Hydrogenation of adipic acid to 1,6-hexanediol by supported bimetallic Ir-Re catalyst

Article abstract of DOI:10.1016/j.mcat.2020.110976

A series of supported Ir-Re catalysts have been synthesized and used for the hydrogenation of adipic acid to 1,6-hexanediol. The influences of supporting materials and the Ir/Re atomic ratio on the catalytic performances have been studied. Results suggested that Ir-Re supported on carbon materials and alumina had appropriate acid sites and better activity for the hydrogenation of adipic acid. Compared to the monometallic catalysts, synergistic interaction was generated and electrons were delivered from Ir to Re. The uniform distribution of metal particles in the Ir-Re catalysts and the well restrained H₂-spillover effect facilitated the transformation of adipic acid and the selective production of 1,6-hexanediol. The selectivity of 1,6-hexanediol was 59% with complete conversion of adipic acid at 180 °C in 10 MPa H₂ after reaction for 16 h. After four times of reaction, the selectivity of 1,6-hexanediol only decreased about 4%.

Full text of DOI:10.1016/j.mcat.2020.110976

Molecular Catalysis 490 (2020) 110976
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Hydrogenation of adipic acid to 1,6-hexanediol by supported bimetallic Ir-
Re catalyst
Xiaoyue Li, Jingjie Luo, Changhai Liang*
State Key Laboratory of Fine Chemicals and Laboratory of Advanced Materials & Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology,
Panjin 124221, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of supported Ir-Re catalysts have been synthesized and used for the hydrogenation of adipic acid to 1,6-
hexanediol. The influences of supporting materials and the Ir/Re atomic ratio on the catalytic performances have
been studied. Results suggested that Ir-Re supported on carbon materials and alumina had appropriate acid sites
and better activity for the hydrogenation of adipic acid. Compared to the monometallic catalysts, synergistic
interaction was generated and electrons were delivered from Ir to Re. The uniform distribution of metal particles
Hydrogenation
Ir-Re catalyst
Adipic acid
1
,6-hexanediol
Synergistic interaction
in the Ir-Re catalysts and the well restrained H
the selective production of 1,6-hexanediol. The selectivity of 1,6-hexanediol was 59% with complete conversion
of adipic acid at 180 °C in 10 MPa H after reaction for 16 h. After four times of reaction, the selectivity of 1,6-
hexanediol only decreased about 4%.
2
-spillover effect facilitated the transformation of adipic acid and
2
Introduction
Considering the availability factor of metal atoms and the possible
minimization of metal pollution, precious metals are even appealing
[15]. Early practical studies confirmed their high-performances to
transform one or even a mixture of dicarboxylic acids into diols
[16,17]. One of the precious metal- rhenium, has high affinity to adsorb
the carboxyl groups and improves effectively the hydrogenation of
benzene ring-containing carboxylic acid, fatty acid, and dicarboxylic
acid, especially if promoted by a secondary precious metal [18–23].
The carboxylic groups in fatty acids could be hydrogenated controllably
to either alcohols or alkanes by the Re-Pd bimetallic catalyst depending
on the reaction conditions [24]. The specific interaction between bi-
metals changed the particle sizes and suppressed the agglomeration of
particles [25]. It was also reported that the Re species was preferred to
be enriched on the surface of Pt, resulting in the enhanced hydro-
genation rate of C = O bond and higher selectively to unsaturated al-
cohol [26]. Takeda and co-workers [27] applied the Re-Pd catalysts
with 14 wt% Re for the selective hydrogenation of adipic acid. The
1,6-hexanediol, as a valuable intermediate to fine chemicals, can be
applied in a variety of industries [1–3]. Especially in recent years, the
market requirement for 1,6-hexanediol increases greatly due to its
raising consumption in the Asia-Pacific region. Conventional produc-
tion of 1,6-hexanediol from adipic acid is processed mainly by a two-
step process: (i) esterification between adipic acid and methanol; (ii)
selective hydrogenation of dimethyl adipate (DMA) to synthesis the 1,6-
hexanediol. However, the second step went through severe reaction
2
conditions under higher H pressure (10-20 MPa) in the temperature
range of 250-350 °C, using traditional copper-chromium or zinc-chro-
mium catalysts with toxicity [4,5]. Although the precious metal based
catalysts were found efficient for the two-step hydrogenation [6–9], the
separation of organic solvent and the pre-esterification significantly
increased the raw cost.
During the past decades, direct hydrogenation of carboxylic acids to
their related alcohols is becoming a preferential pathway [10–12].
Recent work suggested that catalyst derived from copper on phyllosi-
licate displayed preferential selectivity to 1,6-hexanediol compared to
2
selectivity of 1,6-hexanediol reached 74% within 96 h under 8 MPa H .
It was also suggested that different from the monocarboxylic acid, the
presence of dicarboxylic acids caused the excessive oxidation of Re
species and as well as the lower activity. Typical research studies by
2
the traditional Cu/SiO by impregnation method [13]. Addition of Ni
with the total Cu + Ni loading of 40 wt% further promoted the turnover
x
Tomishige and co-workers have been focused on the Ir-ReO catalysts
for a variety of hydrogenation reactions. It was previously reported that
monometallic Ir catalyst usually performed lower activity for
frequency (TOF) to 15.1 h- [14]. The positively charged Cu species in
1
+
the reduced sample was claimed responsible for the higher TOF value.
⁎
Corresponding author.
E-mail address: changhai@dlut.edu.cn (C. Liang).
https://doi.org/10.1016/j.mcat.2020.110976
Received 18 February 2020; Received in revised form 23 March 2020; Accepted 20 April 2020
2468-8231/ © 2020 Elsevier B.V. All rights reserved.

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
hydrogenation reactions than other precious metals, e.g. Rh, Ru, Pt, and
Pd [28]. Current work revealed the wide range of hydroconversion
ability of Ir species for the unsaturated aldehydes, ketones and its ex-
cellent activation performance for C = O bonds [29–31]. Compared to
the monometallic Ir sample, the bimetallic samples displayed much
enhanced synergistic interaction between metals, with superiority for
was subsequently carried out and recorded from 50 °C to 900 °C. Before
each experimental test, the sample was pre-reduced in H
2
/Ar stream at
400 °C for 2 h except the H -TPR, to get the identical state of the cat-
2
alyst as those for the catalytic reaction.
Evaluation of catalytic performance
the activation and selective hydrogenation of C = O, -O-C-CH
2
OH, C-O
n+
[
32–36]. Liu et al. regarded the Re species as the main active sites to
The aqueous phase hydrogenation of adipic acid was operated in a
hastelloy autoclave equipped with magnetic stirrer, heater and con-
troller. The reaction solution consisted of 0.5 g adipic acid dissolved in
19.5 g deionized water. 0.2 g catalyst pre-reduced at 400 °C for 4 h in
adsorb the reactant and deactivation could be happened if the Re sites
were covered by Ir species [33]. No matter how, the direct hydro-
genation of adipic acid- a six-carbon dicarboxylic acids, brings great
challenges compared to monocarboxylic acids or dicarboxylic acids
such as succinic acid and pentanedioic acid. The longer chains of adipic
acid make the easier formation of by-products such as gaseous products
H /Ar stream was consumed for each reaction time. After the reactor
2
was charging and discharging with hydrogen for three times, the re-
actor was finally filled with hydrogen under the desired pressure. The
stirring speed was kept at 800 rpm. After the reaction was finished, the
reactor was cooled down to room temperature. The composition of the
final products in the liquid phase solution were analyzed by Shimadzu
high-performance liquid chromatography (HPLC, RID-20A) and the gas
chromatography (Agilent GC-7890).
(
2 4 2 2
e.g. CO , CH , C H etc.), cyclization compounds, and macromolecular
compounds by condensation/polymerization processes [27]. The func-
tional roles of bimetallic Ir-Re catalysts with higher atomic utilization
during the hydrogenation of adipic acid are still needing investigation.
In this work, a series of Ir-Re bimetallic samples have been syn-
thesized and used for the hydrogenation of adipic acid to 1,6-hex-
anediol. The influences of the supporting material and the Ir/Re atomic
ratio on the hydrogenation process have been revealed. The functional
roles of the chemical composition and the surface property of catalyst
have been studied and correlated with the catalytic performances.
The conversion of adipic acid (x ), selectivity (Si) and the relative
A
yield s (Yi) of different products were calculated by the following
equations:
n
0
− n
t
Conversion(x
A
) =
× 100%
n
0
n
i
Materials and methods
Selectivity(S
i
) =
× 100%
n
0
− n
t
Catalyst preparation
Yield(Yi) = xA × Si × 100%
A series of Ir-Re bimetallic catalysts were prepared by impregnation
0 t
Where n represents the initial number of moles of adipic acid and n
method. 0.05 M aqueous solution of ammonium perrhenate (NH
4
ReO
4
)
represents the number of moles of adipic acid after reaction. n
presents the number of moles of product i.
i
re-
and 0.02 M acetylacetonate (C15 Ir) in acetone were used as the
21 6
H O
metal precursors. Certain amounts of the above solutions were mixed
and dissolved in 50 ml distilled water to get the required Ir/Re atomic
ratio with total metal loading of 3 wt%. After mixed uniformly, 1 g of
support was impregnated in the aqueous solution. After sonication
under stirring for 30 min, the sample was aged at room temperature for
Results and discussion
Influences of supporting materials for the Ir-Re catalysts
1
1
6 h. The catalyst was dried subsequently in an oven at 110 °C for
0 hours without wash or filter process. The as-synthesized bimetallic
A series of bimetallic Ir-Re samples on different supports have been
synthesized with theoretical metal loading of 3 wt%. The structures and
crystal phases of typical samples are detected by the XRD patterns as
shown in Fig. 1a.
samples were denoted as Ir
Ir/Re as x: y. M denoted the supporting materials including the AC-p
x y
Re /M with the theoretical atomic ratio of
(
(
Ir
pristine activated carbon), AC (HNO
carbon nanotubes), SBA-15, and Al
/AC and Re /AC samples were also prepared by the same method.
Before each characterization and reaction evaluation, the tested sample
was pre-reduced in 40% H /Ar gas mixture at 400 °C for 4 hours, with
3
oxidized activated carbon), CNT
1 1
In the Ir Re /SBA-15 catalyst, two diffraction peaks can be observed
2
O
3
. For comparison, monometallic
in the range of 20-60°. The broad diffraction peak at 23.5° is related to
the amorphous silica. The diffraction peaks of Ir (111) plane and Re
(101) plane should appear at 40.5° (JCPDS 06-0598) and 42.9° (JCPDS
05-0702), respectively [32,37]. However, only one small and dispersive
3
3
2
the heating rate of 5 °C/min.
1 1
peak at about 42.5° is observed in the Ir Re /SBA-15, which is a sign of
the co-existence of interacted Ir-Re compositions [32,38]. Two broad
diffraction peaks at 20-30° and 40-50° related to the (002) and (100)
crystal faces of amorphous carbon can be observed in carbon supported
samples [39,40]. It is not easy to identify the formation of Ir and Re
species due to the overlap of diffraction peaks by amorphous carbon.
Catalysts characterization
Powder X-ray diffraction (XRD) patterns were obtained using a
000S at 40 kV and 100 mA with scan speed of 10 °/min.
7
a
Transmission electron microscopy (TEM) was performed with the
Tecnai G2 F30 transmission electron microscope to investigate the
morphology and particle sizes of the catalysts. XPS was conducted by
the ESCALAB™250Xi with monochromatic Al Kα radiation. The tem-
perature-programmed reactions were performed by the Quantachrome
ChemStar3000 equipment. The catalyst was preliminarily kept at 30 °C
3 3
For more confirmation, monometallic Ir /AC and Re /AC samples are
also synthesized and detected by XRD as shown in Fig. 1b. Although the
intensive carbon peaks in the range of 40-45° overlap the metal peaks,
the different shapes of peaks are observed probably due to the forma-
tion of metallic species. A shoulder peak appears at 25-27° in both the
3 3
monometallic Ir /AC and Re /AC samples, indicating the possible ex-
for 30 min in Ar to remove the impurity. In the H
grammed reduction (H -TPR), after the temperature was cooled down,
the reduction process was carried out in 10% H /Ar and the data was
recorded from 50 °C to 900 °C at the heating rate of 5 °C/min. In the
ammonia-temperature-programmed desorption (NH -TPD) and H -TPD
experiments, after the pre-purging of Ar, the tested sample was kept in
the adsorption gas atmosphere (5% NH /He for NH -TPD and 10% H
Ar for H -TPD) at 50 °C for the full adsorption. The desorption process
2
-temperature-pro-
istence of metal species in oxidation states [41,42].
TEM images was carried out to further confirm the surface in-
formation (Fig. 2) and the distribution of the metal species. The TEM
2
2
images of Ir
surface of carbon materials. The average particle sizes of Ir
and Ir Re /AC are 4.2 nm and 4.7 nm, respectively. The TEM image of
the Ir Re /SBA-15 clearly illustrates the ordered mesoporous structure
of the SBA-15. However, particles are aligned or even aggregated in the
1
Re
1
/CNT and Ir
1
Re
1
/AC suggest the fine particles on the
3
2
1
Re /CNT
1
1
1
3
3
2
/
1
1
2
2

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
1 1
Fig. 1. XRD patterns of Ir Re catalysts on different supports (a) and monometallic Ir and Re samples supported by activated carbon (b).
Ir
1
Re
1
/SBA-15 sample. Simultaneously, very few numbers of particles
Re /SBA-15, which is mainly due to the sin-
selectivity of 1,6-hexanediol and n-hexanol is less than 10% with most
reactants converted into gaseous products and macromolecular de-
can be observed in the Ir
1
1
tering of particles during thermal treatment. It is worth to note that the
carbon materials and SBA-15 are both supports with acidity (point of
zero charge < 7.0) [43]. In fact, the acidity of carbon materials should
be related with its surface oxygen groups as reported everywhere
posits. Ir
1
Re
1
/AC and Ir
1
Re
1
/AC-p samples supported on carbon ma-
Re /CNT sup-
terials display better performances. However, the Ir
1
1
ported on carbon nanotubes performs unsatisfied performance with
only 40% conversion of adipic acid and 35% selectivity of 1,6-hex-
anediol. The adipic acid conversion and 1,6-hexanediol selectivity by
[
43,44], which may be responsible to interact with the metal precursor
and cause the fine dispersion of metal species.
Ir
the surface acidity of supported Ir
with the catalytic performances. The Ir
samples with abundant acid sites deserve the better activity and yield of
1,6-hexanediol. Pretreated activated carbon supported Ir Re /AC dis-
1
Re
1
/Al
2
O
3
reach 99% and 49%, respectively. It is remarkable that
Re catalysts seems to be relevant
Re /AC and Ir Re /Al
The pure supports such as silica, alumina and carbon materials do
not possess activity for the selective hydrogenation of adipic acid.
However, the supporting materials do result in distinctive catalytic
performances for the hydrogenation process, even if the bimetallic
1
1
1
1
1
1
2 3
O
1
1
particle sizes are similar (except the Ir
the surface property especially the surface acidity of different sup-
porting materials may also influence the adsorption/activation of H
and reactants [45], which further facilitates or restrains the catalytic
activity. Under such consideration, the NH -TPD results are conducted
to reveal the surface acid sites as shown in Fig. 3. Several overlapped
peaks appear by different catalysts. The desorption peak at lower
3
temperature of 50-300 °C is related to NH adsorbed on the surface
1
Re
1
/SBA-15). It is inferred that
plays the better catalytic performance than other samples, the yield of
1,6-hexanediol reaches 59% with the complete conversion of adipic
acid.
The Ir-Re bimetallic catalyst supported on pretreated activated
carbon displays appropriate surface chemistry with better catalytic
performance. Herein, pretreated activated carbon is used as typical
support to understand the role of Ir/Re atomic ratio in promoting the
chemical and surface properties as well as the catalytic behavior.
2
3
weak acid sites. The peaks in the range of 300-500 °C should be as-
signed to the medium strong acid sites [46,47]. The desorption peaks at
x y
Surface and chemical properties of Ir Re /AC
1 1 1 1 2 3
500-700 °C in Ir Re /SBA-15 and Ir Re /Al O are resulted from the
strong acid sites due to the acidic surface of the support. There are also
obvious desorption peaks in the carbon supported bimetallic catalysts at
Ir-Re based catalysts with different Ir/Re atomic ratios have been
synthesized and the XRD patterns are shown in Fig. 5. Different from
the Ir-Re catalysts supported on other materials, an additional diffrac-
tion peak at 26.2° appears, which is attributed to the metal species in
oxidation states. Unique interaction between Ir-Re is possibly induced
on the pre-functionalized activated carbon. And distinctive surface
compositions and metal distribution are able to be generated in these
samples.
7
00 °C and even higher temperature, which should be ascribed to the
carbon reduced by NH . In fact, only the Ir Re /Al , Ir Re /AC, and
Ir Re /AC-p samples have the desorption peaks related to the weak acid
sites at 100-200 °C. Whilst no obvious consumption peaks of weak or
medium strong acid sites can be observed in the Ir Re /SBA-15 and
Ir Re /CNT samples.
The catalytic performances to hydrogenate adipic acid by Ir Re
1
3
1
1
2
O
3
1
1
1
1
1
1
1
1
1
1 1
The morphology of the Ir Re /AC sample is observed by HAADF-
STEM EDS elemental mappings. Small and highly dispersed metal
particles can be revealed as the shining spots in the STEM images
catalysts supported on different materials were investigated (Fig. 4),
including the carbon materials (activated carbon and CNT), alumina
and silica (SBA-15). Ir-Re bimetallic composition supported on SBA-15
displays poor activity with only 23% conversion of adipic acid. The
(
Fig. 6). It is clearly seen that both the Re and Ir elements are uniformly
distributed on the surface of activated carbon. There is no aggregation
1 1 1 1 1 1
Fig. 2. TEM images of the Ir Re /CNT (a), Ir Re /AC (b) and Ir Re /SBA-15 (c) samples.
3

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
3
Fig. 3. NH -TPD spectra of Ir-Re bimetallic samples supported by different materials.
or sintering of particles observed from the elemental mappings.
The TEM images of monometallic Ir /AC and Re /AC samples are
displayed in Fig. 7 for comparison. The average particle sizes of Ir /AC
and Re /AC samples are 2.6 and 1.8 nm, respectively. It is suggested
that both fine particles of Ir and Re species on the surface can be easily
formed and anchored on the surface of carbon. Compared to the Re /AC
3
3
3
3
3
catalyst, Ir-containing particles with narrower size distribution are
generated. No matter how, the bimetallic particles are becoming even
uniformly distributed on the surface with slightly increased sizes
compared to the monometallic particles, mainly due to the complexity
of contact interface and interaction between the bimetallic composi-
tions.
The XPS spectra in the Re4f and Ir4f regions are displayed in Fig. 8.
Two broad peaks can be observed in the Re4f region and deconvoluted
into several overlapped peaks. Two pairs of peaks related to Re4f7/2 and
4f5/2 are locating at 40.5 and 45.2 eV, which should be corresponding
6
+
to the presence of surface Re° and Re
species.
In the 3 wt% monometallic Re
3
/AC sample, Re6+ as the major
Fig. 4. Catalytic performances of different carriers supported Ir
Reaction conditions: 200 mg catalyst with 0.5 g adipic acid in 19.5 g H
reaction at 180 °C in 8 MPa H for 16 h).
1
Re
1
catalysts
(
2
O,
component (93%) are detected with only few Re° species (7%). After
small amount of Ir decoration, the Re4+ species (13%) at binding en-
2
ergy of 42.6 eV appears in the Ir
mation of novel metal sites in the bimetallic particles. The total per-
centage of reduced Re species (Re4+ and Re°) is 25% in the Ir
Re /AC
sample. As the Ir content further raises, the Re6+ species on the surface
become 84% and 86% in Ir Re /AC and Ir Re /AC samples, respec-
1 2
Re /AC sample, suggesting the for-
1
2
1
1
2
1
tively. It is interesting that appropriate Ir decoration leads to the for-
mation of more reduced Re species. It is also worth to note the binding
energy of Re6+ species slightly shifts to higher B.E. with the additional
Ir species. Meanwhile, two pairs of peaks can be observed in the Ir4f
4
+
3
XPS spectra of Ir /AC, which should be resulted from the Ir (61.5 and
4.5 eV) and Ir5 (62.2 and 65.2 eV) species [48,49]. 78% of Ir
species is present in the monometallic Ir/AC sample. After the co-dec-
oration of small amount of Re species in Ir Re /AC, the surface per-
+
4+
6
2
1
centage of Ir4+ species greatly decreases to 44%, whilst the metallic Ir°
species generates with the appearance of binding energy at 60.9 eV and
accounts for 32% of the surface Ir species [50]. Further increase of the
Re amount causes the decrease of the reduced Ir species. It is suggested
that the formation of metal species obviously depends on the atomic
ratio between Ir and Re. New species generated on the surface in the Ir-
5
+
Re catalysts. Considering the formation of reduced Re species and Ir
Fig. 5. XRD patterns of the Ir-Re catalysts supported by pretreated activated
carbon.
species in the bimetallic samples, it is inferred that specific interaction
between Re and Ir is constructed. Electrons are likely to be transformed
from Ir atoms to the Re atoms. The synergistic interaction between Re
4

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
1 1
Fig. 6. The STEM-EDS elemental mappings of the Ir Re /AC catalyst.
and Ir may also adjust the morphology of bimetallic particles, causing
the rearrangement of different compositions, as revealed by the much
uniform particles distributed on the surface in bimetallic samples.
In order to understand the reducibility of precursor and the redox
2
properties of the active sites in different catalysts, H -TPR has been
operated and the results are shown in Fig. 9a. Broad reduction peaks
exist in the range of 100-400 °C and 500-750 °C. The peaks at higher
temperature are resulted from the methanation of carbon in the pre-
2 3 3
sence of H [51]. In the monometallic Ir /AC and Re /AC samples, huge
desorption peaks are observed with the maximum at 280 °C and 350 °C,
respectively, which should be mainly resulted from the hydrogen
overflow usually happened on the surface of precious metal, and was
reported to be efficient for the hydrogen evolution reaction [52].
However, in the current hydrogenation reaction, the hydrogen overflow
of monometallic catalyst may largely impair their ability to adsorb and
2
dissociate H molecules, resulting in the restrained hydrogenation
ability of catalysts. In the Ir-Re bimetallic catalysts, there is no intensive
reduction peaks observed before 400 °C. Only small and broad reduc-
tion peak at about 100 °C appears in the Ir Re /AC sample and should
1 1
be related to the reduction of surface metal species in oxidation states.
The remarkable hydrogen spillover effect can be efficiently restrained
in the bimetallic Ir-Re samples. It has also to be noted that very few
2 1 2 2 1
amount of H consumption in the Ir Re /AC and Ir Re /AC is detected
Fig. 7. TEM images of monometallic Ir
3
/AC (a-b) and Re
3
/AC (c-d) samples.
in the range of 100-400 °C, although XPS spectra suggested the ex-
istence of surface metal species in oxidation states. The intensive in-
teraction between Ir and Re may also hinder the co-reduction of the
2
bimetallic compositions. Fig. 9b is the H -TPD profiles of different
Fig. 8. XPS spectra in the Re4f region (a) and Ir4f region (b) of different sample.
5

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
2 2 x y
Fig. 9. H -TPR (a) and H -TPD profiles of the monometallic and bimetallic Ir Re /C samples.
samples. Observable desorption peaks are locating at 90 °C and 105 °C
of Re /AC and Ir /AC, respectively. It can be ascribed to the adsorbed
molecules by surface precious metal species. Only slight variation of
peak maximum to the higher temperature can be found with the raising
ratio of additional Ir species.
The possible roles of Ir-Re during hydrogenation
3
3
H
2
Based on the characterization and experimental results, bimetallic
Ir-Re particles supported on pre-treated AC deserve better catalytic
performances. In fact, both the Ir and Re species could act as the active
sites for the current hydrogenation process, although their activity is
relatively mean. In a typical bimetallic sample, rhenium species in
oxidation states with electron-deficient surface are easily acting as the
acid sites to adsorb/activate the C = O bond in the adipic acid [53].
x y
Catalytic performances of Ir Re /AC
The catalytic performances to hydrogenate adipic acid by a series of
Re /AC catalysts have been displayed in Fig. 10. The monometallic
/AC and Re /AC samples also possess the catalytic performances.
/AC and Re /AC are only 35% and
9%, respectively. The selectivity of 1,6-hexanediol is as low as 2% and
0% by the Ir /AC and Re /AC samples. It is suggested that mono-
NH
3
-TPD also suggested the positive correlation between the catalytic
Re samples and the number of weak acid sites. H
2
Ir
Ir
x
y
performances of Ir
1
1
3
3
molecules are ready to be adsorbed, activated and dissociated on the
surface of reduced metal species [13,53]. However, the functional roles
of Ir and Re species in the bimetallic particles are more than physical
collaboration. Different from the monometallic samples, H
restrained evidently in the Ir Re bimetallic samples. Whilst the acti-
vation and adsorption ability of H are only slightly influenced in the
The conversions of adipic acid by Ir
1
1
3
3
3
3
2
-spillover is
metallic samples did not have enough catalytic activity to activate or
dissociate of the reactant.
x
y
2
Whilst after the addition of secondary metal composition, the Ir-Re/
AC samples display much enhanced activity of both the conversion of
adipic acid and the yield of 1,6-hexanediol. The atomic ratio between Ir
and Re significantly influences the catalytic performance. Compared to
bimetallic Ir-Re samples compared with the monometallic samples. It is
also consistent with the previous suggestion that the activation ability
of H
dissociation of H
confirmed more predominant in the bimetallic Ir-ReO
x
2
were barely influenced after ReO
to hydride and protons stimulated by the ReO
samples. The
x
addition [53]. The heterolytic
2
x
was
the Ir
1
Re
2
/AC and Ir
2
Re
1
/AC, the Ir
1
Re
1
/AC has better performance.
The conversion reaches 77% with 52% selectivity of 1,6-hexanediol. No
matter how, challenges still exist considering the harsh conditions such
as high pressure and the strict acid atmosphere of the catalyst during
the reaction. Herein, cycle reactions have been investigated using
Ir Re /AC as shown in Fig. 10b. After four times of reaction, although
1 1
the conversion of adipic acid decreases from 77% to 66%, only 4% loss
of 1,6-hexanediol selectivity can be observed.
surface structure and chemical compositions of the bimetallic particles
have been greatly modified compared to the monometallic samples. In
the bimetallic sample such as Ir Re /AC, the particles are more uni-
1 1
formly distributed on the surface with slightly larger sizes around 4 nm.
It is believed that the metal elements in the bimetallic samples may go
through re-arrangement due to the co-existence of Ir and Re species.
Surface information detected by XPS spectra confirms the generation of
bimetallic synergy. The chemical compositions change along with the
x y 1 1
Fig. 10. Catalytic performances for hydrogenation of adipic acid by Ir Re /AC samples (a) and the cycle reaction ability of the typical Ir Re /AC sample (b). Reaction
conditions: 200 mg catalyst with 0.5 g adipic acid in 19.5 g H
2
2
O, reaction at 180 °C at 8 MPa H for 8 h.
6

X. Li, et al.
Molecular Catalysis 490 (2020) 110976
Ir/Re atomic ratio, and electrons are delivered between the Ir and Re
species. Specific Ir-Re bimetallic particles with Ir/Re ratio of 1:1 facil-
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tween metals. The H
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loaded TiO
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2
: A highly versatile and chemoselective catalyst for the hydrogenation
2
and CO
2
,
Xiaoyue Li: Data curation, Investigation, Writing - original draft.
Jingjie Luo: Conceptualization, Formal analysis, Funding acquisition.
Changhai Liang: Writing - review & editing, Supervision, Project ad-
ministration, Funding acquisition.
2
2
aqueous-phase selective hydrogenation of succinic Acid to 1,4-butanediol as a
function of palladium and rhenium deposition methods, ChemCatChem 7 (14)
(2015) 2161–2178, https://doi.org/10.1002/cctc.201500197.
Declaration of Competing Interest
[
[
21] Z.F. Shao, C. Li, X. Di, Z.H. Xiao, C.H. Liang, Aqueous-phase hydrogenation of
succinic acid to gamma-butyrolactone and tetrahydrofuran over Pd/C, Re/C, and
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There is no conflict to be declared.
Acknowledgements
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(22) (2017) 5212–5223, https://doi.org/10.1039/c7cy01039g.
This work was supported by the National Natural Science
Foundation of China (21573031, 21978031 & 21802015), and Program
for Excellent Talents in Dalian City (2016RD09, 2016RD04).
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