Efficient single-atom Ni for catalytic transfer hydrogenation of furfural to furfuryl alcohol

Article abstract of DOI:10.1039/d0ta10838c

The employment of single-atom catalysts in the catalytic transfer hydrogenation (CTH) of furfural (FF) to furfuryl alcohol (FAL) has never been effectively explored. Herein, a catalyst of Ni single-atoms supported on nitrogen doped carbon (Ni-SAs/NC) is synthesized and first ever utilized in the CTH of FF to FAL. Atomically dispersed Ni-N4 sites change the electron density at the metal center and exhibit specific adsorption and desorption to FF and FAL, promoting an outstanding catalytic performance with a turnover frequency (TOF) of 832 h-1 and selectivity as high as 97.1% at 130 °C for 2 h. Such performance is 9-fold higher than that of supported Ni nanocatalysts. The Ni-SAs/NC catalyst also exhibits superior stability for the CTH of FF and excellent catalytic activity for other α,β-unsaturated aldehydes. This work provides a new strategy of producing green chemical compounds using catalytic biomass conversion and suggests the future application of long-lasting single-atom catalysts for emerging sustainable technologies.

Full text of DOI:10.1039/d0ta10838c

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DOI: 10.1039/D0TA10838C
ARTICLE
Efficient single-atom Ni for catalytic transfer hydrogenation of
furfural to furfuryl alcohol
Received 00th January 20xx,
Accepted 00th January 20xx
Yafei Fan, Changfu Zhuang, Shangjing Li, Ying Wang, Xiaoqin Zou, Xiaoteng Liu,c* Weimin
a
a*
a
a
b*
d
b
Huang and Guangshan Zhu
DOI: 10.1039/x0xx00000x
The employment of single-atom catalysts in the catalytic transfer hydrogenation (CTH) of furfural (FF) to furfuryl alcohol
FAL) have never been effectively explored. Herein, Ni single-atoms supported on nitrogen doped carbon (Ni-SAs/NC)
catalyst is synthesized and first ever utilized in CTH of FF to FAL. Atomically dispersed Ni-N sites change the electron
(
4
density at the metal center and exhibit specific adsorption and desorption to FF and FAL, promoting an outstanding
catalytic performance with turnover frequency (TOF) of 832 h-1 and selectivity as high as 97.1% at 130 C for 2 h. Such
performance is 9-fold higher than that of supported Ni nanocatalysts. The Ni-SAs/NC catalyst also exhibits superior
stability for CTH of FF and excellent catalytic activity for other α,β-unsaturated aldehydes. This work provides a new
strategy of producing green chemical compounds using catalytic biomass conversion and suggests the future application of
long-lasting single-atom catalysts for emerging sustainable technologies.
o
A variety of noble catalysts have been studied for CTH
2
0
21
22
1
. Introduction
reactions of FF, including Pt , Pd , Ru etc. High catalytic
activity has been received using noble metal catalysts, but it
often accompanied with side reactions of over hydrogenation
resulting in low selectivity. Outstanding performance for
hydrogen dissociation has been achieved using low-cost non-
Selective conversion of biomass resources to biofuels and
platform molecules has attracted extensive attention for
chemical production industry. Furfural (FF), as a versatile
biomass-derived platform compound, serving as a precursor to
produce over 80 different industrial chemicals, owing to the
presence of C=O and C=C bonds. Furfuryl alcohol (FAL) is an
important hydrogenation product of FF and it is widely used in
synthetic resins, reactive solvent and production of
chemicals.1 Traditional biorefinery of FF to produce furfuryl
alcohol (FAL) is constrained by high pollution and toxicity, low
selectivity and high-cost etc., which made this process not
economically and environmentally viable. In addition, high-
pressure reaction with H as H-donor has many drawbacks,
such as highly flammable, explosive and high instrumental
requirements.1
transfer hydrogenation (CTH) has received increasing
attention, due to it uses low cost and readily available alcohol
compounds as H-donor ， so CTH offers a greener, more
1
-5
2
3-24
noble Ni-based catalysts for selective hydrogenation of FF,
but the main problem is the low transformation efficiency,
and Ni species tend to leach and aggregate in the reaction
process. Therefore, development of efficient and stable Ni-
based catalysts for CTH of FF to FAL has become the research
challenge.
Single-atom catalysts (SACs) have been revealed for their
advanced performance towards multiple catalytic reactions.
6
-9
2
5
0-12
2
6-
1
3
2
9
Unlike conventional metal nanoparticles (NPs) catalysts,
2
isolated metal atoms supported on substrate materials display
unique features, i.e. maximized exposure of the active sites,
ultra-uniform distribution, quantum size effect, altered atom-
substrate interaction, unsaturated coordination configuration
SACs derived from zeolite imidazolate frameworks
ZIFs) are emerging investigations, which have demonstrated
4-15
2
Advantages over H as H-donor, catalytic
3
0-32
etc.
(
1
6-19
effective and safer hydrogenation reaction process.
3
3
good catalytic activity and selectivity in the electrocatalysis ,
3
4
35
photocatalysis and thermocatalytic. When ZIFs precursor is
a. Key Laboratory of Forest Resources Conservation and Utilization in the Southwest pyrolyzed, the evaporation of zinc at high temperature can
Mountains of China Ministry of Education, School of Chemical Engineering,
prevent metal atoms from agglomerating to obtain stable
Southwest Forestry University, Kunming 650051, P. R. China. Email:
3
6
cfzhuang@swfu.edu.cn.
Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R.
China. E-mail: zouxq100@nenu.edu.cn.
Department of Mechanical and Construction Engineering, Faculty of Engineering
and Environment, Northumbria University, Newcastle upon Tyne, NE1 8ST, United
Kingdom. Email: terence.liu@northumbria.ac.uk
SACs. Nitrogen doping can also regulate the nanostructure
and coordination bonds of metal sites and improve the
thermal stability of metal atoms.
anchored on the N-doped carbon to form the M-N structure
after pyrolysis and different coordination structure can induce
b.
c.
3
7-38
The metal atoms are
x
d.
College of Chemistry, Jilin University, Changchun 130012, P. R. China.
Electronic Supplementary Information (ESI) available: [details of any the change of electronic structure at the active site of the
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
metal, leading to the difference of catalytic activity and
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Journal Name
selectivity.39-40 In addition, ZIFs derived SACs have excellent probe-II Scanning XPS Microprobe system. The morphology of
View Article Online
DOI: 10.1039/D0TA10838C
pore structure, which can effectively promote the transport of the catalyst was researched using a scanning electron
4
1
reactants and improve the catalytic performance.
Nevertheless, SACs derived from ZIFs have never been Transmission electron microscopy (TEM) images were
effectively explored in CTH of FF to FAL. obtained with a JEM-2100 working at 200 kV. The high-angle
microscope (SEM, Hitachi SU8010). Abbreviated correction
Herein, we report Ni single-atoms supported on N-doped annular dark-field scanning transmission electron microscopy
porous carbon (Ni-SAs/NC) catalyst, which was first ever (HAADF-STEM) images were imaged by using a FEI Themis Z at
utilized in CTH of FF to FAL. The catalyst exhibits an excellent 300 kV, equipped with a probe spherical aberration corrector.
activity for CTH of FF with TOF is as high as 832 h- and The X-ray absorption fine structure spectra (Ni K-edge) were
selectivity is 97.1% for hydrogenation of FF, and this catalyst collected at 1W1B station in Beijing Synchrotron Radiation
significantly improved catalytic performance for 9 times than Facility (BSRF). The storage rings of BSRF were operated at 1.5
Ni nanoparticles/N-doped porous carbon (Ni-NPs/NC). The GeV with a maximum current of 200 mA. The metal contents
mechanism of this reaction was investigated in detail, in the samples were measured by the inductively coupled
including solvent, reaction time and temperature. In addition, plasma optical emission spectrometry (ICP-OES) using a VISTA-
Ni-SAs/NC exhibits good stability of FF hydrogenation for four MPX instrument.
1
cycles of reuse.
2.4 Catalysis Evaluation
The catalytic hydrogenation performance of FF was measured
in a 25 mL autoclave reactor with a reaction solution of 1
mmol of FF, 25 mg of catalyst, and 8 mL of 2-propyl alcohol (2-
2
2
. Experimental
2
.1 Synthesis of Ni@ZIF-8
ProOH). The reactor was purged three times with N and
pressurized with 2 MPa N
2
, and then the reactor was heated to
An encapsulation method was used to prepare Ni@ZIF8. Brief
details include 2-methylimidazole (16 mmol) was dissolved in
o
the set temperature of 120 C while being magnetically stirred
at a rate of 800 rpm. At the end of the reaction, the liquid
samples were collected for analysis. The used catalyst was
filtered, washed with ethanol and then dried at 80 °C in
vacuum. Then the catalysts are reduced at 400 °C with 10%
1
5 mL methanol with stirring in flask A. Zn(NO
3 2 2
) ·6H O (4
mmol) and Ni(acac) ·2H O (0.4 mmol) were dissolved in 30 mL
2
2
methanol under ultrasonic mixing for 15 min to form a clear
solution in flask B. Then, flask A was subsequently added into
flask B with vigorous stirring for 1 h at room temperature. The
solution was transferred into a 100 mL Teflon lined stainless-
steel autoclave and heated at 120 °C for 4 h. The obtained
product was separated by centrifugation and washed
subsequently with CH OH for third and finally dried at 70 °C
3
under vacuum for overnight. For the purpose of synthesis Ni-
NPs/NC, we also synthesized 5Ni@ZIF-8 using the same
2
H /Ar for further reactions and characterizations.
The liquid products were diluted in methanol then analyzed
using gas chromatography-mass spectrometry (GC-MS,
QP2010 SE, Shimadzu) and quantitatively analyzed using GC
(GC-7890A, Agilent), equipped with FID and a (30 m × 0.25 mm
×
0.25 μm) KB-WAX capillary column (Kromat Corporation,
USA). 1.0 uL sample was injected into the injection port with a
split ratio of 30:1. The column temperature was initially
maintained at 40 °C for 1 min followed by heating up to 220 °C
at a ramp rate of 20 °C/min and the temperature was held at
procedure, but the extra 2 mmol of Ni(acac)
Zn(NO ·6H O were dissolved in 30 mL methanol under
ultrasonic mixing for 15 min to form a clear solution in flask B.
.2 Synthesis of Ni-SAs/NC
2
and 4 mmol
)
3 2
2
2
50 °C for 1 min using nitrogen as the carrier gas.
2
The conversion of reactant is defined with the following
The powder of Ni@ZIF-8 was transferred into a crucible boat
and placed in a tube furnace. The sample was heated to 900 °C
equation:
Mole of FF converted
-1
Conversion (%) =
× 100%
with a heating rate of 5 °C min and kept at 900 °C for 2 h
under argon gas flow and then cooled to room temperature.
The obtained material was directly used without further
Mole of FF loaded
The selectivity of product is defined with the following
treatment. NC and Ni-NPs/NC are synthesized in the same way equation:
with Ni-SAs/NC, and their precursor is ZIF-8 and 5Ni/ZIF-8,
Mole of FAL produced
Selectivity (%) =
× 100%
respectively. Ni/NC and Ni/AC were synthesized by
impregnation with NC and activated carbon (AC) as carriers. A
schematic diagram shows the whole preparation procedure is
illustrated in Figure 1a.
Mole of product produced
3. Results and discussion
2
.3 Characterization
XRD was used to determine the structural information of the
Powder X-ray diffraction (XRD) patterns were analyzed using a catalyst. Figure 1b shows the XRD patterns of the Ni@ZIF-8
Rigaku Ultima X-ray diffraction spectrometer with Cu Kα and 5Ni@ZIF-8, which were matched with ZIF-8. Ni-SAs/NC
o
radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Brunauer-Emmet- shows a peak located around 26 corresponding to the (002)
Teller (BET) specific area and pore size distribution of the plane of graphite carbon, which was the same as that of Ni-
2
catalysts was characterized by N adsorption-desorption NPs/NC and NC in Figure 1c. Ni-NPs/NC did not show the
measurement at 77 K using ASAP 2020 Plus Physisorption characteristic diffraction peak of Ni due to its relatively low Ni
instrument. XPS analysis was performed on a PHI5000 Versa content and single Ni atoms could be under the detection limit
2
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of the technique. Table S1 listed the pore structure data and elemental mappings clearly showed C, N and VNieiw uArnticifleoOrmnlinlye
metal content of materials. BET surface area measurement distributed throughout the particles (FiguDrOeI:21c0)..10T3h9e/DN0Ti Alo10a8d3i8nCg
was used to determine the catalysts. The
N
2
was 0.12 wt% in Ni-SAs/NC, according to the ICP-OES analysis.
adsorption/desorption isotherm indicates that the content of In Figure 2d, Ni single-atoms were clearly observed as isolated
Ni had an important effect on the structure of porous carbon. bright dots, marked in red circles in the Spherical Aberration
In Figure 1d, all samples follow type-IV isotherms, indicating Corrected Transmission Electron Microscope (AC-TEM) image.
that the micro-mesoporous co-exist in the porous carbon. The above results indicate that the Ni was loaded on the N-
2
−1
Compared to precursor Ni@ZIF-8 (1910 m g ), the specific doped carbon as single atoms. In addition, the morphology of
2
−1
surface area of Ni-SAs/NC (1037 m g ) was significantly Ni-NPs/NC was severely damaged in Figure S4. The Ni
reduced. During the pyrolysis process, the pore structure of nanoparticles of Ni- NPs/NC were observed on the HR-TEM
the precursor partially collapsed, resulting in a significant (Figure S5). Meanwhile, Figure S6 TEM images revealed that Ni
decrease in the specific surface area of the derived Ni catalyst. nanoparticles of Ni/NC and Ni/AC were uniformly loaded on
As the Ni contents increase, more Ni is confined in the pore the NC and activated carbon with a mean size of 4.9 and 6.4
structure, so the BET surface area gradually decreased from nm, respectively.
2
−1
2
−1
2
−1
1
183 m g (NC) to 1037 m g (Ni-SAs/NC) and 954 m g
(
0
Ni-NPs/NC). A similar change in the pore volume 0.37 (NC),
3
-1
.32 (Ni-SAs/NC) and 0.31 (Ni-NPs/NC) cm
g
was also
observed. Figure 1e shows the pore size distribution of the
material. Interestingly, Ni-SAs/NC showed the maximum
average pore size (2.07 nm) compared with NC (1.79 nm) and
Ni-NPs/NC (1.87 nm), which also contribute to the transfer of
substrates and products during catalysis.
Figure 2. (a) SEM image of Ni-SAs/NC. (b) TEM image of Ni-SAs/NC. (c) HAADF-STEM
images of Ni-SAs/NC and corresponding element maps showing the distribution of C
(green), N (yellow), Ni(red). (d) AC-TEM images of Ni-SAs/NC. Single Ni atoms
highlighted by red circles.
X-ray photoelectron spectroscopy (XPS) was conducted to
investigate the elemental composition and valence states in
Ni-SAs/NC. The XPS element analysis of the samples is shown
in Table S2. In Figure S7d, the XPS results indicated the
presence of C, N, O and Ni for Ni-SAs/NC. The Ni-SAs/NC
included three main peaks located at 284.6 eV (C=C), 285.8 eV
4
2
(
C=N) and 288.9 eV (C-C) in the one of C 1s. The high-
resolution XPS spectra of N 1s revealed the pyridinic-N (398.7
eV), Ni-N (399.2 eV), pyrrolic-N (399.4 eV), graphitic-N (401.0
eV) and oxide-N (403.4 eV) in Figure S7f. Part of the nitrogen in
Figure 1. (a) Schematic illustration for the preparation of Ni-SAs/NC catalyst. (b) XRD
patterns of precursors. (c) XRD patterns of NC, Ni-SAs/NC and Ni-NPs/NC. (d) N
2
sorption isotherms for Ni@ZIF-8, NC, Ni-SAs/NC and Ni-NPs/NC samples. (e) Pore size the pyridine-N will coordinate with Ni to form Ni-N (399.2
distribution.
43
eV). The advantage of pyridine N can be coordinated with
metal atom and used as anchor points to more effectively
stabilize the atomically dispersed nickel sites. In Figure S8,
The morphology of Ni-SAs/NC was observed by SEM and
44
TEM. In Figure 2a-b, Ni-SAs/NC retained the dodecahedral
structure of the precursor Ni@ZIF-8 (Figure S1), but the
surface became rougher after pyrolysis. However, no particles
were observed in the HR-TEM of Ni-SAs/NC (Figure S3). High-
angle annular dark-field scanning TEM (HAADF-STEM)
the pyridinic-N binding energy of NC was 398.3 eV and 0.4 eV
less binding energy relative to Ni-SAs/NC. We think that
electron transfer occurs at the coordination of Ni and pyridine-
N, caused an increase in the electron density at the center of
45
the Ni. In order to further study the chemical state and
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coordination structure of Ni species in Ni-SAs/NC, XANES and 3.2 Catalytic Performance
View Article Online
DOI: 10.1039/D0TA10838C
EXAFS spectroscopy were performed and results are shown in
Table 1. Catalytic transfer hydrogenation of furfural with
Figure 3. As we can see from the XANES spectrum (Figure 3a),
the Ni-SAs/NC near edge absorption energy is located between
NiO and Ni foil, indicating that the valence state of Ni was
between 0 and +2, and atomically dispersed Ni is loaded on
the NC. EXAFS exhibited that Ni-SAs/NC had a major peak at
various catalysts.a
b
c
Entry
Catalyst
Conv.
Sel.
(%)
0
93.6
97.1
96.8
95.1
96.4
71.5
0
TOF
R
rate
-1
-1 -1
(
%)
0
(h )
-
(mmol g h )
1
2
3
4
5
6
7
8
9
Blank
NC
Ni-SAs/NC
Ni-SAs/NC
Ni-NPs/NC
5% Ni/NC
5% Ni/AC
5% Pd/C
-
6.3
-
0.84
17.02
12.75
7.63
11.98
2.91
1.69
2.55
-
~1.31 Å, which is smaller than the Ni-Ni peak at ~2.15 Å for the
d
85.1
95.6
57.2
89.9
21.8
12.7
19.1
99.2
832
623
69
16
3.3
-
Ni foil and Ni-O peak at ~1.56 Å for the NiO. This could be due
to a mixture of Ni-N and Ni-O bonding, indicating atomic
dispersion of the Ni species. Furthermore, EXAFS wavelet
transform analysis further revealed the atomic distribution of
Ni-SAs/NC. From the Figure 3d-f, Ni foil and NiO standards
-1
indicated that the intensity maxima at ~7.9 and ~7.2 Å .
10% Pt/C
0
93.6
-
-
Notably, Ni-SAs/NC exhibited the intensity maximum at ~5.2 Å
,
1
0
M-MOF-
17.6
1
shows Ni atoms are in the form of coordination which
e
8
08
associated with the Ni-N(O), this also confirmed the major
peak at ~1.31 Å attributes to Ni-N(O) bond (Figure 3b). In
addition, least-squares EXAFS curve-fitting analysis was
performed to obtain quantitative structural data. The best
fitting analysis of Ni-SAs/NC showed the coordination number
of Ni is 4.46 and the mean bond length is 1.98 Å (Figure 3c,
Figure S10 and Table S3). Above evidence indicates Ni presents
f
1
1
1
2
Zr-LS
92.2
72.6
99.4
96.7
4.2
2.6
9.16
3.51
NiO(P)-
g
3
00
h
1
1
3
4
BZC
Cu/SO
97.8
100
98.9
99
0.37
0.2
-
2-/Ci
-
3
a
b
◦
Reaction conditions: 1 mmol FF, 8 mL 2-ProOH, 25 mg catalyst, 130 C, 3 h, 2 MPa N
TOF = (converted moles of FF)/(moles of active sites × time).
2
.
c
Rrate = mmol (converted
as isolated single atom state, and was anchored in a N-doped FF) / [g (catalyst amount) × h (time)]. 130 C, 2 h. From ref 18. f From ref 11. g From
d
o
e
h
i
ref 49. From ref 50. From ref 14.
porous carbon and the Ni atoms and N atoms exist in the form
of quaternary coordination (Ni-N ). The atomic structure
model was shown in Figure 3c (inset) and O molecules were
4
Table 1 shows the catalytic properties of FF hydrogenation
2
thought to attach to Ni atoms. The best-fit results of Ni foil and to FAL of NC (Entry 2), Ni-SAs/NC (Entry 3, 4), Ni-NPs/NC (Entry
NiO were shown in Figure S9, respectively.
5), Ni/NC and Ni/AC (Entry 6, 7), together with several
catalysts prepared by other researchers as comparison. A
reaction without catalyst was performance (Table 1, Entry 1)
and it can be noticed that there is no FF was converted into
FAL. NC had a conversion rate of 6.3% and selectivity of 93.6%
because the doping of nitrogen atoms change the local
electron cloud density of delocalized bonds, which leads to the
4
6
metal catalytic properties (Entry 2). The catalytic activity of
the Ni-SAs/NC was carried out for CTH of FF under different
reaction time (Entry 3, 4). Ni-SAs/NC exhibited 85.1%
conversion when the reaction time at 2 h and the TOF reached
-1
an astonishing 832 h . This is because atomically dispersed Ni-
sites may change the electron cloud density at the metal
center and exhibit specific adsorption and desorption to FF
N
4
4
7-48
and FAL, promoting efficient hydrogen transfer.
The ultra-
high atom utilization has provided maximised active sites,
which could improve the reaction efficiency to the highest
level. Meanwhile the excellent pore structure of Ni-SAs/NC
2
-1
(
1037 m g ) leads to a significant exposure of Ni monatomic
active sites, such pore size (2.07 nm) is greatly beneficial to the
mass transfer between substrates and products. Ni-SAs/NC
exhibited 95.6% conversion and 96.8% selectivity when the
reaction time increased to 3 h. While the selectivity of FAL
decreased slightly with the extension of reaction time, this is
due to etherification between FF and the solvent. As a
Figure 3. (a) XANES spectra. (b) Fourier transform (FT) of the Ni K-edge EXAFS. (c) EXAFS
fitting curve for Ni-SAs/NC samples (inset schematic model of Ni-SAs/NC). (d) Wavelet comparison, the catalytic performance of Ni nanoparticles on
transform (WT) of Ni foil. (e) Wavelet transform (WT) of Ni-SAs/NC. (f) Wavelet N-doped carbon (Ni-NPs/NC) was also studied. The conversion
transform (WT) of NiO.
rate of 57.2% and selectivity of 95.1% were shown over Ni-
NPs/NC in 130 C (Entry 5). The Ni in the Ni-NPs/NC was mainly
o
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in the form of Ni nanoparticles and Ni load was 0.65 wt% reason could be the average particle size is 4.9VienwmArtic(lFeiOgnulirnee
Table S1), which resulting in low atomic utilization. In addition, S6a), which is significantly larger tDhOanI: 10s.1in03g9le/D-a0tToAm1083N8Ci.
it confirmed the results from XPS analysis which revealed a Activated carbon (AC) was used as the support, and surface
(
2
-1
4
lack of clear Ni-N active sites for Ni-NPs/NC. Compared to area of Ni/AC is 1275 m g , which is obviously greater than
2
-1
2
-1
1
037 m g and 2.07 nm Ni-SAs/NC, the specific surface area 954 m g of Ni-NPs/NC. Although advanced pore structures
2
-1
and pore size of Ni-NPs/NC are as low as 954 m g and 1.87 facilitated material transfer, the conversion of FF was 25%
nm, which led to less exposure of catalytic active sites and the over Ni/AC and the TOF is as low as 3.3 due to larger
release of hydrogenated products. Therefore, Ni-NPs/NC has nanoparticle sizes (6.4 nm, Figure S6b) result in incomplete
lower catalytic activity and the TOF was only 69 h , which is exposure of active sites and the lack of nitrogen doping (Entry
/9 of that of the single-atom Ni-SAs/NC catalyst. Although the 6 and 7). In conclusion, it was proved once again that the
-1
1
metal loading of the Ni-NPs/NC is 5.4 times that of the Ni- activity of single-atoms Ni catalyst has much higher catalytic
SAs/NC, but the conversion rate (Rrate) of Ni-SAs/NC catalyst is activity towards CTH of FF than other Ni catalysts, it also shows
1
.7 times that of Ni-NPs/NC catalyst. In order to better higher performance over commercially available Pd/C and Pt/C
illustrate the excellent performance of Ni-SAs/NC, we also catalysts as shown in Entry 8-9 (conversion is only 12.7% and
synthesized Ni nanocatalysts supported on nitrogen carbon 19.1%, with zero selectivity). Moreover, compared to catalysts
materials (Entry 6). The catalyst is Ni nanoparticles with previously reported in literatures, Ni-SAs/NC demonstrated
uniform distribution supported by impregnation method excellent TOF and higher catalytic activity under mild
(
1
Ni/NC), it shows 89.9% FF conversion, but the TOF was only conditions for CTH of FF to FAL (Table 1, Entry 10-14 and Table
6 h (Entry 6), which was 1/39 of that of Ni-SAs/NC. The main S4).
-1
Figure 4. (a) Effect of solvents on hydrogenation of FAL over Ni-SAs/NC. Reaction conditions: 1.0 mmol FF, 25 mg catalyst, 120 oC, 2 MPa N
, 3 h; (b) Effect of
2
, 3 h; (c) Effect of reaction time on
2
temperature on hydrogenation of FAL over Ni-SAs/NC. Reaction conditions: 1.0 mmol FF, 25 mg catalyst, 8 mL 2-ProOH, 2 MPa N
o
hydrogenation of FAL over Ni-SAs/NC. Reaction conditions: 1.0 mmol FF, 25 mg catalyst, 8 mL 2-ProOH, 120 C, 2 MPa N
2
. (d) Kinetic studies of FF hydrogenation over
Ni-SAs/NC at different temperatures (the inset represents the Arrhenius plot for the FF hydrogenation). Reaction conditions: 1 mmol FF, 25 mg catalyst, 8 mL 2-ProOH,
o
2
MPa N
2
, 100-130 C.
activity and selectivity of the catalyst. Therefore, the effect of
the H-donor for CTH of FF was researched, including ethanol
3
.3. Reaction Mechanism
(
EtOH), n-propyl alcohol (PrOH), 2-propyl alcohol (2-ProOH), n-
butyl alcohol (n-BuOH) and 2-butyl alcohol (2-BuOH). In Figure
a, we can see that reaction shows good catalytic activity using
3
.3.1. H-donor Effect
In order to better understand the reaction mechanism of FF
hydrogenation to FAL on single-atoms catalyst, a series of
catalytic experiments were carried out using Ni-SAs/NC
catalysts. Solvent effect plays a very important role in
heterogeneous catalysis, so it had a great influence on the
4
alcohols as H-donor. That is because hydrogen bonding
between C=O of FF and hydroxyl group of the alcohol
contributed to the activation of the FF molecule. When EtOH
29
was used as hydrogen donor, the FF conversion rate was 53.1%
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and the selectivity of FAL was 70.8%. Furthermore, secondary reaction rate constants (k) in this experiment were calculated
View Article Online
alcohols exhibited better catalytic activity than primary to be 0.2489, 0.3026 and 0.4567 min- for the temperatures
1
DOI: 10.1039/D0TA10838C
alcohols. For example, 2-ProOH had 79.1% conversion and 100, 110, and 120 °C, respectively. According to the Arrhenius
9
a
6.5 selectivity higher than 63.8%, 95.3% of ProOH, and similar plot, the activation energy (E ) was determined to be 36.86 kJ
−1
results is 67.4% and 95.1% of 2-BuOH better than 48.7%, 92.0% mol over the Ni-SAs/NC catalyst for CTH of FF into FAL using
of n-BuOH. -H on secondary alcohols is more likely prone to 2-ProOH as hydrogen donor. This value is higher than FF
1
8

-H elimination than on primary alcohols. In addition, with adsorption energy, which shows that the reaction was
5
1
a
the increase of the number of carbon in secondary alcohols, controlled by surface dynamics. Meanwhile, this E value is
the conversion rate of FF decreases obviously. The possible lower than the recently reported literatures for selective
reason for this is that the longer carbon chain of secondary conversion of FF into FAL (Table S4). The low activation energy
alcohols may lead to a more pronounced negative steric may be related to the chemical environment of the atom-
4
hindrance effect, which prevents FF diffusion in the Ni-SAs/NC dispersed Ni-N structure. So, the catalytic activity of Ni-
channel.11
.3.2. Reaction Temperature and Duration Influence
SAs/NC is higher than most catalysts in Table S4 under the mild
conditions.
3
3
.3.4. Reaction mechanism studies
Figure 5. Possible reaction mechanism for catalytic transfer hydrogenation of FF.
In Figure 4b, the influence of reaction temperature to the
hydrogenation of FF was investigated over Ni-SAs/NC using 2-
o
ProOH as H-donor from 110-140 C for 3 h. The FF conversion
o
o
significantly increased from 64.1% at 110 C to 95.6% at 130 C,
while the selectivity decreased from 97.4% to 96.8%. Notably,
with the increase of reaction temperature, the etherification
of FF and 2-ProOH to form isopropoxymethylfuran resulted in
a decrease of selectivity of FAL. Further increase in reaction
o
temperature e.g. 140 C has no obvious effect on conversion.
Figure 6. (a) Reusability of Ni-SAs/NC catalysts for the hydrogenation of furfural.
Figure 4c showed the effect of reaction time on the catalytic
activity. Low conversion of 58.6% and 97.1% selectivity was
obtained at 2 h. Selectivity decreased from 97.1% to 96.3% at
the reaction time of 5 h. Obviously, prolongation of the
reaction time can increase the conversion of FF, but the slight
decrease in selectivity also occurs resulting from etherification
between the FF and 2-ProOH.
o
Reaction conditions: 1 mmol FF, 8 mL 2-ProOH, 25mg catalyst, 110 C, 3 h, 2 MPa N
2
.
(b) Metal leaching experiment with catalyst (black) and with filtrate (red).
According to previous reports, Lewis acid and basic sites of the
catalyst are the main effective factors to make the catalytic
52-53
transfer of hydrogenated carbonyl compounds.
The XANES
test could prove that the Ni valence is 0 to +2 in the Ni-SAs/NC
Figure 2a), so Ni is considered to be Lewis acid site. In
The reaction was conducted at three different reaction addition, XPS also proved that Ni-SAs/NC catalyst contains a
3
.3.3. Reaction Kinetics Study
(
5
4
temperatures of 100, 110, and 120 °C to study kinetic large number of pyridine-N, which created Lewis base sites.
behaviors of the CTH of FF to FAL. Since the amount of catalyst To verify the frustrated Lewis pairs (FLP) mechanism, we
is fixed and 2-ProOH is excessive, we assumed that this conducted two controlled experiments. When the formic acid
reaction is a first-order reaction of the FF concentration was added as Lewis acid in the FF hydrogenation reaction, it
function. Obviously, as shown in Figure 4d, ln (C
t 0
/C ) is linearly was found that the conversion rate dropped sharply from 85.1%
related to reaction time at different temperatures. The to 3.7%, and the selectivity completely dropped to 0. Then we
6
| J. Name., 2012, 00, 1-3
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added potassium thiocyanate (KSCN) to poison the Ni (Lewis time, the carbonyl group of FF is also adsorbed aVniedw AartcictlievOantleinde
acid sites), and the conversion rate decreased to 3.0% without by the Lewis acid sites (Ni cations). DSOuIb: 1s0e.1q0u3e9n/Dtl0yT,A1a083s8ixC-
any formation of FAL. The above results indicate that Lewis membered ring transition state forms through the
acid base position is necessary for hydrogenation reaction. In coordination between the Ni bound isopropoxide and the
1
4, 18, 23, 25
combination with relevant literature,
a mechanism for activated carbonyl group, and the -H of Ni bound
FF transfer hydrogenation catalyzed by Ni-SAs/NC catalyst was isopropoxide is directly transferred to the carbonyl group of
1
8
proposed (Figure 5). First, 2-ProOH adsorbed on the surface of FF, meanwhile the acetone forms and then releases. Finally,
the Ni-SAs/NC catalyst interacts with the acid-base sites (Ni the formation of FF can easily be obtained through proton
cations and neighboring non-coordinated pyridinic N) on the transfer from another 2-ProOH to deprotonated FF along the
catalyst surface, resulting in 2-ProOH dissociation forming the hydrogen bonding, in which the transition state has a four
+
25
14
corresponding Ni bound isopropoxide and N-H . At the same membered-ring.
Table 2. Catalytic transfer hydrogenation of Various Carbonyl Compounds over Ni-SAs/NC.a
Temp
(oC)
Time
(h)
Conv.
(%)
Sel.
(%)
TOF b
Entry
1
Substrate
Product
-1
(h )
1
1
40
40
3
3
74.7
56.3
98.6
98.3
243
183
2
3
1
1
40
80
3
3
48.6
22.6
98.9
96.5
158
74
4
5
1
60
3
74.5
100
242
6
7
8
1
1
80
40
3
4
81.8
93.2
63.9
100
266
228
1
1
40
70
4
3
51.6
29.6
100
100
126
96
9
a
Reaction conditions: 0.5 mmol substrate, 25mg catalyst, 8mL 2-ProOH, 2 MPa N
2
. b TOF = (moles of substrate)/(moles of “Ni” × time).
cycles. The high catalytic stability is attributed to the stable Ni-
structure formed by N-doped carbon, which plays an
important role in preventing metal atoms from reunion. The
XRD results show that the characteristic peaks of the used
N
4
3
.3.5. Catalysts Stability
In Figure 6a, the activity and selectivity of Ni-SAs/NC were not
significantly reduced when the catalyst has been used for 4
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catalyst and the fresh catalyst have not changed, and no
characteristic peaks of Ni appear (Figure S12). Moreover, the
Ni atoms did not agglomerate and remained atomically
dispersed according to the AC-TEM (Figure S13). In addition,
leaching experiments were conducted to further verify the
stability of the Ni-SAs/NC in Figure 6b. After filtration, the
reaction of the solution basically stopped at a conversion rate
of 58.6%, and Ni was not observed as a detectable limit of ICP-
OES in the filtrate after the reaction, indicating that the Ni
metal was not leachable. It is also proved that the Ni-SAs/NC
catalyst is heterogeneous and stable.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No.31760190, 21531003, 21501024,
View Article Online
DOI: 10.1039/D0TA10838C
2
1971035), Special project of agricultural basic research in
Yunnan (No.2017FG001026, 2017FG001055), Jilin Scientific
and Technological Development Program (20170101198JC,
2
0190103017JH), Jilin Education Office (JJKH20180015KJ). Key
Laboratory of State Forestry Adminstration for Highly-Efficient
Utilization of Forestry Biomass Resources in Southwest China
(2019-KF06).
3
.3.6. Substrate Scope
The excellent catalytic activity of Ni-SAs/NC has been discussed Notes and references
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Journal of Materials Chemistry A
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DOI: 10.1039/D0TA10838C
Graphical Abstract:
Single-atoms Ni catalyst is first utilized in the catalytic transfer hydrogenation
of furfural to furfuryl alcohol.