Selective Hydrodeoxygenation of Vegetable Oils and Waste Cooking Oils to Green Diesel Using a Silica-Supported Ir–ReOx Bimetallic Catalyst

Article abstract of DOI:10.1002/cssc.201800321

High yields of diesel-range alkanes are prepared by hydrodeoxygenation of vegetable oils and waste cooking oils over ReO_x-modified Ir/SiO₂ catalysts under mild reaction conditions. The catalyst containing a Re/Ir molar ratio of 3 exhibits the best performance, achieving 79–85 wt % yield of diesel-range alkanes at 453 K and 2 MPa H₂. The yield is nearly quantitative for the theoretical possible long-chain alkanes on the basis of weight of the converted oils. The catalyst retains comparable activity upon regeneration through calcination. Control experiments using probe molecules as model substrates suggest that C=C bonds of unsaturated triglycerides and free fatty acids are first hydrogenated to their corresponding saturated intermediates, which are then converted to aldehyde intermediates through hydrogenolysis of acyl C?O bonds and subsequently hydrogenated to fatty alcohols. Finally, long-chain alkanes without any carbon loss are formed by direct hydrogenolysis of the fatty alcohols. Small amounts of alkanes with one carbon fewer are also formed by decarbonylation of the aldehyde intermediates. A synergy between Ir and partially reduced ReO_x sites is discussed to elucidate the high activity of Ir–ReO_x/SiO_2.

Full text of DOI:10.1002/cssc.201800321

Accepted Article
Title: Selective Hydrodeoxygenation of Vegetable Oils and Waste
Cooking Oils to Green Diesel Using a Silica Supported Ir-ReOx
Bimetallic Catalyst
Authors: Basudeb Saha, Sibao Liu, Trent Simonetti, and Weiqing
Zheng
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To be cited as: ChemSusChem 10.1002/cssc.201800321
Link to VoR: http://dx.doi.org/10.1002/cssc.201800321
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10.1002/cssc.201800321
ChemSusChem
Selective Hydrodeoxygenation of Vegetable Oils and Waste Cooking Oils to Green
Diesel Using a Silica Supported Ir-ReOx Bimetallic Catalyst
Sibao Liu, Trent Simonetti, Weiqing Zheng and Basudeb Saha*
Catalysis Center for Energy Innovation and Department of Chemical and Biomolecular
Engineering, University of Delaware, Newark, DE 19716, USA
Corresponding authors’ email: bsaha@udel.edu
Abstract
High yields of diesel ranged alkanes are achieved by hydrodeoxygenation of vegetable oils
and waste cooking oils over ReOx modified Ir/SiO2 catalysts under mild reaction conditions.
The catalyst containing a Re/Ir molar ratio of 3 exhibits the best performance, achieving
79-85 wt% yield of diesel ranged alkanes at 453 K and 2 MPa H2. The yield is nearly
quantitative to the theoretical possible long-chain alkanes on the basis of weight of converted
oils. The catalyst retains comparable activity upon regeneration via calcination. Controlled
experiments using probe molecules as model substrates suggest that C=C bonds of
unsaturated triglycerides and free fatty acids are first hydrogenate to their corresponding
saturated intermediates, which then convert to aldehyde intermediates via hydrogenolysis of
acyl C-O bonds and subsequently hydrogenate to fatty alcohols. Finally, long-chain alkanes
without any carbon loss are formed by direct hydrogenolysis of the fatty alcohols. Small
amounts of alkanes with one carbon less are also formed via decarbonylation of the aldehyde
intermediates. A synergy between Ir and partially reduced ReOx sites is discussed to elucidate
the high activity of Ir-ReOx/SiO2.
Keywords: Vegetable oils, iridium, rhenium oxide, hydrodeoxygenation, green diesel
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1. Introduction
High volatility of crude oil price due to demand-supply imbalance and global climate
change concerns necessitate the search for sustainable resources to meet the future energy
demand and combat current challenges[1-2]. Lignocellulosic biomass and food wastes
constitute inexpensive and sustainable renewable carbon source to upgrade into high quality
diesel fuels via catalysts.[3-7] The catalytic conversion of lignocellulisoc biomass to diesel fuel
ranged alkanes has been reported, which involves several steps[8-11]. In contrast, vegetable
oils and waste cooking oils (WCO), constituting triglycerides and free fatty acids as the main
components, conversion to renewable diesel involves less processing steps and can be more
energy-efficient[12-14]. Estimated supply of waste lipids, including WCO, in the U.S. exceeds
10 billion pounds/year[15]. Compared to vegetable oils, WCO conversion is more complex
because of its high water content, high acid value, and unknown impurities[15]
.
Several strategies have been reported to upgrade vegetable oils and WCO into diesel.
The transesterification of vegetable oils with alcohols gives fatty acid esters, known as the
first generation biodiesel [12]. Poor flow properties of fatty acid esters at low temperature
limit its application. The catalytic hydrotreating of vegetable oils over zeolites, such as
HZSM-5, to produce gasoline and diesel is an alternative strategy[16-17]. However, the low
selectivity of desired products and coking of the catalysts are the drawbacks of the later
process. Hydrodeoxgenation (HDO) of vegetable oils and WCO into long chain hydrocarbons
(C12~C22) with high conversion and high selectivity is the most attractive strategy. The long
chain alkanes have better flow properties at low temperature, better adaptation as diesel fuels,
and better compatibility with current infrastructures and engines. Sulfided Ni-Mo and Co-Mo
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are conventional hydrotreating catalysts[18-20], but harsh reaction conditions and sulfide
leaching into alkanes hinders their application. Supported transition-metal catalysts
containing Ni-[21-26], Fe-[27], Ru-[28], Pt-[29] and Mo carbides[30] have been used for the HDO of
oils. The latter catalysts required harsh reaction conditions (>523 K and 5 MPa H2) to achieve
reasonable performance, and also caused some carbon loss because decarboxylation and/or
decarbonylation of fatty acids and esters are favored as high temperatures. .
For energy- and process-efficiency, the catalytic conversion of vegetable oils and
WCO under milder conditions is desirable. Recently, Guo et al. reported good activities of
Ru/La(OH)3[31] and Ru-Hydroxyapatite (HAP)[32] catalysts in the HDO of different vegetable
oils and WCO at 453K, achieving ~80% yield of diesel ranged alkanes. However, the
decarbonylation reaction, resulting in carbon loss in the final products, was a dominated
pathway. Although a bi-functional Pd/NbOPO4 catalyzed conversion of vegetable oils
produced hydrocarbon with higher carbon efficiency (~84 wt% yield) at reasonably mild
conditions (453 K, 3 MPa H2) [33], long reaction time (24 h) due to slow reaction rate was a
drawback. Therefore, utilization of efficient catalyst for the HDO of vegetable oils and WCO
to diesel hydrocarbons with high yield and selectivity at mild conditions is desirable.
Ir-ReOx/SiO2 with different Ir/Re ratios have been reported to be effective catalysts
for the HDO of polyols[34], sugars[34], hemicellulose[35], cellulose[36-37], angelica lactone
dimer[38] and high carbon furylmethane[39] to hydrocarbons with very low C-C cracking.
These catalysts are also effective for selective hydrogenation of unsaturated aldehydes to
unsaturated alcohols[40-41]. Spectroscopic characterizations indicated that Ir metal sites are
partially covered with three-dimensional ReOx clusters[42] and the catalyst mainly has Lewis
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acidity[39], which can weaken the carbonyl group of carboxylic acid and ester, and promote
C-O bond cleavage during HDO or hydrogenation of fatty acid and ester[43-46]. Thus, we
hypothesize that Ir-ReOx/SiO2 can be an effective catalyst for the HDO of vegetable oils and
WCO to produce alkanes with high carbon efficiency.
In this paper, we report for the first time that Ir-ReOx/SiO2 is effective to
hydrodeoxygenate vegetable oils (palm oil, soybean oil, rapeseed oil, sunflower oil) and
WCO at low temperature (453 K) and low hydrogen pressure (2 MPa), and can produce
diesel ranged alkanes with high yield (up to 85%) and selectivity. Controlled experiments
using model substrates as probe molecules are conducted to elucidate the reaction pathway
involving hydrogenation, hydrogenolysis and decarbonylation steps.
2. Experimental
2.1. Catalyst Preparation
The Ir-ReOx/SiO2 catalysts with varying Re to Ir ratios (0 to 3) were prepared by a
sequential impregnation method described previously[39]. First, Ir/SiO2 was prepared by
impregnating Ir on SiO2 (Fuji Silysia G-6 which was pre-calcined in air at 973 K for 1 h at a
heating rate of 10 K /min, BET surface area 535 m2/g) using an aqueous solution of H2IrCl6
(Sigma-Aldrich). After evaporating the solvent at 348 K and drying at 383 K for 12 h in an
oven, the resulting Ir/SiO2 was impregnated with ReOx using an aqueous solution of
NH4ReO4 (Alfa-Aesar). The catalysts were calcined in a crucible in air at 773 K for 3 h using
a 10 K /min temperature ramp. Other metals impregnated catalysts, M-ReOx/SiO2 (M=Rh, Ru,
Pt and Pd, Re/M=3), were prepared with the same method. The metal precursors of the
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Rh-ReOx, Ru-ReOx, Pt-ReOx and Pd-ReOx catalysts were RhCl3 (Alfa-Aesar), RuCl3
(Sigma-Aldrich), H2PtCl6 (Sigma-Aldrich) and PdCl2 (Sigma-Aldrich), respectively. The
reported metal loadings are based on the theoretical amount of metals used for impregnation.
All the catalysts were used in powdered form with a granule size of < 400 mesh.
2.2. Catalysis
The conversion of stearic acid and different oils over catalysts was performed in a 50
mL Parr reactor with an inserted Teflon liner and a magnetic stirrer. First, catalyst (0.05 g)
and solvent (cyclohexane, 10 mL) were added to the reactor for the catalyst pre-reduction and
the reactor was sealed with the reactor head equipped with a thermocouple, a rupture disk, a
pressure gauge, and a gas release valve. The mixture was heated at 473 K and 2 MPa H2 for 1
h at 240 rpm. Upon pre-reduction, the reactor was cooled to room temperature and H2 was
released. Then, stearic acid or oils (0.25 g) was added, closed the reactor head immediately,
purged the reactor with H2 (1 MPa) for three times, pressurized to 2.0 MPa H2, and heated the
reaction mixture to the desired temperature with continuous stirring at 500 rpm. The heating
time to reach the set temperature was about 25 min. After reaction for the set time, the reactor
was immediately transferred to a water bath. The catalyst was separated from the solution by
centrifugation or filtration.
The products were analysed by a gas chromatograph (GC, Agilent 7890A) equipped
with an HP-1 column and a flame ionization detector (FID) using icosane (C20) as an internal
standard. A GC (Agilent 7890B) mass spectrometer (Agilent 5977A with a triple-axis
detector) equipped with a DB-5 column was used for identification of unknown peaks. Before
analysis, samples were silylated using BSTFA (N, O-bis(trimethylsilyl)-trifluoroacetamide)
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as a silylation agent. Controlled experiments suggested the conversion a small amount of
solvent (cyclohexane) to n-hexane and low carbon alkanes over Ir-ReOx/SiO2. Thus, C1-C6
alkanes were not quantified in the present study.
The conversion and the yield of all products from the HDO of stearic acid, methyl
stearate and n-octadecanol were calculated on carbon basis using the following equations:
The yields of all products from the conversion of various vegetable oils and WCO were
calculated on weight basis using the following equation:
where m is weight
Spent catalysts were washed with excess cyclohexane, dried in air, and calcined at 773 K
for 3 h before to reuse in the next cycle. A slight loss in the catalyst mass (<12%) was
observed during the recovery in each cycle.
2.4. Catalyst Characterization
X-ray diffraction (XRD) patterns of the reduced and used catalysts were recorded by
a diffractometer (Bruker D8) equipped with a Cu Kα (λ . 54 ) 4 kV
and 40 mA. XPS experiments were conducted using a Thermo-Fisher K-alpha+X-ray
photoelectron spectrometer equipped with a monochromatic aluminum K-alpha X-ray source
(400 µm). Before XPS analysis, the Ir-ReOx/SiO2 catalyst was reduced in cyclohexane at 473
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K for 1 h following the procedure detailed in the catalysis section. The pre-reduced sample
was transferred into the K-Alpha+ Vacuum Transfer Module (VTM, ThermoScientific)
without being exposed to air. The microstructures of pre-reduced fresh catalyst and the
recovered catalyst after reaction were examined using a field emission transmission electron
microscope JEM-2010F FasTEM at 200 kV.
3. Results and Discussion
3.1 Catalyst screening
Because vegetable oils and WCO contain triglycerides and fatty acids and their
conversion involve several sequential steps, first stearic acid was selected as a model
compound to screen the HDO performance of the M-ReOx/SiO2 (M=Rh, Ru, Pt and Pd)
catalysts (Table 1). After 1 h reaction, the main products are n-octadecane, n-heptadecane,
n-hexanedecane and 1-octadecanol. Ir-ReOx/SiO2 and Ru-ReOx/SiO2 achieve quantitative
stearic acid conversion and high selectivity to C18 alkane (94%) as the sole product over the
Ir-ReOx/SiO2 catalyst (entry 1) or a mixture of C18 and C17 alkanes (total 93%) using the
Ru-ReOx/SiO2 catalyst (entry 2). Pt-ReOx/SiO2 and Pd-ReOx/SiO2 are less active and give
1-octadecanol as the dominate product (entries 3 and 4). The results of the later two catalysts
are consistent with prior reports of Pd-Re and Pt-Re bimetallic catalyzed hydrogenation of
carboxylic acid and ester[47-50]. Rh-ReOx/SiO2 shows moderate activity, giving 29% total
selectivity to C18 and C17 alkanes with C17 being the major alkane (entry 5). Similar to
Rh-ReOx/SiO2, the Ru-ReOx/SiO2 catalyzed reaction forms a significant amount of C17 alkane,
likely via decarbonylation and decarboxylation of stearic acid. Prior studies using Ru/HAP[32]
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Table 1. HDO of stearic acid over different catalysts.
Selectivity (%)
n-C17
Re/M
Conv.
(%)
Entry
Catalysts
molar
n-C18
alkane
94
n-C16
alkane
0.6
4.1
0.0
0.2
0.2
0.2
0.5
0.3
0.3
0
Stearic
alcohol
0.0
0.0
95
ratio
3
alkane
5.8
41
1
2
Ir-ReOx/SiO2
Ru-ReOx/SiO2
Pd-ReOx/SiO2
Pt-ReOx/SiO2
Rh-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Rh-ReOx/SiO2
Ir/SiO2
100
100
15
3
52
3
3
2.2
12
2.6
4.1
22
4
3
11
84
5
3
48
7.2
28
70
6
0.5
1
25
8.2
6.9
5.3
5.0
47
64
7
32
27
66
8
2
70
43
52
9
2.5
0.5
0
99
47
47
10
11
12
5.2
0.4
5.6
5.6
5.6
49
48
37
0.0
0.0
0.0
14
ReOx/SiO2
-
6.0
22
1.3
7.0
93
Ir/SiO2
-
71
13a
+ReOx/SiO2
Reaction conditions: catalyst (0.05 g), M: Ir, Rh, Ru, Pt and Pd (4 wt%), stearic acid (0.25 g), cyclohexane
(10 mL), initial H2 (2 MPa), 453 K, 1 h. a: physical mixture of Ir/SiO2 and ReOx/SiO2 with the same metal
amount and ratio as Ir-ReOx/SiO2 (Re/Ir=3).
and Ru/La(OH)3[31] catalysts demonstrated C17 alkane formation. Ru-catalysts also caused
high C-C cracking[51-53]. In contrast, Ir-ReOx/SiO2 formed C18 alkane with very high
selectivity.
Next, the catalytic activity of Ir-ReOx/SiO2 was assessed at different Re/Ir molar
ratios (entries 1 and 6-9). The conversion of stearic acid and the yields of C18 alkane
increased with an increase in Re/Ir molar ratio, indicating the higher amount of Re is
beneficial to the HDO activity and consistent with previous studies of Ir-ReOx/SiO2 catalyzed
cellulose[36] and high carbon furylmethanes[39] HDO. The stearic acid is fully converted to
alkanes using the catalyst containing Re/Ir molar ratio of 3 (entry 1), which was chosen for
subsequent experiments. The effect of Re amount in the Ir-ReOx/SiO2 and Rh-ReOx/SiO2
catalysts has been studied for the hydrogenolysis of polyols and cyclic ethers in aqueous
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phase[54-55]. The Ir-ReOx catalyst achieved optimal activity at Re/Ir molar ratios of 1 - 2[56-57]
,
while the Rh-ReOx catalyst reported maximum activity at Re/Rh molar ratio of 0.5.[58-62] In
the HDO of stearic acid, higher ratio of Re/Ir likely ensures enough surface active sites
(Lewis acid sites from partially reduced ReOx as discussed below) for absorption of carbonyl
group of oils to activate the reaction. This interpretation is consistent with prior studies of
carboxylic acids or esters over Pd-Re[47-49] and Pt-Re[50] catalysts. Likewise, the
Rh-ReOx/SiO2 catalyst also showed higher performance at higher molar ratio of Re/Rh (3)
(entries 5 and 10).
Ir/SiO2 alone is ineffective (entry 11) while ReOx/SiO2 achieves only 5.6% conversion
with 1-octadecanol as the main product (entry 12). A physical mixture of Ir/SiO2 and
ReOx/SiO2 at the same Re/Ir ratio as Ir-ReOx/SiO2 achieves similar stearic acid conversion
(5.6%) as ReOx/SiO2. The later reaction improved selectivity to C18 alkane, suggesting
Ir/SiO2 is beneficial to promote hydrogenolysis of 1-octadecanol. Importantly, Ir-ReOx/SiO2
outperformed the physical mixture of Ir/SiO2 and ReOx/SiO2, indicating a strong synergy
between Ir and ReOx.
3.3 Effect of Reaction Temperature and H2 Pressure
The effect of the reaction temperature on the HDO of stearic acid is shown in Table
2. The conversion of stearic acid and the yield of C18 alkane increase as the reaction
temperature increases from 413 K to 453 K (entries 1, 3 and 5). Stearic acid is fully converted
≥ 453 K within 1 h at 2 MPa of H2 (entries 5 and 6). At lower temperature, complete
conversion of stearic acid took longer time, e.g., 5 h at 413 K (entry 2) and 3 h at 423 K
(entry 4). The results show a slight decrease of C18 alkane selectivity (from 97% to 92%) and
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a slight increase of C17 alkane selectivity (from 2% to 7%; entries 2, 4, 5 and 6) as the
temperature increases from 413 K to 473 K, which is likely due to enhanced decarbonylation
reaction at higher reaction temperatures. Initial hydrogen pressure (0.6 MPa to 2 MPa) has a
significant effect on the conversion of stearic acid (53% to 100%) and the selectivity of C18
Table 2. The effects of temperature, H2 pressure and reaction time on HDO of stearic acid over
Ir-ReOx/SiO2 (Re/Ir = 3).
T
P
Selectivity (%)
Time
(h)
Conv.
(%)
(K) (MPa)
Entry
Catalysts
n-C18
alkane
21
n-C17
n-C16
alkane
0.2
Stearic
alcohol
76
alkane
2.1
2.3
2.9
3.9
5.8
7.3
6.8
10
1
2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
Ir-ReOx/SiO2
1
5
1
3
1
1
1
4
1
2
1
413
413
433
433
453
473
453
453
453
453
453
2
2
38
100
68
97
0.4
0.2
3
2
28
0.1
67
4
2
100
100
100
53
96
0.4
0.0
5
2
94
0.6
0.0
6
2
92
0.8
0.1
7
0.6
0.6
1
38
0.3
55
8
100
74
89
0.8
0.3
9
41
6.0
7.6
4.6
0.3
53
10
11
1
100
100
92
0.6
0.3
3
95
0.4
0.0
Reaction conditions: catalyst (0.05 g), stearic acid (0.25 g), cyclohexane (10 mL).
alkanes (38% to 94%) (entries 7, 9 and 5). The C-C cracking products (mainly C17 alkane)
decrease with increasing H2 pressure (entries 8, 10, 5 and11) due to the positive reaction
order with respect to H2 pressure, consistent with prior reports[39]. The results suggest lower
reaction temperature and higher hydrogen pressure are beneficial for selective production of
C18 alkane.
3.4 HDO of vegetable oils and WCO
Different vegetable oils, e.g. palm oil, soybean oil, sunflower oil and rapeseed oil,
and WCO were hydrotreated with Ir-ReOx/SiO2 at 453 K and 2 MPa H2 (Table 3). Fatty acids
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compositions of these oils are shown in Table S1. After 3 h reaction, vegetable oils give high
yields of diesel-range alkanes (79-85%; entries 1-4 in Table 3).
Table 3. HDO of vegetable oils and WCO over Ir-ReOx/SiO2 (Re/Ir = 3).
Time
(h)
Yield (wt%)
n-C17
alkane
1.9
Entry
Oils
Total
alkanes
85
n-C18
alkane
46
n-C16
alkane
35
n-C15~C9
alkane
2.5
1
2
Palm oil
Soybean oil
Sunflower oil
Rapeseed oil
WCO
3
3
84
71
3.1
9.2
0.8
3
3
85
75
3.4
5.5
0.7
4
3
79
71
3.6
4.3
0.6
5
3
20
17
1.1
2.2
0.2
6
7a
WCO
18
82
69
3.9
7.9
1.1
Palm oil
3
54
28
1.3
23
1.7
Reaction condition: catalyst (0.05 g), oil (0.25 g), cyclohexane (10 ml), 453 K, initial H2 (2 MPa), a1 mg
NaCl
In contrast, WCO conversion achieves only 21 wt% yield of diesel-ranged alkanes in 3 h
(entry 5, Table 3). We hypothesize that impurities in WCO could inhibit the reaction. The
HDO activity of Ir-ReOx/SiO2 has been reported to be severely deactivated by the alkali
metal and alkali earth metal cations such as Na+ and Ca2+.[35] During cooking, various
seasonings including salt (NaCl), are added in the oils, which might slow down the HDO rate
of Ir-ReOx/SiO2. To evaluate the effect of seasonings, we repeat palm oil conversion (entry 1,
Table 3) in the presence of 1 mg NaCl, which yielded significantly less alkanes (54%; entry 7
in Table 3) as opposed to 85% in the absence of NaCl. A reaction for a longer time improves
the yield of diesel ranged alkanes (82 wt%, entry 6), which is comparable to the results of
vegetable oils achieved in 3 h. These values (yields) are nearly quantitative to the theoretical
possible yield of long chain alkanes (~85 wt%) on the basis of weight of converted oils. Our
maximum alkanes yields are comparable to the results obtained by a Pd/NbOPO4 catalyst,
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however, our reaction time (3 h) is much shorter than that used for the Pd/NbOPO4 catalysis
(24 h).[33] In addition, the Ir-ReOx/SiO2 catalysis gives alkanes with higher selectivity and
slightly higher yields than that reported by a Ru/HAP catalyst[32] (Table S2) under
comparable reaction conditions (Table S2, entries 26 and 27). Furthermore, our reaction
temperature (453 K) is much lower than those used in prior studies (≥ 523 K)[21-30] over Ni,
Mo, Fe and Ru based catalysts, underscoring the effectiveness and high selectivity of
Ir-ReOx/SiO2 for the HDO of vegetable oils and WCO.
3.5 Reaction network and proposed mechanism
To map the reaction network, the time-course experiments of palm oil and WCO
were conducted. The reaction profiles of both oils are similar (Figure 1). The dominant
products are C18 and C16 alkanes, which are formed simultaneously. Small amount of C17
alkane is also formed. Several saturated esters (n-propyl stearate, iso-propyl stearate, n-propyl
palmitate and iso-propyl palmitate), stearic acid, and alcohol (stearic alcohol) intermediates
are detected. The concentrations of these intermediates are significantly low at any given time
(within 5 wt%). The ester is present in higher quantity than fatty acid or alcohol in the
solution suggesting that fatty acid or alcohol are converted faster after they are formed.
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90
80
70
60
50
40
30
20
10
0
5
4
3
2
1
0
(b)
(a)
0
0.5
1
1.5
Time / h
2
2.5
3
0
0.5
1
1.5
Time / h
2
2.5
3
90
80
70
60
50
40
30
20
10
0
5
4
3
2
1
0
(c)
(d)
0
3
6
9
12
15
18
0
3
6
9
12
15
18
Time / h
C18
Time / h
Total alkanes
C16
C17
C18OH
C17COOH
Esters
C15-C9
Figure 1. Time-profiles of palm oil (a, b) and WCO (c, d) conversions. Reaction condition:
catalyst (0.05 g), oil (0.25 g), cyclohexane (10 mL), 453 K, initial H2 (2 MPa). The esters
included n-propyl stearate, iso-propyl stearate, n-propyl palmitate and iso-propyl palmitate.
Palm oil and WCO mainly contain triglycerides and a very small amount of free
fatty acids (0.4 wt% and 1.4 wt%, respectively). As saturated esters and alcohol intermediates
are detected during the reaction, in addition to fatty acid, we used different oxygenates,
n-octadecanol, stearic acid and methyl stearate as probe substrates to understand the reaction
network. The reaction of n-octadecanol as a probe substrate shows its full hydrogenation to
n-octadecane within 10 minutes, suggesting the primarily alcohol hydrogenolysis by
Ir-ReOx/SiO2 in alkane solvent is fast as reported previously by the Tomishige group[63]
13
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ChemSusChem
Mechanistically, the primary alcohol conversion over the Ir-ReOx/SiO2 catalyst has been
proposed to proceed via a direct C-O hydrogenolysis route[63]. Pure stearic acid conversion
gives two main products, 1-octadecanol and n-octadecane, and a trace amount of
n-heptadecane (Figure 2a). In the beginning, 1-octadecanol selectivity is high because of its
slow hydrogenolysis in the present of stearic acid. The hydrogenolysis rate of 1-octadecanol
speeds up after complete conversion of stearic acid. This could be attributed to stronger
adsorption of
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
(a)
(b)
0
10
20
30
40
50
60
0
30
60
90
120 150 180
Time / min
Time / min
C17
Conv.
C18
C17
C16
C18OH
Conv.
C18
C16
C18OH
Figure 2. Time-profiles of stearic acid (a) and methyl stearate (b) conversions. Reaction
conditions: catalyst (0.05 g), stearic acid or methyl stearate (0.25 g), cyclohexane (10 mL), 453
K, initial H2 (2 MPa).
stearic acid on the catalyst surface and blocking the active sites of Ir-ReOx/SiO2. Pure methyl
stearate conversion (Figure 2b) is slower than that of stearic acid. The main product is
n-octadecane and its selectivity is within 88-92%, with balance carbon being small amounts
of C17 alkane and C18-alcohol, at different conversions of methyl stearate. Stearic acid was
not detected as an intermediate, indicating a cleavage of acyl C-O bond likely occurred and
the resulting 1-octadecanal is converted to C18 alkane via its hydrogenation to 1-octadecanol
followed by hydrogenolysis. Similar to methyl stearate, oils conversion to the dominant
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alkanes is likely occur via direct hydrogenolysis of acyl C-O bonds of esters. Trace amount of
detected stearic acid could be formed from hydrogenation of free fatty acids present in oils.
To understand whether C17 alkane is formed via decarboxylation or decarbonylation, stearic
acid and methyl stearate conversions were conducted under nitrogen atmosphere in which the
conversion of these substrates to the corresponding aldehyde can be precluded in the absence
of H2 and only decarboxylation is expected. We observed no decarboxylation products (C17
alkane and CO2) under N2 suggesting C17 alkane in our reaction is likely formed via
decarbonylation pathway.
Based on the results of controlled experiments, a reaction pathway for the HDO of
vegetable oils and WCO is proposed (Scheme 1). The first step is the hydrogenation of C=C
bonds of unsaturated triglycerides and free fatty acids to their corresponding saturated
analogues. Saturated triglycerides convert to alkanes without any carbon loss via direct
hydrogenolysis of acyl C-O bonds of three ester groups, followed by sequential
hydrogenation of the resulting aldehyde to alcohols and hydrogenolysis of the alcohol.
Concurrently, 1,2-propane diester and 1,3-propane diester and their mono-ester intermediates
(e.g. n-propyl stearate, iso-propyl stearate, n-propyl palmitate and iso-propyl palmitate) are
formed and they convert to long chain alkanes and propane in the same manner. In the case of
fatty acids, the hydrogenation of the carbonyl groups of acids results in formation of the
corresponding fatty alcohols via aldehyde intermediates, which are then hydrogenolyzed to
alkanes. A minor route of decarbonylation of the aldehyde intermediates formed alkanes with
one carbon atom less. Prior studies reported the conversion of triglycerides to fatty acids
occurs via hydrolysis in the first step[21-22, 31-32]
15
.
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H2
H2
H2
H2
H2
H2
Free fatty acids
H2
H2
- CO
R=: unsaturated alkyl chain (mainly C15 and C17
)
Scheme 1. Proposed reaction route for the HDO of vegetable oils and WCO.
To elucidate the relationship between the structures of the catalysts and their
performances, we characterized pre-reduced fresh and recovered Ir-ReOx/SiO2 by XRD,
TEM and XPS. The XRD patterns (Figure 3), show characterized Re metal peaks of
ReOx/SiO2 2θ 37.5o, 40.4o, 42.8o 56.4o and 67.5o.[39] However, these peaks for Re are
absent in Ir-ReOx/SiO2 (Re/Ir=3), suggesting Re species could be highly dispersed in the
presence of Ir and the high dispersity provides enhanced performance of the bimetallic
catalyst. The peak position of Ir in Ir-ReOx/SiO2 is almost the same as Ir/SiO2 without any
peak shift, which indicates no alloy formation between Re and Ir in Ir-ReOx/SiO2. The XRD
pattern of Ir-ReOx/SiO2 after the reaction is very similar to that of pre-reduced fresh
Ir-ReOx/SiO2. TEM images (Figure 4) of the fresh (pre-reduced) and spent catalysts (Re/Ir=3)
evidenced the presence of Ir particles of average particle size of 2.8 nm and 3.0 nm,
respectively.
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10.1002/cssc.201800321
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Ir
Re
d
c
b
a
30
40
50
60
70
2 Theta / degree
Figure 3. XRD patterns of (a) ReOx/SiO2, (b) Ir/SiO2, (c) pre-reduced fresh Ir-ReOx/SiO2
(Re/Ir=3), and (d) spent catalyst after the HDO of palm oil at 453 K and 2MPa H2 for 3 h.
a
b
Figure 4. TEM images of Ir-ReOx/SiO2 (Re/Ir=3): (a) fresh (pre-reduced) and (b) spent
catalysts after the HDO of palm oil at 453 K for 3 h under 2 MPa H2.
X-ray photoelectron spectroscopy was carried out to ascertain the oxidation states of
Ir and Re in pre-reduced Ir-ReOx/SiO2 (Figure 4). The Ir 4f spectrum reveals the binding
energies for 4f7/2 and 4f5/2 peaks of ~60.8 and 63.8 eV, respectively, indicating Ir exists in
metallic state[64-65]. The Re 4f spectrum shows peaks for Re0, Re2+, Re4+, Re6+, and Re7+ as
per their literature BE values[66]. The analysis suggests a significant portion of Re is partially
reduced to Re2+, Re4+ and Re6+ (27%, 17% and 4%, respectively), which is consistent with
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10.1002/cssc.201800321
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our prior report of the Ir-ReOx/SiO2 catalyzed HDO of high carbon furylmethanes[35]. After
reaction, the iridium is still in metallic state while a small amount of Re7+ is partially reduced
to Re2+, Re4+ and Re6+ with their respective amount of 28%, 25% and 3%, which are slight
higher than that of the pre-reduced fresh catalyst. However, the amount of metallic Re (Re0)
is the same in the fresh and spent catalysts. In-situ FTIR analysis of Py-adsorbed
Ir-ReOx/SiO2 mainly exhibit Lewis acidity, due to partially reduced ReOx species[39]
.
Re 4f
(b)
Ir 4f
(a)
Ir0 4f7/2
ReVI
ReIV
Ir0 4f5/2
ReII
Re0
ReVII
Ir 4f
(c)
Re 4f
(d)
68 67 66 65 64 63 62 61 60 59 58
Binding Energy / eV
54 52 50 48 46 44 42 40 38
Binding Energy / eV
Figure 5. Deconvoluted core-level Ir 4f and Re 4f XPS spectra of Ir-ReOx/SiO2 (Re/Ir=3).
(a,b) for the pre-reduced fresh catalyst, and (c, d) for the spent catalyst after the HDO of palm
oil at 453 K and 2MPa H2 for 3 h.
Based on the reaction performance and characterization of the catalyst, we propose a
concerted catalysis by Ir and highly dispersed ReOx (Scheme 2). The partially reduced ReOx,
exhibiting Lewis acidity, interacts with‘O’of carbonyl group of fatty acids or esters. Similar
interaction between ReOx and carbonyl group of unsaturated aldehydes has been demonstrated by
in-situ FT-IR[41] study. This interaction weakens the C=O bond and promotes acyl C-O bond
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10.1002/cssc.201800321
ChemSusChem
cleavage. Then the adjacent Ir metal provides the dissociated hydrogen and cleavages the acyl C-O
bond of fatty acids or esters and form a fatty aldehyde and water or an alcohol. This synergy
between hydrogenation metal (e. g., Pt and Ru) and partially reduced metal oxides (e.g., ReOx,
MoOx, SnOx and NbOx) has been recently demonstrated for the hydrogenation/HDO of carboxylic
acids and esters[43-46, 67-68]. The fatty aldehyde converts to alkane by pathways discussed above
(Scheme 1).
Hydrogenolysis
+
Ren+
Ir
Scheme 2. Proposed cooperativity between Ir and reduced ReOx in cleaving acyl C-O bond
of carboxylic acid or ester.
3.4 Catalytic stability
Catalyst reusability is important in determining its potential for practical applications.
The results of the reused Ir-ReOx/SiO2 (Re/Ir = 3) are shown in Figure 5. After the 1st cycle,
the catalyst is separated from the products by centrifugation, washed with cyclohexane, dried
at 383 K overnight and then calcined at 773 K in air for 3 h before to reuse in the next cycle.
A small loss of catalyst mass (~12 wt%) is observed during recovery. We decreased the scale
of each experiment to keep the ratios of catalyst, substrate and solvent same at each run. The
recovered catalyst was pre-reduced before every cycle, as discussed in the experimental
section. Palm oil was used as a substrate for the catalyst stability study. The total yield of
diesel ranged alkanes decreased from 85 wt% to 77 wt% after first recycle and remained
comparable in the third and fourth cycles. Thus, the catalyst has good stability. The XRD
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10.1002/cssc.201800321
ChemSusChem
peaks of the recovered catalyst assigned to the metallic Ir phase became sharper (Figure S2).
TEM images of the catalyst after the fourth cycle show an increase of average particle size
(~4.2 nm; Figure S3), indicating the sintering of the catalyst during the reaction and
regeneration, which is consistent with previous reports[35-39]. The sintering of the catalyst
could cause a slight activity loss.
90
80
70
60
50
Esters
40
30
20
10
0
C18OOH
C18OH
C9-C15
C16
C17
C18
1
2
3
4
Run
Figure 5. The performance of the catalyst in the recyclability experiments. Reaction
conditions: catalyst (0.025 g), palm oil (0.25 g), cyclohexane (10 mL), 453 K, initial H2 (2
MPa) and 3 h.
4. Conclusions
We reported an efficient catalytic system for hydrodeoxygenation of various vegetable oils
(palm oil, soybean oil, rapeseed oil and sunflower oil) and waste cooking oil to diesel-ranged
alkanes under mild conditions over the Ir-ReOx/SiO2 catalyst. High yield (79~85 wt%) of
alkanes is obtained at an optimal Re/Ir molar ratio of 3. The reaction follows a consecutive
pathway in which unsaturated triglycerides and free fatty acids first hydrogenate to saturated
triglycerides and saturated fatty acids, and then convert to fatty alcohols via acyl C-O bonds
hydrogenolysis of esters or acids, followed by hydrogenation of the resulting aldehyde
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intermediates. Finally, alkanes (without carbon loss) are formed by hydrogenolysis of the
alcohol. A small amount of alkane with one carbon less is formed via decarbonylation. Ir and
ReOx sites of the catalyst act synergistically in which Ir dissociates H2 and partially reduced
ReOx activates carbonyl groups of carboxylic acids or esters of substrates.
Acknowledgements
This work was supported as part of the Catalysis Center for Energy Innovation, an
Energy Frontier Research Center funded by the U. S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Award Number DE-SC0001004. The authors
acknowledge the Advanced Materials Characterization Laboratory at the University of
Delaware for providing XRD facility and Jeffrey Everhart for helping with GC/GCMS
analysis of the reaction products. We also acknowledge instrumentation support for XPS
(funded by NSF award number 142149).
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Table of Content
HDO
Ir-ReOx/SiO2
Waste cooking
/vegetable oils
Diesel-range
alkane
27
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