Effective deoxygenation of fatty acids over Ni(OAc)2 in the absence of H2 and solvent

Article abstract of DOI:10.1039/c5gc01147g

Different metal acetate salts were systematically examined for the catalytic deoxygenation of stearic acid in the absence of H₂ and solvent for the first time. Ni(OAc)₂ exhibited the highest activity with 62% yield achieved at 350°C for 4.5 h with only 1 mol% (0.2 wt%) of the catalyst. Even with 0.25 mol% (0.05 wt%) catalyst, around 28% yield was achieved within 2 h at 350°C with 89% selectivity to C17 hydrocarbons. The activity based on C17 yields per Ni was 14.5 mol mol^-1 h^-1, considerably higher than that in previous reports. The catalytically active species were identified to be in situ generated Ni nanoparticles (8-10 nm) formed from the decomposition of the metal precursor with stearic acid as a stabilizer. A new reaction pathway of alkane formation from stearic acid via anhydride intermediate decarbonylation under an inert gas atmosphere was proposed. The excellent stability of the catalyst was demonstrated by re-adding a substrate to the system, during which the activity remained constant through four consecutive runs. The novel catalytic system was found to be applicable to a range of fatty acids and triglycerides with varying activities.

Full text of DOI:10.1039/c5gc01147g

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Effective deoxygenation of fatty acids over Ni(OAc)2 in the
absence of H2 and solvent
Wenjing Li,a,b Yongjun Gao,a Siyu Yao,b Ding Ma*b and Ning Yan*a
ceived 00th January 20xx,
Accepted 00th January 20xx
Different metal acetate salts were systematically examined for the catalytic deoxygenation of stearic acid in the absence of
H2 and solvent for the first time. Ni(OAc)2 exhibited the highest activity with 62% yield achieved at 350 ℃ for 4.5 h with
only 1 mol% (0.2 wt%) of catalyst. Even with 0.25 mol% (0.05 wt%) catalyst, around 28% yield was achieved within 2 h at
350 ℃ with 89% selectivity to C17 hydrocarbons. The activity based on C17 yields per Ni was 14.5 mol·mol-1·h-1,
considerably higher than previous reports. The catalytically active species were identified to be in situ generated Ni
nanoparticles (8-10 nm) formed from the decomposition of the metal precursor with stearic acid as a stabilizer. A new
reaction pathway of alkane formation from stearic acid via anhydride intermediate decarbonylation under an inert gas
atmosphere was proposed. The excellent stability of catalyst was demonstrated by re-adding substrate to the system,
during which the activity remained constant through four consecutive runs. The novel catalytic system was applicable to a
range of fatty acids and triglycerides with varying activities.
DOI: 10.1039/x0xx00000x
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deposits and lubricant thickening.6,11,12 In recent years, much
effort has been made to upgrade these renewable sources to
biodiesel.13-15 Cracking of triglycerides with zeolites can
1. Introduction
The rising concern of the decline of fossil fuel reserves has
created increasing interest toward the search for renewable
chemicals and fuels for future society, such as the use of lignin,
cellulose and chitin for fine chemicals1-5 and solar, wind and
nuclear energy for fossil fuels replacement. However, the
application of solar energy and wind energy is limited by the
surrounding environment and nuclear energy suffers from the
problems associated with residue disposal and radiation
pollution. One of the most suitable renewable sources for
future transportation fuel are natural oils and fats, derived
from such sources as palm, microalgae, jatropha plants and
kitchen waste oil, due to their high energy density, structural
similarity to fossil fuels (long alkane chain) and as well as the
widespread and rich content in nature.6-9
remove the oxygen moieties, but together with a significant
portion of carbon lose, causing energy loss.16-19
Transesterification of triglycerides with methanol and ethanol
using homogenous base catalyzed systems has been
commercialized. Unfortunately, the products still contain high
oxygen content, so that they are not fully compatible with a
diesel engine. To make things worse, this process cannot
tolerant high free fatty acid content as acids undergo
saponification causing the deactivation of catalysts.11,12
Alternatively, the oxygen in triglycerides and free fatty acids
can be selectively removed in the form of CO2, CO or H2O via a
deoxygenation process. As such, the feedstock is transformed
into liquid hydrocarbons, which are ideal replacements of
conventional fossil fuels. Deoxygenation of fatty acids (as
model compounds for fats and oils) have been studied with
various metal catalysts such as Pd, Pt, Ni, Cu, and W supported
on ZrO2, SiO2, CeO2, TiO2, Al2O3, carbon and zeolitic materials
under H2 atmosphere with excellent performance achieved.20-
Natural oils and fats are comprised of triglycerides and free
fatty acids with a long linear hydrocarbon chain ranging from
C4 to C24, in particular C16 and C18.10 The high viscosity and
low volatility of these feedstocks cause severe problems when
directly used as fuels in engines such as injector coking, carbon
29
However, the main disadvantage of this process is the
consumption of H2, which mainly comes from fossil fuels with
high costs and difficulties in storage and transportation.
Ideally, the fully renewable biofuel needs to be obtained
without additional H2 input. Several reports concerning the
deoxygenation process under inert gas or atmospheric
conditions have been examined, mainly focused on noble
metal catalysts such as Pd30-33 and Pt,34,35 which are not highly
appealing in the industry due to their high cost and low
a.Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive 4, 117585, Singapore. E-mail:
ning.yan@nus.edu.sg; Fax: +65 6779 1936; Phone: +65 6516 2886
b.
Beijing National Laboratory for Molecular Sciences, College of Chemistry and
Molecular Engineering, Peking University, Beijing 100871, China. E-mail:
dma@pku.edu.cn; Fax: +86-10-62758603; Phone: +86-10-62758603
† Electronic Supplementary Informaꢀon (ESI) available: Details of additional
characterizations and reaction optimizations. Supporting Information, Figure S1-S6
and Table S1-3. See DOI: 10.1039/x0xx00000x
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abundancy. More recently, Ni based catalysts,36-39 after pre- on an Agilent HPLC-1200 with an Agilent ZORBAX C18 column
activation with high temperature H2, were also examined for using methanol and water as mobile phase.
this deoxygenation process. Most of these reactions were
2.3 Ni nanoparticles isolation
carried out in dodecane, toluene or water, with low activity
being observed (for example, 18.1% and 17.8% conversion of After the catalytic deoxygenation of stearic acid was carried
stearic acid could be achieved over 60% Ni/SiO2 and 16% out over 1 mol% Ni(OAc)2 at 350 °C for 2 h, the reaction
Ni/Al2O3 under 300
℃
for 6 h, respectively).40 New catalytic mixture was collected and centrifuged at 60 °C to remove
systems for the deoxygenation of fatty acids/triglycerides in stearic acid. The solid was then washed with CH2Cl2 for three
the absence of auxiliary reagent over cheap and easily times to remove additional stearic acid and dried for use.
available metal catalysts are still highly desirable.
2.4 Characterization
In the present study, stearic acid was used as a model
compound to identify superior catalyst for the deoxygenation X-ray diffraction (XRD) analysis was performed on a Bruker D8
reaction in the absence of solvent and additional H2. A Advanced Diffractometer with Cu kα (λ = 1.5418 Å) at 40 kV.
systematic catalyst screening was carried out over 11 metal Transmission electronic microscopic (TEM) images were
acetate salts as pre-catalysts. Ni(OAc)2 exhibited highest obtained on a JEOL 2100 under 200 kV. Thermogravimetric
activity with heptadecene and heptadecane as the major analysis (TGA) was carried out on a Shimadzu DTG-60AH with a
products. Different nickel salts were further examined, and nitrogen stream of 100 mL/min from 25 ℃ to 1000 ℃ at a rate
Ni(NO3)2 and Ni(acac)2 outperformed NiCl2 and NiSO4. In-depth of 10 /min. Fourier transformed infrared (FT-IR) spectra was
℃
investigations were conducted to identify the real catalytic determined on a Bio-Rad FTS-3500 ARX instrument. X-ray
active species and the reaction pathways. The recyclability and absorption spectroscopy (XAS) on the Ni K-edge was measured
the substrate scope were also evaluated.
at the 1W1B beamline at the Beijing Synchrotron Radiation
Facility (BSRF) in transmission mode using ion chamber
detector to collect data. Ni foil, NiO and all Ni precursors were
measured as standards.
2. Experimental
2.1 Materials
3. Results and discussion
Free fatty acids (including stearic acid, palmtic acid,
dodecanoic acid, oleic acid and linoleic acid), tristearin,
dodecane, Ni(NO3)2·6H2O and Pd(OAc)2 were purchased from
3.1 Catalysts screening
Sigma-Aldrich. [Rh(OAc)2]2 and Au(OAc)3 were purchased from The deoxygenation process of stearic acid at 350
℃ for 2 h
Alfa Aesar. Ni(OAc)2·4H2O, Ni(acac)2, NiCl2·6H2O, NaOAc, under a nitrogen atmosphere did not occur in the absence of a
Ca(OAc)2·H2O, Cu(OAc)2·H2O, Zn(OAc)2, Co(OAc)2·4H2O, catalyst—only 4.8% products were generated, among which a
Mn(OAc)2·4H2O, AgOAc were purchased from Sinopharm majority (78%) was heavy products (mainly stearone and 2-
Chemical Reagent (SCR). NiSO4·6H2O was purchased from C19-one). At higher temperature, the auto-decomposition of
Merck. CH2Cl2 was purchased from Fisher and CH3OH was stearic acid was more pronounced, but the selectivity was
purchased from Tedia. All chemicals were used without any largely directly to cracking products (C8-C16) (Table S1). The
further purification.
catalytic activity of a series of metal acetate salts was
evaluated as pre-catalysts for the reaction as shown in Table 1
(entries 2-12). These acetate salts were selected as they are all
2.2 Reaction setup and analytical techniques
In a typical experiment, stearic acid (200 mg, 0.7 mmol) and easily available and more importantly, acetate represents the
the catalyst precursor (0.07 mmol) were charged in a high- simplest fatty acid carboxylate so that the anion effect is
pressure glass vessel. The vessel was sealed in a glovebox minimized. The catalytic performance of these 11 acetate salts
under nitrogen atmosphere. The glass vessel was immerged differed drastically from each other, with Ni(OAc)2 being the
into a furnace at the desired reaction temperature and time most effective and NaOAc/Ca(OAc)2 being almost inert. TGA
with continuous stirring. After reaction, the reactor was cooled results (Fig. S1a) revealed that all salts decomposed under
down to room temperature and CH2Cl2 containing dodecane as reaction conditions except NaOAc and Ca(OAc)2 which only
the internal standard were added into the system. The reactor decomposed above 400
was sealed and transferred to the furnace again at 70 for 20 their original state in the reaction, can hardly catalyze the
min, in order to wash all products into CH2Cl2 solution. Finally, deoxygenation of stearic acid. The other acetate salts
the reaction mixture was transferred to a volumetric flask and decomposed at 350 to form heterogeneous catalytic
diluted to 10 mL. systems. Ag, Pd and Rh acetate salts decomposed into metallic
℃. The latter two salts, maintained
℃
℃
The CH2Cl2 solution was passed by a filter to remove any metals according to the weight of the residues obtained from
solid. The products were qualified with gas-chromatography TGA results, but these metal catalysts were not effective in the
mass spectrometer (GC-MS) on an Agilent 7890-5975C and reaction. For example, only 10.1% of products were obtained
quantified by gas chromatograph (GC) on an Agilent GC-7890 over Pd(OAc)2, although high activity of supported Pd(0)
equipped with a HP-5 capillary column and flame ionization catalyst has been achieved in previous reports. This may result
detector (FID). The conversion of stearic acid was determined from the large size and low metal dispersion of Pd in the
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a
Table 1. Performance of screened catalysts in the deoxygenation of stearic acid without H2
GC yield / %b
HPLC Conv. /
Entry
Catalyst
Cracking
product
0.7
c
%
Heptadecene
Heptadecane
Stearaldehyde
Heavy product
Total
1
2
Blank
NaOAc
Ca(OAc)
0.2
-
< 0.1
0.2
-
0.1
-
3.8
0.2
4.8
0.4
7.4
2.9
-
3
1.7
1.0
0.5
< 0.1
0.3
0.6
22.4
1.4
5.7
3.9
0.4
21.4
14.6
4.6
-
-
1.2
3.9
4.0
2
4
Cu(OAc)
Zn(OAc)
1.5
1.5
0.1
0.3
1.5
18.9
3.6
1.3
1.0
0.4
18.4
13.1
5.5
1.0
29.6
-
0.2
3.7
11.3
23.3
26.5
31.3
72.0
12.7
16.7
9.6
2
5
1.8
-
3.5
5.4
2
6
Mn(OAc)
2.3
-
15.6
12.8
13.8
1.5
18.4
16.7
57.4
10.0
10.1
8.0
2
7
Co(OAc)2
1.8
-
8
Ni(OAc)
2.3
-
2
9
AgOAc
Pd(OAc)2
[Rh(OAc)2]2
Au(OAc)3
Ni(acac)2
Ni(NO )
3.5
-
10
11
12
13
14
15
16
17
1.9
-
1.2
2.5
-
0.6
2.7
-
2.0
5.5
11.6
68.8
62.6
13.5
4.2
3.8
-
13.7
10.9
0.6
57.3
47.5
11.2
2.4
8.9
-
3
2
NiCl
0.5
-
2
NiSO
1.3
-
0.1
4
Ni-isolatedd
5.1
19.6
-
2.0
56.3
58.2
a Reaction conditions: stearic acid (200 mg); metal salt (10 mol%); T = 350 ℃; t = 2 h. b Determined by GC and calculated according to effective carbon number with
dodecane as internal standard. Cracking: C8-C16 alkenes and alkanes; Heavy product: 2-C19-one and stearone; -: not detected. c Determined by HPLC. d Ni NPs isolated
from catalytic system after reaction (Ni(OAc)2 as pre-catalyst).
present study as the metal dispersion will affect the
From these observations, it was clear that Ni2+ in the salt
deoxygenation activity of fatty acid considerably.41 Mn(OAc)2 form was inactive for the H2 free deoxygenation reaction of
and Co(OAc)2 exhibited slightly better activity than Cu, Zn salts, fatty acid, and decomposed Ni salts were the active catalytic
enabling 18.4% and 16.7% product yield respectively, but species. TGA results, however, could not reveal whether the
these two catalysts led to the formation of a considerable decomposed Ni species was metallic Ni or NiO. Noteworthy, Ni
portion of undesired cracking and heavy products (ca. 97% and species were exposed to an inert atmosphere in TGA whereas
87% combined selectivity in Mn(OAc)2 and Co(OAc)2, Ni species were in contact with fatty acids and their derivatives
respectively) such as 2-C19-one and stearate ketone (entries 4 - during the reaction.
7). Among all the catalysts screened, Ni(OAc)2 was the most
Several combinations between Ni(OAc)2 and other metal
active for the deoxygenation of stearic acid with 72.0% acetate as the pre-catalysts were further tested (Table S2), but
conversion and 57.4% product yield, among which 72% the product yield was not enhanced. As such, further studies
belongs to C17 hydrocarbons (Table 1, entry 8). The were focused on single component Ni salts.
exceptional performance of Ni(OAc)2 prompted us to examine
3.2 Active species identification
other nickel salts such as Ni(NO3)2, Ni(acac)2, NiCl2 and NiSO4
as pre-catalysts under identical conditions (Table 1, entries 13- Several characterization techniques were performed to
16). Ni(NO3)2 and Ni(acac)2 were similarly active compared identify the true catalytic active species. XRD patterns of
with Ni(OAc)2, achieving 62.6% and 68.8% stearic acid different nickel salts with stearic acid before and after reaction
conversion with 47.5% and 57.3% product yield, respectively. are shown in Fig. 1a and Fig. S2. Before reaction, 10 mol%
On the other hand, NiCl2 and NiSO4 exhibited little catalytic nickel salts and stearic acid were mixed together under
activity. We further analyzed these four Ni salts using TGA. grinding conditions and then used for XRD characterization.
Similar to Ni(OAc)2, Ni(acac)2 and Ni(NO3)2 fully decomposed The results in Fig. S2a suggested that these samples all
before 400
℃
decomposed only at a much higher temperature (ca. 800
, whereas NiCl2 and NiSO4 were stable and exhibited the peaks belong to stearic acid. The spectra also
showed the peaks from nickel salts, although these peaks were
℃,
Fig. S1b). These TGA results indicated that Ni(OAc)2, Ni(acac)2 very weak and difficult to identify due to the complexity and
and Ni(NO3)2 were decomposed under the applied reaction strong intensity of peaks from stearic acid. After reaction, the
conditions, while NiCl2 and NiSO4 remained in the form of a peaks from Ni(OAc)2, Ni(acac)2 and Ni(NO3)2 salts disappeared
metal salt.
(Fig. S2b) whereas the diffraction peak of metallic Ni (at ca.
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44.6 º) appeared in the enlarged XRD patterns (Fig. 1a). in situ formed, nanosized Ni metal particles were responsible
Plausibly, these Ni salts decomposed and were reduced to for the high activity for the H2-free deoxygenation of stearic
Ni(0) over reducing species such as CO and H2 generated in the acid. We further isolated the in situ formed Ni NPs after the
reaction. Not unexpected, the peak belonging to Ni(0) was not reaction (Ni(OAc)2 as pre-catalyst) for a second batch of
observed in catalytic systems where NiCl2 and NiSO4 were used reaction. The product yield remained essentially the same
as pre-catalysts, in agreement with TGA results.
while the stearic acid conversion slightly dropped (Table 1
entry 17). Interestingly, these Ni nanoparticles were
unsupported and unprotected by any additional stabilizer, yet
they were small in size, stable and catalytically active under
harsh conditions. We propose that stearic acid acted as a
stabilizer and ligand for the Ni nanoparticles. Noteworthy the
use of fatty acids as surfactant for the synthesis of different
kinds of nanoparticles has been previously reported, despite
different synthetic conditions.42-44
Fig.1 (a) Typical XRD patterns of stearic acid and nickel salts after reaction (enlarge
spectra); and TEM images of (b) Ni(OAc)2, (c) Ni(acac)2 and (d) Ni(NO3)2 catalytic
systems after reaction.
Besides XRD, XAS was also applied to investigate the state of
nickel after reaction. The samples were sealed in a chamber
with Kapton film windows under Ar protection in a glovebox
after reaction. In agreement with XRD data, metallic nickel was
observed in the Ni(OAc)2, Ni(acac)2 and Ni(NO3)2 systems (Fig.
S3a), whereas NiCl2 and NiSO4 still maintained the same
valence state as the precursor (Fig. S3b and Fig. S3c). Nickel
oxide was also observed in the Ni(OAc)2, Ni(acac)2 and Ni(NO3)2
systems, which indicated some of the surface Ni was oxidized
during the sample preparation process.
Fig. 2 (a) Effects of catalyst loading on catalytic activity; reaction conditions: stearic acid
(200 mg); catalyst: Ni(OAc)2; T = 350 ℃; t = 2 h. (b) Effects of substrate amount on
catalytic activity; reaction conditions: substrate, stearic acid; Ni(OAc)2 (0.007 mmol); T =
350 ℃; t = 2 h. Note: The activity data of blank was taken from Table 1, entry1; the 10
mol% catalyst amount and 200 mg of stearic acid was taken from Table 1, entry 8.
Since XRD patterns and XAS results both suggested the
existence of metallic Ni after reaction, TEM was applied to
characterize the size and morphology of these particles. TEM
images revealed the formation of highly dispersed, 8-10 nm
nanoparticles during the reaction in the Ni(OAc)2, Ni(acac)2 and
Ni(NO3)2 catalytic systems (Fig. 1b-1d). The nanoparticles in
the Ni(OAc)2 catalyst system have been further analyzed with
TEM-EDX and TEM-EDX mapping, confirming the existence of
Ni in these nanoparticles (Fig. S4a). On the other hand, only
bulk structures of aggregates were observed in the NiCl2 and
NiSO4 systems. TEM-EDX mapping exhibited approximately the
same atomic ratio between Ni:Cl and Ni:S as nickel precursors
in these two samples, respectively. This suggested that these
two salts did not decompose under the catalytic conditions
(Fig. S4b and Fig. S4c). Combining TGA, XRD, XAS and TEM
analysis, Ni salt precursors are inactive in the reaction. Instead,
3.3 Catalyst loading test
Next, we tested the catalytic performance of the Ni(OAc)2 with
varying loadings. As illustrated in Fig. 2a, the deoxygenation
process can hardly occur without catalyst (the yield of
heptadecene and heptadecane was less than 1%). However,
when 1 mol% of Ni(OAc)2 was added into the system, the
conversion of stearic acid increased dramatically to 59% and
the selectivity to C17 hydrocarbons increased from 3% to 77%.
This result indicated the necessity of the catalyst to improve
the deoxygenation activity and selectivity to C17 hydrocarbons.
With an increase of the catalyst amount from 1 mol% to 5
mol%, the conversion of stearic acid increased steadily from
59% to 71%. This number maintained almost unchanged when
the catalyst loading exceeded 5 mol%. Reducing the catalyst
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loading below 1 mol% while maintain the same stearic acid used in the kinetic study. The product yields with respect to
amount was not practical as the measurement of small reaction time at different reaction temperatures was
quantities of Ni(OAc)2 could not be accurately determined. determined (Fig. 3a). As discussed previously, the metallic
Therefore, the amount of substrate was increased, while nickel nanoparticles formed during the reaction catalyzed the
keeping the Ni(OAc)2 amount constant to reduce the catalyst stearic acid deoxygenation, however, no induction period can
loading (Fig. 2b). The conversion decreased with the increase be observed from the kinetic curve and suggested that the
of stearic acid amount. Nevertheless, even with 0.25 mol% formation of nickel nanoparticles was very fast. According to
Ni(OAc)2, 39% of the stearic acid could be converted with 64% previous reports,35,45 the deoxygenation of stearic acid was
selectivity towards C17 hydrocarbons. The activity of Ni considered as a first order reaction with respect to liquid
nanoparticles formed in situ, based on C17 hydrocarbon yields, products. As expected, the reaction rate increased with
was calculated to be 14.5 mol·mol-1·h-1, which was two orders reaction temperature and the rate constants of the
of magnitude higher than that obtained in Al2O3 supported deoxygenation at each temperature were summarized in Table
nickel catalysts carried out under He (0.079 mol·mol-1·h-1).40 S3. Each temperature followed a similar trend between
Furthermore, this activity was 30 times higher than supported product yield and time, in which at the beginning of the
nickel catalysts operated under H2 atmosphere (0.48 mol·mol- reaction the yield increased with time and then reached a
1·h-1).25 These results suggested that the in situ formed Ni steady state. Higher temperatures increased the reaction rate
nanoparticles were superior to supported Ni catalysts in the and a shorter time was required to reach a steady state.
deoxygenation of stearic acid, presumably due to the very Higher temperatures also led to lower final product yields. For
small size of the Ni NPs in the system.
example, the catalytic reactions carried out at 370
could reach the steady state within 60 and 30 min,
respectively, but only 45% and 42% product yields were
℃ and 390
℃
3.4 Kinetic study
Since 1 mol% of Ni(OAc)2 exhibited the highest selectivity to achieved.
deoxygenation products at a relatively high conversion, it was
Fig. 3 The variation of product yields with time at different temperatures. (a) Total product yield at each temperature; (b) Product yield with time at 350 ℃; (c) Product yield with
time at 390 ℃; (d) Arrhenius plot of rate constants and temperature.
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To better understand the reaction sequence, the yield of can be directly produced from stearic anhydride without being
each product as a function of time was analyzed. At 330 and hydrolyzed to stearic acid. The gas phase product was also
350 , all products followed the same trend as the total analyzed to provide additional information on the reaction
product yield in the entire duration (Fig. S5a and Fig. 3b). pathway. After carrying out the deoxygenation reaction at 350
When reaction was conducted at 370 , all products showed with stearic acid as substrate for 2.5 h, the gas products
℃
℃
℃
℃
similar trend in the first 60 min. Afterwards, the total product were collected and examined with FT-IR. As expected, both
yield kept unchanged while the increase in the cracking CO2 and CO peaks appeared in the spectra (Fig. S6) with an
product yield was observed at the expense of heavy products approximate concentration ratio of 1:10. At 350 ℃, the reverse
(Fig. S5b). This phenomenon was particularly obvious in water gas shift (RWGS) reaction was thermodynamically
catalytic system at 390
℃ after 30 min (Fig. 3c). The increase in unfavorable with △G = 16.7 kJ/mol, suggesting CO was mainly
cracking product yield from heavy ones was more likely to formed from a decarbonylation process rather than from the
produce carbon coking on catalysts and cause catalyst RWGS. On the contrary, the conversion of CO to CO2 through
deactivation, which may result in the lower final yields at 370 the water gas shift reaction was likely to occur under applied
℃
and 390
℃
.
conditions. In the literature, it was proposed that alkane was
The variation of the rate constants with temperature (Table mainly generated by decarboxylation,46 however, it appears
S3) followed the Arrhenius Equation prediction. The apparent heptadecane in our system is generated mainly via a two-stage
activation energy of this reaction over nickel catalyst was route starting from stearic anhydride. The first stage was the
112.1 kJ/mol, based on fitting the parameters with a linear hydrogenation of stearic anhydride to form stearaldehyde
correlation of ln k~1/T (Fig. 3d). The activation energy of this which subsequently underwent a decarbonylation reaction to
catalytic system was significantly lower than supported nickel heptadecane. H2 was confirmed to exist in system through the
catalysts employed under hydrogen atmosphere, suggesting processes of cracking, dehydrogenation and water gas shift
again the much higher efficiency of the in situ formed Ni reaction under N2 atmosphere.38,47 Further, stearic aldehyde
nanoparticle catalysts in the present system.45
was also detected (Table 1, entry 1 and Table 2, entry 1) and
provided additional evidence for the proposed pathway.
Combined the reaction network of stearic acid deoxygenation
3.5 Reaction pathway
Since stearate ketone can be obtained from the over Ni(OAc)2 as pre-catalyst under inert gas atmosphere is
decarboxylation of stearic anhydride and stearone was illustrated in Fig. 4.
detected in current catalytic system, stearic anhydride was
proposed as an intermediate of the deoxygenation process of
stearic acid as reported in literature.46 When stearic anhydride
was used as the substrate in the absence of catalyst, the major
product was the stearate ketone and stearic acid. Only trace
amounts of C17 hydrocarbons were observed (Table 2,
entry1). These results indicated that the formation of stearic
acid and stearate ketone from anhydride can be proceeded
readily without catalyst. When 1 mol% of the nickel catalyst
was added into the reaction system, the yield of C17
hydrocarbons increased sharply within 10 min. The yield of
heptadecene and heptadecane increased to 15.6% and 22.2%,
respectively, when further prolonging the reaction to 30 min,
which is much higher than that obtained from stearic acid
Fig.4 Reaction pathway of stearic acid under N2 atmosphere. The dot line indicated the
under the same reaction conditions (Table 2, entries 2-4). This
limited reaction pathway.
phenomenon suggested that heptadecene and heptadecane
Table 2. Stearic anhydride as substrate for the deoxygenation processa
Yield / %b
Entry
Substrate
Catalyst
Time / min
Stearic
aldehyde
Cracking
0.88
Heptadecene
1.24
Heptadecane
0.83
Heavy
10.3
Stearic acidc
37.4
Stearic
anhydride
Stearic
anhydride
Stearic
1
2
-
30
10
0.03
Ni(OAc)2
0.87
8.12
11.3
10.5
42.4
3
4
Ni(OAc)2
Ni(OAc)2
30
30
1.89
1.47
15.6
5.82
22.2
7.77
10.9
0.81
29.5
-
anhydride
Stearic acid
a Reaction conditions: substrate (200 mg); catalyst amount (1 mol%); T = 350 ℃. b Determined by GC. c Determined by HPLC.
6 | J. Name., 2012, 00, 1-3
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3.6 Recyclability
ARTICLE
solvent. There was
a clear trend that a decrease in
deoxygenation catalytic activity was observed with the
decrease of carbon number in the fatty acid. This observation
was inconsistent with previous reports carried out over Pd/C
catalyst indicating that the carbon chain length did not have
any effect on the activity.48,49 This could be rationalized by
considering that the real catalyst was formed in situ under
interaction with the fatty acid substrate. When different free
fatty acids were used, the Ni nanoparticles formed in situ
varied from one another as the surfactant properties of the
system were altered, which influenced the size and
morphology of the nanoparticles,50 and ultimately the
deoxygenation activity. For unsaturated feedstocks such as
oleic acid and linoleic acid, the deoxygenation activity was low,
which was in agreement with the literature that the
adsorption of double bonds onto metal catalysts inhibit the
deoxygenation reaction.47,51 Encouragingly, Ni(OAc)2 was also
applicable in deoxygenating tristearin, which was the major
component of oil and fats (Table 3, entry 8), with comparable
yields of C16-acid and C18-acid substrates.
As shown by entry 17 in Table 1, the isolated Ni nanoparticles
after reaction was reused as catalyst and the catalytic
performance was almost same to the Ni(OAc)2. However, it is
difficult to further continue the recycling due to the very small
amount of Ni used (< 0.5 mg). To conveniently assess the long
term stability and recyclability of the catalyst, 200 mg of
stearic acid was re-added to the system after a 2 h reaction at
350
℃ without separation, and then kept under the same
conditions for another 2 h. After four repeats (Fig. 5), the
activity of nanoparticles could be maintained at around 52% ~
55% of yield without any loss. As such these Ni nanoparticles
formed in situ were very active and stable for the
deoxygenation of stearic acid to produce liquid hydrocarbons.
Conclusions
Highly effective deoxygenation of stearic acid into alkanes and
other products was successfully achieved over a small loading
of commercially available and cheap Ni(OAc)2 as pre-catalyst
under solvent and hydrogen-free conditions. The real catalytic
active species were Ni nanoparticles formed in situ, with fatty
acids acting as
a stabilizer and capping agent. These
nanoparticles are much more active than supported Ni
catalysts in previous reports and can be reused for at least four
times without any loss in activity. A new reaction pathway for
alkane formation via the intermediate of stearic anhydride
under inert gas was proposed. This study provides an
alternative way for the production of transportation fuels
featured by low cost, high activity and stability of the catalyst.
Fig. 5 Stability study of nickel catalyst for the stearic acid deoxygenation. Reaction
conditions: substrate, stearic acid; Ni(OAc)2 (1 mol%); T = 350 ℃, t = 2 h.
3.7 Substrate scope
Finally, deoxygenation of free fatty acids bearing different
carbon chain length and double bonds were studied over
Ni(OAc)2 as pre-catalyst at 350
℃ for 4.5 h without H2 and
Table 3. Catalytic activity of Ni(OAc)2 on different substratesa
Yield / %b
Heavy
Conversion /
c
Entry
Substrate
Cracking
Palmitic acidc Stearic acidc
Total
%
C
-ene
C -ane
n-1
n-1
1
2
3
4
5
6
7
8
C18-acid
C16-acid
C12-acid
4.40
2.83
1.71
0.90
2.41
1.83
3.38
14.2
23.0
19.7
5.85
1.16
30.0
18.8
3.72
1.17
4.64
4.97
4.63
1.09
-
-
-
-
-
-
-
-
-
-
62.1
46.3
15.9
4.32
28.0
14.7
3.38
59.7
67.5
53.1
15.3
3.18
73.9
32.8
5.36
-
C10-acid
Oleic acid
25.6
12.9
-
Linoleic acid
Methyl stearate
Tristearin
-
-
-
-
-
-
-
-
16.0
7.14
13.6
2.30
6.10
a
Reaction conditions: substrate (0.7 mmol); Ni(OAc)2, (1 mol%); T = 350
℃
; t = 4.5 h. b Determined by GC. c Determined by HPLC.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
27 Sun, K.; Wilson, A. R.; Thompson, S. T.; Lamb, H. H. ACS Catal.
2015, 5, 1939.
Acknowledgements
28 Kandel, K.; Frederickson, C.; Smith, E. A.; Lee, Y. J.; Slowing, I.
I. ACS Catal. 2013, 3, 2750.
29 Yang, Y.; Ochoa-Hernández, C.; de la Peña O’Shea, V. A.;
Coronado, J. M.; Serrano, D. P. ACS Catal. 2012, 2, 592.
30 P. Mäki-Arvela, M. Snåre, K. Eränen, J. Myllyoja and D. Y.
We thank the MOE Tier-1 project (WBS R-279-000-438-112),
Natural Science Foundation of China (21173009, 21222306)
and 973 Project (2011CB201402, 2013CB933100) for the
financial support. W. J. Li thanks the China Scholarship Council
for the fellowship. We also thank Dr. Kylie Luska for the proof
reading.
Murzin, Fuel, 2008, 87, 354 3.
31 H. Bernas, K. Eränen, I. Simakova, A. R. Leino, K. Kordás, J.
Myllyoja, P. Mäki-Arvela, T. Salmi and D. Y. Murzin, Fuel,
2010, 89, 2033.
32 E. W. Ping, R. Wallace, J. Pierson, T. F. Fuller and C. W. Jones
Microporous Mesoporous Mater., 2010, 132, 174.
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DOI: 10.1039/C5GC01147G
Effective deoxygenation of fatty acids over Ni(OAc)2 in the absence of H2 and solvent