Catalytic transfer hydrogenation of levulinic acid to γ-valerolactone over a bifunctional tin catalyst

Article abstract of DOI:10.1039/c6ra25594a

The conversion of biomass-derived levulinic acid (LA) to γ-valerolactone (GVL) is important but extremely challenging due to the use of noble metal catalysts and high-pressure gaseous hydrogen. Here we describe the transfer hydrogenation of LA to GVL using 2-propanol as hydrogen donor over a Sn modified silica catalyst (SnO₂/SBA-15) which gives high LA conversion (85.1%) and GVL selectivity (95.2%). Key to this success is employing a simply synthesized Sn-modified silica catalyst (SnO₂/SBA-15) with both Lewis and Br?nsted acidity, which could synergistically catalyze the tandem steps of hydrogen transfer and esterification, leading to successful transformation of LA to GVL.

Full text of DOI:10.1039/c6ra25594a

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Catalytic transfer hydrogenation of levulinic acid to
g-valerolactone over a bifunctional tin catalyst†
Cite this: RSC Adv., 2017, 7, 1026
Shaodan Xu,*ab Deqing Yu, Tao Ye and Panpan Tian
a
ac
a
The conversion of biomass-derived levulinic acid (LA) to g-valerolactone (GVL) is important but extremely
challenging due to the use of noble metal catalysts and high-pressure gaseous hydrogen. Here we describe
the transfer hydrogenation of LA to GVL using 2-propanol as hydrogen donor over a Sn modified silica
Received 21st October 2016
Accepted 28th November 2016
2
catalyst (SnO /SBA-15) which gives high LA conversion (85.1%) and GVL selectivity (95.2%). Key to this
success is employing a simply synthesized Sn-modified silica catalyst (SnO
2
/SBA-15) with both Lewis and
DOI: 10.1039/c6ra25594a
Brønsted acidity, which could synergistically catalyze the tandem steps of hydrogen transfer and
www.rsc.org/advances
esterification, leading to successful transformation of LA to GVL.
make their further application difficult due to the pricy cost of
noble metals and hazardous gaseous hydrogen. Therefore,
Introduction
Biomass, which is regarded as a renewable feedstock, has efficient production of GVL from LA without using noble
attracted much attention for the production of ne chemicals metal catalysts and gaseous hydrogen is still challenging.
1
–5
and fuels. Many economically viable processes have been Recently, transfer hydrogenation of LA using the organic
2
,6
developed for the conversion of biomass. One efficient molecules (e.g. formic acid, 2-propanol) as hydrogen sources
route is the fast pyrolysis and/or acidolysis of the cellulose have been extensively studied, which is regarded to be
and lignin feedstocks to various platform molecules, and potentially important for LA conversion. In the transfer
then transformation of the platform molecules to the desired hydrogenation of LA, the Zr- and Ni-based catalysts have
7
–12
fuels and chemicals.
molecules have high oxygen content and various functional and high efficiency.
However, the biomass platform attracted much attention because of their non-noble feature
35–38
In these cases, the metal atoms were
groups, which makes the transformation route complex and attached into the zeolite or organic polymer framework to
1
nonselective. Therefore, developing an efficient strategy to form Lewis acid sites, which benet the transfer hydrogena-
efficiently transform the biomass platform molecules is tion reaction. Considering the importance of LA conversion,
strongly desirable for producing high-quality fuels and developing more efficient catalysts for this process is still
chemicals.
Levulinic acid (LA), which can be easily produced from
cellulosic feedstocks by acidolysis, has been recognized as one (SnO
of the top-10 most promising platform molecules derived on the surface of silica, could efficiently catalyse the transfer
necessary.
In the present work, we reported a Sn modied silica catalyst
/SBA-15), which is simply synthesized by graing Sn sites
2
1
3
from biomass by the U.S. Department of Energy. One of the hydrogenation of LA to GVL using 2-propanol as hydrogen
most important use of LA is to produce g-valerolactone (GVL), donor. For SnO /SBA-15 catalyst, the Sn sites are uniformly
which has been used as a high-quality solvent and fuel addi- distributed on the SiO support and exhibited both Lewis and
Recently, many catalysts, such as homogeneous Pd Brønsted acidity. It is observed that the Lewis and Brønsted
2
2
1
4–20
tive.
complex, heterogeneous Ru/C,
2
1
22,23
24
25,26
RuNi/C, Au/ZrO
graphene, have successfully catalysed the transformation of genation and dehydration, respectively, giving GVL yield at as
LA to GVL by a hydrogenation and subsequent dehydration high as 81.0%. Importantly, the SnO /SBA-15 catalyst is very
However, it should be noted that noble metals and stable and recyclable, showing constant GVL yield in long
2
,
Ru/ acidity are benet for the tandem reactions of transfer hydro-
2
7
2
2
8–34
steps.
gaseous hydrogen are necessary in most of these cases, which reaction period in a xed-bed reactor.
a
Zhejiang Institute of Quality Inspection Science, Hangzhou 310018, China. E-mail:
Experimental
Sample preparation
xushaodan6677@126.com
b
Department of Chemistry, Zhejiang University, Hangzhou 310027, China
c
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department
All chemicals were commercially available and directly used
without further purication. The SBA-15 support was synthe-
sized according to the method described in literature.
of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c6ra25594a
39
1
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Synthesis of SnO
SnO /SBA-15)
2
2
/SBA-15 and HL-SnO
2
/SBA-15 (high-loading
Results and discussion
ꢀ
It is well known that ultra-small metal nanoclusters (e.g. Al, Zr,
5
.0 g of SBA-15 was dried in vacuum at 80 C for 24 h, and then
Sn) could create Brønsted acidic sites from the surface hydroxyl
dispersed in 150 ml of p-xylene. Aer ultrasonic treatment for
h, a desired amount of dimethyldichlorostannane and 22 ml
of triethylamine was added, and the mixture was continuously
stirred for another 10 h at room temperature. Aer washing
with p-xylene and ethanol, and calcining at 600 C for 2 h in
40
groups. Therefore, it is possible to synthesize a metal catalyst
with both Lewis and Brønsted acidity by loading uniformly
dispersed metal species on an inert silica support. In this case,
the mesoporous SBA-15 was employed as silica support because
of the obvious advantage of the large surface area, which is
expected to be favorable for the dispersion of acid sites as well
as the diffusion of reactants. The Sn species are loaded on the
1
ꢀ

owing oxygen (5% in N ), the SBA-15 with graed Sn species
2
could be obtained. By adjusting the amount of dimethyldi-
chlorostannane in the starting solution, the SnO /SBA-15 and
HL-SnO /SBA-15 with different Si/Sn ratio could be obtained.
The SnO /MCM-41 catalyst was synthesized in the same method
as synthesizing SnO /SBA-15 except the use of MCM-41 as
2
40,41
SBA-15 support by a graing method,
thyldichlorostannane was graed the mesoporous silica of SBA-
5 by a condensation reaction with the silanol groups, followed
where the dime-
2
2
1
2
by calcination to form hydroxyl group on Sn species (Scheme 1).
support instead of SBA-15.
The obtained SnO /SBA-15 has a Si/Sn ratio at 53, as determined
2
by inductively coupled plasma optical emission spectrometry
Sample characterization
(ICP-OES) (Table 1).
Fig. 1A shows the small-angle X-ray diffraction (XRD) pattern
Nitrogen sorption isotherms were measured using a Micro-
ꢀ
of SnO
2
/SBA-15. In the region at 0.6–2 , three peaks obviously
meritics ASAP 2020M system. The samples were outgassed for
ꢀ
appeared indexing to the (100), (110) and (200) reections of the
1
0 h at 200 C before the measurements. The pore-size distri-
39
hexagonal mesoporous arrays. Notably, the peaks assigning to
bution was calculated using the Barrett–Joyner–Halenda (BJH)
model. Titan ChemiSTEM and FEI Tecnai G2 F20 microscopes
equipped with an EDAX detector were employed to acquire
TEM, HAADF-STEM images, and energy dispersive X-ray spec-
trum (EDS) elemental maps. Powder X-ray diffraction patterns
SnO
2
crystals at ꢁ34 and ꢁ52ꢀ are completely absent in the
wide-angle XRD pattern of SnO /SBA-15 (Fig. 1B), which might
2
be due to the low metal loading. Fig. 1C shows the nitrogen
sorption isotherm of SnO /SBA-15, which exhibit typical IV
2
curve with a hysteresis loop, assigning to typical mesoporous
(XRD) were obtained with a Rigaku D/MAX 2550 diffractometer
structure. Additionally, the SnO
2
/SBA-15 exhibits a surface area
and uniform pore diameter distribution at
.5 nm (Fig. 1D, Table 1), which are slightly smaller than those
with CuKa radiation (l ¼ 0.1542 nm). The FTIR spectra of
pyridine-adsorption were recorded on a Thermo-Fisher FTIR
spectrometer. 15 mg of sample in a cell was used for the
pyridine-adsorption IR test. The pyridine vapor was admitted
into the cell and adsorption lasted for 1 h. Subsequently, the
2
ꢂ1
at 359 cm
g
6
of the as-synthesized SBA-15 (Fig. S1†). This phenomenon
might be due to the successful graing of Sn species in the
mesopores of SBA-15.
ꢀ
samples were heated up to 220 C under vacuum and cooled
Electronic microscopy images give direct observation of the
down to RT for collecting the signal.
2
SnO /SBA-15. Fig. 2a shows the high-angle annular dark-eld
scanning transmission electron microscopy (HAADF-STEM) by
the Z-contrast, which exhibits high-quality mesoporous struc-
Catalytic tests
The reactions were performed in a 50 ml high-pressure auto- ture, in good agreement with the results of XRD pattern and N2
clave with a magnetic stirrer. The substrate, catalyst, and
solvent were mixed in the reactor, then pure N
duced to remove the air in autoclave and keep the N
2
was intro-
pressure
2
at 0.7–0.8 MPa. Then, the autoclave was heated to a given
temperature. Aer the reaction, the product was taken out
from the reaction system and analyzed by gas chromatography
(
ame ionization detector) with a exible quartz capillary
column (100 m in length) coated with FFAP. The recyclability
2
Scheme 1 Synthesis of SnO /SBA-15 by grafting Sn species on the
of the catalyst was tested by separating the catalyst solids from SBA-15 support.
the reaction system by centrifugation, washing with large
ꢀ
quantity of methanol and water, drying at 60 C, then the
catalyst was reused in the next reaction. The reactions in the Table 1 Structural parameters of various samples

xed-bed reactor were carried in a 10 ml quartz tube with an
oven to control the temperature. The catalysts were made into
–2 mm particles and mixed with quartz particles with similar
a
2
ꢂ1
Sample
Si/Sn ratio
S
BET (m g
)
Pore width (nm)
1
SBA-15
—
53
13
565
359
320
7.5
6.5
6.3
sizes, then the mixture was localized in the middle of the tube. SnO /SBA-15
2
The liquid substrates were injected into the reactor by a pump, HL-SnO
and N owed in the reactor during the whole reaction
process.
2
/SBA-15
a
2
By ICP analysis.
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assigning to the aggregated Sn species because of high Sn
loading.
The acidity of SnO /SBA-15 and HL-SnO /SBA-15 was studied
2
2
by pyridine-adsorption IR spectra. As shown in Fig. 3, SnO /SBA-
2
ꢂ1
1
5 gave the obvious bands at 1453 and 1614 cm , which are
assigned to the coordinative bound of pyridine on Lewis acid
4
2–44
sites.
Interestingly, the spectrum of SnO
2
/SBA-15 also gives
ꢂ1
a band at 1531 cm , which is characteristic of pyridine adsorbed
on the Brønsted acid sites. The band at 1491 cm is from the
ꢂ1
contribution of pyridine adsorbed on both Lewis and Brønsted
acid sites. These results indicate that SnO /SBA-15 has both Lewis
2
and Brønsted acid sites, which should origin from the Sn atoms
3
1,45
and silanolgroups, respectively.
1
In contrast, the HL-SnO
2
/SBA-
ꢂ1
5 exhibited very weak band at 1531 cm , indicating the sparsely
2
Brønsted acid on HL-SnO /SBA-15, which might be due to the
aggregation of Sn species could eliminate most of the Sn–OH
2
Fig. 1 (A) Small-angle and (B) wide angle XRD patterns, (C) N sorption
groups. Furthermore, the band of pyridine on the Lewis acid sites
isotherms, and (D) mesopore size distribution of SnO
2
/SBA-15.
ꢂ1
ꢂ1
of HL-SnO /SBA-15 appeared at 1446 cm , with a red shi of 7
2
ꢂ1
cm from that of SnO /SBA-15 (1453 cm ). The red shi in IR
2
band suggests the weaker pyridine-Lewis acid interaction on HL-
SnO
2
/SBA-15 than SnO
2
/SBA-15. Similar phenomenon could also
ꢂ1
be observed in the band at 1491 cm on SnO
shied to 1480 cm on HL-SnO
the signicant advantages of SnO
Brønsted acidity compared with HL-SnO
2
/SBA-15, which
ꢂ1
2
/SBA-15. These results indicate
/SBA-15 in both Lewis and
2
46
2
/SBA-15.
The catalytic study starts from the conversion of LA to GVL in
-propanol solvent. As summarized in Table 2, SBA-15 support
2
without any Sn species is inactive for the formation of GVL
entry 1). Interestingly, when SnO /SBA-15 was used as catalyst,
the conversion of LA could reach 85.1% with GVL selectivity at
as high as 95.2% (entry 2). In contrast, the HL-SnO /SiO , which
has the same individuals of Sn species and SiO support to
SnO /SBA-15 catalyst, exhibits much lower LA conversion at only
.9% (entry 3). Additionally, we also tested the catalytic perfor-
(
2
2
2
2
2
4
mances of various transition metal oxides with Lewis acidity,
including Al O , SnO , ZrO , which also give low LA conversion
2
3
2
2
(
1.9–14.4%) and GVL selectivity (<42%) (entries 4–6). These
results indicate that the catalysts with only Lewis acidity is
active for the transfer hydrogenation of LA to GVL, but SnO
SBA-15 with strong Lewis and Brønsted acidity is more efficient.
2
/
Fig. 2 (a and b) STEM images of SnO
elemental maps of the red square in (b). (f) STEM image and (g) Si and
h) Sn EDS elemental maps of HL-SnO /SBA-15.
2
/SBA-15; (c) O, (d) Si and (e) Sn
(
2
sorption isotherms. Notably, it is difficult to directly observe the
Sn species in the high-resolution STEM image (Fig. 2b), which
might be due to the uniform dispersion of Sn species on the
surface of SBA-15. The energy dispersive X-ray (EDX) mapping
analysis gives clear observation of the distribution of Sn
element. As shown in Fig. 2c–e, the Sn element has similar
distribution to Si and O elements. Furthermore, energy
2
dispersive spectra of randomly selected regions on SnO /SBA-15
give similar signals (Fig. S2†). These results indicate the
uniform dispersion of Sn on SnO /SBA-15 sample. For
2
comparison, Fig. 2f–h show the electron microscopy charac-
terization of HL-SnO
2
/SBA-15. Clearly, many nanoparticles
2
Fig. 3 Pyridine adsorption IR spectra of (a) SnO /SBA-15 and (b) HL-
could be obviously observed in the mesopores of the SBA-15, SnO
2
/SBA-15.
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Table 2 Catalytic data in conversion of LA to GVL using 2-propanol pentanol with n-pentanoic acid to ester. This method has been
a
over various catalysts
widely used to study the contribution of catalysts in the trans-
48
formation with multiple steps, which benet the investigation
on the catalytic activity of the catalyst in the individual steps,
thus to understand the rate control step and the origin of high
activity. Table 3 lists the reaction rates of the Sn-based catalysts
in the individual reactions of (I) hydrogen transfer from 2-
propanol to reduce the C]O group and (II) esterication of C–
OH and COOH groups and the combined reaction of (III)
transformation of LA to GVL using 2-propanol as hydrogen
donor (Scheme 2). The reaction rates are obtained by the cata-
lytic conversion at the beginning of each reaction within a short
time of 20 min. It is worth emphasizing that the intramolecular
esterication to form GVL in the LA-to-GVL process could occur
Entry
Catalyst
SBA-15
LA conv. (%)
GVL Sel. (%)
1
2
3
4
5
6
—b
85.1
4.9
1.9
3.9
—b
95.2
c
SnO
HL-SnO
Al
SnO
ZrO
2
/SBA-15
2
/SBA-15
96.5
b
2
O
3
—
b
2
—
2
14.4
42.1
a
Reaction conditions: 3 mmol of LA, 50 mmol of 2-propanol, 140 mg of
ꢀ
b
c
catalyst, 110 C, 8 h. Undetectable. The by-products are mainly the
lipid from the esterication of acid with 2-propanol.
37,38
even in the absence of Brønsted acid catalyst,
here the
According to the well-known Meerwein–Ponndorf–Verley
intermolecular esterication (reaction II) was employed to slow
down the reaction rate for clarifying the different activities of
various catalysts. Clearly, the SBA-15 without Sn species failed to
catalyze all the reactions (entry 1 in Table 3). Interestingly,
reduction using 2-propanol as hydrogen donor over Lewis acid
47
sites, it is reasonable to assume the conversion of LA to GVL
should process the tandem steps of (I) transfer hydrogenation
from 2-propanol to reduce the C]O to C–OH group on LA and
SnO /SBA-15 is very active for both the hydrogen transfer
2
(
(
II) esterication of the C–OH and COOH groups to form GVL
Scheme 2). On the basis of the proposed reaction process, we
(
reaction I, Scheme 2) and esterication (reaction II, Scheme 2),
ꢂ1 ꢂ1
giving reaction rate at 16.7 and 28.1 mmol g
h , respectively.
discussed the reasons why SnO /SBA-15 shows better catalytic
2
In the conversion of LA to GVL (reaction III, Scheme 2), the rate
2
performances than the HL-SnO /SBA-15 by studying the reac-
of producing GVL over Sn/SiO
2
could reach as high as
ꢂ1 ꢂ1
tion rate of each individual step, because the reaction rates in
the beginning of the reactions at low conversion are sensitive to
the activities of the catalysts. In order to understand the
performances of the catalysts in each of the steps for transfer
hydrogenation and esterication in LA-to-GVL reaction
12.1 mmol g
h
2
(entry 2 in Table 3). In contrast, the HL-SnO /
SBA-15 which has only Lewis acidity, could solely catalyze the
hydrogen transfer reaction and exhibit reaction rate at 3.1 mmol
ꢂ1 ꢂ1
g
h
(entry 3 in Table 3). Considering hydrogen transfer is
a control step in the reaction, the obviously higher activities of
SnO /SBA-15 than HL-SnO /SBA-15 in hydrogen transfer step
(Scheme 3), we employ different substrates to split the LA-to-
2
2
GVL process into two individual reactions, as presented in
Scheme 2, which are (I) transfer hydrogenation of C]O in 2-
pentanone with 2-propanol to C–OH and (II) esterication of 2-
play important role to make it an efficient catalyst. As well, the
high activity in the esterication step might also contribute to
the good catalytic performances of SnO /SBA-15.
2
Importantly, the SnO /SBA-15 catalyst is reusable. Aer each
2
reaction run, the catalyst can be easily separated from the
reaction system and reused in the next run by simple washing
and drying. As shown in Fig. 4, in the recycling process,
a constant LA conversion (ꢁ85.0%) and GVL selectivity
(
ꢁ95.0%) could be obtained. Moreover, the performances of the
th
fresh catalyst and used catalysts aer the 6 run in the transfer
hydrogenation reaction also have been tested in a shortened
Table 3 Reactions rates of various catalysts in the model reactions
listed in Scheme 2
ꢂ1 ꢂ1
Reaction rate (r
0
, mmol g
h
)
Scheme 2 The model reactions of (I) hydrogen transfer from 2-pro-
ponal to reduce the C]O group, (II) esterification of C–OH and Entry
COOH groups, and (III) transformation of LA to GVL using 2-pentonal
a
b
c
Catalyst
SBA-15
Reaction I
Reaction II
Reaction III
1
2
3
—d
16.7
4.9
<1.0
28.1
<1.0
—d
12.1
<1.0
as hydrogen donor.
SnO
2
/SBA-15
/SBA-15
HL-SnO
2
a
Reaction conditions: 3 mmol of 2-pentanone, 50 mmol of 2-propanol,
40 mg of catalyst, 110 C, 30 min. Reaction conditions: 5 mmol of 2-
ꢀ
b
1
pentanol, 5 mmol of pentanoic acid, 4 g of toluene solvent, 100 mg of
ꢀ
c
catalyst, 110 C, 20 min. Reaction conditions: 3 mmol of LA,
ꢀ
50
mmol of 2-propanol, 110 mg of catalyst, 140 C, 20 min.
d
Undetectable.
Scheme 3 The two steps in LA-to-GVL process.
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By TEM analysis (Fig. 5), the SnO
2
/SBA-15 catalyst aer a 50 h
run still exhibited high-quality mesoporous structure. More
importantly, no aggregation of Sn species was observed in the
used SnO /SBA-15 catalyst, indicating the good stability of the
2
catalysts. The high activity and selectivity combined with the
good stability of SnO /SBA-15 catalyst make it potentially
2
important for wide applications in the future.
Conclusions
In summary, we reported the transfer hydrogenation of LA to
GVL using 2-propanol as hydrogen donor over a Sn modied
Fig. 4 Catalytic data of SnO
2
/SBA-15 in the recyclability test. Reaction silica catalyst (SnO /SBA-15) which gives high LA conversion
2
ꢀ
at 100 C, reactor volume at 10 ml, 0.2 g of SnO
2
/SBA-15, 2.0 wt% LA in (85.1%) and GVL selectivity (95.2%). The good catalytic perfor-
ꢂ1
2
-propanol solution, 0.2 ml min
2
N .
mance is reasonably attributed to the strong Lewis acid sites on
2
SnO /SBA-15, which could efficiently catalyze the individual
reactions of hydrogen transfer, as well as the Brønsted acidity,
which is benet for the esterication step, thus leading to an
efficient LA-to-GVL process without using noble metals and
gaseous hydrogen. Although it is still difficult to conclude that
the transfer hydrogenation excelled in LA conversion efficiency,
because the LA concentration in transfer hydrogenation tests is
usually lower than that in direct hydrogenation cases, the
transfer hydrogenation strategy indeed offers new ideas in
developing alternative pathways in LA conversion. It is expected
that this work can be potentially used to develop efficient
catalysts and processes for the conversion of other biomass
platform molecules.
reaction time, where similar catalytic performances were ob-
tained, indicating the high stability of the Sn catalyst (entries 1,
2
in Table 4). By ICP analysis, the Sn leaching is negligible
during the recycling process. The conversion of LA to GVL was
also performed in a xed-bed reactor using a mixture of LA and
2-propanol as feed (Fig. S3A†). In the catalytic tests, no obvious
decrease in LA conversion and GVL selectivity was observed for
a 50 h reaction (Fig. S3B†). On the other hand, the use of SBA-15
as supports has been compared with another typical silica
support of MCM-41. The SnO
comparable performance to that of SnO
2
/MCM-41 catalyst exhibits very
/SBA-15 (entry 3 in
2
Table 4), suggesting that the silica support have little inuence
on the catalytic performances of Sn catalysts.
Acknowledgements
This work is supported by the Science and Technology Project of
Table 4 Catalytic data in conversion of LA to GVL using 2-propanol General Administration of Quality Supervision, Inspection and
a
over various catalysts in a shorten reaction time
Quarantine of the People's Republic of China (AQSIQ, No.
2014QK203).
Entry
Catalyst
LA conv. (%)
GVL Sel. (%)
1
2
3
Fresh SnO
2
/SBA-15b
59.5
56.2
63.4
91.0
93.7
88.1
Notes and references
2
Used SnO /SBA-15
SnO
2
/MCM-41
1
2
3
4
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Reaction conditions: 3 mmol of LA, 50 mmol of 2-propanol, 140 mg of
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ꢀ
b
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