The Low Dimensional Co-Based Nanorods as a Novel Platform for Selective Hydrogenation of Cinnamaldehyde

Article abstract of DOI:10.1007/s10562-019-02787-7

Abstract: Since hydrogenation of C=C bond in the cinnamaldehyde is thermodynamically favored, the selective hydrogenation of C=O group is challenging. Developing effective catalysts for this transformation has been hindered by the intrinsic disadvantages of traditional materials for decades. Hereby, we report the synthesis of the low dimensional Co based nanorods (NRs) as the effective platform for C=O groups hydrogenation in the conjugated compounds. The Pt/Co-NRs catalyst is simply fabricated by loading the Pt nano-particles (NPs) on the Co-NRs and the stability of the Co-NRs support is improved by coordination between the Pt NPs and the pyridinic N ring. Resorting to XRD, FT-IR, XPS, HRTEM, DTG-TG characterization methods, the catalytic mechanism for C=O bond hydrogenation has been proposed. The synergistic effects of K⁺ and OH^? enhance the polarization of C=O group, leading to more adsorption of C=O groups on the Co-NRs so as to promote its hydrogenation performance. In the absence of spatial micropores in low dimensional Co based nanorods, the Pt/Co-NRs catalyst is more advantageous for mass transfer. Under optimal conditions, the conversion of cinnamaldehyde is 97.9% with 92.7% selectivity of cinnamyl alcohol within 3 h. In addition, the selectivity of cinnamyl alcohol changes slightly (only 2.4% fluctuations) after five recycle tests. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02787-7

Catalysis Letters
https://doi.org/10.1007/s10562-019-02787-7
The Low Dimensional Co‑Based Nanorods as a Novel Platform
for Selective Hydrogenation of Cinnamaldehyde
Tao Yuan1 · Derong Liu1 · Jianshan Gu1 · Yongde Xia2 · Yue Pan1 · Wei Xiong1
Received: 27 January 2019 / Accepted: 11 April 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Since hydrogenation of C=C bond in the cinnamaldehyde is thermodynamically favored, the selective hydrogenation of C=O
group is challenging. Developing efective catalysts for this transformation has been hindered by the intrinsic disadvantages
of traditional materials for decades. Hereby, we report the synthesis of the low dimensional Co based nanorods (NRs) as the
efective platform for C=O groups hydrogenation in the conjugated compounds. The Pt/Co-NRs catalyst is simply fabricated
by loading the Pt nano-particles (NPs) on the Co-NRs and the stability of the Co-NRs support is improved by coordination
between the Pt NPs and the pyridinic N ring. Resorting to XRD, FT-IR, XPS, HRTEM, DTG-TG characterization methods,
the catalytic mechanism for C=O bond hydrogenation has been proposed. The synergistic efects of K+ and OH− enhance the
polarization of C=O group, leading to more adsorption of C=O groups on the Co-NRs so as to promote its hydrogenation
performance. In the absence of spatial micropores in low dimensional Co based nanorods, the Pt/Co-NRs catalyst is more
advantageous for mass transfer. Under optimal conditions, the conversion of cinnamaldehyde is 97.9% with 92.7% selectiv-
ity of cinnamyl alcohol within 3 h. In addition, the selectivity of cinnamyl alcohol changes slightly (only 2.4% fuctuations)
after fve recycle tests.
Keywords Low dimensional · Co based nanorods · Cinnamaldehyde · Selective hydrogenation · Cinnamyl alcohol
1 Introduction
sieves [13] and polymers [14] could not compensate the
thermodynamical diference in hydrogenation between C=O
bond and C=C bond (in favour of the C=C group hydrogen-
ation) in the conjugated compounds, which leads to the low
catalytic efciency for single C=O hydrogenation. There-
fore, the way of modifcation traditional support by adding
activated sites has been developed to deal with low catalytic
efciency for single C=O bond hydrogenation [15, 16].
Three dimensional Metal Organic Framework struc-
ture (MOFs) with intrinsic unsaturated metal sites has
drawn much attention by virtue of the ordered and tune-
able pores, large surface area, diverse structure which
ofer great potential to versatile applications especially in
selective hydrogenation feld [17, 18]. Those unsaturated
metal sites in MOFs can be acted as Lewis acid sites that
activate and lower the C=O bond energy of hydrogenation.
Up to now, MOFs containing metal NPs have been inten-
sively investigated on account of versatile and synergistic
functionalities. Chen et al. [19] found that noble metal
nanoparticles embedded in MOFs demonstrated novel
properties due to the confnement efect and the structural
fexibility of MOFs. The efect of the MOFs aperture size
Although cinnamyl alcohol is the important raw materials to
produce favouring, perfume and medicine [1, 2], the trans-
formation of the selective hydrogenation of cinnamaldehyde
is challenging. The preparation process of the catalyst for
this transformation is also complex and tedious, which often
involves high temperature or uncommon methods [3–6].
Besides MgO [7], high-valence metal oxides [8–10], GO
[11] could be directly used as loading nano-scale metal par-
ticles materials without postmodifcations and achieve a high
selectivity to cinnamyl alcohol. In absence of activated sites
that interact with and activate the C=O bond, most of tra-
ditional carriers including carbon materials [12], molecular
* Wei Xiong
18875024416@163.com
1
Institute of Chemistry and Chemical Engineering,
Chongqing University of Science and Technology,
Chongqing 401331, China
2
College of Engineering, Mathematics and Physical Sciences,
University of Exeter, Exeter EX4 4QF, UK
Vol.:(0123456789)
1 3

T. Yuan et al.
and fexible structure on the fnal reaction yield distribu-
tion of selective hydrogenation to cinnamaldehyde was
investigated by comparing Pt@ZIF-71 with Pt@ZIF-8
catalysts. Moreover, to induce better selectivity control
on the encapsulated metal NPs@MOFs in catalytic selec-
tive hydrogenation reaction, Guo et al. [20] invented UIO-
66-NH2 confned Pt nanoclusters catalysts. The beneft of
confning Pt NPs inside UIO-66-NH2 is the high selectiv-
ity of 91.7% in contrast to less than 72% of the deposi-
tion of Pt NPs on the external surface of the UIO-66-NH2.
Besides, the confned Pt@UIO-66-NH2 catalytic perfor-
mance remains highly stable after repeated experiments.
The only issue lies in the blockage of some micropores for
the confned Pt@UIO-66-NH2, thereby resulting in poor
mass transport. To overcome this problem, Zhao et al.
[21] developed the MOFs@NPs@MOFs catalyst, utiliz-
ing the epitaxial growth shell MIL-101 (Fe, Cr) layer as
the selectivity regulator for the hydrogenation reactions.
Exact regulation of the shell MIL-101 precursor concen-
tration can achieve efective mass transport between the
reactants and the products. In addition, those coordination
unsaturated metal sites (CUSs) inside MIL-101 can act as
Lewis acid sites that readily interact with the C=O bond
and lower the reaction-barrier energy, which contributes
to the C=O selective hydrogenation. Considering that
the hydrophilic shell MIL-101 (Fe) is difcult to gather
hydrophobic substrate, Yuan et al. [22] have applied con-
jugated micro- and mesoporous polymers with Iron(III)
porphyrin (FeP-CMPs) to replace the shell MIL-101(Fe)
and have innovated hydrophobic MIL-101@Pt@FeP-
CMP. The porous and hydrophobic shell is superior to the
hydrophilic MIL-101 (Fe) in gathering the hydrophobic
reactants. Furthermore, Fe(III) ions in the FeP-CMPs is
similar to the CUSs in MIL-101 that interacts with C=O
bond and benefcially breaks the conjugated C=O bond.
More interestingly, MIL-101@Pt@FeP-CMP can reach
ultrahigh turnover frequency (TOF) of 1516.1 h−1 with
97.3% selectivity towards cinnamyl alcohol and conver-
sion of 97.6%.
C=C and C=O bond over low dimensional MOFs materi-
als such as Co based nanorods.
Herein, the cinnamaldehyde selective hydrogenation is
chosen as the model reaction, which is shown in Scheme 1.
The low dimensional Co based nanorods with Pt NPs load-
ing have been fabricated by a simple wet impregnation
method. In contrast to Metal@MOFs type catalysts, the
reaction transformation efficiency could be improved
over the low dimensional Pt/Co-NRs catalyst. Moreover,
the Pt/Co-NRs catalyst can achieve efective hydrogena-
tion of single C=O bond without the aid of stereoscopic
micropores.
2 Experimental
2.1 Chemicals
Cinnamaldehyde (AR, 95%) were used without purifca-
tion, Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, AR,
Pt 37.5%), Cobalt nitrate hexahydrate (Co(NO3)2·6H2O,
AR, 99%), 4,4-Bipyridine (AR, 98%), PVP (K-30) were
purchased from Mackin, Potassium hydroxide (KOH, AR,
90%), Ethanol (AR, 99%) were purchased from ChengDu
Chron Chemicals.
2.2 Synthesis of Pt/Co‑NRs Catalyst
2.2.1 Synthesis of Pt Nanoparticles (NPs)
Pt NPs were synthesized according to the literature [27].
In a typical procedure, 50 mg PVP (K-30) was dissolved
in 72 mL ethanol, and then 8.0 mL H2PtCl6 aqueous solu-
tion (10 mM) was added drop by drop. After stirring for
10 min at room temperature, the solution was refuxed in
150 mL fask for 3 h in air to obtain the PVP-stabilized Pt
NPs. The concentration of the as-synthesized Pt NPs was
about 1 mM, which were used without further treatment.
Although the encapsulation of active metal nanopar-
ticles on selected MOFs (Metal@MOFs) is an excellent
way to promote the selectivity of unsaturated alcohol, the
instability of the three dimensional MOFs structure, the
complexity of catalyst-making and the high mass trans-
fer resistance are important issues to be solved. To tackle
these problems, low dimensional MOFs (nanosheet,
nanorods, nanofber) have been fabricated in the presence
of PVP. Without space stereoscopic structure, the superi-
ority of low dimensional MOFs in structure stability and
mass transfer without loss of metal particles have been
gaining extensive attention in catalysis [23–26]. To our
knowledge, there has been no study in the open literature
exploring the selective hydrogenation of the conjugated
Scheme 1 Reaction pathways for selective hydrogenation of CAL
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The Low Dimensional Co-Based Nanorods as a Novel Platform for Selective Hydrogenation of…
2.2.2 Synthesis of Co‑NRs
2.4 Selective Hydrogenation Reaction
Co based nanorods were synthesized according to the lit-
erature with minor modifcations [28]. In a typical pro-
cedure, 400 mg PVP and 2 mmol 4,4-bipyridine were
dissolved in 15 mL Ethanol, and another 15 mL ethanol
containing 1 mmol Co(NO3)2·6H2O was added into the
above solution. The obtained aqueous mixture was stirred
for 10 min. Subsequently, the solution was placed in a
microwave vessel and sealed. The vessel was then rapidly
heated to 100 °C, and was held at this temperature for 24 h.
After cooling to room temperature, the solid products were
collected by centrifugation at 12,000 rpm for 2 min and
washed twice with ethanol. Finally, the obtained Co-NRs
were placed in a vacuum drying oven at 80 °C overnight.
The hydrogenation reaction was performed in a 0.1 L
stainless steel autoclave equipped with a magnetic stirrer.
0.8 mmol cinnamaldehyde, 1 mL catalyst dosage (100 mg
Pt/Co-NRs in 10 mL ethanol), 0.1 mL KOH (0.02 M) and
5 mL ethanol were added into the stainless autoclave in
turn. The autoclave was kept sealed under hydrogen by
purging with H2 for 5 times. Then the autoclave was heated
to 30 °C to start the reaction. After a period of reaction
time, the autoclave was decompressed and the fnal liquid
was analysed by an Agilent GC-6890N gas chromatography
equipped with a FID detector and an Rtx-1 capillary column
(30 m×0.25 mm×0.25 µm) with N2 as the carrier gas.
3 Results and Discussion
2.2.3 Preparation of Pt/Co‑NRs Catalyst
3.1 Catalyst Analysis
A quota as-synthesized Pt NPs solution (1 mM) was added
drop by drop into 20 mL ethanol containing 100 mg Co-
NRs under vigorous stirring. Then the mixed solution
was further stirred at room temperature for 2 h. Subse-
quently, the Pt/Co-NRs was collected by centrifugation
at 12,000 rpm for 2 min and washed twice with ethanol.
Finally, the obtained Pt/Co-NRs solid was redispersed in
10 mL ethanol, which was used as the catalyst for the cin-
namaldehyde liquid-phase selective hydrogenation.
To gain a better comprehensive understanding of the cor-
relation between Pt/Co-NRs catalytic activity and structural
features, the Pt/Co-NRs catalyst has been fully characterized
by XRD, SEM, XPS, HRTEM, DTG-TG. The XRD pat-
terns of Pt/Co-NRs catalyst with diferent Pt loadings are
shown in Fig. 1. The peaks appearing at around 9.3°, 10.0°,
14.8°, 17.2°, 17.5°, 18.8°, 20.4°, 23.7° and 24.0° are origi-
nated from the Co-NRs. After 3% Pt NPs was loaded on the
surface of Co-NRs, those peaks at 2θ=10.0°, 17.2°, 18.8°,
20.4° and 23.7° disappear, and the characteristic peaks for
Co-NRs (2θ=9.3°, 17.5°) turn weak, which completely dis-
appear at 5% Pt NPs loading. The intensity of peak locating
2.3 Catalyst Characterization
The Powder X-Ray difraction (PXRD) scans of the syn-
thesized Pt/Co-NRs catalysts were carried out from 5°–50°
with an XRD-7000 X-ray difractometer (CuKα radiation
at 40 kV and 30 mA, Japan). The spectra of functional
groups of the Pt/Co-NRs catalyst were confrmed using
a Fourier transform infrared spectrum (FT-IR, KBr,
2000–500 cm−1) BRUKE TENS OR27 (Germany). The
elemental valence state and content of Pt and Co nanopar-
ticles of the Pt/Co-NRs catalyst were measured by X-ray
photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD,
UK) by a double anode Mg Ka X-ray source (75 W) and
an analyzer pass energy of 20 eV. The morphology and
microstructure of the sample, the HAADF-STEM imaging
and EDX mapping of Pt/Co-NRs catalysts were recorded
by a high resolution electron microscopy (200 kV, FEI
Talos F200x, America) and the samples were supported
by carbon coated copper grids. The DTA-TG data of Pt/
Co-NRs catalyst (10°C/min, 20–800°C, Ar) were recorded
by STA 449 F3 instrument (Netzsch, Germany).
Fig. 1 The XRD patterns of the Pt/Co-NRs catalyst with various Pt
loadings
1 3

T. Yuan et al.
at 14.8° increases as the Pt loading adds. Interestingly, the
peak appears in 24° that is ofset by 0.5° when Pt loading
exceeds 5%. Moreover, with the addition of Pt, the occur-
rence of new peaks at 2θ = 10.7°, 15.5°, 31.3° and 34.2°
indicates the possible formation of new crystalline phase
(Pt–N) between the Pt NPs and the Co-NRs carrier [29, 30].
Furthermore, the Peak appears at 2θ=40.1° is from the Pt
NPs (PDF No. 04-0802) and the Pt peak position is right of-
set by 0.3°, which also demonstrates the Pt NPs coordination
with N atom in pyridine ring. The strong difraction peak
of Pt NPs reveals that the Pt NPs were successfully loaded
on the surface of Co-NRs which is in accordance with the
HRTEM result.
Furthermore, to study the electron transfer between the
Pt NPs and the Co-NRs, the XPS has been applied. In
Fig. 3c, The N 1s peaks of Co-NRs occur at 399.7 and
406.8 eV. The 406.4 eV represents the N 1s shake-up peak,
which is feature of the conjugated π system based pyri-
dinic N [34]. But as Pt NPs are loaded, the disappearance
of the unique shake-up peak (406.4 eV) reveals the transfer
of the pyridinic N based conjugated π electron to Pt NPs
[35]. Meanwhile, the binding energy of Pt 4f7/2 and Pt 4f5/2
(70.3 eV and 73.5 eV) is lower than that of metal Pt0 (Pt0
4f7/2 = 70.9–71.3 eV; Pt0 4f5/2 = 74.2–74.4 eV from XPS
standard reference data in NIST), which implies that the
Pt NPs obtains the electron from the conjugated π sys-
tem in Pyridine ring. In addition, From the N 1s and Co
2p analysis, the binding energy of the Co–N coordinated
system reduces by 0.8 eV compared with the original Co-
NRs, which also suggests that part of the electrons transfer
from the conjugated π system in pyridine ring to the Co–N
coordinated system.
To further validate the interaction between Pt and the
pyridinic N ring in Co-NRs, the FT-IR has been applied to
confrm the coordination between the Pt NPs and the free
pyridinic N atoms in Co-NRs. In Fig. 2, these peaks occur-
ring at 1605 cm−1 are attributed to the C–N stretching vibra-
tions of pyridine ring [31]. The strong peak at 1384 cm−1
is assigned to the NO3− antisymmetric stretching vibration
of pyridine ring [32, 33]. After Pt NPs loading, the peak
intensity attenuation of the C–N stretching vibration indi-
cates the coordination of Pt NPs on the Co-NRs and the free
N atoms from pyridine ring. Interestingly, the intensity of
antisymmetric stretching vibration peak of NO3− ( appearing
at 1384 cm−1) signifcantly decreases and then splits into
another separate peaks at 1411 cm−1, probably because the
In order to elucidate the micro structure, components and
active ingredient (Pt NPs) dispersion of the Pt/Co-NRs cata-
lyst, the HRTEM with element mapping has been employed.
The Co-NRs support shows rod-like structure with 88.1 m2/g
surface area in Fig. 4a. The nanorod structure remains
unchanged and the surface area is 84.7 m2/g after 7% Pt NPs
are loaded on its surface. Moreover, the Pt NPs distribute
homogeneously on the surface of Co-NRs and the size of
Pt NPs closes to 1.8 0.5 nm (Fig. 4c). The lattice spacing
of d=0.227 nm is from the Pt NPs. The element mapping
images (Fig. 4f–i) clearly show the presence of the Pt, Co,
N three elements in the Pt/Co-NRs catalyst and all of them
are uniformly existed in the Pt/Co-NRs catalyst.
−
surface NO3 groups were concealed by the Pt NPs load-
ing and herein reduces the intensity of NO3− antisymmetric
stretching vibration in Co-NRs.
By comparing the DTG-TG profles of Co-NRs and Pt/
Co-NRs catalyst (Fig. 5), three weight losses are observable
in Co-NRs TG profle at 250.2, 280.3 and 514.1 °C, respec-
tively. The weight loss at 250.2 °C at a rate of − 5%/min is
due to the Co-NRs degradation. The main weight loss of
Co-NRs starts at 280.3 °C at a rate of − 23.4%/min, which
declares that the Co-NRs matrix is completely degraded
[36]. The third weight loss represents the carbonization of
4,4-bipyridine and PVP. The fnal materials contain Co, C,
N elements at temperature higher than 514.1 °C. However,
the DTG-TG curve of Pt/Co-NRs shows weight loss at much
lower rate. As shown in Fig. 5, the frst weight loss occurs
at 480.9 °C at a rate of − 7.2%/min that its matrix starts
to degrade and carbonization. Higher than 529.7 °C, the
ligands are carbonized into N doped carbon, which results
in the weight loss and the residual weight is 34.67%. These
results suggest that the Co-NRs stability is signifcantly
strengthened by the addition of the Pt NPs, as a consequence
of collective interaction between the Pt NPs and free pyri-
dinic N, and between Pt NPs and the conjugated π system
in pyridinic N ring.
Fig. 2 The FT-IR spectra of the Co-NRs support and Pt/Co-NRs cata-
lyst
1 3

The Low Dimensional Co-Based Nanorods as a Novel Platform for Selective Hydrogenation of…
a
Co-NRs
7% Pt/Co-NRs
b
N 1s
399.7 eV
C 1s
O 1s
406.8 eV
N 1s
Co 2p
398.9 eV
Co-NRs
Pt/Co-NRs
Pt 4f
800
600
400
200
0
410
408
406
404
402
400
398
396
B.E.(eV)
B.E.(eV)
70.3 eV
c
d
Co 2p
Pt 4f
73.5 eV
781.0 eV
Co-NRs
Pt/Co-NRs
780.2 eV
810
805
800
795
790
785
780
775
80
78
76
74
72
70
68
B.E.(eV)
B.E.(eV)
Fig. 3 The XPS profles of the Co-NRs support and 7% Pt/Co-NRs catalyst (a); N 1s (b); Co 2p (c); Pt 4f (d)
has employed to test the K+ role in the polarization of C=O
bond via the interaction between K+ and the lone electron
pair of oxygen atom in the C=O group. The COL selectivity
increases from 12.9 to 47.9%, which is lower than OH− in
promotion the C=O bond hydrogenation. The OH− mainly
plays role in weakening the competitive adsorption of C=C
bonds by the repulsion between π electron in C=C bond and
OH− anion in base solution, thus making the polarized C=O
easily absorbed over Pt/Co-NRs catalyst [37].
3.2 Optimization of Reaction Conditions
After the micro structure features of the Pt/Co-NRs catalyst
was analyzed, the catalytic performance of the Pt/Co-NRs
was further studied. The results are summarized in Table 1,
from which it can be observed that the Pt NPs mainly turn
CAL into HCOL without single C=O bond hydrogenation.
Conversely, the Co-NRs can provide much higher COL
selectivity (83.3%) (Entry 2, in Table 1). With increasing
addition of Pt, the conversion of CAL gradually increases
but the COL selectivity shows a slow downward trend from
91.5 to 87.3%. So the 7% Pt loading (ICP test result is 6.8%)
is chosen for the subsequent studies.
Various reaction conditions including KOH concentra-
tion, H2 pressure, reaction duration and solvent types were
investigated for optimization of experimental conditions.
The infuence of KOH concentration on the 7% Pt/Co-NRs
catalytic activity is presented in Fig. 6a. The CAL conver-
sion declines from 97.9 to 56.1% with increasing KOH
concentration because the base solution is unfavorable for
the H2 dissociation over the Pt NPs. Conversely, the COL
selectivity increases from 12.9 to 91.6%. The HCAL and
HCOL selectivity have a similarly downward trend. As the
H2 pressure goes up (Fig. 6b), the CAL conversion increases
Table 2 lists the efect of base type on the catalytic activ-
ity. By the addition of KOH, the COL selectivity signif-
cantly increases from 12.9 to 90.5%. However, there is no
major diference in the CAL conversion and COL selectiv-
ity among those inorganic and organic bases (Entry 2, 3,
4, 6, in Table 2). To fnd out the role of K+ and OH− in the
promotion of the C=O bond hydrogenation, KCl solution
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T. Yuan et al.
Fig. 4 The SEM images of Co-NRs (a), 7% Pt/Co-NRs (b–d) [the inserted fgure in (c): the Pt NPs size distribution diagram]; the HAADF-
STEM image (e), and element mapping of N (f), Co (g), Pt (h), all (i)
Table 1 The infuence of Pt loading on catalytic performance
Entry
Catalyst
Conversion
Selectivity
COL
HCAL
HCOL
1
2
3
4
5
6
Pt NPs
Co-NRs
3%Pt/Co-NRs
5%Pt/Co-NRs
7%Pt/Co-NRs
9%Pt/Co-NRs
100
0.1
83.3
91.5
90.9
90.5
87.3
24.3
4.8
8.4
5.6
4.1
5.3
51.6
11.9
0
3.4
5.4
7.3
13.0
19.9
56.3
80.5
85.6
Reaction conditions: 0.45 mg Pt (Entry 1); 1 mL catalyst dosage
(100 mg catalyst dispersion into 10 mL ethanol) (from Entry 2 to
Entry 6), 0.8 mmol cinnamaldehyde, 30 °C, 2 MPa H2, 0.1 mL KOH
(0.02 M), 5 mL ethanol, 120 min
Table 2 The comparison of diferent base type for infuence catalytic
Fig. 5 The DTG-TG profles of the Co-NRs support and 7% Pt/Co-
activity
NRs catalyst
Entry Base type
Conversion Selectivity
COL HCAL HCOL
from 52.5 to 90.2%. Surprisingly, the COL selectivity does
not change signifcantly and hold at 88.5–90.5%. However
the HCAL and HCOL selectivity have an opposite trend
that H2 pressure increase results in further hydrogenation
transformation of the HCAL and COL to HCOL, so that the
HCOL selectivity increases from 4.4 to 10.9%. Furthermore,
the reaction duration mainly afects the CAL conversion
(Fig. 6c). With the time extending to 180 min, the CAL con-
version increases from 60.5 to 97.9%, while the COL selec-
tivity changes slightly (only 5.8 fuctuations). In addition,
1
2
3
4
5
6
No
97.9
76.7
80.5
77.9
96.2
12.9
89.5
90.5
89.1
47.9
86.0
6.4
4.6
4.1
4.8
20.8
6.5
79.6
5.8
5.4
6.1
31.3
7.5
NaOH
KOH
K2CO3
KCl
Ethylenediamine 78.9
Reaction conditions: 1 mL catalyst dosage (100 mg 7% Pt/Co-NRs
catalyst dispersion into 10 mL ethanol and each 1 mL ethanol con-
taining 0.68 mg Pt NPs), 0.8 mmol cinnamaldehyde, 30 °C, 2 MPa
H2, base concentration (0.02 M), 5 mL ethanol, 120 min
1 3

The Low Dimensional Co-Based Nanorods as a Novel Platform for Selective Hydrogenation of…
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(mol/L)
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n-BuOH
n-PrOH
i-PrOH
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Time(min)
Solutions
Fig. 6 a Reaction condition: 1 mL catalyst dosage (100 mg 7% Pt/Co-
NRs catalyst dispersion into 10 mL ethanol and each 1 mL ethanol
containing 0.68 mg Pt NPs), 0.8 mmol cinnamadehyde, 30 °C, 2 MPa
(H2), 0.1 mL KOH (diferent concentration), 5 mL ethanol, 180 min;
b reaction condition: 1 mL Catalyst dosage (100 mg 7% Pt/Co-NRs
catalyst dispersion into 10 mL ethanol and each 1 mL ethanol con-
taining 0.68 mg Pt NPs), 0.8 mmol cinnamaldehyde, 30 °C, 0.1 KOH
(0.02 M), 5 mL ethanol, 180 min; c reaction condition: 1 mL catalyst
dosage (100 mg 7% Pt/Co-NRs catalyst dispersion into 10 mL etha-
nol and each 1mL ethanol containing 0.68 mg Pt NPs), 0.8 mmol cin-
namadehyde, 30 °C, 2 MPa H2, 0.1 KOH (0.02 M), 5 mL ethanol;
d reaction condition: 1 mL catalyst dosage (100 mg 7% Pt/Co-NRs
catalyst dispersion into 10 mL ethanol and each 1mL ethanol contain-
ing 0.68 mg Pt NPs), 0.8 mmol cinnamaldehyde, 30 °C, 2 MPa H2,
0.1 KOH (0.02 M), 180 min
the time prolonging is benefcial to the HCOL conversion
at a sacrifce of the HCAL selectivity. Finally, we studied
the infuence of polar solvents on the Pt/Co-NRs catalytic
activity (Fig. 6d). The polar solvents including methanol,
ethanol and i-PrOH are benefcial to both the conversion and
selectivity. It has been confrmed that C=O group polariza-
tion could be enhanced via the hydrogen bond formation in
low-molecular weight alcohols, thereby increase the COL
selectivity [38, 39]. Among these solvents, the ethanol
gives 80.5% CAL conversion and 90.5% COL selectivity.
Table 3 lists the Pt/Co-NRs catalytic activity for various α,
β-unsaturated aldehyde (ketone). From Table 3, it is clear
that α, β-unsaturated aldehyde with diferent substituents and
the position of substituent greatly afect the reaction conver-
sion and selectivity. The cinnamaldehyde conversion is up to
97.9%, but the 3-(2-furyl) acrolein and furfural conversion
only gains 59.0% and 69.9%, respectively, which indicates
that the presence of electron donating substitute (the electron
cloud density of furan group is higher than that of benzene)
is adverse for the reaction conversion. The electron donat-
ing substitute leads to the electron cloud density increase
in the conjugated C=C and C=O system that is hard for
reduction. Steric hindrance is an important factor to afect
the selectivity of C=O group hydrogenation. By contrast to
Entries 1 and 4, there exists larger spatial steric hindrance
close to the C=C bond end, which is benefcial for the C=O
group hydrogenation.
As a consequence, the position of the electron-donating
substitute next to the C=O bond end is extremely hostile for
the benzalacetone hydrogenation and results in low conver-
sion (45.8%) and selectivity (12.1%).
In Fig. 7, the catalytic mechanism has been proposed to
explain the efcient hydrogenation of C=O group over the
Pt/Co-NRs catalyst. Based on above experimental data, the
Co-NRs mainly determine the selectivity of C=O hydro-
genation. And the Pt NPs take charge of H2 dissociation
1 3

T. Yuan et al.
Table 3 Selective hydrogenation of diferent α, β-unsaturated alde-
and control the conversion of cinnamaldehyde transforma-
tion. The XPS results show that the Co-NRs with partial
negative charge is favored for the polarized C=O group
adsorption. Meanwhile, the π-electron in the C=C bond is
easily rejected by the surface OH− anion and the Ptδ− with
enhanced electronic density that favor a d-electron feedback
into the π* orbit of the polar C=O group [40, 41] and leads
to less hydrogenation of the C=C group on the surface of
Pt/Co-NRs catalyst. Besides, the synergistic efects of K+
and OH− enhance the C=O bond polarization and thereby
increase the C=O group hydrogenation.
hyde (ketone) with 7% Pt/Co-NRs catalyst
Entry Substrates
Conversion Selectivity
C=O C=C All
1
97.9
92.7
1.8
5.6
O
O
2
3
59.0
92.8
3.5
5.6
O
O
69.9
100
0
0
O
Among the reported catalysts, selecting microporous and
mesoporous materials with the shape-selective efect such as
MOFs, meso-SiO2, SBA as the catalyst carrier (like metals
encapsulation) is an excellent method to solve the low selec-
tivity of C=O hydrogenation in the cinnamaldehyde. These
materials can not only efectively control the size of noble
metal in the pore, but also regulate the way of substrate
adsorption that allows the low steric hindrance to pass into
the pore and reacts with the active ingredients. However, the
metals encapsulation structure is bad for the mass transfer
and prolongs the reaction time. Besides, the complexity of
catalyst-making and instability of the material structure are
urgent issues to cope with. Therefore, the low dimensional
Co based nanorods reported in this study could provide a
novel platform for the selective hydrogenation of cinnamal-
dehyde, which is compared with other reported catalysts for
the CAL hydrogenation and presented in Table 4.
4
5
64.5
48.5
69.1
12.1
25.2 5.7
78.4 6.7
O
O
Reaction conditions: 1 mL catalyst dosage (100 mg 7% Pt/Co-NRs
catalyst dispersion into 10 mL ethanol and each 1 mL ethanol con-
taining 0.68 mg Pt NPs), 0.8 mmol substrates, 30 °C, 2 MPa (H2),
0.1 mL KOH (0.02 M), 5 mL ethanol, 180 min
Figure 8 shows that the 7% Pt/Co-NRs catalyst maintains
the COL selectivity of 90.3–92.7% (only 2.4% fuctuations)
and CAL conversion of 77.8% after fve recycle tests. The
HRTEM has been applied to study the reason for the loss of
catalytic activity of Pt/Co-NRs catalyst.
The HRTEM image in Fig. 9 shows that the Co-
based nanorods keep integrity and there is not any Pt
NPs agglomeration. The Pt NPs particle size is around
2.17 0.38 nm that is slightly larger than the initial Pt
NPs size (1.8 0.5 nm). It is likely that the migration of Pt
NPs accounts for the Pt NPs slight aggregation during the
Fig. 7 The proposed catalytic mechanism for the 7% Pt/Co-NRs cata-
lyst
Table 4 Comparison of the
Catalysts
Pt content
Reaction time Conv. (CAL) Sel. (COL) (%)
Ref.
activity of reported catalysts for
the CAL hydrogenation to COL
(wt%)
(h)
(%)
Pt@ZIF-8
1
1
10.7
1
5
6
6
42
10
20
58.6
11.2
85.9
52
88.7
96.9
87.9
93
[19]
[19]
[20]
[42]
[21]
Pt@ZIF-71
Pt@UIO-66-NH2
Pt/mSiO2
MIL-101(Cr)@Pt@
90.6
94.3
MIL-101(Fe)
Pt/mTiO2-SiO2
Co–Pt/SBA-15
Pt/Co-NRs
2
3
6.8
5
2
3
99.0
71
97.5
63.8
91
92.1
[43]
[44]
This work
1 3

The Low Dimensional Co-Based Nanorods as a Novel Platform for Selective Hydrogenation of…
condition, the Pt/Co-NRs catalyst can achieve 97.5% CAL
conversion with 92.7% COL selectivity within 3 h. After fve
recycle tests, the Pt/Co-NRs catalyst can still maintain the
high COL selectivity (only 2.4% fuctuations).
Acknowledgements The authors are grateful for the fnancial support
from the Basic and Frontier Research Project of Chongqing in China
(No. cstc2016jcyjA0139). I gratefully acknowledge myself and Pro Xia
for having revised this manuscript on New year’s Day in 2019.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no confict of
interest.
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