Hydrogen-free synthesis of 1,2-propanediol from glycerol over Cu-Mg-Al catalysts

Article abstract of DOI:10.1039/c3ra42543f

Hydrogen-free synthesis of 1,2-propanediol (1,2-PDO) from glycerol was carried out over a series of Cu-Mg-Al catalysts with different (Cu + Mg)/Al ratios. Several hydrogen donors were tested and it was found that ethanol was the best donor for this process, the efficiency of the ethanol donor reached 79.8%. Cu_0.4/Mg_6.28Al_1.32O_8.26 showed the best performance among these tested catalysts. The conversion of glycerol and selectivity of 1,2-PDO reached 95.1% and 92.2%, respectively, at 210 °C. Characterizations indicated that the activity of Cu-Mg-Al for this hydrogen-free synthesis of 1,2-PDO depended mainly on the basicity of the catalysts. Both the conversion of glycerol and selectivity to 1,2-PDO could be improved with prolonged time, reaction temperature and the amount of hydrogen donor. The reaction mechanism was proposed. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42543f

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PAPER
Hydrogen-free synthesis of 1,2-propanediol from
glycerol over Cu–Mg–Al catalysts
Cite this: RSC Advances, 2013, 3,
16569
Shuixin Xia,a Liping Zheng,a Lina Wang,b Ping Chena and Zhaoyin Hou*a
Hydrogen-free synthesis of 1,2-propanediol (1,2-PDO) from glycerol was carried out over a series of Cu–
Mg–Al catalysts with different (Cu + Mg)/Al ratios. Several hydrogen donors were tested and it was found
that ethanol was the best donor for this process, the efficiency of the ethanol donor reached 79.8%. Cu0.4
/
Mg6.28Al1.32O8.26 showed the best performance among these tested catalysts. The conversion of glycerol
and selectivity of 1,2-PDO reached 95.1% and 92.2%, respectively, at 210 uC. Characterizations indicated
that the activity of Cu–Mg–Al for this hydrogen-free synthesis of 1,2-PDO depended mainly on the basicity
of the catalysts. Both the conversion of glycerol and selectivity to 1,2-PDO could be improved with
prolonged time, reaction temperature and the amount of hydrogen donor. The reaction mechanism was
proposed.
Received 23rd May 2013,
Accepted 9th July 2013
DOI: 10.1039/c3ra42543f
www.rsc.org/advances
sources.40 To avoid the inherent drawbacks of working with
molecular hydrogen, one promising alternative route is to
produce the required hydrogen in situ directly in the reaction
mixture under the process conditions.
1. Introduction
Nowadays, biomass-derived energy has received more and
more attention due to the dwindling supply of fossil fuels.1
Biodiesel which comes from 100% renewable resources
provides an alternative fuel option for the future. As an
emerging new green fuel, it is produced via transesterification
of vegetable oil with methanol or ethanol, accompanied with
glycerol as a major byproduct (10% in weight).2 In recent
decades, with the rapid development of the biodiesel industry,
glycerol has been oversupplied in the market, resulting in a
significant decrease in the glycerol price. And disposal of the
excess glycerol by incineration is expensive and environmen-
tally problematic.3,4 Converting glycerol into value-added
chemicals will contribute to the cost competitiveness of the
biodiesel process.5–9 Particularly, much attention has been
paid to the catalytic hydrogenolysis of glycerol to 1,2-
propanediol (1,2-PDO)10–34 and 1,3-propanediol (1,3-PDO).35–39
Among them, the catalytic hydrogenolysis of glycerol to 1,2-PDO
has more potential for commercialization in the near future.6
In published works, the hydrogenolysis reactions were
carried out at high hydrogen pressure and high temperature in
order to reach an acceptable glycerol conversion and product
selectivity, owing to the low solubility of hydrogen in aqueous
glycerol solutions.17 And thus, it inevitably raised the require-
ment of the equipment because molecular hydrogen is easily
ignited when it contacts with air. In addition, molecular
hydrogen is produced in energy intensive processes from fossil
The in situ generation of hydrogen usually involved aqueous
phase reforming (APR) of glycerol6,41–43 and catalytic transfer
hydrogenation (CTH). CTH is a process in which hydrogen is
transferred from a hydrogen donor molecule to an acceptor
and CTH reactions can be of industrial importance as the
transportation and storage of hydrogen donors can be cheaper
than those for molecular hydrogen.40 It was reported that
glycerol hydrogenolysis via CTH under an inert atmosphere
was more feasible since its reaction conditions are less harsh
than APR.40 Also, molecular hydrogen has high diffusibility,
being easily ignited and presents considerable hazards on a
large scale; the use of a hydrogen donor can overcome these
difficulties.12,44 For the hydrogen donor selection, the first
criterion to be fulfilled is that the donor molecules should be
soluble in glycerol. Reported hydrogen donors include formic
acid,17 2-propanol (2-PO),12,13,16 methanol16 and ethanol13 etc.
Musolino et al. found that the selective conversion of
glycerol to 1,2-PDO performed efficiently via CTH in the
presence of Pd/Fe2O3 catalyst, using ethanol and 2-PO as
solvents and hydrogen donor molecules at 180 uC, 5 bar of
inert atmosphere. It was proposed that glycerol was firstly
dehydrated to acetol and then it reacted with the in situ formed
hydrogen to form 1,2-PDO.13 Under nitrogen pressure,
Gandarias et al. found that CTH with 2-propanediol was more
efficient for the production of 1,2-PDO than that in glycerol
APR over Ni–Cu/Al2O3 under N2 at 493 K, 24 h. They believed
that glycerol could be converted directly to 1,2-PDO through
intermediate alkoxide formation.12
aInstitute of Catalysis, Department of Chemistry, Zhejiang University, Hangzhou
310028, China. E-mail: zyhou@zju.edu.cn; Fax: +86-571-88273283;
Tel: +86-571-88273272
bEngineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of
Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China
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In this work, a series of Cu–Mg–Al catalysts with different
(Cu + Mg)/Al ratios have been prepared by the coprecipitation
method. It was confirmed that the basicity of these catalysts
could be manipulated by adjusting the (Cu + Mg)/Al ratio and
the hydrogen-free synthesis of 1,2-PDO performed efficiently
over the catalyst with an appropriate basicity. The reaction
mechanism was also discussed.
RSC Advances
min21 and cooled to room temperature. A reduction agent
(10% H2–N2 mixture, 30 mL min21) was shifted and the
reactor was heated to 450 uC at a ramp of 10 uC min21. Effluent
gas was dried by powder KOH and the consumption of
hydrogen was recorded by TCD. The amount of reduced Cu in
these samples was calibrated with pure CuO (AR, Sinopharm
Chemical Reagent Co., Ltd, China) of known amount.
Catalysts were first reduced in the procedure described above,
and the amount of hydrogen consumption was denoted as X.
And then the reactor was purged with He to 50 uC. 20% N2O–
N2 (30 mL min21) was shifted to oxidize surface copper atoms
to Cu2O at 50 uC for 0.5 h. The reactor was flushed with He to
remove the oxidant. Finally, another reduction experiment was
2. Methods and experimental
2.1 Catalyst preparation
performed in 10% H2–N2 at a flow rate of 30 mL min21
.
Cu–Mg–Al catalysts were prepared in
a coprecipitation
method. In a typical synthesis, 0.01 mol Cu(NO3)2?3H2O, 0.15
mol Mg(NO3)2?9H2O and 0.04 mol Al(NO3)3?9H2O (AR,
Sinopharm Chemical Reagent Co., Ltd, China) were dissolved
together in 400 mL deionized water, which was referred as
solution A. Solution B was a mixture of Na2CO3 and NaOH (AR,
Sinopharm Chemical Reagent Co., Ltd, China) with concentra-
tions of 0.25 and 0.80 mol L21, respectively. Solution A and B
were simultaneously added into a glass reactor under vigorous
stirring at room temperature and a pH value of 9.5. The
amount of solution B that was added to solution A was about
480 mL. The slurry was aged at 90 uC for 20 h, filtered off, and
washed thoroughly with distilled water until the effluent was
neutral. The precipitate was then dried at 80 uC overnight.
Keeping the amount of Cu(NO3)2?3H2O constant and by
varying the amount of Mg(NO3)2?9H2O and Al(NO3)3?9H2O, a
series of Cu–Mg–Al catalysts with different (Cu + Mg)/Al ratios
were prepared and denoted as Cu0.4MgxAl7.62x(OH)21.62xCO3.
Before catalytic reaction, these catalysts were heated slowly
(2 uC min21) to 400 uC and calcined at 400 uC in stationary air
for 4 h. The calcined product catalysts were identified as
Cu0.4MgxAl7.62xO11.820.5x. At last, the calcined catalysts were
further reduced in a hydrogen flow (30 mL min21) at 350 uC for
1 h before the reaction, and these reduced catalysts were
Hydrogen consumption in the second reduction was denoted
as Y. The dispersion and surface area of the surface Cu were
calculated according to the equations reported by Van Der
Grift et al.,45 which are shown below:
Reduction of all copper atoms:
CuOzH2?CuzH2O, hydrogen
consumption in the first TPR~X
Reduction of surface copper atoms only:
Cu2OzH2?2CuzH2O, hydrogen
consumption in this TPR~Y
And the dispersion and surface area of the surface Cu were
calculated as:
D = (2 6 Y/X) 6 100%
S = 2 6 Y 6 Nav/(X 6 MCu 6 1.4 6 1019) =
1353 6 Y/X (m2 Cu per g Cu)
In these equations, Nav is Avogadro’s constant, MCu is the relative
atomic mass of copper (63.46 g mol21), 1.4 6 1019 is the number
of copper atoms per square meter because the average surface area
of a copper atom is assigned to 7.11 6 1022 nm2.
The average volume–surface diameter can be expressed as a
function:
identified as Cu0.4/MgxAl7.62xO11.420.5x. As references, Cu0.4
Mg6.28Al1.32O8.26-IM (prepared by impregnation) and Cu0.4
/
/
Mg6.28Al1.32O8.26-IE (prepared by ion-exchange) were also
prepared.
2.2 Catalyst characterization
N2 adsorption was measured at its normal boiling point using
an ASAP 2010 analyzer (Micromeritics) after pretreatment at
250 uC for 4 h in a vacuum. Surface area and pore size
distribution were calculated using their adsorption isotherms.
Powder X-ray diffraction (XRD) patterns of Cu–Mg–Al
samples were detected at room temperature on a Rigaku D/
WAX-2500 diffractometer using Cu-Ka radiation (l = 1.5406 Å)
with a 2h step of 0.02u.
Scanning electron microscopy (SEM) images were detected
on a Leo Evo Series SEM (VP 1430, Germany). Samples were
coated with gold using sputter coating to avoid charging.
Analysis was carried out at an accelerating voltage of 15 kV.
The amount of surface metallic copper was determined by
N2O oxidation and followed H2 titration using the procedure
described by Van Der Grift et al.45 Samples were first
pretreated at 400 uC for 1 h under N2 at a flow rate of 30 mL
dv.s. = 6/(S 6 rCu) # 0.5 6 X/Y (nm)
rCu in the above equation is the density of copper (8.92 g cm23).
The basicity of the catalyst was determined via temperature-
programmed desorption (TPD) of CO2 (CO2-TPD). The sample
was first reduced at 350 uC in a H2 flow of 30 mL min21 for 1 h,
purged by purified Ar and further treated at 450 uC for 0.5 h in
Ar. Then the reactor was cooled to 50 uC in Ar, exposed to 20%
CO2–Ar for 30 min, and purged by Ar for 5 h at 50 uC in order
to eliminate the physically adsorbed CO2. CO2-TPD was
conducted by ramping to 550 uC at 10 uC min21 and CO2 (m/
z = 44) in the effluent was detected and recorded as a function
of temperature on
a
quadrupole mass spectrometer
(OmniStarTM, GSD301, Switzerland).
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2.3 Catalytic reaction
Hydrogen-free synthesis of 1,2-PDO reactions were performed
in a custom-designed stainless steel autoclave equipped with a
Teflon inner layer. Before reaction, the catalyst was pretreated
in a flow of H2 at 350 uC for 1 h. The reaction mixture included
0.087 mol hydrogen donor, 0.022 mol glycerol and 0.25 g
reduced catalyst. Then the autoclave was purged with N2 to 3.0
MPa and placed in an oil bath preheated to the required
temperature and maintained at that temperature under
vigorous stirring (MAG-NEO, RV-06 M, Japan). After reaction,
the reactor was cooled to room temperature, the vapor phase
was collected by
a gas-bag and analyzed using a gas
chromatograph (Shimadzu, 8A) with a thermal conductivity
detector. Solid catalyst powder in the liquid phase was
separated via centrifugation and washed with deionized water
(5 6 5 mL). These eluent and reaction mixture were collected
and charged into a 50 mL volumetric flask and analyzed using
a flame ionization detector gas chromatograph (Shimadzu,
14B) equipped with a 30 m capillary column (DB-WAX 52 CB,
USA).
Fig. 2 SEM images of the fresh Cu–Mg–Al catalysts. (a) Cu0.4Mg5.6Al2(OH)16CO3;
(b) Cu0.4Mg6Al1.6(OH)15.6CO3; (c) Cu0.4Mg6.28Al1.32(OH)15.32CO3; (d)
Cu0.4Mg6.46Al1.14(OH)15.14CO3.
All products detected in the liquid were verified by a gas
chromatography and mass spectrometry system (GC-MS,
Agilent 6890) and quantified via an external calibration
method. The selectivity of product was calculated on the basis
of carbon.
ratio > 4, and the intensity of its characteristic peaks enhanced
with the increase of the (Cu + Mg)/Al ratio. The SEM patterns
of fresh precipitated Cu–Mg–Al catalysts are shown in Fig. 2. It
can be found that the orderliness of these LDH crystals
decreased and curved lamellar was detected with the increase
of the (Cu + Mg)/Al ratio, which is in accordance with the
observation from the XRD pattern of the catalysts. The average
thickness of solid lamellar of Cu0.4Mg5.6Al2(OH)16CO3 is 20
nm. However, with the increase of the (Cu + Mg)/Al ratio, a
slight decrease in the thickness of lamellar could be observed.
The thicknesses of solid lamellar in Cu0.4Mg6Al1.6(OH)15.6CO3,
Cu0.4Mg6.28Al1.32(OH)15.32CO3, Cu0.4Mg6.46Al1.14(OH)15.14CO3
are 17, 15 and 16 nm, respectively (see Fig. 2). The decrease
in the thickness of lamellar could be ascribed to the decline of
orderliness of these LDH crystals with the increasing (Cu +
Mg)/Al ratio.
3. Results and discussion
3.1 Catalyst characterization
Fig. 1 shows the XRD patterns of fresh precipitated Cu–Mg–Al
catalysts. Peaks at 11.7, 23.6, 35.0, 39.7, 47.1, 60.9 and 62.4u
were observed in all these catalysts, which were assigned to the
(003), (006), (009), (105), (108), (110) and (113) diffractions of
layered double hydroxides (LDHs) (JCPDS 35-0965). However,
these peaks became broad and their intensity decreased with
increasing the molar ratio of M2+ to M3+. Another phase
Mg(OH)2 (JCPDS 44-1482) could be detected when (Cu + Mg)/Al
Nitrogen adsorption–desorption isotherms of calcined Cu–
Mg–Al are shown in Fig. 3. All of them are a type IV pattern
according to the International Union of Pure and Applied
Chemistry (IUPAC) classification. These results indicated that
there was a large amount of slit-shaped pore channels formed
because of the aggregation of plate-like layers. The closure
points of hysteresis loops occurred at a relative pressure of 0.5
in Cu0.4Mg5.6Al2O9 and Cu0.4Mg6Al1.6O8.8, suggesting that they
have similar pore diameters. However, the hysteresis loops of
Cu0.4Mg6.28Al1.32O8.66 and Cu0.4Mg6.46Al1.14O8.57 closed at 0.60
and 0.65, respectively, which suggested that they have larger
pore diameters. BET surface area, pore volume and average
pore size of the calcined samples (calculated from their
adsorption isotherms) are summarized in Table 1. The
calculated BET surface areas of these catalysts decreased
continuously from 204.6 m2 g21 (for Cu0.4Mg5.6Al2O9) to 66.4
m2 g21 (of Cu0.4Mg6.46Al1.14O8.57) (Table 1), which might be
ascribed to the decline of orderliness of LDH crystals and the
formation of Mg(OH)2 when (Cu + Mg)/Al ratio > 4.
Fig. 1 XRD patterns of fresh Cu–Mg–Al catalysts. (a) Cu0.4Mg5.6Al2(OH)16CO3;
(b) Cu0.4Mg6Al1.6(OH)15.6CO3; (c) Cu0.4Mg6.28Al1.32(OH)15.32CO3; (d)
Cu0.4Mg6.46Al1.14(OH)15.14CO3.
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Fig. 3 N2 adsorption isotherms of calcined catalysts. (a) Cu0.4Mg5.6Al2O9; (b)
Fig. 4 CO2-TPD profiles of calcined catalysts. (a) Cu0.4Mg5.6Al2O9; (b)
Cu0.4Mg6Al1.6O8.8; (c) Cu0.4Mg6.28Al1.32O8.66; (d) Cu0.4Mg6.46Al1.14O8.57
Cu0.4Mg6Al1.6O8.8; (c) Cu0.4Mg6.28Al1.32O8.66; (d) Cu0.4Mg6.46Al1.14O8.57
.
.
Copper dispersion and average particle size detected in N2O
oxidation and following H2 titration are summarized in Table 1.
The dispersion and average particle size of copper in reduced
Cu0.4Mg5.6Al2O9 catalyst (calcined at 400 uC) are 43.2% and 2.31
nm. The detected dispersion of Cu decreased with increasing
the (Cu + Mg)/Al ratio, and the calculated average diameter of
adsorbed CO2 has been calculated and summarized in Table 1.
All the results showed the basicity of Cu–Mg–Al catalysts
enhanced with the increasing content of Mg.
3.2 Catalytic reactions with different hydrogen donors
Table
2
summarizes the performance of the Cu0.4/
copper increased to 3.83, 5.10 and 6.41 nm in Cu0.4Mg6Al1.6O8.8
,
Mg6.28Al1.32O8.26 catalyst for hydrogen-free synthesis of 1,2-
PDO with different hydrogen donor molecules at 200 uC. It was
found that glycerol achieved its highest conversion (86.5%)
and high selectivity to 1,2-PDO (92.5%) when ethanol was
selected as hydrogen donor. And the calculated turnover
frequency (TOF, defined as (mol of converted glycerol)/(mol of
exposed Cu atom)/(reaction time, h)) reached 32.9 h21. In the
liquid phase, acetol, 1-propanol and ethylene glycol were also
detected besides 1,2-PDO. The carbon balance, which was
defined as (the total amount of 1,2-PDO, acetol, ethylene glycol
and glycerol in the liquid phase)/(the amount of added
glycerol) 6 100%, is higher than 96.5% in all experiments.
The loss of carbon balance might be attributed to the
dehydration of glycerol, because acrolein was also detected
in the gas phase in most experiments. At the same time, a
large amount of ethyl acetate and 1,1-diethoxyethane were
detected in liquid phase, which are the dehydrogenated
Cu0.4Mg6.28Al1.32O8.66 and Cu0.4Mg6.46Al1.14O8.57, respectively.
The decreased dispersion of Cu might be attributed to the lower
surface area of corresponding catalyst. The average diameters of
copper in Cu0.4Mg6.28Al1.32O8.66-IM and Cu0.4Mg6.28Al1.32O8.66
-
IE are larger than that of Cu0.4Mg6.28Al1.32O8.66, which are 10.3
and 8.1 nm, respectively.
CO2-TPD profiles of calcined Cu–Mg–Al are shown in Fig. 4.
All these desorption profiles can be deconvoluted into a peak
at 100 uC and an increasing peak (from 170 to 230 uC). The first
peak could be assigned to the desorbed CO2 from the surface
of catalysts and the second peak could be ascribed to the
desorbed CO2 that adsorbed onto the Lewis basic sites of
MgO.46–52 It can be found that the second peak shifted to
higher temperature with the increase of (Cu + Mg)/Al ratio,
which indicated the strength of this basic site enhanced with
increasing the Mg content. At the same time, the amount of
Table 1 The structure of calcined Cu–Mg–Al and Cu dispersion in the reduced catalysts
Structurea
Cu dispersionb
Pore volume
Pore size
(nm)
SBET
Dispersion
(%)
Cu particle
size (nm)
Adsorbed CO2
(mmol g21 c
Catalysts
(cm3 g21
)
(m2 g21
)
)
Cu0.4Mg5.6Al2O9
Cu0.4Mg6Al1.6O8.8
Cu0.4Mg6.28Al1.32O8.66
Cu0.4Mg6.46Al1.14O8.57
Cu0.4Mg6.28Al1.32O8.66-IM
Cu0.4Mg6.28Al1.32O8.66-IE
0.543
0.937
0.632
0.368
—
8.8
9.9
17.7
18.9
—
204.6
145.1
102.1
66.4
—
43.2
26.1
19.6
15.6
13.4
15.1
2.31
3.83
5.10
6.41
10.3
8.1
0.225
0.241
0.323
0.561
—
—
—
—
—
a
b
All of the catalysts are calcined at 400 uC, 4 h. Calculated from N2O chemical adsorption after the catalysts were reduced in H2 at 400 uC.
c
The quantitative adsorbed CO2 in moles per g calculated from the TPD of CO2.
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a
Table 2 Hydrogen donor selection for glycerol hydrogenolysis over Cu0.4/Mg6.28Al1.32O8.26
Selectivity (%)
Carbon
Hydrogen donor
conv. (%)
Hydrogen donor
efficiency (%)e
Hydrogen donor
Conv. (%)
TOFb (h21
)
1,2-PDO
Acetol
Othersc
balanced (%)
Methanol
Ethanol
1-Propanol
Butanol
41.2
86.5
64.5
83.4
50.8
15.7
32.9
24.6
31.8
19.4
91.2
92.5
87.7
94.8
89.0
7.5
7.1
11.8
4.4
1.3
0.4
0.5
0.8
0.7
96.7
97.0
96.5
96.8
97.2
19.2
25.1
23.4
24.8
23.1
46.7
79.8
62.0
78.2
51.7
Isopropanol
10.3
a
Reaction conditions: 0.087 mol hydrogen donor and 0.022 mol glycerol, 0.25 g catalyst, 3.0 MPa N2, 200 uC, 10 h. The catalysts were reduced
b
c
under H2 for 1 h at 350 uC. Defined as (mol of converted glycerol)/(mol of exposed Cu atom)/(reaction time, h). Ethylene glycol, methanol,
1-propanol etc. Carbon balance was defined as (the total amount of 1,2-PDO, acetol, 1-propanol, ethylene glycol and glycerol in the liquid
phase)/(the amount of added glycerol). Some amount of acrolein was also detected in the gas phase. Hydrogen donor efficiency was defined
d
e
as (the amount of formed 1,2-PDO, mol)/(the amount of consumed hydrogen donor, mol).
products of ethanol. The conversion of the hydrogen donor
and the efficiency of the hydrogen donor were also calculated.
It was found that the efficiency of the tested hydrogen donor
varied from 46.7% (methanol) to 79.8% (ethanol), and ethanol
is the best hydrogen donor for this reaction. This higher
efficiency of ethanol could be attributed to that Cu is also a
good catalyst for the dehydrogenation of ethanol. There are
several works about the performance of Cu-based catalysts for
dehydrogenation of ethanol, such as, Carotenuto et al. found
ethanol can be dehydrogenated to acetaldehyde and ethyl
acetate efficiently at 200–260 uC, over a copper/copper-
chromite based catalyst in a gas–solid continuous flow
reactor.53 At the same time, they also found that the first step
was the dissociative adsorption of ethanol on the surface,
giving an adsorbed ethoxy group, then two other consecutive
steps take place to give respectively acetaldehyde as an
intermediate and ethyl acetate. Chang et al. found that ethanol
conversion depended mainly on the Cu surface area, and was
independent of calcination temperature and had little effect
on Cu loading.54
Fig. 5 shows the time course of hydrogen-free synthesis of
1,2-PDO over Cu0.4/Mg6.28Al1.32O8.26 catalyst at 200 uC. The
conversion of glycerol increased quickly to 71.4% in the first 6
h, and then increased smoothly to 91.2% (14 h). It is quite
interesting to note that the selectivity towards 1,2-PDO
increased continuously from 82.7 (2 h) to 93.5% (14 h) with
increasing the reaction time. The selectivity of acetol, the
common reported intermediate in the hydrogenolysis of
glycerol, decreased with the prolonged reaction time.
According to the published works of ethanol dehydrogena-
tion over Cu-based catalysts,53,54 glycerol hydrogenolysis34–36
and glycerol hydrogenolysis using formic acid,17 2-propanol (2-
PO),12,13,16 methanol,16 ethanol13 and glycerol6,55 as hydrogen
donors, we think that the reaction mechanism could be
deduced as in Scheme 1. The first step was the dehydration of
glycerol and acetol was formed as the intermediate product.
This step was performed mainly on the basic sites of the Cu–
Mg–Al catalyst. Then the acetol was further hydrogenated to
1,2-PDO over Cu using the active hydrogen atom originating
from the dehydrogenation of ethanol. Considering no dihy-
drogen molecule was detected in the gas phase, we suppose
that hydrogen dehydrogenated from ethanol might exist
mainly in the form of active hydrogen atom which adsorbed
on the surface of the catalyst, and then it reacts with acetol
quickly. And no hydrogen molecule was released from the
reaction mixture. On the other side, the dehydrogenation of
ethanol yielded acetaldehyde, which could be further con-
verted to ethyl acetate, 1-butanol, 1,1-diethoxyethane etc.
Apart from ethanol, butanol is also a good hydrogen donor
for this reaction at 200 uC, in which the conversion of glycerol
Fig. 5 Time courses of glycerol hydrogenolysis on Cu0.4/Mg6.28Al1.32O8.26
catalyst under N2 atmosphere. (Reaction conditions: 0.087 mol ethanol + 0.022
mol glycerol, 0.25 g reduced catalyst, 200 uC, 3.0 MPa N2).
Scheme 1 The proposed reaction mechanism of the reaction.
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Table 3 Glycerol hydrogenolysis over catalysts with different (Cu + Mg)/Al ratiosa
Selectivity (%)
1,2-PDO
Catalysts
Conv. (%)
TOF (h21 b
)
Acetol
Othersc
Carbon balanced
Cu0.4/Mg5.6Al2O8.6
Cu0.4/Mg6Al1.6O8.4
Cu0.4/Mg6.28Al1.32O8.26
Cu0.4/Mg6.46Al1.14O8.17
Mg6.68Al1.32O8.66
Cu0.4/Mg6.28Al1.32O8.26-IM
Cu0.4/Mg6.28Al1.32O8.26-IE
33.1
46.7
86.5
66.9
1.1
56.3
62.6
5.9
91.7
91.1
92.5
90.1
0.8
88.9
88.6
6.1
6.9
7.1
7.0
7.0
7.4
8.7
2.2
2.0
0.4
2.9
92.2
3.7
96.6
96.5
97.0
96.8
97.1
96.5
96.7
13.5
32.9
31.9
—
31.4
30.9
2.7
a
Reaction conditions: 0.087 mol ethanol and 0.022 mol glycerol, 0.25 g catalyst, 3.0 MPa N2, 200 uC, 10 h. The catalysts were reduced under
b
c
H2 for 1 h at 350 uC. Defined as (mol of converted glycerol)/(mol of exposed Cu atom)/(reaction time, h). Ethylene glycol, methanol,
1-propanol etc. Carbon balance was defined as (the total amount of 1,2-PDO, acetol, 1-propanol, ethylene glycol and glycerol in the liquid
d
phase)/(the amount of added glycerol). Some amount of acrolein was also detected in the gas phase.
and the selectivity of 1,2-PDO reached 83.4% and 94.8%,
respectively. Propanol, isopropanol and methanol can also be
employed as the hydrogen donors for hydrogen-free synthesis
of 1,2-PDO (see Table 2), while the detected conversions of
glycerol in these mixtures are slightly lower. The activity of
Cu0.4/Mg6.28Al1.32O8.26 in ethanol is higher than those of Ni–
Cu/Al2O3 catalyst16 or Cu:Al catalyst.56 Also, the temperature
we employed (200 uC) was lower than those published data
(220 uC).
On the basis of the above results, a series of Cu–Mg–Al
catalysts with different (Cu + Mg)/Al ratios were tested in
hydrogen-free synthesis of 1,2-PDO with ethanol hydrogen
donor at 200 uC and these results are summarized in Table 3. It
can be found that the conversion of glycerol increased from
Mg6.28Al1.32O8.26-IE were 86.5, 56.3 and 62.6%, respectively.
But the calculated TOF (on the basis of surface Cu atoms) of
these catalysts were 32.9, 31.4 and 30.9 h21, respectively. These
results indicated that the activity of these catalysts increased
with their basicity, and this conclusion fit well with our
previous works.52 The lower catalytic activity of Cu0.4
/
Mg6.28Al1.32O8.26-IM and Cu0.4/Mg6.28Al1.32O8.26-IE was ascribed
to their lower dispersion of Cu than that of Cu0.4
/
Mg6.28Al1.32O8.26. Cu dispersed highly in Cu0.4Mg5.6Al2O9, but
its TOF is the lowest (5.9 h21). However, almost no 1,2-PDO
formed over Cu-free Mg6.68Al1.32O8.66, and the conversion of
glycerol was quite low. This data and those published works on
ethanol dehydrogenation indicated that Cu was also indis-
pensable for this reaction because ethanol dehydrogenation
was performed mainly on Cu surfaces.53,54
33.1% (on Cu0.4/Mg5.6Al2O8.6
)
and 46.7% (on Cu0.4/
Mg6Al1.6O8.4) to 86.5% (on Cu0.4/Mg6.28Al1.32O8.26), the selec-
tivity of 1,2-PDO over these catalysts remained higher than
Table 4 summarizes the results of hydrogen-free synthesis of
1,2-PDO over Cu0.4/Mg6.28Al1.32O8.26 catalyst using ethanol as
the hydrogen donor at different temperatures. As was
expected, the conversion of glycerol increased with the rising
temperature. The reaction could be undertaken at 180 uC, and
the conversion of glycerol reached 95.1% at 210 uC. And the
calculated TOF of surface Cu reached 36.3 h21. At the same
time, it is interesting to note that the selectivity towards 1,2-
PDO increased from 87.5 (at 180 uC) to 92.2% (at 210 uC). The
selectivity to acetol decreased from 11.8 (at 180 uC) to 7.1% (at
200 uC), which indicated high temperature favored the
hydrogenation of acetol to 1,2-PDO.
91.0%. The calculated TOF of surface Cu over Cu0.4
/
/
Mg5.6Al2O8.6 was 5.9 h21
,
but this value for Cu0.4
Mg6.28Al1.32O8.26 increased to 32.9 h21. The lower activity of
Cu0.4/Mg6.46Al1.14O8.17 might be attributed to its low surface
area and/or large Cu particles formed. In order to separate the
role of basic sites and Cu dispersion, Cu0.4/Mg6.28Al1.32O8.26-IM
(prepared by impregnation) and Cu0.4/Mg6.28Al1.32O8.26-IE
(prepared by ion-exchange) were prepared and tested in this
reaction. The conversions of glycerol over Cu0.4
Mg6.28Al1.32O8.26 Cu0.4/Mg6.28Al1.32O8.26-IM and Cu0.4
/
/
,
Table 4 Hydrogenolysis of glycerol at different temperatures over Cu0.4/Mg6.28Al1.32O8.26 catalysta
Selectivity (%)
Temperature (uC)
Conv. (%)
TOF (h21 b
)
1,2-PDO
Acetol
Othersc
Carbon balanced
180
190
200
210
43.2
73.4
86.5
95.1
16.5
27.9
32.9
36.3
87.5
92.3
92.5
92.2
11.8
7.2
7.1
7.4
0.7
0.5
0.4
0.4
97.3
97.1
97.0
96.5
a
Reaction conditions: 0.087 mol ethanol and 0.022 mol glycerol, 0.25 g catalyst, 3.0 MPa N2, 10 h. The catalysts were reduced under H2 for 1
b
c
h at 350 uC. Defined as (mol of converted glycerol)/(mol of exposed Cu atom)/(reaction time, h). Ethylene glycol, methanol, 1-propanol etc.
d
Carbon balance was defined as (the total amount of 1,2-PDO, acetol, 1-propanol, ethylene glycol and glycerol in the liquid phase)/(the
amount of added glycerol). Some amount of acrolein was also detected in the gas phase.
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a
Table 5 The influence of ethanol/glycerol molar ratio over Cu0.4/Mg6.28Al1.32O8.26
Selectivity (%)
1,2-PDO
Ethanol/glycerol molar ratio
TOFb (h21
)
Conv. (%)
Acetol
Othersc
Carbon balanced
4
8
12
32.9
34.4
35.8
86.5
90.3
93.9
92.5
92.8
93.1
7.1
6.7
6.5
0.4
0.5
0.4
97.0
97.2
96.9
a
Reaction conditions: 0.022 mol glycerol, 0.25 g reduced catalyst, 3.0 MPa N2, 200 uC, 10 h. The catalysts were reduced under H2 for 1 h at
b
c
350 uC. Defined as (mol of converted glycerol)/(mol of exposed Cu atom)/(reaction time, h). Ethylene glycol, methanol, 1-propanol etc.
d
Carbon balance was defined as (the total amount of 1,2-PDO, acetol, 1-propanol, ethylene glycol and glycerol in the liquid phase)/(the
amount of added glycerol). Some amount of acrolein was also detected in the gas phase.
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A series of Cu–Mg–Al catalysts with different (Cu + Mg)/Al
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Cu0.4/Mg6.28Al1.32O8.26 reached 95.1% at 210 uC, and the
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Acknowledgements
This research work was supported by the National Natural
Science Foundation of China (Contract No. 21273198,
21073159), Zhejiang Provincial Natural Science Foundation
(Grant No. LZ12B030001) and the Pre-research Ocean
Foundation of Zhejiang University (2012HY025B).
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