Catalytic transfer hydrogenation of oleic acid to octadecanol over magnetic recoverable cobalt catalysts

Article abstract of DOI:10.1039/c8gc03075h

Efficient transformation of biomass into fuel and chemicals under mild conditions with cost-effective and environmentally friendly characters is highly desirable but still challenging. Herein, a scalable and Earth-abundant cobalt catalyst was used for selective catalytic transfer hydrogenation (CTH) of unsaturated fatty acids to fatty alcohols with sustainable isopropanol as a hydrogen donor. By tuning the surface Co composition by varying the reduction temperature, the catalytic performance could be easily boosted. At 200 °C in 4 h, the optimal catalyst Co-350 (reduced at 350 °C) gives 100% oleic acid conversion with 91.9% octadecanol selectivity. Various characterization studies reveal that the co-existence of Co^δ+ and Co⁰ over the cobalt core might be responsible for its high performance for CTH of oleic acid. This catalyst could be magnetically separated and is highly stable for reusing ten times. Moreover, this cobalt catalyst is relatively cheap and easy to scale-up, thus achieving a low-cost transformation of biomass into high value-added chemicals.

Full text of DOI:10.1039/c8gc03075h

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Catalytic transfer hydrogenation of oleic acid to
octadecanol over magnetic recoverable cobalt
catalysts†
Cite this: DOI: 10.1039/c8gc03075h
Juncheng Wang,a Renfeng Nie, *b Ling Xu,a Xilei Lyua and Xiuyang Lu*a
Efficient transformation of biomass into fuel and chemicals under mild conditions with cost-effective and
environmentally friendly characters is highly desirable but still challenging. Herein, a scalable and Earth-
abundant cobalt catalyst was used for selective catalytic transfer hydrogenation (CTH) of unsaturated fatty
acids to fatty alcohols with sustainable isopropanol as a hydrogen donor. By tuning the surface Co com-
position by varying the reduction temperature, the catalytic performance could be easily boosted.
At 200 °C in 4 h, the optimal catalyst Co-350 (reduced at 350 °C) gives 100% oleic acid conversion with
91.9% octadecanol selectivity. Various characterization studies reveal that the co-existence of Coδ+ and
Co0 over the cobalt core might be responsible for its high performance for CTH of oleic acid. This catalyst
could be magnetically separated and is highly stable for reusing ten times. Moreover, this cobalt catalyst is
relatively cheap and easy to scale-up, thus achieving a low-cost transformation of biomass into high
value-added chemicals.
Received 1st October 2018,
Accepted 3rd December 2018
DOI: 10.1039/c8gc03075h
rsc.li/greenchem
fatty acids to alcohols,8–12 but their widespread applications
are limited by their low Earth-abundance and high cost. As for
Introduction
Using biomass to replace fossil-fuel resources is a desirable non-noble metals, Cu–Cr based catalysts have also been
strategy for solving the energy and environmental crisis.1,2 As applied for hydrogenation of fatty acids.13 In order to achieve
one of the most promising renewable molecules, fatty acids or high performance, extreme conditions of temperature
their esters could be produced via hydrolysis of biomass- (250–400 °C) and pressure (20–30 MPa) are sometimes
derived triglycerides.3 Hydrodeoxygenation (HDO) of fatty required for these catalytic systems.14 What’s more, environ-
acids or their esters is of particular importance,4,5 which could mental pollution related to the disposal of Cr makes them less
reduce the high-oxygen content and related acidity of fatty suitable for large-scale application in industry. Recently, Cr-
acids, yielding products suitable for renewable fuels and high free Cu-based catalysts (such as CuZn, CuFe, etc.) for the pro-
added-value chemicals.5,6 Due to their important applications duction of fatty alcohols have drawn considerable atten-
in the synthesis of plasticizers, surfactants, food additives, cos- tion.15,16 In general, HDO processes consume large amounts
metics and pharmaceuticals,6,7 fatty alcohols have attracted of fossil-derived H2 (at least an ∼3–4 molar ratio of
growing attention in the past few years, and the global H2/reactant), together with high pressures.17,18 This excess high-
demand has reached 3.1 million tons per year with a projected pressure H2 is difficult and dangerous to control, often resulting
growth rate of 3.2%.4 Therefore, selective HDO of fatty acids to in hydrogen waste or over hydrogenation.19 Therefore, seeking
fatty alcohols has great potential for industrial applications.
alternative strategies for selective HDO of fatty acids under con-
Generally, traditional hydrogenation catalysts based on trollable and mild conditions is highly desirable.
noble metals, including Ru, Pt and Re, are active for HDO of
Catalytic transfer hydrogenation (CTH) with renewable
alcohols as hydrogen donors has become the center of
attention.20–23 In comparison to H2-involving hydrogenation,
this process is much safer and easier to operate, and does not
require special experimental setups.24 In these aspects, Cu/
ZnO/Al2O3 and Cu/Al2O3 have been reported for selectively
hydrogenating fatty acids and their esters to alcohols in a
methanol/H2O system,4,25 but they are unstable due to their
low tolerance to water. Although Cu/SiO2 is also capable of
CTH of aliphatic ester to alcohol using methanol as a hydro-
gen donor,26 the optimized reaction temperature is above
aKey Laboratory of Biomass Chemical Engineering of Ministry of Education, College
of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027,
China. E-mail: luxiuyang@zju.edu.cn
bHubei Collaborative Innovation Center for Advanced Organic Chemical Materials, &
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, School of Chemistry and Chemical Engineering,
Hubei University, Wuhan 430062, P.R. China. E-mail: refinenie@163.com
†Electronic supplementary information (ESI) available: Additional experimental
results. See DOI: 10.1039/c8gc03075h
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the reactor was quenched in cold water to quickly stop the
reaction. The solid catalyst and liquid products were separated
by centrifugation. The liquid was diluted with acetone and
analyzed by GC (Agilent 7890A) with a 30 m capillary column
(HP-5) using a flame ionization detector (FID). All the products
were confirmed by GC-MS (Agilent 5977A MSD).
The conversion of oleic acid and the yield of products were
defined as follows:
Scheme 1 Representative work on the transformation of fatty acids
into fatty alcohols.
nOA ꢀ n′OA
Conversion=% ¼
ꢁ 100%
nOA
240 °C. Thus, developing non-noble metals with superior
activity and chemoselectivity for low-temperature CTH of fatty
acids to alcohols is highly desirable.
n
Yield=% ¼ ðmole of productÞ ꢁ 100%
nOA
Herein, we report the selective CTH of unsaturated oleic
acid into octadecanol with isopropanol over cheap and scal-
able cobalt catalysts under mild conditions, presenting a sig-
nificant advance in the conversion of fatty acid into fatty
alcohol as the major product (Scheme 1). By tuning the
surface Co composition through the use of different reduction
temperatures, the conversion and selectivity could be easily
tuned. The effects of catalyst loading, substrate concentration,
reaction temperature and reaction time were also investigated.
Finally, combined with various characterization methods and
control experiments, a plausible reaction route for CTH of
oleic acid over cobalt is proposed.
where nOA is the initial mol of oleic acid and n′OA is the
residual mol of oleic acid after the reaction.
Results and discussion
Characterization of cobalt catalysts
Co-T catalysts were prepared via a traditional precipitation
method. After reduction at different temperatures in a 10%
H2/Ar flow, the surface Co species could be easily tuned.
Firstly, the morphology and structure of cobalt catalysts were
studied by XRD (Fig. 1). Characteristic diffraction peaks are
observed at 19.0°, 31.3°, 36.8°, 44.8°, 59.4° and 65.2° for Co–O,
which could be attributed to the (111), (220), (311), (400),
(511), and (440) reflections of Co3O4 (PDF 43-1003), respect-
ively.27 The crystalline phase of cobalt starts to change when
the reduction temperature is raised to 250 °C, in which the
diffraction peaks of Co3O4 weaken, but the diffraction peaks of
CoO (PDF 48-1719) and hcp-structured Co (PDF 05-0727)
emerge.28 When the reduction temperature is raised to 350 °C,
no Co3O4 and CoO could be observed. Further increasing the
reduction temperature to 450 and 550 °C would generate fcc-
structured Co (PDF 15-806) and sharpen the diffraction peaks
of Co.29 The reduction behaviour of Co species was investi-
gated by H2-TPR. As shown in Fig. S1,† two H2 consumption
peaks are located at 283 and 353 °C. The former one (283 °C)
could be attributed to the reduction of Co3O4 to CoO
Experimental
Catalyst preparation
All cobalt catalysts were prepared via a traditional precipitation
method. Typically, a certain amount of Co(NO3)2·6H2O was dis-
solved in 200 mL DI water, which was marked as solution A. A
mixed solution of NaOH (0.2 mol L−1) and Na2CO3 (0.1 mol
L−1) was marked as solution B. Then, solutions A and B were
added dropwise to 100 mL DI water under vigorous stirring at
30 °C and pH 9.5. After that, the suspension was aged at 80 °C
for 1 h with stirring and for another 3 h without disturbance.
The solid was separated by filtration and washed thoroughly
with DI water. The final sample was dried at 100 °C overnight
and calcined at 350 °C in air for 3 h. The obtained sample was
marked as Co–O. Before the catalytic test, Co–O was reduced
for 2 h in a 10% H2/Ar flow at a certain temperature, and the
resulting catalyst was denoted as Co-T (T represents the
reduction temperature). Other metallic catalysts were prepared
by similar methods. The calcined precursor Fe–O was activated
in a 10% H2/Ar flow at 500 °C for 2 h, denoted as Fe-A. Cu-A
and Ni-A were obtained via the reduction of Cu–O and Ni–O at
240 °C and 400 °C, respectively.
Typical experiment and product analysis
The CTH of oleic acid was carried out in a 14 mL micro-batch
cylindrical stainless steel high-pressure reactor. In a typical
run, 7 mL isopropanol, 70 mg oleic acid and a certain amount
of catalyst were added into the reactor. Then the reactor was
Fig. 1 XRD patterns of (a) Co–O, (b) Co-150, (c) Co-250, (d) Co-350, (e)
sealed and heated to a certain temperature. After the reaction, Co-450 and (f) Co-550.
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(Co3+ → Co2+), while the latter peak (353 °C) could be attribu- percentages are 14.7, 53.2 and 32.7%, respectively.30 On
ted to the reduction of CoO to metallic Co (Co2+ → Co0),27 well further increasing the reduction temperature to 550 °C
in agreement with the XRD results.
(Fig. 2d), however, these CoOx phases are almost reduced to
The HRTEM images of cobalt catalysts are shown in Fig. 2. metallic cobalt, well in agreement with the XRD and TPR
Co–O shows (Fig. 2a) distinct lattice planes with lattice results.
spacings of 0.122, 0.201, 0.242 and 0.287 nm, which could be
ascribed to the (622), (400), (311) and (220) planes of Co3O4,
CTH of oleic acid over cobalt catalysts
respectively. As for Co-250 (Fig. 2b), Co and CoO start to
emerge according to the confirmation of the lattice spacing.
CoO, Co3O4 and Co phases coexist throughout the catalyst,
indicating the partial reduction of Co3O4 in Co-250. When the
reduction temperature is raised to 350 °C (Fig. 2c), the core of
the particle is reduced to metallic cobalt, while the surface of
the particle still consists of Co, CoO and Co3O4. The Co 2p XPS
spectrum of Co-350 (Fig. 3) shows that three Co species, Co0,
Co(III) and Co(II), exist in the catalyst, and their corresponding
In order to study the influence of reduction temperature on
the catalytic activity of cobalt catalysts, CTH of oleic acid using
isopropanol as a hydrogen donor was conducted. As shown in
Fig. 4, the catalytic performance is found to be strongly depen-
dent on the reduction temperature. All catalysts afford
almost full conversion at 200 °C in 4 h. When Co–O is used
directly, nearly no octadecanol could be generated, and the
main products are isopropyl esters. The octadecanol yield
increases quickly from 6.9 to 91.9% when raising the reduction
temperature from 150 to 350 °C. Apart from GC-MS (Fig. S3†),
1H and 13C NMR (Fig. S4 and S5†) were also used to confirm
the generation of octadecanol. Further increasing the tempera-
ture to 550 °C, however, will lead to a significant decrease of
octadecanol yield to 5.1%. Furthermore, the catalytic perform-
ance of catalysts based on iron, nickel and copper was also
investigated under the same reaction conditions (Fig. S6†). For
instance, Fe-A, Cu-A and Ni-A give 0, 0 and 5.8% yields of octa-
decanol, which are much lower than that of cobalt catalysts.
Traditional CTH of fatty acid to fatty alcohol was commonly
catalyzed by Cu, Ni and Ru catalysts with methanol, Fe and Zn
as reductants.4,25,26,31,32 Fe and Zn are not desirable because
they are not atom efficient and will cause environmental pol-
lution due to the disposal of solid residues. In order to achieve
a high yield of fatty alcohol, these catalysts need to be operated
at a high temperature (>240 °C) or for a long reaction time
(Table S1†). Comparatively, the use of metal-free and sustain-
able reductants at low temperature (200 °C) demonstrates their
promising potential in industrial applications from an econ-
omic, environmental, and safety viewpoint.
In order to figure out the active centers of cobalt catalysts,
several control experiments were carried out (Table 1). No octa-
decanol was produced over commercial metallic cobalt since
Fig. 2 HRTEM images of (a) Co–O, (b) Co-250, (c) Co-350 and
(d) Co-550 catalysts.
Fig. 4 CTH of oleic acid over different cobalt catalysts. Reaction con-
ditions: Oleic acid (70 mg), catalyst (10 mg), isopropanol (7 mL), 200 °C,
4 h. a Isopropyl esters refer to the sum of isopropyl oleate and isopropyl
stearate.
Fig. 3 XPS spectra of (a) fresh Co-350 and (b) reused Co-350 catalysts.
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Table 1 Controlled CTH of oleic acid over different cobalt catalysts
Yield/%
Entry
Catalysts
Conv./%
Stearic acid
Octadecanol
Alkanese
Isopropyl esters
1
Coa
Co-350
Co/CoO
CoO/Co3O4
Co/CoO/Co3O4
100
100
90.0
46.7
80.9
23.9
n.d
22.0
n.d
n.d
91.9
2.5
n.d
6.1
n.d
2.5
n.d
n.d
n.d
49.3
5.7
34.5
27.5
21.8
2
3b
4c
5d
n.d
Reaction conditions: Oleic acid (70 mg), catalyst (10 mg), isopropanol (7 mL), 200 °C, 4 h. a Commercial metallic cobalt. b The physical mixture of
5 mg commercial Co and 5 mg commercial CoO. c The physical mixture of 5 mg commercial CoO and 5 mg commercial Co3O4. d The physical
mixture of 3.3 mg commercial Co, 3.3 mg commercial CoO and 3.3 mg commercial Co3O4. e Alkanes are the sum of heptadecane (n-C17) and octa-
decane (n-C18). n.d: not detected.
full conversion could be achieved, and the main products are
stearic acid and isopropyl esters. When using a mixture of
commercial CoO and Co3O4 as a catalyst, only 46.7% conver-
sion and a low yield of products could be observed. On com-
bining commercial Co with CoO or CoO/Co3O4, the oleic acid
conversion increases to >80%, and 6.1 and 2.5% octadecanol
yields could be obtained over Co/CoO/Co3O4 and Co/CoO,
respectively. Although Co3O4 and metallic Co have been
reported for CTH or hydrogenation of biomass,28,30,33 they are
not a good choice for the hydrogenation of oleic acid to octa-
decanol. On using Co-350 as a catalyst, 26.8% isopropanol is
consumed according to HPLC analysis, which is accompanied
by the generation of 1.54 mmol hydrogen (4.9 bar in a reactor)
Fig. 6 Influence of catalyst loading on CTH of oleic acid. Reaction con-
ditions: Oleic acid (70 mg), Co-350, isopropanol (7 mL), 200 °C, 4 h.
determined via GC analysis (Fig. S7†). These results indicate
that a possible synergistic effect between metallic Co and CoOx
would favor the dehydrogenation of isopropanol for oleic acid
hydrogenation, and even favor the generation of octadecanol.30
Although the yield of isopropyl esters remains almost constant,
With Co-350 as the optimized catalyst, several alcohols were
increasing the loading from 5 to 20 mg could lead to the
also used as hydrogen donors. As shown in Fig. 5, methanol
increase of octadecanol yield, while an excessive loading (e.g.
40 mg) would decrease their yield due to the over HDO of octa-
decanol to alkanes. Furthermore, keeping catalyst loading at
and ethanol afford 0 and 10.5% yield of octadecanol, which
are much lower than that of isopropanol. The effect of catalyst
loading was studied as shown in Fig. 6. It is found that full
conversion could be achieved in each case, but the product dis-
10 mg (Fig. S8†) and increasing the oleic acid concentration
from 10 to 40 mg mL−1 would give rise to a low yield of octade-
tribution varies significantly at different catalyst loadings.
canol, which could be ascribed to the lack of active sites.
The reaction temperature and time have a significant influ-
ence on conversion and product distribution (Fig. 7). Full con-
version could be obtained at 160 °C in 6 h. A higher tempera-
ture will accelerate the reaction rate, and full conversion could
be achieved at >200 °C within 1 h. When the CTH reaction is
operated at 160 °C, the yield of octadecanol increases slowly
with the increasing reaction time. Increasing the temperature
to 180 °C would afford 92.9% octadecanol yield in 8 h. The
maximal octadecanol yield (91.9%) could be achieved in 4 h
when operating at 200 °C. Although raising the temperature to
220 °C would shorten the reaction time to 2 h for achieving a
maximal yield (85.7%) of octadecanol, further prolongation of
time would lead to a quick drop in octadecanol yield.
To further explore the reaction mechanism of CTH towards
oleic acid, the evolution of the reactant and products with
Fig. 5 Effect of the hydrogen donor on the CTH of oleic acid. Reaction
time over Co-350 was investigated at 200 °C (Fig. 8). At the
initial stage of the reaction, oleic acid is consumed quickly
conditions: Oleic acid (70 mg), Co-350 (10 mg), hydrogen donor (7 mL),
200 °C, 4 h.
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kept almost constant, while the alkane yield increases continu-
ally with a low reaction rate, which would finally cause a drop
in octadecanol yield. Notably, as an intermediate during the
hydrogenation of stearic acid to octadecanol, trace octadecanal
is detected. A carbon balance plot (Fig. S10†) shows that a
short reaction time would cause relatively low carbon balance,
which could be mainly ascribed to the generation of stearyl
stearate, while a long reaction time (more than 5 h) will cause
the carbon balance to drop again due to the generation of CO
and CO2 (Fig. S7 and S11†). Combined with the above results,
a possible reaction mechanism for CTH of oleic acid with iso-
propanol is proposed in Fig. 9.35–37 Meanwhile, Co-350 is also
suitable for scale-up of CTH of oleic acid and affords >80%
yield of octadecanol (Table S2†).
Catalyst recycling is an important issue for sustainable
development of chemicals. Co-350 is easy to synthesize via a
coprecipitation-reduction method and can be handled and
stored under air. As shown in Fig. 10, Co-350 could be recycled
ten times without a significant loss of efficiency, and the yield
of octadecanol is always above 90%. This catalyst could be
easily recovered via magnetic separation (Fig. S12†) and
washed with 2-propanol, followed by vacuum drying without
any reduction in hydrogen flow. The XRD pattern of the
recycled Co-350 (Fig. S13†) shows that only diffraction peaks of
Co0 are observed and its crystalline state does not change sig-
Fig. 7 (a) Time-conversion and (b) time–yield profiles for CTH of oleic
acid to octadecanol. Reaction conditions: Oleic acid (70 mg), Co-350
(10 mg), isopropanol (7 mL).
Fig. 9 Possible reaction route for CTH of oleic acid with isopropanol
over cobalt catalysts.
Fig. 8 Time course for CTH of oleic acid over Co-350. Reaction con-
ditions: Oleic acid (70 mg), Co-350 (10 mg), isopropanol (7 mL), 200 °C.
and full conversion is obtained within 1 h. Stearic acid is gen-
erated quickly due to the easy hydrogenation of the CvC
bond, the maximal yield of 31.7% is achieved at 1 h, and then
the stearic acid yield quickly drops down with a further
increase in reaction time to 4 h. Octadecanol is generated at a
relatively moderate rate, and the maximal yield of 91.9% is
obtained at 4 h. Although the as-formed octadecanol easily
reacts with stearic acid to generate stearyl stearate (Fig. S9†),34
it turns to acid again and disappears as the reaction time is
Fig. 10 Recycling of the Co-350 catalyst for CTH of oleic acid.
Reaction conditions: Oleic acid (70 mg), Co-350 (10 mg), isopropanol
prolonged. In the whole process, the isopropyl ester yield is (7 mL), 200 °C, 4 h.
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nificantly. XPS of the recycled Co-350 (Fig. 3b) indicates that
the percentages of Co0, Co(III) and Co(II) are 16.3, 65.5 and
18.3%, respectively, which are similar to those of the fresh
one. These results indicate that Co-350 is highly stable for
CTH of oleic acid to octadecanol under the optimized
conditions.
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