Highly efficient catalytic conversion of cellulose into acetol over Ni-Sn supported on nanosilica and the mechanism study

Article abstract of DOI:10.1039/c9gc02449b

Selective conversion of cellulose into high value-added C3 chemicals is a great challenge in biorefinery due to the complicated reaction process. In this work, 61.6% yield of acetol was obtained by one pot conversion of cellulose using Ni-Sn/SiO₂ catalysts. A series of characterization methods including TEM, STEM-HAADF, EDS, AAS, XRD, XPS, H₂-TPR, Py-FTIR, and CO₂-TPD were carried out to explore the structure-activity relationship. The strong basicity of the catalysts was a key factor affecting the production of acetol. In addition, catalysts with the hydrothermally stable L-acid sites and no B-acid sites inhibited side reactions and ensured efficient conversion of cellulose into small molecules. Further studies showed that the formation of the Ni₃Sn₄ alloy significantly promoted the acetol production, and its weak hydrogenation activity inhibited further conversion of acetol. Noninteger valence tin species (Sn^δ+ and SnO_x) were formed both in Ni₃Sn₄ and Sn/SiO₂. These Sn species were the source of basic sites and the active sites for catalyzing cellulose to acetol. Under the synergistic catalysis of Sn/SiO₂ and the Ni₃Sn₄ alloy, cellulose was efficiently converted into acetol. This work provides guidance for the selective conversion of cellulose into C3 products.

Full text of DOI:10.1039/c9gc02449b

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Highly Efficient Catalytic Conversion of Cellulose to Acetol over Ni-
Sn Supported on Nanosilica and the Mechanism Study
Xiaohao Liua,c, Xiaodong Liub, Guangyue Xub, Ying Zhangb *, Chenguang Wanga, Qiang Lud, Longlong
Received 00th January 20xx,
Accepted 00th January 20xx
Maa,c*
DOI: 10.1039/x0xx00000x
Selective conversion of cellulose to high value-added C3 chemicals is a great challenge in biorefinery due to the complicated
reaction process. In this work, 61.6% yield of acetol was obtained by one pot conversion of cellulose using Ni-Sn/SiO2
catalysts. A series of characterizations including TEM, STEM-HAADF, EDS, AAS, XRD, XPS, H2-TPR, Py-FTIR, CO2-TPD were
carried out to explore the structure-activity relationship. The strong basicity of the catalysts was a key factor affecting the
production of acetol. In addition, catalysts with the hydrothermally stable L-acid sites and absent B-acid sites inhibited side
reactions and ensured efficient conversion of cellulose to small molecules. Further studies showed that the formation of
Ni3Sn4 alloy significantly promoted the acetol production, and its weak hydrogenation activity inhibited the further
conversion of acetol. Noninteger valence tin species (Snδ+ and SnOx) formed both in Ni3Sn4 and Sn/SiO2. These Sn species
were the source of basic sites and the active sites for catalyzing cellulose to acetol. Under the synergistic catalysis of Sn/SiO2
and Ni3Sn4 alloy, cellulose was efficiently converted to acetol. This work provides guidance for the selective conversion of
cellulose to C3 products.
compounds, or directly used as an additive for foods and
cosmetics.13,14 Currently, acetol can be prepared from the selective
oxidation of 1,2-propanediol (1,2-P). The selective conversion of
cellulose to 1,2-P, followed by oxidation could be one of the methods
Introduction
The conversion of abundant, inedible and renewable lignocellulosic
biomass to prepare chemicals and fuels is very critical to building a
green and sustainable chemical industry.1-4 As a major component of
lignocellulosic biomass, cellulose conversion and utilization has
attracted worldwide attention.5,6 Hydrogenolysis of cellulose is one
of the important ways of cellulose conversion. However, the reaction
process is very complicated, including hydrolysis, retro-aldol
condensation, isomerization, hydrogenation, dehydration, etc.
(Scheme 1), so it is not easy to obtain a target product selectively by
a specific route.7 At present, in addition to selective preparation of
small molecules such as ethylene glycol (EG), ethanol, lactic acid (LA)
or complete hydrodeoxygenation to produce alkanes, few other
for producing acetol from cellulose. However, the yield of 1,2-P is still
low (less than 37%) due to the complicated reaction process (Scheme
1), although the researches have continued for many years.15,16
Moreover, it is also uneconomical to prepare acetol by the two-step
process. In fact, as shown in Scheme 1, acetol is one of the
intermediates in the reaction of preparing 1,2-P from cellulose. It can
be obtained if the catalyst can suppress its hydrogenation. Some
studies have reported the conversion of cellulose to acetol, but the
yield was low.17-19 Rodella et al. used Ni–WxC/C and 23% yield of
acetol was obtained from cellulose.17 Deng et al. used Ni-SnOx/Al2O3
with different atomic ratios of Sn/Ni as catalyst to catalyze the
conversion of saccharide to acetol. Although 73% and 53% yield of
acetol was obtained from fructose and glucose, respectively, when
cellulose was used as a raw material, the yield of acetol was reduced
to 35% with low carbon balance.18 It could be because each
additional reaction step (such as glucose isomerization and cellulose
hydrolysis) may cause more side reactions (such as the production of
humins) to reduce acetol yield. Recently, our group used Sn-Ni@C
catalysts to catalyze cellulose conversion. A low yield (33.2%) of
acetol was obtained, and the mechanism study was insufficient.19
Therefore, further research is necessary to increase the yield of
acetol from cellulose. Moreover, further mechanistic study is also
necessary on cellulose conversion, not only for production of acetol
but also for other C3 products such as 1,2-propanediol, propanol,
lactic acid, etc.
products were directly prepared from cellulose with high selectivity.
8-11
As one of the α-hydroxy ketones (HK), acetol is widely used in
pharmaceutical preparation.12 Moreover, it is also an important
intermediate for the preparation of acrolein and heterocyclic
a. CAS Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and
Renewable Energy Research and Development, Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences, Guangzhou 510640, China.
b. Department of Chemistry, University of Science and Technology of China, Hefei,
230026, China.
c. University of Chinese Academy of Sciences, Beijing 100049, China.
d. National Engineering Laboratory for Biomass Power Generation Equipment,
North China Electric Power University, Beijing 102206, China
* E-mail: zhzhying@ustc.edu.cn
mall@ms.giec.ac.cn.
In this work, nanosilica was chosen as the carrier to support Sn and
the hydrogenation metals because of its good hydrophilicity and
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Scheme 1. Reaction routes of cellulose conversion to acetol and other C2-C4 products.
of heating rate) for 2 h in H2 flow. When cooling down to room
temperature, the catalyst was purged with nitrogen for 2 h and then
treated with 1% O2/N2 for another 0.5 h. In Supporting Information,
5%Ni-15%Sn/SiO2-500C catalyst was prepared by first calcining
SnO2/SiO2 (the metal content of Sn was about 15 wt%) at 500 °C and
reducing it at 600 ° C after loading Ni (5 wt%). The preparation
process of 5% Ni-15%Sn/SiO2-600C was the same as that of 5% Ni-
15% Sn/SiO2-500C except that the calcination temperature was 600
°C. 5%Ni-15%Sn/SiO2-400R was prepared by first calcining SnO2/SiO2
at 700 °C and reducing it at 400 °C after loading Ni. The preparation
process of 5% Ni-15%Sn/SiO2-500R was the same except that the
reduction temperature was 500 °C.
smaller particle size making it easier to disperse in the aqueous
solvent and contact with the substrates. A high yield of acetol
(61.6%), which has never been reported before, was achieved when
5%Ni-15%Sn/SiO2 was used as the catalyst. A series of catalyst
characterizations were performed to explore the factors influencing
acetol production. Moreover, in-depth research was carried out to
explore the catalytic mechanism, which contributed to the
development of research on cellulose conversion to produce C3
products.
Experiment section
SnNi alloys were prepared according to previous reports.20 The
typical procedure (Ni3Sn4) was as follows: 7.2 mmol of NiCl2·6H2O
was dissolved in 50 mL of deionized water (Solution A). 9.6 mmol of
SnCl2·2H2O was dissolved in 30 mL of ethanol/2-methoxy ethanol
(2:1) mixture solution (Solution B). The Solution A and Solution B
were mixed at room temperature and stirred at 50 °C for 12 h.
Thereafter, the pH was adjusted to 12 using 3.1 M of NaOH solution.
The mixture was then placed in a hydrothermal autoclave with a
Teflon-lined at 150 °C for 24 h. The obtained black precipitate was
centrifuged, washed and dried overnight. The obtained powder was
reduced at 600 °C for 2 h. When cooling down to room temperature,
the powder was purged with N2 for 2 h and then treated with 1%
O2/N2 for another 0.5 h.
Reagents.
Nanosilica (15 ± 5 nm), α-cellulose (25 μm) and acetol were
purchased from Aladdin Chemistry Co., Ltd.; 1-hydroxy-2-butanone
and dihydroxyacetone were purchased from Bidepharm Technology
Co., Ltd.; Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Co(NO3)2·6H2O, SnCl4·5H2O
，NiCl2·6H2O, SnCl2·2H2O, NaOH, NH3·H2O, methanol and acetone
were purchased from Sinopharm Chemical Reagent Co., Ltd.
Catalysts preparation
The typical procedure of SnO2/SiO2 preparation is described as
follows: 2.0 g of nanosilica was added into a round-bottom flask
which contained 110 mL of deionized water. A certain amount of
SnCl4·5H2O was dissolved in 10 mL of water and then dripped into
the suspension above. After stirring at 45 °C for 24 h, the pH of the
solution was adjusted to 8.5 with NH3·H2O. After stirring for another
6 h, the solid was centrifuged, washed and dried at 105 °C overnight.
SnO2/SiO2 was obtained after calcination at a certain temperature
(usually 700 °C) in a muffle furnace.
Catalysts characterization
Transmission electron microscopy (TEM) images were taken with a
JEOL Model JEM-2010 LaB6 TEM system. High-angle annular dark-
field scanning transmission electron microscopy (HAADF-STEM)
images and the energy dispersive X-ray spectroscopy (EDS) were
obtained with a JEOL JEM-2100F field emission transmission electron
microscope. X-ray photoelectron spectroscopy (XPS) were
conducted on an ESCALAB250 X-ray photoelectron spectrometer. X-
ray diffraction (XRD) patterns were measured by a TTR-III X-ray
diffractometer using Cu Kα radiation. Metal content was determined
on an AA800 atomic absorption spectrophotometer (AAS). The
sample handling process was as follows: a certain amount of catalyst
M-Sn/SiO2 was prepared by wetness impregnation method. Taking
Ni-Sn/SiO2 as an example, 1.0 g of SnO2/SiO2 (calcination at 700 °C)
was added into 80 g of acetone in a round-bottom flask. A certain
amount of Ni(NO3)2·6H2O was dissolved in 10 mL of water and then
dripped into the suspension above. After stirring at 45 °C for 24 h,
the solvent was removed using rotary evaporation and dried at 105
°C overnight. Then the solid powder was reduced at 600 °C (1 °C/min
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Catalyst recycle
was added into a round-bottom flask which containing 4 mL of aqua
regia. After stirring at 80 °C overnight, the mixture was diluted to 100
mL for testing.
Temperature-programmed reduction (TPR) was measured by a
home-built reactor coupled to a gas chromatograph. Before testing,
the catalysts were pretreated at 500 °C for 1 h in an Ar flow. 5% H2/Ar
mixture gas flow was used for TPR test from 40 °C to 800 °C at a
heating rate of 10 °C/min.
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DOI: 10.1039/C9GC02449B
After the reaction, the catalyst was separated by centrifugation, and
the recovered catalyst was directly added into reactor for the next
run.
Results and discussion
Temperature-programmed desorption of CO2 (CO2-TPD) was
conducted on a ChemBET Pulsar Automatic Chemisorption Analyzer
with a thermal conductivity detector (TCD). Before testing, the
catalysts were pretreated at 500 °C for 1 h in He flow. The catalysts
adsorbed CO2 at 45 °C for 1 h, then purged with He for another1 h to
remove the physisorbed CO2. After that, a temperature programmed
from 45 °C to 900 °C at a heating rate of 10 °C/min was performed.
Pyridine- Fourier Transform Infrared spectroscopy (Py-FTIR) was
measured by a FTIR Spectrometer (Thermo Scientific Nicolet iS 50).
The samples were activated at 300 °C, 10-4 Pa for 1 h. The background
spectrum was recorded when the temperature was decreased to 30
°C. After adsorption of pyridine vapor for 0.5 h, the physically
adsorbed pyridine was removed under vacuum and the spectrum
was recorded.
Catalytic Activity Test
A series of catalysts were prepared and used for cellulose
conversion. As shown in Table 1 (entry1-3), three non-noble
metals (Cu, Co, Ni) were selected as hydrogenation metals due
to their abundant resourses and proper hydrogenation activity.
All of the catalysts showed good activity for acetol production.
The yield of acetol was higher than 40%, which is higher than
that reported so far. Especially for 5% Ni-20%Sn/SiO2, 54.3%
yield of acetol was obtained after reaction for 1 h. In addition,
12.6% yield of 1-hydroxy-2-butanone (HB) was obtained, which
is also a kind of HK and widely used as a pharmaceutical
intermediate. Although the total yield of C3 products (73.4%)
was higher when 5%Co-20% Sn/SiO2 was used as catalyst, the
yield of acetol was only 41.1%. More lactic acid was produced,
which may be due to the lower hydrogenation activity of Co
than Ni, so that the pyruvaldehyde intermediate was not
efficiently hydrogenated to acetol (Scheme 1). In order to
obtain more acetol, a series of 5%Ni-x%Sn/SiO2 with different
Sn/Ni ratios were prepared and tested. When Ni/SiO2 was used
as catalyst, only 0.8% of acetol was obtained. The main products
were C2-C4 diols and sorbitol. When 5% Sn was incorporated
into the catalyst, the yield of acetol increased to 2.7% and the
main product was still polyols. Further increasing the Sn content
to 10%, the yield of acetol increased dramatically to 38.6%, and
the yield of HB increased to 9.9%. At the same time, the diols
and sorbitol yield reduced significantly. An unprecedented
reported yield of acetol (59.6%) was obtained when 5%Ni-
15%Sn/SiO2 was used as the catalyst and the yields of α-HK and
C3 products reached 74% and 70.8%, respectively. They
reduced slightly when the Sn content was further increased.
Catalyst test
All of the reactions were performed in a 25 mL autoclave with
magnetic stirring (Anhui Kemi Machinery Technology Co., Ltd). In a
typical experiment, 50 mg of cellulose, 34 mg of catalyst and 10 mL
of deionized water were added in the autoclave. After purging with
H2 for several times, 4 MPa H2 was charged at room temperature.
After reaction, the reaction solution was filtered, and the filtrate was
analyzed by high-performance liquid chromatography (HPLC, Waters
1525) with a Shodex RI-201H detector and Bio-Rad Aminex HPX-87H
column (300 x 7.8 mm) using 0.005M H2SO4 aqueous solution as the
mobile phase. The formula for calculating the yields of products is as
follows：
Masses of carbon in the product
Masses of carbon in the cellulose added to reactor
The yields of products =
Table1. Cellulose conversion over a series of metals supported on Sn/SiO2 catalysts.a
Yield %
Entry
Catalyst
Acetol
45.2
41.1
54.3
0.8
HB
15.4
5.7
C2-C4 diols
3.8
LA
Sorbitol
trace
trace
trace
10.5
10.7
trace
trace
--
α-HK
60.6
46.8
66.9
0.8
Total C3
48.8
73.4
66.4
8.7
1
2
10%Cu-20%Sn/SiO2
5%Co-20%Sn/SiO2
5%Ni-20%Sn/SiO2
5%Ni/SiO2
2.4
2.6
30.9
10.8
--
3
12.6
--
2.2
4
20.9
23.5
9.9
5
5%Ni-5%Sn/SiO2
5%Ni-10%Sn/SiO2
5%Ni-15%Sn/SiO2
5%Ni-25%Sn/SiO2
5%Ni-15%Sn/SiO2
2.7
2.7
5.4
5.4
21.2
57.7
70.8
68.1
37
6
38.6
59.6
53.7
16.6
9.9
13.4
10.5
12.0
20.4
48.5
74
7
14.4
12.7
1.9
1.2
8
4.1
66.4
18.5
9b
0
--
Reaction conditions: a 50 mg of cellulose, 34 mg of catalyst, 10 mL of H2O, 250 °C, 4 MPa H2, 1 h. b the reaction was carried out under 1
MPa N2. HB, LA and α-HK represent 1-hydroxy-2-butanone, lactic acid and α-hydroxy ketones, respectively.
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Figure 2. Products distribution of cellulose conversion as a
function of reaction time. Reaction conditions: 50 mg of cellulose,
34 mg of catalyst, 10 mL of H2O, 4 MPa H2 250 °C. Cellulose
conversion was calculated by the change in cellulose weight
before and after the reaction.
85%. However, as the hydrogen pressure increased, acetol
increased with lactic acid decreased slowly. This could be due to
the fact that high hydrogen pressure favors hydrogenation of
the pyruvicaldehyde intermediate rather than hydration. When
the hydrogen pressure was 5 MPa, the yield of acetol reached
61.6%.
The products distribution at different reaction time was
also analyzed. As shown in Figure 2, at 0 min (The reactor was
just heated to 250 °C), 23.8% yield of acetol was obtained with
more than half of the cellulose conversion. After reaction for 15
min, 50% yield of acetol was produced. Then, the acetol yield
slowly increased with prolong the reaction time. 59.6% yield of
acetol was achieved after reaction for 60 min. The yields of
lactic acid and HB did not change significantly after reaction for
15 minutes. During the reaction, no or only traces of
intermediates including glucose, fructose, dihydroxyacetone,
glyceraldehyde were detected, indicating that the catalyst is
efficient for intermediates conversion including isomerization
of glucose, the retro-aldol condensation of fructose and the
conversion of dihydroxyacetone and glyceraldehyde.
Figure 1. Effect of a) temperature and b) H2 pressure on cellulose
conversion. Reaction conditions: 50 mg of cellulose, 34 mg of
catalyst, 10 mL of H2O, 1 h. 4 MPa H2 for Figure 1a or 250 °C for
Figure 1b.
Based on the above results, it can be clearly found that when
the Sn content was more than 10%, the hydrogenation activity
of the catalyst was lowered and the isomerization and retro-
aldol condensation activity were improved significantly. All of
these are very important factors for preparing acetol from
cellulose. 5%Ni-15%Sn/SiO2 with different preparation
conditions including different calcination temperatures and
reduction temperatures were also prepared and used for
cellulose conversion (Table S1, Figure S1). The preparation
conditions had a slight effect on the catalytic activity, and the
yield of acetol was >52%.
The effect of temperature and hydrogen pressure on
cellulose conversion was investigated. As shown in Figure 1a,
when the reaction was carried out at 220 °C, 30.8% yield of
acetol and 8.7% yield of HB were obtained. When the
temperature reached 230 °C, the yield of acetol rapidly
increased to 53%, and the yield of HB increased to 14.5%.
Further increasing the temperature, the acetol yield increased
slowly, while the yield of HB remained stable. The yield of LA
increased as the temperature increased, and at 250 °C, 12.3%
yield of LA was obtained. The hydrogen pressure from 3 MPa to
5 MPa has no obvious effect on the reaction (Figure 1b), and the
total yield of the three small molecules maintained at about
The stability of 5%Ni-15%Sn/SiO2 was tested. After 5 runs,
there was no significant decrease in the yield of acetol. The AAS
characterization showed that only a small amount of metal was
lost after 5 runs (Figure S5, Table S5)
Catalysts Characterization and Discussion
To investigate the structure-activity relationship, a series of
characterization methods were performed. The metal contents
of the catalysts were shown in Table S2. In general, the content
of the two metals and the Sn/Ni ratio in the catalysts were
consistent with the theoretical values. The morphologies and
particle distributions of the catalysts were obtained by TEM
images. As shown in Figure 3, the metal particles were well
dispersed on the nanosilica for all of the catalysts. For Ni/SiO2,
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Figure 3. TEM images of 5%Ni-x%Sn/SiO2 catalysts. (a) 5%Ni/SiO2, (b)5%Ni-5%Sn/SiO2, (c) 5%Ni-10%Sn/SiO2, (d) 5%Ni-15%Sn/SiO2,
(e) 5%Ni-20%Sn/SiO2, (f) 5%Ni-25%Sn/SiO2.
broad indicating that the metal particles well disperse on the
nanosilica, which is consistent with the TEM results. For Ni/SiO2,
a small diffraction peak at 44.5° was attributed to (111) plane of
Ni. When 5% Sn was incorporated, the diffraction peak of Ni
disappeared, and an overlapping peak appeared near 44°. As
the Sn content further increased, the peak shifted slightly
toward higher angle. The diffraction peak of 5%Ni-5%Sn/SiO2
could be attributed to Ni3Sn2 alloy (JCPDS No. 65-9460), and the
diffraction peak of 5%Ni-10%Sn/SiO2 was attributed to Ni3Sn4
alloy (JCPDS No. 65-4310). For 5%Ni-15%Sn/SiO2 the tailing
peak at 30.7° (Overlapping peaks of Ni3Sn4(111), (-401), and
(310) plane) and the diffraction peaks at 39.2° and 55.3° also
illustrated the formation of Ni3Sn4. For 5%Ni-15%Sn/SiO2 and
5%Ni-20%Sn/SiO2, in addition to the peaks of Ni3Sn4, no other
diffraction peaks such as Sn, SnO, and SnO2 were observed.
However, the Sn/Ni ratio in the catalyst was larger than that in
the alloy, indicating that in addition to Ni3Sn4, other Sn species
could be well dispersed on the support. For 5%Ni-25%Sn/SiO2,
the diffraction peaks of metal Sn were observed due to the
excessive Sn content.
XPS analysis was performed to detect the chemical state of
the metals on the catalyst surface. As shown in Figure 4c, the
Figure 4. (a) STEM-HADDF-EDS elemental mapping of 5%Ni-
15%Sn/SiO2, (b) XRD patterns and (c,d) XPS spectra of Ni 2p and
Sn 3d of 5%Ni-x%Sn/SiO2 catalysts with different Sn/Ni ratio.
binding energy at around 852.5 eV was attributed to Ni0.21 As
the content of Sn increased, the Ni0 content increased,
indicating that the formation of alloy (Figure 4b) can inhibit the
oxidation of Ni. For the Sn 3d spectra (Figure 4d), it cannot
distinguish Sn2+ and Sn4+ by peak fitting due to their similar
binding energies.22 Therefore, Sn-O was used here to represent
the oxidation state of Sn species. In addition to Sn0, a large
number of Sn-O species existed. They were not detected by
XRD, probably because the catalyst surface was oxidized when
the catalyst was treated in 1% O2/N2. Surface elemental analysis
showed that the Sn/Ni ratio of the catalyst surface was larger
than the bulk (Table S2). These Sn species were important for
the isomerization and retro-aldol condensation reaction.
the average particle size of Ni particles was only 2.3 nm. As the
content of Sn increased, the size of metal particles increased
gradually. The average size of metal particles on 5%Ni-
15%Sn/SiO2 was 5.2 nm. For 5% Ni-25% Sn/SiO2, some particles
larger than 50 nm appeared, which may be due to excessive Sn
content resulting in metal agglomeration. The HADDF-EDS
elemental images of 5%Ni-15%Sn/SiO2 (Figure 4a) showed that
the two metals were well dispersed on the support. XRD
analysis was performed to study the crystal structure of the
catalysts. As shown in Figure 4b, all of the diffraction peaks are
H2-TPR was performed to investigate the reducibility of the
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Figure 5. H2-TPR profiles for 5%Ni-x%Sn/SiO2 catalysts.
catalysts. As shown in Figure 5, for 15% Sn/SiO2, two distinct
reduction peaks showed at around 600 °C and 660 °C. However,
according to previous studies, the unsupported SnO2 has only
one reduction peak.23-25 Therefore, we speculated that the
reduction peak at 600 °C was attributed to the reduction of
SnO2, while the peak at higher temperature could be attributed
to the reduction of tin oxides interacted with silica. As Ni was
added, the position of the reduction peaks moved toward lower
temperature. This could be because Ni promoted the
dissociation of H2 to active H, and the active H migrated onto tin
oxides to promote its reduction.26 For 5%Ni-x%Sn/SiO2
catalysts, the highest temperature of the reduction peaks is 585
°C, indicating that the reduction temperature (600 °C) of the
catalysts in this work was sufficient.
The acidity and basicity of the catalyst are essential for the
selective preparation of C2-C4 small molecules from cellulose.
Therefore, Py-FTIR was performed to analyze the surface acidity
of the catalysts (Figure 6a). According to previous researches,
the bands at around 1447, 1577 and 1607 cm-1 were ascribed to
the vibrations of pyridine adsorbed on Lewis acid (L-acid) sites,
and the band at 1597 cm-1 was attributed to the vibration of
hydrogen-bonded pyridine.27,28 The bands of the Sn-containing
catalysts were much stronger than that of Ni/SiO2, indicating
that the incorporation of Sn significantly increased the L-acid of
the catalyst. The 15% Sn/SiO2 spectrum indicated that the L-acid
sites of the 5%Ni-x%Sn/SiO2 were mainly derived from the
Sn/SiO2. The L-acid sites can catalyze glucose isomerization and
retro-aldol condensation. However, after calculation, 5%Ni-
10%Sn/SiO2 had the most L-acid sites concentration (Table S3).
This was inconsistent with the results in Table 1, that 5%Ni-
15%Sn/SiO2 rather than 5%Ni-10%Sn/SiO2 showed the best
catalytic activity. This suggested that L-acid was not the most
important factor for acetol production. All of the catalysts
showed no band around 1540 cm-1, which was attributed to the
vibrations of pyridine adsorbed on Bronsted acid (B-acid)
sites,27,28 indicating that there are no B-acid sites in these
catalysts. In the aqueous reaction system, some L-acid sites are
unstable and easily converted to B-acid sites. Therefore, the
Figure 6. Pyridine-FTIR spectra of (a) 5%Ni-x%Sn/SiO2 with
different Sn/Ni ratio (b) 5%Ni-15%Sn/SiO2 before and after
hydrothermal treatment. Treatment conditions: 250 °C, 4 MPa H2,
15 min.
5%Ni-15%Sn/SiO2 with the best activity was treated under
hydrothermal conditions at 250 °C. As shown in the Figure 6b,
there was no significant change after hydrothermal treatment,
indicating that L-acid was stable under the reaction conditions.
According to previous studies, B-acid tends to catalyze
unwanted side reactions, such as the formation of humins and
levulinic acid etc.29,30 The 5%Ni-15%Sn/SiO2 catalyst with weak
or no B-acid can suppress side reactions and promote the
conversion of cellulose to C2-C4 small molecules.
The basicity of the catalysts was characterized by CO2-TPD.
Different CO2 desorption peaks represent different strength of
basic sites. The peak below 200 °C was related to the bidentate
carbonate species desorbed from weaker basic sites. The high-
temperature peak above 300 °C was assigned to the desorption
of unidentate species on strong base sites.18,31 As shown in
Figure 7, 5% Ni/SiO2 has a weaker basicity. When Sn was
incorporated, the desorption peak at 600 °C - 850 °C increased,
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View Article Online
DOI: 10.1039/C9GC02449B
Figure 7. CO2-TPD profiles for 5%Ni-x%Sn/SiO2 catalysts.
Figure 8. XRD patterns of fresh 15%Sn/SiO2, fresh 5%Ni/SiO2 and
indicating that the incorporation of Sn significantly increased
the basic sites of the catalyst. Moreover, the CO2 desorption
temperature in this work is higher than that in previous studies,
which Sn was supported on other carriers like Al2O3 and
ZrO2.18,32 This demonstrated the catalysts in this work had
stronger basicity. The difference in basicity of the Sn based
catalysts could be attributed to the difference in the interaction
between Sn and the carriers. As the content of Sn increased, the
amount of base sites (based on areas of the CO2 desorption
peaks) increased first and then decreased (Table S4). This is
consistent with the trend of catalytic activity (Table 1). 5%Ni-
15%Sn/SiO2 with the best catalytic activity has the most basic
sites and the strongest basicity. The base sites can catalyze the
glucose isomerization and retro-aldol condensation.33 The
basicity of 5%Ni-15%Sn/SiO2 could be the main factor for the
high selectivity of the C3 products.
their mixture after reaction.
obtained. The total yield of the C3 products was 51%. This
indicated that without the participation of Ni, 15%Sn/SiO2
already has better activity for catalyzing isomerization of
glucose and retro-aldol condensation of fructose. However, due
to the weak hydrogenation activity of metal Sn, more lactic acid
was produced (20.2%). When 5%Ni/SiO2 combined with
15%Sn/SiO2 as catalysts, the yield of acetol significantly
increased to 43.5% and the total yield of the C3 products was
increased to 61.5%. Ni/SiO2 generally has good hydrogenation
activity, and acetol as a carbonyl compound should be easily
hydrogenated to form 1,2-propanediol (Table 2, entry4).
However, the resulting acetol was not further hydrogenated as
physically mixed 5%Ni/SiO2 with 15%Sn/SiO2 as catalysts. To
explore the reasons, XRD characterization was carried out. As
shown in Figure 8, the main diffraction peaks of 15%Sn/SiO2 was
metal Sn, and small peaks of SnO and SnO2 were also observed.
After the reaction, the diffraction peaks of the metal Sn and SnO
disappeared, and the diffraction peak of Ni3Sn4 appeared,
indicating that the Sn species in 15%Sn/SiO2 could transfer to
Ni/SiO2 to form the Ni3Sn4 alloy under reaction conditions. The
Mechanism study
In order to explore the role of each component in cellulose
conversion,
a series of comparative experiments were
conducted. As shown in Table 2, when 15%Sn/SiO2 was used as
the catalyst, 29.5% yield of acetol and 8.2% yield of HB were
Table 2. Comparative experiments of cellulose conversion over different Sn based catalysts.a
Yield %
1,2-B
Entry
Catalyst
15%Sn/SiO2
Acetol
29.5
43.5
4
HB
8.2
EG
2.3
2.7
7.4
--
1,2-P
1.4
LA
20.2
15.5
trace
--
Sorbitol
--
Total C3
51.1
61.5
8.2
1
2
trace
trace
trace
--
5%Ni/SiO2+15%Sn/SiO2
5%Ni/SiO2+15%SnO2/SiO2
5%Ni/SiO2
10.2
trace
--
2.5
trace
10.4
--
3
4.2
4b
5
12.3
trace
79.2
6.2
--
5%Ni/SiO2+Sn powder
trace
9.6
2.8
--
13.2
6.2
Reaction conditions: a 50 mg of cellulose, 34 mg of catalyst, 10 mL of H2O, 250 °C, 4 MPa H2, 1 h. For the physical mixture of catalysts,
each catalyst was 34 mg. b Substrate was acetol. HB, EG, 1,2-P, 1,2-B and LA represent 1-hydroxy-2-butanone, ethylene glycol, 1,2-
propanediol, 1,2-butanediol and lactic acid, respectively.
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formation of the Ni3Sn4 alloy reduced the hydrogenation activity
of the catalyst and thereby inhibited further hydrogenation of
the acetol. Ni3Sn4 alloy also significantly promoted the
formation of acetol and C3 products. This is consistent with the
above results, that when the Sn content was more than 10%,
Ni3Sn4 was formed (Figure 4b) and the yields of the acetol and
C3 products were improved remarkably (Table 1).
View Article Online
DOI: 10.1039/C9GC02449B
When 5%Ni/SiO2 combined with 15%SnO2/SiO2 as catalysts,
only 4% yield of acetol was obtained, and the total yield of C3
products was only 8.4% (Table 2, entry 3), indicating that 15%
SnO2/SiO2 could not catalyze isomerization and retro-aldol
condensation reaction. The CO2-TPD analysis showed that the
basic sites of 15%Sn/SiO2 was much more than 15%SnO2/SiO2
(Figure S4), which could be the main reason why 15%Sn/SiO2
could catalyze the cellulose conversion to C3 products (Table 2,
entry 1) but 15%SnO2/SiO2 could not, and the basicity of the
catalyst should be derived from the reduced Sn species.
However, when 5%Ni/SiO2 mixing with Sn powder was used as
catalyst, only 6.3% yield of C3 products were formed, indicating
that Sn powder could not catalyze isomerization and retro-aldol
condensation reactions. Sun et al. also used Ni/SiO2 + Sn powder
as catalysts to catalyze cellulose conversion, and similar results
were obtained.34 However, when they used Ni/AC +Sn powder
as catalyst, 57.6% yield of ethylene glycol (EG) was obtained. It
was found that Sn in Sn powder could transfer to Ni/AC to form
NiSn alloy (atomic ratio 1:1) but could not transfer to Ni/SiO2
due to the different interaction between Ni and carriers, which
is the main reason for their difference in catalytic activity. In our
work, 5%Ni/SiO2 mixing with 15%Sn/SiO2 could form the Ni3Sn4
alloy but mixing with Sn powder could not, which could be
because the Sn dispersed on SiO2 with higher surface area was
easier transfer to Ni/SiO2 and form Ni3Sn4 alloy.
Figure 9. XPS spectra of Sn 3d5/2 of different Sn based catalysts.
to acetol formation. When NiSn (atomic ratio 1:1) was used as
the catalyst, 43.2% of acetol and 11% of HB were obtained, and
the total C3 products yield was 58.4%. Similar result was
obtained when Ni3Sn4 was used as the catalyst. It can be seen
that the SnNi alloy has better isomerization and retro-aldol
condensation activity when Sn/Ni≥ 1. When the dosage of
Ni3Sn4 catalyst was doubled, the yield of acetol (45.3%) was only
slightly increased and significantly lower than that of 5%Ni-
15%Sn/SiO2 as catalyst (59.3%), indicating that Ni3Sn4 alone was
insufficient.
XPS analyses of Sn powder, Ni3Sn4, 15%Sn/SiO2, and 5%Ni-
15%Sn/SiO2 were conducted to investigate the electron state
and chemical properties of different Sn species (Figure 9). For
Sn powder, the binding energy at 484.3 eV was attributed to
Sn0,35 and the binding energy at 486.7 eV was attributed to Sn4+
and Sn2+.22,34,36 For unsupported Ni3Sn4, the binding energy of
Sn0 increased to 484.6 eV, and the binding energy of Ni0
decreased (Figure S3), indicating that some Sn orbital electrons
To further confirm the catalytic activity of Ni3Sn4, Ni3Sn4 alloy
and other SnNi alloys with different Sn/Ni ratios were
synthesized and used for the cellulose conversion (Table 3). XRD
analysis showed that Ni3Sn2, NiSn and Ni3Sn4 were successfully
prepared (Figure S2). When Ni3Sn2 was used as the catalyst, only
4.5% of acetol was obtained, with EG (19.6%) as the main
product, and the total yield of the C3 products was only 21.9%.
This was consistent with the 5%Ni-5%Sn/SiO2 result (Table 1,
entry 5), suggesting that Ni3Sn2 has low glucose isomerization
activity and high hydrogenation activity, which is not conducive
Table 3. Cellulose conversion over different SnNi alloy catalysts.a
Yield %
Entry
Catalyst
Ni3Sn2
NiSn
Acetol
4.5
HB
1.9
11
EG
19.6
0.8
1,2-P
5.7
1,2-B
4.3
LA
Sorbitol
Total C3
21.9
1
2
11.7
14.7
15.8
10.8
1.0
--
43.2
41.2
45.3
0.5
trace
trace
trace
58.4
3
Ni3Sn4
Ni3Sn4
9.7
13.4
0.8
trace
0.4
--
57.2
4b
0.6
--
56.5
Reaction conditions: a 50 mg of cellulose, 34 mg of catalyst, 10 mL of H2O, 250 °C, 4 MPa H2, 1 h. b The dosage of catalyst was 68
mg. HB, EG, 1,2-P, 1,2-B and LA represent 1-hydroxy-2-butanone, ethylene glycol, 1,2-propanediol, 1,2-butanediol and lactic acid,
respectively.
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transferred to Ni. Therefore, the Sn in the alloy actually showed
slightly positive (Snδ+) instead of zero valence.34 In addition, the
binding energy (486.3 eV) of the oxidation state of Sn species
on the alloy surface was significantly lower than that of on Sn
powder, suggesting that the electronic effect between Ni and
Sn inhibited the deep oxidation of the surface Sn when it
exposed in air, also making the Sn species showed noninteger
valence (SnOx). These noninteger valence Sn species (Snδ+ and
SnOx) in Ni3Sn4 alloy may be the important reason that it can
catalyze the conversion of cellulose to C3 molecules, but Sn
powder cannot catalyze this reaction due to the lack of these Sn
species. In previous work, the in-situ NiSn alloy exhibited
catalytic retro-aldo condensation activity but no catalytic
isomerization activity. The Sn in the in-situ alloy showed a little
positive valence (Snδ+), which was the active site for catalyzing
the retro-aldo condensation reaction.34 Therefore, the
isomerization activity in this work could be derived from the
SnOx species. When Sn was supported on SiO2, the binding
energy of Snδ+ was even higher than that in Ni3Sn4 (484.9 eV vs
484.6 eV), indicating that the Snδ+ had lower electron
density..Moreover, compared with Sn powder and Ni3Sn4, the
XPS spectrum of supported Sn was broader, suggesting that
there should be other Sn species that combined with carrier.
This agreed with the characterization of H2-TPR. An apparent
shoulder peak at around 486.1 eV was found in 15% Sn/SiO2,
which could be the noninteger valence (low-valent) tin oxides
species combined with the SiO2 carrier (SnOx-Si). According to
the results of CO2-TPD (Figure S4), the basicity of 15% Sn/SiO2
may be derived from these Sn species (Snδ+ and SnOx-Si). The
binding energy of Snδ+ in 5%Ni-15%Sn/SiO2 was further
increased (485.0 eV) as Sn transferred electrons to Ni. From the
above characterization results, it can be seen that the formation
of the Ni3Sn4 alloy and the combination of Sn and SiO2 carrier
significantly change the electronic state and chemical
properties of the Sn species. The noninteger valence Sn species
formation alter the acid-base properties of the catalyst and also
form the catalytic sites for the conversion of cellulose to the
acetol.
Conclusion
View Article Online
DOI: 10.1039/C9GC02449B
A series of Ni-Sn/SiO2 catalysts with different Sn/Ni ratios were
prepared and used for cellulose conversion. Unprecedented
yield (61.6%) of acetol was obtained by using 5%Ni-15%Sn/SiO2
catalyst. A series of characterizations indicated that the basicity
of the catalyst was the main factor affecting acetol production.
The Ni-Sn/SiO2 catalysts have only L-acid sites and absent B-acid
sites. The L-acid sites of the catalysts are stable even after the
hydrothermal treatment. The absent B-acid can suppress
humins formation and other unwanted side reactions, thereby
facilitating conversion of cellulose to small molecules.
Mechanism studies suggested that the formation of Ni3Sn4 alloy
significantly promoted the acetol production, and its weak
hydrogenation activity inhibits the further hydrogenation of
acetol. Sn in Ni3Sn4 alloy formed noninteger valence Sn species
(Snδ+ and SnOx), which could be the reason for its activity to
convert cellulose to C3 products. Similar phenomena occurred
when Sn was loaded on SiO2 due to the interaction between
each other. These noninteger valence Sn species were the
source of the basic sites and the active sites for cellulose
conversion. Under the synergistic catalysis of Sn/SiO2 and Ni3Sn4
alloy, cellulose was efficiently converted to acetol. This work
provides guidance for the selective conversion of cellulose to C3
products.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 51536009 and 51876200).
National Key R&D Program of China (2018 YFB1501402).
References
According to the above characterization and experimental
results, the catalytic mechanism was proposed. First, cellulose
was hydrolyzed to glucose under high temperature
hydrothermal conditions. Under the catalysis of strong basic
sites (SnOx and Snδ+), glucose was efficiently isomerized to
fructose, and further retro-aldol condensation to
dihydroxyacetone and glyceraldehyde. After dehydration, the
C3 intermediate (pyruvaldehyde) was hydrogenated to acetol
under the catalysis of Ni3Sn4. Due to the weak hydrogenation
activity of Ni3Sn4, acetol would not be further hydrogenated.
The weak hydrogenation activity also inhibited the formation of
sorbitol and glycerol (Scheme 1). The strong basicity of the
catalyst promoted the isomerization of glucose, thereby
reducing the formation of C2 and C4 products and increasing
the selectivity of the C3 products. The absent B-acid of the
catalyst inhibited the formation of humins and other unwanted
products. All of these factors result in efficient conversion of
cellulose to acetol.
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Table of contents
61.6% yield of acetol was obtained by one pot conversion of cellulose using Ni-Sn/SiO2
catalysts, and the catalytic mechanism was studied.