Reductive amination of furfural toward furfurylamine with aqueous ammonia under hydrogen over Ru-supported catalyst

Article abstract of DOI:10.1007/s11164-015-2334-5

Poly(N-vinyl-2-pyrrolidone)-capped ruthenium-supported hydroxyapatite (Ru-PVP/HAP) shows significant activity for the synthesis of furfurylamine (FAM) via the reductive amination of furfural. As-prepared 5 wt% Ru-PVP/HAP affords 50 % yield of FAM in 25 % NH₃ aqueous solution under pressurized H₂ gas (2.5 atm), and the highest yield approaches 60 % at 4.0 H₂ atm. Comparison between the activities over four Ru-supported HAP catalysts prepared with different methods and the results of X-ray absorption spectroscopy suggested that the metallic Ru cluster is the active center for the reductive amination of furfural. Transmission electron microscope and inductively-coupled plasma analysis indicated that the as-prepared 5 wt% Ru-PVP/HAP catalyst possessed 4.0 wt% PVP-capped Ru clusters with average diameter of 1.7 ± 0.3 nm on HAP support. It was also demonstrated that the reductive amination approach with Ru-PVP/HAP catalyst, NH₃ aq. and pressurized H₂ gas has capability for transformation of aromatic aldehydes to the corresponding aromatic amines. According to these results, it is concluded that Ru(0) cluster supported on HAP will represent a suitable catalyst for widely-usable reductive amination to convert an aldehyde functionality towards an amine.

Full text of DOI:10.1007/s11164-015-2334-5

Res Chem Intermed (2016) 42:19–30
DOI 10.1007/s11164-015-2334-5
Reductive amination of furfural toward furfurylamine
with aqueous ammonia under hydrogen over
Ru-supported catalyst
Shun Nishimura1 Kunihiko Mizuhori1
•
•
Kohki Ebitani1
Received: 8 July 2015 / Accepted: 22 October 2015 / Published online: 12 November 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract Poly(N-vinyl-2-pyrrolidone)-capped ruthenium-supported hydroxyap-
atite (Ru-PVP/HAP) shows significant activity for the synthesis of furfurylamine
(FAM) via the reductive amination of furfural. As-prepared 5 wt% Ru-PVP/HAP
affords 50 % yield of FAM in 25 % NH3 aqueous solution under pressurized H2 gas
(2.5 atm), and the highest yield approaches 60 % at 4.0 H2 atm. Comparison
between the activities over four Ru-supported HAP catalysts prepared with different
methods and the results of X-ray absorption spectroscopy suggested that the
metallic Ru cluster is the active center for the reductive amination of furfural.
Transmission electron microscope and inductively-coupled plasma analysis indi-
cated that the as-prepared 5 wt% Ru-PVP/HAP catalyst possessed 4.0 wt% PVP-
capped Ru clusters with average diameter of 1.7 0.3 nm on HAP support. It was
also demonstrated that the reductive amination approach with Ru-PVP/HAP cata-
lyst, NH3 aq. and pressurized H2 gas has capability for transformation of aromatic
aldehydes to the corresponding aromatic amines. According to these results, it is
concluded that Ru(0) cluster supported on HAP will represent a suitable catalyst for
widely-usable reductive amination to convert an aldehyde functionality towards an
amine.
Keywords Furfural Á Reductive amination Á Furfurylamine Á Ru cluster Á Aqueous
ammonia solution
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2334-5)
contains supplementary material, which is available to authorized users.
&
Kohki Ebitani
ebitani@jaist.ac.jp
1
School of Materials Science, Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
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S. Nishimura et al.
Introduction
Catalytic transformation of renewable biomass resources into chemical intermedi-
ates and/or fuels has been a key technology to slow global warming and the
depletion of fossil resources. Furfural (2-furaldehyde) obtained by dehydration of
xylose, an inedible biomass extensively distributed in nature, is the one of expected
terminal materials in bio-refineries [1, 2]. So far, hydrogenation to furfuryl alcohol,
hydrogenolysis to 2-methylfuran, oxidation to succinic acid, Knoevenagel conden-
sation with active methylene compounds, aldol condensation with acetone, and
other catalytic routs have been widely examined for furfural utilizations; however,
studies for the reductive amination of furfural to furfurylamine (FAM) have rarely
been reported in the literature. Amine-functionalization is an important fundamental
chemical transformation for agrochemicals, dyes and pharmaceuticals. Reduction of
petroleum-based nitrile- or nitro- compounds (e.g., benzonitrile and nitrobenzene)
[3–5] under pressurized conditions is a common method for aromatic amines
production.
The product, FAM, is known as a widely-useful compound in the manufacture of
pharmaceutical drugs, pesticides, fibers and perfumes, etc. [6–8]. Ayedi et al.
reported the synthesis of primary amines from the corresponding aldehyde with
hydroxylammonium chloride (H2NOHÁHCl) and a Zn dust/HCl mixture as a
nitrogen and a hydrogen source, respectively, which produces a 99 % yield of FAM
[9, 10]. Ayusawa et al. [11] investigated a method over Raney catalysts in liquid
ammonium and a high-pressure H2 (87 atm), producing a [97 % yield of FAM.
Binggeng reported that Raney-Ni produced an 85 % yield of FAM from furfural
with similar reaction conditions (30 % NH3 aq. and 30 atm H2 gas) [12]. For future
challenges, it is better to replace such a highly reactive agent or catalyst (explosive
and flammable materials) and a high-pressure condition (special reactor required)
for a greener and a more easily-handled transformation process.
On the basis of a bio-refinery approach, here we report the reductive amination of
furfural towards FAM over Ru-supported catalyst to open up further ways for
furfural utilization. We achieved a 60 % yield of FAM via a reductive amination of
furfural using a PVP-capped Ru-supported HAP catalyst in 25 % aqueous NH3
solution under pressurized H2 gas at 373 K (Scheme 1).
XAS and TEM analyses were performed to estimate active Ru species. We
propose the reaction pathway via amide and applied the present catalytic system to
amination of aromatic aldehydes into aromatic amines.
Scheme 1 Reductive amination
of furfural to furfurylamine
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Reductive amination of furfural toward furfurylamine with…
21
Experimental
Catalyst preparation
Ru-supported hydroxyapatite (HAP; Ca10(PO4)6F2; Wako) catalyst was prepared by
the polyol reduction method according to previous reports [13, 14]. RuCl3ÁnH2O
(Sigma-Aldrich), 0.58 g of poly(N-vinyl-2-pyrrolidone) (PVP; Sigma-Aldrich K29-32),
0.1 g of KCl (KANTO) and 50 mL of ethylene glycol (Wako) were solved in an
aqueous solution (50 mL), and then refluxed for 2 h under vigorous stirring
(600 rpm). Thereafter, 1.0 g of HAP was added with successive heating for 1 h. The
obtained slurry was separated by filtration, washed and dried in vacuum at room
temperature (RT). The loading amounts of Ru were varied in the range of 1–7 wt%.
The prepared catalyst was denoted as Ru-PVP/HAP. To compare the support effects
on reactivity, 5 wt% Ru-supported hydrotalcite (HT, Mg/Al = 3.0, TOMITA), c-
Al2O3 (JRC-ALO-8), TiO2 (KANTO; anatase), CeO2 (Wako), and ZrO2 (KANTO)
were also prepared in the same way.
In the case of the 5 wt% Ru-supported HAP catalyst, the effect of three different
preparation methods was also examined. First, Ru-none/HAP was prepared in
accordance with the polyol reduction method mentioned above in the absence of
PVP agent. Second, Ru/HAP(ad.) was made by an adsorption method; briefly, 1.0 g
of HAP was stirred in an aqueous RuCl3 solution (50 mL) at 353 K for 24 h, and
then filtered, washed with distilled water (2 L) and dried at 383 K overnight. And,
third, Ru-PVP/HAP(red.) was produced with NaBH4 reductant: 50 mL of an
aqueous solution of RuCl3 and PVP (0.58 g) was stirred for 20 min at RT, and then
0.1 M NaBH4 aq. (16 mL) was injected into the mixture to reduce Ru(?3) ions.
After an additional 30 min with vigorous stirring at RT, a successive mixing for 1 h
at 393 K was performed in the presence of HAP support (1.0 g) for stabilization.
Finally, the obtained slurry was filtered off, washed with distilled water (2 L), and
dried overnight in vacuum at RT.
Catalyst characterization techniques
Inductively-coupled plasma (ICP) analysis was performed with ICPS-7000 v.2
(Shimadzu). The wavelength of 240.3 nm was selected for calculation of the actual
Ru amount on the basis of the standard with commercial Ru solutions (Wako). To
prepare the sample, the Ru-supported catalyst was treated with sodium hypochlorite
(KANTO) under ultrasonic wave for 10 min, and then diluted in a measuring
cylinder to 50 mL total, leading to about 10 ppm order. Before introduction into the
feed port, the support was filtered off with a Millex filter (0.20 lm).
A transmission electron microscope (TEM) image was observed by an H-7100
(Hitachi) operated at 100 kV. The particle size distribution was pictured in terms of
300–500 particles with random selection by using the ImageJ software (v.1.48).
Ru-K edge X-ray absorption spectroscopy (XAS) analyses were performed at
BL01B1 in SPring-8 under the proposal Nos. 2014B1472 and 2015A1389. Si(311)
double single crystals were used for division of energy. All samples were grained
123

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S. Nishimura et al.
and pressed to a pellet with 10u diameter for measurement. The obtained spectra
were analyzed with REX2000 v.2.5.92. software (Rigaku). Fourier-transformation
(FT) of the k3-weighted EXAFS was performed in the range of k = 3.0–13.0.
Curve-fitting analysis in FT at R = 1.903–2.823 was examined with McKale
simulation in the fitting range at k = 4.0–12.0. The Debye–Waller factor was
tentatively fixed at r2 = 0.01145 for fitting calculations. A reference of Ru(OH)x
was prepared with the procedure reported in the Ref. [15].
Reaction procedure
Reductive amination of furfural into FAM was performed in a sealed autoclave
reactor (50 mL vol.; TAIATSU Technol., Japan). Furfural (1 mmol; Sigma-
Aldrich), 25 % NH3 aq. (3 mL; Wako) and Ru-supported catalyst (100 mg) were
placed into the reactor and mixed together (500 rpm) for 1 h at RT, and then heated
at 373 K for 2 h under pressurized H2 gas condition (2.5 atm). After the reaction,
the products were cooled to RT at static state, and filtered by a Millex filter
(0.20 lm). The products were analyzed by a GC-FID (Shimadzu GC-2014)
equipped with a non-polar column (Agilent J&W DB-1; 0.320 mm 9 50 m 9
0.25 lm) using 1-butanol (Wako) as an internal standard. The 1-buthanol, furfural
and furfurylamine (TCI) were observed at 4.2, 9.5 and 10.3 min in the GC chart,
respectively. A peak derived from the compound with molecular weight of 268 was
also detected at 26.5 min in GC chart.1
Results and discussion
Effect of support
First, we surveyed the activity of Ru, Pd, Pt and Rh using a commercial carbon-
supported catalyst (Figure S1). The activity (FAM yield) was in the order: Ru/C
(11 %) [ Pd/C (7 %) [ Pt/C (2 %) [ Rh/C (0 %) & Blank (0 %). Thus, we
focused on the Ru-supported catalysts in this study.
Figure 1 shows the activities of Ru on different supports for the reductive
amination of furfural to FAM. The basic supports of HAP and HT shows significant
activity of 50 and 36 %, respectively. Among the acidic supports, Al2O3 and TiO2
possess middle activity of 33 and 27 %, whereas CeO2 and ZrO2 gave only 9 %
FAM in both cases. We attempted to prepare 5 wt% Ru catalyst; however, the actual
amount of Ru was very different depending on the support: Ru-PVP/HAP
(3.3 wt%), Ru-PVP/HT (4.1 wt%), Ru-PVP/Al2O3 (3.1 wt%), Ru-PVP/TiO2
(1.4 wt%), Ru-PVP/CeO2 (0.8 wt%), and Ru-PVP/ZrO2 (0.4 wt%). TEM images
and particle size distributions of Ru-PVP stabilized on different supports are shown
in Fig. 2. The Ru-PVP immobilized HAP, HT and Al2O3 support possessed Ru
clusters with 1.5–2.2 nm diameters whereas TiO2 and CeO2 showed larger-sized Ru
1
As intermediate, hydrofuramide (Exact Mass: 268.08) is the principal candidate estimated by GC–
MS(EI) analysis (see Fig. S7).
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Reductive amination of furfural toward furfurylamine with…
23
Fig. 1 Activities on PVP-
stabilized Ru on different
supports for the reductive
amination of furfural to FAM:
5 wt% Ru-PVP-supported
catalyst (100 mg), furfural
(1 mmol), 25 % NH3 aq.
(3 mL), H2 gas (2.5 atm),
373 K, 2 h, 500 rpm
HAP
HT
Al2O3
TiO2
CeO2
ZrO2
(a)
(c)
(e)
(b)
(d)
(f)
250
200
150
100
50
120
100
80
60
40
20
0
Mean = 2.0
0.2 nm
Mean = 1.7
0.3 nm
N = 480
N = 377
50 nm
50 nm
50 nm
50 nm
50 nm
50 nm
0
0
1
2
3
4
5
1 Particle 2size /nm 3
0
4
Particle size /nm
2
100
80
60
40
20
0
120
100
80
60
40
20
0
Mean = 1.8
0.3 nm
N = 480
Mean = 3.5
0.7 nm
N = 326
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0
1
2
3
4
5
6
7
8
9
10
Particle size /nm
Particle size /nm
100
80
60
40
20
0
Mean = 4.1
0.9 nm
N = 336
Ru particles were
hardly identified
0
1
2
3
4
5
6
7
8
9
10
Particle size /nm
Fig. 2 TEM images and particle size distributions of 5 wt% Ru-PVP-stabilized on different supports.
a Ru-PVP/HAP. b Ru-PVP/HT. c Ru-PVP/Al2O3. d Ru-PVP/TiO2. e Ru-PVP/CeO2. f Ru-PVP/ZrO2
nanoparticles (NPs) of 2.8–5.0 nm. It is hard to distinguish between the Ru particles
and support from TEM images in the case of ZrO2. The oxidation states on Ru
species estimated by Ru-K edge XANES hardly showed any differences (Fig. S2).
The small-sized Ru clusters over HAP support were attributes to the highest FAM
yield.
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S. Nishimura et al.
Effect of preparation method for Ru-supported HAP
To optimize the preparation for the Ru-supported HAP catalysts, Ru-PVP/HAP, Ru-
PVP/HAP(red.), Ru-none/HAP and Ru/HAP(ad.) were applied for the reaction.
Figure 3 shows the activities of Ru-supported HAP catalysts prepared with different
methods. It is worth noting that Ru/HAP(ad.) exhibited no activity (0 % yield) for
the reaction even though it possessed 4.0 wt% Ru determined by ICP. Ru-none/
HAP and Ru-PVP/HAP(red.) also kept a large quantity of Ru (ICP), 4.8 and
3.0 wt%, respectively; however, they showed mild activity with ca. 30 % yields.
HAP support alone (without Ru loading) showed 0 % yield (not shown in Fig. 3).
Accordingly, it was envisaged that not only the Ru amount but also the size and/or
oxidation state would contribute to the catalytic activity for the amination.
Figure 4 shows the data for Ru-K edge XAS studies on Ru-supported HAP
prepared by different methods. The k3-weighted EXAFS spectra are shown in
Fig. S3. Interestingly, four Ru-supported HAP catalysts possessed similarity with
RuO2 and/or Ru(OH)x in the XANES area (Fig. 4a). The first derivatives of XANES
clearly showed that the Ru metal (Ru foil) component was also included in those
catalysts except for Ru/HAP(ad.), i.e. the peak at around 22.11 keV was observed in
Fig. 4b for those three samples. Therefore, the formation of metallic Ru would be an
important factor for the activity. TEM analyses were not suitable for comparison
with the size of the formed Ru(0) because of some difficulties in the observations
(Fig. S4). On the other hand, both the intensity of FT and the estimated
coordination-number (CN, McKale) for Ru–Ru suggested that the size of the
constructed Ru(0) NPs was the smallest in the case of Ru-PVP/HAP; e.g., the CN
was Ru-PVP/HAP (3.20) \ Ru-PVP/HAP(red.) (6.55) \ Ru-none/HAP (8.10)
(Fig. 4c). According to these results, the presence of small Ru(0) clusters leads to
a high activity for the reductive amination of furfural with NH3 and H2. The PVP
capping agent could act as a protector to preserve the Ru(0) cluster on HAP support
against its growth/aggregation during catalyst preparation [16–18].
Optimizations of Ru loading for Ru-PVP/HAP and reaction conditions
Figure 5 represents the yield of FAM over the various amounts of Ru-PVP NPs
supported on HAP. All Ru-PVP/HAP catalysts were prepared with the same
procedure; however, the activity showed drastic differences depending on the Ru
amount: 1–2 wt% Ru-loaded HAP(s) were inactive for the reaction (0 % yield),
while, above 3 wt% Ru, loading leads to good activity, and the 5 wt% Ru-PVP-
loaded HAP catalyst gave the maximum yield for the reaction (50 %). A larger
amount of Ru loadings ([5 wt%) exhibited a small decrease in activity. XAS results
indicated that Ru(OH)x was the dominant phase below 3 wt% Ru loading, and then
the Ru(0) phase was produced at above 4 wt% (Fig. S5). These phenomena also
supported that Ru(0) clusters were the active center for the reaction.
The effect of reaction temperature was also investigated over the 5 wt% Ru-PVP/
HAP catalyst. As shown in Fig. 6, the reactions at higher than 373 K gave a large
diminishment of FAM yield. At the same time, unidentified peaks were detected in
the GC chart at 27.3 and 28.5 min. According to the previous reports [11, 19, 20],
123

Reductive amination of furfural toward furfurylamine with…
25
Fig. 3 Activities of Ru-supported HAP catalysts prepared with different methods: 5 wt% Ru-PVP-
supported catalyst (100 mg), furfural (1 mmol), 25 % NH3 aq. (3 mL), H2 gas (2.5 atm), 373 K, 2 h,
500 rpm
(a)
(b)
(c)
RuCl3
1/2
RuO2
Ru(OH)x
Ru metal
1/4
Ru/HAP(ad.)
Ru-none/HAP
Ru-PVP/HAP(red.)
Ru-PVP/HAP
0
1
2
3
4
5
Distance /Å
Fig. 4 a XANES spectra, b first derivatives of XANES and c |FT| of the k3-weighted EXAFS of Ru-
supported HAP catalysts prepared by different methods. The results of McKale simulation are illustrated
with red lines in c (For interpre-tation of the references to color in this legend, the reader is referred to the
webversion of the article)
the hydrofuramide, an expected intermediate (vide infra), is converted to an isomer
(furfurine) via thermal cyclization, which is not hydrogenated to FAM, and/or FAM
is hydrogenated into tetrahydro-FAM. These plausible side-reactions could be the
reason for low FAM yields at higher reaction temperatures.2
When we applied the high pressured H2 gas, the FAM yield increased to 60 % at
373 K under 4.0 atm H2 (Fig. S6).
2
The peaks at 27.3 and 28.5 min were not originated from tetrahydro-FAM (Exact mass: 101.08) nor di-
furfurylamine (Exact mass: 177.08) estimated by GC–MS(EI) analysis. Thus, side-reactions in our
condition have not been identified at this moment.
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S. Nishimura et al.
Fig. 5 Yields of FAM over the
various amounts of Ru-PVP NPs
supported on HAP: Ru-PVP/
HAP catalyst (100 mg), furfural
(1 mmol), 25 % NH3 aq.
50
40
30
20
10
0
(3 mL), H2 gas (2.5 atm),
373 K, 2 h, 500 rpm
0
1
2
3
4
5
6
7
Ru loading /wt%
Fig. 6 Yields of FAM over
5 wt% Ru-PVP/HAP at various
reaction temperatures: catalyst
(100 mg), furfural (1 mmol),
25 % NH3 aq. (3 mL), H2 gas
(2.5 atm), 2 h, 500 rpm
50
40
30
20
10
0
353
373
393
413
Reaction temperature /K
Possible reaction pathway for the reductive amination of furfural
to furfurylamine in the presence of NH3 and H2
Time-courses of the FAM yield and furfural conversion are plotted in Fig. 7. The
relative area of the peak at 26.5 min in GC-chart (expected intermediate) (see
Footnote 1) to the peak of internal standard (1-butanol) is also inserted as a function
of reaction time, because it appeared in all cases and frequently changed its area
during the reaction. Conversion of furfural was almost 100 % from the initial stage
of the reaction, meaning that the conversion of furfural to intermediate state is so
fast in NH3 aqueous solution. While, the yield of FAM showed the top-peak at 2 h
reaction, thereafter it was gradually decreased with the appearance of an
unidentified peak at 27.3 min in the GC chart. Thus, it was expected that a longer
123

Reductive amination of furfural toward furfurylamine with…
27
Fig. 7 Time-course of reductive amination of furfural over 5 wt% Ru-PVP/HAP: catalyst (100 mg),
furfural (1 mmol), 25 % NH3 aq. (3 mL), H2 gas (2.5 atm), 2 h, 500 rpm
Scheme 2 Possible reaction pathway for reductive amination of furfural to furfurylamine with NH3 in
the presence of H2 gas
reaction would enhance the successive reaction which consumed the produced
FAM.
It is noteworthy that the plot for the relative area was the highest value at 0 h
reaction (before heating), and then it was subsequently decreased. It is well known
that furfural readily reacts with ammonia to form furfuramide [21, 22]. In our
observation, both high conversion of furfural and a large peak at 26.5 min for the
GC chart were observed at the initial stage of the reaction (see Footnote 1).
According to these results, we expected that our catalytic amination of furfural to
furfurylamine proceeded as described in Scheme 2, which is in line with the
proposals reported in the literature [11, 23].
123

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S. Nishimura et al.
Application for the synthesis of aromatic amines via reductive
amination over Ru-PVP/HAP catalyst
Aromatic amines are currently receiving a great deal of attention for important
imines and amides production [24–26]. Thus, instead of the conventional method
for the reduction of nitroarenes to aminoarenes [27, 28], further investigations for
the reductive amination of aromatic aldehyde towards aromatic amine are being
performed. There is little literature on the reductive amination of aldehydes with
ammonia; e.g., Ir and/or Rh complexes [29, 30] or Pd-based heterogeneous catalysts
[31, 32] have been previouslyreported.
Results for the reductive amination of benzaldehyde and its derivatives over the
Ru-PVP/HAP catalyst with NH3 and H2 gas are listed in Table 1. The reaction was
conducted with the same conditions optimized for the furfural amination. In all four
cases, approximately 30–40 % yields of the corresponding amines were obtained
from each benzaldehyde. Accordingly, it was found that Ru-PVP/HAP possesses
suitable performances for the transformation of aromatic aldehydes into aromatic
amines via reductive amination reaction.
Conclusion
Reductive amination of furfural in NH3 aqueous solution under pressurized H2 gas
has been performed by using a PVP-capped Ru-supported catalyst prepared by
various methods. It was found that 5 wt% Ru-PVP-supported HAP catalyst, which
Table 1 Reductive amination of benzaldehyde and its derivatives over Ru-PVP/HAP catalyst with NH3
and H2 gas
Entry
1
Substrate
Product
Yield of amine/ %
32
2
3
4
36
36
41
Reaction conditions: aromatic aldehyde (1 mmol), 25 % NH3 aq. (3 mL), 5 wt% Ru-PVP/HAP
(100 mg), H2 gas (2.5 atm), 373 K, 2 h, 500 rpm
123

Reductive amination of furfural toward furfurylamine with…
29
contained 4.0 wt% Ru clusters with an average diameter of 1.7 0.3 nm showed
the highest yield for FAM (60 %) in 25 % NH3 aqueous solution at 373 K for 2 h
reaction under 4.0 atm H2. The importance of the Ru metallic phase as the active
center was suggested by means of XAS spectroscopy. The Ru-PVP/HAP was also
applied for the reductive amination of benzaldehyde derivatives under the same
reaction condition to give the corresponding aromatic amines with moderate yields
(30–40 %). In conclusion, we anticipate that Ru-PVP/HAP represents a proper
catalyst for the reductive amination of aldehydes to amines with NH3 as a nitrogen
source under a moderately pressurized H2 condition.
Acknowledgments This work is supported by the Grant-in-Aid for Scientific Research (C) (No.
25420825) and Young Scientist (B) (No. 25820392) by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan. The authors also appreciate the contribution of Photon Factory
at High Energy Accelerator Research Organization (KEK-PF), Tsukuba, Japan, on XAFS discussions at
BL-9A under the Proposal No. 2013G586.
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