Promoting effect of SnOx on selective conversion of cellulose to polyols over bimetallic Pt-SnOx/Al2O3 catalysts

Article abstract of DOI:10.1039/c2gc36088h

Cellulose is the most abundant source of biomass in nature, and its selective conversion into polyols provides a viable route towards the sustainable synthesis of fuels and chemicals. Here, we report the marked change in the distribution of polyols in the cellulose reaction with the Sn/Pt atomic ratios in a wide range of 0.1-3.8 on the SnO_x-modified Pt/Al ₂O₃ catalysts. Such a change was found to be closely related to the effects of the Sn/Pt ratios on the activity for the hydrogenation of glucose and other C₆ sugar intermediates involved in the cellulose reaction as well as to the notable activity of the segregated SnO _x species for the selective degradation of the sugar intermediates on the Pt-SnO_x/Al₂O₃ catalysts. At lower Sn/Pt ratios of 0.1-1.0, there existed electron transfer from the SnO_x species to the Pt sites and strong interaction between the catalysts, as characterized by temperature-programmed reduction in H₂ and infrared spectroscopy for CO adsorption, which led to their superior hydrogenation activity (per exposed Pt atom), and in-parallel higher selectivity to hexitols (e.g. sorbitol) in the cellulose reaction, as compared to Pt/Al ₂O₃. The hexitol selectivity reached the greatest value of 82.7% at the Sn/Pt ratio of 0.5, nearly two times that of Pt/Al ₂O₃ at similar cellulose conversions (～20%). As the Sn/Pt ratios exceeded 1.5, the Pt-SnO_x/Al₂O₃ catalysts exhibited inferior hydrogenation activity (per exposed Pt atom), due to the formation of the crystalline Pt-Sn alloy, which led to the preferential conversion of cellulose to C₂ and especially C₃ products (e.g. acetol) over hexitols, most likely involving the isomerization of glucose to fructose and retro-aldol condensation of these sugars on the segregated SnO_x species, apparently in the form of Sn(OH)₂. These findings clearly demonstrate the feasibility for rational control of the cellulose conversion into the target polyols (e.g. acetol or propylene glycol), for example, by the design of efficient catalysts based on the catalytic functions of the SnO_x species with tunable hydrogenation activity.

Full text of DOI:10.1039/c2gc36088h

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Promoting effect of SnOx on selective conversion of
cellulose to polyols over bimetallic Pt–SnOx/Al2O3
catalysts†
Cite this: Green Chem., 2013, 15, 116
Tianyin Deng and Haichao Liu*
Cellulose is the most abundant source of biomass in nature, and its selective conversion into polyols pro-
vides a viable route towards the sustainable synthesis of fuels and chemicals. Here, we report the marked
change in the distribution of polyols in the cellulose reaction with the Sn/Pt atomic ratios in a wide
range of 0.1–3.8 on the SnOx-modified Pt/Al2O3 catalysts. Such a change was found to be closely related
to the effects of the Sn/Pt ratios on the activity for the hydrogenation of glucose and other C6 sugar
intermediates involved in the cellulose reaction as well as to the notable activity of the segregated SnOx
species for the selective degradation of the sugar intermediates on the Pt–SnOx/Al2O3 catalysts. At lower
Sn/Pt ratios of 0.1–1.0, there existed electron transfer from the SnOx species to the Pt sites and strong
interaction between the catalysts, as characterized by temperature-programmed reduction in H2 and
infrared spectroscopy for CO adsorption, which led to their superior hydrogenation activity (per exposed
Pt atom), and in-parallel higher selectivity to hexitols (e.g. sorbitol) in the cellulose reaction, as compared
to Pt/Al2O3. The hexitol selectivity reached the greatest value of 82.7% at the Sn/Pt ratio of 0.5, nearly
two times that of Pt/Al2O3 at similar cellulose conversions (∼20%). As the Sn/Pt ratios exceeded 1.5, the
Pt–SnOx/Al2O3 catalysts exhibited inferior hydrogenation activity (per exposed Pt atom), due to the for-
mation of the crystalline Pt–Sn alloy, which led to the preferential conversion of cellulose to C2 and
especially C3 products (e.g. acetol) over hexitols, most likely involving the isomerization of glucose to fruc-
tose and retro-aldol condensation of these sugars on the segregated SnOx species, apparently in the
form of Sn(OH)2. These findings clearly demonstrate the feasibility for rational control of the cellulose
conversion into the target polyols (e.g. acetol or propylene glycol), for example, by the design of efficient
catalysts based on the catalytic functions of the SnOx species with tunable hydrogenation activity.
Received 13th July 2012,
Accepted 28th September 2012
DOI: 10.1039/c2gc36088h
www.rsc.org/greenchem
to avoid the use of mineral acids and to improve polyol
productivity.
1 Introduction
Catalytic conversion of cellulose to polyols has attracted great
Fukuoka and Dhepe reported the direct conversion of cellu-
attention as one of the most viable primary routes for cellulose lose into hexitols (i.e. sorbitol and mannitol) in a 31% yield at
utilization because of the versatile uses of polyols in the syn- 463 K on Pt/Al2O3 in the absence of mineral acids.10 By taking
thesis of fuels and value-added chemicals.1–7 This conversion advantage of the unique properties of water under near-critical
route traditionally involves the use of mineral acids for cellu- conditions, Luo et al. obtained a ∼40% yield of hexitols on Ru/
lose hydrolysis to sugars, which are subsequently hydrogen- C in combination with the hydrolysis role of H+ ions reversibly
ated to polyols on metal-based catalysts.6,8,9 Since the formed from hot water at 513 K.12 Further studies on different
pioneering work on Pt/Al2O3 in water and Ru nanoclusters in hydrogenation catalysts led to improved yields.13–15 For
an ionic liquid,10,11 recent studies on this cellulose reaction example, Pang et al. achieved a 59.8% yield of hexitols using a
have been dedicated to the development of greener approaches mesoporous carbon supported Ni–Rh catalyst;13 Sels and co-
workers reported a >90% yield of hexitols at a 100% conversion
of ball-milled cellulose on bifunctional Ru/H-USY catalyst in
the presence of a trace amount of HCl, and even the quantita-
Beijing National Laboratory for Molecular Sciences, College of Chemistry and
tive conversion of ball-milled cellulose into hexitols on Ru/C
Molecular Engineering, Peking University, Beijing 100871, China.
together with heteropolyacids.14b,c More noticeably, Zhang and
E-mail: hcliu@pku.edu.cn; Fax: +86-10-6275-4031
co-workers found that cellulose can even be converted directly
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
to ethylene glycol on Ni–W2C/C.16 These results demonstrate
c2gc36088h
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that the polyol selectivities are strongly dependent on the prop-
erties of the hydrogenation catalysts, which accordingly influ-
ence the relative rates between the competitive hydrogenation
of the sugar intermediates and their degradation. Therefore, it
is necessary to further understand the underlying reasons for
such a dependence in order to design more efficient catalysts
towards the controllable conversion of cellulose to polyols.
Compared to monometallic catalysts, bimetallic catalysts
frequently exhibit superior activity and selectivity in many reac-
tions. Concerning the aforementioned Pt/Al2O3 catalyst, its
modification with SnOx as an example leads to wide appli-
cations in industry.17–19 Extensive studies on the SnOx-modi-
fied Pt/Al2O3 (Pt–SnOx/Al2O3) catalysts have revealed the
drastically electronic and geometric effects of SnOx on the cata-
lytic properties of the Pt sites.20,21 Moreover, the presence of
SnOx as a Lewis acid in the Pt catalysts can largely improve the
hydrogenation of unsaturated aldehydes (e.g. citral, crotonalde-
hyde and carvone) via its close interaction with the carbonyl
groups, to form desirable products of unsaturated
alcohols.19,22–24 Therefore, we herein study the Pt–SnOx/Al2O3
catalysts with varying Sn/Pt atomic ratios in the range of 0–3.8
in the cellulose reaction and for comparison in the reactions
of glucose and fructose. Their structures are characterized by
X-ray powder diffraction (XRD), X-ray photoelectron spec-
troscopy (XPS), transmission electron microscopy (TEM), temp-
erature-programmed reduction in H2 (H2-TPR), Fourier
transform infrared (FT-IR) spectroscopy using a CO probe mol-
ecule and H2 chemisorption. These detailed studies lead to
our understanding of the SnOx modification effects on the
structures of the Pt–SnOx/Al2O3 catalysts and their hydrogen-
ation activities, and consequently on the polyol distribution in
the cellulose reaction.
Table 1 Pt loading, particle size and dispersion on Pt/Al2O3 and Pt–SnOx/
Al2O3 with different Sn/Pt ratios in the range of 0.1–3.8
Catalyst
Pt loadinga/wt%
db/nm
Dc/%
Pt/Al2O3
2.07
2.07
2.04
1.92
1.88
1.98
1.88
1.90
0.8 0.2
1.1 0.2
0.9 0.2
—
1.0 0.3
—
100.0
83.6
94.4
76.1
55.9
44.9
38.5
31.8
Pt–SnOx(0.1)/Al2O3
Pt–SnOx(0.3)/Al2O3
Pt–SnOx(0.5)/Al2O3
Pt–SnOx(1.0)/Al2O3
Pt–SnOx(1.5)/Al2O3
Pt–SnOx(2.1)/Al2O3
Pt–SnOx(3.8)/Al2O3
0.9 0.2
1.2 0.4
a Measured by ICP. b Measured by TEM. c Measured by H2
chemisorption.
ReOx, SbOx and GaOx-modified Pt/Al2O3 catalysts were pre-
pared in the same way using HReO4 (75–80% aqueous solu-
tion, Alfa Aesar), Ga(NO3)3·xH2O (99.9%, Alfa Aesar) and SbCl3
(AR, Sinopharm Chemical), respectively. They contained
1.98–1.99 wt% Pt, and their Re/Pt, Sb/Pt or Ga/Pt atomic ratios
were each 0.3.
In a similar way, SnOx/Al2O3 (0.36 wt% Sn) was prepared by
the incipient wetness impregnation of Al2O3 with ethanol solu-
tions of SnCl2·2H2O. SnOx (4.87 wt% Sn)/Al2O3 was prepared
using tri-n-butyltin acetate (AR, Alfa Aesar). Sn(OH)2 and
Sn(OH)4 were prepared by hydrolysis of aqueous SnCl2·2H2O
and SnCl4·5H2O (AR, Shantou Xilong Chemical) with stoichio-
metric NH3·2H2O (AR, 25%–28%, Beijing Chemical), respecti-
vely, followed by washing the precipitates with deionized water
until the filtrate was neutral. The precipitates were dried at
room temperature under vacuum.
Catalyst characterization
The contents of Pt, Sn, Re, Sb, and Ga in the catalysts were
measured by ICP-AES after they were dissolved in the aqua
regia solutions.
2 Experimental
Catalyst preparation
XRD patterns were recorded on a Rigaku D/MAX-2400 diffr-
Pt/Al2O3 and SnOx-modified Pt/Al2O3 (denoted as Pt–SnOx/ actometer using Cu Kα1 radiation (λ = 1.5406 Å) operated at
Al2O3) catalysts with seven different nominal Sn/Pt molar 40 kV and 100 mA. The 2θ angle was scanned in the range of
ratios in the range of 0.1–3.8, were prepared by the incipient 25–50° at a rate of 4° min−1
.
wetness impregnation of γ-Al2O3 (Alfa Aesar, 208 m2 g−1) with
XPS studies were conducted on an Axis Ultra spectrometer
aqueous solutions of H2PtCl6·6H2O (AR, Beijing Chemical), (Kratos Analytical Ltd.) using monochromatic Al Kα radiation
H2PtCl6·6H2O (AR, Beijing Chemical) and SnCl2·2H2O (AR, at a source power of 15 kV. The binding energies were refer-
Shantou Xilong Chemical), respectively. The impregnated enced to the Al 2p peak of the Al2O3 support at 74.5 eV.
samples were dried at room temperature for about 12 h and
TEM images were taken on a FEI Tecnai G2 T20 instrument
then at 383 K overnight. Afterwards, they were calcined in operated at 300 kV. Samples were prepared by uniformly dis-
flowing air at 673 K for 4 h, and then reduced in a 1 : 4 H2/N2 persing in ethanol and then placing onto carbon-coated
flow at 673 K for 4 h. The compositions of these samples were copper grids. The average sizes of metal particles and their
measured by profile spec inductively coupled plasma-atomic size distributions were determined by measuring more than
emission spectrometry (ICP-AES), showing that they contained 300 particles randomly distributed in the images. Energy dis-
1.88–2.07 wt% Pt and that the Sn/Pt atomic ratios for the Pt– persive X-ray (EDX) analysis was carried out on a Philips
SnOx/Al2O3 samples were 0, 0.1, 0.3, 0.5, 1.0, 1.5, 2.1, and 3.8 Tecnai F30 FEI TEM equipped with an EDX spectrometer oper-
(Table 1). The Pt–SnOx/Al2O3 samples were denoted as Pt– ated at 300 kV.
SnOx(m)/Al2O3, where the m in parentheses represents their
Temperature programmed reduction (TPR) experiments
measured Sn/Pt atomic ratio. For example, Pt–SnOx(0.1)/Al2O3 were performed on the TP-5000 flow unit (Tianjin Xianquan)
represents the sample with a Sn/Pt atomic ratio of 0.1.
by ramping the temperature from 298 to 1073 K at 10 K min−1
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in a 5.02% H2/N2 flow (30 mL min−1). Prior to the measure-
ments, the samples were calcined at 673 K in an air flow for
4 h. The amounts of consumed H2 for the samples were
measured by a TCD detector and then estimated by referring
to the amount obtained from the CuO reduction.
Hydrogen chemisorption measurements were taken on a
Quantachrome instrument (Autosorb-1). Calcined samples
were reduced in situ at 673 K under a 5.02% H2/N2 flow for 4 h,
then out-gassed at 673 K for 6 h and finally cooled to room
temperature. The total H2 chemisorption isotherm was per-
formed at 303 K between 4 and 43 kPa H2.
FT-IR spectra for CO adsorption were taken in the wave-
number range 4800–600 cm−1 at 298 K on a Bruker Tensor 27
spectrometer equipped with a MCT detector with a resolution
of 4 cm−1 by averaging 100 scans per spectrum. The samples
were pressed into self-supported thin wafers and placed into a
quartz cell with CaF2 windows. Prior to each measurement,
the samples were treated in the cell in a 5.02% H2/N2 flow
(40 mL min−1) at 673 K for 30 min, followed by evacuation at
673 K for 30 min. Upon cooling to ambient temperature in the
vacuum, the background spectra of the samples were recorded.
Afterwards, the samples were exposed to ca. 4 kPa CO for
10 min, and then evacuated for 30 min at 298 K for taking
their spectra. All spectra were obtained as difference spectra by
subtracting the background spectra of the samples from the
spectra after exposure to CO.
Fig. 1 XPS spectra of Pt 4d for Pt/Al2O3 and Pt–SnOx/Al2O3 (∼2 wt% Pt) with
Sn/Pt atomic ratios in the range of 0.1–3.8.
Cellulose (microcrystalline, Alfa Aesar) reactions were
carried out in a stainless steel autoclave (100 mL) typically at
473 K and 6 MPa H2 for 30 min with vigorous stirring at a
speed of 800 rpm. In a typical run, 1 g cellulose and 0.4 g cata-
lyst were introduced to the autoclave containing 50 mL H2O.
Afterwards, the reactor was fully purged with H2 (>99.999%,
Beijing Longhui Jingcheng), pressurized with H2 to 6.0 MPa
and then heated to 473 K which was kept constant during the
reaction. After cooling to room temperature in water, the reac-
tion mixture was filtrated and the solids were washed several
times with deionized water. The filtrate was diluted with de-
ionized water to 100 mL. The solids including the catalyst and
remaining cellulose were washed with acetone three times and
then fully dried in an oven at 333 K for 24 h. Cellulose conver-
sions were determined by the change in the weight of cellulose
loaded before and after the reactions. The products in the
liquid phase (e.g. polyols) were analyzed by high-performance
liquid chromatography (Shimadzu LC-20A) using Bio-Rad
Aminex HPX-87H and HPX-87C columns with an RID detector.
The products in the gas phase (e.g. methane) were analyzed by
gas chromatography (Shimadzu 2010) using a HJ-OV-101
column connected to a FID detector. The product selectivities
were reported on a carbon basis.
Fig. 2 XPS spectra of Sn 3d for Pt–SnOx/Al2O3 (∼2 wt% Pt) with Sn/Pt atomic
ratios in the range of 0.1–3.8.
lines were analyzed instead of the most intense Pt 4f lines
because of their superimposition with the strong Al 2p peak.
Only two peaks were observed at binding energies of around
332.5 and 315.0 eV, the characteristic peaks of Pt 4d3/2 and
Pt 4d5/2 for metallic Pt, respectively, which indicates that the
surface Pt species were present as zero-valent Pt in these cata-
lysts.25,26 In the Sn 3d region, as shown in Fig. 2, the bimetallic
Pt–SnOx/Al2O3 samples exhibited two symmetrical signals at
binding energies of 486.3–487.1 and 494.8–495.6 eV, which
can be respectively assigned to Sn3d5/2 and Sn 3d3/2 for oxi-
dized tin, Sn2+ and Sn4+, although XPS cannot distinguish
between the two oxidation states.25–27 These results indicate
that the SnOx species, i.e. SnO and/or SnO2, were the main Sn
species at the surfaces of the bimetallic samples. The SnOx
species did not reveal XRD peaks (Fig. 3), suggesting that they
3 Results and discussion
Fig. 1 shows the XPS spectra of the Pt 4d core level for the
monometallic Pt/Al2O3 and bimetallic Pt–SnOx/Al2O3 catalysts
with different Sn/Pt atomic ratios between 0.1–3.8. The Pt 4d
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Fig. 3 XRD patterns of Pt/Al2O3 and Pt–SnOx/Al2O3 (∼2 wt% Pt) with different
Sn/Pt atomic ratios in the range of 0.1–3.8.
were highly dispersed. But a diffraction peak at 2θ = 41.7°
appeared when the Sn/Pt molar ratios exceeded 1.5, and the
intensity increased with increasing the Sn/Pt ratios (Fig. 3).
This peak is tentatively assigned to the Pt–Sn (Sn/Pt atomic
ratio = 1 : 1) alloy (JCPDS: 25-0614 and JCPDS: 65-0959), which
is confirmed by the representative TEM image for the Sn-rich
Pt–SnOx(3.8)/Al2O3 catalyst with a Sn/Pt ratio of 3.8, showing a
parallel variation of the relative Pt and Sn compositions
against the line-scan positions in the energy-dispersive X-ray
spectrum (Fig. S1 in the ESI†).21,28,29 No detection of zero-
valent Sn in the XPS spectra (Fig. 2) may be due to the facile
surface oxidation of Sn after exposure to air induced by the
known strong affinity of tin for oxygen.30
The Pt particle sizes for the Pt/Al2O3 and Pt–SnOx/Al2O3
samples were characterized by TEM. As shown in Fig. 4 and
Fig. S2,† these samples possessed similar Pt particle sizes
(0.8–1.2 nm), irrespective of the Sn/Pt ratios. However, their Pt
dispersions measured by H2 chemisorption at 303 K changed
with the Sn/Pt ratios (Table 1). An almost 100% Pt dispersion
was obtained on Pt/Al2O3. But the dispersion decreased to ca.
83–94% on the bimetallic samples with the Sn/Pt ratios of 0.1
Fig. 4 TEM images and histograms of Pt particle size distributions of Pt/Al2O3
(a) and Pt–SnOx/Al2O3 (∼2 wt% Pt) with different Sn/Pt atomic ratios of (b) 0.3,
and (c) 3.8.
and 0.3, which then sharply decreased to 31.8% with increas- which is similar to the result (43.0%) reported by Fukuoka and
ing the Sn/Pt ratio up to 3.8. Such a decrease in the Pt dis- co-workers.31 The other identified products mainly included
persion is obviously not due to the change in the Pt particle pentitols (7.7%), tetritols (3.4%), glycerol (7.2%), 1,2-propane-
sizes, which may arise from the partial blockage of the Pt sites diol (6.3%), and ethylene glycol (3.5%). Glucose (2.9%), cello-
by the SnOx species and the formation of the large PtSn alloy biose alcohol (0.6%) and methane (3.2%) were also detected,
crystallites at the higher Sn/Pt ratios.19,20
which are listed in Table S1 in the ESI† for clarity. The total
These catalysts were examined in the cellulose reaction. As selectivity of the identified products was 78.1%, not including
shown in Table 2, they gave similar cellulose conversions the unidentified compounds most likely derived from the con-
(16.3–22.5%) after 30 min at 473 K and 6 MPa H2, reflecting densation or degradation of the sugar intermediates. Upon
the fact that the cellulose conversions are controlled by the cel- addition of a small amount of SnOx to Pt/Al2O3 (Sn/Pt = 0.1),
lulose hydrolysis step that is catalyzed mainly by the H+ ions the hexitol selectivity sharply increased from 43.3 to 78.0%
generated from hot water under such reaction conditions.12 while the selectivity to the cracking products, e.g. C2 and C3
However, the selectivities to polyols on these catalysts changed polyols, decreased concurrently from 17.0 to 3.3%. The total
significantly with their Sn/Pt ratios (Table 2). The hexitol selec- selectivity to the identified products reached 96.0%. With
tivity on Pt/Al2O3 was 43.3% (including 34.5% sorbitol, 8.0% increasing the Sn/Pt ratios to 0.3 and 0.5, the hexitol selectivity
mannitol and trace amount of iditol, see Table S1 in ESI†), increased to 81.3 and 82.7%, respectively, nearly twice the
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Table 2 Cellulose conversion and polyol selectivity on Pt/Al2O3 and Pt–SnOx/Al2O3 with different Sn/Pt ratios of 0.1–3.8a
Selectivity%
Ethylene
Hexitolsb Pentitolsc Tetritolsd Glycerol 1,2-Propanediol Acetol glycol
Catalyst
Conversion% C6
C5
C4
C3
C3
C3
C2
Pt/Al2O3
17.6
16.3
18.9
17.1
17.8
19.6
18.3
22.5
23.8
19.2
18.0
20.1
17.7
43.3
7.7
6.9
5.5
4.1
4.3
2.6
1.6
1.6
1.5
0.9
14.1
1.6
4.5
3.4
1.7
1.3
1.1
3.8
5.2
4.6
1.1
2.4
2.5
3.2
2.4
2.6
7.2
2.2
1.4
1.5
3.2
4.8
3.9
2.0
2.4
3.2
2.8
7.3
7.5
6.3
0.5
0.3
0.3
1.4
5.8
5.6
1.4
13.5
16.3
1.0
5.6
7.3
0.0
0.0
0.0
0.0
0.0
0.0
19.6
25.2
12.3
14.2
0.0
3.5
0.6
0.3
0.4
2.9
8.7
17.0
7.4
22.0
14.2
0.7
2.7
3.3
Pt–SnOx(0.1)/Al2O3
Pt–SnOx(0.3)/Al2O3
Pt–SnOx(0.5)/Al2O3
Pt–SnOx(1.0)/Al2O3
Pt–SnOx(1.5)/Al2O3
Pt–SnOx(2.1)/Al2O3
Pt–SnOx(3.8)/Al2O3
Pt–SnOx(0.3)/Al2O3 + Pt–SnOx(3.8)/Al2O3
78.0
81.6
82.8
73.4
43.2
3.2
0.8
3.9
5.7
e
f
Pt/Al2O3 + SnOx/Al2O3
Pt–ReOx(0.3)/Al2O3
Pt–SbOx(0.3)/Al2O3
Pt–GaOx(0.3)/Al2O3
63.7
51.6
37.1
0.0
0.0
a 1 g cellulose, 0.4 g catalyst, 473 K, 50 mL H2O, 6 MPa H2, 30 min. b Sorbitol, mannitol and iditol. c Xylitol and arabitol. d Erythritol and threitol.
e Physical mixture of Pt–SnOx(0.3)/Al2O3 and Pt–SnOx(3.8)/Al2O3. f Physical mixture of Pt/Al2O3 and SnOx/Al2O3, Sn = 0.36 wt%.
Table 3 Glucose hydrogenation on Pt/Al2O3 and Pt–SnOx/Al2O3 with different
Sn/Pt ratios in the range of 0.1–3.8a
an increase in their turnover frequency (TOF) from 4.6 to
11.4 min−1 (normalized per exposed surface Pt atom). After-
wards, a further increase in the Sn/Pt ratio up to 3.8 led to a
Selectivity%
monotonic decline in the glucose conversion and TOF to only
Sn/Pt
Conversion% TOFb/min−1 Hexitols Fructose
3.8% and 0.3 min−1, respectively. The Sn/Pt ratios also influ-
enced the product selectivities in the glucose hydrogenation.
Notably, the catalyst with the highest Sn/Pt ratio of 3.8 exhib-
ited essentially no hydrogenation activity, and it catalyzed
mainly the glucose isomerization to fructose with a selectivity
of 82.3% (at a low conversion of only 3.8%). Clearly, the Pt–
SnOx/Al2O3 samples with the Sn/Pt ratios of 0.1–1.0 afforded
the superior hydrogenation activities, and thus favored the
efficient synthesis of hexitols from the hydrogenation of
glucose and other C6 sugar intermediates involved in the cellu-
lose hydrolysis step, which would otherwise undergo degra-
dation or condensation reactions, as observed at the Sn/Pt
ratios higher than 1.0 (Table 2). To understand the underlying
Pt/Al2O3
35.2
4.6
7.1
7.4
11.4
9.6
7.6
2.2
0.3
94.0
96.5
94.8
96.7
98.3
93.3
63.0
0.0
1.4
0.0
0.0
0.0
0.0
4.3
30.1
82.3
Pt–SnOx(0.1)/Al2O3 43.2
Pt–SnOx(0.3)/Al2O3 50.9
Pt–SnOx(0.5)/Al2O3 63.3
Pt–SnOx(1.0)/Al2O3 38.9
Pt–SnOx(1.5)/Al2O3 25.7
Pt–SnOx(2.1)/Al2O3 6.1
Pt–SnOx(3.8)/Al2O3 3.8
a 1 g glucose, 0.4 g catalyst, 393 K, 50 mL H2O, 6 MPa H2, 10 min.
b TOF was calculated from the amount of glucose converted to polyols
normalized per exposed Pt atom.
value of Pt/Al2O3. However, further increasing the Sn/Pt ratios reason of such SnOx effects on the catalytic properties of the
led to a concurrent decline in the hexitol selectivity and an Pt–SnOx/Al2O3 catalysts, they were characterized by TPR in H2
increase in the C2–C5 polyols. Specifically, as the Sn/Pt ratio (H2-TPR) and FT-IR of CO adsorption (CO-IR).
increased to 2.1, the hexitol selectivity dramatically decreased
Fig. 5 shows the H2-TPR profiles of the Pt–SnOx/Al2O3
to as low as 3.2%, whereas the selectivity to the C2 and C3 samples. For comparison, the profiles of Pt/Al2O3 and SnOx/
polyols increased to as high as 46.1%. It is noted that acetol, Al2O3 are also included. SnOx/Al2O3 presents a broad SnOx
an important chemical intermediate,32,33 was formed with a reduction feature around 450 °C, as reported previously.26 For
selectivity of 19.6%, which further increased to 25.2% by Pt/Al2O3, two reduction peaks were observed at 227 and
increasing the Sn/Pt ratio from 2.1 to 3.8.
351 °C.34,35 The first peak is characteristic of the reduction of
Such a change in the polyol selectivities reflects the effect of the PtOx species interacting weakly with the Al2O3 support,
SnOx on the hydrogenation property of Pt on Al2O3, as reported while the second one can be assigned to the PtOx or oxy-
previously in many other reactions such as the hydrogenation chlorided Pt species interacting strongly with Al2O3.34,35 The
of crotonaldehyde or citral.19 This effect is further examined total consumption of H2 corresponds to the stoichiometric
here in the hydrogenation of glucose, the key primary inter- reduction of Pt4+ to Pt0. This TPR feature confirms the pres-
mediate in the cellulose conversion into polyols. As shown in ence of merely metallic Pt on Pt/Al2O3 after the general treat-
Table 3, the glucose conversion was 35.2% on Pt/Al2O3 after ment with H2 at 400 °C employed in this work, which is
10 min at 393 K and 6 MPa H2, which increased to 63.3% on consistent with the XPS result (Fig. 1). Compared to Pt/Al2O3,
Pt–SnOx(0.5)/Al2O3 with a Sn/Pt ratio of 0.5, corresponding to an additional peak appeared around 400 °C on the Pt–SnOx/
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Fig. 6 FT-IR spectra of CO adsorption on Pt/Al2O3 and Pt–SnOx/Al2O3 with Sn/
Pt atomic ratios in the range of 0.1–3.8.
Fig. 5 TPR profiles of Pt–SnOx/Al2O3 with different Sn/Pt atomic ratios in the
range of 0.1–3.8, and for comparison Pt/Al2O3, SnOx/Al2O3 (0.36 wt% Sn) and
Al2O3.
It is known that the electron transfer from SnOx to Pt can
increase the electron density of the Pt sites, and consequently
promote H2 dissociation, required for the hydrogenation of
glucose and other C6 sugar intermediates involved in the cellu-
lose conversion. Moreover, the electron transfer can make the
SnOx surfaces more positively charged and enhance their
Lewis acidity, which, as reported by Gallezot et al., can favor
the polarization of the carbonyl group of glucose via withdraw-
ing electrons from the oxygen atom in its carbonyl group.40
Such enhanced polarization then facilitates the CvO hydro-
genation by H2 dissociatively adsorbed on the Pt sites that are
in intimate contact with the SnOx species, as proposed in
Scheme 1. Taken together, these electronic effects of the SnOx
species led to the superior hydrogenation activities and
efficiency for the synthesis of hexitols in the cellulose reaction
on the Pt–SnOx/Al2O3 catalysts with the Sn/Pt ratios of 0.1–1.0,
relative to Pt/Al2O3 (Tables 2 and 3).
Al2O3 catalysts due to the reduction of their SnOx species. The
PtOx reduction peak shifted to lower temperatures around
200 °C with increasing the Sn/Pt ratios to 0.3 and 0.5. It then
shifted to the higher temperature around 225 °C, approaching
that for Pt/Al2O3, as the Sn/Pt ratios exceeded 1.0. The higher
reducibility of the PtOx species observed at the lower Sn/Pt
ratios of 0.1–0.5 is related to the strong interaction between
PtOx and SnOx, as reported previously, via the electron transfer
from SnOx to PtOx on Pt–SnOx/Al2O3.36,37 The observed weak
effect of SnOx on the PtOx reduction at the higher Sn/Pt ratios
may be due to the partial coverage of the PtOx species by SnOx,
as inferred from the decreased Pt dispersion (Table 1), and the
formation of larger PtSn alloy particles with low ability of
hydrogen activation.38 These results are in agreement with
those obtained from the in situ FT-IR characterization of CO
adsorption, as shown below in Fig. 6.
Such electronic effects on Pt hydrogenation activity were
further confirmed by the Pt/Al2O3 catalysts modified with
ReOx, SbOx and GaOx in the cellulose reaction. These catalysts
(∼2 wt% Pt) possessed similar Pt dispersions (∼72–75%) with
It is known that the SnOx surfaces do not significantly
adsorb CO.29,34 Therefore, the CO adsorption bands observed
on the bimetallic Pt–SnOx/Al2O3 catalysts (Fig. 6) arose from
the CO adsorption on the Pt sites. For Pt/Al2O3, an intense
band was detected at 2081 cm−1, which corresponds to the
linear adsorption of CO on the metallic Pt sites.34 This band
shifted to ca. 2073 cm−1 on the Pt–SnOx/Al2O3 samples with
Sn/Pt ratios of 0.1–1.0. Such a red-shift on the Pt–SnOx cata-
lysts can be explained by the geometric effect of the SnOx
species that block or dilute the Pt sites, leading to the weaken-
ing of dipolar coupling between the adsorbed CO molecules,
or by the electronic effect of the SnOx species that transfer elec-
trons to the Pt sites, leading to increasing electron back-
donation from the d orbitals of Pt to the 2π* states of CO.29,39
However, a further increase in the Sn/Pt ratios to over 1.5 led
to the appearance of the CO adsorption band at ca. 2081 cm−1
,
as observed on Pt/Al2O3, which excludes the geometric effect
of the SnOx species. Therefore, the red-shift of the CO adsorp-
tion band on the samples with Sn/Pt ratios of 0.1–1.0 is attrib-
uted to the electron transfer from SnOx to Pt.37
Scheme 1 Schematic description of the promoting role of SnOx in glucose
hydrogenation on Pt–SnOx/Al2O3 catalysts with atomic ratios of 0.1–1.5.
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Table 4 Cellulose conversion and polyol selectivity on different SnOx samplesa
Selectivity%
Conversion
%
Lactic
Glucose Glycerol Acetol acid
Catalyst
SnO
SnO2
Sn(OH)4
Sn(OH)2
SnOx/Al2O3
17.1
19.9
12.0
15.5
8.2
5.3
10.2
0.6
1.7
12.5
11.8
2.6
2.1
1.8
5.1
3.6
3.2
8.4
6.5
4.7
21.1
12.2
9.9
2.2
5.7
4.3
8.2
13.0
17.3
b
SnOx/Al2O3-
6.8
recycled b
a 1 g cellulose, 0.4 g catalyst, 473 K, 50 mL H2O, 6 MPa H2, 30 min.
b Sn = 4.87 wt%.
the C2 and C3 products, which apparently occurs more readily
on the SnOx species than the sugar hydrogenation to hexitols
on the Pt sites. This proposition was indeed confirmed by the
catalytic performance of another physical mixture containing
Pt/Al2O3 and SnOx/Al2O3 (0.36 wt% Sn). As shown in Table 2,
the addition of SnOx/Al2O3 to Pt/Al2O3 even at a low Sn/Pt ratio
of 0.3, led to a sharp increase in the selectivity to the C2 and
C3 polyols from 9.8 to 48.8% and a concurrent decline in the
selectivity to hexitols from 43.3 to 5.7% at similar cellulose
conversions. Such catalytic properties of the SnOx species were
more directly evidenced from the cellulose reaction catalyzed
only by SnOx, as discussed below.
To understand the nature of the active SnOx species respon-
sible for breaking the C–C bonds, different tin samples includ-
ing bulk SnO, SnO2, Sn(OH)2 and Sn(OH)4 were examined for
the cellulose reaction in the absence of the Pt sites (Table 4).
On these samples, three C3 products: glycerol, acetol and lactic
acid, were detected. Other products included glucose and trace
amounts of C2 compounds (e.g. glycolic acid). SnO, SnO2 and
Sn(OH)4 exhibited low selectivities to glycerol, acetol and lactic
acid in the range of 10–15%, which increased significantly to
34.4% on Sn(OH)2 at similar cellulose conversions under iden-
tical conditions. These results indicate that Sn(OH)2 is the
active SnOx species for catalyzing the retro-aldol condensation
of glucose or other C6 sugars to form the C2 and C3 products
in the cellulose reaction. To further understand the catalytic
properties of the SnOx species on the Pt–SnOx/Al2O3 catalysts,
SnOx/Al2O3 (4.87 wt% Sn) was examined. Similar to Sn(OH)2, it
catalyzed cellulose conversion to the C3 products at a selectiv-
ity of 28.8%, which remained essentially unaltered (i.e. 30.4%)
when the SnOx/Al2O3 sample was recycled. These results imply
that Sn(OH)2 species were formed on SnOx/Al2O3 and
remained stable during the cellulose reaction.
Fig. 7 FT-IR spectra of CO adsorption on ReOx, SbOx, and GaOx-modified Pt/
Al2O3 catalysts with atomic ratios of Sb, Re, or Ga to Pt of 0.3, and for compari-
son, on Pt/Al2O3 and Pt–SnOx(0.3)/Al2O3 with a Sn/Pt atomic ratio of 0.3.
low atomic ratios of Re, Sb or Ga to Pt of 0.3. Their CO-IR
spectra show that the CO adsorption band shifted from
2081 cm−1 on Pt/Al2O3 to 2076 and 2074 cm−1 on Pt–ReOx/
Al2O3 and Pt–SbOx/Al2O3, respectively, while it remained at
2081 cm−1 on Pt–GaOx/Al2O3 (Fig. 7). Such different effects on
the CO adsorption reflect the electron transfer to Pt from ReOx
and SbOx, but not from GaOx, which is consistent with their
effects on the Pt hydrogenation activity probed by the cellulose
reaction. As shown in Table 2, the selectivity to hexitols
increased from 43.3% on Pt/Al2O3 to 63.7 and 51.6% on Pt–
ReOx/Al2O3 and Pt–SbOx/Al2O3, respectively, but it was as low
as 37.1% on Pt–GaOx/Al2O3 at the similar cellulose conversions
(17.6–20%). These results reveal that ReOx and SbOx, similar to
SnOx, can donate electrons to the Pt sites on Al2O3, and
accordingly improve their hydrogenation activity.
Next, we examine the underlying basis for the preferential
conversion of cellulose to the C2 and C3 polyols at the expense
of hexitols on the Pt–SnOx/Al2O3 catalysts with Sn/Pt ratios
above 1.5. These catalysts exhibited inferior hydrogenation
activity, which appears to lead to the degradation of glucose or
other C6 sugar intermediates to form the C2 and C3 products,
rather than the sugar hydrogenation to hexitols. However, our
control experiments show that the low hydrogenation activity
cannot fully account for the formation of the degradation pro-
ducts. For example, as aforementioned, the total selectivity of
the C2 and C3 products was ∼36% on the Pt–SnOx(3.8)/Al2O3
catalyst with the Sn/Pt ratio of 3.8 (Table 2). After physically
mixing it with the active hydrogenation catalyst of Pt–
SnOx(0.3)/Al2O3, the selectivity to hexitols only increased
slightly from 0.6 to 3.9%, but the C2 and C3 polyols were still
the dominant products (50.2% selectivity) at similar cellulose
conversions (Table 2). This result suggests that the SnOx
species not in contact with the Pt sites on Pt–SnOx(3.8)/Al2O3
contributes to the cleavage of the C–C bonds in glucose or
other sugar intermediates in the cellulose reaction, most likely
following the retro-aldol condensation mechanism, to form
Furthermore, it is noted that the C3 products were domi-
nant over the C2 products on the SnOx and Pt–SnOx/Al2O3
(with the higher Sn/Pt ratios) catalysts, as shown in Tables 2
and 4. Following the retro-aldol condensation mechanism,
glucose tends to convert to the C2 products while the C3 pro-
ducts dominate in the fructose reaction. Therefore, we suggest
that the formation of the C3 products in the cellulose reaction
involves the isomerization of glucose to fructose prior to its
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retro-aldol condensation on the SnOx species, which is consist- subsequent retro-aldol condensation on SnOx. Taken together,
ent with the observed conversion of glucose to fructose, as these results demonstrate the bifunctional role of the SnOx
shown in Table 3, on the Pt–SnOx/Al2O3 catalysts with high Sn/ species in tuning the reaction rate of the hydrogenation of
Pt ratios of 2.1 and 3.8 at 393 K. This suggestion can account glucose and other C6 sugar intermediates to hexitols and of
for the formation of the C3 products. The retro-aldol conden- their selective degradation to C2 and C3 products, which is
sation of fructose is known to form the glyceraldehyde and crucial for the rational synthesis of target polyols in the cellu-
dihydroxyacetone intermediates which undergo hydrogenation lose reaction.
to glycerol (or further to 1,2-propanediol), or dehydration to
pyruvaldehyde. The pyruvaldehyde intermediate then converts
to acetol and lactic acid by hydrogenation and benzilic acid
Acknowledgements
rearrangement, respectively.41 This suggestion is further con-
firmed by the reactions of glucose and fructose on Pt– This work was supported by the National Natural Science
SnOx(3.8)/Al2O3, predominantly forming acetol and lactic acid. Foundation of China (20825310, 21173008 and 51121091) and
For example, the yields to acetol and lactic acid were ∼40.0 and the National Basic Research Project of China (2011CB201400
∼55.0% from glucose and fructose, respectively, at 473 K and 6 and 2011CB808700).
MPa H2 (at a 100% conversion), in which the yields to acetol
were 32.9 and 38.2%. Such high acetol yields, although
obtained under non-optimized conditions, show the potential
reactivity of the SnOx species for the selective cleavage of the
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