Production of Bio Hydrofined Diesel, Jet Fuel, and Carbon Monoxide from Fatty Acids Using a Silicon Nanowire Array-Supported Rhodium Nanoparticle Catalyst under Microwave Conditions

Article abstract of DOI:10.1021/acscatal.9b04784

Biodiesel was efficiently produced from biomass fatty acids using renewable gas H₂ and a reusable heterogeneous catalyst under low-energy-consumption microwave conditions. As the decarboxylation of fatty acids to alkanes is an important transformation in the production of bio hydrofined diesel (BHD) and jet fuel, we herein report the development of a highly active and reusable Rh nanoparticle catalyst supported by a silicon nanowire array (SiNA-Rh) and its application in the decarboxylation of fatty acids to alkanes under mild conditions. More specifically, SiNA-Rh (500 mol ppm) selectively promoted the hydrogenative decarboxylation reaction at 200 °C under microwave irradiation (～40 W) in a H₂ atmosphere (10 bar) to afford the corresponding alkanes in high yields selectively. The only coproduct observed was carbon monoxide, an important and essential staple for the chemical industry. Importantly, carbon dioxide formation was not observed. Moreover, the aldehydes were efficiently converted to alkanes by SiNA-Rh, and this catalyst was reused 20 times without any loss in catalytic activity. Finally, to investigate the effects of microwave irradiation on the enhancement of this chemical transformation based on the Si nanorod structures present in the SiNA-Rh catalyst, the effect of the microwave electric field and magnetic field in the microwave to the reaction was experimentally investigated, and the spatial distribution of the electric field intensity around the surface of the Si nanostructure was simulated using the finite element method.

Full text of DOI:10.1021/acscatal.9b04784

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Article
Production of Bio Hydrofined Diesel, Jet Fuel, and Carbon Monoxide
from Fatty Acids Using a Silicon Nanowire Array-Supported
Rhodium Nanoparticle Catalyst under Microwave Conditions
Heeyoel Baek, Keiichiro Kashimura, Takashi Fujii, Shuntaro Tsubaki, Yuji Wada,
Shigenori Fujikawa, Takuma Sato, Yasuhiro Uozumi, and Yoichi M. A. Yamada
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b04784 • Publication Date (Web): 02 Jan 2020
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ACS Catalysis
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Production of Bio Hydrofined Diesel, Jet Fuel, and Carbon Monoxide
from Fatty Acids Using a Silicon Nanowire Array-Supported Rhodium
Nanoparticle Catalyst under Microwave Conditions
Heeyoel Baek,† Keiichiro Kashimura,§ Takashi Fujii,§ Shuntaro Tsubaki,∏ Yuji Wada,∏ Shigenori Fujikawa,¶
Takuma Sato,† Yasuhiro Uozumi,†‡Yoichi M. A. Yamada†*
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† RIKEN Center for Sustainable Resource Science, Wako, Saitama 351-0198, Japan
§ Faculty of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan
∏ School of Materials and Chemical Technology, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
¶ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan
‡ Institute for Molecular Science, Okazaki, Aichi 444-8787, Japan
Keyword: rhodium, silicon, nanowire, decarboxylation, microwave, fatty acids, biodiesel fuel
E-mail: ymayamada@riken.jp
ABSTRACT Biodiesel was efficiently produced from biomass fatty acids using renewable gas H2 and a reusable
heterogeneous catalyst under low-energy-consumption microwave conditions. As the decarboxylation of fatty acids to
alkanes is an important transformation in the production of bio hydrofined diesel (BHD) and jet
fuel, we herein report the development of a highly active and reusable Rh nanoparticle catalyst
supported by a silicon nanowire array (SiNA-Rh) and its application in the decarboxylation of
fatty acids to alkanes under mild conditions. More specifically, SiNA-Rh (500 mol ppm)
selectively promoted the hydrogenative decarboxylation reaction at 200 °C under microwave
irradiation (~40 W) in a H2 atmosphere (10 bar) to afford the corresponding alkanes in high
yields selectively. The only co-product observed was carbon monoxide, an important and
essential staple for the chemical industry. Importantly, carbon dioxide formation was not
observed. Moreover, the aldehydes were efficiently converted to alkanes by SiNA-Rh, and this
catalyst was reused 20 times without any loss in catalytic activity. Finally, to investigate the effects of microwave
irradiation on the enhancement of this chemical transformation based on the Si nanorod structures present in the SiNA-
Rh catalyst, the effect of the microwave electric field and magnetic field in the microwave to the reaction was
experimentally investigated, and the spatial distribution of the electric field intensity around the surface of the Si
nanostructure was simulated using the Finite Element Method.
INTRODUCTION
since the stability and combustibility of FAME are poorer
than that of BHD, the development of suitable catalysts for
the efficient production of BHD is of particular importance
in the context of renewable energy sources. In this context,
a number of homogeneous12 and heterogeneous catalysts
have been reported.13–21 For example, Vardon et al.22
reported that the decarboxylation of stearic acid using a Pt-
Re/C catalyst resulted in the highly selective formation of
C17 alkanes, while Kaneda et al.23 and Zhang et al.24 reported
the decarboxylation of fatty acids using Ru-CeO2 and Ru-
HAP catalysts, respectively, to yield BHD. However, the
development of highly active, reusable, and selective
The decarboxylation of fatty acids to yield alkanes is an
important reaction not only in organic synthesis but also in
the production of biodiesel fuel (BDF).1–4 Indeed, the use of
free fatty acids is particularly desirable, because they are
readily available through the decomposition of vegetable
oils by enzymes. For example, following the harvest of palm
fruit from palm trees, the palm oil present in the fruit was
decomposed over several hours to form free fatty acids.5
The efficient conversion of free fatty acids to high quality
BDF under mild conditions is therefore desirable. In terms of
the composition of BDF itself, it can be categorized into
first-generation biodiesel fuel, i.e., fatty acid methyl esters
(FAME), and second-generation biodiesel fuel, i.e., bio
hydrofined diesel (BHD, hydrocarbons). To date, FAME has
been synthesized by a variety of basic (and sometimes acid)
catalysts, including our polymeric acid catalyst.6–11 However,
supported catalysts for BHD production remains
challenge.
a
In this context, we previously reported the development
silicon nanowire array-stabilized Pd nanoparticle
catalyst for organic transformations such as cross-coupling
of
a
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reactions, where the nanospace within the support worked
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efficiently as a nano reaction field.25 We expected that this
nanospace structure composed of a silicon nanowire array
(i.e., silicon nanowire array-stabilized Rh nanoparticle
catalyst (SiNA-Rh)) would provide a highly efficient and
reusable catalyst for the production of BHD via the
hydrogenative decarboxylation of fatty acids (i.e. carboxylic
acids).
Moreover, we believed that the use of microwave
irradiation for activation of the heterogeneous catalyst
should be key to lowering the electric power consumption
of this chemical catalytic process, as previously reported by
coauthor Wada.26 As microwaves improve the efficiency of
industrial processes and have been recognized as a novel
heating source in industry, Wang et al. investigated the
effect of microwave drying on the behavior of biomass
pyrolysis on a bench scale. They concluded that microwave
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Scheme 1. Selective bio hydrofined diesel formation
accompanied by the production of carbon monoxide from
abundant biomass fatty acids and renewable hydrogen gas,
catalyzed by a low loading of a reusable heterogeneous catalyst
under microwave irradiation.
drying is
a
technically and economically feasible
pretreatment method for biomass fast pyrolysis when
Thus, we herein developed a novel reusable supported
compared to conventional heating at 105 °C.27 In addition,
nanospace catalyst, namely
a silicon nanowire array-
Vongpradubchaihi et al. established
a
continuous
stabilized Rh nanoparticle catalyst (SiNA-Rh), which
efficiently produced biodiesel fuel from biomass fatty acids
using renewable H2 gas under low energy consumption
microwave conditions. Indeed, we found that the use of
SiNA-Rh containing 0.05 mol% Rh (i.e., 500 mol ppm Rh)
promoted the decarboxylation of a variety of carboxylic
acids at 200 °C under microwave irradiation (~40 W) and a
hydrogen pressure of 10 bar to give the corresponding
alkanes, including BHD, in high yields. In this transformation,
a one-carbon degraded alkane (A) was selectively produced
without the formation of hydrocarbons such as B1 and B2
(Scheme 1). In addition, although carbon monoxide and
carbon dioxide are possible co-products in this process,
only carbon monoxide was obtained, which could be
employed as an important and essential staple for the
chemical industry. In addition, the developed SiNA-Rh
catalyst was reused 20 times without any loss in catalytic
activity (i.e., consecutive turnover number = 35000). SiNA-
Rh was also applied in the decarbonylation of aldehydes in
an Ar atmosphere to give the corresponding alkanes
(including BHD) and carbon monoxide in high yields.
Moreover, based on numerical calculations, the electric field
distribution of the small area (nano order) of the catalyst
surface was estimated to determine the effects of
microwave irradiation on the functional groups of the SiNA-
Rh catalyst.
microwave applicator technology and reported several
advantages, including shortening of the processing time
and a greater energy efficiency over various the product
scales. 28 Indeed, it has been reported that microwaves can
reduce the energy cost of the drying process by 35% and
reduce the defective rate by 50%.29,30 Furthermore, when
Scuracchio et al. employed microwave heating in the
preparation of pulverized rubber, they reported shorter
process times at low temperature, and concluded that the
glass transition temperature was lowered by microwave
heating.31 Moreover, Sombatsompop et al. experimentally
monitored the physical and mechanical properties of
vulcanized natural rubber/carbon black/recycled rubber
formulations,32 where they compared microwave heating
with conventional heating in the vulcanization process. They
found that the chemical structure of the two obtained
rubber products differed. More specifically, with microwave
heating, the vulcanization process was more efficient, and as
a result, numerous practical applications and patents have
been established.33 In terms of sustainable development
goals set by the United Nations, industrial heterogeneous
catalytic processes based on microwave irradiation should
be applied to not only biomass and polymer chemistry but
also to chemical and catalytic reactions. As such, microwave
irradiation
has
received
significant
attention
in
heterogeneously-catalyzed organic processes, although its
industrial application is still under development.34
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ACS Catalysis
array-stabilized rhodium nanoparticle catalyst was then
1
readily prepared as follows. Initially, a silicon nanowire array
(SiNA)36–38 (2” wafer) was treated with hydrofluoric acid to
form a Si-H surface, which was dipped in an aqueous
acetone solution of rhodium chloride to afford SiNA-Rh as a
black plate (Figure 1(a)). Analysis of a cross-section of SiNA-
Rh by scanning electron microscopy (SEM) showed the
presence of a ‘nano-forest’ measuring <360 nm in width by
9 µm in height although we were not able to control the
size (Figures 1(b) and 1(c)). In addition, transmission
electron microscopy (TEM) observations showed that the Rh
nanoparticles on the silicon array had a mean diameter of
4.29±1.14 nm (Figures 1(d)–1(f)). Furthermore, analysis by
combined energy-dispersive X-ray spectrscopy and
scanning transmission electron microscopy (EDX/STEM)
indicated that the Rh species was well dispersed on the
silicon nanowire array (Figures 1(g)–1(h)). In addition, Rh K-
edge X-ray absorption near edge structure (XANES) analysis
of SiNA-Rh was employed to investigate formation of the
metallic Rh nanoparticles (Figure 1(i)). Indeed, the spectrum
of Rh2O3 showed quite different features from those of Rh
foil and SiNA-Rh. More specifically, the broader peaks of
SiNA-Rh compared with those of Rh foil were attributed to
the nanoparticle size. The best fit for extended X-ray
absorption fine structure (EXAFS) analysis of SiNA-Rh was
then obtained for the Rh-Rh first coordination shell (Figure
1(j)), and the fitted parameters were as follows: R factor:
0.001, coordination number: 7.3, Rh-Rh distance: 2.68 Å, and
2: 0.006 Å2. Since the ideal coordination number of the first
coordination shell of bulk metallic Rh is 12, the lower
coordination number in SiNA-Rh also indicated the
formation of Rh nanoparticles. These observations thereby
suggest that the desired silicon nanowire array-supported
rhodium nanoparticle catalyst was successfully prepared.
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With SiNA-Rh in hand, the hydrogenative decarboxylation
of stearic acid (1a) was investigated using the nanoparticle
catalysts, and the results are presented in Table 1. More
specifically, the reaction of 1a was carried out in the
presence of 500 mol ppm Rh of SiNA-Rh in a hydrogen
atmosphere (10 bar) at 200 °C (confirmed by a fiberoptic
thermometer) with microwave irradiation (~40 W power to
maintain
a
temperature of 200 °C) over 6 h. The
Figure 1. a) Photographic image of SiNA-Rh. b) SEM image of
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decarboxylated BHD product heptadecane (2a) was
selectively obtained in 47% yield (entry 1), but
deoxygenated products such as octadecane (3a) and
octadecanol were not detected. In addition, carbon
monoxide, an important and essential staple for the
chemical industry, was selectively obtained as a co-product
without the formation of the undesired carbon dioxide. The
formation of CO/CO2 was checked with GC/PDHID. Since
water was not detectable with GC/PDHID, we cannot check
the mass balance of the formation of CO but the ratio of
CO/CO2. The recovered SiNA-Rh was then reused without
any loss in catalytic activity, giving 2a in 41% and 57% yields
after the 2nd and 3rd uses, respectively. In contrast, the
reaction with SiNA-Pd was performed under similar
conditions to give 2a in 32% yield (entry 2), while a
SiNA-Rh. c) Schematic representation of SiNA-Rh. d) and e)
TEM images of the Rh nanoparticles on SiNA-Rh (scale bars =
20 and 5 nm, respectively). f) Particle size distribution of the Rh
nanoparticles in SiNA-Rh ( = 4.29±1.14 nm). g) STEM image of
SiNA-Rh. h) EDX image of the Rh nanoparticles in SiNA-Rh. i) Rh
K-edge XANES spectra of Rh foil, Rh2O3, and SiNA-Rh. j) Fourier
transform of the EXAFS of SiNA-Rh and the corresponding best
fit.
RESULTS AND DISCUSSION
The silicon nanowire array was previously developed for
silicon-based optoelectronics, fuel cells, solar cells, and
photoelectrodes, and was prepared by metal-assisted
catalytic etching to silicon wafers.35 The silicon nanowire
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(300)
significantly lower yield was obtained in the second cycle
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following catalyst recovery (i.e., 12% 2a). Several other
silicon nanowire array-stabilized metal nanoparticles were
prepared for testing, and these were based on Pt (SiNA-Pt),
bimetallic Pd and Pt (SiNA-PdPt), bimetallic PdRh (SiNA-
PdRh), and bimetallic Pd and Ir (SiNA-PdIr) (entries 3–7). The
reaction with SiNA-Pt was carried out under similar
conditions to give 2a in a 38% yield (Entry 3). Although
SiNA-PdPt gave 2a in a 49% yield, its reusability was
extremely poor (i.e., 15% 2a on the 2nd use, entry 4).
Moreover, the yields obtained using SiNA-PdRh and SiNA-
PdIr were relatively poor, i.e. 37 and 36%, respectively
(entries 6 and 7), and no reaction was observed when the
SiNA catalyst without metal deposition was employed (entry
8). Other rhodium catalysts were also investigated for this
transformation (entries 9–12); however, no reaction took
place in the presence of Rh/Si (entry 9). This result suggests
that the silicon nanostructure of the prepared SiNA is key to
the catalytic process. Moreover, the general heterogeneous
catalysts Rh/C, Rh/Al2O3, and the homogeneous RhCl3•3H2O
catalyst gave 2a in low yields (22, 0, and 0%, respectively)
(entries 10–12), and reused Rh/C gave a <1% yield of 2a,
indicating that the heterogeneous SiNA-Rh catalyst
exhibited a higher catalytic activity than its homogeneous
RhCl3 counterpart and common heterogeneous catalysts. In
addition, to indicate the importance of hydrogen gas and
microwave heating in this reaction, a number of control
experiments were conducted. More specifically, the reaction
under N2 without the use of H2 did not proceed (Entry 13).
Surprisingly, when the reactions were performed at 200 and
300 °C under traditional thermal heating conditions (oil bath
at 200 °C, sand bath at 300°C), no decarboxylative products
were obtained (0% yield) (entries 14 and 15). Rhodium
particles on an original p-type silicon wafer (Rh/Flat Si) were
also prepared for investigating significance of silicon
nanowire array in the reaction. Reactivity and reusability of
Rh/Flat Si was lower than those of SiNA-Rh (Entry 16).
7
SiNA-Pd0.98Ir0.02 (680)
SiNA (no metal)
Rh/Si (6500)
24, -, -
0, -, -
0, -, -
0, -, -
0, -, -
1, 0, -
0, -, -
0, -, -
0, -, -
100:0
8
-
9
0, -, -
-
10
11
12
13b
Rh/C (500)
22, <1, -
0, -, -
100:0
Rh/Al2O3 (500)
RhCl3•3H2O (560)
-
-
-
0, -, -
SiNA-Rh (500)
(no H2)
0, -, -
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14c
15d
16
SiNA-Rh (500)
(oil bath at 200°C)
0, -, -
0, -, -
0, -, -
0, -, -
-
-
SiNA-Rh (500)
(sand bath at 300 °C)
Rh/Flat Si (500)
34, 12, 2
7, 19, 0
(not
checked)
a
1a (1 mmol equiv), 200 °C, 6 h, H2 (10 bar), microwave
irradiation (~40 W; using a CEM microwave reactor) in a
glass vial, b under N2 without H2 gas, c thermal heating at 200
°C, and d thermal heating at 300 °C.
Since the catalytic activity of SiNA-Rh was superior to
those of the other catalysts, it was selected for application in
the hydrogenative decarboxylation of various carboxylic
acids (1). Thus, when the reaction of 1a was carried out over
24 h under the above conditions, complete conversion was
achieved to afford 2a in 94% yield without by-product
formation being observed (Table 2, entry 1). In this reaction,
only carbon monoxide was produced, with no carbon
dioxide being detected. Arachidic acid (1b) and
heptadecanoic acid (1c) were also converted to nonadecane
(2b) and hexadecane (2c) in yields of 93% and 95%,
respectively (entries 2 and 3). In addition, palmitic acid (1d),
pentadecanoic acid (1e), and myristic acid (1f), which are
the most common fatty acids in animals and plants, were
quantitatively converted to the corresponding BHDs,
pentadecane (2d), tetradecane (2e), and dodecane (2f)
without the formation of by-products (entries 4–6).
Furthermore, oleic acid (1g) and linoleic acid (1h), the most
abundant unsaturated fatty acids, were decarboxylated to
yield BHD 2a in 98% and 92% yields, respectively (entries 7
and 8). Moreover, the reaction of a dicarboxylic acid, namely
octadecanedioic acid (1i), also proceeded effectively to give
the desired hexadecane in 92% yield (entry 9), while the
carboxylic ester methyl stearate (1j) was selectively
converted to heptadecane (2a) in the presence of water
(79% yield, entry 10). These results indicate that the
preparation of jet and rocket fuel (kerosene = C10–C15) can
be readily prepared using this system. It was also found that
a secondary carboxylic acid 1k was applicable as a substrate
to afford 2d in 78% yield (entry 11). Importantly, it should
be noted that carbon monoxide was the only by-product
detected in the reactions listed in Table 2.
Table 1. Catalytic screening for the hydrogenative
decarboxylation of 1a using SiNA-metal nanoparticles (MNPs) a
Entry
Catalyst
(mol ppm)
2a yield (%)
1st, 2nd, 3rd
use of
3a yield (%)
1st, 2nd, 3rd
use of
CO:CO2
catalysts
catalysts
1
2
3
4
5
SiNA-Rh (500)
47, 41, 57
32, 12, -
38, 22, -
49, 15, -
37, 33, 33
0, 0, 0.2
0, 0, -
0, 3, -
0, 0, -
0, 0, 0
100:0
100:0
100:0
100:0
100:0
SiNA-Pd (670)
SiNA-Pt (690)
SiNA-Pd0.65Pt0.35 (660)
Table 2. Hydrogenative decarboxylation of carboxylic acids (1)
using SiNA-Rh a
SiNA-Pd0.81Rh0.19
(410)
6
SiNA-Pd0.92Ru0.08
36, 25, 0
0, 0, 0
100:0
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ACS Catalysis
1
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6
Entry
Substrate 1 (R)
1a C17H35
2 yield
(%)
3
yield
(%)
CO:CO2
7
8
1
94
0
(<1)
100:0
9
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3
4
5
6
7
1b C19H39
1c C16H33
1d C15H31
1e C14H29
1f C13H27
93
95
94
90
95
2
100:0
100:0
100:0
100:0
100:0
100:0
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2
0.2
3
2
Figure 2. Reusability of the SiNA-Rh catalyst in the
hydrogenative decarboxylation of 1a. Reaction conditions:
0.5 mmol stearic acid, H2 (10 bar), 200 °C, 24 h, microwave
irradiation (~40 W) in a glass vial.
1g
98
(R =
C17H35
0.3
(Z)-CH3(CH2)7CH=CH(CH2)7
)
)
)
8
9
1h (9-Z, 12-Z)
CH3(CH2)4CH=CHCH2CH=CH(CH2)7
92
(R =
C17H35
0.4
0
100:0
100:0
As shown in Entries 1, 13, and 14 in Table 1, we were
surprised to find that microwave heating promoted the
reaction while traditional thermal heating did not. The effect
of microwave irradiation was therefore investigated by
altering the electric power of the microwave irradiation at
200 °C (Scheme 2). As shown in Table 1, entry 1, the reaction
using 40 W of microwave irradiation gave 2a in a 57% yield
when a glass vial was employed. As quartz is known to
absorb very little microwave irradiation, the use of a quartz
vial was also examined. In this case, it was necessary to
irradiate with 100 W of microwave irradiation to maintain
the reaction temperature at 200 °C; however, the reaction
proceeded to afford 2a in 73% yield. These results suggest
the non-thermal effect of microwave and/or selective
heating to SiNA-Rh in this catalytic transformation.
1i HOOC(CH2)16
92
(R =
C16H33
10 b
11
1j = CH3(CH2)16-COOCH3
79
78
1
0
100:0
100:0
1k CH3(CH2)7CH(CO2H)(CH2)5CH3
a
1 (0.5 mmol equiv), SiNA-Rh (0.05 mol%), 200 °C, 24 h,
H2 (10 bar), microwave irradiation (~40 W; using a CEM
microwave reactor) in a glass vial. Methyl stearate (1j)
(0.5 mmol equiv), H2O (58.9 mg), 200 °C, 48 h, H2 (10 bar),
microwave irradiation (~40 W).
b
Following confirmation of the acceptable catalytic activity
and substrate scope, the reusability of SiNA-Rh was
investigated for the reaction of stearic acid (Figure 2) under
the conditions outlined in Table 1. Surprisingly, SiNA-Rh was
suitable for reuse 20 times without any loss in catalytic
activity: 2a was obtained in yields of 89, 85, 84, 86, and 82%
following the 2nd, 3rd, 10th, 15th, and 20th uses,
respectively. To the best of our knowledge, this is the first
successful example of a high catalyst reusability in the
hydrogenative decarboxylation reaction. This result indicates
that BHD can be readily and sustainably produced by the
reusable SiNA-Rh catalyst under mild conditions while
lowering the power consumption required for this
transformation. Leaching tests were then conducted on the
reaction mixture, where only 0.019 ppm Rh was detected by
ICP-MS (for further details, see SI section 4). This indicates
that only 0.0066% of the Rh present in SiNA-Rh was leached
out. Hot filtration tests were also performed, and the filtrate
was not observed to promote the reaction (see SI section 4).
Moreover, although the interval between 6th use and 7th use
of the catalytic reusability investigation was eight months
(SiNA-Rh was left under atmospheric conditions during 8
months.), the reusability of SiNA-Rh was fine to give 2a in
83% (6th use) and 88% (7th use) yield.
Scheme 2. Decarboxylation of 1a by thermal heating,
microwave irradiation at 40 W (in a glass vial), and at 100 W (in
a quartz vial) (using a CEM microwave reactor).
Since microwaves are a type of electromagnetic wave
composed of an electric field and a magnetic field, an
electric- and magnetic-field separable microwave reactor
was used for the purpose of this study (Scheme 3). In
general, an electric field supplies energy to molecular
polarization, while a magnetic field supplies energy to spins
and free electrons. Therefore, if an electric field and a
magnetic field are separated and heated, we can study
which energy path is effective for reaction acceleration. By
employing cavity resonator, it was possible to irradiate the
sample with either a pure electrical or pure magnetic field.
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When such reactions under electric- and magnetic-field
Indeed, this phenomenon has been discussed in the
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irradiation conditions (employing E-max in TM010 mode and
H-max in TM110 mode,39 respectively) were carried out at
200 °C for 6 h, 12 and 25 W microwave irradiation were
required to produce 2a in yields of 65 and 46% yield,
respectively. These results suggest that the electric field is
key to efficient heating (12 W vs. 25 W) and also to
promoting the reaction (65% vs. 46% yield). For comparison,
when the reaction of 1a was carried out using a CEM
microwave reactor at 200 °C for 6 h in a quartz vial, 100 W
microwave irradiation was required to maintain the
temperature at 200 °C and to afford 2a in a 73% yield.
microwave chemistry community, and it can be explained by
considering that the microwave electric field selectively
supplies energy to functional groups or rotation levels.40,41
To investigate the contribution of this phenomenon on our
system, the spatial distribution of the electric field intensity
around the surface of the Si nanostructure was simulated
using the Finite Element Method. The simulation model
contains the tetragonally-aligned Si rods of 50 nm diameter
as shown in Figure 3.42 The distance (d) between the rod
was varied between 5 and 180 nm, and a microwave electric
field with a frequency of 2.45 GHz and an intensity of 1
[V/m] was introduced along the z direction in the model.
Although the wavelength of the incident electromagnetic
wave (which is ~122.36 [mm] in free space) is significantly
larger than the size of the Si rod, the incident microwave
generates enhanced electric fields. Similar phenomena are
known in the light region.43,44 Numerical calculations
therefore indicated that the nanostructure amplifies the
intensity of the electric field, and its amplification rate
increases with a decreased rod spacing.
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The relationship between the amplified electric field and
the enhanced chemical reaction rate was then considered in
more detail. Thus, the SiNA surface is decorated with Rh
nanoparticles, and Si–OH, Si–O, and Rh–Si bonds are pre-
sent on its surface and these functional groups present have
polarities. The microwave electric field is enhanced several
times by the structure of the Si rod as calculated in Figure 3,
the enhanced microwave field combined with these
functional groups and charge transfers were driven at this
surface. For the physical image, the motion of the electric
charge is not like direct current, but like rotation as
observed in microwave heating of water 45. This charge
Scheme 3. Decarboxylation of 1a using electric and magnetic
field microwave irradiation (in 6 h) in a quartz vial under
electric- and magnetic-field irradiation conditions
(employing E-max in TM010 mode and H-max in TM110
mode, respectively), and using a CEM microwave reactor.
Table 3. Complex dielectric constants for the various
components at 1.0 GHza
Entry
Real part
44.43
Imaginary part
9.695
Tan 
Silicon
0.2182
0.0007
transfer is well known as the primary heating mechanism of
46
microwaves,
which forms local heating. It is well known
Stearic
2.08
0.001
acid (1a)
that high selectivity of microwave energy creates local
heating on the order of nm, 47-49 and since there is thought
to be a reaction where a H2 become into protons and Rh-H
on the surface, the local heating accelerates the reaction. In
addition, the amplified electric field may also enhance these
functional group activities to improve the reaction rate.
a
Complex dielectric constant measured by
cavity perturbation methods (Measured by
KEYCOM Corp., Tokyo, Japan).
As outlined in Table 3, the dielectric loss tangents of
stearic acid and the silicon substrate are 0.0007 and 0.2182,
respectively. Moreover, the microwave heating of stearic
acid (1a) and the silicon wafer in a quartz vial at 40W using
a CEM microwave reactor gave temperatures of 50 and 145
°C, respectively, as detected by an infrared thermometer.
These results indicate that the microwave electric field
mainly heats the silicon substrate, and so the presence of
hot spots in the solution is unlikely since stearic acid
absorbs little microwave energy from the viewpoint of heat
transfer theory. The energy concentration structures of the
silicon substrate and the stearic acid interface were
therefore considered by numerical calculations. It was
assumed that some phenomenon responsible for
accelerating the reaction exists at the silicon-stearic acid
interface.
As mentioned above, numerical simulations indicate that
the enhancement of the electric field becomes larger upon
decreasing the distance between the rods (Figure 3). If this
reaction is accelerated as a result of enhancement of
electrons and protons by an electric field, the reaction rate
of our system would be expected to be enhanced by
increasing the microwave electric field or decreasing the
distance between rods. This increasing of reaction yield is
50
often observed and known as microwave effect
. These
results thereby account for the fact that only the silicon
nanowire array-stabilized Rh catalyst (SiNA-Rh) efficiently
promoted the reactions (entry 1 vs. entries 9–12, Table 1).
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Figure 4. Plausible catalytic pathway for the decarboxylation
reaction using SiNA-Rh.
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Since carboxylic anhydride B is a possible intermediate
for the catalytic pathway, the decarboxylation of 1a was
performed using pivalic anhydride (Piv2O), which should
react with 1a to form pivalic stearic anhydride. Indeed, the
reaction of 1a with 1.5 mol equiv of Piv2O in the presence of
500 mol ppm Rh of SiNA-Rh in a hydrogen atmosphere
(10 bar) at 200 °C with microwave irradiation (~40 W power
to maintain a temperature of 200 °C) gave 2a in 89% yield
after 6 h (Scheme 4), indicating that the reactivity of this
system was higher than that in the absence of Piv2O (89%
vs. 47%, Table 1, entry 1). The formation of a carboxylic
anhydride therefore appears to be key to this reaction
process. In addition, a suitable catalyst reusability was
achieved under these reaction conditions, with 2a being
obtained in yields of 89, 88, 87, 87, 89, 86, and 86% after the
2nd, 3rd, 4th, 5th, 6th, 8th, and 10th uses, respectively. As in
the above reactions, only carbon monoxide was formed as a
co-product.
Figure 3. Numerical calculation results for the electric field
strength between Si rods ( φ 50 [nm] × h) in electromagnetic
waves (scattered electric field: 1 [V/m], wave vector of the
background field: [0,0,−1], rod length h: 10000 [nm]). The
shorter the rod spacing, the stronger the microwave electric
field.
A plausible catalytic pathway for this decarboxylation
process has been proposed, as shown in Figure 4.51–53 Two
carboxylic acids (A) are converted to a single carboxylic
anhydride B, whose oxidative addition to SiNA-Rh
(Rh/Sinano) gives an acyl rhodium carboxylate C. The
insertion of hydrogen to C gives the acyl rhodium hydride D
in addition to regenerating A. Subsequent decarbonylation
followed by -hydride elimination gives E, and
intramolecular hydrogen transfer in E affords the desired
alkane (BHD) F and carbon monoxide as a co-product, while
also regenerating the SiNA-Rh (Rh/Sinano) catalyst.
Scheme 4. Decarboxylation of 1a in the presence pivalic
anhydride (Piv2O).
As indicated previously in Figure 2, compound D is also a
possible intermediate in the proposed catalytic cycle;
therefore, we envisioned that the decarbonylation of
aldehydes should also proceed to produce BHD and CO.54,55
We therefore attempted the decarbonylation of octadecanal
(4a) using 500 mol ppm Rh of SiNA-Rh at 150 °C over 24 h
under Ar. This reaction afforded heptadecane (2a) in a 99%
yield after the 1st catalyst cycle and 98% after the 2nd use
(Scheme 5). In this reaction, microwave heating was not
essential to promote the reaction although microwave
heating provided higher reactivity (In 6 h: with microwave,
2a 70% yield; with conventional heating, 2a 11% yield. In 24
h: with microwave, 2a 95% yield; with conventional heating,
2a 99% yield)). These results suggest that microwave
irradiation contributes especially to the process at some
point between compounds B and D (and between D and C
a little bit). Finally, we note that E could also be an
intermediate in the plausible catalytic pathway since
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heptadecane was detected in the course of the optimization
reaction using stearic acid (1a).
Yoichi M. A. Yamada:
https://orcid.org/0000-0002-3901-5926
Yasuhiro Uozumi:
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2
3
4
https://orcid.org/0000-0001-6805-3422
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6
7
8
9
ACKNOWLEDGMENT
We would like to thank Prof. Naoto Chatani (Osaka
University, Japan) for useful suggestions on the catalytic
Scheme 5. SiNA-Rh-catalyzed decarbonylation of aldehyde 4
under Ar.
pathway. We would also like to thank RIKEN for assistance
with the XPS (Dr. Akiko Nakao, RIKEN), ICP-AES (Ms.
Chieko Kariya; materials Characterization Support Team,
CEMS, RIKEN), and ICP-MS (Ms. Rikako Ishii in our team)
measurements, and the Institute for Molecular Science for
TEM/EDX measurements (Prof. Go Hamasaka). We
gratefully acknowledge financial support from the JST
ACT-C (#JPMJCR12ZC), the JST ACCEL (#JPMJAC1401),
the JSPS (#24550126, #20655035, and #15K05510), the
Takeda Science Foundation, the Naito Foundation, and
RIKEN.
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CONCLUSION
We herein reported the development of a novel silicon
nanowire-stabilized rhodium nanoparticle catalyst (SiNA-
Rh), which smoothly and selectively promoted the
hydrogenative decarboxylation of fatty acids under mild
conditions to yield the corresponding alkanes for producing
bio hydrofined diesel (BHD) in high yields with the
formation of only carbon monoxide (an important and
essential staple for the chemical industry) as a co-product.
More specifically, SiNA-Rh (500 mol ppm) selectively
promoted the hydrogenative decarboxylation reaction at
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AUTHOR INFORMATION
Corresponding Author
E-mail: ymayamaada@riken.jp
ORCID
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Scheme 1
101x70mm (301 x 301 DPI)
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Figure 1
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Table 1: Scheme
116x24mm (300 x 303 DPI)
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Table 2: Scheme
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Figure 2: Scheme
1016x624mm (72 x 72 DPI)
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Scheme 2
72x38mm (301 x 301 DPI)
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Scheme 3
112x49mm (301 x 301 DPI)
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Figure 3
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Figure 4
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Scheme 4
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Scheme 5
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