
Battelle Blvd., Richland, Washington 99352, USA
Scheme 1 Schematic representation of the already reported strategy (a) and our
Electronic Supplementary Information (ESI) available: [details of any supplementary
proposed (b) strategy.12, 16, 17
information available should be included here]. See DOI: 10.1039/x0xx00000x
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In the present study, therefore, we developed for the first time,
a partially reduced, non-noble metal oxide-based (MoOx) catalyst for
highly efficient and selective production of 1-heptadecene through
decarbonylative dehydration of stearic acid in the presence of Ac2O
and phosphine ligands. The reduction temperature of the MoO3
precursor was found to have a profound influence on the catalytic
activity of the resultant MoOx-T-based catalyst for the production of
both heptadecene and 1-heptadecene. A series of characterization
techniques including the powder X-ray diffraction (PXRD),
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DOI: 10.1039/D0CC01307B
temperature-programmed
reduction
measurements,
high-
resolution transmission electron microscopy (HR-TEM), and X-ray
photoelectron spectroscopy (XPS), were used to investigate the
structure, composition, surface and bulk chemical properties of the
MoOx-T catalysts. Through the variation of different reaction times
and the mass of catalyst added, the catalytic performance of our
partially reduced MoOx-T catalyst was optimized for the production
of both heptadecene and 1-heptadecene.
To understand the influence of reduction temperature on the
crystal structure of the as-synthesized MoOx-T, the X-ray diffraction
(XRD) patterns of the catalysts were recorded and are shown in Fig.
1. The XRD pattern of MoOx (obtained from the calcination of
ammonium molybdate at 500 °C) shows the characteristic peaks of
MoO3 (PDF #05-0508) before reduction. Upon exposure of MoOx to
H2 reducing environment at 150 °C (MoOx-150) and 300 °C (MoOx-
300), the diffraction patterns of MoOx-150 and MoOx-300 both very
closely resemble that of unreduced MoOx, indicating that MoO3
reduction is quite difficult to accomplish at a temperature below 300
°C. The temperature-programmed reduction (H2-TPR) analysis profile
of MoO3, which has no H2 consumption peak below 300 °C (Fig. S2,
ESI†) additionally collaborates this claim. As the reduction
temperature was increased to 450 °C (MoOx-450), the characteristic
MoO2 diffraction peaks (PDF #32-0671) appeared at the expense of
the peak intensity of MoO3. The occurrence of new diffraction peaks
at approximate 2θ value of 22.5° is attributed to the formation of a
small amount of Mo4O11 (with Mo valence between +4 and +6), as
consistent with the previous report.19 Upon further increase of the
reduction temperature to 600 °C (MoOx-600 ), the diffraction peaks
of MoO3 disappeared completely as characteristic diffraction peaks
of both MoO2 and Mo emerged, showing a complete reduction of
bulk MoO3 precursor at 600 °C.
Fig. 2 TEM images of the (a) MoO3, (c) MoOx-600, and (e) MoOx-700 samples and
their corresponding HRTEM images (b, d, and f).
The XRD pattern of the sample following a reduction in the H2
environment at 700 °C (MoOx-700) contains the characteristic peaks
of a metallic Mo (PDF #42-1120). The physical appearance of all six
samples (MoO3, MoOx-150, MoOx-300, MoOx-450, MoOx-600, and
MoOx-700) during these transition stages was noted and is presented
(Fig. S3, ESI†).
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) images of MoO3, MoOx-600, and MoOx-700 are shown
in Fig. 2. The TEM image of MoO3 (Fig. 2a) shows a portion of strip-
like particles, which are invisible in the MoOx-600 and MoOx-700
samples. Notably, all three samples are composed of a disorderly
accumulation of granular compounds. The HRTEM image of MoO3
shown in Fig. 2b shows the distinct (110) lattice planes of MoO3 with
lattice spacings of 3.7 Å. For the MoOx-600 sample, the lattice planes
ascribed to Mo (110) and MoO2 (-111) with the lattice spacings of 2.3
Å and 3.5 Å, respectively, co-exist, suggesting the partial reduction of
Mo6+ in its bulk phase. The HRTEM image of MoOx-700 (Fig. 2f) has
only the metallic Mo lattice planes, which indicates a fairly complete
reduction of all suboxide intermediates in the bulk phase. This
observation is consistent with the XRD results.
Whereas XRD and TEM results provide comprehensive insights
into the composition of MoOx catalysts in the bulk phase, the surface
composition and chemical state of a catalyst usually have more direct
and profound influence on its catalytic activity, hence, the X-ray
photoelectron spectroscopy (XPS) (high-resolution) of Mo 3d was
used to investigate the surface properties of our MoOx catalysts, as
shown in Fig. 3. For the MoO3, MoOx-150, and MoOx-300 catalysts
(Fig. 3a-3c), two obvious characteristic peaks from 3d 5/2 and 3d 3/2
are located at 232.5 and 235.7 eV, respectively, suggesting the sole
existence of Mo6+ species on the catalyst surface.20 It should be
MoOx-700
MoOx-600
MoOx-450
MoOx-300
MoOx-150
MoO3
10
20
30
40
50
60
70
80
90
2
Fig. 1 XRD patterns of the MoO3 and MoOx-T catalysts; Reference: Mo (orange),
MoO2 (pink), and MoO3 (black).
2 | J. Name., 2012, 00, 1-3
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Mo6+ 3d5/2
Mo6+ 3d5/2
(a)
(b)
80
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DOI: 10.1039/D0CC01307B
heptadecene
1-heptadecene
60
40
20
240 238 236 234 232 230 228 226 224 222
Binding energy(eV)
240 238 236 234 232 230 228 226 224 222
Binding energy(eV)
Mo6+ 3d5/2
(d)
(c)
Mo6+ 3d5/2
0
MoO -700
MoO -600
MoO3 MoOx-150 MoOx-300 MoOx-450
x
x
Mo5+3d5/2
Fig. 4 Yields of heptadecene and 1-heptadecene from conversion of stearic acid
over the MoOx-T catalysts; Reaction conditions: Stearic acid = 50 mg, catalyst = 30
mg, T= 190 °C, t = 16 h, Ac2O = 30 μL, DPEphos = 30 mg.
Mo4+3d5/2
Biomass conversion is an efficient route to produce the renwable
biofuel and value-added chemicals.23-25 In this work, a series of
MoOx-T-based catalytic system was investigated for decarbonylative
dehydration of stearic acid at 190 °C for 16 h in the presence of
DPEphos (Fig. S1, ESI†) and acetic anhydride (Ac2O). The detailed
reaction conditions are summarized in Fig. 4. This DPEphos and Ac2O-
assisted catalytic system was first reported by Jensen and co-
workers, who utilized a commercial, heterogeneous Pd/C catalyst to
achieve an α-olefins yield above 70%.12 In DPEphos-AcO2 catalytic
system, the catalyst with attached DPEphos ligand can react with
anhydride obtained from the conversion of stearic acid and Ac2O to
produce acyl carboxylate. Subsequent decarbonylation and β-
hydride elimination of the acyl carboxylate proceed to obtain 1-
heptadecene. The reaction steps are shown in Fig. S4.12 In the
present MoOx catalytic system, the significant influence of reduction
temperature on catalytic performance towards decarbonylative
dehydration of stearic acid is clearly visible in Fig. 4. For the non-
reduced catalysts (MoO3, MoOx-150, and MoOx-300), both the yields
of heptadecene and 1-heptadecene were below 18% (the yield of
heptadecene/1-heptadecene was calculated by dividing the moles of
heptadecene/1-heptadecene recovered by the initial moles of
stearic acid). We then investigate the uncatalyzed stearic acid
conversion of stearic acid and obtained yields of 1-heptadecene is
only ~16%. However, an increase in the yield of heptadecene from
17.4% to 33.2% was observed upon increasing the reduction from
300 °C to 450 °C. This improvement in catalytic performance can be
attributed to the formation of a small amount of under-coordinated
Mo (Mo4+ and Mo5+) in MoOx-450 resulting from the removal of
oxygen. This implies that the formation of under-coordinated Mo4+
and Mo5+ accelerates decarbonylative dehydration of stearic acid.
Interestingly, the yield of heptadecene increased significantly up to
74.0% in MoOx-600 catalyst with selectivity to 1-heptadecene as high
as 95%. The carbon balance is same as the yield of heptadecene since
the product obtained from the reaction of stearic acid and Ac2O can
not be detected with gas chromatography. The MoOx-700 catalyst,
on the other hand, afforded a relatively lower yield of heptadecene
as compared to MoOx-600, which indicates that excessive reduction
of the MoO3 precursor is equally detrimental to heptadecene
production. In addition to stearic acid, MoOx-600 catalyst also
displays the comparable catalytic performance for the conversion of
240 238 236 234 232 230 228 226 224 222 240 238 236 234 232 230 228 226 224 222
Binding energy (eV)
Binding energy (eV)
Mo03d5/2
(e)
(f)
Mo5+3d5/2
Mo6+ 3d5/2
Mo4+3d5/2 Mo03d5/2
Mo5+3d5/2
Mo4+3d5/2
240 238 236 234 232 230 228 226 224 222 240 238 236 234 232 230 228 226 224 222
Binding energy (eV) Binding energy(eV)
Fig. 3 High-resolution XPS spectra of Mo 3d in (a) MoO3, (b) MoOx-150, (c) MoOx-
300, (d) MoOx-450, (e) MoOx-600, and (f) MoOx-700 samples.
recalled that the lone existence of Mo6+ species on these catalysts
was equally reported for the material bulk (XRD results) since the
reduction temperatures (≤ 300 °C) are quite insufficient to induce
partial reduction. For the MoOx-450 catalyst, the deconvoluted peaks
in Fig. 3d unambiguously revealed the co-existence of Mo4+, Mo5+,
and Mo6+.21, 22 The surface state of the MoOx-450 catalyst is similar
to that in the bulk phase since the peaks ascribed to MoO2, Mo4O11,
and MoO3 were equally observed in the XRD pattern. The XPS
spectrum of MoOx-600 (Fig. 3e) has new peaks at 227.6 eV and 230.8
eV, that suggests the existence of molybdenum in zero oxidation
state (Mo0). Although MoO2 and Mo crystalline phases are both
present in the MoOx-600 catalyst, the Mo phase with a valence state
above +4 is only visible in the XPS profile and not the XRD pattern.
This observation indicates that the surface composition of MoOx-600
has a wide distribution of Mo in 0, +4 and/or ~+6 valence states. For
the MoOx-700 sample, the disappearance of Mo6+ species was
observed in the XPS profile, with increased production of Mo0 (Fig.
3f). The XPS binding energy values of different Mo valence in MoOx-
450, MoOx-600, and MoOx-700 are summarized in Table. S1. In both
MoOx-600 and MoOx-700 samples, the difference in the oxidation
state of Mo on the catalyst surface relative to the bulk phase, is
possibly due to the relative instability of MoOx on the surface
containing a lower Mo valency. Overall, the distribution of various
Mo species can be controllably tuned by different reduction
temperature.
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5. C. Lu, F. Shen, S. Wang, Y. Wang, J. Liu, W. J. Bai,VXie.wWAraticnleg,OAnlCinSe
lauric acid, myristic acid, palmitic acid and eicosanoic acid with
different carbon numbers (Fig. S5, ESI†).
Catal., 2018, 8, 5794-5798.
DOI: 10.1039/D0CC01307B
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Further, the stearic acid conversion was studied over a range of
reaction times (4 h, 8 h, 12 h, 16 h, 20 h, and 24 h) at 190 °C to
determine the optimum reaction time. As seen in Fig. S4, ESI†, the
obtained yields of heptadecene progressively increased from 4 h to
16 h of reaction times (Fig. S6, ESI†). Further increase in the reaction
time beyond 16 h resulted in no additional increase in the yield of
heptadecene, revealing that 16 h of reaction time was enough for
maximum utilization of stearic acid. The effect of the mass of the
catalyst added on heptadecene was equally investigated through
variation of the catalyst amount from 0 - 40 mg. Among the different
masses of the catalyst investigated, the 30 mg of MoOx-600 catalyst
was found to be the most ideal concentration (Fig. S7, ESI†). By
introducing DPEphos and Ac2O into the heterogeneous catalytic
system, high selectivity to 1-heptadecene can be guaranteed over
different catalysts under different reaction conditions. It should be
noted that that inability of the yield of 1-heptadecene to increase
beyond 70% despite the prolonged reaction time and optimized
mass of the catalyst added, is mainly due to the formation of highly
concentrated unsaturated olefins (near 70% yield) and decarbonated
product CO, which occludes the catalyst pores and poisons the active
sites, as previously reported.12 More importantly, the MoOx-600
catalyst can be reused for at least three times without the significant
loss of catalytic activity (Fig. S8, ESI†).
Summarily, a partially reduced MoOx-600 catalyst that shows the
co-existence of different Mo species (0, +4 ~+6) was successfully
developed and investigated for decarbonylative dehydration of
stearic acid. This catalyst exhibited an outstanding catalytic
performance toward decarbonylative dehydration of stearic acid and
achieved a yield of heptadecene close to 75.0% with selectivity to 1-
heptadecene production close 95%. Notably, the catalytic activity of
this partially reduced MoOx-600 catalyst is higher than that of the
non-reduced MoO3 and over-reduced MoOx-700 catalysts, making it
suitable for the production of linear α-olefins from biomass-derived
fatty acids.
J. Fu was supported by the National Natural Science Foundation
of China (No. 21978259), the Zhejiang Provincial Natural Science
Foundation of China (No. LR17B060002), and the Fundamental
Research Funds for the Central Universities.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Mater. Interfaces, 2018, 10, 43578-43587.
4 | J. Name., 2012, 00, 1-3
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