One-step hydrogenolysis of glycerol to biopropanols over Pt-H 4SiW12O40/ZrO2 catalysts

Article abstract of DOI:10.1039/c2gc35564g

The one-step hydrogenolysis of biomass-derived glycerol to propanols (1-propanol + 2-propanol), which are known as biopropanols, was investigated over different supported Pt-H₄SiW₁₂O₄₀ (HSiW) bi-functional catalysts in aqueous media. Among the catalysts/supports tested, Pt-HSiW supported over ZrO₂ converted glycerol to biopropanols with high selectivity and high yield (94.1%), while exhibiting long-term stability (160 h). In addition, this catalyst can be resistant to the impurities present in crude glycerol. The reaction pathway to propanols from glycerol is proposed to proceed mainly through 1,2-propanediol. With the strategy toward one-step hydrogenolysis of glycerol to biopropanols sustainably, the biomass can be readily transformed to biodiesel and biopropanols via glycerol, which will bring about the benign development of the biodiesel industry.

Full text of DOI:10.1039/c2gc35564g

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PAPER
One-step hydrogenolysis of glycerol to biopropanols over
Pt–H SiW O /ZrO catalysts†
4
12 40
2
a,b
a,c
c
c
a,b
a,c
Shanhui Zhu, Yulei Zhu,* Shunli Hao, Hongyan Zheng, Tao Mo and Yongwang Li
Received 15th April 2012, Accepted 9th July 2012
DOI: 10.1039/c2gc35564g
The one-step hydrogenolysis of biomass-derived glycerol to propanols (1-propanol + 2-propanol),
which are known as biopropanols, was investigated over different supported Pt–H SiW O (HSiW)
4
12 40
bi-functional catalysts in aqueous media. Among the catalysts/supports tested, Pt–HSiW supported over
ZrO converted glycerol to biopropanols with high selectivity and high yield (94.1%), while exhibiting
2
long-term stability (160 h). In addition, this catalyst can be resistant to the impurities present in crude
glycerol. The reaction pathway to propanols from glycerol is proposed to proceed mainly through
1
,2-propanediol. With the strategy toward one-step hydrogenolysis of glycerol to biopropanols
sustainably, the biomass can be readily transformed to biodiesel and biopropanols via glycerol,
which will bring about the benign development of the biodiesel industry.
1
6
1
. Introduction
of n-propyl acetate. 2-Propanol (2-PO) has been synthesized
by the hydration of propylene and is applied extensively as an
17
As fossil resources deplete and concerns about environmental
issues increase, the utilization of biomass as a sustainable
alternative is stimulating growing interest.
biomass-derived glycerol has gained considerable attention
because of its continuous availability in huge amounts from bio-
diesel processes and its capability of being a prominent platform
chemical to synthesize value-added products. Significant efforts
have been devoted to the transformation of glycerol by various
catalytic processes involving oxidation, dehydration, reforming,
etherification, esterification and so on. In this context, one of
the most promising approaches is the catalytic hydrogenolysis of
glycerol to produce 1,2-propanediol (1,2-PDO), 1,3-propanediol
industrial solvent. Additionally, due to its non-toxicity, 2-PO is
widely used in the pharmaceutical industry, in disinfectants,
hand sanitizers, as an alternative to formaldehyde to preserve
1
–3
In this context,
17
specimens, and in the auto industry as a gas dryer and deicer.
Both 1-PO and 2-PO are important chemical intermediates and
can be converted to a number of valuable chemicals, which is
summarized in Scheme S1.† Recently, a new process has been
relatively well developed in which glycerol-derived 1,2-PDO is
converted to biopropanols.
formation of 1,2-PDO to 1-PO in sulfolane at 110 °C over a
homogeneous ruthenium complex catalyst; the yield of 1-PO
was 54%. Tomishige and coworkers conducted the hydrogeno-
lysis of 1,2-PDO in water at 120 °C and 8.0 MPa with the dis-
continuous operation, in which the optimized Rh–ReO /SiO2
4
18,19
18
Schlaf et al. exhibited the trans-
4
–9
19
(1,3-PDO) and biopropanols.
Abundant work has been done towards glycerol hydrogenolysis
x
to 1,2-PDO and 1,3-PDO in recent years, and high yields
catalyst gave high yields of 1-PO (66%) and biopropanols
1
0–15
have been achieved in some literature.
,2-PDO and 1,3-PDO, biopropanols are also expensive com-
In comparison with
(85%).
1
In comparison with the process based on petroleum-derived
modity chemicals while there are scarcely any available reports
on the potential utilization of glycerol for production of bio-
propanols. 1-Propanol (1-PO), which has been currently pro-
duced via hydroformylation of ethylene to form propanal
followed by hydrogenation, is utilized mainly as an industrial
solvent, printing ink and chemical intermediate for the synthesis
ethylene, propylene or glycerol-derived 1,2-PDO, the one-step
hydrogenolysis of glycerol to biopropanols would be preferential
in terms of sustainability and energy efficiency.
The propanols are usually by-products of the glycerol hydro-
genolysis to propanediols (1,2-PDO + 1,3-PDO) and the yields
20
of propanols are rather low. Thibault et al. obtained 18% yield
to 1-PO from glycerol hydrogenolysis at 200 °C and 3.45 MPa
in a water–sulfolane mixed solvent in the presence of a homo-
genous ruthenium complex and methane sulfonic acid. Kurosaka
a
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan 030001, PR China.
E-mail: zhuyulei@sxicc.ac.cn; Fax: +86-351-7560668;
Tel: +86-351-7117097
14
et al. found that glycerol can be hydrogenolyzed to 1,3-PDO,
1,2-PDO and 1-PO in yields of 24%, 13% and 28%, respec-
tively, at 130 °C and 4.0 MPa in 1,3-dimethyl-2-imidazolidinone
b
Graduate University of Chinese Academy of Sciences, Beijing 100039,
PR China
15
c
on Pt/WO /ZrO . Furikado et al. achieved high selectivities of
3
2
Synfuels China Co. Ltd., Taiyuan 030032, PR China
4
1.3% to 1-PO and 17.0% to 2-PO over Rh/SiO at 120 °C and
2
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c2gc35564g
8.0 MPa in the presence of Amberlyst during glycerol
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Table 1 Physicochemical properties of supports
using a method similar to that of Pt–HSiW/ZrO , except for
2
Pt–WO /ZrO and Pt–MoO /ZrO where the latter calcination
3
2
3
2
Surface area
Pore size
(nm)
Pore volume
2
−1
3
−1
temperature was 600 °C. The nominal weight loadings of metal
Pt and Pd) and acid components (HSiW, HPW, HPMo, WO3
Supports
(m g
)
(cm g
)
(
ZrO
AC
γ-Al
2
59.7
10.5
3.1
9.1
8.8
0.22
0.08
0.57
0.27
and MoO ) in all the catalysts were 2% and 15%, respectively,
except in the study of HSiW loading. The nominal weight load-
3
1091.8
197.6
94.2
2 3
O
ings of metal components (Ni and Cu) in Cu–HSiW/ZrO and
TiO
2
2
Ni–HSiW/ZrO were increased to 10% in order to improve the
2
reactivity of glycerol hydrogenolysis.
2
1,22
hydrogenolysis. Tomishige and coworkers
reported that Ir/
SiO modified with a Re species was an effective heterogeneous
2
2.2 Catalyst characterization
catalyst for the direct chemoselective hydrogenolysis of glycerol
to 1,3-PDO with high selectivity to 1-PO. Recently, van Ryne-
veld et al. have reported the conversion of glycerol into lower
alcohols over commercial Ni/SiO2 catalysts. Through this
process, 1-PO and ethanol were produced with 42.8% and
The BET surface area measurements were performed at −196 °C
on a Micromeritics ASAP 2420 instrument. All the catalyst
samples were degassed under vacuum at 350 °C for 8 h before
the measurements.
23
2
0.2% yields, respectively.
Nowadays, plants that produce glycerol are closing down
CO chemisorption was conducted using Auto Chem.II2920
equipment (Mircromeritics, USA). In a typical run, about 0.2 g
of sample was placed in a U-shaped quartz tube. The sample
while others that utilize glycerol as a raw material are opening,
this is a result of the tremendous surplus of glycerol that is
formed as a by-product in the production of biodiesel. The
global capacity for refined glycerol has increased at an average
annual rate of 8.7% over recent years and evidently a glycerol
was first reduced by H at 200 °C for 2 h, flushed with He for
2
1 h and then cooled to 30 °C. The CO chemisorption was
operated by pulse injection of pure CO at 30 °C. The Pt particle
size was calculated by assuming an adsorption of one CO
molecule per surface platinum atom.
“lake” is being formed, which causes the price of glycerol to
23
decline sharply. Consequently, it is highly desirable to convert
low-cost glycerol into value-added chemicals or materials. Thus,
in the present investigation, we developed a catalytic strategy for
the one-step hydrogenolysis of glycerol to valuable biopropanols
over bi-functional Pt-HSiW/ZrO2 catalysts without employing
organic solvent in a fixed-bed reactor. With this strategy, the
biomass can be readily transformed to biodiesel and biopropa-
nols via glycerol, which will contribute to the sustainable devel-
opment of the biodiesel industry.
Temperature-programmed desorption of ammonia (NH -TPD)
3
was performed using the same apparatus as CO chemisorption.
Prior to each test, about 0.3 g of sample was pretreated in He at
350 °C for 1 h, cooled to 100 °C and then was saturated with
pure NH for 30 min before being purged with He for 30 min.
3
Then the sample was heated from 100 °C to 700 °C at a ramp of
10 °C min− and the NH
1
desorption was monitored with a TCD
3
detector.
Powder X-ray diffraction (XRD) patterns were obtained on a
D2/max-RA X-ray diffractometer (Bruker, Germany), with
Cu Kα radiation at 30 kV and 10 mA. The X-ray patterns were
recorded in 2θ values ranging from 10° to 90°.
Raman spectra were recorded on a LabRAM HR800 System
equipped with a CCD detector at room temperature. A He–Cd
laser operating at 325 nm was used as the exciting source with a
power of 30 MW.
2
. Experimental
2
.1 Catalyst preparation
ZrO (Jiangsu Qianye Co., Ltd, China), TiO (anatase, Jiangsu
Qianye Co., Ltd, China), γ-Al O (Shandong Aluminum Co.,
Ltd, China) and activated carbon (AC for short, Liyang Zhuxi
Carbon Co., Ltd, China) were used as support materials. Some
physicochemical properties of these supports are listed in
Table 1. HSiW, H PW O (HPW), H PMo O (HPMo),
2
2
2
3
Finally, the morphologies of the fresh and spent samples were
investigated by scanning electron microscopy (SEM) (Quanta
4
00F).
3
12 40
3
12 40
H PtCl ·6H O, Pd(NO ) , Cu(NO ) ·3H O, (NH ) W O ·
2
6
2
3 2
3 2
2
4 6
7 24
2
.3 Catalytic tests
6
H O and (NH ) Mo O ·4H O were purchased from Sino-
2 4 6 7 24 2
pharm Chemical Reagent Co., Ltd. Ni(NO ) ·3H O was pro-
Hydrogenolysis of glycerol was carried out in a vertical fixed-
bed reactor (i.d. 12 mm, length 600 mm) with an ice–water trap.
In a typical run, 4.0 g catalyst (20–40 mesh) was charged in the
constant temperature section of the reactor, with quartz sand
3
2
2
vided by the Tianjin Chemical Co., Ltd. All the catalysts
were prepared by an incipient wetness impregnation method.
Taking Pt-HSiW/ZrO as an example, the Pt/ZrO catalyst was
2
2
prepared by impregnation of 8.30 g zirconia support with
packed in both ends. Prior to the reaction, the catalyst was in situ
−1
−1
1
3.61 ml aqueous solution of 77.23 mmol l H PtCl ·6H O.
reduced in a stream of pure H (100 ml min ) at 200 °C for 2 h.
2
6
2
2
The impregnated sample was dried overnight at 110 °C and then
After reduction, a glycerol aqueous solution was continuously
introduced into the reactor with an HPLC pump. The liquid and
gas products were cooled and collected in a gas–liquid separator
immersed in an ice–water trap. The products were obtained
when the reaction reached the steady state. The standard reaction
conditions were as follows: 200 °C, 5.0 MPa, 10 wt% glycerol
calcined at 400 °C in static air for 4 h. The Pt-HSiW/ZrO cata-
2
lyst was prepared by impregnation of Pt/ZrO with 12.0 ml
2
−1
aqueous solution of 43.43 mmol l HSiW. After impregnation,
the sample was dried overnight at 110 °C and then calcined at
3
50 °C in static air for 4 h. The other catalysts were prepared
2608 | Green Chem., 2012, 14, 2607–2616
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aqueous solution, H –glycerol = 137 : 1 (molar ratio), WHSV =
them, Pt–HSiW/ZrO was the best catalyst for glycerol hydroge-
nolysis to propanols.
2
2
−
1
0
.045 h . The reaction conditions were changed to investigate
the dependence of the parameters on glycerol hydrogenolysis.
Additionally, the reaction performance of 1,3-PDO, 1,2-PDO,
Meanwhile, glycerol hydrogenolysis was also performed over
ZrO . There is no conversion of glycerol, indicating that ZrO2
2
1
-PO and 2-PO was also evaluated in order to reveal the reaction
cannot promote this reaction. In the case of the HSiW/ZrO cata-
2
mechanism of glycerol hydrogenolysis.
The liquid products were analyzed by gas chromatography
lyst, glycerol conversion was only 10.6%. The primary products
were acrolein and acetol, from the dehydration of glycerol,
(Ruihong chromatogram analysis Co., Ltd, China) with a FID
whereas the production of propanols was very low. Pt/ZrO and
2
using a DB-WAX capillary column. The tail gas was off-line
analyzed using a gas chromatograph (Huaai chromatogram
analysis Co., Ltd, China) equipped with a TCD and OV-101
column. The products were identified by GC-MS. The mass
balances were 100 ± 5%.
The identified products were 1-PO, 2-PO, 1,3-PDO, 1,2-PDO,
ethanol, propanal, acetaldehyde, acetone, acetol, propionic acid,
acetic acid, ethylene glycol, methanol, propane, methane and
HSiW/ZrO did not show comparable results to Pt–HSiW/ZrO .
The glycerol conversion and propanols selectivity over Pt–
HSiW/ZrO2 was much higher than the sum of Pt/ZrO2 and
2
2
HSiW/ZrO . The combined results suggest that the hydrogen-
2
ation active sites and dehydration acidic sites are indispensable
for effective glycerol hydrogenolysis, which acts in accordance
with the dehydration–hydrogenation mechanism, as reported in
24
our earlier study.
CO . The conversion of glycerol and the selectivity of the pro-
2
ducts were quantified according to the following equations:
Conversion ð%Þ
3.2 Influence of support
moles of glycerol ðinÞ ꢀ moles of glycerol ðoutÞ
Table 3 presents the performance of glycerol hydrogenolysis
over various supported Pt-HSiW bi-functional catalysts at
¼
ꢁ 100
moles of glycerol ðinÞ
2
00 °C, 5.0 MPa. The tested supports include ZrO , AC, TiO2
2
moles of one product
and γ-Al O with a wide range of surface properties such as
2
3
Selectivity ð%Þ ¼
ꢁ 100
moles of all product
textural properties and acid-basicity in favor of checking the
support effect. ZrO and TiO are amphoteric and stable under
2
2
hydrothermal conditions, which have been used widely as the
support of heteropolyacids due to the strong interaction with
3. Results and discussion
25,26
them.
AC is inert and also hydrothermally stable with a
highly developed capillary structure. γ-Al O is acidic and exten-
sively applied in industrial processes.
2
3
3
.1 Optimization of the catalyst active components
A series of ZrO supported metal–acid bi-functional catalysts
As shown in Table 3, the Pt-HSiW/ZrO catalyst gave excel-
2
2
were prepared and investigated for glycerol hydrogenolysis.
Pt/ZrO and HSiW/ZrO were also tested for comparison and the
lent performance. The conversion of glycerol over Pt–HSiW/
ZrO reached 99.7%, which was greater than the other three
2
2
2
results are listed in Table 2. Among the metal (Pt, Pd, Cu and
Ni)-HSiW catalysts tested, Pt–HSiW/ZrO2 exhibited the best
activity and propanols selectivity, while the others showed poor
performance. Pt–HPW/ZrO and Pt–WO /ZrO were also quite
catalysts, especially Pt–HSiW/TiO2 (57.6%) and Pt–HSiW/
γ–Al O (53.5%). Regarding the selectivity, Pt–HSiW/ZrO was
more selective, relative to the other catalysts, towards 1-PO and
2-PO with a combined selectivity of 90.9%. The selectivity to
2
3
2
2
3
2
active for glycerol hydrogenolysis, exhibiting superior propanols
propanols over Pt–HSiW/ZrO was much higher than those over
2
selectivity. In the case of Pt–HPMo/ZrO and Pt–MoO /ZrO ,
Pt–HSiW/γ–Al O , Pt–HSiW/AC and Pt–HSiW/TiO , being
2
3
2
2
3
2
both showed moderate activity and propanols selectivity. Among
63.5%, 59.1% and 39.5%, respectively. In contrast, the
a
Table 2 Performance of glycerol hydrogenolysis over various catalysts
Selectivity (%)
b
Catalysts
Conversion (%)
1-PO
2-PO
1,3-PDO
1,2-PDO
Others
Pt–HSiW/ZrO
Pd–HSiW/ZrO
Cu–HSiW/ZrO
2
99.7
25.3
15.2
24.7
92.4
72.3
90.7
60.4
52.1
10.6
80.0
27.4
11.3
16.1
65.2
54.1
64.0
40.5
8.2
10.9
3.3
3.7
4.2
8.7
10.8
11.8
6.2
0.6
1.1
0.9
10.9
7.2
5.7
8.6
4.6
9.3
5.0
0.5
2.6
5.1
49.2
70.9
52.6
9.2
18.0
8.7
36.8
75.1
8.5
3.1
9.2
6.9
21.4
8.3
12.5
6.2
11.5
15.6d
82.7
2
c
2
c
Ni–HSiW/ZrO
Pt–HPW/ZrO
Pt–HPMo/ZrO
Pt–WO /ZrO
Pt–MoO /ZrO
Pt/ZrO
2
2
2
3
2
3
2
2
HSiW/ZrO
2
5.1
a
–glycerol = 137 : 1 (molar ratio), WHSV = 0.045 h−1
b
Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol aqueous solution, H
2
.
Others include
c
d
ethanol, methanol, propane, etc. Reduction temperature was 300 °C. Acrolein (64.6), acetol (11.7), etc.
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a
Table 3 Performance of glycerol hydrogenolysis over different supported catalysts
Selectivity (%)
Catalysts
Conversion (%)
1-PO
2-PO
1,3-PDO
1,2-PDO
Others
Pt–HSiW/ZrO
Pt–HSiW/AC
Pt–HSiW/γ-Al O
2 3
2
99.7
70.6
53.5
57.6
80.0
56.1
55.8
38.7
10.9
3.0
7.7
0.8
0.9
6.1
2.6
5.9
5.1
24.6
19.1
43.5
3.1
10.2
14.8
11.1
Pt–HSiW/TiO
2
a
–glycerol = 137 : 1 (molar ratio), WHSV = 0.045 h−1
.
Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol aqueous solution, H
2
Table 4 Physicochemical properties of different catalysts
Surface area
Pore size
(nm)
Pore volume
Pt size
(nm)
Pt dispersion
(%)
Total acidity
2
−1
3
−1
−1 a
Catalysts
(m g
)
(cm g
)
(mmol NH
3
g
cat
)
Pt–HSiW/ZrO
Pt–HSiW/AC
Pt–HSiW/γ-Al O
2 3
2
54.8
949.4
187.9
89.1
9.8
3.0
8.7
8.2
0.16
0.07
0.51
0.22
3.2
3.5
5.4
6.8
35.8
32.8
21.0
16.7
0.53
0.47
1.20
1.51
Pt–HSiW/TiO
2
a
3 3
The amount of acid sites was calculated by quantifying the desorbed NH from NH -TPD.
selectivity to propanediols increased from 6.0% over Pt–HSiW/
ZrO to 49.4% over Pt–HSiW/TiO .
2
2
The textural properties of different supported catalysts and
Pt particle size determined by CO-chemisorption are listed in
Table 4. The order of surface area of catalysts is as follows:
Pt–HSiW/AC > Pt–HSiW/γ–Al O > Pt–HSiW/TiO > Pt–HSiW/
2
3
2
ZrO , which is not in good coincidence with the catalytic per-
2
formance. However, there is a good relationship between Pt dis-
persion and the performance of the supported Pt-HSiW catalysts
2
7,28
with various supports. According to the literature,
ZrO2
interacts strongly with Pt sites, which is instrumental in prevent-
ing the crystallization of Pt nanoparticles in calcination at high
temperatures. Thus, it is speculated that the superior performance
of Pt–HSiW/ZrO was related to the small Pt particle size with
2
maximal exposure of the fraction of catalytically Pt active
species on the catalyst surface. Besides immobilizing and disper-
sing Pt active species, the support also affects the surface acidic
properties which can play a key role in glycerol hydrogenolysis
Fig. 1 Effect of HSiW loading on glycerol hydrogenolysis over
Pt–HSiW/ZrO . Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol
aqueous solution, H –glycerol = 137 : 1 (molar ratio), WHSV =
2
2
.045 h−
.
1
0
24
due to the dehydration–hydrogenation bi-functional process.
The acidities of these catalysts determined by NH -TPD are
3
summarized in Table 4. It is assumed that the Pt-HSiW/ZrO2
considerably. For example, over the Pt–5%HSiW/ZrO catalyst,
2
catalyst could provide appropriate acidity to catalyze dehydration
of glycerol and further hydrogenation.
the selectivity of 1-PO and 2-PO was 59.2% and 7.5%, respec-
tively, whereas that of 1,2-PDO decreased to a lower level. The
glycerol conversion and propanols selectivity improved with
HSiW amount and reached a maximum at a loading of 15%
HSiW. However, further increase of HSiW amount led to the
decrease of activity and propanols selectivity. Thus, a suitable
balance between acid sites and the active hydrogen species is
responsible for the excellent performance of Pt–HSiW/ZrO2
with 15% HSiW loading.
3
.3 Influence of HSiW loading
Fig. 1 displays the effect of HSiW loading on the catalytic per-
formance of glycerol hydrogenolysis over Pt–HSiW/ZrO . For
2
the HSiW-free catalyst, the primary product was 1,2-PDO with a
selectivity of 75.1%, whereas only minor propanols were
formed. This result suggested that the sequential hydrogenolysis
capability of Pt/ZrO2 was not high enough to catalyze the
sequential hydrogenolysis of 1,2-PDO to propanols. In contrast,
with the promotion of HSiW, even in a minor amount, the
sequential hydrogenolysis capability of the catalyst was elevated
3
.4 Influence of reaction temperature
Since the Pt–HSiW/ZrO2 catalyst with 15% HSiW loading
exhibited promising catalytic performance, the following
2610 | Green Chem., 2012, 14, 2607–2616
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Fig. 2 Effect of reaction temperature on glycerol hydrogenolysis over
Pt–HSiW/ZrO . Reaction conditions: 5.0 MPa, 10 wt% glycerol
aqueous solution, H –glycerol = 137 : 1 (molar ratio), WHSV =
Fig. 3 Effect of hydrogen pressure on glycerol hydrogenolysis over
Pt–HSiW/ZrO
2
. Reaction conditions: 200 °C, 10 wt% glycerol aqueous
2
−1
solution, H –glycerol = 137 : 1 (molar ratio), WHSV = 0.045 h .
2
2
−1
0
.045 h .
investigations were conducted on this sample. The reaction
temperature dependence of glycerol hydrogenolysis over
Pt–HSiW/ZrO is illustrated in Fig. 2. As expected, glycerol con-
2
version improved dramatically from 56.1% (160 °C) to 99.7%
(200 °C) and then remained at 100% as the temperature elevated.
While, the selectivity to 1-PO and 2-PO first increased remark-
ably to a maximum at 200 °C and then decreased. It is assumed
that the low selectivity at lower temperature (<200 °C) is due to
the existence of partial intermediate propanediols, whereas the
decrease of selectivity at higher temperature (>200 °C) is related
to the formation of large amounts of undesired by-products, such
as the overhydrogenolysis product propane, and the degradation
products methanol, ethanol and ethylene glycerol. The selec-
tivity to 1,3-PDO and 1,2-PDO monotonically declined with the
increase of temperature, indicating that it was favorable to
promote the sequential hydrogenolysis of propanediols to form
Fig. 4 Effect of WHSV on glycerol hydrogenolysis over Pt–HSiW/
ZrO . Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol aqueous
solution, H –glycerol = 137 : 1 (molar ratio).
2
2
10,24
propanols at elevated temperature, as described previously.
demonstrated that Pt–HSiW/ZrO can be a good catalyst for the
2
one-step hydrogenolysis of glycerol to propanols.
3
.5 Influence of hydrogen pressure
Fig. 3 shows the effect of hydrogen pressure on the catalytic
behavior of glycerol hydrogenolysis over Pt–HSiW/ZrO2 at
3
.6 Influence of weight hourly space velocity (WHSV)
2
00 °C. Compared to the temperature effect, hydrogen pressure
The influence of WHSV on the performance of glycerol hydro-
did not have a notable influence on the glycerol hydrogenolysis
genolysis over Pt–HSiW/ZrO at 200 °C and 5 MPa was studied;
2
2
3
as similarly reported by van Ryneveld et al. The conversion of
glycerol enhanced slightly from 95.3% to 100% when increasing
the hydrogen pressure from 3.0 MPa to 6.0 MPa, revealing that
this catalyst was highly active for glycerol hydrogenolysis,
even at low hydrogen pressure. With respect to the selectivity,
the increase of hydrogen pressure favored the sequential hydro-
genolysis of propanediols to produce propanols, which is in
the results are shown in Fig. 4. The activity dropped obviously
with increasing WHSV because the residence time shortened
correspondingly. Concurrently, the selectivity to propanediols
increased along with the decrease of propanols, suggesting that
the short residence time favored the suppression of the sequential
hydrogenolysis of propanediols to propanols.
2
9,30
good agreement with previous literature.
The increase of
3
.7 Influence of catalyst weight
hydrogen pressure is favorable to produce propanols, which
might be ascribed to the enhancement of the active hydrogen
species formed from hydrogen on Pt–HSiW/ZrO2. The
maximum yield of 1-PO reached 83.9% at 6 MPa and that of
propanols was up to 94.1%. As far as we know, the yield of
Fig. 5 shows the influence of catalyst loading on the catalytic
performance of glycerol hydrogenolysis over Pt–HSiW/ZrO . As
2
the catalyst amount is increased from 1.0 g to 4.0 g, a uniform
enhancement in glycerol conversion is observed from 30.6% to
99.7%. Concurrently, the selectivity to 1-PO and 2-PO improved
with the rising catalyst amount at the expense of propanediols,
9
4.1% is the highest value reported for the one-step production
of propanols from glycerol. Evidently, these results have
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The main impurities present in crude glycerol consist of metha-
nol (from an incomplete removal by distillation), water (from
washing procedures) and salts, such as sodium chloride or
sodium sulphate (from neutralization of the basic catalysts with
31,32
HCl or H SO ).
As a consequence, in most cases crude gly-
2
4
cerol must be purified by expensive separation steps to satisfy
the requirement of the downstream processes. Accordingly, it is
more profitable and environmental to utilize crude glycerol
directly. Thus, the hydrogenolysis of glycerol doped with metha-
nol, sodium chloride or sodium sulphate over Pt–HSiW/ZrO2
was performed to check its tolerance towards impurities. In
addition, crude glycerol diluted with deionized water was used
to make a comparison and the results are summarized in Table 5.
As can be seen, the addition of NaCl or Na SO to glycerol
2
4
Fig. 5 Effect of catalyst weight on glycerol hydrogenolysis over Pt–
HSiW/ZrO . Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol
aqueous solution, H –glycerol = 137 : 1 (molar ratio).
aqueous solution did not affect the reactivity and product distri-
bution. Nevertheless, the addition of methanol to the solution
slightly declined the activity, probably because methanol com-
peted for the active sites of the Pt–HSiW/ZrO2 catalyst.
Additionally, more methane was detected in gas products,
suggesting that methanol can be hydrogenolyzed to form
2
2
methane by the Pt–HSiW/ZrO catalyst. The observed results for
2
the hydrogenolysis of crude glycerol were similar to the results
obtained using analytical grade glycerol. It is noted that the Pt–
HSiW/ZrO catalyst is resistant to the impurities present in crude
2
glycerol.
3
.10 Long-term performance of glycerol hydrogenolysis
The long-term performance of glycerol hydrogenolysis over
Pt–HSiW/ZrO was conducted at 200 °C and 5 MPa, and the
2
results are shown in Fig. 7. No sign of any decline in the activity
was observed; the catalyst can be stable up to 160 h, despite the
harsh reaction conditions. Meanwhile, the product distribution
did not show any appreciable change during the complete test.
This result demonstrates that the Pt–HSiW/ZrO2 catalyst was
rather durable under hydrothermal conditions for glycerol hydro-
genolysis and is suitable for practical applications.
Fig. 6 Effect of glycerol concentration on glycerol hydrogenolysis
over Pt–HSiW/ZrO . Reaction conditions: 200 °C, 5.0 MPa, H –glycerol
137 : 1 (molar ratio), WHSV = 0.045 h
2
2
−1
=
.
indicating that the propanols were produced via sequential
hydrogenolysis of propanediols.
The spent catalyst was characterized by XRD, Raman, H -che-
2
misorption, SEM and NH -TPD; the results were compared with
3
those of the fresh catalyst. As shown in Fig. 8a, the spent and
fresh catalysts showed similar XRD patterns in which no crystal-
line phase related to the Pt or HSiW species was observed. This
can be presumably ascribed to the fact that the Pt and HSiW
species were homogeneously dispersed on the support surface
and did not present obvious agglomeration during the reaction.
The CO-chemisorption result also confirmed that the Pt particle
size had no remarkable change during the reaction. The SEM
3
.8 Influence of glycerol concentration
It is well-known that crude glycerol from biodiesel processes
unavoidably contains water, and water is also a by-product of the
reaction sequence, which makes water the desired solvent for
glycerol hydrogenolysis in terms of environmental and economi-
cal viability. Fig. 6 shows the effect of glycerol concentration or
initial water content on glycerol hydrogenolysis with constant
WHSV. Both the glycerol conversion and selectivity to propa-
nols presented a trend of rapid increase with the increase of
glycerol concentration from 5% to 10%. When the glycerol con-
centration was above 10%, it was not as influential, indicating
that the glycerol on the catalyst surface became saturated.
(Fig. 8b) images of the fresh and spent samples showed similar
morphologies, implying that the structure of this catalyst was
rather stable during the hydrothermal environment, in spite of
the high pressure. Raman spectroscopy was employed to check
the state of HSiW before and after reaction. As shown in Fig. 8c,
the fresh and spent catalysts displayed similar characteristic
bands of HSiW and monoclinic ZrO , suggesting that the
2
Keggin structure of HSiW remained intact during the glycerol
3
.9 Influence of impurities
hydrogenolysis. As illustrated in Fig. 8d, the NH -TPD profile of
3
Crude glycerol, which is to be obtained directly from the bio-
spent Pt–HSiW/ZrO after calcination at 350 °C was also similar
2
diesel industry by transesterification, contains many impurities.
to the fresh one.
2612 | Green Chem., 2012, 14, 2607–2616
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a
Table 5 Performance of glycerol hydrogenolysis in the presence of impurities over Pt–HSiW/ZrO
2
Selectivity (%)
Feed composition
Glycerol
Conversion (%)
1-PO
2-PO
1,3-PDO
1,2-PDO
Others
99.7
98.4
96.5
90.1
97.2
80.0
78.9
79.2
72.5
80.4
10.9
9.3
9.1
7.5
10.4
0.9
1.3
1.5
3.8
1.2
5.1
7.0
6.5
8.8
4.7
3.1
3.5
3.7
7.4
3.3
b
Glycerol + NaCl
c
4
Glycerol + Na
2
SO
d
Glycerol + methanol
e
Crude glycerol
a
–glycerol = 137 : 1 (molar ratio), WHSV = 0.045 h−1. b Glycerol with
Reaction conditions: 200 °C, 5.0 MPa, 10 wt% glycerol aqueous solution, H
2
c
d
e
2
8
wt% NaCl. Glycerol with 2 wt% Na
2
4
SO .
Glycerol with 1 wt% methanol. Crude glycerol (glycerol–NaCl–KCl–methanol (mass ratio) =
0 : 8 : 3 : 1).
higher than that of 2-PO in 1,2-PDO hydrogenolysis and the
ratio of 1-PO to 2-PO was about 7. As discussed in Section 3.6
and 3.7, the propanols were formed by sequential hydrogenolysis
of propanediols from glycerol. These combined results can
explain that the selectivity to 1-PO was much more than 2-PO,
and that 2-PO is produced via 1,2-PDO in glycerol hydrogenoly-
sis. The ratio of 1-PO to 2-PO in glycerol hydrogenolysis was
similar to that in 1,2-PDO hydrogenolysis, which confirmed that
the propanols were mainly produced through 1,2-PDO in gly-
1
0
cerol hydrogenolysis, particularly at low temperatures. This be-
30
havior differed from the report proposed by Miyazawa et al.
wherein propanols were mainly formed via 1,3-PDO in glycerol
hydrogenolysis over a Ru/C + Amberlyst catalyst. On the other
10
hand, Qin et al. have acclaimed that the sequential hydrogeno-
lysis of propanediols to propanols occurred mainly via 1,2-PDO
over Pt/WO /ZrO . Apparently, both offer plausible pathways
which are highly related to the reaction conditions and catalytic
system used. In the case of 1-PO and 2-PO hydrogenolysis over
Fig. 7 Long-term performance of glycerol hydrogenolysis over the Pt–
HSiW/ZrO catalyst. Reaction conditions: 200 °C, 5.0 MPa, 10 wt%
glycerol aqueous solution, H –glycerol = 137 : 1 (molar ratio), WHSV =
2
3
2
2
−1
0
.045 h .
Pt–HSiW/ZrO , the conversion of 1-PO and 2-PO was only
2
1
8.4% and 10.7% at 200 °C, respectively, which was much
The superior long-term performance of Pt–HSiW/ZrO was
2
lower than that of glycerol, 1,2-PDO and 1,3-PDO. The lower
reactivity of 1-PO and 2-PO can explain the high yield of propa-
nols in glycerol hydrogenolysis. As shown in Table 6, it can be
seen that the suppression of overhydrogenolysis of propanols to
propane is favored at lower reaction temperatures.
mainly attributed to the strong interaction of active sites with
ZrO2 and its good hydrothermal stability. Devassy and
Halligudi reported the strong interaction between HSiW and
33
ZrO based on XPS and Raman spectroscopy and acclaimed the
2
zirconia-anchored mono-oxotungstate as the major tungsten
23,30,34–36
Based on the literature
and our experimental results,
species was shown over HSiW/ZrO with the optimum HSiW
2
the possible reaction route involved in glycerol hydrogenolysis is
proposed in Fig. 9. The acid-catalyzed dehydration of glycerol
initially proceeds to form acetol and 3-hydroxypropionaldehyde
loading (15 wt%) due to the monolayer coverage of HSiW on
ZrO . The strong HSiW interaction with uZr–OH groups is an
2
acid–base reaction where the uZr–OH groups act as a base and
HSiW as a Brønsted acid. On the other hand, the strong inter-
(3-HPA), which can subsequently hydrogenate to 1,2-PDO and
1
,3-PDO over metal catalysts, respectively. The unstable inter-
action between Pt and ZrO can prevent sintering and leaching
2
mediate 3-HPA would decompose to formaldehyde and acet-
aldehyde; a subsequent oxidation of acetaldehyde would lead to
the formation of acetic acid. The propanediols can further
undergo sequential hydrogenolysis to form 1-PO and 2-PO.
When dehydration occurs at the central hydroxyl group of
1,2-PDO or the terminal hydroxyl group of 1,3-PDO, propanal is
produced and subsequently hydrogenates to form 1-PO. In con-
trast, when dehydration undergoes at the terminal hydroxyl
group of 1,2-PDO, acetone is produced and is then further
hydrogenated to yield 2-PO. Furthermore, propionic acid was
also detected as a by-product, which was probably produced by
of Pt nanoparticles during the long-term test.
3
.11 Reaction route and kinetics of glycerol hydrogenolysis
In order to elucidate the reaction sequence of glycerol hydroge-
nolysis, the hydrogenolysis of 1,2-PDO, 1,3-PDO, 1-PO and
2
-PO over Pt–HSiW/ZrO was also evaluated under conditions
2
similar to that of glycerol and the results are presented in
Table 6. The conversion of 1,3-PDO was lower than that of
glycerol and 1,2-PDO, particularly at low temperatures. In the
37
1,3-PDO hydrogenolysis, no 2-PO was produced but the selectivity
the oxidation of propanal. With respect to the formation of
ethylene glycol, it was obtained in glycerol hydrogenolysis.
to 1-PO was more than 95%. The selectivity to 1-PO was much
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Fig. 8 Characterization of fresh and spent catalysts (a) XRD; (b) SEM images; (c) Raman; (d) NH
3
-TPD.
a
2
Table 6 Performance of hydrogenolysis of glycerol, 1,2-PDO, 1,3-PDO, 1-PO and 2-PO over Pt–HSiW/ZrO
Selectivity (%)
Reactant
Glycerol
T/°C
Conversion (%)
1-PO
2-PO
1,3-PDO
1,2-PDO
Others
180
85.2
99.7
97.2
100
55.6
92.6
62.2
80.0
83.7
84.9
99.5
95.4
8.0
10.9
11.2
11.7
0.0
22.1
0.9
0.0
0.0
—
5.7
5.1
—
—
0.0
0.0
2.0
3.1
5.1
3.4
0.5
4.6
2
00
180
00
180
00
1
,2-PDO
,3-PDO
2
1
2
0.0
—
Propane
>99.0
1
2
-PO
-PO
180
00
180
00
6.1
18.4
4.9
2
>99.0
>99.0
>99.0
2
10.7
a
Reaction conditions: 5.0 MPa, 10 wt% aqueous solution of reactant, WHSV = 0.045 h−1
.
2614 | Green Chem., 2012, 14, 2607–2616
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2
Fig. 9 Proposed reaction routes involved in glycerol hydrogenolysis over Pt–HSiW/ZrO .
However, it was not detected in the 1,2-PDO and 1,3-PDO
hydrogenolysis, indicating that ethylene glycol was produced
directly from glycerol by a C–C bond cleavage reaction. In the
reaction of glycerol and 1,2-PDO, ethanol was observed which
can be formed via sequential hydrogenolysis of ethylene glycol
or decomposition of 1,2-PDO. Finally, the other products such
as propane, methane and methanol are formed through over-
hydrogenolysis or decomposition reactions.
This study has demonstrated a strategy for developing an
active, selective and robust catalyst for the one-step hydrogenoly-
sis of biomass-derived glycerol to biopropanols in aqueous
media. The 94.1% yield of propanols was achieved under the
optimized conditions, which is the best yield reported to date.
The reaction pathways were proposed to help understand the
reaction mechanism. The strategy in the present work to develop
an effective and green process might provide guidance for the
sustainable production of valuable chemicals from polyols.
We also performed a kinetic study of glycerol hydrogenolysis,
which is shown in Fig. S1 and S2.† Glycerol conversion linearly
improved with increasing H pressure from 2 MPa to 5 MPa,
2
indicating that the reaction order on H pressure was first, which
was consistent with the conclusion reported by Amada et al.
2
Acknowledgements
21
As shown in Fig. S2,† glycerol conversion was enhanced when
increasing glycerol concentration from 5% to 10%, while it was
almost saturated with further increase of glycerol concentration.
To obtain high yields of propanols, it is essential to perform this
reaction at higher hydrogen pressure and moderate glycerol con-
centration. On the other hand, the suppression of overhydrogen-
olysis of propanols is necessary to improve the propanols yield,
and the balance between the hydrogenation active species and
acidic sites may be the key. The decrease of Pt nanoparticles size
and regulation of HSiW acidity can be instrumental in suppres-
sing of the overhydrogenolysis reactions.
The authors gratefully acknowledge the financial support of the
Natural Science Foundation of China (no. 20976185). This work
was also supported by the Major State Basic Research Develop-
ment Program of China (973 Program) (no. 2012CB215305).
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